The Impact of Immobilization on Pt/Metal-Organic Framework-Based Electrocatalysis: An NMR Study

A commercial catalyst (Pt/Vulcan XC 72R) was transformed into a selective electrocatalyst for alcohol oxidation by functionalization of Pt particles with membrane-like metal-organic framework (MOF) coatings of UiO-66 and UiO-66-NO2 materials. The electrochemical performance of the MOF-coated electrocatalysts was evaluated using cyclic voltammetric studies in alkaline electrolyte for various alcohols (methanol, ethanol, n-propanol, and n-butanol). The coating modulates the catalytic selectivity, depending on the surface polarity of the MOF material favoring the oxidation of more hydrophilic or more hydrophobic molecules. The adsorption state of alcohols in MOFs was investigated at the molecular level by solid-state NMR experiments. C-13 MAS NMR spectroscopy was applied to UiO-66 and UiO-66-NO2 loaded with aqueous solutions of isotope-enriched alcohols. To quantitatively detect C-13 MAS NMR spectra, single pulse (SP) excitation was used. Cross-polarization was applied to investigate the CP buildup behavior of alcohols in MOFs at different contact times in order to characterize the adsorption state, i.e., the mobility of alcohols in the pores. H-1-C-13 HETCOR spectra were measured to further characterize the adsorption complexes of alcohols in UiO-66. The experiments demonstrated that the electrocatalytic selectivity originates from the differences in the adsorption strength of primary alcohols in the pores of the MOFs.