Showing papers on "Chemisorption published in 1972"

Chemisorption on Copper

Abstract: Although it is much less active than the transition metals copper does catalyse reactions of hydrogen and carbon monoxide. The nature of the weak adsorptions of these gases has been investigated by surface potentials, low energy electron diffraction, and infrared spectroscopy. Hydrogen is dissociatively adsorbed by copper but the adsorption is activated. Surface potential measurements have provided isotherms and isosteric heats of adsorption. Carbon monoxide is adsorbed in two distinct stages giving positive and negative surface potential changes. The positive stage is accompanied by a sharp infrared absorption band near 2100 cm−1 which is sufficiently strong to permit reflection spectroscopic studies on single crystal surfaces. Frequency shifts occur during the negative stage. Direct comparisons of surface potentials, diffraction patterns, and vibrational frequencies have been made and will be discussed in relation to the chemisorption bond.

Chemisorption of Hydrogen on the (111) Plane of Nickel

Abstract: The adsorption of hydrogen on the (111) plane of nickel at room temperature has been investigated using the technique of flash desorption spectroscopy. The residual CO partial pressure has been reduced below 1 × 10−11 torr to minimize the effect of H2 and CO coadsorption. The experimental apparatus and crystal cleaning procedure are described. Hydrogen was observed to desorb in a single pressure burst for all the experimental conditions examined. Four techniques have been used to analyze the experimental results and a coherent picture emerges for low surface populations. The adsorption energy, Arrhenius preexponential factor and sticking coefficient have been determined for surface hydrogen populations less than 1013 atoms/cm2, the values obtained being 22.7 kcal/mole, 2×10−1 cm2 atoms−1 · sec−1, and 0.1, respectively. Furthermore it is shown that the adsorption energy is constant at low coverages. For surface populations greater than 1013 atoms/cm2, we have been able to reproduce the desorption spectra with a model in which either the adsorption energy decreased with adsorbed population or the pre‐exponential factor increased with population. It is not possible to distinguish between the two possibilities. Such a model does not however describe very well the observed adsorption kinetics. We have proposed that this discrepancy may be due to diffusion of hydrogen into the nickel when the crystal is heated.

Chemisorption of hydrogen on evaporated copper films

Abstract: Surface potentials (s.p.) of atomic hydrogen adsorbed at low temperature on clean copper films show wide variations from one film to another. This is believed to reflect varying proportions of different crystal planes. Initial adsorption with a negative s.p. may be followed by a weaker adsorption with a positive effect. The negative s.p. corresponds to hydrogen which can be in equilibrium with gaseous molecular hydrogen. This spontaneous dissociative chemisorption has been studied between 242 and 337 K at pressures up to 60 Torr, using vibrating capacitor s.p. measurements to follow the equilibrium concentration of adatoms. Isosteric heats (typically 40–50 kJ mol–1) are almost independent of coverage on some films, and a major part of the adsorption can then be described approximately by the Langmuir isotherm for dissociative adsorption.

Oxygen species adsorbed on oxides. Part 1.—Formation and reactivity of (O–)s on MgO

Abstract: Nitrous oxide reacts with Fs+(H) centres (surface oxygen ion vacancies with a single trapped electron) to give (O–)s+ ions adsorbed at the anion vacancy. The reaction, Fs+(H)+ N2O →(O–)s++ N2 is quantitative and the reflectance spectrum indicates a band centred at 2 eV which is ascribed to the (O–)s+ centre. Comparison of the optical transition energy calculated from the e.s.r. data with the observed transition shows that the normally accepted value of 135 cm–1 for the spin-orbit coupling of the O– ion is too low and that a value of >200 cm–1 is more appropriate for the solid state. Nitrous oxide also reacts with other surface centres to give an adsorbed species which may be (O2–2)s or (O–2)s. The reaction of (O–)s+ with hydrogen reforms the Fs+(H) centre which then reacts with more nitrous oxide; the reaction can be cycled several times. The reactions of (O–)s+ with O2, CO2, alcohols and olefins are also investigated. To facilitate reference to various types of surface defects and adsorbed species a nomenclature is proposed which is compatible with a recently suggested notation for defects in bulk lattice.

