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

Abstract

Adsorption of IR probe molecules (CO and NO) has been used for characterization of a Pt–Cr₂O₃/WO₃–ZrO₂ sample. For better assignments of the results obtained ‘blank’ experiments with WO₃–ZrO₂, Pt/WO₃–ZrO₂ and Cr₂O₃/WO₃–ZrO₂ were also performed. The Cr₂O₃/WO₃–ZrO₂ 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.) Zr⁴⁺ cations. Reduction of the sample generates Cr³⁺ 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 Zr⁴⁺ and Cr³⁺ sites, NO is selectively strongly adsorbed on Cr³⁺ cations which allows a more precise monitoring of the evolution of the Cr³⁺ sites during reduction. The activated Pt–Cr₂O₃/WO₃–ZrO₂ 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/WO₃–ZrO₂ sample shows that chromium and/or sulfates stabilize the cationic form of platinum.