Structural Characterization and Selective Catalytic Reduction of Nitrogen Oxides with Ammonia: A Comparison between Co/ZSM-5 and Co/SBA-15

Abstract

Cobalt-containing catalysts supported on ZSM-5 zeolite and mesoporous siliceous SBA-15 were prepared and characterized by nitrogen sorption, X-ray diffraction, scanning electron and transmission electron microscopies, energy-dispersive X-ray, Fourier transform infrared, ultraviolet–visible diffuse reflectance, X-ray photoelectron spectroscopies, and temperature-programmed desorption of ammonia measurement. The effect of cobalt loading ratio on the selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia was investigated. The existing Brønsted acid sites contributed to the cobalt species finely dispersed within the ZSM-5 zeolite, either as isolated cobalt ions anchored at α, β, and γ sites, or as amorphous cobalt oxides enriched on the ZSM-5 surface. NOx conversion profiles of Co/ZSM-5 exhibited two peaks. The low-temperature peak (<300 °C) was induced by cobalt ions at β and γ sites, while the high-temperature peak (>300 °C) was assigned to the amorphous and crystalline cobalt oxides. With increasing cobalt content, the intensity of low-temperature peak was enhanced monotonously, and the peak position remained constant. Increasing cobalt content promoted the high-temperature peak to shift toward lower temperatures. NOx conversion profiles of Co/SBA-15 only exhibited a high-temperature peak. For Co/SBA-15, the poor dispersion of cobalt species was derived from the absence of Brønsted acid sites. The activity of Co/SBA-15 catalysts was lower than that of the Co/ZSM-5 catalysts due to inactive cobalt ions anchored on isolated Si–OH groups, and agglomerated cobalt oxides within the SBA-15 channels blocking the reactant pathway to active sites.