Nanoscale Engineering of the Metal-Support Interface Reveals Its Crucial Role in Ceria-Based Catalysts

Nanoscale engineering of the metal-support interface reveals its crucial role in ceria-based catalysts

Matteo Cargnello,^1,2 Vicky Doan-Nguyen,³ Thomas R. Gordon,¹ Rosa E Diaz,⁴ Eric A. Stach,⁴  Raymond J. Gorte,⁵ Paolo Fornasiero,² and Christopher B. Murray^1,3

¹ Department of Chemistry, University of Pennsylvania, 231 S. 34^th Street, Philadelphia, PA 19104, USA

²Department of Chemical and Pharmaceutical Sciences, ICCOM-CNR, Consortium INSTM, University of Trieste, 34127 Trieste, Italy

³Deparment of Materials Science and Engineering, University of Pennsylvania, 19104 Philadelphia, Pennsylvania, United States

⁴Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States

⁵Department of Chemical and Biomolecular Engineering, University of Pennsylvania, 19104 Philadelphia, Pennsylvania, United States

The synergy between support and supported phases is key for determining the properties of heterogeneous catalysts. Ceria (CeO₂) is an example of an “active support” in that it plays a critical role in the reaction mechanism of many industrially important catalytic processes by greatly increasing rates for reactions involving redox steps, such as CO oxidation, water-gas shift (WGS), steam reforming, and organic transformations, compared to rates observed on “inert” supports such as alumina. The increased activity of metal-ceria catalysts is typically attributed to the ability of ceria to store and release lattice oxygen, which then participates in the catalytic cycle. The observed enhancement is assumed to result from active sites at the metal-ceria interface, since rates can be much greater than the sum of rates over ceria and the metal individually. Although there is evidence that oxygen migration from the support to the metal particles occurs, this evidence comes only from model systems and this has not been demonstrated under industrially relevant reaction conditions. Clearly, understanding size-activity relationships for ceria-based catalysts is important for improving catalyst performance. The oxidation of CO has been thoroughly investigated as a model reaction and is very well understood on pure metals. It is generally accepted that the turnover rate for this reaction is independent of metal particle size. Therefore, this reaction is ideal for studying the role that the metal-support interface plays by measuring changes in rates upon varying the concentration of interfacial sites.

In this contribution, the role of the metal-support interface in ceria-based systems is unambiguously demonstrated through the use of monodisperse, size-tunable d⁸ metal nanocrystals (Ni, Pd and Pt). Synthesis of these catalysts allows the relative fraction of interfacial sites to be controlled and varied so that the special role of ceria in enhancing oxidation rates could be examined under realistic conditions in comparison to an inert alumina support. The results confirm that CO oxidation is insensitive to particle size for alumina-supported samples, and, for the first time, clearly demonstrate size-dependency for ceria-supported catalysts, for three important catalytic metals. A physical model unambiguously demonstrates that the metal atoms at the interface with ceria are responsible for the activity. The approach we developed represents a powerful tool for the preparation of model catalysts with tailored interfaces and provides parameters useful for understanding the special reactivity of particular materials, with the goal of preparing improved catalytic systems.