Chemisorption of CO on tungsten (100): Combined flash desorption and electron stimulated desorption study. II

The chemisorption of CO on W(100) at ~ 100K has been studied using a combination of flash desorption and electron stimulated desorption (ESD) techniques. This is an extension of a similar study made for CO adsorption on W(100) at temperatures in the range 200–300K. As in the 200–300 K CO layer, both α₁-CO and α₂-CO are formed in addition to more strongly bound CO species upon adsorption at ~ 100K; the α-CO states yield CO⁺ and O⁺ respectively upon ESD. At low CO coverages, the α₁ and α₂-CO states are postulated to convert to β-CO or other strongly bound CO species upon heating. At higher CO coverages, α₁-CO converts to α₂-CO upon thermal desorption or electron stimulated desorption. There is evidence for the presence of other weakly-bound states in the low temperature CO layer having low surface concentration at saturation. The ESD behavior of the CO layer coadsorbed with hydrogen on W(100) is reported, and it is found that H(ads) suppresses either the concentration or the ionic cross section for α₁ and α₂-CO states.     

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.     

Ion angular distributions in electron stimulated desorption: Oxygen and CO on W(111)

The ion angular distributions resulting from electron stimulated desorption (ESD) of oxygen and carbon monoxide chemisorbed on a tungsten (111) crystal have been determined. The O⁺ ions released during ESD of adsorbed oxygen exhibit three-fold symmetric angular distributions in orientational registry with the W(111) substrate. The CO⁺ and O⁺ ions released during ESD of a monolayer of CO are desorbed normal to the (111) surface. Models for both oxygen and CO adsorption are discussed. The data for CO are consistent with adsorption of CO in “standing up” carbonyl structures in the virgin and α-CO binding states.     

Adsorbate characterisation by correlation of various photoelectron spectroscopies: CO on W(110)

As a test of the value of various electron spectroscopies and their combination for the characterisation of adsorption states, UPS valence spectra, XPS core spectra (O (1s) and C (1s)) and core satellite spectra (O (1s)), and X-ray induced Auger spectra (O KLL) were measured for various adsorption layers of CO on W(110) prepared at and above room temperature and, for comparison, of oxygen on the same surface. Virgin- and β-CO can readily be distinguished in all four kinds of spectra, while α-CO shows spectra very similar to those of virgin-CO. The conversion of virgin- to β-CO and their desorption can be followed in some detail. For all four techniques, the oxygen derived spectra of β-CO are identical to those of adsorbed oxygen, at about half the intensity. This makes it very likely that CO is dissociated in the β-layer on W(110). Virgin- and α-CO show the typical features of molecularly-adsorbed CO.     

Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

The adsorption and coadsorption of CO and H₂ have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as β₂, β₁, and β₀ were detected after adsorption at 250 K. The population of β₂ and β₁ states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD O⁺ and CO⁺ ions observed was discussed. Hydrogen is dissociatively adsorbed on Pd(111) and no ESD H⁺ signal was recorded following H₂ adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H⁺ ESD signal, a decrease of hydrogen surface population and an arisement of a broad H₂ TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO β₂ state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO coverages, the “cooperative” species being responsible for the H⁺ ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.     

ESCA study of carbon monoxide and oxygen adsorption on tungsten

The chemisorption of both CO and O₂ on a clean tungsten ribbon has been studied using an ultrahigh vacuum X-ray photoelectron spectrometer. For CO, the energy and intensity of photoemission from O(1s) and C(1s) core levels have been studied for various adsorption temperatures.At adsorption temperatures of ～100 K., the “virgin”-CO state was the dominant adsorbed species. Conversion of this state to more strongly-bound β-CO is observed upon heating the adsorbed layer to ～320K. Thermal desorption of CO at 300?T?640 K causes sequential loss of α₁-CO and α₂-CO as judged by the disappearance of O(1s) and C(1s) photoelectron peaks characteristic of these states.Oxygen adsorption at 300K gives a single main O(ls) peak at all coverages, although at high oxygen coverages there exist small auxiliary peaks at ～2eV lower kinetic energy. The photoelectron C(1s) and O(1s) binding energies observed for these adsorbed species are all lower than for gaseous molecules containing C and O atoms. For CO adsorption states there is a systematic decrease in photoelectron binding energy as the strength of adsorption increases. These observations are in general accord with expectations based on electronic relaxation effects in condensed materials.     

