Reactivity of Chemisorbed Oxygen Atoms and Their Catalytic Consequences during CH4-O2 Catalysis on Supported Pt Clusters
Reactivity of Chemisorbed Oxygen Atoms and Their Catalytic Consequences during CH4-O2 Catalysis on Supported Pt Clusters
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DOI：
10.1021/ja202411v
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发表时间：
2011-10-12
影响因子：
15
通讯作者：
Iglesia, Enrique
中科院分区：
文献类型：
--
作者：
Chin, Ya-Huei (Cathy);Buda, Corneliu;Iglesia, Enrique
动力学和同位素数据以及密度泛函 1 理论处理为 CH4、使用 O-2、H2O 或 CO2 作为 Pt 簇氧化剂的氧化反应期间观察到的四种不同动力学状态所涉及的基本步骤和活性位点要求提供了证据。由于涉及不同的动力学相关步骤、主要吸附物质以及不同基本步骤的速率和平衡常数，这四种状态表现出不同的速率方程。随着 Pt 簇上化学吸附氧 (O*) 覆盖度的变化，会发生 3 种状态之间的转变。 O* 覆盖率又由虚拟 O-2 压力给出，该压力代表在保持吸附-解吸平衡的情况下给出普遍稳态 O* 覆盖率的压力。虚拟 O-2 压力作为催化表面氧化学势的替代，反映了 C H 和 O=O 活化步骤之间的动力学耦合。当 O-2 活化平衡时，O* 覆盖率和虚拟压力取决于 O-2 压力；当该步骤由于 CH4 衍生中间体快速清除 O* 而变得不可逆时，则取决于 O-2/CH4 比率。在这三个动力学机制中，C H 键激活是唯一的动力学相关步骤，但发生在不同的活性位点上，随着 O* 覆盖率的降低，这些活性位点从氧氧 (O*-O*) 演变为氧氧空位 (O*-*) 和空位空位 (*-*) 位点对。在 O* 饱和簇表面上，O*-O* 位点对通过均裂夺氢步骤激活 CH4 中的 C-H 键，形成具有显着自由基特征且在过渡态与表面弱相互作用的 CH3 基团。在这种情况下，速率与 CH4 压力线性相关，但与 O-2 压力无关。观察到的正常 CH4/CD4 动力学同位素效应与 C-H 键激活的动力学相关性一致；在存在或不存在 CH4 的情况下相同的 O-16(2)-O-18(2) 同位素交换率表明 O-2 活化步骤在催化过程中是准平衡的。测量的和 DFT 衍生的 C-H。由于过渡态 CH3 片段的稳定性较弱，键激活势垒很大，但可以通过这些类自由基物质的高熵来补偿。这种状态下的周转率随着 Pt 分散度的增加而降低，因为小团簇上的低配位暴露 Pt 原子与 O* 的结合比大团簇上低折射率面的 Pt 原子结合得更强烈，从而使 O* 在 H 抽象中的效率较低。当 O* 覆盖的表面上出现空位（*，也是暴露的 Pt 原子）时，O*-* 位点对通过协同氧化加成和过渡态的 H 抽象激活 C-H 键，而过渡态通过 CH3 与空位的相互作用有效稳定，这导致比 O*-O* 对高得多的周转率。在这种情况下，O-2 活化变得不可逆，因为快速 C-H 键活化步骤会在 O* 形成时清除它。因此，O* 覆盖范围由普遍的 O-2/CH4 比率而不是 O-2 压力决定。对于 O*-* 介导的转换，CH4/CD4 动力学同位素效应比 O*-O* 位点对大得多，因为需要 C-H（和 C-D）激活步骤来形成参与 C-H 键激活的 * 位点。 O*-* 对介导的 CH4-O-2 反应的转换速率随着 Pt 分散度的增加而降低，就像 O*-O* 活性结构的情况一样，因为小簇上更强的 O* 结合不仅导致 O* 反应性降低原子，还可以降低团簇表面的空位浓度。随着 O-2/CH4 比率和 O* 覆盖范围变小，裸 Pt 团簇上的 O-2 激活成为唯一的动力学相关步骤；周转率与 O-2 压力成正比，与 CH4、压力无关，并且未观察到 CH4/CD4 动力学同位素效应。在这种情况下，周转率几乎与 Pt 分散无关，因为 O-2 活化步骤基本上是无障碍的。在不存在 O-2 的情况下，替代较弱的氧化剂（例如 H2O 或 CO2）会导致最终的动力学状态，其中裸簇表面 *-* 对上的 C-H 键解离限制了 CH 转化率。 CH4 中的速率变为一级，并且与共反应物无关，并且观察到正常的 CH4/CD4 动力学同位素效应。在这种情况下，周转率随着分散度的增加而增加，因为低配位 Pt 原子通过与 CH3 和 H 片段的更强结合更有效地稳定 C-H 键激活过渡态。这些发现及其机制解释与所有速率和同位素数据以及激活势垒和簇尺寸对过渡态影响的理论估计一致。它们证明了化学吸附氧的覆盖率和反应性在确定 CH4 表面结构的类型和有效性、使用 O-2、H2O 或 CO2 作为氧化剂的氧化反应以及这些反应中普遍存在的速率依赖性、活化能和熵以及簇尺寸效应的多样性方面的重要作用。这些结果还表明理论和实验如何在实际条件下阐明现实催化剂上的复杂表面化学，并通过由此产生的机理见解提供簇尺寸和表面配位对周转率影响的具体预测，其趋势和幅度敏感地取决于主要吸附中间体的性质和动力学相关步骤。
Kinetic and isotopic data and density functional 1 theory treatments provide evidence for the elementary steps and the active site requirements involved in the four distinct kinetic regimes observed during CH4, oxidation reactions using O-2, H2O, or CO2 as oxidants on Pt clusters. These four regimes exhibit distinct rate equations because of the involvement of different kinetically relevant steps, predominant adsorbed species, and rate and equilibrium constants for different elementary steps. Transitions among 3 regimes occur as chemisorbed oxygen (O*) coverages change on Pt clusters. O* coverages are given, in turn, by a virtual O-2 pressure, which represents the pressure that would give the prevalent steady-state O* coverages if their adsorption-desorption equilibrium was maintained. The virtual O-2 pressure acts as a surrogate for oxygen chemical potentials at catalytic surfaces and reflects the kinetic coupling between C H and O=O activation steps. O* coverages and virtual pressures depend on O-2 pressure when O-2 activation is equilibrated and on O-2/CH4 ratios when this step becomes irreversible as a result of fast scavenging of O* by CH4-derived intermediates. In three of these kinetic regimes, C H bond activation is the sole kinetically relevant step, but occurs on different active sites, which evolve from oxygen oxygen (O*-O*), to oxygen oxygen vacancy (O*-*), and to vacancy vacancy (*-*) site pairs as O* coverages decrease. On O*-saturated cluster surfaces, O*-O* site pairs activate C-H bonds in CH4, via homolytic hydrogen abstraction steps