Pt Electrodes In Acid Solutions Research Articles

Electrochemical study on hydrogen evolution and CO2 reduction on Pt electrode in acid solutions with different pH

Hydrogen evolution reaction (HER) is the major cathodic reaction which competes CO2 reduction reaction (CO2 RR) on Pt electrode. Molecular level understanding on how these two reactions interact with each other and what the key factors are of CO2 RR kinetics and selectivity will be of great help in optimizing electrolysers for CO2 reduction. In this work, we report our results of hydrogen evolution and CO2 reduction on Pt(111) and Pt film electrodes in CO2 saturated acid solution by cyclic voltammetry and infrared spectroscopy. In solution with pH>2, the major process is HER and the interfacial pH increases abruptly during HER; COad is the only adsorbed intermediate detected in CO2 reduction by infrared spectroscopy; the rate for COad formation increases with the coverage of UPD-H and reaches maximum at the onset potential for HER; the decrease of COad formation under HER is attributed to the available limited sites and the limited residence time for the reduction intermediate (Had), which is necessary for CO2 adsorption and reduction.

Formation of volatile products during nitrate reduction on a Sn-modified Pt electrode in acid solution

Nitrate reduction is investigated in acid solution on Sn-modified Pt electrodes with different Sn coverages. Volatile products and intermediates were detected by means of on-line electrochemical mass spectroscopy. N 2O is found as the main product for intermediate coverages of Sn whereas NO is the dominant product for high Sn coverages. By comparison to N 2O reduction on Pt and Sn-modified Pt electrodes with intermediate and high Sn coverages, respectively, N 2 formation is proposed to result from the consecutive reduction of N 2O on free Pt sites on the PtSn surface. In addition, chemical reactions between ammonia, hydroxylamine and nitrite are also briefly studied as possible reaction steps leading to N 2O and N 2 during nitrate reduction.

Electrochemical reduction of oxygen on thin-film Pt electrodes in acid solutions

The electrochemical reduction of oxygen on thin-film platinum electrodes in 0.1 M HClO 4 and 0.05 M H 2SO 4 solutions has been investigated using the rotating disk electrode (RDE) method. Thin films of Pt (0.25–20 nm thick) were prepared by vacuum evaporation onto glassy carbon substrate. The surface morphology of Pt films was examined by transmission electron microscopy (TEM). The specific activity of O 2 reduction was higher in HClO 4 and decreased with decreasing film thickness. In H 2SO 4, the specific activity was lower and appeared to be independent of the Pt loading. The values of Tafel slopes close to −120 mV dec −1 in high current density range and −60 mV dec −1 in low current density range were obtained for all electrodes in both solutions, indicating that the mechanism of O 2 reduction is the same for thin-film electrodes as for bulk Pt. The number of electrons transferred per O 2 molecule was close to four for all thin Pt films studied.

In situ ATR-SEIRAS study of electrooxidation of dimethyl ether on a Pt electrode in acid solutions

The electrooxidation of dimethyl ether (DME) was studied on a Pt film electrode in HClO 4 solution using surface enhanced infrared absorption spectroscopy (SEIRAS) and cyclic voltammetry. (CH 2OCH 3) ad and η 2(O, C)-H 2CO are inferred as the adsorbed intermediates resulting from the dissociative adsorption of DME in the hydrogen adsorption region. Adsorbed CO was also found, with coverages increasing rapidly upon DME adsorption, probably due to the further dissociation of (CH 2OCH 3) ad and η 2(O, C)-H 2CO. In the main oxidation region no adsorbed species, excluding CO, were found. However, for low DME concentration (0.025 M), vibration bands associated with adsorbed CH 3COO or CH 3OCOO were observed between 0.3 and 0.95 V. These results lead to the conclusion that the reaction products could depend on the concentration of the fuel. Additional discussion of the reaction mechanism is also presented.

