Temperature Programmed Desorption/Reaction of Ammonia over V2O5/TiO2 De-NOxing Catalysts

Abstract

Temperature programmed desorption (TPD) of ammonia, temperature programmed surface reaction (TPSR) of adsorbed ammonia with gas phase NO, temperature programmed reaction (TPR), and steady state reaction (SSR) experiments of NH3 + NO under diluted gas conditions in the presence and in the absence of oxygen are performed on a series of V2O5/TiO2 (anatase) catalysts with V2O5 loadings ≤ 3.56% w/w that are comparable to those of commercial De-NOxing catalysts. Both isolated vanadyls and polymeric metavanadate species are observed on the surface of submonolayer vanadia/titania samples, the relative concentration of the latter species increasing with the vanadia content. TPR, TPSR, and SSR experiments performed in the presence and in the absence of oxygen provide new and more direct evidence than previously reported in the literature fora greater reactivity of polymeric metavanadate species as compared to isolated vanadyls and for a faster reduction by NH3 and a faster reoxidation by gaseous oxygen of the polymeric metavanadate groups. This has been related to the presence of more labile oxygen atoms, that is beneficial for the activity but is also detrimental for the selectivity of the SCR reaction. The turnover frequency measured at 500 K under steady state conditions over high-vanadia loading catalysts is six times larger than that of low-vanadia loading catalysts, due to the higher amounts of polyvanadate species. The temperature window of the SCR reaction, corresponding to high NO conversion and complete selectivity to nitrogen, is shifted towards lower temperatures on increasing the vanadia loading and extends towards higher temperatures under steady state conditions and in the presence of oxygen due to a proper oxidation level of the catalyst.