Size effects of Pt-Re bimetallic catalysts for glycerol hydrogenolysis

Abstract

A series of Pt-Re/CNTs catalysts with different particle sizes have been synthesized and applied in glycerol hydrogenolysis to elucidate the size effects. The trend of turnover frequency (TOF) for these different sized Pt-Re/CNTs catalysts follows a volcanic curve, in which the TOF for Pt-Re/CNTs catalyst with particle size of 1.9 nm is ca. 7.5 times higher than that of 4.9 nm. X-ray photoelectron spectroscopy analysis revealed that the surface Pt/Re ratio decreased with the decrease of particle size. The enrichment of surface rhenium species on smaller particles probably led to the increase in the surface acidity, which could be one reason for the enhanced activity over smaller sized Pt-Re/CNTs catalysts. However, too small sized Pt-Re/CNTs catalyst (e.g., 1.5 nm) suffered from severe coking, resulting in the lower activity. Moreover, the hydrogenolysis of different substrates (glycerol, 1,2-propanediol and 1,3-propanediol) over different sized Pt-Re/CNTs catalysts was also investigated. It was revealed that the reactivity declined in the following order: glycerol > 1,3-propanediol > 1,2-propanediol, and the cleavage of secondary C–O bond was favored over larger sized Pt-Re/CNTs catalyst. Based on these results, a possible reaction pathway depending on Pt-Re particle size was proposed.