Scanning electrochemical microscopy of hydrogen electro-oxidation. Rate constant measurements and carbon monoxide poisoning on platinum

We describe an application of the scanning electrochemical microscope that uses tip–sample feedback to characterize the electro-oxidation of hydrogen on a polycrystalline platinum electrode in sulfuric acid solutions in the presence and absence of adsorbed carbon monoxide. The hydrogen oxidation reaction is probed by reducing protons at a diffusion-limited rate at the microscope's tip electrode while it is positioned near a platinum substrate. A series of approach curves measured as a function of the substrate potential provides hydrogen oxidation rate constant values over a wide range of substrate conditions. In the absence of CO, the rate of hydrogen oxidation exceeds 1 cm s⁻¹ at potentials within the hydrogen adsorption and double layer charging regions. A Tafel slope of 30 mV per decade is determined near the reversible potential. At increasingly positive substrate potentials, the hydrogen oxidation rate decreases exponentially with increasing potential as the surface is covered with an oxide layer. The adsorption of solution-phase carbon monoxide completely deactivates the platinum substrate towards steady-state hydrogen oxidation over a large range of substrate potentials. Approach curves indicate a near-zero rate constant for hydrogen oxidation on CO-covered platinum at potentials below oxide formation. An increase in the hydrogen oxidation rate is seen at potentials sufficiently positive that CO fails to adsorb and the platinum oxide forms. In comparison, dynamic tip–substrate voltammetry depicts a complex substrate response whereby the adsorbed carbon monoxide layer transforms from a weakly adsorbed state at low potentials to a strongly adsorbed state at high potentials. Although steady-state approach curve measurements depict the complete deactivation of catalytic activity at these potentials, a significant hydrogen oxidation current is observed during the potential-induced transformation between these weakly and strongly adsorbed CO states. The rate of hydrogen oxidation approaches that of a pristine platinum surface during this surface transformation before returning to the poisoned state.     

Pt-Mo电极上CO、甲醛和甲醇的电催化氧化-微分电化学质谱研究

利用微分电化学质谱（DEMS）研究了Mo修饰的Pt电极上CO、甲醛和甲醇的电催 化氧化,证实了Mo(IV)是催化活性样品,而且它只对弱吸附CO的氧化起催化作用, 对强吸附CO的氧化没有催化活性。在低于0.4 V的电位下,吸附在Pt电极上的Mo结 甲醇和甲醛的催化氧化是通过弱吸附CO的氧化路径进行的。     

Electrocatalytic oxidation of carbon monoxide and methanol at Pt nanoparticles confined in SBA-15: voltammetric and in situ infrared spectroscopic studies

Electrocatalytic oxidation of carbon monoxide and methanol at Pt nanoparticles confined in mesoporous molecular sieve SBA-15 was studied by using cyclic voltammetry and in situ FTIR spectroscopy. Cyclic voltammetric studies revealed that the Pt nanoparticles confined in SBA-15 exhibit a high activity in the presence of hydrated phase consisting of SiO₂ in the SBA-15. In situ FTIR spectroscopy results discovered that IR absorption of CO adsorbed on Pt nanoparticles confined in SBA-15 has been enhanced 11-fold, and the full-width at half-maximum of the CO band is significantly increased, in comparison with IR feature of CO adsorbed on a bulk Pt electrode. The linearly adsorbed CO species is the only intermediate derived from dissociative adsorption of methanol, which is more readily oxidized to form CO₂ in the aid of the active oxide in SBA-15.This paper is dedicated to Professor G. Horanyi on the occasion of his 70th birthday and in recognition of his outstanding contribution to electrochemistry     

Modification of a Pt surface by spontaneous Sn deposition for electrocatalytic applications. 2. Oxidation of CO,formaldehyde, formic acid,and methanol

