WO2024131415A1

WO2024131415A1 - Tammann temperature-guided synthesis of core@shell catalysts

Info

Publication number: WO2024131415A1
Application number: PCT/CN2023/132881
Authority: WO
Inventors: Molly Meng-Jung LI; Shu Ping Lau; Ye Zhu; Pei XIONG; Zhi hang XU
Original assignee: The Hong Kong Polytechnic University
Priority date: 2022-12-22
Filing date: 2023-11-21
Publication date: 2024-06-27

Abstract

A method of preparing core-shell nanoparticles, the method comprising: contacting a mixture comprising a first metal oxide, a first encapsulating material comprising an alkali metal or alkaline earth metal, and a second encapsulating material comprising a second metal oxide with hydrogen gas at a temperature higher than the hydrogen reduction temperature of the first metal oxide and lower than the decomposition temperature of the first encapsulating material resulting in the reduction the first metal oxide and the formation of metal nanoparticles that are at least partially encapsulated with a bimetallic oxide comprising the alkali metal or the alkaline earth metal and the second metal oxide thereby forming the core-shell nanoparticles, wherein the Tammann temperature of the first encapsulating material is lower than the hydrogen reduction temperature of the first metal oxide.

Description

TAMMANN TEMPERATURE-GUIDED SYNTHESIS OF CORE@SHELL CATALYSTS

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. Provisional Patent Application No. 63/434,899, filed on December 22, 2022, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure generally relates to a method of preparing core@shell catalysts with improved stability.

BACKGROUND

Transition metal nanoparticles (M_T NPs) exhibit excellent activities for many important catalytic reactions, such as methane (CH₄) or ammonia (NH₃) conversions, yet stability drawbacks hinder their practical applications. This is because the highly active surface atoms are generally thermodynamically unstable, leading to severe NPs sintering via secondary nucleation, recrystallization, ripening, particle migration, and coalescence. To preserve the NPs’ stability, different approaches have been developed. Among them, the core@shell architecture offers advantageous structure by isolating the active nanoparticle cores with outer shells to prevent sintering during high-temperature catalytic reactions. The most commonly used core@shell synthesis method is to physically confine the NPs in porous materials (e.g., silica, alumina, zeolites, etc. ) . Despite various choices and vast sources of confining materials, the limitation of active site blockage by the inert supports is often observed, thereby diminishing the catalytic performance.

Alternatively, strong metal-support interactions (SMSI) have been engineered to produce core@shell architecture for stabilizing and modifying active metal NPs. For example, on the important industrial Cu/ZnO/Al₂O₃ methanol synthesis catalyst, it is found that the partially reduced zinc oxides can interact with metallic Cu NPs via SMSI effect, forming a protective layer to prevent the sintering of Cu NPs. Due to the unique geometric and sintering-resistant interfacial interactions, various SMSI metal and support compositions have been explored and studied. It is observed that at the metal-support interfaces, the reducible metal oxide substrates (e.g., TiO₂, V₂O₃, CeO₂, and Ta₂O₅) can be thermally activated and migrated onto metal NPs surface (typically platinum group metals like Pt, Pd, etc. ) under hydrogen (H₂) atmosphere at high temperature, forming complete encapsulation of the NPs by the oxides. Such unique SMSI metal@support configurations not only dramatically improve the thermodynamic stability of the NPs against sintering but also impart tunable versatility of the catalytic properties via modulating the core@shell interface.

Unfortunately, the high temperature required in the thermal activation step of SMSI restricts the application of this strategy to NPs with low melting points. For those with inherently low melting temperatures, such as Fe, Co, Ni, Cu, Au, etc., the NPs tend to sinter before the encapsulation occurs. Hence, to achieve successful encapsulation of M_T NPs through SMSI, it is critical to activate the supporting metal oxides with sufficient mobility before the M_T NPs sinter. Several successes have been reported, such as HCO_x-adsorbate-mediated SMSI encapsulation on TiO₂-and Nb₂O₅-supported Rh NPs, CO₂-enhanced redox-inert MgO encapsulation on Au NPs, and the adsorbate-induced SMSI reconstruction of the commercial Cu/ZnO/Al₂O₃ that maximizes the activity and stability. However, these methods are highly material-specific and thus hard to be extended to other systems. Therefore, there is still a need for general methods for core@shell synthesis that address or overcome at least some of the disadvantages described above.

SUMMARY

Provided herein is a general and efficient approach for achieving thermal-induced SMSI encapsulation on M_T NPs. The approach involves utilizing the empirical Tammann temperature (T_Tam) phenomenon, which describes the atoms in the solid-state materials becoming loosened with higher mobility and reactivity at temperatures higher than c. a. 1/2 of the melting point. The advantage of the low T_Tam compounds that can be thermally activated and reacted at relatively low temperatures is leveraged. Once a coating of low T_Tam compounds is formed on M_T NPs, further overlayer functionalization can be realized through a solid-state reaction, resulting in the formation of the highly thermally stable protective layer of the catalyst support.

In a first aspect, provided herein is a method of preparing core-shell nanoparticles, the method comprising: contacting a mixture comprising a first metal oxide, a first encapsulating material comprising an alkali metal or an alkaline earth metal, and a second encapsulating material comprising a second metal oxide with hydrogen gas at a temperature higher than the hydrogen reduction temperature of the first metal oxide and lower than the decomposition temperature of the first encapsulating material resulting in the reduction the first metal oxide and the formation of metal nanoparticles that are at least partially encapsulated with a bimetallic oxide comprising the alkali metal or the alkaline earth metal and the second metal oxide thereby forming the core-shell nanoparticles, wherein the Tammann temperature of the first encapsulating material is lower than the hydrogen reduction temperature of the first metal oxide.

In certain embodiments, the method further comprises providing screening data of changes in enthalpy of possible reaction products of a reaction of a candidate first metal oxide, a candidate first encapsulating material, a candidate second encapsulating material, and hydrogen gas; and selecting the first encapsulating material and the second encapsulating material based on the screening data and the decomposition temperature of the candidate first encapsulating material.

In certain embodiments, the screening data is generated using a computer.

In certain embodiments, the method further comprises selecting the first encapsulating material and the second encapsulating material based on the hydrogen reduction temperature of the first metal oxide; the decomposition temperature of the first encapsulating material and the Tammann temperature of the first encapsulating material.

In certain embodiments, the first metal oxide comprises a metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Se, Sn, Pt, Ru, Pd, W, Ir, Os, Rh, Nb, Ta, Pb, Bi, Au, Ag, Sc, and Y.

In certain embodiments, the first metal oxide is selected from the group consisting of CoO₃, Co₃O₄, CoO, CuO, Cu₂O, NiO, Ni₂O₃, Ag₂O, AgO, FeO, Fe₂O₃, Fe₃O₄, ZnO, PdO, PdO₂, PtO, and PtO₂.

In certain embodiments, the first encapsulating material comprises an alkali metal or an alkaline earth metal salt comprising one or more anions selected from the group consisting of carbonate, sulfate, nitrate, chloride, phosphate, and acetylacetonate.

In certain embodiments, the first encapsulating material comprises CaCO₃, MgCO₃, SrCO₃, BaCO₃, NaCl, KCl, RbCl, Na₂CO₃, K₂CO₃, or Rb₂CO₃.

In certain embodiments, the second metal oxide comprises Al₂O₃, TiO₂, silica, Ga₂O₃, In₂O₃, ZrO₂, or a mixture thereof.

In certain embodiments, the first metal oxide is selected from the group consisting of CoO₃, Co₃O₄, CuO, Cu₂O, NiO, Ni₂O₃, Ag₂O, AgO, FeO, Fe₂O₃, Fe₃O₄, ZnO, PdO, PdO₂, PtO, and PtO₂.

In certain embodiments, the bimetallic oxide comprises MgAl₂O₄, CaAl₂O₄, SrAl₂O₄, BaAl₂O₄, MgTiO₃, CaTiO₃, SrTiO₃, BaTiO₃, MgGa₂O₄, CaGa₂O₄, SrGa₂O₄, BaGa₂O₄, MgIn₂O₄, CaIn₂O₄, SrIn₂O₄, BaIn₂O₄, MgZrO₃, CaZrO₃, SrZrO₃, or BaZrO₃.

In certain embodiments, each of the core-shell nanoparticles comprise a shell comprising MgAl₂O₄, CaAl₂O₄, SrAl₂O₄, BaAl₂O₄, MgTiO₃, CaTiO₃, SrTiO₃, BaTiO₃, MgGa₂O₄, CaGa₂O₄, SrGa₂O₄, BaGa₂O₄, MgIn₂O₄, CaIn₂O₄, SrIn₂O₄, BaIn₂O₄, MgZrO₃, CaZrO₃, SrZrO₃, or BaZrO₃; and a core comprising Co, Cu, Ni, Ag, Fe, Zn, Pd, or Pt.

In certain embodiments, the core-shell nanoparticles are selected from the group consisting of Co@BaAl₂O₄, Cu@BaAl₂O₄, Ni@BaAl₂O₄, Co@BaTiO₃, Fe@MgGa₂O₄, Co@MgGa₂O₄, and Ni@BaZrO₃.

In certain embodiments, the core-shell nanoparticles have an average diameter of 10-15 nm.

In certain embodiments, the method further comprises calcining the mixture prior to the step of contacting the mixture with hydrogen gas.

In certain embodiments, calcining is conducted below the decomposition temperature of the first encapsulating material.

In certain embodiments, the method further comprises contacting a first metal nitrate, an alkali metal or an alkaline earth metal nitrate, a second metal nitrate, and an alkali metal carbonate thereby forming a coprecipitate; and calcining the coprecipitate thereby forming the mixture.

In certain embodiments, the method further comprises providing screening data of changes in enthalpy of possible reaction products of a reaction of a candidate first metal oxide, a candidate first encapsulating material, a candidate second encapsulating material, and hydrogen gas; and selecting the first encapsulating material and the second encapsulating material based on the screening data and the decomposition temperature of the candidate first encapsulating material; and selecting the first encapsulating material and the second encapsulating material based on the hydrogen reduction temperature of the first metal oxide; the decomposition temperature of the first encapsulating material and the Tammann temperature of the first encapsulating material.

Core-shell nanoparticles prepared in accordance with the methods described herein exhibit high thermal resistance, good hydrothermal stability, and excellent catalytic properties in a broad range of catalytic reactions, such as catalytic ammonia decomposition, methane dry reforming, methane steam reforming, methanol reforming, CO₂ hydrogenation, selective catalytic reduction, etc.

Herein, M_T NPs are synthesized by in-situ reduction from the corresponding M_T oxides (M_TO_x) to avoid the troublesome multi-step processes and handling of active M_T NPs materials. To establish SMSI encapsulation, the T_Tam as an indicator is considered to select suitable encapsulating materials, i.e., supports, for the targeted metal catalysts. More specifically, as shown in Fig. 1a, the T_Tam of the encapsulating materials, denoted as T_Tam (support) , should be lower than the reduction temperature (T_red) of M_TO_x, denoted as T_red (M_TO_x) , so encapsulation can occur during the M_T NP formation at T_red (M_TO_x) to inhibit M_T particle overgrowth. Meanwhile, the decomposition temperature (T_dec) of the encapsulating materials, denoted as T_dec (support) , must be higher than the T_red (M_TO_x) to avoid the loss of high mobility due to the phase/structure change. Based on these considerations, if one support material can fit in the simple rule of T_Tam (support) < T_red (M_TO_x) < T_dec (support) , then it is expected to exhibit high mobility to achieve SMSI encapsulation on M_T NPs at T_red (M_TO_x) .

