CO Saturation Coverage Research Articles

Correlating CO Coverage and CO Electroreduction on Cu via High-Pressure in Situ Spectroscopic and Reactivity Investigations.

The absolute coverage of CO has been a missing piece in the mechanistic puzzle of the CO reduction reaction (CORR) on Cu. For the first time, we revealed the upper bound of the CO coverage under electrocatalytic conditions to be 0.05 monolayer at atmospheric pressure and the saturation CO coverage to be ∼0.25 monolayer by conducting surface enhanced infrared spectroscopy at CO pressures up to 60 barg in a custom-designed spectroelectrochemical cell. CORR activities on Cu were also determined in the same pressure range. Calculated reaction orders of C2+ products with respect to adsorbed CO are substantially less than unity, clearly indicating that the coupling of adsorbed CO is not the rate-determining step leading to multicarbon products. The increase in CO coverage can reduce the C affinity on the Cu surface and favor the selectivity towards oxygenates, especially acetate, over ethylene. Uncommon products, including ethane, glycolaldehyde, and ethylene glycol, were detected in appreciable amounts, likely due to a new C-C coupling mechanism taking place at elevated CO pressures.

Reduced Carbon Monoxide Saturation Coverage on Vicinal Palladium Surfaces: the Importance of the Adsorption Site.

Steps at metal surfaces may influence energetics and kinetics of catalytic reactions in unexpected ways. Here, we report a significant reduction of the CO saturation coverage in Pd vicinal surfaces, which in turn is relevant for the light-off of the CO oxidation reaction. The study is based on a systematic investigation of CO adsorption on vicinal Pd(111) surfaces making use of a curved Pd crystal. A combined X-ray Photoelectron Spectroscopy and DFT analysis allows us to demonstrate that an entire row of atomic sites under Pd steps remains free of CO upon saturation at 300 K, leading to a step-density-dependent reduction of CO coverage that correlates with the observed decrease of the light-off temperature during CO oxidation in vicinal Pd surfaces.

CO adsorption on nanoislands: Ni on Au(111).

The adsorption behavior of CO on Ni islands grown on Au(111) was studied with a combination of temperature programmed desorption, Fourier transform infrared spectroscopy, and surface resistivity measurements. The Au(111) herringbone reconstruction provides a template for the growth of ordered Ni islands, with evidence for the presence of strain and Au atoms within the islands. The islands grow radially until θNi ≈ 0.3 ML, after which subsequent Ni atoms contribute primarily to a second layer. We study saturated CO adsorption at 227 K over a range of Ni island sizes and find layer-dependent adsorption properties. For single-layer islands at low Ni coverage, CO adsorbs primarily in the atop position and desorbs at lower temperatures than on pure Ni, with a saturation CO coverage of about 0.5 CO/Ni. As second layer Ni grows, saturated CO coverages on the Ni approach unity, with higher desorption temperatures, but still with primarily atop CO. Based on previous studies, we propose that in the first Ni layer, ligand effects from the Au substrate and possibly Au in the islands and strain due to the Ni/Au lattice mismatch affect the Ni-CO bonds. CO adsorption behavior on the two-layer islands is qualitatively explained by a decrease in Au nearest neighbors and the presence of a more expanded/corrugated structure.

DFT study on bimetallic Pt/Cu(1 1 1) as efficient catalyst for H2 dissociation

To design a catalyst for the dissociation of H2 with better CO-tolerance performance is very important for proton exchange membrane fuel cells (PEMFCs) towards high efficiency. With slab model, the catalytic properties of overlayer Pt on Cu substrate (Pt/Cu) are analyzed by first-principle calculations. The CO saturation coverage (40%) on Pt2/Cu is found to be lower than that of pure Pt (about 75%). The dissociation barrier from H2 to H is less than 0.4 eV under the saturation coverage of CO. On the basis of kinetics of proton formation, the CO-tolerance ability on double-layer Pt with Cu is found to be greatly improved compared with that on pure Pt. It is expected that Pt overlayer on Cu(1 1 1) is a potential anode material with lower cost for PEMFCs.

Quantitative and Atomic-Scale View of CO-Induced Pt Nanoparticle Surface Reconstruction at Saturation Coverage via DFT Calculations Coupled with in Situ TEM and IR.

