Controlled-Release Mechanism Regulates Rhodium Migration and Size Redistribution Boosting Catalytic Methane Conversion

Abstract

The migration of Rh atoms under a gas/reactive environment has a great impact on the dynamic restructuring and size redistribution of Rh catalysts in a variety of structure-sensitive catalytic reactions. To date, regulating the size distribution of active Rh species via controlled atomic migration remains challenging. Here, we show a controlled-release mechanism to regulate Rh atom migration through two-dimensional (2D) zeolite nanosheets, enabling quasi-continuous size redistribution of active Rh species from a single atom to nanoparticles with reversibility. Utilizing state-of-the-art in situ characterizations, a reversible aggregation/redispersion of Rh catalysts in/out of the 2D zeolite was directly observed under H2 or CO environment. The interplay between gas environment and support confinement was disclosed that substantially restrained the dynamic migration of Rh species and enabled a quasi-continuous control of Rh size distribution over a wide temperature window and size range. The catalytic testing for mild oxidation of methane demonstrated a volcano correlation of methanol activity with increasing particle size. The sub-nanometer Rh clusters with an average diameter of 0.9 nm exhibited the highest methanol activity of 39.7 molCH3OOH·molRh–1·h–1 with remarkable selectivity as high as 73.2%, far beyond that of single atom species and larger particles. Density functional theory calculations and electron paramagnetic resonance spectroscopy further revealed that this size dependency is related to the preferential formation of •OH radicals on Rh clusters with different sizes, which acts as a key initiator for methane activation. Our results thus provide a practical approach to synthesize size-specific metal catalysts via controlled atomic migration.