Surface Diffusion Of CO Research Articles

Assessing the Intrinsic Activity of Pt‐Group Electrocatalysts for Carbon Monoxide Oxidation: Best Practices and Benchmarking Parameters

AbstractPt‐group elements are the state‐of‐the‐art electrocatalysts for various fuel cells. However, their CO‐poising is a critical hitch for large‐scale applications, so the researchers are exerting huge efforts to solve this issue. The exponentially increasing attention in this field pressures the researchers to publish their findings quickly, leading to unavoidable flawed evaluation parameters for the intrinsic activity of electrocatalysts. The CO oxidation (COOxid) is highly sensitive to various factors. Thus, it is urgent to afford a deeper understanding of the inherent COOxid activity of state‐of‐the‐art electrocatalysts and adopt accurate guidelines for researchers to test, optimize, and compare their electrocatalysts. This review provides exactitude in the evaluation and precise assessment of the key descriptors related to electrocatalysts (i. e., effect of both size, shape, and support) and CO oxidation (i. e., effect of electrolyte, working electrode, and CO surface diffusion). This is besides the fundamental aspects (i. e., COOxid Process, mechanism, measurements, calculations, thermodynamics, and kinetics). Various experimental results from our group and others besides in‐situ analysis were provided to support our deep discussion. Finally, we provide a brief synopsis of the relevant milestones of the up‐to‐date challenges and perspectives.

Vibrational response and motion of carbon monoxide on Cu(100) driven by femtosecond laser pulses: Molecular dynamics with electronic friction

Carbon monoxide on copper surfaces continues to be a fascinating, rich microlab for many questions evolving in surface science. Recently, hot-electron mediated, femtosecond-laser pulse induced dynamics of CO molecules on Cu(100) were the focus of experiments [Inoue et al., Phys. Rev. Lett. 117, 186101 (2016)] and theory [Novko et al., Phys. Rev. Lett. 122, 016806 (2019)], unraveling details of the vibrational nonequilibrium dynamics on ultrashort (subpicoseconds) timescales. In the present work, full-dimensional time-resolved hot-electron driven dynamics are studied by molecular dynamics with electronic friction (MDEF). Dissipation is included by a friction term in a Langevin equation which describes the coupling of molecular degrees of freedom to electron-hole pairs in the copper surface, calculated from gradient-corrected density functional theory (DFT) via a local density friction approximation (LDFA). Relaxation due to surface phonons is included by a generalized Langevin oscillator model. The hot-electron induced excitation is described via a time-dependent electronic temperature, the latter derived from an improved two-temperature model. Our parameter-free simulations on a precomputed potential energy surface allow for excellent statistics, and the observed trends are confirmed by on-the-fly ab initio molecular dynamics with electronic friction (AIMDEF) calculations. By computing time-resolved frequency maps for selected molecular vibrations, instantaneous frequencies, probability distributions, and correlation functions, we gain microscopic insight into hot-electron driven dynamics and we can relate the time evolution of vibrational internal CO stretch-mode frequencies to measured data, notably an observed redshift. Quantitatively, the latter is found to be larger in MDEF than in experiment and possible reasons are discussed for this observation. In our model, in addition we observe the excitation and time evolution of large-amplitude low-frequency modes, lateral CO surface diffusion, and molecular desorption. Effects of surface atom motion and of the laser fluence are also discussed.

Evolution of adsorbed CO on Pt and Pt/Au surface

In this work, we studied in detail one of the possible limiting-steps during CO oxidation at Pt and Pt/Au electrodes, namely CO adsorption and diffusion at the electrode's surface. With this aim, we used an electroless method for Pt surface modification, which entails the spontaneous deposition of gold at Pt electrodes. We also use a home-made fast-response (four electrodes) flow cell that allows an accurate control of the electrode exposition time to CO (tad) and of the time interval (tw) before stripping of the adsorbed CO (COad). Thus two key experimental parameters of the electro-oxidation process can be independently modified and the results evaluated. Our results are consistent with a mechanism in which CO is initially adsorbed onto active sites and then moves away by surface diffusion. As a result, the subsequent oxidative stripping elimination of CO starts with COad far away from the most active sites. The experiments carried out for Pt/Au electrodes with different fraction of electrode area covered by CO (θCO) demonstrate that Au acts as a barrier, hindering the surface diffusion of CO from/to the active sites, giving raise to the multiple pecks observed when tw is low.

