CO Adlayer Research Articles

CO oxidation on stepped Rh[ n(1 1 1) × (1 1 1)] single crystal electrodes: a chronoamperometric study

The CO electro-oxidation reaction has been studied on Rh[ n(1 1 1) × (1 1 1)]-type electrodes in 0.5 M H 2SO 4 using chronoamperometry. The transients recorded on Rh(1 1 1), Rh(5 5 4), Rh(5 5 3) and Rh(3 3 1) are characterized by a current plateau, visible directly after charging of the double layer, followed by a main oxidation feature, which consists of two peaks, a pre-peak and a main peak. The current density in the plateau region is nearly constant over time and, thus, is of (quasi) zeroth order in CO coverage. A plot of log( j plateau) vs. E final gives a linear relationship with a slope of ca. 45 mV dec −1, suggesting a second electron transfer as the rate determining step. Analogously to platinum, the current density plateau was ascribed to a Langmuir–Hinshelwood type reaction between CO ads and OH ads with no effective freeing of sites for OH adsorption due to relaxation of the CO adlayer. The presence of two peaks, rather than one, in the main oxidation feature can be explained by assuming a low surface mobility of CO and high oxidizability of rhodium surfaces. Indeed, dual step chronoamperometry shows that the mobility of CO on rhodium surfaces in aqueous media is very low. Since rhodium surfaces are known to oxidize readily, the pre-peak and main peak can be ascribed to CO reacting with OH, which adsorbs fast at the steps and more slowly at terrace sites. Since the geometry of the steps is nearly the same on each surface, the pre-peak appears structure insensitive, while the main peak shifts considerably with the step density. Introducing randomly distributed crystalline defects by cycling the electrodes repeatedly up to the surface oxidation region prior to each potential step experiment, results in a negative shift of the main peak, while the position of the pre-peak remains fixed. From the data presented, we conclude that the reaction nucleates at the steps and grows in the direction of the terraces.

From atomic scale reactant ordering to mesoscale reaction front propagation: CO oxidation on Pd(100)

We utilize a heterogeneous coupled lattice-gas (HCLG) approach to connect the length scales from a realistic atomistic description of surface reactions to the associated mesoscale spatiotemporal behavior. This method is applied to describe reaction front structure in a model for CO oxidation on Pd(100) which incorporates complex ordering of CO and O adlayers, and a precise treatment of the chemical diffusion for interacting CO adlayers in an environment of coadsorbed O.

Lattice-gas modeling of CO adlayers on Pd(100).

Using a lattice-gas model with pairwise interactions, we study the ordered structures, coverage dependence of the heat of adsorption, and other experimentally observable behavior of adsorbed CO overlayers on Pd(100) single crystal surfaces. Transfer matrix and Monte Carlo methods give accurate information regarding the lattice-gas model that often contradicts simple mean-field-like analysis. We demonstrate the usefulness of the model by reproducing experimental results over a large range of pressures and temperatures.

Mechanism of Stationary Bulk CO Oxidation on Pt(111) Electrodes

Carbon monoxide oxidation at platinum electrodes in acid solutions presents a characteristic behavior depending on the presence or not of dissolved CO in the bulk of the solution. In the present paper, bulk CO electro-oxidation is studied on a Pt(111) electrode. A change of the (2 × 2)-3CO overlayer, formed by admitting CO at 0.05 V, into a (√19 × √19)-13CO structure was observed at around 0.4 V. These changes were monitored by in situ Fourier transform IR spectroscopy through the bands for on-top, 2-fold, and 3-fold bonded CO. If the solution is saturated with CO, oxidation of the CO adlayer formed at 0.05 V begins at ca. 0.4 V and presents a “quasistationary” behavior. This process involves weakly bonded CO. The presence of such species is rationalized in terms of the steep decrease of the desorption energy at high degrees of coverage, reported for CO adlayers on Pt(111) in UHV. When CO is adsorbed at high potentials (above 0.6 V), segregated adlayers of CO and H-bonded H2O are formed. The H-bonded H2O ...

