(265g) First Principles Analysis of Selectivity and Durability of Pt-Based Alloys for Light Alkane Dehydrogenation

Exploitation of shale gas resources necessitates the development of new catalytic techniques to efficiently convert the ethane and propane available in the shale gas to value-added fuels and chemicals. A central catalytic technology in the conversion of such light alkanes is catalytic dehydrogenation, which produces alkenes that are, in turn, the key building blocks for many other fuels and chemicals. Pt-based bimetallic alloy nanoparticles (NPs) have been shown to be effective catalysts for this chemistry and have demonstrated higher selectivity to alkenes than pure Pt in numerous experimental studies¹. Although the improvements in selectivity are attributed to an interplay of electronic and geometric effects due to the introduction of alloying atoms, the mechanistic details of this improvement remain unclear. To probe these details, and to unravel the molecular pathways by which catalysts deactivate via coking, a detailed mechanistic study is necessary.

In this work, periodic density functional theory (DFT) calculations are employed to elucidate the catalytic properties of pure Pt NPs and their alloy counterparts for propane dehydrogenation (PDH). We begin with the comparisons between a pure Pt catalyst and a Pt₃Mn alloy, since the latter demonstrates a reasonably higher selectivity to propylene production than many other alloys in our experimental collaborators' study². An extensive reaction network is analyzed, including both propylene production and multiple side reactions such as C-C bond breaking, deep dehydrogenation, and coke formation. For the latter process, a critical nucleus model of graphene is used to estimate the rate of coke nucleation, and lattice matching technique enables an accurate estimation of the most stable binding configuration of coke-like precursors on a NP surface³.

Analysis of the C₂ and C₃ species on pure Pt(111) and Pt₃Mn(111) surfaces shows that the molecules adsorb more weakly on the alloy surface than on pure Pt. The energy difference between propylene desorption and the undesired dehydrogenation of propylene is more pronounced for the alloy surface, suggesting that the desorption step is more favorable. The smaller Pt ensemble in the alloy surface leads to a higher activation barrier of C-C bond breaking for all C₂ and C₃ species. These results are consistent with the higher selectivity to propylene, using Pt₃Mn alloy NP, that is observed in our experiments². The coking/graphene island analysis, in turn, demonstrates that graphene can bind to Pt(111) surface with both zigzag and armchair geometries. The stability of the binding configurations is driven by a moiré pattern, of which different cuts generate stable edge structures. Finally, the combined kinetic and coking analyses are incorporated into a microkinetic study to identify rate- and selectivity-determining steps on both Pt(111) and the Pt₃Mn alloy.

References:

1. Jesper J. H. B. Sattler et al. “Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides”. In: Chemical Reviews 114.20 (2014), pp. 10613–10653. DOI: 10.1021/ cr5002436. URL: https://doi.org/10.1021/cr5002436.
2. Zhenwei Wu et al. “Changes in Catalytic and Adsorptive Properties of 2 nm Pt3Mn Nanoparticles by Subsurface Atoms”. In: Journal of the American Chemical Society 140.44 (2018), pp. 14870–14877. DOI: 10.1021/jacs.8b08162. URL: https://doi. org/10.1021/jacs.8b08162.
3. Souheil Saadi et al. “On the Role of Metal Step-Edges in Graphene Growth”. In: The Journal of Physical Chemistry C 114.25 (2010), pp. 11221–11227. DOI: 10 . 1021 / jp1033596. URL: https://doi.org/10.1021/jp1033596.