Adsorption/Desorption Studies of NO_X on Well-Mixed Oxides Derived from Co-Mg/Al Hydrotalcite-like Compounds

摘要

Co_xMg_(3-x)/Al hydrotalcite-like compounds (where x=0.0,0.5,1.0,1.5,2.0,2.5,3.0) were synthesized by the coprecipitation method and characterized by the XRD and TGA techniques.Incorporation of Co for x=0.0-3.0 gradually decreased the transformation temperature of the hydrotalcites to the corresponding oxides from 444 to 246 deg C and also decreased the surface area from 162.7 to 21.6 m~2/g upon calcination at 800 deg C for 4 h in air.The resultant oxide was generally composed of a poor MgO phase and a spinel phase,with more spinel phase at higher Co incorporation.The derived oxides were tested as the storage/reduction catalysts for NO_x adsorption/desorption.The storage capacity for NO_x was highly dependent on the catalyst composition and storage temperature.In general,more NO_x was stored at lower temperature (100 deg C) than that at higher temperature (300 deg C),and tertiary catalysts (x=0.5-2.5) stored more NO_x than binary catalyst (x=0.0 or 3.0).The catalytic conversion of NO to NO_2 and the catalytic decomposition of NO_x were observed on the tertiary catalysts during NO_x adsorption at 300 deg C,which was highly related to the loading of cobalt.The reducibility of catalysts was determined by TPR experiments,and the reduction of cobalt cations started at 150-200 deg C in H_x.In situ IR spectra of catalysts adsorbing NO_x revealed that the major NO_x species formed on the catalysts were various kinds of nitrites and nitrates,together with some forms of dimers,such as N_2O_2~(2-) and N_2O_4 (or NO~+NO_3~-).The storage/reduction mechanism and the function of Co in the mixed oxides are proposed and discussed on the basis of these observations.