ATR-SEIRAS––an approach to probe the reactivity of Pd-modified quasi-single crystal gold film electrodes

Quasi-single crystalline gold films of 20 nm thickness and preferential (1 1 1) orientation on Si hemispheres were modified by controlled potentiostatic deposition of Pd (sub-ML, ML, multi-L) from sulphate and/or chloride-containing electrolyte. The electrochemical properties of these model electrodes were characterised for hydrogen and (hydrogen-) sulphate adsorption as well as for surface oxide formation by cyclic voltammetry. Conditions were developed to fabricate defined and stable Pd monolayers. In situ ATR-SEIRAS (Attenuated Total Reflection Surface Enhanced Infrared Reflection Absorption Spectroscopy) experiments were carried out to describe the electrochemical double layer of Pd modified gold film electrodes in contact with aqueous 0.1 M H 2SO 4 with focus on interfacial water and anion adsorption. Based on an analysis of the non-resonant IR background signal the potential of zero charge is estimated to 0.10–0.20 V (vs. RHE). CO was found to be weakly physisorbed in atop sites on Au(1 1 1-20 nm)/0.1 M H 2SO 4 only in CO saturated electrolyte. CO, deposited on a quasi-single crystal gold film modified with 1 ML Pd, is chemisorbed with preferential occupation of bridge sites and atop positions at step edges. Saturated CO adlayers, as obtained by deposition at 0.10 V, contain isolated water species and are covered by a second layer of hydrogen bonded water. Potentiodynamic SEIRAS experiments of CO electro-oxidation on Pd-modified gold film electrodes demonstrate clearly the existence of a “pre-oxidation” region. They also provide spectroscopic evidence that isolated water and weakly hydrogen bonded water are consumed during the reaction and that atop CO on defect sites is a preferential reactant. The simultaneous in situ monitoring of the potential- and time-dependent evolution of characteristic vibrational modes in the OH- and CO-stretching regions are in agreement with the Gilman (“reactant pair”) mechanism of CO oxidation.

Surface electrochemistry of CO on reconstructed gold single crystal surfaces studied by infrared reflection absorption spectroscopy and rotating disk electrode.

The electrooxidation of CO has been studied on reconstructed gold single-crystal surfaces by a combination of electrochemical (EC) and infrared reflection absorption spectroscopy (IRAS) measurements. Emphasis is placed on relating the vibrational properties of the CO adlayer to the voltammetric and other macroscopic electrochemical responses, including rotating disk electrode measurements of the catalytic activity. The IRAS data show that the C-O stretching frequencies are strongly dependent on the surface orientation and can be observed in the range 1940-1990 cm(-1) for the 3-fold bridging, 2005-2070 cm(-1) for the 2-fold bridging, and 2115-2140 for the terminal position. The most complex CO spectra are found for the Au(110)-(1 x 2) surface, i.e., a band near 1965 cm(-1), with the second, weaker band shifted positively by about 45 cm(-1) and, finally, a weak band near 2115 cm(-1). While the C-O stretching frequencies for a CO adlayer adsorbed on Au(111)-(1 x 23) show nu(CO) bands at 2029-2069 cm(-1) and at 1944-1986 cm(-1), on the Au(100)-"hex" surface a single CO band is observed at 2004-2029 cm(-1). In the "argon-purged" solution, the terminal nu(CO) band on Au(110)-(1 x 2) and the 3-fold bridging band on the Au(111)-(1 x 23) disappear entirely. The IRAS/EC data show that the kinetics of CO oxidation are structure sensitive; i.e., the onset of CO oxidation increases in the order Au(110)-(1 x 2) > or = Au(100)-"hex" > Au(111)-(1 x 23). Possible explanations for the structure sensitivity are discussed.

