Platinum Single Crystal Surfaces Research Articles

Pt single crystal surfaces in electrochemistry and electrocatalysis

In this review, recent advances in the use of platinum single-crystal surfaces in electrochemistry are addressed.

Adsorption/Desorption Behaviors of Sulfur Species at Pt Single Crystal Surfaces

Platinum is an excellent electrocatalyst for polymer electrolyte membrane (PEM) fuel cells because it drives hydrogen oxidation and oxygen reduction with very high efficiency. However, when sulfur species contained as a trace amount of impurity in hydrogen fuel and/or pollutant in air adsorb on platinum surfaces, power output significantly decrease because of reduced electrochemically active surface area. Such negative effect caused by the adsorption of sulfur species, so-called sulfur poisoning, is a serious problem because interaction between sulfur species and platinum surfaces is very strong. Thus, understanding and control of adsorption behavior of sulfur species at platinum surfaces are very important to maintain the cell performance of PEM fuel cells. In the present study, adsorption/desorption behaviors of sulfur species at well-defined platinum single crystal surfaces in acidic aqueous solutions are studied by electrochemical measurements, x-ray photoelectron spectroscopy (XPS), and surface x-ray diffraction (SXRD).Prior to each experiment, platinum single crystal surfaces were annealed by induction heating method [1, 2, 3] at 1600°C more than 1 h under the flowing of Ar and H2 mixed gas, followed by cooling under the Ar and H2 flow for 7 min. The surface was immersed in a 1 mM Na2S aqueous solution saturated with Ar and H2 mixed gas, for 1 h. After rinsing with ultrapure water, electrochemical measurements were carried out in an aqueous solution of 0.1 M HClO4 or H2SO4 deaerated with pure N2 gas.At the bare Pt(111) surface, characteristic current responses such as adsorption/desorption of hydrogen and hydroxyl group were observed. After immersing in the sulfur-containing solution, however, those characteristic current responses disappeared, confirming the adsorption of sulfur species and decrease of electrochemically active surface area. XPS in the S 2p region of the Pt(111) surface immersed in the sulfur-containing solution showed three major adsorbed species; adsorbed hydrosulfide ion (Pt-HS- ad) at 161.8 eV, adsorbed sulfur (Pt-Sad) at162.7 eV, and adsorbed polysulfide (Pt-Sn) at 164.3 eV. While Pt-HS- ad reductively desorbed from the surface by the potential sweeping up to -0.2 V vs. Ag/AgCl, Pt-Sad and Pt-Sn oxidatively desorbed by the potential sweeping up to the potential more positive than 0.7 V vs. Ag/AgCl. SXRD showed that the sulfur species adsorbed at the Pt(111) surface in the (√3×√3)R30° structure.At the Pt(100) surface, the oxidative desorption of sulfur species occurred at more positive potential than that at the Pt(111) surface. This indicates that the recovery from sulfur poisoning at the Pt(100) surface is more difficult than that at the Pt(111) surface. Unlike the Pt(111)-(√3×√3)R30° structure, SXRD showed that the sulfur species adsorbed at the Pt(100) surface in the (√2×√2)R45° structure. While the sulfur species occupies a three-fold hollow site of the Pt(111)-(√3×√3)R30° structure, it occupies a four-fold hollow site of the Pt(100)-(√2×√2)R45° structure. The different coordination number of adsorbed sulfur species is considered as the origin of different oxidative desorption behaviors.

Inverse kinetic isotope effects in the oxygen reduction reaction at platinum single crystals.

Although the oxygen reduction reaction (ORR) involves multiple proton-coupled electron transfer processes, early studies reported the absence of kinetic isotope effects (KIEs) on polycrystalline platinum, probably due to the use of unpurified D2O. Here we developed a methodology to prepare ultra-pure D2O, which is indispensable for reliably investigating extremely surface-sensitive platinum single crystals. We find that Pt(111) exhibits much higher ORR activity in D2O than in H2O, with potential-dependent inverse KIEs of ~0.5, whereas Pt(100) and Pt(110) exhibit potential-independent inverse KIEs of ~0.8. Such inverse KIEs are closely correlated to the lower *OD coverage and weakened *OD binding strength relative to *OH, which, based on theoretical calculations, are attributed to the differences in their zero-point energies. This study suggests that the competing adsorption between *OH/*OD and *O2 probably plays an important role in the ORR rate-determining steps that involve a chemical step preceding an electrochemical step (CE mechanism).

