CO Surface Mobility Research Articles

On the behavior of CO oxidation on shape-controlled Pt nanoparticles in alkaline medium

In this work, the behavior of the CO electro-oxidation reaction on shape-controlled Pt nanoparticles in alkaline medium was examined in order to understand the effect of the surface structure on this reaction. A series of experiments using Pt nanoparticles of different surface structures/shapes was used and the results obtained were compared with the previous knowledge gained from stepped platinum single crystal electrodes. Independently of the preferential orientation of the nanoparticles, the CO oxidation voltammetry exhibits two main peaks: one at ca. 0.56–0.59V and the second one at 0.66–0.67V, being the intensity of the peaks dependent on the shape of the nanoparticle. These two peaks have been assigned to the oxidation of CO on the (111) terraces and on the rest of the sites, respectively. The appearance of two differentiated peaks reveals that these (111) terraces and the rest of the sites on the nanoparticle surface behave independently of the presence of the other type of sites, that is, they are not connected. The results are discussed considering the effects of the surface mobility of CO and of the OH adsorption properties on the different sites in the oxidation peaks.

Comparison of CO on Carbon-supported Pt Catalysts Prepared by CO Gas Bubbling and Methanol Dehydrogenation

CO adsorbates on the surface of Pt supported on carbon catalysts (Pt/C) were investigated by CO stripping voltammetry. Three types of CO adsorbed samples were prepared: by methanol dehydrogenation only (CO m ), by CO gas bubbling only (COg), and by methanol dehydrogenation followed by CO gas bubbling (CO m+g ). Our coverage data show that CO gas can be adsorbed on Pt/C catalyst already saturated with CO adsorbates by methanol dehydrogenation. The CO m+g sample showed the properties of both CO m as well as COg samples in terms of the potential although the CO adsorbed by dehydrogenation was completely exchanged with CO in the electrolyte solution. Therefore, the oxidation pathways of CO on Pt/C were observed to depend on the initial adsorption conditions of CO more strongly than on the CO coverage. Our results imply that an initial CO poisoning condition in fuel cell operation is an important factor to determine the difficulty in removing the adsorbed CO and confirm that the properties of the adsorbed CO do not change even with chemical replacement with CO in different conditions. In addition, our results indicate a low CO surface mobility on the Pt in an electrolyte solution.

CO oxidation on stepped single crystal electrodes: A dynamic Monte Carlo study

Abstract We investigate a simple model for the mechanism of the electrooxidation of CO on stepped electrodes by dynamic Monte Carlo simulations. The model is studied in the limits of no and fast CO surface diffusion. In the absence of CO diffusion, the reaction initiates at step sites by adsorption of OH on the steps and reaction with a neighboring CO molecule, resulting in a pre-peak, followed by growth onto the terraces by OH adsorption on the terraces reacting with neighboring terrace adsorbed CO, resulting in a main peak. The presence of these two peaks in both the simulated cyclic voltammetry and chronoamperometry is a direct consequence of the low CO surface mobility, and resembles the experimental voltammetric and chronoamperometric profiles on stepped Rh[n(1 1 1) × (1 1 1)] electrodes [T.H.M. Housmans, J.M. Feliu, M.T.M. Koper, J. Electroanal. Chem. 572 (2004) 79, T.H.M. Housmans, M.T.M. Koper, J. Electroanal. Chem. 575 (2005) 39, T.H.M. Housmans, M.T.M. Koper, Electrochem. Commun. 7 (2005) 581]. The shape and position of the pre-peak are determined largely by a reaction in one dimension at the steps. Initiation of the reaction at empty sites results in the formation of two reaction fronts proceeding to the left and right of the initiation point, leading to a strong initial current growth followed by a gradual decay as multiple “shrinking rows” extinguish one another. After stripping of the step and the first row of CO molecules adsorbed on the terraces, the reaction grows over the terraces by slower adsorption of OH. For high CO surface mobility the simulated voltammograms and transients resemble the experimental results on stepped rhodium surfaces in perchloric acid and stepped Pt electrodes in sulfuric acid.

Transient and steady state CO oxidation kinetics on nanolithographically prepared supported Pd model catalysts: Experiments and simulations

Applying molecular-beam methods to a nanolithographically prepared planar PdSiO2 model catalyst, we have performed a detailed study of the kinetics of CO oxidation. The model catalyst was prepared by electron-beam lithography, allowing individual control of particle size and position. The sample was structurally characterized by atomic force microscopy and scanning electron microscopy before and after reaction. In the kinetic experiments, the O-rich and CO-rich regimes were investigated systematically with respect to their transient and steady-state behaviors, both under bistable and monostable reaction conditions. Separate molecular beams were used in order to supply the reactants, allowing individual control over the reactant fluxes. The desorbing CO2 was detected by both angle-resolved and angle-integrated mass spectrometries. The experimental results were analyzed using different types of microkinetic models, including a detailed reaction-diffusion model, which takes into account the structural parameters of the catalyst as well as scattering of the reactants from the support. The model quantitatively reproduces the results as a function of the reactant fluxes and the surface temperature. Various kinetic effects observed are discussed in detail on the basis of the model. Specifically, it is shown that under conditions of limited oxygen mobility, the switching behavior between the kinetic regimes is largely driven by the surface mobility of CO.

