B3LYP study of water adsorption on cluster models of Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0): Effect of applied electric field

A density functional theory (DFT) study of the adsorption of a water molecule on Pt(1 1 1), Pt(1 0 0) and Pt(1 1 0) surfaces has been carried out using cluster models, at the B3LYP/LANL2DZ,6-311++G(d,p) level. The water molecule binds preferentially at the top site on Pt(1 1 1) and Pt(1 0 0) with adsorption energy around −27 kJ mol⁻¹, and is oriented with the molecular plane nearly parallel to the metal surface and the H atoms pointing away from it. On Pt(1 1 0) a hollow site is preferred, with adsorption energy of −32 kJ mol⁻¹. Potential energy barriers for the rotation around an axis normal to the surface have been estimated to be below 1 kJ mol⁻¹ for Pt(1 1 1) and Pt(1 0 0) when water is adsorbed on top. Upon application of an external electric field (inducing positive charge density on the metal) adsorbed water is additionally stabilized on the three surfaces, especially at the top adsorption site, and adsorption on Pt(1 1 1) and Pt(1 0 0) becomes more favoured than on Pt(1 1 0). Good agreement has been found between harmonic vibrational frequencies calculated at the B3LYP/LANL2DZ,6-311++G(d,p) level and experimental frequencies for adsorbed water monomers on Pt(h k l) surfaces.     

Vibrational Stark tuning rates from periodic DFT calculations: CO/Pt(1 1 1)

DFT periodic calculations have been used to study the influence of an external electric field on the adsorption of CO on Pt(1 1 1). Particular attention has been focused on the determination of the CO and metal-CO vibrational Stark tuning rates. Stark tuning rates have been calculated at various CO coverages; a linear dependence between the CO Stark tuning rate and the CO surface coverage has been found. We have calculated a value of 68.94 cm⁻¹/(V/Å) for the zero-coverage limit CO Stark tuning rate, in good agreement with the experimental value of 75 ± 9 cm⁻¹/(V/Å). Like the CO Stark tuning rate, the metal-CO vibrational Stark tuning rate also increases as CO surface coverage decreases. In addition, we have found (at 0.25 ML) that the CO Stark tuning rate is similar at different adsorption sites, being only slightly larger at high-coordinated sites. CO vibrational Stark tuning rates of 45.58, 47.96, 47.61 and 48.49 cm⁻¹/(V/Å) have been calculated for ontop, bridge, hcp and fcc hollow sites, respectively. Calculations at high coverage using a (2 × 2)-3CO model yield a CO Stark tuning rate of 21.08 and 25.93 cm⁻¹/(V/Å) for ontop and three-fold hollow CO, respectively. These results show that the CO Stark tuning rate for CO adsorbed at high coordinated sites is only slightly larger than that at ontop sites. This result is in contradiction with experiments, which reported larger CO Stark tuning rates at high-coordinates sites than at ontop sites. Furthermore, the calculated metal-CO stretch is larger for ontop sites than for high-coordinated sites; this result is in disagreement with previous DFT cluster model calculations. Unfortunately, there is not experimental information available to support either result. Finally, we have also studied the CO adsorption site preference dependence on electric fields. We have found that CO adsorbs preferentially at high coordinated sites at more negative fields, and at ontop sites at more positive fields, in agreement with previous experiments and DFT cluster model calculations.     

