The mechanism of hydrazine electro-oxidation revealed by platinum microelectrodes: role of residual oxides

The electrochemistry of hydrazine at platinum has been re-evaluated by an investigation using microelectrodes. Platinum oxides remaining from preceding oxidative scans results in hydrazine oxidation occurring up to ca. 400 mV more cathodic than at an oxide-free Pt electrode. The observed voltammetry at oxidised or 'activated' platinum electrodes was found to be a function of the immersion time (time since 'activation') and pH. Differences between phosphate, sulphate and acetate-based electrolytes are noted. The anodic hydrazine oxidation features at 'activated' electrodes occurred as a prewave or a prepeak, depending upon the electrolyte and scan rate employed. Although hydrazine is known to react with bulk Pt oxide, the loss of activation with time was found to be independent of hydrazine concentration and was instead a function of pH and supporting electrolyte, therefore the 'activation' corresponds to residual rather than bulk platinum oxide. The condition of platinum was examined by X-ray photoelectron spectroscopy (XPS), which demonstrated an increase in oxygen coverage with cycling and the absence of any strongly adsorbed or poisoning species. The facile oxidation of hydrazine has implications with regards to hydrogen storage, generation and fuel cells. The different effects corresponding to insufficient buffering, which has relevance to the electroanalytical detection of hydrazine, was also investigated.     

An investigation of the oxidation of formaldehyde on noble metal electrodes in alkaline solutions by electrochemically modulated infrared spectroscopy (emirs)

In-situ spectroscopy was used to detect adsorbed species formed during the oxidation of formaldehyde at polycrystalline Au, Pt, Rh and Ir electrodes in alkaline solution. At the gold electrode, the spectra showed the oxidation of the gem-diol form of formaldehyde and the formation of an adsorbed formate species. At platinum, rhodium and iridium, adsorbed CO was observed in the hydrogen adsorption region where the oxidation of formaldehyde is strongly inhibited on these metals. Alternative routes for producing the poison are discussed.     

酸性溶液中NaH2PO2在铂电极上氧化的原位红外光谱研究

用SNIFTIRS和循环伏安法研究酸性溶液中次亚磷酸钠在多晶铂电极上的电氧化机理.分析了0.5mol/LH2SO4+0.1mol/LNaH2PO2溶液中原位红外反射谱图与电极电位的关系,发现在发生反应的电位下Pt电极上的吸附物种有氢原子和H2PO2,最终的氧化产物是H3PO4而不是H2PO3-,据此提出了酸性介质中次亚磷酸根离子在Pt上氧化的新机理.     

Vibrational spectroscopy on platinum single-crystal electrodes: Part I. In-situ infrared spectroscopic studies of the adsorption and oxidation of CO on Pt (111) in sulphuric acid

Platinum single-crystal electrodes of 5 mm diameter were prepared for in situ infrared spectroscopic measurements by melting platinum wires. The linear potential sweep voltammograms of hydrogen adsorption/desorption on Pt (111), (110) and (100) in 0.5 M sulphuric acid are in excellent agreement with those observed on smaller platinum single-crystal surfaces.The adsorption and oxidation of CO on Pt (111) in 0.5 M sulphuric acid was studied by in situ polarization modulated infrared reflection absorption spectroscopy. The effects of the initial adsorption potential and surface reconstruction on the nature and oxidation mechanism of the adsorbed CO layer are reported.     

Revealing structural effects, part II: the influence of molecular structure on the adsorption of butanol isomers on platinum

The nature of the adsorbates formed when butanol isomers interact with platinum electrodes in a perchloric acid medium was investigated by the application of on-line differential electrochemical mass spectrometry (DEMS) and cyclic voltammetry. In this way, the reactivity of the residues remaining on the electrode surface after a flow-cell experiment was established for the different molecules. It was found that the four isomers form strongly adsorbed species on the electrode, which undergo both electro-oxidation and electroreduction, depending on the potential applied at the electrode. Oxidative stripping of the adsorbates produces CO2 as the only oxidation product, whereas propane and the corresponding butane isomer are obtained on platinum in the hydrogen adsorption potential region. The yields of these hydrocarbons were found to depend strongly on the nature of the butanol isomer and on the adsorption potential. According to these results, it can be concluded that fragmentation of the butanol isomers occurs during adsorption and reduction reactions. C4 alkene and acetyl species are proposed as the adsorbed intermediates in all cases.     

