Structural investigation by electrochemically modulated infrared spectroscopy of adsorbed hydrogen on rhodium

Measurements were made, in situ, by IR reflectance spectroscopy at a rhodium electrode in 1 M H₂SO₄ and mixed H₂SO₄/D₂SO₄ electrolytes to give spectra corresponding to the difference between the state of the electrode at a potential in the double layer region and at a potential in the hydrogen adsorption region. A model for weakly adsorbed hydrogen in which the adsorbed atom is bonded to an oriented water structure is deduced from the spectral data. The spectra also show the presence of a strongly bound hydrogen species although the voltammetry shows no separate feature for its formation.     

Catalytic influence of underpotential-deposited submonolayers of heavy metals on d-glucose oxidation on various noble metal electrodes in alkaline media

The catalytic influence of underpotential-deposited (upd) submonolayers of heavy metals (i.e. Pb, Tl, Bi) on the electrooxidation of D-glucose on various noble metal electrodes (i.e. Pd, Rh, Ir) is studied in alkaline media, and the results are compared with those observed for these systems when Pt was used as the electrode. In the case of Rh and Ir electrodes the catalytic activity is expressed mainly through the considerable increase of the respective current peaks as well as through the negative shift of the oxidation peak in the double-layer region, while in the case of the Pd electrode no significant catalytic action of upd ad-layers on the oxidation of D-glucose is observed. An explanation of the enhancement of the catalytic action of these electrodes (except Pd) is given on the basis that the upd ad-atoms decrease the electrode poisoning, due to an intermediate gluconolactone-type adsorbate, according to the third body mechanism in electrocatalysis. Finally, from a volcano-type diagram it is found that for the catalysis of D-glucose oxidation by upd Pb, Tl and Bi ad-atoms the catalytic activity of the electrode metal used decreases according to the order: Pt > Pd > Rh > Ir.     

涂覆在堇青石上的负载贵金属催化剂催化氧化甲醛的性能研究

将催化性能优异的负载贵金属催化剂与拟薄水铝石(Al OOH)溶胶混合,制备得到均匀的料浆,采用料浆涂覆法将其负载到堇青石蜂窝陶瓷基体上,考察了Al OOH含量对涂层形貌、负载量、涂层牢固度的影响,并对合成的整体式催化剂进行催化氧化甲醛性能测试。结果表明,合成的负载贵金属催化剂Pt粒子粒径为1.14nm时,可在17℃完全降解甲醛;涂覆过程中Al OOH含量越高,载体负载量越大,但过高易导致孔道堵塞,涂覆不均和涂层龟裂;超声振荡测试表明,Al OOH含量对涂层牢固度影响不大,当Al OOH含量为8(wt)%时,负载型蜂窝陶瓷的催化效果和涂覆效果最佳,可以在60℃下完全降解甲醛。     

Kinetics and mechanisms of oxidation by metal ions,part XII [1] oxidation of formaldehyde by alkaline osmium(VIII)

The stopped-flow measurements on the disappearance of alkaline osmium(VIII) at 402 nm indicated a first-order dependence each in [Os(VIII)] and [HCHO]. The pseudo first-order rate constant k_obs ([HCHO] ? [Os(VIII)]) decreased with increasing [OH^?]. The ionic strength, however, had no effect on k_obs. The rapid scan spectra of the reaction mixture indicated the formation of an inert complex which absorbs at 319 nm. Therefore the rate determining step is considered to involve the bimolecular collision between OsO₄(OH) and hydrated formaldehyde. The values of the rate limiting constant k and the equilibrium constant K_ha for the formation of the alkoxide ion from the reaction of hydrated formaldehyde with OH^? are evaluated. The equilibrium constant K_ha, within the experimental limits, is independent of temperature. The pK_a value, calculated from K_ha, is 13.62 ± 0.05 which is in good agreement with the pK_a value 13.27 for formaldehyde. The activation parameters, ΔH? = 40 ± 2 kJ mol^?1 and ΔS? = ?51 ± 6 JK^?1 mol^?1, for the rate limiting constant k are determined.     

