A. Aldaz

An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes

In this work we have studied the electrochemical behaviour of a bismuth surface compound formed spontaneously without applying external potential when platinum is put in contact with a solution of a Bi(III) salt. When studied in a sulphuric acid solution, the surface compound undergoes a redox reaction where oxidized and reduced species are both adsorbed at the platinum surface. The redox reaction depends on the crystalline surface structure of the platinum substrate. The behaviour of the adsorbate at the platinum (111), (100), (110) and (332) faces as well as at the surface of a spherical platinum single crystal has been investigated. The redox reaction has been found to be highly reversible at the Pt (111) electrode and has been studied as a function of pH and sweep rate at this orientation. This reaction appears as a good probe for detection of (111) surface domains on the platinum surface, as shown by the results obtained with Pt (332), otherwise noted in the terrace-step notation as 5 (111) × (110). The results show also that steps may be blocked selectively by the bismuth adsorbate while terrace sites remain free for hydrogen adsorption. The stability of the bismuth surface compound at the various surface orientations has been investigated. The voltammetric behaviour of the surface compound at Pt (100) has been compared to the voltammetric UPD of bismuth. The comparison allows the assignment of peaks in the voltammogram of the UPD of bismuth at this orientation to a change in the valency of the surface bismuth compound without change in the coverage because the adsorbate is already present at the surface. A similar conclusion is valid for other orientations. The spontaneous deposition has been assumed to be due to an incipient underpotential deposition of bismuth at potentials higher than that of incipient Pt-OH formation. Three surface compounds have been identified at the surface from the reversible behaviour of the adsorbate at Pt (111): Biad at low potential, (BiOH)ad as an intermediate in the redox process and (BiO)ad or (Bi(OH)2)ad, the stable species, at higher potential.

Electrochemical characterisation of platinum nanoparticles prepared by microemulsion: how to clean them without loss of crystalline surface structure

Abstract A new process to clean nanoparticles of platinum obtained by a water-in-oil (W/O) microemulsion without loss of crystalline structure was studied. Polyethyleneglycol–dodecylether (BRIJ®30) was the surfactant used for the preparation of the platinum particles. Pt(111), Pt(110), Pt(100) and polyoriented platinum electrodes were used to check the effectiveness of this new procedure. Preliminary results obtained with nanoparticles of platinum are also presented. The voltammetric profiles of platinum particles show the adsorption states associated with (110), (100) and (111) domains. The ratio Pt SURF /Pt TOT was found to be 0.1 for an upper potential limit of 1 V and 0.12 for an upper potential limit of 1.5 V.

Imaging Structure Sensitive Catalysis on Different Shape-Controlled Platinum Nanoparticles

The structure sensitive catalytic activity for oxygen reduction reaction (ORR) on shape-controlled Pt nanoparticles (NPs) is directly imaged using scanning electrochemical microscopy (SECM). We synthesize and compare four types of Pt NPs: spherical, cubic, hexagonal, and tetrahedral-octahedral. Our SECM images show the hexagonal Pt NPs displaying the highest activity for ORR in two acid electrolytes. Meanwhile, cubic and tetrahedral-octahedral NPs drastically change their activity depending on specific adsorption of the different anions in solution. The NPs morphology produces predominant crystallographic planes at the surface of these shape-controlled Pt NPs, which are responsible for their different catalytic activity. Our results translate the studies on Pt single crystal electrodes present in the literature into Pt NPs that are useful as a catalyst in real fuel cells.

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.

Poison formation reaction from formic acid and methanol on Pt(111) electrodes modified by irreversibly adsorbed Bi and As

Abstract Dissociative adsorption of formic acid and methanol on adatom-modified Pt(111) electrodes has been carried out as a way of studying poison formation reactions on these surfaces. The electrodes were prepared using irreversible adsorption of Bi and As. A modification of the dissociative adsorption technique used for poison formation studies has been employed. For clean Pt(111) surfaces the behaviour of the poison formation reaction of the two organic molecules is almost the same, but on adatom-modified Pt(111) electrodes different results are found. Bi and As show an important long-range electronic effect which inhibits poison formation from the dissociative adsorption of formic acid at very low adatom coverage. However, for methanol, the inhibition due to the presence of Bi adatoms on the surface can be explained by a simple third-body effect. A computer simulation of the dissociative adsorption of formic acid on Bi-modified Pt(111) electrodes has been carried out in order to calculate the width of the domains affected by the presence of Bi adatoms.

