Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.     

Equilibrium shape of nickel crystal

The equilibrium shape of pure nickel and the effect of carbon on changes in the equilibrium shape at 1200°C were investigated. A statistical observation on the size-dependent, time-dependent and carbon-induced morphological evolution of crystallites suggested that the equilibrium crystal shape (ECS) of pure nickel is a polyhedron consisting of {111}, {100}, {110} and {210} surfaces. However, crystals with an extensive proportion of {320} surfaces were also frequently observed. The appearance of {320} surfaces was interpreted as kinetically stabilized metastable surfaces, which survived during the thermal equilibrating process, possibly due to a high nucleation energy barrier for their removal. On the other hand, the ECS of pure nickel was observed to change dramatically into a spherical shape with facets of {111}, {100}, {110} and {210} without exception under a carburized atmosphere, which indicates that carbon not only facilitates surface diffusion by which energetically more stable surfaces can be easily developed but also decreases the surface energy anisotropy. Together with X-ray photoelectron spectroscopy studies, it was proposed that the carbon-induced changes in the ECS are possibly due to a solid solution effect, which could lead to a reduction in the binding energy among atoms in the bulk as well as on the surfaces.     

Oxygen and carbon monoxide adsorption on W(111): An investigation with angular resolved ESD,AES and LEED

Oxygen and carbon monoxide adsorption on clean W(111) surfaces have been studied by angular resolved ESD emission (ESDIAD). In addition, the specimen could be characterized in situ with AES and LEED. Adsorption was performed at room temperature. The electron stimulated desorption yielded O⁺ ions from the two investigated adsorption layers. Upon oxygen adsorption followed by subsequent annealing at least eight different ESDIAD patterns have been obtained. However, a convincing interpretation on the basis of the surface geometry can only be presented for three patterns produced without annealing as well as for one pattern at a very high annealing temperature. The difficulties are a consequence of complex structure changes which the surface undergoes in the intermediate annealing temperature range. This may influence the little known neutralisation probability of the desorbing ions. In this special case ESDIAD probably reflects in contrast to LEED a picture of some specific adsorption sites (minority species) and therefore, no clear correlation of the two techniques can be seen. ESDIAD from carbon monoxide shows four different patterns and supports the model of linear bonded CO molecules at room temperature with oxygen in the “standing up” position. At T > 900 K, CO starts to dissociate and results in similar ESDIAD patterns as obtained from O₂ adsorption.     

Electron energy-loss spectra of clean and gas-covered Ni(100) surfaces

Electron energy-loss spectra have been measured on Ni(100) surfaces, clean and following oxygen and carbon monoxide adsorption, at primary energies of 40–300 eV. The observed peaks at 9.1, 14 and 19 eV in the clean-surface spectrum are ascribed to the bulk plasmon of the 4s electrons, the surface plasmon, and the bulk plasmon of the coupled 3d + 4s electron, respectively, and the weak but sharp peak at 33 eV is tentatively attributed to the localized many-body effect in the final state. Assignments of the loss structures on the gas-covered surfaces have been attempted.     

Experimental aspects of angle-resolved photoemission from clean and adsorbate-covered metal surfaces

Instrumental techniques for the observation of angle-resolved photoelectron spectra from surfaces are reviewed. Results from the work of various groups on clean metals, particularly tungsten, copper, nickel and palladium, are described. Spectra induced by adsorption of hydrogen, oxygen, carbon monoxide and benzene on some of these metals are discussed.     

Crystallographic anisotropy in chemisorption: Nitrogen on tungsten single crystal planes

A molecular beam technique for the determination of sticking probabilities and surface coverages was used in earlier work to investigate the adsorption of nitrogen on tungsten {110}, {111} and {100} single crystal planes. In the present paper these studies have been extended to the {310}, {320} and {411} planes. Absolute sticking probabilities and adatom surface coverages are reported for crystal temperatures between 90 K and 960 K. Crystallographic anisotropy in this system is exemplified by zero coverage sticking probabilities with the crystal at room temperature: {110}, 1̃0^?2; {111}, 0.08; {411}, 0.4; {100}, 0.59; {310}, 0.72; {320}, 0.73. Results for planes on the [001] zone are quantitatively described by a general model developed for adsorption on stepped planes as an extension to the precursor-state order-disorder model for adsorption kinetics of King and Wells. It is shown that nitrogen dissociation only takes place at vacant pairs of {100} sites, but that subsequently the chemisorbed adatoms so formed may migrate out onto {110} terraces. The results are critically analysed in terms of the available LEED and work function data for nitrogen on tungsten single crystal planes, and the general model developed by Adams and Germer.     

