CO Adlayer Research Articles

Oxidation of CO adsorbed from CO saturated solutions on the Pt(111)/Ru electrode

Using Pt(111) surfaces covered by ruthenium by a spontaneous deposition method we conducted voltammetric oxidation of chemisorbed CO in clean sulfuric acid electrolyte at 50 mV s −1 and under variable sweep rate conditions. The deposition yields Pt(111) surfaces decorated by two-dimensional nanosized ruthenium islands previously reported. At 50 mV s −1, the CO stripping produces two well-resolved current peaks, at 0.55 and 0.67 V versus RHE. The first peak originates from CO chemisorbed on (and strictly around) ruthenium islands deposited on Pt(111), while another comes from CO chemisorbed on ‘pure’ Pt(111) phases of the Pt(111)/Ru electrode, away from the Ru islands. Using double-potential step chronoamperometry we investigated the CO oxidation on these two surface phases, and found that the low potential oxidation of chemisorbed CO occurred via the Langmuir–Hinshelwood mechanism. This shows that diffusion of CO on ruthenium to the Ru edge is not needed for a complete stripping of CO from such islands, i.e. the oxidation process is activated uniformly across an island. In contrast, CO chemisorbed on Pt sites not occupied by Ru is oxidized at the Pt(111)/Ru edge, and there may be a surface diffusion contribution to a complete removal of CO from the surface. However, a part of the CO adlayer oxidation in the 0.67 V peak area occurs with no participation of surface diffusion, according to a pseudo-first order surface reaction kinetics, and the reaction may involve a ‘reactant pair’ (PtOH⋯OCPt) breakdown on the electrode surface.

Electrooxidation of small organic molecules on mesoporous precious metal catalysts I: CO and methanol on platinum

Nanostructured platinum prepared by the chemical reduction of hexachloroplatinic acid dissolved in aqueous domains of the liquid crystalline phases of oligoethylene oxide surfactants has been examined as an innovative model electrocatalyst. Owing to its high specific surface area and periodic mesoporous nanostructure, electrooxidation of both carbon monoxide and methanol has been investigated using cyclic voltammetry and chronoamperometry as a probe of the electrocatalytic properties of this material. The oxidation of CO adsorbed on the new type of platinum is determined by the rate of an adsorption and nucleation-growth controlled phase transformation occurring inside the CO adlayer. This mechanism appears to be different from the traditional Langmuir–Hinshelwood model for CO oxidation. The methanol electrooxidation reaction appears to proceed mainly through a direct pathway with CO 2 as the final product and the potentiostatic current attains a steady-state value even at low potentials (<0.5 V). These results are very important since the nanostructured platinum shows a much lower poisoning rate at potentials relevant to fuel cell operation. Enhanced electrocatalytic durability to the methanol oxidation reaction is obtained on this new type of platinum in comparison to one high surface area platinum from Johnson–Matthey plc.

Role of Crystalline Defects in Electrocatalysis: CO Adsorption and Oxidation on Stepped Platinum Electrodes As Studied by in situ Infrared Spectroscopy

The adsorption of CO and the electrochemical oxidation of a CO adlayer on stepped Pt electrodes, Pt(443), Pt(332), and Pt(322), has been studied using in situ infrared reflection−absorption spectroscopy (IRAS). Coverage-dependent and potential-dependent spectra of CO adlayers on stepped Pt surfaces are reported. Infrared spectra acquired during oxidation of the CO adlayer provide information on the mechanism of the reaction and the structure of the operational catalytic active site. CO adsorbed on the (111) terraces is found to be more reactive compared to that adsorbed on either (110) or (100) steps. The step trough of either (110) or (100) step is concluded to be the active site for the electrocatalytic oxidation of the CO adlayer, the most reactive combination involving CO from the terrace and an oxygen-containing species in the step trough.