Electrooxidation of ethanol on a Pt electrode in acid solutions: in situ ATR-SEIRAS study

The electrooxidation of ethanol was investigated on a Pt thin film electrode in a HClO 4 solution using surface enhanced infrared absorption spectroscopy (SEIRAS) with the attenuated total reflection (ATR) technique. The spectra indicate that during this reaction acetate and CO adsorbates are formed. The intensity of symmetric OCO stretching band of adsorbed acetate correlates well with voltammetry in the potential range between −0.1 and 0.85 V. The C O stretching band for adsorbed acetaldehyde and/or acetyl also was observed; these compounds are the reaction intermediates whose oxidation generates CO ad and acetic acid. We also explored the oxidation behavior of adsorbed residues. The oxidation of acetaldehyde was studied for comparison.

Ethyne oxidation and hydrogenation on porous Pt electrode in acidic solution

Electrocatalytic ethyne oxidation and reduction on Pt was studied by on-line mass spectrometry (DEMS). It was shown that the initial step in both processes involves a rapid reactant adsorption on the metal surface. The adsorbate consists of molecular ethyne, partially dehydrogenated hydrocarbon residues C 2H x and C 2-oxygen-containing species in a ratio dependent on the electrode potential. The maximum electrode coverage is achieved in the potential range 0.2–0.4 V, whereas the major part of the adsorbate is ethyne itself. Ethane was identified as the main volatile product of the cathodic hydrogenation of the surface species. A trace amount of 1-butene was formed in a slow side reaction. Evolution of a mixture of ethane and ethene was observed in the presence of bulk reactant. Measurements at various sweep rates revealed that electrooxidation of the preadsorbed ethyne to CO 2 at E>0.8 V is much faster than its electroreduction in the hydrogen potential region. The optimal conditions for monitoring and/or utilization of ethyne in an electrochemical cell working in a programmed step potential mode are specified.

Bistability and excitability in the electrochemical oxidation of ethanol

Results are presented from experiments on the oxidation of ethanol at a Pt electrode in acid solution. The experiments were conducted under conditions of constant current. The results show that the oxidation process exhibits a bistable response in which two different branches of steady-state potential values exist under the same conditions. Measurements also reveal that the process exhibits excitability: a relatively small change or perturbation causes the potential to undergo a large and long excursion before it returns to its steady-state value. Oscillations are also examined with a focus on the correlation between characteristics of the oscillations and the length of time the system is allowed to relax. Bistability is discussed in terms of a model previously used to describe multistability in the oxidation of methanol. Comparisons are made between the nonlinear behavior observed in the oxidation of ethanol and that observed in the oxidation of methanol.

The possible existence of subsurface H-atom adsorbates and H2 electrochemical evolution reaction intermediates on platinum in acid solutions

The possible existence of H-subsurface states on Pt electrodes in acid solutions was studied using Pt electrodes of different topographies which were characterized by ex-situ scanning tunnelling microscopy (STM). The H-adatom electrodesorption current peak at ca. 0.22 V versus RHE resulting from combined voltammetric and potentiostatic potential routines applied in the H-adatom potential range was assigned to H-subsurface states. Seemingly, the complementary electroreduction occurs at potentials slightly more positive than the hydrogen evolution reaction threshold potential and the reactant, in this case, appears to be related to the hydrogen evolution reaction. The charge density involved in these current contributions is only a small fraction of the H-adatom monolayer charge density. The formation of H-subsurface states appears to be favoured for those Pt surfaces exhibiting a larger contribution of weak H-atom electrosorption. A reasonable correlation between electrochemical and chemisorption data on H on Pt surfaces with different topographies and crystallographic orientations can be established. This fact provides further support for the existence of adsorbed H atom precursor states related to the formation of subsurface H adatoms and to the hydrogen evolution reaction on Pt in acid solutions.

Influence of changes in the total surface area and in the crystalline surface composition of Pt electrodes on their electrocatalytic properties with respect to the electro-oxidation of hydrazine

The electro-oxidation of hydrazine on electrodispersed and electrofacetted Pt electrodes in acid solutions takes place without noticeable changes in the mechanism with respect to that followed on smooth polycrystalline Pt electrodes. Nonetheless, a negative shift in the oxidation peak potential is observed with increasing roughness factor, R, of the electrodes. By working with preferentially oriented electrodes it was possible to demonstrate that the shift towards more negative potential values of the hydrazine oxidation peak is due to a slightly higher proportion on the electrodispersed electrodes of (100) microfacets.

