Surface and Subsurface Oxygen on Platinum in a Perchloric Acid Solution

The oxidation of polycrystalline platinum in perchloric acid is studied by cyclic voltammetry at a potential scan rate of 0.1 V s^–1 in various potential cycling ranges. The earlier model for the formation of a barrier layer of strong complexes consisting of subsurface oxygen O_ss, platinum atoms, and anions adsorbed on the latter is shown to correctly describe experimental results on the platinum oxidation in sulfuric and perchloric acids. The regularities in these acids are on the whole similar. A weaker adsorption of perchlorate anions as compared with bisulfate facilitates chemisorption of oxygen at 0.7–0.85 V and hinders exchange by sites Pt O at 0.85–1.35 V. A prolonged potential cycling with a cathodic limit of 0.27 V and low anodic limits leads to the accumulation of surface complexes O_ss–Pt _n –ClO₄, which hinder both the oxygen chemisorption and the exchange Pt O below 1 V. At more positive potentials, the complexes are destroyed and oxygen penetrates into subsurface platinum layers.     

Surface and Subsurface Oxygen on Platinum in a 0.5 M H2SO4 Solution

The oxidation of polycrystalline platinum in 0.5 M H₂SO₄ is studied by cyclic voltammetry at potential scan rates of 5–500 mV s^–1 while varying the potential cycling range. The scheme, which is proposed for explaining the observed acceleration and deceleration of oxygen sorption at 0.75–1.0 V, accounts for the presence of oxygen in the subsurface layers of platinum (O_ss) and the formation of a barrier layer comprising complexes O_ss–Pt _n –SO₄. Cycling platinum secures certain steady-state contents of O_ss at 0.01–1.35 V. In an anodic scan, O_ss accumulates at E > 0.85 V (slow post-electrochemical stage) due to exchange of platinum and oxygen atom sites. In a cathodic scan, the desorption of most oxygen gives way to the adsorption of anions, which prevent residual O_ss from appearing on the surface. The residual O_ss disappears at E < 0.1 V after a sufficiently complete desorption of anions and the destruction of stable complexes O_ss–Pt _n –SO₄. Varying the potential cyclic limit leads, after a delay, to other steady-state O_ss contents.     

Electrocatalytic and adsorption properties of platinum microparticles electrodeposited onto glassy carbon and into Nafion® films

A comparative investigation of electrocatalytic and adsorption properties of platinum microparticles electrodeposited onto a glassy carbon surface (Pt/GC) and within a thin Nafion® film formed on a GC electrode (Pt/Nf/GC) is described. As test reaction the methanol oxidation in sulfuric acid solutions is used. Dependences of the steady-state specific reaction rates upon potential and methanol concentration were established, as well as those of the platinum surface coverage with methanol chemisorption products upon concentration. It was shown that at higher platinum loadings (above 60 μg cm⁻²) the specific activities of Pt/GC and Pt/Nf/GC are nearly the same and close to that of smooth platinum. At such loadings the surface coverage of the platinum deposit surface with organic particles does not differ from that of smooth platinum. At very low platinum loadings in the polymeric matrix (10–30 μg cm⁻²) a considerable decrease in the adsorption of strongly chemisorbed methanol particles is observed. These deposits are characterized by a low specific activity, which may be caused by the decrease of the platinum particle’s size, leading to a decrease in the amount of weakly bound methanol particles participating in the limiting reaction step.     

Cyclic voltammetric behavior of Pd–Pt–Rh ternary alloys

The electrochemical behavior of Pd–Pt–Rh alloys has been investigated using cyclic voltammetry (CV). The alloys were prepared by electrochemical codeposition as limited volume electrodes (less than 1 m in thickness). The morphology of the alloy surface and bulk compositions were examined by the SEM/EDAX method. Surface oxides generation (oxygen adsorption) and oxides reduction (oxygen desorption) currents together with hydrogen adsorption and hydrogen absorption signals can be distinguished on CV curves. During potential cycling through the full hydrogen–oxygen potential range Rh and Pd are preferentially dissolved, which is reflected in a dramatic transformation in the voltammogram shape. The composition changes involve not only the surface but also some atomic layers beneath the surface.     

