CN108573124B - Method for quantitatively analyzing hydrogen evolution activity of metal-embedded carbon nanotube catalyst - Google Patents
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Abstract
The invention discloses a method for quantitatively analyzing hydrogen evolution activity of a metal-embedded carbon nanotube (MCNT) catalyst. The method is used for quantitatively analyzing the hydrogen evolution activity of the MCNT catalyst based on the quantum chemical density functional theory simulation. Determining the stable existence of the model in the hydrogen evolution reaction process through the stability research of the MCNT model; screening according to the adsorption sites, and determining the specific position of the surface of the catalyst for hydrogen evolution reaction; and then, the hydrogen adsorption free energy and the hydrogen evolution reaction path are combined for calculation, and the hydrogen evolution activity of the MCNT catalyst is quantitatively analyzed. The method comprises the steps of MCNT catalyst model construction, model structure optimization, stability calculation, adsorption site screening, hydrogen adsorption free energy and hydrogen evolution reaction path calculation, and hydrogen evolution activity analysis and characterization. The method can quantitatively analyze the hydrogen evolution activity of the MCNT catalyst without carrying out actual experiments and actual synthesis of the catalyst.
Description
Technical Field
The invention relates to the determination of hydrogen evolution activity of a carbon nanotube material, in particular to a method for quantitatively analyzing the hydrogen evolution activity of a metal-embedded carbon nanotube (MCNT) catalyst.
Background
Fossil energy is a non-renewable energy source, and combustion of fossil energy generates carbon dioxide and dust, which are major factors causing global warming and environmental pollution. The hydrogen energy is used as clean energy, and the hydrogen can be regenerated through hydrogen evolution reaction. The existing hydrogen production methods mainly comprise a coal gasification hydrogen production technology, a natural gas hydrogen production technology, a biomass hydrogen production technology and an electrolytic water hydrogen production technology. Compared with other hydrogen production technologies, the water electrolysis hydrogen production technology is clean and pollution-free, the yield is stable, and the purity of the produced hydrogen is high, so the water electrolysis hydrogen production technology is mostly adopted in commercial application. The water electrolysis hydrogen production technology is a process of introducing current into water to decompose water molecules into hydrogen and oxygen, and a stable and efficient electrode catalyst needs to be designed for reducing electric quantity consumption and improving hydrogen yield. At present, the method for evaluating the hydrogen evolution activity of the electrode catalyst is mainly an experimental method, namely, the catalyst is synthesized by the experimental method, and then the hydrogen evolution experimental test is carried out. However, the method has the disadvantages of harsh experimental operating environment requirements, complex experimental instruments, high experimental cost and blindness to the selection and preparation of new catalysts.
With the development of computer technology, quantum chemical computational simulation is increasingly applied to the design and research of new materials, and becomes a research means which is the same as an experimental method. It can carry out analog calculation on the reaction generated on the surface of the catalyst and analyze the catalytic activity of the catalyst. The quantum chemical computation simulation method is based on the quantum mechanics theory, only needs to carry out computer simulation computation, does not need to carry out real experiments, and has the advantages of high efficiency, low cost, short computation period, high repeatability and accurate result, thereby having general guiding significance. Therefore, the quantum chemical computation simulation technology can predict the performance of a new material, has high efficiency which cannot be obtained by an experimental method, and can guide the design, preparation and application of the catalyst. However, no research case for detailed determination and characterization of hydrogen evolution activity of metal-embedded carbon nanotube (MCNT) catalysts by quantum chemical computational simulation is available internationally.
