单个金或银原子掺杂的氧化钒团簇上的甲烷活化反应

Methane Activation on(Au/Ag)1-Doped Vanadium Oxide Clusters

运用密度泛函理论系统研究了甲烷在MV3Oyq (M=Au/Ag,y=6–8,q=0或±1)团簇上的吸附和活化。研究得到了吸附体系的微观几何构型、吸附能、电荷分布等性质,找到了5个可以明显活化甲烷分子的含Au团簇。在这些体系中,Au均吸附在基底团簇V3Oyq的O位置,而CH4均在Au原子上被活化。团簇电荷对活化能力有明显影响,阳离子团簇的活化能力最强,中性体系次之,阴离子团簇的活化能力很弱。测试计算表明引入D3色散矫正对于体系结构和能量的计算结果影响不大。本文作为单原子催化剂上甲烷吸附和活化反应的团簇模型研究,为进一步研究单原子催化剂上甲烷的活化机理提供了基础,也为合理设计低温下甲烷转化的单原子催化剂提供了有益的线索。

The activation of methane(CH4)is a key step in its conversion to more valuable products.The activation mechanisms of CH4 on catalyst surfaces have been widely studied using gas-phase cluster models,which can be operated on systems with a precise number of atoms and determined structures.Herein,we have used MV3Oyq(M=Au/Ag,y=6–8,q=0 or±1)clusters,in which a single Au or Ag atom was supported on vanadium oxide clusters,as simple models to mimic the properties of newly developed single-atom catalysts.The adsorption and activation of CH4 on these MV3Oyq clusters were systematically studied via density functional theory calculations at the B3LYP/Def2-TZVP level,which provided insights into the geometric structures,adsorption energies,and charge distributions of the adsorption systems.Five Au-containing clusters,AuV3O6,AuV3O7,AuV3O8,AuV3O6+,and AuV3O7+,were able to activate CH4,while other clusters,including all Ag-containing clusters,were inert.In the active clusters,all Au atoms were adsorbed on the O-atom sites of the supporting V3Oyq cluster and served as the active sites for CH4 activation.The activation of CH4 was characterized by the lengthened C―H bond(approximately 115 pm),short distances between CH4 and Au(approximately 184 pm),relatively high adsorption energies of CH4(~0.590–1.145 eV),and significant electron transfer from CH4 to the clusters(above 0.08 e).In particular,AuV3 O8,which is a neutral cluster with a close-shell electronic state,can activate CH4 with a C―H bond length of 115 pm,Au―H bond length of 183 pm,the adsorption energy of CH4 of 0.853 eV,and the charge on CH4 of+0.088 e.The charge state of the cluster has a significant effect on the activation ability:cationic clusters are the most active,followed by neutral clusters,while anionic clusters have the lowest activities toward CH4.Consistently,the local charge on the M atom has a positive correction with the activation ability of MV3Oyq clusters with a certain M.However,as compared to Au-containing clusters,Ag-containing clusters have lower activities despite the higher local charges on Ag in each MV3Oyq cluster.The results indicate that the inclusion of D3 dispersion correction has a small effect on structures and energies.This study may serve as a foundation for further research on the activation of CH4 on singleatom catalysts and provides useful information on rational designing of single-atom catalysts for CH4 conversion at low temperatures.