CO Saturation Coverage Research Articles

Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammograms. Part 2: Pt(100)

Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammograms. Part 2: Pt(100)

Gold Cluster Carbonyls: Vibrational Spectroscopy of the Anions and the Effects of Cluster Size, Charge, and Coverage on the CO Stretching Frequency

We report the vibrational spectra of the carbonyl complexes of anionic gold clusters in the range of the CO stretching frequency as measured in the gas phase using IR multiple photon dissociation spectroscopy. The investigated complexes contain between 3 and 14 Au atoms and up to 7 CO ligands. Special attention is given to the complexes that exhibit saturation CO coverage as well as to the monocarbonyl species. In conjunction with data from the corresponding cationic complexes we quantify how the CO stretching frequency varies with the charge state of the gold cluster. Our results provide a size- and charge-dependent basis to interpret values of the CO stretching frequency measured for CO on deposited gold clusters in terms of the charge states of the clusters.

Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammograms. Part 1: Pt(1 1 1)

Combined CO-stripping cyclic voltammetry and FT-IR spectroscopy measurements have allowed us to monitor changes in the coverage and structure of CO adlayers on Pt(1 1 1) electrodes in 0.1 M H 2SO 4 as a function of potential. Our results show that in CO-free solutions the maximum coverage is θ CO = 0.68 and that higher coverages can only be achieved in the presence of CO in the solution. Saturation coverages can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, E d) is more negative than 0.30 V vs. RHE. The lowest CO coverage at which hydrogen adsorption on the Pt(1 1 1) electrode is completely blocked is θ CO = 0.63, which corresponds to an E d = 0.50 V vs. RHE. Our results suggest that the process at the pre-peak (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of adsorbed CO by reaction with oxygenated species nucleating at steps, the main peak appearing when nucleation of oxygenated species at the terraces also occurs.

Potential dependence of the saturation CO coverage of Pt electrodes: The origin of the pre-peak in CO-stripping voltammograms. Part 3: Pt(poly)

Abstract Combined CO-stripping cyclic voltammetry and FT-IR spectroscopy measurements have allowed us to monitor changes in the coverage and structure of CO adlayers on Pt(1 1 1) electrodes in 0.1 M H2SO4 as a function of potential. Our results show that in CO-free solutions the maximum coverage is θCO = 0.68 and that higher coverages can only be achieved in the presence of CO in the solution. Saturation coverages can only be reached if the potential at which the electrode is held during CO adsorption (dosing potential, Ed) is more negative than 0.30 V vs. RHE. The lowest CO coverage at which hydrogen adsorption on the Pt(1 1 1) electrode is completely blocked is θCO = 0.63, which corresponds to an Ed = 0.50 V vs. RHE. Our results suggest that the process at the pre-peak (and, hence, the oxidation at low overpotentials of bulk CO in CO-saturated solutions), corresponds to the oxidation of adsorbed CO by reaction with oxygenated species nucleating at steps, the main peak appearing when nucleation of oxygenated species at the terraces also occurs.

Sulfur poisoning of the CO adsorption on Co(0 0 0 1)

CO adsorption on a sulfur covered cobalt surface at 185 K has been studied using XPS, TDS, LEED, and WF measurements. As in the case of CO adsorption on the clean Co(0 0 0 1) surface, CO adsorbs and desorbs molecularly and no dissociation was observed. The saturation coverage of CO decreases linearly from 0.54 ML to 0.27 ML when the S pre-coverage increases to 0.25 ML. The WF increased during CO adsorption, but did not reach the value obtained for CO adsorption on the clean surface. The smaller work function change is explained by the reduced adsorption of CO on the sulfur-precovered surface. A reduction in the activation energy of desorption for CO from 113 kJ/mol to 88 kJ/mol was observed indicating weaker bonding of the CO molecules to the surface. The behavior of the CO/S/Co(0 0 0 1) system was explained by a combination of steric and electronic effects.

