On the thermodynamics of hydrogen adsorption over Pt(111) in 0.05M NaOH.

The reasons for the sluggish kinetics of the hydrogen adsorption reaction in alkaline media remain a question still to be solved. This information is important to achieve a complete understanding of the mechanistic details that could lead to the production of key catalytic materials necessary for the development of a future hydrogen economy. For a better understanding of this reaction, it is important to acquire information about the thermodynamic parameters characteristic of the different steps in the reaction. Among these, the hydrogen adsorption is a key step in the process of hydrogen evolution. Although some debate still remains about the difference between adsorbed hydrogen in the underpotential deposition (UPD) region and at the overpotential deposition region, there is no doubt that understanding the former can help in the understanding of the latter. Making use of charge density measurements, we report on this paper a thermodynamic study of the hydrogen UPD process on Pt(111) in 0.05M NaOH over the range of temperatures from 283 ≤ T/K ≤ 313. The coulometric features corresponding to HUPD allow for the calculation of the hydrogen coverage and a fit to a Generalized Frumkin isotherm. From these values, different thermodynamic functions for the UPD reaction have been calculated: ΔGads, ΔSads, ΔHads, and the Pt-H bond energy. From extrapolation, a value of ΔSads ◦=-7.5±4Jmol-1K-1 was found, which is very close to 0, much lower than previously reported measurements both in acid and in alkaline solutions. Such value has an effect on the enthalpy and bond energy calculations, the latter having a decreasing tendency with pH and coverage. This tendency is completely different from the acidic systems and implies that the change in the thermodynamic functions due to the formation of the double layer and the reorganization of interfacial water has a strong influence on the process in high pH solutions.

Glycerol electrooxidation on Pd modified Au surfaces in alkaline media: Effect of the deposition method.

The catalytic effect of Pd on gold electrodes for glycerol oxidation is evaluated for Pd-Au surfaces prepared using three different methods: irreversible adsorption of palladium by a simple immersion of a gold electrode in palladium solution, the deposition of palladium on the gold substrate by a step potential from 1 to 0.75 V, and the forced deposition of palladium on the gold electrode with the help of a reducing hydrogen atmosphere. Voltammetry has been used for the electrochemical characterization of the Pd-Au deposits and to determine its reactivity towards glycerol oxidation, whereas FTIR experiments have allowed detecting adsorbed species and products formed during the oxidation reaction. Pd-Au surfaces prepared by irreversible adsorption are the electrodes that show the highest activity for the glycerol complete oxidation to carbonate, whereas Pd-Au surfaces made by the step potential are the catalyst that exhibits the highest rate for the formation and adsorption CO before carbonate production, poisoning the surface and diminishing their electrocatalytic properties. In addition to carbonate, glycerate, glycolate, and formate are detected as oxidation products. The integrated bands of the spectra are used to give quantitative information for comparing the product distribution of the different Pd-Au deposits prepared.

In situ surface characterization and oxygen reduction reaction on shape-controlled gold nanoparticles.

Gold nanoparticles of different shapes/surface structures were synthesized and electrochemically characterized. An in-situ surface characterization of the Au nanoparticles, which was able to obtain qualitative information about the type and relative sizes of the different facets present in the surface of the Au nanoparticles, was carried out by using Pb Under Potential Deposition (UPD) in alkaline solutions as a surface sensitive tool. The results obtained show that the final atomic arrangement on the surface can be different from that expected from the bulk structure of the well-defined shape Au nanoparticles. In this way, the development of precise in-situ methods to measure the distribution of the different sites on the nanoparticle surface, as lead UPD on gold surfaces, is highlighted. Oxygen Reduction Reaction (ORR) was performed on the different Au nanoparticles. In agreement with the particular sensitivity of the oxygen reduction to the presence of Au(100) surface domains, cubic Au nanoparticles show much better electrocatalytic activity for ORR than small spherical particles and long nanorods, in agreement with the presence of a great fraction of (100) terrace sites on the surface of cubic gold nanoparticles.

Shape-dependent electrocatalysis: methanol and formic acid electrooxidation on preferentially oriented Pt nanoparticles.

