Stability and reactivity of the nitrogen-oxo species formed after NO adsorption and NO+O-2 coadsorption on Co-ZSM-5: An FTIR spectroscopic study

Adsorption of NO on Co-ZSM-5 leads to formation of Co2+(NO)(2) dinitrosyls [nu(s)(NO) at 1895 cm(-1) and nu(as)(NO) at 1812 cm(-1)], Co3+-NO linear sp ecies [nu(NO) at 1950 and 1937 cm(-1)] and NO+ occupying cationic zeolite p ositions [nu(NO) at 2133 cm(-1)]. The NO+ and Co3+-NO species are of low st ability and can be removed by evacuation at ambient temperature. The Co2+(N O)(2) species start to decompose at 200 degrees C, thus forming Co2+-NO com pounds [nu(NO) at 1857 cm(-1)]. The latter are stable up to 350 degrees C. Water strongly suppresses the formation of NO+ and Co3+-NO. Its effect on t he Co2+(NO)(2) dinitrosyls is, however, weaker and even small amounts of wa ter favour the formation of Co2+(NO)(2) species. The dinitrosyls of Co2+ do not react with oxygen at ambient temperature but are oxidized in an O-2 at mosphere above 100 degrees C. However, they react easily with a NO+O-2 mixt ure forming surface nitrates as final products. This process is almost unaf fected by water. The Co2+(NO)(2) species start to interact with ethane at 1 00 degrees C and water has been detected as a reaction product. These resul ts account for the (although low) activity of Co-ZSM-5 in the reduction of NO in the absence of oxygen. The only stable species formed in a NO+O-2 atm osphere are different kinds of surface nitrates (observed in the 1650-1500 cm(-1) region). The latter start to interact with ethane at 100 degrees C a nd nitriles are suggested as interaction intermediates. The role of differe nt surface species and the effect of the reactants and reaction products on the SCR of NO over Co-ZSM-5 are discussed.