CO Saturation Coverage Research Articles

Microcalorimetric and Reaction Kinetic Studies of Alkali Metals on Pt Powder and Pt/SiO2 and Pt/Sn/SiO2 Catalysts

Microcalorimetric measurements of CO adsorption at 403 K on reduced Pt powder gave an initial heat of 140 kJ/mol and a saturation CO coverage of 30 μmol/g Pt. The addition of metallic Rb and Cs to Pt increased the initial heats of CO adsorption by 20 and 40 kJ/mol and extended the CO adsorption capacity to 110 and 60 μmol/g Pt, respectively. The increase in CO adsorption capacity is caused by formation of an alkali−CO complex at 403 K, with an interaction heat near 120 kJ/mol. Microcalorimetric measurements of CO adsorption at 403 K on Pt/SiO2 (0.85 wt % Pt) and Pt/Sn/SiO2 (2.61 wt % Pt) catalysts showed initial heats of 140 and 130 kJ/mol and saturation CO coverages of 25 and 65 μmol/g, respectively. The addition of K, Rb, and Cs salts (1:5 atomic Pt/alkali) to Pt/SiO2 followed by treatment in H2 at 773 K did not alter the properties of these materials for CO adsorption at 403 K or for reactions with isobutane at 673 K. In contrast, the addition of Na and Cs salts (1:3 atomic Pt/alkali) to Pt/Sn/SiO2 sig...

Carbon Monoxide Adsorption on a Cu3Pt(111) Single Crystal Alloy

The adsorption of CO on CU3Pt(111) was investigated by low energy He+ ion scattering. The alloy surface was characterised by low energy electron diffraction (LEED) and work function change (Δφ) measurements. No surface ordering of the alloy was evidenced by LEED. The CO induced work function changes were small. Under normal incidence of the beam, only the scattered He+ ions from 0, Cu and Pt were detected, indicating the orientation of the CO molecule with 0 facing away from the surface. In the initial stages of CO adsorption at 200 K, the Pt intensity in a Cu-rich surface was not detectable at an exposure of ~5 L CO, indicating that all the Pt atoms were covered by CO. At a saturation coverage of CO, the Pt intensity in a Pt-rich surface, however, fell to ~12% of its clean surface intensity. This observation strongly suggested that not all the Pt sites were covered by the CO molecules. These results are discussed in relation with the structure and composition of the surface and compared to data for CO adsorption on Pt(111).

Electrochemical and in situ FTIRS studies of the CO adsorption at palladium and rhodium multilayers deposited on platinum single crystal surfaces II. Pt(100) substrate

Multilayers of rhodium and palladium have been deposited on Pt(100) electrodes and subsequently characterized by means of cyclic voltammetry and charge displacement experiments. In addition, the saturated adlayer of CO on PdPt(100) and RhPt(100) has been studied by both electrochemical and infrared techniques. The voltammetric profile corresponding to these multilayers is due to a coupled adsorption/desorption process involving both anions and hydrogen. The set of different results seems to indicate that the multilayer is epitaxial in the case of palladium, thus behaving as a bulk Pd(100) electrode, whereas it is disordered in the case of rhodium. The spectra for the saturated CO adlayer on RhPt(100) and PdPt(100) also reflect the presence of the foreign metal deposit and give some indication about the bonding geometry and order of the CO adlayer. The CO saturation coverage for the PdPt(100) system is around 0.63.

CO adsorption on Cu3Pt(111): a LEIS study

The interaction of CO with the Cu3Pt(111) surface has been studied by low energy He+ ion scattering. The intensities of He+ ions scattered off O, Cu and Pt were only detected, indicating the orientation of the CO molecule with O facing away from the surface. In the initial stages of CO adsorption at 200 K, the Pt intensity in a Cu-enriched surface was not detectable at an exposure of 6 L CO. This was interpreted as a preferential chemisorption of CO onto Pt sites. At a saturation coverage of CO, however, the Pt intensity in a Pt-enriched surface fell to ∼ 15% of its clean surface intensity, indicating not all the Pt sites were covered by the CO molecules. The results also suggested that the presence of the CO adsorbed surface layer does not induce surface segregation of Pt. The distance between the adsorbed CO along the [01̄1] azimuth below a saturation of CO coverage has also been tested.

