Characterization and methanol electrooxidation studies of Pt(111)/Os surfaces prepared by spontaneous deposition

Catalytic activity of the Pt(111)/Os surface toward methanol electrooxidation was optimized by exploring a wide range of Os coverage. Various methods of surface analyses were used, including electroanalytical, STM, and XPS methods. The Pt(111) surface was decorated with nanosized Os islands by spontaneous deposition, and the Os coverage was controlled by changing the exposure time to the Os-containing electrolyte. The structure of Os deposits on Pt(111) was characterized and quantified by in situ STM and stripping voltammetry. We found that the optimal Os surface coverage of Pt(111) for methanol electrooxidation was 0.7 +/- 0.1 ML, close to 1.0 +/- 0.1 Os packing density. Apparently, the high osmium coverage Pt(111)/Os surface provides more of the necessary oxygen-containing species (e.g., Os-OH) for effective methanol electrooxidation than the Pt(111)/Os surfaces with lower Os coverage (vs e.g., Ru-OH). Supporting evidence for this conjecture comes from the CO electrooxidation data, which show that the onset potential for CO stripping is lowered from 0.53 to 0.45 V when the Os coverage is increased from 0.2 to 0.7 ML. However, the activity of Pt(111)/Os for methanol electrooxidation decreases when the Os coverage is higher than 0.7 +/- 0.1 ML, indicating that Pt sites uncovered by Os are necessary for sustaining significant methanol oxidation rates. Furthermore, osmium is inactive for methanol electrooxidation when the platinum substrate is absent: Os deposits on Au(111), a bulk Os ingot, and thick films of electrodeposited Os on Pt(111), all compare poorly to Pt(111)/Os. We conclude that a bifunctional mechanism applies to the methanol electrooxidation similarly to Pt(111)/Ru, although with fewer available Pt sites. Finally, the potential window for methanol electrooxidation on Pt(111)/Os was observed to shift positively versus Pt(111)/Ru. Because of the difference in the Os and Ru oxophilicity under electrochemical conditions, the Os deposit provides fewer oxygen-containing species, at least below 0.5 V vs RHE. Both higher coverage of Os than Ru and the higher potentials are required to provide a sufficient number of active oxygen-containing species for the effective removal of the site-blocking CO from the catalyst surface when the methanol electrooxidation process occurs.     

Preoxidation of CO on Os-modified Pt(111): a comparison with Ru-modified Pt(111)

The variation in CO adsorption structures during the preoxidation of CO on Os-modified Pt(111) (Pt(111)/Os) was investigated using cyclic voltammetry and electrochemical scanning tunneling microscopy. The spontaneous deposition of Os on Pt(111) resulted in randomly scattered islands with a coverage range of 0.13-0.54. During preoxidation on Pt(111)/Os, a phase transition from (2 × 2)-α to (√19 × √19) via the transient structures of (2 × 2)-β and (1 × 1) took place as on unmodified Pt(111). As the amount of Os increased, however, the transient structures of (2 × 2)-β and (1 × 1) appeared at lower potentials with higher populations. When the population of the transient structures was greater than 50%, an oxidative CO stripping process took place to the structure of (√19 × √19), completing the preoxidation. These observations strongly support the idea that the presence of Os increases the mobility of adsorbed CO by electronic modification of the Pt(111) surface (electronic effect). In addition, the results obtained with Pt(111)/Os were compared with those of Pt(111)/Ru.     

In situ STM study of Au(111)/Os bimetallic surfaces: spontaneous deposition and electrochemical dissolution

We provide an electrochemical and structural characterization by in situ STM of Au(111)/Os electrodes prepared by spontaneous deposition of Os on Au(111). Surfaces with Os coverage values up to the saturation coverage were examined, from 10%. Using comparisons to previous work on Au(111)/Ru, Pt(111)/Ru, and Pt(111)/Os, we find that we may now generalize that Os deposits spontaneously faster than Ru and has a greater tendency to form 3-D structures. Additionally, the Au(111) substrate shows preferential step and near-step decoration in both cases, although it is less pronounced for Os than Ru. We also investigated the incremental dissolution of the Os from Au(111), to better understand electrochemical dissolution processes in general and to better control the Os deposit structure. The application of controlled electrochemical treatments (cyclic voltammetry up to increasingly positive values) significantly increased the dispersion of the Os deposit by generating smaller, more widely spaced islands. Upon voltammetry up to 0.75 V, the Au(111)/Os surface showed evidence of alloying and the formation of 3-D structures suggestive of strong Os-Os (oxidized) species interactions. The CO stripping results show the Au(111)/Os is not particularly effective for this reaction, but such results help to complete the overall picture of NM-NM catalytic combinations. Although the Au(111)/Os system itself is not catalytically active, the electrochemical manipulation of the deposit structure demonstrated here may be applied to other noble metal/noble metal (NM/NM) catalytic substrates to find optimal deposit morphologies.     

