IR study of the low temperature adsorption of CO on tetragonal zirconia and sulfated tetragonal zirconia

Abstract

In situ FTIR spectroscopy has been used to study the adsorption at ∼77 K of CO on (i) tetragonal zirconia (t-ZrO 2), (ii) non-calcined sulfated tetragonal zirconia (t-SZ) and (iii) calcined sulfated tetragonal zirconia ([t-SZ] C). CO adsorption at low temperature turned out to be a suitable probe to test the surface charge-withdrawing properties of the various zirconia-based systems. On t-ZrO 2 it has been possible to distinguish the H-bonding adsorption of CO on at least two families of acidic surface OH groups and the coordinative adsorption of CO on surface Lewis acidic sites (i.e. coordinatively unsaturated Zr 4+ surface centres) located either in regular patches of low-index crystal planes (the `top' termination of ZrO 2 crystallites) or in defective terminations of the particles (the `side' termination of ZrO 2 crystallites). On t-SZ, virtually only CO uptake by H-bonding on some surface OH groups is observed, as virtually all of the non-hydroxylated parts 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 crystal terminations. As a consequence CO uptake reveals, besides two well resolved families of OH groups yielding adsorption by H-bonding, two well resolved families of Lewis acid sites located in defective crystal terminations (strong Lewis sites), whereas no CO uptake occurs in the regular terminations of the crystallites. The latter crystal positions remain occupied by a family of sulfates, that turn out to be responsible for the catalytic activity of [t-SZ] C systems.