The electrocatalytic activity of a spontaneously tin-modified Pt catalyst, fabricated through a simple dip-coating method under open-circuit conditions and characterized using surface analysis methods, was studied in electrooxidation reactions of a preadsorbed CO monolayer and continuous oxidation of methanol, formic acid, and formaldehyde in the potentiodynamic and potentiostatic modes. The catalytic activity of the tin-modified Pt surface is compared with that of a polycrystalline Pt electrode. Spontaneously Sn-modified Pt catalyst shows a superior activity toward adsorbed CO oxidation and thus can be promising for PEFC applications. The methanol oxidation rate is not enhanced on the Sn-modified Pt surface, compared to the Pt electrode. Formic acid oxidation is enhanced in the low potential region on the Sn-modified surface, compared to the Pt electrode. The formaldehyde oxidation rate is dramatically increased by modifying tin species at the most negative potentials, where anodic formaldehyde oxidation is completely suppressed on the pure Pt electrode. The results are discussed in terms of poisoning CO intermediate formation resulting from dehydrogenation of organic molecules on Pt sites, and oxidation of poisoning adsorbed CO species via the surface reaction with OH adsorbed on neighboring Sn sites.     

Surface-enhanced Raman spectroscopic evidence of methanol oxidation on ruthenium electrodes

Electrooxidation of methanol on Ru surfaces was investigated using in situ surface-enhanced Raman spectroscopy. Although the cyclic voltammogram did not show a significant methanol oxidation current on Ru, a Raman band at approximately 1970-1992 cm(-1) was observed from 0.4 to 0.8 V in 0.1 M HClO(4) + 1 M methanol. By comparing with the C-O stretching band (nu(CO)) of carbon monoxide (CO) adsorbed on RuO(2)(110) in the ultrahigh vacuum and on oxidized Ru electrodes, the observed spectral feature is assigned to nu(CO) of adsorbed CO (CO(ads)) on RuO(2). The formation of CO(ads) suggests that methanol oxidation does occur on Ru at room temperature, which is in contrast to the perception that Ru is not active for the reaction. The lack of significant methanol oxidation current is attributed to the competing rapid surface oxidation, which forms inactive surface oxides and therefore inhibits the methanol oxidation.     

Carbon Corrosion in Proton-Exchange Membrane Fuel Cells: Spectrometric Evidence for Pt-Catalysed Decarboxylation at Anode-Relevant Potentials

The carbon oxidation reaction (COR) is a critical issue in proton-exchange membrane fuel cells (PEMFCs), as carbon in various forms is the most used electrocatalyst support material. The COR is thermodynamically possible above the C/CO₂ standard potential, but its rate becomes significantly important only at high overpotential (e. g. PEMFC cathode potential). Herein, using on-line differential electrochemical mass spectrometry, we show that oxygen-containing carbon surface groups present on high-surface aera carbon, Vulcan XC72 or reinforced graphite are oxidized at PEMFC anode-relevant potential (E=0.1 V vs. the reversible hydrogen electrode, RHE), but not at E=0.4 V vs. RHE. We rationalized our findings by considering a Pt-catalysed decarboxylation mechanism in which Pt nanoparticles provide adsorbed hydrogen species to the oxygen-containing carbon surface groups, eventually leading to evolution of carbon dioxide and carbon monoxide. These results shed fundamental light on an unexpected degradation mechanism and facilitate the understanding of the long-term stability of PEMFC anode nanocatalysts.     

Towards Mixed Fuels: The Electrochemistry of Hydrazine in the Presence of Methanol and Formic Acid

The electrochemistry of formic acid, carbon monoxide and methanol have been investigated and evaluated in combination with hydrazine. Hydrazine was observed to display the anticipated steady‐state oxidation waves at platinum (Pt) microelectrodes by cyclic voltammetry, and upon introduction of carbon monoxide (CO) gas, the Pt surface was fully passivated (prior to CO oxidation). However, the two individual responses of hydrazine and formic acid (HCOOH) are to be additive when combined in solution. No detrimental effects were observed upon the hydrazine voltammetry, even in the presence of excess formic acid, despite formic acid clearly displaying characteristic self‐poisoning tendencies (primarily due to the formation of CO) in its own voltammetry. Effects intermediate to those of CO and formic acid were observed when methanol was present. Currents were essentially additive at low methanol content, but hydrazine oxidation current decreased by about 40 % when an 100‐fold excess of methanol was present, corresponding to poisoning by methanol dehydrogenation intermediates. These results are discussed with relevance to mixed fuels for more flexible or powerful fuel cells, and the possible formation of a random microelectrode array (templated by strongly adsorbed poison) on the microelectrode surface.     

