Ceria with the Active Noble Metals

Interaction of Ceria with the Active Noble Metals 

Interaction of ceria with the active noble metals is fundamental to the provision of oxygen storage, or buffering, in three-way catalysts, and it depends first and 

For reactions in which ceria is a source of oxygen (as opposed to being a sink for oxygen), the detailed mechanism is thought to involve adsorption of the reductant on the noble metal, extraction of oxygen from the ceria, and reaction between the reductant and oxygen on (or in the periphery of) the metal. This view is consistent with the results of numerous model studies conducted in both UHV, such as CO and H2 TPD/TPR, as well as at normal pressure, such as transient and steady-state CO oxidation, WGS, and steam-reforming reactions (e.g., [8]). 

Pd films may be oxidized by their ceria-zirconia supports merely by heating to 470 K in UHV reinforces Yang et al. s result [10]. 

Deactivation of oxygen storage occurs by both thermal and chemical modes. 

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In the 1990s and the first part of the 2000s, noble metal and mixed oxide-based catalysts were mostly investigated, with interest first focused on alternative materials, especially metal oxides and protonic and metal zeohtes. More recently, efforts have been focused on formulations with enhanced redox properties and oxygen storage capacity, such as combinations of ceria, zirconia, titania and vanadia. Noble metals (Pt, Pd) show higher specific activity when they are prepared in a highly... 

Catalyst systems for the WGS reaction that have recently received significant attention are the cerium oxides, mostly loaded with noble metals, especially platinum 42—46]. Jacobs et al. [44] even claim that it is probable that promoted ceria catalysts with the right development should realize higher CO conversions than the commercial Cu0-Zn0-Al203 catalysts. Ceria doped with transition metals such as Ni, Cu, Fe, and Co are also very interesting catalysts 37,43—471, especially the copper-ceria catalysts that have been found to perform excellently in the WGS reaction, as reported by Li et al. [37], They have found that the copper-ceria catalysts are more stable than other Cu-based LT WGS catalysts and at least as active as the precious metal-ceria catalysts. 

Most of the current converters consist of a flow-through ceramic monolith with its channel walls covered with a high-surface-area 7-AI2O3 layer (the washcoat) which contains the active catalyst particles. The monolith is composed of cordicrite, a mineral with the composition 2MgO 2AI2O3 5Si02. The chemical composition of a modern TWC is quite complex. In addition to alumina, the washcoat contains up to 30 wt% base metal oxide additives, added for many purposes. The most common additives are ceria and lanthana in many formulations BaO and Zr02 are used, and in some converters NiO is present. The major active constituents of the washcoat are the noble metis Pt, Pd, and Rh (typically 1-3 g). Most of the TWC systems in use today are still based on Pt and Rh in a ratio of about 10 1. 

Hardacre el al. (7 75, 174) investigated the properties, structure, and composition of cerium oxide films prepared by cerium deposition on Pt(lll), finding that the activity for CO oxidation is enhanced on Pt(lll) that is partially covered by ceria. It was suggested that new sites at the Pt-oxide interface become available for reaction. A remarkable observation is the high activity for CO oxidation when the Pt(lll) sample is fully encapsulated by ceria (Pt was undetectable by XPS and AES). It was proposed that an ultrathin, disordered ceria film becomes the active catalyst. It was also demonstrated by XPS and AES that Pt dramatically increases the reducibility of cerium oxide that is in intimate contact with Pt. This result suggests that intimate contact between the noble metal and oxide phases is indeed crucial to facile oxygen release from ceria. High-resolution electron microscopy demonstrated the presence of direct contact between ceria and noble metal for supported Pt-Rh catalysts (775). Hardacre et al. (173,174) related the catalytic activity of the ceria phase to partially reduced cerium oxide. 

In conclusion, there is overwhelming evidence for the beneficial effect of cerium oxide on the activity of the noble metal catalyst. However, the nature of the promoter effect of ceria is not fully understood. Most likely, the noble metal-cerium oxide interface is of crucial importance for some of the effects observed. More studies with model systems are needed for a better understanding of the promoter effects of ceria. 

Additional measurements on a full series of ceria and cerium-zirconium mixed oxides supported noble metals showed that Ru was at least 10000 times more active than Pd and about 20 times more active than Rh for the activation of oxygen [70]. Up to now, most results were obtained with Rh catalysts but Ru could be a good candidate for surface diffusion measurements. 

In all these applications and related studies, CeO is seldom used alone but it often employed in combination with other oxides or in conjunction with active metals (nobles metals or Ni, Ag. Cu) and thermally stable supports. Therefore strictly speaking, ceria may function either as structural/electronic promoter or as cocatalyst, depending on the type of application, but not as a true catalyst, although the amount of cerium oxide which is used in certain catalyst formulations may easily exceed 20-30 wt. %. 

Cerium-based catalysts have been successfully used in several processes. For example, ceria (Ce02) is used as an additive [ 1,2] in modem automotive exhaust catalysts. Ceria acts as an excellent oxygen store [3-5] in the catalyst, which is thus rendered a very effective catalyst for combustion [6]. Moreover, addition of ceria to the automotive exhaust catalysts minimises the thermally induced sintering of the alumina support and stabilises the noble metal dispersion [7]. Ceria also enhances nitric oxide dissociation when added to various supported metal catalysts [8], which is another important function of the automotive exhaust catalyst. Recent investigations by Harrison et al have shown that ceria doped with certain lanthanides and promoted with copper and chromium have catalytic activities comparable to that of the noble metal catalysts [9]... 

A lot of studies have been published concerning ceria, pure or doped, associated with a noble metal for boosting activity in catalytic combustion. The main metals studied are Pt and Pd essentially, but some studies concern Rh and Ag. 

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