Oxygen monolayer

On Au(lll) with o =10, a single H2O desorption peak is observed at 168 K, as shown in the right panel of Figure 6. This peak is due to physisorbed water on the oxygen monolayer. This coverage of oxygen blocks the high temperature desorption state, which indicates that the direct adsorption of water to the Au(lll) surface is blocked. 

Chemisorption measurements have shown that ethylene does not adsorb on pure silver, but only on a silver surface which has been preoxidized [339]. Complete coverage with an oxygen monolayer, however, also seems to destroy the capacity to adsorb ethylene, as was demonstrated by Force and Bell [114,116] (favouring the idea of adsorption on silver). Consequently, partial oxygen coverage seems to be a necessary condition for catalytic activity. 

The average heat of formation of an oxygen monolayer varied from sample to sample and correlated linearly with the intrinsic activity of the catalysts for ethene... 

Fig. 44. Dynamic scaling in the growth of Bragg peaks for LEED scattering from oxygen monolayers at 0 = 1/2 and T — 297 K for adsorption in the p(2xl) structure on W(110). Different symbols denote different times t in seconds) after the adsorption has taken place. The halfwidth (used for normalization exactly as in fig. 43) is denoted by FWHM ( full width at half maximum ). From Wu et al. (1989).

Inactivity is not readily ascribed to a scarcity of conduction-band electrons for negative ion formation (10). Such an explanation is valid only for n-type semiconductors, and Te films at least seem likely to be p-type and should be on the basis of this theory form a complete oxygen monolayer with fair ease. 

A quantitative comparison between these two sets of experiments cannot be made since they were performed under very different conditions, one under static 10 torr O2, the other after the injection of a pulse containing 1% CH4 and 10% O2. However, a qualitative comparison of the shapes of the two curves implies important differences. The most important one is that while the oxygen uptake curve presented a clear break when a monolayer was completed, no break was present in the conversion curve From this behavior the authors have concluded that chemisorbed oxygen was not the active species. They proposed that oxidized Pd is much more active and that the activity continuously increases with the extent of oxidation until a plateau is reached after formation of several oxygen monolayers. 

Figure 3. A typical current-step charging curve of Pt in acid solution. Segments of the curve correspond to A, hydrogen desorption B, double layer charging C, oxygen adsorption D, end of oxygen monolayer buildup C, oxygen desorption.

Vanadium nitride has high electrical conductivity (1.67 xlO S.mi) compared to the limiting conductivity of vanadium oxide powders (V20s-nH20, 1 X 10 S.m ) [95]. Unlike titanium nitride, which does not exhibit any oxygen functionalities, vanadium nitride was confirmed by FTIR spectroscopy to have an oxygen monolayer (V2O5 0.5 nm) on the surface of the spherical nitride crystal (400°C sample 6.33 nm crystal size, 38 m. gr BET area). The... 

One valuable chemisorption technique is the hydrogen-oxygen titration reaction. First proposed by Benson and Boudart for Pt catalysts [59], it represents the titration of an oxygen monolayer by hydrogen near 300 K and its stoichiometry for Pt was predicted based on equations 3.1 and... 

In a typical TPIE experiment, the equivalent number of monolayers of atoms exchanged can be calculated as a function of time, and the time elapsed for which the first oxygen monolayer is exchanged can be also determined. Prior to this time < 1), the apparent activation energy for surface oxygen mobility kj-mol ) can... 

---