522—Catalytic oxydation of biological components on platinum electrodes modified by adsorbed metals: Anodic oxidation of glucose

The catalytic oxidation of glucose on Pt electrodes modified by adsorbed metals was studied in 1 M HClO₄ by linear sweep voltammetry. The adsorbed metals (denoted as M_ad, such as Bi_ad and Pb_ad) formed on Pt in the potential region more positive than the reversible potential of an M⁼⁺/M^o couple, lead to a marked increase in the anodic c?urrent of glucose by about one order of magnitude. The catalytic activity depends on the surface coverage by the M_ad. The strongly adsorbed species of lactone type, which are responsible for blocking the successive oxidation, are formed on the electrode surface in the anodic processes of glucose on a bare Pt electrode. The formation of such poisonous species is accelerated in the presence of adsorbed hydrogen on Pt. The effects of M_ad were discussed on the basis that M_ad plays its major role on the Pt electrode surface in removal of the adsorbed hydrogen which initiates the formation of the poisonous species.     

Electrocatalysis and pH (a review)

The factors determining pH effects on principal catalytic reactions in low-temperature fuel cells (oxygen reduction, hydrogen oxidation, and primary alcohols oxidation) are analyzed. The decreasing of hydrogen oxidation rate when passing from acidic electrolytes to basic ones was shown to be due to the electrode surface blocking by oxygen-containing species and changes in the adsorbed hydrogen energy state. In the case of oxygen reduction, the key factors determining the process’ kinetics and mechanism are: the O₂ adsorption energy, the adsorbed molecule protonation, and the oxygen reaction thermodynamics. The process’ high selectivity in acidic electrolytes at platinum electrodes is caused by rather high Pt-O₂ bond energy and its protonation. The passing from acidic electrolytes to basic ones involves a decrease in the oxygen adsorption energy, both at platinum and nonplatinum catalysts, hence, in the selectivity of the oxygen-to-water reduction reaction. The increase in the methanol and ethanol oxidation rate in basic media, as compared with acidic ones, is due to changes in the reacting species’ structure (because of the alcohol molecules dissociation) on the one hand, and active OH_ads species inflow to the reaction zone, on the other hand. In the case of ethanol, the above-listed factors determine the process’ increased selectivity with respect to CO₂ at higher pHs. Based on the survey and valuation, priority guidelines in the electrocatalysis of commercially important reactions are formulated, in particular, concepts of electrocatalysis at nonplatinum electrode materials that are stable in basic electrolytes, and approaches to the practical control of the rate and selectivity of oxygen reduction and primary alcohols oxidation over wide pH range.     

Anodic Oxidation of Serine Anion on Smooth and Platinized Platinum

The anodic oxidation of serine anion on smooth (Pt) and platinized (Pt(Pt)) platinum electrodes is studied by the methods of cyclic and linear voltammetry, rotating disk electrode, coulometry, and reflectance IR spectroscopy. On both electrodes, the potential regions of electrochemical transformation of this amino acid are determined. It is shown that electrooxidation of serine proceeds with abstraction of 4 and 2 electrons on Pt and Pt(Pt) electrodes, respectively. It is found that the anodic oxidation of serine anions proceeds from the adsorbed state; a possible kinetic scheme of this process is proposed.     

Size-controlled synthesis of colloidal platinum nanoparticles and their activity for the electrocatalytic oxidation of carbon monoxide

Using polyvinylpyrrolidone (PVP) as a stabilizing agent, stable colloidal solutions of platinum nanoparticles of different size distributions have been prepared by reducing H2PtCl6 with hydrogen. The UV-vis adsorption peaks at 258 nm due to the adsorption of Pt(IV) species disappear completely, indicating that the Pt(IV) species has been used up and colloidal Pt has been formed. The electrodes have been prepared from aqueous Pt colloids and glassy carbon (GC). The effect of platinum particle size of Pt/GC catalyst electrode on the electrocatalytic oxidation of carbon monoxide has been investigated. The voltammetry shows that a higher potential is needed for the oxidation of absorbed carbon monoxide with a decrease of the platinum particle size for particle sizes larger than 1 nm. But for particle sizes smaller than 1 nm, the potential remains constant while the activity decreases with decreasing the size. The snowlike, well-dispersed, and highly ordered platinum nanoparticles demonstrate high activity in the oxidation reaction of carbon monoxide. The reason may be due to the geometric structure of platinum nanoparticles.     