Influence of alloyed Sn atoms on the chemisorption properties of Ni(111) as probed by RAIRS and TPD studies of CO adsorption

The chemisorption of CO on NiSn alloys formed on a Ni(111) surface has been studied with reflection-absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD). Formation of the (√3 × √3) R30°- Sn Ni(111) surface alloy strongly suppresses CO adsorption. Only 0.04 ML CO can be chemisorbed on this surface at 110 K, exclusively at atop sites. The binding energy of this adsorbed CO is reduced to only about 15 kcal/mol. Additionally, no significant effect of subsurface Sn on CO chemisorption on this alloy was observed. These results are in sharp contrast to our previous results for CO chemisorption on the (√3 × √3) R30°- Sn Pt(111) surface alloy, where the chemisorbed CO saturation coverage, adsorption site distribution and desorption temperature were quite similar to those properties of the Pt(111) surface. We ascribe the influence of alloyed Sn atoms on the CO chemisorption properties of this Sn Ni(111) surface alloy to be due to repulsive SnCO interactions at the NiCO distance required for chemisorption. The differences in the chemisorption properties of these PtSn and NiSn alloys are rationalized by considering the different sizes of the surface unit cells and the location of Sn with respect to the surface plane (the Sn buckling distance). These results have important implications for the discussion and determination of ensemble sizes at bimetallic surfaces since the surface chemistry depends strongly on the vertical position of the modifier.

STM investigations of CO coadsorption with O and with S on Ni(110)

The interaction between CO coadsorbed with oxygen and sulfur on Ni (110) has been studied with room temperature STM and LEED. In the case of CO/O/Ni(110)−(θ o⩽0.3 ML), it is found that due to a large local repulsion between the differing species, the coadsorbed species phase separate into large domains of O−(3 × 1) and CO−p2mg(2 × 1) structure. Similarly in the case of CO/S/Ni(110)−(θ s⩽0.4 ML), at low local coverages of coadsorbed CO, island segregation of CO and S-c(2 × 2) is observed. At locally saturated CO coverage, the S-c(2 × 2) structure transforms into long -S-S- chains running predominantly along the [ 1 10 ] direction and separated by a local p2mg(2 × 1)-CO structure; this transformation is attributed to the large CO-CO repulsion in the condensed overlayer structure.

Modification of the electronic structure of Pd by U films: chemisorption of CO

X-Ray and ultraviolet photoelectron spectroscopies (XPS and UPS), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) have been used to study the coadsorption of CO and U on polycrystalline Pd. We investigated the modification of the electronic structure of Pd at low U coverages and the mode of CO chemisorption (dissociative vs. associative) as a function of surface U concentration. U adsorption results in the narrowing of the Pd 4d band. At low U coverages the density of states (DOS) at the Fermi level decreases and the U 5f electrons are localized. The CO saturation coverage at room temperature decreases with increasing U surface concentration, while CO adsorption at –165 °C is less affected. This is attributed mainly to a decrease of CO chemisorption energy but also to blocking of Pd adsorption sites by U. A. decrease in the heat of chemisorption for CO is explained by a change of the electronic structure of Pd by U. At low dosage U itself loses most of its reactivity, probably because of U—Pd solid-state bonding and below a critical U surface concentration CO dissociation becomes an activated process. High-temperature reaction between CO and U-covered surfaces leads to the partial transformation of U into a surface oxycarbide. In the presence of this compound some CO is chemisorbed strongly on the surface and is stable even at 300 °C, in contrast to CO on Pd metal. A similar effect has also been observed for classical promoters such as the alkali metals.

Auger electron spectroscopy, low-energy electron diffraction, and electrochemistry of carbon monoxide on a platinum(100) electrode

We report on carbon monoxide adsorption on the Pt(100) electrode in CO-saturated perchloric and sulfuric acid solutions studied by a combination of low-energy electron diffraction, Auger electron spectroscopy, and electrochemistry. We have developed an experimental method that produces accurate CO coverages making use of a capacitive current generated due to iodine adsorption at a constant electrode potential. Using this method, we have obtained the saturation CO coverage equal to 0.77±0.04 ML, in a good agreement with the quantitative Auger electron spectroscopy result of 0.79±0.09 ML. The LEED analysis of the saturation CO structure that corresponds to the 0.77 ML coverage gives rise to the Pt(100)c(4×2) surface structure, as in earlier gas-phase studies of the equivalent surface system

