Thermal reduction and reoxidation of spinel-type Cu1−xZnxAl2O4

The thermochemical reactivity of the spinel-type quaternary metal oxide Cu_1–xZn_xAl₂O₄ has been investigated for different Cu:Zn ratios. In oxygen or inert gas atmospheres no considerable reduction is observed. In molecular hydrogen metal selective reduction of the Cu is found at relatively high temperature. The solid reduction product is made up of sintered, poorly dispersed metallic copper on a Zn-Al-O metal oxide support, a potential catalyst for the methanol synthesis. Owing to the measured high reduction temperature leading to the mentioned sintering of the metallic copper, the activity of this system cannot be high.Financial support by the Swiss National Science Foundation under project Nr. 2027933.89 is gratefully acknowledged.     

Probing reoxidation sites by in situ Raman spectroscopy: differences between reduced CeO2 and Pt/CeO2

Raman spectroscopy is a powerful technique for detecting peroxo (O₂)^2– and superoxo (O₂)^– species adsorbed on defect sites of ceria. These sites are probed by reducing CeO₂ at high temperature and then chemisorbing oxygen species at low temperature. In the present study, it is shown for the first time that such Raman characterization has to be achieved at very low laser power to avoid formation of oxygen species by photolysis and analyze only the chemisorbed species. Respecting this requirement, the (O₂)^2– and (O₂)^– species formed on 0.7% Pt/CeO₂ compound, and the CeO₂ support used to prepare it were compared after reduction for various times and at various temperatures. Superoxo species were more stabilized on reduced 0.7% Pt/CeO₂ after short reduction at 773 K than on reduced CeO₂. Additionally, the distributions of peroxo species adsorbed on defect sites of Pt/CeO₂ and CeO₂ were significantly different after long reduction at 773 K in spite of similar amounts. Indeed, less stable species were formed during the reduction of 0.7% Pt/CeO_2. These two features revealed that new sites were created during the preparation and reduction of Pt/CeO₂ compared to its bare support. Copyright © 2012 John Wiley & Sons, Ltd.     

The Impact of Grain Boundary Diffusion on the Low Temperature Reoxidation Mechanism in Nanocrystalline TiO2−δ Films

The redox reaction TiO₂ TiO_2- + /2O_2(gas) can modify the oxygen deficiency and electrical conductivity of TiO_2– upon changing the ambient oxygen pressure. It is found that in nanocrystalline films this leads to significant changes in even at relatively low temperatures (200–325°C) that were previously considered too low to modify TiO₂ defect chemistry. This unusual phenomenon is attributed to the fine-grained structure and the important role of grain boundary (GB) diffusion in these films. A phenomenological model of the low temperature redox mechanism in nanocrystalline TiO_2– films is elaborated. Taking into account the impact of GB diffusion and considering time-dependent decay of the volume fraction of GB diffusion sites, we derive a modified parabolic law for the redox reaction kinetics. It is demonstrated that this law describes very well the electrical response kinetics of nanocrystalline TiO_2– thin films during exposure to oxygen between 200 and 325°C. From the fitting between the experimental results and this formula the activation energy of chemical diffusion in TiO_2– bulk and GBs is evaluated, obtaining 0.69 and 0.52 eV, respectively.