Ceria/zirconia-supported catalysts

The presence of oxygen enhances the catalyst stability. Breen et al. [187] investigated SRE over a range of oxide-supported metal catalysts. They concluded that the support plays an important role in the reaction. In fact, they observed that alumina-supported catalysts are very active at low temperatures for dehydration of ethanol to ethylene, which at higher temperatures (550 °C) is converted into H2, CO and CO2 as major products and CH4 as a minor product. The activities of the metal decrease in the order of Rh > Pd > Ni PS Pt. Ceria/zirconia-supported catalysts are more active and exhibit 100% conversion of ethanol at high space velocity and high temperature (650 °C). 

These results evidence that OSC is larger for Ir-based catalysts than for Rh catalysts in the whole temperature range from 200 to 500°C. Additionally, OSC for ceria-supported catalysts varies slightly with temperature and reaches a maximum after full reduction of the surface. In this case, OSC appears to be limited by surface diffusion. On the opposite, for ceria-zirconia supported catalysts, OSC is multiplied by a factor of 3 to 4 - depending on the metal - between 200 and 500°C. Bulk reduction is then responsible for such a large increase of the OSC. In that case, oxygen storage would be limited by bulk diffusion. 

Table 7. Coefficients of diffusion determined with the PLATINA model specially developed for ceria-zirconia supported catalysts (after Ref. 151).

C. Bozo, N. Guilhaume, and J.-M. Herrmann, The role of the ceria-zirconia support in the reactivity of platinum and palladium catalysts for methane total oxidation under lean conditions, J. Catal. 393, 393 06 (2001). 

The order of activity at higher temperatures is Pt fs Rh > Pd. By using a combination ofa ceria/zirconia-supported metal catalyst with the alumina support, it was observed that the formation of ethene does not inhibit the steam reforming reaction at higher temperatures. 

For the other three catalysts it was shown that below 250X the OSC is approximately equal to the OSCC and that (he storage process is certainly restricted to the metal itself or to its close perimeter. Above 250 0, farther oxygen atoms of the support become activated and may react. This oxygen mobilization step is easier on ceria-containing supports so that the increase in OSC with temperature is faster even at lower temperature than on alumina. The influence of temperature on the OSC was also examined on four different ceria- or ceria-zirconia-supported Rh or Ir catalysts. The results are presented on Fig. 7.5. 

Similar results were found by Bozo [44]. Palladium deposited onto ceria-zirconia Ceo67Zro3302 solid solution showed very high activity in methane combustion (T50 close to 300 C) but similar to that of palladium deposited onto alumina. Like for the case of platinum a deactivation is observed during tests at temperatures comprised between 200°C and 400 C (Fig. 13.3). However when aged at 1000 C under an air+water mixture this catalysts showed superior resistance compared to classical catalysts as far as activity is considered. Despite a severe sintering of both metal (dispersion is now 1%) and support, whose surface area is close to 4 mVg, T50 was shifted to 420 C, i.e. 120°C only, still much lower for platinum deposited on the same support which showed a TSO close to 620°C. Calculation of specific activities in the 200-300°C range have clearly evidenced that ceria-zirconia support does not have any influence upon performance of PdO in... 

A. A. Fonsecaa, J. M. Fishera, D. Ozkayaa, M. D. Shannon, D. Thompsett, Ceria-zirconia supported Au as highly active low temperature water-gas shift catalysts. Top. Catal. 44 (2007) 223-235. 

However, ceria/zirconia supports may improve the activity of platinum catalysts especially in a temperature range below 100 °C [329]. 

Early reports on this reaction were focused on iron oxide-based catalysts, supported on different supports alumina, zeolite, and active carbon, among others. Later, hydrotalcite-like compounds, and the topical V-Mg-0 catalysts were explored. Park and his group, one of the most active in this field, first reported the use of zirconia as a catalyst, " and later on that of ceria-zirconia-based catalysts and vanadium-antimony oxide-based catalysts. Publications up to 2007 have been subject of several reviews. Therefore, the most relevant feamres will be revised hereinafter, as well as more recent publications, and the reader is invited to get further details in the aforementioned reviews. 

These final comments connect perfectly with the topic addressed in the next section the interaction of CO with ceria-zirconia-supported gold catalysts. 

Step 2 The catalyst sample is further treated with several pulses of Og at Tqsc until saturation. During this step, oxygen is stored on both the Pd metal and the ceria-zirconia support. 

By loading low-surface-area perovskite species on high-surface-area active supports, activity can be improved. Alifanti et loaded 10 and 20 wt% LaCoOg perovskites on Cei xZr 02 (0ceria-zirconia-supported LaCoOg catalyst was found to be very active in the combustion of benzene and toluene in diluted streams. This catalyst decreased the light-off temperatures and increased the... 

The redox properties of ceria-zirconia mixed oxides are interesting, because these materials find applications as electrolytes for solid oxide fuel cells, supports for catalysts for H2 production, and components in three-way automobile exhaust conversion catalysts. The group of Kaspar and Fornasiero (Montini et al., 2004, 2005) used TPR/TPO-Raman spectroscopy to identify the structural features of more easily reducible zirconia-ceria oxides and the best method for their preparation by suitable treatments. TPR/TPO experiments and Raman spectra recorded during redox cycles demonstrated that a pyrochlore-type cation ordering in Ce2Zr2Og facilitates low temperature reduction. 

One example is a catalyst consisting of gold on cobalt oxide particles supported on a mechanical mixture of zirconia-stabilised ceria, zirconia and titania, that survived 773 K for 157 h, with some deactivation [145]. Grisel and Nieuwenhuys [38,127] have shown that the addition of transition metal oxides to form Au/MO j/ADOs catalysts, massively suppresses Au particle sintering in methane oxidation tests up to 973 K. Also, Seker and Gulari [30] found that Au/ADOs catalysts survive rigorous pre-treatments of 873 K in air for 24 h, followed by several cycles of 423-773 K and they were then kept... 

Also, Marsh and co-workers [145] showed that gold on cobalt oxide particles, supported on a mechanical mixture of zirconia-stabilised ceria, zirconia and titania remains active in a gas stream containing 15 ppm SO2. Haruta and co-workers [207] found that although the low-temperature CO oxidation activity of Ti02-supported Au can be inhibited by exposure to SO2, the effect on the activity for the oxidation of H2 or propane is quite small. Venezia and co-workers [208] reported that bimetallic Pd-Au catalysts supported on silica/alumina are resistant to sulphur poisoning (up to 113 ppm S in the form of dibenzothiophene) in the simultaneous hydrogenation of toluene and naphthalene at 523 K. 

M. Haruta, M. Date, Advances in the catalysis of Aunanopartides, Appl. Catal. A Gen. 222(2001)427. A. Wootsch, C. Descorme, D. Duprez, Preferential oxidation of carbon monoxide in the presence of hydrogen (PROX) over ceria-zirconia and alumina-supported Pt catalysts, /. Catal. 225 (2004) 259. 

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