Atomic-scale insights into how supported metal nanoparticles catalyze chemical reactions are critical for the optimization of chemical conversion processes. It is well-known that different geometric configurations of surface atoms on supported metal nanoparticles have different catalytic reactivity and that the adsorption of reactive species can cause reconstruction of metal surfaces. Thus, characterizing metallic surface structures under reaction conditions at atomic scale is critical for understanding reactivity. Elucidation of such insights on high surface area oxide supported metal nanoparticles has been limited by less than atomic resolution typically achieved by environmental transmission electron microscopy (TEM) when operated under realistic conditions and a lack of correlated experimental measurements providing quantitative information about the distribution of exposed surface atoms under relevant reaction conditions. We overcome these limitations by correlating density functional theory predictions of adsorbate-induced surface reconstruction visually with atom-resolved imaging by in situ TEM and quantitatively with sample-averaged measurements of surface atom configurations by in situ infrared spectroscopy all at identical saturation adsorbate coverage. This is demonstrated for platinum (Pt) nanoparticle surface reconstruction induced by CO adsorption at saturation coverage and elevated (>400 K) temperature, which is relevant for the CO oxidation reaction under cold-start conditions in the catalytic convertor. Through our correlated approach, it is observed that the truncated octahedron shape adopted by bare Pt nanoparticles undergoes a reversible, facet selective reconstruction due to saturation CO coverage, where {100} facets roughen into vicinal stepped high Miller index facets, while {111} facets remain intact.

Kinetic Monte Carlo Simulation of CO Adsorption on Sulfur-Covered Pd(100)

The use of atomistic Kinetic Monte Carlo method was explored to examine the influence of sulfur poisoning on CO adsorption on Pd(100) surface. The model explicitly incorporates key elementary processes such as CO adsorption and CO desorption including diffusion of surface CO and S species. Relevant energetic and kinetic parameters were derived using information calculated from density functional theory as a starting point. Kinetic Monte Carlo simulation was performed to determine relevant observables such as CO saturation coverage as a function of amount of preadsorbed sulfur and to predict temperature programmed desorption spectra.

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.

Carbon monoxide dissociation to oxidation surface reaction pathway shift on Mo(110) upon alloying with boron

Comparative study of coadsorption and reaction of carbon monoxide and oxygen on bare Mo(110) and borided Mo(110) has been done by means of temperature programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The B–Mo(110) alloy has been in-situ prepared by depositing of 2 monolayers of atomic boron on Mo(110) and subsequent annealing at 1300 K. The dominant surface reaction on bare Mo(110) is CO dissociation, with no CO2 formation, whereas the B–Mo(110) system exhibits rather high activity for carbon monoxide oxidation. Reaction is efficient at lower than saturation CO coverage, pointing at a key role of free surface sites. It was shown that the electronic state of both carbon monoxide and oxygen adsorbed on B-Mo(110) is notably different from that on bare Mo(110), which is presumably the cause of the observed (CO + O2) reaction pathway shift upon B–Mo(110) alloying.

Structural, electronic and adsorption properties of V–Rh(111) subsurface alloy

V–Rh(111) bimetallic systems were prepared by deposition of 0.4 and 2.5ML of vanadium on Rh(111) surface at 823K and by annealing up to 1023K. Subsurface alloys of similar composition and electronic structure were formed. On the latter one, (2×2) surface reconstruction was observed. Electronic structure of the V–Rh(111) subsurface alloys was studied by photoelectron spectroscopy utilizing synchrotron radiation. Atomic structure was studied by means of LEED and X-ray photoelectron diffraction. Chemisorption properties were probed by CO and O2 adsorption. The topmost atomic layer with (2×2) reconstruction consisted exclusively of Rh atoms. The obtained results indicate formation of alloy with Rh3V composition and L12-type structure. Changes observed in the electronic structure were correlated with lower reactivity towards CO adsorption. The photoemission data indicate that the CO saturation coverage at 300K is reduced to less than half on the alloy surface as compared to clean Rh(111). Oxygen adsorbed on the alloy surface strongly interacted with the subsurface vanadium and induced formation of a surface vanadium oxide.

