Oxygen adsorption/desorption

Considering the above discussion, the currents in Fig. 15.5 are normalized to the surface areas estimated from CO stripping. Similar to massive electrodes, CVs for Pt nanoparticles show three characteristic potential regions (all potentials vs. RHE) (i) an Hupd region 0.05 Y < E < 0.40 V, followed by (ii) the so-called doublelayer region 0.40V < E< 0.60V and (iii) for E > 0.7 V, the oxygen adsorption/ desorption region. 

Figures. (A)VoltammogramsofPt(lI0)cooledinH2 + Arand0.1Af HCIO4. (B) Voltammogram of Pt( 110) cooled in H2 + Ar and 0.5M H2SO4, where the dotted curve was observed after the first oxygen adsorption-desorption cycle. The sweep rate was 50 mV s. (From Ref. 26.)...

The potential cycling illustrated by Figure 5.20 is a commonly used pre-treatment procedure for attainment of a reproducible active surface. Less widely known is the fact that in aqueous solution this cycling procedure causes the dissolution of appreciable quantities of metal. The discrepancy between the integrated anodic and cathodic oxygen adsorption-desorption peaks has been shown to be due to dissolution of the metal. Typical values are given in Ref. 68 and indicate that platinum and gold dissolve to a much lesser extent than do palladium and rhodium. 

As a hemoglobin model, using imidazole attached polystyrene) with the picket fence porphyrin-Co(H) (XIV) the reversible oxygen adsorption-desorption reaction is observed (148,149). 

Rochoux, M., Guo, Y., Schuurman, Y., and Farrusseng, D. (2015) Determination of oxygen adsorption-desorption rates and diffusion rate coefficients in perovskites at different oxygen partial pressures by a microkinetic approach. Phys. Chem. Chem. Phys., 17,1469-1481. 

This principle is illustrated in Figure 10 (45). Water adsorption at low pressures is markedly reduced on a poly(vinyhdene chloride)-based activated carbon after removal of surface oxygenated groups by degassing at 1000°C. Following this treatment, water adsorption is dominated by capillary condensation in mesopores, and the si2e of the adsorption-desorption hysteresis loop increases, because the pore volume previously occupied by water at the lower pressures now remains empty until the water pressure reaches pressures 0.3 to 0.4 times the vapor pressure) at which capillary condensation can occur. 

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.

At low temperatures the reaction is negatively affected by the lack of oxygen on the surface, while at higher temperatures the adsorption/desorption equilibrium of CO shifts towards the gas phase side, resulting in low coverages of CO. As discussed in Chapter 2, this type of non-Arrhenius-like behavior with temperature is generally the case for catalytic reactions. 

CO oxidation is often quoted as a structure-insensitive reaction, implying that the turnover frequency on a certain metal is the same for every type of site, or for every crystallographic surface plane. Figure 10.7 shows that the rates on Rh(lll) and Rh(llO) are indeed similar on the low-temperature side of the maximum, but that they differ at higher temperatures. This is because on the low-temperature side the surface is mainly covered by CO. Hence the rate at which the reaction produces CO2 becomes determined by the probability that CO desorbs to release sites for the oxygen. As the heats of adsorption of CO on the two surfaces are very similar, the resulting rates for CO oxidation are very similar for the two surfaces. However, at temperatures where the CO adsorption-desorption equilibrium lies more towards the gas phase, the surface reaction between O and CO determines the rate, and here the two rhodium surfaces show a difference (Fig. 10.7). The apparent structure insensitivity of the CO oxidation appears to be a coincidence that is not necessarily caused by equality of sites or ensembles thereof on the different surfaces. 

Oxygen adsorption that occurs at platinum at potentials more positive than 0.9 to 1.0 V is irreversible, in contrast to hydrogen adsorption. Oxygen can be removed from the surface by cathodic current, but the curves obtained in the anodic and cathodic scan do not coincide cathodic oxygen desorption occurs within a narrower region of potentials, and these potentials are more negative than the region where the... 

The effect of oxidizing atmospheres on the reduction of NO over rhodium surfaces has been investigated by kinetic and IR characterization studies with NO + CO + 02 mixtures on Rh(lll) [63], Similar kinetics was observed in the absence of oxygen in the gas phase, and the same adsorbed species were detected on the surface as well. This result contrasts with that from the molecular beam work [44], where 02 inhibits the reaction, perhaps because of the different relative adsorption probabilities of the three gas-phase species in the two types of experiments. On the other hand, it was also determined that the consumption of 02 is rate limited by the NO + CO adsorption-desorption... 

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