Effects of Oxygen on Silver Surface Structure

Abstract: The chemisorption of oxygen on the (100), (110), and (111) faces of silver single crystals was studied with LEED, secondary electron spectroscopy, and other auxiliary techniques. The effect of exposures to oxygen at pressures up to 1 Torr and temperatures up to 500 °C was investigated using a crystal isolation valve. The chemisorbed oxygen causes mainly a strong faceting of the (100) and (110) faces, and a 4×4 superstructure on the (111) face. The effect of the internally diffused oxygen on the surface behavior was also investigated.

The Reactivity of Low Index [(111) and (100)] and Stepped Platinum Single Crystal Surfaces

Abstract: Several high Miller index crystal surfaces of platinum have been shown to consist of low index (111) or (100) terraces of constant width, linked by steps of monatomic height. The surface structures that form in the presence of diatomic molecules (H 2 , O 2 , CO, NO), aliphatic and aromatic hydrocarbons on the (111), (100) and on these stepped platinum surfaces were studied by low energy diffraction. Several catalytic reactions (H 2 + D 2 ; H 2 + O 2 ; dehydrocyclization of n -heptane) that take place on the various platinum crystal surfaces at low pressures were monitored by means of a mass spectrometer and the surface composition by Auger electron spectroscopy. The chemisorption characteristics of diatomic molecules on stepped platinum surfaces are markedly different from those on low index [(111) and (100)] platinum surfaces. Organic molecules of different types form ordered surface structures on low index faces of platinum, but decompose rapidly on stepped surfaces under identical experimental conditions. Surface chemical reactions of diatomic molecules that were studied take place only on stepped surfaces at a detectable rate. The dehydrocyclization of n -heptane to toluene occurs slowly on the Pt(lll) face. The rate of decomposition of n -heptane on stepped platinum surfaces is rapid and the carbon deposit that forms prevents dehydrocyclization. In the presence of hydrogen, however, dehydrocyclization occurs at a rapid rate on stepped surfaces with atomic terraces of (111) orientation, while at a slower rate on stepped surfaces with atomic terraces of (100) orientation. It appears that catalytic reactions can readily be studied using one face of a single crystal of small surface area. Dissociation of diatomic molecules and breaking of C—C and C—H bonds occur preferentially at atomic steps, while the structure of the atomic terraces plays an important role in the more complex dehydrocyclization reaction.

Metal Catalysts Supported on Zeolite. I. A Study on Platinum Particles Distribution

Abstract: Zeolite 13Y, loaded with platinum cations by ion exchange, was calcined in the air and subsequently reduced in a hydrogen stream. Hydrogen and carbon monoxide adsorption studies, together with an electron microscopic observation of the catalyst, revealed that the size distribution of platinum particles is controlled by the calcination temperature. The hydrogen chemisorption method gave the average particle diameters, which were in fairly good agreement with the observations made by means of an electron microscope on the catalyst with a platinum particle diameter larger than 20 A, while trace amounts of hydrogen chemisorption were observed for platinum particles less than 20 A.

Infra-red studies of reactions on oxide surfaces. Part 1.—Boron trifluoride on silica

Abstract: Infra-red spectroscopy has been used to study the adsorption of boron trifluoride on silica. On highly dehydroxylated samples chemisorption is rapid and the major product initially appears to be a surface SiOBF2 species. This species is also formed when BF3 interacts with totally dehydroxylated silicas which have been prepared by heating silica in vacuum at about 1250°C. Spectroscopic evidence indicates that on highly or totally dehydroxylated silica, BF3 preferably reacts with a new reactive site, probably a siloxane bridge site, which is formed as a result of the removal of the surface silanol groups during heating. Only after the reaction is complete on the “siloxane” sites does chemisorption take place with silanol groups if these are present. During prolonged evacuation the surface SiOBF2 groups react further with trace amounts of water to produce (SiO)2BF groups; the latter are also produced during the initial stages of chemisorption when the silanol concentration is high. When water or air is admitted after chemisorption of BF3, a spectrum characteristic of boric acid is observed.