The effect of sulphation on the catalytic activity of CoO[sub]x/ZrO[sub]2 for NO reduction with NH[sub]3 in the presence of O[sub]2

Selective catalytic reduction (SCR) of NO with NH[sub]3 in the presence of O[sub]2, and NH[sub]3 oxidation with O[sub]2 (NH[sub]3 + O[sub]2) were studied on CoO[sub]x/ZrO[sub]2 and sulphated- CoO[sub]x/ZrO[sub]2 catalysts. Monoclinic CoO[sub]x/ZrO[sub]2 (Co/Z[sub]m) containing 2.0 or 4.6 Co-atoms nm[super]-2 were prepared by dry impregnation of monoclinic ZrO[sub]2 (Z[sub]m) with Co(CH[sub]3COO)[sub]2 aqueous solutions. Three sulphated catalysts, all having roughly the same surface density of cobalt (2 Co- atoms nm[super]-2) were prepared by (i) exposure of Co/Z[sub]m to a gaseous stream SO[sub]2 + O[sub]2 (Co/Z[sub]mS[sub]g), (ii) impregnation of Z[sub]m with CoSO[sub]4 aqueous solution (CoS/Z[sub]m), and (iii) impregnation of Zr(OH)[sub]4 with CoSO[sub]4 aqueous solution (CoS/Z[sub]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[sub]m, isolated Co[super]2+ anchored to the ZrO[sub]2 surface and small Co[sub]3O[sub]4 particles coexisted. In Co/Z[sub]m, upon exposure to CO at 298 K, isolated Co[super]2+ and cobalt ions on the surface of Co[sub]3O[sub]4 easily underwent reduction, yielding Co[super]n+-carbonyls (n < 2). Conversely, in sulphated samples, nearly all cobalt was present as isolated Co[super]2+, which was far less reducible with CO at 298 K than isolated Co[super]2+ in Co/Z[sub]m. Specifically, the reducibility of isolated Co[super]2+decreased in the order Co/Z[sub]m > CoS/Z[sub]t > CoS/Z[sub]m [congruent with] Co/Z[sub]mS[sub]g. On all sulphated samples, surface covalent sulphates formed. The structure of surface covalent sulphates on CoS/Z[sub]t differed from those on CoS/Z[sub]m and Co/Z[sub]mS[sub]g. On Co/Z[sub]mS[sub]g, covalent and ionic sulphates coexisted. In the temperature range 425-600 K, Co/Z[sub]m samples were poorly active for NO reduction and highly active for NH[sub]3 oxidation. In this temperature range, sulphated samples were inactive for both reactions. They became highly active for NO reduction above 600 K, and for NH[sub]3 oxidation above 675 K. We conclude that the catalytic activity and selectivity of CoO[sub]x/ZrO[sub]2 depend crucially on (i) the presence of isolated Co[super]2+ endowed with the proper redox behaviour and (ii) the absence of Co[sub]3O[sub]4.