Theoretical Investigations On The Catalytic Performance Of Methanol Synthesis From Syngas On Zn/Ga-doping Cu Sites

Cu-based catalysts are widely applied in industrial methanol synthesis reaction from syngas?CO/CO₂/H₂?,because of its lower price,longer catalysis lifetime.The excellent catalysis performance of Cu-based catalyst is partially attributed to the existence of ZnO.The possible functions of ZnO are explored by lots of experimental and theoretical investigations.As reported,ZnO in Cu-based catalyst can effectively avoid the aggregation of active component Cu species,act as the spillover of H atom as well as exhibit stronger interaction with Cu.On the other hand,the addition of promoter Ga can also improve the activity of Cu-based catalyst in methanol synthesis reaction from syngas.This is because Ga can not only help enhance the dispersion of Cu site,but also modify the electronic environment of components,such as enhancing the reductivity of Cu and the semiconductivity of ZnO.However,due to the limition of experimental methods,the exact effect of Zn/Ga on the performance of Cu site for methanol synthesis reaction has not been investigated.Therefore,based on the density function theory,our work are focused on the effect of Zn/Ga on the catalysis performance in mehtanol synthesis reaction from syngas by investigating the adsorption status of intermediates and analyzing the potential energy profiles of possible basic reactions in methanol synthesis reaction over Cu?211?,ZnCu?211?and GaCu?211?models,which can represent pure Cu site,Zn decorating Cu site and Ga decorating Cu site.The main conclusions are as follows.?1?The addition of Zn/Ga cannot evidently change the adsorption capcity of the possible intermediate over Cu site.Therefore,the doping of Zn/Ga can't modify the catalysis performance of Cu site by evidently changing the adsorption states of possible intermediates over Cu site.?2?For methanol synthesis from CO/H₂ reaction,the preferential route over Cu?211?is CO+H?CHO?CHO+H?CH₂O?CH₂O+H?CH₂OH?CH₂OH+H?CH₃OH.The relevant rate-limiting step is CH₂OH+H ? CH₃ OH,with the activation energy of 158.9 kJ/mol.After the addition of Zn/Ga,the optimal reaction route on Cu site turns into: CO+H?CHO?CHO+H?CH₂O?CH₂O+H?CH₃O?CH₃O+H?CH₃OH.Over ZnCu?211?surface,the rate-limiting step is CO+H?CHO,with the lower barrier energy of 120.8 kJ/mol.While for GaCu?211?model,the rate-limiting step is CHO+H?CH₂O and the activation energy is the lowest,only 80.5 kJ/mol.Therefore,the addition of Zn/Ga can evidently enhance the catalysis activity of Cu site in mehtanol synthesis from CO/H₂ reaction.In comparison with Zn doping,Ga doping Cu site is of the highest reaction activity.?3?In the reaction of methanol synthesis from CO₂/H₂,the preferential route on Cu?211?surface is CO₂+H?HCOO ? HCOO+H?HCOOH ?HCOOH+H?H₂COOH ? H₂COOH?CH₂O+OH ? CH₂O+H?CH₂OH ?CH₂OH+H?CH₃OH,and the rate-limiting step is CO₂+H?HCOO,with the highest activation energy of 269.3 kJ/mol.Over ZnCu?211?and GaCu?211?,the best reaction route becomes CO₂+H?HCOO ? HCOO+H?HCOOH ?HCOOH+H?H₂COOH ? H₂COOH?CH₂O+OH ? CH₂O+H?CH₃O ?CH₃O+H?CH₃OH.After the addition of Zn,the rate-limiting step is HCOOH+H?H₂COOH and the barrier energy is 202.5 kJ/mol.Over Ga decorating Cu site,the rate-limiting step is also CO₂+H?HCOO,the same as pure Cu site,while the barrier energy is a little reduced,264.0 kJ/mol.?4?In methanol synthesis reaction,over pure Cu site,the hydrogenation reaction of CO₂ have to overcome the highest barrier energy.However,the decoration of Zn species can effectively reduce the reactions: CO₂+H?HCOO,CO₂+H?COOH as well as CH₃O+H?CH₃OH.Although the hydrogenation reaction of CO₂ molecule can't be well catalyzed over Ga decorating Cu site,the other basic steps are more easily processed with lower barrier energies than pure Cu site.Therefore,both Zn doping and Ga doping can help improve the catalysis performance of Cu site for methanol synthesis reaction from syngas.