Partially Oxidized Palladium Nanodots for Enhanced Electrocatalytic Carbon Dioxide Reduction

Here we report a partially oxidized palladium nanodot (Pd/PdO_x) catalyst with a diameter of around 4.5 nm. In aqueous CO₂‐saturated 0.5 m KHCO₃, the catalyst displays a Faradaic efficiency (FE) of 90 % at −0.55 V vs. reversible hydrogen electrode (RHE) for carbon monoxide (CO) production, and the activity can be retained for at least 24 h. The improved catalytic activity can be attributed to the strong adsorption of CO₂^.− intermediate on the Pd/PdO_x electrode, wherein the presence of Pd²⁺ during the electroreduction reaction of CO₂ may play an important role in accelerating the carbon dioxide reduction reaction (CO₂RR). This study explores the catalytic mechanism of a partially oxidized nanostructured Pd electrocatalyst and provides new opportunities for improving the CO₂RR performance of metal systems.     

Kinetics and mechanism of the oxidation of dibenzothiophene in hydrocarbon solution Oxidation by aqueous hydrogen peroxide-acetic acid mixtures

The oxidation of white oil solutions of dibenzothiophene (DBT) by aqueous hydrogen peroxide-acetic acid solutions was studied kinetically at 50–100°. Under these conditions, the rate of DBT oxidation was found to be first order in acetic acid, second order in hydrogen peroxide, and inversely proportional to the water concentration. The activation energy between 50–100° in 64·5% acetic acid was 14 kcal/mole. We have also found that the monoxide is oxidized about 1·4 times faster than DBT. A mechanism consistent with the kinetic data has been postulated. The rate-determining step appears to be attack of a peracetic acid-hydrogen peroxide dimer on the sulfur atom of DBT.     

Solvent dependent dimercaptothiadiazole monolayers on gold electrode for the simultaneous determination of uric acid and ascorbic acid

Dimercaptothiadiazole compound, 2,5-dimercapto-1,3,4-thiadiazole (DMcT) forms ‘thin’ monolayers on Au electrode when it was adsorbed from methanol, ethanol or DMSO solutions while it forms ‘thick’ layers on Au electrode from an aqueous solution under identical experimental conditions. Thick DMcT layers formed from aqueous solution effectively blocks the redox reaction of couple in contrast to thin DMcT monolayers. The monolayer thickness did not vary when structurally related DMcT compounds, 5-methyl-1,3,4-thiadiazole-2-thiol or 5-amino-1,3,4-thiadiazole-2-thiol was adsorbed from aqueous and non-aqueous solutions. This indicates that the presence of two thiol groups in DMcT plays a crucial role in the formation of thick and thin DMcT layers on Au electrode when it was adsorbed from aqueous and non-aqueous solutions. Methanol, ethanol, or DMSO solution of DMcT is considered as strong acid because these solvents are able to deprotonate DMcT into DMcT⁻ and thus thin monolayers formed on Au electrode. The deprotonating ability of these solvents was further verified from the observed absorption spectrum characteristic of DMcT⁻ species. On the other hand, an aqueous solution of DMcT is less acidic due to weak deprotonation of DMcT by water and thus DMcT forms thick layer on Au electrode. Interestingly, thin DMcT monolayers formed from non-aqueous solvents separates the voltammetric signals of uric acid and ascorbic acid while thick DMcT layers formed from aqueous solution fails to separate them.     

Nanostructured platinum as an electrocatalyst for the electrooxidation of formic acid

Nanostructured platinum prepared by the chemical reduction of hexachloroplatinic acid dissolved in aqueous domains of the liquid crystalline phases of oligoethylene oxide surfactants, was examined as an electrocatalyst for the electrooxidation of formic acid. The electrocatalytic properties of the catalyst combining highly specific surface areas and a periodic mesoporous nanostructure were accessed in sulfuric acid solution containing 0.5 mol dm⁻³ formic acid using cyclic voltammetry (CV) and chronoamperometry. The electrocatalytic activity of the material at 60 °C, is characterised by a mass activity of 8.6 A g⁻¹ and a specific surface area activity of 26 μA cm⁻² at 0.376 V (vs. RHE). The resistance to CO poisoning was found to depend upon electrode potential. At hydrogen adsorption potentials, the material is easily poisoned, while the material shows high resistance to CO poisoning at potentials positive of the hydrogen region. These facts suggest that the decomposition of HCOOH on the mesoporous platinum is likely to proceed through a dual-path mechanism and the high surface area material is a potential electocatalyst towards the electrooxidation of small organic molecules.     