Disclosure of Invention
The invention aims to overcome the defects of the experimental technology and provide a method for quantitatively analyzing the hydrogen evolution activity of a metal-embedded carbon nanotube (MCNT) catalyst. The method is based on density functional theory calculation in quantum chemical calculation to characterize and measure the hydrogen evolution activity of the MCNT catalyst. Determining the stability of the designed MCNT catalyst through calculation of formed energy, phonon dispersion spectrum and first principle molecular dynamics; quantitatively analyzing active sites generated by hydrogen evolution reaction through calculation of adsorption energy; the hydrogen evolution activity of the MCNT catalyst is analyzed by calculating the hydrogen evolution reaction path and combining the hydrogen adsorption free energy. The purpose of the invention is realized by the following technical scheme:
a method for quantitatively analyzing hydrogen evolution activity of a metal-embedded carbon nanotube catalyst comprises the following steps:
(1) catalyst model construction
Introducing a Carbon Nanotube (CNT) model into Materials Studio 7.0 according to CNT model parameters to create defects on the CNT, embedding metal atoms (M) into the defects to construct an MCNTAnd (4) modeling. In order to fully consider the action of hydrogen bonds among water molecules, a water layer model is established by adding a plurality of water molecules. Model docking of the water layer model and the MCNT model, and then placing the whole model to a vacuum layer with a minimum thickness
In a periodic box, a model of a metal-embedded carbon nanotube (MCNT) catalyst was obtained for the simulation calculations. According to the method, an adsorption structure model of the adsorbates such as hydrated protons, hydrogen atoms and hydrogen molecules on the surface of the MCNT is sequentially established.
(2) Model structure optimization
And the model structure optimization is to carry out structure optimization on the initially constructed model through quantum chemical density functional theory calculation to obtain a stable structure. The hydrogen evolution reaction process comprises three elementary reactions of a Walmer reaction, a Heliowski reaction and a Tafel reaction. Model structure optimization involves possible reactants, intermediates and products during the hydrogen evolution reaction, including atoms, free radicals and molecules; the simultaneous calculation also involves the co-adsorption structure of the relevant species in the Initial State (IS) and the Final State (FS) of each elementary reaction. In the structure optimization process, the electron exchange function adopts a Generalized Gradient Approximation (GGA) PBE and DNP base group, and the convergence standard is 0.00001 Ha.
(3) Stability calculation
The stability calculations involved formation energy, phonon dispersion spectroscopy, and first principles molecular dynamics calculations for the MCNT catalyst model. The formation energy is the energy that needs to be overcome when decomposing a condensed material into isolated monatomics, on average, to each atom. When the formation energy of the material is less than zero, a stable structure can be formed, and the more negative the formation energy, the more stable the structure. The phonon dispersion spectrum reflects the relationship between phonon energy and momentum, and when the frequencies of phonon vibration are all real frequencies, the structure is shown to be at the local minimum point of the relevant potential energy surface, and the stable structure is correspondingly arranged. The first principle molecular dynamics is a method combining a density functional theory and molecular dynamics, and can combine the temperature in the molecular dynamics with the structure calculated by the density functional theory, predict the structural state of a material at the corresponding temperature by observing the change of the bond length between atoms of the material, prove that the structure can exist stably if the bond length fluctuates at an equilibrium position, and prove that the structural stability is damaged if the bond length deviates from the equilibrium position.
(4) Adsorption site screening calculation
Determining the adsorption position of the adsorbate on the MCNT surface based on the stable adsorption structure of the adsorbate (such as hydrated protons, hydrogen atoms and hydrogen molecules) on the MCNT surface after structure optimization, calculating the energy of each adsorption structure, and obtaining the adsorption energy of the adsorbate at the adsorption position by calculating the difference between the energy of the adsorption structure and the energy of the adsorbate before adsorption and the energy of the MCNT. The more negative the adsorption energy, the more easily the adsorbate is adsorbed, and the more favorable the hydrogen evolution reaction occurs. And determining the optimal adsorption position, namely the optimal position of the hydrogen evolution reaction on the surface of the MCNT catalyst by comparing the adsorption energy of each adsorption position.
(5) Hydrogen adsorption free energy and hydrogen evolution reaction path calculation
The hydrogen adsorption free energy is calculated by subtracting the hydrogen atom (i.e., adsorbate) free energy and the MCNT free energy before adsorption from the free energy of the hydrogen adsorption structure. The hydrogen evolution reaction path involves three elementary reactions, the Walderm reaction, the Hellofski reaction and the Tafel reaction. Starting from the stable structures of the initial state and the final state of the reaction, carrying out atom pairing on the initial state and the final state of each reaction, then carrying out search calculation on a Transition State (TS) of the reaction to obtain the transition state in each reaction, and then carrying out energy calculation on the initial state, the transition state and the final state. And carrying out frequency calculation on the initial state, the transition state and the final state, and carrying out zero energy and entropy change correction on the obtained energy value to finally obtain the whole reaction path network and the structure and energy change in the reaction.
