Catalyst copper/ceria/alumina

The [MSR 6] reactor type (see below) was applied for methanol steam reforming over Cu/Ce02/Al203 catalysts by Men et al. [34, 35], Wash coating of the alumina was performed, followed by subsequent impregnation steps with ceria and copper salt solutions. At 250 °C reaction temperature and a water/methanol molar ratio of 0.9, the copper/ceria atomic ratio was varied from 0 to 0.9, revealing the lowest conversion for pure ceria and a sharp maximum for a ratio of 0.1 (see Figure 2.13). 

Since classical Cu/ZnO catalysts exhibited a poor stability while the addition of alumina resulted in much better systems, it was tempting to add alumina to Cu-Ce intermetallic compounds. Jennings et al. (1992a), prepared ternary Cu-Ce-Al alloys of various compositions and also tried a variety of other metals (Ca, Cr, Mn, Pd, Zn). Among these ternary alloys aluminum-containing catalysts were the best. In spite of lower initial activities as compared to binary alloys, they exhibited a much better long-term stability. It is believed that the role of aluminum is to stabilize the disperse copper-ceria phases responsible for methanol synthesis activity, although the mechanism for such a process remains unclear. 

It is noteworthy that components other than alumina often have detrimental chemical and physical effects on the catalyst. For example, Herman et al. (77) reported that addition of ceria to the Cu/ZnO catalyst lowered methanol conversion by a factor of 5, despite the presence of a large concentration of microparticulate copper metal. This effect was explained by the ability of ceria to drive copper from the active state in zinc oxide solution to inactive metallic copper. Chromia, which had been used as a component of catalysts for methanol for a considerable period of time, is a suitable structural promoter, but some preparations result in an increase of concentration of side products such as higher alcohols (39), dimethyl ether (47), or even hydrocarbons. 

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]... 

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