Pt, Pd, and Rh Nanoparticles Supported on Polydopamine Nanospheres as Catalysts for Transfer Hydrogenolysis

Sub-5 nm nanoparticles of the palladium-group platinum-group (PPG) noble metal elements, specifically Pt, Pd, and Rh, dispersed on solid-state support materials have been of particular interest to the heterogeneous catalysis community because they play indispensable roles in catalyzing a library of industrially important molecule-transforming processes. Although Pt, Pd, and Rh share a lot of similarities in terms of crystalline structures and physicochemical properties, they may behave differently during their nucleation and growth on the surface of a support material. Here, we systematically compare the nucleation and growth behaviors of Pt, Pd, and Rh nanocrystals on the intrinsically adhesive surfaces of polydopamine supports in a capping-ligand-free reaction environment. Through polydopamine-mediated metal deposition, a submonolayer of Pt, Pd, and Rh nanocatalysts with narrowly distributed sizes in the sub-5 nm range can be deposited and uniformly dispersed onto the surfaces of colloidal polydopamine supports. We choose the catalytic transfer hydrogenolysis of methyl orange with formic acid and formate serving as the hydrogen donors as a model reaction system for detailed kinetic studies, which enables us to shed light on not only the mechanistic multiplexity of catalytic transfer hydrogenolysis reactions but also the composition-dependent multimodal catalytic behaviors of nanoparticulate PPG metals. Upon alteration of the PPG metal elements or the hydrogen-donating species, the reactions selectively follow either the Langmuir-Hinshelwood or the Eley-Rideal mechanism, while the hydrogen donors may interswitch among cooperative, noncooperative, and anticooperative surface-adsorption modes when interacting with the metal nanocatalyst surfaces. The reaction pathway selection and the surface adsorption cooperativity are found to be two key factors that synergistically determine the overall kinetic features of the catalytic reactions.