As a proof-of-concept demonstration, low T_Tam alkaline earth metal (common catalyst promoters) in the carbonate form (M_AECO₃) were selected as the candidate support. The phase-changing temperatures of the common non-noble M_TO_x (M_T = Fe, Co, Ni, Cu) and M_AECO₃ (M_AE = Mg, Ca, Sr, Ba) combinations are mapped in Fig. 1b. The T_dec (M_AECO₃) and T_red (M_TO_x) are measured by H₂ temperature-programmed decomposition and reduction experiments, respectively (see details in Figs. 10-11 and Fig. 36) , while the T_Tam (M_AECO₃) are retrieved from literature. In Fig. 1b, the region sandwiched by T_dec (M_AECO₃) and T_Tam (M_AECO₃) (shadowed area) indicates the suitable temperature window (or encapsulable window) to theoretically allow for the facile SMSI encapsulation on the M_T NPs if the corresponding T_red (M_TO_x) falls into this range. A test synthesis on the Co and BaCO₃ combination has been used to demonstrate this concept. Uniformly distributed Co NPs encapsulated in BaCO₃ (i.e., Co@BaCO₃) were obtained through one-step thermal treatment at the temperature around T_red (Co₃O₄) in H₂ atmosphere (see details in Fig. 1c, Fig. 12 and Fig. 38) . This result demonstrates the viability of the proposed strategy for convenient thermal encapsulation of M_T NPs guided by Tammann temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated by reference to the following detailed description, when taken in conjunction with the accompanying drawings.

Figure 1 Theoretical synthetic rationale for the thermal encapsulation based on the Tammann temperature. a, Schematic illustration of the designed synthesis rationale for this thermal-induced encapsulation strategy guided by Tammann temperature. b, Schematic of the estimated suitable temperature range (or encapsulable window, shadowed) to enable encapsulation on in-situ formed M_T NPs with M_AECO₃ through thermal-induced SMSI. c, TEM, STEM images and element distribution maps of Co@BaCO₃.

Figure 2 Metal encapsulation and overlayer functionalization from one-step thermal treatment. a, Schematic illustration of the subsequent overlayer functionalization step following the thermal-induced SMSI encapsulation. b, Rapid computational screening of reaction enthalpy changes (ΔH) for forming possible products from solid-state reactions between BaCO₃, Al₂O₃ and Co₃O₄ in H₂. ΔH were obtained from the Materials Project database. The lower the ΔH, the more stable the reaction product. c, STEM image and element distribution maps of Co@BaAl₂O₄. d, EXAFS fitting curves in R-space for the Co K-edge (upper panel) and Ba L₃-edge (lower panel) of Co@BaAl₂O₄. e, Rietveld refinement of the SXRD patternof Co@BaAl₂O₄.

Figure 3 Real-time observation and mechanism study for the encapsulation. a-d, In-situ STEM images at the initial state at room temperature (a) , BaCO₃ diffusion stage when the temperature is above T_Tam (BaCO₃) (b) (the dotted lines represent the expansion of the BaCO₃ boundary, and the dashed circles show the vacancies inside the BaCO₃) , Co NP formation stage when the temperature is above the T_red (Co₃O₄) (c) (the circles indicate Co nucleation and growth) , and Co@BaAl₂O₄ formation stage after the solid-state reaction between BaCO₃ and Al₂O₃ (d) . e, Schematic illustration of the mechanism for this thermal-induced encapsulation strategy guided by Tammann temperature. f, XRD patterns of the 40Co: 20Ba: 40Al reduced at different temperatures ranging from room temperature to 700 ℃. g, In-situ Raman spectra (left panel) of the 40Co: 20Ba: 40Al reduced at different temperatures ranging from room temperature to 700 ℃, and MS signals (right panel) of the outlet gas monitored during the H₂ reduction process (tracking m/z = 2 for H₂, m/z = 18 for H₂O and m/z = 28 for CO) .

Figure 4 The encapsulated nanocatalyst enables excellent catalytic performance. a, Comparison of H₂ production rate at 500 ℃ between the Co@BaAl₂O₄ and the selected core@shell catalysts used for NH₃ decomposition reaction (details see Fig. 46) . b, H₂ production rate according to time-on-stream during the NH₃ decomposition reaction over the core@shell Co@BaAl₂O₄, and the non-encapsulated catalysts including Co/Al₂O₃, and Co/BaAl₂O₄ samples prepared by co-precipitation and polyol method. The performance was evaluated at weight hourly space velocity (WHSV) of 30,000 mL g_cat ^-1 h^-1. c, Co NP size distribution histograms of the non-encapsulated Co/BaAl₂O₄-coprecipitation before and after 80-hour NH₃ decomposition reaction, inset shows the TEM images of corresponding samples. d, Co NP size distribution histograms of core@shell Co@BaAl₂O₄ before and after 80-hour NH₃ decomposition reaction, inset shows the TEM images of corresponding samples. e, STEM images (upper panel) and element distribution maps (lower panel) of Cu@MgAl₂O₄, Ni@BaAl₂O₄, and Co@BaTiO₃.

Figure 5 Precursors preparation via co-precipitation synthesis method and thermal treatment. Schematic representation of the mM_T: nM_AE: 2nAl precursor synthesis procedure and subsequent thermal treatment process.

Figure 6 XRD measurement of air-sensitive samples. Air-tight XRD holder for air-sensitive samples.

Figure 7 In-situ Raman setup. (a) in-situ Raman gas cell, with gas flow directions marked. (b-c) Experimental setup for the in-situ example showing the gas cell mounted on the stage of a Raman imaging microscope. (d) Temperature and flow control units of the in-situ Raman setup.

Figure 8 In-situ STEM setup. (a) Schematic showing the assembly of the Protochips Atmosphere gas holder. (b) An E-chip used in the Protochips Atmosphere gas holder. (c) Atmosphere system for the in-situ STEM experiment.

Figure 9 Schematic of the NH₃ decomposition performance testing setup.

Figure 10 Determination of the decomposition temperature (T_dec) of M_AECO₃. MS signals during the H₂ temperature-programmed decomposition of (a) MgCO₃, (b) CaCO₃, (c) SrCO₃, and (d) BaCO₃. When it reached 800 ℃, the temperature was kept at 800 ℃ for 3 hours (950 ℃ for BaCO₃) . The unshadow areas (x = 50 to 800 ℃) show the MS signals as a function of temperature. The blue-shadow areas (x = 0 to 180 min) show the MS signals as a function of the retention time at 800℃ (950 ℃ for BaCO₃) . The maximum of each decomposition profile is determined as the corresponding T_dec (M_AECO₃) .

Figure 11 Determination of the reduction temperature (T_red) of M_TO_x. MS signals during the H₂ temperature-programmed reduction of (a) Fe₂O₃, (b) Co₃O₄, (c) NiO, and (d) CuO. When it reached 800 ℃, the temperature was kept at 800 ℃ for 3 hours (950 ℃ for Fe₂O₃) . The unshadow areas (x = 50 to 800 ℃) show the MS signals as a function of temperature. The blue-shadow areas (x = 0 to 180 min) show the MS signals as a function of the retention time at 800℃ (950 ℃ for Fe₂O₃) . The orange-shadow areas are the literature-reported H₂ temperature-programmed reduction temperature ranges, indicating that our measured values are all in reasonable ranges according to previous reports (Detailed reported data can be found in Fig. 37) . The maximum of each reduction profile is determined as the corresponding T_red (M_TO_x) .

Figure 12 Proof-of-concept demonstration by constructing Co@BaCO₃ encapsulation configuration. Structure and configuration identification of C (500) -R (500) -67Co: 33Ba: (a) XRD pattern, and (b) Co NP size distribution measured from low magnification TEM images.

Figure 13 High-throughput screening of the possible products from solid-state reactions. All (upper panel) and top 20 (lower panel) possible products and corresponding reaction enthalpy change (ΔH) of the solid-state reactions between BaCO₃ + Al₂O₃ + Co₃O₄ +H₂.

Figure 14 Construction of Co@BaAl₂O₄ encapsulation configuration. (a) Upper panels: XRD pattern of C (500) -40Co: 20Ba: 40Al. Lower panels: XRD pattern of C (500) -R (700) -40Co: 20Ba: 40Al. It can be observed that Co₃O₄ has been reduced to Co NPs during the H₂ thermal treatment, and BaAl₂O₄ is formed via the solid-state reaction between BaCO₃ and Al₂O₃. (b) Size distributions of Co NPs measured from low magnification TEM images of C (500) -R (700) -40Co: 20Ba: 40Al.

Figure 15 Morphology and structural investigation of Co@BaAl₂O₄ from TEM. (a) High-resolution TEM image of Co@BaAl₂O₄. The dashed boxes denote the regions in which the fast Fourier transform (FFT) was performed. (b) FFT pattern calculated from the dashed box, which shows the fcc Co phase diffraction pattern along [011] zone axis. (c) FFT pattern calculated from the dashed box, which shows the BaAl₂O₄ phase diffraction pattern along zone axis.

Figure 16 Electronic properties and local structures of Co in Co@BaAl₂O₄. (a) Co K-edge XANES spectra of 40Co: 20Ba: 40Al, C (500) -40Co: 20Ba: 40Al, and C (500) -R (700) -40Co: 20Ba: 40Al, along with the Co foil for comparison purpose. (b) k³-weighted Co K-edge Fourier transform (FT) EXAFS of 40Co: 20Ba: 40Al, C (500) -40Co: 20Ba: 40Al, and C (500) -R (700) -40Co: 20Ba: 40Al. (c) Co K-edge EXAFS fitting curves of C (500) -40Co: 20Ba: 40Al. (d) Co K-edge EXAFS fitting curves of C (500) -R (700) -40Co: 20Ba: 40Al. (e-h) WT representations of the Co K-edge EXAFS signals for: (e) C (500) -40Co: 20Ba: 40Al, (f) C (500) -R (700) -40Co: 20Ba: 40Al, (g) theoretical Co₃O₄ model, and (h) Co foil standard.

Figure 17 Electronic properties and local structures of Ba in Co@BaAl₂O₄. (a) Ba L₃-edge XANES spectra of 40Co: 20Ba: 40Al, C (500) -40Co: 20Ba: 40Al, and C (500) -R (700) -40Co: 20Ba: 40Al. (b) k³-weighted Ba L₃-edge Fourier transform (FT) EXAFS of 40Co: 20Ba: 40Al, C (500) -40Co: 20Ba: 40Al, and C (500) -R (700) -40Co: 20Ba: 40Al. (c) Ba L₃-edge EXAFS fitting curves of 40Co: 20Ba: 40Al. (d) Ba L₃-edge EXAFS fitting curves of C (500) -40Co: 20Ba: 40Al. (e) Ba L₃-edge EXAFS fitting curves of C (500) -R (700) -40Co: 20Ba: 40Al. (f-j) WT representations of the Ba L₃-edge EXAFS signals for: (f) 40Co: 20Ba: 40Al, (g) C (500) -40Co: 20Ba: 40Al, (h) C (500) -R (700) -40Co: 20Ba: 40Al, (i) theoretical BaCO₃ model, and (j) theoretical BaAl₂O₄ model.

Figure 18 Oxygen vacancies in the BaAl₂O₄ overlayer of Co@BaAl₂O₄. (a) XPS spectra of the O1s core level for Co@BaAl₂O₄, which shows a peak at binding energies of 532.2 eV attributing to the oxygen vacancies in the BaAl₂O₄ lattice. (b) EPR spectra of BaAl₂O₄, which shows a signal with the splitting factor values (g) of 1.9806, corresponding to oxygen vacancies with a single trapped electron (V_O ⁺) . (c) Ar adsorption-desorption isotherms of Co@BaAl₂O₄ and pristine BaAl₂O₄. (d) The density functional theory (DFT) pore size distribution curves of Co@BaAl₂O₄ and pristine BaAl₂O₄.

Figure 19 Necessity of low-T_Tam M_AECO₃ as the facilitator. (a-d) Structure and configuration identification of Co/BaAl₂O₄-coprecipitation: (a) XRD pattern of Co/BaAl₂O₄-coprecipitation, (b-c) TEM images of Co/BaAl₂O₄-coprecipitation, and (d) Co NP size distribution of Co/BaAl₂O₄-coprecipitation measured from low magnification TEM images. (e-h) Structure and configuration identification of Co/BaAl₂O₄-polyol: (e) XRD pattern of Co/BaAl₂O₄-polyol, (f-g) TEM images of Co/BaAl₂O₄-polyol, and (h) Co NP size distribution of Co/BaAl₂O₄-polyol measured from low magnification TEM images.

