Ceria supported

Bunluesin, T., Gorte, R.J., and Graham, G.W. 1998. Studies of the water-gas-shift reaction on ceria-supported Pt, Pd, and Rh Implications for oxygen-storage properties. Appl. Catal. B Environ. 15 107-14. 

Figure 30 H2 yields from ATR of diesel (a) over alumina supported Pd and/or Pt catalysts (b) over ceria supported Pt and/or Ni catalysts (Reprinted from Cheekatamarla and Lane, copyright (2003), with permission from authors)...

Conventional Ni-based catalysts still dominate in SR applications however, ceria-supported noble metal catalysts have also attracted interest reeently. The study of Rh for both POX and ATR has increased sinee Rh is in general more active for reforming and is less prone to form carbon. H2 and CO selectivities in Rh-based catalysts have been shown to be affected by catalyst geometry. This indicates that feed mixing and mass transfer can play an important role. 

In addition to titania, alumina and ceria supports (Table 4.4), the DPU method has been applied to other supports. Gold particles supported on ferric oxide were quite small (3-7 nm) after thermal treatment is static air at 623 K.80 Another study reported the use of the DPU method to deposit gold onto other supports (MgO, CaO, SrC>2 and BaO).81 After calcination at 673 K, gold particles of moderate size were obtained on magnesia (8 nm) and on calcia (6nm). In all cases, all the gold in solution was deposited on the support. 

The reducibility of the ceria support when gold is present also occurs at lower temperatures than in its absence, but only the surface layer is reduced.9 27 49 Redox processes are thought to occur with Au/CeC>2 even at room temperature 24 water appears to re-oxidise anion vacancies close to the metal, and the failure of XANES measurements to show any change in the Ce3+/Ce4+ ratio on exposure of a reduced catalyst to water vapour even at 623 K27 may be due to the limited number of sites participating in redox reactions. Re-oxidation of reduced ceria surfaces by water in the absence of metal has long been known.56... 

Most of the supported oxides referred to above are reduced under the reducing conditions mentioned however, contrasting behavior has been observed for ceria-supported vanadia. Ce4+ is reduced to Ce3+ while vanadium remains fully oxidized it is assumed that, as a consequence of the strong interaction between vanadia surface species and the ceria support, reduction of ceria occurs at the interface (Martinez-Huerta et al., 2004). A TPR/TPO-Raman investigation by Martinez-Huerta et al. showed that the solid-state reaction between surface vanadia and the ceria support to form CeV04 is facilitated in a reducing atmosphere at approximately 200 °C (Martinez-Huerta et al., 2007). 

Ceria/noble metal (such as Ru, Rh, and Pd) catalysts are composed of noble metal species such as nanoparticles and clusters dispersed on the ceria supports. The catalysts show typical strong metal-support interactions (SMSI) (Bernal et al., 1999), that is, the catalysts exhibit a number of features for SMSI effects including (1) reducible supports (2) "high temperature" reduction treatments (3) heavily disturbed chemical properties and significant changes in catalytic behavior of the dispersed metal phase (4) reversible for recovering the conventional behavior of the supported metal phase. In these cases, the reducibility of ceria NPs is greatly enhanced by the noble metal species and the catalytic activities of the noble metals are enhanced by ceria NPs. 

Table 4.2. Some reacitons investigated on ceria-supported metal catalysts and retaled systems.

In the case of the ceria supported non-noble metal catalysts, both conventional impregnation techniques (16,32,264,292) and precipitation of the metal precursor onto the ceria support have been used (43). Some P CeO catalysts have also been prepared in the latter way (284,293). Co-precipitation from a mixed solutions containing both... 

Regarding the Rh/Ce02 (N) catalyst. Table 4.3 shows that the overall concentration of species is actually determined by the addition of two different components. They have been referred to as reversible and irreversible contributions (195). The first one, is associated with the hydrogen chemisorbed on the ceria support. This hydrogen form may be eliminated, with inherent reoxidation of the catalyst, by simple evacuation, it being restored by a further treatment with H2. In accordance with Table 4.3, for a mean surface area catalyst (49 m. g ), reduced at a moderate temperature (623 K), the reversible contribution represents about two thirds of the total concentration of species. This observation is in full agreement with the magnetic... 

Figure 4J. TPR-(TCD) study of a scries of NM(1 wt%yCe02 catalysis. Traces corresponding to Rh/CeQ2, Oj), Ir/Cc02 ( ), Ru/Ce (d), Pt/CcQj (s) and Pd/CcOz (D The diagram for the bare ceria support (trace has also been included for comparison. Reducing gas mixture 5.22% Hj/Ar. Results taken from (27).

Very recently, H-NMR technique has been applied to the investigation of the hydrogen chemisorption on Rh/CeO catalysts prepared from both Rh(N03)3 and RhCla precursors (163). Because of the different chemical shift shown by the hydrogen species adsorbed on the metal and the ceria support, the evolution of the rhodium... 

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