CO Monolayer Research Articles

Grain and phase boundary segregation in WC–Co with TiC, ZrC, NbC or TaC additions

The microstructure of interfaces in WC–Co based cemented carbides with TiC, ZrC, NbC or TaC additions was analysed using energy dispersive X-ray analysis in a transmission electron microscope and using atom probe tomography. Segregation to WC/WC grain boundaries and WC/(M,W)C phase boundaries corresponding to between 0.4 and 1.2 atomic layers of close packed monolayers of Co was observed in all the materials. In addition to Co, Ti, Zr and Nb, but not Ta, segregate to the WC/WC grain boundaries. Segregation was also observed for B, P, Fe and Cr, but not Ni, to the boundaries. These are impurities that originate from the material production. Segregation of Ti, Zr, Nb and Ta to WC/binder phase boundaries was observed. If formation of a MC phase at the interface is assumed, the segregation corresponded to a thickness less than a monolayer.

Oxygen Adsorption and Magnetic Properties of Ultrathin Co/Ir(111) Films

During oxygen exposure to ultrathin Co/Ir(111) films, both the oxidative process and physical adsorption occur. The diffusion length of oxygen is around two monolayers of Co. Within this thickness, Co films are almost oxidative and ferromagnetism disappears. For thicker films with nanometer thickness, after completing the oxidative process at the beginning of oxygen exposure, the physical adsorption becomes a major process. By controlling the amount of further exposure to oxygen, the coercive force in the polar configuration can be adjusted while keeping the Kerr rotation at a nearly constant value.

Substrate-induced structural modulation of a CoO(111) bilayer on Ir(100)

We report on an unusual structural modulation of a single CoO(111) bilayer grown on $\text{Ir}(100)\text{\ensuremath{-}}(1\ifmmode\times\else\texttimes\fi{}1)$ by oxidation of slightly less than one monolayer of Co deposited on the substrate. Quantitative low-energy electron diffraction and scanning tunneling microscopy in combination with standard x-ray photoelectron spectroscopy and thermal-desorption spectroscopy reveal a cobalt layer next to the substrate covered by an oxygen layer. Both layers' hexagonal atomic arrangements are, however, strongly distorted by the quadratic substrate and form a $c(10\ifmmode\times\else\texttimes\fi{}2)$ superstructure on that. The Co layer's buckling amplitudes and atomic bond lengths to Ir atoms are consistent with the hard-sphere radius of metallic Co. The oxide's binding to the substrate appears to be further characterized by two types of oxygen ions. One of them is close to the expected rocksalt-type stacking with respect to the cobalt layer while the other type resides nearly on top of Ir atoms. Its hard-sphere radius is only $0.77\text{ }\text{\AA{}}$ (in contrast to $1.25\text{ }\text{\AA{}}$ in the CoO bulk) and it is by about $1\text{ }\text{\AA{}}$ closer to the substrate than the other type. Being so almost coplanar with the Co layer, it locally forms a hexagonal boron-nitride-type oxide. The oxygen bond to Ir can be interpreted as local pinning of the oxide to the substrate so modulating the entire oxide bilayer.

The potential of zero total charge of Pt nanoparticles and polycrystalline electrodes with different surface structure: The role of anion adsorption in fundamental electrocatalysis

In the present paper anion effects are discussed on different shape-controlled Pt nanoparticles as well as on polycrystalline Pt electrodes of different surface structures. In particular, the potentials of zero total charge (pztc) were determined by using the CO displacement approach, both in sulphuric and perchloric media. The CO monolayer oxidative stripping was studied on the different samples. CO coverages were independent of the electrolyte provided that the true CO stripping charges, corrected for double layer contributions are compared to the charge involved only in the hydrogen adsorption region. The classical charge density normalization process is discussed and small changes are proposed. Then, criteria to evaluate the real surface area of platinum electrodes from charge measurements are proposed in both electrolytes.