Elementary reaction modeling of reversible CO/CO2 electrochemical conversion on patterned nickel electrodes

CO/CO2 are the major gas reactant/product in the fuel electrode of reversible solid oxide cells (RSOC). This study proposes a two-charge-transfer-step mechanism to describe the reaction and transfer processes of CO-CO2 electrochemical conversion on a patterned Ni electrode of RSOC. An elementary reaction model is developed to couple two charge transfer reactions, C(Ni)+O2−(YSZ) ↔ CO(Ni)+(YSZ) +2e− and CO(Ni)+O2−(YSZ) ↔ CO2(Ni)+(YSZ)+2e−, with adsorption/desorption, surface chemical reactions and surface diffusion. This model well validates in both solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) modes by the experimental data from a patterned Ni electrode with 10 μm stripe width at different pCO (0–0.25 atm), pCO2 (0–0.35 atm) and operating temperature (600-700 °C). This model indicates SOEC mode is dominated by charge transfer step C(Ni)+O2−(YSZ)↔CO(Ni)+(YSZ) +2e−, while SOFC mode by CO(Ni)+ O2−(YSZ)↔CO2(Ni)+(YSZ)+2e− on the patterned Ni electrode. The sensitivity analysis shows charge transfer step is the major rate-determining step for RSOC, besides, surface diffusion of CO and CO2 as well as CO2 adsorption also plays a significant role in the electrochemical reaction of SOEC while surface diffusion of CO and CO2 desorption could be co-limiting in SOFC.

3-Aminopropyltriethoxysilane functionalized inorganic membranes for high temperature CO 2/N 2 separation

High temperature inorganic membranes may play an important role in the development of economical processes for pre-combustion and/or post-combustion CO 2 capture. The CO 2/N 2 selectivity of mesoporous silica membranes is enhanced by surface modification using APTS (3-aminopropyl-triethoxy silane). Unmodified silica membranes exhibit Knudsen diffusion behavior for most gases but also have some contribution from surface diffusion of heavier or interacting gases like CO 2, CH 4, etc. Gas separation experiments were performed on the modified membranes for both pure gas and mixed gases for a range of temperatures and feed gas compositions. Mixed gas separation factors as high as 10 for CO 2 over N 2 were observed at 393 K and CO 2 partial pressures of 15 kPa; whereas for pure gases (CO 2 partial pressure of 303 kPa), no separation was seen. The hypothesized transport mechanism is the reaction of CO 2 with surface amine groups to form a carbamate species and subsequent surface “hopping” of carbon dioxide. The CO 2 binding at ambient conditions is sufficiently strong to greatly inhibit the surface diffusion of CO 2, however, as the temperature increases, the CO 2 permeance increases and selective transport of CO 2 is observed. The permeance of CO 2 is highly non-linear, and increases with decreasing CO 2 partial pressure in the feed gas. NMR studies performed on the bare support and modified membrane shows the support has been modified by APTS. Pore size analysis on the unmodified and modified silica substrates show that the pore diameter of modified substrates are lower than that of the unmodified substrate indicating that the amine silanes are anchored on the pore walls of the substrate. The NMR measurement for an APTS/silica membrane with adsorbed CO 2 shows the presence of a carbamate species and thus supports the hypothesized reaction mechanism or facilitated solid-state CO 2 transport. The binding energy for CO 2 adsorption is 15.5 kcal/mol and activation energy for CO 2 diffusion/hopping from one amine group to another of APTS is 7.2 kcal/mol as computed using ab initio Density Functional Theory (DFT).

