Unconventional promoters of catalytic activity in electrocatalysis

The classic models of metal electrode-electrolyte interfaces generally focus on either covalent interactions between adsorbates and solid surfaces or on long-range electrolyte-metal electrostatic interactions. Here we demonstrate that these traditional models are insufficient. To understand electrocatalytic trends in the oxygen reduction reaction (ORR), the hydrogen oxidation reaction (HOR) and the oxidation of methanol on platinum surfaces in alkaline electrolytes, non-covalent interactions must be considered. We find that non-covalent interactions between hydrated alkali metal cations M(+)(H(2)O)(x) and adsorbed OH (OH(ad)) species increase in the same order as the hydration energies of the corresponding cations (Li(+) > Na(+) > K(+) > Cs(+)) and also correspond to an increase in the concentration of OH(ad)-M(+)(H(2)O)(x) clusters at the interface. These trends are inversely proportional to the activities of the ORR, the HOR and the oxidation of methanol on platinum (Cs(+) > K(+) > Na(+) > Li(+)), which suggests that the clusters block the platinum active sites for electrocatalytic reactions.

The particle size effect on the formation of OH adlayer, the CO bulk oxidation, and the oxygen reduction reaction (ORR) have been studied on Pt nanoparticles in perchloric acid electrolyte. From measurements of the CO displacement charge at controlled potential, the corresponding surface charge density versus potential curves yielded the potentials of total zero charge (pztc), which shifts approximately 35 mV negative by decreasing the particle size from 30 nm down to 1 nm. As a consequence, the energy of adsorption of OH is more enhanced, that is, at the same potential the surface coverage with OH increases by decreasing the particle size, which in turn affects the catalytic reactions thereon. The impact of the electronically induced potential shift in the OH adsorption is demonstrated at the CO bulk oxidation, in which adsorbed OH is an educt species and promotes the reaction, and the ORR, where it can act as a surface site blocking species and inhibits the reaction.