Electrocatalytic properties of Au(111)-Pd quasi-single-crystal film electrodes as probed by ATR-SEIRAS

Electrochemical and electrocatalytic properties of thin films Au(111-25 nm), which are quasi-single-crystal electrodes 25 nm thick made of gold with the (111) preferential orientation, and same electrodes modified with a monolayer (ML) of palladium are studied in 0.1 M solutions of HClO₄ and H₂SO₄ employing voltammetric techniques and surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS). Spectroscopic experiments demonstrate strong adsorption of electrolyte species (H₂O, OH_ads, anions) on the Pd surface. The weak and reversible adsorption of CO on Au(111-25 nm) does not change the interfacial-water structure. Adsorption of CO on the Pd-modified film results in an irreversibly adsorbed CO adlayer stabilized by co-adsorbed isolated water species. Various electrooxidation mechanisms are discussed. Electrochemical and spectroscopic investigations on the adsorption and electrooxidation of HCOOH on bare and 1 ML Pd-Au(111-25 nm) electrodes reveal that electrooxidation proceeds in both cases via a direct or dehydrogenation pathway. This mechanism involves the formation of formate as intermediate, which is detected by in situ ATR-SEIRAS. The reactivity on Pd-modified surfaces is higher than on bare gold. The specifically adsorbed anions (sulfate/bisulfate) and the oxide formation on the substrate surface lower the reactivity for CO and HCOOH on both surfaces. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1312–1329. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Anion-exchange separation of Pt and Pd using perchloric and hydrochloric acid solutions

On Biorad Ag-1X8 anion-exchange resin (200–400 mesh), Pd and Pt may be separated from one another by elution with 0.2M HClO₄, and 5M HClO₄, respectively. If present, Au may be retained by making the elutriants 0.003M in HCl. Alternatively, reduction by H₂SO₃ enables elution of Pt²⁺ with 6M HCl before recovery of Pd²⁺ with 0.2M HClO₄·Ir⁴⁺ is reduced to Ir³⁺ by H₂SO₃ and may be eluted ahead of Pt²⁺ by 2M HCl.     

Electrostatically controlled nanostructure of cationic porphyrin diacid on sulfate/bisulfate adlayer at electrochemical interface

Two different cationic tetraphenyl porphyrins, one with two carboxyphenyl groups in cis-position and the other in trans-position (cis- and trans-H(4)DCPP(2+)), have been examined to control the structure of their 2D supramolecular assemblies in 0.05 M H(2)SO(4) at electrochemical interfaces. Electrochemical scanning tunneling microscopy (EC-STM) images revealed the formation of supramolecularly organized nanostructures of cis-H(4)DCPP(2+) such as dimer, trimer, and tetramer on the (square root(3) x square root(7)) sulfate/bisulfate adlayer, suggesting the importance of both electrostatic interaction between cationic porphyrin core and sulfate/bisulfate adlayer and the hydrogen bond formation between carboxyl groups of the nearest neighbor cationic porphyrins. Trans-H(4)DCPP(4+) ions were also found to be aligned in the square root(3) direction of the sulfate/bisulfate adlayer. The structure of these cationic porphyrin adlayers was found to depend upon the electrode potential; i.e., when the potential was changed in the negative direction, the (square root(3) x square root(7)) sulfate/bisulfate adlayer disappeared, and no ordered arrays were formed. In contrast, when 0.1 M HClO(4) was used as an electrolyte solution, only a disordered array was observed. The results of the present study indicate that the (square root(3) x square root(7)) sulfate/bisulfate adlayer formed on Au(111) in 0.05 M H(2)SO(4) plays a significant role as a nanorail template in the control of electrostatically assembled diacid porphyrin dicarboxylic acid derivative. In addition, the high-resolution STM clearly distinguished between cis-H(4)DCPP(2+) ion and cis-H(2)DCPP molecule. The cis-H(2)DCPP molecules on Au(111) provided an adlayer structure and an electrochemical behavior which are different from those of cis-H(4)DCPP(2+) ions.     

