Mechanism of interaction of molecular hydrogen with adsorbed oxygen on platinum electrodes under open circuit conditions

Transients of open-circuit potential observed at the reaction of hydrogen molecules with oxygen preliminarily adsorbed (O_ads) on the smooth polycrystalline (pc Pt) and platinized platinum (Pt/Pt) electrodes are measured under conditions of controlled stirring of solution (0.5 M H₂ SO₄). The dependence of the surface coverage with O_ads(θ_O) on the potential in the cause of the potential decay on pc Pt are determined. It is found that for Pt/Pt, the reaction kinetics is largely determined by diffusion of H₂. For pc Pt in the range of high θ_O, the Eley-Ridiel mechanism is realized. For medium θ_O, the regions where the reaction obeys the mechanisms of Eley-Ridiel, “conjugated reactions”, and diffusion control are observed to overlap (even at the most intense stirring possible). The rate of H₂ reaction with O_ads is substantially higher compared with analogous reactions of CO, HCOOH, and CH₃OH.     

Specific features of interaction between formic acid and oxygen adsorbed on smooth polycrystalline platinum: Transients of the open-circuit potential

Transients of the open-circuit potential, which are observed when formic acid is interacting wit adsorbed oxygen (O_ads) preliminarily accumulated on polycrystalline “smooth” platinum (pcPt), are measure in an aqueous solution of sulfuric acid. It is shown that, as with platinized platinum (Pt/Pt), at large coverage by adsorbed oxygen (θ_O = 1?0.8), adsorbed oxygen interacts directly with molecules of formic acid from solution. In the region of medium coverages (θ_O = 0.8?0.2), on the other hand, a mechanism of “conjugated reactions” is realized. It is established that, in the case of pcPt, the direct interaction of O_ads with molecules of HCOOH from solution proceeds slower by nearly three times and the interaction via the mechanism of “conjugated reactions,” faster by about three times, as compared with Pt/Pt.     

Transients of the Open-Circuit Potential Observed during the Interaction of Formic Acid with Preliminarily Adsorbed Oxygen on a Platinized Platinum Electrode

Transients of the open-circuit potential, which are observed during the interaction of formic acid with preliminarily adsorbed oxygen (O_ads) on a Pt/Pt electrode in 0.5 M H₂SO₄, are measured. It is established, by means of the method of cathodic potentiodynamic pulses, that the slowest interaction of formic acid with O_ads occurs in the region of large coverages of the electrode surface by oxygen (θ_O ∼1–0.8). A presumption is put forward that the process rate in this region is defined by a direct reaction of O_ads with molecules of formic acid from the bulk solution. It is shown that the interaction of formic acid with O_ads in the region of intermediate coverages (θ_O ∼ 0.8–0.2) proceeds via a mechanism of “conjugated reactions.” Transients of the open-circuit potential for formic acid are compared to transients for carbon monoxide obtained in analogous conditions. The substantially shorter overall time of potential decay in the case of CO (at the same concentrations) is caused by a faster reaction of CO with adsorbed oxygen in the region of large θ_O. The difference is explained by assuming that the HCOOH adsorption as opposed to CO bears a dissociative character.__________Translated from Elektrokhimiya, Vol. 41, No. 8, 2005, pp. 936–942.Original Russian Text Copyright © 2005 by Manzhos, Maksimov, Podlovchenko.     

Concurrent oxygen reduction and methanol oxidation in sulfuric acid solutions on platinized platinum electrodes

Steady-state polarization curves are compared in solutions of 0.5 M H₂SO₄ + O₂ (saturated), 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH, and 0.5 M H₂SO₄ + (0.005–0.1) M CH₃OH + O₂ (saturated) on a Pt/Pt electrode. A considerable difference is found between the currents in mixed solutions and those expected based on the principle of additivity of currents in CH₃OH and O₂ individual solutions. The surface coverages with the CH₃OH and O₂ adsorption products are determined in the potential range of 0.2–0.9 V (RHE). Open-circuit potentials are measured in mixed solutions. The obtained results suggest that the direct heterogeneous interaction between methanol and oxygen occurs alongside with faradaic reactions. This is assumed to lead to a decrease in methanol electrooxidation currents at E ≥ 0.8 V and their increase at E ≤ 0.65 V.     