Adsorption of CO on W(001) by high resolution electron spectroscopy

The vibrational modes induced by CO on W(001) at temperatures ? 350 K are detected by means of electron energy loss spectroscopy with resolution in the 6–7 meV range. Two β adsorption regimes are identified depending on coverage. Heating at various increasing temperatures reveals coverage dependant irreversible surface structure modifications. The β spectra after adsorption or desorption are discussed in terms of the usual questions of multiple β states, dissociation, and reconstruction. The α₁ and α₂ states are detected both by their WC and CO frequencies. A small signal is assigned to a new a-state, named α₃, which may explain some thermal desorption results.     

The adsorption of CO on the (100) plane of tungsten; thermal and work function measurements

Thermal desorption and work function measurements indicate that a largely molecular layer, with some dissociation, is formed at 80–100 K, with an increase in work function of 0.55 eV. The coverage in this layer is 11.5 × 10¹⁴ molecules/cm², or CO/W = 1.15. On heating, equal amounts of a β precursor, possibly dissociated, and a molecular α species are formed at ≈300 K, with abundances of 5 × 10¹⁴ molecules/cm² each. The α desorption is complete at 360 K. The β precursor evolves on heating without desorption in the range 400–700 K as indicated by work function decreases, to β-CO, which is almost certainly dissociated. This change occurs at lower temperatures for low coverages. Thermal desorption shows 3 peaks, which have been traditionally labelled β_{1, β2}, and β₃ at 930, 1070, and 1375 K. Of these only β₃ corresponds to a well defined state. Readsorption after heating to 950 or 1150 K results in a doubly peaked spectrum at 1070 and 1375 K. The β₁ and β₂ peaks obey complex desorption kinetics, probably corresponding to desorption and rearrangement. The coverage of β₃ is 2.5 × 10¹⁴ molecules/cm², suggesting that the c(2 × 2) LEED pattern corresponds to occupany of every other unit cell by a C or an O atom. For coverages ? 1.5 × 10¹⁴ molecules/cm² β₃ desorption obeys second order kinetics with an activation energy of 83 ± 3 kcal/mole. For β₃ the work function decreases from the clean W value by 0.1 eV, suggesting adsorption of C and O in the center of the W unit mesh, below the surface layer of W atoms. Readsorption on β and β precursor layers leads to formation of electropositive α-CO, with a multiply peaked thermal desorption spectrum, indicating the existence of different binding sites. Adsorption-heatingreadsorption, -heating-readsorption sequences indicate that additional changes in the α desorption spectrum occur, suggesting reconstruction in the β layer.     

Surface vibrations of CO on W(100)

High resolution electron-energy-loss spectroscopy has been used to study the surface vibrations of CO on a W(100) surface at 300 K. For small exposures (β-CO) two losses at ～68 meV and ～78 meV are observed. This vibrational spectrum of β-CO is a clear indication of dissociative adsorption with the carbon and oxygen atoms in fourfold coordination sites each. With further exposure to CO two additional losses at 45 meV and 258 meV are observed, which represent the vibration of undissociated α-CO in upright position on top of a W atom. Furtheron results of coadsorption of H₂/CO and O₂/CO on W(100) are reported.     

