Effect Of Ceria Crystal Planes And Valence States On Dimethyl Carbonate Formation From Carbon Dioxide And Methanol

More and more attention has been paid on dimethyl carbonate?DMC?formation from carbon dioxide and methanol because of the high atomic economy,and remission of excess CO₂ which leads environment promblem.Ceria exposing different crystal planes and valence states exhibit different activity in DMC formation.In this paper,we combine in-situ FTIR with other characterizations to investigate the effect of crystal planes and valence states on DMC formation from CO₂ and methanol over ceria catalyst.Three well-shaped CeO₂ catalysts,nanorod,nanocube and octahedron,which are enclosed with different crystal planes?{111},{110},{100} on nanorod;{100} on nanocube;{111} on octahedron?,are prepared through hydrothermal method.Ceria with different morphologies show different activities in DMC formation from CO₂ and CH₃OH: rod > octa > cube.According to XRD,HR-TEM and in situ FTIR characterization,it's speculated that quantity of –OH groups and different kinds of CO₂ adsorptions on ceria surface are decisive factor in this system,which are closely related to ceria crystal planes.Insufficient –OH groups on octahedron ceria surface limit its activity.Bidentate carbonate formed on {110} and {111} crystal surface is nesessary for the reaction with –OCH₃ to itermediate product,monodentate methyl carbonate.Ceria nanorods have a large amount of –OH groups and mainly enclosed by {111} and {110} crystal planes,which leads to more activation of CH₃ OH and CO₂,furthermore,to the highest activity.Bidentate carbonate can't be formed on cube surface,as ceria nanocube enclosed by {100} crystal planes.As a result,ceria cubes nearly show none activiy.As a heterogeneous catalyst for DMC formation from carbon dioxide and methanol,ceria calcined at different temperature show different valence states and catalytic activities.The concentration of Ce⁴⁺ increases with the calcination temperature rising,which leads to larger distribution of activity methanol adsorptions?on top methoxy?type I?and bridging methoxy without connection with oxygen vacancy?type II??and CO₂ adsorptions?bidentate carbonate?.At the same time,high calcination temperature leads to the minishing of surface area and amounts of –OH groups on surface dehydrating.As a result,with the increasing calcination temperature,the ceria activation of methanol and CO₂ increases firstly and then decreases.For DMC yield,ceria calcined at 773 K shows the highest activity and the sample calciend at 973 K shows nearly none activity.