Ammonia selective oxidation on Pt(100) sites in an alkaline medium

The oxidation of ammonia on platinum surfaces is a structure sensitive reaction that takes place almost exclusively on Pt(100) sites. This report shows how dependent the activity is on different arrangements of (100) sites of platinum. The effect of two-dimensional domains has been addressed by using stepped surfaces having terraces with (100) geometry, either with (111) and (110) steps. The results were compared with those obtained from stepped surfaces having terraces with (111) or (110) symmetry and monatomic (100) steps, thus representing monodimensional (100) domains. The observed behavior confirms the extreme sensitivity of the reaction to the different arrangement of this type of square sites.     

Adsorption of hydrogen on Pt(1 1 1) and Pt(1 0 0) surfaces and its role in the HOR

Hydrogen adsorption isotherms, evaluated by combination of cyclic voltammetry and chronoamperometry, are reported on Pt(1 1 1) and Pt(1 0 0) surfaces in 0.1 M HClO₄. We found that at E > 0.05 V Pt(1 1 1) and Pt(1 0 0) are only partially covered by the adsorbed hydrogen (H_ad). On both surfaces, a full monolayer of the adsorbed hydrogen is completed at −0.1 V, i.e. the adsorption of atomic hydrogen is observed in the hydrogen evolution potential region. We also found, that the activity of the hydrogen oxidation reaction is mirrored by the shape of the hydrogen adsorption isotherms, implying that H_ad is in fact a spectator in the HOR.     

Preferential oxidation of carbon monoxide adsorbed on Pd submonolayer films deposited on Pt(1 0 0)

The electrochemical behavior of palladium modified Pt(1 0 0) in perchloric acid solution has been studied by means of cyclic voltammetry, CO bulk oxidation and in-situ Fourier transform infrared (FTIR) spectroscopy. FTIR spectra on Pd submonolayers reveal that at low potentials (about 0.4 V) Pd-bounded CO is oxidized preferentially, whereas at the same time the Pt–CO oxidation rate is rather slow.     

Synthesis and characterization of preferentially oriented (1 0 0) Pt nanowires

Two types of platinum deposits were obtained by potentiostatic deposition onto Ti substrates, namely platinum black (Pt B) and platinum nanowires (Pt NW) with the latter being achieved through a porous anodic aluminum oxide (AAO) membrane at the solution/substrate interface. Surface characterization of these deposits was performed using scanning electron microscopy (SEM) and cyclic voltammetry (CV) in sulphuric acid solution. Surface properties for Pt NW revealed a predominant presence of (1 0 0) crystallographic planes, not present in Pt B deposit grown in the same conditions. Also, Pt NW exhibits an increased resistance to electrochemically active surface area (EASA) loss upon potential cycling in acidic solution, as compared to Pt B.     

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.     

Long-range effects on palladium deposited on Pt(1 1 1)

Two-dimensional long-range effects have been pointed out electrochemically for palladium adlayers on Pt(1 1 1) in the absence of anion specific adsorption. The observation of these effects requires well ordered electrode substrates. In this way, the characteristic features are not observed on stepped electrodes with 20 atoms wide terraces. The formation of a homogeneous adlayer is also required and the effects are pointed out only when the palladium coverage is close to the completion of the full monolayer.     

Preparation and electrocatalytic activity of Rh adlayers on Pt(1 0 0) electrodes: reduction of nitrous oxide

Rhodium adlayers (submonolayer range) have been prepared on Pt(1 0 0) electrodes by electrodeposition from acidic solutions containing an excess of chloride. These Rh/Pt(1 0 0) electrodes give a well-defined voltammetric signal in the hydrogen adsorption region, which gives evidence of a high level of order in the Rh adlayer and allow a reliable estimation of the coverage. The voltammetric behavior of the Rh/Pt(1 0 0) electrodes points to an epitaxial growth with formation of rhodium islands. The well-ordered bimetallic surfaces freshly prepared were tested as electrocatalysts for nitrous oxide reduction and the responses were compared with those of the bulk Pt(1 0 0) and Rh(1 0 0) electrodes. The voltammogram for the bimetallic surface showed well separated N₂O reduction signals for Rh and Pt surface zones. An exceptionally high electrocatalytic activity for the Rh adlayer was found for low coverages. This behavior is explained on the basis of a high activity of the rhodium adatoms in the periphery of the islands.     