Reflection-absorption infrared spectrum of α-CO chemisorbed on polycrystalline tungsten

The infrared spectrum of chemisorbed α-CO on polycrystalline tungsten has been studied using ultrahigh vacuum techniques. The α-CO state has been spectroscopically resolved into two states, designated α₁-CO (wavenumber ～2128 cm^?1) and α₂-CO (wavenumber ～2090 cm^?1). α₂-CO adsorbs predominantly in the first stages of α-CO adsorption; α₁-CO forms primarily at high α-CO coverages at the partial expense of α₂-CO. α₁-CO is found to desorb at a slightly lower temperature than α₂-CO. Both α-CO states are postulated to involve sp-hybridized carbon which is bonded to the tungsten surface. These states have previously been detected in electron impact desorption measurements, where α₁-CO was shown to liberate CO⁺ and α₂-CO to liberate O⁺.     

Electron beam interactions with CO on W {100} studied by Auger electron spectroscopy

The interaction of 2500 eV electrons with carbon monoxide chemisorbed on tungsten {100} was investigated by rapid-scan Auger electron spectroscopy. When no α state was present the O and C signals from the β state of CO were invariant during electron bombardment, giving an upper limit estimate for the electron stimulated desorption cross section, Q_β of 2 × 10^?21 cm². With the crystal at room temperature and saturated with CO, however, electron-beam induced accumulation of carbon was observed and characterised, the rate of the process being independent of CO pressure at pressures above 2 × 10^?8 Torr. At 450 K the rate was found to be pressure dependent up to at least 6 × 10^?7 Torr. A model is proposed for the accumulation process, which is based on electron beam dissociation of α₂-CO to form adsorbed carbon and gaseous O and the creation of new sites for further α₂-CO adsorption; it is in quantitative agreement with the results and yields a cross section for ESD of α₂-CO (Q_α2 = 1.55×10^?18cm²) in close agreement with direct measurements.     

CO chemisorption on PtCu surfaces I. CO on (1 × 3) Pt0.98Cu0.02(110)

Thermal desorption and photoemission spectroscopy (PES) have been used to investigate the chemisorption of CO on an annealed Pt_0.98Cu_0.02(110) surface. The clean surface shows 9.1 ± 2.6% Cu within the top 4 Å, and is (1 × 3) reconstructed. Thermal desorption of CO has revealed the existence of various adsorption states with these respective heats of adsorption: (α) 35.2 to 37.8 kcal/mol and (β) 24.5 to 26.3 kcal/mol on Pt sites, (γ) 16.0 to 17.2 kcal/mol on PtCu “mixed” sited, and (δ) 12.9 to 13.9 kcal/mol on Cu sites. PES observation of Cu 3d-derived states (using hv = 150 eV) and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels (using Mg Kα radiation) shows that the electronic structure of the Cu constituent is changed only when CO adsorbs on the Pt-Cu “mixed” sites or the Cu sites. Furthermore, the CO states associated with Pt sites reflect the structural difference between the (1 × 3) alloy surface and the (1 × 2) pure Pt(110) surface: α-CO on the alloy surface desorbs at a temperature 17 to 21 K. higher than the maximum desorption temperature of CO from pure Pt(110), and the ratio of β-CO to α-CO desorption from the alloy surface is larger than the ratio of low temperature to high temperature peaks in the desorption of CO from pure Pt(110).     

A comparative study of ESD of positive and negative ions from CO on tungsten

Electron stimulated desorption of O^? ions has been demonstrated from the CO/W system. Ion yields are shown to be approximately a factor of 50 smaller for O^? than for O⁺. The O^? species is shown to originate from both α-CO adsorption states with an estimated 80% of the O^? signal coming from the CO⁺ producing α-state. Preliminary results indicate that the ion energy distribution for O^? is broader and peaks at a higher energy than for the O⁺ ions.     

