Adsorption of oxygen

The AES spectra for chemisorbed and incorporated oxygen on Ir(110) are shown in Fig. 20, together with the assignments of the transitions observed (124). Although differences are observed indicating that the chemical bonding differs in the two cases, no substantial features that would allow 

A second criterion for distinguishing between chemisorbed and incorporated oxygen is their reactivity towards H2 or CO, Chemisorbed oxygen is 

Conversion of chemisorbed oxygen to an incorporated species that can be reduced by CO has been studied by Taylor et al. (124) on Ir(l 10). The results (shown in Fig. 22) were obtained by annealing an oxygen covered surface to various temperatures for a fixed time after which CO was adsorbed and the C02 evolved during thermal desorption with a linear temperature rise was registered. It is seen that C02 is evolved at two distinct temperatures that 

However, given the uncertainty in the experimental determinations and the necessary assumption that the best LEED pattern corresponds to complete occupation of all available sites, the unambiguous assignment of a coverage to the 2 x 2 structure can only be done with a LEED intensity calculation. Order-disorder transitions at elevated temperatures have been reported for the 2 x 2 structures formed on Ru(001) (148) as well as on Rh(l 11) (146). In the latter case this process is irreversible. 

Adsorption on the (100) plane is complicated through the reconstruction observed for the Pt and Ir surfaces. On the Rh(100) surface which is unreconstructed, Castner et al. (89) have observed that room temperature adsorption leads to a 2 x 2 pattern at low coverages and a c2 x 2 pattern at higher coverages that presumably correspond to 0 = 0.25 and 9 = 0.5, respectively. Adsorption of 02 on the reconstructed clean Pt(100) surface has not been seen (69, 98, 154), whereas adsorption on the metastable 1 x 1 surface occurs rapidly with the formation of 5 x 1 and 2 x 1 patterns (69,152). The 5 x 1 pattern is formed directly upon exposure to oxygen whereas the 2 x 1 pattern is formed from the 5 x 1 structure after partial reduction with H2 (152). 

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If adsorption of oxygen on such an oxide involves the process... 

An example of the fomiation of a new reconstmction is given by certain fee (110) metal surfaces. The clean surfaces have (1x1) synunetry, but become (2x1) upon adsorption of oxygen [16, 38]. The (2x1) synuiietry is not just due to oxygen being adsorbed into a (2 x 1) surface unit cell, but also because the substrate atoms rearrange themselves... 

Fig. 3.58. Dependence of peak intensities of Nb and Ta on the oxygen peak intensity for adsorption of oxygen on the pure metais [3.144],...

Two other methods worth discussing are wet air oxidation and regeneration by steam. Wet oxidation may be defined as a process in which a substance in aqueous solution or suspension is oxidized by oxygen transferred from a gas phase in intimate contact with the liquid phase. The substance may be organic or inorganic in nature. In this broad definition, both the well known oxidation of ferrous salts to ferric salts by exposure of a solution to air at room temperature and the adsorption of oxygen by alkaline pyrogallol in the classical Orsat gas analysis would be considered wet oxidations. 

Physical adsorption of oxygen resulting in the formation of one or more monolayers of oxide and requiring no activation energy. 

The rate and mechanism are different on the basal plane and edge sites of carbon. The reactions involving oxygen are two to three orders of magnitude slower on the basal plane than on the edge sites, because of the weak adsorption of oxygen molecules on the basal plane surface [34]. 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

The maximum oxygen uptake as well as the heat of adsorption also vary significantly with varying substrate. The adsorption of oxygen is accompanied... 

M. Ayyoob, and M.S. Hegde, An XPS study of the adsorption of oxygen on silver and platinum surfaces covered with potassium or cesium, Surf. Sci. 133, 516-532 (1983). 

The simplest example of oxygen spillover is found in the adsorption of oxygen on carbon. The spillover oxygen migrates from the basal carbon (donor) to carbon atoms exposed at steps between layers of the graphite surface, where it reacts with the edge carbons (acceptor).71 In this case the donor and acceptor phase consist of the same material with different surface properties. 

Thus if the working and reference electrodes are made of different metals, and there is negligible adsorption of oxygen on the two metals, in which case Or=o,m and Ow=eO0lM then equation (7.19) gives the cpd (contact potential difference) of the two metals ... 

The reaction mechanism is fairly straightforward. It starts with the dissociative adsorption of oxygen on silver ... 

MacDonald, W.R. and Hayes, K.E. (1970) Comparative study of the rapid adsorption of oxygen by silver and gold. Journal of Catalysis, 18, 115-118. 

Types of aging processes such as those described for the adsorption of oxygen and methanol on platinum are also characteristic of the adsorption of other substances and of adsorption on other electrodes. In some cases these changes manifest themselves as a change (mostly an increase) in the adsorption energy with time. In... 

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