Coadsorption of electropositive and electronegative elements: I. Cs and H2 on W(100)

A systematic experimental study of Cs and CsH₂ adsorption on W (100) surfaces by LEED and work function measurements has been made. It was found that at high coverages the electropositive and electronegative adsorbates formed a double layer. The arrangement of the adsorbates was independent of the sequence of deposition. The two adsorbates affected the structures of each other: a Cs layer caused an increase in ordering and binding energy of H on W and the existence of H on W(100) caused an increase in the maximum density of Cs atoms that can be deposited on the surface. This increase in Cs density was accompanied by an increase in work function. The coverage of Cs at the work function minimum was also shifted to a smaller value with increasing amount of initially adsorbed H₂. The work function result was compared with a recent non-uniform electron gas theory of work function. The adsorption of H₂ on Cs-covered W was found to be an activated process and the sticking coefficient decreased exponentially with increasing Cs coverage. It appears that adsorption of Cs did not form a duolayer on W (100), as reported by MacRae et al., rather it formed a single layer on top of some patches of hydrogen impurity. The small amount of H impurity was not ordered unless there was a layer of Cs on top.     

Oxidation of ethylene on Pd(100); bonding of ethylene and scavenging of dehydrogenation fragments by surface oxygen

The adsorption and reaction of C₂H₄ on oxygen covered Pd(100) was studied with high resolution electron energy loss spectroscopy (EELS) and temperature programmed reaction spectroscopy (TPRS). The clean Pd(100) surface at 300 K was exposed to O₂ to produce atomic oxygen in the p(2×2) structure for coverages between 0.05 and 0.25. The EELS and TPRS measurements were conducted following saturation coverage of the oxygen covered surface by C₂H₄ at 80 K. Both the di-σ- and π-bonded forms of C₂H₄ were stable on the surface for θ_O less than 0.25. The π-bonded form desorbed without reaction between 100 and 300 K, but the di-σ-bonded form underwent dehydrogenation above 250 K. The C₂H₄ dehydrogenation products were reactive towards atomic oxygen and produced H₂, H₂O, CO, CO₂, and adsorbed C. Oxygen preadsorption inhibited C₂H₄ Oxidation by limiting the formation of di-σ-bonded C₂H₄, and the fully developed p(2×2)O overlayer, corresponding to θ_O = 0.25, was sufficient to block completely the reaction of ethylene. The extent of reaction decreased in a 2:1 ratio to the increase in oxygen coverage, and indicated that oxygen islands blocked C₂H₄ dissociation. Only the π-bonded form of C₂H₄ was stable on the surface for θ_O greater than 0.25; the saturation coverage of π-bonded C₂H₄ of 0.25 was the same as for clean Pd(100).     

Chemisorption of H2 on W(100): Absolute sticking probability,coverage, and electron stimulated desorption

Measurements of both the absolute sticking probability near normal incidence and the coverage of H₂ adsorbed on W(100) at ~ 300K have been made using a precision gas dosing system; a known fraction of the molecules entering the vacuum chamber struck the sample crystal before reaching a mass spectrometer detector. The initial sticking probability S₀ for H₂/W(100) is 0.51 ± 0.03; the hydrogen coverage extrapolated to S = 0 is 2.0 × 10¹⁵ atoms cm^?2. The initial sticking probability S₀ for D₂/W(100) is 0.57 ± 0.03; the isotope effect for sticking probability is smaller than previously reported. Electron stimulated desorption (ESD) studies reveal that the low coverage β₂ hydrogen state on W(100) yields H⁺ ions upon bombardment by 100 eV electrons; the ion desorption cross section is ~ 1.8 × 10^?23 cm². The H⁺ ion cross section at saturation hydrogen coverage when the β₁ state is fully populated is ? 10^?25 cm². An isotope effect in electron stimulated desorption of H⁺ and D⁺ has been found. The H⁺ ion yield is ? 100 × greater than the D⁺ ion yield, in agreement with theory.     

Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

The co-adsorption of copper and oxygen on a tungsten {100} surface

The co-adsorption of Cu on O₂ and a W{100}surface is studied by Auger electron spectroscopy (AES), thermal desorption (TD), low energy electron diffraction (LEED) and by work function change (δø) measurements. It is shown that the presence of Cu on the surface initially decreases s_O, the sticking coefficient of O₂. For longer oxygen exposures and for higher adsorption temperatures, θ_O reaches values larger than those on the clean surface for the same O₂ exposure. Except at the highest θ_O values and temperatures, the sticcking coefficient for copper, s_Cu, is unity and is independent of the oxygen coverage θ_O in the range studied (0 ? θ_O ? 2). Co-adsorption at room temperatures does not produce any long range order while co-adsorption at elevated temperature leads to the ordered structures (1 × 1), p(2 × 1), p(2 × 2) and c(2 × 2). The saturation coverage of the two dimensional co-adsorbate at 800 K is given by the relation $\text{θ}{}_{\mathrm{Cu}}\text{+}\text{8}\text{5}\text{θ}{}_{\mathrm{O}}\text{= 2}$. The work function is a complicated function of θ_O and θ_Cu and is determined predominantly by the temperature at which oxygen is adsorbed. At high temperatures the sequence of adsorption has no influence, in contrast to the room temperature behavior.     

Photoemission studies of clean and oxygen-covered Pt 6(111) × (100)

Employing the enhanced sensitivity obtained by using synchrotron radiation near the Cooper minimum for the 5d valence electrons, we have located the oxygen 2p and 2s levels for oxygen chemisorbed on a Pt 6(111) × (100) crystal. We find the oxygen 2p level located ?6 eV with a FWHM of 3 eV and the 2s at ?21.6 eV. A factor of four difference in saturation coverage is measured between temperatures of 300 and 120 K, but the position and width of the 2p level is independent of temperature. We observe also the 1b₁ orbital of weakly adsorbed H₂O molecules, which has pure O 2p parentage; from the intensity of this orbital, we are able to suggest why it is difficult to observe the oxygen 2p signal at low photon energies. In addition, we note a strong preferential attenuation in the Pt states near E_f for the adsorbed H₂O in spite of the weak nature of the bond.     

Coadsorption of cesium and oxygen on Ni(100): II. Cesium enhanced chemisorption and oxidation

The effects of adsorbed Cs atoms on the chemisorption and oxidation of Ni(100) surfaces have been studied with low energy electron diffraction and work function measurements. In addition to the c(2×2) structure of O on clean Ni(100), the preadsorption of Cs caused the formation of a (3×3) and a c(4×2) structure. The experimental results suggest that these new structures were due to ordered arrays of chemisorbed O atoms underneath the Cs layer, with O densities higher than that of c(2×2). It is found that a Cs overlayer increased drastically the rate of O chemisorption and NiO formation. Depending on the initial Cs coverage, the NiO formed in the (100) and (111) crystallographic rientations. During the enhanced oxidation the Cs layer remained on top of the oxide.     

Electronic structure and adsorption properties of the system Cs/O/W(110)

Comparative studies are carried out of the Cs/O/W(110) and Cs/W(110) adsorption systems. The method of threshold photoemission spectroscopy is used to study the work function and electronic structure in the energy region near the Fermi level as functions of the sub-monolayer cesium coverage. A significant increase of the saturation cesium coverage is observed on the O/W(110) surface. A new adsorption-induced surface band is observed in the electronic spectrum of the system Cs/O/W(110) with a binding energy ∼0.7 eV. For coverages of about one monolayer metallization of the adsorbed layer is observed. It is shown that the electronic structures of the systems Cs/O/W(110) and Cs/W(110) are similar for low coverages. A difference in the adsorption properties for these two systems occurs for coverages close to one monolayer, which is explained by the creation of new interaction centers of the Cs adatoms on the W(110) surface in the presence of oxygen. Fiz. Tverd. Tela (St. Petersburg) 39, 1683–1686 (September 1997)     

Adsorption of oxygen on the tungsten (110) plane at low temperatures; Spectroscopic measurements

The adsorption of oxygen on the W(110) plane was carried out at 26 K and investigated by means of ultraviolet and X-ray photoelectron spectroscopy. It was found that atomic oxygen is adsorbed first to essentially saturation coverage (O/W = 0.6) before adsorption of molecular oxygen occurs. The spectrum of the latter is very similar to that of gas phase O₂ but the shift to weaker binding energies is greater for the 1 s level than for the valence orbitals.     