Interactions between chemisorbed CO and oxygen on Ni(111)

Infrared reflection absorption spectroscopy (IRAS), low energy electron diffraction (LEED), work function (ΔΦ), and temperature programmed desorption (TPD) have been used to characterize the interactions between chemisorbed CO and oxygen on Ni(111). It has been found that chemisorbed oxygen exhibits both a long range and a short range effect on chemisorbed CO on Ni(111). The short range interaction between chemisorbed oxygen and chemisorbed CO produces a perturbed terminal-CO species, designated as CO∗, with a singleton frequency 41 cm −1 higher than that of the unperturbed terminal-CO. The CO∗ species is found to adsorb on Ni sites which are the nearest neighbors to the oxygen adatoms adsorbed in the threefold sites. The long range interaction between the two coadsorbates also produces a blue-shift in the singleton frequency of the bridged-CO, and a reduction in the CO saturation coverage. A comparison of the interactional effect of oxygen and sulfur on chemisorbed CO indicates that the size of an electronegative species and its projection above the surface is more important than the electronegativity in determining the strength of the poisoning effect of the electronegative species on chemisorbed CO.

Infrared spectroscopic studies of bimetallic electrochemical interfaces: Carbon monoxide coadsorbed with metal overlayers on ordered low-index rhodium and platinum surfaces

The influence of underpotential deposited (upd) copper on Rh(111) and (110) electrodes and of predosed antimony on Pt(111) electrodes upon coadsorbed carbon monoxide in aqueous 0.1 M HClO4 has been examined by means of in situ Fourier transform infrared spectroscopy in the C-O stretching (νCO) frequency region as illustrative examples of the use of infrared spectra to discern microscopic interactions in such binary electrochemical systems. Saturation dosages of CO in the presence of small upd copper coverages on Rh(111) yielded segregated coadsorbate domains as evidenced by νCO spectra. In contrast, the corresponding data for CO and copper on Rh(110) are consistent with the presence of intermixed adlayer structures as indicated by the marked decreases in the terminal νCO frequency and the selective removal of the CO bridge band. The irreversibly adsorbed nature of antimony adlayers on Pt(111) enabled the effect of known and adjustable metal coverages, θSb, upon the νCO spectra to be examined for various postdosed CO coverages. Increasing θSb yields approximately commensurate decreases in the saturation CO coverage, along with a progressive shift in site occupancy from twofold bridging to terminal coordination as deduced from the relative infrared band intensities. At the saturation antimony coverage, θSb=0.3, the CO binding is exclusively terminal. These results are consistent with adsorption of antimony chiefly in bridging sites, yielding a microscopically intermixed adlayer with CO. The observed greater tendencies to form intermixed adlayers for Sb–CO than for Cu–CO, and on (110) vs (111) surfaces are consistent with steric and electronic expectations.

Quantum-resolved angular distributions of neutral products in electron-stimulated processes: desorption of CO ∗ from Pt(111)

The first quantum-resolved angular distribution measurements of electronically-excited neutral molecules which have undergone electron stimulated desorption (ESD) are presented. Two-dimensional imaging of laser resonance-enhanced multiphoton ionization (REMPI) is used to obtain angular distributions of CO ∗ in the v=0 vibrational level of the metastable a 3Π r state desorbed from CO/Pt(111) by 350 eV electrons. For saturation CO coverages at 90 K, sharp Gaussian distributions peaked about the surface normal (6° ± 0.5° half-width at half maximum) are observed, consistent with previously reported data acquired by ESDIAD (ESD ion angular distributions). The (1 + 1) photon REMPI scheme for state-specific CO ∗ detection involves the b 3Σ( v = 0) ← a 3Π( v = 0) transition at 283 nm, followed by photoionization at the same wavelength. In this paper, the overall experimental technique for REMPI imaging of products from electron stimulated processes is discussed. Thus specific CO ∗ data as a function of coverage and temperature is presented for comparison with the ESDIAD results.

