Development of Nitric Oxide Oxidation Catalysts for the Fast SCR Reaction

This study was undertaken in order to assess the potential for oxidizing NO to NO{sub 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 (1) proven (or likely) activity for NO oxidation, (2) low activity for SO2 oxidation (where data were available), and (3) 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: [NO] = 250 ppm, [SO{sub 2}] = 0 or 2800 ppm, [H{sub 2}O] = 7%, [CO{sub 2}] = 12%, [O{sub 2}] = 3.5%, balance = N{sub 2}; T = 275-375 C. Studies conducted in the absence of SO{sub 2} revealed a number of supported and unsupported metal oxides to be extremely active for NO oxidation to NO{sub 2}. These included known catalysts (Co{sub 3}O{sub 4}/SiO{sub 2}, FeMnO{sub 3}, Cr{sub 2}O{sub 3}/TiO{sub 2}), as well as a new one identified in this work, CrFeO{sub x}/SiO{sub 2}. However, in the presence of SO{sub 2}, all the catalysts tested were found to be severely deactivated with respect to NO oxidation. Of these, Co{sub 3}O{sub 4}/SiO{sub 2}, Pt/ZSM-5 and Pt/CeO{sub 2} showed the highest activity for NO oxidation in the presence of SO{sub 2} (based on peak NO conversions to NO{sub 2}), although in no cases did the NO conversion exceed 7%. Reactor studies indicate there are two components to SO{sub 2}-induced deactivation of Co{sub 3}O{sub 4}/SiO{sub 2}, corresponding to an irreversible deactivation due to sulfation of the surface of the Co{sub 3}O{sub 4} phase, together with a reversible inhibition due to competitive adsorption of SO{sub 2} with NO on the catalyst. In an effort to minimize the deactivating effect of SO{sub 2} on Co{sub 3}O{sub 4}/SiO{sub 2}, two synthetic approaches were briefly examined. These consisted of (1) the incorporation of highly dispersed Co(II) ions in silica, as a non-sulfating matrix, via the sol-gel preparation of CoO-SiO{sub 2}; and (2) the sol-gel preparation of a mixed metal oxide, CoO-Nb{sub 2}O{sub 5}-SiO{sub 2}, with the aim of exploiting the acidity of the niobium oxide to minimize SO2 adsorption. While both catalysts showed almost no activity for NO oxidation in the absence of SO{sub 2}, when SO{sub 2} was present low activity was observed, indicating that SO{sub 2} acts as a promoter for NO oxidation over these materials. The kinetics of NO oxidation over Co{sub 3}O{sub 4}/SiO{sub 2}, Pt/SiO{sub 2} and Pt/CeO{sub 2} were also examined. Co{sub 3}O{sub 4}/SiO{sub 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{sub 2} for the three catalysts was very close to one. CO{sub 2} was found to have no effect on the kinetics of NO oxidation over these catalysts. The presence of H{sub 2}O caused a decrease in NO conversion for both Co{sub 3}O{sub 4}/SiO{sub 2} and Pt/CeO{sub 2} catalysts, while no effect was observed for Pt/SiO{sub 2}. The inhibiting effect of water was reversible and is attributed to competitive adsorption with the reactants. In sum, this study has shown that a variety of base metal catalysts are very active for NO oxidation. However, all of the catalysts studied are strongly deactivated in the presence of 2800 ppm SO{sub 2} at typical flue gas temperatures; consequently improving catalyst resistance to SO{sub x} will be a pre-requisite if the fast SCR concept is to be applied to coal-fired flue gas conditions