CO surface diffusion on platinum fuel cell catalysts by electrochemical NMR

We report on surface CO diffusion processes in relation to properties of nanoparticle Pt and Pt/Ru fuel cell catalysts. The CO ad diffusion was studied by the use of 13C electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy. Measurements were carried out in the temperature range 253–293 K, where the solution side of the nanoparticle–electrolyte interface is liquid, in contrast to previous measurements, in ice. We offer a concerted view of the effect of particle size and surface coverage on CO ad diffusion, and find that both are important. We also found that the diffusion parameters were influenced by the variations in the distribution of chemisorption energies on particles of different sizes, and by the CO–CO lateral interactions. On all Pt nanoparticle surfaces investigated, we conclude that CO surface diffusion is too fast to be considered as the rate-limiting factor in methanol reactivity. The addition of Ru to Pt increases the surface diffusion rates of CO, and there is a direct correlation between the Fermi level local density of states ( E f-LDOS) of the 2π* molecular orbital of adsorbed CO and the activation energy for surface diffusion. These results are of interest since they improve our knowledge of surface dynamics of molecules at electrochemical interfaces, and may help to formulate better models for the electrooxidation of adsorbed CO on nanoparticle surfaces.

Stripping voltammetry of carbon monoxide oxidation on stepped platinum single-crystal electrodes in alkaline solution

The electrochemical oxidation of a CO adlayer on Pt[n(111)x(111)] electrodes, with n = 30, 10, and 5, Pt(111), Pt(110) as well as a Pt(553) electrode (with steps of (100) orientation) in alkaline solution (0.1 M NaOH) has been studied using stripping voltammetry. On these electrodes, it is possible to distinguish CO oxidation at four different active oxidation sites on the surface, i.e. sites with (111), (110) and (100) orientation, and kink sites. The least active site for CO oxidation is the (111) terrace site. Steps sites are more active than the (111) terrace sites, the (110) site oxidizing CO at lower potential than the (100) site. The CO oxidation feature with the lowest overpotential (oxidation potential as low as 0.35 V vs. RHE) was ascribed to oxidation of CO at kink sites. The amount of CO oxidized at the active step or kink sites vs. the amount of CO oxidized at the (111) terrace sites depends on the concentration of the active sites and the time given for the terrace-bound CO to reach the active site. By performing CO stripping on the stepped surfaces at different scan rates, the role of CO surface diffusion is probed. The possible role of electronic effects in explaining the unusual activity and dynamics of CO adlayer oxidation in alkaline solution is discussed.

Coverage Dependence of CO Surface Diffusion on Pt Nanoparticles: An EC-NMR Study

We have studied the effects of CO surface coverage on the diffusion rates of CO adsorbed on commercial Pt-black in sulfuric acid media by using 13C electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy in the temperature range 253−293 K. The temperature range chosen for these measurements was such that the electrolyte is in a liquid-like and liquid environment. For CO coverage between θ = 1.0 and 0.36, the CO diffusion coefficients (DCO) follow a typical Arrhenius behavior and both the activation energies (Ed) as well as the pre-exponential factors ( ) show CO coverage dependence. For partially CO covered samples, Ed decreases linearly with increasing CO coverage, indicating that the repulsive CO−CO interactions exert a stronger influence on the coverage dependence of the activation energy than does the nature of the CO adlayer structure. On the other hand, shows an exponential decrease with increasing CO coverage, consistent with the free site hopping model [Gomer, R. Rep. Prog. Phys. 1990, 53, 917] as the major mechanism for surface diffusion of CO at partial coverages, unlike the situation found with a fully CO covered surface [Kobayashi et al., J. Am. Chem. Soc., 2005, 127, 14164]. Overall, these results are of interest since they improve our understanding of the surface dynamics of molecules at electrochemical interfaces, and may help facilitate better control of fuel cell reactions in which the presence of surface CO plays a crucial role in controlling electrocatalytic reaction rates.

