Propane dehydrogenation (PDH) is one of the most promising technologies for on-purpose propylene production. Pt-based catalysts, while active for this process, require periodic regeneration and redispersion to reverse carbon accumulation and nanoparticle sintering. Atomic layer deposition (ALD) of alumina atop Pt/Al₂O₃ catalysts has been shown to improve catalyst stability through mitigation of Pt sintering. These overcoats are also observed to enhance propylene selectivity through a mechanism that remains unknown and not widely studied. C₃H₆-TPD-MS was performed on samples with increasing ALD cycles and revealed that propylene desorbs not only as hydrocarbon species, but also in the form of CO_x and water from a reduced catalyst. Further, in Temporal Analysis of Products (TAP) experiments pulsing nanomole quantities of propane, we found that small amounts of oxygen cannot be removed by hydrogen from the gas phase but are accessible to the adsorbed hydrocarbon. For an atomistic mechanistic understanding, we conducted first principles Density Functional Theory (DFT) calculations to simulate a modified γ-Al₂O₃/Pt configuration, mimicking the ALD alumina over Pt/Al₂O₃ catalysts. Our simulations of the modified γ-Al₂O₃/Pt, featuring exposed γ-Al₂O₃ edges for PDH, reveal significant interactions between hydrocarbons and oxygen atoms along these edges arising from the undercoordination of oxygen atoms along the edges. TAP experiments demonstrate that prior to the onset of propylene production, a small amount of surface oxygen must be removed through CO_x formation. The amount was observed to decrease with increasing ALD-modification, indicating that ALD may be decreasing the perimeter at the metal/alumina interface. The transient probe molecule characterizations presented here provide a quantitative assessment of the changes in PDH kinetics with the addition of alumina ALD overcoats. These results are used to rationalize performance data collected in a bench-scale flow reactor under steady state conditions.

Brooklyne Thompson

Appearances