Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammograms. Part 1: Pt(111)

Abstract

Combined CO-stripping cyclic voltammetry and FT-IR spectroscopy measurements have allowed us to monitor changes in the coverage and structure of CO adlayers on Pt(1 1 1) electrodes in 0.1 M H 2 SO 4 as a function of potential. Our results show that in CO-free solutions the maximum coverage is h CO = 0.68 and that higher coverages can only be achieved in the presence of CO in the solution. Saturation coverages can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, E d ) is more negative than 0.30 V vs. RHE. The lowest CO coverage at which hydrogen adsorption on the Pt(1 1 1) electrode is completely blocked is h CO = 0.63, which corresponds to an E d = 0.50 V vs. RHE. Our results suggest that the process at the pre-peak (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of adsorbed CO by reaction with oxygenated species nucleating at steps, the main peak appearing when nucleation of oxygenated species at the terraces also occurs.

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

“…The coverage of the saturated CO ads adlayer relative to the H upd layer was calculated to be 0.676 with a 50 standard deviation of 1.2% over 5 experiments; assuming a charge density of 210 C cm -2 for a monolayer of adsorbed hydrogen 34 . This is in good agreement with similar experiments in the literature for Pt(111) electrodes, for which the maximum coverage in CO-free 0.1 mol dm -3 H 2 SO 4 was 55 shown to be 0.68 42 . During CO ads oxidation experiments it is also important to correct for any residual oxygen reduction 59 .…”

“…The early onset is exhibited in potential sweep experiments by both a pre-peak and a lower peak potential of the main oxidation peak compared to adlayers prepared in the double layer region. Currently the most popular theory to explain the early onset of 75 CO ads oxidation for CO adlayers on platinum at room temperature prepared in the potential region at which adsorbed hydrogen is present (<350 mV), is that during CO adsorption most, but not all, H ads is displaced 9, 42-47 .Oxidation of this remaining H ads produces CO-free sites which promote 80 the early onset of CO ads oxidation 9,[42][43][44][45][46][47] . This seems remarkably similar to the approach offered by Burke et al 48 .…”

“…If the diffusion of either of the species on the surface is 15 assumed to be relatively fast then the reaction can be modelled as proportional to the coverage of CO ads and OH ads known as the mean field approximation. For this approximation when OH ads formation is assumed to be fast the rate of reaction is then relative to the rate of formation of 20 OH ads 54,79 , which is effectively equal to the number of free sites, coupled with the effective diffusion rate of the CO ads 2, 12, 75, 76, 80-84 , any segregation of adsorbed species that may occur 85,86 , and any effect of the redistribution or reorganisation of the CO ads adlayer during oxidation 9,12,40,42,25 78, 80, 87 . However the mean field approximation is misleading as it assumes that OH ads will be formed spontaneously upon the adsorption of water, which is unlikely except at very high potentials (>800 mV).…”

On the behavior of CO oxidation on shape-controlled Pt nanoparticles in alkaline medium

“…The stripping voltammogram for partial CO coverages can be compared with that measured for the full CO coverage (black line). The partial stripping experiment is also shown for the kinked Pt (17,13,11) electrode in Fig. 7C.…”

“…Generally, this single CO stripping peak shifts towards lower potentials as the step density increases thus evidencing that the CO electro-oxidation is influenced by well-ordered [6] or randomly [13] distributed step/kink sites on the electrode surface. Once assumed that the surface diffusion rate of CO on (111) terrace domains is higher than the rate of CO electrooxidation reaction [14], it has been accepted that CO electro-oxidation occurs only in step/kink sites as nucleation center of oxygen-containing species required for catalyzing CO oxidation [6].…”