S. Affrossman

X-ray photoelectron studies of the reaction of clean metals (Mg, Al, Cr, Mn) with oxygen and water vapour

Abstract Variation in the X-ray excited photoelectron spectrum during exposure of clean magnesium, aluminium, chromium and manganese to oxygen and water vapour, provide data on the kinetics of reaction with these gases. With exposures of up to 10 −2 torr sec the intensity of the oxygen 1s peak is approximately proportional to the quantity of oxygen adsorbed on clean surfaces of magnesium, aluminium and chromium at room temperature. The X-ray photoelectron spectra are shown to differentiate between oxides, hydroxides and adsorbed water. The metals investigated give principally oxides in their reaction with water vapour at room temperature but hydroxides are commonly formed at low temperatures. On reaction with oxygen the surfaces of magnesium, aluminium and chromium become protected by the first layers of oxide formed, which is not the case for manganese. Small shifts in the O 1s peak are observed during the reaction of Al and Mg with oxygen.

X-ray excited Auger and photoelectron spectra of magnesium, some alloys of magnesium and its oxide

KLL, and KLM Auger and X-ray photoelectron spectra for pure magnesium, for Mg2Cu, Mg3Au and Mg3Bi2, and for MgO are reported. Assignments are made for almost all of the observed features in the Auger spectra. For the pure metal, internal photoemission peaks and a plasmon gain peak are shown to be present in the X-ray excited spectra.

The reaction of oxygen and water with iron films studied by X-Ray photoelectron spectroscopy

Abstract Adsorption of oxygen on iron at ambient temperature and low pressure is shown by XPS to give a chemisorbed species and the oxide, Fe 2 O 3 . At low temperatures a further adsorbed species is detected, similar to the nickel-oxygen system. Correction of the intensity of the oxygen signal for depth results in an oxidation curve in agreement with reported work using other techniques, i.e. oxidation is fast until about four layers of oxide are formed, at an exposure of ca. 10 2 L, and then proceeds slowly to about ten layers. Adsorption of water vapour produces an overlayer less than one layer in depth at an exposure of 10 5 L. Comparison of the overlayer depths calculated from the decrease in unoxidised iron signal intensity and from the increase in oxygen intensity gives good agreement for the thick oxide film produced by oxygen adsorption, but not for the thin overlayer formed by exposure to water vapour. This suggests a difference in packing of the ions in the thin overlayer compared to the arrangement in the bulk oxide.

The reactions of oxygen and water with the rare-earth metals terbium to lutetium studied by x-ray photoelectron spectroscopy

Abstract The X-ray excited oxygen 1s photoelectron spectra of water adsorbed on clean evaporated films of the heavy rare-earth metals, terbium to lutetium, is characterised by two peaks: (1) at 531.0 ± 0.5 eV binding energy, and (2) at 533.0 ± 0.5 eV binding energy, assigned respectively to oxide and hydroxide species. Variation of the relative intensities of these peaks with exposure to water leads to the postulate that the oxidation mechanism is island growth with a layer of hydroxide at the surface of the oxide island. At low temperatures, adsorption of water gives two additional peaks: (3) at 534.5 ± 0.5 eV, and (4) at 535.7 ± 0.2 eV, assigned respectively to chemisorbed and condensed water. On adsorption of small amounts of dry oxygen the O 1s spectra exhibit solely peak (1), whose intensity increases with further oxygen treatment to reach a steady value after ∼40 L exposure. The kinetics of reaction with oxygen follow a logarithmic relation, once correction is made for the effect of escape-depth on peak intensity. However, ytterbium, with a closed 4f shell in the metallic state, exhibits oxidation characteristics different from the other rare-earth metals; its oxygen 1s intensity increases linearly with exposure, and the steady plateau level is reached sharply rather than asymptotically. Island growth with a limited number of nucleation sites may explain this behaviour.

X-ray photoelectron studies of scandium and its hydride and oxide

Binding energies for the core levels of scandium metal and its oxide and hydride are recorded. Plasmon energy-loss peaks are observed for the metal and show a large shift on hydriding, approximately 3.7 eV, compared with a shift of approximately 0.2 eV for the 2p core levels. The large plasmon shift is consistent with the view that scandium forms a metallic hydride in which the H 1s electrons contribute to the Sc valence band.

Reactions of carbon monoxide, methanol and ethanol with clean scandium films

Abstract Scandium dissociates carbon monoxide to form a carbide and sorbed oxygen. Undissociated CO is also present at high coverages. Methanol or ethanol dissociate to leave oxygen preferentially at the surface at low exposures. As adsorption continues, a carbide is also formed, and the carbide and oxide form a β-CO type layer. Before saturation of this layer, alcohol is also adsorbed intact, or as an alkoxy species, as an overlayer. There are good agreements for the saturation adsorptions of carbide, oxide and alkoxy species from methanol and ethanol.

Oxidation of scandium by oxygen and water studied by XPS

Abstract Reaction of scandium with water at 293 K gives initially the oxide and, above ∼50 L, also the hydroxide. The ratio of hydroxide to oxide is approximately constant in the later stages of reaction. The reaction of water with scandium is slower and less extensive then with the lanthanides. At low exposures of water at 80 K only the hydroxide and chemisorbed water are produced, again in the contrast to the lanthanides. Oxidation with oxygen is complex. For small adsorptions of oxygen, the O 1s BE does not agree with the value for the bulk oxide, whereas for a large single step adsorption, they are in good agreement. The overlayer depths calculated from the metal and from the oxygen signals also differ for stepwise additions of oxygen. The results are tentatively interpreted as dissolution of the oxygen in the scandium, with the oxygen concentration below the value required for nucleation of oxide.

Oxygen-hydrogen titrations of platinum black partially poisoned by mercury

Abstract Oxygen-hydrogen titrations of a platinum black show that the stoichiometry of the reaction is variable. Blocking of the surface by mercury reduces the titre. The formation of the alloy PtHg is postulated, with the platinum at the surface of the alloy retaining reactivity for the titration reaction. Further mercury adsorbed on top of this platinum can be removed by heating at 200 °C. The amount of mercury remaining after heating tends to a monolayer of PtHg. The results suggest that the titration method of estimating the area of Pt catalysts is not critically dependent on the surface cleanliness.

Interaction of ozone with nickel ions adsorbed on alumina

Abstract Nickel ions adsorbed on alumina react strongly with surface oxygen species. Electron donation from the alumina facilitates oxidation of the nickel by ozone, producing an active oxide which is brown at low concentrations and black at high concentrations. The variation in color is attributed to a particle size effect. At very high concentrations, the interaction of nickel ions with alumina is reduced because the nickel compound used for impregnation crystallizes at the surface. Exposure of these samples to ozone does not produce the active oxide.

Surface acidity of trisodium hydrogen disulfate

Abstract The surface acidity of trisodium hydrogen disulfate is measured by titration with butylamine vapor at −78 °C. Titration at low temperature overcomes the problem of diffusion and reaction within the bulk material. Trisodium hydrogen disulfate is not hygroscopic, unlike sodium hydrogen sulfate, and is more readily prepared with a higher specific area than the single salt.