Oxygen sorption

In recent years, research on catalysts for the ATR of hydrocarbons has paid considerable attention to perovskite systems of general formula ABO3. In the perovskite stmcture, both A and B ions can be partially substituted, leading to a wide variety of mixed oxides, characterized by structural and electronic defects. The oxidation activity of perovskites has been ascribed to ionic conductivity, oxygen mobility within the lattice [64], reducibility and oxygen sorption properties [65, 66]. 

A simple topochemical model for the growth of NiO islands on the Ni surface during the reaction of oxygen with a Ni(lll) crystal is clearly described by Holloway and Hudson [112]. They considered three cases in which the rate-determining step is, respectively (a) oxygen sorption from the gas phase (surface diffusion is fast), (b) surface diffusion of oxygen, and (c) oxygen insertion over the island boundary. 

Because of these slow processes, a ZnO surface remembers for some time whether and where it has been illuminated. The memory keeps the properties of the surface caused by illumination for some time after the illumination. This has been observed with respect to sorption and catalysis 41 >42), surface potential, as just discussed, and conductivity. It was been observed also on other semiconductors, e.g. the exchange of oxygen isotopes and oxygen sorption on illuminated MgO 43>- The memory effect has been treated extensively by the electronic theory of catalysis 41.44-45). 

Con(tetren)]2+-NaY oxygen sorption is irreversible, due to p-peroxo formation, possibly related to the presence of a ligand with extremely high electron density (tetren, 10). In contrast to Co, with [Ni(II)cyclam]Y the trans-form dominates, keeping the axial positions available for ligand exchange. 

When pure and doped nickel oxides, prepared in vacuo, are heated in oxygen at 250°, their electrical conductivity increases and their color changes from yellowish green or green to black (77). Increase of electrical conductivity is associated with the increase of the number of Ni + ions resulting from the oxygen sorption. The electrical conductivity of the lithium-doped sample [NiO(10 Li)(250°)] is larger (0250° = 1-86 x IO-2... 

H.-M. Zhang, Y. Shimizu, Y. Teraoka, M. Miura and N. Yamazoe, Oxygen sorption and catalytic properties of Lai-xSrxCoi-yFcyOs-s perovskite-type oxides. /. Catal., 121 (1990) 1367-1370. 

The corresponding oxygen sorption isotherm of Lai cSr cCoi yFej03, 5 perovskite oxide sorbents that were calculated from the oxygen nonstoichiometry data are shown in Fig. From these oxygen... 

Yang, Z. Lin, Y.S. Equilibrium of oxygen sorption on perovskite type ceramic sorbents. AIChE J. 2003, 49, 793-798. 

Yang, Z.H. Lin, Y.S. High temperature oxygen sorption in fixed-bed packed with perovskite-type ceramic sorbents. Ind. Eng. Chem. Res. 2003, 42, 4376 381. 

An axially coordinated base in case of Co(smdpt) enhanced the oxygen sorption capacity [58],... 

The adsorption of oxygen on certain semiconductors, such as NiO and Cu20 was studied in fair detail. The activation energies, heats of adsorption and kinetic laws for oxygen sorption on simple semiconducting catalysts are summarized in Table VI. 

K, desorbed a large amount of oxygen comparable to those observed when samples were pretreated at 773 and 1073 K. These results suggest that oxygen sorption proceeds easily and rapidly even at room temperature on the oxides with such mixed A and B compositions. 

In the present study we have aimed at the preparation and examination of catalysts with different oxygen sorption capacities. For that purpose the characteristics of the tin(IV)oxide had to be altered and thus, firstly, a more general investigation of the modification of tin(IV)oxide powder by an adequate temperature treatment was performed. Subsequently, reaction rates for the carbon monoxide oxidation were determined with catalysts of different oxygen sorption capacities in order to obtain additional information concerning the synergism between the noble metal and tin(IV)oxide. 

The oxygen sorption capacities of the four catalysts vary in a broad range. They cannot be correlated with the carbon monoxide sorption capacities, independent of the fact whether the latter are related to the catalyst mass or to the platinum mass. No explanation could be found for the comparatively low oxygen sorption capacity of the catalyst prepared by use of the cordierite carrier (cat. 1). The results obtained with the two catalysts prepared using a-AljOs carriers (cat. 2 and cat. 3) clearly indicate that a prolonged calcination leads to both a diminuation of the tin(IV)oxide surface area and a lower oxygen sorption capacity while the platinum dispersion obtained is not markedly influenced. 

Figure 6. The dependence of the reaction rate in the second regime on the oxygen sorption capacity at a temperature of 60 °C and an oxygen content of 6 vol.% (cat. 1 to cat. 4).

The variation of both the temperature pretreatment and the ceramic carrier of platinum - tin(rV)oxide catalysts allowed the preparation of catalysts with nearly identical platinum content and dispersion, but broadly differing oxygen sorption capeicity. 

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