Role of Different Active Sites in Heterogeneous Alkene Hydrogenation on Platinum Catalysts Revealed by Means of Parahydrogen-Induced Polarization

Substantial NMR signal enhancements provided by parahydrogen-induced polarization (PHIP) are associated with the ability of a catalyst to incorporate both H atoms of a dihydrogen molecule into the same product molecule. Therefore, PHIP can provide valuable information about the mechanisms and kinetics of catalytic hydrogenation reactions as well as produce hyperpolarized molecules for sensitivity enhancement in NMR. In this work, the PHIP technique was applied to study the structure sensitivity and the support effects on the degree of pairwise H 2 addition in propene hydrogenation over supported platinum catalysts. Four series of Pt catalysts supported on Al 2 O 3, SiO 2, ZrO 2, and TiO 2 were examined. A nontrivial dependence of the selectivity toward pairwise H 2 addition on the Pt particle size was found. Its analysis indicates that at least three types of different active sites coexist on the catalysts surface. Among them, the major one is responsible for the nonpairwise H 2 addition to the double bond, whereas pairwise addition can proceed on the other two minor active sites. An explanation of the nature of these active sites is proposed. A substantial increase in the pairwise addition selectivity was found for Pt/TiO 2 catalysts as compared to other catalyst series, possibly due to a strong metal–support interaction taking place even after low temperature catalyst reduction.