Enhanced CO monolayer electro-oxidation reaction on sulfide-adsorbed Pt nanoparticles: A combined electrochemical and in situ ATR-SEIRAS spectroscopic study

The influence of adsorbed sulfide (S2−ads) on CO monolayer electro-oxidation reaction (CMOR with M for monolayer to emphasize that oxidation always starts with a saturated CO coverage) on carbon-supported Pt nanoparticles (Pt/C NPs) was investigated using electrochemical and in situ ATR-SEIRAS (attenuated total reflection-surface enhanced IR absorption spectroscopy) techniques. An enhanced kinetics of CMOR on the S2−ads-Pt/C was observed by potentiodynamic CO stripping and potentiostatic CO oxidation analyses. Electrochemical impedance measurements revealed a weakened bonding of oxygen-containing species with the S2−ads-Pt/C surface as compared with the clean Pt/C surface, which we believe was the main reason for the observed enhanced CMOR by the S2−ads. The in situ SEIRAS data showed the existence of both linear- and bridge-bonded CO (COL and COB respectively) in all samples and that the sulfide adsorption increased substantially the fraction of the former on the available Pt sites and facilitated the conversion between them. Our results also indicate that the frequently observed pre-CMOR current arises most likely from oxidation of weakly-bonded CO rather than from reactions taking place at defect sites.

CO adsorption and desorption on size-selected Pdn/TiO2(110) model catalysts: Size dependence of binding sites and energies, and support-mediated adsorption

The nature of CO adsorption on Pd(n)/TiO(2)(110) (n = 1, 2, 7, 20) has been examined using temperature-programmed desorption (TPD), temperature-dependent helium ion scattering (TD-ISS), and X-ray photoelectron spectroscopy (XPS). All samples contain the same number of Pd atoms (0.10 ML-equivalent) deposited as different size clusters. The TPD and TD-ISS show that CO binds in two types of sites associated with the Pd clusters. The most stable sites are on top of the Pd clusters ("on-top" sites), however, there are also less stable sites, in which CO is bound in association with, but not on top of the Pd ("peripheral" sites). For saturation CO coverage over a fixed atomic concentration of Pd (present in the form of Pd(n) clusters of varying size), the population of CO in peripheral sites decreases with increasing cluster size, while the on-top site population is size-independent. This is consistent with what geometric considerations would predict for the density of the two types of sites, provided the clusters adsorb predominantly as 2D islands, which ISS results suggest to be the case. The XPS analysis indicates that CO-Pd binding is dominated by π-backbonding to the Pd(n) clusters. The results also show evidence for efficient support-mediated adsorption (reverse-spillover) of CO initially impinging on TiO(2) to binding sites associated with the Pd clusters.

Adsorption Dynamics of CO on Silica Supported CuOx Clusters: Utilizing Electron Beam Lithography To Study Methanol Synthesis Model Systems

Electron beam lithography was used to nanofabricate 12 and 63 nm Cu clusters supported on silica (model nanoarray catalysts). The Cu clusters could reversibly be oxidized and reduced at ultrahigh vacuum conditions. The chemical activity of these clusters was probed by Auger and X-ray photoelectron spectroscopy, thermal desorption spectroscopy, and molecular beam scattering. CO was used as the probe molecule. Scanning electron microscopy was used to obtain cluster size distributions. CO adsorption is molecular and nonactivated. CO binding energies on the oxidic clusters are larger than for the metallic clusters. Adsorption transients, recorded as a function of surface temperature and CO impact energy, are consistent with precursor models, as expected from the so-called capture zone model (CZM). Cluster size effects are evident, as predicted by the CZM. However, unexpectedly, the CO saturation coverage does not simply scale with the cluster area but depends also on the rim length of the deposits. Metallic Cu clusters are more reactive than oxidic clusters, in part not only due to the cluster size effect but apparently also because of the electronic effect.

