Cerium-based catalysts

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]... 

Table 7.8. 02 + 02 equilibration at 200°C - Particle size effect on cerium-based catalysts [69]. 

ELECTRONIC STATE OF CERIUM-BASED CATALYSTS STUDIED BY SPECTROSCOPIC METHODS (XPS, XAS)... 

The 3d core level spectra of Pd-Ce/ y A O catalysts are compared to "the Ce02 spectrum (see fig. 1). We referred the binding energies to A12p at 119.8 eV. For CeC, some A Oj was added mechanically. Apart from the spin-orbit coupling, the 3d 5/2 transition of cerium exhibits three main contributions for ceria. We observed two transitions for the cerium based catalysts, as previously reported in the literature (ref. 8,9)... 

XAS and XPS have proved to be powerful probes for the study of the electronic configuration of cerium based catalysts. Thus they can give us useful informations on the nature of the cerium compounds. In the case of Pd-Ce/ y Al O (chloride) and Ce/ yA Oj catalysts we have evidenced ... 

Electronic State of Cerium-Based Catalysts Studied by Spectroscopic Methods (XPS, XAS)",... 

Dehydrogenation, Ammoxidation, and Other Heterogeneous Catalysts. Cerium has minor uses in other commercial catalysts (41) where the element s role is probably related to Ce(III)/Ce(IV) chemistry. Styrene is made from ethylbenzene by an alkah-promoted iron oxide-based catalyst. The addition of a few percent of cerium oxide improves this catalyst s activity for styrene formation presumably because of a beneficial interaction between the Fe(II)/Fe(III) and Ce(III)/Ce(IV) redox couples. The ammoxidation of propjiene to produce acrylonitrile is carried out over catalyticaHy active complex molybdates. Cerium, a component of several patented compositions (42), functions as an oxygen and electron transfer through its redox couple. 

Figure 17 shows a comparison of the fresh SO2 removal ability for these five major types of commercially available SOx catalysts. The materials were tested at 1350 F at various concentrations with a very low capacity cracking catalyst. The magnesia-based catalyst is much better than lanthanum-based catalyst followed by platinum or cerium on alumina and finally alumina alone. The reverse order in activity observed for the lanthanum-based and cerium additives, compared to the relative results given previously for lanthanum and cerium, was not investigated, but may be related to the presence of cerium on the lanthanum-based additive (27). 

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

Over the past 20 years, The U.S. Department of Energy s (DOE s) Pacific Northwest National Laboratory (PNNL) has developed the patented MEO technology using cerium as a metal catalyst. CerOx Corporation holds as exclusive, world-wide license to market the proprietary, cerium-based MEO process. The first commercial CerOx system was sold to the University of Nevada in Reno. The technology is commercially available. 

From the early 1960s onwards, the use of lanthanide (Ln) based catalysts for the polymerization of conjugated dienes came to be the focus of fundamental studies [31]. The first patent on the use of lanthanides for diene polymerization originates from 1964 and was submitted by Union Carbide Corporation (UCC) [32,33]. In this patent the use of binary lanthanum and cerium catalysts is claimed. Soon after this discovery by UCC, Throckmorton (Goodyear) revealed the superiority of ternary lanthanide catalyst systems over binary catalyst systems. The ternary systems introduced by Throckmorton comprise a lanthanide compound, an aluminum alkyl cocatalyst and a halide donor [34], Out of the whole series of lanthanides Throckmorton... 

In ceria-based catalysts, one main aspect studied with XPS is the cerium redox state (i.e. the proportion of Ce and Ce " ) for different preparations or treatments. Peak u, located at Eb==916.5 eV, which appears to be typical of and absent for Ce, and consequently decreases upon reduction, has been used frequently with this... 

The conventional nickel-based catalysts could be modified by adding oxide promoters such as potassium, lanthanam, cerium, and molybdenum in the catalyst formulations. It is believed that the added promoters improve the dispersion of nickel metal on the catalyst surface, thereby reducing the chance of carbon accumulation. Noble metals such as Pd, Pt, Ru, and Ir have been found to be more carbon tolerant as the solubility of carbon is less in these metals.54-57 However, they are more expensive than nickel-based catalyst, and as a consequence, they are less attractive for large-scale commercial applications. Alloying of nickel with other base metals such as Cu, Co, or noble metals such as Au, Pt, and Re has also been found to decrease... 

No XPS signal for Mn or Ce is detected after reaction. No new phases were detected by XRD. The carbon at the catalyst surface is probably present as a carbonaeeous polymer, as has been reported previously for copper based catalysts [2]. At this stage of the investigation, we concentrated on polymer formation. As no new phase (neither another manganese or cerium oxide... 

In ceria-based catalysts, one main aspect studied with XPS is the cerium redox state (i.e. the proportion of Ce and Ce " ) for different preparations or treatments. Peak u, located at Eb 916.5 eV, which appears to be typical of Ce and absent for Ce, and consequently decreases upon reduction, has been used frequently with this aim. The percentage %(u ) of the total integrated spectrum intensity (after baseline subtraction) lying within this peak in pure, nonreduced Ce02 has been computed to amount to 13.7% [108]. The %(u ) value of an analyzed material is frequently used in the literature to quantify its redox state, assuming e.g. that the the quotient betwen it and the value for Ce02 gives the fraction of Ce in the material. It has been however pointed out in early works [109], that this value is not proportional to the amount of Ce present specifically Romeo et al. [110] claimed that peaks v... 

The use of CeOa-based materials in catalysis has attracted considerable attention in recent years, particularly in applications like environmental catalysis, where ceria has shown great potential. This book critically reviews the most recent advances in the field, with the focus on both fundamental and applied issues. The first few chapters cover structural and chemical properties of ceria and related materials, i.e. phase stability, reduction behaviour, synthesis, interaction with probe molecules (CO, Os, NO), and metal-support interaction — all presented from the viewpoint of catalytic applications. The use of computational techniques and ceria surfaces and films for model catalytic studies are also reviewed. The second part of the book provides a critical evaluation of fhe role of ceria in the most important catalytic processes three-way catalysis, catalytic wet oxidation and fluid catalytic cracking. Other topics include oxidation-combustion catalysts, electrocatalysis and the use of cerium i catalysts/additives in diesel soot abatement technology,... 

The nature of the outer-layer of the Cu-Zn based catalysts and the role of the different active sites are still a topic of investigation. Metallic copper is implicated as being the dominant oxidation state of the metal during the reaction. However, the presence of Cu+ is also important as a small amount of oxygen increases the reaction rate.51,66 Shen et al.67 found on ceria supported copper catalyst that in spite of the reductive reaction atmosphere, metallic copper particles on cerium oxide were oxidised during reaction and the catalyst was activated. The formation of the copper oxide species was considered indispensable for the onset of high catalytic activity. Synergy between Cu and ZnO in the catalysis of methanol synthesis... 

Cerium oxide based catalysts have been widely studied during the last two decades. Main catalytic application was the elimination of automotive exhaust emissions (1,2). The catalytic properties of this oxide has often been related with the mobility of oxygen vacancies in the solid (3,4) and hence with its capacity to release stored oxygen under reducing conditions tests (5,6).Moreover, A.F. Ahlstrom and C.U.I. Odenbrand (7) reported the deactivation by sulphur dioxide of supported copper oxide during the oxidation of soot. 

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