Electrocatalytic oxidation of sugars on silver-UPD single crystal gold electrodes in alkaline solutions

A highly catalytic system for sugar oxidation in alkaline media is presented, for the first time, in which glucose oxidation takes place at ca. −0.44 V (vs. Ag|AgCl). Modification of Au(1 1 1) single crystal surface by under potential deposition (UPD) was carried out for a variety of metals and catalytic effect for sugar oxidation has been studied in 0.1 M NaOH. UPD of Ag ad-atoms on Au electrodes were of the best catalytic activity compared to other metals (Cu, Co, Ru, Cd, Ir, and Pt, etc.). For aldose type monosaccharide studied (glucose, mannose and xylose) as well as for aldose-containing disaccharides (maltose and lactose), one significant oxidation peak was obtained, however, no significant oxidation current was observed for disaccharides like sucrose. Gluconolactone and mannolactone gave no oxidation current at negative potentials at which glucose was oxidized, indicating no more than two-electron oxidation took place. With Ag ad-atoms coverage of ca. 0.3 monolayer leads to a positive catalytic effect expressed through a negative shift of ca. 0.14 V (glucose case) on the oxidation potential and a slight increase in peak current. At the Au(1 0 0) surface similar results to those at an Au(1 1 1) electrode were also observed.     

Kinetics of the Ru(III) catalyzed oxidation of formaldehyde and acetaldehyde by alkaline hexacyanoferrate(III)

The kinetics of ruthenium(III) catalyzed oxidation of formaldehyde and acetaldehyde by alkaline hexacyanoferrate(III) has been studied spectrophotometrically. The rate of oxidation of formaldehyde is directly proportional to [Fe(CN) _3– ⁶ ] while that of acetaldehyde is proportional tok[Fe(CN) _3– ⁶ ]/{k +k[Fe(CN) _3– ⁶ ]}, wherek, k andk are rate constants. The order of reaction in acetylaldehyde is unity while that in formaldehyde falls from 1 to 0. The rate of reaction is proportional to [Ru(III)] _T in each case. A suitable mechanism is proposed and discussed.

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Die Kinetik der Ru(III)-katalysierten Oxidation von Formaldehyd und Acetaldehyd mittels alkalischem Hexacyanoferrat(III)

Zusammenfassung Die Untersuchung der Kinetik erfolgte spektrophotometrisch. Die Geschwindigkeitskonstante der Oxidation von Formaldehyd ist direkt proportional zu [Fe(CN) _3– ⁶ ], währenddessen die entsprechende Konstante für Acetaldehyd proportional zuk[Fe(CN) _3– ⁶ ]/{k +k[Fe(CN) _3– ⁶ ]} ist, wobeik,k undk Geschwindigkeitskonstanten sind. Die Reaktionsordnung für Acetaldehyd ist eine erste, die für Formaldehyd fällt von erster bis zu nullter Ordnung. Die Geschwindigkeitskonstante ist in jedem Fall proportional zu [Ru(III)] _T . Es wird ein passender Mechanismus vorgeschlagen.

     

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.     

Voltammetric and in situ infrared spectroscopy studies of hydroxyurea electrooxidation at Au(111) electrodes in HClO4 solutions

The oxidation of hydroxyurea (H₂NCONHOH, HU) at Au(111) single crystal and Au(111)-25 nm thin film electrodes is studied spectroelectrochemically in HClO₄ solutions using external reflection infrared and Surface Enhanced Infrared Reflection Absorption Spectroscopy under Attenuated Total Reflection conditions (ATR-SEIRAS). The in situ external reflection spectra prove the formation of dissolved carbon dioxide and adsorbed cyanate during the electrooxidation of HU. The enhanced surface sensitivity of the ATR-SEIRAS experiments facilitates the detection of adsorbed cyanate and allows the observation of other adsorbate bands tentatively ascribed to reaction intermediates involving partial oxidation of the HU molecules, some of them corresponding to nitrosyl groups.     

An investigation of the electrochemical responses of superactivated gold electrodes in alkaline solution

It is now well established that gold, in the form of oxide-supported microparticles or even as conventional macroelectrodes, displays inexplicably high catalytic activity for some reactions. In the present work, gold surfaces were superactivated by a combination of thermal and cathodic pretreatment and such electrodes in base yielded up to five distinct, and quite marked, premonolayer oxidation responses within the double layer region, over the range 0.0–1.0 V (RHE). As outlined in earlier publications, such unusual behaviour is important from an electrocatalytic (and heterogeneous catalysis) viewpoint. A new mode of active site adsorption, involving highly localized electron transfer from active surface atoms to either the external circuit (in electrocatalysis) or an adsorbing reactant (in heterogeneous catalysis), is proposed. Such localized (active site) adsorption, which is based on surface quantum confinement effects, is virtually independent of (or only indirectly related to) the electronic properties of the bulk metal.     

