Atomic-Scale Understanding of Catalyst Activation: Carboxylic Acid Solutions, but Not the Acid Itself, Increase the Reactivity of Anatase (001) Faceted Nanocatalysts
Atomic-Scale Understanding of Catalyst Activation: Carboxylic Acid Solutions, but Not the Acid Itself, Increase the Reactivity of Anatase (001) Faceted Nanocatalysts
复制标题
对催化剂活化的原子尺度理解：羧酸溶液而非酸本身可以提高锐钛矿 (001) 多面纳米催化剂的反应性
DOI：
10.1021/acs.jpcc.7b11054
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发表时间：
2018
期刊：
影响因子：
--
通讯作者：
Hines, Melissa A.
中科院分区：
文献类型：
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作者：
DeBenedetti, William J.;Skibinski, Erik S.;Jing, Dapeng;Song, Anqi;Hines, Melissa A.
由于缺乏对纳米催化剂表面结构的原子尺度理解，我们预测纳米催化剂反应性的能力受到了阻碍。纳米催化剂表面是采用体端结构，还是像在真空中观察到的那样，通过重构使其自由能最小化，从而降低其反应活性？同样，在高温下处理的纳米催化剂在低温下使用时是否保持其低反应性，重建表面？利用一种制备适合原子尺度成像和表面光谱的锐钛矿纳米催化剂的新技术，我们发现溶液制备的锐钛矿被一层单层氟所终止，这层氟作为原子尺度的疏油涂层，防止了非定形碳的积累。我们进一步表明，最常见的tio2功能化化学，羧酸溶液，导致重构锐钛矿纳米催化剂的自发重组，导致反应位点增加五倍。当羧酸从气相沉积时，没有观察到这种重组，这表明真空环境下的模型实验可能导致非平衡，动力学捕获状态，可能与催化无关。羧酸水溶液产生密集堆积的羧酸盐单层，具有比先前预测的更丰富的吸附几何形状。从头算模拟表明，虽然羧酸终止在去除表面应力方面的效果略低于重建，但在降低表面能方面更有效。这一观察结果表明，体端金属氧化物纳米晶体可能在反应环境中很常见，即使使用高温来处理纳米催化剂或稍后将反应物冲洗掉。因此，假设本体端接表面可能是计算工程纳米催化剂的“设计材料”方法的合理起点。
Our ability to predict nanocatalyst reactivity has been hindered by our lack of atomic-scale understanding of nanocatalyst surface structure. Do nanocatalyst surfaces adopt a bulk-terminated structure or do they reconstruct to minimize their free energy, thereby lowering their reactivity as often observed in vacuum? Similarly, do nanocatalysts processed at high temperatures maintain their low reactivity, reconstructed surfaces when used at low temperatures? Using a new technique for the preparation of anatase nanocatalysts suitable for atomic-scale imaging and surface spectroscopy, we show that solution-prepared anatase is terminated by a monolayer of fluorine, which acts as an atomic-scale oleophobic coating, preventing the accumulation of adventitious carbon. We further show that the most common TiO2functionalization chemistry, a carboxylic acid solution, causes the spontaneous reorganization of a reconstructed anatase nanocatalyst, leading to a five-fold increase in reactive sites. This reorganization is not observed when carboxylic acids are deposited from the gas phase, suggesting that model experiments in vacuum environments can lead to a nonequilibrium, kinetically trapped state that may not be catalytically relevant. Aqueous carboxylic acid solutions produce densely packed carboxylate monolayers with richer adsorption geometries than previously predicted. Ab initio simulations show that although the carboxylate termination is somewhat less effective at removing surface stress than the reconstruction, it is more effective in lowering the surface energy. This observation suggests that bulk-terminated metal-oxide nanocrystals may be common in reactive environments, even if high temperatures are used to process the nanocatalyst or if the reactant is later rinsed off. As such, the assumption of a bulk-terminated surface may be a reasonable starting point for “materials-by-design” approaches to computationally engineered nanocatalysts.