Dennis S. Corrigan

Mechanisms of formic acid, methanol, and carbon monoxide electrooxidation at platinum as examined by single potential alteration infrared spectroscopy

Abstract Surface infrared spectra have been obtained as a function of potential and time during the voltammetric oxidation of formic acid and methanol on polycrystalline platinum in order to probe the possible role of adsorbed carbon monoxide in the electrooxidation mechanisms. The procedure involves obtaining a sequence of single-beam infrared spectra using an FTIR spectrometer during potential sweep or following potential-step perturbations, and referencing these to a spectrum obtained at the initial potential or after complete oxidation had occurred. Using this “single potential alteration infrared” (SPAIR) technique, individual spectra for the C-O stretch of adsorbed carbon monoxide, νCO, as well as the O-C-O stretch for the CO2 product could be obtained under voltammetric conditions in as little as 3.5 s, and repetitively every 7 s. Such SPAIR spectra obtained during either potential sweep or step excursions indicate that the onset of both formic acid and methanol oxidation coincides with the oxidative removal of adsorbed carbon monoxide. The electrooxidation of CO irreversibly adsorbed from solution carbon monoxide was also examined using this approach. The integrated absorbance of the νCO band was generally found to be proportional to the CO coverage (determined from either the CO2 band intensity or the voltammetric charge) during electrooxidation. The dependence of the νCO peak frequency upon coverage during potentiostatic oxidation, however, is sensitive to the timescale over which the process proceeds. The adsorption kinetics of CO formed from formic acid and methanol were evaluated from the time dependence of the νCO band intensity following suitable potential step sequences to potentials where CO electrooxidation is suitably slow. These data suggest that while adsorbed CO may act as an adsorbed intermediate for methanol oxidation, it probably acts as a chemisorbed poison for formic acid oxidation.

The interpretation of solution electrolyte vibrational bands in potential-difference infrared spectroscopy

Abstract The influence of ionic migration to and from the surrounding solution reservoir upon potential-difference infrared (PDIR) spectra is examined for some cases involving anionic adsorption in order to elucidate its consequences upon the net potential-induced compositional changes in the thin-layer solution. Representative PDIR spectra for the adsorption of azide anions on gold are compared in the absence and presence of excess alkali perchlorate supporting electrolyte. In the latter, the loss of solution azide in the spectral thin layer upon stepping to a more positive potential, resulting from increased azide adsorption, is accompanied by extensive migration of perchlorate into the thin layer. The form of the spectra induced by potential-dependent azide specific adsorption differs in these two circumstances since in the former the ionic migration between the thin-layer cavity and the solution reservoir necessary for charge compensation is provided by the azide electrolyte itself, whereas in the latter case migration of the supporting electrolyte yields a fixed quantity of azide in the thin layer. The intensities and sign of the PDIR bands arising from solution-phase azide and perchlorate enable the extent of the potential-dependent anionic redistribution in the thin-layer cavity to be quantified. In the absence of added perchlorate, the magnitude of the solution azide band is diminished substantially, inferring that replenishment of the thin-layer solution concentration occurs predominantly via N − 3 migration from the surrounding solution reservoir. Similar results were also obtained for cyanate adsorption on gold. The influence of cation as well as anion migration on this thin-layer charge redistribution was examined by employing an infrared-active cation, NH + 4 , as well as from the addition of H 3 O + . While the results indicate that cation migration can contribute substantially to this charge redistribution, anion migration typically appears to predominate when specific anion adsorption is encountered. Some general consequences of such ion migration effects to the interpretation of PDIR spectra are noted.

Device for computer‐controlled potential modulation in electrochemical Fourier transform infrared spectroscopy

A circuit has been designed for the automatic control of potential modulation for subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) using an IBM IR/90 series FTIR spectrometer. This device enables the potential modulation to remain synchronized with the spectral data collection under all conditions.

