The Impact of Geometric and Surface Electronic Properties of Pt-Catalysts on the Particle Size Effect in Electrocatalysis

Abstract

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.

The role of adsorbed hydroxyl species in the electrocatalytic carbon monoxide oxidation reaction on platinum

“…Further papers by Mayrhofer et al 38,39 highlighting the role of defects in the particle size effect of Pt nanoparticles also suggest that the catalytic activity for CO ads oxidation is 25 predominantly influenced by the ability of the Pt surface to dissociate water and form OH ads on defect sites, with the onset of CO ads oxidation seen at approximately 250 mV, suggesting the formation of OH ads at 250 mV. We repeated these experiments for our system and measured a 30 potential of zero total charge of 234 ±5 mV for polycrystalline platinum in 0.5 mol dm -3 sulphuric acid, which is less than the value of ~285 mV reported by Mayrhofer et al for polycrystalline Pt in 0.1 mol dm -3 perchloric acid solution 35 . This is not surprising considering the increased effect of anion 35 adsorption in sulphuric acid compared to perchloric acid.…”

“…7 in 35 ). 110 Although the onset of inhibition of the ORR in this example 35 does not begin to occur until 700 mV this does not necessarily…”

“…This is used by electrochemists to describe the lowest potential at which anions begin to predominate over cations on a surface. Bockris and Argade 17 35 reviewed the determination of the pzc on solid metals, including platinum, gold, silver, and nickel in 1969. They assessed the pzc, both experimentally and by comparing a large number of authors using 3 different techniques; adsorption in the double layer of a neutral organic compound 40 as a function of the concentration of the supporting electrolyte; double-layer capacitance as a function of potential; and static friction between surfaces in solution.…”

“…For a thorough review of this subject the reader is referred to Markovic et al 7 . In contrast, a number of studies of the formation of oxides on 30 platinum assume that the onset of formation of oxygen containing species on polycrystalline or nanostructured Pt does not occur until above 700 mV [18][19][20][21][22][23] and some have even claimed that it has been 'unambiguously proven' 21 that the initial oxide growth on Pt leads directly to the formation of 35 PtO 20,21,23 , contrary to others who suggest it is via the formation of OH ads 22 . This is despite the fact that it is known that the formation of OH ads on Pt(111) and Pt(100) takes place in wide potential regions positive to the potential of zero charge of these surfaces but significantly less positive than the 40 threshold for the formation of the surface oxide phases 24 .…”

“…7 in 35 ). 110 Although the onset of inhibition of the ORR in this example 35 does not begin to occur until 700 mV this does not necessarilyThis journal is © The Royal Society of Chemistry [year] mean that there is no hydroxyl present below these potentials. Indeed, OH ads may be an intermediate in the oxygen reduction reaction.…”