Stability and reactivity of the nitrogen-oxo species formed after NO adsorption and NO+O2 coadsorption on Co-ZSM-5: An FTIR spectroscopic study

Abstract

Adsorption of NO on Co-ZSM-5 leads to formation of Co²⁺(NO)₂ dinitrosyls [ν_s(NO) at 1895 cm^-1 and ν_as(NO) at 1812 cm^-1], Co³⁺–NO linear species [ν(NO) at 1950 and 1937 cm^-1] and NO⁺ occupying cationic zeolite positions [ν(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°C, thus forming Co²⁺–NO compounds [ν(NO) at 1857 cm^-1]. The latter are stable up to 350°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 O₂ atmosphere above 100°C. However, they react easily with a NO+O₂ 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°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+O₂ atmosphere are different kinds of surface nitrates (observed in the 1650–1500 cm^-1 region). The latter start to interact with ethane at 100°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.