CO Admolecules Research Articles

Mobility and Oxidation of Adsorbed CO on Shape-Controlled Pt Nanoparticles in Acidic Medium.

The knowledge about how CO occupies and detaches from specific surface sites on well-structured Pt surfaces provides outstanding information on both dynamics/mobility of COads and oxidation of this molecule under electrochemical conditions. This work reports how the potentiostatic growth of different coverage CO adlayers evolves with time on both cubic and octahedral Pt nanoparticles in acidic medium. Data suggest that during the growth of the CO adlayer, COads molecules slightly shift toward low coordination sites only on octahedral Pt nanoparticles, so that these undercoordinated sites are the first filled on octahedral Pt nanoparticles. Conversely, on cubic Pt nanoparticles, adsorbed CO behaves as an immobile species, and low coordinated sites as well as (100) terraces are apparently filled uniformly and simultaneously. However, once the adlayer is complete, irrespectively of whether the CO is oxidized in a single step or in a sequence of different potential steps, results suggest that COads behaves as an immobile species during its oxidation on both octahedral and cubic Pt nanoparticles.

Size-Selective Kinetics of Nanostructured Pt/GC Model Electrocatalysts for CO Stripping

We studied using dynamic Monte Carlo (DMC) models the size-selective kinetic characteristics of CO monolayer oxidation (CO stripping) on multiscale nanostructured Pt/GC model electrodes comprising nanodisks with diameters of 120 nm and nanoparticles with diameters of 6 nm. We used the DMC models to simulate preadsorbed CO (COad) oxidation peaks and voltammetry responses for the two types of nanostructures and compared them to experiments. Our DMC simulations showed that the different CO stripping voltammetry peaks for the nanodisks and the nanoparticles observed in experiments result from different surface motilities of the COad molecules on the catalyst surfaces and from different initial COad configurations.

ChemInform Abstract: Adsorption of NO and CO and Specific Features of Their Interaction on the Pt(100) Surface

AbstractReview: 127 refs.

Specific features of the formation of co-adsorption layers on metal surfaces with the participation of simple molecules: NH3 + CO on Pt(111)

The co-adsorption of ammonia and carbon monoxide on the Pt(111) surface was studied at temperatures <300 K using high-resolution electron energy loss spectroscopy (HREELS). The state of ammonia and carbon monoxide molecules in the co-adsorption layer was established to differ significantly from their state in individual adsorption layers. The adsorption of CO on a clean surface occurs with the primary filling of single-bound terminal sites, whereas the bridging sites are filled preferably by CO molecules in the presence of NH3,ads. The symmetry axis of ammonia molecules adsorbed on the clean surface is parallel to the normal to the surface, whereas in the co-adsorption layers the interaction with COads molecules results in the deviation of the symmetry axis toward the surface. Presumably, the observed changes in the state of adsorbed molecules are due to the donor-acceptor interaction inducing the electron density transfer from ammonia molecules across the metal surface to CO molecules.

Adsorption of NO and CO and specific features of their interaction on the Pt(100) surface

This article presents an analytical review of the author’s results and the literature concerning the nature of species resulting from NO and CO adsorption on the unreconstructed (1 × 1) and reconstructed hexagonal (hex) Pt(100) surfaces, including specific features of the reactions between these species. At 300 K, both surfaces adsorb NO and CO mainly in their molecular states. When adsorbed on Pt(100)-1 × 1, the NOads and COads molecules are uniformly distributed on the surface. Under the same conditions, the hexagonal surface undergoes adsorption-induced reconstruction with the formation of NOads/1 × 1 and COads/1 × 1 islands, which are areas of the unreconstructed phase saturated with adsorbed molecules and surrounded with the adsorbate-free hex phase. In adsorption on structurally heterogeneous surfaces containing both hex and 1 × 1 areas, the 1 × 1 and hex phases are occupied in succession, the latter undergoing reconstruction into the 1 × 1 phase. The reaction between NO and CO on the unreconstructed surfaces occurs even at room temperature and results in the formation of N2 and CO2 in quantitative yield. On the hexagonal surface, a stable layer of adsorbed molecules as (NOads + COads)/1 × 1 mixed islands forms under these conditions. Above 350 K, the reaction in the mixed islands is initiated by the desorption of small amounts of the initial compounds, and this is followed by rapid self-acceleration leading to a surface explosion yielding N2, CO2, and N2O (minor product). These products show themselves as very narrow desorption peaks in the temperature-programmed reaction spectrum.