(6) Analysis and characterization of hydrogen evolution activity of catalyst
According to the data of the hydrogen adsorption free energy, the more positive the hydrogen adsorption free energy is, the more unfavorable the occurrence of the Waldermer reaction in the hydrogen evolution reaction process, and the more negative the hydrogen adsorption free energy is, the more unfavorable the occurrence of the Helofsky reaction and the Tafel reaction in the hydrogen evolution reaction process. Thus, the closer the hydrogen adsorption free energy is to zero, the higher the MCNT catalyst hydrogen evolution activity. And determining a reaction energy barrier through the reaction path, and determining a rate control step for controlling the hydrogen evolution reaction rate by combining with the hydrogen evolution reaction path network. The lower the reaction energy barrier, the easier the reaction proceeds, and the higher the hydrogen evolution activity of the MCNT catalyst.
The invention has the advantages and effects that: the invention does not need to carry out actual experiments, and solves the problems of complicated hydrogen evolution experimental equipment and corresponding operation and blindness of experimental synthesis of the novel hydrogen evolution catalyst in the prior art. The method provided by the invention is used for performing simulation calculation on the reaction path in the hydrogen evolution reaction process by applying a method based on a quantum chemical density functional theory, and performing system analysis on the hydrogen evolution activity of the novel nanotube catalyst by combining the hydrogen adsorption free energy and the reaction energy barrier in the reaction process.
Drawings
FIG. 1 is a detailed step diagram of the present invention for quantitative analysis of hydrogen evolution activity of metal-embedded carbon nanocatalysts.
Fig. 2 is a modeling process, wherein (a) is a MnCNT (5,5) modeling process; (b) is a water layer model; (c) is a MnCNT (5,5) catalyst model.
Fig. 3 is a structurally optimized stable structure, wherein (a) is a top view and a side view of mncnts (5, 5); (b) (c), (d), (e) and (f) are possible adsorption structures for a single hydrogen atom; (g) (h), (i), (j), and (k) are possible adsorption structures of the second hydrogen atom after the first hydrogen atom determines the adsorption site; (l) And (m) are the initial state and the final state of the Walmer reaction in the process of catalyzing hydrogen evolution by MnCNT (5, 5); (n) and (o) are respectively the initial and final structures of the herofski reaction in the process of MnCNT (5,5) catalyzing hydrogen evolution; and (p) and (q) are respectively the initial state structure and the final state structure of Tafel reaction in the process of catalyzing hydrogen evolution by MnCNT (5, 5).
Fig. 4 is MnCNT (5,5) stability calculation data, where (a) is the phonon dispersion spectrum of MnCNT (5, 5); (b) the (C) and (d) are the variation trends of the Mn-C bond length within 1ps of the first-principle molecular dynamics simulation at 300K, 500K and 800K, respectively.
FIG. 5 is a model of adsorption site screening calculations, wherein (a) represents all possible adsorption sites for a single hydrogen atom and the arrows represent the hydrogen atom transfer events during the structural optimization process; (b) represents all possible adsorption sites of a second hydrogen atom after one hydrogen atom has been adsorbed at the adsorption site T, and the arrows represent the transfer of hydrogen atoms during the structure.
FIG. 6 is a hydrogen evolution reaction pathway wherein (a) is a hydrogen evolution reaction pathway network; (b) is a Walmer reaction pathway; (c) a helofsky reaction pathway; (d) is a tafel reaction pathway.
Fig. 7 is a graph of curvature of Carbon Nanotubes (CNTs) of mncnts as a function of hydrogen adsorption free energy.
FIG. 8 is a graph showing the relationship between the hydrogen adsorption free energy of the intercalation metal species (M) and MCNT.
