Subsurface oxide

It is interesting to compare our results on single crystal surfaces with those of Turner and coworkers for Pt, Pd, and Ir. In this study wires of Pt formed less than one layer of oxide under CO oxidation conditions. Considering that the Pt wires were known to have substantial Si impurities, which form subsurface oxides , it is not surprising that some oxide was formed. The absence of impurities on the rigorously cleaned, Pt single crystal surface used in this study precluded the formation of any oxides during CO oxidation. 

Due to the tendency of hydrocarbons in the soil to undergo subsurface oxidation, measuring COj levels in the soil gas could be used as a cost-effective field screening tool. In one soil-gas survey, COj levels in soil gas correlated well with petroleum hydrocarbons in the soil (Diem et al. 1988). 

The effect of metal structure and phase formation on the kinetics of catalytic oxidation reactions was treated in detail by Savchenko et al. (see, for example, refs. 83, 84, 117 and 118). In metal surface layers both reconstruction of the metal proper (faceting) and processes associated with the formation of surface oxides can take place. In this case the first to form can be chemisorption structures (without breaking the metal-metal bond) and then the formation of two-dimensional surface oxides is observed. Finally, three-dimensional subsurface oxides are produced. An important role is played by the temperature of disordering the adsorbed layer. 

Riekert in his theoretical investigation [173] showed that, in oxidation reactions over metals, critical effects can arise if, under reaction conditions, the existence of both a pure metal surface and a subsurface oxide that can contact with the reactants is thermodynamically possible. 

Adsorption methods may be used to provide information about the total surface area of a catalyst, the surface area of the phase carrying the active sites, or possibly even the type and number of active sites. The interaction between the adsorbate and the adsorbent may be chemical (chemisorption) or physical (phys-isorption) in nature and ideally should be a surface-specific interaction. It is necessary to be aware, however, that in some cases the interaction between the adsorbate and the adsorbent can lead to a chemical reaction in which more than just the surface layer of the adsorbent is involved. For example, when using oxidizing compounds as adsorbates (O2 or N2O) with metals such as copper or nickel or sulfides, subsurface oxidation may occur. 

Problems in the determination of the surface area of active phases can arise from a number of sources. An overestimation of the amount of active surface can be caused by spillover of the adsorbing species on the support, solubility in the adsorbent, subsurface oxidation (when using O2 or N20), or as a result of additional physical adsorption on the support. The extent to which these factors affect the accuracy of the results depends on the nature of the active phase, the support, and the conditions of the experiment. Problems can occur with adsorptives which may corrode the surface. For example, CO can remove Ni atoms from the surface of small Ni particles even at ambient temperatures. 

The use of N2O to determine Cu surface areas requires great care to avoid subsurface oxidation. The frontal chromatography method, in which a dilute mixture of N2O and He (typically, 2% N2O) is passed over a large bed of catalyst until no further N2O is reacted, appears to be the most reliable method. In the pulse method the extent of subsurface oxidation depends on the temperature (a very serious problem above about 100 °C), the size of the N2O pulse, the size of the catalyst sample, the metal loading of the sample, and the geometry of the catalyst bed. In general, small pulses of N2O should be used, at temperatures below about 60 °C, with a deep catalyst bed (>1 cm). 

The composition of the surface may also depend on gas pressure, for example, a surface may change from that of a metal with adsorbed oxygen to a surface metal oxide (JJ-JP) or to a metastable (subsurface) oxide that cannot be identified in UHV or by other analysis (60,61). It is apparent that such pressure effects have a strong impact on the catalytic properties and that measurements under elevated pressure are desirable. 

In certain environments, localized anomalously low concentrations of soil O2 have been used by exploration geologists to indicate the presence of a large body of chemically reduced metal sulfides in the subsurface. Oxidation of sulfide minerals during weathering and soil formation draws down soil gas po below regional average. Oxidation of sulfide minerals generates solid and aqueous-phase oxidation products (i.e., sulfate... 

Munoz-Marquez MA, Tanner RE, Woodruff DP (2004) Surface and subsurface oxide formation on Ni(lOO) and Ni(lll). Surf Sci 565 1... 

Wilson, T. R. S., J. Thomson, S. Colley et al. (1985) Early organic diagenesis the significance of progressive subsurface oxidation fronts in jtelagic sediments. Geochim. Cosmochim. Acta 49, 811-22. 

Sato and Seo have studied the electronic properties of the subsurface oxide film by monitoring the continuous exo-electron emission which occurs on silver catalysts during an epoxidation reaction. They interpreted this effect as a thermo-electron emission from a non-stoicheiometric semi-conducting oxide film present on silver, the work function of which is lowered by the adsorption of ethylene. The heat of formation of the film was calculated to be 45 kJ mol L No exo-electron emission was observed on non-epoxidation catalysts, including copper. 

One way in which the adsorption of a chlorine atom could affect more than one adsorption site is if the chlorine is incorporated into the subsurface oxide layer. No direct evidence of chloride accumulation in the catalyst subsurface has been published. However, there is at 373 K an apparent competition between chlorine and oxygen for adsorption sites which, we have argued above, correspond to the formation of the first monolayer of the oxide film. In view of this it would be surprising if chloride accumulation in the subsurface did not occur under practical epoxidation conditions. The net result would be to modify the electronic properties of this semi-conducting layer and hence the adsorptive properties of the surface. The chloride catalysed reorganization of surface silver atoms is perhaps indirect evidence of such an affect. ... 

Reactions (7) and (8) represent the formation of a chloride-doped, oxygen-deficient, subsurface oxide film, which we believe portrays the true nature of the catalyst. Oxygen is then adsorbed on this surface as in reaction (9). The presence of surface and subsurface chloride will tend to inhibit the dissociative adsorption, leaving the associative form as the major reactive species. Ethylene can be reversibly adsorbed on Ag" or irreversibly adsorbed on the two oxygen species [reactions (10), (11), and (13)]. Reactions (11) and (12) lead to ethylene oxide via the intermediates observed by Kilty et al. and also Foice and Bell. With propylene, the hydroperoxide can be formed, which subsequently combusts... 

Herz and Shinouskis (ref.9), it is reasonable to assign the more reactive peak to surface 0, and the later peak to oxygen from subsurface oxide. Since the formation of bulk oxide is thermodynamically favored over the temperature range of the present work, the increasing oxygen isotherm of Fig. 5 is explained by the kinetics of this activated process. 

Characterization. The mechanism of subsurface oxidation of ultrahigh-molecular-weight polyethylene (UHMWPE) was characterized by EPR and NMR spectroscopy and imaging studies and Fourier transform IR... 

Fig. 7.13 Pd3d5/2 (left) and O li/Pd3/ 3/2 (right) XPS regions corresponding to different stages in the oxidation of Pd(lll). (a) Surface oxide, (b) subsurface oxide, and (c) bulk PdO. Peaks are normalized to the total Pd3rf5/2 and Pd3/ 3/2 area, respectively...

Alam TM, Celina M, Collier IP, Currier BH, Currier JH, Jackson SK, Kuethe DO, Timmins GS. -irradiation of ultrahigh-molecular-weight polythylene Electron paramagnetic resonance and nuclear magnetic resonance spectroscopy and imaging studies of the mechanism of subsurface oxidation. J Polym Sci Part A Polym Chem 2004 42 5929-59. 

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