Bimetallic electrochemical interfaces: carbon monoxide coadsorbed with copper and lead on ordered low-index rhodium surfaces

The influence of predosed underpotential deposited (UPD) copper and lead upon carbon monoxide adsorbed on ordered Rh(111), (100), and (110) electrodes in aqueous 0.1M HClO 4 has been examined by means of in-situ FTIR spectroscopy in the C-O stretching (ν CO) frequency region. The presence of submonolayer coverages of Cu and Pb results in roughly commensurate decreases in the saturated CO coverage. Nevertheless, even low copper coverages on Rh(100) diminish markedly the intensities of the terminal and bridging ν CO features, attributed to screening of neighboring C-O dipoles by the metal adlayer atoms. On Rh(110), copper attenuates preferentially the bridging ν CO feature, signaling the formation of a microscopically intermixed Cu-CO adlayer. While the behavior of the Pb-CO and Cu-CO adlayers on Rh(111) does not differ greatly, the presence of lead on Rh(100) and especially (110) blocks multifold sites in a highly selective fashion as discerned from the loss of the bridging ν CO feature. The observed greater tendencies to form intermixed adlayers for Pb-CO than for Cu-CO, and in the crystallographic sequence Rh(111) < (100) < (110), are consistent with steric and electronic expectations.

Effects of K, O, and H adatoms on the adsorption kinetics of CO on Pt(111)

The sticking coefficient, S, of CO on clean and K-, O- and H-modified Pt(111) surfaces has been measured directly using a simple kinetic uptake method, as a function of CO and modifier coverage and surface temperature. A simple method is also introduced to determine the coverage-dependent desorption energy from these measurements without the need to know the pre-exponential factor or desorption order for those systems where S is only a function of coverage or only weakly dependent on temperature. The initial sticking coefficient of CO on clean Pt(111) is 0.9 and is independent of substrate temperature. The major influence of adsorbed K on the initial sticking coefficient of CO on Pt(111) at 320 K is site blocking, and the magnitude of this effect correlates well with the size of the adsorbed K ion or atom. The shape of S as a function of CO coverage for low K coverages in the ionic regime is similar to that of CO on clean Pt(111). A dramatic change occurs for K coverages near or greater than 0.24 ML, where S passes through a maximum at about case1 3 of the saturation CO coverage and coincident CO and K desorption is observed. This behavior of S, which has not been seen before, can be fit quite adequately by a simple model that incorporates the reduced site blocking effects of K due to the CO-induced transition of K from a metallic neutral to an ionic state and the accompanying size change of K atoms. These results are best described by a coverage-dependent alkali-CO interaction. In additional studies, we find that adsorption of 0.25 ML O on Pt(111) has no effect on S for low CO coverages at 100 K, and only the saturation CO coverage is reduced from 0.59 to 0.31 ML. In contrast, adsorbed H reduces not only the CO saturation coverage but also S even at low CO coverages.

Influence of coadsorbed bismuth and copper on carbon monoxide adlayer structures at ordered low-index platinum-aqueous interfaces

The influence of predosed bismuth and underpotential deposited (UPD) copper on the adlayer structure of carbon monoxide on ordered Pt(111), (100), and (110) electrodes in 0.1 M HClO 4 has been examined by means of in situ FTIR spectroscopy in the CO stretching ( v CO) region. The irreversibly adsorbed and electroactive nature of the bismuth adlayers enabled the effect to be examined of known and adjustable metal coverages, θ Bi, upon the v CO spectra for various dosed CO coverages, θ CO. On Pt(111), increasing θ Bi yields approximately commensurate decreases in the saturation CO coverage, along with a progressive shift in the CO site occupancy from twofold bridging to terminal coordination as deduced from the relative v CO, band intensities. At the “saturation” bismuth coverage, θ Bi(sat) = 0.21, the CO binding is exclusively terminal throughout the θ CO range, 0.05 ≲ θ CO < 0.37. These results are consistent with adsorption of bismuth chiefly in bridging sites, yielding a microscopically intermixed adlayer with CO. A similar effect of increasing θ Bi upon θ CO [up to θ Bi(sat) = 0.43] is seen on Pt(100); the bridging v CO feature is again preferentiall yet incompletely, removed by θ Bi(sat). These findings indicate that an intermixed layer is formed on Pt(100) with bismuth again present at twofold bridging sites. By contrast, saturated CO adlayers in the presence of UPD copper on Pt(111) form segregated domains as evidenced by v CO spectra. The latter results indicate the presence of compressed CO island structures even down to low θ CO. Corresponding data for CO and copper on Pt(100), however, are consistent with the presence of intermixed structures, as deduced from the marked decreases in the terminal v CO frequency for larger copper coverages. Spectra for CO on Pt(110) with either coadsorbed bismuth or copper are also indicative of the presence of microscopically intermixed adlayers. The contrasting occurrence of segregated and intermixed copper/CO adlayers on Pt(111), and on Pt(100) and Pt(110), respectively, are consistent with the more open structure of the latter faces (cf. ref. [18]). For comparison, spectral data are also reported for CO/Bi adlayers on polycrystalline platinum.

