文献类型：期刊文献

中文题名：ZnCr_(2)O_(4)(111)表面合成气选择性加氢反应机制的理论研究

英文题名：Mechanism of Syngas Selective Hydrogenation on ZnCr_(2)O_(4)(111)Surface:DFT+U

作者：刘慧慧[1];来壮壮[1];魏呵呵[1];王志强[1];郭耘[1];胡培君[1,2]

机构：[1]华东理工大学化学与分子工程学院,绿色化学工程与工业催化国家重点实验室,工业催化研究所,上海200237;[2]上海科技大学物质科学与技术学院,上海201210

年份：2024

卷号：48

期号：7

起止页码：955

中文期刊名：稀有金属

外文期刊名：Chinese Journal of Rare Metals

收录：CSTPCD;;EI(收录号：20243516936786);北大核心:【北大核心2023】；CSCD:【CSCD2023_2024】；

基金：国家自然科学基金青年基金项目(22203030)资助。

语种：中文

中文关键词：ZnCr_(2)O_(4)(111);氢负物种;合成气;选择性加氢;密度泛函理论

外文关键词：ZnCr_(2)O_(4)(111);hydride;syngas;selective hydrogenation;density functional of theory

摘要：尖晶石相ZnCr_(2)O_(4)作为CO/CO_(2)加氢反应的重要催化材料,其表面活性氢物种的产生机制及其对CO加氢反应的作用一直备受争议。首先计算了最稳定的ZnCr_(2)O_(4)(111)表面H_(2)物种的活化,结果表明H_(2)分子在ZnCr_(2)O_(4)(111)表面异裂解离产生的Zn-H和O-H物种比H_(2)均裂解离产生两个O-H物种更加容易,且H_(2)通过异裂解离可以有效产生高活性的Zn-H物种,这主要归因于ZnCr_(2)O_(4)(111)表面Zn物种具有灵活的3d轨道,同时由于Zn 3d轨道具有较高的对称性,因此有利于H负物种的稳定。最后,研究了ZnCr_(2)O_(4)表面CO选择性加氢的反应机制,发现ZnH物种进攻CO分子的C~(δ+)/O~(δ-)生成COH或HCO物种能垒均比O-H物种作为氢源产生相同的关键中间物种能垒更低,说明Zn-H物种是CO选择性加氢反应具有高活性的关键物种;其次,还可以发现Zn-H物种活化CO产生HCO物种能垒最低,表明Zn-H物种对CO选择性加氢反应也体现出较高的活性。实验结果可为Zn基合成气选择性加氢催化材料的理性设计提供一些指导。
The spinel-phase ZnCr_(2)O_(4) as an important catalytic material for CO/CO_(2) 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 ZnCr_(2)O_(4)(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 adsorbed H species was-0.31|e|).This was mainly attributed to the fact that the stoichiometric ZnCr_(2)O_(4)(111)surface had a lower coordination number of surface Zn site(Zn3c),leading to surface polarization,while the 3d orbitals of the Zn3c species on ZnCr_(2)O_(4)(111)surface were flexible,and there was also a matching of orbital orientations that allowed for strong bond when interact with adsorbed species.Therefore,when there was an H species adsorbed at the Zn site on ZnCr_(2)O_(4)(111)surface,the hydride species could be efficiently generated due to the existence of special storage electron properties of Zn.Secondly,the activation of the most stable H_(2) species on ZnCr_(2)O_(4)(111)surface was also calculated,and the results showed that the heterolytic H_(2) dissociation to produce Zn-H and O-H species on ZnCr_(2)O_(4)(111)surface was easier than the homolytic H_(2) dissociation to produce the two O-H species,and that the heterolytic H_(2) dissociation could form the highly active and stable Zn-H species,which was mainly attributed to the fact that the flexible 3d orbitals of the Zn species on ZnCr_(2)O_(4)(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 H_(2) dissociation-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 H_(2) participated in the selective hydrogenation reaction on ZnCr_(2)O_(4)(111)surface mainly through heterolytic activation,and the Zn-H species was key intermediate.Thirdly,the reaction mechanism of CO selective hydrogenation on ZnCr_(2)O_(4)(111)surface was also systematically investigated,and it could be found that the energy barriers of the Zn-H species attack with the Cδ+/Oδ-of 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 hydrogenation.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 H_(2) dissociation and then it reacted with Cδ+in CO molecule to form the HCO species,this process only required overcoming an energy barrier of 2.15 eV and required endothermic about 1.29 eV.Obviously,this phenomenon was contrary to the conventional chemical knowledge,and by the analysis of the structure of OH species attacking the Cδ+/Oδ-of CO_(2) to form HCO or COH intermediates,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 favourable than attacking Oδ-reaction to generate COH species.Finally,the following inspiration could be gotten,for the activity of CO-selective hydrogenation reaction on ZnCr_(2)O_(4)(111)surface,the catalytic activity might be higher when using reduced ZnCr_(2)O_(4) catalytic materials.Moreover,other metal oxides that could effectively improve the CO conversion instead of ZnCr_(2)O_(4) should be searched for,and such catalysts should also have the following characteristics:moderate oxygen vacancy formation energy and strong ability to activate CO.In summary,the present work systematically elucidated the intrinsic properties of the active hydrogen species on ZnCr_(2)O_(4)(111)surface and their effects on the CO-selective hydrogenation reaction from the thermodynamic and geometry perspectives,and explained 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.