The behaviour of carbon species produced by CO disproportionation on Ru(1, 1, 10) and Ru(001) surfaces

The behaviour of adsorbed CO on Ru(001) flat and Ru(l,1,10) stepped surfaces in the CO pressure range between 10^?6 and 10¹ Pa has been investigated by TDS, AES, LEED and UPS. The disproportionation of CO proceeds rapidly on the stepped surface and its apparent activation energy was obtained to be 20 kJ mol^?1 at nearly zero coverage. The carbon species produced by CO disproportionation show non-uniform reactivity with ¹⁸O₂ and provide four CO desorption peaks in TPR spectra, which are assigned to α-C¹⁸O,ß-C¹⁸O and those derived from carbidic and graphitic carbons. At smaller carbon coverage, only α-CO and β-CO were observed, but with increasing coverage the amount of ß-CO reaches a maximum and carbidic carbon is newly formed. Further increase of carbon deposition gives graphitic carbon. The conversion from carbidic to graphitic carbon and the dissolution into the bulk took place upon heating to 1000 K. It is remarkable that very active carbon species are converted to molecular CO through the reaction with O₂ even at low temperature such as 200 K. It was also confirmed that active carbon species are formed on Ru surface during COH₂ reaction.     

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

Isotope labelling experiments have established that the adsorption of O₂ on the W(110) plane at 20 K leads first to the formation of a dissociated atomic layer. A weakly bound molecular species, α-O₂, forms only when the atomic layer is essentially complete (O/W = 0.6). The desorption of α-O₂ was found to be first order with an activation energy of $\text{E = 1.9}\text{kcal}\text{mole}$ and a frequency factor γ = 3 × 10⁹ s^?1. The activation energy is shown to be less than the enthalpy of desorption and the meaning of this result is discussed.     

Oxygen chemisorption and the carbon monoxide-oxygen interaction on Ru(101)

The adsorption of oxygen and the interaction of carbon monoxide with oxygen on Ru(101) have been studied by LEED, Auger spectroscopy and thermal desorption. Oxygen chemisorbs at 300 K via a precursor state and with an initial sticking probability of ~0.004, the enthalpy of adsorption being ~300 kJ mol^?1. As coverage increases a well ordered ¦11,30¦ phase is formed which at higher coverages undergoes compression along [010] to form a ¦21,50¦ structure, and the surface eventually saturates at 0 ~ $\text{8}\text{9}$. Incorporation of oxygen into the subsurface region of the crystal leads to drastic changes in the surface chemistry of CO. A new high; temperature peak (γ CO, E_d ~ 800 kJ mol^?1) appears in the desorption spectra, in addition to the α and β CO peaks which are characteristic of the clean surface. Coadsorption experiments using ¹⁸O₂ indicate that γ CO is not dissociatively adsorbed, and this species is also shown to be in competition with β CO for a common adsorption site. The unusual temperature dependence of the LEED intensities of the ¦11,30¦-O phase and the nature of α, β, and β CO are discussed. Oxygen does not displace adsorbed CO at 300 K and the converse is also true, neither do any Eley-Rideal or Langmuir-Hinshelwood reactions occur under these conditions. Such processes do occur at higher temperatures, and in particular the reaction CO(g) + O(a) → CO₂(g) appears to occur with much greater collisional efficiency than on Ru(001). The oxidation of CO has been examined under steady state conditions, and the reaction was found to proceed with an apparent activation energy of 39 kJ mol^?. This result rules out the commonly accepted explanation that CO desorption is rate determining, and is compared with the findings of other authors.     

Adsorption behaviors of CO on W(1 1 0) and Mo(1 1 0) surfaces in the β-state are still not clear

Adsorption structure of CO on W and Mo at above ～800 K (β-CO) has been extensively studied in the history of surface science. Most of the previous studies concluded that CO is dissociated in the β-CO, and a tilted structure plays a role as a precursor state of the dissociation. We have recently studied valence band spectra of the β-CO on W(1 1 0), oxygen-precovered W(1 1 0) and Mo(1 1 0) using synchrotron radiation. CO-derived states with binding energies close to those of the 4σ-CO can be observed, implying a non-dissociative chemisorption in this high-temperature state. We suggest that still some additional works need to be done in order to understand adsorption structure of β-CO completely.     

