The electronic structure of a catalyst, and thereby its performance, can be tuned via several known avenues, including particle morphology, particle geometry, and catalyst coverage. In heterogeneous catalysis, controlling the charge transfer between a semiconducting catalyst support and its metal active phase offers an additional strategy to tailor a metal catalyst’s behavior. These electronic metal-support interactions (EMSI) remain virtually unexplored for carbon supports due to complexities in their nanoscale structure. This work serves to prove that, carbon supports exhibit EMSI that alter the nature of the metal’s active sites in Pd/graphene systems. Additionally, such interactions yield catalysts with a series of heterogeneous metal sites, in contrast with what is presented by Langmuirian models. Stacked cup carbon nanotube supports were functionalized using nitric acid, followed by thermal defunctionalization in flowing nitrogen at varying temperatures to obtain a range of functionalized supports. Pd nanoparticles were subsequently added via incipient wetness impregnation using an aqueous solution of palladium nitrate. The obtained catalyst/support systems were tested for the hydrogenation of cinnamaldehyde, revealing shifts in selectivity depending on the carbon support. Plane wave density functional theory calculations were coupled with experimental work to compare binding energies of probe adsorbates (H*, O*, and CH*) on Pd on a pristine graphene support and Pd on graphene functionalized with oxygen and hydroxide. These tests revealed that smaller Pd particles (~100 atoms) are influenced by the support, showing shifts in adsorbate binding energies of ~50 kJ mol−1 from the pure graphene to the functionalized graphene. Binding energies for the adsorbates were also compared on the top and side of Pd particles, showing shifts in adsorbate binding energy of ~10 kJ mol−1 for both graphene supports. Furthermore, the hydrogenation of but-2-enal was similarly modeled on the top and side of a 4-layer, 119 atom Pd particle, revealing similar trends in energetic shifts. The average intrinsic barrier for H-addition to but-2-enal decreased by 8 kJ mol−1 when modeled on the side of the Pd particle supported on functionalized graphene. This decrease in activation barrier for reactions occurring near the Pd/graphene interface represents an increase in reactivity for these sites, as demonstrated by the shifts in binding energies for the adsorbates studied.

Elucidating electronic metal-support interactions for Pd-graphene systems

2019 Spring National Meeting

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