Figure 20 Elemental composition analyses during the synthesis process. (a) SEM image of 40Co: 20Ba: 40Al, (b) SEM image of C (500) -40Co: 20Ba: 40Al, and (c) SEM image of C (500) -R (700) -40Co: 20Ba: 40Al. (d) EDX chemical composition analyses of 40Co: 20Ba: 40Al, (e) EDX chemical composition analyses of C (500) -40Co: 20Ba: 40Al, and (f) EDX chemical composition analyses of C (500) -R (700) -40Co: 20Ba: 40Al. It can be observed that the atomic ratio of Co, Ba, and Al remains unchanged at about 2: 1: 2 in each step during the whole synthesis process without significant loss, which is consistent with the original precursor metal composition.

Figure 21 Morphology evolution during the synthesis process. TEM images of the samples at different stages of the synthesis process (i.e., as-precipitated, as-calcined, and thermally treated in H₂) .

Figure 22 Morphology of mix-metal precursor before H₂ thermal treatment. (a) Low magnification TEM image of the precursor C (500) -40Co: 20Ba: 40Al. (b) TEM image of BaCO₃ region. Inset is the corresponding FFT pattern of the whole area, which corresponds to the (230) , (111) , (220) , and (011) planes of BaCO₃ phase. (c-d) High-resolution TEM images of Co/Al oxides region. Inset of (d) is the FFT pattern calculated from marked Al₂O₃ rods, which shows the Al₂O₃ phase diffraction pattern alongzone axis.

Figure 23 Real-time observation of the element distribution evolution by in-situ STEM-EDX. EDX mapping in the same area of (a) the as-calcined precursor C (500) -40Co: 20Ba: 40Al, (b) the specimen after 500 ℃ in-situ reduction for 30 min, and (c) the specimen after 800 ℃ in-situ reduction for 30 min. (d) EELS mapping and corresponding selected area electron diffraction (SAED) pattern of the specimen after 800 ℃ in-situ reduction for 30 min.

Figure 24 Evaluation of the electron beam influence on the observed sample. (a) EDX mapping of the mixed metal precursor outside the in-situ tracked region. (b) EELS mapping of the specimen outside the in-situ tracked region after 800 ℃ reduction for 30 min.

Figure 25 The reliability of NH₃ decomposition performance testing equipment and the accuracy of product composition analyses. (a) NH₃ conversions comparison between reported benchmark catalysts and our verification experiment results. (b) NH₃ conversions obtained from MS analyses and titration analyses.

Figure 26 Catalytic performances on NH₃ decomposition of Co@BaAl₂O₄ catalysts. NH₃ decomposition conversion and H₂ production rate over Co@BaAl₂O₄ catalysts as a function of reaction temperature. The performance was evaluated at weight hourly space velocity (WHSV) of 30,000 mL g_cat ^-1 h^-1 and 1 atm.

Figure 27 Co@BaAl₂O₄ represents one of the most effective catalysts in both non-noble and noble catalysts records. Comparison of NH₃ conversion between the Co@BaAl₂O₄ (red balls) and the reported Ru-based and non-noble metal-based catalysts. The sizes of the balls represent the value of WHSV. The larger the ball diameter, the higher the WHSV. Detailed data can be found in Fig. 47.

Figure 28 S24. Sintering of non-encapsulation structure in harsh thermal environments: Co/BaAl₂O₄-coprecipitation sample. Structure and configuration identification of Co/BaAl₂O₄-coprecipitation (see Fig. 21 for synthesis details) before (blue) and after (green) 80-hour NH₃ decomposition reaction: (a) XRD patterns before and after the reaction. (b-c) TEM images before the reaction. (d) Co NP size distribution before the reaction measured from low magnification TEM. (e-f) TEM images after the reaction. (g) Co NP size distribution after the reaction measured from low magnification TEM images.

Figure 29 Sintering of non-encapsulation structure in harsh thermal environments: Co/Al₂O₃ sample. Structure and configuration identification of Co/Al₂O₃ before and after 80-hour NH₃ decomposition reaction: (a) XRD patterns before and after the reaction. (b-c) TEM images before the reaction. (d) Co NP size distribution before the reaction measured from low magnification TEM. (e-f) TEM images after the reaction. (g) Co NP size distribution after the reaction measured from low magnification TEM images. The Co/Al₂O₃ was synthesised by the following steps: (i) co-precipitation of Co (NO₃) ₂ and Al (NO₃) ₃ with the molar ratio of 2: 3, i.e., 40Co: 60Al; (ii) air calcination at 500 ℃, i.e., C (500) -40Co: 60Al; (iii) H₂ thermal treatment at 500 ℃, i.e., C (500) -R (500) -40Co: 60Al.

Figure 30 Structural stability of Co@BaAl₂O₄ with encapsulation structure in harsh thermal environments. (a) XRD pattern of Co@BaAl₂O₄ catalysts before (blue) and after 80-hour NH₃ decomposition reaction, (b) Co3p XPS spectra of Co@BaAl₂O₄ catalysts before and after 80-hour NH₃ decomposition reaction, blank circles represent experimental XPS data, and solid lines are the convolution of Gaussians and Lorentzians, (c) Ba4d XPS spectra of Co@BaAl₂O₄ catalysts before and after 80-hour NH₃ decomposition reaction, blank circles represent experimental XPS data, and solid lines are the convolution of Gaussians and Lorentzians, and (d) k³-weighted Fourier transforms (FT) Co K-edge (upper panel) and Ba L₃- edge (lower panel) EXAFS of post-reaction Co@BaAl₂O₄ and corresponding EXAFS fitting: circles represent experimental data, and solid lines represent the EXAFS fitting results.

Figure 31 Excellent environment tolerance and reusability for practical application. NH₃ conversion during the five-month-long testing events, including temperature fluctuations, system ONs/OFFs, and catalyst offload/reload.

Figure 32 Good stability of Co@BaAl₂O₄ with encapsulation structure in CH₄ dry-reforming reaction. CH₄ conversion, CO₂ conversion, H₂ selectivity and H₂/CO ratio in different time-on-stream during the CH₄ dry-reforming reaction over the Co@BaAl₂O₄ catalyst. The performance was evaluated at WHSV of 31, 765 mL g_cat ^-1 h^-1 and a temperature of 750 ℃. The molar ratio of CH₄: CO₂: Ar in inlet gas is 0.45: 0.45: 0.10. The dotted lines indicate the thermodynamic equilibrium of CH₄ and CO₂ conversions.

Figure 33 Ni@BaAl₂O₄ core@shell synthesis. (a) All (upper panel) and top 20 (lower panel) possible products and corresponding ΔH of the solid-state reactions between BaCO₃ +Al₂O₃ + NiO + H₂. (b) XRD pattern of C (500) -40Ni: 20Ba: 40Al and Ni@BaAl₂O₄. (c) Size distribution of Ni NPs measured from low magnification TEM images of Ni@BaAl₂O₄.

Figure 34 Cu@MgAl₂O₄ core@shell synthesis. (a) All (upper panel) and top 20 (lower panel) possible products and corresponding reaction enthalpy change (ΔH) of the solid-state reactions between MgCO₃ + Al₂O₃ + CuO + H₂. (b) XRD pattern of C (300) -40Cu: 20Mg: 40Al and Cu@MgAl₂O₄. (c) Size distribution of Cu NPs measured from low magnification TEM images of Cu@MgAl₂O₄.

Figure 35 Co@BaTiO₃ core@shell synthesis. (a) All (upper panel) and top 20 (lower panel) possible products and corresponding reaction enthalpy change (ΔH) of the solid-state reactions between BaCO₃ + TiO₂ + CoO + H₂. (b) XRD pattern of C (500) -50Co: 25Ba: 25Ti and Co@BaTiO₃. (c) Size distribution of Co NPs measured from low magnification TEM images of Co@BaTiO₃.

Figure 36 Melting points (T_melt) and Tammann temperatures (T_Tam) of M_AECO₃ (M_AE = Mg, Ca, Sr, Ba) and M_T (M_T = Fe, Co, Ni, Cu) retrieved from literature. The experimental decomposition temperatures (T_dec) of M_AECO₃ and reduction temperatures (T_red) of M_TO_x. ^*The crystalline lattice of a solid compound maintains constant vibrational motion at normal room temperature. When the temperature increases, the ions’ motion amplitude increases until they reach the melting point and transition into a liquid phase. Tammann temperature (T_Tam) is an absolute temperature at which atoms in the crystal lattice of a solid bulk material become ‘loosened’ and, therefore, more reactive or susceptible to diffusion by other molecules. T_Tam is reported as approximately half of a compound’s absolute melting point (T_melt) .

Figure 37 Reduction temperatures of M_TO_x to M_T NPs in H₂ temperature-programmed reduction spectra retrieved from literature.

Figure 38 Estimation of Co NP crystallite size in C (500) -R (500) -67Co: 33Ba. 67Co: 33Ba precursor was synthesized by co-precipitating Co (NO₃) ₂ and Ba (NO₃) ₂ solutions in BaCO₃ solutions, followed by calcination in air at 500 ℃ and thermal treatment in H₂ at 500 ℃. The final obtained product was denoted C (500) -R (500) -67Co: 33Ba. Fig. 12a shows the XRD pattern of C (500) -R (500) -67Co: 33Ba, in which BaCO₃ and fcc Co are observed, indicating that Co₃O₄ has been reduced to fcc Co while keeping BaCO₃ undecomposed.

Figure 39 Estimation of Co NP crystallite sizes.

Figure 40 Co K-edge EXAFS fitting parameters of C (500) -40Co: 20Ba: 40Al and C (500) -R (700) -40Co: 20Ba: 40Al. *Co K-edge EXAFS fitting results, in which CN is the average coordination number, R is the distance from the absorber atom, and σ² the Debye–Waller factor. R-factor denotes the quality factor of the fitting, and ΔE₀ the energy shift from the absorption edge energy E₀.

Figure 41 Coordination environments of the absorbing Co atoms in theoretical Co₃O₄ and Co metal model.

Figure 42 Ba L₃-edge EXAFS fitting parameters of 40Co: 20Ba: 40Al, C (500) -40Co: 20Ba: 40Al, and C (500) -R (700) -40Co: 20Ba: 40Al.

Figure 43 Coordination environments of the absorbing Ba atoms in theoretical BaCO₃ and BaAl₂O₄ models.

Figure 44 Estimation of Co NP crystallite sizes in Co/BaAl₂O₄-coprecipitation and Co/BaAl₂O₄-polyol.

Figure 45 The atomic ratio of Co, Ba, and Al elements in 40Co: 20Ba: 40Al, C (500) -40Co: 20Ba: 40Al, and C (500) -R (700) -40Co: 20Ba: 40Al measured by SEM-EDX.

Figure 46 Catalytic performances on NH₃ decomposition of Co@BaAl₂O₄ and other reported core@shell catalysts at 1 atm.

Figure 47 Selected catalysts from literature with high NH₃ decomposition performances (NH₃ conversion ≥ 95%) at 1 atm and high WHSV (> 15,000 mL g_cat ^-1 h^-1) .

Figure 48 Estimation of Co NP crystallite sizes in Co/BaAl₂O₄-coprecipitation before and after 80-hour NH₃ decomposition reaction.

Figure 49 Estimation of Co NP crystallite size in Co/Al₂O₃ before and after 80-hour NH₃ decomposition reaction.

Figure 50 Estimation of Co NP crystallite sizes in Co@BaAl₂O₄ before and after 80-hour NH₃ decomposition reaction.

Figure 51 Co K-edge EXAFS fitting parameters of post-reaction Co@BaAl₂O₄.

Figure 52 Ba L₃-edge EXAFS fitting parameters of post-reaction Co@BaAl₂O₄.