Adsorption/oxidation of CO on highly dispersed Pt catalyst studied by combined electrochemical and ATR-FTIRAS methods: Part 1. ATR-FTIRAS spectra of CO adsorbed on highly dispersed Pt catalyst on carbon black and carbon un-supported Pt black

Elecrochemical ATR-FTIRAS measurements were conducted for the first time to investigate nature of CO adsorbed under potential control on a highly dispersed Pt catalyst with average particle size of 2.6 nm supported on carbon black (Pt/C) and carbon un-supported Pt black catalyst (Pt-B). Each catalyst was uniformly dispersed by 10 μg Pt/cm² and fixed by Nafion^® film of 0.05 μm thick on a gold film chemically deposited on a Si ATR prism window. Adsorption of CO was conducted at 0.05 V on the catalysts in 1 and 100% CO atmospheres, for which CO coverage, θ_CO, was 0.69 and 1, respectively. Two well-defined ν(CO) bands free from band anomalies assigned to atop CO (CO(L)) and symmetrically bridge bonded CO (CO(B)_sym.) were observed. It was newly found that the CO(L) band was spitted into two well-defined peaks, particularly in 1% CO, from very early stage of adsorption, which was interpreted in terms of simultaneous occupation of terrace and step-edge sites, denoted as CO(L)_terrace and CO(L)_edge, respectively. This simultaneous occupation was commonly observed in our work both on Pt/C and Pt-B. A new band was also observed around 1950 cm⁻¹ in addition to the bands of CO(L) and CO(B)_sym., which was assigned to asymmetric bridge CO, CO(B)_asym., adsorbed on (1 0 0) terraces, based on our previous ECSTM observation of CO adsorption structures on (1 0 0) facet. The CO(B)_asym. on the Pt/C, particularly in 100% CO atmosphere, results in growth of a sharp band at 3650 cm⁻¹ accompanied by a concomitant development of a band around 3500 cm⁻¹. The former and the latter are assigned to ν(OH) vibrations of non-hydrogen bonded and hydrogen bonded water molecules adsorbed on Pt, respectively, interpreted in term of results from a bond scission of the existing hydrogen bonded networks by CO(L)s and from a promotion of new hydrogen bonding among water molecules presumably by CO(B)_asym..It was found that the frequency ν(CO) of CO(L) both on Pt/C and Pt-B is lower than that on bulky polycrystalline electrode Pt(poly) or different crystal planes of Pt single-crystal electrodes by 30-40 cm⁻¹ at corresponding potentials, which implies a stronger electronic interaction between CO and Pt nano-particles and/or an increased contribution of step-edge sites on the particles. Determination of the band intensities of CO(L), CO(B)_asym. and CO(B)_sym. has led us to conclude a much higher bridged occupation of sites at Pt nano-particles than Pt(poly) electrodes.     

Electrochemical characterization of irreversibly adsorbed germanium on platinum stepped surfaces vicinal to Pt(1 0 0)

The electrochemical behavior of germanium irreversibly adsorbed at stepped surfaces vicinal to the Pt(1 0 0) pole is reported. The process taking part on the (1 0 0) terraces is evaluated from charge density measurements and calibration lines versus the terrace dimension are plotted. On the series Pt(2n − 1,1,1) having (1 1 1) monoatomic steps, the charge involved in the redox process undergone by the irreversibly adsorbed germanium is able to account for (n − 0.5) terrace atoms, thus suggesting some steric difficulties in the growth of the adlayer on the (1 0 0) terraces. Conversely, no steric problems are apparent in the series Pt(n,1,0) in which more open (1 0 0) steps are present on the (1 0 0) terraces. In this latter case the charge density under the germanium redox peaks is proportional to the number of terrace atoms. Some comparison is made with other stepped surfaces to understand the behavior and stability of germanium irreversibly adsorbed on the different platinum surface sites.     

Kinetics of dissociative adsorption of ethylene glycol on Pt(1 0 0) electrode surface in sulfuric acid solutions

