Oxygen surface coverage

Time-dependent evolution of the gas-phase O atom signal at Etrans —0.19eV. At t = 0 the monitoring starts and is completed at t = 600 s when the oxygen surface coverage at the Al(lll) surface is 0.2. (Reproduced from Ref. 40). 

At high temperatures (> 170 K), the water desorbs and so the autocatalytic reaction cannot be sustained and is an explanation for why the H2 + 02 reaction slows, the formation of OH species now being solely dependent on the H(a) + O(a) reaction, which is the slowest step in the above scheme. That the water + oxygen reaction was fast and facile was evident from the spectroscopic studies at both nickel and zinc surfaces, when the oxygen surface coverage was low and involving isolated oxygen adatoms. 

Figure 12. (a) Comparison of ethylene-d4 oxide selectivity and model-predicted average subsurface oxygen concentration as a function of pulse number, (b) Comparison of carbon dioxide production and model-predicted average oxygen surface coverage as a function of pulse number. 

Here i is the formation rate of new phase nucleates on a unit surface. It can be shown [111] that the overall oxygen surface coverage, without taking into account the above assumptions, is... 

After substituting the expression for u and i in the equation for the determination of oxygen surface coverage and integrating by parts, we obtain (assuming that vt is very high)... 

Fig. 4. Sustained oscillations that occur during the oxidation of CO over Pt(lOO) at 500 K, Poj = 5 X 10 Pa, Pco = 5 X lo Pa. The dashed vertical lines that coincide with the sudden decrease in the c(2 X 2) signal occur at the maximum work function (oxygen coverage), which also corresponds to the maximum production rate of CO2. The next vertical line corresponds to the maximum in the hex signal, and the simultaneous minima for the oxygen surface coverage and the CO2 production rate (after 82).

Fig. 2. Temperature-programmed desorption (TPD) spectra from 4.0 L of 2-C3H7I adsorbed on Ni(lOO) surfaces predosed with various amounts of oxygen. Three regimes are observed for this system (1) that for the clean nickel, where only the hytbogenation-dehydrogenation steps typical of transition metals are seen (left) (2) that for nickel oxide, where there is little reactivity, and where only complete oxidation is observed (right) and (3) that for an intermediate oxygen surface coverage, where some partial oxidation is manifested by the appearance of a TPD peak for acetone around 350 K (center).

This moving reaction front behavior implies that not the reaction rate as a function of a varying oxygen surface coverage is measured, as was plarmed, but the reaction rate as a function of the location of the reaction front in the reaction. 

The results from the simple analytical model (sect. 2.2.) allow us to define a rather simple ignition criterium for the simulations in terms of a vertical tangent of the temperature derivative of the oxygen surface coverage at the ignition point, i.e. OOoldT —> oc. It is... 

However, this reaction can only occm at relatively low oxygen surface coverage, because the nitrogen/oxygen bond of NO needs to be broken. For that reason, this route to nitrogen formation can be activated after the initial desorption of N2O and H2O from the smface. Eventually, platinmn is reduced and the oxygen smface coverage is lowered. 

Ammonia oxidation is conducted on a pre-oxidised platinum sponge catalyst. Figure 20 shows the conversion and selectivity at 373 K. The same selectivity characteristics as on the reduced platinum sponge catalyst are observed (Fig. 14). Thus, a high oxygen surface coverage does not favour initial nitrous oxide formation. The main difference with the reduced platinum sponge is the faster deactivation of the pre-oxidised catalyst below 413 K. 

The formation of nitrogen via an NO(a) intermediate is not favourable at low temperatures. At temperatures below 380 K, it has been reported on Pt(lOO) that the dissociation of NO is prohibited.Both proposed reactions for the formation of N(a) via NO(a) require the dissociation of nitric oxide, and therefore the recombination of N ad-atoms is a more feasible option. Moreover, the formation of NO at these conditions is apparently not favourable, because the deactivated catalyst is mainly covered with NHx species (NH and NH2) instead of with NO. Thus, at low surface coverage, NO seems not to be the dominant species, probably due to the low oxygen surface coverage which disfavours the formation of NO. 

The TPO experiment (Fig. 18) showed that NO desorbs from platinum from about 423 K, but only at high oxygen surface coverage. In Fig. 14, a drastic decrease of nitrogen and N2O formation is observed, which can be explained in terms of the moving reaction front through the catalyst bed. As the reaction zone arrives at the last positions, N2O cannot decompose anymore, since there is no fresh platinum surface left. As the last positions are deactivated, the catalyst s activity sharply decreases and the surface remains mainly covered with NH and NH2. This is supported by XPS N(ls) measurement and indirectly by NO pulse experiments. 

Fig. 2. Pxilsing oxygen at 523 K shortly after reduction, 2.5 10 molecules/pulse, 2 s/pulse. A. Raw data, B. 02-conversion as a function of pulse number, C. 02-conversion as a function of oxygen surface coverage before pulse, assuming 0o= 0 before first pulse, 6o= 1 before last pulse...

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