In-situ soft X-ray studies of CO oxidation on the Pt(111) surface

Temperature-programmed and isothermal carbon–K-edge fluorescence yield measurements have been used to characterize carbon monoxide oxidation on the Pt(1 1 1) surface in pressures of flowing oxygen ranging from 0.00002 to 0.02 Torr over the 200–600 K temperature range. Temperature programmed-fluorescence yield near edge spectroscopy (TP-FYNES) of the oxidation of pre-adsorbed CO coverages in O2 pressures up to 0.02 Torr has been used to directly characterize surface reaction rates and to focus on the effects of temperature and oxygen flux during oxidation. For a saturated CO coverage, oxidation does not begin until significant CO desorption from the surface occurs. This finding is supported by the observation that the onset temperature for oxidation remains at 305 K for the CO saturated surface even for O2 pressures up to 0.02 Torr. For large CO coverages below full saturation, the onset temperature for oxidation decreases with increasing oxygen pressure, indicating oxygen adsorption is not completely inhibited. After oxidation begins for large CO coverages, oxidation rapidly decreases the CO surface concentration, as the activation energy for CO oxidation is smaller than the activation energy for CO desorption. Isothermal oxidation experiments clearly show that the oxidation activation energies observed depend on both the CO coverage and the O2 pressure. Oxidation activation energies are larger for small CO coverages, with a clear increase below 70% of a saturated CO coverage. For instance, at an oxygen pressure of 0.002 Torr, the activation energy for oxidation increases from 11.9 to 16.8 kcal/mol as the CO coverage decreases below 70% of a saturated coverage. In addition, oxidation activation energies decrease with increasing oxygen pressure. For example, as the oxygen pressure is increased from 0.002 to 0.02 Torr, the activation energy for oxidation of CO coverages below 70% of a saturated coverage is decreased from 16.8 to 12.3 kcal/mol. For the fully saturated CO coverage, the inhibition of oxygen adsorption completely suppresses oxidation below the CO desorption temperature. For large CO coverages below saturation where oxidation proceeds at a measurable rate, the order in oxygen is approximately zero (0.08), suggesting that oxidation is generally not limited by oxygen adsorption. Taken together, these findings clearly confirm that CO inhibition of O2 adsorption is crucial for the saturated coverage of CO. For CO coverages below saturation, interactions within the adsorbed layer significantly alter both the activation energy and pre-factor for CO oxidation on the low-index Pt(1 1 1) surface.

CO Adsorption on Ru-Modified Pt(100) Surfaces: Infrared Reflection Absorption Studies in Ultrahigh Vacuum

The mode of bonding of CO to Ru-modified Pt(100) surfaces prepared by chemical vapor deposition was investigated using infrared reflection absorption spectroscopy (IRAS) in ultrahigh vacuum (UHV). Spectra recorded in the CO stretching region for freshly prepared Ru(θRu ca. 0.52)/Pt(100) exposed to saturation coverage of CO displayed, in addition to a rather weak peak ascribed to CO bound to bridging sites, a single asymmetric peak centered at ca. 2092 cm-1. Statistical analyses of IRAS spectra recorded in experiments in which a Ru(θRu ca. 0.52)/Pt(100) surface was annealed sequentially to 650 K for ca. 2.5 h prior to a saturation exposure of CO yielded two peaks at 2100 cm-1 and 2092 cm-1 with a clearly identifiable isosbestic point, characteristic of a quantitative conversion of one type of species into the other. This behavior has been attributed to a thermally induced surface incorporation of Ru into Pt, as has been shown with low-energy ion-scattering spectroscopy (LEIS) for other Ru-modified low-inde...

Adsorption and reaction of CO on a ceria–Rh(1 1 1) “inverse model catalyst” surface

