Mechanism of Syngas Selective Hydrogenation on ZnCr2O4(111)Surface:DFT+U
The spinel-phase ZnCr2O4 as an important catalytic material for CO/CO2 hydrogenation,the mechanism of the generation of active hydrogen species and its role of the CO hydrogenation reaction mechanism on this surface has been much debate.In this work,the adsorption of H species on ZnCr2O4(111)surface was first systematically calculated,the results showed that the adsorption of H species at the Zn site with endothermic about of 0.99 eV.Interestingly,Bader charge calculations illustrated that the adsorbed single H species at the Zn site accepted some of electrons from the Zn site and form a hydride species(the corresponding Bader charge of the ad-sorbed H species was-0.31|e|).This was mainly attributed to the fact that the stoichiometric ZnCr2O4(111)surface had a lower coordi-nation number of surface Zn site(Zn3c),leading to surface polarization,while the 3d orbitals of the Zn3c species on ZnCr2O4(111)sur-face were flexible,and there was also a matching of orbital orientations that allowed for strong bond when interact with adsorbed spe-cies.Therefore,when there was an H species adsorbed at the Zn site on ZnCr2O4(111)surface,the hydride species could be efficient-ly generated due to the existence of special storage electron properties of Zn.Secondly,the activation of the most stable H2 species on ZnCr2O4(111)surface was also calculated,and the results showed that the heterolytic H2 dissociation to produce Zn-H and O-H spe-cies on ZnCr2O4(111)surface was easier than the homolytic H2 dissociation to produce the two O-H species,and that the heterolytic H2 dissociation could form the highly active and stable Zn-H species,which was mainly attributed to the fact that the flexible 3d orbit-als of the Zn species on ZnCr2O4(111)surface and also favored the stabilization of hydride species due to the high symmetry of the Zn 3d orbitals.In addition,Bader charge calculate also revealed that Bader charge of the Zn-H species formed by the heterolytic H2 disso-ciation-0.35|e|,which was only half of the value of Bader charge of the O-H species(the corresponding Bader charge was 0.66|e|),these results suggested again that Zn species possessed a special property of storing electrons,and that at the same time the produced Zn-H species was relatively active and might effectively modulate the activity of the selective hydrogenation reaction.Therefore,it could be found that H2 participated in the selective hydrogenation reaction on ZnCr2O4(111)surface mainly through heterolytic activa-tion,and the Zn-H species was key intermediate.Thirdly,the reaction mechanism of CO selective hydrogenation on ZnCr2O4(111)surface was also systematically investigated,and it could be found that the energy barriers of the Zn-H species attack with the Cδ+/Oδof CO to generate HCO/COH species were both lower than the energies barriers of the OH species as a hydrogen source to react with CO produce the same key intermediate species,these results suggested that the Zn-H species had high activity for CO selective hydro-genation.It could also be found that the Zn-H species activation of CO to produce HCO species with the lowest energy barriers,thus indicating that Zn-H species also reflected high activity and selectivity for CO hydrogenation.Notably,when the OH species produced by the homolytic H2 dissociation and then it reacted with Cδ+in CO molecule to form the HCO species,this process only required over-coming an energy barrier of 2.15 eV and required endothermic about 1.29 eV.Obviously,this phenomenon was contrary to the conven-tional chemical knowledge,and by the analysis of the structure of OH species attacking the Cδ+/Oδ-of CO2 to form HCO or COH inter-mediates,it could be found that in the transition state for the formation of HCO species,the structure is closer to the final state.While in the transition state for the formation of COH species,the H in the transition state existed in the form of radicals.Therefore,from the above discussion,it could be learnt that when using OH species to attack Cδ+reaction of CO to generate HCO species was more favour-able than attacking Oδ-reaction to generate COH species.Finally,the following inspiration could be gotten,for the activity of CO-se-lective hydrogenation reaction on ZnCr2O4(111)surface,the catalytic activity might be higher when using reduced ZnCr2O4 catalytic materials.Moreover,other metal oxides that could effectively improve the CO conversion instead of ZnCr2O4 should be searched for,and such catalysts should also have the following characteristics:moderate oxygen vacancy formation energy and strong ability to acti-vate CO.In summary,the present work systematically elucidated the intrinsic properties of the active hydrogen species on ZnCr2O4(111)surface and their effects on the CO-selective hydrogenation reaction from the thermodynamic and geometry perspectives,and ex-plained the intrinsic reason for the high activity of the CO-selective hydrogenation reaction under the presence of the hydride species,which provided some theoretical guidance for the rational design of Zn-based syngas selective hydrogenation catalytic materials.
ZnCr2O4(111)hydridesyngasselective hydrogenationdensity functional of theory
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