Electrochemical nanogravimetric studies of platinum in acid media

Electrochemical quartz crystal nanobalance (EQCN) is one of the most powerful tools to obtain information on the events occurring at the electrode surface. This method has been exploited to monitor the surface mass changes and hence to draw conclusions in respect of the formation and removal of adsorbed species and oxides as well as changes in the electrochemical double layer also in the case of platinum electrodes. However, the results that had been obtained so far are somewhat contradictory, and consequently diverse interpretations can be found in the literature. Therefore, it is worth to review the knowledge accumulated and to carry out systematic study in this respect. In this work smooth and platinized platinum electrodes in contact with acidic solutions were studied using EQCN technique. The effects of temperature, the nature of cations and anions, pH, concentrations, potential range were investigated on the electrochemical, and the simultaneously detected nanogravimetric responses. It is shown that in the underpotential deposition (upd) of hydrogen the adsorption/desorption of species from the solution phase is governed by the oxidative desorption/reductive adsorption of hydrogen; however, unambiguos conclusions cannot be drawn regarding the actual participation of anions and water molecules in the surface coverage. In the hydrogen evolution region a weak cation adsorption can be assumed and the potential of zero charge can be estimated. Cs⁺ cations affect the EQCN response in the hydrogen upd region. In some cases, e.g., in the case of upd of zinc the mass change can be explained by an induced anion adsorption. Two types of dissolution processes have been observed. A platinum loss was detected during the reduction of platinum oxide, the extent of which depends on the positive potential limit and the scan rate, and to a lesser extent on the temperature. The platinum dissolution during the electroreduction of oxide is related to the interfacial place exchange of the oxygen and platinum atoms in the oxide region. At elevated temperatures two competitive processes take place at high positive potentials: a dissolution of platinum and platinum oxide formation.     

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.     

The electrochemical behaviour of irreversibly adsorbed palladium on Pt(111) in acid media

The irreversible adsorption of submonolayer and monolayer coverages of palladium on Pt(111) has been investigated by means of cyclic voltammetry in sulphuric and perchloric acid. The so-called anomalous and normal hydrogen regions are always observed, irrespective of the number of palladium atoms adsorbed. However, subtle changes in the distribution of charge between the two regions and changes in their fine structure appear to contradict previous assertions concerning strongly bonded hydrogen on clean Pt(111). Depending on the electrolyte used, slight differences are also observed in the electrochemical characteristics of these features in the presence of palladium, particularly with reference to the reversibility of the peaks. This suggests that the anomalous peaks in perchloric and sulphuric acid have different origins. Coincidence of thermal Pd-O and Pt-O desorption with the anomalous peak in perchloric acid implies strongly that this feature arises from the adsorption and desorption of some oxygenated species, probably OH(ads) interacting weakly with the background electrolyte. The nature of the anomalous region in sulphuric acid is discussed in the light of these results. Finally, the role played by specifically adsorbed anions in facilitating surface mobility is again emphasised, particularly in relation to the stability of the palladium overlayer in acid media and the removal of surface heterogeneity.     

Two forms of chemisorbed sulfur on platinum and related studies

The effect of temperature on the potentiodynamic oxidation of adsorbed sulfur layers on platinum, obtained from H₂S or SO₂ was studied. The broad oxidation peak at 1.2–1.3 V observed at room temperature is resolved at 80°C. Two distinct peaks are observed at 80°C, oxidation peak I (at 0.97 V) corresponding to the weakly bound sulfur and oxidation peak II (at 1.10 V) corresponding to the strongly bound sulfur. Evidence is adduced to show that these two forms of chemisorbed sulfur are distinguished by the number of platinum sites they occupy. At elevated temperatures an extension of the hydrogen region was observed during cathodic charging in the presence of adsorbed sulfur. This phenomenon was found to be reversible with respect to temperature and does not correspond to a desorption of sulfur.     