Electrocatalytic oxidation of methanol on Pt/NiZn electrode in alkaline medium

In this study, a platinum electrode was coated with NiZn layer (Pt/NiZn) in a nickel-zinc bath by electrodeposition for use as anode material for methanol electrooxidation in alkaline solution. The electrode prepared was etched in a concentrated alkaline solution (30% NaOH) to produce a porous and electrocatalytic surface suitable for use in the methanol electrooxidation (Pt/NiZn). The surface morphologies and compositions of coating before and after alkaline leaching were determined by energy dispersive X-ray (EDX) and scanning electron microscopy (SEM) techniques. The effect of NiZn coated platinum electrode for methanol electrooxidation was investigated in 1 M NaOH solution by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. Methanol electrooxidation on Pt/NiZn electrode was studied at various temperatures and potential scan rates. The results showed that Pt/NiZn electrode behaved as an efficient catalyst for the electrooxidation of methanol in alkaline medium.     

A possible approach towards spin-polarized transport through single molecule magnets: Mn12 on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0)

The possibility to use the Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) system as a substrate for future spin-polarized transport measurements on Mn₁₂ single molecule magnets has been investigated by means of scanning tunneling microscopy and X-ray photoelectron spectroscopy at room temperature. In particular, the stability of the iron layer during a wet chemical preparation of Mn₁₂ monolayers was studied. The results demonstrate that Mn₁₂ can be deposited on Au(1 0 0)/Fe(1 0 0)/MgO(1 0 0) while preserving the metallic nature of the ferromagnetic iron layer which is required as a possible source of spin-polarized electrons in future studies.     

Adsorption of alkali metals on Ge(0 0 1)(2 × 1) surface

Ab initio total energy calculations have been performed for Na, K and Rb adsorption on Ge(0 0 1)(2 × 1) surface. It was found that the adsorption site of AM is AM size dependent. Structural analysis showed that the Ge–Ge dimer bond becomes stronger with increasing AM size. As the coverage increases from 0.5 to 1 ML it turns out that no depolarization effect occurs upon Na adsorption, while this effect becomes more important with increasing AM size. We also found that for all adsorption systems investigated the germanium surface is metallic and semiconducting for the coverage of 0.5 and 1 ML, respectively.     

CI Study of CO adsorption on MgO(1 0 0)

Using CI embedding method, we have studied the adsorption of CO on MgO(1 0 0). The MgO(1 0 0) substrate is described by a Mg₉O₉ (3 × 3 × 2) core cluster, embedded in ionic (Mg²⁺/O²⁻) core potentials. The adsorption energy is calculated to be 0.11 eV at the CI level with a blue shift of 19 cm⁻¹ for CO stretching on MgO(1 0 0). The dispersion accounts only 35% of the total binding energy of CO on MgO(1 0 0). The CO/MgO(1 0 0) interaction is weak and mainly of the van der Waals type with only slight chemical bonding characters.     

Mechanism of CO oxidation on Pt(111) in alkaline media

Electrochemical techniques, coupled with in situ scanning tunneling microscopy, have been used to examine the mechanism of CO oxidation and the role of surface structure in promoting CO oxidation on well-ordered and disordered Pt(111) in aqueous NaOH solutions. Oxidation of CO occurs in two distinct potential regions: the prepeak (0.25-0.70 V) and the main peak (0.70 V and higher). The mechanism of reaction is Langmuir-Hinshelwood in both regions, but the OH adsorption site is different. In the prepeak, CO oxidation occurs through reaction with OH that is strongly adsorbed at defect sites. Adsorption of OH on defects at low potentials has been verified using charge displacement measurements. Not all CO can be oxidized in the prepeak, since the Pt-CO bond strength increases as the CO coverage decreases. Below theta(CO) = 0.2 monolayer, CO is too strongly bound to react with defect-bound OH. Oxidation of CO at low coverage occurs in the main peak through reaction with OH adsorbed on (111) terraces, where the Pt-OH bond is weaker than on defects. The enhanced oxidation of CO in alkaline media is attributed to the higher affinity of the Pt(111) surface for adsorption of OH at low potentials in alkaline media as compared with acidic media.     

Nitrate reduction on Pt(1 1 1) surfaces modified by Bi adatoms

This work presents the effect of Bi modification of Pt(1 1 1) electrodes on the electroreduction of nitrate anions by using voltammetric and FTIR experiments. On Pt(1 1 1) nitrate consumption occurs at potentials lower than 0.35 V, but with Pt(1 1 1)/Bi this process is shifted to significantly higher potentials (0.6–0.7 V). In the latter surface N₂O was observed as the main product in solution. Different forms of adsorbed NO were detected on the adatom covered surfaces as well as on clean Pt(1 1 1).     

