Reduction kinetics of ceria surface by hydrogen
Powdered cerium dioxide (ceria, CeO2) as compressed, sintered pellets, of porosity 16.4% and density 5.99 g cm?3, were treated in hydrogen flow at 1 atm and various temperatures to effect reduction. The electrical conductivity was measured in situ during the reduction process. The conductivity increased continuously during the hydrogen treatment because of the creation of anion vacancies and accompanying small polaron electrons. The conductivity�time relationship exhibits three distinct regions indicated as I, II, and III. For each of steps I and II, the conductivity increases exponentially with the reduction. It is suggested from the kinetic analysis of the data that region I is due to desorption of adsorbed oxygen states. Region II appears to be the reduction of surface lattice oxygen. The kinetics of the reactions in both regions I and II obey first-order rate laws with similar activation energies of 86 and 115 kJ mol?1, respectively. Thermogravimetric experiments were used to determine the time needed to remove one monolayer of adsorbed oxygen from the surface. This could be used to estimate the activation energy of the desorption process at 95 kJ mol?1�close to the value measured by conductivity measurements. After completing the surface reduction the electrical conductivity subsequently increased slowly during region III. This step is assigned to a diffusion-controlled process during which the bulk of the pellets are reduced. 1H MASNMR and in situ PXRD experiments confirmed the chemical nature of each of the three steps.
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