(79g) 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.