The displacement of hydrogen by carbon monoxide on the (100) face of tungsten: A photoemission and thermal desorption study

Photoelectron spectra (hv = 21.22 eV) and thermal desorption data were obtained for CO and H coadsorbed on W(100) at 80 K. When the clean surface is exposed to a saturation dose of H₂, subsequent exposure to CO results in the formation of a state whose emission spectrum is similar to that of molecular α-CO. Upon heating to ~280 K, a structural rearrangement occurs in which most of the adsorbed CO is converted to the strongly bound β form as the hydrogen is simultaneously desorbed. These data plus the observation that H₂ cannot be adsorbed to any significant degree on a saturated layer of β-CO suggest that adsorbed β-CO and H occupy the same atomic sites on the W(100) surface. The distinction between long and short range repulsive COH interactions is discussed. For CO adsorbed on clean W(100), the range of activation energies for vigin to β conversion is calculated from the UPS data to be 45–62 kJ/mol.     

Interaction of NO with a Ni (111) surface

The interaction of NO with a Ni (111) surface was studied by means of LEED, AES, UPS and flash desorption spectroscopy. NO adsorbs with a high sticking probability and may form two ordered structures (c4 × 2 and hexagonal) from (undissociated) NO_ad. The mean adsorption energy is about 25 $\text{kcal}\text{mole}$. Dissociation of adsorbed NO starts already at ?120°C, but the activation energy for this process increases with increasing coverage (and even by the presence of preadsorbed oxygen) up to the value for the activation energy of NO desorption. The recombination of adsorbed nitrogen atoms and desorption of N₂ occurs around 600 °C with an activation energy of about 52 $\text{kcal}\text{mole}$. A chemisorbed oxygen layer converts upon further increase of the oxygen concentration into epitaxial NiO. A mixed layer consisting of N_ad + O_ad (after thermal decomposition of NO) exhibits a complex LEED pattern and can be stripped of adsorbed oxygen by reduction with H₂. This yields an N_ad overlayer exhibiting a 6 × 2 LEED pattern. A series of new maxima at ≈ ?2, ?8.8 and ?14.6 eV is observed in the UV photoelectron spectra from adsorbed NO which are identified with surface states derived from molecular orbitals of free NO. N_ad as well as O_ad causes a peak at ?5.6 eV which is derived from the 2p electrons of the adsorbate. The photoelectron spectrum from NiO agrees closely with a recent theoretical evaluation.     

Interaction of O2, CO2, CO,C2H4 and C2H4O with Ag(110)

The interaction of O₂, CO₂, CO, C₂H₄ AND C₂H₄O with Ag(110) has been studied by low energy electron diffraction (LEED), temperature programmed desorption (TPD) and electron energy loss spectroscopy (EELS). For adsorbed oxygen the EELS and TPD signals are measured as a function of coverage (θ). Up to θ = 0.25 the EELS signal is proportional to coverage; above 0.25 evidence is found for dipole-dipole interaction as the EELS signal is no longer proportional to coverage. The TPD signal is not directly proportional to the oxygen coverage, which is explained by diffusion of part of the adsorbed oxygen into the bulk. Oxygen has been adsorbed both at pressures of less than 10^-4 Pa in an ultrahigh vacuum chamber and at pressures up to 10³ Pa in a preparation chamber. After desorption at 10³ Pa a new type of weakly bound subsurface oxygen is identified, which can be transferred to the surface by heating the crystal to 470 K. CO₂ is not adsorbed as such on clean silver at 300 K. However, it is adsorbed in the form of a carbonate ion if the surface is first exposed to oxygen. If the crystal is heated this complex decomposes into O_ad and CO₂ with an activation energy of 27 kcal/mol(1 kcal = 4.187 kJ). Up to an oxygen coverage of 0.25 one CO₂ molecule is adsorbed per two oxygen atoms on the surface. At higher oxygen coverages the amount of CO₂ adsorbed becomes smaller. CO readily reacts with O_ad at room temperature to form CO₂. This reaction has been used to measure the number of O atoms present on the surface at 300 K relative to the amount of CO₂ that is adsorbed at 300 K by the formation of a carbonate ion. Weakly bound subsurface oxygen does not react with CO at 300 K. Adsorption of C₂H₄O at 110 K is promoted by the presence of atomic oxygen. The activation energy for desorption of C₂H₄O from clean silver is ～ 9 kcal/mol, whereas on the oxygen-precovered surface two states are found with activation energies of 8.5 and 12.5 kcal/mol. The results are discussed in terms of the mechanism of ethylene epoxidation over unpromoted and unmoderated silver.     