Novel dynamic effects in electrocatalysis of methanol oxidation on supported nanoporous TiO 2 bimetallic nanocatalysts

New dynamic aspects of the catalysis of methanol oxidation reaction (MOR) have been studied using quantum mechanical calculations applied to the support–catalyst cluster interactions and surface diffusivity of adsorbed intermediates. For very small catalyst–support clusters, we have found a strong enhancement of the ligand effect for bimetallic catalysts of the type Pt n M m attributed to the decreased local density of states near the Fermi level of Pt atoms neighboring the additive metal atom M. This enhancement results in a decreased barrier for surface diffusion of adsorbed CO ad through the cooperative diffusion mechanism, based on structural relaxation of the catalyst–support cluster, proposed in this work. The strong ligand effect dominates over the Schwoebel potential and trapping well effects, being responsible for accumulation of poisoning intermediates at step sites on the catalyst surface and gradual decrease of catalytic activity with decreasing size of catalyst nanoparticles. The lattice relaxation and strong ligand effects in small catalyst–support clusters lead to lower adsorption energy for CO ad and thus, to higher reactivity and mobility of reactants and intermediates. The experimental investigations included submonolayer films of bi-functional catalysts (PtRu, PtFe) deposited on novel nanostructured supporting materials, designed with the goal of achieving high variability of their electronic and chemical properties to influence the catalytic activity of sub-monolayer catalyst. The mesoscopic TiO 2 supporting film formation was investigated using EQCN, pulse voltammetric and AFM techniques. The conditions for the formation of monodispersed TiO 2 nanoparticles with regular nanopores (nanotubes), 20–80 nm in diameter, were described. It follows from EQCN and voltammetric measurements and AFM image analysis that the nanopores are formed by a dissolution-precipitation mechanism. The catalysts, Pt and PtRu, deposited on supporting nanoporous TiO 2− x films, were used to study MOR. A lower poisoning effect for cluster PtRu on a TiO 2− x support film than that for unsupported PtRu or bare Pt catalysts has been observed. These effects have been attributed to differences in CO ad binding energy and lowering of activation energy for surface mobility leading to a more facile 2D diffusion of CO ad from Pt sites to Ru and the supporting TiO 2− x . The substrate–catalyst interactions were further investigated using quantum mechanical calculations performed for a model TiO 2 nano-ring (representing an orifice of a TiO 2− x nanotube studied experimentally) with adlayers of Pt, Ru and Fe catalysts. We have found unusually strong electron delocalization effects for Pt 2Fe 2 clusters on (TiO 2) 4 as compared to (TiO 2− x ) 4Pt 2Ru 2. We have also analyzed various states in surface diffusion of CO ad on bimetal clusters supported on (TiO 2) n and observed considerable dynamic widening of metal-to-metal atom distances induced by CO adsorption (up to 9% for Pt–Pt distance and up to 15% for Fe–Fe distance). We propose that this new dynamic effect leading to cooperative surface diffusion may be further explored in designing novel nanoparticle catalysts.

CO oxidation on stepped single crystal electrodes: A dynamic Monte Carlo study

Abstract We investigate a simple model for the mechanism of the electrooxidation of CO on stepped electrodes by dynamic Monte Carlo simulations. The model is studied in the limits of no and fast CO surface diffusion. In the absence of CO diffusion, the reaction initiates at step sites by adsorption of OH on the steps and reaction with a neighboring CO molecule, resulting in a pre-peak, followed by growth onto the terraces by OH adsorption on the terraces reacting with neighboring terrace adsorbed CO, resulting in a main peak. The presence of these two peaks in both the simulated cyclic voltammetry and chronoamperometry is a direct consequence of the low CO surface mobility, and resembles the experimental voltammetric and chronoamperometric profiles on stepped Rh[n(1 1 1) × (1 1 1)] electrodes [T.H.M. Housmans, J.M. Feliu, M.T.M. Koper, J. Electroanal. Chem. 572 (2004) 79, T.H.M. Housmans, M.T.M. Koper, J. Electroanal. Chem. 575 (2005) 39, T.H.M. Housmans, M.T.M. Koper, Electrochem. Commun. 7 (2005) 581]. The shape and position of the pre-peak are determined largely by a reaction in one dimension at the steps. Initiation of the reaction at empty sites results in the formation of two reaction fronts proceeding to the left and right of the initiation point, leading to a strong initial current growth followed by a gradual decay as multiple “shrinking rows” extinguish one another. After stripping of the step and the first row of CO molecules adsorbed on the terraces, the reaction grows over the terraces by slower adsorption of OH. For high CO surface mobility the simulated voltammograms and transients resemble the experimental results on stepped rhodium surfaces in perchloric acid and stepped Pt electrodes in sulfuric acid.

