Mesoscale Behavior Of Adlayer In Heterogeneous Catalysis

In heterogeneous catalysis,reactants and products can form complex adlayer structures on the catalyst surface,which play an important role on the macroscopic performance of heterogeneous catalysis.At present,there are three catogries of models describing heterogeneous catalysis,namely macroscopic,microscopic,and mesoscopic models.The macroscopic model adopts the mean field assumption,neglecting heterogeneous distributions of adsorbates,and thus probably leading to unacceptable deviations from actual physics;On the other end,the microscopic model contains too much redundant information in order to reflect practical processes of atoms or molecules,contributing to huge computational cost and thus unpractical within the involved scales in appliation.In recent years,although the mesoscopic model is introduced by correlating kinetic rates with the local coverage,the influence of adlayer structures on the dynamic behavior cannot reasonably get rid of the macroscopic model because the mean field assumption is still adopted locally.Therefore,the current mesoscopic model is still not accurate to reflect the effects of adlayer structures on the system.In the study of gas-solid fluidized systems,the energy-minimization multiscale(EMMS)model proves to be effective for correlating microscale and macroscale.The extended EMMS principle reckons that the stability condition of a complex system can be expressed as the compromise in competition between different dominant mechanisms,that is to say,the compromise in competition between dominant mechanisms is indeed a governing rule for dynamic heterogeneous structures.Therefore,this thesis concretely depicts the structural characteristics of adlayer in heterogeneous catalysis,and relevant dominant mechanisms are explored to correlate structural characteristics in the mesoscale,accounting for the existence of the stability condition resulted from the compromise in competition between dominant mechanisms according to EMMS principle.Based on these work,a good foundation for the establishment of the mesoscale model is prepared.It can be found that the kinetic factors have important influence on the adlayer structures on the catalyst.The mesoscale model combining the stability condition with conservation equations is an efficient and accurate way to correlating the macroscale and microscale,showing the structural characteristics in mesoscale.Thus,four questions are answered in this thesis.namely,what are the adlayer structures on the catalyst,why should we consider the influence of adlayer structures,how to accurately characterize the structures and how to find the competing mechanisms corresponding to different adlayer structures.Based on this research thinking,this thesis can be divided into four chapters,namely,the introduction,analysis on the A-B model system,analysis on the A-B2 model system,and the conclusions and prospects.The first chapter introduces the research background and the research thinking of this thesis,and explains the effects of adlayer structures on the macroscopic dynamic behavior on the catalyst in detail,and the differences of existing models is studied in comparison.The second and the third chapters are the main body of this thesis.The influence of the kinetic factors on the adlayer structures on the catalyst is studied in different systems,A-B system and A-B2 system,respectively,and the corresponding dominant mechanisms are extracted,indicating that the heterogeneous structures arises from the stability condition induced from compromise in competition between dominant mechanisms.At the same time,due to the effect of dissociation,the differences in the influence of the kinetic factors on the adlayer structures are compared in detail in Chapter 3,showing the general conclusion about structural characteristics of adlayer in mesoscale.Chapter 4 is about conclusions and prospects.which is divided into two parts.firstly some conclusions are summarized based on the thesis,and then the further work is promoted.