Simulation Of Chemical Reactions In Heterogeneous Catalysis:Hydrogen On Pd(111) Surface

Under real conditions,surface reactions in heterogeneous catalysis take place always at finite surface coverage.However,early theoretical works focus on studying the reaction on clean surface.The main reasons are the complexity of the coverage effect and the computational efficient.In addition,most phenomena we truly interested are more complex to clean surface.For example,adsorbate diffuses to subsurface and adsorbate desorbs from surface into gas.Although we can get some useful information by studying the clean surface,it also needs to get insight into the complex system.The development of a more efficient method for studying complex rare event is urgent.The surface reaction usually is a very complex process.It can be consider as a sequence of elementary steps such as adsorption,diffusion,desorption,and reaction.Among the four elementary steps,adsorption and diffusion can be a rate limiting step in surface catalysis.These two processes were considered as our research objects.The interaction of hydrogen with Pd surfaces represents one of the most extensively investigated subjects among the studies on surface reaction.One of the reasons is the catalytic properties of this metal in hydrogenation reactions,but more important reason lies probably in the fact that this system has been considered to be simple and therefore to be well suited for fundamental research on adsorption and diffusion phenomena.In the present work,we studied the dissociative adsorption of hydrogen on Pd(111)in terms of the coverage effect by using density functional theory(DFT),the diffusion process of the hydrogen on Pd(111)with transition state theory(TST)and molecular dynamic method,and developing a accelerated molecular dynamics method based on the studying of hydrogen diffusion on Pd(111).This thesis includes six chapters:In Chapter 1,a short review of the recent progresses for hydrogen dissociative adsorption and diffusion on Palladium surfaces was given.For hydrogen dissociative adsorption,previous experiments are focused on investigating the incident energy effect of impinging molecule and the coverage effect on the dissociative adsorption of hydrogen,and previous dynamics simulations are focused on studying the sticking curves with a very limited number of H adatom configurations on the substrate.Such simulation cannot be compared directly to any experiment since statistical average over H adatom configurations is lacking.For hydrogen diffusion,the variety of method development was reviewed.Benefit the development of reactive force field(RFF),it is possible that the molecular dynamics(MD)can be used to study the complex systems.In Chapter 2,we provided a general presentation of a variety of theoretical approaches and approximations which underline the different calculations we have performed.These approaches are classified into three categories,i)electronic structure calculation;ii)MD simulation;iii)TST.Some important aspects of these approaches would be recalled.In Chapter 3,a systematic investigation based on density-functional-theory calculations was presented for elucidating the effect of adatoms on the energetics of H2 dissociation on H-precovered Pd(111)surfaces.Surprisingly,we found that the H-adatoms do not only have a poisoning effect but can also promote H2 dissociation when they are adsorbed on sites which are sufficiently far from the dissociating H2 molecule.The presence of these antagonistic effects produces many crossovers of the minimum energy profiles for H2 dissociation when coverage is varied and thus leads to a quite perplexing picture for the adatom effects.We devised a sorting procedure which allows for rationalizing nicely the influence of H-adatoms on the energetics of H2 dissociation on H-precovered Pd(111)surfaces.In Chapter 4,we adopted a general method based on Einstein formula to study the diffusion of a hydrogen atom on the Pd(111)surface at T=250K/300K/500 K.The diffusion coefficient is determined from the long time asymptotic slope of MSD as function of time.We performed MD simulations in which the inter-atomic forces are calculated from a RFF.Our simulations show that the correlated multiple hopping plays a non negligible role.The TST does not give accurate results for the diffusion coefficient for H/Pd(111)system with multiple moderate energy barriers.We found that two hopping steps can be considered as a smallest repeat unit of the whole diffusion process which can reasonably involve the correlation effect.Based on this smallest repeat unit model,we obtained the analytical TST formula for calculating macroscopic diffusion coefficient which is in good agreement with that from MD simulation.In Chapter 5,We firstly tried to employ a variant of the Bond-Boost method to simulate the diffusion of hydrogen atoms on Pd(111).However,the construction of bias potential for the pre-covered surface is very complex due to the interaction between adatoms break the symmetry of potential energy surface.For solving the problem,we developed a new accelerate method.Compared with hyperdynamics method which raises the potential well through filling the basins with a boost potential,we adopted another approach to achieve the accelerating——Increase the kinetic Energy of diffusing atom by choosing velocity of diffusing atom at random from a Maxwell-Boltzmann velocity distribution of higher temperature.We checked the validity of our method by simulating the diffusion dynamics of H adatom on Pd(111)surface at T=300K and T=100K.And the physicaltime scale can be extended at the magnitude of microsecond.In Chapter 6,some conclusions were given and we pointed out also some important interesting directions along which the study can be pursued.For example,absorption of H atoms into the bulk of Pd through different surfaces [(111),(100)or(110)] or thermal programmed desorption H2 from different Pd surfaces.