Journal of Molecular Catalysis A: Chemical 163 (2000) 221–232 In consideration of precursor states, spillover and Boudart’s ‘collection zone’ and of their role in catalytic processes M. Bowker * , L.J. Bowker, R.A. Bennett, P. Stone, A. Ramirez-Cuesta Department of Chemistry, Centre for Surface Science and Catalysis, University of Reading, Reading RG6 6AD, UK Received 23 February 2000; accepted 5 June 2000 Abstract Various aspects of ‘spillover’ and ‘reverse spillover’ in catalysis are considered. The ideas of Kisliuk regarding the kinetic role of weakly held adsorbed states as precursors to stronger chemisorption are shown to be important for reverse spillover, as highlighted by Boudart’s concept of the ‘collection zone’. STM has proved to be an important tool for the understanding of the formation and thermal evolution of nanoparticles on surfaces. Here we present direct, in situ observations of spillover for the reaction of oxygen with Pd nanoparticles on TiO 2 (1 1 0). In turn, this is connected to a form of SMSI, and it is proposed that this spillover may also be the mechanism of the oxygen storage phenomenon for CeO 2 -doped automobile catalysts. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Collection zone; Spillover; Boudart; Oxygen storage; SMSI; Catalysis; Nanoparticles; STM 1. Introduction In recent years, surface science studies related to catalysis have advanced from single crystal work to the fabrication of model nanoparticulate catalysts (for re- views, see [1] and [20]). In some ways these bridge the materials gap perceived to exist between bulk samples (single crystals, for example) and industrial catalysts. This is important in the context of catalysis since, as shown in Fig. 1, it is possible that very small particles do not behave like single crystals for a variety of rea- sons. These include the fact that if the particle is very small (say <2 nm), the average surface co-ordination is very low (surface energy is high). Also, there are so few atoms in the particle that a metallic-like con- duction band is not formed because there is not a ‘continuum’ of energy levels. The equation in Fig. 1 * Corresponding author. E-mail address: m.bowker@rdg.ac.uk (M. Bowker). gives an approximate relationship between the num- ber of atoms in a particle and the spacing of energy levels in the particle, and is related to the Fermi en- ergy of the metal. Thus, a 2 nm hemispherical particle (which will consist of approximately n = 120 atoms), with a typical metal work function of 4.5 eV, will have levels spaced by approximately 4 kJ mol -1 . However, a particle of half that size has a spacing of around 30 kJ mol -1 , making electron conduction highly acti- vated and difficult at room temperature. Additional factors, which may modify small par- ticle behaviour, include the possibilities of spillover, that is diffusion of reactive species from the metal to the support or vice-versa (reverse spillover). This can include diffusion of inorganic oxides from the support onto the metal, the proposed source of the SMSI ef- fect in catalysis. Such diffusion effects are of limited range and are, therefore, likely to be of greatest sig- nificance for the behaviour of small particles. Also the particle-support interface may be the location of the 1381-1169/00/$ – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S1381-1169(00)00388-5