SDS‐modified Nanoporous Silver as an Efficient Electrocatalyst for Selectively Converting CO2 to CO in Aqueous Solution

Selectively electrochemical conversion of CO₂ into organic fuel using renewable electricity is one of the most sought‐after processes. In this paper, we report the electrochemical reduction of CO₂ (CO₂RR) on the nanoporous Ag electrodes made of compacted Ag nanoparticles (AgNPs), which were prepared by one‐step reduction in the water phase with or without the surfactant sodium dodecyl sulfate (SDS). The scanning electron microscope (SEM) characterizations show that the compacted Ag electrodes have the nanoporous morphology formed by stacking AgNPs. Compared with the nanoporous Ag electrode without SDS modification (C‐AgNPs), the SDS‐modified AgNPs electrode (C‐AgNPs‐SDS) is highly effective in improving selective CO production in a wide range of potentials (–0.69 V — –1.19 V, vs. RHE), with a Faradaic efficiency of 92.2% and a current density of –8.23 mA·cm^–2 for CO production at –0.79 V (vs. RHE). C‐AgNPs‐SDS is also catalytically stable with only less than 7% deactivation after 8 h of continuous electrolysis.     

Adsorption of TTF,TCNQ and TTF-TCNQ on Au(111): An <Emphasis Type="Italic">in situ</Emphasis> ECSTM study

The adsorption and adlayer structures of tetrathiofulvalene (TTF), tetracyanoquinodimethane (TCNQ) and TTF-TCNQ on Au(111) have been systematically investigated by in situ electrochemical scanning tunneling microscopy (ECSTM) and cyclic voltammetry in 0.1 mol L⁻¹ HClO₄. All the three molecules were found to form well-ordered adlayers in the double-layer potential region of Au(111). For TTF and TCNQ adlayers, (6×3) and (4×7) structures have been observed, respectively. A structural transition was observed on TCNQ adlayer at potential negative of 0.08 V vs. the reversible hydrogen electrode (RHE), and induced a new phase with (3 × 12) structure. On the other hand, the charge transfer complex, TTF-TCNQ, self-organized into ordered domains with a lamellar structure different from those of the pure TTF and TCNQ adlayers on Au(111). Its packing arrangement was comparable to surface structures of either single crystal or thin film of TTF-TCNQ. Supported by the National Natural Science Foundation of China (Grant Nos. 20673121, 20733004 & 20821003), the National Key Project for Basic Research (Grant Nos. 2006CB806101 & 2006CB932100) and Chinese Academy of Sciences     

Polycrystalline Gold Electrodes: A Comparative Study of Pretreatment Procedures Used for Cleaning and Thiol Self‐Assembly Monolayer Formation

The influence of different surface pretreatment procedures on the electrochemical response of a polycrystalline gold electrode was evaluated. Mechanical polishing with slurry alumina (M), chemical oxidation with H₂SO₄/H₂O₂ (C), electrochemical polishing (potential cycling between ?0.1 V and 1.2 V vs. SCE) (E), chemical reduction with ethanol, and combinations among these treatments were employed to change the surface electrode characteristics. The efficiency of the proposed pretreatments was evaluated by electrochemical responses towards the redox couple ferri(II/III)‐ammonium sulfate and by the formation of a self‐assembly monolayer of 3‐mercaptopropionic acid (3 MPA SAM) on gold electrodes. The procedure (C) allowed important gold surfaces activation. Using procedures (C) and (E) the roughness of polycrystalline gold surfaces was significantly minimized and more reproducible surfaces could be obtained. From the profile of reductive desorption of 3 MPA SAM it was possible to verify that reduced gold surfaces generated better packed monolayers than oxidized ones and a comparative study using CV and DPV techniques showed that between the two desorption peaks, the one localized at more negative potential values corresponds to the cleavage of Au‐S bond from the chemisorbed thiol. In general, the improvement in the studied electrochemical responses could not only be attributed to an increase in the real surface area of the electrode, but to the chemical surface states set off by the pretreatment procedure.     

