Structure of Selective and Nonselective Dicopper (II) Sites in CuMFI for Methane Oxidation to Methanol

The one-step valorization of natural gas remains a challenge. Methane conversion to methanol via chemical looping over copper-containing zeolites is a promising route, and CuMFI is among the earliest successfully applied. However, the structure of the active sites in CuMFI, as well as the effect of copper loading and Si/Al ratio on the copper speciation, are yet to be understood. We found that for CuMFI, the Cu/Al ratio determines the selectivity of methane conversion by governing the structure of the active dicopper sites. At a Cu/Al ratio below 0.3, copper-containing MFI materials host dimeric centers with a Cu–Cu separation of 2.9 Å and a UV/vis absorption band at 27 200 cm–1 capable of selective oxidation of methane to methanol in a wide temperature range (450–550 K). A higher Cu/Al ratio leads to the formation of mono-μ-oxo dicopper sites with Cu–Cu = 3.2 Å, which exhibit a characteristic band at 21 900 cm–1 and react with methane at lower temperatures (<450 K), yielding overoxidation products. Identifying distinctions in the structure of selective and nonselective copper sites will aid in the design of better-performing materials

The Extent of Platinum-Induced Hydrogen Spillover on Cerium Dioxide

Hydrogen spillover from metal nanoparticles to oxides is an essential process in hydrogenation catalysis and other applications such as hydrogen storage. It is important to understand how far this process is reaching over the surface of the oxide. Here, we present a combination of advanced sample fabrication of a model system and in situ X-ray photoelectron spectroscopy to disentangle local and far-reaching effects of hydrogen spillover in a platinum–ceria catalyst. At low temperatures (25–100 °C and 1 mbar H2) surface O–H formed by hydrogen spillover on the whole ceria surface extending microns away from the platinum, leading to a reduction of Ce4+ to Ce3+. This process and structures were strongly temperature dependent. At temperatures above 150 °C (at 1 mbar H2), O–H partially disappeared from the surface due to its decreasing thermodynamic stability. This resulted in a ceria reoxidation. Higher hydrogen pressures are likely to shift these transition temperatures upward due to the increasing chemical potential. The findings reveal that on a catalyst containing a structure capable to promote spillover, hydrogen can affect the whole catalyst surface and be involved in catalysis and restructuring