The dissociative adsorption of ethylene glycol (EG) on Pt(1 0 0) electrode surface cooled in air after flame annealing was investigated by using programmed potential step technique and in situ FTIR spectroscopy. The stable adsorbates derived from EG dissociative adsorption on Pt(1 0 0) were determined by in situ FTIR spectroscopy as linear- and bridge-bonded CO. The quantitative results demonstrated that the average rate of dissociative adsorption of EG on Pt(1 0 0) surface varies with electrode potential, yielding a volcano-type distribution with a maximum value located near 0.10 V versus SCE. From the variation of the quantity of CO adsorbates generated in EG dissociative adsorption with the adsorption time t_ad, the initial rate (ν_i) of this surface reaction was evaluated quantitatively. The maximum value of ν_i has been determined to be 2.64 × 10⁻¹¹ mol cm⁻² s⁻¹ in a solution containing 2 × 10⁻³ mol L⁻¹ EG. The influence of the surface structure of Pt(1 0 0) electrode obtained by different pretreatment as well as of the specific adsorption of (bi)sulfate anions on the kinetics of EG dissociative adsorption has been also investigated and discussed. In comparison with a Pt(1 0 0) surface cooled in air atmosphere after flame treatment, the Pt(1 0 0) surface cooled in an Ar-H₂ stream or subjected to a treatment of fast potential cycling decreased significantly the initial rate ν_i of EG dissociative adsorption. Similar effect was also observed for the specific adsorption of (bi)sulfate anions. However, the maximum attainable coverage () of adsorbates derived from EG dissociative adsorption is not affected either by the surface structure of Pt(1 0 0) or by (bi)sulfate anions adsorption.     

Adsorption of phosphate species on poly-oriented Pt and Pt(1 1 1) electrodes over a wide range of pH

The adsorption of phosphate anions from phosphate solutions at poly-oriented and single-crystal platinum electrodes, primarily Pt(1 1 1), was studied over a wide range of pH by cyclic voltammetry. The features observed at the poly-oriented Pt electrode in phosphate solution may be related to the different crystalline facets, the (1 1 1) orientation presenting the most significant behavior in terms of phosphate adsorption. On the reversible hydrogen electrode (RHE) scale, the phosphate adsorption strength decreases with increasing alkalinity of the solution. Qualitatively, three different pH regions can be distinguished. At pH < 6 only a broad reversible peak is observed, corresponding to the adsorption of H₂PO₄⁻ and further deprotonation to adsorbed HPO₄⁻. For 6 < pH < 11 a butterfly feature followed by one or two anodic peaks (depending on scan rate) is observed, ascribed to the adsorption of HPO₄⁻ followed by its subsequent deprotonation to adsorbed PO₄³⁻. The splitting into two or three voltammetric features, and the irreversibility of the two features at more positive potential, is ascribed to the deprotonation reaction leading to a surface species (i.e. phosphate) which needs to change its surface coordination. At pH > 11 a reversible pre-wave and a sharp spike are observed, ascribed to the co-adsorption of phosphate and hydroxide.     

Kinetic study of nitrate reduction on Pt(1 1 0) electrode in perchloric acid solution

The kinetics of electrocatalytic reduction of nitrate on Pt(1 1 0) in perchloric acid was studied with cyclic voltammetry at a very low sweep rate of 1 mV s⁻¹, where pseudo-steady state condition was assumed to be achieved at each electrode potential. Stationary current-potential curves in perchloric acid in the absence of nitrate showed two peaks at 0.13 V and 0.23 V (RHE) in the so-called adsorbed hydrogen region. The nitrate reduction proceeded in the potential region of the latter peak in the pH range studied. The reaction orders with respect to NO₃⁻ and H⁺ were observed to be close to 0 and 1, respectively. The former value means that the adsorbed NO₃⁻ at a saturated coverage is one of the reactants in the rate-determining step (rds). The latter value means that hydrogen species is also a reactant above or on the rds. The Tafel slope of nitrate reduction was −66 mV per decade, which is taken to be approximately −59 mV per decade, indicating that the rds is a pure chemical reaction following electron transfer. We discuss two possible reaction schemes including bimolecular and monomolecular reactions in the rds to explain the kinetics and suggest that the reactants in the rds are adsorbed hydrogen and adsorbed NO₃⁻ with the assistance of the results in our recent report for nitrate reduction on Pt(S)[n(1 1 1) × (1 1 1)] electrodes: the nitrate reduction mechanism can be classified within the framework of the Langmuir-Hinshelwood mechanism.     

