Mtm Marc Koper

Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions

A comparative study was performed to determine the reactivity of nitrate ions at 0.1 M on eight different polycrystalline electrodes (platinum, palladium, rhodium, ruthenium, iridium, copper, silver and gold) in acidic solution using cyclic voltammetry (CV), chronoamperometry and differential electrochemical mass spectroscopy (DEMS). Cyclic voltammetry shows that the current densities for nitrate reduction depend strongly on the nature of the electrode. The activities decrease in the order Rh>Ru>Ir>Pd and Pt for the transition-metal electrodes and in the order Cu>Ag>Au for the coinage metals. The rate-determining step on Ru, Rh, Ir, Pt, Cu, and Ag is concluded to be the reduction of nitrate to nitrite, as is evident from the Tafel slope, the kinetic reaction order in nitrate, and the anion effect. Transfer experiments with Pt suggest that chemisorbed nitric oxide is the key surface intermediate in the nitrate reduction. Since on-line mass spectrometry (DEMS) measurements on Pt and Rh show no formation of gaseous products such as nitric oxide (NO), nitrous oxide (N2O) or nitrogen (N2), it is suggested that ammonia and hydroxylamine are the main products on transition-metal electrodes. This is in agreement with the known mechanism for NO reduction, which forms N2O or N2 only if NO is in solution. On Cu, DEMS measurements show the production of gaseous NO, which is explained by the weaker binding of NO to Cu as compared to the transition metals.

The role of adsorbates in the electrochemical oxidation of ammonia on noble and transition metal electrodes

Abstract The activity for ammonia oxidation and the intermediates formed during the reaction have been studied on platinum, palladium, rhodium, ruthenium, iridium, copper, silver and gold electrodes. The activity in the selective oxidation to N2 is related directly to the nature of the species at the surface: the electrode is active if NHads (or NH2,ads) is present and deactivates when Nads is present. The potential at which NHads or Nads is formed is metal dependent. The observed trend in the strength of adsorption of Nads is Ru>Rh>Pd>Ir>Pt ≫ Au, Ag, Cu . This trend corresponds well with the trend observed in the calculated heat of adsorption of atomic nitrogen, with iridium being an exception. Platinum is the best catalyst for this reaction because Nads is formed at high potential, compared to rhodium and palladium, but seems to stabilize NHads rather well. Gold, silver and copper do not form NHads or Nads, and show only an activity when the surface becomes oxidized. The metal electrodissolution is enhanced by ammonia under these conditions. Most metals produce oxygen-containing products, like NO and N2O, at potentials where the surface becomes oxidized.

CO oxidation on stepped Pt[n(111) x (111)] electrodes

Abstract The oxidation of CO adlayers, formed by direct dosing from a CO-saturated solution, and bulk CO has been studied on Pt[ n (111)×(111)] single crystals in 0.5 M H 2 SO 4 . For the stepped Pt surfaces studied, CO is found to adsorb preferentially on the steps, blocking the electrochemical hydrogen adsorption there. A pronounced effect of electrode surface structure on CO oxidation has been observed. The overpotential for the oxidation of a saturated CO adlayer, as well as of submonolayer CO coverages, is found to increase in the sequence Pt(553)

Quantum-chemical calculations of CO and OH interacting with bimetallic surfaces

In this work we present results of a periodic density-functional theory study of the adsorption of carbon monoxide (CO) and hydroxyl (OH) on platinum–ruthenium, platinum–molybdenum and platinum–tin alloys as well as the adsorption of CO on a series of transition metals modified with a Pt overlayer. The surfaces are modelled as four-layer slabs (three-layer slab in case of Pt3Sn(111)). The binding energies and geometries of CO and OH are computed. In the case of PtRu, the mixing of Pt by Ru leads to a weaker bond of both CO and OH to the Pt sites, whereas mixing of Ru by Pt causes a stronger bond of CO and OH to the Ru sites. The binding energy trends for CO do not show a clear-cut relationship with its vibrational characteristics. The mixing of Pt by Mo leads to weakly adsorbed CO on both Pt and Mo sites, and OH strongly adsorbed only on Mo sites. This suggests that PtMo could be a better bifunctional catalyst for CO oxidation then PtRu. On Pt3Sn(111) the calculations show that CO binds only to Pt and not to the Sn, whereas OH has an energetic preference for the Sn sites. This also implies that PtSn should be a good CO oxidation catalyst. For Pt–monolayer systems, we demonstrate a relationship between the PtPt distance in the monolayer and the changes in the CO binding energy. The nature of the substrate seems to be of secondary importance.

Monte Carlo simulations of a simple model for the electrocatalytic CO oxidation on platinum

A simple lattice-gas model for the electrocatalytic carbon monoxide oxidation on a platinum electrode is studied by dynamic Monte Carlo simulations. The CO oxidation takes place through a Langmuir–Hinshelwood reaction between adsorbed CO and an adsorbed OH radical resulting from the dissociative adsorption of water. The model enables the investigation of the role of CO surface mobility on the macroscopic electrochemical response such as linear sweep voltammetry and potential step chronoamperometry. Our results show that the mean-field approximation, the traditional but often tacitly made assumption in electrochemistry, breaks down severely in the limit of vanishing CO surface mobility. Comparison of the simulated and experimental voltammetry suggests that on platinum CO oxidation is the intrinsically fastest reaction on the surface and that CO has a high surface mobility. However, under the same conditions, the model predicts some interesting deviations from the potential step current transients derived f...

