The Nature of Active Sites of Co/Al2O3 for the Selective Catalytic Reduction of NO with C2H4

The effects of Co loading and calcination temperatures on the catalytic activity of Co/Al₂O₃ for selective catalytic reduction (SCR) of NO with ethylene in excess oxygen were investigated. Co/Al₂O₃ showed high and low activities when calcined at high (800 °C) and low (350 °C) temperatures, respectively. The formation and dispersion of cobalt species for catalysts calcined at 350 and 800 °C as well as for Al₂O₃ were studied by XRD, UV–vis and FTIR spectra. Combined with DRIFTS results of ad-species and reaction experiments, it allowed us to correlate the catalytic activity with active sites of Co/Al₂O₃, and the catalytic functions of active cobalt species and support were clarified. Co₃O₄ species contributed to the oxidation of NO to various nitrates and of C₂H₄ to reactive formate species, even in the absence of O₂, whereas the side reaction of ethylene combustion occurred simultaneously when excess oxygen was present. Tetrahedral Co²⁺ ions in CoAl₂O₄, which acted as the active sites, were responsible for the reaction between formate and nitrate species to form organic nitro compound.