Surface-enhanced IR spectroscopy investigation on the electro-oxidation of CO adlayer at a polycrystalline Pt film electrode in Cl-containing HClO4

The electro-oxidation of CO adlayer on Pt electrode in Cl⁻-containing 0.1 M HClO₄ has been investigated with in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). Two potentials were selected for predosing CO on the Pt electrode: one is in the H-UPD region, i.e., 0.1 V (vs. RHE) and the other is in the double-layer region, i.e., 0.45 V (vs. RHE). The broadening of the prewave and the main peak for the CO oxidation is observed, in addition to the positively shifted oxidation potentials. The spectroelectrochemical data suggest the specific adsorption of Cl⁻ starts at a potential as negative as 0.1 V which may compete with the adsorption of OH at CO-unoccupied sites (including but not limited to defect sites) and/or hinder the diffusion of CO to OH adsorption sites on Pt electrode, slowing down the CO oxidation. This competitive Cl⁻ adsorption at lower potentials disrupts the interfacial free H₂O structure on the top of CO adlayer, signaled by a reduced OH stretching band intensity.     

Temperature dependence of co-adsorption of carbon monoxide and water on highly dispersed Pt/C and PtRu/C electrodes studied by in-situ ATR-FTIRAS

ATR-FTIRAS measurements were conducted to investigate nature of water molecules co-adsorbed with CO on highly dispersed PtRu alloy and Pt catalysts supported on carbon black in the temperature range between 23 °C and 60 °C. Each catalyst was uniformly dispersed and fixed by Nafion^® film of 0.0125 μm thickness on a chemically deposited gold film. Adsorption of CO was conducted and monitored by ATR-FTIRAS for 30 min in 1% CO saturated 0.1 M HClO₄ after stepping the potential from 1.2 V and 1.0 V to 0.05 V on Pt/C and PtRu/C, respectively. Similar atop and bridge bonded CO bands were observed on both PtRu/C and Pt/C, but a smaller relative band intensity, bridge bonded vs. atop CO, was observed on PtRu/C compared to Pt/C. A distinct O-H stretching band was found around 3643 cm⁻¹ and 3630 cm⁻¹ on PtRu/C and Pt/C, respectively, upon CO adsorption. They are assigned to non-hydrogen bonded water molecules co-adsorbed with CO on these catalysts. We found that the number of non-hydrogen bonded water molecules co-adsorbed with a given number of CO molecules decreases with increasing temperature and is higher on PtRu/C than Pt/C at each temperature. We interpret the higher ability of water co-adsorption at PtRu/C over Pt/C is due to stronger H₂O-metal interactions on the alloy surface. We present a model of the CO-H₂O co-adsorbed layer based on the bilayer model of water on metal surfaces.     

Electrochemical reduction of nitrate on Pt(S)[n(1 1 1) × (1 1 1)] electrodes in perchloric acid solution

The electrochemical reduction of nitrate ion was studied by cyclic voltammetry on Pt(1 1 1) and [n(1 1 1) × (1 1 1)] stepped Pt surfaces, where n (=14, 10, 7, 6, 5, 4, 3, 2) is the number of terrace atoms, in 0.1 M HClO₄ + 10 mM KNO₃. The electrocatalytic nitrate reduction was found to hardly proceed on Pt(1 1 1) in the hydrogen adsorption region, while the electrocatalytic activity was improved with the increase in the step density. Inactivation was observed in the presence of adsorbed hydrogen or nitrate-derived reduced adsorbate, i.e. adsorbed NO, on (1 1 1) step sites. It was, therefore, concluded that the electrocatalytically active NO₃⁻ species does not adsorb on the (1 1 1) terraces but on the (1 1 1) monoatomic steps. The nitrate reduction current increased with the step density in a non-linear relationship. The overall current density at 0.21 V (RHE) corresponding to the peak potential of the main electrocatalytic nitrate reduction wave which was maximum at n = 2, abruptly increased with short terraces, i.e. n < 5, where the current wave of adsorbed hydrogen on the Pt stepped surface with comparatively narrow (1 1 1) terraces, denoted as H_nt, also appeared unmodified for n < 5 on voltammograms recorded in 0.1 M HClO₄ in the absence of nitrate.     