Oxidation of formic acid and carbon monoxide on gold electrodes studied by surface-enhanced Raman spectroscopy and DFT.

The oxidation of formic acid and carbon monoxide was studied at a gold electrode by a combination of electrochemistry, in situ surface-enhanced Raman spectroscopy (SERS), differential electrochemical mass spectrometry, and first-principles DFT calculations. Comparison of the SERS results and the (field-dependent) DFT calculations strongly suggests that the relevant surface-bonded intermediate during oxidation of formic acid on gold is formate HCOO- ad*. Formate reacts to form carbon dioxide via two pathways: at low potentials, with a nearby water to produce carbon dioxide and a hydronium ion; at higher potentials, with surface-bonded hydroxyl (or oxide) to give carbon dioxide and water. In the former pathway, the rate-determining step is probably related to the reaction of surface-bonded formate with water, as measurements of the reaction order imply a surface almost completely saturated with adsorbate. The potential dependence of the rate of the low-potential pathway is presumably governed by the potential dependence of formate coverage. There is no evidence for CO formation on gold during oxidation of formic acid. The oxidation of carbon monoxide must involve the carboxyhydroxyl intermediate, but SERS measurements do not reveal this intermediate during CO oxidation, most likely because of its low surface coverage, as it is formed after the rate-determining step. Based on inconclusive spectroscopic evidence for the formation of surface-bonded OH at potentials substantially below the surface oxidation region, the question whether surface-bonded carbon monoxide reacts with surface hydroxyl or with water to form carboxyhydroxyl and carbon dioxide remains open. The SERS measurements show the existence of both atop and bridge-bonded CO on gold from two distinguishable low-frequency modes that agree very well with DFT calculations.     

Kinetic studies of adsorbed CO electrochemical oxidation on Pt(335) at full and sub-saturation coverages

Electrochemical measurements were performed to characterize the kinetics of adsorbed CO oxidation on the surface of the stepped Pt(s)-[4(111)x(100)][triple bond, length half m-dash]Pt(335) single crystal electrode. For CO adsorbed to full coverage at 0.1 V (versus the reversible hydrogen electrode, RHE) in 0.5 M H(2)SO(4) at ambient temperature (23 degrees C), oxidation of the layer gave 7.6 x 10(14) +/- 0.3 CO/cm(2) as the saturation CO coverage, just below the average value reported for CO on Pt(335) in ultra high vacuum (8.3 x 10(14) +/- 0.6 CO/cm(2)). In potential step measurements carried out between 0.75 and 0.9 V, the peak region in the current-time transient was consistent with the surface reaction between adsorbed CO and adsorbed oxide as rate limiting. Plotting the log of the rate constant for the surface reaction versus potential gave a Tafel slope of 79 mV per decade, consistent with responses for CO electrochemical oxidation on structurally related stepped Pt electrodes. For CO coverages below saturation, current-time transients were more stable in 0.05 M H(2)SO(4) than in the higher concentration electrolyte. Numerically solving the rate equations to the Langmuir-Hinshelwood model of adsorbed CO electrochemical oxidation reproduced the main features in current-time transients measured at 0.7 V in 0.05 M H(2)SO(4) for sub-saturation CO coverages. The results provide new insights into CO oxidation on Pt at sub-saturation coverage and confirm that anions play a role in CO surface chemistry.     

