SURFACES OF DOPED NANOPHASE CERIUM OXIDE CATALYSTS A.E.C. Palmqvist 1, †, M. Wirde 2 , U. Gelius 2 and M. Muhammed 1 1 Department of Materials Chemistry, Royal Institute of Technology-KTH, 100 44 Stockholm, Sweden 2 Department of Physics, Uppsala University, 751 21 Uppsala, Sweden (Received January 3, 2000) (Accepted January 4, 2000) Abstract—Solid solutions of nanophase cerium oxides have been prepared and the relationship between their bulk crystal structure and surface characteristics has been studied at room temperature with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Dopants with a valence lower than +4, such as Ca 2+ , Nd 3+ , and Pb 2+ , introduce structural defects (oxygen vacancies) in the cerium oxide lattice, which has been found to affect the redox catalytic activity of the materials. The introduction of oxygen vacancies leads to the appearance of an O1s core level peak with a shift of 2.0 –2.5 eV to higher binding energies as compared to the core level peak of the lattice oxygen in CeO 2 . The intensity of this high binding energy O1s peak varies with the expected concentration of oxygen vacancies in the surface, the type of dopant cation and its concentration in the solid solution. It was found that carbonate and hydroxide species are responsible for the appearance of this O1s peak, presumably as a result of capping of oxygen vacancies at the surface. The implications of these surface groups on the catalytic activity of the materials are discussed. ©2000 Acta Metallurgica Inc. 1. Introduction The importance of X-ray photoelectron spectroscopy (XPS) for studying catalytic materials has long been well recognized. In catalysis, it is often more important to perform a careful characterization of elemental composition, oxidation states, coordination numbers, and diffusivities of the catalyst surface than of the bulk. Furthermore, it is often advantageous to use materials having high surface areas, e.g. materials with very small particle sizes. Solid solutions of nanophase CeO 2 powders have been prepared for a large number of different dopants (1). The catalytic oxidation of methane over some Ca-, Mn-, Nd-, and Pb-doped nanophase solid solutions of CeO 2 was studied at temperatures between 400 and 600°C. The catalytic activity was found to increase upon introduction of cations of Ca and Nd. Also the Mn-doped sample showed increased activity compared to undoped CeO 2 , but normalized to surface area, this increase was small (2). In contrast, the Pb-doped CeO 2 sample had lower activity than the undoped CeO 2 . The Ca-doped CeO 2 sample has further shown a higher catalytic activity than the undoped CeO 2 for the reduction of SO 2 by CO, forming elementary sulfur and CO 2 (3). The Ca- and Nd-doped samples showing high catalytic activity were found to have a higher intensity of a high binding energy O1s XPS peak than the undoped CeO 2 sample (4). The Pb-doped sample showed only slightly higher intensity of this peak than the undoped sample. One of the aims of the present paper was to identify the chemical species present on the “technical” catalyst surfaces giving rise to this O1s peak. Theoretical modelling using static minimizations and molecular dynamics (MD) simulations in combination with ab initio quantum chemical cluster models has been performed on the CeO 2 and † Formerly named A.E. Persson. Pergamon NanoStructured Materials, Vol. 11, No. 8, pp. 995–1007, 1999 Elsevier Science Ltd Copyright © 2000 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter PII S0965-9773(00)00431-1 995