Figure 53 Reported catalysts used for CH₄ dry-reforming and their catalytic performances at corresponding conditions.

Figure 54 Estimation of M_T NPs crystallite sizes.

DETAILED DESCRIPTION

Definitions

Throughout the present disclosure, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising" , will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. It is also noted that in this disclosure and particularly in the claims and/or paragraphs, terms such as “comprises” , “comprised” , “comprising” and the like can have the meaning attributed to it in U.S. Patent law; e.g., they can mean “includes” , “included” , “including” , and the like; and that terms such as “consisting essentially of” and “consists essentially of” have the meaning ascribed to them in U.S. Patent law, e.g., they allow for elements not explicitly recited, but exclude elements that are found in the prior art or that affect a basic or novel characteristic of the present invention.

Furthermore, throughout the present disclosure and claims, unless the context requires otherwise, the word “include” or variations such as “includes” or “including” , will be understood to imply the inclusion of a stated integer or group of integers, but not the exclusion of any other integer or group of integers.

The use of the singular herein includes the plural (and vice versa) unless specifically stated otherwise. In addition, where the use of the term "about" is before a quantitative value, the present teachings also include the specific quantitative value itself, unless specifically stated otherwise. As used herein, the term "about" refers to a ±10%, ±7%, ±5%, ±3%, ±1%, or ±0%variation from the nominal value unless otherwise indicated or inferred.

The present disclosure provides a method of preparing core-shell nanoparticles, the method comprising: contacting a mixture comprising a first metal oxide, a first encapsulating material comprising an alkali metal or an alkaline earth metal, and a second encapsulating material comprising a second metal oxide with hydrogen gas at a temperature higher than the hydrogen reduction temperature of the first metal oxide and lower than the decomposition temperature of the first encapsulating material resulting in the reduction the first metal oxide and the formation of metal nanoparticles that are at least partially encapsulated with a bimetallic oxide comprising the alkali metal or the alkaline earth metal and the second metal oxide thereby forming the core-shell nanoparticles, wherein the Tammann temperature of the first encapsulating material is lower than the hydrogen reduction temperature of the first metal oxide.

The first metal oxide can comprise a metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Se, Sn, Pt, Ru, Pd, W, Ir, Os, Rh, Nb, Ta, Pb, Bi, Au, Ag, Sc, and Y. The oxidation state of the metal can be 1+, 2+, 3+, 4+, 5+, or 6+. In certain embodiments, the metal oxide comprises V²⁺, V³⁺, V⁴⁺, V⁵⁺, Cr²⁺, Cr³⁺, Cr⁶⁺, Fe²⁺, Fe³⁺, Co²⁺, Co³⁺, Ni²⁺, Ni³⁺, Cu¹⁺, Cu²⁺, Zn¹⁺, Mo²⁺, Mo³⁺, Mo⁴⁺, Mo⁵⁺, Mo⁶⁺, Se²⁺, Se⁴⁺, Sn²⁺, Sn⁴⁺, Pt²⁺, Pt⁴⁺, Ru²⁺, Ru³⁺, Ru⁴⁺, Ru⁵⁺, Ru⁶⁺, Pd²⁺, Pd⁴⁺, W²⁺, W³⁺, W⁴⁺, W⁵⁺, W⁶⁺, Ir¹⁺, Ir³⁺, Os³⁺, Os⁴⁺, Os⁵⁺, Os⁶⁺, Rh¹⁺, Rh³⁺, Nb³⁺, Nb⁴⁺, Nb⁵⁺, Ta³⁺, Ta⁴⁺, Ta⁵⁺, Pb²⁺, Pb⁴⁺, Bi¹⁺, Bi²⁺, Au¹⁺, Au³⁺, Ag¹⁺, Sc³⁺, or Y³⁺. Exemplary metal oxides include, but are not limited to, CoO₃, Co₃O₄, CoO, CuO, Cu₂O, NiO, Ni₂O₃, Ag₂O, AgO, FeO, Fe₂O₃, Fe₃O₄, ZnO, PdO, PdO₂, PtO, and PtO₂.

The alkali metal can be lithium, sodium, potassium, rubidium, or caesium. The alkaline earth metal can be beryllium, magnesium, calcium, strontium, or barium. In certain embodiments, the first encapsulating material can comprise an alkali metal salt or an alkaline earth metal salt. In certain embodiments, the alkali metal salt or an alkaline earth metal salt comprises one or more anions selected from the group consisting of carbonate, sulfate, nitrate, and chloride. Exemplary alkali metal salt or an alkaline earth metal salts include, but are not limited to, NaCl, KCl, RbCl, Na₂CO₃, K₂CO₃, Rb₂CO₃, CaCO₃, MgCO₃, SrCO₃, and BaCO₃.

The first encapsulating material can be any alkali metal salt or alkaline earth metal salt with a Tammann temperature lower than the hydrogen reduction temperature of the first metal oxide and a decomposition temperature higher than the hydrogen reduction temperature of the first metal oxide. The Tammann temperature and the of the first encapsulating material can be readily determined based on the melting point of the first encapsulating material. The decomposition temperature of the first encapsulating material can be determined experimentally by identifying the temperature that the first encapsulating material begins to decompose under the reaction conditions that the mixture and hydrogen are contacted.

The second encapsulating agent can comprise a second metal oxide selected from the group consisting of Al₂O₃, TiO₂, silica, Ga₂O₃, In₂O₃, ZrO₂, and mixtures thereof.

Under the reaction conditions the first encapsulating material and the second encapsulating material can undergo a solid-state reaction at temperatures above the Tammann temperature of the first encapsulating material whereby a bimetallic oxide forms, which at least partially encapsulates the metal nanoparticle, wherein the bimetallic oxide comprises the alkali metal or the alkaline earth metal and the second metal oxide. In certain embodiments, the bimetallic oxide partially encompasses a plurality of metal nanoparticles, e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or more metal nanoparticles. The composition of the bimetallic oxide shell will vary based on the selection of the first encapsulating material and the second encapsulating material. In certain embodiments, the bimetallic oxide is MgAl₂O₄, CaAl₂O₄, SrAl₂O₄, BaAl₂O₄, MgTiO₃, CaTiO₃, SrTiO₃, BaTiO₃, MgGa₂O₄, CaGa₂O₄, SrGa₂O₄, BaGa₂O₄, MgIn₂O₄, CaIn₂O₄, SrIn₂O₄, BaIn₂O₄, MgZrO₃, CaZrO₃, SrZrO₃, or BaZrO₃.

In certain embodiments, the metal nanoparticles comprise V⁰, Cr⁰, Mn⁰, Fe⁰, Co⁰, Ni⁰, Cu⁰, Zn⁰, Mo⁰, Se⁰, Sn⁰, Pt⁰, Ru⁰, Pd⁰, W⁰, Ir⁰, Os⁰, Rh⁰, Nb⁰, Ta⁰, Pb⁰, Bi⁰, Au⁰, Ag⁰, Sc⁰, or Y⁰.

The hydrogen reduction temperature of the first metal oxide is the temperature required to reduce at least a portion of the first metal oxide thereby forming zero-valent metal nanoparticles under the reaction conditions that the mixture and hydrogen gas are contacted. The hydrogen reduction temperature of the first metal oxide can be experimentally determined by a person of ordinary skill in the art based on well-known experimental techniques. In certain embodiments, reduction of the first metal oxide yields a mixture of reduction products including, partially reduced metal species. The core can comprise at least 5%wt/wt, at least 10%wt/wt, at least 20%wt/wt, at least 30%wt/wt, at least 40%wt/wt, at least 50%wt/wt, at least 60%wt/wt, at least 70%wt/wt, at least 80%wt/wt, at least 90%wt/wt, at least 95%wt/wt, at least 97%wt/wt, at least 98%wt/wt, at least 99%wt/wt, or at least 99.9%wt/wt of zero-valent metal relative to the total weight of vero-valent metal, partial first metal oxide reduction products, and unreacted first metal oxide. In certain embodiments, the core comprises between 5-99.9%wt/wt, 10-99.9%wt/wt, 20-99.9%wt/wt, 30-99.9%wt/wt, 40-99.9%wt/wt, 50-99.9%wt/wt, 60-99.9%wt/wt, 70-99.9%wt/wt, 80-99.9%wt/wt, 90-99.9%wt/wt, 95-99.9%wt/wt, 30-99%wt/wt, 30-90%wt/wt, 30-80%wt/wt, 30-70%wt/wt, 30-60%wt/wt, 40-60%wt/wt, or 40-50%wt/wt of zero-valent metal relative to the total weight of vero-valent metal, partial first metal oxide reduction products, and unreacted first metal oxide.

The core-shell nanoparticles prepared in accordance with the methods described herein can comprise one or more cores and a shell at least partially encompassing the one or more cores. In certain embodiments, the core-shell nanoparticles comprise one or more cores comprising V⁰, Cr⁰, Mn⁰, Fe⁰, Co⁰, Ni⁰, Cu⁰, Zn⁰, Mo⁰, Se⁰, Sn⁰, Pt⁰, Ru⁰, Pd⁰, W⁰, Ir⁰, Os⁰, Rh⁰, Nb⁰, Ta⁰, Pb⁰, Bi⁰, Au⁰, Ag⁰, Sc⁰, or Y⁰; and a shell comprising MgAl₂O₄, CaAl₂O₄, SrAl₂O₄, BaAl₂O₄, MgTiO₃, CaTiO₃, SrTiO₃, BaTiO₃, MgGa₂O₄, CaGa₂O₄, SrGa₂O₄, BaGa₂O₄, MgIn₂O₄, CaIn₂O₄, SrIn₂O₄, BaIn₂O₄, MgZrO₃, CaZrO₃, SrZrO₃, or BaZrO₃. Exemplary core-shell nanoparticles include, but are not limited to Co@BaAl₂O₄, Cu@BaAl₂O₄, Ni@BaAl₂O₄, Co@BaTiO_3, Fe@MgGa₂O₄, Co@MgGa₂O₄, and Ni@BaZrO₃.

In certain embodiments, the selection of the first encapsulating material is based on the hydrogen reduction temperature of the first metal oxide; the decomposition temperature of the first encapsulating material and the Tammann temperature of the first encapsulating material. More particularly, for a given first metal oxide, suitable first encapsulating materials can have a Tammann temperature below the hydrogen reduction temperature of the given first metal oxide and a decomposition temperature higher than the given first metal oxide. In certain embodiments, the selection of suitable first encapsulating materials for a given first metal oxide is determined by screening a list comprising Tammann temperatures of candidate first encapsulating agents and decomposition temperatures of candidate first encapsulating agents against a given first metal oxide’s hydrogen reduction temperature thereby identifying suitable first encapsulating materials from the list having a Tammann temperature below the hydrogen reduction temperature of the given first metal oxide and a decomposition temperature higher than the given first metal oxide. Screening can be conducted manually, e.g., by reference to tables or lists, or can be conducted using a computer.

The Tammann temperature of the first encapsulating material, the decomposition temperature of the first encapsulating material, and the hydrogen reduction temperature of the first metal oxide can be determined experimentally and/or by reference to literature values.

The selection of the fist encapsulating material and the second encapsulating material can be based on data collected from screening changes in enthalpy of potential reaction products of the reaction of a candidate first metal oxide, a candidate first encapsulating material, a candidate second encapsulating material, and hydrogen gas in view of the decomposition temperature of the candidate first encapsulating material. Enthalpically favorable reactions that yield bimetallic oxides at reaction temperatures below the decomposition temperature of the candidate first encapsulating material can be selected, which determines the choice of the first encapsulating material and the second encapsulating material. This process is illustrated in Fig. 13, wherein the possible reaction products of the solid-state reaction of BaCO₃ + Al₂O₃ + Co₃O₄ + H₂ are ordered from most stable to least stable based on the possible product’s reaction enthalpy change (ΔH) . Of the most stable possible products, BaAl₂O₄ is selected, because the reaction temperature required for its production is lower than the decomposition temperature of BaCO₃. In certain embodiments, the step of screening changes in enthalpy of potential reaction products is conducted using the Materials Project database in a computer.

