552132 Density Functional Theory Analysis of Propane Dehydrogenation on Palladium Alloys

Monday, June 3, 2019: 10:15 AM
Republic ABC (Grand Hyatt San Antonio)
Ranga Rohit Seemakurthi, Yinan Xu, Brandon C. Bukowski, Fabio H. Ribeiro and Jeffrey Greeley, Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN

Alkane dehydrogenation has the potential to convert large amounts of ethane and propane available in shale gas to more valuable alkenes and hydrogen. Bimetallic alloy catalysts have been used for this purpose, as they have been found to achieve higher selectivity than their pure metal counterparts.1 In addition to the industrially used PtSn catalyst, work by our collaborators have shown that various other Pt, Pd alloys (In, Zn) are also highly selective and active for propane dehydrogenation (PDH).2,3 This improvement in performance has been attributed to the electronic and geometric effects due to alloying, but the exact mechanistic details remain poorly understood.

In the present study, first-principles periodic density functional theory (DFT) calculations, in conjunction with microkinetic modelling, are used to elucidate the characteristics of nanoparticle alloys that underlie their selectivity and improvement in turnover rates. We focus, in particular, on PdIn alloys, for which the experimental results from our collaborators have shown to have high selectivity to propylene. We additionally investigate reaction pathways and catalyst structures that lead to deactivation and coke formation, and we leverage these insights to point towards electronic and geometric descriptors for catalyst activity, selectivity and stability trends across a wider space of alloy catalysts.

We describe a comprehensive thermodynamic analysis on pure Pd (111) and PdIn (110) alloys, which demonstrates that the binding of all of the reaction intermediates is weaker on the alloy compared to the pure metal. Comparison of desorption energy with the dehydrogenation barrier, the proposed selectivity descriptor on Pt alloys,4 suggests that the desorption of propylene is far more favorable than dehydrogenation on the PdIn alloy compared to Pd. These results point to an explanation for the measured selectivity of the alloy. To complement the thermodynamic analysis, we also calculate activation barriers for the elementary reactions (C-H and C-C bond breaking), including deep dehydrogenation pathways of propylene. We include these results in a comprehensive microkinetic model, and we use this model to predict likely coverages of spectator coking species on the pure metal and alloy surfaces on both terraces and step defects. We then evaluate the effect of these spectator species on the thermodynamic and kinetic barriers of the reaction mechanism, and we discuss how the results can be used to formulate descriptors for subsequent catalyst screening studies.

References:

  1. Sattler, J. J. H. B., Ruiz-Martinez, J., Santillan-Jimenez, E. & Weckhuysen, B. M. Catalytic Dehydrogenation of Light Alkanes on Metals and Metal Oxides. Chem. Rev. 114, 10613–10653 (2014).
  2. Wegener, E. C. et al. Structure and reactivity of Pt–In intermetallic alloy nanoparticles: Highly selective catalysts for ethane dehydrogenation. Catalysis Today 299, 146–153 (2018).
  3. Wu, Z. et al. Pd–In intermetallic alloy nanoparticles: highly selective ethane dehydrogenation catalysts. Catalysis Science & Technology 6, 6965–6976 (2017).
  4. Nykänen, L. & Honkala, K. Selectivity in Propene Dehydrogenation on Pt and Pt3Sn Surfaces from First Principles. ACS Catal. 3, 3026–3030 (2013).

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