The adsorption of CO and the reaction of CO with preadsorbed oxygen at room temperature has been studied on the Rh(1 1 1)2 × 1-O surface and on ceria–Rh(1 1 1) “inverse model catalyst” surfaces using C 1s and O 1s core level and valence band photoelectron spectroscopy with synchrotron radiation. The adsorption of CO on the oxygen-precovered (2 × 1)-O surface was found to proceed slower than on the clean Rh(1 1 1) surface, because of a kinetic limitation which is introduced by the O + CO clean-off reaction. The latter removes adsorbed oxygen as CO 2 at 300–320 K, which desorbs into the gas phase. On the ceria–Rh(1 1 1) surfaces the O + CO oxidation reaction is much faster than on the Rh(1 1 1)2 × 1-O surface suggesting a catalytically active role of the ceria–Rh interface. The XPS spectra indicate a preferential occupation of hollow-type CO adsorption sites on the ceria–Rh(1 1 1) surface, which might be located at the oxide–metal interface. No CO dissociation has been detected on the Rh(1 1 1) supported ceria inverse catalyst surfaces. On a CeRh 3 alloy surface, prepared by thermal decomposition of the ceria in ultrahigh vacuum, the hollow-type CO adsorption sites are energetically favoured suppressing the on-top sites almost completely, but the global adsorption energy is lower than on Rh(1 1 1) as indicated by the reduced CO saturation coverage.

Chemical reactivity of the V–Pd(1 1 1) subsurface alloy: adsorption of CO

The reactivity of a V–Pd(111)(√3×√3)R30° subsurface alloy has been investigated, using the adsorption of CO as a test case, by photoelectron spectroscopy with synchrotron radiation of the valence and core states and by kinetic analysis of temperature-programmed thermal desorption spectra. The overall conclusion from this study is that the chemical reactivity towards CO is lowered on the subsurface alloy surface as compared to unmodified Pd(111). The photoemission data indicate that the room-temperature CO saturation coverage is reduced from 0.5 monolayer (ML) on the clean Pd(111) surface to ∼0.28–0.30 ML on the alloy surface, in excellent agreement with the thermal desorption results. This is explained in terms of a reduced CO adsorption energy, as revealed by the kinetic analysis of the thermal desorption spectra. The balance between threefold hollow and bridge CO adsorption sites at saturation coverage is changed in favour of the preferential occupation of hollow sites on the alloy surface. The observed modifications of the electronic structure of valence states upon alloying and on CO adsorption are well reproduced by a recent ab initio DFT study of CO on V–Pd alloys by Hirschl and Hafner [Surf. Sci. 498 (2002) 37].

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.

CO dissociation on Mo(1 1 0) studied by high-resolution core-level spectroscopy

We have studied the CO/Mo(1 1 0) adsorption system at different sample temperatures by high-resolution core-level spectroscopy and near-edge X-ray-absorption fine-structure measurements. We demonstrate that CO molecules having different tilt angles with respect to the surface normal may be assigned to different components of the C 1s core-level spectra. The tilt of the CO molecules is shown to depend on the CO coverage. The thermally induced dissociation of CO is studied and is shown to differ at low and high initial CO coverages. At low coverage CO dissociation starts already at temperatures as low as 125 K whereas at saturation CO coverage no dissociation occurs below 280 K. The reasons for this difference in dissociation behavior are discussed and it is concluded that steric effects play a large role.

The modification of Pt(110) by ruthenium: CO adsorption and electro-oxidation

The adsorption and incorporation of ruthenium on Pt(110) have been investigated using the techniques of low energy ion scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (ARXPS) and low energy electron diffraction (LEED). Ruthenium lifts the Pt(110)-(1×2) reconstruction by a coverage of 0.5, with evidence of only a small amount of clustering following deposition at 300 K. LEISS and XPS measurements indicate that ruthenium can be excluded from the top layer by flash annealing, but incorporated into the second and third layers. After annealing to over 1000 K, bulk dissolution of ruthenium is observed. Temperature programmed desorption (TPD) of CO on the adsorbed phases of ruthenium indicates that CO coverages remain relatively high, and CO is adsorbed on both the platinum and ruthenium. TPD of CO on the alloyed phases has demonstrated that second layer ruthenium has a significant effect on the binding of CO on the top platinum atoms. The maximum saturation coverage of CO at 325 K observed on these alloyed phases is significantly reduced. Surfaces in which ruthenium is incorporated in the top surface layer provide a phase which reduces the overpotential of CO electro-oxidation to 0.6 V RHE. No promotion is observed when ruthenium is excluded from the top layer, but substantial concentrations are present in the second and third layers (the conditions that gave rise to the strongest perturbation in CO adsorption behaviour). This suggests that the effect of ruthenium is primarily to reduce the overpotential in the activation of water to produce the surface oxidising species in the electro-oxidation process, and that it is water activation that is rate limiting in the overall reaction. The incorporated ruthenium phase in the top layer is significantly more stable with respect to oxidative dissolution than the adsorbed phase. Oxidative cycling of surfaces with ruthenium in the second and third layers results in the segregation of ruthenium to the top layer, and such modified surfaces result in overpotentials for electro-oxidation as low as 0.52 V RHE.

