Oxygen adsorption-desorption peaks

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. 

The results of adsorption and desorption of CO mentioned above suggest that for the reaction at low temperature, the sites for relatively weakly chemisorbed CO are covered by the deposited carbon and the reaction occurs between molecularly adsorbed CO and oxygen on the carbon-free sites which are the sites for relatively strongly chemisorbed CO. Therefore, the definition of the turnover rate at 445 K remains as given in Equation 1. For the reaction at 518 K, however, this definition becomes inappropriate for the smaller particles. Indeed, to obtain the total number of Pd sites available for reaction, we now need to take into consideration the number Trp of CO molecules under the desorption peak. Furthermore, let us assume that disproportionation of CO takes place through reaction between two CO molecules adsorbed on two adjacent sites, and let us also assume that the coverage is unity for the CO molecules responsible for the LT desorption peak, since this was found to be approximately correct on 1.5 nm Pd on 1012 a-A O (1). Then, the number Np of palladium sites available for reaction at 518 K is given by HT/0 + NC0 LT s nce t ie co molecules under the LT desorption peak count only half of the available sites. Consequently, the turnover rate at 518 K should be defined as ... 

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. 

The adsorption of CO on pristine V=O terminated V2O3 was limited (Fig. 17.10a). The small TPD desorption peak at 105 K was most likely due to the adsorption of CO on a small number of defects (sites where the vanadyl oxygen was missing... 

Line-shape analysis of the desorption peaks shown in Fig. 7.2 was carried out by Atkins and co-workers from a plot of In (desorption rate/coverage ) versus 1/T from which a value of 140 1 kj/mol was obtained for the desorption activation energy [3]. The oti peak, therefore, derived from desorption of oxygen from the Ag (111) face and the heat of adsorption of oxygen on that face is 123 kJ/mol. 

---