RU2532756C1 - METHOD OF SYNTHESIS OF NANOSTRUCTURAL COMPOSITE CeO2-PdO MATERIAL - Google Patents

Abstract

FIELD: nanotechnology.

SUBSTANCE: invention relates to the field of nanotechnologies, namely to flame-arc technology of synthesis of nanostructural composite materials. The proposed method of synthesis of nanostructural composite CeO₂-PdO material in plasma of electric discharge comprises evacuation of vacuum chamber, filling it with inert gas, ignition of the electric direct-current arc between the graphite electrode and the metal-carbon composite electrode which is a graphite rod with the cavity perforated in the centre, and spraying the composite electrode. At that in the plasma of electric arc discharge the metal-carbon composite electrode is sprayed, in which cavity drilled in the centre the rod of cerium is mounted, which is wrapped in palladium foil, and the weight ratio Pd/Ce is from 3 to 9%. Then, annealing of the synthesised material is performed, which comprises Ce₂O₃nanocrystals and a small amount of Pd nanocrystals with characteristic sizes of 2-5 nm, by heating it in the oxygen-containing environment with the atmospheric pressure to the temperature of 600, 700, 800, 900°C, with exposure for 2 hours and slow cooling.

EFFECT: method enables to obtain nanostructural composite material CeO₂-PdO which can be used as a catalyst having high activity at low temperatures as well as improved thermal stability and resistance to corrosion in aggressive environments.

2 cl, 6 dwg

Description

The invention relates to the field of nanotechnology. The invention relates to a plasma-arc technology for the synthesis of nanostructured composite materials, in particular, a CeO ₂ -PdO catalyst, designed not only to neutralize automobile exhausts, but also to a number of industrial processes for purification, afterburning, cracking of heavy hydrocarbons on zeolites, reforming of CH ₄ with CO ₂ and H ₂ O, steam reforming CO, etc.

The PGM / CeO ₂ systems, where PGM are the platinum group metals or the active component, are studied extensively and intensively, and fundamental principles on the structure of low temperature oxidation of carbon monoxide (NTO CO) catalysts have been formulated [R. Bera, KC Patil, V. Jayaram, GN Subbanna, MS Hegde, Journal of Catalysis 196 (2000) 293-301; M. Cargnello, T. Montini, S. Polizzi, NL Wieder, RJ Gorte, M. Graziani, P. Fornasiero, Dalton Transactions 39 (2010) 2122-2127; G. Glaspell, HMA Hassan, A. Elzatahry, V. Abdalsayed, M. Samy El-Shall, Topics in Catalysis 47 (2008) 22-31, etc.], the main of which is the need for strong interaction of PGM - CeO ₂ with the transfer of PGM (or part thereof) to an oxidized state.

Based on the formulated provisions, a modern tendency is clearly outlined in the synthesis of NTO CO catalysts, which consists in depositing the active component on the surface of the support (oxides of elements with variable valency (OEPV)) in the maximum ionic form, which, of course, can only be achieved due to the strong interactions with the surface of OEPV, with the formation of joint oxides of PGM-OEPV in the form of surface solid solutions, or their mixed clustered (nanostructured) forms.

At present, the methods for the formation of CO afterburning catalysts are usually associated with the deposition of PGM salts on the surface of a previously prepared carrier, followed by their reduction, drying, annealing, etc., i.e. according to the standard methodology of impregnation. In this case, the absence of stability of the formed surface phases of the PGM and OEPV catalyst, both at elevated reaction temperatures and during local overheating of the catalyst particles, cannot be ruled out. The instability of the surface phases can lead to both their separation into the constituent phases of individual oxides and the diffusion of PGM ions into the OEPV volume with a loss of concentration on the OEPV surface. Both processes will inevitably lead to a decrease in catalytic activity.

To obtain mixed phases of PGMs and OEPVs, completely new methods of preparing catalysts have recently been created. In this regard, the "solution-combustion" method [P. Bera, K.C. Patil, V. Jayaram, G.N. Subbanna, M.S. Hegde, Journal of Catalysis 196 (2000) 293-301; K.R. Priolkar, P. Bera, P.R. Sarode, M.S. Hegde, S. Emura, R. Kumashiro, N.P. Lalla, Chemistry of Materials 14 (2002) 2120-2128; M.S. Hegde, G. Madras, K.C. Patil, Accounts of Chemical Research 42 (2009) 704-712], which consists in the fact that the formation of the active component and the carrier occurs simultaneously in one stage and leads to the formation of ionic states of the active component both on the surface and in the bulk of the carrier. Despite the fact that this method is simple and gives good results in catalytic activity, it leads to a strong morphological and microstructural heterogeneity, as well as a diffuse electronic structure for several states.

