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Rh modified Au(111-25 nm) electrodes, prepared by electron beam evaporation and galvanostatic deposition, were employed to study adsorption and electro-oxidation of CO on Rh in 0.1 M sulfuric acid solution by in situ attenuated total reflection surface enhanced infrared absorption spectroscopy (ATR-SEIRAS). The results of ATR-SEIRAS experiments were compared with those obtained by infrared reflection absorption spectroscopy on three low-index Rh single crystal surfaces. The Rh film deposited on Au(111-25 nm) electrode consists of 3D clusters forming a highly stepped [n(111) × (111)]-like surface with narrow (111) terraces. When CO was dosed at the hydrogen adsorption potential region, CO adsorbed in both atop (COL) and bridge (COB) configurations, as well as coadsorbed water species, were detected on the Rh film electrode. A partial interconversion of spectroscopic bands due to the CO displacement from bridge to atop sites was found during the anodic potential scan, revealing that there is a potential-dependent preference of CO adsorption sites on Rh surfaces. Our data indicate that CO oxidation on Rh electrode surface in acidic media involves coadsorbed water and follows the nucleation and growth model of a Langmuir-Hinshelwood type reaction. The work was supported by the Research Center Jülich, the University of Bern, Swiss National Science Foundation (200020_144471, 200021-124643), the Spanish Ministerio de Economía y Competitividad (project CTQ2013-44083-P) and University of Alicante. QX acknowledges fellowships of the Research Center Jülich; IP acknowledges support by COST Action TD 1002; and AK acknowledges the financial support by CTI Swiss Competence Centers for Energy Research (SCCER Heat and Electricity Storage).

Ministerio de Educacion y Ciencia of Spain[CTQ2010-16271]; China Scholarship Council; NSFC[20833005]

In the present work the electrooxidation of methanol, 2-propanol and different mixtures of both alcohols has been studied on the three platinum basal planes in three different electrolytes (H 2 SO 4 , HClO 4 and NaOH). The results indicate that, like in the case of both individual alcohols, the electrooxidation of the mixture is a structure sensitive reaction and that Pt(1 1 1) leads to higher current densities for some mixture compositions as compared to what could be expected from the contribution of the individual compounds. The effect of the methanol concentration in the mixture points out that 2-propanol is the main reacting fuel at the Pt(1 1 1) surface. Interestingly, the addition of methanol clearly has a positive effect on 2-propanol oxidation. For a specific mixture composition, while results from cyclic voltammetry indicate a modest twofold increase in current density, chronoamperometric results after 10 min experiment show a nearly 200 times higher current density. IR experiments have been performed to gain information about the enhancement mechanism. Nevertheless, we have found that both methanol and 2-propanol seem to follow the same mechanism as they follow in the absence of the other alcohol, and therefore the enhancement could be probably related to a competitive adsorption for the active surface sites.

The variation of electrode nature and surface structure (the use of stepped single crystal faces with controlled width of (1 1 1) terraces and monoatomic steps of (1 0 0) or (1 1 0) orientation) allows to determine peculiarities of co-adsorption of acetonitrile molecules, hydrogen adatoms and (bi)sulfate anions. It has been shown that first of all acetonitrile blocks adsorption sites at the steps. Anion adsorption at terraces of stepped platinum surfaces in 0.5 M H2SO4 solution with additions of acetonitrile depends on terrace width and the step orientation. This demonstrates the important role of structural factors in competitive adsorption processes. The decrease in adsorption of hydrogen and anions on narrow terraces is substantially due to the influence of acetonitrile molecules placed at the steps or nearby sites. At E < 1.0 V, electrochemical conversion of acetonitrile has not been detected at single crystal Pt surfaces. However, acetonitrile oxidation might proceed on polycrystalline platinum followed by product desorption. On Au(1 1 1) surface acetonitrile adsorption is considerably weaker than that on platinum electrodes.

