纳米尺寸氧化钇团簇阴离子与正丁烷反应的研究

Study on the Reaction of Nanosized Yttrium Oxide Cluster Anions with n-Butane in the Gas Phase

在单一原子量分辨水平上研究纳米尺寸过渡金属氧化物团簇(MxOyq)与小分子的反应不仅能够获得氧化物纳米颗粒的反应性随原子组成和尺寸连续变化的演变规律,而且对认识其结构特征以及表面活性氧物种(如O−•自由基)的产生机制等具有重要意义.本工作分别采用耦合快速流动反应管和耦合四极质量过滤器-线形离子阱的两套反射式飞行时间质谱研究了不同“氧缺陷指数”(Δ)的氧化钇团簇YxOy−(x≤50,y≤76;Δ≡2y-1-3x,Δ=0~5)和掺杂氟(F)原子团簇YxOyF−[x≤49,y≤74;Δ≡(2y+1)-1-3x,Δ=1]与n-C4H10分子的反应.实验观测到Δ=1系列团簇(Y_(2)O_(3))NO−(N=1~25)、(Y_(2)O_(3))NYO_(2)F−(N=1~24)及Δ=4系列团簇(Y_(2)O_(3))NYO4−(N=1,3~24)具有氢抽取反应活性,N≥2时其它Δ系列团簇(Δ=0,2,3,5)在相同实验条件下没有表现出明显的反应性.密度泛函理论研究Δ=1或4系列小尺寸团簇(Y_(2)O_(3))NYxOyF0,1−(N≤4;x=0,1)的结构揭示O−•自由基是氢抽取反应的活性位点,结合实验可推测Δ=1或4系列纳米尺寸团簇(Y_(2)O_(3))NYxOyF0,1−(x=0,1)结构中也含有O−•自由基.这些结果表明Δ=0系列惰性纳米尺寸团簇(Y_(2)O_(3))NYO^(2-)可以通过吸附一个O_(2)分子发生电子转移生成O−•自由基(O^(2-)+O_(2)→O−•+O^(2-)•),也可以通过掺入F原子的方式生成O−•自由基(O^(2-)+F•→O−•+F−).

Investigation on the reactions of atomically precise transition metal oxide nanoparticles with small molecules can not only identify the property evolution of nanoparticles with respect to the continuum change of size and composition,but also improve our understanding of the formation mechanism of reactive oxygen species[e.g.,atomic oxygen radical anions(O−•radicals)]over oxide surface.Yttrium oxide cluster anions YxOy−(x≤50,y≤76)with different oxygen deficiencies(Δ≡2y-1-3x,Δ=0~5)and YxOyF−(x≤49,y≤74)withΔ=1[Δ≡(2y+1)-1-3x]have been prepared by laser ablation in an O_(2) background and reacted with alkane molecules(n-butane)in a fast flow reactor.The reactions of mass-selected small-sized YxOy−(x≤8)clusters with n-butane were investigated in a linear ion trap reactor.Time-of-flight mass spectrome-ter was used to detect the cluster distribution before and after the reactions.The observation of(Y_(2)O_(3))NOH−(N=1~25),(Y_(2)O_(3))NYO4H−(N=1,3~24),and(Y_(2)O_(3))NYO_(2)FH−(N=1~24)products suggest that(Y_(2)O_(3))NO−(Δ=1;N=1~25),(Y_(2)O_(3))NYO4−(Δ=4;N=1,3~24)and(Y_(2)O_(3))NYO_(2)F−(Δ=1;N=1~24)can bring about hydrogen-atom abstraction(HAA)from n-butane.The reactivity of(Y_(2)O_(3))NO−,(Y_(2)O_(3))NYO_(2)F−and(Y_(2)O_(3))NYO4−clusters are significantly size-dependent and the highest reactivity was observed for N=3(Y6O10−,Y7O11F−,and Y7O13−).Density functional theory(DFT)calculations were performed to study the geometrical structures and unpaired spin densities of(Y_(2)O_(3))NO−(N=1~4),(Y_(2)O_(3))NYO_(2)F−(N=1~3),and(Y_(2)O_(3))NYO4−(N=1,3,4)clusters that were found to contain O−•radicals as active centers to abstract hydrogen atoms from n-butane,in agreement with the experiments.The results imply that the O−•radicals which had been proved to be present in the small yttrium oxide clusters withΔ=1 or 4 are well preserved in the yttrium oxide nanoparticles.(Y_(2)O_(3))NYO4−(Δ=4;N=1,3~24)and(Y_(2)O_(3))NYO_(2)F−(Δ=1;N=1~24)can be considered to be generated by the adsorption of an O_(2) molecule or F onto the unreactive singlet(Y_(2)O_(3))NYO^(2-)(Δ=0;N=1~24).The studies on gas-phase clusters suggest that adsorption of O_(2) molecule or F atom onto unreactive metal oxide clusters will result in the generation of O−•radical(O^(2-)+O_(2)→O−•+O^(2-)•and O^(2-)+F•→O−•+F−).This work not only reveals the new mechanisms of the formation of O−•radical on met-al oxide surfaces,but also provides new insights into the design of novel transition metal oxide-based catalysts.