Spill-Over Effects on Bimetallic Pt/Ru(0001) Surfaces

The concept of spill-over of adsorbed species has a long tradition in Heterogeneous Catalysis and has been explored also for adsorption on bimetallic surfaces, in particular by the Goodman group. In the present paper, we report results of a comprehensive temperature programmed desorption (TPD) and infrared reflection absorption spectroscopy study on spill-over effects in the adsorption and desorption of CO on structurally well defined bimetallic Pt/Ru(0001) surfaces, where part of the substrate is covered by monolayer Pt islands. While upon adsorption at 90 K, the mobility of COad molecules on the surface is very limited, it is activated when the adlayer is annealed to 150 K or, more directly, if CO exposure is done at 150 K or higher temperatures. This enables diffusion of COad molecules to the Pt free Ru(0001) areas, even at local COad coverages which preclude further adsorption from the gas phase on the Ru parts of the surface. Spill-over processes are shown to have significant impact on the TPD spectra; furthermore they provide an additional adsorption channel for adsorption on the bare Ru(0001) areas, allowing uptake of CO at local coverages where adsorption from the gas phase is precluded. This indicates that the apparent CO saturation coverage of 0.68 ML determined for direct adsorption on Ru(0001) under UHV conditions is limited by kinetics rather than thermodynamics. The data are discussed in comparison with results and interpretations in earlier studies, which indicate that these effects are not limited to the Pt/Ru(0001) surface, but may be found on a wide range of bimetallic systems.

Efficient Pt/carbon electrocatalysts for proton exchange membrane fuel cells: Avoid chloride-based Pt salts!

The catalytic layers of Proton Exchange Membrane (PEM) fuel cells use carbon-supported Pt-based nanoparticles, which are classically synthesized by impregnation with a metal precursor usually based on chloride counter-ions. However, the literature points out that chloride ions poison the catalytic activity. The purpose of this work is to compare the surface reactivity and the electrocatalytic activity of Pt/carbon xerogel nanoparticles contaminated with different amount of chlorine. The catalysts were synthesized using the Strong Electrostatic Adsorption (SEA) method and H2PtCl6 as a metal precursor. The Cl content in the catalysts was changed by varying the gas atmosphere and the duration of the reduction step. The effect of Cl contamination was investigated on two model reactions: the electrochemical COads oxidation and the O2 reduction. Evidences are provided that the Cl− ions issued from the incomplete decomposition/removal of the metal precursor partially block the Pt catalytic sites, and limit the rate of the COads + OHads recombination and of the diffusion of COads molecules in CO stripping experiments. The adsorption of Cl− ions also depreciates the ORR kinetics. In search for efficient catalysts, chloride-containing metal precursors must therefore be avoided.

Experimental study of intermediates and wave phenomena in co oxidation on platinum metal (Pt, Pd) surfaces

The mechanism of catalytic CO oxidation on Pt(100) and Pd(110) single-crystal surfaces and on Pt and Pd sharp tip (∼103 A) surfaces has been studied experimentally by temperature-programmed reaction, temperature desorption spectroscopy, field electron microscopy, and molecular beam techniques. Using the density functional theory the equilibrium states and stretching vibrations of oxygen atoms adsorbed on the Pt(100) surface have been calculated. The character of the mixed adsorption layer was established by high resolution electron energy loss spectroscopy—molecular adsorption (O2ads, COads) on Pt(100)-hex and dissociative adsorption (Oads, COads) on Pt(100)-(1×1). The origin of kinetic self-oscillations for the isothermal oxidation of CO in situ was studied in detail on the Pt and Pd tips by field electron microscopy. The initiating role of the reversible phase transition (hex) ↔ (1 × 1) of the Pt(100) nanoplane in the generation of regular chemical waves was established. The origination of self-oscillations and waves on the Pt(100) nanoplane was shown to be caused by the spontaneous periodical transition of the metal from the low-active state (hex) to the highly active catalytic state (1 × 1). A relationship between the reactivity of oxygen atoms (Oads) and the concentration of COads molecules was revealed for the Pd(110) surface. Studies using the isotope label 18Oads demonstrated that the low-temperature formation of CO2 at 150 K is a result of the reaction of CO with the highly reactive state of atomic oxygen (Oads). The possibility of the low-temperature oxidation of CO via interaction with the so-called “hot” oxygen atoms (Ohot) appearing on the surface at the instant of dissociation of O2ads molecules was studied by the molecular beam techniques.