Conventional electrochemical methods have been applied to study the oxidation of a possible alternative fuel for a direct oxidation fuel cell. The electrooxidation of dimethoxymethane (DMM) was investigated on the three low index planes, (1 0 0), (1 1 0) and (1 1 1) of platinum single crystals and compared with its oxidation on a platinum polycrystalline electrode. Among platinum electrodes, electroreactivity of DMM observed is Pt(1 1 1) > Pt(1 0 0) > Pt(1 1 0) ∼ Pt poly. Hydrogen adsorption is limited by the presence of DMM, except for Pt(1 1 1) plane. In situ IR experiments show the presence of bands of COads with all electrodes except Pt(1 1 1). This work shows that the mechanism of DMM electrooxidation is structure sensitive. A path takes place on Pt(1 0 0) and Pt(1 1 0) which is favourable to the formation of COads. Another path proceeds on Pt(1 1 1), where COads is not present and reaction does not occur at low potential. Results indicate that peak intensities are higher in perchloric acid than in sulphuric acid. So DMM adsorption is dependent on the specific adsorption of the anions. In situ IR reflectance spectroscopy identified some intermediates and reaction products of DMM adsorption and electrooxidation on Pt electrodes: COL (linearly bonded) and COB (bridge bonded), adsorbed CHO and CH3O species, methanol and CO2. The electrochemical and spectroelectrochemical experiments suggest a complex mechanism of DMM electrooxidation.

Acetate adsorption at gold electrodes is studied in perchloric acid solutions by cyclic voltammetry and in-situ infrared spectroscopy. External reflection measurements, performed with gold single crystal electrodes, are combined with Surface Enhanced Infrared Reflection Absorption Spectroscopy experiments under attenuated total reflection conditions (ATR-SEIRAS) carried out with sputtered gold thin-film electrodes. Theoretical harmonic IR frequencies of acetate species adsorbed with different geometries on Au clusters with (1 1 1), (1 0 0) and (1 1 0) orientations have been obtained from B3LYP/LANL2DZ, 6–31 + G* calculations. The theoretical and experimental results confirm that, irrespective of the surface crystallographic orientation, bonding of acetate to the surface involves the two oxygen atoms of the carboxylate group, with the OCO plane perpendicular to the metal surface. DFT calculations reveal also that the total charge of the metal cluster–acetate supermolecule has small effect on the vibrational frequencies of adsorbed acetate species. Both the external and the internal reflection measurements show the co-adsorption of acetate and perchlorate anions. Step-scan measurements carried out with the gold thin-film electrodes have allowed the monitoring of the time-dependent behaviour of perchlorate, acetate and water bands in potential step experiments. Acetate adsorption under those conditions is shown to involve perchlorate desorption and to follow a Langmuir-type kinetics. The step-scan spectra also show the rise and decay of transient water structures with parallel time-dependent shifts of the background intensity in the infrared spectra.

Oxalic acid adsorption has been studied at platinum single crystal electrodes belonging to the [011] zone. Cyclic voltammetry and charge displacement experiments have been combined with in situ external reflection infrared experiments and results compared with those previously reported for the Pt(1 1 1) and Pt(1 0 0) electrodes. The spectroscopic data confirm the potential dependent behaviour of oxalic acid adsorption derived from the electrochemical experiments. At the same time, the infrared spectra show distinct CO stretching bands for adsorbates on (1 0 0) and (1 1 1) terraces. As previously concluded for Pt(1 0 0) and Pt(1 1 1) electrodes, oxalate and bioxalate anions are adsorbed, respectively, at the (1 0 0) and (1 1 1) terraces for potentials above 0.30 V. Oxalate and bioxalate anions adsorb at edge and step sites at potentials lower than at terrace sites. For Pt(2n−1,1,1) = Pt(S)[n(1 0 0) × (1 1 1)], bands are observed at potentials higher than 0.20 V that can be assigned to oxalate anions adsorbed at (1 0 0) edge sites and bioxalate adsorbed at (1 1 1) step sites. In the case of Pt(n+1,n−1,n−1) = Pt(S)[n(1 1 1) × (1 0 0)] electrodes, bands for oxalate anions adsorbed at the (1 0 0) step sites appears at potentials lower than bands for bioxalate adsorption at the (1 1 1) terrace sites. For all the stepped surfaces, adsorbed CO is slowly formed at the step sites in the hydrogen adsorption region. The extent of this process can be minimised during the collection of the infrared spectra by taking the reference spectrum at 0.20 V.