Chikashi Egawa

Adsorption of Na atoms and oxygen-containing molecules on MgO(100) and (111) surfaces

The adsorption of Na atoms and oxygen-containing molecules such as H 2 O, CH 3 OH, CO 2 , HCOOH and HCOOCH 3 has been studied on the annealed MgO(100) and (111) surfaces by means of XPS, UPS and LEED. Annealing a (111) surface at 1000 K yielded microfacets ~ 20 A across. This faceted (111) surface showed a high adsorption activity compared with the flat (100) surface. Hence it is suggested that the coordinatively unsaturated edge-sites or the multicentered valley-sites on the microfaceted surface play important roles in the chemisorption.

Adsorption of CH3OH, HCOOH and SO2 on TiO2(110) and stepped TiO2(441) surfaces

Abstract A TiO2(441) surface was prepared whose electronic states and chemisorption properties for CH3OH, HCOOH and SO2 were compared with those of a TiO2(110) surface by means of XPS, UPS and LEED at 298 K. The (441) surface had a regular step structure which is indexed as [3(110) × (111)]. Its work function was smaller by 0.7 eV than that of the (110) surface due to the pinned Fermi level originated from a small amount of Ti3+ species. CH3OH and HCOOH were molecularly adsorbed on both the surfaces, giving a p(2×1) structure for HCOOH adsorbed on (110). It was suggested from the change of the work function that they were absorbed with their dipole axes normal to the local crystal plane on the step sites, thus the adsorbed species at step sites being more inclined as compared with those on the (110) terrace. SO2 was absorbed on (441) forming SO2-3 and also reacted with Ti3+ probably at the step to form S2-, while only SO2-3 was detected on the (110) surface.

Modification of surface electronic structure on TiO2(110) and TiO2(441) by Na deposition

Abstract The geometric and electronic properties of Na overlayers deposited on TiO 2 (rutile) (110) and stepped (441) surfaces have been examined by means of XPS, UPS, XAES (X-ray excited Auger electron spectroscopy), EELS and LEED. Na atoms form a smooth atomic layer interacting with surface oxygen atoms on TiO 2 (110). A surface model consisting of ordered Na 2 O dimers is proposed for a c(4×2) structure which appears at 0.5 ML Na coverage. At the first monolayer, the Na 2 O units are considered to complete an array along the [001] direction in (1×1) periodicity. Bonding of Na with oxygen causes charge transfer to the substrate, resulting in a downward band bending toward the surface. With the increase of the band bending, the Fermi level crosses an unfilled surface state localized in five-fold coordinated Ti 4+ ions, which leads to the reduction of these Ti 4+ to Ti 3+ . The strong interaction of the TiO 2 (110) surface toward the Na overlayer is ascribed to surface states just above the valence band maximum localized on protruded ridge oxygen atoms, from comparison with previous results for a flat MgO(100) surface. Almost the same picture is drawn for a stepped TiO 2 (441) surface.

Photoelectron spectroscopic study of clean and CO adsorbed NI/TiO2(110) interfaces

Abstract Nickel deposited on a rutile TiO 2 (110) surface was examined by XPS, AES, UPS and LEED. The nickel deposits formed an atomic layer at 300 K with a density of 8 × 10 14 cm −2 . A three-dimensional agglomeration was detected above 1 ML, by analyzing the forward scattering of photoelectrons from the Ni 2p core level. The work function deduced from UPS decreased with Ni coverage with a minimum at 0.5 ML. From its initial decrease by 0.7 eV we evaluated the outward dipole to be 0.5 debye per Ni atom. This dipole was attributed to an electron transfer from the Ni deposits into the substrate. The charge transferred was estimated to be not more than 0.1 electron per Ni adatom. As the overlayer grew more dense, the lateral interaction between Ni adatoms predominated and inhibited the electron transfer through the interface. XPS and UPS data revealed that CO was adsorbed molecularly on the Ni/TiO 2 (110) surfaces at 300 K for all coverages. The uptake of CO increased proportionally to the Ni coverage below 1 ML.

The behaviour of CO adsorbed on Ru(1,1,10) and Ru(001); the dissociation of CO at the step sites of the Ru(1,1,10) surface

A multimethod investigation (TDS, AES, XPS and angle-resolved UPS) has been performed to elucidate the behaviour of CO adsorbed on Ru(1, 1, 10) and Ru(001) and the effect of the step structure on the surface. The thermal dissociation of CO was observed firstly on the stepped Ru surface as a new desorption peak ( β ) at ∼ 580 K. The atomic oxygen was confirmed with AES, XPS and UPS for the β -state of CO on Ru(1, 1, 10) even though molecularly adsorbed CO was not observed with UPS. The β -peak in TDS spectra were assigned to the associative desorption of C and O on the basis of TDS experiments with isotopes. On the flat Ru(001) surface, on the other hand, the oxygen formed from the dissociation of CO coult not be detected with electron spectroscopies. It was found with angle-resolved UPS that CO molecules adsorbed vertically on the (001) planes of both Ru(001) and Ru(1, 1, 10) surface. CO molecules which adsorbed around step sites were found to be oriented in a direction with an angle of ∼ 30° from [001] in the direction of [551]. The inclined CO molecules around steps were decreased by the presence of dissociated CO, which suggested that the dissociation of CO occurred at the step sites.

