Quantum Dynamics Study Of Small Molecules Reactions In Gas Phase And On Metal Surfaces

The calculation of thermal rate constants for chemical reactions remains one of the central tasks of theoretical chemistry.The quantum instanton method is a newly developed method for calculating the rate constants of chemical reactions.Since this method only involves calculating the Boltzmann operator,the method can be extended to chemical reactions with large numbers of degrees of freedom.Together with path integral Monte Carlo and adaptive umbrella sampling techniques,the O（³P）+CH₄ reaction and the dissociation of H₂ on the H precovered Ni（111）surfaces are investigated with the quantum instanton method.The O（³P）+CH₄ reaction is substantially important in hydrogencarbon combustion.With the quantum instanton method,the rate constants and the kinetic isotope effects of the O（³P）+CH₄ reaction are calculated in full dimensionality.Above 400 K,the calculated rate constants are consistent with the experimental values,and the calculated values are in the range of experimentally measured values below 400 K.Compared with other theoretical methods,the quantum instanton method gives the largest quantum tunneling effect,so its rate constant is the largest at low temperatures.The calculated kinetic isotope effect is always much greater than 1 and increases with decreasing temperature,which are caused by the zero point energy and the quantum tunneling effect.The calculated results on different potential energy surfaces reveal that the width of the potential energy barrier not only determines the quantum tunneling effect,but also has a great influence on the kinetic isotope effect.The dissociation of hydrogen molecule on metal surfaces is a prototype reaction for investigating gas-surface interaction.The dissociation and recombination rates of H₂ on the H precovered Ni（111）surface are calculated with the quantum instanton method.In order to investigate the effect of relative position of the preadsorbed H atom and H₂ on the dissociation and recombination rates,four arrangements are considered（H₂/H¹-Ni（111）,H₂/H²-Ni（111）,H₂/H³-Ni（111）and H₂/H⁴-Ni（111））.The dissociation rates of H₂ on H¹-Ni（111）are smaller that those on a clean Ni（111）surface.For example,at 300 K,the latter is 5.22 times larger than the former.This is because the strong interaction between H₂ and the preadsorbed H atom hinders the dissociation of H₂.The dissociation rates of H₂ increase in the following order:H₂/H¹-Ni（111）<H₂/H²-Ni（111）<H₂/H³-Ni（111）<H₂/H⁴-Ni（111）.For example,at 300 K,the rate constant ratio of H₂/H²-Ni（111）to H₂/H¹-Ni（111）is equal to 4.40.This condition further indicates that the repulsive interaction rapidly decreases with the increase of the distance between H₂ and preadsorbed H atom.In addition,the dissociation rates on the non-rigid lattice are larger than those on the rigid lattice.For example,at 300 K,the lattice motion increases the dissociation rate by 29%.The calculated kinetic isotope effect is greater than 1 and increases rapidly with decreasing temperature,which reveals that the quantum tunneling effect is very significant.The kinetic isotopic effects of H₂/Ni（111）,H₂/H¹-Ni（111）,H₂/H²-Ni（111）,H₂/H³-Ni（111）and H₂/H⁴-Ni（111）are similar,which shows that the surface coverage,lattice motion,and the relative position of H atom and H₂ have little effect on the kinetic isotope effect.