STUDIES ON PROMOTION OF UPD OF. LEAD FOR ELECTRO-OXIDATION OF FORMIC ACID ON PLATINUM

Using potential sweep and potential-time sequence techniques,we have studied the promotion of the UPD of lead for the electrocata-lytic oxidation of formic aeid on platinum in the HClO4 aqueous solution.The results show that the first oxidation peak in the anodic direction and the oxidation peak in the cathodic direction in the cyclic voitammogram are attributed to the oxid-ation of the same weakly adsorbed intermediate,while the second and third oxidation peaks in the anodic direction are attributed to the oxidation of the two different strongly adsorbed intermediates.Furthermore,all these intermediates appear as the linear adsorption on the platinum surface and they are probably single bonded species,Pt-HCOO,and multiple bonded species,Pt=CO and Pt=COH,respectively.So the function of the UPD of lead for the electro-oxidation of formic acid is neither a third-body effect nor the decrease of nydrogen adsorption thus preventing the formation of poisoning species.The role of the UPD of lead should be an e     

Effect of Cs+ ions on the electrochemical nanogravimetric response of platinum electrode in acid media

Platinum electrodes have been investigated in sulfuric acid solutions in the presence and absence of Cs⁺ ions by electrochemical quartz crystal nanobalance (EQCN). An unusual potential dependence of the quartz crystal frequency response has been observed in the presence of Cs⁺ ions. The frequency decrease is more pronounced in the region of the underpotential deposition of hydrogen, and the frequency decrease in the double layer region diminishes as the concentration ratio of Cs⁺ and H⁺ ions increases. After immersion in Cs₂SO₄ solutions the frequency change was higher than that expected taking into account the density and viscosity. The effects observed can be explained by the specific adsorption of Cs⁺ ions on the Pt surface, which competes with the hydrogen adsorption. At more positive potentials than the potential of zero charge (pzc) a desorption of the Cs⁺ ions starts. In this potential region both Cs⁺ and HSO₄^? ions are adsorbed at the platinum surface. In the double layer region the mass change caused by the desorption of Cs⁺ ions and the starting adsorption of sulfate ions compensates each other.     

Overlap of the oxide and hydrogen regions of platinum electrodes in aqueous acid solution

Two unusual features of noble metal electrode surfaces, active states of the metal and their anodic oxidation products (hydrous oxides), are of increasing interest at the present time owing to the important role of such species in electrocatalysis. The extent to which the hydrous oxide reduction process overlaps with the hydrogen adsorption region was investigated for platinum in acid solution. At least three distinct hydrous oxide reduction peaks (or regions) were observed and in some cases one of these peaks commenced at ca. 0.0 V, i.e. it was almost totally within the hydrogen gas evolution region. Following repeated hydrous oxide growth and reduction, which disrupted and thus activated the metal surface, a sequence of four low-level premonolayer oxidation peaks (each of which has been noted earlier by other authors) appeared in the positive sweep. As discussed earlier for copper in base, the transitions giving rise to such peaks are assumed to be mediator generation reactions, which strongly influence electrocatalytic processes occurring on platinum at low potentials. Electronic Publication     

UPD铅对甲酸在铂上电氧化的助催化功用的研究

应用电位扫描和电位-时间程序技术研究了在UPD铅不存在和存在的情况下,各种因素对甲酸在铂上和酸介质中电催化氧化的影响。结果表明,在甲酸的CV图上,阳极方向的第一氧化峰和阴极方向的氧化峰是由于同一的弱吸附中间物的氧化,而阳极方向的第二和第三氧化峰是由于两种不同的强吸附中间物的氧化。cps值的测定和电极表面宏观结构的影响等实验表明,这些吸附中间物在电极表面上都呈线性吸附,它们分别可能是单键吸附的HCOO_ad双键吸附的CO_ad和叁键吸附的COH_ad。据此可以推论,UPD铅的主要功用是一种电子效应。     

Adsorption of atomic hydrogen at a nanostructured electrode of polyacrylate-capped Pt nanoparticles in polyelectrolyte

Atomic hydrogen electrosorption is reported at crystallite sites of polyacrylate-capped Pt nanoparticles (d = 2.5 +/- 0.6 nm), by assembling nanostructured electrodes of polyacrylate-Pt nanocrystallites layer-by-layer in a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). Cyclic voltammetry in 1 M H2SO4 revealed a strongly adsorbed hydrogen state and a weakly adsorbed hydrogen state assigned to adsorption at (100) and (110) sites of the modified nanocrystallites, respectively. Resolving hydrogen adsorption states signifies that surface capping by the carboxylate groups is not irreversibly blocking hydrogen adsorption sites at the modified Pt nanoparticle surface. Adsorption peak currents increased with increasing the number of layers up to 16 bilayers, indicating the feasibility of nanoparticle charging via interparticle charge hopping and the accessibility of adsorption states within the thickness of the nanoparticle/polyelectrolyte multilayers. Despite similarity in hydrogen adsorption in the cyclic voltammorgrams in 1 M H2SO4, negative shifts in adsorption potentials were measured at the nanocrystallite Pt-polyelectrolyte multilayers relative to a polycrystalline bulk Pt surface. This potential shift is attributed to a kinetic limitation in the reductive hydrogen adsorption as a result of the Pt nanoparticle surface modification and the polyelectrolyte environment.     

