(349ak) Methane Oxidation on Pd/SSZ-13 Active Sites: A Computational Study

Pd-based catalysts supported on various zeolite materials have gained recent interest for the application of low-temperature methane oxidation due to their good thermostability, hydrophobicity, and tunable features. On SSZ-13 specifically, there exist few theoretical studies for methane oxidation. This theoretical work aims to determine the methane oxidation mechanism over active sites within Pd/SSZ-13. First a wide variety of Pd_xO_y active sites were created and their free energies of formation were evaluated to determine which sites are most likely to be formed as shown in Fig. 1a using a modified Langmuir competitive adsorption model.¹ A likely active site which is both stable at low temperatures and activates methane is ZPdOPdZ, a bridge type active site. From the chosen active sites, an extensive oxidation mechanism was derived which includes gas phase adsorption and desorption events, Mars-Van Krevelen steps, active site regeneration (through O₂ adsorption), and possible byproducts of the methane oxidation reaction. Transition state searches were conducted using the climbing nudged elastic band method (cNEB)² as shown in Fig. 1b. Various possible active sites and different reaction pathways will be discussed and their feasibility analyzed at different temperatures and conditions.

References:

1 C. Horvatits, D. Li, M. Dupuis, E. A. Kyriakidou and E. A. Walker, J. Phys. Chem. C, 2020, 124, 7295–7306.

2 G. Henkelman, B. P. Uberuaga and H. Jónsson, J. Chem. Phys., 2000, 113, 9901–9904.

Figure 1 (below): (a) Estimated coverage of bridged Pd₂O_x active sites on SSZ-13 at altering temperature and O₂ volume. Fractional coverages estimated using a modified Langmuir competitive adsorption model. (b) Transition state search of methane activation using the cNEB method of dissociative adsorption of methane on a bridged PdO₂Pd active site.