Quantum effects in thermal reaction rates at metal surfaces.

There is wide interest in developing accurate theories for predicting rates of chemical reactions that occur at metal surfaces, especially for applications in industrial catalysis. Conventional methods contain many approximations that lack experimental validation. In practice, there are few reactions where sufficiently accurate experimental data exist to even allow meaningful comparisons to theory. Here, we present experimentally derived thermal rate constants for hydrogen atom recombination on platinum single-crystal surfaces, which are accurate enough to test established theoretical approximations. A quantum rate model is also presented, making possible a direct evaluation of the accuracy of commonly used approximations to adsorbate entropy. We find that neglecting the wave nature of adsorbed hydrogen atoms and their electronic spin degeneracy leads to a 10× to 1000× overestimation of the rate constant for temperatures relevant to heterogeneous catalysis. These quantum effects are also found to be important for nanoparticle catalysts.

Platinum single crystal electrodes: Prediction of the surface structures of low and high Miller indexes faces

Platinum is widely used as electrode in electrocatalysis studies; however, the use of polycrystalline surfaces introduces a series of variables in the electrochemical system, due to the random contribution of all crystallographic orientations with different superficial packing of atoms. On the other hand, low Miller index platinum single crystal electrodes present surface structure with high regularity and serve as models for establishing the correlation between atomic arrangements and macroscopic properties, among which their electrochemical behavior. Here we predict the different contributions of each low Miller indexes planes in the size of terraces and number of steps of the high Miller indexes faces of platinum (called stepped surfaces), establish​ the correlation between the structural properties of these surfaces and their electrochemical behavior, and discuss the surface reconstruction process of each platinum low Miller index face.

Single Crystal Electrochemistry as an In Situ Analytical Characterization Tool.

The electrochemical behavior of platinum single crystal surfaces can be taken as a model response for the interpretation of the activity of heterogeneous electrodes. The cyclic voltammogram of a given platinum electrode can be considered a fingerprint characteristic of the distribution of sites on its surface. We start this review by providing some simple mathematical descriptions of the voltammetric response in the presence of adsorption processes. We then describe the voltammogram of platinum basal planes, followed by the response of stepped surfaces. The voltammogram of polycrystalline materials can be understood as a composition of the response of the different basal contributions. Further resolution in the discrimination of different surface sites can be achieved with the aid of surface modification using adatoms such as bismuth or germanium. The application of these ideas is exemplified with the consideration of real catalysts composed of platinum nanoparticles with preferential shapes.

Site-specific reactivity of molecules with surface defects-the case of H2 dissociation on Pt.

The classic system that describes weakly activated dissociation in heterogeneous catalysis has been explained by two dynamical models that are fundamentally at odds. Whereas one model for hydrogen dissociation on platinum(111) invokes a preequilibrium and diffusion toward defects, the other is based on direct and local reaction. We resolve this dispute by quantifying site-specific reactivity using a curved platinum single-crystal surface. Reactivity is step-type dependent and varies linearly with step density. Only the model that relies on localized dissociation is consistent with our results. Our approach provides absolute, site-specific reaction cross sections.