Preparation of Core-Shell SiO<sub>2</sub>/Pt Particles via Layer-by-Layer Assembly and Their Electrooxidation and In-situ Electrochemical FTIR Spectra for CO

Large-scale core-shell SiO2/Pt particles were synthesized via layer-by-layer assembly.The platinum shell was found to be about 26 nm thick and consisted of Pt nanoparticle aggregates.Cyclic voltammetry (CV) was employed to evaluate the electrocatalytic activity of the as-prepared core-shell SiO2/Pt particles for CO detection.Compared with the bulk Pt catalyst, the main oxidation potential was more negative at about 0.49 V (vs SCE).Moreover, CO adsorption behavior was also examined using in-situ electrochemical Fourier transform infrared (FTIR) spectrum.Interestingly, the FTIR spectra showed inverted IR bands of linearly bonded CO and each IR band was split into two bands with an interval of ca 14 cm-1, which was difficult to discern at saturation coverage of CO on the Pt metal surface.Those anomalous phenomena can most probably be attributed to the structural effects of core-shell SiO2/Pt particles.

Cluster size effects on sintering, CO adsorption, and implantation in Ir/SiO2

A series of planar model catalysts have been prepared via deposition of Ir(n) (+) on thermally grown amorphous SiO(2)/Si(100) and ion scattering spectroscopy was used to probe surface structure as a function of cluster size, impact energy, and surface temperature. Deposition of Ir(2) or Ir(10) at low energies and room temperature results in stable clusters forming one- or two-dimensional single layer islands on the oxide surface. Heating the samples to 750 K leads to agglomeration, forming multilayer structures on the surface. Ir(1) deposited under similar conditions sinters into large clusters at room temperature. Deposition at 110 K at least partially stabilizes the Ir atoms with respect to diffusion and sintering. At higher deposition energies, partial implantation into the surface is observed, but this appears to be insufficient to stabilize the clusters against sintering at elevated temperature. At low temperatures, substrate-mediated adsorption of CO is found to be highly efficient, leading to near saturation coverages of CO bound atop the Ir(n) clusters. The CO can be removed by careful He(+) sputtering. The deposition/binding behavior of Ir(n) on SiO(2) is quite different from Ir(n)/TiO(2)(110), for which the clusters bind in three-dimensional morphology, starting at Ir(5). That system also shows substrate-mediated adsorption of CO, but the CO preferentially binds at the periphery of the clusters rather than on top.

Coverage-Dependent CO Adsorption Energy from First-Principles Calculations

CO saturation coverage on Pt(111) is crucially important in diesel oxidation catalysis. We systematically studied coverage-dependent CO adsorption on the Pt(111) surface using density functional theory (DFT) calculations and classical Monte-Carlo (MC) simulations. The zero-coverage-limit CO-adsorption energies at different binding sites are almost degenerate at the revised Perdew−Burke−Erzernhof functional (RPBE) level. As the CO coverage increases, strong through-space repulsion and substrate-mediated metal-sharing effects tend to dominate the stability of adsorbates and alter their binding preferences. The calculated differential binding energy curve and adsorption patterns compare well with experiments.

A photoemission study of carbon monoxide interaction with the Ga–Pd(110) system

We have investigated the Ga–Pd(110) system by means of soft X-ray photoelectron spectroscopy and low energy electron diffraction, and tested its chemical properties by co-adsorbing CO. On adsorption on Pd, the Ga 3d and 2p 3/2 core levels showed small shifts with respect to metallic Ga, and the density of Pd 4d states below the Fermi level was reduced. The Ga–Pd surface alloy exhibited a (1 × 2) structure. The presence of gallium reduced the CO saturation coverage and bond strength in comparison with clean Pd(110).

Kinetic studies of adsorbed CO electrochemical oxidation on Pt(335) at full and sub-saturation coverages

Electrochemical measurements were performed to characterize the kinetics of adsorbed CO oxidation on the surface of the stepped Pt(s)-[4(111)x(100)][triple bond, length half m-dash]Pt(335) single crystal electrode. For CO adsorbed to full coverage at 0.1 V (versus the reversible hydrogen electrode, RHE) in 0.5 M H(2)SO(4) at ambient temperature (23 degrees C), oxidation of the layer gave 7.6 x 10(14) +/- 0.3 CO/cm(2) as the saturation CO coverage, just below the average value reported for CO on Pt(335) in ultra high vacuum (8.3 x 10(14) +/- 0.6 CO/cm(2)). In potential step measurements carried out between 0.75 and 0.9 V, the peak region in the current-time transient was consistent with the surface reaction between adsorbed CO and adsorbed oxide as rate limiting. Plotting the log of the rate constant for the surface reaction versus potential gave a Tafel slope of 79 mV per decade, consistent with responses for CO electrochemical oxidation on structurally related stepped Pt electrodes. For CO coverages below saturation, current-time transients were more stable in 0.05 M H(2)SO(4) than in the higher concentration electrolyte. Numerically solving the rate equations to the Langmuir-Hinshelwood model of adsorbed CO electrochemical oxidation reproduced the main features in current-time transients measured at 0.7 V in 0.05 M H(2)SO(4) for sub-saturation CO coverages. The results provide new insights into CO oxidation on Pt at sub-saturation coverage and confirm that anions play a role in CO surface chemistry.