Relationship between the reactions of hydrogen sorption/desorption and methanol oxidation on bifunctional Ni/Pd electrode in alkaline solution

The present paper is aimed at studying the influence of the hydrogen sorption/desorption process occurring on the layered nickel–palladium (Ni/Pd) electrode on the kinetics of the reaction of methanol oxidation in strong alkaline KOH solution. The electrodes were prepared by chemical deposition of a thin layer of porous palladium on a nickel foam support. A scanning electron microscope was used for studying the morphology of both the nickel support and the porous palladium layer. The mechanism of the anodic desorption of hydrogen changes depending on whether or not 6 M KOH electrolyte is admixed with methanol. It was shown that, in the first cycle of the cyclic voltammetry (CV) measurements, the anodic peak current and peak charge related to the oxidative desorption of hydrogen significantly decrease due to the presence of methanol in KOH. This effect is attributed to the obstacles in hydrogen sorption due to the formation of a passivating layer on the Pd surface composed of both adsorbed methanol molecules and the intermediate products involving adsorbed CO. On the other hand, hydrogen desorbing from Pd electrode exerts influence on the kinetics of the reaction of methanol oxidation. Ni/Pd electrode undergoes considerable reactivation due to the potentiostatic saturation with hydrogen at ?1.1 V, followed by the ease in hydrogen desorption. The CV measurements proved that, after such a treatment, the peak of hydrogen desorption partially overlaps the double peak of methanol oxidation and, in consequence, the rate of methanol oxidation is enhanced. The positive effect of hydrogen releasing from the electrode on the kinetics of the reaction of methanol oxidation is ascribed to the anti-poison behavior consisting in the reaction of hydrogen radicals with intermediates adsorbed on the Pd surface.     

In situ transmission difference FTIR spectroscopic investigation on anodic oxidation of methanol in aqueous solution

In situ transmission difference FTIR spectroscopy method was introduced for studying the anodic oxidation of methanol in acid aqueous solution. A minigrid Pt optically transparent thin layer electrode was used as working electrode. This method has the ability to clarify the identity of species involved in the oxidation process both in solution and adsorbed at the surface of electrode. From the results of in situ transmission difference FTIR spectroscopy measurement it was found that HCHO, HCOOH, HCOOCH₃ and CH₂(OCH₃)₂ could be formed in the oxidation process of methanol. The final product was CO₂. The adsorbed poisonous intermediate CO was detected. It was formed at near 0.6 V and became significant at 0.9 V, where the oxidation current was inhibited. The in situ transmission difference FTIR spectroscopy method is a very convenient, relative simplicity and efficient method for investigating the electrochemical process, and could be as a good candidate for further application.     

Aqueous Photoelectrochemical CO2 Reduction to CO and Methanol over a Silicon Photocathode Functionalized with a Cobalt Phthalocyanine Molecular Catalyst

We report a precious-metal-free molecular catalyst-based photocathode that is active for aqueous CO₂ reduction to CO and methanol. The photoelectrode is composed of cobalt phthalocyanine molecules anchored on graphene oxide which is integrated via a (3-aminopropyl)triethoxysilane linker to p-type silicon protected by a thin film of titanium dioxide. The photocathode reduces CO₂ to CO with high selectivity at potentials as mild as 0 V versus the reversible hydrogen electrode (vs RHE). Methanol production is observed at an onset potential of −0.36 V vs RHE, and reaches a peak turnover frequency of 0.18 s⁻¹. To date, this is the only molecular catalyst-based photoelectrode that is active for the six-electron reduction of CO₂ to methanol. This work puts forth a strategy for interfacing molecular catalysts to p-type semiconductors and demonstrates state-of-the-art performance for photoelectrochemical CO₂ reduction to CO and methanol.     

Scanning electrochemical mapping of spatially localized electrochemical reactions induced by surface potential gradients

The influence of a surface potential gradient on the location and extent of electrochemical reactions was examined using a scanning electrochemical microscope. A linear potential gradient was imposed on the surface of a platinum-coated indium tin oxide electrode by applying two different potential values at the edges of the electrode. The applied potentials were used to control the location and extent of several electrochemical reactions, including the oxidation of Ru(NH3)6(2+), the oxidation of H2, and the oxidation of H2 in the presence of adsorbed CO. Scanning electrochemical mapping of these reactions was achieved by probing the feedback current associated with the oxidation products. The oxidation of Ru(NH3)6(2+) occurred at locations where the applied potential was positive of the formal potential of the Ru(NH3)6(2+/3+) redox couple. The position of this reaction on the surface could be spatially translated by manipulating the terminal potentials. The rate of hydrogen oxidation on the platinum-coated electrode varied spatially in the presence of a potential gradient and correlated with the nature of the electrode surface. High oxidation rates occurred at low potentials, with decreasing rates observed as the potential increased to values where platinum oxides formed. The extent of oxide formation versus position was confirmed with in-situ ellipsometry mapping. In the presence of adsorbed carbon monoxide, a potential gradient created a localized region of high activity for hydrogen oxidation at potentials between where carbon monoxide was adsorbed and platinum oxides formed. The position of this localized region of activity could be readily translated along the surface by changing the terminal potential values. The ability to manipulate electrochemical reactions spatially on a surface has potential application in microscale analytical devices as well as in the discovery and analysis of electrocatalytic systems.     