Preparation and Electrochemical Properties of Ternary Catalyst PtRuCo/C for Direct Methanol Fuel Cell

Using sodium citrate as a stabilizer, sodium borohydride as a reductant, 20%(w) Pt60Ru30Co10/C catalyst was prepared by impregnating reduction. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to characterize the catalysts. We studied the catalytic activity and stability in a methanol solution at different pH values and pre-adsorbed CO monolayer stripping was used to investigate its anti-poisoning ability. The results showed that PtRuCo/C had the highest catalytic activity toward methanol at pH 8, and its catalytic activity and stability were much higher than that of commercial Pt50Ru50/C. Compared with Pt50Ru50/C, the pre-adsorbed CO monolayer oxidation potential showed a significant negative shift.

The role of the support in COadsmonolayer electrooxidation on Ptnanoparticles: Pt/WOxvs.Pt/C

The electrocatalytic properties of home-made Pt nanoparticles supported onto WO(x) were determined for the electrooxidation of a CO(ads) monolayer and compared with that of a commercial Pt/C having the same Pt particle size. By combining electrochemical and spectroscopic techniques, we found that Pt/WO(x) nanoparticles exhibit a very high tolerance to CO at low electrode potentials (E = 0.1 V vs. RHE), which was never reported in the literature before. CO adsorption at E = 0.1 V vs. RHE on Pt/WO(x) yields CO(2) production as observed by Fourier-transform infrared spectroscopy (FTIR). When the gas bubbling in solution changes from CO to Ar, the current attenuates and the CO(2) production vanishes. This points towards a limited number of "active sites" and a slow step in the electrocatalytic process. When H(2) is used to purge the electrolyte from CO, a steep and continuous increase of the H(2) electrooxidation current is observed pointing towards continuous liberation of the Pt catalytic sites. The high tolerance to CO of Pt/WO(x) is discussed in terms of strong metal-support interaction (SMSI), which involves formation of a metal-oxide film partially covering the Pt nanoparticles (encapsulation) and creation of W-OH groups upon H(+) insertion at low electrode potentials.

Active Area Determination for Porous Pt-Electrodes used in PEM Fuel Cells - Temperature And Humidity Effects

This paper aim to discuss a proper measure of the electrochemical active area of carbon supported Pt nanoparticle catalyst used in polymer electrolyte membrane (PEM) fuel cells. The cyclic voltammetric determination of hydrogen under potential deposition (Hupd) and carbon monoxide monolayer oxidation (CO stripping) performed in a fuel cell at fuel cell relevant conditions are compared and the influences of operation temperature (25-80 {degree sign}C) and relative humidity (RH) (40-90% RH) are discussed. The results show that both the shape and the charge of the Hupd are strongly dependent on operating conditions. However, for the CO stripping experiments only a negative shift in potential with increasing temperature or humidity was observed in accordance with results from aqueous electrolytes, whereas the charges for CO monolayer oxidation were almost constant for the temperatures investigated.

Fully relativisticab initiodescription of spin-polarized STM experiments in terms of magnetic-field-dependent tunneling currents

An approach is introduced to interpret spin-polarized scanning tunnel microscope experiments in terms of differences in the tunneling conductivity with respect to different magnetic configurations and the corresponding band energy contribution to the magnetic anisotropy energy, the former being considered to be proportional to the applied external magnetic field. This approach is illustrated for Cu 111 coated with two monolayers of Co considering two types of Cr/W tips. It is found that the reorientation transition of the magnetization in the Co layers can unambiguously be identified even in cases when the tip contributes considerably to the tunneling current difference.

