NO–NH3 coadsorption on vanadia/titania catalysts: determination of the reduction degree of vanadium

Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH₃ and NO adsorption over a 15% w/w vanadia/titania catalyst. NH₃ is adsorbed as coordinate NH₃ and NH₄⁺ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300°C, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the V=O sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced V=O groups, resulting in N₂, according to a NO+V_□→1/2N₂+V=O reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2ν(V=O) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1.