Theoretical Studies On Reaction Dynamics Of Methane And Carbon Dioxide And Other Small Molecules On Transition Metal Surfaces

In recent years,the conversion and utilization of carbon-based energy sources have been the key research focus in many subjects,such as catalytic chemistry,energy chemistry and environmental chemistry.Dynamical processes of carbon-based small molecules on transition metals,which are commonly used as commercial catalysts,are crucial factors to control the catalytic product formation and efficiency.It is important to study the dynamical processes,such as adsorption,diffusion and dissociation of carbon-based molecules on transition metal surfaces,so as to better understand catalytic reactions.On the other hand,understanding these processes can also provide fundamental ideas for the design of catalysts and control of practical catalytic processes.Along with the development of computer technologies and theoretical approaches,first principles calculations become an increasingly popular way to describe the behaviors of carbon-based molecules on transition metal surfaces at atomic and molecular level,providing detailed information of reaction mechanisms,and help us further understand the intrinsic properties of elementary reactions on surfaces together with experimental studies on surface reaction dynamics.As a major component of the natural gas,methane is of high interest in energy science.The dissociative chemisorption(DC)of methane is the rate-controlling step in steam reforming to produce hydrogen.In this thesis,we firstly studied the dynamical mechanism of methane DC on metal surfaces in detail.Based on the SRP32-vdW functional proposed by our cooperators,we applied the permutation invariant polynomial-neural network(PIP-NN)approach to construct a fifteen dimensional reactive potential energy surface(PES)of CH4/Ni(111)system,on which quasi-classical trajectory(QCT)calculations reproduced the measured initial sticking probabilities(S0)of CHD3 DC on Ni(111)for a wide range of incident energy(Ei)within chemical accuracy(<4.2 kJ/mol).Furthermore,we analyzed mode specificity and bond selectivity of deuterated methane,i.e.CHD3 and CH2D2.Our calculated results on this PES qualitatively reproduced the experimental results when Ei were lower than 50 kJ/mol.On CHD3(v1)excitation state,our results very well reproduced the C-H/C-D branching ratio observed in previous experiment,which emphasized again the non-statistical characters of methane DC.In addition,taking CHD3/Ni(111)system as an example,we proposed a modified generalized Langevin oscillator(MGLO)model.This model was based on the original GLO model,incorporated with the concept of lattice relaxed sudden(LRS)model.This model introduces the couplings between molecule and surface into the dynamical calculations.It was found that the MGLO model performed well in CHD3 DC on Ni(111).The calculated results well reproduced the previous ab initio molecular dynamics(AIMD)and experimental data,and accurately described the energy transfer dynamics,illustrating the applicability of MGLO model in highly activated gas-surface reactions.AIMD,albeit very expensive,is a powerful approach to account for lattice motion effects in surface reactions by including degrees of freedom(DOFs)of surface atoms and mimic real conditions in gas-surface system dynamical calculations.Thus,this method is increasingly popular in theoretical studies of heterogeneous catalysis recent years.Using this approach,we simulated the dynamical processes of CH4 on Ir(332),to reveal the site selectivity and effects of lattice motion and defects on CH4 DC.AIMD simulations confirmed the direct mechanism of CH4 DC at relatively high-Ei.In addition,our calculations also shed light on the“trapping-precursor”mechanism at low-Ei(<0.15 eV)proposed in previous experiments and provided valuable data for further studies.Beside CH4,CO2 is also an important carbon-based energy molecule,as the major component of greenhouse gas.Studying dynamics of CO2 adsorption and dissociation on metal surfaces is of great importance in CO2 conversion and utilization.Mimicking the experimental conditions,we used AIMD approach to study the adsorption,dissociation,and scattering dynamical processes of CO2 on Ni(100).Our calculated dissociative probabilities reproduced the experimental tendency and we found an indirect mechanism of CO2 DC on Ni(100).In this system,the dynamical behavior is not solely determined by the reaction barrier,highlighting the important influence of PES topography in surface reaction dynamics.Finally,in a system that is closely related to hetergeneous catalysis,i.e.Fischer-Tropsch synthesis,we investigated the dynamical processes of H atom impinging on the Cu(111)surface pre-covered by CO molecule(1/4 ML)via the AIMD approach,with the focus on the energy transfer dynamics and non-adiabatic effects.Our results agreed reasonably well with the experimentally measured cross section,product energy,and angular distributions of desorbed CO.Our results clearly demonstrated that the dynamical displacement of CO takes place via the hot-atom mechanism.These studies presented in this thesis will help us better understand the interactions between carbon-based energy small molecules and metal surfaces,and the energy transfer between molecular and surface DOFs,eventually guide us to reveal the reactive mechanisms of relevant heterogeneous catalytic processes.