M.L. Marcos

The behaviour of electrodispersed platinum electrodes for the voltammetric electrooxidation of reduced carbon dioxide

The voltammetric electrooxidation of reduced CO2 adsorbate is studied on large active area (electrodispersed) platinum electrodes. This reaction is used as a test reaction in the absence of readsorption to establish a surface roughness/electrocatalytic activity relationship. In addition, the depolarizing effect of electrodispersed electrodes for the reaction is discussed in terms of the structural model recently derived for these electrodes from scanning tunneling microscopy imaging.

Kinetics and mechanism of “reduced CO2” electrooxidation on electrodispersed platinum in different acid solutions

Abstract The electrooxidation of “reduced CO2” electroadsorbates on electrodispersed platinum electrodes has been investigated in 0.05 M HClO4, 1 M HClO4, 0.5 M H2SO4 and 1 M H3PO4 at 25° C through voltammetry and potential step techniques. The overall reaction comprises three distinguishable processes, namely a first order triggering process, and two second order surface processes. The latter are influenced remarkably by the solution composition (anions). The second order reaction mechanism involves two distinguishable “ reduced CO2” electroadsorbates which react independently with the OH species formed from H2O electrooxidation on the bare platinum sites as the reaction proceeds. An interaction term has to be included in the rate equations to account for the experimental results. The mechanistic interpretation accounts for the values of the number of electrons per site ranging from 1 to 2.

Influence of changes in the total surface area and in the crystalline surface composition of Pt electrodes on their electrocatalytic properties with respect to the electro-oxidation of hydrazine

Abstract The electro-oxidation of hydrazine on electrodispersed and electrofacetted Pt electrodes in acid solutions takes place without noticeable changes in the mechanism with respect to that followed on smooth polycrystalline Pt electrodes. Nonetheless, a negative shift in the oxidation peak potential is observed with increasing roughness factor, R , of the electrodes. By working with preferentially oriented electrodes it was possible to demonstrate that the shift towards more negative potential values of the hydrazine oxidation peak is due to a slightly higher proportion on the electrodispersed electrodes of (100) microfacets.

In situ FTIRS study of ‘reduced’ CO2 on columnar-structured platinum electrodes in different acid media

In situ Fourier transform infrared spectra (FTIRS) of species related to ‘reduced’ CO2 (r-CO2) adsorbates formed on columnar-structured Pt electrodes in aqueous 0.5 M HClO4, 0.5 M H2SO4 and 0.5 M H3PO4 at 25°C are presented. From spectral, electrochemical and electrode topography data, the appearance of different types of complex adsorbates involving CO2, Hads, multi, bridge-, and linearly bonded CO has been concluded. The influence of anions on the CO adsorbate adsorption band is tentatively interpreted through changes in the water-electrolyte structure caused by different types of anions.

On the surface diffusion of Pt adatoms in acidic solutions

Abstract Surface diffusion coefficients, D s , of platinum adatoms were determined by means of in situ electrochemical methods on columnar-structured Pt electrodes immersed in 1M HClO 4 , 1M H 3 PO 4 and 0.5M H 2 SO 4 electrolytic solutions. Measurements were made at different potential values covering the range from hydrogen coverage formation to that in which platinum oxides begin to be formed. The D s values observed confirm an adsorbate induced relaxation of the Pt surface atoms and could explain why restructuring of Pt electrodes has been observed under some experimental conditions. A study of the variation of D s with temperature leads to values of ΔH ≢ f and ΔS ≢ at three potentials, which can be explained according to interactions between adsorbates and Pt surface sites. According to the data obtained, a surface diffusion mechanism in three steps is formulated, in which different steps become rate determining at different potential ranges.