Electrooxidation of adsorbed CO on Pt(111) and Pt(111)/Ru in alkaline media and comparison with results from acidic media

Oxidation of adsorbed CO on Pt(1 1 1) and Pt(1 1 1)/Ru electrodes was studied in 0.1 M NaOH by the use of cyclic voltammetry and chronoamperometry. The data obtained in alkali are compared to those in 0.1 M H 2 SO 4 , and additional relevant information obtained with Pt(1 1 0) and Pt(1 0 0) surfaces is presented. We demonstrate, for the first time, that the voltammetric oxidation of CO on Pt(1 1 1)/Ru in alkali shows two clearly resolved CO stripping peaks, reminiscent of similar behavior in acids. On pure Pt(1 1 1), we confirm previous work that the voltammetric oxidation of a saturated CO adlayer in 0.1 M H 2 SO 4 and in 0.1 M NaOH yields a single and a split peak, respectively. Notably, we have found that while the CO oxidation on the Pt(1 1 1) electrode in acid occurs via a Langmuir–Hinshelwood mechanism, the CO oxidation in alkali in the pre-peak region is controlled predominantly via an Eley–Rideal (E–R) mechanism. On Pt(1 1 1)/Ru, the low potential CO oxidation in alkali occurs simultaneously through the L–H mechanism and the E–R mechanism. The addition of Ru to clean Pt(1 1 1) further improves the CO tolerance by promoting the oxidation of a greater fraction of the CO adlayer at low potentials. Therefore, the use of Pt/Ru catalysts in acidic and alkaline media may substantially reduce the CO poisoning that has so far limited low temperature fuel cell electrocatalyst performance, thus contributing to the development of more efficient direct oxidation fuel cells.

CO site conversion during electro-oxidation on Pt(1 0 0): a vibrational spectroscopic study

We have investigated the structure of the CO adlayer produced through methanol dissociative adsorption on Pt(1 0 0) in acidic solution by in situ IR-visible sum frequency generation (SFG) vibrational spectroscopy. Our results provide evidence of a site conversion from bridging CO to atop CO as the electrode potential varies from 350 to 450 mV versus Ag|AgCl|KCl sat. A new broad band is detected above 450 mV. This spectral feature is tentatively assigned to an electronic excitation at the sum frequency between the metal Fermi level and unoccupied electronic states of OH ads. The CO site migration could be triggered by OH ads formation on terraces, the latter process perturbing the balance between COCO and COH 2O lateral dipolar interactions.

Electrooxidation of adsorbed CO on Pt(1 1 1) and Pt(1 1 1)/Ru in alkaline media and comparison with results from acidic media

Oxidation of adsorbed CO on Pt(1 1 1) and Pt(1 1 1)/Ru electrodes was studied in 0.1 M NaOH by the use of cyclic voltammetry and chronoamperometry. The data obtained in alkali are compared to those in 0.1 M H 2SO 4, and additional relevant information obtained with Pt(1 1 0) and Pt(1 0 0) surfaces is presented. We demonstrate, for the first time, that the voltammetric oxidation of CO on Pt(1 1 1)/Ru in alkali shows two clearly resolved CO stripping peaks, reminiscent of similar behavior in acids. On pure Pt(1 1 1), we confirm previous work that the voltammetric oxidation of a saturated CO adlayer in 0.1 M H 2SO 4 and in 0.1 M NaOH yields a single and a split peak, respectively. Notably, we have found that while the CO oxidation on the Pt(1 1 1) electrode in acid occurs via a Langmuir–Hinshelwood mechanism, the CO oxidation in alkali in the pre-peak region is controlled predominantly via an Eley–Rideal (E–R) mechanism. On Pt(1 1 1)/Ru, the low potential CO oxidation in alkali occurs simultaneously through the L–H mechanism and the E–R mechanism. The addition of Ru to clean Pt(1 1 1) further improves the CO tolerance by promoting the oxidation of a greater fraction of the CO adlayer at low potentials. Therefore, the use of Pt/Ru catalysts in acidic and alkaline media may substantially reduce the CO poisoning that has so far limited low temperature fuel cell electrocatalyst performance, thus contributing to the development of more efficient direct oxidation fuel cells.

