Chemical, structural and mechanistic aspects on NOx SCR over commercial and model oxide catalysts

The chemico-physical characteristics and the catalytic activity of commercial and home-made V2O5-WO3/TiO2 catalysts has been investigated in this work. The samples are constituted by TiO2 anatase that supports the V and W components (and S in the case of commercial catalysts). The V+W estimated surface coverage is below that corresponding to the theoretical monolayer, but when surface sulfates are also taken into account the monolayer capacity of the samples is exceeded. V, W and sulfates are present on the dry catalyst surface in the form of isolated vanadyl, wolframyl and sulfate species, all in a mono-oxo-type form. The adsorption-desorption study showed that NO does not adsorb on the catalyst surface, whereas NH3 adsorbs on both Lewis and Bronsted acid sites. Lewis-bonded NH3 species are thermally more stable than ammonium ions, and upon heating, a weak band is observed at 1540 cm(-1), that has been assigned to an amide species NH2. When a NH3-covered surface is heated in the presence of NO, ammonia is activated on Lewis acid sites and then reacts with gas-phase NO to give N-2. Mechanistic features of the selective catalytic reduction (SCR) reaction have also been collected by means of transient methods, including the temperature programmed desorption/reaction techniques and the transient response analysis (TRA). These experiments proved that: (i) the reaction occurs between adsorbed ammonia and gas-phase or weakly adsorbed NO; (ii) NH3 can not only adsorb over the active V-sites but also on the surface W- and Ti-sites and on surface sulfates as well, hence acting as an ammonia "reservoir"; (iii) the mechanism is of the redox type, i.e. oxygen oxidizes the surface sites reduced by the other reactants. A mechanistic model for the SCR reaction has thus been derived that is consistent with our data and with literature indication as well. (C) 1998 Elsevier Science B.V. All rights reserved.