An Electrochemical in Situ Surface-Enhanced Raman Spectroscopic Study of Carbon Monoxide Chemisorption at a Gold Core−Platinum Shell Nanoparticle Electrode with a Flow Cell

The adsorption behavior of a CO adlayer at a Au core−Pt shell (Au@Pt) nanoparticle electrode as a function of CO coverage, partial pressure, and electrolyte composition is examined by electrochemical in situ surface-enhanced Raman spectroscopy (SERS) combined with a thin layer flow cell, which allows time-resolved spectro-electrochemical measurements upon sudden exchange of the electrolyte under potential control. We find (i) a clear decrease (increase) in the Pt−COL (C−OL) peak frequency and an increase of both bands’ intensities with COad coverage; (ii) a slight increase in the Pt−COL and decrease in the C−OL stretching frequencies and an increase in both intensities of the COad-saturated layer when switching from CO-saturated to CO-free solution; (iii) a clear decrease (up to 50%) of intensities of all of the Pt−CO and C−O bands and the SERS background during the solution switch from H2SO4 or NaOH to Na2SO4; (iv) a slightly higher C−OL (lower Pt−COL) peak frequency in H2SO4 than that in Na2SO4 and NaOH. The contributions of different factors such as SERS enhancement, Pt−CO binding strength, CO molecular orientation, and dynamic dipole−dipole coupling interactions among the COad molecules to the SERS spectral behavior are discussed on the basis of the complementary information of both Pt−CO and C−O stretching vibrations.

Adsorption of Formaldehyde on Pt(111) and Pt(100) Electrodes: Cyclic Voltammetry and Scanning Tunneling Microscopy

The adsorption of formaldehyde (HCHO) on Pt(111) and Pt(100) electrodes was examined by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) in 0.1 M HClO(4). The extent of HCHO adsorption at both Pt electrodes was evaluated by comparing the CVs, particularly for the hydrogen adsorption and desorption between 0.05 and 0.4 V, obtained in 0.1 M HClO(4) with and without HCHO. The adsorption of HCHO on these Pt electrodes was significant only when [HCHO] >/= 10 mM. Adsorbed organic intermediate species acted as poisons, blocking Pt surfaces and causing delays in the oxidation of HCHO. Compared to Pt(111), Pt(100) was more prone to poisoning, as indicated by a 200 mV positive shift of the onset of HCHO oxidation. However, Pt(100) exhibited an activity 3 times higher than that of Pt(111), as indicated by the difference in peak current density of HCHO oxidation. Molecular resolution STM revealed highly ordered structures of Pt(111)-( radical7 x radical7)R19.1 degrees and Pt(100)-( radical2 x radical2) in the potential region between 0.1 and 0.3 V. Voltammetric measurements further showed that the organic poisons produced by HCHO adsorption behaved differently from the intentionally dosed CO admolecules, which supports the assumption for the formation of HCO or COH adspecies, rather than CO, as the poison. On both Pt electrodes, HCHO oxidation commenced preferentially at step sites at the onset potential of this reaction, but it occurred uniformly at the peak potentials.

CO oxidation on Pt(111)—Scanning tunneling microscopy experiments and Monte Carlo simulations

The catalytic reaction between adsorbed oxygen atoms and CO molecules on Pt(111) was investigated by scanning tunneling microscopy and modeled by Monte Carlo simulations. Experiments were performed by dosing preadsorbed Oad layers with CO between 237 and 274 K. Two stages were observed during dosing with CO, an initial reordering and compression of (2×2)Oad islands, and a subsequent shrinking of the islands by the reaction of Oad to give CO2. The reaction occurs exclusively at boundaries between (2×2)Oad and c(4×2)COad domains. The reaction order with respect to the oxygen coverage is 0.5; the reactivity of the boundary increases during the reaction. The Monte Carlo simulations included surface diffusion of Oad atoms, attractive interactions between Oad atoms, the Oad–COad reaction probability (with parameters from quantitative scanning tunneling microscopy measurements), adsorption/desorption of CO, and a high mobility of COad. The experimentally observed domain shapes, the reaction order of 0.5, and the increasing boundary reactivity could only be reproduced by additionally including an Oad coordination-dependent activation energy ΔEreact* of 25 meV per Oad neighbor that accounts for the attractive Oad–Oad interactions. The initial ordering stage could be modeled by incorporating an additional repulsive interaction between Oad and COad. The fact that no reaction occurs in the interior of the (2×2)Oad domains, although they are covered by a layer of interstitial COad molecules, is attributed to the crucial role of reactive Oad–COad configurations that only exist at the domain boundaries.

