O2 adsorption

Figure A3.10.25 Arrhenius plots of CO oxidation by O2 over Rli single crystals and supported Rli/Al203 at PCO = PO2 = 0.01 atm [43]. The dashed line in the figure is the predicted behaviour based on the rate constants for CO and O2 adsorption and desorption on Rli under UHV conditions.

Isotherms for H2O and / -hexane adsorption at room temperature and for O2 adsorption at Hquid oxygen temperature on 13X (NaX) zeoHte and on the crystalline Si02 molecular sieve siHcaHte are are shown in Figure 8 (43). SiHcaHte adsorbs water very weaMy. Further modification of siHcaHte by fluoride incorporation provides an extremely hydrophobic adsorbent, shown in Figure 9 (44). These examples illustrate the broad range of properties of crystalline molecular sieves. 

A TWC catalyst must be able to partition enough CO present in the exhaust for each of these reactions and provide a surface that has preference for NO adsorption. Rhodium has a slight preference for NO adsorption rather than O2 adsorption Pt prefers O2. Rh also does not cataly2e the unwanted NH reaction as does Pt, and Rh is more sinter-resistant than Pt (6). However, the concentrations of O2 and NO have to be balanced for the preferred maximum reduction of NO and oxidation of CO. This occurs at approximately the stoichiometric point with just enough oxidants (O2 and NO ) and reductants (CO, HC, and H2). If the mixture is too rich there is not enough O2 and no matter how active the catalyst, some CO and HC is not converted. If the mixture is too lean, there is too much O2 and the NO caimot effectively compete for the catalyst sites (53—58). 

Figure 5.22 reveals the ability of solid state electrochemistry to create new types of adsorption on metal catalyst electrodes. Here oxygen has been supplied not from the gas phase but electrochemically, as 02 via current application for a time, denoted tj, of 1=15 pA at 673 K, i.e. at the same temperature used for gaseous O2 adsorption (Fig. 5.21). Figure 5.23 shows the effect of mixed gaseous-electrochemical adsorption. The Pt surface has been initially exposed to po2 =4x1 O 6 Torr for 1800 s (7.2 kL) followed by electrochemical O2 supply (1=15 pA) for various time periods ti shown on the figure, in order to simulate NEMCA conditions. 

Adzic RR, Wang JX. 1998. Configuration and site of O2 adsorption on the Pt(lll) electrode surface. J Phys Chem B 102 8988-8993. 

O are being formed predominantly on the surface of oxides as a results of O2 adsorption. According to estimates of the authors of study [60] the fraction of oxygen chemisorbed on ZnO as 0 does not exceed 8%. It is assumed that formation of O2 and O at low temperatures are weakly connected processes. At temperatures higher than 200 C O2... 

In our view the final verification was given to this conclusion in paper [66] in which simultaneous O2 adsorption on partially reduced ZnO and resultant change in electric conductivity was studied. It was established in this paper that the energies of activation of chemisorption and that of the change of electric conductivity fully coincide. The latter is plausible only in case when localization of free electron on SS is not linked with penetration through the surface energy barrier which is inherent to the model of the surface-adjacent depleted layer. 

Table 2. Heats offormation, H (kcal), magnitude of electron density, Q (Of), displace from the cluster to adsorbed oxygen molecules, equilibrium interatomic distances R (A-B) and bond orders P (A-B), corresponding to them, in PANI-O2 adsorption complexes.

O2 adsorption, 38 227-228 olefin bond, strength, 30 348-349 one-component, 24 173-213 output, 24 222, 223 palladium, see Palladium particle size... 

O2 adsorption, 28 38 surface modility, 28 34 surface structure, 28 30, 31 oxidation of CO on, 28 65 oxide-supported metal catalysts, 41 10, 11 -phosphine catalysts achiral, 25 83-85 recovery, 32 354-356, 367-369 selectivity, 30 348 platinum, 30 355 -silica catalysts... 

Table 9.3 Summary of results for the FeS2 (100)/O2 adsorption system...

O2 species formed over LaFeo SCU0.2O3 after O2 adsorption were investigated via O2-TPD experiments as described in Table 9, showing a-02 peaks at 253 and 671 °C and P-O2 peak at 793 °C. In the presence of 20 ppm SO2 in the adsorption gas, a diminution of adsorbed O2 species (especially ai-02) formed over LaFeo 8CU0 2O3 was found (see Table 9), indicating a competitive adsorption between gaseous SO2 and O2 at the same site. Acidic SO2 was believed preferentially adsorbed on the surface of perovskite compared to O2 due to the basicity of this solid. A process similar to a-02 adsorption upon anion vacancies is involved ... 

FT-IR data recorded upon NO-O2 adsorption in the presence of CO2 showed that nitrites are formed first over the carbonated surface. On increasing the time of contact, carbonates are partially displaced, while nitrites evolve to nitrate species [126]. Notably, for a given time of contact, the amount of surface nitrites is lower in the presence of CO2, which suggests that the displacement of CO2 from the carbonated Ba sites to give Ba nitrite is rate determining in the nitrite route, whereas the kinetics of the nitrate route are only marginally affected by the presence of CO2. This may indicate that the nitrate route prevails over the nitrite route in the presence of CO2, that is, under real operating conditions the nitrate route is the most important. 

Figure 13.16 TPD in He and TPSR in H2 (2000ppm) + He after NO-O2 adsorption at 35O C over Pt-Ba/Al2O3 (1 20 100 w/w) catalyst. Adapted from ref. [119].

To analyze these aspects better, H2 TPSR and NH3 TPSR experiments were performed in the presence of 1% v/v water over the Ba-Pt/AbO (1 20 100 w/w) catalyst with stored NO after NO-O2 adsorption at 350 °C [135]. The results are presented in Figures 13.18. 

As pointed out in part 1, ° ID oxide nanostructures do not represent only particles with high elongation ratio, but possess different spedfie eharaeteristics related to the nanostructure. For example, O2 adsorption is different from that observed for Ti02 single erystals and this has consequenees on the catalytic behaviour. In order to exploit the properties of ID oxide nanostructure, however, it is essential to orient nanotubes on substrates and to create ordered arrays. 

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