FTIR study of defects produced in ZrO2 samples by thermal treatment -: Residual species into cavities and surface defects

The effect of two different thermal/chemical pretreatments on a high surface area zirconia sample has been studied using FTIR spectroscopy. As revealed by high resolution electron microscopy (HREM), the sample exhibits a significant concentration of small pores and cavities with size ranging from 1 to 2 nm. When pretreatment 1 was applied, (step-by-step heating under O-2 followed by evacuation at increasing temperatures up to 873 K), the oxide appeared clean, and free from impurities detectable by IR spectroscopy. Likewise, no O-2(-) species resulting from further O-2 chemisorption at 298 K were observed. In contrast, the IR spectrum for the oxide exposed to pretreatment 2 (heating under O-2 at 873 K, followed by fast evacuation at 873 K), shows a number of features that can be interpreted as due to oxalate-like species, as well as adsorbed CO. Additionally, superoxide species could be identified upon O-2 chemisorption at room temperature. Further reoxidation treatment at 673 K induces the disappearance of CO features and the transformation of the oxalate-like species. The initial spectrum can be recovered by subsequent evacuation at 873 K, thus indicating the reversible nature of the chemical processes involved. In accordance with the HREM images, we propose that a redox interconversion reaction of chemisorbed species trapped in the oxide cavities is responsible for what we observe on the zirconia sample exposed to pretreatment 2.