Effect of CO on hydrogen thermal desorption from Fe(100)

The adsorption and coadsorption of H and CO was studied in order to illuminate the enhanced reactivity of H toward olefin hydrogenation induced by coadsorbed CO. The hydrogen saturation coverage on clean Fe(100) is 1.0 ± 0.1 ML, and H2 desorbs in two states, β1 and β2, with activation energies of 17 and 23 kcalmol, respectively. Low coverages of CO adsorbed on hydrogen-presaturated Fe(100) broaden and reduce the peak temperatures of the β1 and β2 desorption states. The dominant effect of coadsorbed CO is to weaken Fe-H bonds, and three new H2 desorption states are induced below 260 K at higher coverages of CO. No displacement of H2 by coadsorption of CO is apparent. The CO saturation coverage is uneffected by preadsorbed hydrogen, but the CO desorption features below 350 K are distinctly different than those for the clean surface. Desorption of CO above 350 K resembles that for CO adsorbed on the H-free surface. When H2 is adsorbed onto a CO-precovered surface at 100 K. the saturation coverage of hydrogen is reduced, and at the CO saturation coverage of 0.58 ML hydrogen adsorption is blocked. H2 desorption from surfaces precovered with a subsaturated adlayer of CO adsorbed at 100 K display only perturbed β1 and β2 states. CO apparently induces low temperature desorption of H2 only when it is adsorbed on top of a locally saturated H adlayer.

Adsorption and catalysis on silicon-modified W(110) surfaces

The adsorption of CO, hydrogen, and benzene was studied at 120 K on silicon-covered W(110) surfaces using temperature-programmed desorption. The saturation coverage of virgin CO is reduced from 0.80 monolayer (ML) on the clean W(110) surface to 0.40 ML in the presence of 0.30 ML of silicon and to 0.21 ML in the presence of 0.56 ML of silicon. No CO adsorption occurs on W(110) covered by a monolayer of silicon. Dissociation of hydrogen, CO, and benzene is strongly inhibited by silicon. The inhibition of CO and hydrogen adsorption by silicon is discussed in terms of the proposed overlayer structures for silicon on W(110). Upon annealing silicon overlayers to 1050 K, silicon diffuses into the bulk to form tungsten silicide, and the resulting increase in the concentration of tungsten atoms at the surface increases the ability of the surface to adsorb CO. Results are compared to those from adsorption studies on silicon-covered polycrystalline tungsten and on a nickeldisilicide surface. The dehydrogenation and hydrogenolysis of cyclohexane were used to test the catalytic properties of the silicon-modified surfaces. Tungsten silicide appears to be a highly selective catalyst for the dehydrogenation of cyclohexane. Comparison with a study of cyclohexane reactions over silane-treated nickel catalysts indicates that high selectivity for dehydrogenation may be a general attribute of silicon-modified metal catalysts. The activity of tungsten silicide for cyclohexane dehydrogenation to benzene is at least as great as that of the Ru(M01) surface.

Chemisorption of carbon monoxide, hydrogen, and oxygen on ordered tin/platinum(111) surface alloys

The chemisorption of CO, H{sub 2}, and O{sub 2} on ordered Pt-Sn surface alloys has been investigated by using thermal desorption mass spectroscopy (TDMS), high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), Auger electron (AES), X-ray photoelectron (XPS) and ultraviolet photoelectron spectroscopy (UPS), and low-energy ion scattering spectroscopy (LEISS). Alloying with Sn causes only slight decreases in the CO desorption peak temperature compared to Pt(111). A (2{radical}3{times}2{radical}3)R30{degree} LEED pattern is observed for the saturation CO coverage on the p(2{times}2) ordered alloy surface. No ordered CO overlays were observed by LEED from the ({radical}3{times}{radical}3)R30{degree} alloy surface. HREELS indicates the atop-bonded CO is the most strongly adsorbed species on both ordered alloy surfaces, as on Pt(111), and that both atop and bridge sites are populated for saturation CO coverage on both ordered alloy surfaces.