Detailed Description
The detailed steps of the method for quantitatively analyzing the hydrogen evolution activity of the metal-embedded carbon nanotube catalyst are shown in fig. 1, and the catalyst model construction, model structure optimization, stability calculation, adsorption site screening, hydrogen adsorption free energy and hydrogen evolution reaction path calculation, and catalyst hydrogen evolution activity analysis and characterization are described in detail below by taking a manganese metal (Mn) embedded (5,5) type carbon nanotube (MnCNT (5,5)) as an example. It is to be understood that the following is illustrative of the present invention only and is not intended to limit the scope of the present invention.
Example 1 quantitative analysis of hydrogen evolution activity of metal-embedded carbon nanotube MnCNT (5,5) catalyst
(1) MnCNT (5,5) catalyst model construction
A model of (5,5) -type carbon nanotubes (CNT (5,5)) was introduced into a Material Studio 7.0, two adjacent carbon atoms in the carbon nanotubes in the tangential direction were removed on the basis of the CNT (5,5) model, and Mn atoms were then embedded in the resulting defects to construct a manganese-embedded carbon nanotube (MnCNT (5,5)) model, the process of which was shown in fig. 2 (a). In order to fully consider the hydrogen bonding between water molecules, a water layer model was created as shown in fig. 2(b), in which the dotted line indicates the hydrogen bonding between water molecules. Completely covering the water layer model above the metal center of the MnCNT (5,5) model to complete the butt joint of the water layer model and the MnCNT (5,5) model, and putting the whole model to the vacuum layer with the minimum thickness
In the periodic box of (a), a simulated MnCNT (5,5) catalyst model was obtained, and the final model is shown in fig. 2 (c). According to the method, an adsorption structure model of the adsorbates such as hydrated protons, hydrogen atoms and hydrogen molecules on the surface of the MnCNT (5,5) catalyst is established in sequence.
(2) Model structure optimization
For all the models established in the step (1), using Dmol of Material Studio 7.03And optimizing the structure of the module. And performing structure optimization calculation by adopting a density functional theory, wherein the electron exchange function is a Generalized Gradient Approximation (GGA) PBE and DNP base group, and the convergence standard is 0.00001 Ha. Fig. 3 is a structurally optimized stable structure, wherein fig. 3(a) is a top view and a side view of MnCNT (5, 5); FIG. 3(b), FIG. 3(c), FIG. 3(d), FIG. 3(e) and FIG. 3(f) are possible adsorption structures for a single hydrogen atom; FIG. 3(g), FIG. 3(h), FIG. 3(i), FIG. 3(j), and FIG. 3(k) are possible adsorption structures for a second hydrogen atom after the first hydrogen atom determines the adsorption site; FIGS. 3(l) and 3(m) are the initial and final structures of the Walmer reaction during the catalytic hydrogen evolution of MnCNT (5,5), respectively; FIGS. 3(n) and 3(o) are the initial and final structures, respectively, of the Helofosky reaction during the catalytic hydrogen evolution of MnCNT (5, 5); fig. 3(p) and 3(q) are the initial and final structures of tafel reaction during the catalytic hydrogen evolution of MnCNT (5,5), respectively.
(3) Stability calculation
Starting from the structurally optimized MnCNT (5,5) (fig. 3(a)), the single-point energy calculation was performed thereon, and then the single-point energy calculation was performed on the manganese atom and the carbon atom in a free state to obtain respective energies, and it was found that the formation energy of MnCNT (5,5) was-7.54 eV, which is the energy required for breaking MnCNT (5,5) into a single atom, and this number is much larger than the energy required for the occurrence of hydrogen evolution reaction on the surface thereof, indicating that MnCNT (5,5) is stable under the reaction conditions.