Electrocatalysis on binary alloys: II. Oxidation of molecular hydrogen on supported Pt+Ru alloys

The electrocatalytic properties of Pt+Ru alloys supported on graphitized carbon have been studied using oxide-free metal alloys that have been well characterized for phase identification, specific metal surface area, and surface composition. The CO tolerance of the Pt+Ru alloys for the oxidation of CO contaminated hydrogen in hot concentrated H₃PO₄ increases monotonically with Ru content of the surface and is a direct result of a decreasing coverage of the alloy by adsorbed CO. Furthermore, the strength of bonding of adsorbed CO with the metal surface decreases dramatically with increasing Ru content in the surface. The absolute activity of Pt+Ru alloys for the oxidation of CO contaminated hydrogen is a complex function of temperature and electrode potential. At 160°C, pure Pt is the most active catalyst at all potentials, but at temperatures lower than 120°C the reaction-limiting current for pure Ru exceeds that of pure Pt. At any temperature from 110–160°C or any electrode potential from 0–0.3V (HE), the variation of electrocatalytic activity with alloy composition indicates only dilution of the activity of the more active component.     

CO adsorption and oxidation at the catalyst-water interface: an investigation by attenuated total reflection infrared spectroscopy

Adsorption of carbon monoxide and oxidation of preadsorbed carbon monoxide from gas and aqueous phases were studied on a platinum catalyst deposited on a ZnSe internal reflection element (IRE) using attenuated total reflection infrared (ATR-IR) spectroscopy. The results of this study convincingly show that it is possible to prepare platinum metal layers strongly attached to an IRE, which are stable for over 3 days in aqueous-phase experiments. It is shown that ATR-IR spectroscopy is a suitable technique to study adsorption and catalytic reactions occurring at the interface of a solid catalyst in an aqueous reaction mixture, even with an extreme low-surface-area catalyst. Clearly, ATR-IR spectroscopy allows for a direct comparison of reactions on a catalytic surface in gas and liquid phases on the same sample. CO was found to adsorb both linearly and bridged on the platinum metal layer when adsorbed from the gas phase, but only linear CO was detected in aqueous solution, although with 5 times higher intensity. Oxidation of preadsorbed CO on platinum occurs in both gas phase, wetted gas, and aqueous media and was found to be 2 times faster in the aqueous phase compared to gas-phase oxidation because of a promoting effect of water. Moreover, during oxidation at room temperature, CO2 adsorbed on Pt/ZnSe was detected in both gas and aqueous phases.     

Electrooxidation Mechanism of Methanol at Pt-Ru Catalyst Modified GC Electrode in Electrolytes with Different pH Using Electrochemical and SERS Techniques

The electrochemical and in-situ surface-enhanced Raman spectroscopy （SERS） techniques were used to investigate the electrooxidation behavior of methanol in acidic, neutral and alkaline media at a Pt-Ru nanoparticle modified glassy carbon （Pt-Ru/GC） electrode. The results showed that methanol could be dissociated spontaneously at the Pt-Ru/GC electrode to produce a strongly adsorbed intermediate, CO. It was found that CO could be oxidized more easily in the alkaline medium than in the acidic and neutral media. The peak potential of methanol oxidation was shifted from 0.663 and 0.708 V in the acidic and neutral media to -0.030 V in the alkaline medium, which is due to that the adsorption strength of CO on the Pt surface in the alkaline medium is weaker than that in the acidic and neutral media. The final product of the methanol oxidation is CO2. However, in the alkaline medium, CO2 produced would form CO3^2- and HCO3^- resulting in the decrease in the alkaline concentration and then in the decrease in the performance of DMFC. Therefore, the performance of the alkaline DMFC is not Stable.