The reactions of formamide and nitromethane, two possible intermediates in the selective catalytic reduction of nitrogen oxides by methane, have been studied over Co-ZSM5, H-ZSM5 and Cu-ZSM5. Formamide, a possible surrogate for nitrosomethane, reacts completely below 250 degrees C over Co-ZSM5 with formation of NH3 and CO by one route and HCN and H2O by another. Inclusion of NO causes partial conversion of NH3 to N-2 at 300 degrees C. H-ZSM5 behaves similarly but with a higher conversion of NH3 in the presence of NO. Cu-ZSM5 gives CO2 and N-2 alone, apparently because of its high oxidation activity. The reaction of nitromethane over H-ZSM5 is similar to that previously established for Co-ZSM5 with NH3 and CO2 as the initial products and subsequent N-2 formation by the NH3-SCR reaction. Again N-2 formation is more extensive with H-ZSM5 than Co-ZSM5 when NO is present while Cu-ZSM5 gives only CO2 and N-2. Deactivation is characteristic of the reaction of nitromethane over all three zeolites at temperatures below approximate to 280 degrees C with eventual breakthrough of isocyanic acid (HNCO) as a product. In situ FTIR measurements with H-ZSM5 indicate that deactivation is due to reactions of HNCO to form deposits of s-triazine compounds which can be removed by NO2. The overall conclusion is that nitromethane and formamide, and by inference nitrosomethane, react in ways which are consistent with the possibility that species of these types could be intermediates in the methane-SCR reaction over zeolite catalysts. Distinction between them is possible only with catalysts of low oxidation capability when CO formation is consistent with the involvement of nitrosomethane and CO2 formation with that of nitromethane. (C)1999 Elsevier Science B.V. All rights reserved.