Oxygen, adsorption

Oxygen adsorbed on Ni(lll) at room temperature produced three LEED patterns depending on the oxygen exposure, as seen in other laboratories. 70 exposure of 1-2 L, a p(2x2) 

Heating race — 5 K/s. The inset is che real space structure of C0-c(4x2) on Ni(lll) surface. 

Oxygen Adsorption. The initial heat of adsorption of oxygen on pre-reduced silver metal is high ( 336 kJ mol As the oxygen coverage increases the 

Under more normal epoxidation conditions the equilibrium amount of adsorbed and absorbed oxygen is equivalent to about 9 monolayers, a similar thickness to that detected by electron diffraction after several cycles of oxidation and reduction with CO. An indirect measurement of the slow response of the subsurface oxygen concentration to changes in gas-phase oxygen pressure and its effect on catalyst performance has been carried out by Levchenko et al. A sudden reduction in oxygen partial pressure results in an immediate loss of activity followed by a much slower response with relaxation times in the range 10—100 min. The activation energy for this process was about 126 kJ mol .  

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. 

The 34 kJ mol activated adsorption process has been ascribed by Kilty et al. to the formation of an associatively adsorbed species. They arrived at this conclusion by studying the effect of pre-adsorbed chloride on oxygen adsorption. Since the activation energy for O2 adsorption, calculated from EPR data, is around 32 kJ mol this interpretation appears to be correct. 

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 

The interpretation of the beneficial effect of alkali modification on oxygen adsorption has to include both stabilization of the adsorbed oxygen atoms on alkali modified sites, due to direct alkali-oxygen interactions, but also 

It follows that in the adsorption state, the O—O bond length varies from 0.121 to 0.135 and 0.137 nm for vertical and parallel adsorption forms, respectively. This bond length is close to the 0—0 bond length in peroxides (0.149 nm). It means that after adsorbed by the FeS2 surface, the oxygen is more like peroxides rather than O2. It is clearly in line with the works reported by Ahlberg and Broo (1996a,b,c). 

The calculated DOS of the parallel O2 adsorption structure is given in Fig. 9.8. It can be seen that the shoulder peaks at -10 eV is the bonding interaction between the 0 (2p) and the Fe (4s), (4p) and (3d) orbital. The antibonding counterpart starts at 0 eV above the Fermi level and is essentially composed of the Fe (3d) orbital and 0 (2p) orbital. The area from -7.5 eV to 0 eV (and above) concerns the interactions of the 0 (2p) with the Fe (4s) and (3d) for the peak. According to the above calculation and discussion, some conclusions can be summarized as follows. 

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In general, then, anion-forming adsorbates should find p-type semiconductors (such as NiO) more active than insulating materials and these, in turn, more active than n-type semiconductors (such as ZnO). It is not necessary that the semiconductor type be determined by an excess or deficiency of a native ion impurities, often deliberately added, can play the same role. Thus if Lr ions are present in NiO, in lattice positions, additional Ni ions must also be present to maintain electroneutrality these now compete for electrons with oxygen and reduce the activity toward oxygen adsorption. 

Heiland W, Iberl F, Taglauer E and Menzel D 1975 Oxygen adsorption on (110) silver Surf. Sc/. 53 383-92... 

DeWit A G J, Bronckers R P N and Fluit J M 1979 Oxygen adsorption on Ou(110) determination of atom positions with low energy ion scattering Surf. Sc/. 82 177-94... 

Duarte H A and Salahub D R 1998 Embedded cluster model for chemisorption using density functional calculations oxygen adsorption on the AI(IOO) surface J. Chem. Phys. 108 743... 

Small changes in impurity content did not affect this rate but the presence of water vapor and changes in partial pressure of oxygen were critical (61,62). Steam and various impurities and binders also affect the oxidation of siUcon carbide (63). Differences have been observed in oxygen adsorption on the different SiC crystal faces (64). 

Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

Reconstruction of the Cu(l 11) close-packed surface at room temperature upon oxygen adsorption has been reported by Niehus.590 This result is in good agreement with data on cyclic voltammetry and second-harmonic generation591 it has been concluded that oxygen-containing spe-... 

M.V. Badani, and M.A. Vannice, Effects of cesium and chlorine on oxygen adsorption on promoted Ag/a AI203 catalysts, Appl. Catal. A 204, 129-142 (2000). 

Figure 4.43. Thermal desorption spectra after gaseous oxygen adsorption on a Pt film deposited on YSZ at 673 K and an 02 pressure of 4x 10"6 Torr for 1800 s (7.2 kL) followed by electrochemical O2 supply (I=+15 pA) for various time periods.29-30 Reprinted from ref. 30 with permission from Academic Press.

These important conclusions are not limited to Pt/YSZ only. Similar is the behaviour of Ag/YSZ31,119 (Fig. 4.44) and Au/YSZ.119 In the last case actually gaseous oxygen adsorption is negligible.119... 

Both the TPD spectra (Fig. 5.2b) and the cyclic voltammograms (Fig. 5.2c) show clearly the creation of two distrinct oxygen adsorption states on the Pt surface (vs. only one state formed upon gas phase 02 adsorption, Fig. 5.2b, t=0). 

The latter acts as a sacrificial promoter. It is a promoter, as it forces oxygen to populate the weakly bonded (and highly reactive) oxygen adsorption state. It is also sacrificed as it is consumed by C2H4 at a rate I/2F, equal to its rate of supply. 

In a broad sense similar effective double layers can be formed via gaseous adsorption or evaporation (e.g. Na evaporated on Pt electrodes deposited on fT-A Ch has been shown to behave similarly to electrochemi-cally supplied Na). In other cases, such as the effective double layer formed upon anodic polarization of Pt deposited on YSZ, the electrochemically created effective double layer appears to be unique and cannot be formed via gaseous oxygen adsorption at least under realistic (<300 bar) oxygen pressure conditions. 

On the fuel-rich side (kadPo2 kpc2H4) 0o is near zero, the oxygen adsorption step is the rls and equation (8.1) reduces to ... 

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