Methane reactions with CO2 are important routes for syngas and hydrogen production. The reactions occur on transition metal clusters via kinetically relevant C-H bond activation steps coupled catalytically with rapid CO2 activation. Experimental and theoretical studies have shown that incorporation of a second metal into the transition metal clusters may alter the rates of methane activation or coke formation, but the nature of the active sites on these bi-metallic clusters and their specific catalytic roles in methane activation have not been unambiguously established. In this contribution, we describe the structural dynamics of Ni based alloy clusters, their surface transformation, and the generation of distinct metal-oxygen site pairs, when a more oxophilic metal such as Co is incorporated into the Ni clusters. The incorporation of a secondary oxophilic metal leads to metal-oxygen site pairs at cluster surfaces that cause a transition in the rate dependencies for C-H bond activation of methane, as confirmed from combined kinetic and isotopic studies, oxygen uptakes, and density functional theory calculations. The C-H bond activation remains as the kinetically relevant step for methane activation on Ni-Co bi-metallic alloy clusters, but occurs preferentially on metal and oxygen site pairs during CH4-CO2 reactions. The surface density of the metal-oxygen site pairs depends strictly on the CO2-to-CO ratios (or H2O-to-H2 ratios), because the CO2 activation is rapid and quasi-equilibrated under all relevant conditions. Oxygen uptake studies indicate that Ni-Co clusters retain their metallic bulk with their surfaces largely decorated with reactive oxygen species. These oxygen species may involve in the kinetically relevant C-H bond activation step via a sigma-bond metathesis route, as found also for methane oxidation reactions on PdO or O* covered Pt surfaces. Our study demonstrates that pairing of bi-elemental metals lead to metal-oxygen as active site pairs that exhibit unique rate dependencies and distinct catalytic pathways unlike those of their monometallic clusters.

Surface dynamics of first row transition metal and alloy clusters and their catalytic consequences for C-H bond activation

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.