In an exemplary embodiment, the reaction enthalpy change (ΔH) of forming different products from the solid-state reactions between BaCO₃ and Al₂O₃, with Co₃O₄ and H₂ coexisting in the system, were theoretically examined by the reaction calculator module in Materials Project. Before H₂ thermal treatment, a mix-metal precursor containing BaCO₃, Al₂O₃, and Co₃O₄ was obtained after calcination in the air. Therefore, the reactants were set as BaCO₃ + Al₂O₃ + Co₃O₄ + H₂ (because the solid-state reaction is under an H₂ atmosphere) , while setting the possible products as all compounds containing Ba element and any one or more of Co, Al, C, O, and H. Searched from the Materials Project database, there are 53 possible different products. Then in the reaction calculator module, the reaction enthalpy change (ΔH) for obtaining each of the above 53 possible products was queried and sorted, as shown in Fig. 13. The stability of the corresponding products is evaluated by comparing the value of ΔH. The lower the ΔH, the more stable the product. Fig. 13 indicates the most stable three products are Ba₇Al₆₄O₁₀₃, BaAl₄O₇, and BaAl₂O₄. Given that the high reported synthesis temperatures of BaAl₄O₇ (c. a. 1100℃) and Ba₇Al₆₄O₁₀₃ (higher than 1100℃, via solid-state reaction of BaAl₄O₇ and Al₂O₃) , we put our focus on BaAl₂O₄ with Ba/Al = 1/2, which has a milder synthesis condition and thus more suitable for the one-pot synthesis of the target Co@BaAl₂O₄ encapsulation structure combined with the Tammann temperature guide.

In another exemplary embodiment, all the possible products and corresponding ΔH of the reactions MgCO₃ + Al₂O₃ + CuO + H₂ were sorted and plotted in Fig. 34a, which indicates that MgAl₂O₄ shows the lowest ΔH among all the product considered in MgCO₃ + Al₂O₃ + CuO + H₂ reactions. Hence MgAl₂O₄ was chosen as the target overlayer for Cu NPs. On the other hand, titanate overlayer formation can also be realised when considering the solid-state reaction between BaCO₃ and TiO₂. Different titanate products with stoichiometric ratios are evaluated using the rapid computational screening method (Fig. 35a) , in which BaTiO₃ is one of the most commonly reported ones. Hence BaTiO₃, with Ba/Ti = 1/1, was chosen as another target overlayer for Co NPs.

Guided by methods described in Fig. 1b, 40Ni: 20Ba: 40Al, 40Cu: 20Mg: 40Al, and 50Co: 25Ba: 25Ti precursors were prepared. After thermal treatment in H₂, the formation of M_T NPs (Ni, Cu, or Co) and the corresponding M_AEAl₂O₄ (BaAl₂O₄, MgAl₂O₄ or BaTiO₃) can be confirmed by the characteristic XRD patterns (Figs. 33b, 34b, and 35b) . STEM and energy dispersive X-ray (EDX) mapping clearly visualize the encapsulation structures of Ni@BaAl₂O₄, Cu@MgAl₂O₄, and Co@BaTiO₃, respectively, with the well-rounded alkali metal aluminates or an alkaline earth metal aluminates overlayer in close contact with the M_T NPs (Fig. 4e) . The encapsulated M_T NPs show relatively uniform size distributions in the range of 4-8, 20-30, and 24-40 nm, for Ni, Cu, and Co, respectively, in line with the crystal size derived from XRD results (Figs. 33c, 34c, 35c, and 54) .

Depending on the choice first metal oxide, first encapsulating material, second encapsulating material, stoichiometries, and reaction conditions the average size of the thus produced core-shell nanoparticles can range from 1-500 nm, 1-400 nm, 1-300 nm, 1-200 nm, 1-100 nm, 1-90 nm, 1-80 nm, 1-70 nm, 1-60 nm, 1-50 nm, 1-40 nm, 1-30 nm, 1-30 nm, 1-20 nm, 5-20 nm, 10-20 nm, 10-15 nm, 10-40 nm, 20-40 nm, 20-30 nm, or 1-10 nm. In certain embodiments, the core-shell nanoparticles have an average size of about 10 to about 15 nm, about 4 to about 8, about 20 to about 30, and about 24 to about 40 nm.

The reaction of the first metal oxide, the first encapsulating material, and the second encapsulating material with hydrogen gas can be conducted autogenic pressure, i.e., pressure generated as a result of heating in a closed system. Alternatively or additionally, the pressure can be applied externally, e.g., by mechanical means. In certain embodiments, the step of heating the reaction solution is conducted at a pressure of 0.1 to 10 MPa or 0.1 to 1 MPa.

To take full advantage of the high reactivity of the low-T_Tam M_AECO₃, it was proposed to further functionalize the BaCO₃ overlayer during the Co NP formation. Considering alkali metal aluminates or an alkaline earth metal aluminates (i.e., (M_AE) _xAl_yO_z) are classic catalyst support materials with high thermal resistance, good hydrothermal stability, and prominent interactions with the active species, it was attempted to introduce Al₂O₃ into the encapsulation system to engineer the overlayer functionalization (Fig. 2a) . By employing rapid computational sorting of the formation energies of different aluminate products from the solid-state reaction between BaCO₃ and Al₂O₃ (Fig. 2b and Fig. 13) , the most suitable overlayer aluminate composition was idenfitied to be BaAl₂O₄, and the more stable BaAl₄O₇ and Ba₇Al₆₄O₁₀₃ were rejected, because they require harsh synthesis conditions (> 1100 ℃) .

To realize Co NP formation, BaCO₃ encapsulation, and BaAl₂O₄ formation in a single thermal treatment, the process began with mixing the Co, Ba and Al metal precursors with a composition of 40Co: 20Ba: 40Al through an automatically pH-controlled co-precipitation method, followed by temperature-and flow-controlled air-calcination (Fig. 5) . After the thermal treatment in H₂, the formation of Co NPs and BaAl₂O₄ can be confirmed by the characteristic X-ray diffraction (XRD) patterns (Fig. 14a) . Scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) mapping (Fig. 2c) clearly show the encapsulation structure with the well-rounded BaAl₂O₄ overlayer in close contact with the Co NPs. The encapsulated Co NPs show relatively uniform size distributions in the range of 10-15 nm, in line with the crystal size derived from XRD (Fig. 14b and Fig. 39) . Transmission electron microscopy (TEM) images and corresponding FFT patterns of Co@BaAl₂O₄ reveal the fcc Co and BaAl₂O₄ lattice fringes (Fig. 15) , which is fully consistent with the Co K-edge and Ba L₃-edge extended X-ray absorption fine structure (EXAFS) (Fig. 2d, Figs. 16-17, and Figs. 40-43) and the synchrotron X-ray diffraction (SXRD) results (Fig. 2e) .

Interestingly, noticeable oxygen vacancies in the BaAl₂O₄ overlayer of Co@BaAl₂O₄ can be detected by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy (Figs. 18a-18b) . In addition, Ar adsorption-desorption analyses reveals the porous structure of Co@BaAl₂O₄ (Figs. 18c-18d) , presumably due to the presence of oxygen vacancies when synthesizing BaAl₂O₄ under H₂ environment. This porous feature is particularly important for catalysis as they impart the good permeability of core@shell structure for the reactant molecules to access the active metal core while maintaining high stability from encapsulation.

It is important to note that the observed Co@BaAl₂O₄ encapsulation cannot be realized by direct synthesis using BaAl₂O₄ (see details in Fig. 19 and Fig. 44) , reflecting the necessity of employing low T_Tam BaCO₃ for our proposed SMSI encapsulation process.

Real-time observation and mechanism study for the encapsulation

The Co@BaAl₂O₄ synthesis process is further investigated to explore the encapsulation formation pathway and mechanism. Starting with the 40Co: 20Ba: 40Al precursor, the elemental composition analyses indicate a similar atomic ratio of Co, Ba, and Al at c. a. 2: 1: 2 at different stages of the synthesis process (i.e., as-precipitated, as-calcined, and thermally treated in H₂, Fig. 20 and Fig. 45) . Yet distinct morphologies are revealed by TEM, where the encapsulation is only formed after the thermal treatment in H₂ (Fig. 21) . Direct real-time visualization of the encapsulation process during the H₂ thermal treatment is then conducted using in-situ atmospheric STEM equipped with an environmental cell (see Fig. 8 for experiment setup and details) . At 25 ℃, the calcined precursor is initially a mixture of plate-like Co₃O₄ and rod-like Al₂O₃ NPs in contact with much larger bulk of BaCO₃ (Fig. 22) . STEM image indicates a clear boundary between the dark Co₃O₄-Al₂O₃ mixture and the bright BaCO₃ bulk particle (Fig. 3a) . When the temperature is ramped to above 400 ℃ (Fig. 3b the boundary of the BaCO₃ particle blurs and some darker pores (marked by dashed circles) appear in the interior, which can be attributed to the increased lattice mobility and subsequent outward diffusion as evidenced by the boundary propagation (the dotted lines) . Raising the temperature to 500 ℃ leads to more obvious diffusion of BaCO₃ with dark pores growing inside, agreeing well with the expanding Ba element distribution observed from EDX elemental mapping (Figs. 23a-23b) . In contrast, no obvious morphology change is observed in the Co/Al-containing area until the bright spheres emerge at 600 ℃ and become more identifiable at 700 ℃ (Fig. 3c) , indicating the in-situ formation of Co NPs from reducing Co₃O₄. The Co NPs are surrounded by highly mobile or liquified BaCO₃, forming the initial encapsulation. When the cell is heated from 700 to 800 ℃, a dynamic reaction can be seen with BaCO₃ decomposition at the expense of the Al₂O₃ rods in proximity, implying the solid-state formation of the functional BaAl₂O₄ overlayer (Fig. 3d, Fig. 23c) , as confirmed by selected area electron diffraction (SAED) (Fig. 23d) . The observed morphology evolution is summarized in Fig. 3e, in full accordance with our proposed SMSI-induced encapsulation strategy guided by Tammann temperature.

To better grasp the morphology evolution during the H₂ thermal treatment, starting morphology of the mix-metal precursor C (500) -40Co: 20Ba: 40Al before H₂ thermal treatment was characterised by TEM. As shown in Fig. 22a, before H₂ thermal treatment, C (500) -40Co: 20Ba: 40Al is initially a mixture of plate-like Co₃O₄ and rod-like Al₂O₃ NPs, in contact with much larger bulk BaCO₃. A clear boundary is observed between the Co/Al-containing and Ba-containing areas. High-resolution TEM image and corresponding FFT shown in Fig. 22b reveal the polycrystalline nature of BaCO₃ structure, consistent with the abovementioned XRD results (Fig. 14a) . The high-resolution TEM image shown in Fig. 22c demonstrates that Al₂O₃ is in rod-like morphology and Co₃O₄ is in plate-like morphology, whose (220) lattice fringes are marked in Fig. 22d.

Fig. 16 shows the XANES features of the samples at different stages of the synthesis process (i.e., as-precipitated, as-calcined, and thermally treated in H₂) . Before H₂ thermal treatment, the C (500) -40Co: 20Ba: 40Al shows a much higher white-line (WL) intensity compared to standard Co foil. After H₂ thermal treatment, the XANES profile for C (500) -R (700) -40Co: 20Ba: 40Al shifts to lower energy compared to C (500) -40Co: 20Ba: 40Al, becoming similar to the Co foil standard. These results indicate the formation of metallic Co from Co₃O₄ reduction during H₂ thermal treatment. Fig. 16b shows the Fourier transforms (FT) magnitude of the Co K-edge EXAFS for the samples at different stages of the synthesis process, along with the Co foil for comparison purpose. The FT magnitude peak atattributing to Co-Co only exists in C (500) -R (700) -40Co: 20Ba: 40Al while being absent in 40Co: 20Ba: 40Al and C (500) -40Co: 20Ba: 40Al. These results prove that Co NPs can only be formed after the H₂ thermal treatment. As shown in Fig. 16c and Fig. 40, the EXAFS spectrum of C (500) -40Co: 20Ba: 40Al can be well-fitted with three groups of paths, corresponding to Co-O (coordination numbers, CN = 4.3) atCo-Co (CN = 2.0) atand Co-Co/O at which agrees well the theoretical Co₃O₄ model (Fig. 41) . In addition, the EXAFS spectrum of C (500) -R (700) -40Co: 20Ba: 40Al can be well-fitted using only the Co-Co path, proving the sole existence of metallic Co for Co species after H₂ thermal treatment (Fig. 16d and 40) .