Recent progress in electrochemical oxidation of saccharides at gold and copper electrodes in alkaline solutions

This article reviews the progress made in the past 10 years, on electrochemical oxidation of saccharides in alkaline media for gold and copper electrodes. The mechanism and processes associated with the electrochemical oxidation of saccharides at native and surface coated electrodes continues to be of great interest. Despite the effort and various mechanisms proposed, still the need for an electrochemically active material that understands the complexity associated with saccharides continues to increase as their detection poses a challenge for bioanalytical chemistry and liquid chromatography.     

Transition metal oxides in the electrocatalytic oxidation of methanol and ethanol on noble metal nanoparticles

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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.     

Kinetics and mechanisms of oxidation by metal ions. Part XIII. Osmium(VIII)-catalysed oxidation of cycloalkanols by alkaline hexacyanoferrate(III)

Summary The kinetics of OsO₄-catalysed oxidation of cyclopentanol, cyclohexanol and cyclooctanol by alkaline hexacyanoferrate(III) have been studied at low [OH^–] so that the equilibrium between alcohol and alkoxide ion is not unduly shifted towards the latter. The reaction shows a first-order dependence in [OH^–]. The order of the reaction with respect to cycloalcohol is fractional, indicating the formation of an intermediate complex with Os^VIII since the order with respect to hexacyanoferrate(III) ion is zero. The order with respect to Os^VIII may be expressed by the equation k_obs=a+b[Os^VIII]. The analysis of the rate data indicates a significant degree of complex formation between [OsO₃(OH)₃]^– and ROH. It was found that the bimolecular rate constant k for the redox reaction between complex and OH^–k₁, the forward rate constant for the formation of alkoxide ion. The activation parameters of these rate constants are reported.     

Stopped-flow Fourier transform infrared spectroscopy of nitromethane oxidation by the diiron(IV) intermediate of methane monooxygenase

The hydroxylase component (MMOH) of soluble methane monooxygenase from Methylococcus capsulatus (Bath) was reduced to the diiron(II) form and then allowed to react with dioxygen to generate the diiron(IV) intermediate Q in the first phase of a double-mixing stopped-flow experiment. CD3NO2 was then introduced in the second phase of the experiment, which was carried out in D2O at 25 degrees C. The kinetics of the reaction of the substrate with Q were monitored by stopped-flow Fourier transform infrared spectroscopy, observing the disappearance of the asymmetric NO2 bending vibration at 1548 cm-1. The data were fit to a single-exponential function, which yielded a kobs of 0.45 +/- 0.07 s-1. This result is in quantitative agreement with a kobs of 0.39 +/- 0.01 s-1 obtained by observing the disappearance of Q by double-mixing stopped-flow optical spectroscopy at its absorption maximum of 420 nm. These results provide for the first time direct monitoring of the hydroxylation of a methane-derived substrate in the MMOH reaction pathway and demonstrate that Q decay occurs concomitantly with substrate consumption.     

Study of the electrochemical oxidation mechanism of formaldehyde on gold electrode in alkaline solution

The oxidation of formaldehyde in alkaline solution was studied by in situ rapid-scan time-resolved IR spectroelectrochemistry（RS-TR-FTIRS） method.In the potential range between -0.7 V and 0.2 V,the gem-diol anions were oxidized(according to the 2765 cm^-1 of-ν_H-O and 1034 cm^-1 ofν_co downward IR bands) and the formate ions appeared(according to the 1588,1357 cm^-1 of the asymmetric and symmetricν_oco and 1380 cm^-1 ofδ_C-H upward IR bands) in aqueous solution.It was also confirmed that gem-diol anion was oxidized(according to the 2026,1034 cm^-1 downward IR bands) to formate ions (according to the 1595,1357,1380 cm^-1 upward IR bands) and water(according to the 3427 cm^-1 ofν_{H- O} upward IR band) in heavy water solution.The results illustrated that formaldehyde formed gem-diol anion in alkaline solution and was absorbed on the electrode surface;then gem-diol anion was oxidized to formate ions and water.