CO oxidation on stepped Rh[ n(1 1 1) × (1 1 1)] single crystal electrodes: Anion effects on CO surface mobility

The influence of anion adsorption on the CO electrooxidation reaction on stepped Rh[ n(1 1 1) × (1 1 1)] electrodes was investigated by comparing voltammetric and chronoamperometric data obtained in 0.5 M HClO 4 with previously published results obtained in 0.5 M H 2SO 4. Compared to sulfuric acid media, complete stripping of the CO adlayer requires fewer cycles in perchloric acid and the resulting stripping voltammetry peaks are shifted to considerably lower potentials, attesting to the reduced influence of more weakly adsorbed anions. The absence of a shoulder prior to the main oxidation peak and the higher symmetry of the main peaks implies that the electrooxidation reaction in perchloric acid is not only faster than in sulfuric acid but probably also not diffusion limited, which suggests the mean field approximation as the best mathematical model for the reaction kinetics rather than nucleation and growth. The chronoamperometric transients recorded at various potentials show only a single oxidation peak with a slight tailing at longer times. Only the Rh(1 1 1) transients display tailing, which is Cottrellian in nature. The surface diffusion coefficient of CO deduced from the Cottrell plots is more than four orders of magnitude larger in perchloric than in sulfuric acid (HClO 4: 1 × 10 −12 < D < 2 × 10 −11 cm 2 s −1 vs. H 2SO 4: 1 × 10 −16 < D < 8 × 10 −16 cm 2 s −1), which also suggests a mean field model for HClO 4 rather than nucleation and growth. Apparently, specific anion adsorption on rhodium surfaces not only affects the rate, but also the dynamics of the CO electrooxidation reaction. Thus, by varying the adsorption strength of the anion, we could, in principle, influence the diffusion rate of adsorbates on the surface and, therefore, the reaction dynamics and the overall reaction rate.

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.

Electrooxidation of Carbon Monoxide at Ruthenium–Modified Platinum Nano‐particles: Evidence for CO Surface Mobility

AbstractRu‐modified Pt nanoparticle surfaces were prepared using Ru electrochemical or spontaneous deposition on commercial‐grade carbon‐supported Pt nanoparticles (Pt‐Vulcan XC72, E‐TEK). Evidence for CO mobility was provided by cyclic voltammetry and FTIR reflectance spectroscopy experiments in weakly specifically adsorbing electrolyte such as 0.1 M HClO4. Since the diffusion of CO is slow, a “two peak” voltammetric behavior was observed for 0 &lt; θRu &lt; 10%. A further increase in Ru coverage leads to the observation of only one CO oxidation peak at a potential ca. 150 mV less positive. FTIR reflectance spectra recorded on Ru‐modified Pt nanoparticles show a band located around 2000–2020 cm–1, which is attributed to adsorbed CO on Ru sites. Exchange between CO species adsorbed on Ru and Pt sites was also observed along with a complete oxidation of a pre‐adsorbed CO monolayer at a potential relevant for DMFC applications. These results confirm that the formation of a PtRu alloy is not a pre‐requisite for obtaining CO‐tolerant electrocatalysts.

Fundamental aspects in electrocatalysis: from the reactivity of single-crystals to fuel cell electrocatalysts

A chemical modification by electrochemical deposition of Ru is performed on Pt(111) yielding mostly monoatomic Ru islands. From the investigation by scanning tunneling microscopy (STM) and by infrared spectroscopy of adsorbed CO it is found that nanostructured Pt–Ru electrodes exhibit distinct characteristics regarding CO oxidation which are due to a cooperative reaction mechanism involving CO surface mobility. The CO mobility can also be directly detected by IR spectroscopy on Pt(111) surfaces that have been modified by Ru deposition only in the center of the crystal. The presence of a second peak in the cyclic voltammogram (CV) for CO monolayer oxidation is most likely due to a dependence of the diffusion rate on the coverage of CO. Carbon supported Pt–Ru catalysts are prepared by the sulfito method and investigated regarding methanol oxidation. Although the structural characterization of the catalysts shows no evidence for alloy formation, a high mass activity is observed which is comparable to commercial catalysts. The catalyst weight loading has been varied systematically to investigate the influence of this parameter on the performance of the catalyst. For a temperature of 25 °C the mass activity increases with increasing catalyst mass loading up to about 55 wt.%. Then a plateau in the mass activity versus weight loading is reached. For 65 °C, however, a maximum mass activity is observed at 60 wt.%. These results show a different pattern of performance relative to loading with changing temperature indicating that some tailoring of the catalyst to different operating conditions could be beneficial.

