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

Uncertainty
analysis, reported experimental literature data, and
density functional theory were synthesized to model the effect of
surface tin coverage on platinum-based catalysts for nonoxidative
propane dehydrogenation to propylene. This study tests four different
platinum–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 corresponding
pure Pt surface sites using an uncertainty analysis methodology that
uses BEEF-vdW with its ensembles (BMwE) to generate the uncertainty
for the energies of the intermediates and transition states. One experimental
data set with two experimental observations, selectivity to propylene
and turnover frequency of propylene, was used as a calibration data
set to evaluate the impact of the experimental data on informing the
models. This study finds that the prior model for Pt3Sn/Pt(100)
is the most active and Pt2Sn/Pt(211) is the most selective
toward propylene. Active sites on the (100) facet have the highest
probability of being responsible for C1 and C2 product formations (C–C bond cleavage). Increasing the Sn
coverage on the (100) surface facet to a PtSn/Pt(100) active site
leads to a significantly reduced rate and might explain the experimentally
observed higher selectivity of Sn-doped catalysts relative to pure
Pt catalysts. Next, this study finds that for all surfaces, except
PtSn/Pt(100), the rate-controlling steps are the initial dehydrogenation
steps alongside some partially rate-controlling second dehydrogenation
steps. For PtSn/Pt(100), only the initial terminal dehydrogenation
step to CH3CH2CH2* and second dehydrogenation
steps are rate-controlling. Next, the calibrated models for all surfaces
were found to be selective toward propylene production and model the
reported turnover frequency successfully. Nevertheless, Pt2Sn/Pt(211) emerges as the active site with some (minor) evidence
as the main active site based on Jeffreys’ scale interpretation
of Bayes factors. This observation agrees with prior studies that
also found step sites to be most likely the most relevant active sites
for pure Pt catalysts. Overall, the results indicate that tin, in
addition to affecting the binding strength of the adsorbed species,
prevents deeper dehydrogenation (reducing coking) and cracking reactions
through increasing activation barriers for unwanted side reactions.