Oxygen surface covering

Figure XVIII-2 shows how a surface reaction may be followed by STM, in this case the reaction on a Ni(llO) surface O(surface) + H2S(g) = H20(g) + S(surface). Figure XVIII-2a shows the oxygen atom covered surface before any reaction, and Fig. XVIII-2h, the surface after exposure to 3 of H2S during which Ni islands and troughs have formed on which sulfur chemisorbs. The technique is powerful in the wealth of detail provided on the other hand, there is so much detail that it is difficult to relate it to macroscopic observation (such as the kinetics of the reaction).

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). 

Fig. 2. Typical curves of the relative changes of the electrical resistance of nickel films as a function of time (a) adsorption of one dose of hydrogen on the surface, partially covered by preadsorbed oxygen (b) adsorption of one dose of oxygen on the surface, covered by preadsorbed hydrogen (both at 300°K).

The clean surface of a Au(lll) crystal at 100 K and a surface covered with o = 0.12, 0.25, 0.75, and 1.0 was dosed with 10 and 50 L CO2. XPS studies of the C(ls) and 0(ls) regions did not reveal any significant new peaks after CO2 exposure. Appreciable CO2 chemisorption does not occur on the clean or oxygen-dosed Au(lll) surface, nor does a stable surface carbonate form under these conditions. 

Equating the rates of formation and removal of C(0) and again letting 0j be the fraction of active surface covered by oxygen atoms. 

A purely bifunctional mechanism was assumed to be operative in the CO oxidation on Pt3Sn surfaces as well as on other Pt alloys with oxophilic transition metals [157-159]. Here, the oxophilic Sn surface atoms are believed to provide nucleation sites for water and, following stepwise hydrogen abstraction, for its subsequent oxygenated surface products OH and O. CO oxidation on Sn atoms is unlikely [131,160] such that no competition for Sn sites occurs between water and CO molecules. All Pt atoms are covered with CO. 

A variety of models have been derived to describe the kinetics of semiconductor photocatalysis, but the most commonly used model is the Langmuir-Hinshel-wood (LH) model [77-79]. The LH model relates the rate of surface-catalyzed reactions to the surface covered by the substrate. The simplest representation of the LH model [Eq. (7)] assumes no competition with reaction by-products and is normally applied to the initial stages of photocatalysis under air- or oxygen-saturated conditions. Assuming that the surface coverage is related to initial concentration of the substrate and to the adsorption equilibrium constant, K, tire initial... 

Fig. 26. STM images of the oxygen pre-covered platinum(l 1 1) surface during reaction with hydrogen. Images were recorded at a temperature of T = 111 K with a time interval of 625 K. The white ring in the upper right corner is associated with a reaction front of OH intermediates from the autocatalytic reaction. The outside is characterized by an oxygen-terminated surface, whereas water molecules from the reaction are identified inside the ring. Adapted with permission from Reference (757).

When hydrogen is adsorbed on a tungsten surface covered with one layer of oxygen, hydrogen and oxygen are not desorbed at 1150°K. even in 20 min, (11). Evidently H is held more firmly on 0-W than it is on clean W. However, at 1200°K. H2O is desorbed from the 100 region in a short time at 1300°K. H2O is desorbed on the 111 plane at 1500°K. clean tungsten is obtained in 2 min. 

Fig. 12. Dependence of the rate of decomposition of NjO on the pretreatment of the surface of a CU2O catalyst at 6()°C, according to Dell, Stone, and Tiley. (I and II are on an evacuated CujO surface, and III and IV are on a CuzO surface covered with chemisorbed oxygen.)...

Fig. 18. Influence of molecular (B) and atomic (C) oxygen on a Pt surface covered with H atoms, investigated by the change of photoelectric emission. Ordinate photoelectric yield I in electrons per light quantum. X = 302.2 rn/i T = 293°K. (a) O2 admitted, po2 = 0.34 mm. Hg (6) O2 pumped off (c) O2 admitted, poz = 10 mm. Hg, Pt spiral kept at yellow heat for 2 min. (d) O2 pumped off [according to (58)).

A fundamental problem in characterizing metal surfaces in oxidation catalysis is that, as with transition metal oxides, the chemistry of the surface is shaped by the reaction conditions. Margolis has taken the plausible position that most metal surfaces in oxygen are covered with oxygen and behave like metal oxides (13, 14). This is true even of platinum, a classical example of a metal catalyst, and here again predictions from bulk thermodynamics are unreliable with respect to the surface. 

We have suggested that t and Da are similar for the surface covered with both chemisorbed oxygen and NiO. If the reaction rate were determined by the impact of gas-phase oxygen molecules with the surface, it would be constant. But in experiments [112] no constant reaction rate was observed, therefore this limiting case has not been considered. 

A lightly sintered compact of monosized 20 nm diameter, Sb205-doped (1 wt%) tin oxide particles is conditioned in an enclosure so that the surface is atomically clean. With the compact held at 300 °C oxygen gas is steadily admitted to the enclosure and the electrical resistance of the compact monitored. At a stage in the process the resistance shows a marked increase. Estimate the fraction of surface covered by the chemically adsorbed species at this stage. [Answer 2%. Hint consider Eq. 4.49]... 

Oxygen adsorption is inhibited by pre-adsorbed CO. At coverages below 0co = V3, Oads and COads form separate domains on the surface. Remarkably, the behaviour is different when O is pre-adsorbed. Then formation of mixed phase of Oads and COads occurs (with local coverages of Go = Geo = 0.5) which are embedded into CO domains. When these mixed phases are present CO2 is produced even below room temperature. Co-adsorption studies on other noble metal surfaces are consistent with this picture pre-adsorbed CO inhibits the dissociative adsorption of oxygen whereas CO does adsorb on a surface covered with O. The CO-covered surface is densely packed with CO, but the oxygen atom-covered surface is less densely packed with CO. 

Figure 12.8 PAES spectra for a Si(100) surface covered with 0.3 ML of Au (a) before and (b) after exposure to 400 Langmuir of 02- The oxygen exposure caused a decreas in the Si peak but not the Au peak. This indicates that the positrons are being pushed away from the Si due to selective adsorption of oxygen at exposed Si sites. A schematic drawing indicating this mechanism is shown in Figure 12.9 below.

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