Department of Chemistry , Trinity University , San Antonio , Texas 78212-7200 , United States.
Pacific Northwest National Laboratory Institute for Integrated Catalysis , Richland , Washington 99352 , United States.
J Am Chem Soc. 2018 Mar 14;140(10):3712-3723. doi: 10.1021/jacs.7b12758. Epub 2018 Mar 5.
The mechanism of CO oxidation over supported gold catalysts has long been debated, with two prevailing mechanisms dominating the discussion: a water-assisted mechanism and a mechanism involving O-defect sites. In this study, we directly address this debate through a kinetic and mechanistic investigation of the role of water in CO oxidation over Au/TiO and Au/AlO catalysts; the results clearly indicate a common water-assisted mechanism to be at work. Water adsorption isotherms were determined with infrared spectroscopy; the extracted equilibrium constant was essentially the same for both catalysts. Added water decreases CO adsorption on Au/TiO, likely by blocking CO binding sites at the metal-support interface. Reaction kinetics (CO, O, and HO reaction orders) were essentially the same for both catalysts, as were measured O-H(D) kinetic isotope effects. These data indicate that the two catalysts operate by essentially the same mechanism under the conditions of these experiments (ambient temperature, significant amounts of water available). A reaction mechanism incorporating the kinetic and thermodynamic data and accounting for different CO and O/COOH binding sites is proposed. The mechanism and kinetic data are treated with an active site (Michaelis-Menten) approach. This indicated that water adsorption does not significantly affect reaction rate constants, only the number of active sites available at a given water pressure. Extracted water and O binding constants are similar on both catalysts and consistent with previous DFT calculations. Water adsorption constants are also similar to independently determined equilibrium constants measured by IR spectroscopy. The likely roles of water, surface carbonates, and oxygen vacancies at the metal-support interface are discussed. The results definitively show that, at least in the presence of added water, O vacancies cannot play an important role in the room-temperature catalysis, and that the water-assisted mechanism is far more consistent with the preponderance of the kinetic data.
负载金催化剂上 CO 氧化的机理长期以来一直存在争议，两种占主导地位的机制主导了讨论：水辅助机制和涉及 O 缺陷位的机制。在这项研究中，我们通过对 Au/TiO 和 Au/AlO 催化剂上 CO 氧化中水作用的动力学和机理研究直接解决了这一争论；结果清楚地表明，起作用的是一种常见的水辅助机制。通过红外光谱测定了水吸附等温线；两种催化剂的提取平衡常数基本相同。添加的水减少了 Au/TiO 上的 CO 吸附，可能是通过阻塞金属-载体界面处的 CO 结合位来实现的。对于两种催化剂，反应动力学（CO、O 和 HO 反应级数）基本相同，O-H(D) 动力学同位素效应也是如此。这些数据表明，在这些实验条件下（环境温度，有大量的水可用），两种催化剂的操作机制基本相同。提出了一种包含动力学和热力学数据并考虑不同 CO 和 O/COOH 结合位的反应机制。该机制和动力学数据采用活性位（米氏门）方法进行处理。这表明，水吸附不会显著影响反应速率常数，而只会影响在给定水压力下可用的活性位数量。两种催化剂上的提取水和 O 结合常数相似，与之前的 DFT 计算结果一致。水吸附常数也与通过 IR 光谱独立确定的平衡常数相似。讨论了水、表面碳酸盐和金属-载体界面处氧空位的可能作用。结果明确表明，至少在添加水的情况下，O 空位不能在室温催化中发挥重要作用，水辅助机制与动力学数据的优势更为一致。
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