Adsorption and solution of H2, D2, N2, O2 and CO by (100) Ta

Sticking coefficients, saturation densities, and solution rates of gases on (100) Ta are obtained by comparing with results on (100) W using Auger electron spectroscopy and flash desorption. Hydrogen has a lower sticking coefficient on (100) Ta than on polycrystalline Ta, but solution occurs readily even at 78°K. Differences between H₂ and D₂ are observed for both adsorption and solution. Nitrogen is confined to the surface of Ta for T < ≈500°K, and adsorbed nitrogen dissolves with an activation energy of ≈2.5 kcal mole^?1 upon heating to higher temperatures. The saturation density of O₂ at 300° K is approximately twice that on (100) W. The first monolayer dissolves at ≈500°K but the second dissolves or desorbs only at much higher temperatures. Carbon monoxide adsorbs without solution of either species at 300°K. At ≈500°K carbon dissolves completely leaving oxygen which desorbs at much higher temperature.     

Coadsorption of oxygen and cesium on Ru(001)

The coadsorption of oxygen and Cs on Ru(001) has been studied by means of thermal desorption, Auger and electron loss spectroscopy and work function measurements. The initial sticking coefficients for oxygen adsorption and oxygen saturation coverages increase with increasing Cs coverage, θ_Cs. Irrespective of the initial θ_Cs, the Cs desorption energy always increases under the influence of the coadsorbed oxygen, the effect becoming stronger with increasing oxygen coverage. At θ_O>0.5and θ_Cs>0.14 the work function, electron loss changes and thermal desorption data give evidence of strong CsO interactions and the formation of a CsO “surface compound”.     

Adsorption of oxygen on W(110): II. The high coverage range

The structure and composition of the interaction layer between oxygen and a W(110) surface for oxygen coverages θ above 0.5 monolayers is studied with LEED, AES, thermal desorption and work function change measurements. Oxygen is adsorbed by depositing WO₂ followed by annealing. The results are interpreted in terms of a topmost layer consisting only of oxygen atoms followed by the formation of isolated three-dimensional WO₃ crystals after saturation of the two-dimensional oxidation layer at 15 × 10¹⁴ O atoms cm^?2. All available experimental evidence is compatible with this interpretation.     

Adsorption of sulfur dioxide and the interaction of coadsorbed oxygen and sulfur on Pt(111)

The adsorption of sulfur dioxide and the interaction of adsorbed oxygen and sulfur on Pt(111) have been studied using flash desorption mass spectrometry and LEED. The reactivity of adsorbed sulfur towards oxygen depends strongly on the sulfur surface concentration. At a sulfur concentration of 5 × 10¹⁴ S atoms cm^?2 ($\text{(}\text{3}\text{×}\text{3}\text{)R30°}$ structure) oxygen exposures of 5 × 10^?5 Torr s do not result in the adsorption of oxygen nor in the formation of SO₂. At concentrations lower than 3.8 × 10¹⁴ S stoms cm^?2 ((2 × 2) structure) the thermal desorption following oxygen dosing at 320 K yields SO₂ and O₂. With decreasing sulfur concentration the amount of desorbing O₂ increases and that of SO₂ passes a maximum. This indicates that sulfur free surface regions, i.e. holes or defects in the (2 × 2) S structure, are required for the adsorption of oxygen and for the reaction of adsorbed sulfur with oxygen. SO₂ is adsorbed with high sticking probability and can be desorbed nearly completely as SO₂ with desorption maxima occurring at 400, 480 and 580 K. The adsorbed SO₂ is highly sensitive to hydrogen. Small H₂ doses remove most of the oxygen and leave adsorbed sulfur on the surface. After adsorption of SO₂ on an oxygen predosed surface small amounts of SO₃ were desorbed in addition to SO₂ and O₂ during heating. Preadsorbed oxygen produces variations of the SO₂ peak intensities which indicate stabilization of an adsorbed species by coadsorbed oxygen.     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

