DFT Studies On The First Dehydrogenation Of Ethanol And Decomposition Of Major Intermediates In Ethanol Oxidation Reaction On Copper-based Catalysts

Direct ethanol fuel cells feature great values of research and application due to their high energy density(8.0 kW·h^-1·kg^-1),portable and easy packaging,non-toxic and renewable characteristics of ethanol.The preparation of battery anode catalysts with high catalytic performances and reasonable cost is one of the focuses and challenges in the research.Copper-based catalysts are considered here because of their relatively low cost and their good catalytic performances in ethanol related reactions.In this thesis,density functional theory method was employed to solve two major problems encountered in copper-based catalysts towards ethanol complete oxidation reaction:Firstly,how to alloy the second metal with Cu catalysts to effectively reduce the energy barriers of the first dehydrogenation of ethanol,thereby enhancing the activity and selectivity of the reaction,and secondly,as the copper in the copper-based catalysts is easily oxidized,how it impacts the complete ethanol oxidation reaction.This work aims to provide an important theoretical understanding and guidance for the development of future direct ethanol fuel cell anode catalysts.Through the study on two kinds of initial dehydrogenation reactions via o-H andα-H of ethanol on four alloy models of Cu₃X（111）（X=Zr,In,Ag,Au）,it was revealed that Cu₃Zr has the greatest catalytic activity for dehydrogenation,exceeding Cu（100）.Besides,the reaction barrier of the o-H dehydrogenation is much lower than that of theα-H dehydrogenation,which means the selectivity of dehydrogenation reactions can be improved greatly.Meanwhile,two key factors that may have great effects on reactivity of dehydrogenation reactions were studied:One is the specific inherent properties of the second alloyed metal,like the ability to activate the O-H or C-H bond in adsorbed ethanol molecules,or indirect changes in the electronic properties of adjacent Cu atoms,and the other one is the electronic distribution on surfaces.More electron density around Cu atoms on surfaces were found to be more beneficial for dehydrogenation reactions occurring.Through the study on the adsorption of important intermediates on Cu₂O（100）and（111）surfaces and the important intermediate decomposition reactions on（111）surfaces,it was revealed that the adsorption energies of most species are pretty large.Cu₂O（111）surface is more stable,and less likely to occur isomerism rearrangement.Besides,the most essential adsorption and reaction site on the surface was the Cu _CUSUS atom.The most possible pathway for CH₃CO to CCO on Cu₂O（111）was the continuous dehydrogenation reaction.Compared with Cu（100）,the reactivity of complete ethanol oxidation reaction on Cu₂O（111）dropped significantly,especially for the C-C cleavage reactions.Furthermore,DFT+U,a more accurate description of the electronic properties of transition metal oxides,was used in the study of the adsorption and decomposition of important intermediates on the Cu₂O（111）surface.It was found that the utilization of the DFT+U method did not have a consistent change in the calculated adsorption energy.It varied between 0.07 eV and 0.82 eV.For the study of reactions,the C-C cleavage barriers were underestimated without+U correction,while the C-H cleavage barriers were overestimated.The reaction barriers and the values for Hubbard U have a nice linear relationship,which further illustrated the importance of the DFT+U method and a proper choice of the Hubbard U values.