Lateral interaction and CO adlayer structures on the Pt( [formula omitted]) surface

We have revisited the structures for CO films adsorbed on the Pt(1 1 1) surface and simulate the diffraction patterns. In particular, we suggest alternative placement of the CO molecules in the unit cell of the c(4×2) structure for CO on Pt(1 1 1) at θ=0.5. To correctly explain the LEED pattern, found from experiment at θ=0.5, the cell ultimately should be asymmetric. This agrees with recent STM results for similar CO structures on Ni(1 1 1). Parameters of the lateral interaction, evaluated from the Monte-Carlo simulations, are in excellent agreement with recent first-principle calculations. Formation of the stable CO adlayer structures on Pt(1 1 1) appears to be dominated by a superposition of the dipole–dipole and indirect interactions, which provide the attraction between the molecules at a characteristic spacing of a√3. Trio interactions are also found to be important and may originate from characteristic pairs of molecules in adsorption sites of different types.

Collision induced phenomena on the CO/H/Ni(100) surface

The effect of the collision of atoms to the coadsorbed system of CO and H on Ni(100) has been studied using Xe atoms energy-controlled by supersonic molecular beam. After the surface was irradiated with Xe atoms, the intensity of the CO stretching mode observed at 2100 cm−1 in the FTIR spectrum decreased and the CO stretching mode at 1950 cm−1 appeared. This indicates that CO was shifted from on-top sites to bridge sites. It is considered that the collision of Xe atoms induces desorption of CO and rearrangement of the CO adlayer.

Mechanism and kinetics of the electrochemical CO adlayer oxidation on Pt(111)

The electrochemical oxidation of saturated and sub-saturated CO adlayers on Pt(111) in 0.5 M H 2SO 4 has been studied using chronoamperometry. For the saturated CO coverage the oxidation is initiated by an apparently zeroth-order process of removing 2–3% of the adlayer, followed by the main oxidation process, which is shown to be of the Langmuir–Hinshelwood type with a competitive adsorption of the two reactants, CO and OH. The Langmuir–Hinshelwood kinetics can be modeled using the mean-field approximation, which implies fast diffusion of adsorbed CO on the Pt(111) surface under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation and its potential dependence are determined by a fitting of the experimental data with the mean-field model. For sub-saturated CO coverages the overall picture is shown to be more complicated and remains to be understood.

Kinetic study of selective CO oxidation in H2-rich gas on a Ru/γ-Al2O3 catalyst

The preferential oxidation (PROX) of CO on Ru/γ-Al2O3 in simulated reformer gas (1.0 kPa CO, 75 kPa H2, rest N2) was investigated over a wide range of CO partial pressures (0.02–1.5 kPa) and O2 excess (pO2∶pCO = 0.5–5.0). Integral flow measurements in a microreactor at λ = 2 show that ∼150 °C is the optimum temperature for the PROX of CO, combining sufficiently high rates with a high selectivity (S ≈ 50%). Ru/γ-Al2O3 shows a significantly higher activity and selectivity under these conditions than the conventionally used Pt/γ-Al2O3 catalyst. Differential flow experiments allow the quantitative determination of CO and O2 oxidation rates at 150 °C, with reaction orders of α = −0.48 for CO and α = 0.85 for O2. In the low temperature regime, 135–200 °C, the apparent activation energy, Ea*, is 95 ± 5 kJ mol−1. Our findings are consistent with a Langmuir–Hinshelwood reaction mechanism in the low-rate branch, where both CO oxidation and H2 oxidation are limited by the presence of a CO adlayer. The saturation of the reaction rate at high oxygen excess (λ 6) as well as previous data support a model where the reaction proceeds on oxygen loaded/surface oxide covered Ru particles. The minimum amount of catalyst and oxygen excess for complete removal (<50 ppm) of CO from methanol reformate in the second stage of a double stage reactor (pCO ⩽ 0.2 kPa) was calculated in reaction simulations, which were based on a plug-flow reactor model and use the kinetic parameters determined in this study. Including contributions from CO and CO2 methanation and from the reverse water gas shift reaction (RWGS) the Ru/γ-Al2O3 catalyst shows a much better performance than Pt/γ-Al2O3, both because of its much lower minimum noble metal mass, lower by a factor of 20, and the much better behavior under dynamic load conditions, where only the Ru catalyst remains below the critical CO level of 50 ppm for load variations to 10% and 1%, respectively.