Spectroscopic investigations of C 3-primary alcohols on platinum electrodes in acid solutions. : Part III. Propargyl alcohol

Electro-oxidation and electroreduction of propargyl alcohol (PA) on Pt electrodes in acid solutions was studied using differential electrochemical mass spectrometry and in-situ Fourier transform IR spectroscopy. The main electro-oxidation product is CO 2. Only very small amounts of propargyl aldehyde are detected at high potentials. Electroreduction of PA yields mainly propane, propylene, propanol, allyl alcohol and small amounts of propyne and ethane. Hydrogenation of the strongly adsorbed residues of PA produces hydrocarbons but not alcohols.

Electro-oxidation of l-ascorbic acid on platinum in acid solutions: an in-situ FTIRRAS study

Studies of in-situ Fourier transform infrared reflection-absorption spectroscopy (FTIRRAS) showed that l-ascorbic acid (AA) undergoes a spontaneous dissociative or destructive adsorption at a Pt electrode in acid solutions at the hydrogen adsorption-desorption region with linearly bonded carbon monoxide (CO) L as the adsorbate. This dissociative adsorption process most likely takes place via an interaction between the side-chain of AA and the Pt surface. The overall electro-oxidation of AA at Pt in acid solutions may involve a contribution from its ethylene glycol (EG)-like side-chain portion as well as its lactone ring portion. This process may consist of three major stages: (1) dissociative adsorption via the EG-like side-chain to form (CO) L on Pt at the hydrogen adsorption-desorption region; (2) direct oxidation via the lactone ring to form dehydro- l-ascorbic acid (DHA) and its hydrated derivatives in the potential region from the onset of oxidation to the current peak; (3) the EG-like side-chain and (CO) L undergo further oxidation to form CO 2 as the final product when the potential is driven to a more positive region. This study indicates that AA undergoes a spontaneous dissociative adsorption at the Pt electrode in acid solution prior to its direct oxidation. This dissociative adsorption probably takes place via an interaction between the side-chain of the AA molecule and the Pt surface, with (CO) L as the adsorbate. The overall electro-oxidation of AA at Pt in acid solutions may consist of three major stages: (1) dissociative adsorption; (2) direct oxidation via the lactone ring to form DHA and its hydrated derivatives in the potential-region from the onset of oxidation to the current peak; (3) further oxidation via side-chain and (CO) L to form CO 2 as the final product when the potential is driven to a more positive region.

On the Kinetics and Mechanism of Oxygen Reduction at Oxide Film‐Covered Pt Electrodes in Acid Solutions: II . Electron Transfer Through the Oxide Film and Mechanism of the Reduction

The kinetics and mechanism of oxygen reduction at oxide film‐covered Pt electrodes in acid solution is analyzed. Two linear regions are observed when it is ensured that the thickness of the oxide film for each point is the same. The kinetics in the high current density (cd) region is characterized by the Tafel slope of −120 mV. In the low cd region, the slope is −60 mV. The reaction order with respect to is and 1 in the high‐ and the low‐cd region, respectively. Because of the observed fractional reaction order in the high‐cd region, the kinetics in either cd region cannot be explained in terms of any classical, commonly used procedure in mechanistic analysis with a simple model of electrified interfaces. Currents at a constant potential in the cd region with the −120 mV slopes decrease exponentially with the thickness of the oxide film. In contrast, currents in the region with the −60 mV slope are invariant with thickness. Three models are used in mechanism analysis to account for the observed pH dependencies. The model of splitting the interfacial potential difference into two components, and the acid‐base equilibria model can explain the observed pH dependencies. However, they do not account for the observed kinetic behavior with respect to the thickness of the oxide film. The model of the distribution of the unoccupied electron energy levels in solution and electron tunneling through the oxide film accounts both for the observed pH dependencies and for the dependence of the rates on oxide film thickness. With this model it is shown that the electron energy level in the ground state of the reacting complex increases 60 meV as pH increases one unit. This dependence of the electron energy levels on pH provides physical significance for the observed pH dependence and fractional reaction order with respect to .

