Oxygen layer formation

Reductive removal of these oxygen layers is a slow kinetic process, commencing at potentials well below the characteristic potential for the layer formation on each metal. Thus, adsorption results based on the commonly used triangular potential sweep method can depend on the anodic potential excursions, the frequency of potential cycling and the number of cycles, that is, the catalyst surface history. Similarly, kinetic studies of oxygen reduction can be influenced by the dependence of the oxygen layer formation... 

Capacity data measured [35] under the same voltammetric conditions on Pt, Pd, Ir, Rh, and Au were used for a comparison of the adsorption of carbonaceous species on these metals. In Fig. 48 the difference Cq=q—Cq is plotted as a function of at the potentials given in parenthesis above the points. The potential of comparison could not be the same for all the metals because the formation of 0 starts at different potentials. A fixed potential which is independent of and is located before the beginning of the oxygen layer formation could be chosen for Pt, Pd and Au. For Ir and Rh the potential of the highest peak of the l/(oC —U curve was taken. This potential shifts slightly with... 

The first step of oxide-layer formation is oxygen adsorption (chemisorption). In the case of platinum, the process stops at this stage, and depending on the conditions, an incomplete or complete monolayer of adsorbed oxygen is present on the platinum surface. In the case of other metals, layer formation continues. When its thickness 5 has attained two to three atomic diameters, the layer is converted to an individual surface phase that is crystalline (more seldom, amorphous) and has properties analogous to those of the corresponding bulk oxides. 

The applicability of this technique is based on four assumptions (1) the number of coulombs per square centimeter used for the anodic formation of an oxygen layer (20H = Oads + H2O + 2e) and the oxygen evolution during the sweep are the... 

The applicability of this technique is based on four assumptions (1) the number of coulombs per square centimeter used for the anodic formation of an oxygen layer (20H=Oads + H20 + 2e) and the oxygen evolution during the sweep is the same in either the absence or presence of methanol (2) the same number of electrons per molecule, independent of the amount of the adsorbed methanol, is used up in the oxidation (3) the double-layer charging current is the same in the presence or absence of methanol and (4) the anodic sweep is sufficiently fast (here 800 V/s) such that oxidation of methanol, which diffuses from the bulk solution to the electrode during the sweep, is negligible (i.e., only the adsorbed methanol is oxidized). 

Aity quantitative inteqjretation is still difficult to make, since the mass variations result from several coupled processes replacement of adsorbed hydrogen, water molecules and supporting anions by strongly adsorbed species from methanol chemisorption, reorganization of the double layer, formation of oxygenated species at the platinum surface, etc. 

The first equation describes the blocked fraction of the catalyst surface and the second describes the catalyst heat balance. The blocking species in this case is oxygen bound as an oxide species. Thus, this model can be considered as a nonisothermal version of the isothermal oxide models discussed above. The experimental support for this model is the variations in CPD measured during the oscillations by other groups (165), indicating the formation and removal of an oxygen layer. 

It can be concluded that the formation of the voids in the center of SnOi particles is mainly a result of the Kirkendall effect [8] associated with a faster outward diffusion of Sn atoms as compared to the inward diffiision of oxygen atoms in the process of the surface oxide layer formation. This produces high density of vacancies at the metal side of the metal/oxide interface. Vacancies transform to vacancy clusters which then aggregate into holes. It might be expected from this model that the increase of oxygen content in the ambiance will result in the promotion of an inward diffusion of oxygen atoms into the Sn particles, and therefore, suppress the formation of holes. 

FIG. 23.16. The formation of composite silicon-aluminium-oxygen or silicon-magnesium-oxygen layers (see text). 

In the potential region from 0 to 3 V S H E the oxygen evolution can be excluded and the anodic current density is purely the oxide formation iox. Thus, the anodic charge ofthe oxide layer formation depends on the texture ofthe underlying Ti. For grain (a),... 

Depending on potential, oxygen is bound on the surface of noble metal electrocatalysts as a chemisorbed species or a surface oxide (7, 99). The formation of these surface oxygen layers is independent of Oj presence (7,779) and irreversible, with the exception of iridium (97,120,121). Irreversibility becomes more pronounced in the order Os < Ru < Rh < Pd Pt... 

PtO2 (752, 757), similar to some electrochemical oxygen layers. Figure 12 shows a possible structure of platinum oxides on various planes (752). The [1(X)] plane has a PtOj composition (752,757), while the bulk corresponds to a PtO oxide. Present information does not unambiguously point toward either surface oxide formation or chemisorption. Because of the apparent similarity of some surface oxygen species on catalysts and electrocatalysts, coordinated efforts in both fields using standardized techniques and procedures could resolve uncertainties. 

J. Camra, E. Bielanska, A. Bemasik, K. Kowalski, M. Zimowska, A. Bialas, M. Najbar, Role of A1 segregation and high affinity to oxygen in formation of adhesive alumina layers on FeCr alloy support, Catal. Today 105 (2005) 629. 

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