Leveraging palladium redox synergy and palladium-activated carbon interfacial interactions for low temperature oxidation of toluene and formaldehyde

초록

Palladium-loaded activated carbon (Pd/AC) catalysts with ultralow metal loadings (0.05-0.5 wt%) are developed to leverage the synergistic potential of microporous supports in low-temperature oxidation of volatile organic compounds (VOCs). The optimized 0.5 wt% Pd/AC catalyst demonstrates high performance, achieving 90% conversion (T90) of 10 ppm toluene at 200 degrees C under a high gas hourly space velocity (52,105 h-1). In contrast, T90 for 100 ppm formaldehyde (FA) requires a significantly elevated temperature of 275 degrees C under the same conditions. Despite the efficient removal of VOCs, the maximum CO2 yield of only 18.7% indicates incomplete mineralization and the build-up of partially oxidized byproducts. Mechanistic insights from in-situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory simulations reveal the divergent partial oxidation pathways and corresponding CO2 forming routes. The oxidation of FA proceeds via dioxymethylene and formate intermediates. In contrast, toluene oxidation initiates with aromatic ring activation, followed by C-H and C-C bond cleavage to form phenolic and carboxylate species. The catalytic behavior of Pd/ AC arises from its mixed Pd0/Pd2+ surface, where the two species likely cooperate in VOC activation and oxygen transfer. Although low-loading Pd/AC achieves substantial parent-VOC conversion, the results indicate a clear need for optimized reaction conditions and thorough byproduct screening to boost COQ selectivity and rule out secondary emissions.

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