Showing papers on "Chemisorption published in 1977"

Kinetics of activated chemisorption. Part 2.—Theoretical models

Abstract: In activated chemisorption the plot of the reciprocal of the rate Z=(dq/dt)–1 against the time t is convex towards the Z axis at low t and concave at high t. This condition is satisfied if both the energy of activation, Et and the number of available sites, Nt decrease with the coverage q and if d2Et/dq2 < 0. Two models are considered:

The theory of grain boundary segregation in terms of surface adsorption analogues

Abstract: The theory of equilibrium solute segregation at solid/solid interfaces can be approached usefully from classical surface/gas adsorption isotherms of varying complexity and encompassing both chemisorption and physisorption behavior. Thus, the McLean segregation theory, the simplest type of behavior, is the counterpart of the Langmuir theory for solid/gas adsorption. Assuming strong pair interactions between segregate neighbors, the McLean theory can be adapted to a more complex form, the analogue of the Fowler isotherm, in which adsorbate clustering or protoprecipitation occurs. Again, multilayer segregation is possible, which can be rationalized by " BET " adsorption analogue behavior. Recent developments show how isotherms may be extended to multicomponent systems, assuming interactions between elements and segregants, with important implications in the behavior of embrittlement-prone engineering steels. The mass action equilibrium approach for surface adsorptive reactions is extended here to grain boundaries and tested on the limited number of isotherms available. This may prove to be a useful analytical procedure, giving a rapid insight into the nature of the segregation process, such as adsorbate interactions and variations in adsorption potential among the various sites.

Surface Composition and Selectivity of Alloy Catalysts

Abstract: Publisher Summary Alloying of a catalytically active metal with an inert component changes the selectivity in hydrocarbon reactions such that C–C bond fission is suppressed compared with C–H bond breaking. Upon alloying, the selectivity for cracking reactions decreases. This chapter shows that the surface composition of macroscopic alloys often differs from the bulk composition. Selectivity changes can be understood in terms of the primary ensemble effect—that is, the dilution of the metal surface with inactive atoms diminishes the probability for metal-adsorbate complexes containing several neighboring metal atoms as required for cracking. The concept of an ensemble of metal atoms, crucial to the understanding of selectivity patterns on alloys, implies that the metal atoms in the surface of the alloy keep their individuality and are only influenced by their immediate environment. The chapter discusses the secondary ensemble effect that ascribes changes in heat of chemisorption of multiply bonded atoms to a decrease in the coordination of these atoms to the surface metal atoms. This effect leads to a decrease in heat of adsorption upon alloying.

Quantitative study of the charge transfer in chemisorption; oxygen chemisorption on ZnO

Abstract: A new experimental approach is presented for studying the charge‐transfer process involved in the chemisorption on polar semiconductors. This approach utilizes the surface piezoelectric effect, contact potential difference measurements, and surface photovoltage spectroscopy. From the study of oxygen adsorption on ZnO it was found that the rate of electron transfer varies exponentially with the surface barrier height and is proportional to the oxygen pressure (from 10−3 to 20 Torr). Furthermore, it was found that the charge transfer is characterized by a thermal activation energy of about 0.72 eV. At room temperature this activation energy constitutes the most significant rate‐limiting factor and is largely responsible for the extremely slow rate of chemisorption. A model for chemisorption was developed in which the thermal activation is treated as an intermediate nonelectronic step involving metastable activated surface states. Upon capturing electrons from the bulk these states become stable surface stat...

Ultraviolet photoemission and flash‐desorption studies of the chemisorption and decomposition of methanol on Ni(111)

Abstract: The chemisorption and decomposition of methanol (CH3OH) on Ni(111) has been studied by ultraviolet photoemission spectroscopy (UPS) and flash‐deposition mass spectroscopy. The UPS spectra suggest that exposure at ∠80 K produces chemisorbed CH3OH in which the chemisorption interaction affects two molecular orbitals, the oxygen lone‐pair oriented perpendicular to the COH plane and an in‐plane orbital having mixed lone‐pair and σ‐bonding character. Upon warming to ∠160 K, some CH3OH desorption occurs and the UPS spectrum changes markedly. Further warming to ∠300 K produces H2 desorption and leaves chemisorbed CO as the final decomposition product on the surface.

Active Sites in Heterogeneous Catalysis

Abstract: Publisher Summary This chapter focuses on the active sites in heterogeneous catalysis. The chapter reviews a surface structure and chemisorption characteristics of crystals cut along different crystallographic orientations. A chemical reaction is studied at low pressure to establish correlations between reactivity and surface structure and composition. The same catalytic reaction is studied at high pressures and the pressure dependence of the reaction rate is determined using the same sample over the nine orders of magnitude range. The rates and product distributions that were determined at high pressures on single-crystal surfaces are compared with the reactivity of polydispersed small-particle catalyst systems. At low pressures, a quadrupole mass spectrometer is used as a detector of both the chemical reaction rates and the product distributions. At high pressures, a gas chromatograph is employed that is as sensitive as a mass spectrometer that is used at low pressures. Small-surface-area single-crystal catalyst samples can readily be used in studies as long as the reaction rate is greater than 10−6 product molecules/surface atom/sec. The rate defined is called “turnover number” in the field of catalysis.

A UPS and LEED/Auger study of adsorbates on Fe(110)

Abstract: Ultraviolet photoemission spectroscopy (UPS) and LEED/Auger were used to study adsorbed species of C, N, O, S, CO, NO, and C2H2 on Fe(110). The complicated “carbon ring” LEED patterns were shown to be due to atomic carbon and/or nitrogen. Molecular nitrogen does not stick at or above room temperature on Fe(110). The optical excitation probability of the 3p electrons of segregated sulphur is found to have a Cooper minimum aroundhω=40.8 eV. Carbon monoxide chemisorbs molecularly at room temperature and then dissociates slowly. Only dissociative CO adsorption was observed atT=385 K. Acetylene also adsorbs molecularly but does not dissociate at room temperature. By contrast, nitric oxide chemisorption is completely dissociative at room temperature.