Interaction of CO,CO2 and O2 with nonpolar,stepped and polar Zn surfaces of ZnO

The nonpolar (10$\text{1}$0), stepped (40$\text{4}$1) and (50$\text{5}$1), and the polar (0001) surfaces of ZnO were prepared. Stable unreconstructed nonpolar and stepped surfaces were obtained. LEED analyses showed that the step height and the step width of the stepped surfaces were similar to the theoretical values. The polar surface showed a 1 × 1 LEED pattern of six-fold symmetry after annealing at 500°C, and evidence of a more complicated pattern at 300–400°C. Temperature programmed desorption of CO resulted in the desorption of CO from the stepped and the polar surfaces. However, desorption of CO₂ was observed from the stoichiometric nonpolar surface, and no desorption from the reduced nonpolar surface. CO₂ was also observed by interacting CO with all surfaces at elevated temperatures. A total of four temperature programmed desorption peaks of CO₂, α, β, γ, and δ were observed. The α and β peaks were observed on the nonpolar and the stepped surfaces, and the γ peak was observed on the polar surface. The α peak was assigned to adsorption on a surface ZnO pair, and the β peak was assigned to adsorption on an anion vacancy or a step. While adsorbed water enhanced the β, preadsorbed methanol reduced it. O₂ adsorption was similar on the nonpolar and the stepped surfaces, but was weak on the polar surface.     

Thermal desorption of cyanogen from Pt(100)

Thermal desorption of cyanogen adsorbed on Pt(100) was studied by flash desorption mass spectrometry. By investigating the parent ion and all possible fragmentation products in the mass spectrometer during desorption it was concluded, that desorption takes place exclusively as molecular C₂N₂. Three desorption peaks were observed at 140, 410 and 480°C denoted as α, β₁ and β₂. The respective surface coverages at saturation were determined by quantitative evaluation of the flash desorption curves to be 2.0 ± 0.2 × 10¹⁴ and 5.5 ± 1.0 × 10¹⁴$\text{molecules}\text{cm}{}^2$ for the α and the β states, respectively. First order desorption kinetics was suggested by the coverage dependencé of the desorption spectra for both α and β states with desorption energies of 12 and 38–42 $\text{kcal}\text{mole}$, respectively. A large difference in the sticking probabilities of α and β states was observed with initial values of 0.06 (α) and 0.9 (β). Adsorption experiments at elevated temperatures led to the assumption, that α and β states coexist on the surface with no or very little interactions between them. The results are discussed in terms of different models for the adsorption states.     

The adsorption of carbon monoxide on rhenium: Basal (0001) and stepped 14 (0001) × (101̄1) planes

The influence of surface defects on the adsorption of CO by rhenium is investigated using LEED, AES and linear temperature programmed desorption. On both surfaces, thermal desorption reveals two adsorption states, the lower temperature α state being resolved into two substates, and one β state, all desorbing with first order kinetics. The α state is unaffected by the surface texture, its maximum population being the same on both surfaces, around 4 × 10¹⁴ molecules cm^?2, similar to the value found for poly crystalline rhenium. On the other hand, the β state is strongly dependent on surface structure. On Re(0001) a maximum of 4 × 10¹³ molecules cm^?2 was found, and 2 × 10¹⁴ molecules cm^?2 on the stepped surface. The adsorption is activated and can be increased, by heating to 550 K, to 2 × 10¹⁴ molecules cm^?2 on the basal plane and 3.5 × 10¹⁴ molecules cm^?2 on the stepped surface. Ordered structures are now seen in LEED. Comparison of these results with previous results from polycrystalline rhenium indicate that the dissociation of β-CO on the latter surface must occur at defects other than steps.