CO adsorption on the Pt/Rh(100) surface studied by high-resolution photoemission

Submonolayers of platinum have been deposited on an Rh(100) substrate. Platinum growth and subsequent CO adsorption on the bimetallic surface has been studied by high-resolution photoemission applying synchrotron radiation. Platinum was found to grow as small, single-layer islands on Rh(100). The CO adsorption site on both metals is very sensitive to compositional changes in the bimetallic surface. A site switch from on-top to bridge sites in the c(2×2) CO overlayer on Rh(100) has been observed in the presence of 0.2 monolayers (ML) of platinum. The CO saturation coverage is found to decrease dramatically with increasing platinum coverages. 0.9 ML of platinum on Rh(100) binds approximately half the CO bound by Pt(100). We explain this in terms of a reduced COPt bond strength through a ‘ligand effect’.

Equilibrated Adsorption of CO on Silica-Supported Pt Catalysts

Microcalorimetric and infrared spectroscopic studies of CO adsorption on Pt/SiO2 were conducted at temperatures from 298 to 673 K. The adsorption of CO on silica-supported Pt is equilibrated at 673 K, and the microcalorimetric and spectroscopic results obtained at this elevated temperature are in agreement with results reported for Pt(111) at 300 K. The heat of CO adsorption decreases with coverage from the initial value of 190−180 kJ/mol to 75 kJ/mol at the saturation CO coverage of 0.7 ML. In addition, the IR band for CO adsorbed on atop sites shifts to higher wavenumbers, and the ratio of integral absorbances of bridge-bonded and atop-bonded species changes with CO coverage. At temperatures lower than 673 K, the adsorption of CO on Pt/SiO2 is not equilibrated, and the microcalorimetric and IR spectroscopic data are essentially independent of the apparent CO coverage. A Monte Carlo simulation model was used to consolidate data for CO adsorption on Pt/SiO2 and Pt(111). The simulation was based on a set o...

Performance evaluation of porous electrocatalysts via normalization of the active surface

When comparing the rate of electrochemical processes at different porous electrocatalysts a surface normalization should be used. It is shown for the case of methanol oxidation at PtRu layers electrodeposited on gold substrates that substantially different data are obtained for current, mass spectrometry signals and integrated IR band intensities of the products, with and without normalization of the catalyst surface used. Using stripping of saturated CO coverage as a normalization tool, cyclic voltammograms, on line MS and in situ FTIR spectroscopy give reasonable agreement of catalytic activity towards methanol oxidation.

Probing the Influence of Alloyed Sn on Pt(100) Surface Chemistry by CO Chemisorption

AbstractThe chemisorption properties of the c(2×2) and (3√2×√2)R45° Sn/Pt(100) alloy surfaces, along with the clean Pt(100) surface, were investigated using CO as a probe molecule. Temperature‐programmed desorption (TPD) studies revealed a reduction in CO desorption peak temperature, and thus the chemisorption bond energy, by alloying Sn into the Pt(100) surface. A large decrease was observed in the saturation coverage of CO on these alloyed surfaces at 150 K compared to the Pt(100) surface. The initial sticking coefficient of CO was found, however, to be nearly independent of the surface Sn concentration. High‐resolution electron energy loss spectroscopy (HREELS) studies showed that CO was only chemisorbed on atop sites on both alloys. Sn incorporation results in isolated Pt atoms at the surface of these two Sn/Pt(100) alloys and eliminates the possibility of CO bonding to multiple Pt centers, i.e., to pure‐Pt 2‐ and 3‐fold bridging sites.