Potential control of the CO adsorption site on Pt(1 0 0) electrodes

A careful choice of the potential at which CO is adsorbed (dosing potential, E d) on Pt(1 0 0) electrodes in 0.1 M H 2SO 4 allows to control the formation of (i) CO adlayers that block all the adsorption sites on the surface (if E d⩽0.40 V vs. RHE), (ii) CO adlayers that only block adsorption on (1 0 0) terraces, allowing hydrogen adsorption on small islands (if 0.45 V vs. RHE ⩽ E d ⩽ 0.60 V vs. RHE), or (iii) subsaturated CO adlayers that leave free adsorption sites of every kind (if E d=0.65 V vs. RHE).

Spatial Distributions of Desorbing Products in 193 nm Photo-induced O2+CO Reaction on Pt(112)

The angular distributions of desorbing products were examined in 193 nm photo-induced reactions of the O2 + CO adlayers on stepped Pt(112) = [(s) 3(111)×(001)]. At high coverage of O2(a) and CO(a), the product (CO2) desorption collimated closely the (111) terrace normal, where the translational temperature was maximized to 3240 K. On the other hand, at low coverage, the CO2 desorption collimated in highly inclined ways in the plane along the surface troughs, where the translational temperature was enhanced, confirming the hot atom mechanism. These results were compared with those in thermal CO oxidation and the origin of the collimation angle shift in the latter was discussed.

Effect of a static electric field on the vibrational and electronic properties of a compressed CO adlayer on Pt(110) in nonaqueous electrolyte as probed by infrared reflection–absorption spectroscopy and infrared-visible sum-frequency generation spectroscopy

The CO–Pt(110) in nonaqueous electrolyte electrochemical interface was studied by infrared reflection–absorption spectroscopy (IRAS) and infrared (IR)-visible sum-frequency generation (SFG) nonlinear vibrational spectroscopy over a wide range of applied potentials (−1.7⩽Φ⩽1 V/NHE). The integrated intensities of the IRAS and SFG peaks associated with resonant excitation of the atop CO internal stretch vibration (AIR and ASFG) showed distinct variations with Φ. The influence of vibrational and electronic properties on the observed variations is discussed. Potential dependent dynamical charge transfer is not sufficient to explain the observed value of ∂AIR/∂Φ. It is shown that screening factors due to dipolar interactions between molecules within the compressed adsorbed layer must be taken into account in order to explain the variations of the IRAS and SFG data and that the observed behavior differences of AIR and ASFG with the potential are not necessarily related to changes in the Raman cross section of the adsorbate.

The adsorption of Sn on Pt(1 1 1) and its influence on CO adsorption as studied by XPS and FTIR

The coverage of Sn on Pt(1 1 1) which is obtained by electrochemical deposition from 5×10 −5 M Sn 2+ in 0.5 M H 2SO 4 has been determined by XPS for different deposition times. Complete suppression of hydrogen adsorption corresponds to a coverage of ϑ max=0.35 (Sn to surface Pt atoms). Co-adsorption of CO with Sn on Pt(1 1 1) has been studied by FTIR spectroscopy. The IR spectra of the stretching vibration of CO can be interpreted in terms of the vibrational signature of the Pt(1 1 1)/CO system and no vibrational bands associated with CO on Sn are detected. At high Sn coverages, the 1840 cm −1 band associated with bridge-bonded CO and the 2070 cm −1 band assigned to on-top CO are present, however, no hollow site adsorption which is characterized by the 1780 cm −1 band is revealed within the resolution of the experiment. This vibrational signature corresponds to a less compressed adlayer compared to the (2×2)–3CO saturation structure on Pt(1 1 1). At lower Sn coverages, signatures from both the compressed and the less compressed CO adlayer structures are seen in the spectra. From earlier structural and electrochemical studies it is known that Sn is adsorbed in 2D islands and influences CO molecules in its neighbourhood electronically. This leads to a disappearance of the IR band from CO adsorbed in the hollow site at high Sn coverages and to higher population of the weakly adsorbed state of CO for all Sn-modified surfaces, i.e. a relative increase of the amount of CO oxidised at low potentials. In addition to this electronic effect, Sn also exerts a co-catalytic effect at low Sn coverages on that part of CO which is adsorbed at a larger distance from Sn due to a bi-functional mechanism. The IR spectra shows for the Sn-modified Pt(1 1 1) surface that the transition from the compressed CO adlayer which is characterized by the hollow site adsorption of CO to the less compressed one which exhibits a characteristic band associated with bridge-bonded CO occurs already at 250 mV instead of 400 mV.

