Kinetics of the hydrogen reduction of NIO/YSZ and associated microstructural changes

The kinetics of hydrogen (4%H2-96%Ar mixture) reduction of porous 75mol%NiO/YSZ was investigated by thermogravimetry at 600°C, 700°C, 750°C and 800°C. to addition, the evolution of structural changes during reduction at 800°C were monitored. It was found that reduction of NiO occurs in two stages. In the first stage, linear reduction kinetics with an activation energy of 22 kJ/mol were observed, to the second stage, the extent of reduction with time was found to decrease significantly and the fraction of reduced NiO at which the transition from the first to the second stage occurred was found to increase with increasing temperature. Using optical and scanning electron microscopy and wavelength dispersive X-ray spectroscopy, it was found that the first stage of the reduction process is associated with the displacement of the reduction front across the thickness of the sample, whereas the second stage involves further reduction of NiO grains behind it. The transition between the first and second stages was found to occur when the reaction fronts meet at the middle of the sample. The porosity of the samples before and after reduction was determined by alcohol immersion and it was found that porosity increases linearly with the fraction of reduced NiO.