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.
作者
刘慧慧
来壮壮
魏呵呵
王志强
郭耘
胡培君
Liu Huihui;Lai Zhuangzhuang;Wei Hehe;Wang Zhiqiang;Guo Yun;Hu Peijun(State Key Laboratory of Green Chemical Engineering and Industrial Catalysis,Research Institute of Industrial Catalysis,School of Chemistry and Molecular Engineering,East China University of Science and Technology,Shanghai 200237,China;School of Physical Science and Technology,Shanghai Tech University,Shanghai 201210,China)
出处
《稀有金属》
EI
CAS
CSCD
北大核心
2024年第7期955-964,共10页
Chinese Journal of Rare Metals
基金
国家自然科学基金青年基金项目(22203030)资助。
作者简介
刘慧慧(1982-),女,辽宁葫芦岛人,博士研究生,研究方向:计算化学,E-mail:liuhuihui@ecust.edu.cn;通信作者:王志强,副研究员,电话:15021580093,E-mail:zhiqiangwang@ecust.edu.cn;通信作者:胡培君,教授,电话:13917069928,E-mail:peijunhu@ecust.edu.cn。
