The effect of sulphation on the catalytic activity of CoOx/ZrO2 for NO reduction with NH3 in the presence of O2

Selective catalytic reduction (SCR) of NO with NH3 in the presence of O-2, and NH3 oxidation with O-2 (NH3 + O-2) were studied on CoOx/ZrO2 and sulphated-CoOx/ZrO2 catalysts. Monoclinic CoOx/ZrO2 (CO/Z(m)) containing 2.0 or 4.6 Co-atoms nm(-2) were prepared by dry impregnation of monoclinic ZrO2 (Z(m)) with Co(CH3COO)(2) aqueous solutions. Three sulphated catalysts, all having roughly the same surface density of cobalt (2 Co-atoms nm(-2)) were prepared by (i) exposure of Co/Z(m) to a gaseous stream SO2 + O-2 (Co/Z(m)S(g)), (ii) impregnation of Z(m) with CoSO4 aqueous solution (CoS/Z(m)), and (iii) impregnation of Zr(OH)(4) with CoSO4 aqueous solution (CoS/Z(t), tetragonal). Calcined samples were characterized by means of XRD, UV-vis DRS and FT-IR (using CO and NO as probe molecules). In Co/Z(m), isolated Co2+ anchored to the ZrO2 surface and small Co3O4 particles coexisted. In Co/Z(m), upon exposure to CO at 298 K, isolated Co2+ and cobalt ions on the surface of Co3O4 easily underwent reduction, yielding Con+-carbonyls (n < 2). Conversely, in sulphated samples, nearly all cobalt was present as isolated Co2+, which was far less reducible with CO at 298 K than isolated Co2+ in Co/Z(m). Specifically, the reducibility of isolated Co2+ decreased in the order Co/Z(m) > CoS/Z(t) > CoS/Z(m) congruent to Co/Z(m)S(g). On all sulphated samples, surface covalent sulphates formed. The structure of surface covalent sulphates on CoS/Z(t) differed from those on CoS/Z(m) and Co/Z(m)S(g). On Co/Z(m)S(g). covalent and ionic sulphates coexisted. In the temperature range 425-600 K, Co/Z(m) samples were poorly active for NO reduction and highly active for NH3 oxidation. In this temperature range, sulphated samples were inactive for both reactions. They became highly active for NO reduction above 600 K, and for NH3 oxidation above 675 K. We conclude that the catalytic activity and selectivity of CoOx/ZrO2 depend crucially on (i) the presence of isolated Co2+ endowed with the proper redox behaviour and (ii) the absence Of Co3O4. (C) 2008 Elsevier B.V. All rights reserved.