法拉第吸脱附偶联过程循环伏安行为的有限元分析

法拉第吸脱附偶联过程的电化学行为较为复杂,难以定量获得其表界面反应动力学信息. 本文通过COMSOL有限元软件对法拉第吸脱附偶联过程的循环伏安行为进行数值分析,研究了反应物或产物不同吸附条件下的循环伏安行为. 结果表明：当反应物或产物弱吸附时,可通过阴、阳极峰电流之差实现饱和吸附量的定量表征. 随着吸附平衡常数的增大,反应由弱吸附向强吸附过渡,峰电流由扩散峰与吸脱附峰相互重叠过渡到相互分离的吸脱附“前波”或“后波”特征. 该吸脱附特征峰的形状和位置与电势依赖的吸附平衡常数有关. 吸附平衡常数及其电势依赖程度越大,吸脱附峰偏离扩散峰越远,吸脱附峰越尖锐. 该模型为法拉第吸脱附偶联过程的循环伏安研究提供了一种定量研究方法,能够帮助研究者从复杂的吸脱附伏安行为中定量获得饱和吸附量和吸附平衡常数等信息,并对涉及吸脱附的电催化研究具有一定指导意义.     

State and properties of ion-exchanged cations in zeolites: 1. IR spectra and chemical activation of adsorbed molecular hydrogen

Adsorption isotherms and of adsorbed molecular hydrogen indicate that H₂ is weakly adsorbed by alkali-metal forms of faujasites, mordenite, and high-silica zeolite ZSM-5. The alkaline-earth forms of the same zeolites adsorb hydrogen somewhat more strongly; nevertheless, the hydrogen molecules adsorbed by the barium form of mordenite are in the hindered rotation state. Molecular hydrogen is most strongly adsorbed by the zinc and cadmium forms of the high-silica zeolite. In this case, molecular hydrogen is strongly polarized and undergoes heterolytic dissociative adsorption, yielding acidic hydroxyl groups and cation-bound hydride ions.     

Electrochemical behavior of Pd–Rh alloys

Pd–Rh alloys were prepared by electrochemical codeposition. Bulk compositions of the alloys were determined by the energy dispersive X-ray analysis method, while surface compositions were determined from the potential of the surface oxide reduction peak. Cyclic voltammograms, recorded in 0.5 M H₂SO₄ for Pd–Rh alloys of different bulk and surface compositions, are intermediate between curves characteristic of Pd and Rh. The influence of potential cycling on electrochemical properties and surface morphologies of the alloys was studied. Due to electrochemical dissolution of metals, both alloy surface and bulk become enriched with Pd. Carbon oxides were adsorbed at a constant potential from the range of hydrogen adsorption. The presence of adsorbed CO₂ causes remarkable diminution of hydrogen adsorption but it does not significantly influence hydrogen insertion into the alloy bulk. On the other hand, in the presence of adsorbed CO, both hydrogen absorption and adsorption are strongly suppressed. Oxidative removal of the adsorbates results in a characteristic voltammetric peak, whose potential increases with the decrease in Rh surface content. Electron per site (eps) values calculated for the oxidation of the adsorbates change with alloy surface composition, more for CO₂ than CO adsorption, indicating the variation of the structure and composition of CO₂ and CO adsorption products. The course of the dependence of eps values on surface composition suggests that the products of CO₂ and CO adsorption on Pd–Rh alloys are similar but not totally identical.     