The influence of temperature on the kinetics and mechanism of reduced CO2 electrooxidation on electrodispersed Pt in 0.5 M H2SO4 and 1 M H3PO4

Abstract The kinetics of reduced CO2 electrooxidation on electrodispersed Pt have been investigated in 0.5 M H2SO4 and 1 M H3PO4 in the 2–80°C range by voltammetric and potentiostatic methods. Kinetic data can be interpreted through a complex reaction pathway involving two different reduced CO2 adsorbates. The reaction mechanism advanced earlier at 25°C is valid as a first approach for interpreting the reaction in the entire temperature range and in different electrolytes. The influence of temperature and potential sweep rate on the kinetic response of the system shows that an interconversion process involving the different CO2 adsorbates should be considered in the reaction mechanism. The interconversion process can explain the voltammetric peak multiplicity change with v and T. The apparent activation energy (80±15 kJ mol−1) is the same for the electrooxidation of both reduced CO2 adsorbates. This figure can be interpreted in terms of a modification of clathrate-type CO2 adsorbates into an intermediate which immediately decomposes electrochemically into H+, H2O and CO2. The influence of CO2 adsorbates on the H-atom electrosorption/electrodesorption reactions is also discussed.

Kinetics of CO2 electroadsorption on electrodispersed platinum electrodes in acid solutions

Abstract The kinetics of CO2 electroadsorption on electrodispersed Pt electrodes have been investigated in different acid electrolytes. The kinetic data obtained at different adsorption potentials, adsorption times and CO2 concentrations in solution have been interpreted in terms of two complex reaction mechanisms involving the formation of strongly and weakly bound reduced CO2 adsorbates through reactions involving s-H and w-H adatoms, respectively, and carbon dioxide. The reaction mechanism between H adatoms and CO2 implies the initial electroadsorption of H atoms, the transport of CO2 from the bulk of the solution to the reaction interface, and the formation of the reduced CO2 adsorbate. Under certain conditions, particularly when s-H adatoms are involved, the entire reaction approaches mass transport rate control. Conversely, the reaction between w-H adatoms and CO2 appears as a rather slow process.

Comparative electrochemical behaviour of CO2 on Pt and Rh electrodes in acid solution

Abstract The electroformation of adsorbed species on Pt and Rh from CO2 dissolved in aqueous 0.5 M H2SO4 and the influence of adsorbed species on the hydrogen evolution reaction (HER) were examined for smooth and columnar structured Pt and smooth and rhodized Rh electrodes at 25|dgC. Different electrosorbed species resulting from the adsorption of CO2 on Rh were found. The electro-oxidation of these adsorbates on Rh takes place over a potential range larger than that for adsorbates on Pt, overlapping the potential range of electroformation of the O-containing layer. A part of the reduced CO2 adsorbates on Rh behaves as CO-like adsorbates. The presence of adsorbed species from CO2 influences differently the stationary HER current potential curves for both types of Pt and Rh electrodes. In the range of low current density (cd), the Tafel slope for Pt is −0.030 V decade−1. Conversely, the presence of a chemisorbed species on Rh changes the Tafel slope for the HER in the range of low cd from −0.030 V decade−1 for CO2-free solutions to −0.120 V decade−1 for CO2-saturated solution. These changes are explained by a modification in the rate-determining step of the HER produced by chemisorbed species.

DEPTH-RESOLVED MICRO-RAMAN STUDY OF POROUS SILICON AT DIFFERENT OXIDATION STATES

Photoluminescence (PL) and Raman spectra were measured along a cross section of porous silicon films at different oxidation times after application of anodic current transients. The average crystallite size was determined from the Raman spectra with the standard phonon confinement model. Before oxidation, the PL emission energy and crystallite size were found to be independent of the layer depth. Also, the integrated PL emission was larger for the middle layers. The effect of oxidation was a blueshift of the PL band and a decrease in the integrated emission for the outer layers. The crystallite size increases for all layers, particularly the outer ones.

Electrochemically induced changes in the surface diffusion in polycrystalline platinum electrodes

Surface diffusion coefficients, DS, of platinum adatoms were measured in situ on polycrystalline platinum electrodes inmersed in 1 M HClO4, 1 M H3PO4 and 0.5 M H2SO4 electrolytic solutions at different potential values. The change in the DS values observed, as well as the rate of change of DS with the potential value, confirm an adsorbate induced relaxation of Pt electrode surfaces.