Study of oxygen and carbon monoxide adsorption on molybdenum (110) by metastable helium de-excitation spectroscopy

The adsorption of oxygen and carbon monoxide on the (110) face of molybdenum is studied at room temperature by metastable helium atom de-excitation spectroscopy (MDS). On the basis of the interaction mechanism described by H.D. Hagstrum, and which was shown to be valid for the He¹?Mo couple in a previous article and again here, the secondary electron energy spectra are interpretated as relating to the self-convolution product of the electron state density of occupied valence band levels of the solid. The results obtained after deconvolution show the variations in this density of states during oxygen and carbon monoxide adsorption, and they are compared with photoemission spectra (UPS) obtained at the same time as the MDS spectra. They show that MDS is a technique well suited to the study of adsorption phenomena.     

Field ion microscope observations of surface rearrangement and oxide formation at clean rhenium surfaces heated in oxygen

The interaction between oxygen and clean, well-ordered rhenium surfaces has been investigated in the field ion microscope over a wide range of temperatures. The surface structure of vacuum annealed specimens was extensively modified by the presence of adsorbed oxygen. At partial monolayer coverages, surface metal atom rearrangement was inhibited, particularly at {112?2}, but at coverages near saturation, facets were produced at {101?0} at a lower temperature than on the clean surface, resulting in the exposure of an increased proportion of closepacked surface structures and suggesting a change in the rearrangement mechanism at high coverages. The production of facets at the major poles was temperature and coverage dependent as a result of the varying influence of adsorbed oxygen on surface free energy. A decrease in oxygen coverage was observed at 1200–1300 K on near-saturated surfaces, which may be associated with the formation of volatile ReO₃ from regions where oxygen concentration exceeds a critical value. There was little evidence of the production of a discrete surface oxide phase formed on specimens heated in the presence of gas phase oxygen, but a reduction in specimen radius as a result of volatile oxide formation was observed at pressures greater than 10^?4 Torr and temperatures above 1400 K. It is concluded that the final structure of rhenium surfaces heated in oxygen is dependent upon the rate of oxide formation, the effective oxygen coverage during oxidation, and the extent of surface rearrangement.     

Décomposition thermique du monoxide de carbone sur une surface de molybdène: Formation de couches superficielles de carbone et de carbure

We have investigated the decomposition of carbon monoxide on polycrystalline and (001), (110) monocrystalline molybdenum surfaces. This study was performed by massspectrometry, for thermal desorption studies, Auger electron spectrometry (AES), low energy electron diffraction (LEED) and photoelectron spectroscopy (ESCA). By heating the clean Mo surface in CO or by heating the Mo surface covered with CO, the dissociation of chemisorbed CO leads to a build-up of carbon layer which inhibits the subsequent adsorption. Two distinct types of fine structure are associated with the KLL line of carbon Auger spectra. If the Mo surface is heated at a temperature between 300 and 1500 K, the Auger peak is characteristic of a “graphite layer”. If the Mo surface is heated at a temperature up to 2000 K, the Auger peak is characteristic of a “carbure” layer. This “carbure layer” give rise to a surstructure which agrees with a Mo₂C surface layer and was also investigated by ESCA. Chemical shifts of (1s) C and (3d) Mo photoemission bands were observed and attributed to the bounding between Mo and C atoms in the Mo₂C layer.     

The adsorption and decomposition of methanol on tungsten. II

A clean tungsten filament adsorbs methanol rapidly at room temperature, the initial sticking probability being 0.8. At saturation, the composition of the adsorbed layer is roughly CO:H = 1:1 and it is suggested that the hydrogen may be in the form of a surface complex. The continuous decomposition of methanol by the hot filament under steady-state conditions, or when the filament had been previously oxygenated, followed a different course from that previously reported for the newly-cleaned filament. Rather than a rapid rise in the rate of decomposition (to CO + H₂) for 600 < T_fil < 1300 K to a high plateau above 1300 K, decompositon to formaldehyde, carbon monoxide and methane was observed. The rates at which these products appeared passed through low maxima between 900 and 1100 K. The change in the relative importance of formaldehyde and carbon monoxide production with filament temperature within this range is attributed to a temperature-dependent life-time of formaldehyde molecules on the oxygenated surface. At the highest temperature (> 1500 K) the reactivity increased rapidly to join that of the clean surface, probably due to the desorption of surface oxygen.     

