Oscillation subsurface oxygen

PbOj anode, 40 155-156 oxygen evolution, 40 109-110 PCE, catalytic synthesis of, l,l,l-trifluoro-2,2-dischloroethane, 39 341-343 7t complex multicenter processes of norboma-diene, 18 373-395 PdfllO), CO oxidation, 37 262-266 CO titration curves, 37 264—266 kinetic model, 37 266 kinetic oscillations, 37 262-263 subsurface oxygen phase, 37 264—265 work function and reaction rate, 37 263-264 Pd (CO) formation, 39 155 PdjCrjCp fCOljPMe, 38 350-351 (J-PdH phase, Pd transformation, 37 79-80 P-dimensional subspace, 32 280-281 Pdf 111) mica film, epitaxially oriented, 37 55-56... 

Additionally, there is some experimental evidence for the crucial role subsurface oxygen is thought to play during the oscillations. Under conditions in which subsurface oxygen is assumed to be formed, complex LEED patterns evolve, temperature-programmed desorption (TPD) experiments yield oxygen coverages that exceed one monolayer, and two different reactivities with CO are observed (248). 

Detailed studies of the coadsorption of oxygen and carbon monoxide, hysteresis phenomena, and oscillatory reaction of CO oxidation on Pt(l 0 0) and Pd(l 1 0) single crystals, Pt- and Pd-tip surfaces have been carried out with the MB, FEM, TPR, XPS, and HREELS techniques. It has been found that the Pt(l 0 0) nanoplane under self-osciUation conditions passes reversibly from a catalytically inactive state (hex) into ahighly active state (1 x 1). The occurrence of kinetic oscillations over Pd nanosurfaces is associated with periodic formation and depletion of subsurface oxygen (Osub)- Transient kinetic experiments show that CO does not react chemically with subsurface oxygen to form CO2 below 300 K. It has been found that CO reacts with an atomic Oads/Osub state beginning at temperature 150 K. Analysis of Pd- and Pt-tip surfaces with a local resolution of 20 A shows the availability of a sharp boundary between the mobile COads and Oads fronts. The study of CO oxidation on Pt(l 0 0) and Pd(l 1 0) nanosurfaces by FEM has shown that the surface phase transition and oxygen penetration into the subsurface can lead to critical phenomena such as hysteresis, self-oscillations, and chemical waves. 

Finally, it may be said that the character of the CO -I- O2 oscillating reaction on Pd differs remarkably from that on Pt because (a) different subsurface oxygen (Pd) and (hex) -o- (1 x 1) phase transition (Pt) mechanisms apply and (b) the oxygen front in CO-I-O2 waves travel in reverse directions on Pd it goes from (1 1 0) to (1 0 0) surface, on Pt it travels in the opposite direction. 

S. Ladas, R. Imbihl, G. Ertl, Kinetic oscillations during the catalytic Co oxidation on Pd(llO)—the role of subsurface oxygen. Surf Sci. 219(1-2), 88-106 (1989)... 

M.R. Bassett, R. Imbihl, Mathematical-modeling of kinetic oscillations in the catalytic co oxidation on Pd(llO)—the subsurface oxygen model. J. Chem. Phys. 93(1), 811-821 (1990)... 

The process of catalyst oxidation and reduction can be treated as a reversible phase transition [136]. It is to this process that the authors of recent investigations [37, 47-49, 85] ascribe critical effects. When studying kinetic self-oscillations in the oxidation of hydrogen over nickel [37] and measuring CPD, the authors established that the reaction performance oscillates between the states in which oxygen is adsorbed either on the reduced or on the oxidized nickel surface. Vayenas et al. [47-49], by using direct measurements of the electrochemical activity of 02 adsorbed on Pt, showed that the isothermal self-oscillations of the ethylene oxidation rate over Pt are due to the periodic formation and decomposition of subsurface Pt oxides. A mathemati-... 

Qualitative studies of this dynamic model with three variables, i.e. surface concentrations of CO and the two forms of oxygen (surface and subsurface), showed [170] the possibility of interpreting self-oscillations in this catalytic system. Recently a comprehensive analysis of this model [170] has been carried out [177], Sales et al. [178, 179] determined experimentally the parameters for the oxidation and reduction of the Pt subsurface layer. The application of these parameters and those for the CO oxidation over Pt that are close to the values measured in high-vacuum experiments, made it possible to perform the quantitative reproduction, by using the model [180], of almost the whole of the experimentally observed characteristics for the self-oscillations in the reaction rate of CO oxidation over Pt. 

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