IR STUDY OF THE LOW-TEMPERATURE ADSORPTION OF CO ON TETRAGONAL ZIRCONIA AND SULFATED TETRAGONAL ZIRCONIA

In situ FTIR spectroscopy has been used to study the adsorption at sim ilar to 77 K of CO on (i) tetragonal zirconia (t-ZrO2), (ii) non-calci ned sulfated tetragonal zirconia (t-SZ) and (iii) calcined sulfated te tragonal zirconia ([t-SZ](C)). CO adsorption at low temperature turned out to be a suitable probe to test the surface charge-withdrawing pro perties of the various zirconia-based systems. On t-ZrO2 it has been p ossible to distinguish the H-bonding adsorption of CO on at least two families of acidic surface OH groups and the coordinative adsorption o f CO on surface Lewis acidic sites (i.e. coordinatively unsaturated Zr 4+ surface centres) located either in regular patches of low-index cry stal planes (the 'top' termination of ZrO2 crystallites) or in defecti ve terminations of the particles (the 'side' termination of ZrO2 cryst allites). On t-SZ, virtually only CO uptake by H-bonding on some surfa ce OH groups is observed, as virtually all of the non-hydroxylated par ts of the surface are occupied and maintained coordinatively saturated by sulfate groups. On [t-SZ](C), the calcination process selectively eliminates the sulfates that were initially located in the defective c rystal terminations. As a consequence CO uptake reveals, besides two w ell resolved families of OH groups yielding adsorption by H-bonding, t wo well resolved families of Lewis acid sites located in defective cry stal terminations (strong Lewis sites), whereas no CO uptake occurs in the regular terminations of the crystallites. The latter crystal posi tions remain occupied by a family of sulfates, that turn out to be res ponsible for the catalytic activity of [t-SZ](C) systems. (C) 1998 Els evier Science B.V.