A known approach associated with the use of plasma technologies in the preparation of surface phases and the so-called inverse catalysts [K. Mašek, M. Václavů, P. Bábor, V. Matolín, Applied Surface Science 255 (2009) 6656-6660; T. Skála, K. Veltruská, L. Sedláček, K. Mašek, I. Matolínová, V. Matolín, Applied Surface Science 253 (2007) 5400-5403; M. Škoda, J. Libra, F. Šutara, N. Tsud, T. Skála, L. Sedláček, V. Cháb, K.C. Prince, V. Matolín, Surface Science 601 (2007) 4958-4965; F. Yang, J. Graciani, J. Evans, P. Liu, J. Hrbek, J.F. Sanz, J.A. Rodriguez, Journal of the American Chemical Society 133 (2011) 3444-3451].

For example, in a method for preparing a catalyst [US 2011257004, 2011-10-20 B01J 23/63; B01J 23/66; B01J 37/34], including cerium oxide and a metal selected from the group: gold, platinum, palladium, ruthenium or nickel, one or more magnetrons are used to spray a thin layer of catalyst onto a substrate, usually a silicone plate. Spraying a cerium oxide plate and a metal wire (Au, Pt, Pd, Sn, Ru or Ni) is carried out at a pressure of 0.6 Pa in an atmosphere of inert gas (argon). During the deposition and growth of a thin CeO ₂ layer on the substrate, it is bombarded with metal ions that disperse into the CeO ₂ layer. Thus, a cerium oxide layer with distributed metal atoms is formed having a high catalytic activity.

The main difference and advantage of the proposed method of synthesis from the specified [US 2011257004, 2011-10-20 B01J 23/63; B01J 23/66; B01J 37/34] is a significantly larger catalyst surface due to the fact that the catalyst is synthesized in the form of nanoparticles, which significantly increases the efficiency of catalytic activity per unit mass of catalyst. The second stage of the synthesis of the catalyst consists in annealing in an oxygen-containing atmosphere at temperatures up to 900 ° C, which characterizes the thermal stability of the catalyst and resistance to corrosion.

Techniques for synthesizing nanostructured materials of various compositions using the gas phase discharge method with atomization of the starting materials and their subsequent thermal calcination in various media are known in the art [John Henry J. Scott and Sara A. Majetich. Morphology, structure, and growth of nanoparticles produced in a carbon arc // 1995. - Phys. Rew. B. - Vol. 52. - No 17. - Pp.12564-12571; Maltsev V.A., Novopashin S.A., Nerushev O.A., Sakhapov S.Z., Smovzh D.V. Synthesis of metal nanoparticles on a carbon matrix // Russian panotechnologies. - 2007, Volume 2, Iss. 5-6. Pp. 85-89]. For the synthesis of metal-carbon nanoparticles, a composite metal-carbon anode is used, which is a graphite rod with a cavity drilled in the center filled with metal material. An effective erosion of the anode occurs in the DC arc, which leads to the formation of atomic components that make up the anode in the high-temperature region. By varying the electrical parameters of the discharge, the composition of the atomized electrode and the pressure of the buffer gas, it is possible to control the morphology and composition of the synthesized material, in particular, it is possible to synthesize nanoparticles with a diameter of 1 to 100 nm.

Most metals can be sprayed in this way. To obtain nanoparticles of refractory metals, metal compounds are subjected to pyrolysis.

In the invention [RU 2433888, 05/21/2010, B82B 3/00, B22F 1/00, B22F 1/02, B22F 9/14, B22F 9/20, B22F 9/02, B22F 9/16, C01B 31/30, C01B 31/00, C01B 31/34] realized the plasma-arc synthesis of tungsten carbide nanoparticles. The method includes pumping out a vacuum chamber, filling it with an inert gas, igniting an electric arc between graphite and composite electrodes, maintaining the conditions of arc burning by varying the interelectrode distance, spraying a composite electrode, which is a graphite rod with a cavity filled with a tungsten compound: WO ₃ , W (CO (CO ) ₆ , the deposition of nanostructured material on a cooled screen.