Diamond deposition on copper treated hardmetal substrates

Abstract The adhesion of diamond coatings onto hardmetal substrates is improved by a copper deposition produced by a cementation from aqueous CuSO 4 solutions. During this reaction Co is dissolved from the substrate surface and copper is deposited. To obtain homogeneous Cu deposits, the influence of CuSO 4 concentration and reaction time on cementation were investigated. During diamond deposition, Cu reduces the surface mobility of Co, which is necessary to decrease deposition of non-diamond carbon and therefore increase adhesion. Indentation tests showed the good adhesion of the diamond coatings qualitatively. Cu precipitation and diamond deposition were examined by scanning electron microscopy (SEM). The diamond quality was detected by Raman spectroscopy. Using secondary ion mass spectroscopy (SIMS) depth profiles the interface and the Cu distribution were characterized indicating that during diamond deposition Cu is dissolved and forms an intermetallic Co–Cu mixed crystal.

Monte Carlo simulations of a simple model for the electrocatalytic CO oxidation on platinum

A simple lattice-gas model for the electrocatalytic carbon monoxide oxidation on a platinum electrode is studied by dynamic Monte Carlo simulations. The CO oxidation takes place through a Langmuir–Hinshelwood reaction between adsorbed CO and an adsorbed OH radical resulting from the dissociative adsorption of water. The model enables the investigation of the role of CO surface mobility on the macroscopic electrochemical response such as linear sweep voltammetry and potential step chronoamperometry. Our results show that the mean-field approximation, the traditional but often tacitly made assumption in electrochemistry, breaks down severely in the limit of vanishing CO surface mobility. Comparison of the simulated and experimental voltammetry suggests that on platinum CO oxidation is the intrinsically fastest reaction on the surface and that CO has a high surface mobility. However, under the same conditions, the model predicts some interesting deviations from the potential step current transients derived from the classical nucleation and growth theories. Such deviations have not been reported experimentally. Furthermore, it is shown that our simple model predicts different Tafel slopes at low and high potential, the qualitative features of which are not strongly influenced by the CO mobility. The comparison of our simulation results to the experimental literature is discussed in some detail.

Surface diffusion of carbon monoxide and potassium coadsorbed on Ru(001): Confirmation of a 1:1 CO:K trapping interaction

The surface mobilities of both CO and K coadsorbed on Ru(001) were studied using laser-induced thermal desorption (LITD) techniques. The LITD measurements revealed that CO was essentially immobile on the potassium-promoted Ru(001) surface when the CO surface coverage, ΘCO, was less than the potassium coverage, ΘK. The CO surface mobility increased dramatically when ΘCO≳ΘK. At ΘK=0.10 ML and 315 K, the CO diffusion coefficient was DCO&amp;lt;4×10−10 cm2/s for ΘCO&amp;lt;0.10 ML and increased to DCO=2.5×10−7 cm2/s for ΘCO≳0.10 ML. At ΘK=0.25 ML and 315 K, the CO surface diffusion coefficient was DCO&amp;lt;4×10−10 cm2/s for ΘCO&amp;lt;0.25 ML and increased to DCO=5×10−9 cm2/s for ΘCO≳0.25 ML. The potassium surface mobility also decreased dramatically as a function of CO coverage at both ΘK=0.10 ML and ΘK=0.25 ML. These diffusion results are consistent with a mutually stabilizing and trapping interaction between coadsorbed CO and K with a 1:1 CO:K stoichiometry. The thermal desorption spectra of both CO and K were also studied using temperature programmed desorption (TPD) techniques. At ΘK=0.10 ML and CO coverages ranging from ΘCO=0.06 ML to ΘCO=0.42 ML, CO TPD peak temperatures were consistently higher than CO peak temperatures obtained on clean Ru(001). These TPD results indicate that CO is stabilized by K on Ru(001). In addition, the potassium TPD spectra at various CO coverages with ΘK=0.10 ML and ΘK=0.25 ML revealed that K was stabilized by coadsorbed CO. The experimental results for CO and K coadsorbed on Ru(001) argue for a mutually stabilizing, short-ranged trapping CO–K interaction with a 1:1 CO:K stoichiometry. Monte Carlo simulations based on this model for the CO:K interaction were also consistent with the coadsorbate surface diffusion results.