Poisoning of the adsorption of CO, O2 and D2 on Pt(111) by preadsorbed H2O

We have measured the influence of adsorbed H2O on the sticking coefficients and saturation coverages of CO, O2 and D2 on Pt(111) at ∼100 K. Strong poisoning is observed for all three gases. For O2 and D2, the surface is essentially totally poisoned at 1 monolayer (ML) water coverage. For CO, the effect is weaker, with some CO adsorption still occurring at 2–3 ML H2O. The influence of these results on the kinetics of the CO and H2 oxidation reactions are discussed briefly. It is concluded that the influence of water must be included in kinetics simulations, at least at low temperatures, when significant humidity levels are present in inlet gas mixtures, or produced by the reactions themselves.

The [formula omitted] layer: preparation, characterization, and analysis of interaction effects in vibrational spectra

The coadsorption system CO + O Ru(001) has been investigated using TDS, IRAS, LEED and XPS. Our work goes beyond preceding investigations in that we show that it is possible to accommodate two CO molecules in an O(2 × 2) unit cell using very high (∼10 5 L) CO exposures. The CO sticking coefficient shows an abrupt decrease by a factor of 60 at the conventional CO saturation coverage of 0.25 ML. A structure model comprising linearly bonded CO molecules on top sites and “bridge”-bonded (in terms of internal stretch frequency) CO molecules in threefold (fcc) sites is proposed. From the line position of the v C−O mode of linearly bonded 13C 16O molecules coverage-dependent chemical shifts of this mode are determined. Also, relative changes of the vibrational polarizability α v of the linear 12C 16O v C−O mode and of the dielectric constant ϵ = 1 + α e θU 0 of the adlayer, as an extra CO molecule is squeezed into the O(2 × 2) unit cell, can be determined; by the presence of the CO molecules in hollow sites the linear 12C 16O v C-O mode suffers a pronounced chemical blue-shift of 25.4 cm −1 · α v is reduced by 22%, and ϵ is increased by 12%. The v M−CO mode is shifted by 15.5 cm −1 to lower energies.

Surface electrochemistry of CO on Pt(110)-(1 × 2) and Pt(110)-(1 × 1) surfaces

A new type of flame-annealing method has been developed for the preparation of surfaces having either fcc(110)-(1 × 1) or fcc(110)-(1 × 2) symmetry. In situ X-ray scattering studies show that both the Pt(110)-(1 × 2) and Pt(110)-(1 × 1) surfaces are stable in the potential region between 0 and 1.0 V. Adsorption of CO on the (1 × 2) surface in solution does not induce the (1 × 2)→(1 × 1) transition that is observed in UHV upon adsorption of CO. Nominally identical saturation coverages of hydrogen (H upd) and CO ad are observed on the two surfaces, implying that two H upd atoms or two CO ad molecules are adsorbed per unit cell of the reconstructed (1 × 2) surface. A significant increase in the interatomic spacing between the first and second layer of the reconstructed (1 × 2) surface occurs upon hydrogen adsorption to full coverage. Occupation of subsurface sites for H upd are inferred from this result. The (1 × 2) surface is found to have a much higher catalytic activity for the electro-oxidation of CO; the anodic oxidation of either CO ad or dissolved CO gas occurs at ≈ 0.3 V lower potential on the (1 × 2) than the (1 × 1) surface.

Simulations of the femtosecond laser-induced desorption of CO from Cu(100) at 0.5 ML coverage

Simulations of the femtosecond laser-induced desorption of CO from Cu(100) at an initial temperature of 95K are reported, employing molecular dynamics with electronic frictions. Results are obtained for saturation (0.5 ML) CO coverage, which are compared with additional simulations in the zero coverage limit. In both cases, the desorption yield depends nonlinearly on laser fluence, with initial nonadiabatic energy flow occurring selectively from hot substrate electrons to the molecule-surface frustrated rotation. There is significant enhancement of the desorption yield at saturation relative to the zero coverage limit. This is due to conversion of momentum in the plane of the surface to normal momentum, by collisions between an energetic adsorbate and its neighbors, and energy pooling between molecules in the overlayer.