Validity of double-layer charge-corrected voltammetry for assaying carbon monoxide coverages on ordered transition metals : comparisons with adlayer structures in electrochemical and ultrahigh vacuum environments

Abstract A coulometric procedure enabling the reliable and accurate evaluation of saturated CO coverages, θ sat CO , on Pt-group transition-metal electrodes is outlined, and applied to CO adlayers on ordered low-index platinum, rhodium, and iridium surfaces in acidic aqueous media. Along with voltammetric data, the method utilizes previously described measurements of the charge displaced upon CO adsorption. The reverse of this charge, Q dis , together with the “background” charge Q b flowing between a suitable pair of electrode potentials in the absence of CO, constitutes the overall “double-layer” correction Q dl to the total voltammetric charge Q tot measured for the electrooxidation of adsorbed CO between the same potentials. Significantly, the Q dis as well as the Q b component of Q dl typically constitutes moderate or even large corrections to Q tot , so that the deduced θ sat CO values are noticeably (20–30%) smaller than some voltammetric-based estimates reported earlier. However, the revised coulometric θ sat CO values are in consistently good agreement with the corresponding coverages obtained by means of an infrared spectrophotometric procedure. These θ sat CO values are compared with adlayer structural information obtained recently from in situ scanning tunneling microscopy along with infrared spectroscopy, and also with structural data for corresponding adlayers in ultrahigh vacuum (UHV). In most cases, the electrochemical and UHV-based θ sat CO values are not greatly different (within 5–10%), even though the CO binding site arrangements are often dissimilar in these two environments. The role of the electrode potential in affecting θ sat CO under some conditions via alterations in binding-site energetics, however, is noted for the Pt(111)/CO system.

Electrochemical characterisation of platinum-palladium nanoparticles prepared in a water-in-oil microemulsion

Abstract A detailed electrochemical characterisation of platinum-palladium nanoparticles prepared by reduction with hydrazine of H 2 PtCl 6 and K 2 PdCl 4 in a water-in-oil microemulsion of water/polyethylenglycol-dodecylether (BRIJ ® 30)/ n -heptane is reported. XPS, AAS and UV–vis experiments were carried out to determine the atomic composition of the bimetallic nanoparticles obtained in the whole composition range. Cyclic voltammograms of clean nanoparticle alloys were obtained after a controlled decontamination procedure of their surfaces. CO adsorption–oxidation and dissociative adsorption of formic acid and methanol were used as test reactions to check the electrocatalytic behaviour of the bimetallic nanoparticles. FTIRS experiments of adsorbed CO were also carried out giving information on the relative amount of linearly and bridge-bonded CO, which is known to depend on the surface distribution of the two elements. Experimental results show how the electrochemical adsorption behaviour and electrocatalytic properties of the particles are under the control of their composition and, in a case studied in this work, of their size.

Influence of chloride ion on electrochemical degradation of phenol in alkaline medium using bismuth doped and pure PbO2 anodes

Electrochemical method at laboratory scale for the treatment of biorefractory solutions with high phenol content--1000 ppm is described. Total degradation of phenol was obtained at alkaline pH when NaCl was present using Bi-doped and pure lead dioxide electrodes. A filter press cell of 63 cm2 geometric area was used for this purpose. Measurements of chemical oxygen demand (COD), phenol, carbon monoxide, carbon dioxide, and volatile organic compounds (VOCs) have been used to characterise the electrochemical process for phenol elimination. It is worth noting that in the absence of NaCl, the charge efficiency of COD removal was independent of the current density in the range studied (50-100 mA cm(-2)) Moreover, when NaCl was present, the current efficiency for COD and phenol removal increase as the chloride concentration increases. Chloroform was the only halocompound detected at the end of reaction. For both electrodes, Bi-doped and pure lead dioxide, the chloroform concentration at the end of the electrolysis decreases, working at low current densities and for low chloride concentrations.

Formic acid oxidation on Pdad + Pt(100) and Pdad + Pt(111) electrodes

Irreversible adsorption of palladium on Pt(100) and Pt(111) is used to form electrocatalysts combining two catalytic elements. Their electrocatalytic activity towards formic acid oxidation is examined. A dramatic effect of the presence of adsorbed palladium on Pt(100) on this reaction is observed with a considerable lowering of the oxidation potential and the absence of self-poisoning on open circuit. In contrast, the activity of the Pt(111) substrate is not greatly changed by adsorbed Pd. A deactivation of the Pdad + Pt(100) electrocatalyst is observed when the oxidation of formic acid takes place, suggesting a slow step in the formation of an adsorbed species which blocks the adsorption of formic acid as a first step of the overall reaction. At all palladium coverages of Pt(100), even with a multilayer of palladium, the activity of the modified electrode towards the electro-oxidation of formic acid was found to be higher than that of bulk palladium.

Selective electrocatalysis of ammonia oxidation on Pt(100) sites in alkaline medium

The oxidation of ammonia on platinum surfaces is a sensitive-structure reaction that takes place almost exclusively on Pt containing (100) sites. However it is strongly inhibited on Pt(111) and Pt(110) surfaces. A study using stepped electrodes with Pt(100) terraces and monoatomic Pt(111) steps, Pt(S)[n(100)x(111)], Miller indices Pt(2[n1,1,1)], shows that the activity of the electrode is strongly dependent on the terrace width, i.e., the wider the terrace the less positive the peak potential for ammonia oxidation. The change in the peak potential from the widest (100) terraces to terraces 6 atoms wide, i.e., from Pt(100) to Pt(11,1,1), is approximately 60 mV. This result is unexpected if we bear in mind the size and geometry of the ammonia molecule.