Heterogeneous reactions involve multiple phases, such as the catalytic reduction of nitrogen oxides on solid surfaces in a catalytic converter. Phase refers to the different immiscible components in the system, so the multiphase reaction process can only occur at the interface between different phases. In addition, surface is also the location where adsorption occurs. During the adsorption process, molecules in adjacent gas or liquid phases are concentrated on the surface through intermolecular forces, such as van der Waals forces (physical adsorption), or chemical bonds (chemical adsorption). These are two reasons why solid surfaces can catalyze reactions. Compared with molecules in free gas or solution, gas and liquid molecules spend more time in close proximity in the adsorption. In addition, chemical adsorption can reduce the activation energy required to break the chemical bonds in the adsorbed molecules, so that the reaction between the adsorbed chemical species can proceed through a different mechanism from that in the free phase. Therefore, in order to make full use of the unique chemical and physical properties of solid surfaces (including gas-solid and liquid-solid interfaces), researchers and engineers use molecular simulations to study surface mass transfer and reaction kinetics in the design of catalytic reactors, biophysics and electrochemistry.
Figure 1. Ab initio molecular dynamics of atomic-scale surface reactions. (Sangiovanni, D. G; et al. 2018)
- Surface dynamics
Our scientists establish a chemical model to simulate the flux of reactants on the catalytic surface. The rate of surface reaction varies with the concentration of reactants and products and other local characteristics (such as temperature or pressure), and the reaction rate is calculated according to the rate law.
- Surface adsorption and transfer
In the adsorption process, the flux of chemical reactants entering the surface is not in equilibrium with the flux of chemical reactants leaving the surface. Therefore, the surface concentration of adsorbed reactants will continue to change. We use molecular simulation to model the adsorption process to predict the adsorption rate of reactants flowing from the solution to the active surface. Moreover, we can draw a 3D schematic diagram to present the surface diffusion rate of adsorbed substances.
At Alfa Chemistry, we mainly apply Reaxff molecular dynamics, ab initio molecular dynamics calculations and molecular dynamics based on density functional theory to carry out molecular simulation of chemical reactions at surfaces. Our fast and high-quality services include the following:
- Reaxff molecular dynamics
The ReaxFF reaction force field is used to study the surface gas adsorption. The influence of temperature, stress and other factors on the gas adsorption effect on the material surface is analyzed through simulation. Our groups construct adsorption model of different gases on the surface of materials, and explain the the relationship between adsorption of the gas on the surface and the temperature. Our simulation process is as follows:
1. Model construction and ReaxFF potential field simulation: the initial state of gas and material surface are established.
2. Relaxation simulation: build an adsorption model, and output the data parameters until the relaxation system reaches a stable equilibrium.
3. Data statistics and analysis: calculate system energy, radial distribution function and adsorption rate.
- First principles method
The statistical nature of surface reactions require calculation of a very large number of trajectories to determine the reaction rate. We therefore adopt the trajectory determined from first principles, in which first the potential energy surface (PES) on which the nucleus moves is determined, and then the dynamical calculations on an appropriate representation of the PES are performed. We conduct a very detailed evaluation of PES using first-principles methods based on density-functional theory. Alfa Chemistry applies a a three-step approach for performing ab initio molecular dynamics calculations:
1. Firstly, a sufficient number of ab initio total-energy calculations is performed.
2. Then, an interpolation scheme is used to fit the ab initio energies and to interpolate between the actual calculated points.
3. Finally, the dynamics calculations are performed on this continuous representation of the ab initio PES.
- Density functional theory
We apply density functional theory at the generalized gradient approximation level to study molecular interactions and chemical reactions at metal surfaces. Our experts calculate electronic structure by using molecular dynamics based on density functional theory, explore transition states and reaction paths, and study chemical reactions on the surface.
Why Choose Us?
- Mode specificity, bond selectivity, steric effects, as well as the lattice effects including the energy transfer between the molecule and surface are able to be investigated, which provide valuable information on how the energy, molecular orientation, and surface motion affect the surface reaction.
- We use various molecular simulation methods to study transition states and reaction paths, providing a molecular level of understanding of the reaction dynamics in surface reaction.
Molecular simulation of chemical reactions at surfaces provides an effective way to optimize the chemical process. Our molecular simulation of chemical reactions at surfaces services remarkably reduce the cost, promote further experiments, and enhance the understanding of chemical process for customers worldwide. Our personalized and all-around services will satisfy your innovative study demands. If you are interested in our services, please don't hesitate to contact us. We are glad to cooperate with you and witness your success!
- Sangiovanni, D. G; et al. Ab initio molecular dynamics of atomic-scale surface reactions: insights into metal organic chemical vapor deposition of aln on graphene. Physical Chemistry Chemical Physics Pccp, 2018, 20, 17751.