Impact of Oxygen Coadsorption on Intercalation of Cobalt under the h-BN Nanomesh

The process of penetration of cobalt atoms through the h-BN nanomesh on Rh(111) is investigated with both spectroscopic and microscopic techniques. It is discovered that oxygen coadsorption can drastically modify the physical properties and behavior of the deposited Co clusters upon postannealing. In the absence of oxygen, Co forms small nanoparticles in the pores (bonding parts) of the h-BN nanomesh, which start to agglomerate at elevated temperatures without any considerable intercalation. However, even a tiny amount of coadsorbed oxygen reduces cobalt agglomeration and greatly promotes its intercalation and trapping under h-BN. The oxygen exposure necessary for a complete intercalation of 1-2 monolayers of Co is very low, and the formation of oxidic species can be easily avoided. The nanomesh structure remains intact upon intercalating submonolayer amounts of Co, while further intercalation gradually distorts and finally destroys the periodic corrugation. Fortunately, this process is not accompanied by damaging the h-BN sheet itself, and the original structure can be restored by removing Co upon annealing at higher temperatures.

Preparation and electrochemical characterization of low-index rhodium single crystal electrodes in sulfuric acid

The electrochemical properties of low-index phase Rh(1 1 1), Rh(1 1 0) and Rh(1 0 0) single crystal bead electrodes, prepared by a novel technique combining electron beam heating with inductive annealing in a controlled atmosphere, have been characterized in 0.1 M H 2SO 4 by cyclic voltammetry and chronoamperometry. Hydrogen and sulfate adsorption as well as surface oxidation depend strongly on the crystallographic orientation of the surface. The potentials of zero total charge ( E pztc) of all three Rh electrodes in 0.1 M H 2SO 4 were determined by the combination of charge displacement and voltammetric experiments. The charge balance reveals unambiguousely that the (√3 × √7) adlayer on Rh(1 1 1) is composed of specifically adsorbed sulfate ions eventually coadsorbed with water molecules. Hydrogen-sulfate coadsorbed with hydronium ions could be excluded. The kinetics of sulfate ion desorption followed by the adsorption of hydrogen at less positive potentials could be represented by a nucleation and growth mechanism coupled with a parallel first order process. The electro-oxidation of irreversibly adsorbed carbon monoxide monolayers was also investigated and revealed distinct structure sensitivity. The reaction pathway on all three low-index phases of Rh proceeds according to a Langmuir–Hinshelwood mechanism and is controlled by nucleation of OH ads at steps and other defect sites followed by a complex growth process on terrace sites. The low surface mobility of CO ads leads to a slow and incomplete CO monolayer electro-oxidation on Rh(1 1 1). The high density of step sites on Rh(1 1 0) and the reversible formation of oxygenated species on Rh(1 0 0) at rather low potentials significantly enhance the electro-oxidation activity leading to the following reactivity sequence: Rh(1 1 1) ≪ Rh(1 1 0) ∼ Rh(1 0 0). The shape of the experimental transients and attempts to model them demonstrate the occurrence of at least two processes occurring in parallel. The long-term response represents clearly a process involving a slow surface-diffusion step.

The influence of support and particle size on the platinum catalysed oxygen reduction reaction

A range of platinum deposits, equivalent thicknesses (delta) 0.2-2.5 nm, have been synthesised on carbon and reduced titania (TiO(x)) supports using physical vapour deposition on (10 x 10) arrays of electrodes. For delta < 1.0 nm, discrete platinum centres are formed and the TiO(x) supported platinum show two distinct characteristics: (a) a strong positive shift in the potential for the oxidation of monolayers of CO with decreasing loading of Pt leading to an inability to oxidise the CO on the lowest loadings and (b) a strong negative shift in the potential for the reduction of oxygen. Both observations can be understood in terms of an increase in the irreversibility of the Pt/PtO couple at such surfaces. The same trends, although significantly weaker, are seen with the carbon supported platinum, delta < 1.0 nm, and it is suggested that the Pt/PtO couple on carbon shows intermediate kinetics between Pt on TiO(x) and bulk Pt. These results have significant implications for understanding the mechanism of oxygen reduction on supported Pt catalysts and hence for the search for alternative supports to platinum for ORR electrocatalysts.