甲醇在铂微粒修饰的聚硫堇电极上的电催化氧化

利用电化学循环伏安和现场ＦＴＩＲ反射光谱等技术研究了甲醇在铂微粒修饰的聚硫堇电极上的电催化氧化。结果表明，循环伏安法制备的铂微粒均匀分散于聚合物膜上，其粒径大小约为３０－１３０ｎｍ；复合修饰电极对甲醇电化学氧化呈现了较高的催化活性，其催化活性的大小依赖于Ｐｔ载量。现场ＦＴＩＲ光谱实验揭示了线性吸附的ＣＯ物种是甲醇在复合电极上氧化的唯一中间体，这种吸附的ＣＯ物种在复合修饰电极上更容易被氧化为最终产物     

Oxidation of glucose at electrodeposited platinum electrodes

The oxidation of glucose at low concentrations was studied at electrodeposited platinum electrodes in 0.1 M HClO₄ using an electrochemical quartz crystal microbalance (EQCM). Experiments were performed over the whole potential range and then over selected regions of potential to investigate the processes giving rise to specific features in the cyclic voltammetry and mass response. In the region of potential where adsorbed hydrogen is present, EQCM experiments reveal the presence of adsorbate derived from glucose through the removal of features associated with the presence of adsorbed anions in the background electrolyte. Cycling over this potential region leads to a decrease in the mass of the electrode as the coverage of poisoning species increases. In the double-layer region of potential an increase in mass is seen as the adsorbate is oxidized and removed from the electrode surface, and is suggested to be a result of the replacement of adsorbed residues (formed from progressive oxidation of glucose) by both anions and fresh reactant. Restriction of the electrode potential to values above 0.2 V (SCE) prevents poison formation and mass responses indicate the presence of adsorbed glucose between 0.2 V and 0.6 V. Mass responses accompanying voltammetry and injection experiments also indicate that glucose or species derived from it can adsorb on an oxidized platinum surface.     

次亚磷酸根离子在多晶铂电极上氧化的原位红外光谱研究

次亚磷酸根离子在多晶铂电极上氧化的原位红外光谱研究;电氧化;电催化;SNIFTIRS     

Borohydride electro-oxidation on Pt single crystal electrodes

The borohydride oxidation reaction on platinum single-crystal electrodes has been studied in sodium hydroxide solution using static and rotating conditions. The results show that borohydride electro-oxidation is a structure sensitive process on Pt surfaces. Significant changes in the measured currents are observed at low potentials. In this region, the Pt(111) electrode exhibits the lowest activity, whereas the highest currents are measured for the Pt(110) electrode. The behavior of the different electrodes is discussed taking as reference the observed behavior on the blank electrolyte and the possible formation of weakly adsorbed intermediates.     

Pt-H_xWO_3复合电极的制备及其对氧还原的电催化

应用电位扫描法制备3种铂与氢钨青铜(HxWO3)的复合电极,即铂上沉积氢钨青铜(HxWO3/Pt),氢钨青铜上沉积铂(Pt/HxWO3)及铂和氢钨青铜共沉积(Pt-HxWO3).测试结果表明,氢钨青铜能显著提高铂电催化对氧还原的活性,其中以复合电极Pt/HxWO3的电催化活性最高,这可能是因为在玻碳基底上先电沉积氢钨青铜再电沉积铂增加了铂的有效比表面,该电极制备为减少铂用量提供一条有效途径.     

Associative and Proton Transfer Effects on the Voltammetric Behaviour of Chemically Grafted Films Bearing Nitrophenyl Groups

The spontaneous decomposition of 4‐nitrophenyldiazonium tetrafluoroborate in acetonitrile solution was used to modify glassy carbon electrodes with films bearing nitrophenyl groups. The modification procedure was performed by dipping non‐connected electrodes into the nitrophenyldiazonium solution. When these modified electrodes were analyzed by cyclic voltammetry in acetonitrile containing traces of water over a wide potential scale, only one reduction wave and two oxidation peaks were observed, which is atypical considering the reversible behaviour of the reduction of nitrobenzene in acetonitrile, however in acetonitrile containing a lower concentration of water, two consecutive reduction waves appear in the same potential scale. The difference between both behaviours is consistent with the presence of water trapped into the film structure, which promotes electron and proton transfer reactions that yield nitrosobenzene as product in surface. The trapping of water inside the film structure is justified by the capability of the nitrophenyl groups to interact with water by surface hydrogen bonding interactions.     

