(462b) Wireless Potentiometry of Thermochemical Heterogeneous Catalysis

Electrochemical potential is a key determinant of interfacial reactivity that is readily measured and controlled in electrochemical systems but is rarely considered in thermochemical catalysis. Recently, however, mechanistic investigation of heterogeneous redox catalysis has demonstrated that (i) electrode-bound thermochemical catalysts spontaneously polarize in liquid media and (ii) the native electrochemical potential of these catalysts has a significant bearing on reactivity and selectivity. The extension of these learnings to conventional thermochemical catalysis is complicated by the electronically-insulating nature of many ubiquitous catalyst supports (e.g. SiO₂, Al₂O₃, MFI, etc.). To overcome this impasse, we establish a strategy for operando measurement of the electrochemical potential of metal catalysts supported on non-conductive materials through use of a molecular redox sensor which acts as an electron shuttle to electronically connect “unwired” catalyst particles with a sensing probe electrode. This strategy is demonstrated to be general and measures the catalyst potential, E_cat, within ~2 mV irrespective of the identity of the metal (Pt, Pd, Au), support (SiO₂, Al₂O₃, TiO₂), solvent (H₂O, CH₃CN), or solution acidity (pH = 0 – 14). Moreover, wireless potentiometry readily senses potential in either oxidizing (E_cat ~ 0.7 V vs RHE) or reducing (E_cat ~ 0 V vs RHE) reaction conditions—engendered by feed of O₂ or H₂ respectively. We demonstrate the utility of these developments for operando catalyst characterization and mechanistic analysis through study of the oxidative dehydrogenation (ODH) of formic acid (HCOOH) on dispersed powder catalysts comprised of 1 wt% Pt supported on SiO₂, Al₂O₃, or TiO₂. The catalyst potential, E_cat, of each material varies by ~100 mV at identical reaction conditions and displays unique transient and steady-state dependencies on reactant activities and the rate of ODH catalysis. These observations are highly enabling for functional catalyst characterization and are impossible to detect through conventional thermochemical analyses alone.