The influence of Pt particle size on the surface oxidation of titania supported platinum

A range of reduced titania (TiO(x)) supported platinum electrocatalysts have been synthesised using physical vapour deposition on arrays of electrodes. Surfaces with equivalent thicknesses of platinum in the range 0.2-2.5 nm on a uniform layer of TiO(x) have been synthesised on 10 x 10 arrays. The arrays have been used to study the surface redox chemistry of the supported platinum as well as the oxidation of a monolayer of carbon monoxide on the platinum. It is shown that below an equivalent thickness of 0.8 nm, there is a positive shift in the potential for the oxidation of the platinum surface and a negative shift for the reduction of the oxide with decrease in the platinum loading. These shifts show that it is the kinetics of the platinum/platinum oxide couple that change with platinum loading; the couple becomes increasingly irreversible with decreasing loading. The peak potential for the oxidation of the monolayer of carbon monoxide also shifts positive and broadens with decreasing platinum loading; these trends are again particularly marked below an equivalent thickness of 0.8 nm while below 0.4 nm no CO oxidation peak is observed although it could be confirmed that CO is adsorbed on such surfaces. Again, these changes with platinum loading are associated with the irreversibility of the platinum/platinum oxide couple. At low equivalent thicknesses, it is impossible to form the oxidised platinum species within the carbon monoxide monolayer essential to the commencement of oxidation of the CO monolayer.

Determination of the Adsorption Sites of tert-Butylamine Molecules on a Cu(111) Surface with a Co-Adsorbed CO Monolayer

Scanning tunneling microscopy (STM) and density functional theory (DFT) were used to investigate the adsorption sites of separable tert-butylamine (t-BA) molecules on a Cu(111) surface at 78 K. We developed a method that uses CO molecules on a co-adsorbed3~(1/2)× 3~(1/2)superstructure as markers for copper atoms on the surface lattice. This method revealed an on-top adsorption for t-BA on the Cu(111) surface. At low coverage, t-BA molecules preferentially adsorbed at the top sites of the Cu(111) surface and this was confirmed using a single CO molecule as a marker for a copper atom. DFT calculations were performed to study the most stable adsorption configuration of t-BA on the terrace of the Cu(111) surface at 78 K. Calculation results indicate that the top site is the most energetically preferred adsorption site for a single t-BA molecule on the terrace, which agrees well with the experimental results.

Confinement of surface state electrons in self-organized Co islands on Au(111)

We report on detailed low temperature scanning tunneling spectroscopy measurements performed on nanoscale Co islands on Au(111) films. At low coverages, Co islands self-organize in arrays of mono- and bilayer nanoscale structures that often have an hexagonal shape. The process of self-organization is induced by the Au(111) ‘herringbone’ reconstruction. By means of mapping of the local density of states with lock-in detection, electron standing wave patterns are resolved on top of the atomically flat Co islands. The surface state electrons are observed to be strongly confined laterally inside the Co nanosized islands, with their wavefunctions reflecting the symmetry of the islands. To complement the experimental work, particle-in-a-box calculations were performed. The calculations are based on a newly developed variational method that can be applied to ‘2D boxes’ of arbitrary polygonal shape. The experimental patterns are found to fit nicely to the calculated wavefunctions for a box having a symmetry corresponding to the experimental island symmetry. The small size of the Co islands under study (down to 7.7 nm2) is observed to induce a strong discretization of the energy levels, with very large energy separations between the eigenstates up to several 100 meV. The observed standing wave patterns are identified either as individual eigenstates or as a ‘mixture’ of two or more energetically close-lying eigenstates of the cobalt island. Additionally, the Co surface state appears not to be limited to mono- and bilayer islands, but this state remains observable for multilayered islands up to five monolayers of Co.

