Supported oxides as combustion catalysts and as humidity sensors. Tuning the surface behavior by inter-phase charge transfer

The changes of the electrical properties of semiconductor oxide catalysts, when measured in situ provide useful information about surface processes as adsorption/desorption/reaction occur with charge transfer. The electrical properties of polycrystalline powders, mainly controlled by inter-grain Schottky-type barriers, are sensible to the "quality" of the surface/inter-grain domains, to sample morphology and to changes in charge and coverage of the surface due to operating conditions. This can be exploited to study oxide catalysts dispersed on supports at nanoscale levels for answering questions like: (i) adsorption ability of the surface layer and (ii) the influence of the active phase and of the support. It is demonstrated that for low loading supported SnO2 or V2O5 catalysts (also important as sensor materials), the conductivity plots show the "fingertip" of the support, which controls the behavior of the supported phase. As an example, the lower propylene combustion activity of SnO2 when dispersed on gamma-Al2O3 is explained by hydrophile/proton conductor properties of the support; gamma-Al2O3 diminishes the oxygen adsorption ability of SnO2 by providing a higher concentration of molecularly adsorbed water and surface OH groups towards its surface. It is proposed that by selecting appropriate n-type semiconductor oxide supports (as TiO2), and by increasing the surface conductivity one can increase the oxygen adsorption ability of the supported phase, with increased activity for combustion/chemical sensing. This suggests the possibility of tuning the surface properties of the supported oxide by interphase charge (electron or ion) transfer. (c) 2006 Elsevier Inc. All rights reserved.