Photocatalytic Decomposition Of NO Over Silver Catalysts

As people's consciousness on environmental quality continues to increase, the indoor air pollution attracts people's attention day by day. The low concentrations of nitrogen oxides mainly come from smoking areas and kitchen, car congested highway. Effective removal of low concentration of nitrogen oxides, i.e, the direct decomposition of NO into N2 and O2, has been a great challenge for many researchers. Photocatalysis is an attractive, clean, safe, low-temperature reaction and a nonenergy-intensitive approach, especilly, for chemical waste removal.In this thesis, the photocatalytic decomposition of NO under UV light was investigated over silver modified H-ZSM-5 catalysts. The characterizations of XRD, UV-vis diffuse spectra, TPD, temperature-programmed decomposition of AgNO3 were employed to discriminate different kinds of Ag species. By correlating silver species with their photo-catalytic behaviors in NO decomposition, the active species for NO decomposition is clarified. Some interesting results were got as follows:1. The photocatalytic decomposition of NO was carried out in a self-made fixed bed reactor. Silver introduced into H-ZSM-5 greatly enhanced the catalytic activity in photocacalytic decomposition of NO. Silver loadings, the initial NO concentration, lamp distance has great affects on the photocatalytic activities.2. Silver is shown to be a structure-sensitive catalyst. Pretreatment has different influence on the catalytic performance. Upon thermal treatment in nitrogen, thermal induced changes in silver morphology leaded to the formation of reduced silver clusters and nano-silver particles to achieve a minimization of surface free energy. With the increase of pretreatment temperatures, catalytic activity decreased. Upon oxygen treatment of silver catalysts at elevated temperatures, the oxygen oxidized silver species into silver ions and depresses the growing of silver patticles. While under thermal treatment in hydrogen, the reduction atmosphere enchanced silver ions self-reduced and aggregated into silver particles. Consequently, the the catalyst lost part of its activity. Using the method of quantitative discrimination of silver species in the zeolite channels and those deposited on the zeolite surface established by Bao Xinhe et.al, it was concluded that silver ions and silver clusters played an important role on the photocatalytic decomposition of NO.3. To improve the photocatalytic reactivity, silver was supported on different kinds of zeolites. It indicated that Ag/H-ZSM-5 catalyst has the best photocatalytic activity. Combined with XRD, UV-vis and in situ temperature-programmed decomposition of AgNO3 techniques, it is found that Ag/HY catalyst had the most active silver species, but the activity of which was worst. This is mainly due to aperture restriction. Size 6-ring window in Y zeolite is smaller than the kinetic diameter of NO. Therefore, NO is difficult to be activated over this catalyst. While aqueous solution of silver ammonia was used as a precursor for Ag to impregnate in H-ZSM-5 zeolite. The Ag(NH)/H-ZSM-5 catalysts with more active Ag species had better activity.4. Study on the TPD results over the fresh and used samples indicates that oxygen produced from NO dissociation strongly adsorbed on silver catalyst, which blocks the active sites. That might be the reason for catalyst deactivation.