Gas-Phase Oxidation of Methane Catalyzed by NO_x

Catalytic conversion and catalytic combustion of CH_4 have received extensive attention because of their potential to synthesize useful chemicals and to reduce pollutant emissions. Both the processes require activation of CH_4, which has been mostly achieved over solid catalysts to date. In the catalytic conversion of CH_4, the promotional effects of NO_x on CH_4 oxidation were reported by some researchers, but the role that NO_x may play in CH_4 activation and the importance of this gas-phase reaction have not received more attention. This work investigated the catalytic effect of NO_x on partial oxidation of CH_4 in the absence of solid catalyst. The product distribution was studied by in-situ FT-IR under different reaction conditions. The results showed that the addition of a small amount of NO into 20%CH_4-10%O_2 system decreased the reaction temperature by 200～300 ℃. At 650～700 ℃, the CH_4 conversion and CO selectivity were 38% and 90%, respectively, and n(H_2)/ n(CO)=0.4～0.7. The optimum conditions for CO formation were n(CH_4)/ n(O_2)=2 and low flow rate. Some reducible substances, such as HCHO, CH_3OH and C_2H_4, were also observed in the products. The HCHO was formed at 460 ℃, then CO (480 ℃), CH_3OH (500 ℃), and C_2H_4 (540 ℃) were formed. Low temperature favours the formation of HCHO and CH_3OH, while high temperature favours C_2H_4. Under strong oxidizing conditions ( n(O_2)/ n(CH_4)=5), NO_x also exhibits considerable catalytic activity and selectivity for CH_4 conversion to CO. For 0.4%CH_4-2%O_2 system, the temperature for CH_4 activation is higher than 800 ℃ in the absence of NO_x. In the presence of NO_x, however, the activation temperature is reduced to below 570 ℃. The addition of 0.06%NO results in the increase of CO yield from 25% to 68%. It was shown that no reaction was observed for CH_4-NO_x system (without O_2) under the reaction conditions, indicating that the NO_2 formation is a prerequisite for CH_4 activation. Also, no decrease of NO/NO_2 concentration was observed up to 950 ℃, indicating that NO_x cannot be reduced by CH_4 in the absence of solid catalyst. Both CH_4 conversion and CO yield reached a maximum when φ(NO)=0.04%～0.06%, while the highest TOF (turnover frequency) of 40 s -1 was obtained when φ(NO)=0.01%.