TEMPERATURE-PROGRAMMED DESORPTION - REACTION OF AMMONIA OVER V2O5 TIO2 DE-NO(X)ING CATALYSTS/

Temperature programmed desorption (TPD) of ammonia, temperature progra mmed surface reaction (TPSR) of adsorbed ammonia with gas phase NO, te mperature programmed reaction (TPR), and steady state reaction (SSR) e xperiments of NH3 + NO under diluted gas conditions in the presence an d in the absence of oxygen are performed on a series of V2O5/TiO2 (ana tase) catalysts with V2O5 loadings less-than-or-equal-to 3.56% w/w tha t are comparable to those of commercial De-NO(x)ing catalysts. Both is olated vanadyls and polymeric metavanadate species are observed on the surface of submonolayer vanadia/titania samples, the relative concent ration of the latter species increasing with the vanadia content. TPR, TPSR, and SSR experiments performed in the presence and in the absenc e of oxygen provide new and more direct evidence than previously repor ted in the literature fora greater reactivity of polymeric metavanadat e species as compared to isolated vanadyls and for a faster reduction by NH3 and a faster reoxidation by gaseous oxygen of the polymeric met avanadate groups. This has been related to the presence of more labile oxygen atoms, that is beneficial for the activity but is also detrime ntal for the selectivity of the SCR reaction. The turnover frequency m easured at 500 K under steady state conditions over high-vanadia loadi ng catalysts is six times larger than that of low-vanadia loading cata lysts, due to the higher amounts of polyvanadate species. The temperat ure window of the SCR reaction, corresponding to high NO conversion an d complete selectivity to nitrogen, is shifted towards lower temperatu res on increasing the vanadia loading and extends towards higher tempe ratures under steady state conditions and in the presence of oxygen du e to a proper oxidation level of the catalyst. (C) 1994 Academic Press , Inc.