Low-temperature catalytic reduction of NO by NH3 over vanadia-based nanoparticles prepared by flame-assisted spray pyrolysis: Influence of various supports

Abstract

A series of V/CeO2, V/TiO2, V/Al2O3, V/ZrO2, V/CeO2–ZrO2, V/TiO2–Al2O3, and V/CeO2–Al2O3 metal oxide nanoparticles were synthesized by a one-step rapid FSP (flame spray pyrolysis) synthesis technique. Benefiting from the short residence time and high quenching rate during the single-step flame spray process, V4+, V3+ ions are successfully incorporated into the crystal lattice of various metal oxide supports. Our XRD, BET studies reveal that the V doping into Al2O3, Al2O3–TiO2, and CeO2–Al2O3 favors the formation of highly dispersed surface vanadia nanoparticles (5.8–9.4nm), whereas, the V doping into ZrO2, CeO2–ZrO2 and CeO2 led to the primary particle size (19–45.5nm, formation of bulk particles) growth and thus inhibition in the catalytic activity. It is remarkable to note that the average particle size (nm) of vanadia in our flame-made catalysts has direct relation with the SCR activity. As can be envisaged from XPS spectra, as-synthesized V/ZrO2 sample primarily consists of V5+ species due to the formation of ZrV2O7 solid solution as a result of zirconia migration into the V2O5 crystallites. Our XPS results imply that the formation of surface V2O3 species is improved enormously by the addition of Al2O3 (V3+/Vn+=0.36, 0.38, and 0.41 for V/TiO2–Al2O3, V/Al2O3, V/CeO2–Al2O3, respectively), whereas, the addition of TiO2 led to the formation of surface VO2 species (V4+/V5+=0.88, 1.57 for V/TiO2, V/TiO2–AlO3, respectively). Among all the catalysts, high surface (V3++V4+)/Vn+, and V4+/Vn+ concentrations were observed for the V/TiO2–Al2O3, V/Al2O3, V/CeO2–Al2O3, respectively. It is highly remarkable to note that the SCR performance of all the as-prepared catalysts is indeed correlated with the surface (V3++V4+)/Vn+, and V4+/V5+ concentrations. The reduction (H2-TPR) profiles reveal that the vanadium oxide reduction peak has shifted to much lower temperatures in Al-modified catalysts, indicating high reduction potential of the high-coverage VOx species. This is primarily due to the penetration of the active component into the pores of the support during the flame spray pyrolysis step itself, which in turn, results in a high dispersion of the active component on the support. Clearly, the close proximity of the support to these sites will mean that the support exerts some influence on the behavior of the oxygen species attached to the vanadium. The catalytic performance of various V/M′ (M′=Ce, Al, Ti, Zr, Ce–Zr, Ti–Al, and Ce–Al) flame-made catalysts (with consistent V content V/M′=0.17) was studied for the low-temperature SCR reaction at a range of temperatures (140–360°C) at gas hourly space velocity (GHSV)=24,000h−1. The intrinsic activity of V/Ti–Al, V/Al, V/Ce–Al catalysts with V/M′=0.17 atomic ratio measured under differential reaction conditions, was found to be highly active, selective toward nitrogen and broadening the temperature window for optimal operation of this reaction.