Anion-induced adsorption of ferrocenated nanoparticles

Au nanoparticles fully coated with omega-ferrocenyl hexanethiolate ligands, with average composition Au225(omega-ferrocenyl hexanethiolate)43, exhibit a unique combination of adsorption properties on Pt electrodes. The adsorbed layer is so robust that electrodes bearing submonolayer, monolayer, and multilayer quantities of these nanoparticles can be transferred to fresh electrolyte solutions and there exhibit stable ferrocene voltammetry over long periods of time. The kinetics of forming the robustly adsorbed layer are slow; monolayer and submonolayer deposition can be described by a rate law that is first order in nanoparticle concentration and in available electrode surface. The adsorption mechanism is proposed to involve entropically enhanced (multiple) ion-pair bridges between oxidized (ferrocenium) sites and certain specifically adsorbed electrolyte anions on the electrode. Adsorption is promoted by scanning to positive potentials (through the ferrocene wave) and by high concentrations of Bu4N+ X- electrolyte (X- = ClO4(-), PF6(-)) in the CH2Cl2 solvent; there is no adsorption if X- = p-toluenesulfonate or if the electrode is coated with an alkanethiolate monolayer. The electrode double layer capacity is not appreciably diminished by the adsorbed ferrocenated nanoparticles, which are gradually desorbed by scanning to potentials more negative than the electrode's potential of zero charge. At very slow scan rates, voltammetric current peaks are symmetrical and nearly reversible, but exhibit E(fwhm) considerably narrower (typically 35 mV) than ideally expected (90.6 mV, at 298 K) for a one-electron transfer or for reactions of multiple, independent redox centers with identical formal potentials. The peak narrowing is qualitatively explicable by a surface-activity effect invoking large, attractive lateral interactions between nanoparticles and, or alternatively, by a model in which ferrocene sites react serially at formal potentials that become successively altered as ion-pair bridges are formed. At faster scan rates, both deltaE(peak) and E(fwhm) increase in a manner consistent with a combination of uncompensated ohmic resistance of the electrolyte solution and of the adsorbed film, as distinct from behavior produced by slow electron transfer.     

A contribution to the voltammetric study of cystine and cysteine at Pt electrodes in 0.5 M H2SO4

Both cysteine and cystine adsorb at the Pt electrode according the Frumkin—Temkin isotherm with the heterogeneity factor f = 51 for cysteine and 21 for cystine. Both the adsorbed cysteine and cystine give in a solution without any dissolved cystine or cysteine almost identical first cyclic voltammetric curves. Each substance dissolved in the electrolyte gives two oxidation peaks which differ when the oxidation is carried out at a “reduced” or an “oxidized” Pt electrode. On the basis of the dependence of the height and potential of the peaks on polarization rate and concentration (in the case of oxidation of dissolved substances) and of coulometric measurements the following conclusions have been made concerning the kinetics and mechanism:(i) Neither cysteine nor cystine change their oxidation state on adsorption at the electrode.(ii) The final oxidation product of both adsorbed cysteine and cystine may be the cysteic acid.(iii) For cysteine there are two adsorbed species, one strongly adsorbed, the other one weakly adsorbed.(iv) The oxidation of dissolved cysteine takes place via the weakly adsorbed species, the surface concentration of which is influenced by the coverage of the strongly adsorbed species. This process is described by an electrode reaction rate equation.(v) In the overall oxidation of cysteine one electron is transferred while the detailed mechanism requires an oxidation by splitting-off two electrons with a subsequent ion—substrate dimerization reaction.     

Anion—surface interactions: Part I. Perchlorate decomposition and sulphate adsorption hysteresis studied by voltammetry

Cyclic voltammetry has been used to investigate the behaviour of sulphate and perchlorate anions adsorbed on Ir{111} and polycrystalline rhodium. In 0.1 M HClO₄, perchlorate anions decompose on polycrystalline rhodium to give a surface intermediate which inhibits hydrogen adsorption. This is in agreement with a previous study by Wieckowski and coworkers, who showed that the surface intermediate was, in fact, adsorbed chloride. The voltammetry of the same electrode in sulphate containing electrolytes depends critically on the concentration of sulphate and the potential sweep rate. We conclude that hydrogen adsorption can only occur after significant desorption of sulphate. Hysteresis in the adsorption and desorption of sulphate results in an asymmetric hydrogen adsorption region with respect to the potential axis. Ir{111} is shown also to cause decomposition of perchlorate anions, although, contrary to the case of rhodium, the process is very much a chemical one in which electric currents, arising from perchlorate decomposition, were not detected. In sulphuric acid, a strong interaction between anions and the Ir{111} surface is observed, which precludes oxide formation at potentials less than 1 V (Pd---H), behaviour characteristic of a Pt{111} surface in the same electrolyte.     

Correlations between hydrogen electrosorption properties and composition of Pd-noble metal alloys

Hydrogen adsorption on and absorption into Pd alloys with other noble metals was studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry. Correlations were found between the potentials of adsorbed/absorbed hydrogen oxidation peaks and surface/bulk compositions of Pd–Rh alloys. The potential of the α–β-phase transition depends linearly on Pd bulk content in Pd–Au, Pd–Rh, Pd–Pt and Pd–Pt–Rh alloys. The obtained relationships can be utilized for the determination of the composition of homogeneous Pd-noble alloys from hydrogen electrosorption experiments.     

