Comparison of Catalyzed and Homogeneous Reactions of Hydrocarbons for Selective Catalytic Reduction (SCR) of NOx

When NO2 (0.21% in He) was passed over CoZSM-5 or HZSM-5 at SVH = 45,000 h-1, equlibrium with NO + O2 was approached at temperatures above 673 K. The reactions of NO2 + CH4 NO2 + CH4 + O2, and NO + CH4 + O2 were compared over these catalysts and in the empty reactor. The latter two reactions yielded essentially identical results when catalyzed, as did NO2 + CH4 up to about 22% conversion of CH4, i.e., to the point where the oxygen supply became exhausted. Without added O2, NO appeared as a reduction product of NO2 along with N2. In the empty reactor, no N2 was formed although NO2 could be quantitatively reduced to NO; combustion of CH4 with O2 (or with NO) alone was not observed at temperatures less than 873 K, but light-off with NO2 or NOx + O2 occurred at about 723 K. In the absence of O2, the homogeneous CH4 conversion was limited to about 22% at 873 K where the conversion of NO2 to NO reached 100%. With added O2, conversion of CH4 reached 62% at 873 K and approached 100% under more severe conditions. These data illustrate the key role played by NO2 in the selective catalytic reduction reaction. They also show that a catalyst is necessary for the formation of N2 and emphasize the importance of O2 in maintaining an adequate supply of NO2, particularly at temperatures above 800 K where equilibrium favors NO. When C3H8 or i-C4H10 was substituted for CH4, the order of reactivity was i-C4H10 > C3H8 > CH4 in both the catalyzed and the homogeneous reactions. Moreover, in the empty reactor dehydrogenation to the corresponding olefins was found to be important with the former two; the mass balance did not close with CH4, possibly due to the formation of formaldehyde.