Ceria, doping

The authors339 found that the best catalysts were those whereby the ceria was impregnated prior to calcination of the hydrotalcite-based Cu-Zn precursors. The addition of ceria led to an important improvement in catalyst stability. For example, during stability tests (0.7 nl/min N2, 0.3 nl/min CO, 0.3 nl/min H20, T = 300 °C, P = 3 atm), the undoped catalyst calcined at 400 °C dropped to 25% of its initial activity within the first couple of hours on-stream. In contrast, the ceria-doped catalyst slowly decreased to 25% of its initial activity in about 50 hours. The activity was linked to the metallic Cu surface. In either case, the deactivation is considered to be very rapid. 

Table 62 Chemical compositions of undoped and ceria-doped Cu/Zn/Al catalysts339...

Catalyst systems for the WGS reaction that have recently received significant attention are the cerium oxides, mostly loaded with noble metals, especially platinum 42—46]. Jacobs et al. [44] even claim that it is probable that promoted ceria catalysts with the right development should realize higher CO conversions than the commercial Cu0-Zn0-Al203 catalysts. Ceria doped with transition metals such as Ni, Cu, Fe, and Co are also very interesting catalysts 37,43—471, especially the copper-ceria catalysts that have been found to perform excellently in the WGS reaction, as reported by Li et al. [37], They have found that the copper-ceria catalysts are more stable than other Cu-based LT WGS catalysts and at least as active as the precious metal-ceria catalysts. 

Rhodium-Based Catalysts. Krause et al. showed that Rh and Ni— which are better SR catalysts than Pt—supported on ceria-doped gadolinium... 

Oxide-ion conducting supports such as ceria, doped ceria (with Sm or Gd), or perovskites are found to be effective for reducing carbon formation on the catalyst during reforming of liquid hydrocarbons. 

The library used was the one based on Pt-ceria-doped catalysts, i.e., the second one designed by DoE (Section 10.2.4). The results are reported in details elsewhere [18, 20]. 

The conductivities of some ceria-containing compounds obtained from the literature have been compiled and shown as solid lines in Fig. 2.12. The main dopants for ceria belong to the alkaline earth or rare-earth metal series and the majority of doped samples exhibit conductivity values which fall into a rather limited band (gray band in Fig. 2.12), which points to a similar behaviour for all doped ceria samples. Exceptions are pure ceria and ceria doped with redox elements like Pr and Tb which give rise to electronic contribution to conductivity. An important requirement is that a homogeneous solid solution forms between the two oxides, which maintains the fluorite structure since the presence of a second phase or phase inhomogeneity due to insufficient solubility can affect ionic conductivity. The very low values of conductivity found for BaO and MgO-doped ceria were in fact attributed to the low solubility of these oxides into the lattice of When the... 

Whereas the ionic conductivity is always much lower than the electronic conductivity in pure reduced ceria, the situation is quite different in ceria doped with oxides of two- or three-valent metals due to the introduction of oxide ion vacancies, cf eqs. 15,2 and 15.3. A high vacancy concentration will shift eq. 15.1 to the left. This means that the ionic domain extended down to 10 atm or even lower in the temperature range of 600 - 1000 0. The electronic conductivity in air may be very low, and the doped cerias are under these conditions excellent electrolytes. The conductivity mechanism is the hopping of oxide ions to vacant sites, and the ionic conductivity, a may be expressed as... 

A large number of studies of the ionic conductivity of ceria doped with alkaline and rare earth metal oxides have been reported. Maximum ionic conductivities in the range of 0.1 - 0.2 S/cm at lOOff C and the activation energies of 0.6 - 1.0 eV... 

