过渡金属氧化物广泛应用在当今能源与环境相关的催化领域,理解其表面化学性质以及结构-反应活性之间的关系对于先进催化材料的进一步发展以至理性设计至关重要.3d后过渡系金属(Mn,Fe,Co,Ni)的氧化物以其中金属离子独特的自旋状态和由此产生的铁磁/反铁磁性为典型特征.研究过渡金属氧化物的自旋状态以及磁性对表面化学的影响将使我们更加完整了解这些材料的表面化学.以NiO为代表的后过渡系金属岩盐结构一元氧化物具有反铁磁性,被经常作为反铁磁研究的模型体系.尽管在低温(低于其Neel温度)下NiO体相的完整晶体具有确定的反铁磁序,但是一系列最新研究表明,在条件变化时NiO表面的Ni离子可以产生不同的磁序.以此为背景,本工作以NiO为模型体系,采用DFT+U的第一性原理方法研究了NiO表面磁序对表面的小分子吸附活性的影响,包括表面吸附活性对各磁性相的表面取向以及吸附物种磁性的依赖关系.我们考察了NiO的5种反铁磁相和一种铁磁相,两个晶面NiO(001)和NiO(011),顺磁性分子NO和非顺磁性分子CO.我们发现表面能受磁性的影响较轻微,NiO(001)面上从49到54 meV/?2,NiO(011)面上从162到172 meV/?2.在NiO(001)面上,CO与NO都倾向于在Ni离子的顶位吸附.对于不同的体相磁序与表面取向,CO吸附能的变化范围为-0.33~-0.37 eV,NO吸附能的变化范围为-0.42~-0.46 eV.在NiO(011)表面,两种分子都倾向于吸附在由两个Ni离子构成的桥位.我们发现相对于NiO不同磁性相的体相长程磁序,吸附位点处构成桥位的两个Ni离子的局部磁矩相对取向对于分子的吸附具有更加显著的影响.计算得到NO在局部磁矩相对取向反平行(↑↓)吸附位点处的吸附能为-0.99~-1.05 eV,在局部磁矩相对取向平行(↑↑)吸附位点处吸附会增强,吸附能为-1.21~-1.30 eV.对于CO,尽管计算的吸附能在(↑↓)吸附位点(-0.73~-0.75 eV)与在(↑↑)吸附位点(-0.71~-0.72 eV)非常接近,两种吸附位点处的CO吸附时分子轨道杂化方式以及吸附后CO的局域电子态密度却具有明显不同的特征.本工作突出揭示了分子在过渡金属氧化物表面的多重吸附位点上吸附时吸附位点的局域磁矩相对取向对吸附性能的影响.%The influence of the magnetism of transition metal oxide, nickel(Ⅱ) oxide (NiO), on its surface reac-tivity and the dependence of surface reactivity on surface orientation and reactant magnetism were studied by density functional theory plus U calculations. We considered five different antiferro-magnetically ordered structures and one ferromagnetically ordered structure, NiO(001) and Ni(011) surfaces, paramagnetic molecule NO, and nonparamagnetic molecule CO. The calculations showed that the dependence of surface energies on magnetism was modest, ranging from 49 to 54 meV/?2 for NiO(001) and from 162 to 172 meV/?2 for NiO(011). On NiO(001), both molecules preferred the top site of the Ni cation exclusively for all NiO magnetic structures considered, and calculated adsorption energies ranged from -0.33 to -0.37 eV for CO and from -0.42 to -0.46 eV for NO. On NiO(011), both molecules preferred the bridge site of two Ni cations irrespective of the NiO magnetism. It was found that rather than the long-range magnetism of bulk NiO, the local magnetic order of two coordinated Ni cations binding to the adsorbed molecule had a pronounced influence on adsorption. The calculated NO adsorption energy at the (↑↓) bridge sites ranged from -0.99 to-1.05 eV, and become stronger at the (↑↑) bridge sites with values of -1.21 to -1.30 eV. For CO, although the calculated adsorption energies at the (↑↓) bridge sites (-0.73 to -0.75 eV) were very close to those at the (↑↑) bridge sites (-0.71 to -0.72 eV), their electron hybridizations were very different. The present work highlights the importance of the local magnetic order of transition metal oxides on molecular adsorption at multi-fold sites.
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