Selective catalytic reduction of NO sub(x) by hydrogen (H sub(2)-SCR) on WO sub(x)-promoted Ce sub(z)Zr sub(1-z)O sub(2 ) solids

The selective catalytic reduction of NO sub(x) by H sub(2) (H sub(2)-SCR) under strongly oxidizing conditions (520ppm NO sub(x)/1% H sub(2)/5% O sub(2)/10% CO sub(2)/He; NO:NO sub(2)-4:1-9:1) in the 150-600 parallel C range has been studied over 3 wt-% W-promoted CeO sub(2)-ZrO sub(2) solids (85 wt-% CeO sub(2) -15 wt-% ZrO sub(2) (CeZr), and 17 wt-% CeO sub(2)-83 wt-% Zr(ZrCe) synthesised by a proprietary method) for the first time. The highest NO, conversion (X sub(NOx), = 54%) was obtained on the W-ZrCe (Zr-rich) solid at 300 parallel C (GHSV of 51,000 h super(-1) ), whereas N sub(2)-selectivity was in the 77-92%-range over both W-ZrCe (Zr-rich) and W-CeZr (Ce-rich) catalysts. Significantly higher integral specific rates (R sub(NO), mu mol NO m super(-2) min super(-1) ) were estimated on the W-ZrCe (Zr-rich) catalyst compared to the W-CeZr (Ce-rich) one in the 250-350 parallel C range. The formation of adsorbed NO sub(x) under 0.1% NO/10% O sub(2)/He gas treatment at 25 parallel C followed by H sub(2)/O sub(2)-TPSR experiments revealed that at least two different kinds of active NO sub(x) of low concentration (4-7 mu mol g super(-1) ) were formed on both catalysts, whereas other inactive (spectator) NO sub(x), species formed were of larger concentration (>160 mu molg super(-1)). UV-vis/DRS studies revealed that deposition of 3 wt-% W on ZrCe (Zr-rich) mixed metal oxide following calcination at 600 parallel C resulted in the formation of both polymeric WO sub(x) and WO sub(3) clusters, whereas on CeZr (Ce-rich) only the latter phase (W super(6+)) was seen. Large differences in the concentration ( mu mol m super(-2)) and strength of surface acid sites between the W-CeZr and W-ZrCe solids were revealed after performing NH sub(3)-TPD and NH sub(3)-DRIFTS. These results were found to correlate with the specific H sub(2)-SCR rate ( mu mol m super(-2) min) 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 2.76 mu mol m super(-2), respectively, whereas the specific reaction rate was 0.14 and 0.046 mu mol m super(-2) min at 300 and 250 parallel C, at which maximum rates were observed, respectively.