Surface structural and chemical characterization of Pt/Ru composite electrodes: a combined study by XPS, STM and IR spectroscopy

Electrochemical Ru deposits on Pt(111) surfaces are investigated by STM; the images of the Ru-modified surfaces show islands of monoatomic height and between 2–5 nm in diameter. The density of islands on the surface depends on the Ru deposition potential (observed by STM and XRSD) and the cyclic voltammograms indicate an increasing Ru coverage for lower deposition potentials. The Ru surface coverage is determined by ex-situ XPS measurements and a linear dependence of the Ru coverage on the deposition potential is demonstrated. IR spectra of a monolayer of adsorbed CO on the Ru-modified Pt(111) surfaces show distinct bands for CO adsorbed on Pt and on Ru. For the integrated band intensity of the CO/Ru vibration a linear dependence on deposition potential is found indicating that lateral dipole interactions between CO adsorbed on Pt and Ru are unimportant and that the CO coverage on the Ru islands is constant for the Ru coverages investigated. The possibility of using adsorbate vibrational bands for the determination of the coverage of deposits is discussed. Received: 24 June 1996 / Revised: 6 December 1996 / Accepted: 12 December 1996     

Decorated Ru/Au(111) and Os/Au(111) surfaces: An in situ STM and electrochemistry study

The structure and reactivity of bimetallic electrodes obtained by spontaneous deposition of Ru and Os on Au(111) single-crystal surfaces are studied. In situ electrochemical STM and cyclic voltammetry are used to characterize a wide range of surface morphologies thus produced. The STM results on Ru/Au(111) demonstrate a pronounced step decoration, while a random distribution of Ru nuclei, quite uniform in size, occurs on terraces. Osmium deposits show a slight preference for deposition on steps, but it also occurs readily on terraces. However, many of the Os islands grow into multilayer heights. The coverage of the Au(111) by the deposited Ru or Os islands for a particular solution concentration depends on the deposition time. Nanostructures of Ru and Os are tested for catalytic behavior and correlated to CO oxidation activity as measured by CO stripping voltammetry. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1385–1392. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Electro-oxidation of carbon monoxide on well-ordered Pt(111)/Sn surface alloys

The electro-oxidation of CO on model platinum-tin alloy catalysts has been studied by ex-situ electrochemical measurements following the preparation of the Pt(111)/Sn(2x2) and Pt(111)/Sn(radical3 x radical3)R30 degrees surfaces. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 V(RHE)/0.15 V(RHE) in H(2)SO(4) electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex-situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 V(RHE) and an oxidized state at potentials above 0.28 V(RHE). Specific adsorption of sulfate anions is not associated with the surface process since there is no evidence from photoemission of sulfate adsorption, and the same surface couple is observed in the HClO(4) electrolyte. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 +/- 0.05 and 0.2 +/- 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites, viz., 0.28 V(RHE). This strong promotional effect is unambiguously associated with the bifunctional mechanism. The Sn-induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 V(RHE). The results are discussed in the light of the requirements for CO-tolerant platinum-based electrodes in hydrogen fuel cell anode catalysts and catalysts for direct methanol electro-oxidation.     

Electrocatalytic oxidation of formic acid and methanol on Pt deposits on Au(111)

This work presents characteristics of Pt deposits on Au(111) obtained by the use of spontaneous deposition and investigated by electrochemical scanning tunneling microscopy (EC-STM). On such prepared and STM characterized Au(111)/Pt surfaces, we studied electrocatalytic oxidation of formic acid and methanol. We show that the first monatomic layer of Pt displays a (square root 3 x square root 3)R30 degrees surface structure, while the second layer is (1 x 1). After prolonged deposition, multilayer Pt deposits are formed selectively on Au(111) surface steps and are 1-20 nm wide and one to five layers thick. On the optimized Au(111)/Pt surface, formic acid oxidation rates are enhanced by a factor of 20 compared to those of pure Pt(111). The (square root 3 x square root 3)R30 degrees-Pt yields very low methanol oxidation rates, but the rates increase significantly with further Pt growth.     

