Assessment of metal-metal interactions and catalytic behavior in platinum-tin bimetallic subnanometric clusters by using reactive characterizations

Abstract

Several reactive characterizations were used to study the effect that reduction duration of PtSn-K/MFI has on hydrocarbon catalysis, including propane dehydrogenation, propane hydrogenolysis, and ethane H/D exchange. Propane dehydrogenation results demonstrate that increased reduction time causes the PtSn-K/MFI to have higher selectivity towards propylene, lower rates of catalyst deactivation, and lower rates of dehydrogenation. Consistent with higher selectivity towards propylene with longer catalyst reduction times, slower rates of hydrogenolysis are also observed over a large range of H2:propane ratios. H/D exchange experiments to assess rates of C–H activation were carried out on PtSn-K/MFI by using ethane and D2, which demonstrate that longer reduction time significantly inhibits the rate of C–H activation on PtSn-K/MFI. The substantial effects observed by using reactive characterizations were difficult to be interpreted by more advanced characterization techniques such as EXAFS, because the structural changes of the PtSn nanoclusters are quite subtle. Indeed, in a previous report on the PtSn-K/MFI material, Pt and Sn EXAFS were statistically indistinguishable with different reduction times, illustrating that combining reactive characterizations, particularly with the application of H/D exchange, provides the ability to differentiate fundamental effects that impact catalyst performance. The importance of C–H activation was confirmed by isotopic experiments for propane dehydrogenation, which indicate that the first C–H activation step is rate-limiting during dehydrogenation, with a kinetic isotope effect of ∼2.5. These results together show that increased reduction time gives a catalytic material with superior catalytic properties (i.e. higher propylene selectivity and lower deactivation rates), and this is proposed to be due to increased platinum-tin interactions, which modifies and weakens hydrocarbon adsorption, thereby limiting strongly adsorbed intermediates that are precursors to hydrogenolysis products and coke.