An updated study on NO catalytic oxidation over activated carbon: The effect of pore structure and a dual-site mechanism

Abstract

NO catalytic oxidation (NOCO) to NO2 is widely believed to be a fundamental step in selective catalytic reduction (SCR) over activated carbon (AC). Thus, the enhancement of NOCO over AC is significant. The effect of the pore structure on NOCO is discussed first. A series of microporous AC samples with various microporous surface areas were prepared, in which a strong positive correlation existed between NO conversion and the microporous surface area. With an increase in the microporous surface area, the NO conversion increased from 13.10% for MAC1 to 41.18% for MAC4. Concurrently, a hierarchical AC sample with an approximate hierarchy degree (the ratio of Vmes+mac to Vt) of 50% demonstrated higher NO conversion than microporous AC when the microporous surface areas were similar. HAC2 exhibited the largest NO conversion (51.58%). Therefore, the microporous surface area and degree of hierarchy are key factors that enhance NO conversion. NOCO has been demonstrated to be an O-enriched process. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) results revealed that carbonyl groups were generated during the NOCO process. Density functional theory calculations were then performed. Carbonaceous models with unsaturated C atom, carbonyl, and lactone groups were constructed, and the corresponding NOCO reaction paths were calculated. A dual-site NOCO mechanism was proposed by combining the experimental and calculation results. Two types of active sites existed: unsaturated C atoms and high-spin density C atoms that affected the kinetic and steady-state stages of NOCO, respectively. The total amount of high-spin density C atoms strongly depends on the microporous surface area. The study of NOCO was updated, which contributed to the preparation of highly efficient AC for NOCO.