H2 activation by FeO

Table 13-11. Computed energies [kcal/mol] of stationary points for the activation of H2 by FeO+ (Dn relative to separated FeO+ (6S+) + H2).

The active sorbent form, which reacts with H2S, is the metallic iron, Fe or the oxide FeO. This is due to the initial rapid reduction of Fe203 by the H2 and the CO present in the fuel gas. On sulfidation of the sorbent a nonstoichiometric pyrrhotite, FeSi i, is formed. This reaction of Fe with H2S is much slower compared to the reduction reaction. 

Therefore, the activity of the absorbed oxygen and hence the rate is proportional to the ratio PcoJPcO- When the ratio becomes higher, the rate of reaction (15) becomes comparable with that of (14) and reaction (14) is no longer in equilibrium, hence the activity of the absorbed oxygen cannot be determined by the ratio PcojPcO A similar analysis was employed for the water gas shift reaction on an FeO surface between 700 and 1000°C [142]. From the previous studies, the step H2 +0(ads)= ... 

Van Damme and Hall (262) studied the oxidation of H2 over LaCo03, MTi03 (M = Sr, Ba), and Ba(Feo.33Tio.67)C>2.67 either in the dark or while being irradiated with band-gap light. No effect of light on the rate of the oxidation was found with any of these materials. These authors described this reaction as a process dominated by charge transfer in the boundary layer of the catalysts. More recently, Futai et al. (266) studied the activity... 

It can be considered that the experimental value of diffusion coefficient is a direct proportion with defect concentration in wiistite (Fig. 3.4). There are certain amount of Fe + cations in the structure of wiistite, which not only ensured the electrical neutrality in lattice, but also offered the favorable conditions for Fe + cation to move along with the cavity and for electrons to transfer (Fe + —> O —> Fe +). As a consequence, the activation energy for diffusion of Fe in FeO is the lowest, only as small as 96kJ mol (Table 3.3), compared with that of other metal oxides. This is the structural factor for Fei xO to be reduced easily by H2. 

The author s research results show that when the catalyst precursor is composed by two different phases, such as FeO and Fe304, two peaks appear on the DTG curve obtained from reduction by H2, which correspond to the reduction of Fei xO and Fc304, respectively (Table 5.10). It indicates that the reduction process is followed by the reduction of phase by phase in turn, and belongs to the competitive mechanism. The easily reduced FeO is first reduced to a-Fe, and then the reduction of Fc304 starts. The result of phased reduction is that the reduction rate of the catalyst is decreased, while the reduction temperature is increased, and also the activity of catalyst after reduction is decreased. 

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