Diffusion with hydrolysis and ion association in aqueous solutions of beryllium sulfate

Taylor dispersion is used to measure mutual diffusion coefficients for aqueous solutions of beryllium sulfate at concentrations from 0.005 to 1 mol-L^–1 at 25°C. Least-squares analysis of the dispersion profiles shows that diffusion of the partially hydrolyzed salt produces a small additional flow of sulfuric acid, about 0.04 mol sulfuric acid per mole of total beryllium sulfate. Ternary diffusion coefficients measured for the aqueous BeSO₄–H₂SO₄ system are qualitatively consistent with Nernst-Planck predictions based on the formation of beryllium sulfate ion pairs, bisulfate ions, and the hydrolysis equilibria 2Be²⁺+H₂O= Be₂OH³⁺+H⁺, 3Be²⁺+2H₂O=Be₃(OH) ₂ ⁴⁺ +2H⁺. Except for very dilute solutions, the predicted flow of sulfuric acid is small compared to the flow of beryllium sulfate because most of the beryllium ions are protected from hydrolysis by the formation of BeSO₄ ion pairs, and most of the hydrogen ions produced by hydrolysis are converted to less-mobile bisulfate ions.     

XPS spectroscopic study of potentiostatic and galvanostatic oxidation of Pt electrodes in H2SO4 and HClO4

The surface oxides produced from potentiostatic and galvanostatic oxidation of Pt electrodes in HClO₄ and H₂SO₄ are examined using X-ray photoelectron spectroscopy. The oxide I species produced as the initial oxidation product by successively more anodic potentiostatic oxidation in 0.2 M HClO₄ is found to have a Pt²⁺ oxidation state, a binding energy characteristic of neither PtO, Pt(OH)₂ or PtO₂, and a limiting thickness of 8 Å. Galvanostatic oxidation in HClO₄ and H₂SO₄ is found to produce PtO₂·H₂O as an unlimiting growth oxide or a limiting growth oxide layer depending on the concentration of the acid electrolyte. The incorporation of the acid electrolyte anion in the surface layer is shown to have an effect on which type of oxide layer is produced. X-ray decomposition and chemical modification by Ar⁺ stripping are shown to produce chemical artifacts complicating any interpretation of a Pt oxide surface layer.     

Effect of ionic composition of solutions on the methanol reaction with adsorbed oxygen on smooth polycrystalline platinum electrodes: Transients of the open-circuit potential

Transients of the open-circuit potential observed in the reaction of methanol with oxygen (O_ads) preliminarily adsorbed on smooth polycrystalline platinum (pcPt) are measured in 0.05 M HClO₄, 0.5 M HClO₄, 0.05 M H₂SO₄, 0.05 M H₂SO₄ + 0.45 M Na₂SO₄, and 0.05 M H₂SO₄ + 0.45 M Cs₂SO₄. It is shown that the solution pH has a weak effect on the transient characteristics (when the reversible hydrogen electrode potential scale is used). This confirms the chemical nature of rate-controlling stages in the reaction mechanism. The changes in the reaction rate, observed upon going from one electrolyte to another, are preferentially associated with the involvement of solution ions in the formation of activated surface complexes that include CH₃OH, O_ads, and supporting-electrolyte components.     

Ion-Selective Electrodes Prepared with Polyaniline Membranes

Polyaniline is a conductive polymer that has electrochemical activity. For this reason, it has many different uses in electrochemical area. In this study, polyaniline was prepared by electrochemical oxidation of aniline in concentrated H₂SO₄ and HCl solutions. Then, membranes were obtained from a mixture of these polymers and paraffin at a weight ratio of 50%. Ion-selective electrodes were prepared with these membranes and AgCl/Ag electrodes. Properties of prepared membranes were investigated with potentiometric measurements made in HCl, KCl, H₂SO₄, and HClO₄ solutions of different concentrations. The E–logc plots obtained with these ion-selective electrodes were found to be linear in a distinct concentration range. Their slopes depend on the membrane type and the doping ion in the test solution. The difference between these membranes was explained according to the morphological structures of polyaniline membranes.     

