Development of Nitric Oxide Oxidation Catalysts for the Fast SCR Reaction

Abstract

This study was undertaken in order to assess the potential for oxidizing NO to NO 2 in flue gas environments, with the aim of promoting the so-called fast SCR reaction. In principle this can result in improved SCR kinetics and reduced SCR catalyst volumes. Prior to commencing experimental work, a literature study was undertaken to identify candidate catalysts for screening. Selection criteria comprised (i) proven (or likely) activity for NO oxidation, (ii) low activity for SO 2 oxidation (where data were available), and (iii) inexpensive component materials. Catalysts identified included supported base metal oxides, supported and unsupported mixed metal oxides, and metal ion exchanged ZSM-5 (Fe, Co, Cu). For comparison purposes, several low loaded Pt catalysts (0.5 wt% Pt) were also included in the study.Screening experiments were conducted using a synthetic feed gas representative of flue gas from coal-fired utility boilers:.5%, balance = N 2 ; T = 275-375 °C. Studies conducted in the absence of SO 2 revealed a number of supported and unsupported metal oxides to be extremely active for NO oxidation to NO 2 . These included known catalysts (Co 3 O 4 /SiO 2 , FeMnO 3 , Cr 2 O 3 /TiO 2 ), as well as a new one identified in this work, CrFeO x /SiO 2 . However, in the presence of SO 2 , all the catalysts tested were found to be severely deactivated with respect to NO oxidation. Of these, Co 3 O 4 /SiO 2 , Pt/ZSM-5 and Pt/CeO 2 showed the highest activity for NO oxidation in the presence of SO 2 (based on peak NO conversions to NO 2 ), although in no cases did the NO conversion exceed 7%.Reactor studies indicate there are two components to SO 2 -induced deactivation of Co 3 O 4 /SiO 2 , corresponding to an irreversible deactivation due to sulfation of the surface of the Co 3 O 4 phase, together with a reversible inhibition due to competitive adsorption of SO 2 with NO on the catalyst. In an effort to minimize the deactivating effect of SO 2 on Co 3 O 4 /SiO 2 , two synthetic approaches were briefly examined. These consisted of (i) the incorporation of highly dispersed Co(II) ions in silica, as a non-sulfating matrix, via the sol-gel preparation of CoO-SiO 2 ; and (ii) the sol-gel preparation of a mixed metal oxide, CoO-Nb 2 O 5 -SiO 2 , with the aim of exploiting the acidity of the niobium oxide to minimize SO 2 adsorption. While both catalysts showed almost no activity for NO oxidation in the absence of SO 2 , when SO 2 was present low activity was observed, indicating that SO 2 acts as a promoter for NO oxidation over these materials.The kinetics of NO oxidation over Co 3 O 4 /SiO 2 , Pt/SiO 2 and Pt/CeO 2 were also examined. Co 3 O 4 /SiO 2 was found to exhibit a higher apparent activation energy for NO oxidation than the Pt catalysts, while the combined reaction order in NO and O 2 for the three catalysts was very close to one. CO 2 was found to have no effect on the kinetics of NO oxidation over these catalysts. The presence of H 2 O caused a decrease in NO conversion for both Co 3 O 4 /SiO 2 and Pt/CeO ...