Formation of mixed (NO ads+CO ads)/(1×1) islands on the Pt(100)-(hex) surface

The coadsorption of NO and CO on the reconstructed Pt(100)-(hex) surface at 300 K was studied by means of high-resolution electron energy-loss spectroscopy (HREELS). NO+CO coadsorption layers prepared in three different ways were investigated: (1) CO adsorption after coverage of the surface with NO ads/(1×1) islands; (2) NO adsorption after coverage of the surface with CO, which was mainly adsorbed on structural defects of the reconstructed (hex) phase (low coverage of CO ads); and (3) NO adsorption after coverage of the surface with CO ads/(1×1) islands (high coverage of CO ads). In all cases the strong mutual interaction between NO ads and CO ads molecules resulted in the formation of mixed (NO ads+CO ads)/(1×1) islands. The mixed islands of NO ads and CO ads covered unreconstructed (1×1) patches. The islands were separated from each other by either free (hex) phase or a region consisting of structural defects, which appeared during the course of a surface (hex)→(1×1) back-reconstruction. In the mixed (NO ads+CO ads)/(1×1) islands, it was found that structural defects were occupied preferentially by molecularly adsorbed CO.

Mechanisms of the CO oxidation on the Pd(110)c(2 × 4)-O surface

The Pd(110)c(2 × 4)-O surface was exposed to various amounts of CO at 90 K, and the reaction-rate-limited CO 2 desorption was studied by thermal desorption spectroscopy (TDS). The results were correlated with those of high resolution electron energy loss spectroscopy (EELS) and low-energy electron diffraction (LEED). As the temperature is increased (heating rate 4 K s −1), the sizes of the c(2 × 4)-O patches become smaller with the remaining O atoms located in the quasi-three-coordinated sites until the CO 2 desorption is completed. The CO molecules are mobile beyond 200 K. The CO 2 desorption occurs as soon as the CO admolecules obtain sufficient translational kinetic energy to approach O adatoms (at the peripheries of the c(2 × 4)-O patches) in close proximity. Three CO 2 desorption peaks were observed which are directly correlated with three different adsorbed sites of CO. These results are quite different from the CO 2 desorptions on Pd(111) and Pd(100) surfaces. The CO 2 desorptions from CO on other oxygen-covered Pd(110) surfaces, i.e. (2 × 3)-1D, c(2 × 6), “complex” and pseudo-(2 × 1), were also briefly investigated. The CO 2 desorption peaks were observed which are correlated with unstable O and subsurface O. A CO 2 desorption peak was also observed which occurs by the direct interaction of O 2 molecules with CO on the c(2 × 4)-O surface.

Theoretical study of interactions among surface defects by He scattering technique: the reason for the failure of the cross section geometric overlap hypothesis

Abstract The study of interactions among surface defects by He scattering technique is mainly based on the hypothesis of geometric overlap (GO) of defect cross sections. Recently, quantum calculations have shown that this hypothesis is not valid. This result has been obtained by considering He scattering from targets consisting of a Pt(111) surface containing two defects, i.e. two CO admolecules or two vacancies arranged at different mutual distances, d def . Quantum cross sections Σ 2 , calculated by sudden approximation for both kinds of defects and in a wide d def range, were in large disagreement — up to 34%, namely more than the 3xperimental and sudden errors on cross section — with the corresponding quantities Σ 2 Δ obtained on a pure geometric ground by the GO hypothesis. Therefore, the main purpose of the present paper is to detect the reason for the GO hypothesis failure. To this end, because a more fruitful comparison can be made between homogeneous quantities, and Σ 2 and Σ 2 Δ are not, suitable target models have been built, containing two CO admolecules or two vacancies on Pt(111) arranged as in the case of the above-mentioned targets. The new targets are such that calculations on He scattering from them give the cross sections σ 2 and σ 2 Δ — both obtained by taking into account He-target interaction, and therefore homogeneous — whose trends versus d def are completely analogous to those of Σ 2 and Σ 2 Δ , respectively. In this way the comparison between σ 2 and σ 2 Δ , is useful to demonstrate that the GO hypothesis fails because it implies — translated in terms of atom-target interaction — that, in the region between the two defects, i.e. where their cross sections overlap, He scattering process could be described by taking into account the interaction of impinging atom with just one of the two defects and neglecting the contemporaneous effect of the other one.

Spatial distribution of reactive carbon dioxide desorption on palladium (100) surfaces

The desorption of CO 2 produced by the interaction of oxygen adatoms and CO admolecules on Pd(100) was studied with angle-resolved thermal desorption. The spectra of CO 2 formation showed four peaks; P 1- (around 380 K), P 2- (∼ 325 K), P 3- (∼ 260 K), and P 4-CO 2 (∼ 220 K). Each CO 2 desorption showed a different angular distribution. The distribution is sharply collimated normal to the surface and isotropic with respect to the crystal azimuth.

