Methods for determining the free energy of dynamic processes on surfaces

Abstract

The present application relates to the field of chemical catalysis technology, and in particular to a method for determining the free energy of a surface dynamic process. The method comprises the following steps to determine the free energy of a surface dynamic process of a heterogeneous catalytic reaction: establishing an interface adsorption model; performing molecular dynamics simulation on the molecular phase of the reactants so that the molecules of the reactants reach an equilibrium state; fixing a series of distances between the adsorbed molecules of the reactants and the adsorption sites of the catalyst, and performing constrained molecular dynamics calculations on the system of each fixed distance; analyzing the results of the constrained molecular dynamics calculations to obtain the mean force potential; integrating the mean force potential and the fixed distance using the trapezoidal integration method to obtain the free energy change of the adsorption process. This method can rigorously determine the energy dynamic change process of the adsorbed molecules from the reactant phase to different sites on the catalyst surface. Through the free energy barrier and thermodynamic evaluation of different sites, the optimal adsorption site of the adsorbed molecules on the catalyst surface can be determined.

Description

Technical Field

[0001] The present application belongs to the field of chemical catalysis technology, and in particular relates to a method for determining the free energy of a surface dynamic process. Background Art

[0002] At present, in the field of theoretical catalysis, great progress has been made in understanding the elementary reactions on catalyst surfaces. However, the determination of the molecular adsorption primitives at the gas-solid (or liquid-solid) interface and the surface adsorption sites containing a variety of different sites is crucial for heterogeneous catalysis and even determines the catalytic mechanism and activity trend. However, there has been a lack of effective methods to quantitatively describe the thermodynamics / kinetics of the adsorption process, which has seriously affected the accurate determination of the rate-limiting step in the entire reaction network.

[0003] Molecular adsorption / desorption at the gas-solid (or liquid-solid) interface is a crucial basic step in heterogeneous catalysis, which usually directly determines the catalytic mechanism and activity trend. However, the quantification of adsorption / desorption kinetics at the molecular dynamics level is far lower than expected, especially for metal oxides / metal sulfides / metal nitrides, which seriously affects the accurate determination of the rate-limiting step in the real catalytic network. Due to the entropy effect that is difficult to describe accurately, it is difficult to obtain the molecular adsorption / desorption barrier by traditional total energy calculation methods. People have to resort to empirical approximation methods, namely mobile / fixed models or collision theory, to roughly estimate the adsorption / desorption energy barrier, which undoubtedly reduces the reliability of kinetic modeling. For important di / multi-site metal oxide / metal sulfide / metal nitride catalytic systems, due to the lack of quantitative adsorption / desorption kinetics, the determination of adsorption sites only from thermodynamic properties is often seriously controversial, which leads to incorrect mechanism / activity determination. For example, in the well-known low-temperature catalytic carbon monoxide (CO) oxidation reaction of cobalt tetroxide Co3O4(110), the adsorption site of CO (metal Co 3+ vs. Nonmetallic O 2c / O 3c ) is a long-standing problem that has hindered the targeted optimization of catalysts. From the perspective of total energy calculation, the di-coordinated oxygen (O 2c ) site is considered to be the adsorption site because it has the strongest binding energy for CO. However, by calculating the adsorption free energy barrier, this is completely wrong; Co 3+ is the adsorption site, and O 2c Sites are reactive sites along the CO oxidation reaction coordinate. Therefore, it is highly desirable to develop quantitative adsorption / desorption kinetics in these systems.

[0004] Calculating the adsorption free energy change and energy barrier by fixing the distance between the adsorbed molecules and the surface is relatively effective for calculating single-element systems. However, it has great limitations for multi-site oxide systems / sulfide systems, because by fixing the distance between the surface and the adsorbed molecules, it is impossible to effectively distinguish the changes in the adsorption energy barrier caused by the changes in the local potential field when the adsorbed molecules are adsorbed at different sites. The average force potential at different sites cannot be accurately calculated, so it is impossible to accurately determine the adsorption active sites. Summary of the invention

[0005] The purpose of this application is to provide a method for determining the free energy of surface dynamic processes, aiming to solve the technical problem of how to describe the potential field changes from reactant molecules to catalyst surface sites in multiphase catalytic reactions, so as to accurately determine the energy dynamic change process of reactant adsorbed molecules adsorbed to different sites on the catalyst surface.

