JOURNAL OF CATALYSIS 134, 204-219 (1992) Microstructure of Rh-Ce Particles on Silica: Interactions between Ce and SiO21 K. R. KRAUSE, 2 P. SCHABES-RETCHKIMAN,* AND L. D. SCHMIDT 3 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455; and *lnstituto de Fisica, UNAM Apdo. Postal 20-364, M~xico, D.F. 01000, Mexico Received August 20, 1991; revised October 18, 1991 We have characterized the microstructure of Rh/Ce on SiO2 after heat treatments in H 2 and 02 using TEM, HREM, XPS, and EELS, focusing on the very stable structures formed after heating in H 2. After initial reduction at 600°C, Rh is present as 50- to 100-A metal particles while the Ce forms a uniform amorphous film of Ce 3+ on the SiO2. After oxidation at 600°C, Rh is oxidized to Rh203 and spreads over the SiO z surface while Ce forms small patches and large (> 1000 A) particles of crystalline CeO 2. After reduction of the oxidized microstructure at 600°C, Rh metal returns with a less uniform particle size distribution, while Ce is reduced to Ce 3 + and structures indicating strong interactions between Ce and Si are formed. Upon reduction in the presence of Rh, the CeO 2particles are reduced to crystalline Ce2Si207 as confirmed by HREM. The Ce silicate nucleates at Rh particles and spreads over the support as large thin (>1000 A diameter and -50 A thick) single-crystal patches. After reoxidation at 650°C, both Ce silicate (Ce 3+) and CeO 2 were identified using EELS chemical shifts, indicating that the crystalline silicate, once formed, is stable in oxygen. Ce on SiO 2 also showed interaction between Ce and Si, but no crystalline species formed after reduction and only small crystalline CeO 2 particles formed after oxidation. Thus, the formation of the Ce silicate and the oxidation of Ce to CeO 2 are catalyzed by Rh. This work represents the first direct evidence for the formation of a Ce silicate in this system, and the combined use of HREM, XPS, and micro- EELS permits chemical characterization of these structures. © 1992 Academic Press,Inc. INTRODUCTION Rh and Ce are major components in the automotive exhaust catalyst. Rh is mainly responsible for the reduction of NOx, while Ce is added in large amounts as a reaction promoter and for support stabilization. On silica, Ce addition to Rh has been shown to improve the selectivity for oxygenates (1) and increase the overall rate of CO hydroge- nation (2) but does not affect the rate of ethane hydrogenolysis (2). On alumina, adding large amounts of Ce to Rh improved both the selectivity toward N 2 (3) and the reaction rate for CO + NO (3, 4). Also, CO oxidation activity by Rh was enhanced after 1This research sponsored by NSF under Grants CBT-882745 and INT-9000511. 2 Supported by a National Science Foundation Grad- uate Fellowship. 3 To whom correspondence should be addressed. 0021-9517/92 $3.00 Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved. Ce addition by reducing the negative-order inhibition effect of CO (5, 6). Evidence for the formation of a Ce aluminate in Rh/Ce on alumina spheres during heat treatment in H2 at 900°C has been found using X-ray absorp- tion (7). H 2chemisorption measurements in- dicated that Ce increased the dispersion of Rh on silica but had no effect on alumina (4). We have previously examined the micro- structure of Rh/Ce on SiO 2 with TEM and electron diffraction (2) and typical results are summarized in Fig. 1. When heated in H2 (Fig. 1A), a thin continuous film of partially reduced cerium formed on the SiOz. The only observed effect of Ce addition on the Rh particles was that Rh/Ce on SiO2 exhib- ited more defects and less faceting than Rh only on SiO2. Heating in 02 (Fig. 1B) oxi- dized the Rh to Rh203 and caused the Ce film to coalesce, forming many small CeO2 patches and some large, flat (> I000 ,~ diam- 2O4