II. Mechanisms of Elevated Temperature Methanol Electro-oxidation and Poisoning on Pt/C-Nafion Catalyst Layers

Half-cell measurements using fuel cell catalysts at elevated temperatures were made in a pulsed reactant electrochemical flow cell. Kinetics of methanol and the poisoning adlayer electro-oxidation were investigated on 20% Pt/Vulcan C-Nafion catalyst layers at 95°C. Transient measurement of the adlayer coverage during methanol electro-oxidation at 0.30 and 95°C showed that the surface reached a steady-state adlayer coverage after 30 s. Almost no change in adlayer coverage or current occurred from 30 to 300 s, indicating that steady-state conditions were maintained at 95°C. With increasing potential from 0.30-0.40 in 0.015 M methanol, the steady-state CO coverage decreased from 0.44 to 0.28 monolayers, while the steady-state current increased from 3.3 to 29 mA/mg Pt. The kinetics of CO adlayer oxidation were determined independently over this potential range for adlayers prepared by repeated, 60 s periods of methanol oxidation. The initial rates of adlayer electro-oxidation were 80-90% of the steady-state methanol electro-oxidation rates throughout the potential range. This indicates high selectivity through the CO adlayer at 95°C. Poisoning adlayer electro-oxidation occurred with faster kinetics on Pt(110) facets relative to Pt(100) facets. Dissociative adsorption of methanol at 0.05 and 95°C was also studied. The adlayer coverage reached 0.36 monolayers after a 60 s exposure to 0.015 M methanol, indicating that the Pt/C electrocatalyst reacts with methanol under these conditions. © 2003 The Electrochemical Society. All rights reserved.

Accurate determination of the CO coverage at saturation on a cyanide-modified Pt(1 1 1) electrode in cyanide-free 0.5 M H 2SO 4

We have used the CO charge-displacement method, in combination with a thermodynamic cycle, to obtain the double-layer correction necessary to determine accurately, using the charge in the corresponding CO-stripping voltammograms, the maximum amount of CO that can adsorb on a cyanide-modified Pt(1 1 1) electrode. The resulting CO coverage at saturation is θ CO=0.25, and corresponds to a mixed CN–CO adlayer where some Pt atoms are still free and consequently can adsorb hydrogen. The hydrogen adsorption charge for the mixed adlayer, obtained from the corresponding cyclic voltammogram, agrees very well with that estimated from the CO and CN coverages, assuming that one hydrogen atom adsorbs on every free Pt atom. Taking into account these data, we propose a structural model for the mixed CN–CO adlayer on Pt(1 1 1).

The effect of chloride on the electrooxidation of adsorbed CO on polycrystalline platinum electrodes

We have used the inhibition of the electrooxidation of adsorbed CO at low overpotentials by chloride ions in the solution to elucidate, in an indirect way, the nature of this process. After a characterisation of the polycrystalline platinum surface by means of cyclic voltammetry, we show that the amount of CO adsorbed on the surface at saturation is independent of the chloride concentration in the solution, ruling out completely the possibility of a mixed CO-chloride adlayer. Our data suggest that the process occurring in the pre-peak in CO-stripping voltammograms corresponds to the oxidation through a Langmuir–Hinshelwood mechanism of a fraction of a compact CO adlayer. The resulting less compact CO adlayer is somewhat more strongly adsorbed because of the lower repulsive interactions between neighbouring CO molecules and is, therefore, more oxidation resistant, its oxidation giving rise to the main CO peak. Chloride adsorption blocks the adsorption sites left free by the first CO molecules oxidised, inhibiting OH adsorption and, therefore, the oxidation of the CO adlayer, which can proceed at higher potentials only.

