The Effect of Rh on the Interaction of Co with Al2O3 and CeO2 Supports

Co and Rh+Co catalysts supported on Al2O3 and CeO2 were investigated by temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The CeO2 defects, resulted from the effects of the metals, were further analyzed by Raman spectroscopy and optical absorption. Although the interaction of Co with these two supports is fairly different, it can be concluded that Rh inhibits the strong Co-support interaction. It was revealed that Co over Al2O3 forms mainly Co2+ compounds, only a smaller fraction of cobalt is in metallic state. After the addition of 0.1 % Rh, Co3O4 like species is dominant, the amount of metallic state increased after reduction. Over CeO2 the Co dissolution into the support was inhibited by Rh. A wide range of TPR results proved the stepwise reduction of Co, which was promoted by the addition of Rh. By Rh the entire mechanism of this process was altered that can be observed even by XPS. On the basis of the Raman and the optical measurements we concluded that the metals induce defect sites on the CeO2 surface, and these appear as similar features on the spectra of Co and Rh containing samples, thus their density depends on the metal loading and not on the metal type. CeO2 has a bandgap of 3.27 eV, which is not altered by the metals, but an electronic contact was detected between the metals and CeO2 by photovoltammetry. The increased number of metallic species is mainly responsible for the higher catalytic activity and for the enhanced hydrogen selectivity in the stream reforming of ethanol.