Bulk CO oxidation on platinum electrodes vicinal to the Pt(111) surface

Bulk CO oxidation has been studied on platinum stepped surfaces belonging to the series Pt(S)[n(111) × (111)], using a hanging meniscus rotating disk electrode (HMRDE) configuration. The general shape of the voltammograms is not significantly affected by the presence of the steps. However, the curves shift towards negative values as the step density increases. Thus, in the positive-going scan, a linear relationship is observed for the dependence of the potential for the ignition peak vs the step density for surfaces with terraces wider than five atoms, shorter terraces deviate from this behavior. In the negative-going scan, a similar situation is observed for the potential where the current drops to zero. In this case, Pt(111) electrode also deviates from the expected behavior because of the formation of the ordered bisulfate adlayer on the electrode. The anion readsorption process is also observed by recording the HRMDE voltammograms at a high scan rate. All these results have been analyzed in light of a common mechanism, discussing the possible role of the steps in the stability and reactivity of the CO adlayer. In memoriam of Francisco C. Nart, an excellent scientist, colleague, and friend.     

Photoinduced surface dynamics of CO adsorbed on a platinum electrode

The surface dynamics of adsorbed CO molecules formed by dissociative adsorption of HCHO at a polycrystalline Pt electrode/electrolyte solution interface was studied by picosecond time-resolved sum-frequency generation (TR-SFG) spectroscopy. A SFG peak at 2050-2060 cm(-1) was observed at the Pt electrode in HClO(4) solution containing HCHO at 0-300 mV (vs Ag/AgCl), indicating the formation of adsorbed CO at an atop site of the Pt surface as a result of dissociative adsorption of HCHO. The peak position varied with potential by approximately 33 cm(-1)/V, as previously found in an infrared reflection absorption spectroscopy (IRAS) study. Irradiation of an intense picosecond visible pulse (25 ps, 532 nm) caused an instant intensity decrease and broadening of the CO peak accompanied by the emergence of a new broad peak at approximately 1980 cm(-1) within the time resolution of the system. These results suggest a decrease and increase in the populations of CO adsorbed on atop and bridge sites, respectively, upon visible pump pulse irradiation.     

The effect of the cooling atmosphere in the preparation of flame-annealed Pt(111) electrodes on CO adlayer oxidation

The effect of the cooling atmosphere on the rate of CO adlayer oxidation on flame-annealed Pt(111) has been studied. Cooling of a flame-annealed Pt(111) electrode in air results in a higher amount of crystalline defects compared to Pt(111) cooled in a hydrogen–argon stream. Although the blank profiles in 0.5 M H₂SO₄ of Pt(111), cooled in air and under oxygen exclusion, are virtually identical, CO adlayer oxidation occurs at significantly lower overpotentials on the former electrode. Three voltammetric peaks are observed for subsaturated CO adlayer oxidation on Pt(111), cooled in Ar+H₂ mixture, while only two peaks develop in the case of a Pt(111) surface cooled in air. Random crystalline defects, introduced via cooling of a flame-annealed Pt(111) in air, enhance CO adlayer oxidation, and apparently also suppress the third high-potential peak observed on a quasi-perfect (111) surface. The high sensitivity of the saturated CO adlayer oxidation to the presence of crystalline defects on Pt(111) can hence be used as a straightforward, sensitive, though qualitative method to assess the degree of crystalline order of the electrode.     

Promotion of the oxidation of carbon monoxide at stepped platinum single-crystal electrodes in alkaline media by lithium and beryllium cations

The role of alkali cations (Li(+), Na(+), K(+), Cs(+), and Be(2+)) on the blank voltammetric response and the oxidative stripping of carbon monoxide from stepped Pt single-crystal electrodes in alkaline media has been investigated by cyclic voltammetry. A strong influence of the nature of the cation on both the blank voltammetric profile and the CO oxidation is observed and related to the influence of the cation on the specific adsorption of OH on the platinum surface. Especially Li(+) and Be(2+) cations markedly affect the adsorption of OH and thereby have a significant promoting effect on CO(ads) oxidation. The voltammetric experiments suggest that, on Pt(111), the influence of Li(+) (and Be(2+)) is primarily through a weakening of the repulsive interactions between the OH in the OH adlayer, whereas in the presence of steps also, the onset of OH adsorption is at a lower potential, both on steps and on terraces.     

