Stability and reactivity of the nitrogen-oxo species formed after NO adsorption and NO+O2 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 species [nu(NO) at 1950 and 1937 cm(-1)] and NO+ occupying cationic zeolite positions [nu(NO) at 2133 cm(-1)]. The NO+ and Co3+-NO species are of low stability and can be removed by evacuation at ambient temperature. The Co2+(NO)(2) species start to decompose at 200 degrees C, thus forming Co2+-NO compounds [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 the Co2+(NO)(2) dinitrosyls is, however, weaker and even small amounts of water 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 atmosphere above 100 degrees C. However, they react easily with a NO+O-2 mixture forming surface nitrates as final products. This process is almost unaffected by water. The Co2+(NO)(2) species start to interact with ethane at 100 degrees C and water has been detected as a reaction product. These results 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 atmosphere 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 and nitriles are suggested as interaction intermediates. The role of different surface species and the effect of the reactants and reaction products on the SCR of NO over Co-ZSM-5 are discussed.