Effect of titania structure on palladium oxide catalysts in the oxidative coupling of 4-methylpyridine

The effects of anatase and rutile titania on PdO/TiO2 catalysts and how these phases influence the activity in the oxidative coupling of 4-methylpyridine to 4,4'-dimethy1-2,2'-bipyridine were investigated. The anatase and rutile concentrations were systematically varied by heat treating a nearly amorphous nanoparticle titania at temperatures between 105 and 800 degrees C. X-ray diffraction (XRD) measurements confirmed that the anatase concentration increased with heat treatment temperature until rutile began to form at 600 degrees C, and at 800 degrees C rutile TiO2 was the only phase observed. X-ray photoelectron spectroscopy (XPS) analysis of the catalysts revealed strong PdO-TiO2 interactions which increased with support treatment temperature. According to XPS, the interactions yielded electron-deficient PdO species (Pds* with 6 >2) and Ti3+ species (likely TiO2 with oxygen vacancies) on the surface of the catalysts, as a result of electron transfer from palladium to titania. These PdO-TiO2 interactions increased with increasing anatase phase in the TiO2 support and the turnover number (TON: number of product molecules formed per surface palladium site) followed the same trend. The strongest interactions (the largest amounts of Pds* with 6 >2 and Ti3+ species) were observed for the catalysts supported on rutile TiO2, which is also the catalyst with the highest TON, revealing the importance of electron transfer from palladium to titania in this reaction. Despite the high TON for PdO on rutile TiO2, the anatase-supported PdO gave a significantly higher product yield due to the very low surface area of rutile TiO2. (C) 2015 Elsevier B.V. All rights reserved.