Structure of a Platinum–Alumina Catalyst Prepared from the Carbonyl Cluster H2[Pt3(CO)6]5

The state of a platinum carbonyl cluster in an initial aqueous acetone solution and its transformations on the surface of aluminum oxide in the course of catalyst preparation were studied by EXAFS spectroscopy. It was found that water enters the polynuclear framework of the dissolved cluster (the Pt–O distance is 2.55 Å, where O is the oxygen atom of water). Structural changes in the supported cluster in the course of catalyst preparation exhibited a strong interaction of platinum with alumina (the Pt–O distance is 1.92–1.95 Å), beginning at the step of H₂[Pt₃(CO)₆]₅ adsorption. This interaction was retained upon the subsequent high-temperature treatments of the catalyst. The structures of samples prepared from platinum carbonyl and chloroplatinic acid were significantly different. In the former case, a surface prototype was formed from the initial cluster; in the latter case, the sample consisted of platinum metal clusters of a considerable size.     

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.     

Kinetics and mechanism of the formation and reduction of oxide layers on platinum part I. Oxidation and reduction of platinum electrodes

Oxidation and reduction of platinum electrodes have been investigated in the potential range up to 3.0 V in acid, alkaline and phosphate buffer solutions with pH from 0 to 14. It is shown that oxygen adsorption and absorption on platinum are usually superimposed, but they may be artificially separated using a strong dependence of the second process upon platinum pretreatment and potential sweep rate. The dependence of oxygen surface coverage on potential has several characteristic regions, each corresponding to the formation of a definite surface oxycompound.     

Kinetic model of catalytic oxidation of carbon monoxide on nickel

A mechanism is proposed for describing the previous disclosed multiplicity of equilibrium states in the oxidation of carbon monoxide on metallic nickel. In contrast to the known mechanism for oxidation of CO oh platinum metals it includes a nonlinear stage of carbon monoxide adsorption and a linear stage of oxygen adsorption. A kinetic model has been obtained and stage velocity constants have been found, providing a basis for obtaining a quantitative agreement between the calculated and experimental relations between the reaction velocity and the reagent concentrations. Opinions are stated in relation to the causes for evolution of the CO oxidation reaction from platinum metals to nickel.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 83–87, January–February, 1986.     

Special features of methanol interaction with adsorbed oxygen at platinized platinum electrode: Transients of the open-circuit potential

Open-circuit potential transients are measured under the conditions of methanol interaction with the pre-adsorbed oxygen at platinized platinum electrode. The time necessary for complete removal of the adsorbed oxygen monolayer appeared being shorter by a factor of ～1.5 as compared with smooth polycrystalline platinum. The dependence of platinum surface coverage with adsorbed oxygen on the potential during its decay is found. It was shown that the reaction of methanol with the adsorbed oxygen is most slow at a high coverage (1–0.8). It is suggested that at these coverages, like the case of polycrystalline platinum, the adsorbed oxygen directly interacts with the methanol molecules from the solution. At moderate coverages (0.8–0.2), the reaction of the adsorbed oxygen with methanol at the platinized platinum is better described by the “conjugated reactions” mechanism. The specific rates of the methanol dissociative adsorption at the platinized platinum turned out to be close to those observed earlier for the polycrystalline platinum.     

Tin underpotential deposition on platinum and its catalytic influence on the kinetics of molecular oxygen electroreduction

The formation and dissolution of tin ad-layers on polycrystalline platinum were analysed by cyclic voltammetry in aqueous 10^–4 M tin(II)/1 M sulfuric acid in the 0.05–0.70 V versus RHE range. At this concentration level it was possible to observe that platinum sites involving (110) planes are mainly related to tin underpotential deposition. In contrast to previous results, no irreversible adsorption was found in the course of the electrodeposition. Thermodynamic adsorption parameters were calculated from the potential dependence of tin surface coverage. Catalytic properties of this new surface were studied on the basis of oxygen electroreduction as a model. Kinetic runs were performed with rotating ring-disk electrodes on bare and tin-modified platinum surfaces. Molecular oxygen reduction on tin-modified platinum takes place through the production of both water and hydrogen peroxide. This interpretation was confirmed by calculating the reaction order with respect to oxygen. Electronic Publication     

Variations in the Overall Charge and Open-Circuit Potential of a Palladium Electrode during the Adsorption of Iodine and Iodide Anions