Mechanism and kinetics of the electrochemical CO adlayer oxidation on Pt(111)

The electrochemical oxidation of saturated and sub-saturated CO adlayers on Pt(111) in 0.5 M H2SO4 has been studied using chronoamperometry. For the saturated CO coverage the oxidation is initiated by an apparently zeroth-order process of removing 2–3% of the adlayer, followed by the main oxidation process, which is shown to be of the Langmuir–Hinshelwood type with a competitive adsorption of the two reactants, CO and OH. The Langmuir–Hinshelwood kinetics can be modeled using the mean-field approximation, which implies fast diffusion of adsorbed CO on the Pt(111) surface under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation and its potential dependence are determined by a fitting of the experimental data with the mean-field model. For sub-saturated CO coverages the overall picture is shown to be more complicated and remains to be understood.

Ab initio calculations of intermediates of oxygen reduction on low-index platinum surfaces

Properties of the oxygen molecule, atomic oxygen, and intermediate products of its reduction, OH, OOH, H 2 O 2 on (111), (100), and (110) Pt surfaces have been investigated using periodic density functional theory. The Pt surfaces are modeled as four-layer slabs. Adsorption energies and geometries, as well as the charge-transfer properties are calculated. Computed characteristics of the adsorbed oxygen reduction intermediates supply known tendencies of the low index Pt surface activities under different experimental conditions. Electric field dependencies of the properties of all species adsorbed on a Pt 9 (111) cluster have been also studied. Lowering the field causes an increase of the O-O bond length of O 2ads , attracting the molecule to the Pt surface and increasing the charge transfer from Pt to 2π* orbitals of the oxygen molecule. The weakening of the O-O bond is evidenced by a decrease of the O-O stretching frequency. The charge-transfer from the Pt 9 cluster to the adsorbates is observed for all species. In our calculations hydrogen peroxide was unstable on all three low-index Pt surfaces and dissociated into two hydroxyls or a water molecule and atomic oxygen. The results of the calculations are discussed in the context of the mechanism of oxygen reduction.

Interaction of H, O and OH with metal surfaces

Abstract The interaction of the primary water dissociation products H, O and OH with various (111) metal surfaces is studied by density functional theory (DFT) calculations using clusters. It is found that H forms an essentially covalent bond with the metal, whereas O and OH form a largely ionic bond. The O and OH adsorbates prefer the high coordination three-fold hollow site on all metals: no such clear trend for H is found, the adsorption energy for on-top and hollow sites being comparable for most metals, especially on transition metals. The O and OH adsorbates are attracted towards, and donate some electronic charge to, the surface when a positive electric field (electrode potential) is applied, whereas the effect of an applied field on H adsorption is much smaller. We also show how the trends in the OH adsorption energies on different metals, as compared with O adsorption, can be explained by a weaker covalent interaction and a stronger Pauli repulsion of the OH with the metal d electrons.

Electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) surfaces

The electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) has been studied using voltammetry, chronoamperometry, and in situ infrared spectroscopy. The oxidative adsorption of ammonia results in the formation of NH(x) (x = 0-2) adsorbates. On Pt(111), ammonia oxidation occurs in the double-layer region and results in the formation of NH and, possibly, N adsorbates. The experimental current transients show a hyperbolic decay (t(-1)), which indicates strong lateral (repulsive) interactions between the (reacting) species. On Pt(100), the NH(2) adsorbed species is the stable intermediate of ammonia oxidation. Stabilization of the NH and NH(2) fragments on Pt(111) and Pt(100), respectively, is in an interesting agreement with recent theoretical predictions. The Pt(111) surface shows extremely low activity in ammonia oxidation to dinitrogen, thus indicating that neither NH nor N (strongly) adsorbed species are active in dinitrogen production. Neither nitrous oxide nor nitric oxide is the product of ammonia oxidation on Pt(111) at potentials up to 0.9 V, as deduced from the in situ infrared spectroscopy measurements. The Pt(100) surface is highly active in dinitrogen production. This process is characterized by a Tafel slope of 30 mV decade(-1), which is explained by a rate-determining dimerization of NH(2) fragments followed by a fast decay of the resulting surface-bound hydrazine to dinitrogen. Therefore, the high activity of the Pt(100) surface for ammonia oxidation to dinitrogen is likely to be related to its ability to stabilize the NH(2) adsorbate.

Modeling the butterfly: the voltammetry of (√3×√3)R30° and p(2×2) overlayers on (111) electrodes

Abstract The voltammetry of the formation of (√3×√3)R30° and p(2×2) overlayers on (111) electrodes is modeled by analytical and Monte Carlo techniques. Both ordered structures are formed by second-order order–disorder phase transitions that lead to sharply-peaked ‘butterfly’ features in the voltammogram. The butterflies for both systems are, however, distinctly different and resemble the voltammetry of Pt(111) in sulfuric and perchloric acid, respectively, even though the simulated adlayer structures are not exactly the same as the experimental ones. Some general features of butterfly peaks in voltammetry and their implications are discussed.