Structural characterization of alumina-supported Rh catalysts: effects of ceriation and zirconiation by using metal-organic precursors

Anna B Kroner; Mark A Newton; Moniek Tromp; Andrea E Russell; Andrew J Dent; John Evans

doi:10.1002/cphc.201300537

Structural characterization of alumina-supported Rh catalysts: effects of ceriation and zirconiation by using metal-organic precursors

Chemphyschem. 2013 Oct 21;14(15):3606-17. doi: 10.1002/cphc.201300537. Epub 2013 Aug 13.

Authors

Anna B Kroner¹, Mark A Newton, Moniek Tromp, Andrea E Russell, Andrew J Dent, John Evans

Affiliation

¹ Diamond Light Source, Chilton, Oxfordshire, OX11 0DE (UK); School of Chemistry, University of Southampton, Southampton, SO17 1BJ (UK).

Abstract

The effects of the addition of ceria and zirconia on the structural properties of supported rhodium catalysts (1.6 and 4 wt % Rh/γ-Al2O3) are studied. Ceria and zirconia are deposited by using two preparation methods. Method I involves the deposition of ceria on γ-Al2O3 from Ce(acac)3, and the rhodium metal is subsequently added, whereas method II is based on a controlled surface reaction technique, that is, the decomposition of metal-organic M(acac)x (in which M=Ce, x=3 and M=Zr, x=4) on Rh/γ-Al2O3. The structures of the prepared catalyst materials are characterized ex situ by using N2 physisorption, transmission electron microscopy, high-angle annular dark-field scanning transmission election microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure spectroscopy (XAFS). All supported rhodium systems readily oxidize in air at room temperature. By using ceriated and zirconiated precursors, a larger rhodium-based metallic core fraction is obtained in comparison to the undoped rhodium catalysts, suggesting that ceria and zirconia protect the rhodium particles against extensive oxidation. XPS results indicate that after the calcination and reduction treatments, a small amount of chlorine is retained on the support of all rhodium catalysts. EXAFS analysis shows significant Rh-Cl interactions for Rh/Al2O3 and Rh/CeOx /Al2O3 (method I) catalysts. After reaction with H2/He in situ, for series of samples with 1.6 wt % Rh, the EXAFS first shell analysis affords a mean size of approximately 30 atoms. A broader spread is evident with a 4 wt % rhodium loading (ca. 30-110 atoms), with the incorporation of zirconium providing the largest particle sizes.

Keywords: EXAFS spectroscopy; cerium; photoelectron spectroscopy; rhodium; zirconium.

© 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.