Surface dynamics at well-defined single crystal microfacetted Pt(111) electrodes: in situ optical studies.

The assembly and electrochemical oxidation of well-defined CO adlayers on Pt(111) microfacets in aqueous CO-saturated 0.1 M H2SO4 were examined by a combination of in situ, simultaneous, time-resolved, reflectance spectroscopy (RS) and second harmonic generation (SHG), and potential step and linear scan techniques. Optical transients were collected following potential steps from a value high enough for a full monolayer of bisulfate (theta = 0.2) to adsorb on the Pt(111) facet, E(ox), to potentials E(ads), at which either the c(2 x 2)-3CO or square root of 19 x square root of 19R23.4 degrees-13CO phase is expected to form once surface saturation is achieved. Similar experiments involving subsequent steps from E(ads) to E(ox) provided unambiguous evidence that the rates of oxidation of c(2 x 2)-3CO on such quasi-perfect Pt(111) facet at constant overpotential are much slower than those of square root of 19 x square root of 19R23.4 degrees -13CO, an effect attributed to the presence of intrinsic vacant sites within the latter, more dilute phase, which are required for oxidation of adsorbed CO to ensue. Furthermore, continuous CO adsorption-oxidation cycles were found to increase the rate of oxidation of the c(2 x 2)-3CO phase. This phenomenon was tentatively ascribed to the progressive emergence of defects along the edge of the facet (and/or within the facet itself) which serve as nucleation sites for the oxidation of adsorbed CO.

Simultaneous in situ reflectance and probe beam deflection measurements at solid electrode-aqueous electrolyte interfaces.

A method is herein described for performing simultaneous in situ normal incidence reflectance spectroscopy (DeltaR/R, lambda = 633 nm) and probe beam deflection (PBD) measurements at solid electrodes in aqueous electrolytes, while scanning the potential linearly between two prescribed limits. Results obtained for Au in 0.1 M HClO4 and for Pt in both 0.1 M HClO4 and 0.1 M NaOH were found to be in excellent agreement with those reported in the literature for each individual spectroelectrochemical technique under otherwise similar conditions. Data collected for Pt electrodes in CO-saturated 0.1 M HClO4 revealed rather sudden changes in both DeltaR/R and PBD signals in the voltammetric region where the characteristic sharp peak associated with the oxidation of adsorbed CO occurs. This behavior was ascribed, respectively, to oxide formation (DeltaR/R) and to changes in the electrolyte composition in region neighboring the electrode, involving predominantly the acid concentration (PBD). In contrast, CO oxidation on Pt in 0.1 M NaOH yielded a PBD response consistent with formation of solution-phase carbonate via the reaction of the product, CO2, with hydroxyl ion. The exquisite sensitivity of DeltaR/R and PBD to interfacial phenomena was further illustrated using a monolayer of hemin irreversibly adsorbed on glassy carbon surfaces in 0.1 M Na2B4O7 (pH approximately 9.2). For this system, DeltaR/R was found to be proportional to the relative fractions of hemin and its reduced counterpart, while the PBD signal could be correlated with corresponding variations in the electrolyte concentration induced by the surface-bound redox process.

In situ, time-resolved reflectance spectroscopy in the microsecond domain: oxidation of adsorbed carbon monoxide on polycrystalline pt microelectrodes in aqueous solutions.

The dynamics of the electrooxidation of adsorbed CO, COads, on polycrystalline Pt microelectrodes has been examined in CO-saturated 0.5 M H2SO4 and 0.5 M HClO4 aqueous solutions, using in situ, time-resolved, normalized differential reflectance spectroscopy lambda = 633 nm). Attention was focused on the unique dependence of COads oxidation on the potential at which the adsorbed full CO monolayer is assembled (i.e., hydrogen adsorption/desorption vs the double-layer region) using both fast linear scan voltammetry and potential step techniques. As evidenced from the data collected, COads oxidation at a fixed potential proceeds at slower rates when the monolayer is formed in the double- layer region compared to when it is formed in the hydrogen adsorption/desorption region. Possible explanations for this effect are discussed.

Oxidation of adsorbed CO on Pt(111) in CO-saturated perchloric acid aqueous solutions: simultaneous in situ time-resolved reflectance spectroscopy and second harmonic generation studies.

Simultaneous normalized differential reflectance spectroscopy (DeltaR/R) and second harmonic generation (SHG) has been employed to follow, independently, OH and adsorbed CO (CO(ads)) on a single Pt(111) microfacet in CO-saturated aqueous perchloric acidic solutions during voltammetric cycles, leading to the oxidation of CO(ads) and subsequent readsorption of CO on the surface. The results obtained are consistent with the disruption of the radical19 x radical19R19.1 degrees phase just prior to the oxidation of adsorbed CO.

Optical determination of differential coverage-potential relations of redox-active species immobilized on electrode surfaces.

Single-wavelength (HeNe laser, lambda = 633 nm), normal incidence UV-visible reflectance spectroscopy was used to monitor the optical properties of the glassy carbon (GC)|0.2 M NaOH(aq) interface as a function of the applied potential, E. Whereas the electroreflectance coefficient for bare GC was found to be small and potential independent, surface functionalization by an irreversibly adsorbed layer of tetrasulfonated cobalt phthalocyanine (CoTSPc) yielded a clearly defined sigmoidally shaped normalized reflectance change (DeltaR/R) vs E curve over the potential region in which the adsorbate displayed redox peaks. Assuming DeltaR/R is proportional to the extent of redox conversion, as has been reported for macrocycles adsorbed on other types of carbon (e.g., Kim, S.; Xu, X.; Bae, I. T.; Wang, Z.; Scherson, D. A. Anal. Chem. 1990, 62, 2647-2650), differential coverage-potential relations were determined based purely on the optical data collected. A similar optical behavior was found for irreversible adsorbed CoPc and tetraamino CoPc (CoTAPc) adsorbed on GC, for which the voltammetric peaks were ill-defined, too small for coulometric analyses to be reliably performed, or both. No detectable changes in the DeltaR/R vs E profiles of either bare or macrocyclic-functionalized surfaces were observed upon addition of hydrazine to the neat 0.2 M NaOH solution at potentials at which these surfaces display electrocatalytic properties for its oxidation. Possible factors responsible for this behavior are discussed.

In situ, time-resolved normal incidence reflectance spectroscopy of polycrystalline platinum microelectrodes in aqueous electrolytes.

In situ normal incidence reflectance spectra of polycrystalline Pt microelectrodes have been monitored as a function of the applied potential in aqueous 0.5 M H(2)SO(4) using a He-Ne laser source (633 nm) and a beam splitter/microscope objective arrangement. Data recorded during voltammetric cycles in Ar-purged solutions revealed a linear correlation between the normalized change in reflectance, DeltaR/R = (R(s) - R(ref))/R(ref) (where R(s) and R(ref) are the light intensities measured by the detector at the sampling, s, and reference potentials, ref, respectively), and the extent of oxidation of the Pt surface over a wide coverage range. Reflectance spectra were also collected in CO-saturated 0.5 M H(2)SO(4) during chronocoulometric measurements involving judiciously selected limits for both the potential step and duty cycle parameters. Analysis of these results made it possible to extract contributions to the current derived from oxide formation during oxidation of adsorbed and bulk CO, based strictly on the optical response.