Growth and crystalline structure of Co layers on Cu(0 0 1)

The crystalline structure of Co layers deposited on the Cu(0 0 1) surface was investigated with the use of the directional elastic peak electron spectroscopy (DEPES). For clean Cu(0 0 1) the experimental DEPES profiles obtained for different energies of the primary electron beam exhibit intensity maxima corresponding to the close packed rows of atoms. The Auger peak kinetics recorded during continuous Co deposition suggest the layer-by-layer growth mode. The DEPES profiles recorded for 10 monolayers (ML) of Co on Cu(0 0 1) reflect a short-range order in the adsorbate. Intensity maxima observed in the DEPES profiles for Co along [1 0 0], [0 1 0], and [1 1 0] azimuths of Cu(0 0 1) are characteristic of the face centered cubic (fcc) Co(0 0 1) layers. Low-intensity reflections and considerable background intensities were found in the low energy electron diffraction (LEED) patterns recorded from 10 ML of Co, which indicates a weak long-range order in the adsorbate. The adsorption of about 20 ML of Co results in considerable background contribution to DEPES. No reflections but a large background were observed with the use of LEED for this layer. The heating of the Co/Cu(0 0 1) system at T = 770 K leads to an increase of the short- and long-range order in the overlayer, observed in the DEPES profiles and LEED patterns, respectively. The theoretical DEPES profiles were obtained with the use of a multiple scattering approximation. A very good agreement between experimental and theoretical scans was found for the clean and covered copper substrate. The latter proves the epitaxial growth of Co layers on Cu(0 0 1).

CO monolayer oxidation at Pt(1 0 0) probed by potential step measurements in comparison to Pt(1 1 1) and Pt nanoparticle catalyst

The oxidation of CO adsorbed to high coverage on Pt(1 0 0) electrodes in 0.5 M H 2SO 4 is investigated through comparative measurements with Pt(1 1 1). In potential step experiments, current–time transients recorded during CO oxidation on Pt(1 1 1) display the same peak times and adherence to a Langmuir–Hinshelwood (LH) model for adsorbed CO electrochemical oxidation as literature benchmarks. For potentials in the vicinity of 0.8–0.9 V (versus a reversible hydrogen electrode reference), CO oxidation was faster on ordered Pt(1 0 0) electrodes than on Pt(1 1 1), and responses for ordered Pt(1 0 0) were close to, but somewhat more complicated than those predicted by the LH model. On Pt(1 0 0) with defects intentionally introduced by eliminating H 2 gas from the cooling atmosphere, current–time transients recorded during CO monolayer oxidation showed tailing at long times and responses similar to those for the reaction over nanometer-scale (<10 nm) Pt catalyst particles. The kinetics for CO monolayer oxidation on Pt(1 0 0) is discussed in terms of properties of Pt(1 0 0)-(1 × 1) islands and possible changes in island size with electrode treatment.

Bifunctional amorphous alloys more tolerant to carbon monoxide

The aim of this work is the study of the electrochemical behaviour of methanol and CO electro-oxidation reaction with amorphous metallic alloys of compositions (NiNb) 99(PtX) 1 (X = Ru, Sn, compositions in at.%), which are obtained by mechanical alloying, that results to in powders used as modified carbon paste electrodes (MCPEs). The presence of tin drastically decreases the reactivity towards methanol electro-oxidation of the alloy, followed by the Ni 59Nb 40Pt 1 and Ni 59Nb 40Pt 0.6Ru 0.4 alloys. Differential electrochemical mass spectrometry (DEMS) studies demonstrate that the Ni 59Nb 40Pt 0.6Ru 0.4 and Ni 59Nb 40Pt 0.6Sn 0.4 electrodes show the best tolerance to CO produced by methanol deprotonation, giving rise to more negative onset potential values for the CO 2 production, although the voltammograms for the stripping of a CO monolayer clearly show that the electrode that possesses a better tolerance to CO adsorbed on the surface of the electrode is Ni 59Nb 40Pt 0.6Sn 0.4, decreasing the onset potential values 281 and 158 mV with regard to the Ni 59Nb 40Pt 1 and Ni 59Nb 40Pt 0.6Ru 0.4, respectively. The catalytic activity of the electrode formed might be attributed to an adsorbed state of CO unique in this alloy.

