Selectively Catalytic Reduction of NO by CH4 over the Silver Modified ZSM-5 zeolite Catalysts

Dr. Chuan Shi(1996-2001)
Directed by Prof. Yang Xuefeng and Prof. Bao Xinhe
Abstract
Present Address: Dalian
    The selectively catalytic reduction (SCR) of NO by CH4 in the excess of oxygen was investigated over silver modified ZSM-5 catalysts. TP spectroscopy was employed to study the adsorption of nitric oxides and the reactions of surface nitric species with CH4 to elucidate the reaction mechanism. The correlation between different silver species with their catalytic behaviors in CH4-SCR of NO was also investigated. The main results presented in this dissertation were summarized as follows:
  1. Silver introduced into ZSM-5 zeolite greatly enhanced the catalytic activity in CH4-SCR of NO in the excess of oxygen and lowered the "light-off" temperatures for both CH4 selective reduction of NO and CH4 direct combustion reactions. The excessive oxygen in the feed greatly increased the catalytic activity of Ag-ZSM-5 catalyst in CH4-SCR of NO.
  2. NO/O2-TPD and TPR results demonstrated that surface nitrate species were formed in NO and O2 co-existing systems and could be effectively reduced by CH4 to N2. The surface nitrates were more reactive than O2, and CH4 could preferentially and selectively reduce surface nitrates to nitrogen, rather than combusted with oxygen directly. The selective reduction of NO in the presence of excess O2 was proposed to pass through the selective reduction of adsorbed nitrate species.
  3. The state and structural changes of silver before and after SCR reaction were characterized by XRD, TEM and UV-vis. Silver existed as highly dispersed silver ions in the zeolite channels and nano-sized metallic silver particles on the surface of the zeolite before SCR reaction (after the pretreatment in He stream at 500°C). While after the reaction, the metallic silver particles were oxidized and a new kind of silver species formed which was characterized with silver dissolved with a certain mount of oxygen. Correlating the state and structural changes of silver with the catalytic behaviors in SCR, it was found that the catalytic activity was decreased but the selectivity was increased during the oxidation of metallic silver. Silver existed as highly dispersed silver ions and aggregated oxidized silver particles after the SCR reaction.
  4. The different catalytic behaviors of silver species in the zeolite or deposited on the zeolite surface in CH4-SCR of NO were investigated. The temperature programmed decomposition of AgNO3 on H-ZSM-5 zeolite was analyzed as an effective way to discriminate the silver in the zeolite or deposited on the zeolite surface qualitatively and quantitatively. Confirmed by XRD, TEM and UV-vis measurements, it was found that silver species were incorporated in ZSM-5 zeolite channels on low Ag loading samples.
    While nano-sized silver particles were formed over Ag loading higher than 7 % samples, and the particle size grew with the increase of Ag loading. The NO conversion to N2 showed a S-shape dependence on Ag loading. The addition of
3-5 % silver on the enhancement of the catalytic activity was so trivial, while the NO conversion to N2 was greatly increased by the silver particles formed. Further increase in loading (>11%), the NO conversion was decreased a little. The temperature programmed desorption of nitric oxides and the surface reaction of nitric oxides with CH4 clearly demonstrated that nitrates adsorbed on the certain size of silver particles could be effectively reduced by CH4. Silver ions in the zeolite channels cooperated with the acid centers of ZSM-5 zeolite may participate the activation of CH4.