An in situ scanning tunneling microscopic study of electrodeposition of bismuth on Au(1 1 1) in a 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid: Precursor adsorption and underpotential deposition

In this article, the electrodeposition of Bi on Au(1 1 1) surface in the underpotential region in BMIBF₄ ionic liquid containing BiCl₃ is studied by cyclic voltammetry and in situ scanning tunneling microscopy (STM). The cyclic voltammogram shows several cathodic and anodic peaks associated with underpotential deposition (UPD) of Bi and dissolution of the UPD deposit, respectively, in the potential region between −0.38 and −0.7 V versus Pt quasi-reference electrode. In situ STM results indicate there is a BiCl₃ precursor adsorption stage prior to the Bi UPD. The adsorption of BiCl₃ leads to the formation of unique hexagonal and trigonal supramolecular assembly with a Au(1 1 1)(10 × 10) structure. The initial stage of Bi UPD proceeds with the formation of Au(1 1 1)(7 × 7) R21.8° adlayer structure composed of Bi trigonal clusters at −0.5 V. A structural transformation occurred at −0.6 V resulting in a unique “zipper-like” double-chain pattern composed of well-aligned Bi trigonal clusters which can be denoted by Au(1 1 1)(5 × 25√3/3) structural model. The trigonal clusters composed of six Bi atoms seem to be the main characteristic elemental units of Bi UPD adlayer regardless of underpotential shift. These features are dramatically different from those observed in Bi(III)-containing acidic aqueous solutions as well as in chloroaluminated ionic liquid, but are similar to those of Sb UPD in BMIBF₄ ionic liquid, which reveals profound solvent effects on the electrodeposition of semimetals.     

Surface X-ray scattering of high index plane of platinum containing kink atoms in solid-liquid interface: Pt(3 1 0) = 3(1 0 0)-(1 1 0)

Surface structure of Pt(3 1 0) = 3(1 0 0)-(1 1 0), which contains kink atoms in the step, has been determined with the use of in situ surface X-ray scattering (SXS) in the double layer region (0.50 V(RHE)) in 0.1 M HClO₄. Clean Pt(3 1 0) surface has pseudo (1 × 1) structure on which lateral displacements of 2-9% and 0.3-1% are found along a and b directions, respectively, whereas the surfaces of Pt(1 1 0) = 2(1 1 1)-(1 1 1) and Pt(3 1 1) = 2(1 0 0)-(1 1 1) are reconstructed to (1 × 2) according to previous reports. Interlayer spacing between the first and the second layers d₁₂ is contracted about 5% compared with the bulk spacing, whereas those between underlying layers are expanded down to fourth layer. Fully adsorbed CO has no effect on the surface structure of Pt(3 1 0). This result differs from that on Pt(1 1 1), where d₁₂ is expanded after CO adsorption.     

In situ radiotracer and voltammetric study of the formation of surface adlayers in the course of Cr(VI) reduction on polycrystalline and (1 1 1) oriented platinum

