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 NHx (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 NH2 adsorbed species is the stable intermediate of ammonia oxidation. Stabilization of the NH and NH2 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 NH2 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 NH2 adsorbate

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

Field-dependent electrode-chemisorbate bonding: sensitivity of vibrational stark effect and binding energetics to nature of surface coordination

Illustrative quantum-chemical calculations for selected atomic and molecular chemisorbates on Pt(111) (modeled as a finite cluster) are undertaken as a function of external field, F, by using Density Functional Theory (DFT) with the aim of ascertaining the sensitivity of the field-dependent metal-adsorbate binding energetics and vibrational frequencies (i.e., the vibrational Stark effect) to the nature of the surface coordination in electrochemical systems. The adsorbates selected - Cl, I, O, N, Na, NH3, and CO - include chemically important examples featuring both electron-withdrawing and -donating characteristics. The direction of metal-adsorbate charge polarization, characterized by the static dipole moment, µS, determines the binding energy-field (Eb-F) slopes, while the corresponding Stark-tuning behavior is controlled primarily by the dynamic dipole moment, µD. Significantly, analysis of the F-dependent sensitivity of µS and µD leads to a general adsorbate classification. For electronegative adsorbates, such as O and Cl, both µS and µD are negative, the opposite being the case for electropositive adsorbates. However, for systems forming dative-covalent rather than ionic bonds, as exemplified here by NH3 and CO, µS and µD have opposite signs. The latter behavior, including electron-donating and -withdrawing categories, arises from diminishing metal-chemisorbate orbital overlap, and hence the extent of charge polarization, as the bond is stretched. A clear-cut distinction between these different types of surface bonding is therefore obtainable by combining vibrational Stark-tuning and Eb-F slopes, as extracted from experimental data and/or DFT calculations. The former behavior is illustrated by means of potential-dependent Raman spectral data obtained in our laboratory

Large-scale computer simulation of an electrochemical bond-breaking reaction

A novel Hamiltonian is employed to explicitly simulate an electrochemical bond-breaking reaction in which an electron-transfer reaction is directly coupled to the dissociation of a molecular species. The free energy surface as a function of both the collective solvation coordinate of the electron transfer and the intramolecular bond length of a CH3Cl molecule is computed by virtue of a classical molecular dynamics (MD) simulation. The method is also easily generalized to treat a variety of electrochemically catalyzed phenomenon. The simulation data show very significant deviations from the predictions of standard analytical theory

Field-dependent chemisorption of carbon monoxide on platinum-group (111) surfaces: relationships between binding energetics, geometries, and vibrational properties as assessed by density functional theory

The field-dependent frequency behavior of the metal-adsorbate (¿M-CO) as well as the intramolecular (¿CO) vibration of carbon monoxide chemisorbed in atop and threefold-hollow sites on three platinum-group (111) metal surfacesPt, Ir, and Pdis explored in relation to the metal-chemisorbate (M-CO) binding energetics and geometries by means of Density Functional Theory (DFT) calculations for finite clusters. This overall objectivehaving particular importance in electrochemical systemsof linking field-dependent vibrational, energetic, and geometric properties of the M-CO bond, prompted by the availability of potential-dependent ¿M-CO data at Pt-group electrodes from Raman spectroscopy, provides an opportunity to assess in quantum-chemical terms these surface-adsorbate binding parameters in relation to the extensively studied intramolecular CO vibration. The binding energies (-Eb) tend to increase toward negative fields (F), especially for hollow-site binding. An energy decomposition into specific orbital and steric interactions shows that this effect is driven primarily by enhanced p-back-donation, although offset by progressively weaker s-donation along with greater surface-chemisorbate steric repulsion. Although these individual orbital and steric interactions exert similar effects on the ¿M-CO frequencies, the overall ¿M-CO-F dependencies are notably different, typically displaying a broad maximum at moderate/large negative fields (ca. -0.3 to -0.5 V Å-1). Unlike the binding-energy behavior, these nonmonotonic ¿M-CO-F dependencies correlate roughly with the corresponding F-dependent M-CO equilibrium bond lengths, rM-CO. A decomposition of the field-dependent ¿M-CO and rM-CO behavior into individual interactions exhibits close parallels, with p-bonding acting to markedly blue-shift ¿M-CO and decrease rM-CO, being offset increasingly toward more negative fields by the effects of s-bonding and steric repulsion. In contrast, the monotonically red-shifted ¿CO frequencies and the correspondingly elongated C-O bond lengths, rCO, found toward negative fields arise chiefly from the well-known effects of dp-2p* back-donation. A common correlation is observed between the field-dependent ¿CO and rCO values for each of the metal-CO systems and even uncoordinated CO. The likely role of electrostatic factors in the ¿M-CO-F dependencies is also considered: the increasing M ¿ CO charge polarization seen toward negative fields can account qualitatively for the ¿M-CO-F maxima. A semiquantitative agreement is evident with electrode potential-dependent ¿M-CO and ¿CO vibrational data, although ¿M-CO-F maxima have yet to be observed experimentally