FTIR spectroscopy of 2-0 band of carbon monoxide confined in silica aerogels with different pore sizes

The absorption spectra of carbon monoxide confined in three aerogel samples with different pore sizes have been recorded within the 4100–4400?cm^-1 spectral region at room temperature. The measurements were made using a Bruker IFS 125HR Fourier-transform infrared spectrometer. Lineshift and half-width values for CO were obtained. The influence of pore sizes on dependence of CO line half-width values on rotational quantum numbers was studied.     

Surface processing techniques using controlled environments and atomically-clean surfaces: Nitrogen-stabilized iron

Purposeful exposure of an atomically clean surface to controlled environments is explored as a technique for obtaining surfaces with desired properties. The incorporation of nitrogen into atomically clean iron at room temperature is discussed as an example of clean surface modification, resulting in significant enhancement of resistance to corrosion at room temperature in dry and moist air, relative to intially air-exposed samples. Treated and untreated sides of samples — subsequently exposed to air and moist air environments — have been characterized by optical and scanning electron microscopy, grazing-incidence electron diffraction, and depth-profiling Auger spectroscopy. Nitrogen treatment was found to inhibit iron from forming crystalline oxides.     

Field emission studies of oxygen on silver tips

Clean [111] oriented silver field emitting tips have been exposed to oxygen at 10^?3 Torr for 1 min at temperatures ranging from ? 170 to 200°C. From 50 to 200°C, an adsorption structure is formed that is stable in oxygen. The structure is characterized by intensely emitting regions on either side of enlarged {110}, {210} and {310} faces and a dark region in the (111)-{100} zone line directions. For adsorption from ? 170 to 200°C, the structure of the patterns depends distinctly on the adsorption temperature because the coverages are different and adsorption is activated. Oxygen adsorption at 10^?3 Torr for 1 min at 0°C causes an increase in the average work function of 1.15 eV. At 0°C, silver was exposed increasingly at 10^?6 Torr until 6100 L was reached. The work function increased progressively by 0.61 eV for this exposure. The {111}, {100}, {311}, {211} and {533} faces are attacked first. Then, the {110} faces are attacked followed by the {210} {310} and {320}. Heating of the adsorption layer formed at 0°C produced no changes in pattern and work function up to 100°C. Between 100 and 200°C, a strong decrease in work function and changes in the pattern result from oxygen penetration into the bulk.     

A study of the adsorption of several oxygen-containing molecules (O2, CO,NO, H2O) on Re(0001) by XPS,UPS and temperature programmed desorption

We have studied the adsorption of oxygen, carbon monoxide, nitric oxide, and water vapour on Re(0001), using X-ray and ultra-violet photo electron spectroscopies (XPS and UPS) and temperature-programmed desorption. As on polycrystalline rhenium, adsorbed oxygen is completely dissociated, even at room temperature. Furthermore, the formation of a superficial oxide at room temperature seems probable. Carbon monoxide is almost completely molecularly adsorbed, only a very small fraction being dissociatively adsorbed in a single β- state. However, an attractive interaction still exists between the adsorbed atoms in this β- state. Nitric oxide is adsorbed in a dissociated β₂ state and a molecular β₁ state. The population is smaller than on polycrystalline rhenium, corresponding to half a monolayer. Mathematical treatment of the desorption spectra allowed us to determine the activation energy for desorption of nitrogen resulting from the decomposition of adsorbed species. These quantities were found to be similar to those measured for polycrystalline rhenium.     

中心波长2.33 μm附近CO和CH4分子的同时探测

 采用分布反馈式半导体激光器作为探测光源,结合程长为100 m的离散型多通吸收池,采用直接吸收光谱技术,对室温下中心波长2.33 μm附近各种低体积分数的CO及混合气体（CO,CHCH₄和N₂）的直接吸收光谱进行了测量。选择CO在4 288.289 8 cm^-1位置的吸收谱线和CH₄在4 287.650 15 cm^-1处的吸收谱线进行痕量探测,在40 698 Pa的总压力下,实验测得CO的探测极限为8.15×10^-6(信噪比约为216）,CH4的探测极限为18.48×10^-6(信噪比约为147）。     

Rh超薄膜在Mo(111)及W(111)上引起{110}小面再构的可能性

用第一性原理总能计算方法,计算了Mo和W表面吸附金属Rh薄膜前后[111],[110]方向的表面能.计算结果表明,清洁Mo和W的（111）面不会发生{111}小面再构,与实验观察一致,当Rh的覆盖厚度达到一物理单层后,Rh/Mo（111）仍不会形成{110}小面;面Rh/W（111）满足小面再构到{110}的热力学条件,在一定条件下可能形成{110}小面. 关键词：     