[0006] In order to achieve the above application purpose, the technical solution adopted in this application is as follows:

[0007] The present application provides a method for determining the surface dynamic process free energy, the method comprising: mixing reactants and catalysts to perform a heterogeneous catalytic reaction, and then determining the surface dynamic process free energy of the heterogeneous catalytic reaction by the following steps:

[0008] Establish an interface adsorption model;

[0009] Performing molecular dynamics simulation on the molecular phase state of the reactants to make the molecules of the reactants reach an equilibrium state;

[0010] A series of distances between the adsorbed molecules of the reactant and the adsorption sites of the catalyst are fixed, and a constrained molecular dynamics calculation is performed on each fixed distance system;

[0011] Analyzing the results of the constrained molecular dynamics calculation to obtain a mean force potential;

[0012] The mean force potential and the fixed distance are integrated using the trapezoidal integration method to obtain the free energy change of the adsorption process.

[0013] The present application provides a method for determining the free energy of a surface dynamic process. The method first establishes an interface adsorption model, and then performs molecular dynamics simulation on the molecular phase of the reactants to allow the molecules to reach an equilibrium state; the mean force potential is obtained by performing constrained molecular dynamics calculations by fixing a series of distances between the adsorbed molecules of the reactants and the adsorption sites of the catalyst, and the free energy change of the adsorption process is obtained by using the trapezoidal integration method. The present application can calculate and accurately describe the potential field change of the molecule from the reactant phase to the catalyst surface site by directly fixing the distance between the adsorbed molecules and the surface adsorption sites, and rigorously determine the energy dynamic change process of the adsorbed molecules from the reactant phase to different sites on the catalyst surface. Through the free energy barrier and thermodynamic evaluation of different sites, the optimal adsorption site of the adsorbed molecule on the catalyst surface can be determined. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0015] Figure 1 It is a schematic diagram of a flow chart of determining the surface dynamic process free energy of a heterogeneous catalytic reaction in the method for determining the surface dynamic process free energy provided in an embodiment of the present application;

[0016] Figure 2 It is a schematic flow chart of determining the surface dynamic process free energy in the multiphase catalytic reaction of Co3O4 and CO provided in the examples of the present application. DETAILED DESCRIPTION

[0017] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0018] In this application, the term "and / or" describes the association relationship of associated objects, indicating that there may be three relationships. For example, A and / or B can mean: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. The character " / " generally indicates that the associated objects are in an "or" relationship.

[0019] In the present application, "at least one" means one or more, "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items.

[0020] It should be understood that in the various embodiments of the present application, the size of the serial numbers of the above-mentioned processes does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0021] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a", "said" and "the" used in the embodiments of the present application and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0022] The present invention provides a method for determining the free energy of a surface dynamic process, which comprises mixing reactants with catalysts for a heterogeneous catalytic reaction, and then using Figure 1 The process steps shown in the figure determine the surface dynamic process free energy of heterogeneous catalytic reactions as follows:

[0023] S01: Establish interface adsorption model;

[0024] S02: Perform molecular dynamics simulation on the molecular phase of the reactants to make the molecules of the reactants reach equilibrium;

[0025] S03: Fix a series of distances between the adsorbed molecules of the reactants and the adsorption sites of the catalysts, and perform constrained molecular dynamics calculations on each fixed distance system;

[0026] S04: Analyze the results of the constrained molecular dynamics calculations to obtain the mean force potential;

[0027] S05: The trapezoidal integration method is used to integrate the mean force potential and the fixed distance to obtain the free energy change of the adsorption process.

[0028] The embodiment of the present application provides a method for determining the free energy of the surface dynamic process in a heterogeneous catalytic reaction. The method first establishes an interface adsorption model, and then performs molecular dynamics simulation on the molecular phase of the reactants to allow the molecules to reach an equilibrium state; the mean force potential is obtained by performing constrained molecular dynamics calculations by fixing the series of distances between the adsorbed molecules of the reactants and the adsorption sites of the catalyst, and the free energy change of the adsorption process is obtained by using the trapezoidal integration method. The embodiment of the present application directly fixes the series of distances between the adsorbed molecules and the surface adsorption sites, and can calculate and accurately describe the potential field changes of the molecules from the reactant phase to the catalyst surface sites, and rigorously determine the energy dynamic change process of the adsorbed molecules from the reactant phase to different sites on the catalyst surface. Through the free energy barrier and thermodynamic evaluation of different sites, the optimal adsorption site of the adsorbed molecules on the catalyst surface can be determined.

