The reaction mechanisms of selective catalytic reduction(SCR)of nitric oxide(NO)by methane(CH4)over solid superacid-based catalysts were proposed and testified by DRIFTS studies on transient reaction as well as by kinetic models.Catalysts derived from different supports would lead to different reaction pathways,and the acidity of solid superacid played an important role in determining the reaction mechanisms and the catalytic activities.Higher ratios of BrØnsted acid sites to Lewis acid sites would lead to stronger oxidation of methane and then could facilitate the step of methane activation.Strong BrØnsted acid sites would not necessarily lead to better catalytic performance,however,since the active surface NO_(y) species and the corresponding reaction routes were determined by the overall acidity strength of the support.The reaction routes where NO_(2)moiety was engaged as an important intermediate involved moderate oxidation of methane,the rate of which could determine the overall activity.The reaction involving NO moiety was likely to be determined by the step of reduction of NO.Therefore,to enhance the SCR activity of solid superacid catalysts,reactions between appropriate couples of active NO_(y)species and activated hydrocarbon intermediates should be realized by modification of the support acidity.
Co_(3)O_(4)-SnO_(2)hybrid oxides were prepared by the coprecipitation method and were used to oxidate methane(CH4)in presence of oxygen.The Co_(3)O_(4)-SnO_(2)with a molar ratio of Co/(Co+Sn)at 0.75 exhibited the highest catalytic activity among all the Co_(3)O_(4)-SnO_(2)hybrid oxides.Experimental results showed that the catalysts were considerably stable in the CH4 combustion reaction,and were verified by X-ray photoelectron spectra(XPS).It was found that Co_(3)O_(4)was the active species,and SnO_(2)acted as a support or a promoting component in the Co_(3)O_(4)-SnO_(2)hybrid oxides.The surface area was not a major factor that affected catalytic activity.The hydrogen temperatureprogrammed reduction(H_(2)-TPR)results demonstrated that the interaction between cobalt and tin oxides accelerated the mobility of oxygen species of Co_(3)O_(4)-SnO_(2),leading to higher catalytic activity.