Broken on impact

Surface Chemistry Two competing models have been proposed for the adsorption of molecular hydrogen on platinum surfaces. Both invoke dissociation at surface defects but differ on whether hydrogen molecules diffuse along the surface before encountering a defect or adsorb only if they initially impact a defect site. Van Lent et al. studied the sticking of hydrogen molecules from a molecular beam scanned across a curved platinum single-crystal surface that varied in the density and type of defects exposed. Modeling of the results was consistent only with the second model invoking direct impact. Science , this issue p. [155][1] [1]: /lookup/doi/10.1126/science.aau6716

In situ Raman spectroscopic evidence for oxygen reduction reaction intermediates at platinum single-crystal surfaces

Developing an understanding of structure–activity relationships and reaction mechanisms of catalytic processes is critical to the successful design of highly efficient catalysts. As a fundamental reaction in fuel cells, elucidation of the oxygen reduction reaction (ORR) mechanism at Pt(hkl) surfaces has remained a significant challenge for researchers. Here, we employ in situ electrochemical surface-enhanced Raman spectroscopy (SERS) and density functional theory (DFT) calculation techniques to examine the ORR process at Pt(hkl) surfaces. Direct spectroscopic evidence for ORR intermediates indicates that, under acidic conditions, the pathway of ORR at Pt(111) occurs through the formation of HO2*, whereas at Pt(110) and Pt(100) it occurs via the generation of OH*. However, we propose that the pathway of the ORR under alkaline conditions at Pt(hkl) surfaces mainly occurs through the formation of O2−. Notably, these results demonstrate that the SERS technique offers an effective and reliable way for real-time investigation of catalytic processes at atomically flat surfaces not normally amenable to study with Raman spectroscopy. The oxygen reduction reaction, catalysed by platinum, is a crucial process in the operation of fuel cells, but the mechanistic pathways through which it occurs remain a matter for debate. Here, the authors use in situ Raman spectroscopy to identify key intermediates for this reaction at different atomically flat platinum surfaces, shedding light on the mechanism.

Selective catalytic reduction of NO in the presence of SO2 and O2: The poisoning effect of SOx under oxygen rich and lean conditions

SOx poisoning has been a critical problem in the chemistry of selective catalytic reduction of NOx, and many fundamental issues surrounding the reactions remain unclear. In this paper, reactions taking place in the 13C2H5OHNO-S18O2O2 system on a platinum single crystal surface were studied. Oxidation of 13C2H5OH by S18O2 proceeds mainly through 18O-H reaction, and oxidation of the backbone carbon atoms can occur at high S18O2 exposures, e.g., 1.6 Ls of S18O2 for 0.4 L of 13C2H5OH. Thus, the presence of S18O2 only slightly suppresses the catalytic reduction of NO by 13C2H5OH. Co-adsorbed O2 evidently exerts a suppressive effect. When oxygen is lean (relative to S18O2), the S18O2O2 reactions mainly give rise to surface sulphoxy species at temperatures between 400 and 500 K and thus, the NO-13C2H5OH reactions can still proceed. The NO reduction reaction is completely suppressed when oxygen is rich, a result of the Pt surface being passivated. This finding provides a valuable guide for optimizing the reaction atmosphere to improve the efficiency of the selective catalytic reduction of NO by hydrocarbons (or oxygen-containing fragments from oxidation of hydrocarbons in this case).

Phosphorus adlayers on Platinum (110)

Platinum is a metal utilized in many applications. Its catalytic activity can be decreased due to chemical poisoning caused e.g. by phosphorus. To gain more understanding of its poisoning, we present a study of phosphorus adsorption on a platinum (110) single crystal surface. Using X-ray photoelectron spectroscopy, we have found that the adsorbate coverage saturates at around 3 monolayers. Annealing the phosphorus-covered platinum surface at 750 °C gives rise to three different ordered adlayer structures, with symmetries of 2 × 3, 11 × 4 and 2×1, from the lowest to the highest coverage, detected with low-energy electron diffraction. We have studied the sample topography with scanning tunnelling microscopy. We also present a tentative model for the observed structures and their evolution.

