J.M. Orts

Study of the charge displacement at constant potential during CO adsorption on Pt(110) and Pt(111) electrodes in contact with a perchloric acid solution

Abstract A new electrochemical approach has been made, employing the current—time transient responses when a CO adlayer is formed at a platinum electrode at various controlled potentials where CO oxidation does not take place. The case of Pt(110) is compared with those of Pt(111) and Pt(111) disordered after ten cycles of oxygen adsorption—desorption. In order to avoid interference with anion-specific adsorption, the study was carried out in a perchloric acid solution. There is good agreement between the charge measured by voltammetry in the absence of CO and the charges measured during the current—time transients. This is indicative that the latter charges are produced by the displacement of the species at the interface as a result of CO adlayer formations. The sign of the current transient has been found to depend on the potential at which CO adsorption is carried out. This dependence may be related to the nature of species which are present in the interfacial region, providing new complementary information that voltammetry cannot yield.

Potentiostatic charge displacement by exchanging adsorbed species on Pt(111) electrodes—acidic electrolytes with specific anion adsorption

Abstract The voltammetric contribution of some specifically adsorbed anions (acetate, oxalate, chloride and bromide) is revised for the case of Pt(111) electrodes by means of experiments of displacement by CO in acidic medium. It is found that the usual states correspond to the reversible adsorption/desorption of hydrogen whereas the so-called unusual states would correspond to the adsorption/desorption of anions. This means that the maximum coverage of hydrogen adatoms prior to hydrogen evolution is around two-thirds of a complete monolayer of Pt(111). From the potential dependence of the voltammetric profiles the electrosorption valency of the different anionic species is calculated.

Electrochemical studies in sulphuric acid solutions of adsorbed CO on Pt (111) electrodes

The electrochemical behaviour of CO adsorbed on Pt (111) from an acidic solution has been studied by voltammetry in 0.5 M H2SO4 as the test electrolyte. From the voltammetric profile two types of CO distribution on the surface were found using hydrogen adsorption as a surface probe for the determination of the size of the CO-free (111) domains. The distribution of type I was observed at low coverage and is the more easily oxidizable form. It corresponds to a dispersion of small islands with small CO-free domains. The distribution of type II appeared at high coverage and co-existed with type I at intermediate coverage. This distribution for a similar CO coverage to that of type I shows large CO-free (111) domains. This type II corresponds to wide compact CO domains. In both types, linearly and bridgebonded CO are present simultaneously. The linearly bonded CO becomes predominant when the CO coverages correspond to complete blocking of the hydrogen adsorption sites. The latter situation may be achieved with various amounts of linearly and bridge-bonded CO. A CO coverage of one CO molecule per platinum surface atom appears to be the upper coverage limit.

Electrochemical behaviour of CO layers formed by solution dosing at open circuit on Pt(111). Voltammetric determination of CO coverages at full hydrogen adsorption blocking in various acid media

The electrochemical oxidation of CO on Pt(111) in sulphuric, perchloric and chloride-containing acid solutions has been investigated by cyclic voltammetry. In all cases the CO coverages correspond to a full blocking of the hydrogen adsorption sites in order to estimate from coulometry the maximum amount of CO which could be adsorbed on Pt(111) from solution dosing. A comprehensive discussion of the correction of the double-layer contribution is given for voltammetric experiments using ultra-clean systems with the lowest amount of surface defects. Two different models for the double-layer correction have been discussed. They differ essentially in the inclusion or omission of the charge corresponding to the unusual adsorption states of Pt(111) in the double-layer correction. The results show that in both cases the CO coverages reached from solution dosing are higher than those obtained in the gas phase in the same temperature range. If equal maximum coverage in the gas or liquid phase were achieved, it would lead to improbably high double-layer corrections for the latter system because some of the usual hydrogen adsorption states should be involved in this correction in a range of potentials where their presence is incompatible. The full blocking of hydrogen adsorption sites on Pt(111) was observed for different CO stripping charges (differing by 105 μC cm−2). This quite large difference, which is independent of the model used for corrections, indicates that 0.22 CO monolayers can be adsorbed additionally on the Pt(111) electrode at the threshold of hydrogen adsorption. In the cases of highest coverages, a CO stripping charge contribution of around 30 μC cm−2 has been observed at lower potentials. A tentative interpretation of this early oxidation step is proposed. After this latter stripping process the electrode surface always remains fully blocked for hydrogen adsorption.

Scanning tunneling microscopy and electrochemical study of the surface structure of Pt(10,10,9) and Pt(11,10,10) electrodes prepared under different cooling conditions

Abstract The effect of three surface electrochemical preparation techniques for single crystal surfaces on two platinum stepped surfaces [Pt(10,10,9) and Pt(11,10,10)] has been investigated by scanning tunneling microscopy and cyclic voltammetry. The preparation techniques consisted of a thermal treatment followed by a cooling step in (a) a hydrogen+argon atmosphere, (b) air (c) iodine vapor+air. For Pt(10,10,9) and Pt(11,10,10) surfaces, the hydrogen+argon treatment provides surfaces that have a very narrow distribution of terrace widths around the nominal value and monatomic steps. On the other hand, facetted surfaces with terrace widths and steps four to five times their nominal values are obtained when the cooling is in the presence of iodine vapor. The air treatment generates surfaces in which the terrace width is not as uniform as in the H2+Ar case and some kink sites are created. The changes in the surface topography can be followed in the voltammetric profile of the surfaces recorded in a sulfuric acid solution.

