Cheng Tao, Xiao Hai, Goddard William A
Joint Center for Artificial Photosynthesis, California Institute of Technology, Pasadena, CA 91125.
Materials and Process Simulation Center (MC139-74), California Institute of Technology, Pasadena, CA 91125.
Proc Natl Acad Sci U S A. 2017 Feb 21;114(8):1795-1800. doi: 10.1073/pnas.1612106114. Epub 2017 Feb 6.
A critical step toward the rational design of new catalysts that achieve selective and efficient reduction of CO to specific hydrocarbons and oxygenates is to determine the detailed reaction mechanism including kinetics and product selectivity as a function of pH and applied potential for known systems. To accomplish this, we apply ab initio molecular metadynamics simulations (AIMμD) for the water/Cu(100) system with five layers of the explicit solvent under a potential of -0.59 V [reversible hydrogen electrode (RHE)] at pH 7 and compare with experiment. From these free-energy calculations, we determined the kinetics and pathways for major products (ethylene and methane) and minor products (ethanol, glyoxal, glycolaldehyde, ethylene glycol, acetaldehyde, ethane, and methanol). For an applied potential () greater than -0.6 V (RHE) ethylene, the major product, is produced via the Eley-Rideal (ER) mechanism using HO + The rate-determining step (RDS) is C-C coupling of two CO, with Δ = 0.69 eV. For an applied potential less than -0.60 V (RHE), the rate of ethylene formation decreases, mainly due to the loss of CO surface sites, which are replaced by H*. The reappearance of CH along with CH at less than -0.85 V arises from CHO formation produced via an ER process of H with nonadsorbed CO (a unique result). This *CHO is the common intermediate for the formation of both CH and CH These results suggest that, to obtain hydrocarbon products selectively and efficiency at pH 7, we need to increase the CO concentration by changing the solvent or alloying the surface.
实现将CO选择性高效还原为特定碳氢化合物和含氧化合物的新型催化剂的合理设计的关键一步，是确定详细的反应机理，包括已知体系中作为pH值和外加电势函数的动力学和产物选择性。为实现这一点，我们对水/Cu(100)体系应用了从头算分子元动力学模拟（AIMμD），该体系在pH值为7、电势为-0.59 V [可逆氢电极（RHE）] 时含有五层显式溶剂，并与实验进行比较。通过这些自由能计算，我们确定了主要产物（乙烯和甲烷）和次要产物（乙醇、乙二醛、乙醇醛、乙二醇、乙醛、乙烷和甲醇）的动力学和反应路径。对于大于-0.6 V（RHE）的外加电势，主要产物乙烯通过使用HO + 的埃里-里德尔（ER）机理生成。速率决定步骤（RDS）是两个CO的C-C偶联，Δ = 0.69 eV。对于小于-0.60 V（RHE）的外加电势，乙烯生成速率降低，主要是由于CO表面位点的损失，这些位点被H取代。在小于-0.85 V时CH与CH一起再次出现，这是由H与未吸附的CO通过ER过程产生的CHO形成所致（一个独特的结果）。这个CHO是CH和CH形成的共同中间体。这些结果表明，为了在pH值为7时选择性且高效地获得碳氢化合物产物，我们需要通过改变溶剂或使表面合金化来提高CO浓度。
