Selective Methane Functionalization via Oxyhalogenation over Supported Noble Metal Nanoparticles

This article investigates SiO2-supported Ru-, Pt-, Ir-, Rh-, and Pd-based catalysts (1 wt % metal loading) as new catalytic systems for the oxychlorination and oxybromination of methane, both pivotal steps in the halogen-mediated production of commodities from natural gas. Furthermore, this article provides insights into the structure-performance relationships and mechanism of these reactions as a function of the metal and the hydrogen halide. In-depth characterization of the equilibrated catalysts by X-ray diffraction, electron microscopy, Raman, and X-ray photoelectron spectroscopies demonstrate a fast restructuring of the starting oxide nanoparticles into metallic, metal silicide, or metal halide phases. The oxychlorination activity, which decreases in the order: Ru/SiO2 > Pt/SiO2 > Ir/SiO2 > Rh/SiO2 approximate to Pd/SiO2, is enhanced in the presence of metal oxides, while the activity order in oxybromination: Ru/SiO2 approximate to Ir/SiO2 approximate to Pd/SiO2 > Pt/SiO2 > Rh/SiO2 is less dependent on the phase composition. The highest selectivity to chloromethanes (78-83%) and bromomethanes (92-98.5%) at moderate methane conversion (20%), rivaling the performance of the best oxyhalogenation catalysts, is attained over Ir/SiO2, Rh/SiO2, and Pd/SiO2, and correlates with their ability to reduce and form metal halides. Catalyst propensity toward halogenation depends on the halide type, although it is less pronounced at higher loadings (up to 5 wt %), while supporting over other inert carriers has a marginal impact on the restructuring patterns. Finally, kinetic analysis coupled with detection of radical intermediates by operando photoelectron photoion coincidence spectroscopy indicates a significant role of gas-phase halogenation in methane activation via oxyhalogenation. In oxychlorination, the latter pathway has a similar contribution as surface activation for Pd/SiO2, Rh/SiO2, and Pt/SiO2, and major contribution for Ir/SiO2 and Ru/SiO2 catalysts, while in oxybromination, it dominates for all the systems, limiting the potential to enhance the selectivity to mono- over dihalomethanes.