A study of the migration and stability of titania on a model Rh catalyst

We have examined growth and stability of titania overlayers which have been vapor deposited onto a Rh foil. Titania was found to grow layer by layer, with approximately 1 × 10 15 TiO x . species/cm 2 in the first layer. Submonolayer coverages of titania were unaffected by heating the sample to temperatures as high as 1320 K. However, heating multilayer coverages to 1320 K resulted in one monolayer remaining on the surface, with the rest migrating into the Rh. The migrating titania could be brought back to the surface by removing the top monolayer and heating the sample above 900 K under vacuum. Saturation coverages of CO were found to decrease linearly with surface titania coverage, independent of sample treatment; however, annealing to high temperatures appears to remove defects in the titania overlayer.

CO on Pt(335): Vibrational Stark effect, mode coupling, and local field effects on a stepped surface

We have studied CO on Pt(335) using infrared diode laser spectroscopy and electric field modulation. The spectra cover the 2000–2160 cm −1 range. The Pt(335) surface is stepped, with (111) terraces four atoms wide. At low coverage only one species is observed: CO adsorbed on step edges. In terms of the applied field, the Stark tuning rate of a low coverage of this species is (6.0 ± 0.8) × 10 −7 cm −1/( V/ cm). At the same coverage, the screened dynamic dipole moment ~ e = (0.94 ± 0.11) e. The rate calculated from ~ e is (7.0 ± 1.1) × 10 −7 cm −1/( V/ cm), agreement with the measured value. At higher coverages te also observed. Terrace CO is similar to CO on Pt(111) in vibrational frequency, dynamic dipole moment and desorption temperature. However, the Stark tuning rate of terrace CO is at least an order of magnitude smaller than expected. We discuss why the tuning rate might be so small. When both species are present, the vibrational spectra are strongly affected by dipole-dipole coupling. As a result, the electroreflectance spectrum is not the derivative of the polarization modulation spectrum. The spectra are analyzed using a phenomenological coupled-oscillator model. Fitting the spectra to the model, we find that for CO adsorbed on step edges, the Stark tuning rate in terms of local field is(7.5 ± 2.0) × 10 −7 cm −1/( V/ cm), independent of coverage. The saturation coverage clean surface at 300 K is (8.3 ± 0.6) × 10 14 cm −2. With saturation coverage of CO the linewidth is very small, only 4.5–5.0 cm −1 FWHM, even though the sample is at 300 K. Also, while cleaning the surface, we observed a correlation between Sn coverage and saturation coverage of CO. One possible explanation for the correlation, although not proven, is that Sn blocks CO adsorption sites. If so, the data suggests that each Sn atom blocks 11 ± 6 CO adsorption sites, which may explain how Sn helps prevent CO poisoning of Pt electrodes in fuel cells.

CO adsorption on stepped Ni(111) surfaces

The CO adsorption on Ni(331) and Ni(221) [referred to as Ni(s)(111)] was studied with low-energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), and work function change measurements. LEED from the clean Ni(s)(111) crystals give evidence for nonreconstructed surfaces with atomic arrangements as expected from the ideal (331) and (221) orientations; these surfaces can be interpreted as stepped Ni(111) surfaces with (111) terraces and (111) steps. CO desorption shows on both surfaces several desorption states, which are sequentially filled with increasing CO exposure. The desorption state with the highest peak temperature [Tm=455 K for Θ(CO)≊0] is identified on both surfaces and related to CO desorption from (111) terraces. For higher coverages, due to the presence of steps, new desorption states fill in with lower desorption temperature in the range of 200–350 K. These desorption states are not present on flat Ni(111). With LEED in a definite regime of CO coverages half and one fourth-order streaks in [1̄10] directions of Ni(s)(111) are observed and related to a one-dimensional ordering of CO in directions parallel to the step directions. Ordered LEED overlayers with c(4×2) [Ni(331) and Ni(221)] and (2×1) [Ni(221)] are established on both surfaces near the CO saturation coverage at 120 K. These overlayers demonstrate that the barrier for lateral interaction introduced by the steps is overcome at higher coverages.