摘要/Abstract

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(M_xO_y^q)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Y_xO_y^- (x≤50,y≤76; Δ≡2y–1–3x, Δ=0~5)和掺杂氟(F)原子团簇Y _xO_yF^- [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(Y₂O₃)_NO^- (N=1~25)、(Y₂O₃)_NYO₂F^- (N=1~24)及Δ=4系列团簇(Y₂O₃)_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(Y₂O₃)_NY_xO_yF_0,1^- (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(Y₂O₃)_NY_xO_yF_0,1^- (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(Y₂O₃)_NYO₂^-可以通过吸附一个O₂分子发生电子转移生成O^-•自由基(O^2-+O₂→O^-•+O₂^-•), 也可以通过掺入F原子的方式生成O^-•自由基(O^2-+F^•→O^-•+F^-).
关键词: 氧化钇团簇, 纳米尺寸, 反应活性, 原子氧自由基, 飞行时间质谱
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 Y_xO_y^- (x≤50,y≤76) with different oxygen deficiencies (Δ≡2y–1–3x, Δ=0~5) and Y _xO_yF^- (x≤49,y≤74) with Δ=1 [Δ≡(2y+1)–1–3x] have been prepared by laser ablation in an O₂ background and reacted with alkane molecules (n-butane) in a fast flow reactor. The reactions of mass-selected small-sized Y_xO_y^- (x≤8) clusters withn-butane were investigated in a linear ion trap reactor. Time-of-flight mass spectrometer was used to detect the cluster distribution before and after the reactions. The observation of (Y₂O₃)_NOH^- (N=1~25), (Y₂O₃)_NYO₄H^- (N=1, 3~24), and (Y₂O₃)_NYO₂FH^- (N=1~24) products suggest that (Y₂O₃)_NO^- (∆=1;N=1~25), (Y₂O₃)_NYO₄^- (∆=4;N=1, 3~24) and (Y₂O₃)_NYO₂F^- (Δ=1;N=1~24) can bring about hydrogen-atom abstraction (HAA) from n-butane. The reactivity of (Y₂O₃)_NO^-, (Y₂O₃)_NYO₂F^- and (Y₂O₃)_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 (Y₂O₃)_NO^- (N=1~4), (Y₂O₃)_NYO₂F^- (N=1~3), and (Y₂O₃)_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 with Δ=1 or 4 are well preserved in the yttrium oxide nanoparticles. (Y ₂O₃)_NYO₄^- (∆=4;N=1, 3~24) and (Y₂O₃)_NYO₂F^- (∆=1;N=1~24) can be considered to be generated by the adsorption of an O₂ molecule or F onto the unreactive singlet (Y₂O₃)_NYO₂^- (∆=0;N=1~24). The studies on gas-phase clusters suggest that adsorption of O₂ molecule or F atom onto unreactive metal oxide clusters will result in the generation of O^-• radical (O^2-+O₂→O^-•+O₂^-• and O^2-+F^•→O^-•+F^-). This work not only reveals the new mechanisms of the formation of O^-• radical on metal oxide surfaces, but also provides new insights into the design of novel transition metal oxide-based catalysts.
Key words: yttrium oxide cluster, nanosize, reactivity, atomic oxygen radical anion, time-of-flight mass spectrometer


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