ECSTM study of adsorption of C60, C70, C86 and Y@C82 on Au(111)

The adsorption of a variety of fullerenes (C₆₀, C₇₀, C₈₆, Y@C₈₂) on Au(111) electrode surfaces was comprehensively investigated in 0.1 M HClO₄ by electrochemical scanning tunneling microscopy (ECSTM). In the ordered C₆₀’s adlayer, C₆₀ molecules formed either (2 $ \sqrt 3 $ ×2 $ \sqrt 3 $ ) or “in-phase” structure. The high resolution STM image shows that the C₆₀ cage is not simply round but shows a bifurcated feature. The adsorption orientation of C₆₀ on Au(111) is tentatively suggested. In the ordered C₇₀’s adlayer, the perpendicular fullerene molecules are the main adsorption mode and form (2 $ \sqrt 3 $ ×2 $ \sqrt 3 $ ) structure. However, for C₈₆ and Y@C₈₂, the ordered adlayer could not be obtained on Au(111) under the present condition. These differences may be due to the different molecular shapes and sizes, and the encapsulated metal atom which affects the lattice matches with the substrate. The adsorption of fullerene molecules on Au(111) from disorder to order could be tuned simply by steering the dimensional sizes or shapes of the fullerenes used.     

Effect of proton donors on direct electron transfer in the system gold electrode–horseradish peroxidase

The effect of proton donors (PD) on the direct electron transfer (ET) reaction between polycrystalline Au electrodes and horseradish peroxidase (HRP) was investigated. HRP was immobilised directly on the bare Au surface. The pH of the contacting solution was varied at a constant ionic strength and the following different PDs were used as additives: H₃O⁺, NH₄⁺, [La(H₂O)]³⁺, [Y(H₂O)]³⁺, [Lu(H₂O)]³⁺. The kinetics of the bioelectrocatalytic reduction of H₂O₂ catalysed by HRP was studied with linear sweep voltammetry (LSV) in the potential range from 700 to −100 mV vs. SCE as well as amperometrically at −50 mV vs. Ag|AgCl with the HRP-modified Au electrodes placed in a wall-jet flow through electrochemical cell. An increase of [H₃O⁺] results in an enhancement of the current of the bioelectroreduction of H₂O₂ due to a more facilitated direct ET between Au and the enzyme over the potential range involved. It is shown that at high overvoltages (E<0.4 V) the PDs do not affect the rate of the enzymatic reduction of H₂O₂ but rather increase significantly the rate of direct ET between Au and HRP and the efficiency of acting as a PD is strongly correlated with their PD properties. The dependence of the apparent heterogeneous rate constant of direct ET, k_s, on [H₃O⁺] makes it possible to suggest that the reaction mechanism involves the participation of a proton in the elementary step of the charge transfer.     

Realization of Rhodium Metal‐Oxide Electrode in Indifferent Electrolytes

The pH‐dependence of the stationary open‐circuit potential E_i=0^st of rhodium electrode with a surface layer of anodically formed insoluble compounds has been studied in sulfate and phosphate solutions by means of cyclic voltammetry and chronopotentiometry. The range of potentials of the investigations performed has been confined to the region of rhodium electrochemical oxidation/reduction, i.e., 0.2<E<1.2 V (RHE) in order to prevent any possible interference of other reactions such as H₂ and O₂ evolution. It has been shown that rhodium electrode with a layer of surface compounds formed anodically at E<<1.23 V (RHE) behaves like a reversible metal‐oxide electrode within the range of pH values from ca. 1.0 to ca. 8.0. It has been presumed that the stationary potential of such electrode is determined by the equilibrium of the following electrochemical reaction: Rh+3H₂O??Rh(OH)₃+3H⁺+3e^?. The pH‐dependence of the reversible potential of Eequation/tex2gif-inf-6.gif electrode has been found to be: Eequation/tex2gif-inf-8.gif=E_i=0^st=0.69?0.059 pH, V. In acid solutions (pH<2.0) rhodium hydroxide dissolves into the electrolyte, therefore, to reach equilibrium, the solution must be saturated with Rh(OH)₃. This has been achieved by adding Rh³⁺ ions in the form of Rh₂(SO₄)₃. The solubility product of Rh(OH)₃, estimated from the experimental Eequation/tex2gif-inf-16.gif?pH dependence obtained, is ca. 1.0×10^?48, which is close to the value given in literature.     