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

Abstract 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 10 1 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 18 O 2 and provide four CO desorption peaks in TPR spectra, which are assigned to α-C 18 O,s-C 18 O 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 s-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 O 2 even at low temperature such as 200 K. It was also confirmed that active carbon species are formed on Ru surface during COH 2 reaction.

Active structures and electronic states for adsorption of CO2 and NO on an Na/TiO2(110) surface

X.p.s. and u.p.s. studies have shown that submonolayer coverages of Na deposited on a rutile TiO2(110) surface remarkably enhance the adsorption of carbon dioxide and nitrogen monoxide forming carbonate and nitride, respectively. The amount of adsorbed CO2 varied with Na coverage showing an S-shaped dependence, where a critical Na coverage of 0.3 monolayer for CO2 adsorption was observed. This threshold coincides with the onset of a c(4 × 2) structure derived from ordered ‘Na2O-dimers’, which suggests that the basicity of oxygen atoms on the TiO2(110) surface is markedly enhanced by the ‘Na2O-dimer’ ensemble of four Na atoms. Single or paired Na atoms play a negligible role in the basic promotion. In contrast, NO decomposes on the Ti3+ cation reduced by Na deposits. The oxidation state of Ti dominates the decomposition of NO.

Adsorption and decomposition of ammonia on W(100); XPS and UPS studies

Abstract The adsorption and decomposition of ammonia on a clean and c(2 × 2)-N ordered W(100) surface has been studied by photoemission spectroscopy (XPS and UPS). At 120 K molecularly adsorbed ammonia was identified by N(1s) core level emission at 400.9 eV and the valence emissions at 7.6 and 11.7 eV. By heating the sample stepwise the N(1s) core level shifted to lower binding energy. In the valence region, the corresponding spectral changes were obtained, where the dependence of the peak intensity on photon energy was observed. These observations were interpreted to demonstrate that adsorbed ammonia dissociates its hydrogen successively to form NH x (a) and finally to atomic nitrogen. On the other hand, ammonia was molecularly adsorbed on a c(2 × 2)-N ordered surface even at temperatures as high as 300 K, although the spectra at 400 K or above were very similar to those under a steady state flow condition, where the tungsten surface was mostly covered by atomic nitrogen. At higher ammonia pressure up to about 100 Pa thicker nitride layers were formed at 700 K, which were characterized by the N(1s) core level at 397.3 eV and a broad emission around 6 eV in the valence level.

Adsorption of N2 and NH3 on Mo(111)

The surface structure of the Mo(111) plane in the adsorption of N 2 and NH 3 has been investigated by means of low energy electron diffraction (LEED). The adsorption of N 2 and NH 3 at room temperature shows no extra new LEED spots except for an increase of the background intensity. Heating of this surface to 850 K or higher caused faceting of the (111) surface to produce the (433) face of molybdenum. On this surface only nitrogen atoms are adsorbed; it exhibits the c(3×2) in-registry overlayer structure of the (433) face from the low nitrogen coverage to the sturated one 0.35, which is determined by the nitrogen Auger intensity. This saturated coverage is consistent with the surface coverage of the c(3×2) overlayer structure (0.33). The sticking coefficient of N 2 adsorption is about 0.05 and it is constant irrespective of the coverage, which suggests island formation of superstructure on the surface. On theother hand, the sticking coefficient at room temperature is only slightly larger than at 850 K. Photoemission spectra obtained from He I or He II also revealed the same atomic nitrogen level at about 5 eV, indicating that the electronic state of the nitrogen atoms is the same at both temperatures.

Ammonia decomposition on (1 1 10) and (0 0 1) surfaces of ruthenium

The decomposition of ammonia on stepped Ru(1 1 10) and flat Ru(0 0 1) surfaces has been investigated by Auger electron spectroscopy, low-energy electron diffraction, thermal-desorption studies and kinetic studies. The reaction takes place at ca. 400 K, and N2 and H2 are formed stoichiometrically. At lower temperatures ( 600 K) the rate of reaction is linearly dependent on ammonia pressure only and independent of hydrogen and nitrogen pressures; the amount of adsorbed hydrogen on the surface was negligible. Since no isotope effect was observed, the reaction is thought to proceed through the recombination of N(ads) on the surface.