Catalytic phenomena during the methanol oxidation at platinum electrodes in contact with solid polymer electrolytes

The methanol electrooxidation at platinum-gauze electrodes contacting a solid polymer electrolyte is studied in water and sulfuric-acid solutions by voltammetry in a broad potential range and by measuring steady-state currents and electrode coverages with chemisorbed species at low anodic potentials. The specific rate of the methanol oxidation in these systems is higher than that of similar platinum electrodes in liquid electrolytes. The catalytic action depends on the measurements conditions and the electrode potential. Reasons for catalytic effects at different potentials are discussed.     

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.     

Comparative study of carbon monoxide adsorption and oxidation of the chemisorbed species at smooth and platinized platinum electrodes

The adsorption of carbon monoxide and the anodic oxidation of the chemisorbed species were investigated at room temperature under the same experimental conditions on foils of smooth and platinized platinum for which hydrogen adsorption is very similar. Both the weakly and strongly bonded species (type II and type I species) were formed to the same extent on the two electrodes in acidic electrolytes. While the freshly platinized platinum electrode behaved like the smooth electrode with predominance of the one-site adsorption of CO, the aging of the platinized electrode led to an increase of the two-site adsorption. The oxidation rate of strongly bonded species at constant potential decreased when the extent of two-site adsorption increased. The electrochemical results are discussed in the light of recent work in the gas phase.     

Decomposition and oxidation of methanol on platinum: A study by in situ X-ray photoelectron spectroscopy and mass spectrometry

The reactions of the catalytic oxidation and decomposition of methanol on the atomically smooth and high-defect Pt(111) single-crystal surfaces were studied using in situ temperature-programmed reaction and X-ray photoelectron spectroscopy. It was found that the decomposition of methanol on both of the surfaces occurred via two reaction pathways: complete dehydrogenation to CO and decomposition with the C-O bond cleavage. Although the rate of reaction via the latter pathway was lower than the rate of dehydrogenation by three orders of magnitude, the carbon formed as a result of the C-O bond cleavage can be accumulated on the surface of platinum to prevent the further course of the reaction. It was shown that oxygen exhibits high activity toward the formed carbon deposits. As a result, the rate of methanol conversion in the presence of oxygen in a gas phase increased by one or two orders of magnitude; in this case, CO₂ and water appeared in the composition of the reaction products as a result of the oxidation of CO and hydrogen, respectively. The high-defect surface of platinum was more active in the reactions of methanol decomposition and oxidation than the atomically smooth Pt(111) single-crystal surface. On the former, selectivity for the formation of methanol dehydrogenation products in oxygen deficiency was higher than on the latter. The main reaction pathways of the decomposition and oxidation of methanol on platinum were considered.     

Electrochemical behavior of acidic acetonitrile solutions at platinum and gold electrodes

Hydrogen evolution reactions from acidic MeCN solutions were studied at Pt and Au electrodes. The acids studied were HCl and HPic in a wide range of concentrations. LiClO₄ and Et₄NClO₄ (0.4 M) were used as supporting electrolytes. Quasi steady state polarization curves show that the reaction is irreversible with a Tafel slope of 2 RT/F after diffusion polarization and pseudo-ohmic drop have been corrected. Non-stationary techniques reveal that the electrochemical reaction is preceded by a chemical reaction attributed to slow dissociation of ACN·2 HCl and ACN·3 HPic species. Equilibria and chemical rate constants for both species were evaluated by chronopotentiometry. The values obtained explain satisfactorily the results accomplished under potentiodynamic conditions. Data at the RDE confirm the existence of a previous chemical reaction. MeCN electroadsorption in the double layer potential region at Pt accounts for the solvent levelling effect towards H adsorption between Pt and Au. Furthermore, an inhibitory effect over hydrogen evolution and hydrogen oxidation is reported at more negative potentials at both Pt and Au electrodes.