Methanol dehydrogenation and oxidation on Pt(111) in alkaline solutions

Adsorption, dehydrogenation, and oxidation of methanol on Pt(111) in alkaline solutions has been examined from a fundamental mechanistic perspective, focusing on the role of adsorbate-adsorbate interactions and the effect of defects on reactivity. CO has been confirmed as the main poisoning species, affecting the rate of methanol dehydrogenation primarily through repulsive interactions with methanol dehydrogenation intermediates. At direct methanol fuel cell (DMFC)-relevant potentials, methanol oxidation occurs almost entirely through a CO intermediate, and the rate of CO oxidation is the main limiting factor in methanol oxidation. Small Pt island defects greatly enhance CO oxidation, though they are effective only when the CO coverage is 0.20 ML or higher. Large Pt islands enhance CO oxidation as well, but unlike small Pt islands, they also promote methanol dehydrogenation. Perturbations in electronic structure are responsible for the CO oxidation effect of defects, but the role of large Pt islands in promoting methanol dehydrogenation is primarily explained by surface geometric structure.     

Comparative theoretical study of the structure and bonding of propyne on the Pt(1 1 1) and Pd(1 1 1) surfaces

The interaction of propyne with the Pt(1 1 1) and Pd(1 1 1) surfaces has been studied by means of the generalised gradient approach of density functional theory using periodic slab models. For both surfaces, the most stable adsorption mode of propyne is di-σ/π mode where the hydrocarbon is σ-bonded to two metal atoms with some additional π bonding to a third adjacent surface atom. The adsorption geometry is a highly distorted propyne with the C₁ and C₂ in a nearly sp² hybridisation. Two equivalent surface structures have been found on Pt and Pd. These correspond to the adsorption on the fcc or hcp hollow sites. The adsorption energies on Pt(1 1 1) and Pd(1 1 1) are predicted to be ∼−197 and −161 kJ mol⁻¹, respectively. The electronic factors that control the chemisorption have been analysed by means of the projected density of states.     

First-principles study of hydrogen adsorption on Mo(1 1 0)

First-principles calculations based on density functional theory-generalized gradient approximation method have been performed for hydrogen (H) adsorption on Mo(1 1 0) surface. For various coverages, the hollow (hol) site was found to be the most stable binding site. The adsorption energy of this site was slightly increased as the increasing of hydrogen coverage. Subsurface (sub) occupation at low and medium coverages was ruled out while it became to be stable at the coverage of 1 ML. This is also supported by the potential energy surface (PES) study for hydrogen diffusing from hol to sub site. It’s interesting to find a surface reconstruction at the coverage of 1 ML, which is characterized by the lateral shift of the topmost layer for the sub adsorption. At higher coverage, the local density of states (LDOS) analysis showed that a new peak was clearly visible which was ascribed to a surface state induced by hydrogen adsorption. This surface state was mostly localized on the hydrogen atom and the first Mo layer, implying the hybridization of the hydrogen 1s states and the Mo metal states.     

New understanding of the nature of OH adsorption on Pt(1 1 1) electrodes

The adsorption of hydroxyl on Pt(1 1 1) single crystal electrodes from aqueous acidic solutions is carefully reinvestigated. The effect of small additions (10⁻⁸–10⁻⁵ M) of chloride and bisulphate anions on the OH adsorption region in perchloric acid solution has been studied. Two regions can be differentiated in the voltammetric profile, that behave differently after the addition of the foreign anion. The initial broad adsorption process is unaffected until the highest concentration is attained. However, the sharper peak at higher potentials is affected even at the lower anion concentration. Since mass transport limitations allow to discard the anion adsorption as the main process giving this peak, we propose that the two processes are due to the dissociative adsorption of two different kinds of water, that are affected by the anion in a different way. From this idea, a new model, based on the Frumkin adsorption isotherm, is proposed, which gives an excellent fit of the experimental results.     

Addition reactions of nitrones on the reconstructed C(1 0 0)-2 × 1 surfaces

In this paper, the reactions of nitrone, N-methyl nitrone, N-phenyl nitrone and their hydroxylamine tautomers (vinyl-hydroxylamine, N-methyl-vinyl-hydroxylamine and N-phenyl-vinyl-hydroxylamine) on the reconstructed C(1 0 0)-2 × 1 surface have been investigated using hybrid density functional theory (B3LYP), Møller–Plesset second-order perturbation (MP2) and multi-configuration complete-active-space self-consistent-field (CASSCF) methods. The calculations showed that all the nitrones can react with the surface “dimer” via facile 1,3-dipolar cycloaddition with small activation barriers (less than 12.0 kJ/mol at B3LYP/6-31g(d) level). The [2+2] cycloaddition of hydroxylamine tautomers on the C(1 0 0) surface follows a diradical mechanism. Hydroxylamine tautomers first form diradical intermediates with the reconstructed C(1 0 0)-2 × 1 surface by overcoming a large activation barrier of 50–60 kJ/mol (B3LYP), then generate [2+2] cycloaddition products via diradical transition states with negligible activation barriers. The surface reactions result in hydroxyl or amino-terminated diamond surfaces, which offers new opportunity for further modifications.