Selective catalytic reduction of NOx by hydrogen (H2-SCR) on WOx-promoted CezZr1-zO2 solids

Abstract

The selective catalytic reduction of NOx by H2 (H2-SCR) under strongly oxidizing conditions (520ppm NOx/1% H2/5% O2/10% CO2/He

NO:NO2-4:1-9:1) in the 150-600°C range has been studied over 3wt-% W-promoted CeO2-ZrO2 solids (85wt-% CeO2-15wt-% ZrO2 (CeZr), and 17wt-% CeO2-83wt-% Zr (ZrCe) synthesised by a proprietary method) for the first time. The highest NOx conversion (XNOx=54%) was obtained on the W-ZrCe (Zr-rich) solid at 300°C (GHSV of 51,000h-1), whereas N2-selectivity was in the 77-92%-range over both W-ZrCe (Zr-rich) and W-CeZr (Ce-rich) catalysts. Significantly higher integral specific rates (RNO, μmolNOm-2min-1) were estimated on the W-ZrCe (Zr-rich) catalyst compared to the W-CeZr (Ce-rich) one in the 250-350°C range. The formation of adsorbed NOx under 0.1% NO/10% O2/He gas treatment at 25°C followed by H2/O2-TPSR experiments revealed that at least two different kinds of active NOx of low concentration (4-7μmolg-1) were formed on both catalysts, whereas other inactive (spectator) NOx species formed were of larger concentration (>160μmolg-1). UV-vis/DRS studies revealed that deposition of 3wt-% W on ZrCe (Zr-rich) mixed metal oxide following calcination at 600°C resulted in the formation of both polymeric WOx and WO3 clusters, whereas on CeZr (Ce-rich) only the latter phase (W6+) was seen. Large differences in the concentration (μmolm-2) and strength of surface acid sites between the W-CeZr and W-ZrCe solids were revealed after performing NH3-TPD and NH3-DRIFTS. These results were found to correlate with the specific H2-SCR rate (μmolm-2min) obtained for the two solids. In particular, the surface acid sites on W-ZrCe and W-CeZr solids were found to be 5.96 and 2.76μmolm-2, respectively, whereas the specific reaction rate was 0.14 and 0.046μmolm-2min at 300 and 250°C, at which maximum rates were observed, respectively. © 2014 Elsevier B.V.