Adsorbate-enhanced chemisorption in the carbon monoxide/rhenium (001) system: experiment and theory

The chemisorption kinetics of CO molecules on Re(001) crystal surface was studied by temperature-programmed desorption (TPD) experiments in the crystal temperature range of 80-360 K. Correlation was found between the population of the tightly bound, partially dissociated, {beta}-CO state and the less tightly bound {alpha}-CO state; basically, the {alpha}-state starts to populate when the {beta}-sites approach saturation. Furthermore, the increase in {beta}-CO coverage is accompanied by an increase in the overall sticking probability, as well as in the nonmonotonic coverage dependence of the peak desorption temperature of the {alpha}-state. The chemisorption kinetics was found to be crystal temperature independent. LEED analysis reveals that the CO overlayer is disordered, as reported previously. A theoretical model is proposed to account for the above observations. In this model, the {beta}-sites are treated as traps for mobile {alpha}-CO admolecules. Occupied {beta}-sites then serve as nucleation centers for enhanced, extrinsic precursor-mediated, chemisorption and island growth. The nonmonotonic variation of {alpha}-CO adsorption energy, and the appearance of a shoulder in the {alpha}-CO TPD peak at high coverages, are explained by a lattice gas model, incorporating repulsive nearest-neighbor and attractive next-nearest-neighbor lateral interactions between the chemisorbed molecules.

Computation of the vibrational modes of infinite two-dimensional ordered overlayers of adsorbed species

The method of theoretical analysis of normal vibrations of infinite two-dimensional ordered arrays of adsorbed molecules has been developed. Using this method, the vibrational state densities as well as dispersion surfaces describing frequency-phase dependences can be easily calculated for these arrays. The method is illustrated by calculating the dispersion surfaces and histograms of vibrational state densities for CO admolecules on Pt(100) at saturated coverages.

Anisotropic angular distribution of desorption of CO 2 produced on Pd(110) surfaces

The desorption of CO 2 produced by the interaction of oxygen adatoms and CO admolecules on Pd(110) was studied with angle-resolved thermal desorption. Four peaks of CO 2 formation appeared in the different coverage and temperature range. The desorption flux shows sharp and anisotropic angular distributions.

The coadsorption of nitrogen with carbon monoxide and oxygen on the Ru(001) surface: Local chemical interactions in mixed overlayers

High resolution electron energy loss spectroscopy and thermal desorption mass spectrometry have been employed to investigate the molecular chemisorption of N2 on both disordered and ordered overlayers of atomic oxygen on the Ru(001) surface, as well as the chemisorption of CO on overlayers of N2 on Ru(001). Pertinent results obtained for the adsorption of N2 on the clean Ru(001) surface are also presented for comparison. Disordered oxygen poisons a fraction of the surface to the subsequent adsorption of N2 whereas the N2 that does adsorb is indistinguishable from N2 on clean Ru(001). The fraction of the surface that is poisoned to the adsorption of N2 is approximately twice the fractional surface coverage of disordered oxygen. The p(2×2) overlayer of ordered oxygen adatoms, which is formed at a fractional surface coverage of 0.25, stabilizes the chemisorption of N2 into a new binding state with a heat of adsorption that is approximately 1.5 kcal/mol greater than any one observed for the adsorption of N2 on the clean surface. Coverage measurements indicate that this state results from the stoichiometric addition of one N2 molecule to each unit cell of the p(2×2)–O overlayer. Electron energy loss spectroscopic results suggest that this N2 binding state results from stabilization of the dominant σ donor contribution to the Ru–N2 bond, due to the presence of the electronegative oxygen adatoms of the p(2×2) overlayer. Measurements of the adsorption of CO on saturated overlayers of N2 show that N2 is displaced from the surface by increasing coverages of subsequently adsorbed CO. For low coverages of CO in the presence of N2, the observed value of ν(CO) is lower than observed under any conditions for the adsorption of CO alone on the Ru(001) surface. The N2 admolecules enhance the ability of the surface ruthenium atoms to backdonate electron density into the 2π orbital of coadsorbed CO under these conditions. At coverages of CO in excess of 0.10 monolayer, the results are consistent with CO island formation and segregation of N2 and CO admolecules into different local regions on the surface.

The mechanism of the CO 2 formation on Pt(111) and polycrystalline surfaces at low temperatures

The mechanism of the reaction of oxygen with CO on Pt(111) and polycrystalline surfaces was studied at low temperatures, with reactive thermal desorption and isotope tracer techniques. Two CO 2 formation processes were observed. The first is the interaction between CO and oxygen admolecules, which takes place below 200 K. The second is the reaction of CO admolecules with oxygen adatoms. The desorption flux of the product CO 2 of both processes shows a very sharp angular distribution along the surface normal.