This paper is focused on the in situ radiotracer and voltammetric studies of the induced HSO₄⁻/SO₄²⁻ adsorption at Pt(poly) and Pt(1 1 1) surfaces in 0.1 mol dm⁻³ HClO₄ solution in the course of Cr(VI) electroreduction. Besides this, the sorption behavior of HSO₄⁻/SO₄²⁻ ions on bare Pt(poly) and Pt(1 1 1) electrodes is compared and discussed. From the experimental results it can be stated that: (i) although the extent of bisulfate/sulfate adsorption is strongly dependent upon the crystallographic orientation of Pt surfaces, the maximum coverage on the Pt(1 1 1) does not exceed 0.2 monolayer; (ii) the Cr(VI) electroreduction on both poly- and (1 1 1) oriented platinum proceeds via a ce (chemical-electron-transfer) mechanism to yield Pt surfaces covered with intermediate surface adlayers containing Cr(VI) particles (and reduced Cr-containing adspecies) and ‘strongly bonded’ HSO₄⁻/SO₄²⁻ ions; (iii) while the coverage of platinum surfaces by the intermediate complexes formed in the course of Cr(VI) electroreduction at E > 0.20 V is basically independent of the crystallographic orientation of the Pt electrode, the onset for rapid Cr(VI) reduction is highly affected by the nature and crystallographic orientation of the electrode.     

CO monolayer oxidation on stepped Pt(S) [(n − 1)(1 0 0) × (1 1 0)] surfaces

The electrochemical oxidation of CO has been studied on Pt(S)[(n − 1)(1 0 0) × (1 1 0)] electrodes to investigate the effect of the step density in the reaction. This series shows two different trends for long (n ≥ 7) and short terraces. For long terraces, the voltammetric peak shifts towards higher potential as the step density increases, unlike the behaviour observed for other stepped surfaces, which exhibit the opposite behaviour in agreement with the Smoluchowski effect. For short terraces, the “normal” behaviour is observed, that is, as the step density increases the peak shifts towards lower potentials. Chronoamperometric measurements were used to determine rate constants and Tafel slopes using the mean field Langmuir-Hinselwood kinetics. Rate constants follow the same trends as the peak potentials in voltammetry. A Tafel slope of 75 ± 4 mV has been obtained for the surfaces with long terraces whereas a value of the surfaces with short terraces showed a value of 100-120 mV is obtained. This change of slopes is interpreted as a change in the electrochemical behaviour of the species involved in the mechanism, probably, a change in the adsorption isotherm of adsorbed OH. Pt(5 1 0) electrode exhibits an intermediate behaviour between those of long and short terraces with two different peaks that can be associated with both behaviours previously described.     

Interactions between interfacial water and CO adsorbed on Pt and Pt-Ru alloy surfaces under electrochemical conditions: Density-functional theory study

The structural and electronic properties of interfacial water and adsorbed CO on platinum and platinum/ruthenium alloy have been studied via density-functional theory calculations to gain insight into the water-adsorbate interaction under electrochemical conditions. The computational simulations reveal a new interpretation for the interaction of adsorbed CO and water at the electrochemical interfaces. The new interaction model rationalizes the observed quantitative relationship between infrared intensities for adsorbed bridging CO and water molecules that impart a high-frequency O-H stretch, ca. 3630-3660 cm⁻¹ on pure Pt and 3600-3620 cm⁻¹ on PtRu alloy. The theoretical modeling indicates that the observed feature common to both pure Pt and PtRu alloy surfaces is due to interfacial water molecules firmly hydrogen-bonded to bridging CO.     

Carbon monoxide oxidation on Au(1 1 1) surface decorated by spontaneously deposited Pt

Platinum is deposited spontaneously on Au(1 1 1) surface from 1 mM H₂PtCl₆ + 1 M HClO₄ solution using multiple deposition procedure. X-ray photoelectron spectroscopy (XPS) analysis has shown that after immersion into the Pt containing solution and rinsing with water, Pt(OH)₂ resides on the Au(1 1 1) substrate. Consecutive depositions as well as in situ scanning tunneling microscopy (STM) and electrochemical measurements are performed on previously electrochemically reduced Pt/Au(1 1 1) surfaces. Only homogeneous distribution of thus deposited Pt islands is observed by in situ STM. With subsequent depositions, the width of deposited Pt islands increases, but stays lower than 10 nm, while a significant increase of Pt islands height is observed, leading to moderate increase of the coverage. Cyclic voltammetry (CV) profiles of obtained Pt/Au(1 1 1) surfaces, and CO stripping curves are recorded in 0.5 M H₂SO₄ solution. CO oxidation takes place only at higher potentials shifting negatively with increasing coverage. This is discussed with respect to Pt islands width and height distributions and to the influence of the Au(1 1 1) substrate surface.     