CO monolayer oxidation at Pt nanoparticles supported on glassy carbon electrodes

CO monolayer oxidation on glassy carbon supported 1–2 nm Pt nanoparticles is studied using potential sweep and potential step methods. The CO stripping peak on the nanoparticles is significantly shifted to positive potentials vs. the corresponding feature at bulk polycrystalline Pt. Current transients at nanoparticulate electrodes are highly asymmetric with a steep rise, maximum at θCO≈0.8–0.9, and a slow decay following t−1/2. The experimental results are compared to the theoretical models of adsorbed CO oxidation described in the literature. A tentative model is suggested to account for the experimental observations, which comprises spatially confined formation of oxygen containing species at active sites, and slow diffusion of CO molecules to the active sites, where they are oxidized. The upper limit of the CO surface diffusion coefficient at Pt nanoparticles is estimated as approximately 4×10−15 cm2 s−1.

Growth and properties of small Co islands on a strained Pt surface

The effect of strain on the surface diffusion of Co on a Pt surface is studied using first-principles density functional theory. We employed the projector augmented wave method in the 7-layer slab geometry. Co tends to occupy FCC sites except for surfaces under large tensile strain where HCP sites are energetically favorable. We find that energy barrier for single atom diffusion increases linearly with strain. The total energy of adatoms decreases with increase of strain. The magnetization of the Co adatom is around 2 μ B. Pt atoms are strongly polarized making the total magnetic moment per Co atom at FCC sites 3.6 μ B. The magnetic moment of the system is sensitive to the adatom position and the strain. For the saddle point, for example, the magnetization changes from 3.1 μ B per Co atom for an unstrained lattice to 3.8 μ B for a 3% compressed lattice. The results suggest that both FCC and HCP islands formation is possible on the strained Pt(1 1 1) surface. Exchange coupling between adatoms is analyzed using Green's function method. There is a strong exchange interaction between Co moments in the small clusters on the surface, suggesting that they should behave like giant moments.

Dynamics of CO at the solid/liquid interface studied by modeling and simulation of CO electro-oxidation on Pt and PtRu electrodes.

We consider theoretical models for CO monolayer oxidation on stepped Pt single-crystal electrodes and Ru-modified Pt(111) electrodes. For both systems, our aim is to assess the importance of CO surface diffusion in reproducing the experimental chronoamperometry or voltammetry. By comparing the simulations with the experimental chronoamperometric transients for CO oxidation on a series of stepped Pt surfaces, it was concluded that mixing of CO on the Pt(111) terrace is good, implying rapid diffusion (N. P. Lebedeva, M. T. M. Koper, J. M. Feliu and R. A. van Santen, J. Phys. Chem. B, submitted). We discuss here a more detailed model in which the CO adsorbed on steps is converted into CO adsorbed on terraces as the oxygen-containing species occupy the steps (as observed experimentally on stepped Pt in UHV), followed by a subsequent oxidation of the latter, to reproduce the observed chronoamperometry on stepped surfaces with a higher step density. On Ru-modified Pt(111), the experimentally observed splitting of the CO stripping voltammetry into two stripping peaks, may suggest a slow diffusion of CO on Pt(111). This apparent contradiction with the conclusions of the experiments on stepped surfaces, is resolved by assuming a weaker CO binding to a Pt atom which has Ru neighbors than to "bulk" Pt(111), in agreement with recent quantum-chemical calculations. This makes the effective diffusion from the uncovered Pt(111) surface to the perimeter of the Ru islands, which are considered to be the active sites in CO oxidation electrocatalysis on PtRu surfaces, very slow. Different models for the reaction are considered, and discussed in terms of their ability to explain experimental observations.