Binding energy of ruthenium submonolayers deposited on a Pt(111) electrode

We investigated the 3d_5/2 core-level binding energy of Ru in Ru nanoislands spontaneously deposited on a Pt(111) electrode [Pt(111)/Ru], and the binding energies of 3d_5/2 iodine and 1s CO adsorbed on Pt(111)/Ru by the use of X-ray photoelectron spectroscopy. Both iodine and CO were used as surface probes of the electronic properties of Pt(111)/Ru. Little difference was found in the binding energy of Ru in Pt(111)/Ru and in Ru(0001). However, the addition of Ru to Pt(111) induces major changes in the core-level binding energies of chemisorbed iodine and CO as referenced to those adsorbed on Ru(0001). We conclude that the iodine 3d_5/2 and CO 1s C core levels experience higher electronic charge on Pt(111)/Ru than on Ru(0001), suggesting a charge transfer from Pt to Ru, or to a Ru-I surface molecule within the deposit. The charge transfer from Pt to Ru is in agreement with the result of previous in situ electrochemical NMR investigations [P.K. Babu, H.S. Kim, A. Wieckowski, E. Oldfield (2003) J. Phys. Chem. B 107:7595] and confirms the general trend of reduction in the density of states of Pt due to alloying with Ru [J. McBreen, S. Mukerjee (1995) J. Electrochem. Soc. 142:3399]. Theoretical calculations are in progress to further interpret the origin of the binding-energy shifts observed in this study.Dedicated to Zbigniew Galus on the occasion of his 70th birthday     

FTIR study of the ethanol electrooxidation on Pt(100) modified by osmium nanodeposits

In the present work, ethanol electrooxidation on a Pt(100) electrode modified by different coverage degrees of osmium nanoislands obtained by spontaneous depositions, was extensively studied employing in situ FTIR spectroscopy. A collection of spectra of the ethanol adsorption and oxidation processes was acquired during the first series of a positive potential step, to determine the intermediate species, as well as the main products formed. The spectroscopic results obtained were correlated with conventional electrochemical results obtained by cyclic voltammetry. It was shown that the catalytic activity of Pt(100) for ethanol oxidation increases significantly after osmium deposition and that the mechanistic pathway for this reaction depends directly on the osmium coverage degree. Thus, for low osmium coverage (theta;( Os) up to 0.15) the formation of CO as an intermediate was favored and hence the full oxidation of adsorbed ethanol to CO(2) was increased. For higher osmium coverages (theta;(Os) up to 0.33), the higher the coverage is, the more the direct ethanol oxidation to acetaldehyde and acetic acid is favored. For osmium coverage degree of 0.40, the catalytic activity of the electrode for ethanol oxidation decreased. On an almost complete osmium layer (theta;(Os) = 0.92) obtained by electrodeposition at 50 mV vs reversible hydrogen electrode, the catalytic activity for ethanol oxidation shows a much lower value.     

A First‐Principle Calculation of Sulfur Oxidation on Metallic Ni(111) and Pt(111), and Bimetallic Ni@Pt(111) and Pt@Ni(111) Surfaces

Sulfur, a pollutant known to poison fuel‐cell electrodes, generally comes from S‐containing species such as hydrogen sulfide (H₂S). The S‐containing species become adsorbed on a metal electrode and leave atomic S strongly bound to the metal surface. This surface sulfur is completely removed typically by oxidation with O₂ into gaseous SO₂. According to our DFT calculations, the oxidation of sulfur at 0.25 ML surface sulfur coverage on pure Pt(111) and Ni(111) metal surfaces is exothermic. The barriers to the formation of SO₂ are 0.41 and 1.07 eV, respectively. Various metals combined to form bimetallic surfaces are reported to tune the catalytic capabilities toward some reactions. Our results show that it is more difficult to remove surface sulfur from a Ni@Pt(111) surface with reaction barrier 1.86 eV for SO₂ formation than from a Pt@Ni(111) surface (0.13 eV). This result is in good agreement with the statement that bimetallic surfaces could demonstrate more or less activity than to pure metal surfaces by comparing electronic and structural effects. Furthermore, by calculating the reaction free energies we found that the sulfur oxidation reaction on the Pt@Ni(111) surface exhibits the best spontaneity of SO₂ desorption at either room temperature or high temperatures.     