Infrared spectroscopic and voltammetric study of adsorbed CO on stepped surfaces of copper monocrystalline electrodes

Voltammetric and infrared (IR) spectroscopic measurements were carried out to study adsorbed CO on two series of copper single crystal electrodes n(1 1 1)-(1 1 1) and n(1 1 1)-(1 0 0) in 0.1 M KH₂PO₄ + 0.1 M K₂HPO₄ at 0 °C. Reversible voltammetric waves were observed below −0.55 V versus SHE for adsorption of CO which displaces preadsorbed phosphate anions. The electric charge of the redox waves is proportional to the step atom density for both single crystal series. This fact indicates that phosphate anions are specifically adsorbed on the step sites below −0.55 V versus SHE. Voltammetric measurements indicated that (1 1 1) terrace of Cu is covered with adsorbed CO below −0.5 V versus SHE. Nevertheless, no IR absorption band of adsorbed CO is detected from (1 1 1) terrace. Presence of adsorbed CO on (1 1 1) terrace is presumed which is not visible by the potential difference spectroscopy used in the present work. IR spectroscopic measurements showed that CO is reversibly adsorbed with an on-top manner on copper single crystal electrodes of n(1 1 1)-(1 1 1) and n(1 1 1)-(1 0 0) with approximately same wavenumber of CO stretching vibration of 2070 cm⁻¹. The IR band intensity is proportional to the step atom density. Thus CO is adsorbed on (1 1 1) or (1 0 0) steps on the single crystal surfaces. An analysis of the IR band intensity suggested that one CO molecule is adsorbed on every two or more Cu step atom of the monocrystalline surface. The spectroscopic data were compared with those reported for uhv system. The CO stretching wavenumber of adsorbed CO in the electrode-electrolyte system is 30-40 cm⁻¹ lower than those in uhv system.     

Irreversible adsorption of Sn adatoms on basal planes of Pt single crystal and its impact on electrooxidation of ethanol

The behaviours of irreversible adsorption (IRA) of Sn adatoms on Pt(1 0 0), Pt(1 1 1) and Pt(1 1 0) electrodes were characterized using cyclic voltammetry. It has revealed that Sn can adsorb irreversibly on Pt(1 0 0) and Pt(1 1 1), while not significantly on Pt(1 1 0) electrode. Quantitative analysis of the relationship between 1 − θ_H and θ_Sn suggests that Sn adatoms may adsorb preferably on hollow sites of Pt(1 1 1) (threefold) and Pt(1 0 0) (fourfold) planes, which is in accordance respectively with the values 0.31 and 0.21 of coverage of IRA Sn adatoms in saturation adsorption determined on these electrodes. The IRA Sn adatoms on different basal planes of Pt single crystal yield different impact on the electrocatalytic oxidation of ethanol. It has revealed that the IRA Sn adatoms on Pt(1 0 0) electrode have declined the activity for ethanol oxidation, while IRA Sn adatoms on Pt(1 1 1) have enhanced remarkably the electrocatalytic activity with Sn coverage θ_Sn between 0.09 and 0.18. The oxidation peak potential E_p and the current density j_p of ethanol oxidation on Pt(1 1 1)/Sn were varied with θ_Sn, and the highest j_p (1258 μA cm⁻²) as well as the lowest E_p (0.20 V) were measured simultaneously at θ_Sn around 0.14. In comparison with the data obtained on a bare Pt(1 1 1), the E_p was shifted negatively by 65 mV and the j_p has been enhanced to about 1.7 times on the Pt(1 1 1)/Sn (θ_Sn = 0.14), which is ascribed to hydroxyl species adsorption at relatively low potentials on Pt(1 1 1)/Sn surfaces. The current study is of importance in revealing the fundamental aspects of modification of the basal planes of Pt single crystal using Sn adatoms, and the impact of this modification on electrocatalytic activity towards ethanol oxidation.     