Structures and vibrational frequencies of CO adlayers on Rh(111) surface

Density functional theory calculations within the generalized gradient approximation (GGA) have been carried out to study the structural and vibrational properties of carbon monoxide adsorption on Rh(111) surface. The optimized geometries, adsorption energies and vibrational frequencies have been obtained and the preferred binding sites have been determined. The results show that at low coverage CO prefers to adsorb at top site and at high coverage one molecule occupies top site while the two other molecules occupy hcp and fcc hollow sites respectively. The investigation of the vibrational properties of CO chemisorption on Rh(111) shows that the top C-O stretching frequency increases along with the increase of the coverage. The site assignments, optimized geometries and calculated vibrational frequencies are found to be in good agreement with the experimental results.

Role of Crystalline Defects in Electrocatalysis: Mechanism and Kinetics of CO Adlayer Oxidation on Stepped Platinum Electrodes

The kinetics of the electrochemical oxidation of a CO adlayer on Pt[n(111)×(111)] electrodes in 0.5 M H2SO4 has been studied using chronoamperometry. The objective is to elucidate the effect of the crystalline defects on the rate of the reaction by using a series of stepped surfaces. The reaction kinetics of the main oxidative process can be modeled using the mean-field approximation for the Langmuir−Hinshelwood mechanism, implying fast diffusion of adsorbed CO on the Pt[n(111)×(111)] surfaces under electrochemical conditions. The apparent rate constant for the electrochemical CO oxidation, determined by a fitting of the experimental data with the mean-field model, is found to be proportional to the step fraction (1/n) for the surfaces with n > 5, proving steps to be the active sites for the CO adlayer oxidation. An apparent intrinsic rate constant is determined. The potential dependence of the apparent rate constants is found to be structure insensitive with a Tafel slope of ca. 80 mV/dec, suggesting the...

The inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported electrocatalysts in the quest for improved CO tolerant PEMFC anodes

The effect of the inclusion of Mo, Nb and Ta in Pt and PtRu carbon supported anode electrocatalysts on CO tolerance in proton exchange membrane fuel cells (PEMFC) has been investigated by cyclic voltammetry and fuel cell tests. CO stripping voltammetry on binary Pt x M/C (M: Mo, Nb, Ta) reveals partial oxidation of the CO adlayer at low potential, with PtMo (4:1)/C exhibiting the lowest value. At 80 °C, the operating temperature of the fuel cell, CO oxidation was observed at potentials close to 0 V versus the reversible hydrogen electrode (RHE). No significant difference for CO electro-oxidation at the lower potential limit, compared to PtRu/C, was observed for PtRuM y /C (M: Mo, Nb). Fuel cell tests demonstrated that while all the prepared catalysts exhibited enhanced performance compared to Pt/C, only the addition of a relatively small amount of Mo to PtRu results in an electrocatalyst with a higher activity, in the presence of carbon monoxide, to PtRu/C, the current catalyst of choice for PEM fuel cell applications.

Influence of steps on the electrochemical oxidation of CO adlayers on Pd(111) and on Pd films electrodeposited onto Au(111)

Electrochemically deposited palladium monolayers on well-ordered and on vicinal Au(111) electrodes have been characterised in sulphuric acid solution by cyclic voltammetry. The electrochemical behaviour of these pseudomorphically grown palladium overlayers is compared with that of a well-ordered and a previously oxidised Pd(111) electrode for hydrogen adsorption, sulphate adsorption as well as for surface oxide formation. A strong impact of the surface defect density was observed for the latter reaction. The electrooxidation of irreversibly adsorbed carbon monoxide monolayers on the various palladium surfaces was studied by cyclic voltammetry and chronoamperometry. The oxidation kinetics are strongly dependent on the number of monoatomic high steps, that act as nucleation centres for adsorbing oxygen-containing species and allow for an anodic stripping of carbon monoxide at more negative potentials than oxidation on terraces. A tailing in the chronoamperometric transients at 0.65 V vs. SCE for the palladium surfaces suggests that diffusion of the reacting species on the terraces is rate-determining.