Bimetallic PtSn/C catalysts promoted by ceria: Application in the nonoxidative dehydrogenation of isobutane

A series of bimetallic platinum–tin catalysts supported on CeO 2/C were prepared and tested in the nonoxidative catalytic dehydrogenation of isobutane. Pt–Sn/CeO 2/C catalysts with varying Sn content (Sn/Pt = 0, 0.25, 0.5, and 0.75) were prepared using a co-impregnation method and chlorinated metal precursors. They were characterized by XPS and by CO and ethylene adsorption microcalorimetry at room temperature after in situ reduction treatments. It was found that the presence of cerium in bimetallic catalysts inhibited reduction of tin species, whereas tin facilitated reduction of cerium(IV) to cerium(III). Reduction at 773 K caused a strong decrease in CO saturation coverage for all catalysts studied. For the bimetallic catalysts, a strong interaction between tin and platinum produced significant changes in the initial heat of CO adsorption. The presence of cerium caused a strong decrease in the heat of ethylene adsorption on platinum in catalysts reduced at high temperature. The monometallic Pt catalyst undergoes a fast deactivation with time on stream in the isobutane dehydrogenation reaction. The addition of tin strongly suppresses the deactivation process. It was found that cerium-containing catalysts showed higher isobutene yields compared with their corresponding cerium-free counterparts. The bimetallic catalyst with Sn/Pt = 0.5 showed the best isobutene yield, and it was studied further. This optimized catalyst demonstrated good deactivation behaviour (comparable with the best catalysts reported in the literature). Furthermore, oxidation treatment at a mild temperature (573 K) allowed recovery of nearly all of its catalytic properties.

Fast methanol oxidation on polycrystalline Pt

Fast cyclic voltammetry and a combined chronoamperometry/cyclic voltammetry pulse-sweep technique are used to study methanol oxidation on platinum in sulphuric acid. Measurement of suppression of hydrogen desorption charges simultaneously with charges for stripping of the CO poison allows for accurate baseline correction of the CO stripping peaks. Saturation coverages of CO formed from methanol, formic acid and dissolved CO are not all the same and differ in their potential dependence. Only dissociative adsorption of methanol to give adsorbed CO and subsequent oxidation of CO to CO 2 occurs during continuous cycling, but in the pulse-sweep method evidence was found for the existence of a parallel pathway occurring independently of adsorbed CO production, provided there is some surface not covered by CO or oxide.

A site-selective in situ study of CO adsorption and desorption on Pt(355)

Using time-dependent high-resolution x-ray photoelectron spectroscopy at BESSY II, the adsorption and desorption processes of CO on stepped Pt(355) = Pt[5(111) x (111)] were investigated. From a quantitative analysis of C 1s data, the distribution of CO on the various adsorption sites can be determined continuously during adsorption and desorption. These unique data show that the terrace sites are only occupied when the step sites are almost saturated, even at temperatures as low as 130 K. The coverage-dependent occupation of on-top and bridge adsorption sites on the (111) terraces of Pt(355) is found to differ from that on Pt(111), which is attributed to the finite width of the terraces and changes in adsorbate-adsorbate interactions. In particular, no long-range order of the adsorbate layer could be observed by low-energy electron diffraction. Further details are derived from sticking coefficient measurements using the method devised by King and Wells [Proc. R. Soc. London, Ser. A 339, 245 (1974)] and temperature-programmed desorption. The CO saturation coverage is found to be slightly smaller on the stepped surface as compared to that on Pt(111). The initial sticking coefficient has the same high value of 0.91 for both surfaces.