The role of adsorbate-layer nonuniformities in catalytic reactor design: multiscale simulations for CO oxidation on Pt

Monte Carlo (MC) simulations under ultra high vacuum (UHV) conditions and multiscale models at atmospheric pressure are compared to the corresponding continuum-based models in order to study the effect of species spatial inhomogeneities on reactor design. These models are applied to the catalytic oxidation of CO on Pt. Under UHV conditions, there are significant deviations in model responses between mean field (MF) and MC models in the absence of surface diffusion, due to adjacency requirements and lateral interactions. The disparity in time scales caused by surface diffusion is also addressed. Sensitivity and partial equilibrium analyses are introduced to delineate when surface diffusion should be included in MC or multiscale simulations. It is shown that including sufficiently fast Fickian surface diffusion of CO* in MC simulations does not always results in the MF limit, especially near extinction and for fuel lean mixtures. In the presence of repulsive interactions, it is shown that thermodynamically driven ordering of CO* affects the ignition temperature. Finally, it is found that in multiscale reactors, continuum-based models more accurately predict design performance, compared to UHV conditions, due to homogenization induced by desorption and readsorption of chemical species.

Lattice Gas Model for CO Electrooxidation on Pt−Ru Bimetallic Surfaces

We propose a lattice gas model for the carbon monoxide oxidation on platinum−ruthenium electrode surfaces. The kinetic model includes the main mechanistic “bifunctional” features as they are generally agreed upon in the literature. The CO stripping voltammetry is solved by dynamic Monte Carlo simulations. For a randomly dispersed alloy of Ru and Pt, the model gives a satisfactory semiquantitative agreement with the experimental CO stripping results of Gasteiger et al. [J. Phys. Chem. 1994, 98, 617]. It is shown how the bifunctional mechanism cannot operate if CO is not mobile on the surface, and a simple Tafel-type experiment with a low concentration of active Pt−Ru sites is suggested to check quantitatively the CO mobility rate. On a surface with large Ru islands, the overpotential for CO oxidation increases, and two CO stripping peaks may appear if the CO mobility is sufficiently low. A mean-field model of the system reproduces the DMC results for high CO mobility but breaks down for a system with large Ru islands and a comparatively low CO surface diffusion constant.

Effect of added basic metal oxides on CO 2 adsorption on alumina at elevated temperatures

In order to improve the ability of CO 2 adsorption for ceramic membrane with the surface diffusion of CO 2, alumina was modified by adding basic metal oxides. Due to such additions, CO 2 was effectively separated from an N 2-containing mixture gas at elevated temperatures. Modifiers examined were the oxides of rare earth metals (La, Ce, Nd, Pr), alkaline earth metals (Mg, Ca, Sr, Ba) and alkali metals (Na, K, Rb, Cs). IR observation of CO 2 species adsorbed on the surface modified alumina revealed that the added elements reacted with the alumina to form surface aluminate layer. The ability of CO 2 adsorption was evaluated by using a high temperature gas chromatograph: the column was packed with modified alumina onto which small amounts of a mixture of CO 2 and N 2 were injected via the helium carrier gas. It was observed that CO 2 was adsorbed on the modified alumina even at elevated temperatures above 773 K, while N 2 was eluted immediately. The effect on CO 2 adsorption varied from one metal oxide to another. The heat of adsorption, determined from the gas chromatogram, increased from 80 to 170 kJ mol −1 with decreasing electronegativity of the cation used for modification. The increase in the heat of adsorption among the oxide groups was in the following sequence: rare-earth metal oxides < alkaline-earth metal oxides < alkali metals oxide. The largest heat (167 kJ mol −1) was obtained for the addition of Rb 2O. The observed increase in the heat of adsorption was discussed in terms of the enhancement in both interactions between the carbon atom of a CO 2 molecule and the oxygen anion of the added metal oxide and between an oxygen atom of a CO 2 molecule and the cation of the oxide.These materials were considered to be the excellent candidate materials for surface diffusion membrane.

