Oxygen subsurface

In this sense subsurface oxygen is also acting as a promoter. The role of the alkali promoter is then to stabilize Cl and anionically bonded O (or nitrate ions) on the catalyst surface, so they can exert their promotional action. Thus alkalis in this system, which requires electronegative promoters according to the mles of section 2.5, are not really promoters but rather promoter stabilizers. This is proven by their inability to promote selectivity in absence of Cl. 

A.van Oertzen, A. Mikhailov, H.-H. Rotermund, and G. Ertl, Subsurface oxygen formation on the Pt(l 10) surface experiment and mathematical modeling, Surf. Sci. 

S. Ladas, R. Imbihl, and G. Ertl, Kinetic Oszillatotions during the catalytic CO oxidation on Pd(110) The role of subsurface oxygen, Surf. Sci. 219, 88-106 (1989). 

The following are now expected first, in the CO oxidation electrode potential range on Pt/Ru surfaces (i.e., at low potentials), entrapped oxygen should be found second, the entrapped (subsurface) oxygen concentration between the Ru layers should increase with increasing multilayer character (and coverage) of the Ru deposit on Pt. Research focusing on these two issues is planned. 

Todorova M, Li WX, Ganduglia-Pirovano MV, Stampfl C, Reuter, Scheffler M. 2002. Role of subsurface oxygen in oxide formation at transition metal surfaces. Phys Rev Lett 8909 6103. 

Figure 3.15 O Is / Ag 3d5/2 XPS intensity ratio as a function of take-off angle for two oxygen species on polycrystalline silver. The data corresponding to an O 1 s binding energy of 528.4 eV are attributed to subsurface oxygen in Ag, the other with a binding energy of 530.5 eV to oxygen atoms adsorbed on the Ag surface (data from Baschenko et al. (39J).

In principle, one can take the interpretation further and calculate what oxygen concentration profile fits the measurements of the O/Ag ratio best. In fact, Baschenko et al. [39] did this and concluded that the subsurface oxygen resides mainly in the third and fourth atomic layer below the surface. Although the result appears plausible, it should be noted that such calculations are only permitted when the surface satisfies the requirements of lateral homogeneity and absence of roughness discussed above. As the O/Ag experiments were done with polycrystalline foils, one might wonder whether too detailed an analysis is warranted. Anyhow, the work forms a nice illustration of what angle-dependent XPS can achieve on catalytically relevant adsorbate systems. 

CO2 production time series, 39 88 equation structure, 39 87-88 Kurtanjek s mechanism, 39 91 oxide models, 39 89-92 subsurface oxygen model, 39 90-91 selective, 30 136-137 small organic molecules, chemical identity of adsorbed intermediates, 38 21 states... 

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... 

The modeling results yield values for the sticking coefficient, and the ratio of surface to subsurface oxygen that are consistent with those of other workers. The results indicate that the activation process involves a concurrent filling of surface and subsurface sites, but that the latter sites are filled much more slowly. This is shown graphically in Figure 11. 

Figure 12. (a) Comparison of ethylene-d4 oxide selectivity and model-predicted average subsurface oxygen concentration as a function of pulse number, (b) Comparison of carbon dioxide production and model-predicted average oxygen surface coverage as a function of pulse number. 

Buildup of Surface Carbon and Subsurface Oxygen and Formation of Stabilized Defect Sites. 

Although these reactions have been researched extensively and are the subjects of numerous patents, the precise reaction mechanism is not fully understood. The controversy has mostly centered on the nature of the oxygen species responsible for ethylene oxide formation (103). The results of various surface characterization studies indicate that there are at least three types of adsorbed oxygen species on silver monoatomic chemisorbed oxygen, diatomic (molecular) oxygen, and subsurface oxygen. The first results from a dissociative adsorption of oxygen on a silver surface ... 

---