Investigation of Potential Active Sites for the Methane Oxidation Reaction on Pd/SSZ-13

It is widely reported that palladium oxide is the active site for methane oxidation on Pd-zeolite catalysts¹, yet there is a lack of understanding of the morphology of the active site and how it interacts with the zeolite structure on the molecular level. This theoretical study aims to identify possible active sites that may exist within Pd/SSZ-13 on the molecular level. A model with two aluminum atoms per unit cell has been chosen to simulate a Si/Al ratio of 17. To ensure that realistic active site candidates are found, a sequence of ordered calculations are performed to yield stable/low energy structures with two aluminum atoms. First, 25 geometrically distinct dual aluminum placements on SSZ-13 are investigated and the lowest energy structure is found (Fig 1A). From this lowest energy aluminum arrangement, all possible dual palladium arrangements are then investigated to find the lowest energy palladium structure. From this final structure, oxygen atoms and palladium atoms are then added to form numerous Pd_xO_x species that may serve as active sites. Lastly, dissociative methane adsorption is investigated for each site to further evaluate it for methane activation (Fig 1B). All geometries are optimized using the PBE functional. Identifying the active sites in this work is critical in the design of zeolite catalysts for the combustion of methane.

Figure 1: (A) Relative energies of 25 distinct dual aluminum placements on SSZ-13 versus distance between aluminum atoms (energies referenced to the lowest energy structure). PBE optimized structures evaluated at single point with HSE06 functional for energy. (B) Dissociative adsorption of methane on a ‘bridged’ dual palladium oxide site.

References

1. Gannouni, A.; Albela, B.; Zina, M. S.; Bonneviot, L., Applied Catalysis A: General 2013, 464-465, 116-127.