Structure-Activity Relationship Of Supported Metal Catalyst Under Realistic Reaction Condition: A Sight From Theoretical Simulation

The study of structure-activity relationship of supported metal catalyst under realistic reaction condition plays a vital role in catalyst design.Despite the present experiment method has reached atomic scale resolution,it still has limitations in some certain time and space domains.Using computational simulation to get a better understanding of the catalytic system has become a trend in catalysis research,especially for the computational method based on the first principles density functional theory?DFT?,which has been widely used.However,there are still two gaps between DFT calculations and realistic system.First is the gap between the catalyst model used in DFT calculations and realistic system.DFT calculations could only simulate the model system with a maximum limit of several hundred atoms?about 2-3nm?,while the catalyst in the realistic system can be as large as several hundred nanometers.Furthermore,the catalyst model needs improve its ability to simulate the complex active sites in real catalyst.Second is the gap between the simulation environment of DFT calculations and the reaction environment of realistic system.In most DFT calculations,the effects of temperature,pressure and solvent on the catalyst structure and catalytic performance are not considered,while these factors are very important in the realistic system.In order to bridge these gaps,four different typical catalytic systems were selected to set up a theoretical study of structure-activity relationship of supported metal catalyst under realistic reaction condition by combining DFT calculations with thermodynamic analysis,microkinetic modeling and ab initio molecular dynamics?AIMD?simulations.?1?As for the gap between the model system used in DFT calculations and realistic system,which hinders our understanding of the intrinsic property of the catalytic reaction,the catalyst system in which the interaction between the support and metal catalyst is very weak was chosen.In this kind of system,metal catalyst plays a main role.A method to associate the reaction sites with nanoparticle size of metal catalyst was proposed in this work.First,different crystal surface models and cluster model were used to represent the different surface sites of metal nanoparticles.Second,DFT calculations and microkinetic modeling were combined to analyze the relationship between reaction site and catalytic selectivity as a function of reaction condition.Finally,according to the relationship between the distribution fractions of reaction sites and metal nanoparticle size,the relationship between metal nanoparticle size and catalytic selectivity as a function of reaction condition was achieved.On the basis of this method,the effect of catalyst size on furfural hydrodeoxygenation conversion over SiO₂ supported Pt catalyst was studied.It found that under experimental operating condition of T=473K and PH2=93kPa,the decarbonylation route dominants on the Pt particle below 1.4nm in size while the furfural hydrogenation prevails on the Pt particles larger than 1.4nm.The modeling results are in good agreement with previous experimental observation.Therefore,the methodology developed here can also be applicated to other size dependent activity/selectivity reactions.?2?For the issue of how to describe the active sites of real catalyst in a better way using a model system,the catalyst system in which the interaction between the support and metal catalyst is strong was chosen.In this kind of system,the active sites are mainly located at the interface between the metal and the support.Thus,a catalyst model was set up by placing a metal cluster over the support,which was represented by a periodic slab model.CO oxidation reaction over supported Au,Pt and Au-Pt alloy catalysts on TiO₂?110?was studied.It found that O₂ adsorption on the interfacial sites is stronger than that on pure metal sites.O₂ dissociation on the interfacial site of Au and TiO₂?110?is very difficult,while O₂ dissociation is very facile on the pure Pt surface sites and the interfacial sites between Pt and TiO₂?110?.CO oxidation mechanism on different reaction site was also investigated.On the interface of Au and TiO₂?110?,CO is directly activated by adsorbed molecular oxygen.Whereas on the interface between Pt and TiO₂?110?,the dissociated oxygen reacts with CO.On the pure Pt sites of Au-Pt alloy,both of the reaction mechanisms work.?3?As for the issue that catalyst structure would be affected by the temperature and pressure conditions,DFT calculations and thermodynamic approach were combined to study the effects of temperature,oxygen partial pressure and Al condition on the structure of spinel MgAl₂O₄.The shapes and surface structures of the spinel MgAl₂O₄ nanoparticles under different environmental conditions were predicted using Wulff construction.It was found that the 100_AlO₂ termination is the most stable surface structure under ultrahigh vacuum condition at T=1100K,while 111_O2?Al?termination becomes the most stable surface in the Al-rich condition as the oxygen pressure increases.The oxygen vacancy formation and its effect on the thermodynamic stability of spinel MgAl₂O₄ were also investigated.The reduced surface terminations of 100_AlO₂ and 111_O2?Al?with oxygen defect sites on the surface were found to be the most dominant surface structures under the atmospheric condition at T=1100K.The theoretical predictions of this work are consistent with previous experimental observations.This work also gives a better understanding of the effects of environmental conditions on the structure of spinel MgAl₂O₄ in the atomic level.?4?As for the issue that catalytic reaction would be affected by the solvent,ab initio molecular dynamics?AIMD?simulations and DFT calculations were used to study the effects of the aqueous water on the acetic acid ketonization reaction over ZrO₂????11?catalyst.The calculations suggested that the presence of water poses both advantages and disadvantages on the reaction.On one hand,the limited available active sites and the hydrophilic nature of reactant inhibit the catalytic reaction.On the other hand,water provides an enhancement of stability for the reactant staying in aqueous phase,thus generating new reaction pathways via Eley-Rideal?ER?mechanism.Water also facilitates the elementary reaction steps involving hydrogen abstraction/addition via the Grotthuss proton transfer mechanism.