Modeling the Effect of Surface Platinum-Tin Alloys on Propane Dehydrogenation on Platinum-Tin Catalysts

Uncertaintyanalysis, reported experimental literature data, anddensity functional theory were synthesized to model the effect ofsurface tin coverage on platinum-based catalysts for nonoxidativepropane dehydrogenation to propylene. This study tests four differentplatinum-tin skin surface models as potential catalytic sites,Pt3Sn/Pt(100), PtSn/Pt(100), Pt3Sn/Pt(111),and Pt2Sn/Pt(211), and compares them to the correspondingpure Pt surface sites using an uncertainty analysis methodology thatuses BEEF-vdW with its ensembles (BMwE) to generate the uncertaintyfor the energies of the intermediates and transition states. One experimentaldata set with two experimental observations, selectivity to propyleneand turnover frequency of propylene, was used as a calibration dataset to evaluate the impact of the experimental data on informing themodels. This study finds that the prior model for Pt3Sn/Pt(100)is the most active and Pt2Sn/Pt(211) is the most selectivetoward propylene. Active sites on the (100) facet have the highestprobability of being responsible for C-1 and C-2 product formations (C-C bond cleavage). Increasing the Sncoverage on the (100) surface facet to a PtSn/Pt(100) active siteleads to a significantly reduced rate and might explain the experimentallyobserved higher selectivity of Sn-doped catalysts relative to purePt catalysts. Next, this study finds that for all surfaces, exceptPtSn/Pt(100), the rate-controlling steps are the initial dehydrogenationsteps alongside some partially rate-controlling second dehydrogenationsteps. For PtSn/Pt(100), only the initial terminal dehydrogenationstep to CH3CH2CH2* and second dehydrogenationsteps are rate-controlling. Next, the calibrated models for all surfaceswere found to be selective toward propylene production and model thereported turnover frequency successfully. Nevertheless, Pt2Sn/Pt(211) emerges as the active site with some (minor) evidenceas the main active site based on Jeffreys' scale interpretationof Bayes factors. This observation agrees with prior studies thatalso found step sites to be most likely the most relevant active sitesfor pure Pt catalysts. Overall, the results indicate that tin, inaddition to affecting the binding strength of the adsorbed species,prevents deeper dehydrogenation (reducing coking) and cracking reactionsthrough increasing activation barriers for unwanted side reactions.