Ce Composite Oxide

In 1985 Imamura et al. developed manganese-cerium composite catalyst independently without noticing aforementioned patent data [25]. They started from modifying Co/Bi composite oxide which was effective for the wet-oxidation of various carboxylic acids [26]. They tried to see the effect of the addition of various oxides of lanthanide at the beginning they didn t appreciate the extraordinary superior function of Ce02. The series of experiments led to the combination of Ce02 and manganese oxide as the best choice. 

The Mn/Ce catalyst was found to be effective for the wet-oxidation of various compounds [29,30]. Table 14.1 shows the activity of the Mn/Ce catalyst (Mn/Ce molar ratio of 7/3) in comparison with those of Cu(NO,)2 and Co/Bi(5/l). Judging from ATOC and the initial reaction rate (Ri), the Mn/Ce catalyst exhibits higher activity than the other two for the oxidation of almost all compounds. When the composition of the Mn/Ce catalysts was varied, the tendency in the change of the activity in the wet-oxidation and that in the decomposition of HOOH roughly coincided. Therefore, it may well be deduced that the redox property plays an important role in the catalyst activity. 

The observed reaction order with respect to the catalyst concentration less than unity (0.61) suggests that a radical chain reaction (Eqs. 14.2 - 14.5) occurs at least partly. 

Different from the usual liquid-phase oxidation in organic solvents, the radical chain cannot be long enough to sustain an efficient autoxidation mode in polar media like water, especially when the concentration of the organic pollutants is small. This is the most serious problem in the wet-oxidation process, and therefore, the role of the catalysts is very important. This point will be addressed later again. 

There is another information on the strong affinity and interaction between CeOj and Mn203[29]. When CeOj and Mn O, are allowed to contact, they interact easily even at a temperature as low as 150 °C, and isolated Mn species (sextet) begins to appear (Fig. 14.4). This means that Mn, although in a small amount, migrates quite easily into CeOj matrix and is reduced. However, it is not known now how this phenomenon is related to the catalytic action of Mn/Ce composite oxide. Thus, the interaction between Mn oxides and CeO is very complex and exact reason for the high activity of this catalyst is not clear. 

---

Hamoudi et al, modified Mn/Ce composite oxide by supj>orijng. Ag iind/or Pi (each I wt%) [34,40]. As shown in Fig. 14.6, un-promoted caialysl shows two reJuclion... 

Yoon et al. tried to improve Mn/Ce based catalyst. They used various support such as SiOj, TiOj, ZrSiO, ZrOj, and ZrO -TiOj and found that TiO, is the best support. They also investigated the effect of the additives on Mn/Ce composite oxide supported on TiO,. As Table 14. 2 shows, the best result is obtained with the catalyst modified with a small amount of Co i.e. Mn (2.7 wt%)-Ce(6.8 wt%)-Co(0.5wt%) on TiO, [36]. 

There is not much work on CeO based catalysts containing elements other than Mn or precious metals. Imamura et al. reported that Co/Ce composite oxide with 13 mol% of Ce [Co/Ce(87/l3)j is active for the oxidation of ammonia, although details are not known [25]. 

Work on Mn/Ce composite oxide catalysts is abundant [31,34-41]. However, deactivation phenomenon is often observed, and deposition of carbonaceous compounds is the main cause for deactivation [31,35,38-40,42]. This situation is shown in Fig. 14. 5 for the oxidation of phenol the higher the reaction temperature, the faster the deposition of carbonaceous compounds [34]. 

As will be addressed later, deposition of carbonaceous compounds is suppressed by supporting a small amount of Ag and/or Pt. Larachi investigated the regeneration of the used Mn/Ce composite oxide catalyst by combustion [43]. 

The shift toward lower reduction temperatures shows the improvement of the low temperature redox properties of the MnO/CeOj catalyst, which indicates the presence of metal-metal as well as metal-support interactions. These interactions are enhanced by the presence of CeO that induces the well-known spillover of hydrogen from Pt/and or Ag to CeOj or an increase in the oxygen mobility within CeO. The promoted catalysts showed higher activity than un-promoted one in the oxidation of phenol, and the amount of carbon deposit on them was small. For example, oxygen uptake by the combustion of carbonaceous compounds after the oxidation of phenol at 80°C for 1 h was 3535pmol/g-cat, 2660pmol/g-cat, and 2941 xmol/g-cat for MnO/CeOj, Pt-MnO /CeOj, and Ag-MnO/CeO, respectively. Larachi also promoted Mn/Ce composite oxide with lwt% of Pt [43]. 

Kawabata and Urano (1985) reported improved biodegradability of aniline and many refractory organic componnds after wet oxidation at 150°C in the presence of Mn/Ce composite oxides. 

---