Electrocatalytic activity of platinum—polyaniline and palladium—polyaniline systems obtained by cycling the electrode potential

Steady-state current densities of electrooxidation of CH₃OH, HCOOH, and CO at the Pt-PAN-GC electrodes (where PAN and GC stand for polyaniline and glassy carbon, respectively) and those of electrooxidation of HCOOH at the Pd-PAN-GC electrodes are measured (per cm₂ of the true metal-catalyst surface area). The found higher activity of Pt and Pd particles incorporated in PAN, as compared with Pt/Pt and Pd/Pt, is attributed to interaction between metal-catalyst particles and the polymeric matrix. The activation effect is the most pronounced for the HCOOH electrooxidation at Pd-PAN-GC. The data concerning hydrogen evolution testify in favor of a decrease in exchange currents of this reaction upon going from Pt to Pt-PAN-GC electrodes     

The effect of hydrocarbon chain length in normal aliphatic alcohols on their reaction with oxygen adsorbed on polycrystalline platinum electrode

The reaction of adsorbed oxygen (O_ads) with aliphatic alcohols n-C _n H_{2n + 1}OH with n = 2–5 is studied by the method of transients of open-circuit potential in combination with potentiodynamic pulses. It is shown that these alcohols react with O_ads by a mechanism the same as for CH₃OH. Kinetic parameters of these reactions are determined in ranges of high and medium surface coverages with O_ads. These data together with analogous results obtained earlier for CH₃OH were studied with the aim of elucidating how the length of the hydrocarbon chain affects the kinetics of interaction of alcohols with O_ads. The complex variations of the reaction rate with n (with a maximum) are explained by several factors among which the energy of the C–H bond at α-carbon atom and the degree hydration of alcohols should be singled out.     

Transients of the Open-Circuit Potential during Carbon Monoxide Interaction with Oxygen Preliminarily Adsorbed on Smooth and Platinized Platinum Electrodes

Variations of potential E in time , observed during the carbon monoxide interaction with preliminarily-adsorbed oxygen O_ads on smooth and platinized platinum electrodes under open-circuit conditions (supporting electrolyte 0.5 M H₂SO₄), are measured. The potential decay rate on smooth Pt is more than ten times that on Pt/Pt; there are some differences in the transients as well. The obtained data suggest that CO interacts with O_ads on smooth Pt and Pt/Pt via different mechanisms. Two models for the process on smooth platinum are considered. In one model, the interaction of O_ads with CO from solution is accepted as the rate-determining step; in the other, the interaction of O_ads with CO_ads. A comparison of theoretical E vs. dependences with experimental data using the MathCad program suggests that CO interacts with O_ads via both mechanisms.     

Infrared studies of the effect of acetylene, hydrogen, and oxygen on CO adsorption on platinum hydrosols

Infrared spectra of CO-treated platinum hydrosols subsequently treated with acetylene, hydrogen, and oxygen reveal that v(CO)_ads decreases from 2070 cm⁻¹ with increasing gas-treatment time. This has been attributed to a reduction in the coverage of adsorbed CO. In Pt sol/CO/C₂H₂ systems, v(CO)_ads decreases to a limiting value of ca. 2060 cm⁻¹ after exposure to acetylene. In the Pt sol/CO/H₂ systems, v(CO)_ads decreases to ca. 2050 cm⁻¹ after exposure to hydrogen gas. The lower frequency in the Pt sol/CO/H₂ system has been attributed to CO adsorption on more active metal sites formed from the reduction of surface platinum oxides. Exposure of the CO-treated platinum hydrosols to O₂ gas was found to cause the eventual disappearance of the v(CO)_ads band in infrared spectra, which was attributed to oxidation of adsorbed CO to CO₂ by weakly bound surface layers of platinum oxides formed by the oxygen treatment.     

