Compensación por Tiempo de Servicios Naturaleza de la CTS.

We have concentrated so far on single crystal surfaces as these are most

commonly studied. However, interest is growing in the characterisation of small supported particles, as these are used widely in industrial catalysis. In this literature, the behaviour o f particles is often compared with that of single

crystals. These studies are of use in this work, particularly in evaluating to what extent the work on single crystals is relevant to our system. In this section, work is discussed in which small metal particles are characterised and their interactions with CO investigated. Characterisation relied almost exclusively on temperature programmed desorption. X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS).

One of the primary goals described in recent literature was to try and pinpoint the transition from localised to bulk metal behaviour as the particle size was increased. Wertheim et al [39] estimated this size in the following way. For simplicity consider alkali or noble metal atoms, which have one outer s electron.

In an n-atom cluster there will be 2n s states, with an interval comparable to the

width of the s band of the infinite solid. When n becomes so large that the

separation between these states becomes comparable to the thermal energy kT,

then the electrons are no longer localised and the cluster has metallic properties. At room temperature, in a metal with a band width of 5 eV, this transition takes

place when n exceeds 100 atoms, corresponding to a spherical cluster with a

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Experimentally, Altman and Gorte [40] have observed that below particle sizes of 2.5 nm, the Pt did not show the electronic structure characteristic of the bulk metal. Mason et al [41] also observed (by XPS) a variation of valence and core level spectra with cluster size, and that as the particle size was increased, the Pt nuclei spectrum changed from resembling the spectrum for atomic Pt to that for bulk Pt. It was observed by Wertheim et al [39, 42], that valence band spectra of small clusters exhibit band narrowing due to the increased localisation. Core­ electron shifts to higher band energies are also noted, where the core-level binding energy can increase by between 0.5 and 1.5 eV.

Altman and Gorte [43, 44] carried out TPD of CO on Pt for particle sizes estimated at 1.1 nm up to a continuous film. The strong dependence of the TPD features on particle size was interpreted as being due to changes in the

crystallographic orientation of the surface sites. The smallest particles, in the range 1.0 to 2.0 nm, exhibited only a single desorption state at 510 K while the larger particles exhibited a second desorption state with a peak temperature at 400 K. The proportion of CO desorbing from these two states also varied

continuously with particle size. The desorption state at 400 K corresponded well with desorption from P t(lll)-ty p e terraces. The state at 510 K corresponded with desorption from highly stepped surfaces like P t ( lll) surfaces with defects or Pt(331). Therefore it was concluded that the two desorption peaks observed on the small particles were related to the crystallographic orientation of the sites available for desorption. In fact the change in the relative populations of the two desorption states with increasing particle size is interpreted as evidence for the formation of (111) type facets on the larger particles. This assignment o f peaks is in agreement with work carried out on single crystal surfaces by Hayden et al [20], and Collins and Spicer [22]. Their work was discussed in section 1.3.1. To summarise, it was found that the desorption states observed on Pt particles are identical to those observed on single crystals, and only the distribution of

desorption states was affected by particle size.

Any differences in bonding should be reflected in differences in the desorption temperatures. The fact that these temperatures are so similar for small Pt particles and bulk Pt metal indicates that the bonding is the same. Altman and Gorte found this to be true for particles so small that all the Pt atoms were exposed to the surface [43]. The smallest metal particles examined by these workers were too small to exhibit the bulk metal band structure, therefore the bonding of CO on Pt must be very localised. This corresponds with work by Shek et al [45] on the single crystal Pt(llO).

It was found that the structure of the supporting material did not directly affect the metal. The final shape of the metal particles after high temperature treatment was not strongly dependent on the substrate structure. Only the temperature required to reach the final particle shape was affected by the use of crystalline or amorphous alumina. This would imply that effects due to the interaction between the metal and the oxide are much weaker than effects due to particle size [44].

To summarize, this work has shown that particles exhibit similar characteristics to single crystals, and can therefore be legitimately compared with them.

Electronically, particles are similar to bulk Pt down to particle sizes of a few nm, below which localised behaviour becomes manifest. Chemically, they act in the same way as single crystals, their exact behaviour depending on which crystal face is exposed.

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1.3.7 THE EFFECT OF PRESSURE AND SURFACE IMPURITIES ON