Fig. 4(a) is a phonon dispersion spectrum of MnCNT (5,5), and the frequencies of phonon oscillations are all real frequencies (i.e., frequencies are positive, greater than zero) indicating that the structure is at a local minimum point of the relevant potential energy surface, and the corresponding MnCNT (5,5) is a stable structure. FIG. 4(b), FIG. 4(c) and FIG. 4(d) are M within 1ps of first principles molecular dynamics simulations at 300K, 500K, 800K, respectivelyTrend graph of Mn-C bond length in nCNT (5, 5). It can be seen that the Mn-C bond length is stabilized at 300K, 500K, 800K
Fluctuating up and down, showing that the Mn-C bond is not broken at 300K, 500K and 800K, and the MnCNT (5,5) structure exists stably.
(4) Adsorption site screening
From the models of the various possible adsorption states on the surface of MnCNT (5,5) in fig. 3, single-point energy calculation was performed, and the adsorption energy corresponding to each adsorption site was calculated using the obtained energy. All possible adsorption sites of a single hydrogen atom on the MnCNT (5,5) surface are shown in fig. 5 (a). The arrows indicate the transfer of hydrogen atoms during the structural optimization. The calculation shows that hydrogen atoms on the adsorption sites of B1, B3 and B4 on the surface of MnCNT (5,5) are transferred to the adsorption site of T2, hydrogen atoms on the adsorption site of H1 are transferred to the adsorption site of T1, hydrogen atoms on the adsorption site of B2 are transferred to the adsorption site of T3, hydrogen atoms on the adsorption site of B5 are transferred to the adsorption site of T4 in the structure optimization process, all possible adsorption states of a single hydrogen atom on the surface of MnCNT (5,5) as shown in FIG. 3 are finally obtained, and the adsorption energy corresponding to each adsorption site is calculated and obtained as shown in Table 1.
TABLE 1 adsorption energy of individual hydrogen atoms at various adsorption sites on the surface of MnCNT (5,5)
The more negative the adsorption energy, the more likely and more likely the hydrogen atom to adsorb at the adsorption site, and it can be seen from table 1 that the T2 adsorption site on the MnCNT (5,5) surface is the optimum adsorption site for a single hydrogen atom. After adsorbing one hydrogen atom based on the monoatomic optimum adsorption site (T adsorption site in fig. 5 (b)), fig. 5(b) shows all possible adsorption sites of the second hydrogen atom on the MnCNT (5,5) surface, and the arrows indicate the transfer of hydrogen atoms occurring during the structure optimization. Calculation shows that hydrogen atoms on the B2 adsorption site on the surface of MnCNT (5,5) are transferred to the T2 adsorption site, hydrogen atoms on the H1 and B1 adsorption sites are transferred to the T1 adsorption site, hydrogen atoms on the B3 and H2 adsorption sites are transferred to the T3 adsorption site in the structure optimization process, all possible hydrogen atom adsorption states shown in FIG. 3 are finally obtained, and the adsorption energy corresponding to each adsorption site is calculated and obtained as shown in Table 2. As can be seen from table 2, after the first hydrogen atom is adsorbed at the T adsorption site of MnCNT (5,5), the second hydrogen atom is most likely adsorbed at the T5 adsorption site.
TABLE 2 adsorption energy of the second hydrogen atom at each adsorption site on the surface of MnCNT (5,5)
(5) Hydrogen adsorption free energy and hydrogen evolution reaction path calculation
Starting from initial state and final state structures of a Walmer reaction, a Hellofski reaction and a Tafel reaction in the hydrogen evolution process on the surface of the MnCNT (5,5) catalyst in the figure 3 respectively, a transition state in the search reaction is calculated, energy calculation is carried out on the transition state, and the whole hydrogen evolution reaction path is finally obtained. The Walmer reaction is a process that one hydrated proton in a water layer is dissociated from water molecules and is adsorbed to the surface of the MnCNT (5,5) catalyst; the Helofosky reaction is a process of combining a hydrated proton in the water layer with hydrogen atoms adsorbed on the surface of MnCNT (5,5) to generate hydrogen gas; the tafel reaction is a process in which two hydrated protons are dissociated from water molecules and adsorbed on the surface of a catalyst at the same time, and are desorbed and combined together to generate hydrogen. FIG. 6(a) shows a reaction path network of a hydrogen evolution reaction, which is divided into two reaction paths, i.e., a Walmer-Heliowski reaction path and a Walmer-Tafel reaction path. (b) Is a Walmer reaction path on the surface of MnCNT (5,5), and the reaction energy barrier is 0.61 eV; (c) is a Hellofski reaction path on the surface of MnCNT (5,5), and the reaction energy barrier is 0.80 eV; (d) is a Tafel reaction path on the surface of MnCNT (5,5), and the reaction energy barrier is 1.72 eV. Further, the calculation revealed that the adsorption free energy of hydrogen atoms on the surface of MnCNT (5,5) was +0.118+ eV.
