NOx-TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO2-Containing Materials

NOx adsorption-desorption experiments were carried out on a series of W-free and W-containing ZrO2 samples. To the best of our knowledge, this study establishes for the first time NOx-TPD (temperature-programmed desorption of NOx) as a reliable method to estimate the accessible ZrO2 surface of complex oxides. The NOx uptake by ZrO2 was found to be 5.8 +/- 0.2 mu mol/m(2). On the tungstated zirconia series of samples, the NOx uptake decreases linearly with W density up to about 5 W/nm(2), which is in excellent agreement with literature data for which the pseudo monolayer coverage was obtained by spectroscopic methods. The accessible ZrO2 surface area of ZrO2/SBA-15 materials, which cannot be ascertained through common spectroscopic methods, was estimated by WO3 thermal spreading and the NOx-TPD method. Both values compare very well but NOx-TPD, unlike WO3 thermal spreading, does not alter the integrity of the investigated sample as the sample is pretreated at lower temperatures (500 instead of 700 degrees C).