Experimental study of intermediates and wave phenomena in co oxidation on platinum metal (Pt,Pd) surfaces

The mechanism of catalytic CO oxidation on Pt(100) and Pd(110) single-crystal surfaces and on Pt and Pd sharp tip (～10³ Å) surfaces has been studied experimentally by temperature-programmed reaction, temperature desorption spectroscopy, field electron microscopy, and molecular beam techniques. Using the density functional theory the equilibrium states and stretching vibrations of oxygen atoms adsorbed on the Pt(100) surface have been calculated. The character of the mixed adsorption layer was established by high resolution electron energy loss spectroscopy—molecular adsorption (O_2ads, CO_ads) on Pt(100)-hex and dissociative adsorption (O_ads, CO_ads) on Pt(100)-(1×1). The origin of kinetic self-oscillations for the isothermal oxidation of CO in situ was studied in detail on the Pt and Pd tips by field electron microscopy. The initiating role of the reversible phase transition (hex) ? (1 × 1) of the Pt(100) nanoplane in the generation of regular chemical waves was established. The origination of self-oscillations and waves on the Pt(100) nanoplane was shown to be caused by the spontaneous periodical transition of the metal from the low-active state (hex) to the highly active catalytic state (1 × 1). A relationship between the reactivity of oxygen atoms (O_ads) and the concentration of CO_ads molecules was revealed for the Pd(110) surface. Studies using the isotope label ¹⁸O_ads demonstrated that the low-temperature formation of CO₂ at 150 K is a result of the reaction of CO with the highly reactive state of atomic oxygen (O_ads). The possibility of the low-temperature oxidation of CO via interaction with the so-called “hot” oxygen atoms (O_hot) appearing on the surface at the instant of dissociation of O_2ads molecules was studied by the molecular beam techniques.     

DFT study of interaction of O, O2, and OH with unreconstructed Pt(hkl) (h, k, l = 0, 1) surfaces—similarities, differences, and universalities

Adsorption of O, O₂, and OH on Pt(111), Pt(100), and Pt(110) surfaces was studied using periodic DFT calculations. It was found that generally adsorbate-surface interaction strengths increase with the decrease in surface packing density. On the Pt(111) surface the dissociation of O₂ molecule was not predicted, but it was predicted on Pt(100) and Pt(110) surfaces. While the strength of the adsorbate-substrate interaction decreases with the rise in surface coverage by O atoms, in the case of OH adsorption adsorbate layer gets stabilized at higher surface coverage through the hydrogen bonding. In spite of all the mentioned differences, single parameter of surface electronic structure was identified, being useful for the explanation of the adsorption trends at different adsorption sites for O and OH adsorption on Pt surfaces of various crystallographic orientations and also provided a deeper understanding of atomic oxygen adsorption as a function of surface coverage.     

Oxidation of carbon monoxide and formic acid on bulk and nanosized Pt–Co alloys

Bulk Pt₃Co and nanosized Pt₃Co and PtCo alloys supported on high area carbon were investigated as the electrocatalysts for the CO_ads and HCOOH oxidation. Pt₃Co alloy with Co electrochemically leached from the surface (Pt skeleton) was employed to separate electronic from ensemble and bifunctional effects of Co. Cyclic voltammetry in 0.1 M HClO₄ showed reduced amount of adsorbed hydrogen on Pt sites on Pt₃Co alloy compared to pure Pt. However, no significant difference in hydrogen adsorption/desorption and Pt-oxide reduction features between Pt₃Co with Pt skeleton structure and bulk Pt was observed. The oxidation of CO_ads on Pt₃Co alloy commenced earlier than on Pt, but this effect on Pt₃Co with Pt skeleton structure was minor indicating that bifunctional mechanism is stronger than the electronic modification of Pt by Co. The HCOOH oxidation rate on Pt₃Co alloy was about seven times higher than on bulk Pt when the reaction rates were compared at 0.4 V, i.e., in the middle of the potential range for the HCOOH oxidation. Like in the case of CO_ads oxidation, Pt skeleton showed similar activity as bulk Pt indicating that the ensemble effect is responsible for the enhanced activity of Pt₃Co alloy toward HCOOH oxidation. The comparison of CO_ads and HCOOH oxidation on Pt₃Co/C and PtCo/C with the same reaction on Pt/C were qualitatively the same as on bulk materials.     