An analysis of the pH dependence of enthalpies and Gibbs energies of activation for O 2 reduction at Pt electrodes in acid solutions

The symmetry factor in oxygen reduction at Pt electrodes in acid solutions in the high current density region is close to 1 2 (=0.45 + 2.3 × 10 −4 T), and Tafel lines obtained at different temperatures intersect in extrapolation at the critical potential E cr = 0.23 V vs she. E cr is the same in solutions of different pH in the acid region. This is the potential at which Gibbs energy of activation is zero. At any constant potential, eg 0.8 V vs rhe, the activation energy decreases only very slowly with pH, ie by 2.8 kJ mol −1 pH. The activation energies at 0.8 V vs rhe obtained from E cr agree with the energies previously reported from Arrhenius plots at the same potential. The energy of the unoccupied electron level in the ground state of the reacting complex in the rate-determining step is −4.72 eV and is pH independent in the acid region.

IR active and inactive CO adsorbed on a Pt electrode in acidic solution observed by reflection absorption spectroscopy

Monolayer and submonolayers of CO adsorbed in the double-layer region on a Pt electrode were investigated in 1M HClO 4 by using in situ polarization-modulated IR reflection absorption spectroscopy (IRRAS) and an electrochemical oxidation. As for the monolayer of CO, it was concluded that linear CO accounted tor only 10% of all the adsorbed CO, and the remainder was RAS-inactive CO (reflection absorption spectroscopically inactive CO). As for the submonolayers of CO, the average number of adsorption sites per CO molecule changed from 1.7 to 1.2 with an increase in the total coverage from 0.1 to 0.9. The bridged CO could not be detected for these coverages by the IRRAS method. The band intensity of linear CO was not proportional to the total coverage. Therefore, it was concluded that the RAS-inactive CO were present, and these were one-site occupying and two-site occupying species. The adsorbed species predominating was the two-site occupying RAS-inactive CO for low coverage; and for high coverage the one-site occupying RAS-inactive CO. The RAS-inactive CO are described by a model in which CO is adsorbed on two Pt atoms with the C-O axis parallel to the electrode surface.

IR active and inactive CO adsorbed on a Pt electrode in acidic solution observed by reflection absorption spectroscopy

Monolayer and submonolayers of CO adsorbed in the double-layer region on a Pt electrode were investigated in 1M HClO4 by using in situ polarization-modulated IR reflection absorption spectroscopy (IRRAS) and an electrochemical oxidation. As for the monolayer of CO, it was concluded that linear CO accounted tor only 10% of all the adsorbed CO, and the remainder was RAS-inactive CO (reflection absorption spectroscopically inactive CO). As for the submonolayers of CO, the average number of adsorption sites per CO molecule changed from 1.7 to 1.2 with an increase in the total coverage from 0.1 to 0.9. The bridged CO could not be detected for these coverages by the IRRAS method. The band intensity of linear CO was not proportional to the total coverage. Therefore, it was concluded that the RAS-inactive CO were present, and these were one-site occupying and two-site occupying species. The adsorbed species predominating was the two-site occupying RAS-inactive CO for low coverage; and for high coverage the one-site occupying RAS-inactive CO. The RAS-inactive CO are described by a model in which CO is adsorbed on two Pt atoms with the C-O axis parallel to the electrode surface.

Energetics of an Electrochemical Reaction Derived from the Intersection of Tafel Lines Determined at Different Temperatures

AbstractUnder most conditions in electrode kinetics, the slopes of Tafel relationships for a given electrode process are dependent on temperature. Tafel lines determined at different temperatures therefore intersect at some singular potential, Ecr, for which the Gibbs energy of activation must be zero. The absolute value of the Gibbs energy of activation can then be evaluated by reference to this critical potential. For the oxygen reduction reaction at Pt electrodes in acid solutions, Ecr = 0.36 E vs. RHE. This critical potential yields a value of the Gibbs energy of activation at 0.8 V vs. RHE of 21 kJ mol‐1. A comparison of this energy with the apparent enthalpies of activation Δ≠Hap (= 21 and 24 kJ mol‐1), obtained previously from the Arrhenius plots at the same potential and in the same solution, shows that TΔS≠ is small. The values for the enthalpic component, βH (= 0.44), and entropic component, Tβs (= 0.07 at room temperature), of the overall symmetry factor, β, are determined. They show that, in this case, the kinetics are controlled mainly by the effect of electrode potential on the enthalpic component of the Gibbs energy of activation. It is shown that the electron energy level, ϵ0, of the reacting complex in the ground‐state can be obtained from Ecr. For oxygen reduction at Pt, it is ‐4.72 eV with respect to the electron energy in vacuum.