Catalytic oxidation of biological components on platinum electrodes modified by adsorbed metals: Anodic oxidation of catecholamines

The catalytic oxidation of catecholamines on Pt electrodes modified by adsorbed metals (denoted as M_ad) was studied in 1 M HClO₄) by linear sweep voltammetry. The anodic peaks of four catecholamines (adrenaline, noradrenaline, iso proterenol and DOPA) shift to the negative potential side in the presence of M_ad, such as Bi_ad and Pb_ad. The anodic oxidation of catecholamines proceeds without the production of poisonous species on a bare Pt electrode. The catalytic activity depends on the surface coverage by M_ad (denoted as θ_M). The maximal effect of M_ad is attained at θ_M = 0.5. The presence of M_ad promotes the formation of the final product through irreversible hydroxylation following the electron transfer. From these results it was suggested that in the catalytic processes M_ad plays the major role in the provision of effective sites to activate water molecules which take part in the subsequent hydroxylation step.     

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

The catalytic oxidation of glucose on Pt electrodes modified by adsorbed metals was studied in 1 M HClO₄ by linear sweep voltammetry. The adsorbed metals (denoted as M_ad, such as Bi_ad, and Pb_ad) formed on Pt in the potential region more positive than the reversible potential of an M_z+/M⁰ couple, lead to a marked increase in the anodic current of glucose by about one order of magnitude. The catalytic activity depends on the surface coverage by the M_ad. The strongly adsorbed species of lactone type, which are responsible for blocking the successive oxidation, are formed on the electrode surface in the anodic processes of glucose on a bare Pt electrode. The formation of such poisonous species is accelerated in the presence of adsorbed hydrogen on Pt. The effects of M_ad were discussed on the basis that M_ad plays its major role on the Pt electrode surface in removal of the adsorbed hydrogen which initiates the formation of the poisonous species.     

Stripping voltammetry of carbon monoxide oxidation on stepped platinum single-crystal electrodes in alkaline solution

The electrochemical oxidation of a CO adlayer on Pt[n(111)x(111)] electrodes, with n = 30, 10, and 5, Pt(111), Pt(110) as well as a Pt(553) electrode (with steps of (100) orientation) in alkaline solution (0.1 M NaOH) has been studied using stripping voltammetry. On these electrodes, it is possible to distinguish CO oxidation at four different active oxidation sites on the surface, i.e. sites with (111), (110) and (100) orientation, and kink sites. The least active site for CO oxidation is the (111) terrace site. Steps sites are more active than the (111) terrace sites, the (110) site oxidizing CO at lower potential than the (100) site. The CO oxidation feature with the lowest overpotential (oxidation potential as low as 0.35 V vs. RHE) was ascribed to oxidation of CO at kink sites. The amount of CO oxidized at the active step or kink sites vs. the amount of CO oxidized at the (111) terrace sites depends on the concentration of the active sites and the time given for the terrace-bound CO to reach the active site. By performing CO stripping on the stepped surfaces at different scan rates, the role of CO surface diffusion is probed. The possible role of electronic effects in explaining the unusual activity and dynamics of CO adlayer oxidation in alkaline solution is discussed.     

Mechanism of CO oxidation on Pt(111) in alkaline media

Electrochemical techniques, coupled with in situ scanning tunneling microscopy, have been used to examine the mechanism of CO oxidation and the role of surface structure in promoting CO oxidation on well-ordered and disordered Pt(111) in aqueous NaOH solutions. Oxidation of CO occurs in two distinct potential regions: the prepeak (0.25-0.70 V) and the main peak (0.70 V and higher). The mechanism of reaction is Langmuir-Hinshelwood in both regions, but the OH adsorption site is different. In the prepeak, CO oxidation occurs through reaction with OH that is strongly adsorbed at defect sites. Adsorption of OH on defects at low potentials has been verified using charge displacement measurements. Not all CO can be oxidized in the prepeak, since the Pt-CO bond strength increases as the CO coverage decreases. Below theta(CO) = 0.2 monolayer, CO is too strongly bound to react with defect-bound OH. Oxidation of CO at low coverage occurs in the main peak through reaction with OH adsorbed on (111) terraces, where the Pt-OH bond is weaker than on defects. The enhanced oxidation of CO in alkaline media is attributed to the higher affinity of the Pt(111) surface for adsorption of OH at low potentials in alkaline media as compared with acidic media.     

