单原子合金对烷基C-H键活化调控的理论研究

Activation of Single Atom Alloys towards Alkyl C-H Bond:A Theoretical Study

单原子合金是指活性金属原子分散在Cu、Ag或Au载体上所构成的催化剂,近年来已成为单原子催化研究中的一颗“新星”.单原子合金上孤立活性位点与载体金属的电子结构不同,具有奇异的电子结构,故通常表现出独特的催化行为。目前尚缺乏一种可靠的单原子合金催化特性描述符.本文系统地考察了甲烷、丙烷和乙苯在15种Rh、Ir、Ni、Pd和Pt掺杂Cu(111)、Ag(111)和Au(111)单原子合金上初始C-H键活化.密度泛函计算表明,烷基C-H键的活化能垒与d带中心和H原子吸附相关较差,而与反应能之间相关性较好.理论分析表明,C原子在顶位的吸附与C-H活化过渡态之间存在着轨道相互作用的相似性,不仅涉及到对d,2轨道给予,也涉及day/dy≥轨道的元反馈.据此,C原子吸附能与甲烷、丙烷和乙苯C-H键活化能也具有很强的相关性(R2>0.9).

Single atom alloys(SAAs),composed of active metal dopants atomically dispersed on the Cu,Ag,or Au host metals,have recently become a“rising star"in single atom catalysis research.SAAs usually display unique catalytic behavior,mainly due to the anomalous electronic structure of isolated active sites,distinguishing from that of the parent metals.As the consequence,there is lack of robust and reliable descriptor of catalytic properties of SAAs.In this work,we present a systematically theoretical study on the first C-H bond activation of methane,propane and ethylbenzene over 15 SAAs comprising of Rh,Ir,Ni,Pd,and Pt doping Cu(111),Ag(111),and Au(111)surfaces.Our DFT calculations demonstrate that not only the d-band centers but also the H atom adsorption energies can not correlate well with the activation barriers of alkyl C-H bond,while enhanced performance is achieved when using the reaction energy as a descriptor.We find that there exists orbital interaction similarity between C atom adsorption on top site and the transition states of C-H activation because both of them involve not only donation with dxy/dyz orbital but also theπback-donation from dry/dyz orbital(s).As a consequence,the C adsorption energies and C-H bond activation energies are very strongly correlated(R2>0.9),not only for methane but also for propane and ethylbenzene.