TEMPERATURE-PROGRAMMED REDUCTION - LIMITATION OF THE TECHNIQUE FOR DETERMINING THE EXTENT OF REDUCTION OF EITHER PURE CERIA OR CERIA MODIFIED BY ADDITIVES

The classical temperature-programmed reduction technique using a therm ally controlled detector and a water vapour trap did not permit the qu antification of the extent of reduction of unsupported ceria with a hi gh surface area. During temperature-programmed reduction of pure ceria with hydrogen, not only is water formed; carbon monoxide and carbon d ioxide desorbing from the sample are also able to reach the thermally controlled detector and contribute to the variations in conductivity o f the actual reduction mixture. When ceria is modified by impregnation with alkaline nitrate, followed by calcination at 673 K, NOx compound s are also formed and contribute both to hydrogen uptake and to variat ions in gas conductivity. A further complication is caused by the stor age of some hydrogen in ceria, below 773 K, followed by the release of hydrogen above this temperature. Temperature-programmed oxidation of the reduced samples is an alternative way to measure the extent of cer ia reduction.