Adsorption of carbon monoxide,oxygen, hydrogen,nitrogen, ethylene and benzene on an iridium (110) surface; Correlation with other iridium crystal faces

The interaction of CO, O₂, H₂, N₂, C₂H₄ and C₆H₆ with an Ir(110) surface has been studied using LEED, Auger electron spectroscopy and flash desorption mass spectroscopy. Adsorption of oxygen at 30°C produces a (1× 2) structure, while a c(2 × 2) structure is formed at 400°C. Two peaks have been detected in the thermal desorption spectrum of oxygen following adsorption at 30°C. The heat of adsorption of hydrogen is slightly higher on Ir(110) than on Ir(111). Adsorption of carbon monoxide at 30°C produces a (2 × 1) surface structure. The main CO desorption peak is found around 230, while two other desorption peaks are observed around 340 and 160°C. At exposures between 250 and 500°C carbon monoxide adsorption yields a c(2 × 2) structure and a desorption peak around 600°C. Carbon monoxide is adsorbed on an Ir(110) surface partly covered with oxygen or carbon in a new binding state with a significantly higher desorption temperature than on the clean surface. Adsorption of nitrogen could not be detected on either clean or on carbon covered Ir(110) surfaces. The hydrocarbon molecules do not form ordered surface structures on Ir(110). The thermal desorption spectra obtained after adsorption of C₆H₆ or C₂H₄ are similar to those reported previously for Ir(111) consisting mostly of hydrogen. Heating the (110) surface above 700°C in the presence of C₆H₆ or C₂H₄ results in the formation of an ordered carbonaceous overlayer with (1 × 1) structure. The results are compared with those obtained previously on the Ir(111) and Ir(755) or stepped [6(111) × (100)] surfaces. The CO adsorption results are discussed in relation to data on similar surfaces of other Group VIII metals.     

Preparation of atomically clean and flat Si(1 0 0) surfaces by low-energy ion sputtering and low-temperature annealing

Si(1 0 0) surfaces were prepared by wet-chemical etching followed by 0.3–1.5 keV Ar ion sputtering, either at elevated or room temperature (RT). After a brief anneal under ultrahigh vacuum (UHV) conditions, the resulting surfaces were examined by scanning tunneling microscopy. We find that wet-chemical etching alone cannot produce a clean and flat Si(1 0 0) surface. However, subsequent 300 eV Ar ion sputtering at room temperature followed by a 700 °C anneal yields atomically clean and flat Si(1 0 0) surfaces suitable for nanoscale device fabrication.     

An investigation of the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1}

Reflection absorption infrared spectroscopy (RAIRS) and temperature programmed desorption (TPD) have been used to investigate the effect of pre-dosed O atoms on the adsorption of NO on Pt{2 1 1} at room temperature. RAIRS experiments show that no new species are formed when NO is adsorbed onto a Pt{2 1 1} surface that has been pre-dosed with oxygen and no species are lost from the spectra, compared to spectra recorded for NO adsorption on the clean Pt{2 1 1} surface. However pre-dosed oxygen atoms do influence the frequency and intensity of several of the observed infrared bands. In stark contrast, pre-dosed O has a large effect on the TPD spectra. In particular N₂ and N₂O desorption, seen following NO adsorption on the clean Pt{2 1 1} surface, is completely inhibited. This effect has been assigned to the blocking of NO dissociation by the pre-adsorbed O atoms. A new NO desorption peak, not seen for NO adsorption on the clean Pt{2 1 1} surface, is also observed in TPD spectra recorded following NO adsorption on an oxygen pre-dosed Pt{2 1 1} surface.     

Auger spectra of lithium hydride

Auger electron emission spectra have been observed for lithium hydride in three conditions : (1) cleaved in vacuum, (2) prepared by the reaction of hydrogen gas with clean lithium metal, and (3) by annealing slightly oxidized lithium hydride single crystals in vacuum. The dominant Auger line (40 ± 1 eV) was found to be a KVV transition involving valence electrons from the anion and was indistinguishable from a similar transition for lithium oxide at room temperature. Lithium hydride surfaces lose hydrogen in vacuum causing the formation of a lithium metal phase at room temperature and a significant reduction in surface hydride stoichiometry at 600°C.