Selective Catalytic Reduction Of NO By Hydrogen Over Pt Supported Catalysts

Nitrogen oxides (NOx) are the main air pollutants leading to acid rain and photochemical smog. Selective catalytic reduction (SCR) is one of the most effective and promising method of NO* abatement in excess oxygen. The SCR of NO by H2 (H2-SCR) has attracted much attention in the recent years because of its high activity at relatively low temperature. In this study, we investigated the H2-SCR reaction over Pt based catalysts. We devoted our efforts to the reaction mechanism of H2-SCR over Pt/HFER catalyst and lower amount of water vapor influenceing the reaction. Pt/Mg-Al-O catalysts prepared from hydrotalcite compounds were also investigated for H2-SCR in details.The performance of Pt based HFER, ZSM-5 and Al2O3 in H2-SCR reaction is compared under the same reaction conditions. Compared to Al2O3 as supports, HFER and ZSM-5 exhibit excellent activity. The most likely reason for this is that the acid sites of zeolites contribute to the existence of metallic state of Pt which plays an important role in H2 activation and then enhanced H2-SCR.On Pt/HFER catalysts, it was found that:(1) Temperature-dependency of NO2 formation was opposite to that of NOx reduction on the catalyst between 80 and 400℃. (2) Increasing the ratio of NO2/(NO+NO2) while keeping the total concentration of NOx in the feed gas mixture drastically decreased the NOx conversion. (3) At the temperatures there the catalyst show low NOx conversion, NO measured in the outlet was much lager than those in the inlet. Combining these results with surface species variation measured by Fourier transforms infrared spectroscopy (FTIR), it was firstly proposed and demonstrated that NOx and nitrate species were not essential intermediates for the H2-SCR over the catalyst. Instead, NOd+ species on Pt were proposed to be the possible species for the efficient elimination of NO in the H2-SCR over the catalyst.Moreover, it was observed that for the H2-SCR reaction over Pt/HFER catalysts, increasing H2 concentration not only enhanced NOx conversion but also H2 conversion. To explain this abnormal phenomenon, the H2-SCR reaction was ivestigated under condition that part H2 was pre-oxidized to water vapor. It was found that the little amount of water (0.2%-0.8%) enhanced not only NOx conversion and N2 selectivity, but also the activation of H2. Based on this result, we gave the explaination for the first time to the phenomenon that the lower H2 concentration for the inlet, the lower H2 conversion obtained in the H2-SCR over Pt/HFER catalyst.The Pt/Mg-Al-0 catalysts prepared from hydrotalcite compounds were also investigated for H2-SCR in details. It was found that the composite oxide obtained by calcining the hydrotalcite at 500℃was better than the parent material for supporting Pt as the H2-SCR catalyst. The NO reduction activitiy of the Pt/Mg-Al-O catalysts was sensitive to pretreatment conditions. Higher NO conversion was obtaind over the catalysts pretreated by H2/N2 in comparison with that pretreated by air. It was found that the states of Pt species on the catalysts was closely related to the the pretreatment conditions. Accordingly, Pt0 was proposed to be the activer site for the H2-SCR on the Pt/Mg-Al-O catalysts. Negative effect on the reaction was observed when Ti was incorporated to the Pt/Mg-Al-O catalysts. Activity of Pt/Mg-Al-0 and Pd/Mg-Al-0 was compared as catalyst for the H2-SCR and it showed that Pt are more active than Pd supported on Mg-Al-O for the reaction.