The change of the local structures of the absorbing Co atoms can be observed more intuitively from wavelet transform (WT) representations of the Co K-edge EXAFS signals. It can be clearly observed that C (500) -40Co: 20Ba: 40Al has an entirely consistent spectrum with the theoretical Co₃O₄ model (Figs. 12e and 16g) , whereas C (500) -R (700) -40Co: 20Ba: 40Al has an entirely consistent spectrum with Co foil (Figs. 16f and 16h) , indicating the reduction of Co₃O₄ to Co metal during the H₂ thermal treatment. This result is consistent with the structural changes observed from the above XRD results (Fig. 14a) .

Fig. 17a shows the Ba L₃-edge XANES spectra of 40Co: 20Ba: 40Al, C (500) -40Co: 20Ba: 40Al, and C (500) -R (700) -40Co: 20Ba: 40Al. It can be observed that the edge positions remain unchanged between the three samples, indicating the Ba valence does not change much after air calcination and H₂ thermal treatment. In addition, the Ba L₃-edge XANES spectrum of C (500) -R (700) -40Co: 20Ba: 40Al shows a distinctive peak between 5250 and 5285 eV (Fig. 17a) , in line with the reported XANES features of BaAl₂O₄ structure. As et al. reported, distinct differences between pure BaAl₂O₄ and pure BaCO₃ spectra are visible in the energy range between 5250 and 5270 eV, i.e., BaCO₃ has an absorption minimum at about 5255 eV and 5264 eV, where BaAl₂O₄ features a visible stronger intensity in this energy range. As can be seen in this comparison, the Ba L₃-edge XANES spectrum for 40Co: 20Ba: 40Al and C (500) -40Co: 20Ba: 40Al samples follow the spectrum of the pure BaCO₃ sample, while C (500) -R (700) -40Co: 20Ba: 40Al follows that of BaAl₂O₄. This observation is consistent with the phase transformation observed from the XRD results of the samples in different stages (Fig. 14a) .

From the Ba L₃-edge EXAFS spectrum at R space, it can be observed that the spectra of 40Co: 20Ba: 40Al and C (500) -40Co: 20Ba: 40Al are similar but much different from C (500) -R (700) -40Co: 20Ba: 40Al (Fig. 14b) . Ba L₃-edge EXAFS fitting was further conducted to identify the Ba local structures. The EXAFS spectra of 40Co: 20Ba: 40Al and C (500) -40Co: 20Ba: 40Al samples can be fitted well using the theoretical BaCO₃ model, with ΔR ≤ 0.1 nm and R-factor < 2% (Figs. 14c-14d, Figs. 42-43) . And the EXAFS spectrum of C (500) -R (700) -40Co: 20Ba: 40Al can be well-fitted using the theoretical BaAl₂O₄ model (Fig. 17e, Figs. 42-43) .

The structure change can be observed more intuitively from Ba L₃-edge WT EXAFS. It can be clearly seen that 40Co: 20Ba: 40Al and C (500) -40Co: 20Ba: 40Al samples have entirely consistent spectra with the theoretical BaCO₃ model (Figs. 17f, 17g, and 17i) . In contrast, the sample C (500) -R (700) -40Co: 20Ba: 40Al has a consistent spectrum with the theoretical BaAl₂O₄ model (Fig. 17h and 17j) , indicating the phase evolution from BaCO₃ to BaAl₂O₄ during H₂ thermal treatment. This result is consistent with the structural changes observed from the above XRD results (Fig. 17a) .

XRD and in-situ Raman spectroscopy equipped with online MS (see details in Fig. 7) are also employed to supplement the real-time phase evolution and the solid-state reactions. At the initial stage, typical XRD patterns of BaCO₃ and Co₃O₄ are recorded (Fig. 3f) , with the absence of Al₂O₃ fingerprint peaks, presumably overwhelmed by the signals of Co₃O₄ or the nano crystallinity of gamma phase Al₂O₃ (Fig. 22d) . As the temperature of thermal treatment increases to 400 ℃, the characteristic peaks of Co₃O₄ at 31.27° and 36.84° fade with the simultaneous appearance of two new peaks at 44.22° and 51.52°, which can be assigned to the fcc Co. At 700 ℃, the characteristic diffraction signal of BaCO₃ is replaced by that of BaAl₂O₄, indicating the completion of the solid-state reaction between BaCO₃ and Al₂O₃. Similarly, in-situ Raman spectroscopy reveals the gradual disappearance of Co₃O₄ fingerprints around ～490, ～525, ～610, and ～690 cm^-1 before 400 ℃, and the replacement of BaCO₃ fingerprints around ～139, ～698, and ～1060 cm^-1 by those of BaAl₂O₄ around ～193, ～271, ～491, ～581, and ～680 cm^-1 at 700 ℃ (Fig. 3g) . Online MS is used to monitor the outlet gas during the thermal treatment under H₂ flow. It can be observed that Co₃O₄ is reduced during 300 -500 ℃, with H₂ consumed and H₂O released. As the temperature is further increased to 700 ℃, both CO and H₂O peaks rise rapidly while consuming H₂, which is consistent with the solid-state reaction between BaCO₃ and Al₂O₃. In other words, these results echo the observed phase change process in the in-situ STEM. Note that the discrepancies in some phase changing temperature recorded by different characterization techniques are due to the distinct in-situ conditions, which does not affect the trend of the identified phase evolution and encapsulation mechanism.

All the evidence collectively indicates that (i) low T_Tam BaCO₃ exhibits sufficient mobility and can migrate onto the in-situ formed Co NPs, and (ii) BaCO₃ could readily react with Al₂O₃ to form BaAl₂O₄ as the final encapsulation overlayer (i.e., BaCO₃ + Al₂O₃ + H₂ →BaAl₂O₄ + CO + H₂O) , thanks to the high reactivity of low-T_Tam BaCO₃ in the H₂ atmosphere at a higher temperature. As a result, the complete encapsulation is realized on the Co NP surface with BaAl₂O₄ by forming a well-defined core@shell configuration.

The encapsulated nanocatalyst enables excellent catalytic performance

To assess the impact of BaAl₂O₄ encapsulation on the catalytic performance of the Co NPs at high temperatures, a series of Co nanocatalysts, both with and without encapsulation, are evaluated for their performance in thermal catalytic NH₃ decomposition reaction. Typically, this reaction requires a temperature of approximately 650 -800 ℃ for non-noble metal catalysts to achieve an NH₃ conversion of over 80%without sacrificing the weight hourly space velocity (WHSV) . Surprisingly, the Co@BaAl₂O₄ achieves almost complete conversion (～99%) at a much lower temperature of only 500 ℃, at a high WHSV of 30,000 mL g_cat ^-1 h^-1 (Fig. 26 and Fig. 46) , making it the best core@shell catalysts ever reported for NH₃ decomposition (Fig. 4a) . Compared to the state-of-the-art NH₃ decomposition catalysts, the encapsulated Co@BaAl₂O₄ also represents one of the most effective catalysts in both Ru-based and non-noble catalysts records (Fig. 27 and Fig. 47) .

The most significant benefit of the core@shell catalyst configuration is demonstrated in long-term performance evaluation. After 50 hours of time-on-stream at 550 ℃, the H₂ production of the non-encapsulated catalysts, Co NPs supported on Al₂O₃ (Co/Al₂O₃) and on BaAl₂O₄ (synthesis details see Fig. 19) , significantly decreases, indicating poor long-term activity due to the severe sintering of Co particles in high-temperature reaction condition (Figs. 4b 4c, Figs. 28-29 and 48-49) . In sharp contrast, the Co@BaAl₂O₄ nanocatalyst maintains a constantly high H₂ production rate over 600 hours under identical testing condition (Fig. 4b) . No significant change in morphology is observed for the post-reaction Co@BaAl₂O₄ (Fig. 4d, Figs. 30, and Fig. 50) , indicating its superior thermochemical stability due to the advantageous core@shell structure. The EXAFS of the post-reaction Co@BaAl₂O₄ reveals the distinct Co and BaAl₂O₄ phases (Fig. 30, 51, and 52) , suggesting no migration of Co species at the metal-support interfaces during the long-term testing. Remarkably, over a five-month-long experiment, the activity of Co@BaAl₂O₄ does not decrease despite intentional temperature fluctuations, system ONs/OFFs, and catalyst offload/reload events (Fig. 31) , demonstrating its excellent environmental tolerance and reusability for practical applications.

Furthermore, Co@BaAl₂O₄ demonstrates promising outcomes in the CH₄ dry-reforming reaction, which typically requires even higher temperatures (> 750 ℃) . As shown in Fig. 32, Co@BaAl₂O₄ achieves the CH₄ and CO₂ conversion rates of 81.8%and 90.1%, respectively, nearing the thermodynamic equilibrium value and sustaining the performance for over 60 hours. In comparison to the benchmark catalysts reported in literature (Fig. 53) , the high efficiency and excellent stability identify Co@BaAl₂O₄ as a promising catalyst for the CH₄ dry-reforming reaction. These results jointly manifest that our new encapsulation strategy not only effectively stabilizes the nanocatalyst against the common sintering issue but also provides further opportunities to modify the performance for good catalytic applicability. Currently, the origin of the exceptional performance of Co@BaAl₂O₄ in the above-mentioned thermal catalytic reactions is under careful investigation, with a focus on the notable synergistic effects of metal–support interfaces in the core@shell structure.

A core@shell catalyst design and synthesis platform with broad applicability

It was successfully demonstrated that low-T_Tam compounds can facilitate thermal-induced SMSI core@shell construction, and subsequent solid-state reactions for overlayer functionalization enable thermal catalytic applicability. Furthermore, to showcase the general applicability of our proposed approach, other M_T and M_AECO₃ combinations in the encapsulable window (shadowed in Fig. 1b) were tested. The results have validated the effectiveness of our thermal encapsulation and sintering prevention strategy guided by Tammann temperature (Fig. 4e, Figs. 33-34, and Fig. 54) . More convincingly, the replacement of Al₂O₃ with TiO₂ for the overlayer solid-state reaction enables us to achieve BaTiO₃ encapsulation on Co NPs (Fig. 4e, Fig. 35, and Fig. 54) , demonstrating the broad applicability of our strategy across various material matrices.

All these results demonstrate the potential of the Tammann temperature-guided SMSI strategy as a design and synthesis platform for the facile encapsulation of M_T NPs. The tunability of both M_T core and functional oxide overlayer shows promise for fine-tuning the performance of encapsulated nanocatalysts. It is anticipated that other low-T_Tam metal compounds, such as chloride, sulfate, nitrate, etc., can also be applied to construct size-controlled encapsulation structures for different transition metal NPs, either from in-situ reduction or pre-made metal NPs. Computational calculations can assist with rapid composition selections from a broad material matrix for overlayer solid-state reactions, thus unlocking a wide range of possibilities for metal oxides (or non-oxides) to act as NP protectors/modifiers. Overall, this novel thermal encapsulation methodology enables future explorations and predictions for rational and dynamic tuning of nanocatalysts to meet diverse requirements in broad energy and environmental applications.

