Wilson, EL;
(2007)
The interaction of Co with Pd supported on CeOx ultrathin films.
Doctoral thesis , UCL (University College London).
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Abstract
A key goal in oxide surface science is to determine the mechanism of catalytic reactions at the atomic level. Here we are ultimately interested in CO oxidation over a model automobile catalyst: Pd supported on Ce02(lll) ultrathin films. The degree of reduction (Ce3+ concentration) is thought to play an important role in determining the catalyst performance. In this work X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) is used to study the degree to which the Ce3+ concentration can be varied in Ce02(l 11) ultrathin films on Pt(lll) and Rh(lll) and how they are modified by Pd. Reflection absorption infrared spectroscopy (RAIRS) has also been used to investigate the adsorption of CO on Pd/Ce02 x/Pt(l 11). The stoichiometry of three layer Ce02_x(lll) grown on Pt(lll) has been varied between CeOi 91 and CeO!M by UHV annealing at 970 K. Pd deposition induces an additional reduction of the film, for instance one monolayer equivalent of Pd on CeOL91 changes the stoichiometry to Ce0187 This reduction, which is dependent on the initial stoichiometry, is thought to be due to charge transfer from Pd to Ce02_x CO adsorbs only in the presence of Pd, independent of the stoichiometry of the original film, without affecting the Ce3+ concentration. Reflection absorption infrared spectroscopy (RAIRS) has been used to investigate the adsorption of CO on Ce02.x-supported Pd nanoparticles at room temperature. RAIRS results show that when Ce02 x is initially grown on Pt(l 11), a small proportion of the surface remains as bare Pt sites. However, when Pd is deposited onto Ce02.x/Pt(lll), most of the Pd grows directly on top of the Ce02.x(l 11). RAIR spectra of CO adsorption on 1 ML Pd/Ce02.x/Pt(l 11) show a broad CO-Pd band, which is inconsistent with a single crystal Pd surface. However, the 5 ML and 10 ML Pd/Ce02 x/Pt(lll) spectra show vibrational bands consistent with the presence of Pd(lll) and (100) faces, suggesting the growth of Pd nanostuctures with well defined facets. XPS studies of Ce02.x films grown on Rh(l 11) have also been carried out. It has been shown that by altering the oxygen pressure during Ce deposition the Ce3+ concentration of the resultant Ce02x thin film can be altered by up to 60%, whilst still observing the same LEED pattern. The effect of dosing Pd onto CeOz.x thin films has been shown to be highly dependent on the initial oxidation state of the Ce02x thin films. On Pd deposition onto CeOL91 and Ce0184 thin films an increase in Ce3+ concentration has been observed. However, in direct contrast, a decrease in Ce3+ concentration is observed when Pd is deposited on CeOl 7l and CeO! 61 thin films. CO adsorbs only in the presence of Pd, independent of the stoichiometry of the original film, without affecting the Ce3+ concentration.
| Type: | Thesis (Doctoral) |
|---|---|
| Title: | The interaction of Co with Pd supported on CeOx ultrathin films |
| Identifier: | PQ ETD:593311 |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Thesis digitised by ProQuest. |
| URI: | https://discovery.ucl.ac.uk/id/eprint/1445985 |
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