Reactivity towards methanol and formic acid electrooxidation on Pt nanoparticles with well characterised surfaces were studied and compared with the behaviour of single crystal electrodes with basal orientations. Polyoriented and preferential (100), (111) and (100)-(111) Pt nanoparticles were synthesised, cleaned preserving its surface structure, characterised and employed to evaluate the influence of the surface structure/shape of the Pt nanoparticles on these two relevant electrochemical reactions. The results pointed out that, in agreement with fundamental studies with Pt single crystal electrodes, the surface structure of the electrodes plays an important role on the reactivity of both oxidation processes, and thus the electrocatalytic properties strongly depend on the surface structure/shape of the nanoparticles, in particular on the presence of sites with (111) symmetry. These findings open the possibility of designing new and better electrocatalytic materials using decorated shape-controlled Pt nanoparticles as previously described with Pt single crystal electrodes.

Ethanol electrooxidation onto stepped surfaces modified by Ru deposition: electrochemical and spectroscopic studies.

Oxidation of ethanol on ruthenium-modified Pt(775) and Pt(332) stepped electrodes has been studied using electrochemical and FTIR techniques. It has been found that the oxidation of ethanol on these electrodes takes place preferentially on the step sites yielding CO(2) as the major final product. The cleavage of the C-C bond, which is the required step to yield CO(2), occurs only on this type of site. The presence of low ruthenium coverages on the step sites promotes the complete oxidation of ethanol since it facilitates the oxidation of CO formed on the step from the cleavage of the C-C bond. However, high ruthenium coverages have an important inhibiting effect since the adatoms block the step sites, which are required for the cleavage of the C-C bond. Under these conditions, the oxidation current diminishes and the major product in the oxidation process is acetic acid, which is the product formed preferentially on the (111) terrace sites.

Tellurium adatoms as an in-situ surface probe of (111) two-dimensional domains at platinum surfaces.

Irreversibly adsorbed tellurium has been studied as a probe to quantify ordered domains in platinum electrodes. The surface redox process of adsorbed tellurium on the Pt(111) electrode and Pt(111) stepped surfaces takes place around 0.85 V in a well-defined peak. The behavior of this redox process on the Pt(111) vicinal surfaces indicates that the tellurium atoms involved in the redox process are only those deposited on the (111) terrace sites. Moreover, the corresponding charge density is proportional to the number of sites on (111) ordered domains (terraces) that are, at least, three atoms wide. Hence, this charge density can be used to measure the number of (111) terrace sites on any given platinum sample. Structural information about tellurium adsorption is obtained from atomic-resolution STM images for the Pt(111) and Pt(10, 10, 9) electrodes. A rectangular structure (2 x radical 3) and a compact hexagonal structure (11 x 8) were identified. However, the redox peak for adsorbed tellurium on (100) domains at 1.03 V overlaps with peaks arising from steps and (110) sites. Therefore, it cannot be used without problems for the determination of (100) sites on a platinum sample. On the (100) terraces, the surface structure of the adsorbed tellurium is c(2 x 2), as revealed by STM. Finally, tellurium irreversible adsorption has been used to estimate the number of (111) ordered domains terrace sites on different polycrystalline platinum samples, and the results are compared to those obtained with bismuth irreversible adsorption.

Effect of deposited bismuth on the potential of maximum entropy of Pt(111) single-crystal electrodes.

The effect of bismuth adsorption on the entropy of formation of the double layer on Pt(111) electrodes has been studied with the laser-induced temperature jump method. The coulostatic response to the temperature change induced by pulsed laser illumination allows the estimation of the sign and magnitude of the thermal coefficient of the potential drop at the interphase. This is related to the entropy of formation of the double layer, and the particular potential where this thermal coefficient becomes zero can be identified with the potential of maximum entropy of double-layer formation (pme). The effect of bismuth adsorption on the pme depends on the adatom coverage. At high coverages, a marked decrease of the pme is observed. This trend follows the change of the potential of zero charge expected from work function measurements, and it is likely due to the change in the orientation of solvent molecules induced by surface dipoles originated between the adatom and the substrate. At low coverage, the pme increases with the bismuth coverage. The disruption of the water structure due to the presence of the bismuth adatoms is tentatively proposed as the most likely explanation for this behavior.

CO oxidation on Pt-modified Rh(111) electrodes.

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.