The Influence Of H₂O On The Selective Catalytic Reduction Of Nitric Oxide With H₂Over Pt/HZSM-5

The nitric oxide is a major pollution in the atmosphere. It makes a big threat to the health of human, animals and plants. Also, it results in a series of problems, such as the hazy weather, photochemical smog, acid rain. In many ways of removing nitric oxide from the atmosphere, selective catalytic reduction of NOx with H2(H2-SCR) is considered as a promising way, due to its better elimination effect for nitrogen oxides at lower temperature. In this thesis, we studied the mechanism of H2-SCR influenced by water vapor at different temperatures over Pt/HZSM-5, as well as the nitrous species on the HZSM-5influenced by temperature in range of80～130℃.It was found that although H2O vapor (1%or5%) presenting in the feed gas promoted the reaction at low temperature (T<100℃), it oppositely suppressed the reaction at high temperature. When5%H2O presenting in the feed gas, the NOx conversions obtained at each tempratue in the range of80～100℃were slightly lower than those in the case of1%H2O presenting in the feed gas. On the other hand, the indroduction of1%H2O into the reaction system notably decreased the N2selectivity at130℃, while it made no change to the N2selectivity below130℃. Nevertheless,5%H2O presenting in the feed gas made N2selectivity markedly decreased in the whole temperature range of80～130℃.In situ FTIR was used to study the mechanism of water vapor influencing the reaction. It indicates that H2is hardly activated at lower temperature, due to more NO2adsorbed on the active sites of Pt. As water vapor with the nitrogen oxides competitively adsorbs on Pt and hence inhibites the NOx adsorption there, it improves the activation of H2on the catalyst. As a result, the water vapor made the NO conversion increase. On the other hand, although the water vapor also improves NH3formation over Pt, the reaction of NOx with NH4+that is derived from NH3adsoption on Bronsted acid sites is inhibited due to the adsorption of NOx on the support decreasing.When the reaction temperature was elevated up, for instance, to130℃, the activation of H? over the catalyst is no longer the key factor determining the NO conversion. In this case, due to the nitrous species decreasing in amount on Pt, the H2combustion is dramatically enhanced. At the same time, the reaction between NH4+and nitrous species is inhibited, due to the adsorption of NO2and NO over the support decrease. Consequently, the water vapor presenting in the feed gas suppresses not only the NOx conversion but also the N2selectivity at higher reaction temperature.