[0029] The present application provides a method for determining the surface dynamic process free energy in a heterogeneous catalytic reaction, wherein the multiple catalytic reactions may be gas-solid catalytic reactions or liquid-solid catalytic reactions, specifically, the reactants are gas-phase reactants or liquid-phase reactants, and the catalyst is a solid-phase catalyst. The determined surface dynamic process free energy is the surface dynamic process free energy of molecular adsorption / desorption at the gas-solid (or liquid-solid) interface in the heterogeneous catalytic reaction.

[0030] In one embodiment, the catalyst is selected from at least one of a metal oxide catalyst, a metal sulfide catalyst, and a metal nitride catalyst. Specifically, the metal oxide catalyst is selected from at least one of cobalt tetraoxide, vanadium pentoxide, molybdenum trioxide, and titanium dioxide; the metal sulfide catalyst is selected from at least one of molybdenum sulfide, tungsten disulfide, and nickel disulfide; the metal nitride catalyst is selected from at least one of lanthanum nitride and cerium nitride. The above-mentioned types of catalysts can catalyze a variety of catalytic reactions, and in the embodiments of the present application, the molecular adsorption / desorption process can be accurately obtained for the multiple catalytic reactions of the above-mentioned types of metal oxide catalysts, metal sulfide catalysts, and metal nitride catalysts.

[0031] The above step S01 is a step of establishing an interface adsorption model, and the interface adsorption model can construct a molecular equilibrium state before the reactant is adsorbed. Specifically, the step includes: placing the adsorbed molecules of the reactant at a height of 0.3 to 0.5 nm in the reactant phase to construct a molecular equilibrium state before adsorption. For example, when the reactant is a gas phase reactant and the catalyst is a solid phase catalyst, when the gas phase is adsorbed to the solid surface of the catalyst, the adsorbed molecules of the gas phase can be placed at a height of 0.4 nm in the gas phase to construct a molecular equilibrium state before adsorption.

[0032] The above step S02 is a molecular dynamics simulation process for the molecular phase of the reactants. The embodiment of the present application can explore the free energy change of the molecular adsorption / desorption process based on advanced constrained ab-initio molecular dynamics (Molecule Dynamics, MD) simulation and coupled thermodynamic integration method.

[0033] The above step S03 is to fix the series of distances between the adsorbed molecules and the surface adsorption sites, and perform constrained molecular dynamics calculation. Specifically, the step of fixing the series of distances between the adsorbed molecules of the reactants and the adsorption sites of the catalyst includes: The adsorption process of the molecules on the interface needs to be studied by fixing the distance between the adsorbed molecules and the adsorption sites on the catalyst surface. The points are scattered until the molecules are adsorbed on the active sites on the interface, and constrained molecular dynamics calculations are performed on each system at a fixed distance, that is, molecular dynamics calculations at a series of distances.

[0034] In step S04, by analyzing the results of the constrained molecular dynamics calculation, a potential of mean force (PMF) can be obtained; for example, the step of analyzing the results of the constrained molecular dynamics calculation includes: according to the following formula (1), the system force of each molecular dynamics calculation system for a period of time after reaching the equilibrium state is sampled and averaged to obtain the potential of mean force, and then a relationship diagram of the potential energy of mean force and the fixed distance is constructed;

[0035] Formula (1):

[0036] See: https: / / www.vasp.at / wiki / index.php / Constrained_molecular_dynamics.

[0037] Furthermore, according to the relationship between the mean force potential energy diagram and the fixed distance constructed above, the phase equilibrium state, adsorption transition state and adsorption state of the reactants can be further determined. Taking the multiphase catalytic reaction of Co3O4 and CO as an example, Figure 2 As shown, point A is the gas phase equilibrium state of the CO molecule, point B is the adsorption transition state, and point C is the adsorption state.