Adsorption of NO and CO on a Ru(1010) surface

A combination of modern surface measurement techniques such as LEED, AES and Thermal Desorption Spectroscopy were used to study the chemisorptive behavior of NO and CO on a $\text{(10}\text{1}\text{0)Ru}$ surface. The experimental evidence strongly favors a model in which NO adsorbs and rapidly dissociates into separate nitrogen and oxygen adsorbed phases, each exhibiting ordered structures: the C(2 × 4) and (2 × 1) structures at one-half and full saturation coveilage, respectively. At temperatures as low as 200°C, the nitrogen phase begins to desorb, and continuous exposure to NO in this temperature range results in an increasing oxygen coverage until the surface is saturated with oxygen and no further NO dissociation can take place. The nitrogen desorption spectrum depends strongly on coverage and exhibits several peaks which are related to structure of the adsorbed phase. There is evidence that once the surface is saturated with the dissociated NO phase further NO adsorption occurs in a molecular state. Carbon monoxide adsorbs in a molecular state and does not exhibit an ordered structure. The implications of the results with respect to the catalytic reduction of NO by H₂ and CO and the N₂ selectivity of Ru catalysts are discussed.     

Adsorption and reaction of nitric oxide and oxygen on Rh(111)

Adsorption of NO and O₂ on Rh(111) has been studied by TPD and XPS. Both gases adsorb molecularly at 120 K. At low coverages (θ_NO < 0.3) NO dissociates completely upon heating to form N₂ and O₂ which have peak desorption temperatures at 710 and 1310 K., respectively. At higher NO coverages NO desorbs at 455 K and a new N₂ state obeying first order kinetics appears at 470 K. At saturation, 55% of the adsorbed NO decomposes. Preadsorbed oxygen inhibits NO decomposition and produces new N₂ and NO desorption states, both at 400 K. The saturation coverage of NO on Rh(111) is approximately 0.67 of the surface atom density. Oxygen on Rh(111) has two strongly bound states with peak temperatures of 840 and 1125 K with a saturation coverage ratio of 1:2. Desorption parameters for the 1125 peak vary strongly with coverage and, assuming second-order kinetics, yield an activation energy of $\text{85 ± 5}\text{kcal}\text{mol}$ and a pre-exponential factor of 2.0 cm² s^?1 in the limit of zero coverage. A molecular state desorbing at 150 K and the 840 K state fill concurrently. The saturation coverage of atomic oxygen on Rh(111) is approximately 0.83 times the surface atom density. The behavior of NO on Rh and Pt low index planes is compared.     

Anomalous coverage behavior of the CsAg distance on CsAg(110)

The position of Cs on the (1 × 2) missing row reconstructed Ag(110) surface was determined by X-ray diffraction for two different Cs-coverages: θ_Cs = 0.2 and θ_Cs = 0.3. The Cs was found to be adsorbed in incommensurate chains in the troughs of the missing row with an average adsorption height of 1.7 Å (θ_Cs = 0.2) and 1.4 Å (θ_Cs = 0.3) above the topmost Ag layer. The apparent contradiction to the classical picture of alkali adsorption, which expects an increase of the Cs adsorption height with coverage, might be partly resolved by introducing a fraction of commensurately adsorbed Cs at θ_Cs = 0.2.