Cerium-based catalysts have been successfully used in several processes. For example, ceria (Ce02) is used as an additive [ 1,2] in modem automotive exhaust catalysts. Ceria acts as an excellent oxygen store [3-5] in the catalyst, which is thus rendered a very effective catalyst for combustion [6]. Moreover, addition of ceria to the automotive exhaust catalysts minimises the thermally induced sintering of the alumina support and stabilises the noble metal dispersion [7]. Ceria also enhances nitric oxide dissociation when added to various supported metal catalysts [8], which is another important function of the automotive exhaust catalyst. Recent investigations by Harrison et al have shown that ceria doped with certain lanthanides and promoted with copper and chromium have catalytic activities comparable to that of the noble metal catalysts [9]... 

In this paper, we present as part of a continuing investigation [19,20] a study of the effect of parameters such as initial concentration of the reactants, drying and calcination temperatures, preparation method, as well as doping levels, on the surface properties of ceria doped with 1+ (alkali) or 2+ (alkaline earths) metallic cations. 

This tendency for reduction restricts the range of oxygen partial pressures over which the ionic transference number remains close to unity. For example, at 800°C, the oxygen partial pressure in the lower limit is restricted to partial pressures over 10 atm. This lower limit has been extended with no loss in conductivity to 10" atm at 700°C in ceria doped with 20 mol% GdaOs by replacing 3% of the gadolinium with praseodymium. 

Fig. 10.7. (a) Defect and (b) conductivity diagram for ceria-doped YSZ at 1000°C. The relevant parameters to construct the diagrams are given in Ref. [119]. The theoretical dependence of ionic transference number tionand oxygai permeability /02 are given in (c). Dashed lines in (c) refer to YSZ. Fm (YzrO represents the aliovalent dopant used. Reproduced (slightly adapted) from Marques et al. 

At 1000°C, significant levels of electronic conduction in titania-doped YSZ, as in ceria-doped specimens, are foimd only under strongly reducing atmospheres. Data of oxygen permeability have been presented for the Zr02-Y203-Ti02 system by Arashi and Naito [127] (see also Table 10.1). By virtue of its... 

The most commonly used electrolyte materials in SOFCs are based on zirconia and ceria doped with a suitable cation, normally a rare earth (see Chapter 9). The properties that make these two materials attractive for use in fuel cells are discussed in Section 4.4.4, and it is sufficient to note that the most important feature is that they are good oxygen ion conductors. We will focus here on some recent investigations of these materials, with emphasis placed on their methods of preparation. 

The development of electrolytes that exhibit a higher conductivity at low temperature. Three candidates have emerged, namely doped ceria, doped lanthanum gallate, and doped barium zirconate. The first two of these are oxygen ion electrolytes, and the latter is a proton conductor. 

K. C. Waugh, M.A. Morris and W.C. Mackrodt, 1996, The surface chemistry of ceria doped with lanthanide oxides, in Catalysis and Surface Characterisation, eds T.J. Dines, C.H. Rochester and J. Thomson (The Royal Society of Chemistry, Cambridge) pp. 76-86. 

We have done an experimental study and a modeling of the surface area decrease of ceria doped with several cations (Mg2+, Y3+, A1 + and SH+) at 943 K. This temperature has been chosen in order to allow a kinetic study, but it is rather low for a catalytic application. To ascertain the results in practical conditions, we have verified the stabilizing effect of the most promising cations at higher temperatures such as 1073 and 1173 K. 

The volumetric isotherms are reported in Figure 5a. In the whole interval of CO pressure examined, the adsorbed amounts per unit surface area are definitely larger on the ceria-doped specimens than on pure alumina, and the difference between doped and non-doped systems increases with increasing CO pressure. (For instance, at pCO = 60 Torr, the uptake is 0.13 CO molecules per nm (0.21 qmol/m2) on pure alumina, whereas it is 0.31 and 0.40 CO molecules per nm2 (0.51 and 0.66 lamol/m ) in the case of ACE3 and ACE20 respectively). 

D. Andreeva, M. Kantcheva, I. Ivanov, L. Ilieva, J. W. Sobczak, W. Lisowskic, Gold supported on ceria doped by Me " (Me = Al and Sm) for water gas shift reaction influence of dopant and preparation method, Catal. Today 158 (2010) 69-77. 

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