[0038] In the above step S05, the trapezoidal integration method is used to perform integration according to the following formula (2):

[0039] Formula (2):

[0040] By using the trapezoidal integration method, according to formula (2), the mean force potential PMF and the fixed distance are integrated to obtain the free energy change of the adsorption process. Figure 2 As shown, the difference between point A and point B is the adsorption free energy barrier, and the difference from point C to point B is the desorption free energy barrier.

[0041] In one embodiment of the present application, in constrained molecular dynamics, a Nose-Hoover thermostat and an NVT ensemble are used to perform structural sampling. The free energy difference ΔA is based on the free gradient The free energy change can be calculated by integrating the free energy gradient over a distance along the adsorption path (1→2). Taking the adsorption of CO on the Co3O4(110) surface as an example, a series of constraint distances (intervals of 1.15) between the adsorption site and the C-terminus of the CO molecule can be used to calculate the free energy change. ) was used to obtain the adsorption free energy by integrating the potential mean force (PMF) based on the partition function.

[0042] In one embodiment of the present application, based on constrained ab initio molecular dynamics simulation, the adsorption / desorption process is systematically studied using CO adsorption / desorption on the Co3O4(110) surface as a case study. Figure 2 As shown, specifically: first establish an interface adsorption model system, and then investigate the adsorption process, by fixing a series of distances between the Co site and the O site on the surface of the catalyst Co3O4 and the CO molecule, and integrating the distance based on the mean force potential PMF, obtain the dynamic change process and free energy change of the CO molecule adsorbed from the gas phase to the two sites. Based on the potential energy diagram, the adsorption energy barrier of CO can be obtained. Through systematic research, the embodiments of the present application clearly demonstrate the existence of a molecular adsorption / desorption free energy (FE) barrier. When analyzing the origin of the adsorption / desorption barrier, the embodiments of the present application quantified that before CO approaches the adsorption TS from the gas phase, a small enthalpy change will result in an increased loss of entropy. By calculating the CO on Co 3+ and O 2c The dynamic adsorption process on the site can clearly determine the adsorption of CO on Co 3+ site instead of O 2c This finding is completely different from the results obtained from total energy calculations that are currently routinely used.

[0043] Therefore, the above method of the embodiment of the present application, (1) quantitatively developed and calculated the adsorption / desorption free energy barrier of adsorbed molecules at different temperatures, and found that the adsorption energy barrier is linearly related to temperature. (1) By capturing the adsorption kinetic process, it is explained that the larger entropy loss caused by the smaller enthalpy change in the initial adsorption process is the reason for the formation of the adsorption free energy barrier; in contrast, the desorption free energy barrier mainly comes from the enthalpy cost. (3) It can be extended to different metal surfaces. Based on this method, the embodiment of the present application found that the adsorption / desorption FE barrier is linearly proportional to the adsorption energy of the adsorbate. This BEP-like relationship can facilitate the direct prediction of the adsorption / desorption process without a lot of calculations; (4) For surface systems with multiple adsorption sites, this method can directly calculate the adsorption barriers of molecules at different sites;

[0044] The above examples of this application use this method to determine the molecular adsorption sites on the surface of the metal oxide Co3O4. Of course, it can also be applied to the study of other metal oxide / metal sulfide / metal nitride systems to determine the adsorption sites of heterogeneous catalytic reactions and the free energy change of the adsorption process from the perspective of free energy rather than total energy at 0K. This method provides a deep understanding of the molecular adsorption / desorption on the catalyst, and the quantification of the adsorption / desorption free energy barrier will improve the microkinetic model and further promote the rational design of catalysts.

[0045] The following describes the invention in conjunction with specific embodiments.

[0046] Example 1

[0047] This embodiment simulates and verifies the method for determining the free energy of the surface dynamic process by CO adsorption on the surface of Co3O4 catalyst, and the specific steps are as follows:

[0048] (1) Simulate the dynamic process of CO molecule adsorption on the surface of the catalyst Co3O4 at 150 Kelvin. Place the CO molecule diagonally above the Co adsorption site on the solid surface. The molecular dynamics simulation was performed with a total time of 6 picoseconds (ps).

[0049] (2) The distance from the Co3O4 surface is Sprinkle a point until the distance between the CO molecule and the active site is the distance in the adsorption state.

[0050] (3) Constrained molecular dynamics calculations are performed for each fixed distance system, such as from the gas phase to the adsorption state, with a total time of 6 ps at each point.