Methanol Oxidation on a Carbon-Supported Pt Fuel Cell CatalystA Kinetic and Mechanistic Study by Differential Electrochemical Mass Spectrometry

Methanol oxidation on a supported Pt fuel cell catalyst was investigated by on-line differential electrochemical mass spectrometry (DEMS) at continuous electrolyte flow and defined catalyst utilization, employing a thin-film electrode setup and a thin-layer flow-through cell. The active surface of the Pt/Vulcan (E-TEK) high surface area catalyst was characterized quantitatively by Hupd and preadsorbed CO monolayer stripping. Methanol stripping DEMS experiments, oxidizing the adsorbed dehydrogenation products formed upon methanol adsorption at potentials in the hydrogen adsorption region, show that the coverage of these products and hence the methanol uptake depend on the electrode potential, in contrast to the potential-independent COad coverage. The dehydrogenation products cannot be displaced by Hupd. The number of close to two electrons used per oxidation of one adsorbed dehydrogenation product identifies this as COad species. Further methanol dehydrogenation is hindered when the CO adlayer reaches a d...

Electrooxidation of CO and H2/CO mixtures on a carbon-supported Pt catalyst—a kinetic and mechanistic study by differential electrochemical mass spectrometry

We report the results of a differential electrochemical mass spectrometry study on the electrooxidation of CO and H2/CO mixtures on a high surface area carbon-supported Pt/Vulcan catalyst. The measurements were performed in a thin-layer flow-cell, allowing the quantitative determination of the individual reaction rates under controlled mass transport conditions as a function of the electrode potential, parallel to the Faraday current. COad monolayer oxidation (stripping) experiments in a 0.5 M H2SO4 solution saturated with H2 or with 2% CO in H2 show that (i) the hydrogen oxidation reaction (HOR) in CO-free, H2-saturated solution starts already at potentials right at the onset of CO oxidation (0.15 VRHE), as evidenced by the simultaneous increase in Faraday current, the increase in the CO2 signal, and the decrease in H2 content in the solution, and that (ii) in CO containing solution the onset of the HOR is shifted to more positive potentials due to continuous CO readsorption. In the former case 90% of the mass transport limited current is reached already at a potential where only ∽5% of the saturated CO adlayer is oxidized, in the latter case the limiting HOR current is reached only in the potential regime of the main CO stripping peak. Furthermore, in the second case CO oxidation in the pre-wave regime is enhanced, while the HOR is suppressed in this potential regime compared to CO stripping in CO-free H2-saturated electrolyte. As a result the ignition of the HOR in the H2/CO(2%) saturated electrolyte occurs only together with the main CO oxidation peak. The data are direct proof that the HOR proceeds in an almost closed CO adlayer, most likely in small (fluctuating) holes of the adlayer and has to compete with CO readsorption. Kinetic measurements at constant electrode potential (0.04 V) in H2/CO(2%)-saturated solution show that CO adsorption from an H2/CO mixture follows a precursor-type adsorption kinetics. For the HOR the data show substantial deviations from both a (1 − θCO) and (1 − θCO)2 dependence of the HOR rate on the CO coverage, pointing to a more complex mechanism than a simple site-blocking mechanism.

X-ray Scattering Studies of Irreversibly Adsorbed Bismuth on the Pt(111) Electrode Surface

The surface electrochemistry of irreversibly adsorbed bismuth (Biir) on the Pt(111) electrode in 0.1 M HClO4 has been studied by in situ surface X-ray scattering to correlate the atomic structure with the voltammetric features. Biir saturates at a coverage of ∼one-third of a monolayer and forms a disordered structure on the Pt(111) surface with some evidence that the Biir is mobile. If the Biir coverage is slightly increased, then there is a reduction in the charge under the reversible peaks in the cyclic voltammetry at a potential of ∼0.68 V (vs Pd/H2). The charge under the reversible peaks is most likely associated with the adsorption of oxygenated species at platinum sites adjacent to Biir atoms. Saturation of the electrolyte with CO and subsequent potential cycling leads to displacement of Biir from the surface and the appearance of an X-ray diffraction pattern due to a p(2 × 2)-3CO adlayer that has previously been observed on the unmodified Pt(111) electrode.