The structure, growth and reactivity of electrodeposited Ru/Au(111) surfaces

In order to elucidate electronic effects on the oxidation of CO on small Ru clusters, we investigated this reaction on well defined Ru/Au(111) model systems via parallel in-situ STM studies of the structure and electrochemical deposition of Ru on Au(111) in H₂SO₄ solution and cyclic voltammetry of CO monolayer oxidation on these surfaces. The Ru deposit consists of nanoscale islands, which coalesce with increasing coverage. The Ru saturation coverage depends on the deposition potential, resulting in Ru submonolayer (>0.1 V), (defective) monolayer (≥−0.1 V), and multilayer films (<−0.1 V). At potentials >0.6 V irreversible formation of Ru oxide/hydroxide species is observed, which can be partly reduced in the range 0.4 to 0.0 V. CO stripping commences at ≈0.1 V and occurs over a broad potential range. From the stripping charge a local CO coverage on the Ru monolayer islands of 0.7 ML was estimated. The observed influence of the morphology of the Ru deposit on the CO stripping voltammetry is explained by (local) variations in the CO adsorption energy due to electronic modifications of the Ru film.     

Methanol dehydrogenation and oxidation on Pt(111) in alkaline solutions

Adsorption, dehydrogenation, and oxidation of methanol on Pt(111) in alkaline solutions has been examined from a fundamental mechanistic perspective, focusing on the role of adsorbate-adsorbate interactions and the effect of defects on reactivity. CO has been confirmed as the main poisoning species, affecting the rate of methanol dehydrogenation primarily through repulsive interactions with methanol dehydrogenation intermediates. At direct methanol fuel cell (DMFC)-relevant potentials, methanol oxidation occurs almost entirely through a CO intermediate, and the rate of CO oxidation is the main limiting factor in methanol oxidation. Small Pt island defects greatly enhance CO oxidation, though they are effective only when the CO coverage is 0.20 ML or higher. Large Pt islands enhance CO oxidation as well, but unlike small Pt islands, they also promote methanol dehydrogenation. Perturbations in electronic structure are responsible for the CO oxidation effect of defects, but the role of large Pt islands in promoting methanol dehydrogenation is primarily explained by surface geometric structure.     

Preparation and structural characterization of RuS2 nanoislands on Au(111)

Among all the transition metal sulfides, ruthenium sulfide (RuS2) has been shown to be the most active catalyst for the hydrodesulfuriztion processes. Using X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM), we have found a novel approach for the preparation of RuS2 nanoislands on an Au(111) substrate. Chemical vapor deposition of Ru3(CO)12 leads to metallic Ru nanoclusters on the gold substrate. Although sulfidation has not been observed on extended Ru (0001) surface, Ru nanoclusters react with S2, forming ruthenium sulfide. While the majority of the sulfide is in the form of nanosized clusters that aggregate into clustered islands, a small fraction of the sulfide is seen as flat islands. When Ru3(CO)12 was deposited on a sulfur-modified gold substrate at elevated temperature, flat islands of ruthenium sulfide are formed exclusively. The flat islands are single-layer RuS2 nanocrystals with a (111) surface termination which exhibits an ordered array of sulfur vacancies. On such RuS2 (111) surfaces, excess sulfur is stable at low temperature and induces surface reconstruction, and desorbs at high temperature. The RuS2(111)/Au system provides an excellent model system for ruthenium sulfide catalysts.     

Atomic arrangements inside Ru and Os nanoislands spontaneously deposited on Pt(111)

The atomic arrangements inside Ru and Os nanoislands spontaneously deposited on Pt(111) electrode surface were observed with electrochemical scanning tunneling microscopy. The surperlattice of the pristine Ru nanodeposits is (square root of 3 x square root of 3)R30 degrees -RuO+. Upon reduction, the Ru nanodeposits are compressed to a uniaxially incommensurate (square root of 3 x square root of 2)R30 degrees -Ru structure, which does not change during the following reoxidation. The atomic arrangement inside the pristine Os nanodeposits is an incommensurate (square root of 2 x square root of 2)-OsO+ structure, which does not transform during the subsequent reduction-oxidation cycles. The structures of the Ru and Os nanodeposits are discussed in terms of removal and insertion of oxygen ions depending on electrode potential.     

