ACTIVITY AND DEACTIVATION OF LA0.8CA0.2CRO3 IN DRY METHANE USING TEMPERATURE-PROGRAMMED TECHNIQUES

The perovskite oxide La0.8Ca0.2CrO3 was studied for application as a direct methane oxidation anode in solid oxide fuel cells (SOFCs) using temperature-programmed (TP) techniques. Activity for methane oxidation, redox behaviour, methane adsorption and extent of deactivation by carbon deposits for the oxide were compared with those for the conventional anode material, Ni-ZrO2 cermet, studied previously. The Ni-ZrO2 exhibited a much higher activity for the formation of carbon deposits than the oxide. Furthermore, the temperature of onset of carbon deposition was much lower on the cermet than on the oxide. Over the oxide, two main methane reaction regimes were identified: complete oxidation at intermediate temperatures (over the pre-oxidised catalyst only) and methane dissociation at high temperatures. Each reaction took place via two mechanisms. Complete oxidation involved reaction with either surface or absorbed oxygen species. Methane dissociation appeared to occur via catalysed (probably at oxygen ion vacancies) and uncatalysed mechanisms. At approximately 400-600 degrees C, where complete oxidation of methane occurred over the oxide, no carbon deposition was detected. This is in contrast with Ni-ZrO2 where carbon deposition was observed over the entire active temperature range of the catalyst. This suggested that a methane direct oxidation SOFC with such an oxide anode material may be operated at these temperatures without becoming deactivated by carbon deposits. The effect of the addition of water to the methane feed was studied and was seen to substantially reduce the amount of carbon deposition.