Electrochemical oxidation of ethylene glycol on Pt single crystal electrodes with basal orientations in acidic medium

Abstract Ethylene glycol electrooxidation on platinum using voltammetry is shown to be a structure sensitive reaction. Among the three basal planes, Pt(111) shows the lowest activity for the oxidation of the organic, although the reaction takes place at lower potential values. The poisoning intermediate can be isolated on each orientation and identified as a CO-like species, its formation taking place in a wide potential range. The stability of the voltammetric profiles during continuous cycling suggests that these CO-like species are the main stable adsorbed residues on Pt(100) and Pt(111). The loss of activity observed with repeated cycling on Pt(110) in a potential range excluding oxygen adsorption is explained by a structural modification of the surface.

Role of the metal and surface structure in the electro-oxidation of hydrazine in acidic media

The adsorption and oxidation of hydrazine in acidic media has been studied on single-crystal electrodes of platinum, gold, and rhodium with basal crystallographic orientations. Voltammetric and charge-displacement experiments indicate the monoelectronic adsorption of hydrazine, either reversible or irreversible, at Pt and Rh electrodes, in the low-potential region. On Au single-crystal electrodes hydrazine adsorption is weak. The electrocatalytic oxidation of hydrazine is sensitive to the nature of the metal used as anode and to its surface structure. In the absence of specific anion adsorption, hydrazine oxidation takes place on rhodium electrodes at lower potentials than on platinum and gold. Under these conditions, the highest catalytic activity is obtained with face-centered cubic (100) orientation for the three metals.

Nitric oxide adsorption at Pt(100) electrode surfaces

Abstract Nitric oxide adlayers formed at Pt(100) electrode surfaces have been characterized in situ by means of electrochemical and FTIR spectroscopic experiments. These adlayers can be reductively stripped from the electrode surface yielding dissolved ammonium as the main reduction product in acidic solution. From the voltammetric charge involved in this process the absolute NO coverage for the saturated adlayers has been calculated to be 0.5. This value agrees with that corresponding to the c(4×2) overlayer found by different authors under UHV conditions. The oxidation of the NO adlayer in acid media proceeds between 0.90 and 1.10 V RHE through the formation of surface species (probably a mixture of nitrite and nitro species), which can be further oxidized to dissolved nitrate anions at higher potentials. The same NO adlayers have been found to oxidize directly to dissolved nitrite anions in alkaline solutions. The potential dependence of the N–O stretching mode, which is shifted towards higher wavenumbers when the electrode potential increases, has been analyzed for the saturated NO adlayer. Linear relations were found both in acidic and neutral solutions with tuning rates of ca. 50 cm −1 V −1 in the potential region between 0.50 and 0.90 V RHE where the saturated NO adlayers are stable. A slow dissolution process has been evidenced at potentials slightly below 0.80 V for the NO adlayer in alkaline solutions.

Comparison of electrosorption at activated polycrystalline and Pt(531) kinked platinum electrodes: surface voltammetry and charge displacement on potentiostatic CO adsorption

Abstract In this work we have tried to reproduce the hydrogen adsorption behaviour of electrochemically activated polycrystalline platinum electrodes by using a kinked platinum single crystal surface subjected only to flame cleaning before checking hydrogen electrosorption at its surface. The Pt(531) surface may be considered as a model for a platinum surface which has been extensively activated by cycles of oxygen adsorption-desorption. In the hard sphere model the surface unit cell for this orientation contains three sites, with four-fold, three-fold and two-fold symmetry respectively. The electrochemical behaviour of this surface, prepared from a platinum bead, was checked in 0.5 M H 2 SO 4 solution by voltammetry after flame cleaning and various cooling conditions. It was compared with that of a rough platinum surface, cut from another bead in a direction chosen at random, either studied after flame cleaning or after electrochemical activation. Experiments on charge displacement by CO adsorption at constant potentials show, for both samples, that the voltammetric charge at the two limits of the hydrogen upd range is made up of two components of opposite sign. Only 0.69 and 0.77 of the voltammetric charge on Pt(531) and rough platinum respectively may be ascribed to adsorbed hydrogen; the remaining part of this charge is involved in a reversible adsorption state formed in an anodic process. These results are discussed in terms of the inadequacy of the conventional method of correcting, from double layer effects, the voltammetric charge of activated platinum electrodes in order to measure their hydrogen surface content.

Study of the conditions for irreversible adsorption of lead at Pt(h,k,l) electrodes

Abstract In contrast to bismuth, lead can be observed in an irreversible adsorption state on platinum single crystal electrodes only by using more restrictive conditions. These may be determined by studying systematically the effect of the pH of the solution from which lead is adsorbed as well as the effect of the pH of the test electrolyte in which the adsorbed layer is characterized electrochemically. Thus acidic and neutral lead solutions were used to prepare the platinum modified electrodes with basal orientations, while acidic and alkaline solutions were used as test electrolytes to check the stability of the layer which was found to increase with pH. Evidence is presented for a surface redox reaction of the lead adlayer whose potential depends on the orientation of the Pt substrate and is independent of pH on a RHE scale. Thus lead undergoes a reversible valence change from zero to the divalent state, both in the adsorbed phase, the latter being in combination with oxygen. The stability with pH of this oxygenated compound determines the possibility or not to observe the irreversible adsorption of lead.