Transients of the current and the open-circuit potential during the adsorption of I^– and I₂ on Pd/Pt (supporting electrolyte 0.5 M H₂SO₄) are measured and so are potentiodynamic and galvanostatic charging curves, in the presence and absence of adlayers. Dependences of the overall electrode charge on potential are constructed. After the adsorption of both I^– and I₂, values of the electrode surface charges at potentials of the double-layer region turn negative and considerably differ in their magnitude. The latter is connected with different character of adlayers, which is shown by experiments on the adsorption of silver atoms at underpotentials after a preliminary adsorption of I^– and I₂ on the electrode surface. Certain discrepancies between experimentally-found and theoretically-expected values of transients are attributed to a substantial increase in the irreversibility of sorption and desorption of hydrogen and oxygen in the presence of iodine atoms adsorbed on the surface.     

Structure-relationships in electrocatalysis: oxygen reduction and hydrogen oxidation reactions on Pt(111) and Pt(100) in solutions containing chloride ions

Kinetics of the oxygen reduction reaction (orr) and the hydrogen evolution–oxidation reactions (her/hor) were studied on the Pt(111) and Pt(100) surfaces in 0.05 M H₂SO₄ containing Cl⁻. The orr is strongly inhibited on the (100) surface modified by adsorbed Cl⁻ (Cl_ad), and it occurs as a 3.5e⁻ reduction via solution phase peroxide formation. In the hydrogen adsorption (H_upd) potential region, the orr is even more inhibited, and corresponds only to a 2 e⁻ reduction at the negative potential limit where the electrode is covered by one monolayer of H_upd and some (unknown) amount of Cl_ad. On the Pt(111)---Cl_ad surface, the orr is inhibited relatively little (in addition to that caused by strong bisulfate anion adsorption on this surface), and the reaction pathway is the same as in Cl⁻ free solution. The kinetics of the hor on Pt(111) are the same in pure solution and in a solution containing Cl⁻, since Cl_ad does not affect platinum sites required for the breaking of the H---H bond. A relatively large inhibition of the hor is observed on the (100) surface, implying that strongly adsorbed Cl_ad is present on the surface even near 0 V.     

Adsorption Kinetics of Normal-Heptanol on the Bismuth Single Crystal Planes

Cyclic voltammetry and impedance methods are employed for a quantitative study of normal-heptanol (n-HepOH) adsorption kinetics at the bismuth single-crystal plane/aqueous Na₂SO₄ solution interface. The results of nonlinear regression analysis show that the Frumkin–Melik-Gaikazyan (FMG) or Frumkin–Melik-Gaikazyan–Randles (FMGR) equivalent circuits can be used for the simulation of experimental impedance data. The dependences of adsorption capacitance (caused by the potential dependence of surface coverage), Warburg diffusion impedance, and adsorption resistance on the electrode potential and organic-compound concentration are established. Analysis of impedance data demonstrates that the adsorption of n-HepOH is mainly limited by the rate of diffusion of organic compound to the electrode surface. Small deviations toward mixed adsorption kinetics are established at very high frequencies. In the region of maximum adsorption in more concentrated n-HepOH solutions, the slow reorganization or two-dimensional association of adsorbed molecules is possible. However, the very low adsorption or partial charge transfer resistance values indicate that then-HepOH adsorption at Bi planes is a practically reversible process and thus there is no noticeable partial charge transfer between adsorbed n-HepOH molecules and Bi surface atoms.     

DFT study of oxygen adsorption on modified nanostructured gold pyramids

Periodic DFT calculations are used to predict and investigate the adsorption behavior of molecular oxygen on Au, Au/Pt, and Pt surfaces. To obtain an array of pyramids containing surface atoms with the lowest possible coordination number, a nano-modified surface consisting of a symmetrically "modified" (100) surface was used. The effect of atom substitution (organized alloying) is investigated. The adsorption of molecular oxygen on a pure gold pyramid is exothermic by 0.77 eV for the end-on adsorption mode. In the case of a pure platinum pyramid, the end-on adsorption mode was found to dissociate; however, a side-on geometry was encountered with an energy of adsorption of 2.3 eV. This value is in line with the fact that the adsorption energy of small molecules does not vary much on Pt surfaces with different indices. Additionally, some geometrically related trends of the surface deformation in relation to its composition and after adsorption of molecular oxygen are highlighted.     