[0051] (4) According to the above formula (1), after the molecular dynamics calculation system energy reaches equilibrium at each distance, the system force of the last 1.5ps in 6ps is sampled and averaged to obtain the mean force potential (PMF), as shown in Table 1, which shows the change of PMF of CO at different distances from the surface of the catalyst Co3O4.

[0052] (5) According to the above formula (2), the trapezoidal integration method is used to integrate the PMF and the fixed distance to obtain the free energy barrier of the CO adsorption process. The desorption free energy barrier is calculated to be 0.76 eV, and the adsorption free energy barrier is 0.19 eV.

[0053] In this embodiment, it can be found that: through constrained molecular dynamics calculation, the CO adsorption energy barrier is between the energy barriers predicted by the mobile model and the immobile model. The CO adsorption energy barriers calculated by the mobile model and the immobile model are 0.07 and 0.29 eV respectively. In addition, the CO adsorption energy barrier is linearly related to the temperature, which is consistent with the contribution of entropy change to the adsorption energy barrier, and the adsorption energy barrier is linearly related to the adsorption energy, which is consistent with the cognition of the effect of strong adsorption and weak adsorption behavior on entropy.

[0054] Table 1

[0055]

[0056] Through the simulation and verification of the above-mentioned embodiments of the present application, it can be known that the method of the present application for determining the free energy of surface dynamic processes in heterogeneous catalytic reactions is technically reliable, has detailed results, and can be applied to most interfacial adsorption systems. It can promote the development of catalysis to a certain extent, especially promote the improvement and development of reaction kinetics.

[0057] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A method for determining the surface dynamic process free energy of a heterogeneous catalytic reaction, characterized in that: The method comprises mixing reactants and catalysts to perform a heterogeneous catalytic reaction, and then determining the surface dynamic process free energy of the heterogeneous catalytic reaction by the following steps: Establish an interface adsorption model; Performing molecular dynamics simulation on the molecular phase state of the reactants to make the molecules of the reactants reach an equilibrium state; A series of distances between the adsorbed molecules of the reactant and the adsorption sites of the catalyst are fixed, and a constrained molecular dynamics calculation is performed on each fixed distance system; Analyzing the results of the constrained molecular dynamics calculation to obtain a mean force potential; The mean force potential and the fixed distance are integrated by using the trapezoidal integration method to obtain the free energy change of the adsorption process; The step of fixing the series distances between the adsorbed molecules of the reactant and the adsorption sites of the catalyst comprises: The dots are scattered at intervals until the molecules of the reactants are adsorbed on the active sites of the interface; The step of analyzing the results of the constrained molecular dynamics calculation comprises: sampling and averaging the system force of each molecular dynamics calculation system for a period of time after the energy reaches a balanced state according to the following formula (1), obtaining the mean force potential, and then constructing a relationship diagram between the mean force potential energy diagram and the fixed distance; Formula (1) The trapezoidal integration method is integrated according to the following formula (2): Formula (2) The reactants are gas phase reactants or liquid phase reactants, and the catalyst is a solid phase catalyst.

2. The method according to claim 1, characterized in that The step of establishing the interface adsorption model includes: placing the adsorbed molecules of the reactant at a height of 0.3 to 0.5 nm in the reactant phase to construct a molecular equilibrium state before adsorption.

3. The method according to claim 1, characterized in that According to the constructed mean force potential energy diagram and the fixed distance relationship diagram, the phase equilibrium state, adsorption transition state and adsorption state of the reactant are determined.

4. The method according to any one of claims 1 to 3, characterized in that: The catalyst is selected from at least one of a metal oxide catalyst, a metal sulfide catalyst and a metal nitride catalyst.

5. The method according to claim 4, characterized in that The metal oxide catalyst is selected from at least one of cobalt tetroxide, vanadium pentoxide, molybdenum trioxide and titanium dioxide; and / or the metal sulfide catalyst is selected from at least one of molybdenum sulfide, tungsten disulfide and nickel disulfide; And / or the metal nitride catalyst is selected from at least one of lanthanum nitride and cerium nitride.

6. The method according to any one of claims 1 to 3, characterized in that: The reactant is carbon monoxide, and the catalyst is selected from cobalt tetroxide.

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