Catalytic Consequences of the Thermodynamic Activities at Metal Cluster Surfaces and Their Periodic Reactivity Trend for Methanol Oxidation

The periodic reactivity trend and the connection of kinetics to the thermodynamic activity of oxygen are established for the oxidation of methanol on metal clusters. First‐order rate coefficients are a single‐valued function of the O₂‐to‐CH₃OH ratio, because this ratio, together with the rate constants for O₂ and CH₃OH activation, determine the oxygen chemical potential, thus the relative abundance of active sites and bulk chemical state of the clusters. CH₃OH activation rate constants on oxygen‐covered Ag, Pt, and Pd and on RuO₂ clusters vary with the metal–oxygen binding strength in a classical volcano‐type relation, because the oxygen‐binding strength directly influences the reactivities of oxygen as H abstractors during the kinetically relevant CH₃OH activation step. The differences in oxygen thermodynamic activity lead to five orders of magnitude variation in rates (Pt>Pd>RuO₂>Ag, 373 K), because of its strong effects on the activation enthalpy and more prominently activation entropy in CH₃OH activation.     

Formation of formic and acetic acids by low-temperature condensation of a mixture of methane and water vapor dissociated in MW discharge

Formic and acetic acids are formed by the low-temperature (77 K) condensation of a mixture of methane and water vapor dissociated by MW discharge at a low pressure. The effect of experimental conditions on the yield of HCOOH and AcOH was studied under different experimental conditions. The yields of H, OH, and O₂ from MW discharge in the CH₄+H₂O mixture were determined by ESR in the gas phase under the experimental conditions used to synthesize HCOOH and AcOH. The kinetics of the gas phase reactions in the connecting channel was simulated. The mechanism of formation of HCOOH and AcOH through the interaction of active species from the gas phase on the condensate surface was suggested. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 379–382, February, 2000.     

Progress in inorganic chemistry,vol. 19 : Edited by S.J. Lippard,Interscience-John Wiley & Sons,New York, 1975, 367 pages, $22.50.

The preparations of [Pt(CH₃)₂(OH)₂(H₂O)₂]_n and the ion Pt(CH₃)₂²⁺(ag) are described. Their reactions with a variety of ligands are outlined.     

The structures and stability of pentacoordinate germylenoid PhCH<Subscript>2</Subscript>(OH)CH<Subscript>3</Subscript>GeLiF

The structures and stability of pentacoordinate germylenoid PhCH₂(OH)CH₃GeLiF were first theoretically studied by density functional theory. Two equilibrium structures, the three-membered ring (1a) and the p-complex (1b) structures, were located. Their energy are in the order of 1b > 1a. The Ge-O coordination energies at the B3LYP/6-311+G(d, p) level are 13.6 and 0.2 kJ/mol in 1a and 1b, respectively. The insertion reactions with CH₃F indicate that germylenoid PhCH₂(OH)CH₃GeLiF is more stable than germylene PhCH₂(OH)CH₃Ge. The insertion barrier of 1a with CH₃F is only 3.1 kJ/mol higher than that of PhCH₃CH₃GeLiF, indicating that the oxygen coordination PhCH₂(OH)CH₃GeLiF has the same stability as PhCH₃CH₃GeLiF.     