The electrooxidation of glucose on platinum electrodes in buffered media

Kinetic studies of glucose electrooxidation are of special interest in fundamental and applied electrochemistry. The electrochemical reaction on bright Pt electrodes is investigated through the application of combined potential/time perturbation programs and electrolytes of different compositions at temperatures ranging from 30 to 64°C. The electrooxidation of glucose should occur through the formation of several intermediate species and the overall reaction pathway should consist of different stages considering electroadsorption, chemical and electrochemical processes. The influence of potential perturbation conditions, anion adsorption, inhibition reactions of products and temperature are analysed through changes in U/I profiles.     

Canard explosion and internal signal stochastic bi-resonance in the CO oxidation on platinum surface

Canard explosion means a dramatic change fromsmall amplitude quasi-harmonic oscillation to largeamplitude relaxation oscillation, accompanied by anexponential increase of period, of a limit cycle withina very narrow interval of a control parameter. Thisphenomenon was first found in the Van Der Pol equa-tions[1], and later has been found also in chemical[2]and biological[3] systems. Generally speaking, it is theresult of multi-time scales in the system, and can bedealt with singular perturbati…     

Canard explosion and coherent biresonance in the rate oscillation of CO oxidation on platinum surface

The relationship between canard explosion and coherent biresonance is analyzed by numerically investigating a temporal dynamical model of CO oxidation on Pt surface. Canard explosion, manifesting itself by a dramatic change in the amplitude and period of a periodic orbit within a very narrow interval of a control parameter, is the result of multiple time scales in a dynamical system and is common in excitable systems. Coherent biresonance, namely, two peaks on the signal-to-noise ratio (SNR) curve when varying noise intensity, is a novel phenomenon of coherent resonance which is well-known in excitable systems. When the control parameter is varied from a stable fixed point, crossing the supercritical Hopf bifurcation, one of the peaks that corresponds to relatively larger noise intensity, keeps a constant height and position, while the other becomes higher and moves to lower noise level. When we consider the case in which two control parameters are perturbed by independent noise simultaneously, an interesting picture of one valley between two ridges appears on the 3D surface of SNR.     

Electrodeposited Pd and PdNi coatings as electrodes for the electrochemical oxidation of ethanol in alkaline media

Journal of Solid State Electrochemistry - Electrodeposition of Pd and PdNi coating samples of different thicknesses, depending on the coating composition and current efficiency, was achieved...     

Studies on the role of oxidation states of the platinum surface in electrocatalytic oxidation of small primary alcohols

The role of the oxidation state of a platinum polycrystalline surface in the electrocatalytic oxidation of C₁ to C₄ primary alcohols has been studied by using electrochemical techniques, in situ FTIR spectroscopy and X-ray photoelectron spectroscopy. The results revealed that the oxidation state of the Pt surface plays a key role in the oxidation of primary alcohols, and demonstrated that the oxidation of C₁ to C₄ primary alcohols on a Pt electrode is controlled by the formation of surface oxides on the Pt electrode at different potentials. It was found that the dependence of the reaction process on the oxidation states of the platinum surface yielded similar features in the cyclic voltammogram for oxidation of different primary alcohols at a Pt electrode. According to the effects in the oxidation of primary alcohols, the surface oxides of platinum may be classified as active and poison species. The Pt surface oxides of higher oxidation states (Pt(OH)₃ and PtO₂) formed at potentials above 1.0 V (SCE) were identified as poison species, while other lower oxidation states of Pt surface oxides such as PtOH, Pt(OH)₂ and PtO may be identified as the possible active species for primary alcohol oxidation.     

Influence of the surface morphology of smooth platinum electrodes for the sodium borohydride oxidation reaction

In this study, we show that the platinum electrode preparation procedure influences its behavior towards the borohydride electrooxidation reaction (BOR) mechanism. Cycling a smooth polycrystalline Pt electrode in alkaline electrolyte within the water stability domain prior to the BOR characterization radically changes the shape of the BOR voltammogram obtained in hydrodynamic conditions using the rotating disk electrode (RDE) setup, compared to the “classical” one measured on a smooth polycrystalline Pt electrode just polished before the BOR RDE study. This particular BOR voltammogram is reversibly brought back to the “classical” one after voltammetric cycling in borohydride alkaline media. These changes in the BOR voltammogram highlight the sensitivity of the BOR mechanism towards the Pt surface morphology. A first comparison of the Pt electrode surface before and after the voltammetric cycling in alkaline media using tapping mode atomic force microscopy (AFM) shows no morphological differences between the two surfaces within the AFM observation range, suggesting a very fine atomic structure disordering of the Pt surface. Such strong dependence of the BOR mechanism on Pt regarding the electrode atomic structuring opens the way to future studies focusing on the BOR on well-defined Pt single crystals.     

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

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