JOURNAL OF CATALYSIS 162, 1–9 (1996) ARTICLE NO. 0254 Reduction of NO over Partially Reduced Metal-Loaded CeO 2 –ZrO 2 Solid Solutions G. Ranga Rao, ∗ P. Fornasiero,† R. Di Monte,‡ J. Kaˇ spar,† ,1 G. Vlaic,† G. Balducci,† S. Meriani,‡ G. Gubitosa,§ A. Cremona,§ and M. Graziani† ∗ International Centre for Science and High Technology, APH Grignano, 34100 Trieste, Italy; †Dipartimento di Scienze Chimiche, Universit` a di Trieste, Via Giorgieri 1, 34127 Trieste, Italy; ‡Dipartimento di Ingegneria dei Materiali e Chimica Applicata, Universit ` a di Trieste, Via Valerio 2, 34127 Trieste, Italy; and §Magneti Marelli D.S.S., Viale Carlo Emanuele II 150, 10078 Venaria Reale, Turin, Italy Received January 4, 1995; revised December 13, 1995; accepted March 30, 1996 CeO 2 –ZrO 2 solid solutions supported with Rh and Pt are inves- tigated as catalysts for reduction of NO by CO. The incorporation of ZrO 2 into the CeO 2 framework strongly promotes the reduction of Ce 4+ in the bulk of the support. The effects of the reduction tem- perature are investigated and it is shown that when bulk oxygen vacancies are formed in the reduced CeO 2 –ZrO 2 solid solution, NO is efficiently decomposed on the support to give N 2 O and N 2 . The presence of bulk oxygen vacancies is indicated as the driving force for NO dissociation, suggesting their indirect participation in the catalytic cycle. c  1996 Academic Press, Inc. INTRODUCTION In recent years much research has been focused on cerium oxide based transition metal catalysts because of their applications in different processes, particularly in the catalytic treatment of automotive exhausts. CeO 2 is indeed extensively added as a promoter to the current three-way catalyst (TWC). Several functions are attributed to this promoter (1, 2), namely (i) stabilization of metal dispersion and its alumina support; (ii) promotion of water gas shift and steam reforming reactions, and (iii) oxygen storage and release capacities under respectively fuel- lean and fuel-rich conditions due to the redox couple 2CeO 2 ⇀ ↽ Ce 2 O 3 + 1/2O 2 . The last point is of relevant tech- nological importance since high oxygen storage capacity (OSC) allows one to enlarge the operating air/fuel window and thus it increases the overall efficiency of TWCs in the working conditions. To increase the catalytic efficiency of TWCs during a cold start, close-coupled locations of the converter are usu- ally employed. Therefore, in the driving conditions the op- erating temperature may easily exceed 1200 K, requiring high thermal stability of the catalyst. Upon ageing at such high temperatures, deactivation of the catalyst may oc- 1 E-mail: kaspar@dschsun1.univ.trieste.it; Fax: 39-40-6763903. cur due to sintering of metal particles, formation of irre- ducible rhodium species, and collapse of the Al 2 O 3 surface area. In addition, the OSC of the CeO 2 strongly decreases upon thermal ageing, due to the growth of CeO 2 crystallites and/or formation of CeAlO 3 (3). Addition of ZrO 2 to the washcoat improves the thermal stability of the catalyst by hindering the sintering of the CeO 2 particles (4). This sug- gests that a close interaction between CeO 2 and ZrO 2 may thermally stabilize the CeO 2 . Therefore, incorporation of CeO 2 into a solid solution with ZrO 2 might provide a suit- able way to improve the thermal resistance of the material due to the ceramic nature of these systems. Furthermore, in the solid solution, the undesirable decline of the OSC due to fixation of the Ce 4+ /Ce 3+ redox couple in the 3+ state (CeAlO 3 , Ce 2 (SO 4 ) 3 ) should be limited. On the basis of these considerations we decided to study CeO 2 –ZrO 2 mixed oxides as supports for noble metal catalysts. Our initial investigation evidenced a remarkable promotion of the reducibility of Rh-loaded CeO 2 –ZrO 2 solid solutions in comparison to Rh/CeO 2 . The OSC as measured by oxygen uptake was strongly enhanced in comparison to Rh/CeO 2 catalysts, independently of the surface area, and this was at- tributed to the participation of lattice oxygen in the redox processes (5). This observation prompted us to investigate the behav- ior of metal-loaded CeO 2 –ZrO 2 solid solutions as cata- lysts for the reduction of NO by CO to give N 2 and CO 2 . This reaction is a key step in the catalytic conversion of au- tomotive exhausts since it is believed that NO is removed by its reaction with CO over rhodium. Rhodium promotes the above reaction due to its ability to efficiently dissociate NO in comparison to other platinum group metals. NO dis- sociation is believed to be the rate-determining step of the reaction, at least at low temperatures (6). Here we report evidence for NO dissociation occurring over reduced ceria- containing catalysts, suggesting a new role of both the metal and ceria in the TWCs under transient conditions. A pre- liminary report of the present results appeared recently (7). 1 0021-9517/96 $18.00 Copyright c  1996 by Academic Press, Inc. All rights of reproduction in any form reserved.