Electrical properties of V 2O 5–WO 3/TiO 2 EUROCAT catalysts evidence for redox process in selective catalytic reduction (SCR) deNO x reaction

Abstract

Electrical conductivity measurements on EUROCAT V 2O 5–WO 3/TiO 2 catalyst and on its precursor without vanadia were performed at 300°C under pure oxygen to characterize the samples, under NO and under NH 3 to determine the mode of reactivity of these reactants and under two reaction mixtures ((i) 2000 ppm NO + 2000 ppm NH 3 without O 2, and (ii) 2000 ppm NO + 2000 ppm NH 3 + 500 ppm O 2) to put in evidence redox processes in SCR deNO x reaction. It was first demonstrated that titania support contains certain amounts of dissolved W 6+ and V 5+ ions, whose dissolution in the lattice of titania creates an n-type doping effect. Electrical conductivity revealed that the so-called reference pure titania monolith was highly doped by heterovalent cations whose valency was higher than +4. Subsequent chemical analyses revealed that so-called pure titania reference catalyst was actually the WO 3/TiO 2 precursor of V 2O 5–WO 3/TiO 2 EUROCAT catalyst. It contained an average amount of 0.37 at.% W 6+dissolved in titania, i.e. 1.07 × 10 20 W 6+ cations dissolved/cm 3 of titania. For the fresh catalyst, the mean amounts of W 6+ and V 5+ ions dissolved in titania were found to be equal to 1.07 × 10 20 and 4.47 × 10 20 cm −3, respectively. For the used catalyst, the mean amounts of W 6+ and V 5+ ions dissolved were found to be equal to 1.07 × 10 20 and 7.42 × 10 20 cm −3, respectively. Since fresh and used catalysts have similar compositions and similar catalytic behaviours, the only manifestation of ageing was a supplementary progressive dissolution of 2.9 × 10 20 additional V 5+ cations in titania. After a prompt removal of oxygen, it appeared that NO alone has an electron acceptor character, linked to its possible ionosorption as NO − and to the filling of anionic vacancies, mostly present on vanadia. Ammonia had a strong reducing behaviour with the formation of singly ionized vacancies. A subsequent introduction of NO indicated a donor character of this molecule, in opposition to its first adsorption. This was ascribed to its reaction with previously adsorbed ammonia strongly bound to acidic sites. Under NO + NH 3 reaction mixture in the absence of oxygen, the increase of electrical conductivity was ascribed to the formation of anionic vacancies, mainly on vanadia, created by dehydroxylation and dehydration of the surface. These anionic vacancies were initially subsequently filled by the oxygen atom of NO. N o atoms, resulting from the dissociation of NO and from ammonia dehydrogenation, recombined into dinitrogen molecules. The reaction corresponded to 3 NO+2 NH 3→ 5 2 N 2+3 H 2 O . In the presence of oxygen, NO did not exhibit anymore its electron acceptor character, since the filling of anionic vacancies was performed by oxygen from the gas phase. NO reacted directly with ammonia strongly bound on acidic sites. A tentative redox mechanism was proposed for both cases.