The specified method is close to the claimed method to the aggregate of essential features, but is intended to obtain another nanostructured material, which is a carbon matrix with nanoparticles of tungsten carbide.

The problem to which the present invention is directed, is to develop a method for producing a CeO ₂ -PdO catalyst having high catalytic activity at low temperatures, as well as increased thermal stability and corrosion resistance in aggressive environments.

The problem is solved by combining known methods, namely, plasma-arc synthesis of a composite metal-carbon material and annealing the synthesized material in an oxygen-containing medium.

The method for synthesizing a nanostructured composite CeO ₂ -PdO material with high catalytic activity at low temperatures, according to the invention, is implemented in two stages, carried out sequentially: the first is a plasma-arc synthesis of a metal-carbon PdCeC composite directly in an arc reactor chamber: the second is annealing the composite in an oxygen-containing medium at temperatures above 600 ° C in order to remove the carbon matrix and obtain CeO ₂ -PdO material.

Plasma-arc synthesis of a nanostructured composite CeO ₂ -PdO material includes pumping out a vacuum chamber, filling it with an inert gas selected from the group: He, Ne, Ar, Kr, Xe, igniting a direct current electric arc between a movable graphite electrode and a stationary composite (metal carbon) electrode, which is a graphite rod, in the center of the cavity drilled a rod of cerium is installed, wrapped with palladium foil, so that the weight ratio of Pd / Ce is from 3 to 9%, maintaining conditions arc burning by varying the interelectrode distance; sputtering in the electric arc of a composite electrode. At the stage of plasma synthesis, high-temperature atomic sputtering of the substances that make up the composite electrode occurs, and subsequent condensation with carbon in a buffer inert gas. The deposition of nanostructured composite (carbon-palladium-cerium) material occurs on a cooled removable screen. The synthesized material is very small particles of cerium and palladium encapsulated in a carbon matrix. The characteristic particle size is from 2 to 5 nm. The obtained X-ray diffraction patterns of the samples indicate that the main components are CeO ₂ and PdO.

Annealing the synthesized material includes placing the synthesized composite (carbon-palladium-cerium) material in a furnace, heating it in an oxygen-containing medium at atmospheric pressure to temperatures of 600, 700, 800, 900 ° C, holding for two hours and slow cooling. Two hours of exposure is enough for uniform heating of the material to the specified temperatures and the completion of all structural (phase) transformations. At temperatures from 600 ° C to 700 ° C, carbon burnout and cerium oxide saturation with oxygen end until cerianite (CeO ₂ ) is formed. Annealing at temperatures above 700 ° C leads to the formation of particles of the mixed oxide PdCeO _x .

The first stage of the process is carried out in a plasma-arc reactor, including a sealed stainless steel vacuum chamber with a fixed expendable composite electrode and a movable graphite electrode, a removable screen for deposition of the synthesized material, a vacuum pumping system, a DC power supply, a water cooling system, a gas supply and discharge system , measuring systems for monitoring pressure, electrical parameters of the discharge, temperature, flow rate of the gas phase.

The second stage of the process is implemented in a furnace made in the form of a pipe installed at an angle into which a slide with the material to be processed is placed. The temperature is controlled by a sensor system.

Experimental studies were carried out in a direct current electric arc, with a buffer gas pressure (helium used) of 25 torr, and an arc current of 100 A. The atomized electrode (anode) was a graphite rod 70 mm long and 7 mm in diameter. A hole was drilled in the center of the electrode into which a cerium rod with a diameter of 2.8 mm was installed, wrapped with palladium foil 50 μm thick to provide a Pd / Ce weight ratio of 3%.

The analysis of both the starting synthesized material and the material obtained as a result of annealing was carried out by transmission electron microscopy (JEOL - 2010) and X-ray diffraction analysis (Bruker D8 Advanced diffractometer) performed in the angle range 10-75 ° with a step of 2θ = 0. 05 ° with an accumulation time of 3 s at each point.

The study of the material (Pd + Ce) using transmission electron microscopy (TEM) and energy dispersive x-ray analysis (EDX) showed that the material mainly consists of amorphous carbon particles with sizes of 10-50 nm with inclusions of crystalline nanoparticles with sizes from 2 to 10 nm containing cerium. The obtained X-ray diffraction patterns of the metal-carbon material indicate that its main constituents are CeO ₂ and PdO.

As a result of the studies, it was determined that the catalytic activity of the Pd _x Ce _1-x O ₂ catalyst is determined by the concentration of palladium in the sample and the annealing temperature of the catalyst, while the optimum temperature is T = 700 ° C.

