Effect of Cr and Pt promoters on the surface properties of tungstated zirconia: FTIR spectroscopy of probe molecules (CO and NO)

Adsorption of IR probe molecules (CO and NO) has been used for characterization of a Pt-Cr(2)O(3)/WO(3)-ZrO(2) sample. For better assignments of the results obtained 'blank' experiments with WO(3)-ZrO(2), Pt/WO(3)-ZrO(2) and Cr(2)O(3)/WO(3)-ZrO(2) were also performed. The Cr(2)O(3)/WO(3)-ZrO(2) sample was prepared via sulfate route and surface sulfates were detected on the activated sample. These species keep ca. 50% of their initial concentration even after reduction by hydrogen at 773 K. The amount of Lewis acid sites on the activated sample, as measured by CO adsorption, is relatively low and is associated mainly with coordiantively unsaturated (c.u.s.) Zr(4+) cations. Reduction of the sample generates Cr(3+) ions and the concentration of the Lewis acid sites continuously increases with the reduction temperature up to 773 K. At this reduction temperature, however, the strength of the acid sites decreases due to partial removal of the sulfates. While CO is not an efficient probe to discriminate between Zr(4+) and Cr(3+) sites, NO is selectively strongly adsorbed on Cr(3+) cations which allows a more precise monitoring of the evolution of the Cr(3+) sites during reduction. The activated Pt-Cr(2)O(3)/WO(3)-ZrO(2) sample also shows presence of sulfates but these species are practically absent after reduction with hydrogen at 773 K. It was found that platinum promotes the reduction of chromium and tungsten species as well. CO adsorption on activated sample reveals the existence of cationic platinum. However, reduction with hydrogen even at room temperature generates metallic Pt. Its amount continuously increases with the reduction temperature up to 573 K and then, due to the SMSI effect and/or agglomeration, the amount of platinum accessible to adsorption decreases. Comparison with the results obtained with the Pt/WO(3)-ZrO(2) sample shows that chromium and/or sulfates stabilize the cationic form of platinum.