Impacts of niobia loading on active sites and surface acidity in NbOx/CeO2–ZrO2 NH3–SCR catalysts

Abstract A series of NbO x /Ce 0.75 Zr 0.25 O 2 catalysts for the selective catalytic reduction of NO with ammonia (NH 3 –SCR) were synthesized using a wetness impregnation method. The effect of niobia loading was studied in relation to the active sites and surface acidity. NH 3 /NO oxidation, X-ray diffraction, Infrared spectroscopy, ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, H 2 temperature-programmed reduction, O 2 /NH 3 temperature-programmed desorption, and diffuse reflectance infrared Fourier transformed spectroscopy experiments were performed to correlate the catalyst structure and surface properties to catalytic performance after Nb 2 O 5 modification. The catalyst with 15 wt.% Nb 2 O 5 loading showed high NH 3 –SCR activity and nearly 100% N 2 selectivity within a broad operation temperature window (190–460 °C) at a high space velocity (300,000 h −1 ). On this catalyst, Nb n + was mainly distributed in the form of typical monomeric and polymeric NbO x species, and was partially incorporated into the Ce 0.75 Zr 0.25 O 2 lattice at the Nb n+ O Ce n + (Zr n + ) interface. The electron redistribution effect arising from the occupation of cerium sites by Nb n + ions promoted the formation of Ce 3+ ions, oxygen vacancies and active oxygen species. This interaction was closely associated with the distribution of NbO x species which varied with niobia loading. NbO x themselves were acid sites and by attracting electrons they enhanced Lewis acid sites on CZ surface, which promoted the adsorption of NH 3 and inhibited the unselective oxidation of NH 3 to NO x . The increased amounts of active oxygen species over NbCZ catalysts promoted the adsorptive oxidation of NH 3 to NH 2 and NO to NO 3 − at low temperatures, and thus facilitated the reaction of ads-NH 3 and ads-NO 3 − /NO 2 − species. This effect as well as the increased amount of acid sites led to good NH 3 –SCR performance of Nb15CZ in a wide temperature range.