Infrared reflection–absorption properties of platinum nanoparticle films on metal electrode substrates: control of anomalous optical effects

A simple method is described for preparing ultrathin films of carbon-supported platinum nanoparticles on gold and other substrates for examination by electrochemical infrared reflection–absorption spectroscopy (EC-IRAS) in which the anomalous “anti-absorbance” properties commonly observed for metal particle arrays can be minimized as well as controlled. The tactic involves physical deposition as an aqueous suspension, followed by drying and thorough rinsing. The Pt monolayer-level films prepared by this means exhibit largely positive unipolar absorbance bands for CO adlayers, whereas an anomalous inverted band component appears for thicker films, especially those featuring high Pt nanoparticle loadings. These findings demonstrate the value of the film preparation tactic for exploring the electrocatalytic properties of C/Pt materials by using combined electrochemical and EC-IRAS measurements.

Adsorption and thermal behavior of CO and NO on Pd{110} and Pd{320}

Chemisorption and thermal behavior of CO and NO on Pd{110} and its vicinal surface, Pd{320} [Pd(S)[3(110)×(100)]], have been investigated using molecular beam surface scattering (MBSS), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). CO is shown to adsorb molecularly on Pd{320} in the temperature range of 250–500 K. No evidence for long-range ordering of CO adlayers on Pd{320} was observed. Saturation coverage for CO/Pd{320} at 300 K is 0.61 ML. The initial sticking probability of CO is 0.91 and is shown to depend only weakly on the surface temperature. It is also found that the initial sticking probability of CO on Pd{110} exhibits a strong dependence on the kinetic energy of CO molecules and the CO adsorption on Pd{320} may also exhibit this same behavior. CO desorbs molecularly from Pd{320} with desorption features near 390 and 500 K. NO is shown to adsorb molecularly below 420 K with a high initial sticking probability close to 0.9. The initial sticking probability is largely temperature independent up to 450 K. No long-range order was observed from NO/Pd{320} in the temperature range of 300–600 K. The saturation coverage of NO is 0.64 ML at 300 K. It is demonstrated that NO partially dissociates upon heating, yielding N2 and N2O. Desorption of molecular NO occurs on both surfaces between 300–600 K, with N2 and N2O desorbing above 450 K. In contrast to Pd{110}, a high temperature N2 peak near 600 K is seen during NO TPD from Pd{320}, which is attributed to tightly bound, adsorbed nitrogen atoms. The relative TPD yields of N2 and N2O from Pd{110} and Pd{320} are found to be strongly dependent on the initial NO coverage with enhanced dissociation at lower coverages. Pd{320} is found relatively unreactive towards the thermal dissociation of NO. The activity of Pd{110} and Pd{320} towards NO dissociation upon adsorption increases with surface temperature. The relative yields of N2 and N2O reach a maximum near 550 K of ∼40% and ∼20%, respectively. By direct comparison to NO/Pd{110}, the present study shows that the presence of (100) steps on the Pd{320} surface does not significantly enhance its reactive properties with respect to adsorption and thermal behavior of CO and NO.

Anion adsorption on Pd–Pt(111) electrodes in sulphuric acid solution

Anion adsorption at palladium-covered Pt(111) electrodes has been studied in sulphuric acid solutions using cyclic voltammetry, charge displacement and in situ FTIR experiments. The infrared spectra obtained for partially covered Pt(111) substrates show distinct bands for adsorbed (bi)sulphate on platinum and palladium surface atoms with frequencies similar to those observed for the bare and fully covered Pt(111) electrode surfaces, respectively. This observation is in agreement with the epitaxial growth of palladium on Pt(111). Increasing the palladium coverage in the (sub)monolayer range is shown to shift (bi)sulphate adsorption towards less positive potentials, whereas the total amount of adsorbed anions on the ensemble of palladium and platinum domains is approximately constant. Combination of CO displacement and voltammetric curves allows the determination of the potential of zero total charge (pztc) as a function of the palladium coverage. The pztc shifts from 0.33 to 0.235 V when the palladium coverage on the Pt(111) is increased up to a monolayer. The oxidation of the CO adlayer resulting from the charge displacement experiments has also been studied. The oxidation of adsorbed CO on palladium-covered Pt(111) electrodes takes place at higher potentials than on Pt(111). The pztc values have been used to calculate the contribution from anion adsorption to the charge obtained from the integration of the CO stripping voltammograms. Coverage values calculated for the CO adlayers generated during the CO displacement experiments at the palladium-covered Pt(111) electrodes diminish as the palladium coverage increases.