(6) Analysis and characterization of hydrogen evolution activity of MCNT catalyst
Theoretically, the closer the adsorption free energy is to zero, the higher the hydrogen evolution activity of the catalyst. The free energy of hydrogen adsorption of MnCNT (5,5) is 0.118eV, which is relatively close to zero, indicating that MnCNT (5,5) has high hydrogen evolution activity. On the other hand, the reaction energy barriers of the womer reaction, the helveto reaction and the tafel reaction on the surface of MnCNT (5,5) are 0.61eV, 0.80eV and 1.72eV, respectively, indicating that the MnCNT (5,5) catalyst surface favors the occurrence of the womer-helveto reaction, and is the main route of the hydrogen evolution reaction, the rate control step of which is the helveto reaction, and the reaction energy barrier is only 0.80eV, further indicating that the MnCNT (5,5) catalyst surface is easy to undergo the hydrogen evolution reaction, and the MnCNT (5,5) catalyst has very high hydrogen evolution activity.
TABLE 3 MnCNT hydrogen evolution reaction energy barriers (unit: eV) for different curvatures
Similar to the calculation and analysis process of the MnCNT (5,5), the hydrogen adsorption free energy and hydrogen evolution reaction path of the MnCNT catalyst having different CNT curvatures were further calculated, and the influence of the CNT curvature on the hydrogen evolution activity of the MnCNT catalyst was analyzed. Mn-embedded (3,3), (5,5), (7,7) and (9,9) type carbon nanotubes are respectively denoted as MnCNT (3,3), MnCNT (5,5), MnCNT (7,7) and MnCNT (9, 9). Table 3 shows hydrogen adsorption free energy and hydrogen evolution reaction energy barriers of MnCNT (3,3), MnCNT (5,5), MnCNT (7,7) and MnCNT (9, 9). Fig. 7 shows the hydrogen adsorption free energy of mncnts of different curvatures. As can be seen from fig. 7, the free energy of hydrogen adsorption of MnCNT (3,3), MnCNT (5,5), MnCNT (7,7) and MnCNT (9,9) are +0.363eV, +0.118eV, +0.184eV and +0.190eV, respectively, which shows that the free energy of hydrogen adsorption of the carbon nanotube shows a "V" type curve relationship as the curvature of the carbon nanotube increases, i.e. the hydrogen adsorption energy shows a tendency of decreasing first and then increasing, wherein the free energy of hydrogen adsorption of MnCNT (5,5) is the smallest and is the closest to zero. This indicates that, as the curvature of the carbon nanotube increases, the hydrogen evolution activity of MnCNT shows a "volcano" type curve relationship, i.e., the hydrogen evolution activity of MnCNT shows a tendency of increasing first and then decreasing, with the highest hydrogen evolution activity of MnCNT (5, 5). Further, the MnCNT hydrogen evolution reaction energy barriers with different curvatures are given in table 3. As can be seen from table 3, similar to the hydrogen evolution reaction path on the surface of MnCNT (5,5), the main path of the hydrogen evolution process of MnCNT (3,3), MnCNT (7,7) and MnCNT (9,9) is also the walmer-heronski reaction, the rate control step of which is the heronski reaction, and the corresponding reaction energy barriers are 1.87eV, 1.08eV and 1.27eV, respectively. As the curvature of the carbon nano tube increases, the hydrogen evolution reaction energy barrier of the MnCNT also presents a V-shaped curve relationship, and the hydrogen evolution reaction energy barrier further indicates that the hydrogen evolution activity of the corresponding MnCNT presents a volcano-shaped curve relationship, wherein the hydrogen evolution activity of the MnCNT (5,5) is the highest.
