Propane Dehydrogenation on Pt_xZn_y Active Sites in Silicalite-1

The improvement of Pt-based catalysts for propane dehydrogenation (PDH) has progressed by recent investigations that have identified Zn as a promising promoter for Pt subnanometer catalysts. It is desirable to gain insights into the structure, stability, and activity of such active sites and the factors that influence them, such as Zn : Pt ratio, Pt coordination and nuclearity. Here, we employ density functional theory and microkinetic simulations to investigate the stability of Pt_xZn_y (x=1-3, y=0-3) active sites grafted on silanols of Silicalite-1 and the PDH activity of Pt. We find that the coordination of a Pt atom to a nest of grafted Zn(II) atoms increases the stability of the Pt₁Zn_y sites, whose activity is similar for y=0-2 and drops dramatically for y>2. We further demonstrate, via linear scaling relations and microkinetic simulations, that the turnover frequency obeys a volcano law as a function of propylene binding strength. The Pt₂Zn₁ and Pt₃Zn₁ sites are stable and exhibit activity similar to Pt₁Zn₂, but only Pt₁Zn₂ manifests reaction kinetics consistent with experimental data, strongly suggesting the active site composition in the synthesized catalyst samples. The methodology presented here suggests a general strategy for deducing active site information such as composition through simple kinetic experiments.