Determination of Site Specific Adsorption Energies of CO on Copper

The binding energies of (isolated) CO molecules adsorbed at several atomic sites (terrace, step, kink) on a number of differently oriented copper surfaces have been measured by thermal desorption spectroscopy (TDS). In addition to the three low-indexed Cu surfaces several regular stepped and kinked single crystal surfaces have been employed. Using LEED measurements together with available data in the literature allowed identification of the various different CO adlayers and to assign the different TDS binding energies to the different adsorbate sites. For the close-packed surfaces binding energies between 47 kJ/mol (Cu(111)) and 51 kJ/mol (Cu(100)) were observed, which increased to 58 kJ/mol for CO molecules bound to step edges. Unexpectedly, for kink sites the same binding energy (to within 1 kJ/mol) as for step edges was observed. Moreover, a very similar binding energy of 58 kJ/mol was also measured for random defect sites on sputtered and on poly-crystalline substrates.

Electrochemical oxidation of carbon monoxide: From platinum single crystals to low temperature fuel cells catalysts. Part I: Carbon monoxide oxidation onto low index platinum single crystals

The electrochemical oxidation of carbon monoxide and the interfacial structure of the CO adlayer (COads) on platinum low index single crystals, Pt(111), Pt(100) and two reconstruction of Pt(110), were examined using the rotation disk electrode method in combination with the in situ surface X-ray diffraction scattering technique. The mechanism of CO oxidation is discussed on the basis of the findings that, depending on the potential, two energetic states ofCOads exist on the platinum surfaces. Thus, at lower potentials, weakly bonded states (COads,w) and at higher potentials strongly bonded states (COads,s) are formed. The mechanism of the oxidation of hydrogen-carbon monoxide mixtures is also proposed.

Potential of zero total charge of palladium modified Pt(111) electrodes in perchloric acid solutions

The electrochemical behaviour of palladium-modified Pt(111) electrodes in perchloric acid solutions has been studied by means of cyclic voltammetry and CO charge displacement. From the results of the charge displacement experiments, values of the potentials of zero total charge (pztc) have been determined and their dependence on the palladium coverage has been studied in solutions without specifically adsorbing anions. These pztc values are compared to those obtained in sulfuric acid solutions. Whilst in the presence of bisulfate anions the pztc decreases significantly as a consequence of the deposited palladium, in the case of perchloric acid solutions the pztc remains constant within experimental error. This constancy has been explained by means of a compensation between the displacement of the potential of zero free charge toward less positive potentials and the increase of hydrogen coverage on the palladium-covered electrode surfaces. The influence of the concentration of specifically adsorbed (bi)sulfate anions at constant pH has been pointed out. The oxidation of the CO adlayer resulting from the charge displacement has also been studied. Despite the apparently easier OH adsorption, the oxidation of adsorbed CO takes place at higher potentials on the modified surfaces. CO coverage values, calculated from knowledge of the pztc value and the CO stripping charge, diminish as the amount of palladium increases.