On the quality and stability of preferentially oriented (100) Pt nanoparticles: An electrochemical insight

Previous knowledge shed by single crystal electrochemistry path the way to obtain better catalysts aiming fuel cell purposes. In this sense, tailoring shape-controlled nanoparticles has been a strategy followed over the past years to optimize the activity, selectivity and stability of electrocatalytic surface sensitive reactions by maximizing the exposure of certain facets of the nanocatalyst. In the present paper, preferentially oriented (100) nanoparticles synthesized by different methodologies have been electrochemically characterized and compared. The voltammetric profile of these cubic nanoparticles has also been compared with different platinum single crystal stepped surfaces, showing a singular fitting to the characteristic response of Pt(510). Subtle changes on the surface structure of platinum nanocubes due to surface degradation upon storage time in aqueous solution are pointed out in this work. The characteristic fingerprints of platinum surfaces and CO oxidation profiles have demonstrated to be powerful surface probes for the in situ detection of these minor structural reorganizations.

Explicit Detection of the Mechanism of Platinum Nanoparticle Shape Control by Polyvinylpyrrolidone

The irreversible adsorption of polyvinylpyrrolidone (PVP) on a series of well-defined platinum single crystal electrode surfaces has been investigated using voltammetry, ex situ XPS and DFT calculations. It is found that the adsorption of PVP is highly structure sensitive with strong adsorption exhibited by step and {100} terrace sites with only weak interactions observed between PVP and Pt{111} terraces, at least at low PVP surface concentrations. Subsequent investigations using CO electrooxidation confirmed that blocking of platinum surface sites by PVP toward CO chemisorption was marked for Pt{100} terraces but hardly occurred at all at Pt{111} terraces. Density Functional Theory calculations also confirmed that the monomer of PVP interacts more strongly with Pt{100} compared to Pt{111} sites (−142 and −125 kJ mol–1 respectively). Ex situ XPS studies suggested that the main PVP–Pt interaction is associated with charge transfer from the carbonyl substituent of PVP toward the metal surface in accordance ...

Kinetics of gas phase formic acid decomposition on platinum single crystal and polycrystalline surfaces

Formic acid dehydrogenation turnover rates (TORs) were measured on Pt(111), Pt(100), and polycrystalline Pt foil surfaces at a total pressure of 800Torr between 413 and 513K in a batch reactor connected to an ultra-high vacuum (UHV) system. The TORs, apparent activation energies, and reaction orders are not sensitive to the structure of the Pt surface, within the precision of the measurements. CO introduced into the batch reactor depressed the formic acid dehydrogenation TOR and increased the reaction's apparent activation energies on Pt(111) and Pt(100), consistent with behavior predicted by the Temkin equation. Two reaction mechanisms were explored which explain the formic acid decomposition mechanism on Pt, both of which include dissociative adsorption of formic acid, rate limiting formate decomposition, and quasi-equilibrated hydrogen recombination and CO adsorption. No evidence was found that catalytic supports used in previous studies altered the reaction kinetics or mechanism.

When Small is Big: The Role of Impurities in Electrocatalysis

Improvements in the fundamental understanding of electrocatalysis have started to revolutionize the development of electrochemical interfaces for the efficient conversion of chemical energy into electricity, as well as for the utilization of electrons to produce new chemicals that then can be re-used in energy conversion systems. Here, some facets of the role of trace level of impurities (from 10−7 to 10−6 M) in electrocatalysis of the oxygen reduction reaction, hydrogen oxidation and evolution reactions, and CO oxidation reactions are explored on well-characterized platinum single crystal surfaces and high surface area materials in alkaline and acidic environments. Of particular interest is the effect of anions (e.g., Cl−, $$ {\text{NO}}_{ 3}^{ - } $$ ) and cations (i.e., Cu2+) present in the supporting electrolytes as well as surface defects (i.e., ad-islands) that are present on metal surfaces. The examples presented are chosen to demonstrate that a small level of impurities may play a crucial role in governing the reactivity of electrochemical interfaces.

Interaction of water with methanesulfonic acid on Pt single crystal electrodes

The electrochemical behavior of methanesulfonic acid on platinum single crystal electrode surfaces is investigated by cyclic voltammetry and infrared spectroscopy measurements. The results are compared with the voltammetric profiles of perchloric and trifluoromethanesulfonic acids. The differences are interpreted in terms of the effect of the anion on the structure of water. No adsorbed species are detected by infrared spectroscopy.