Embedding Amorphous Molybdenum Sulfide within a Porous Poly(3,4‐ethylenedioxythiophene) Matrix to Enhance its H2‐evolving Catalytic Activity and Robustness

Amorphous molybdenum sulfide (MoS_x) is a promising alternative to Pt catalyst for the H₂ evolution in water. However, it is suffered of an electrochemical corrosion. In this report, we present a strategy to tack this issue by embedding the MoS_x catalyst within a porous poly(3,4‐ethylenedioxythiophene) (PEDOT) matrix. The PEDOT host is firstly grown onto a fluorine‐doped tin oxide (FTO) electrode by electrochemical polymerization of EDOT monomer in an acetonitrile solution to perform a porous structure. The MoS_x catalyst is subsequently deposited onto the PEDOT by an electrochemical oxidation of [MoS₄]^2? monomer. In a 0.5 M H₂SO₄ electrolyte solution, the MoS_x/PEDOT shows higher H₂‐evolving catalytic activities (current density of 34.2 mA/cm² at ?0.4 V vs RHE) in comparison to a pristine MoS_x grown on a planar FTO electrode having similar catalyst loading (24.2 mA/cm²). The PEDOT matrix contributes to enhance the stability of MoS_x catalyst by a significant manner. As such, the MoS_x/PEDOT retains 81 % of its best catalytic activity after 1000 potential scans from 0 to ?0.4 V vs. RHE, whereas a planar MoS_x catalyst is completely degraded after about 240 potential scans, due to its complete corrosion.     

Electrochemical and surface science investigations of PtCr alloy electrodes

Electrodes of supported Pt, modified with Cr, have shown an increase in electrochemical activity for oxygen reduction in phosphoric acid fuel cells over supported Pt only electrodes. To clarify the role of chromium and its chemical nature at the electrode surface, we have characterized a series of Pt_xCr(_1-x) bulk alloys (x = 0.9, 0.65, 0.5, 0.2) by electrochemical and ex-situ surface science methods. In this paper we report the surface characterization of native and post-electrochemical electrodes by XPS, cyclic voltammetry in 0.05 M H₂SO₄ and 85% H₃PO₄, and analysis of 0.05 M H₂SO₄ electrolyte following electrochemical treatment. The surface Cr(1 to 2 nm) was oxidized to Cr³⁺ oxide for surfaces at open circuit and those exposed to potentials < + 1.3 V vs DHE in 0.05 M H₂SO₄ and < + 1.55 V vs. DHE in 85% H₂PO₄. In 0.05 M H₂SO₄ the Cr component was electrooxidized to solube Cr⁶⁺ species at potentials > +1.3 V with the extent of Cr dissolution dependent on initial alloy stoichiometry. Alloys with Cr content 0.5 are capable of producing (dependent on time spent at potentials above +1.3 V in 0.05 M H₂SO₄) very porous Pt-rich surfaces. Loss of Cr was also observed in 85% H₃PO₄ for the alloys with Cr content 0.5, although at the more positive potential limit of +1.55 V. For the Pt_0.2Cr_0.8, treatment in 85% H₃PO₄ at +1.4 V and above led to the appearance of Pt⁴⁺ and Cr⁶⁺ species, apparently stabilized in a porous phosphate overlayer up to 5 nm thick (dependent on time spent at potentials above this limit). The enhancement reported for supported Pt+Cr oxygen cathodes is discussed in the light of these results.     

Potentiodynamic electrochemical impedance spectroscopy of lead upd on polycrystalline gold and on selenium atomic underlayer

Pb upd on polycrystalline Au and on Au coated with Se atomic layer was investigated by potentiodynamic electrochemical impedance spectroscopy. Faradaic and double layer responses have disclosed two distinct stages in Pb upd on Au: a partly irreversible stage, attributed to formation and growth of Pb 2D islands, and a reversible phase transition in the final stage of a monolayer deposition. The completion of a continuous monolayer formation in the potential scan was signalised by a sharp minimum in double layer pseudocapacitance Q_dl. Pb²⁺ reduction, which was monitored concurrently by parameters of Faradaic response, continued shortly after the Q_dl minimum and showed sharp maxima of adsorption capacitance and inverse Warburg constant at 40 mV below Q_dl minimum. This was explained by surface free energy minimisation that forced continuous atomic layer formation with inclusion of some lead cations into Pb monolayer. The two-stage Pb upd transformed into a single-stage strongly irreversible upd as a result of Se atomic underlayer deposition on Au.     

