Cerium oxide stoichiometry alteration via Sn deposition: Influence of temperature

Abstract

Cerium oxide layers grown on Cu(1 1 1) were studied by conventional X-ray and resonant photoelectron spectroscopy with synchrotron radiation. A quantitative method of determining the cerium chemical state from the Ce 3d photoelectron spectra is described in detail. After the preparation of the ceria layer, Sn films of different thickness were evaporated onto the surface at temperatures of 120, 300 and 520 K. In all three cases, the deposited Sn was oxidized, CeO₂ was partially reduced, and a mixed Sn–Ce–O oxide was formed. The quantitative extent of these reactions was found to be determined by limited diffusion of the deposited Sn atoms into the ceria layer at low temperature. The excess of tin formed a metallic overlayer on the sample surface.

Introduction

Besides alumina, one of the most important ceramic materials nowadays is cerium oxide (ceria; CeO₂). Various sectors of industry use ceria for its excellent properties in electronic, physical, chemical and optical applications. The catalytic behaviour of cerium oxide makes it an important part of solid-oxide fuel cells and three-way catalysts in the automotive industry [1], [2].

A very important property of ceria is its oxygen storage capacity. Under reducing conditions, e.g. during catalytic reaction on the surface, ceria can release a part of its oxygen content and provide it to the reaction. This process includes a transformation from CeO₂ to Ce₂O₃ and a change of the crystal structure from cubic fluorite to hexagonal. For this transition, relatively low activation energy is needed and oxygen vacancies exhibit a high mobility. The transition is reversible and re-oxidation to stoichiometric CeO₂ is possible in the presence of oxygen in the ambient atmosphere [3].

Some dopants may further improve the catalytic properties of cerium oxide. They can be either catalytically active transition metals forming active metal particles on the ceria surface [4] or metals strongly interacting with cerium and oxygen [5], [6]. In the latter case the bond strength between cerium and oxygen is decreased, a large number of defects are introduced into the crystal structure acting as active sites for the catalyzed reaction, the activation energy of the cerium chemical state alteration is decreased and the oxygen storage capacity is improved. Tin is such a metal, and tin dioxide, SnO₂, is an important compound used in heterogeneous catalysts and semiconductor gas sensors. Sn forms with Ce several bulk intermetallic compounds exhibiting high melting points in the range from 1435 to 1673 K (CeSn₃, Ce₂Sn₃, Ce₂Sn), much higher than that of the single components (Ce 1071 K, Sn 505 K) [7]. Such a strong interaction leads to formation of a mixed Sn–Ce–O oxide that exhibits higher catalytic activity than both pure individual oxides [6], [8], [9].

From the behaviour described above it can be seen that it is important to study the interaction of ceria with metals, other oxides and gases, and the chemical state of cerium is a key parameter in these studies. It is most usually determined by conventional X-ray photoelectron spectroscopy (XPS) of the Ce 3d line [10], [11]. The electronic configuration of CeO₂ (Ce⁴⁺) is characterized by unoccupied 4f orbitals (4f⁰) and Ce₂O₃ (Ce³⁺) by a 4f¹ configuration. The differences influence the shape of both core level and valence band photoelectron spectra. Using tunable synchrotron radiation allows the acquisition of valence band spectra with high surface sensitivity and energy resolution, and sensitivity to the fingerprints of the cerium chemical state can be hugely amplified using resonance effects in the Ce 4d–4f photoabsorption region [12].

For model studies, ceria is commercially available in the form of single crystals or a nanopowder, but both suffer from charging effects in electron spectroscopy. To avoid charging caused by their low electrical conductivity, thin films have been grown on different single-crystalline substrates, such as Pt(1 1 1) [11], Re(0 0 0 1) [3], or Cu(1 1 1) [13]. We have developed a procedure for growth of continuous oriented CeO₂ layers on Cu(1 1 1) with a very low number of defects in the structure and good stoichiometry [14].

In our previous papers [15], [16], the interaction of Sn with these CeO₂ layers was studied after tin deposition on the oxide surface at 520 K (i.e. the same temperature as used for the CeO₂ growth on a Cu(1 1 1) single crystal) and immediate formation of the mixed Sn–Ce–O compound was found. The aim of the present work was to check how the temperature influences the tin-induced ceria reduction. We suspected that the phase transition from Ce⁴⁺ to Ce³⁺ compound accompanied by the oxidation of Sn, i.e. the formation of the mixed Sn–Ce–O oxide from metallic Sn and CeO₂ layers may occur at temperatures below 520 K, and here we report experiments performed at 120, 300 and 520 K.