Evidence for the Formation of W5+ Centers and OH Groups upon Hydrogen Reduction of Platinum-Promoted Tungstated Zirconia Catalysts

Abstract

The redox behavior of Pt-promoted tungstated zirconia catalysts which are active in the isomerization of light alkanes was investigated. For this purpose, the catalysts were reduced in hydrogen and characterized by EPR, FTIR, and diffuse reflectance UV−Vis. EPR shows evidence of formation of W5+ at reduction temperatures above 473 K for the unpromoted catalyst and room temperature when platinum is present, indicating the increased reducibility of the catalyst by the latter. Diffuse reflectance UV−Vis spectroscopy confirms the formation of W5+ on the unpromoted catalyst. FTIR spectroscopy carried out with D2 shows that hydroxyl groups are formed upon reduction of W6+. Low-temperature CO adsorption on these hydroxyl groups reveals that the acid strength is lower than that of the hydroxyl groups present prior to reduction. The higher reducibility of the tungstate phase due to platinum is potentially responsible for the strong enhancement of the catalytic activity and selectivity observed when hydrogen is present in the feed during the isomerization of small alkanes.