Oxygen catalyst surface, desorption

Subsequently one plots InNo vs tHe and extrapolates to tHe=0. This plot provides the 02 desorption kinetics at the chosen temperature T. The intersect with the N0 axis gives the desired catalyst surface area NG (Fig. 4.8) from which AG can also be computed. More precisely NG is the maximum reactive oxygen uptake of the catalyst-electrode but this value is sufficient for catalyst-electrode characterization. 

It can be seen from the figure that the qra(02) is much higher than qeoxygen from the catalyst surface is the slowest step in this reaction. 

The adsorption of NO, under lean conditions was studied by imposing a step change of NO and NO2 feed concentrations in the presence and absence of excess oxygen over the reference catalysts in a fixed-bed flow microreactor operated at 350 ° C and analyzing the transient response in the outlet concentrations of reactants and products [transient response method (TRM)[. The adsorption/desorption sequence was repeated several times in order to condition the catalytic systems fully due to the regeneration procedure adopted (either reduction with 2000 ppm H2 + He or TPD in flowing He), BaO was the most Ba-abundant species present on the catalyst surface. FT-IR spectroscopy was used as a complementary technique to investigate the nature of the stored NO species. 

More imeresiing is the analysis of ihe peaJc shape which reflects both the rate of formation and the rate of desorption. Assuming the latter constant for MA (a reasonable assumption, because the rate of desorption depends primarly on the nature of the moiecule), the shift in the maAtmum and especially the mictal slope deviation indicate a decrease in the turnover number of the specific sites (or cluster of sites) responsible for maleic anhydride formation. In these conditions, in fact, no iiuemiediate products desorb from the catalyst surface. Probably, this derives frc m the inhibition in the availability of neighboring oxygen sites as discussed above. 

The authors also studied the N2O decomposition on the same catalysts and found it to be much faster than NO decomposition. Since both reactions leave Oacatalyst surface it was concluded that oxygen desorption is not the rate-determining step of NO decomposition on these solids. On the other hand, the retarding effect of O2 gas is consistent with the idea that O2 <-> 20 (ads) is in equilibrium. In brief, all their data are consistent with a mechanism in which the active sites for NO decomposition are coordinatively unsaturated Cu2+ ions on the surface that can be easily oxidized to Cu3+ upon NO adsorption ... 

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