On the Kinetics and Mechanism of O 2 Reduction at Oxide Film Covered Pt Electrodes: I . Effect of Oxide Film Thickness on Kinetics

Oxygen reduction reaction is examined at oxide‐covered Pt electrodes in acid solutions. In the high current density region characterized by the Tafel slope of −120 mV, the reaction is affected by the thickness of the oxide film. As the film thickness increases, current at the same potential decreases. In the low current density region characterized by the Tafel slope of −60 mV, the rate of reduction is unaffected by film thickness. In both current density regions, the potential at a given current decreases 60 mV as pH increases one unit. This leads to the fractional reaction order of with respect to in the high current density region. In the low current density region, the order is 1. Possible mechanisms compatible with the observed kinetic parameters are discussed.

Adsorption of CO on a Pt electrode in acidic solution: Ir-acttve and -inactive CO'S adsorbed in the double-layer region

Carbon monoxide adsorbed on a smooth platinum electrode in the double-layer region was investigated in 1 M HClO 4 solution by using in situ polarization modulation IR reflection spectroscopy and an electrochemical oxidation. From the electrochemical oxidation, the adsorbed CO could be distinguished to be comprised of stable and unstable adsorbed CO's. The unstable adsorbed CO constituted about 1 3 of the adsorbed CO, and corresponded to linearly adsorbed CO, but the band intensity of the linearly adsorbed CO was not proportional to the amount of unstable adsorbed CO. The stable adsorbed CO constituted about 2 3 ; it was one-site adsorbed, and was an IR-inactive species. It is presumed that the IR-inactive species is adsorbed on two Pt atoms with the C-O axis parallel to the electrode surface and one of the Pt atoms bound to two CO molecules.

Kinetics and mechanism of H and OH electrosorption on faceted (100) Pt electrodes in acid solutions

The multiplicity of voltammetric peaks related to H-adatom and OH electroadsorption/electrodesorption reactions on faceted (100) Pt in 1 M H 2SO 4 and 0.5 M HClO 4 are investigated at different temperatures by using linear sweep voltammetry with triangular modulation covering a relatively large frequency range of the modulating signal. The voltammetric multiplicity related to H-adatom reactions in acid electrolytes is sensitive to both the surface structure and the structure of the solution side of the edl. The kinetics of the reactions are apparently influenced in a cooperative way by the water-H-adatom-an-ion ensemble formed at each site of a particular crystallographic plane. The main three reactions exhibit different relaxation time constants from which one can infer that the process occurring in the potential range presumably closer to the potential of zero charge is the slowest one. Anion adsorption produces a surface energy leveling effect and changes the kinetics of the electrochemical reactions. The H-adatom reactions can be interpreted through a formalism comprising the surface diffusion of H-adatom among different adsorption sites followed by a relatively fast electrochemical step involving only one kind of H-adatom. Data obtained for the OH-adsorbed reaction show, in principle, the same trend as that of the H-adatom reactions, although in this respect more extensive data are required.

Invariance with pH of enthalpies of activation for O 2 reduction at Pt electrodes in acid solutions

The apparent enthalpies of activation, Δ H * a, for O 2 reduction at Pt electrodes are determined in acid solutions of different pH. In the high current density region, characterized by a Tafel slope close to − 120 mV, Δ H * a,h is lower than Δ H * a,l in the low current density region characterized by a Tafel slope close to − 60 mV. Although Δ H * a,l > Δ H * a,h it is concluded that at every pH the enthalpies of activation at the zero Galvani potential difference are the same in both current density regions. Therefore, irrespective of the different Tafel slopes, the same mechanism is operative in both regions. In all solutions, Δ H * a for either current density region are independent of pH when they are determined at constant potentials vs rhe. This invariance with pH is unexpected in view of the suggested variance of the Galvani potential difference with pH. Alternative causes for the invariance are discussed.