Adsorption and first stages of polymerization of aniline on platinum single crystal electrodes

The present communication studies the adsorption of aniline on platinum single crystal electrodes and the electrochemical properties of the first layers of polyaniline(PANI) grown on those platinum surfaces. The adsorption process was studied in aqueous acidic solution (0.1M HClO4) and the electrochemical properties of thin films of PANI in both aqueous (1M HClO4) and non-aqueous media (tetrabutyl ammonium hexafluorophosphate (TBAPF6) with additions of methanesulphonic acid in acetonitrile). First of all, it was found that the adsorption of aniline on platinum single crystal surfaces is a surface sensitive process, and even more important that the adsorption features found at low concentrations (5×10−5M) can be directly correlated to the electrochemical properties of thin films of PANI in the very early stages of polymerization. The Pt(110) surface was found to be more suitable to obtain polymers with more reversible redox transitions when studied in aqueous media (1M HClO4). This is in good agreement with the higher polymerization rates found on this surface compared to Pt(100) and Pt(111). Finally the differences in ionic exchange rate were greatly enhanced when they were studied in organic media. The AC 250Hz response in the case of the thin films synthesized on Pt(110) is about twice greater than that obtained in the other basal planes using polymer layers with the same thickness.

Comparative study of the catalytic hydroconversion of cyclopentane over iridium and platinum single-crystal surfaces

Abstract In the context of fuel upgrading by selective ring opening of naphthenes, we have investigated the catalytic conversion of cyclopentane in large hydrogen excess over iridium and platinum single-crystal surfaces. Both (111) and (112) orientations have been considered. The catalytic tests have been performed at 1 kPa and 25–600 °C using a recently developed surface reactor equipped with laser heating and online gas chromatography. Only cyclopentene and C 1 –C 4 cracking products are formed on iridium, while platinum additionally catalyzes the formation of pentane around 200 °C, which dehydrogenates to pentene at 250 °C. Noticeably, on both metals, the surface steps prevent hydrocarbon cracking (up to 400 °C) at the benefit of dehydrogenation. In all cases, a carbon overlayer is formed on the surfaces in the course of the reaction.

Oxygen reduction reaction at Pt single crystals: a critical overview

Oxygen reduction reaction (ORR) dynamics at platinum single crystal surfaces is reviewed, and experimental results in acid and alkaline solutions are discussed in the framework of theoretical studies. Special emphasis is devoted to point out the role of the surface charge, water structure and adsorbed oxygen containing species. Additionally, discussion about the possible relevance of hydrogen peroxide as an intermediate species is also included. It is shown that the ORR is a complex process affected by many different factors and so neither surface charge nor oxygen-containing species coverage alone are a determining factor of the electrode activity. Instead, adlayer structures and the relation between adsorbed water, H2Oads, water dissociation products, OHads or Oads, and PtO oxide species coverage affect the whole energetics of the adsorption processes and may determine the surface reactivity. Finally, if H2O2 is an intermediate product in the ORR mechanism, it would be crucial to find a suitable catalyst able to effectively reduce H2O2 at high potentials and inhibit its oxidation.

Fundamental aspects of HCOOH oxidation at platinum single crystal surfaces with basal orientations and modified by irreversibly adsorbed adatoms

Oxidation of formic acid at Pt(hkl) electrodes with basal orientations and modified by irreversibly adsorbed adatoms is revisited. It was shown that the adatoms of the nitrogen group enhance the electrocatalytic activity through long-range electronic effects in the case of Pt(111) substrates or through shorter third-body effects when adsorbed at Pt(100) electrodes. In both cases, it appears that free platinum sites were required. Special attention is given here to the reactivity at Pt(110) substrates. It is found that maximum electrocatalysis is observed at fully blocked surfaces. This result points out again structure sensitivity effects for this characteristic reaction. Unlike the more compact planes, in which the enhancement of the oxidation rate was explained through essentially rigid models for the adlayer, it is suggested that the resulting enhancement observed at Pt(110) substrates could be explained if some adatom mobility is considered.