Electrooxidation of Organic Substances on Platinum Modified with Oxotungstate Adlayers

The behavior of a platinized platinum electrode (roughness factor 500) in 0.5 M H₂SO₄ containing Na₃PW₁₂O₄₀ (PW₁₂) is studied by cyclic voltammetry. It is established that the effect of the PW₁₂ additive on the adsorption of hydrogen and oxygen is similar to the influence exerted by specifically adsorbed anions: a predominant displacement of strongly-bound hydrogen and a heavy suppression of oxygen adsorption are observed. It is shown that the regularities of oxidation of methanol, ethylene glycol, and formic acid at platinized platinum undergoe substantial alteration in the presence of PW₁₂. In the region of potentials E _r = 0.6–0.8 V, where the limiting stage of these processes is the adsorption of organic molecules, the oxidation rates substantially decrease. The run of steady-state polarization curves in the region E _r = 0.40–0.55 V depends on the direction of potential change and the extreme value of E _r. With E _r varied from 0.6–0.8 V in the direction of lower values in the interval E _r < 0.45–0.50 V, the oxidation processes accelerate as compared with the rates in the absence of PW₁₂. The acceleration occurs if conditions are created for a preliminary adsorption of the tungstate.     

Surface and Subsurface Oxygen on Platinum in a 0.5 M H2SO4 + 0.01 M CuSO4 Solution

The formation of a copper adatom layer on polycrystalline platinum in a copper sulfate solution is studied by cyclic voltammetry in different cycling ranges at 0.1 V s^–1. The copper adatom deposition kinetics is controlled by the following factors. The substrate's top layer structure during the oxygen exit onto the surface may be unstable at anodic limits E _a = 0.90–1.35 V. The concentration of copper oxides (active centers) may be higher at E _a = 0.8–0.95 V. The balance between different adsorption sites differs in different cycling conditions. Of importance is the number of complexes O_ss–Pt _n –SO₄ and O_ss–Pt _n –O_c, where O_ss is subsurface oxygen and O_c is chemisorbed oxygen.     

Surface and Subsurface Oxygen on Platinum in a Copper Perchlorate Solution

The formation of an adatom layer on polycrystalline platinum and the three-dimensional nucleation of copper in a copper perchlorate solution are studied by cyclic voltammetry at 0.1 V s^–1 while varying potential ranges and by recording potentiostatic current transients. About 0.6 monolayers of copper adatoms are deposited when cycling with anodic limit E _a = 1.35 V, the process is slower than that in an acid sulfate solution. Decreasing E _a accelerates the process (nearly one monolayer forms for E _a = 0.80–0.95 V in a cathodic scan) due to an increased number of active centers (metastable copper oxides) and, probably, to a change in the platinum surface microstructure. Oxygen for copper oxides is presumably supplied by water molecules adsorbed on a monolayer of copper adsorbed atoms and by subsurface oxygen (O_ss), which appears on the platinum surface after the destruction of complexes O_ss–Pt _n –ClO₄. Both the copper nucleation and the deposit growth accelerate at higher concentrations of copper oxides, which form at low E _a. High cathodic overvoltages decrease the number of active crystallization centers due to reduction or removal of copper oxides.     

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.     

Adsorption of anions on ultra-thin metal deposits on single-crystal electrodes: II: Voltammetric and radiochemical study of bisulfate adsorption on Pt(111) and Pt(poly) electrodes containing copper adatoms

The formation of ultra-thin metal deposits of copper on Pt(111) and polycrystalline platinum electrodes, as well as the adsorption of bisulfate on the copper-covered platinum surfaces, were studied by cyclic voltammetry and radioactive labeling. The highest charge obtained by voltammetry in the underpotential stripping range nearly corresponds to a close-packed monolayer of copper. The radioactive labeling data indicate that there are inactive and active copper adlayers toward bisulfate adsorption. The transition from inactive to active behavior is interpreted in terms of an increase in surface—bisulfate interactions at the expense of surface—perchlorate interactions. Based on recent X-ray absorption near-edge spectroscopy (XANES) analysis of copper deposition onto platinum, the site for bisulfate adsorption is most probably a Cu⁺ surface species. Combining this spectroscopic information with coulometry shows that an additional electron is confined to surface platinum atom(s) covered by the copper species. The copper film attains bulk copper properties when approximately 2.5 monolayers of copper are deposited.