Examples

Materials and Instruments

Iron nitrate nonahydrate (Fe (NO₃) ₃·9H₂O, 99.0%) , cobalt nitrate hexahydrate (Co (NO₃) ₂·6H₂O, 99.0%) , nickel nitrate hexahydrate (Ni (NO₃) ₂·6H₂O, 99.0%) , copper nitrate (Cu (NO₃) ₂, 99.0%) , magnesium carbonate (MgCO₃, AR) , calcium carbonate (CaCO₃, AR) , strontium carbonate (SrCO₃, AR) , barium carbonate anhydrous (BaCO₃, AR) , sodium carbonate (NaCO₃, 99.0%) and sodium hydroxide (NaOH, 96.0%) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Barium nitrate (Ba (NO₃) ₂, 99.0%) and aluminium nitrate nonahydrate (Al (NO₃) ₃·9H₂O, 99.0%) were purchased from Sigma-Aldrich. Nano iron oxide (Fe₂O₃, 99.0%) , nano cobalt oxide (Co₃O₄, 99.0%) , nano nickel oxide (NiO, 99%) and nano copper oxide (CuO, 99%) were purchased from Shanghai Macklin Biochemical Co., Ltd. All chemicals with analytical-grade purity were used directly without further treatment unless otherwise noted. All solutions were prepared with deionized (DI) water. Anhydrous grade ammonia (NH3, ≥99.99%) and H₂/Ar (v/v, 5/95) were purchased from Scientific Gas Engineering Co., Ltd.

Precursors coprecipitation synthesis methods and thermal treatment

The precursor mixture, mMT: nMAE: 2nAl, was synthesized by an automatic pH controlled co-precipitation method. For a typical preparation procedure, an aqueous solution that contained M_T ^m+, M_AE ²⁺ and Al³⁺ with target molar ratios was prepared by dissolving the corresponding metal nitrates in aqueous solution. At room temperature, it was added dropwise into a stirred tank reactor (capacity of 500 mL-2L) with a 0.5 M NaCO₃ solution under feeding rates of 0.1-2.0 mL min^-1 regulated by a syringe pump. The mixture was stirred vigorously to ensure efficient mixing, while the pH of the precipitating solution was carefully maintained at a constant by dropwise addition of a 4.0 M NaOH solution using another syringe pump. The pH of the solution is maintained lower than the precipitation pKsp of M_AE (OH) ₂, thereby favoring theformation of M_AECO₃ rather than M_AE (OH) ₂ precipitates. The pKsp of Ba (OH) ₂ is 12.9 and Mg (OH) ₂ is 10.10. Once the premeasured amount of solutions was added into the tank, the liquid was aged for 16 h, then the mixture was filtered and washed with DI water until the pH was close to 7.0. The obtained wet cake solid sample was re-dispersed in 200 mL acetone and stirred at room temperature for 2 h. Then, the resultant solid was vacuum filtered, washed thoroughly with acetone and dried overnight in a vacuum oven at room temperature. The precursors were named by their nominal mixed-metal ratios, denoted as mM_T: nM_AE: 2nAl, wherein m and n represent the nominal molar ratios of M_T and M_AE in the co-precipitation process, respectively. After getting the mM_T: nM_AE: 2nAl precipitate, it was calcined in air (denoted as C (temp. ) -mM_T: nM_AE: 2nAl) and treated in H₂/Ar (v/v, 5/95, 160 mL min-1 per 50 mg samples) at specified temperatures to finally form the possible encapsulation structure, denoted as C (temp. ) -R (temp. ) -mM_T: nM_AE: 2nAl (To intuitively indicate the sample structure via sample name, if the final sample had a core-shell structure, it is also denoted as M_T@M_AEAl₂O₄. C (temp. ) and R (temp. ) to indicate the air-calcination and H₂ treatment processes, and the highest temperature (℃) reached in each process is listed in the brackets. Fig. 5 depicts a schematic representation of the steps involved in above synthesis process.

Measurement of M_AECO₃ decomposition temperature (T_dec)

Temperature-programmed decomposition measurements of commercial alkali metal carbonates or alkaline earth metal carbonates (M_AECO₃, M_AE = Na, K, Rb, Cs, Mg, Ca, Sr, Ba, etc) were conducted using a tube furnace (GSL-1100X, Kejing) combined with a quadrupole mass spectrometer (MS, HPR-20 EGA, Hiden) . Inside the quartz tube, c. a. 50 mg of the M_AECO₃ sample was sandwiched between two layers of quartz wool with a thermocouple in contact with the sample. The quartz tube was then inserted into the tube furnace for argon (Ar) pretreatment, with the Ar flow rate of 10 mL min^-1. The temperature was raised from 25 to 50 ℃ with a ramp rate of 5 ℃ min^-1, then held at 50 ℃ for 30 min. The Ar pretreatment cleaned the catalyst surface by removing adsorbed ambient gas molecules. After the pretreatment, to decompose M_AECO₃, H₂/Ar (5/95, v/v) gas flowed through the quartz tube with the flow rate of 10 mL min^-1, and the temperature was raised from 25 to 800 ℃ (950 ℃ for BaCO₃) with a ramp rate of 5 ℃ min^-1. The outlet gas products, including H₂O (m/z = 18) , CO (m/z = 28) and CO₂ (m/z = 44) , were recorded by MS as a function of temperature.

Measurement of M_TO_x reduction temperature (T_red)

Temperature-programmed reduction measurements of nanosized commercial metal oxides (M_TO_x, M_T = Fe, Co, Ni, Cu) were conducted using a tube furnace (GSL-1100X, Kejing) combined with a quadrupole MS (HPR-20 EGA, Hiden) . Inside the quartz tube, c. a. 50 mg of the M_TO_x sample was sandwiched between two layers of quartz wool with a thermocouple in contact with the sample. The quartz tube was inserted into the tube furnace for Ar pretreatment, with the Ar flow rate of 10 mL min^-1. The temperature was raised from 25 to 50 ℃ with a ramp rate of 5 ℃ min^-1, then held at 50 ℃ for 30 min. The Ar pretreatment cleaned the catalyst surface by removing adsorbed ambient gas molecules. After the pretreatment, to reduce M_TO_x, H₂/Ar (5/95, v/v) gas flowed through the quartz tube with the flow rate of 10 mL min^-1, and the temperature was raised from 25 to 800 ℃ (950 ℃ for Fe₂O₃) with a ramp rate of 5 ℃ min^-1. M_TO_x was reduced to M_T NPs, with H₂O produced during the heating treatment. The gas product H₂O (m/z = 18) were recorded by MS as a function of temperature.

Theoretical estimation of solid-state reaction energy

Reaction energies for forming different alkali metal aluminates or alkaline earth metal aluminates (M_AEAl₂O₄) from M_AECO₃ and Al₂O₃ were estimated using the reaction calculator module in the Materials Project. Details can be found in the descriptions of Fig. 13.

Material characterisations

X-ray diffraction (XRD) Phases and crystallographic structures of mentioned samples were characterised by powder X-ray diffraction (XRD) using a parallel-beam XRD instrument (Rigaku SmartLab 9kW -Advance, with Cu Kα of wavelength) . For the measurements of the reduced and post-reaction samples, the samples were protected in the reaction tube with both ends sealed using closed ball valves, and were directly transferred into the inert gas glove box. In the inert gas glove box, the samples were taken from the reaction tube and loaded into a quartz holder, which was then covered by Kapton film (Fig. 6) . In this air-tight XRD holder, the air-sensitive samples were kept isolated from the air and can be directly measured by the XRD facility.

The crystallite size (D) was estimated by XRD using the Debye-Scherrer equation where k is the shape coefficient for the reciprocal lattice point and shape coefficient for crystal in the direct space (k = 0.9) , λ is the wavelength of the incident radiation, β is the full width at half-maximum (FWHM) of the peak, and θ is the Bragg angle.

Synchrotron X-ray diffraction (SXRD) SXRD patterns were collected using the Powder Diffraction (PD) beamline at the Australian Synchrotron (AS) , which was set to provide a photon energy of 21.0005 keVEach capillary was mounted on a crystallographic goniometer head and rotated at 15 rpm. Patterns were measured using Debye-Scherrer geometry with a goniometer diameter = 152 cm, over the 2θ range of 1.01398° to 80.8344° and with an effective step size of 0.00375°. The SXRD incident and diffracted beams were fully polarized.

Each sample was measured twice using 450 seconds acquisitions, with the Mythen detector being off-set by 0.5° for the second data acquisition. The two off-set patterns were merged using the program PDViPeR. An additional pattern was measured for a NIST SRM-660b LaB₆ powder under the same conditions to determine the instrument wavelength, the 2θ₀ correction and the instrument FWHM widths for the Bragg peaks.

The diffraction patterns were analysed by Rietveld refinement methods based on TOPAS (V6) to obtain structural details. The Thompson-Cox-Hastings pseudo-Voigt peak function was applied to describe the diffraction peaks. The scale factor and lattice parameters were allowed to refine for all the diffraction patterns. The quality of the Rietveld refinements of synchrotron data has been assured with a low goodness-of-fit (GoF) factor, a low weighted profile factor (Rwp) and a well-fitted pattern with an acceptable temperature factor (Beq) within experimental errors.

Transmission electron microscopy (TEM) The microstructure and phase information of the samples were characterised by scanning transmission electron microscopy (STEM) in high angle annular dark field (HAADF) mode and transmission electron microscopy (TEM) , using double-Cs-corrected STEM (Spectra 300, TFS, USA) equipped with Super-X energy-dispersive X-ray spectroscopy (EDX) and STEM/TEM (JEM-2100F, JEOL, Japan) combined with a Gatan Enfina electron spectrometer (USA) . TEM samples were prepared by sonicating a suitable amount of material in 1 mL ethanol for 180 seconds before the dropwise addition of the solution onto the copper grids. For the reduced and post-reaction samples, the samples were protected in the reaction tube with both ends sealed using closed ball valves and were directly transferred into the inert gas glove box, where the samples can be extracted from the reaction tube and dispersed in ethanol solution for further deposition onto copper grids.

Scanning electron microscopy with energy dispersive X-ray analyses (SEM-EDX) Scanning electron microscopy (SEM) images were recorded on a Tescan VEGA3 Model GMH microscope at an accelerating voltage of 5 kV. The atomic ratios for metal elements including Co, Ba, and Al of mentioned samples were measured by EDX.

Electron paramagnetic resonance (EPR) The electron paramagnetic resonance (EPR) analyses was performed by a Bruker EMX Plus spectrometer equipped with a dual-mode cavity (ER 4116DM) . The microwave frequency and power were 9.83 GHz and 3.17 mW, respectively. The weight of the sample was 50.0 mg.

X-ray absorption fine structure (XAFS) X-ray absorption fine structure (XAFS) measurements were performed in fluorescence mode using a Lytel detector at beamline BL01C of Taiwan Light Source, National Synchrotron Radiation Research Center (NSRRC) . A Si (111) double crystal monochromator (DCM) was used to scan the photon energy. To ascertain the reproducibility of the experimental data, at least two scans were collected and compared for each sample. X-ray absorption near edge structure (XANES) data were background-subtracted and normalised using the AUTOBK routine in Athena software. Quantitative information on the radial distribution of neighbouring atoms surrounding Co and Ba atoms was derived from the extended absorption fine structure (EXAFS) data. An established data reduction method was used to extract the EXAFS χ-functions from the raw experimental data using the IFEFFIT software. For the wavelet transform, k³-weighted EXAFS spectra were used. The Morlet wavelet mother function was used for the wavelet transform (WT) of all spectra within the k-range offor Co K-edge andfor Ba L₃-edge, respectively. The selection of this wavelet was governed by the fast oscillatory part localised in a Gaussian envelope, making its real and imaginary parts similar to an EXAFS spectrum.

X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) experiments were carried out using a Thermo Scientific Nexsa instrument. The instrument was equipped with an electron flood and scanning ion gun. The adventitious carbonaceous C1 s line (284.8 eV) was used to calibrate the binding energy (BE) . The XPS spectra were deconvoluted using Gaussian-Lorentzian functions after the Shirley background subtraction, with the integrated peak areas being used to estimate the surface chemical compositions.

