Ceria sizes

Relaxations in the double layers between two interacting particles can retard aggregation rates and cause them to be independent of particle size [101-103]. Discrepancies between theoretical predictions and experimental observations of heterocoagulation between polymer latices, silica particles, and ceria particles [104] have promptetl Mati-jevic and co-workers to propose that the charge on these particles may not be uniformly distributed over the surface [105, 106]. Similar behavior has been seen in the heterocoagulation of cationic and anionic polymer latices [107]. 

These results lead us to the conclusion that nano-sized metallic gold and strong interaction between gold and ceria contribute enhanced catalytic activity for CO oxidation in the absence and presence of water vapor. 

As shown in Figures 1.36(a) and 1.36(b), the glycine nitrate process-derived powder is highly porous. The pore size ranges from tens of nanometers to several micrometers. The powder is thus named as foam powder, which shows extremely low fill densities of less than 1/100 of the theoretical density value. The low fill density makes it possible to prepare thin films of doped ceria by dry pressing, in which the films are processed by means of punches in a hardened metal die. Figures 1.36(c) and 1.36(d) show a cross-sectional view of an 8 /tm-thick GDC film fabricated by dry pressing. 

The catalysts were synthesized as films, with ceria prepared by spray pyrolysis of 0.1 M solutions of Ce(N03)3 onto nonporous alumina wafers held at 250 °C. The ceria was then calcined at 300 °C, resulting in a crystallite size of 10 nm. Pt, Pd, or Rh was vapor deposited onto the oxide film. For kinetics testing, the temperature was 300 °C. To determine the reaction order of H20, Pco was maintained constant at 0.026 atm. For the reaction order on CO, Ph2o was maintained constant at 0.02 atm. The kinetic parameters are tabulated in Table 69. 

Catalyst Conditions Rate (mol/g cat per second) BET SA (m2/g) Au crystallite size (nm) Ceria crystallite size (nm)... 

Owing to low copper content, copper-ceria catalysts are nonpyrophoric and stable, showing little or no deactivation during the experiments. The Cu0 2Ce0 x02 r catalyst prepared by coprecipitation method showed good catalytic activity for the WGS reaction. The Cu01Cc()9O2, catalyst prepared by sol-gel method was found to be less active, which could be due to lower number of active copper sites, or to different crystallite size and structure of copper-containing species. The copper-ceria catalysts were shown to be selective for the WGS reaction and no methanation reactions were observed over any catalyst under the experimental conditions used. 

A marked effect of the Ce02/Zr02 composition (in samples containing 40 wt.% NiO) on the catalytic activity was noticed. The catalysts with Ce Zr =1 1 (6A) were not only more active (than 7A and 8A) but were also stable during the reaction. Sample 8A containing no zirconia in the support showed a low activity. The NiO crystallite size (Table 11.2) in these compositions varied in the order 7A < 6A < 8A. It may be recalled that on ceria-based catalysts the crystallite size of nickel metal was similar to that of NiO. The higher activity for 6A than 7A indicates that in addition to accessibility of... 

The activation energy for oxide ion conduction in the various zirconia-, thoria- and ceria-based materials is usually at least 0.8 eV. A significant fraction of this is due to the association of oxide vacancies and aliovalent dopants (ion trapping effects). Calculations have shown that the association enthalpy can be reduced and hence the conductivity optimised, when the ionic radius of the aliovalent substituting ion matches that of the host ion. A good example of this effect is seen in Gd-doped ceria in which Gd is the optimum size to substitute for Ce these materials are amongst the best oxide ion conductors. Fig. 2.11. 

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