CO oxidation on nanostructured SnO(x)/Pt(111) surfaces: unique properties of reduced SnO(x)

We have investigated surface CO oxidation on "inverse catalysts" composed of SnO(x) nanostructures supported on Pt(111) using X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (LEISS) and temperature-programmed desorption (TPD). Nanostructures of SnO(x) were prepared by depositing Sn on Pt(111) pre-covered by NO(2) layers at low temperatures. XPS data show that the SnO(x) nanoparticles are highly reduced with Sn(II)O being the dominant oxide species, but the relative concentration of Sn(II) in the SnO(x) nanoparticles decreases with increasing Sn coverage. We find that the most active SnO(x)/Pt(111) surface for CO oxidation has smallest SnO(x) coverage. Increasing the surface coverage of SnO(x) reduces CO oxidation activity and eventually suppresses it altogether. The study suggests that reduced Sn(II)O, rather than Sn(IV)O(2), is responsible for surface CO oxidation. The occurrence of a non-CO oxidation reaction path involving reduced Sn(II)O species at higher SnO(x) coverages accounts for the decreased CO oxidation activity. From these results, we conclude that the efficacy of CO oxidation is strongly dependent on the availability of reduced tin oxide sites at the Pt-SnO(x) interface, as well as unique chemical properties of the SnO(x) nanoparticles.     

Kinetics of CO oxidation on high-concentration phases of atomic oxygen on Pt(111)

Temperature-programmed reaction spectroscopy (TPRS) and direct, isothermal reaction-rate measurements were employed to investigate the oxidation of CO on Pt(111) covered with high concentrations of atomic oxygen. The TPRS results show that oxygen atoms chemisorbed on Pt(111) at coverages just above 0.25 ML (monolayers) are reactive toward coadsorbed CO, producing CO(2) at about 295 K. The uptake of CO on Pt(111) is found to decrease with increasing oxygen coverage beyond 0.25 ML and becomes immeasurable at a surface temperature of 100 K when Pt(111) is partially covered with Pt oxide domains at oxygen coverages above 1.5 ML. The rate of CO oxidation measured as a function of CO beam exposure to the surface exhibits a nearly linear increase toward a maximum for initial oxygen coverages between 0.25 and 0.50 ML and constant surface temperatures between 300 and 500 K. At a fixed CO incident flux, the time required to reach the maximum reaction rate increases as the initial oxygen coverage is increased to 0.50 ML. A time lag prior to the reaction-rate maximum is also observed when Pt oxide domains are present on the surface, but the reaction rate increases more slowly with CO exposure and much longer time lags are observed, indicating that the oxide phase is less reactive toward CO than are chemisorbed oxygen atoms on Pt(111). On the partially oxidized surface, the CO exposure needed to reach the rate maximum increases significantly with increases in both the initial oxygen coverage and the surface temperature. A kinetic model is developed that reproduces the qualitative dependence of the CO oxidation rate on the atomic oxygen coverage and the surface temperature. The model assumes that CO chemisorption and reaction occur only on regions of the surface covered by chemisorbed oxygen atoms and describes the CO chemisorption probability as a decreasing function of the atomic oxygen coverage in the chemisorbed phase. The model also takes into account the migration of oxygen atoms from oxide domains to domains with chemisorbed oxygen atoms. According to the model, the reaction rate initially increases with the CO exposure because the rate of CO chemisorption is enhanced as the coverage of chemisorbed oxygen atoms decreases during reaction. Longer rate delays are predicted for the partially oxidized surface because oxygen migration from the oxide phase maintains high oxygen coverages in the coexisting chemisorbed oxygen phase that hinder CO chemisorption. It is shown that the time evolution of the CO oxidation rate is determined by the relative rates of CO chemisorption and oxygen migration, R(ad) and R(m), respectively, with an increase in the relative rate of oxygen migration acting to inhibit the reaction. We find that the time lag in the reaction rate increases nearly exponentially with the initial oxygen coverage [O](i) (tot) when [O](i) (tot) exceeds a critical value, which is defined as the coverage above which R(ad)R(m) is less than unity at fixed CO incident flux and surface temperature. These results demonstrate that the kinetics for CO oxidation on oxidized Pt(111) is governed by the sensitivity of CO binding and chemisorption on the atomic oxygen coverage and the distribution of surface oxygen phases.     

