Reduction kinetics of ceria surface by hydrogen

Abstract

Sintered ceria pellets of porosity 16.4% and density 5.99 g cm−3 were treated in a hydrogen flow at 1 atm and various temperatures. The electrical conductivity was measured in situ while hydrogen gas was flowing over the CeO2. The conductivity increased continuously during the hydrogen treatment due to the continuous generation of electron carriers. The conductivity–time relationship exhibits two distinct regions labeled as I and II. In the initial region there are two consecutive steps labeled as 1 and 2, during which the conductivity increased exponentially with time of hydrogen flow, however, with a change in the slope after a relatively short time. From the kinetic analysis region I it is suggested that the first step 1 is due to oxygen desorption, and that the second step 2 is due to surface reduction. The kinetics of steps 1 and 2 in both cases obey first-order rate law with activation energies of 86 and 115 kJ mol−1 for the first and the second step, respectively. These values of the activation energies from the conductivity measurements were further supported by one more value from thermogravimetry measurements. The activation energy of surface reduction from thermogravimetry was about an average value of the above two activation energies (95 kJ mol−1). Scanning electron microscopy (SEM) studies showed that surface grains were broken down into smaller ones due to reduction. These breakages did not extend towards the bulk of the pellet; revealing that reduction was limited to surface region. After completing the surface reduction, presumably, by the end of region I, the electrical conductivity subsequently increased slowly during region II. This region is assigned to a diffusion-controlled process during which the bulk of the pellet is reduced.