To analyze the effect of the intercalation metal species on the hydrogen evolution activity of MCNTs, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn were respectively intercalated into (5,5) type carbon nanotubes to obtain TiCNT (5,5), VCNT (5,5), CrCNT (5,5), MnCNT (5,5), FeCNT (5,5), CoCNT (5,5), NiCNT (5,5), CuCNT (5,5) and ZnCNT (5, 5). Fig. 8 shows the hydrogen adsorption free energy of different metal embedded carbon nanotubes and pure Pt. As can be seen from the figure, the hydrogen adsorption free energy sequence is: pure Pt (-0.090eV) < MnCNT (5,5) (+0.118eV) < FeCNT (5,5) (-0.146eV) < VCNT (5,5) (-0.273eV) < CrCNT (5,5) (-0.351eV) < CoCNT (5,5) (-0.424eV) < CuCNT (5,5) (-0.484eV) < TiCNT (5,5) (-0.540eV) < NiCNT (5,5) (-0.926) < ZnCNT (5,5) (-1.103 eV). The results show that pure Pt has the smallest free energy of hydrogen adsorption (-0.090eV), and the corresponding highest hydrogen evolution activity. The hydrogen adsorption free energy of MnCNT (5,5) is second only to Pt and close to zero in various metal-embedded carbon nanotubes, indicating that the MnCNT (5,5) catalyst has the highest hydrogen evolution activity among various metal-embedded carbon nanotubes.
Table 4 gives the hydrogen evolution reaction energy barriers for MCNT (5,5) catalysts with different metal atom insertions. As can be seen from table 4, all the metal atom intercalated MCNT (5,5) catalysts involved follow the walmer-hewski reaction, the corresponding rate controlling step being the hewski reaction. The hydrogen evolution reaction barriers of TiCNT (5,5), VCNT (5,5), CrCNT (5,5), MnCNT (5,5), FeCNT (5,5), CoCNT (5,5), NiCNT (5,5), CuCNT (5,5) and ZnCNT (5,5) are 1.88eV,1.22eV,1.32eV,0.80eV,1.20eV,1.39eV,2.03eV,1.52eV and 2.83eV, respectively, indicating that the MnCNT (5,5) catalyst has the highest hydrogen evolution activity, which is consistent with the results of hydrogen adsorption free energy analysis.
TABLE 4 MCNT (5,5) Hydrogen evolution reaction energy barriers (Unit: eV) for different metal atom insertions
Claims (4)
1. A method for quantitatively analyzing hydrogen evolution activity of a metal-embedded carbon nanotube catalyst is characterized by comprising the following steps of:
a catalyst model construction
Introducing a CNT model into Materials Studio 7.0 according to the CNT model parameters of the carbon nano tube, manufacturing defects on the CNT, and embedding metal atoms M into the defects to construct an MCNT model; in order to fully consider the action of hydrogen bonds among water molecules, a water layer model is established by adding a plurality of water molecules; model docking of the water layer model and the MCNT model, and then placing the whole model to a vacuum layer with a minimum thickness
Obtaining a metal-embedded carbon nanotube MCNT catalyst model for simulation calculation in the periodic box; according to the method, an adsorption structure model of hydrated protons, hydrogen atoms and hydrogen molecular adsorbates on the surface of the MCNT is sequentially established;
b model structure optimization
The model structure optimization is to carry out structure optimization on the initially constructed model through quantum chemical density functional theory calculation to obtain a stable structure; the hydrogen evolution reaction process comprises three elementary reactions of a Walmer reaction, a Heliowski reaction and a Tafel reaction; model structure optimization involves possible reactants, intermediates and products during the hydrogen evolution reaction, including atoms, radicals and molecules; meanwhile, calculating the co-adsorption structure of related species in the initial IS and the final FS of each elementary reaction; in the structure optimization process, the electronic exchange function adopts generalized gradient approximate GGA PBE and DNP base groups, and the convergence standard is 0.00001 Ha;
c stability calculation
