Ceria-based oxides

B. Sintering Inhibition Mechanism of Platinum Supported on Ceria-based Oxide... 

Computer Simulation Studies of Ceria-based Oxides 281... 

The sol-gel synthesis of some rare earths oxides has been carried out for the first time in 1971. For the preparation of ceria-based oxides, cerium isopropoxide, cerium acetylacetonate, cerium nitrate are used as the precursors. The water necessary for the hydrolysis reactions is brought in by adding directly or by the hydrated cerium nitrate. Using this method, Ce02, Ce02-PrOx, Ce02-Zr02, and CeOj-... 

Sc, Al, Th Zr", and Si, have been prepared according to the wetness impregnation method to investigate their effects on thermal stability of ceria. These impregnation methods are. of course, useful to synthesize ceria-based oxides supported on another oxides such as silica and alumina. The most advantage of the method is that highly dispersion is obtained. 

High Resolution Electron Microscopy (HREM) has proven as a very useful technique in the structural characterisation of supported metal catalysts (383-386) in general and, in particular, of noble metal catalysts supported on ceria-based oxides (52,70,72,97,105,109,117,124,135,137,139,144,147,155,171,182-184.194.203,209, 210,218,226,234,235,387) ... 

COMPUTER SIMULATION STUDIES OF CERIA-BASED OXIDES... 

Computer simulation studies of ceria based oxides... 

It is considered that the bulk area specific resistance i o must be lower than l o = k/<7 = 0.15 Qcm, where L is the electrolyte thickness and a is its total conductivity, predominantly ionic [39]. At present, fabrication technology allows the preparation of reliable supported structures with film thicknesses in the range 10-15 pm consequently, the electrolyte ionic conductivity must be higher than 10 Scm. As shown in Figure 12.9, a few electrolytes (ceria-based oxides, stabihzed zirconias, and doped gallates) exceed this minimum ionic conductivity above 500 °C. 

Ceria-based oxides can be obtained by the decomposition of some compound precursor, such as hydroxide, nitrate, halides, sulfates, carbonates, formates, oxalates, acetates, and citrates.For example, nanosize or porous cerium oxide particles have been prepared at low temperatures by pyrolysis of amorphous citrate, which is prepared by the evaporation of the solvent from the aqueous solution containing cerium nitrate (or oxalate) and citric acid. In the case of mixed oxides, the precursor containing some cations in the same solid salts is prepared. In the same manner of ceria particles, the precursors complexing some cations with citrates are useful to synthsize ceria-zirconia mixed oxides and their derivatives. Also, Ce02-Ln203 solid solutions, where Ln = La, Pr, Sm, Gd, and Tb, have been synthesized from the precursors obtained by the evaporation of nitrate solutions at 353 K in air from an intimate mixture of their respective metal nitrates. The precursors are dried and then heated at 673 K to remove nitrates, followed by calcination at 1073 K for 12h. 

An important aspect concerning the defects in ceria-based materials is their tendency to associate to form clusters of various complexity, which was mentioned in the previous section. The most studied defect clusters have been those formed by lower-valent (acceptor) dopant cations and oxygen vacancies. The reason for this is that these associations are widely considered to be responsible for the observed variation of the electrical conductivity of ceria-based oxides with dopant size, concentration and temperature. ... 

The main experimental techniques used to study oxygen migration in doped eerias are based on the AC impedance analysis of the measured eleetrical conductivity. It is found that the oxygen ion conductivity of ceria-based oxides depends strongly upon the dopant size and concentration. Both these faetors are related to defect association between oxygen vacancies (the charge carriers) and other defects (mainly dopant substitutionals) which were discussed in section 8.3.2.. 

This survey has aimed to demonstrate that computational techniques can play a valuable role in contemporary studies of ceria-based oxides, which complement related experimental work, and provide information that is relevant to catalytic and fuel cell applications. Materials that were investigated include pure Ce02, doped Ce02 (where, for example, M = Sc, Y, Gd) and the mixed Cei-xZrx02 system. 

Develop synthesis methods for highly active water-gas-shift (WGS) catalysts based on platinum supported on nanoscale ceria-based oxides. 

Use wet-chemical synthesis methods to prepare aqueous suspensions of nanoscale ceria-based oxides and to incorporate platinum onto ceria nano-particle surfaces. 

Ceria first, and since the mid 1990s ceria-zirconia mixed oxides, are key components in the formulation of TWCs [18,19]. A variety of functions are attributed to them, those related to their redox properties being particularly relevant [247]. Under the usual TWC operation conditions, the chemical composition of the exhaust gases rapidly oscillates between net reducing and net oxidizing conditions [8]. This implies deviations from the optimum stoichiometric air to fuel (A/F) ratio (A/F = 14.63), and therefore, loss of efficiency in the auto-exhaust catalyst. To attenuate these oscillations, oxygen buffer materials are required, ceria-based oxides constituting the best option at present available [19]. 

For the practical application of the oxide ion conducting solid electrolytes for the SOFCs, there is another problem that they usually need high operation temperature over 800°C. In fact, the SOFCs based on the YSZ thin film electrolyte cannot provide acceptable power output due to the fundamental limit of YSZ that it is difficult to obtain enough conductivity below 650°C. The ceria-based oxides are. 

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