Electrosorbed carbon monoxide monolayers on Pt(1 1 1)

We review structures of high-density CO monolayers on Pt(1 1 1) surfaces in CO-saturated electrolytes or in gaseous CO at near atmospheric pressure, using surface X-ray scattering (SXS) and scanning tunneling microscopy (STM). In electrolytes, we confirmed the well-known (2 × 2)-3CO and (√19 × √19)-13CO structures and were able to study the transition between them. For gas-phase studies, we were able to stabilize extremely well-ordered CO monolayers by emersion transfer from an electrochemical cell. We found that the hexagonal close-packed (2 × 2)-3CO structure is the equilibrium phase at room temperature in ∼1 atm CO gas pressure. This commensurate (C) phase transforms continuously to an incommensurate (IC) phase at elevated temperature (a second-order phase transition). We also confirm that the (√19 × √19)-13CO structure is stable at lower CO partial pressure. This C phase transforms discontinuously to an IC phase (a first-order phase transition). A tentative phase diagram and a brief review of structure details of the (2 × 2)-3CO and (√19 × √19)-13CO phases will be presented.

In situ ATR-FTIRS coupled with on-line DEMS under controlled mass transport conditions—A novel tool for electrocatalytic reaction studies

We present a novel combination of in situ attenuated total reflection-infrared spectroscopy (ATR-FTIRS) and on-line differential electrochemical mass spectrometry (DEMS) via a dual thin-layer electrochemical flow-cell, which allows: (i) simultaneous detection of stable adsorbed species and adsorbed reactive intermediates and side products by ATR-FTIR spectroscopy, (ii) quantitative determination of volatile reaction products and by-products by DEMS, and (iii) determination of the overall reaction kinetics by measuring the Faradaic current under controlled mass transport of reactants and reaction products to and from the electrode. The wealth of information and the insight into reaction mechanism and kinetics accessible from these complementing techniques are demonstrated taking the oxidation of CO on a Pt thin film electrode as example, including the electrooxidation of a pre-adsorbed CO monolayer at constant potential (0.6 V), the open-circuit reduction of pre-formed PtO by chemical reaction with CO, and potentiodynamic continuous CO bulk electrooxidation.

High-density electrosorbed carbon monoxide monolayers on Pt(111) under atmospheric pressure

We report structure studies of high-density CO monolayers on Pt(111) surfaces in equilibrium with gaseous CO near atmospheric pressure, using surface x-ray scattering (SXS) and scanning tunneling microscopy (STM). We were able to stabilize extremely well-ordered CO monolayers by emersion transfer from an electrochemical cell. We found the hexagonal close-packed $(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}3\mathrm{C}\mathrm{O}$ structure at room temperature in $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{atm}$ CO gas pressure. We also confirm the $(\sqrt{19}\ifmmode\times\else\texttimes\fi{}\sqrt{19})\text{\ensuremath{-}}13\mathrm{C}\mathrm{O}$ structure previously proposed in STM studies [Vestergaard et al., Phys. Rev. Lett. 88, 259601 (2002)]. Based on SXS and STM measurements, a tentative phase diagram is proposed. Detailed SXS structure studies of the $(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}3\mathrm{C}\mathrm{O}$ structure suggest a surface structure and substrate reconstruction less symmetric than previously studies suggested. We also find that $(\sqrt{19}\ifmmode\times\else\texttimes\fi{}\sqrt{19})\text{\ensuremath{-}}13\mathrm{C}\mathrm{O}$ structure induces substrate reconstructions including the dimerization of Pt bonded to bridging CO and the rotation of the Pt cluster with atop CO.