The stability calculation relates to the formation energy of the MCNT catalyst model, phonon dispersion spectrum and first-principle molecular dynamics calculation; the formation energy is the energy that the condensed material needs to overcome when decomposed into isolated monatomics on average to each atom; when the formation energy of the material is less than zero, a stable structure can be formed, and the more negative the formation energy is, the more stable the structure is; the phonon dispersion spectrum reflects the relationship between phonon energy and momentum, and when the frequencies of phonon vibration are all real frequencies, the structure is shown to be at the local minimum point of a relevant potential energy surface, and a stable structure is correspondingly arranged; the first principle molecular dynamics is a method combining a density functional theory and molecular dynamics, and can combine the temperature in the molecular dynamics with the structure calculated by the density functional theory, predict the structural state of a material at corresponding temperature by observing the change of the bond length between atoms of the material, prove that the structure can exist stably if the bond length fluctuates up and down at an equilibrium position, and prove that the structural stability is damaged if the bond length deviates from the equilibrium position;
d adsorption site screening
Determining the adsorption position of the adsorbate on the MCNT surface based on the stable adsorption structure of the adsorbate on the MCNT surface after the structure optimization, such as hydrated protons, hydrogen atoms and hydrogen molecules, calculating the energy of each adsorption structure, and obtaining the adsorption energy of the adsorbate at the adsorption position by calculating the difference between the energy of the adsorption structure and the energy of the adsorbate before adsorption and the energy of the MCNT; the more negative the adsorption energy, the easier the adsorption site is to adsorb adsorbate, and the more favorable the hydrogen evolution reaction is; determining the optimal adsorption position, namely the optimal position of the hydrogen evolution reaction on the surface of the MCNT catalyst by comparing the adsorption energy of each adsorption position;
e calculation of hydrogen adsorption free energy and hydrogen evolution reaction path
Calculating the free energy of hydrogen adsorption by subtracting the free energy of hydrogen atoms before adsorption, namely the free energy of adsorbates and the free energy of MCNT from the free energy of the hydrogen adsorption structure; the hydrogen evolution reaction path relates to three elementary reactions of a Walmer reaction, a Heliowski reaction and a Tafel reaction; starting from stable structures of initial states and final states of the reactions, carrying out atom pairing on the initial states and the final states of the reactions, then carrying out search calculation on a transition state TS of the reactions to obtain transition states in the reactions, and then carrying out energy calculation on the initial states, the transition states and the final states; carrying out frequency calculation on the initial state, the transition state and the final state, and carrying out zero energy and entropy change correction on the obtained energy value to finally obtain the whole reaction path network and the structure and energy change in the reaction;
f analysis and characterization of hydrogen evolution activity of catalyst
Calculating data according to the hydrogen adsorption free energy, wherein the more positive the hydrogen adsorption free energy is, the more adverse to the occurrence of the Waldermer reaction in the hydrogen evolution reaction process, and the more negative the hydrogen adsorption free energy is, the more adverse to the occurrence of the Helofsky reaction and the Tafel reaction in the hydrogen evolution reaction process; therefore, the closer the hydrogen adsorption free energy is to zero, the higher the hydrogen evolution activity of the MCNT catalyst is; determining a reaction energy barrier through a reaction path, and determining a rate control step for controlling the hydrogen evolution reaction rate by combining a hydrogen evolution reaction path network; the lower the reaction energy barrier, the easier the reaction proceeds, and the higher the hydrogen evolution activity of the MCNT catalyst.
2. The method of claim 1, wherein the method comprises the steps of: the characterization means comprises formation energy, phonon dispersion spectrum, first principle molecular dynamics, hydrogen adsorption free energy and a hydrogen evolution reaction path.
3. The method of claim 1, wherein the method comprises the steps of: the metal M is Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn.
4. The method of claim 1, wherein the method comprises the steps of: the carbon nano tube CNT is a CNT (3,3), a CNT (5,5), a CNT (7,7) and a CNT (9,9) type carbon nano tube.
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