Theoretical Studies Of H₂and CH₄Dissociation On Metal Surfaces

The dissociation of polyatomic gas molecule is not only of fundamental importance in physics,chemistry and the related disciplines, but also can improve the industrial process, e.g., hydrogenproduction. It is a challenging task to understand its reaction mechanism and dynamics in the involvedreaction （i.e., breaking and forming of chemical bonds）. As a benchmark system of polyatomicgas-surface reaction, methane dissociative chemisorptions on Ni surfaces have been extensivelyinvestigated to understand the energy flow and reaction pathways. Some recent state-resolvedmolecular beam experiments have been shown mode-and bond-specific reactivity of CH₄on Ni （111）and Ni （100） surfaces. In theoretical viewpoint, most previous dynamics studies have been based on asimplified model which treats CH₄as a pseudo-diatomic molecule. The concept of the spectatormethyl group introduced in such a model imposes some severe restrictions. For example, theindistinguishability of the four C-H bonds in CH₄is violated by singularizing one bond as cleavablebut keeping the three others unbreakable. In reality, any one of the four H atoms can be dissociated.Moreover, the single bond stretching mode of the pseudo-diatomic molecule does not resemble any ofthe four fundamental vibrational modes of CH₄which are all the collective motion of several atoms.When all the degrees of freedom （DOF） of CH₄are taken into account, the dimension of the potentialenergy surface （PES） for CH₄/Ni（111） system is very high （15DOFs for CH₄+some DOFs of thesubstrate）. Constructing a reliable and accurate PES with such a high dimension is itself a verychallenging task. To our knowledge, no PES with a so high dimension for any reaction of apolyatomic molecule on a metal surface has ever been reported until now. In the present work, using areactive bond order （REBO） force field, we first developed a full dimensional PES for CH₄/Ni（111）including all the15DOFs of CH₄and those of the three top layers of Ni（111）. Some strikingdynamics results are found from the classical （or quasi-classical） molecular dynamics simulations onthe reactivity of methane in ground state and excited states. This thesis includes six chapters:In Chapter1, a short review of the recent progresses for methane dissociation on transition metalsurfaces is given. It mainly focuses on previous state-resolved experiments and quantum dynamicssimulations using potential energy surfaces of reduced dimension. Recent state-resolved experimentshave revealed some important aspects for the dissociation of methane on Ni surfaces, e.g., mode-andbond-specific reactivity, surface temperature effect, steric effect, rotationally excited effect, etc. Someimportant theoretical works are reviewed as well.In Chapter2, we give a general presentation of a variety of theoretical approaches and approximations which underline the different calculations we have performed. These approaches areclassified into two categories, i) electronic structure calculation; ii) molecular dynamics simulation.Some important aspects of these approaches will be recalled.In Chapter3, we have explored a benchmark system, H₂/Pd（111）, using reactive force fieldsbased on second moment approximation （SMA） and reactive bond order force field （REBO）. Thevalidity of reactive force fields （RFFs） for surface chemical reaction has been established. Someinfluences on the parameterization of RFFs are discussed in detail, i.e., the size of effective database,the repulsive potential and the long range adsorbate-substrate interaction. Some useful technicaldetails concerning the parameterization are provided for studying more complex surface reaction.In Chapter4, a full dimensional potential energy surface （PES） for CH₄/Ni （111） is firstdeveloped by using an approach based on reactive bond order （REBO） force field. The appraisal onthe quality of the REBO PES is given, e.g., the comparison of transition state （TS） configurationsfrom REBO PES with those from DFT calculations. Moreover, some direct molecular dynamicssimulations on the reactivity of methane in ground state have been performed to further validate thereliability of REBO PES.In Chapter5, we present a series of molecular dynamics simulations with the full dimensionalPES which allowed us to discover some subtle dynamics features not revealed previously. We haveintensively investigated the reactivity of CH₄in ground state （v=0,J=0）, vibrationally antisymmetricstretching state v3（v=1,J=0） and rotationally excited state （v=0,J=1-12） on Ni （111） surface. For CH₄in ground state （v=0,J=0）, the significant effects of surface impact position and surface temperaturehave been quantitatively and qualitatively studied. For the enhanced reactivity of CH₄v3（v=1,J=0）, itis shown that the translational energy does not flow into rotational DOFs while energy flow fromvibrational DOFs to rotational ones takes place easily. A significant coupling between vibration androtation has been demonstrated. Moreover, for CH₄in （v=0,J=12）, we found that rotation enhancessignificantly CH₄reactivity on Ni（111） with a deposited rotational energy amounting only to12%ofthe dissociation barrier. More surprisingly, striking evidences are found that CH₄rotation can promoteeven better its dissociation on Ni （111） than vibration. In a vibrationally excited CH₄, its C-H bondsundergo alternate stretching and compressing and the latter hinders dissociation. In this case, thereactivity is inevitably modulated by vibration phase. However, the centrifugal force due to rotationthat tends always to stretch the C-H bonds for CH₄in rotationally excited states.In Chapter6, some conclusions are given and we point out also some important interestingdirections along which the study can be pursued.