Effects of surface impurities on surface diffusion of CO on Ni(110)

A small amount of coadsorbed impurity species could significantly alter the surface diffusion of CO on Ni(110). Three impurity species, sulfur, oxygen, and potassium are studied here. The former two are known as ``poisons'' and the latter as a ``promoter'' for CO hydrogenation on Ni. All three are found to impede CO diffusion drastically. The apparent diffusion activation energy ${E}_{D}$ increases from the clean-surface value of 2--3 kcal/mol, to a saturation value of 7--8 kcal/mol at sufficiently high impurity coverages. The impeding effect decreases from S to O to K. Mechanisms responsible for the effect are discussed in detail. With S and O, the impurity-covered step-controlled diffusion appears to be the dominant mechanism. With K, the nearest-neighbor attractive interaction between CO and K seems to be most important.

Ab initio diffusional potential energy surface for CO chemisorption on Pd{110} at high coverage: Coupled translation and rotation

The ground state potential energy surface for CO chemisorption across Pd{110} has been calculated using density functional theory with gradient corrections at monolayer coverage. The most stable site corresponds well with the experimental adsorption heat, and it is found that the strength of binding to sites is in the following order: pseudo-short-bridge&amp;gt;atop&amp;gt;long-bridge&amp;gt;hollow. Pathways and transition states for CO surface diffusion, involving a correlation between translation and orientation, are proposed and discussed.

Surfactant-mediated growth of CoSi 2 on Si(100)

Thin co-deposited CoSi 2 films grown at 500–600°C on Si(100) are studied by scanning tunneling microscopy (STM). With a Si rich deposition we observe initially the formation of elongated three-dimensional CoSi 2 islands. The use of one preadsorbed atomic layer of As as a surfactant results in a drastic increase of the island density. This effect appears to be a consequence of a decreased rate of surface diffusion of Co and Si. At higher coverages the roughness of the CoSi 2 film is reduced considerably by the surfactant. The results are discussed with regard to the method of allotaxy which allows the fabrication of buried silicide layers. Here, the requirements for small precipitates and high growth temperature can possibly be met more efficiently using As as a surfactant.

Surface diffusion of carbon monoxide and potassium coadsorbed on Ru(001): Confirmation of a 1:1 CO:K trapping interaction

The surface mobilities of both CO and K coadsorbed on Ru(001) were studied using laser-induced thermal desorption (LITD) techniques. The LITD measurements revealed that CO was essentially immobile on the potassium-promoted Ru(001) surface when the CO surface coverage, ΘCO, was less than the potassium coverage, ΘK. The CO surface mobility increased dramatically when ΘCO≳ΘK. At ΘK=0.10 ML and 315 K, the CO diffusion coefficient was DCO&amp;lt;4×10−10 cm2/s for ΘCO&amp;lt;0.10 ML and increased to DCO=2.5×10−7 cm2/s for ΘCO≳0.10 ML. At ΘK=0.25 ML and 315 K, the CO surface diffusion coefficient was DCO&amp;lt;4×10−10 cm2/s for ΘCO&amp;lt;0.25 ML and increased to DCO=5×10−9 cm2/s for ΘCO≳0.25 ML. The potassium surface mobility also decreased dramatically as a function of CO coverage at both ΘK=0.10 ML and ΘK=0.25 ML. These diffusion results are consistent with a mutually stabilizing and trapping interaction between coadsorbed CO and K with a 1:1 CO:K stoichiometry. The thermal desorption spectra of both CO and K were also studied using temperature programmed desorption (TPD) techniques. At ΘK=0.10 ML and CO coverages ranging from ΘCO=0.06 ML to ΘCO=0.42 ML, CO TPD peak temperatures were consistently higher than CO peak temperatures obtained on clean Ru(001). These TPD results indicate that CO is stabilized by K on Ru(001). In addition, the potassium TPD spectra at various CO coverages with ΘK=0.10 ML and ΘK=0.25 ML revealed that K was stabilized by coadsorbed CO. The experimental results for CO and K coadsorbed on Ru(001) argue for a mutually stabilizing, short-ranged trapping CO–K interaction with a 1:1 CO:K stoichiometry. Monte Carlo simulations based on this model for the CO:K interaction were also consistent with the coadsorbate surface diffusion results.