Kinetics and mechanism of interaction between methanol and adsorbed oxygen on a smooth polycrystalline platinum electrode: Transients of the open-circuit potential

The method of transients of the open-circuit potential, combined with cathodic potentiodynamic pulses, is used for studying the methanol interaction with preliminarily adsorbed oxygen (O_ads) in 0.5 M H₂SO₄. It is established that, for the larger part of the time period required for a full reduction of a monolayer of O_ads in solutions of methanol on polycrystalline platinum, the process occurs at large coverages of the surface by O_ads (?_O). In the region of medium coverages, transients of the open-circuit potential are accurately described by the equation that corresponds to the mechanism of “conjugated reactions.” The mechanism of the methanol interaction with O_ads happens to be close to that for formic acid, which is explained by a dissociative character of these HCO compounds. Kinetic parameters characterizing the methanol reaction with O_ads in the region of large and medium coverages are determined and analyzed.     

Oxygen dissolution in polycrystalline palladium at O2 pressures of 0.1 to 100 Pa

Oxygen dissolution in polycrystalline palladium Pd(poly) at O₂ pressures ( $P_{O_2 } $ ) of 0.1 to 100 Pa and a temperature of 600 K has been investigated by temperature-programmed desorption. The dissolution process under these conditions includes O₂ chemisorption on the oxide film surface, the insertion of Oads atoms under the oxide layer, and their diffusion into the subsurface layers of palladium. During chemisorption, a structure ensuring that the O_ads coverage of the surface increases with increasing $P_{O_2 } $ forms on the surface of the oxide film. This is favorable for O_ads penetration through the oxide film and increases the amount of absorbed oxygen. The O_ads coverage of the surface calculated via the Langmuir equation at an O₂ desorption activation energy of E _des = 125 kJ/mol correlates with the number of absorbed oxygen monolayers (n). At n ≥ 1, oxygen absorption by Pd(poly) is due to the diffusion of O atoms in the palladium lattice. After the accumulation of 14–18 oxygen monolayers in the subsurface layers of palladium, oxygen absorption practically stops depending on $P_{O_2 } $ . Thus, the acceleration of oxygen dissolution in palladium is due to the formation of the surface oxide film and the increase in the O_ads coverage of this film, which facilitates the insertion of O_ads atoms into the subsurface layers of palladium.     

A Theoretical Investigation of the Decomposition Reactions of Ethyl Formate in the S0 State

This study revisits the stability of the possible conformations and the decomposition reactions of ethyl formate in the S₀ state using the (U)MP2, MP4SDTQ, CCSD(T), and (U)B3LYP methods with various basis sets. The transition states of the decomposition channels to HCOOH + C₂H₄, CO + CH₃CH₂OH, CH₂O + CH₃CHO, HCOH + CH₃CHO, C₂H₆ + CO₂, and H₂ + CH₂CHOCHO are determined. The microcanonical rate constants derived from the RRKM theory are calculated for each of the decomposition reactions. The high‐pressure limit rate constants are calculated for the decomposition channels to HCOOH + C₂H₄, CO + CH₃CH₂OH, and CH₂O + CH₃CHO.     

Radiation stability of trivalent antimony in the presence of organic acids

The gamma ray induced oxidation of Sb(III) in sulfuric acid solutions was studied. A simplified method depending on selective extraction of the different valency states and radiometric counting was elaborated for oxidation yield determination. The effect of increasing amounts of HCOOH, CH₃COOH, NH₂CH₂COOH, CH₃CHOH COOH and H₂C₂O₄ on G[-Sb(III)] was examined. The study enabled a determination of rate constant values for reactions of the used additives with the OH radical in the working solutions.     

Effect of the Pt/Pt Electrode Roughness on the Interaction of CO with Oads

Transients of potential E during the CO-O_ads interaction on Pt/Pt electrodes of different roughness (f = 20-1200), measured in 0.5 M H₂SO₄ under open-circuit conditions, slow down with increasing f. Dependence of specific activity of Pt/Pt on f cannot be described by one CO-O_ads interaction mechanism in a wide range of f. At f < 100, more acceptable is the mechanism suggested earlier for polycrystalline (smooth) Pt, and at larger f—mechanism of conjugated reactions. Presumably, increasing f at small f reduces intrinsic electrocatalytic activity of the Pt surface, while at large f inner-diffusion limitations can arise.