In situ ATR-FTIR study of bulk CO oxidation on a polycrystalline Pt electrode

Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) measurements were conducted to analyze the electrochemical oxidation of CO dissolved in bulk electrolyte solution on a polycrystalline Pt electrode during linear sweep (cyclic) voltammetry (CV) in 1% and 100% CO-saturated 0.1 M HClO₄. A CO adlayer was first formed on Pt at 0.05 V vs. the reversible hydrogen electrode (RHE) during CO bubbling in each electrolyte, followed by the CV measurement. In 1% CO-saturated 0.1 M HClO₄, a well-developed prepeak was found, showing an onset and a peak at around 0.25–0.3 V and 0.42 V, respectively. A major contribution of bridge-bonded CO, CO(B), to the prepeak is concluded based on a decrease in the corresponding band intensity. In 100% CO-saturated 0.1 M HClO₄, the onset of bulk CO oxidation takes place around 0.25–0.3 V, which is closely associated with a band intensity decrease of CO(B), whereas atop (linear) CO, CO(L), did not exhibit intensity change in this potential region. This suggests that vacant sites made available upon oxidation of CO(B) serve as active sites for bulk CO oxidation. The oxidation of CO(B) at such low potentials is interpreted in terms of an adsorption energy on Pt that is lower than that for CO(L) and also of the specific structure of an adlayer that consists of intermixed CO(L) and CO(B). The bulk CO oxidation becomes diffusion-limited by dissolved CO above ca. 0.72 V, where we observed hardly any infrared spectral features ascribed to reaction intermediates. During a negative-going scan back to 0.05 V from 1.2 V, a steep decrease of the bulk CO oxidation current takes place around 0.66 V, at which preferential adsorption of CO(L) is observed. A rigid CO(L) island formation is strongly suggested from its high CO stretching frequency vs. its very small initial coverage and from its subsequent dependence on potential, with a linear Stark shift characterized by a slope of −29 cm⁻¹/V. Such an island formation is in marked contrast to the adlayer structure with intermixed CO(L) and CO(B) initially formed at 0.05 V during CO bubbling. It is concluded that the Pt surface saturated with the CO adlayer formed initially at 0.05 V exhibits a low overpotential for bulk CO oxidation owing to its adlayer structure with intermixed atop and bridge-bonded CO.     

Adsorption of camphor and 2,2′-bipyridine on Bi(1 1 1) electrode surface

Impedance spectroscopy and in situ STM methods have been used for investigation of the camphor and 2,2′-bipyridine (2,2′-BP) adsorption at the electrochemically polished Bi(1 1 1) electrode from weakly acidified Na₂SO₄ supporting electrolyte solution. The influence of electrode potential on the adsorption kinetics of camphor and 2,2′-BP on Bi(1 1 1) has been demonstrated. In the region of maximal adsorption, i.e. capacitance pit in the differential capacitance versus electrode potential curve, the heterogeneous adsorption and diffusion steps are the rate determining stages for camphor and 2,2′-BP adsorption at the Bi(1 1 1) electrode. It was found that for camphor | Bi(1 1 1) interface the stable adsorbate adlayer detectable by using the in situ STM method has been observed only at the positively charged electrode surface, where the weak co-adsorption of SO₄²⁻ anions and camphor molecules is possible. At the weakly negatively charged Bi(1 1 1) electrode surface there are only physically adsorbed camphor molecules forming the compact adsorption layer. The in situ STM data in a good agreement with impedance data indicate that a very well detectable 2,2′-BP adsorption layer is formed at Bi(1 1 1) electrode in the wide region of charge densities around the zero charge potential.