Lattice ceria

Although the dopant dissolves in the ceria lattice, we cannot rule out the presence of an amorphous dopant-rich phase at the surface of the catalyst (even after severe calcining). XPS + XRD measurements show a dopant-lean bulk and a dopant-rich surface. The structural similarity of the different catalysts is supported by the surface area-pore volume relationship (Figure 3). 

Depending on the electronegativity and the cation s oxidation state, the oxide can be either acidic or basic. Also, doping the ceria lattice does not automatically mean a substitution of a host cerium cation. Inomata and coworkers showed, in the case of Ce(i x)FexOy, that Fe3+ ions were located not only at Ce4+ sites, but also at interstitial sites (49). These interstitial ions bring about an increased barrier for electrons moving between Ce3+ and Ce4+, and influence the redox potential and the ease of reducibility of the material. 

It has been noted that the conductivity and activation energy can be correlated with the ionic radius of the dopant ions, with a minimum in activation energy occurring for those dopants whose radius most closely matches that of Ce4+. Kilner et al. [83] suggested that it would be more appropriate to evaluate the relative ion mismatch of dopant and host by comparing the cubic lattice parameter of the relevant rare-earth oxide. Kim [84] extended this approach by a systematic analysis of the effect of dopant ionic radius upon the relevant host lattice and gave the following empirical relation between the lattice constant of doped-ceria solid solutions and the ionic radius of the dopants. 

There are certain similarities between ceria and ferric oxide the cations Ce4+ and Fe3+ are both quite easily reduced, and in the case of ceria, the surface lattice oxide ions are readily mobilised, so that cation vacancies are... 

---