(743a) Exploring Ruthenium Metal-Support Dynamics for the Low-Temperature Partial Oxidation of Methane
Exploring Ruthenium Metal-Support Dynamics for the Low-Temperature Partial Oxidation of Methane
Department of Chemical Engineering and SUNCAT Center for Interface Science and Catalysis, Stanford University, Stanford, CA 94305, USA
Industrially, the transformation of methane to syngas stands as an invaluable reaction, the latter products being converted downstream into valuable bulk chemicals such as methanol, ammonia, and synthetic fuels. Steam or bireforming are common routes for methane conversion to syngas, but work at high temperatures because of their endothermic nature. Methane partial oxidation (CH4 + ½ O2 = CO + 2 H2) is exothermic and benefits from more favorable thermodynamics. Thus, there are significant incentives to explore catalysts that can partially oxidize methane at low temperatures, with high selectivity and yield, into an industrially relevant feedstock.
Methane partial oxidation (MPO) is a complex reaction, in which cycling catalyst oxidation states dramatically influences catalyst structure, activity, and selectivity. In order to rationally design a catalyst for MPO, it is important to understand how interactions between catalyst active phases and underlying support affect catalyst structure and subsequent behavior. However, in non-uniform systems with active phases of a variety of sizes and shapes, it is often difficult to back out such structure-property relationships. By colloidally synthesizing ruthenium nanoparticles prior to support impregnation, we may make a fair comparison of activity, selectivity, and stability across supports and understand how catalyst support affects the morphological and catalytic evolution of ruthenium active phase(s). We report highly active 0.5 wt. % ruthenium catalysts prepared from monodisperse nanocrystals, which show >90% CH4 conversion and >98% CO selectivity at ~650Â oC.
Mechanistically, at low temperatures, RuO2 slowly catalyzes complete combustion; upon oxygen depletion in the reactor, and reduction to metallic Ru, we ignite partial oxidation, which tends toward equilibrium values. This study thus identifies two design rules for igniting low-temperature formation of synthesis gas: oxygen depletion in the reaction feed, and the effect of metal-support interaction on oxidation state. We find that selectivity and yield are strongly dependent on the support type, with a size dependence on certain âactiveâ supports. These effects are related to how certain supports preferentially stabilize different oxidation states of ruthenium. Using this information, we can gain fundamental mechanistic insight regarding methane partial oxidation on supported ruthenium systems, and work to design more effective catalysts. Taking advantage of these design rules, we suggest the development of highly active bifunctional catalysts, which take advantage of a palladium phase to deplete feed oxygen, and a reduced ruthenium phase to form partial oxidation products.