Selective Catalytic Reduction Of NO By Hydrogen Over Pt/Zeolites

Nitrogen oxides (NOx) are the main air pollutants. Selective catalytic reduction (SCR) is the most promising way of NOx abatement. Recently, hydrogen with the advantage of clean and high activity at relatively low temperature attracts much attention. In this study, ZSM-35 zeolite is used as support. Metal based ZSM-35 catalysts are investigated in selective catalytic reduction of NO by hydrogen (H2-SCR). Metal additives, composite zeolites and fast SCR reaction are introduced to improve the activity and selectivity. Many characterizations are used to get deep insight into the reaction mechanism, the effect of additives and the advantage of fast SCR reaction.The performance of Pt based ZSM-35, ZSM-5 and Beta in H2-SCR reaction is compared under the same reaction conditions. At 120℃, Pt/ZSM-35 catalyst exhibits attractive activity (80.8%) and selectivity (68.5%), which is better than those on Pt/ZSM-5 and Pt/Beta. XRD and NH3-TPD results suggest that metallic Pt species exist in all catalysts; the introduction of Pt does not influence the framework of zeolite support, but decreases the number of strong acid sites. Adsorbed ammonium species are formed during in situ H2-SCR reaction. The effect of Pt loading on catalytic activity is studied secondly. XRD、TEM、XPS results suggest that Pt species on Pt/ZSM-35 catalyst exist mainly in metallic (80%) and only 20% in+2 oxidation state, and the increase of Pt loading has no effect on the valance state of Pt, but causes the aggregation of metallic Pt. In situ IR results indicate that adsorbed nitrates and ammonia are key intermediates in H2-SCR reaction.Several metals are used to modify Pt/ZSM-35 catalyst. An attractive enhancement of NO conversion from 80.5% to 94.7% is obtained at 120℃on Cr modified sample. Further studies show that 1 wt% is the optimal loading of Cr, and co-impregnation is the optimal introducing way. H2-TPR results suggest that comparing with Cr/ZSM-35, the main reduction peak of Cr species on Pt-Cr/ZSM-35 shifted to low temperature. Moreover, the introduction of Cr enhances (1) the adsorption of NO-O2 by forming new Pt-NOδ+ and NO species adsorbed on Pt were detected upon NO+O2 adsorption; (2) the formation of ammonia species under reaction condition. It indicates that the enhanced adsorption of NOx and formation of surface NH4+ species is the origin of promotional effect of Cr on Pt/ZSM-35 for H2-SCR reaction.Composite zeolites are used firstly in NOx elimination under excess oxygen. Pt based ZSM-35/MCM-49 catalysts exhibit high activity in H2-SCR reaction. Further study suggests that the ratio of ZSM-35 and MCM-49 in composite zeolitic support has little influence on the catalytic activity. XRD and H2-TPR results, coupled with those in CO adsorption show that Pt species in Pt/35(x%)/49 catalysts are in metallic, +2 and+4 states. In situ IR results show that during H2-SCR reaction same surface species are formed on Pt/35(x%)/49 catalysts; the ratio of ZSM-35 and MCM-49 has some effect on the formation of ammonia species.Fast SCR is introduced to H2-SCR reaction for the first time. The performance of 0.5%Pt/ZSM-35 and Pd/Al2O3, Pd/SiO2, Pd/MgO in standard and fast H2-SCR reaction is compared respectively. Results show that compared with the normal H2-SCR process, a significant enhancement of deNOx efficiency is achieved in the fast H2-SCR process. Pd/Al2O3 and Pd/SiO2 catalysts exhibited attractive activity and gave 100%NOx conversion above 200℃. In situ DRIFT results show that same surface species are formed during standard and fast H2-SCR reaction, and much more ammonia species are formed during fast H2-SCR reaction than those in standard one.