Controlling the charge state of supported nanoparticles in catalysis: lessons from model systems

Abstract

Model systems are very important to identify the working principles of real catalysts, and to develop concepts that can be used in the design of new catalytic materials. In this review we report examples of the use of model systems to better understand and control the occurrence of charge transfer at the interface between supported metal nanoparticles and oxide surfaces. In the first part of this article we concentrate on the nature of the support, and on the basic difference in metal/oxide bonding going from a wide-gap non-reducible oxide material to reducible oxide semiconductors. The roles of oxide nanostructuring, bulk and surface defectiveness, and doping with hetero-atoms are also addressed, as they are all aspects that severely affect the metal/oxide interaction. Particular attention is given to the experimental measures of the occurrence of charge transfer at the metal/oxide interface. In this respect, systems based on oxide ultrathin films are particularly important as they allow the use of scanning probe spectroscopies which, often in combination with other measurements and with first principles theoretical simulations, allow full characterization of small supported nanoparticles and their charge state. In a few selected cases, a precise count of the electrons transferred between the oxide and the supported nanoparticle has been possible. Charge transfer can occur through thin, two-dimensional oxide layers also thanks to their structural flexibility. The flow of charge through the oxide film and the formation of charged adsorbates are accompanied in fact by a substantial polaronic relaxation of the film surface which can be rationalized based on electrostatic arguments. In the final part of this review the relationships between model systems and real catalysts are addressed by discussing some examples of how lessons learned from model systems have helped in rationalizing the behavior of real catalysts under working conditions.

Nature and Topology of Metal–Oxygen Binding Sites in Zeolite Materials:17O High‐Resolution EPR Spectroscopy of Metal‐Loaded ZSM‐5

“…The nature of the metal–oxide bonding interaction is one of the primary factors affecting the catalytic activity of heterogeneous catalysts based on metals stabilized on inorganic supports . Tiny metal nanoparticles 2–10 nm in size, small clusters, and even single atoms, stabilized on oxide surfaces or in the cages and channels of zeotype materials exhibit surprising catalytic properties, intimately connected with the nature of the chemical interactions between the metal and the oxide support.…”

Nanoparticles Supported on Sub‐Nanometer Oxide Films: Scaling Model Systems to Bulk Materials

“…Many nanoparticulate catalysts are prepared by wet impregnation on an oxidic support. The oxidic surface is often regarded as an “inert” support assuring stabilization and dispersion while hampering Ostwald ripening [1] . Recent results show, however, that the right choice of support can have significant influence on the selectivity and activity of catalysts [2] .…”

Nanopartikel auf subnanometer dünnen oxidischen Filmen: Skalierung von Modellsystemen

“…Eine häufig genutzte Methode zur Generierung von Nanopartikeln auf oxidischen Tr ägern ist die Imprägnierung mit Präkursoren, gefolgt von chemischer oder thermischer Behandlung.I nv ielen Fällen wird der oxidische Tr äger als inert angesehen und dient zur Dispergierung und Stabilisierung der Nanopartikel gegen Ostwald Reifung. [1] Allerdings kann die geschickte Auswahl des richtigen Tr ägers die Selektivitätu nd Aktivitätd es nanopartikulären Katalysators signifikant verändern. [2] Es konnte gezeigt werden, dass sogenannte Elektronische-Metall-Träger-Wechselwirkungen (EMSI) Einfluss auf die katalytische Aktivitätv on Metallen nehmen kçnnen.…”