Sulfate-enhanced catalytic destruction of 1,1,1-trichlorethane over Pt(111)

The catalytic destruction of 1,1,1-trichloroethane (TCA) over model sulfated Pt(111) surfaces has been investigated by fast X-ray photoelectron spectroscopy and mass spectrometry. TCA adsorbs molecularly over SO4 precovered Pt(111) at 100 K, with a saturation coverage of 0.4 monolayer (ML) comparable to that on the bare surface. Surface crowding perturbs both TCA and SO4 species within the mixed adlayer, evidenced by strong, coverage-dependent C 1s and Cl and S 2p core-level shifts. TCA undergoes complete dechlorination above 170 K, accompanied by C-C bond cleavage to form surface CH3, CO, and Cl moieties. These in turn react between 170 and 350 K to evolve gaseous CO2, C2H6, and H2O. Subsequent CH3 dehydrogenation and combustion occurs between 350 and 450 K, again liberating CO2 and water. Combustion is accompanied by SO4 reduction, with the coincident evolution of gas phase SO2 and CO2 suggesting the formation of a CO-SOx surface complex. Reactively formed HCl desorbs in a single state at 400 K. Only trace (<0.06 ML) residual atomic carbon and chlorine remain on the surface by 500 K.     

Reforming of oxygenates for H2 production: correlating reactivity of ethylene glycol and ethanol on Pt(111) and Ni/Pt(111) with surface d-band center

The dehydrogenation and decarbonylation of ethylene glycol and ethanol were studied using temperature programmed desorption (TPD) on Pt(111) and Ni/Pt(111) bimetallic surfaces, as probe reactions for the reforming of oxygenates for the production of H2 for fuel cells. Ethylene glycol reacted via dehydrogenation to form CO and H2, corresponding to the desired reforming reaction, and via total decomposition to produce C(ad), O(ad), and H2. Ethanol reacted by three reaction pathways, dehydrogenation, decarbonylation, and total decomposition, producing CO, H2, CH4, C(ad), and O(ad). Surfaces prepared by deposition of a monolayer of Ni on Pt(111) at 300 K, designated Ni-Pt-Pt(111), displayed increased reforming activity compared to Pt(111), subsurface monolayer Pt-Ni-Pt(111), and thick Ni/Pt(111). Reforming activity was correlated with the d-band center of the surfaces and displayed a linear trend for both ethylene glycol and ethanol, with activity increasing as the surface d-band center moved closer to the Fermi level. This trend was opposite to that previously observed for hydrogenation reactions, where increased activity occurred on subsurface monolayers as the d-band center shifted away from the Fermi level. Extrapolation of the correlation between activity and the surface d-band center of bimetallic systems may provide useful predictions for the selection and rational design of bimetallic catalysts for the reforming of oxygenates.     

NO Chemisorption on Pt(111), Rh/Pt(111), and Pd/Pt(111)

The chemisorption of NO on clean Pt(111), Rh/Pt(111) alloy, and Pd/Pt(111) alloy surfaces has been studied by first principles density functional theory (DFT) computations. It was found that the surface compositions of the surface alloys have very different effects on the adsorption of NO on Rh/Pt(111) versus that on Pd/Pt(111). This is due to the different bond strength between the two metals in each alloy system. A complex d-band center weighting model developed by authors in a previous study for SO2 adsorption is demonstrated to be necessary for quantifying NO adsorption on Pd/Pt(111). A strong linear relationship between the weighted positions of the d states of the surfaces and the molecular NO adsorption energies shows the closer the weighted d-band center is shifted to the Fermi energy level, the stronger the adsorption of NO will be. The consequences of this study for the optimized design of three-way automotive catalysts, (TWC) are also discussed.     

Kinetically Limited CO Adsorption: Spill-Over as a Highly Effective Adsorption Pathway on Bimetallic Surfaces

We demonstrate that the (local) adsorbed carbon monoxide, CO_ad, coverage on the Pt-free areas of bimetallic Pt/Ru(0001) surfaces (a Ru(0001) substrate partly covered by Pt monolayer islands) can be increased to ∼0.80 monolayers (ML), well above the established saturation CO_ad coverage of 0.68 ML, even under ultrahigh vacuum conditions by using spill-over of CO adsorbed on the Pt islands to the Ru areas as an highly effective adsorption channel. The apparent CO_ad saturation coverage of 0.68 ML on pure Ru(0001) is identified as due to kinetic limitations, hindering further uptake from the gas phase, rather than being caused by thermodynamic reasons. This spill-over mechanism is proposed to be a general phenomenon for adsorption on bimetallic surfaces.