<i>In Situ</i> Surfaced Mn–Mn Dimeric\nSites Dictate CO Hydrogenation Activity and C<sub>2</sub> Selectivity\nover MnRh Binary Catalysts

Massive ethanol production has long been a dream of human\nsociety.\nDespite extensive research in past decades, only a few systems have\nthe potential of industrialization: specifically, Mn-promoted Rh (MnRh)\nbinary heterogeneous catalysts were shown to achieve up to 60% C<sub>2</sub> oxygenates selectivity in converting syngas (CO/H<sub>2</sub>) to ethanol. However, the active site of the binary system has remained\npoorly characterized. Here, large-scale machine-learning global optimization\nis utilized to identify the most stable Mn phases on Rh metal surfaces\nunder reaction conditions by exploring millions of likely structures.\nWe demonstrate that Mn prefers the subsurface sites of Rh metal surfaces\nand is able to emerge onto the surface forming MnRh surface alloy\nonce the oxidative O/OH adsorbates are present. Our machine-learning-based\ntransition state exploration further helps to resolve automatedly\nthe whole reaction network, including 74 elementary reactions on various\nMnRh surface sites, and reveals that the Mn–Mn dimeric site\nat the monatomic step edge is the true active site for C<sub>2</sub> oxygenate formation. The turnover frequency of the C<sub>2</sub> product on the Mn–Mn dimeric site at MnRh steps is at least\n10<sup>7</sup> higher than that on pure Rh steps from our microkinetic\nsimulations, with the selectivity to the C<sub>2</sub> product being\n52% at 523 K. Our results demonstrate the key catalytic role of Mn–Mn\ndimeric sites in allowing C–O bond cleavage and facilitating\nthe hydrogenation of O-terminating C<sub>2</sub> intermediates, and\nrule out Rh metal by itself as the active site for CO hydrogenation\nto C<sub>2</sub> oxygenates.