Department of Electrical and Computer Engineering , University of Toronto , 35 St. George Street , Toronto , Ontario M5S 1A4 , Canada.
Department of Mechanical and Industrial Engineering , University of Toronto , 5 King's College Road , Toronto , Ontario M5S 3G8 , Canada.
J Am Chem Soc. 2018 Mar 21;140(11):3833-3837. doi: 10.1021/jacs.7b13542. Epub 2018 Mar 9.
Over a broad range of operating conditions, many CO electroreduction catalysts can maintain selectivity toward certain reduction products, leading to materials and surfaces being categorized according to their products; here we ask, is product selectivity truly a property of the catalyst? Silver is among the best electrocatalysts for CO in aqueous electrolytes, where it reaches near-unity selectivity. We consider the hydrogenations of the oxygen and carbon atoms via the two proton-coupled-electron-transfer processes as chief determinants of product selectivity; and find using density functional theory (DFT) that the hydronium (HO) intermediate plays a key role in the first oxygen hydrogenation step and lowers the activation energy barrier for CO formation. When this hydronium influence is removed, the activation energy barrier for oxygen hydrogenation increases significantly, and the barrier for carbon hydrogenation is reduced. These effects make the formate reaction pathway more favorable than CO. Experimentally, we then carry out CO reduction in highly concentrated potassium hydroxide (KOH), limiting the hydronium concentration in the aqueous electrolyte. The product selectivity of a silver catalyst switches from entirely CO under neutral conditions to over 50% formate in the alkaline environment. The simulated and experimentally observed selectivity shift provides new insights into the role of hydronium on CO electroreduction processes and the ability for electrolyte manipulation to directly influence transition state (TS) kinetics, altering favored CO reaction pathways. We argue that selectivity should be considered less of an intrinsic catalyst property, and rather a combined product of the catalyst and reaction environment.
在广泛的操作条件下，许多 CO 电还原催化剂可以对某些还原产物保持选择性，从而根据产物对材料和表面进行分类；在这里，我们要问的是，产物选择性真的是催化剂的固有特性吗？银是水相电解质中 CO 的最佳电催化剂之一，在这种电解质中，它的选择性接近 100%。我们认为，通过两个质子耦合电子转移过程，对氧原子和碳原子的氢化是决定产物选择性的主要因素；并通过密度泛函理论（DFT）发现，水合氢离子（HO）中间体在第一个氧氢化步骤中起着关键作用，并降低了 CO 形成的活化能垒。当去除这种水合氢离子的影响时，氧氢化的活化能垒显著增加，而碳氢化的活化能垒降低。这些影响使甲酸盐反应途径比 CO 更有利。实验上，我们在高浓度氢氧化钾（KOH）中进行 CO 还原，限制水相电解质中的水合氢离子浓度。银催化剂的产物选择性从中性条件下完全 CO 转变为碱性环境下超过 50%的甲酸盐。模拟和实验观察到的选择性转变为 CO 电还原过程中水合氢离子的作用以及电解质操纵直接影响过渡态（TS）动力学从而改变有利的 CO 反应途径提供了新的见解。我们认为，选择性不应被视为催化剂的固有特性，而应是催化剂和反应环境的综合产物。
