CHEMICAL, STRUCTURAL AND MECHANISTIC ASPECTS ON NOX SCR OVER COMMERCIAL AND MODEL OXIDE CATALYSTS

The chemico-physical characteristics and the catalytic activity of com mercial 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). Th e V+W estimated surface coverage is below that corresponding to the th eoretical monolayer, but when surface sulfates are also taken into acc ount the monolayer capacity of the samples is exceeded. V, W and sulfa tes are present on the dry catalyst surface in the form of isolated va nadyl, wolframyl and sulfate species, all in a mono-oxo-type form. The adsorption-desorption study showed that NO does not adsorb on the cat alyst surface, whereas NH3 adsorbs on both Lewis and Bronsted acid sit es. Lewis-bonded NH3 species are thermally more stable than ammonium i ons, and upon heating, a weak band is observed at 1540 cm(-1), that ha s 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 site s and then reacts with gas-phase NO to give N-2. Mechanistic features of the selective catalytic reduction (SCR) reaction have also been col lected by means of transient methods, including the temperature progra mmed desorption/reaction techniques and the transient response analysi s (TRA). These experiments proved that: (i) the reaction occurs betwee n adsorbed ammonia and gas-phase or weakly adsorbed NO; (ii) NH3 can n ot 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 ox idizes the surface sites reduced by the other reactants. A mechanistic model for the SCR reaction has thus been derived that is consistent w ith our data and with literature indication as well. (C) 1998 Elsevier Science B.V. All rights reserved.