In Situ FTIR Studies of the Effect of Temperature on the Adsorption and Electrooxidation of CO at the Ru(0001) Electrode Surface

The adsorption and electrooxidation of CO at a Ru(0001) electrode in perchloric acid solution have been investigated as a function of temperature, potential and time using in situ FTIR spectroscopy. This builds upon and extends previous work on the same system carried out at room temperature. As was observed at room temperature, both linear (COL) and 3-fold-hollow (COH) binding CO adsorbates (bands at 2000−2045 cm-1 and 1768−1805 cm-1, respectively) were detected on the Ru(0001) electrode at 10 °C and 50 °C. However, the temperature of the Ru(0001) electrode had a significant effect upon the structure and behavior of the CO adlayer. At 10 °C, the in-situ FTIR data showed that the adsorbed CO species still remain in rather compact islands up to ca. 1100 mV vs Ag/AgCl as the CO oxidation reaction proceeds, with oxidation occurring only at the boundaries between the COad and active surface oxide/hydroxide domains. However, the IR data collected at 50 °C strongly suggest that the adsorbed CO species are present as relatively looser and weaker structures, which are more easily electro-oxidized. The temperature-, potential-, and coverage-dependent relaxation and compression of the COL adlayer at low coverages are also discussed.

Adlayer structures of CO adsorbed on Ni(111) electrode surfaces studied by in situ STM combined with IRAS

In situ scanning tunneling microscopy (STM) combined with infrared reflection absorption spectroscopy (IRAS) has been used to investigate the coverage-dependent structural changes of CO adlayers adsorbed on Ni(111) in solution. Two different structures were obtained in a CO-containing Na 2SO 4 (pH 3) solution depending on the CO coverage. At low CO coverage, it was found that a ( 3 × 3 )R30° structure with a coverage of 0.33 ML started to form, where all the CO molecules occupy the bridge sites. A further increase in the CO coverage gave rise to the formation of a new c(4×2) structure with a coverage of 0.5 ML until adlayer saturation. The combined in situ STM and IRAS results showed equal occupations of the on-top and bridge CO species in the c(4×2) phase.

Potential-dependence of CO adlayer structures on Pt(111) electrodes in acid solution: Evidence for a site selective charge transfer

The oxidation of carbon monoxide on a Pt(111) electrode surface is studied under transient and stationary reaction conditions in a 0.1 M HClO4 aqueous solution as a function of the applied electrode potential. The optical response is investigated by optical second harmonic generation (SHG) at 280 nm and infrared absorption spectroscopy (IRAS) starting at low potentials where the electrode is not reactive [e.g., 0.1 V vs the reversible hydrogen electrode (RHE)] towards more positive potentials where CO oxidation is initiated. For transient reaction conditions, in the absence of CO dissolved in solution, the oxidation starts at about 0.5 V vs. RHE and consists of a fast oxidation of about 10% of the adlayer and of a second reaction with slower kinetics involving the removal of the complete CO layer (overlayer stripping). Under steady-state conditions in CO-saturated solution the CO adlayer is stable up to 0.9 V vs RHE. At 0.63 V an overlayer phase transition is indicated by a 20% increase of the isotropic component of the second harmonic (SH) intensity (pp-polarization). The same potential region for this phase transition in the adlayer structure is derived from IRAS spectra after correcting for the effect of the thin layer electrolyte in IRAS measurements. The disappearance of hollow sites, the appearance of bridge sites, as well as an increased occupation of on-top sites at 0.63 V is interpreted as corresponding to the adlayer phase transition. The observations are consistent with a transition from the c(2×2) to the (√19×√19) adlayer structure of CO. The high sensitivity of SHG with regard to structural phase transitions of the CO adlayer is explained by distinct charge transfer contributions to the second-order surface susceptibility at different coordination sites.

Adlayer structures of CO adsorbed on Ru(0001) electrode studied by in-situ STM combined with IRAS

In-situ scanning tunneling microscopy (STM) combined with infrared reflection absorption spectroscopy (IRAS) has been used to investigate the structural changes of the CO adlayers adsorbed on Ru(0001) in solution. At a low CO coverage, we found that a ( 3 × 3 ) R30° structure with a coverage of 0.33 ML formed, where all the CO molecules occupy on-top sites. A further increase in the CO coverage gave rise to the formation of a new c(2×2)-2CO structure with a coverage of 0.5 ML as the saturation phase where CO molecules occupy both the on-top and threefold hollow sites. Our STM image clearly demonstrates that one of the threefold hollow sites remains unoccupied in the c(2×2)-2CO saturation adlayer.