Kinetic data are presented for the electrooxidation of aqueous
solution carbon monoxide to carbon dioxide
on two monocrystalline gold surfaces, Au(210) and (110), with the
objective of elucidating the reaction
mechanism, especially regarding the nature of adsorbed
intermediate(s). Tafel plots (i.e., log rate
versus
electrode potential) were obtained by means of linear sweep
voltammetry, particularly as a function of the
solution reactant concentration and over a wide range (0−13.5) of the
electrolyte pH. Under most conditions,
the reaction order in CO was found to be near unity, as anticipated
from the low coverages of adsorbed CO
ascertained from infrared spectroscopy. Interestingly, the log
rate−pH dependence observed on both surfaces
display three distinct regions. At low (≤2) and higher (≥4) pH
values, essentially unit slopes were obtained
(i.e., a unity reaction order in [OH-]), these regions
being separated by one displaying apparently
pH-independent kinetics. The potential region over which
conveniently measurable electrooxidation kinetics
occur lies substantially (ca. 0.8 V) below the onset of gold surface
oxidation throughout the entire pH range.
The pH-dependent kinetic behavior is consistent with a reaction
pathway featuring the involvement of an
adsorbed hydroxycarbonyl intermediate. While such intermediates
have been identified in a number of metal
complex-catalyzed CO oxidations in homogeneous solution, they
apparently have not been considered
previously for such electrocatalytic processes. The observed unity
hydroxide reaction order at higher pH
values is indicative of a rate-determining step (rds) involving
OH- discharge onto adsorbed CO sites to form
the hydroxycarbonyl species, while the apparent transition to
zero-order kinetics at lower pH is consistent
with water rather than OH- becoming the preferred
reactant. This picture is supported by solvent
isotope
measurements which display the onset of a substantial H/D isotope
effect below pH 4, signaling the occurrence
of proton transfer within the rds. The emergence of another
pH-dependent reaction pathway at the lowest
pH values is attributed to a rds involving hydroxycarbonyl
decomposition to form CO2. The
mechanistic
opportunities provided by the analysis of electrocatalytic
rate−potential data over wide pH ranges are pointed
out, along with the possibility that the proposed hydroxycarbonyl
pathway occurs for a wide range of related
processes on transition-metal surfaces.
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