Vinyl polymerization by cobaltic ions in aqueous solution. Part II. Polymerization of acrylonitrile and methyl acrylate

Polymerization of methyl acrylate in HClO₄ and HNO₃ was studied in the temperature range 10–15°C. The kinetics of the polymerization were found to be very simple, involving initiation and termination by cobaltic ions. Kinetic studies on polymerization of acrylonitrile in HClO₄ and HNO₃ revealed that water oxidation, and monomer oxidation were side reactions as in the case of methyl methacrylate. Experimental evidence favored the simultaneous initiation by Co³⁺ and CoOH²⁺ species. In H₂SO₄, certain unusual features were encountered. At low [Co³⁺], linear termination as well as termination by mutual combination occurred. Another interesting aspect was that CoSO₄⁺ initiated at low [Co³⁺]. This was unlike the case of other monomers in H₂SO₄. The rates of polymerization and rates of cobaltic ion disappearance were measured with respect to changes in [M], [Co³⁺], [H⁺], temperature, etc. The various rate constants were evaluated.     

A new simple preparation of platinum-nickel alloy nanoparticles and their characterization as an electrocatalyst for methanol oxidation

Pt-Ni alloy nanoparticles were produced by casting 2 or 10 mM H₂PtCl₆ solutions on a Ni column. The apparent particle size for the resultant Pt-Ni alloys increased with the concentration of the H₂PtCl₆ solution, while the content of Pt in the alloy decreased. The potential sweeps of 5 cycles in an H₂SO₄ aqueous solution for Pt-Ni (2 mM)/Ni and Pt-Ni (10 mM)/Ni electrodes led to electrochemical behavior similar to a polycrystalline Pt electrode, suggesting the formation of a few thin Pt layers on each Pt-Ni alloy surface. In electrochemical measurements, both Pt-Ni/Ni electrodes showed more negative onset potential of methanol oxidation and slower degradation of oxidation current of methanol than the polycrystalline Pt electrode. X-ray photoelectron spectroscopy of both Pt-Ni/Ni electrodes showed the shift of Pt4f peaks to a higher binding energy, suggesting that the increase in the d vacancy in the balance band 5d orbital of Pt contributed to the improved electrocatalytic activity and durability of the Pt-Ni/Ni electrodes.     

Substoichiometric Extraction of Mercury by Diethyldithiocarbamic Acid

The substoichiometric extraction of Hg²⁺ using diethyldithiocarbamic acid and ²⁰³Hg tracer was studied. Chloroform was employed to remove the complexes from the aqueous media which were 0.5M H₂SO₄ or 1 M HClO₄ and 0 to 5 M NaCL. - Systems containing Cl^? allowed extraction of Hg²⁺ for all DDC/Hg molar ratios, the extracted complexes being HgCl(DDC) and Hg(DDC)₂. Their exchange constant was determined. - In the absence of Cl^?, no extraction could be effected in either system if the DDC/Hg molar ratio was < 1; the H₂SO₄ system remained clear, whereas a precipitate of HgClO₄(DDC) formed in the HClO₄ system. For molar ratios > 1, the extraction of Hg²⁺ increased linearly with the addition of DDC, the extracted complex being Hg(DDC)₂.     

Anodic Stripping Voltammetric Detection of Arsenic(III) at Gold Nanoparticle‐Modified Glassy Carbon Electrodes Prepared by Electrodeposition in the Presence of Various Additives

The simple, fast and highly sensitive anodic stripping voltammetric detection of As(III) at a gold (Au) nanoparticle‐modified glassy carbon (GC) (nano‐Au/GC) electrode in HCl solution was extensively studied. The Au nanoparticles were electrodeposited onto GC electrode using chronocoulometric technique via a potential step from 1.1 to 0 V vs. Ag|AgCl|NaCl (sat.) in 0.5 M H₂SO₄ containing Na[AuCl₄] in the presence of KI, KBr, Na₂S and cysteine additives. Surfaces of the resulting nano‐Au/GC electrodes were characterized with cyclic voltammetry. The performances of the nano‐Au/GC electrodes, which were prepared using different concentrations of Na[AuCl₄] (0.05–0.5 mM) and KI additive (0.01–1.0 mM) at various deposition times (10–30 s), for the voltammetric detection of As(III) were examined. After the optimization, a high sensitivity of 0.32 mA cm^?2 μM^?1 and detection limit of 0.024 μM (1.8 ppb) were obtained using linear sweep voltammetry.