that form CH3 groups with significant radical character and weak interactions with the surface at the transition state. In this regime, rates depend linearly on CH4, pressure but are independent of O-2 pressure. The observed normal CH4/CD4 kinetic isotope effects are consistent with the kinetic-relevance of C-H bond activation; identical O-16(2)-O-18(2) isotopic exchange rates in the presence or absence of CH4, show that O-2 activation steps are quasi-equilibrated during catalysis. Measured and DFT-derived C-H. bond activation barriers are large, because of the weak stabilization of the CH3 fragments at transition states, but are compensated by the high entropy of these radical-like species. Turnover rates in this regime decrease with increasing Pt dispersion, because low-coordination exposed Pt atoms on small clusters bind O* more strongly than those that reside at low-index facets on large clusters, thus making O* less effective in H-abstraction. As vacancies (*, also exposed Pt atoms) become available on O*-covered surfaces, O*-* site pairs activate C-H bonds via concerted oxidative addition and H-abstraction in transition states effectively stabilized by CH3 interactions with the vacancies, which lead to much higher turnover rates than on O*-O* pairs. In this regime, O-2 activation becomes irreversible, because fast C-H bond activation steps scavenge O* as it forms. Thus, O* coverages are set by the prevalent O-2/CH4 ratios instead of the O-2 pressures. CH4/CD4 kinetic isotope effects are much larger for turnovers mediated by O*-* than by O*-O* site pairs, because C-H (and C-D) activation steps are required to form the * sites involved in C-H bond activation.Turnove rates for CH4-O-2 reactions mediated by O*-* pairs decrease with increasing Pt dispersion, as in the case of O*-O* active structures, because stronger O* binding on small clusters leads not only to less reactive O* atoms, but also to lower vacancy concentrations at cluster surfaces. As O-2/CH4 ratios and O* coverages become smaller, O-2 activation on bare Pt clusters becomes the sole kinetically relevant step; turnover rates are proportional to O-2 pressures and independent of CH4, pressure and no CH4/CD4 kinetic isotope effects are observed. In this regime, turnover rates become nearly independent of Pt dispersion, because the O-2 activation step is essentially barrierless. In the absence of O-2, alternate weaker oxidants, such as H2O or CO2, lead to a final kinetic regime in which C-H bond dissociation on *-* pairs at bare cluster surfaces limit CH, conversion rates. Rates become first-order in CH4, and independent of coreactant and normal CH4/CD4 kinetic isotope effects are observed. In this case, turnover rates increase with increasing dispersion, because low-coordination Pt atoms stabilize the C-H bond activation transition states more effectively via stronger binding to CH3 and H fragments. These findings and their mechanistic interpretations are consistent with all rate and isotopic data and with theoretical estimates of activation barriers and of cluster size effects on transition states. They serve to demonstrate the essential role of the coverage and reactivity of chemisorbed oxygen in determining the type and effectiveness of surface structures in CH4, oxidation reactions using O-2, H2O, or CO2 as oxidants, as well as the diversity of rate dependencies, activation energies and entropies, and cluster size effects that prevail in these reactions. These results also show how theory and experiments can unravel complex surface chemistries on realistic catalysts under practical conditions and provide through the resulting mechanistic insights specific predictions for the effects of cluster size and surface coordination on turnover rates, the trends and magnitude of which depend sensitively on the nature of the predominant adsorbed intermediates and the kinetically relevant steps.