In-situ Raman Spectroscopy During a typical in-situ Raman measurement, c. a. 5 mg samples were loaded into the in-situ Raman cell, with H₂/Ar (5/95, v/v) passing through the samples. A series of Raman spectra were obtained at different temperatures, from room temperature to 700 ℃ with an interval of 50 ℃, to investigate the phase transformation during the H₂ thermal treatment process. Each temperature was set dwelling for 40 min, and the spectra were collected at the 30^th min to get a steady state. In-situ Raman spectra were recorded on a WITEC confocal microscopy system with a laser diode at 532 nm. A 50× objective lens was used to focus the laser on the sample, and the laser spot size was 1 μm. The Raman measurements were performed under an accumulation time of 60 s and an accumulation number of 10 times by illuminating 3 mW laser power. Fig. 11 shows the in-situ Raman setup. Equipped with a quadrupole MS (HPR-20 EGA, Hiden) , the composition and concentration of the outlet gas were analysed, with H₂ (m/z = 2) , H₂O (m/z = 18) and CO (m/z = 28) recorded as a function of temperature.

In-situ STEM Direct visualisation of the encapsulation process of Co@BaAl₂O₄ was achieved using the in-situ atmospheric STEM study (JEOL JEM-2100F) . During the in-situ STEM observation, a Protochips atmosphere gas holder was applied (Fig. 8a) , in which the flowing gas can be injected, and the temperature can be controlled. In this experiment: 380 Torr H₂/N₂ (5/5, v/v) with 0.1 mL/min flow rate were used, and the temperature was set from 25 to 800 ℃.

The effect of the electron beam was carefully evaluated by comparing the sample state in the observation region with that out of the observation region. The initial precursor, C (500) -40Co: 20Ba: 40Al, was firstly prepared on the bottom cell and characterised by STEM and EDX before heating the in-situ cell. The bottom cell was then assembled in the in-situ holder, followed by injecting an H₂/N₂ mixture gas into it. The temperature was gradually increased to 300, 400, 500, 600, 700, and 800 ℃ at a rate of 20 ℃/min. Each target temperature was kept for 30 min for a thorough reaction. Fig. 8 shows the in-situ STEM setup. The introduction of gas into the vacuum environment of the TEM is made possible by injecting and sealing the gas in the space between two windowed silicon slabs (the E-chips product from Protochips, shown in Fig. 8b) . The windowed slabs have notches near their centres and are covered with amorphous silicon nitride thin films of thickness 80 nm that serve as windows.

NH₃ decomposition performance testing method and setup

Fig. 9 is the schematic of the NH₃ decomposition performance testing setup. To measure the catalytic NH₃ decomposition performance of designed catalysts, inside a quartz tube (diameter of 4.5 mm) , c. a. 50 mg sieved (45-80 mesh) sample was sandwiched between two layers of quartz wool with a thermocouple placed in contact with the sample. Then high-purity NH₃ (≥ 99.99%) was passed through the catalyst bed with the flow rate controlled by a mass flow controller. The weight hourly space velocity (WHSV) was set as 30,000 mL g_cat ^-1 h^- ¹ at atmospheric pressure. The concentrations of outlet N₂, H₂ and NH₃ after the reaction were measured online by MS (HPR-20 EGA, Hiden) , which was equipped with a quadrupole probe and a secondary electron multiplier detector (850 eV) . The accuracy of product analyses was further verified by back titration (Fig. 25B) . The measured temperatures range from 450 to 650 ℃ with 50 ℃ as an interval, and a steady state was reached by maintaining each temperature for 60 min.

The NH₃ conversion was calculated using the following Eq. 1

where [NH₃] _inlet and [NH₃] _outlet refer to the measured concentrations of NH₃ fed into and flowing out of the reactor.

The H₂ production rate, with the unit of mmol H₂ g_cat ^-1 min^-1, was calculated from the NH₃ conversion (X_NH3) by the Eq. 2 below:

where WHSV is the weight hourly space velocity (30,000 mL g_cat ^-1 h^-1) , X_NH3 is the conversion of NH₃, and V_m is the molar volume of gas at 25 ℃ and 1 atm (24 mL mmol^-1) .

To verify the reliability of the performance testing equipment, two benchmark catalysts were synthesised following the reported methods, those are Ru/MgO by polyol method and Ru/Al₂O₃ by co-precipitation method. Their NH₃ decomposition performances were tested using our equipment at 500 to 650 ℃ and WHSV of 30,000 mL g_cat ^-1 h^-1. The obtained result was compared to the reported values. As shown in Fig. 25a, our measured NH₃ conversions are almost the same as those reported in the literature, indicating that our performance testing equipment is reliable. In addition, to guarantee the accuracy of the reaction product analyses method, both MS analyses and back titration experiment were applied to analyse the NH₃ content in the outlet gas (Fig. 25b) . The experimental details of back titration analyses have been reported in our previous studies. It can be observed that the results of the two analytical methods are consistent, only with a tiny difference of less than 3%in NH₃ conversion, hence confirming the accuracy of our analyses method for reaction product compositions.

Methane dry reforming performance testing

Catalytic methane (CH₄) dry reforming activity testing was performed under atmospheric pressure in a fixed-bed quartz reactor (5 mm inner diameter) . Before each reaction, 100 mg of catalyst was placed in the reactor tube between quartz wool plugs and reduced in situ at 700 ℃ under flowing H₂/Ar (5/95, v/v, 60 mL min^-1) for 2.5 h. Dry reforming of CH₄ was then undertaken at 750 ℃ using a mixture of CO₂/CH₄ in Ar (45/45/10, v/v/v, 53 mL min^- ¹ total flow rate) to generate a weight hourly space velocity (WHSV) of 31, 765 mL g_cat ^-1 h^-1. Reactant and product concentrations were analysed online by a MS (QGA, Hiden) equipped with a quadrupole probe and a secondary electron multiplier detector. CH₄ and CO₂ conversions were calculated respectively according to the following Eqs. 3 and 4:

Where [CH₄] and [CO₂] are the respective mole fractions of CH₄ and CO₂ in the stream, and F_inlet and F_outlet represent the total gas flow rate (mL min^-1) of the inlet and outlet, respectively.

The selectivity of H₂ and CO are presented by the following Eqs. 5 and 6:

Claims

A method of preparing core-shell nanoparticles, the method comprising: contacting a mixture comprising a first metal oxide, a first encapsulating material comprising an alkali metal or an alkaline earth metal, and a second encapsulating material comprising a second metal oxide with hydrogen gas at a temperature higher than the hydrogen reduction temperature of the first metal oxide and lower than the decomposition temperature of the first encapsulating material resulting in the reduction the first metal oxide and the formation of metal nanoparticles that are at least partially encapsulated with a bimetallic oxide comprising the alkali metal or the alkaline earth metal and the second metal oxide thereby forming the core-shell nanoparticles, wherein the Tammann temperature of the first encapsulating material is lower than the hydrogen reduction temperature of the first metal oxide.
The method of claim 1 further comprising providing screening data of changes in enthalpy of possible reaction products of a reaction of a candidate first metal oxide, a candidate first encapsulating material, a candidate second encapsulating material, and hydrogen gas; and selecting the first encapsulating material and the second encapsulating material based on the screening data and the decomposition temperature of the candidate first encapsulating material.
The method of claim 2, wherein the screening data is generated using a computer.
The method of claim 1 further comprising selecting the first encapsulating material and the second encapsulating material based on the hydrogen reduction temperature of the first metal oxide; the decomposition temperature of the first encapsulating material and the Tammann temperature of the first encapsulating material.
The method of claim 1, wherein the first metal oxide comprises a metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Se, Sn, Pt, Ru, Pd, W, Ir, Os, Rh, Nb, Ta, Pb, Bi, Au, Ag, Sc, and Y.
The method of claim 1, wherein the first metal oxide is selected from the group consisting of CoO₃, Co₃O₄, CoO, CuO, Cu₂O, NiO, Ni₂O₃, Ag₂O, AgO, FeO, Fe₂O₃, Fe₃O₄, ZnO, PdO, PdO₂, PtO, and PtO₂.
The method of claim 1, wherein the first encapsulating material comprises an alkali metal or an alkaline earth metal salt comprising one or more anions selected from the group consisting of carbonate, sulfate, nitrate, chloride, phosphate, and acetylacetonate.
The method of claim 1, wherein the first encapsulating material comprises CaCO₃, MgCO₃, SrCO₃, BaCO₃, NaCl, KCl, RbCl, Na₂CO₃, K₂CO₃, or Rb₂CO₃.
The method of claim 1, wherein the second metal oxide comprises Al₂O₃, TiO₂, silica, Ga₂O₃, In₂O₃, ZrO₂, or a mixture thereof.
The method of claim 8, wherein the first metal oxide is selected from the group consisting of CoO₃, Co₃O₄, CuO, Cu₂O, NiO, Ni₂O₃, Ag₂O, AgO, FeO, Fe₂O₃, Fe₃O₄, ZnO, PdO, PdO₂, PtO, and PtO₂.
The method of claim 1, wherein the bimetallic oxide comprises MgAl₂O₄, CaAl₂O₄, SrAl₂O₄, BaAl₂O₄, MgTiO₃, CaTiO₃, SrTiO₃, BaTiO₃, MgGa₂O₄, CaGa₂O₄, SrGa₂O₄, BaGa₂O₄, MgIn₂O₄, CaIn₂O₄, SrIn₂O₄, BaIn₂O₄, MgZrO₃, CaZrO₃, SrZrO₃, or BaZrO₃.
The method of claim 1, wherein each of the core-shell nanoparticles comprise a shell comprising MgAl₂O₄, CaAl₂O₄, SrAl₂O₄, BaAl₂O₄, MgTiO₃, CaTiO₃, SrTiO₃, BaTiO₃, MgGa₂O₄, CaGa₂O₄, SrGa₂O₄, BaGa₂O₄, MgIn₂O₄, CaIn₂O₄, SrIn₂O₄, BaIn₂O₄, MgZrO₃, CaZrO₃, SrZrO₃, or BaZrO₃; and a core comprising Co, Cu, Ni, Ag, Fe, Zn, Pd, or Pt.
The method of claim 1, wherein the core-shell nanoparticles are selected from the group consisting of Co@BaAl₂O₄, Cu@BaAl₂O₄, Ni@BaAl₂O₄, Co@BaTiO₃, Fe@MgGa₂O₄, Co@MgGa₂O₄, and Ni@BaZrO₃.
The method of claim 13, wherein the core-shell nanoparticles have an average diameter of 10-15 nm.
The method of claim 1 further comprising calcining the mixture prior to the step of contacting the mixture with hydrogen gas.
The method of claim 15, wherein calcining is conducted below the decomposition temperature of the first encapsulating material.
The method of claim 1 further comprising contacting a first metal nitrate, an alkali metal or an alkaline earth metal nitrate, a second metal nitrate, and an alkali metal carbonate thereby forming a coprecipitate; and calcining the coprecipitate thereby forming the mixture.
The method of claim 7 further comprising providing screening data of changes in enthalpy of possible reaction products of a reaction of a candidate first metal oxide, a candidate first encapsulating material, a candidate second encapsulating material, and hydrogen gas; and selecting the first encapsulating material and the second encapsulating material based on the screening data and the decomposition temperature of the candidate first encapsulating material; and selecting the first encapsulating material and the second encapsulating material based on the hydrogen reduction temperature of the first metal oxide; the decomposition temperature of the first encapsulating material and the Tammann temperature of the first encapsulating material.
The method of claim 18, wherein the first metal oxide comprises a metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Se, Sn, Pt, Ru, Pd, W, Ir, Os, Rh, Nb, Ta, Pb, Bi, Au, Ag, Sc, and Y.
The method of claim 19 further comprising contacting a first metal nitrate, an alkali metal or an alkaline earth metal nitrate, a second metal nitrate, and an alkali metal carbonate thereby forming a coprecipitate; and calcining the coprecipitate thereby forming the mixture.