Some applications of surface Raman and infrared spectroscopies to mechanistic electrochemistry involving adsorbed species

Abstract Some methods are outlined by which surface-enhanced Raman Spectroscopy (SERS) and infrared reflection-absorption Spectroscopy (IRRAS) can be applied to gain mechanistic information for multistep electrode processes involving adsorbed species. Emphasis is placed on approaches in which time-resolved spectra can be obtained in conjunction with conventional electrochemical techniques, enabling simultaneous vibrational and electrochemical kinetic (current-potential-time) information to be obtained during the evolution of irreversible electrode processes. Specific applications to irreversible electroorganic reactions are outlined, employing optical multichannel analyzer and Fourier transform instrumentation for SERS and IRRAS, respectively, in conjunction with linear sweep voltammetry.

Effect of surface crystallographic orientation upon specific adsorption of azide at silver electrodes as examined by potential-difference infrared spectroscopy

The specific adsorption of azide anions on the three low-index faces of silver, having (111), (100), and (110) orientations, has been studied by means of potential-difference infrared spectroscopy using an FTIR spectrometer in order to examine the influence of surface crystallography upon the adsorbate binding. Bipolar infrared spectra were obtained in the 2050–2100 cm−1 region, with a negative-going (i.e., decreased transmittance) feature, va (sur), at 2075–2100 cm−1 due to the asymmetric N-N-N vibration for adsorbed azide, and its positive-going band partner, va (sol), at 2048 cm−1 arising from the corresponding loss of an equal amount of solution azide in the thin-layer cavity brought about by the positive alteration in electrode potential. Information on the adsorbate structure was obtained from the relative intensities, Isur and Isol, as well as frequencies, of va(sur) and va(sol). For the Ag (111) and (100) faces, Isur⪡Isol small azide surface concentrations (Γ ⩽ 4×10−10 mol cm−2), whereas Isur≈ Isol at higher coverages. These findings are interpreted in terms of the surface infrared selection rule as indicting a prominence of “flat” and “vertical” orientations, respectively, of adsorbed azide under these conditions; the presence of the latter, end-on bonded, form is also consistent with the observed va(sur) frequencies. In contrast, for the Ag (110) face, Isur⪡Isol over the entire accessible coverage range (up to Γ ≈ 1×10−9 mol cm−2, indicating that a flat azide orientation is especially favored at this surface. This is attributed to the adsorption of the linear N3− ions along the atomic-scale “furrows” which characterize the (110) face. Polycrystalline silver exhibited spectral behavior that is in some respects intermediate between that observed for the low-index single-crystal faces. Comparisons are also made between the surface infrared results and corresponding capacitance-potential data.

Laser-induced electron ejection at metal surfaces Evidence for photoelectroreduction of cobalt(III) and chromium(III) ammines by direct heterogeneous electron transfer

Abstract Photocurrents for the reduction of selected Co(III) and Cr(II) ammine complexes at silver, gold, and mercury electrodes induced by laser irradiation at 514.5 and 647.1 nm have been measured as a function of electrode potential and double-layer structure, and compared with corresponding thermal electroreduction data in order to provide information diagnostic of the photoelectroreduction mechanism. Photocurrents were observed at potentials considerably (up to 2 V) positive of the threshold for conventional photoemission. Similar responses were obtained in the photo- and thermal currents for outer-sphere Co(NH 3 ) 3+ 6 reduction brought about by variations in the reactant double-layer concentration by means of specific ionic adsorption at silver and mercury electrodes. This suggests that the effective electron-tunneling distance is similarly small (⪅ 0.6–0.8 nm) for the photoassisted as well as unassisted electroreduction pathways. A relationship was seen between the electrode potentials at which photoassisted and unassisted reduction occurred for the various reactants, including those following inner- as well as outer-sphere thermal reduction mechanisms. The photoelectroreduction mechanism appears to involve direct transfer of photoexcited electrons at the metal surface to acceptor species located within the double-layer region. The photocurrent-potential behavior is consistent with the presence of only a small Gibbs energy barrier, due to nuclear reorganization, as a result of the high reaction exoergicity. The possible value of such photoassisted pathways for examining highly exoergic electrode reactions is pointed out, along with complications from their presence when utilizing laser spectroscopic methods for examining thermal electrochemical processes.