Heat of Adsorption of Pure Sulfur Hexafluoride on Micro-Mesoporous Adsorbents

The isotherms and the isosteric heats of adsorption of pure SF₆ were measured on two microporous zeolites (NaX and Silicalite), one mesoporous alumina, and two activated carbons (BPL and PCB) at 305 K. The adsorption isotherms were Type I by Brunauer classification. The PCB carbon adsorbed SF₆ most strongly and the alumina adsorbed SF₆ most weakly. The adsorption of SF₆ on the other three materials were comparable in the low pressure region despite their drastic differences in the physicochemical properties. The heat of adsorption of SF₆ on the silicalite and the alumina remained practically constant over a large range of coverage. The heat of adsorption of SF₆ increased with increasing adsorbate loading on the NaX zeolite in the high coverage region. The heat of adsorption of SF₆ on the activated carbons decreased with increasing adsorbate loading before leveling off in the high coverage region.     

CO electrooxidation on a polycrystalline Pt electrode: A wall-jet EQCN study

Pre-adsorbed and bulk (continuous) CO oxidation on a polycrystalline Pt electrode were examined in a wall-jet electrochemical quartz crystal nanobalance (EQCN) setup, using both differential and integral evaluation of the EQCN data, to get further insights into the kinetics and mechanism of this important fuel-cell related electrocatalytic reaction. The hydrogen underpotential adsorption–desorption features in the base cyclic voltammogram of a Pt film are accompanied by significant changes in the electrode mass due H-upd induced desorption–adsorption of anion. In the double-layer region small capacitive currents are accompanied by comparatively large reversible mass changes indicating anion adsorption/desorption (96.5 g mol⁻¹ assigned to bisulfate). OH and oxygen electrosorption from water at potentials more positive of 1.0 V result in relatively small variations in the electrode mass (16 g mol⁻¹ for PtOH and ca. 9 g mol⁻¹ for PtO formation, respectively). The CO-adlayer stripping first leads to the electrode mass decrease in the “pre-peak” region, followed by a fast mass increase within the main stripping peak due to re-adsorption of bisulfate anion (91 g mol⁻¹). A mass-transport limited current for bulk CO oxidation under continuous flow of CO-saturated electrolyte leads to negligible mass changes (0–1 g mol⁻¹) in the PtO region, suggesting that bulk CO oxidation is mediated by electroformed PtO.     

Adsorption of CO on Ru and Pt blacks and catalysts and the possibility of its utilization for the determination of the ruthenium-free surface

Electrochemical voltammetric curves on Ru and Pt blacks of a different surface area were measured in potential intervals 0.05–1.05 V in pure 0.5 M H₂SO₄ and after CO adsorption. It was proved that after the CO adsorption, the outset of ruthenium oxidation is shifted by about 150 mV towards the positive potentials, e.g. to the region of oxidation of adsorbed CO. This fact made possible the determination of a double-layer charging current of Ru electrodes and, subsequently, also the determination of the amount of adsorbed hydrogen on the Ru surface. An evaluation of the amount of CO and hydrogen adsorption showed that the ratio of adsorbed CO:H on the Pt surface was about 1:1, while on Ru electrodes this ratio was around twice as large. The amount of hydrogen adsorbed on Ru blacks depends on the preliminary preparation of the electrodes. The CO adsorption could also be employed in the determination of a charging current of electrode double-layers during voltammetric oxidation of adsorbed hydrogen on ruthenium supported on Al₂O₃, SiO₂, or TiO₂ carriers. However, a similar determination of hydrogen adsorbed on the tin-modified Ru catalysts is not very reliable.     

Specific Features of the Adsorption of Carbon Monoxide on a Smooth Polycrystalline Platinum Electrode Modified with Adsorbed Silver Atoms

The adsorption of carbon monoxide at the surface of smooth polycrystalline platinum (smPt) is studied in conditions of a preliminary accumulation of various quantities of silver (θ_Ag) on the surface. A comparison with similar data obtained previously for Pt/Pt is conducted. It is discovered that on smPt, exactly as in the case of Pt/Pt, carbon monoxide undergoes adsorption at sites that are not occupied by adsorbed silver, without forcing the preliminarily adsorbed silver out. At small and intermediate Ag_ad, as opposed to Pt/Pt, a mere two peaks are observed in a voltametric curve in the region of electrodesorption of the mixed layer on smPt. It is shown that, in the region of potentials of the first peak, there occurs practically no transition of silver into solution in the course of oxidation of the mixed layer. Specific features that characterize the behavior of the CO_ads + Ag_ad mixed layer are discussed under the assumption about an “islet” character of the adsorption of silver.