After annealing it in an oxygen-containing medium, the composite acquired the properties of an effective catalyst, not inferior in activity to NTO CO catalysts synthesized using IWI (incipient wetness impregnation) technologies.

Figure 1 presents a TEM photograph of a synthesized material consisting of amorphous carbon with sizes with inclusions of crystalline nanoparticles with sizes from 2 to 5 nm.

Figure 2 shows the Fourier image of the crystal lattices of the synthesized material. The positions of the reflections are identified with reference data of Ce ₂ O ₃ with a trigonal crystal lattice, which has reference data of interplanar distances of 0.303 nm {002}, 0.22502 nm {012}, 0.33602 nm {100}, 0.18476 nm {111 } and 0.29387 nm {011} [Match! Entry # 96-101-0280]. The set of equivalent planes {100} includes a family of parallel planes (010), while {011} contains (101),

Figure 3 presents a TEM photograph of the synthesized material after annealing at a temperature of 700 C. Elemental analysis showed that the material consists of cerium, palladium and oxygen.

Figure 4 shows the Fourier image of the crystal lattices of the annealed material. According to the relative position of the reflections, the interplanar distances corresponding to these reflections (0.3183 nm, 0.1904 nm, 0.3125 nm and 0.2707 nm), it was determined that the crystal lattice corresponds to cerianite (CeO ₂ ), which corresponds to the reference interplanar distances 0.3124 nm {111}, 0.19131 nm {202}, 0.27055 {200}. Since the set of equivalent {111} planes includes the (111) and (-111) planes, and (022) is included in {202}, and (- 200) in {200}, it can be concluded that the parameters of the Fourier picture are good consistent with the reference data, and the crystal lattice shown in Fig.4, belongs to the crystal CeO ₂ .

Figure 5 shows the experimental x-ray of the annealed sample. All peaks correspond to CeO ₂ reflections. At 34 ° along the 2θ, the shoulder appears, associated with the crystallization of PdO. The data confirm the measurements on the analysis of interplanar distances obtained by processing TEM images.

Figure 6 shows the TPR-CO-O ₂ curves for 3PdCeC, 3PdCe-450, 3PdCe-700 catalysts obtained upon first heating in the reaction mixture to 450 ° C (1) and upon reheating after cooling in the reaction mixture (2) . It is seen that the initial carbon-containing composite 3PdCe turns out to be inactive in the region of NTO CO, and the temperature of 50% conversion (T ₅₀ ) is about 260 ° C. Cooling in the reaction mixture and reheating lead to a sharp increase in catalytic activity, and the T ₅₀ value decreases to 150 ° C. This is due to carbon burnout during the first heating, which leads to the formation of an accessible surface of the active component for the reactants of the reaction mixture. However, the catalyst remains inactive at low temperatures. Calcination of the composite in air at 450 ° C, and then at 700 ° C, leads to a further increase in activity (T ₁₀ = 25 ° C, T ₅₀ = 60 ° C), and the catalyst, according to these parameters of NTO CO, approaches the efficient catalysts obtained traditional methods of impregnation.

The performed experimental studies showed that the metal-carbon composite after annealing acquires a uniform microstructure, which affects the properties of the obtained material. The resulting material is characterized by high catalytic activity at low temperatures, increased thermal stability and corrosion resistance in aggressive environments.

Claims (2)

1. A method of synthesizing a nanostructured composite CeO ₂ -PdO material in an electric discharge plasma, including pumping out a vacuum chamber, filling it with an inert gas, igniting a direct current electric arc between a graphite electrode and a metal-carbon composite electrode, which is a graphite rod with a center-drilled cavity and spraying a composite electrode, characterized in that a metal-carbon composite electrode is sprayed in a drilled plasma in an electric arc discharge plasma Central cavity whereof a rod of cerium, palladium foil wrapped, wherein the weight ratio of Pd / Ce is between 3 and 9%, and then annealing is performed synthesized material comprising nanocrystals Ce ₂ O ₃ and small amounts of Pd nanocrystals with characteristic sizes of 2-5 nm , by heating it in an oxygen-containing medium at atmospheric pressure to temperatures of 600, 700, 800, 900 ° C, holding for 2 hours and slow cooling. 2. The method according to claim 1, characterized in that the inert gas is selected from the group: He, Ne, Ar, Kr, Xe.

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