Ceria-zirconia catalysts

In a more recent demonstration that titration methods do not properly account for OSC, it has been shown that Pd/ceria and Pd/ceria-zirconia catalysts are capable of reversibly releasing more oxygen after they have been poisoned with SO2 than they are before being poisoned with SOj [17]. An example of this is given in the pulse data from a Pd/ceria-zirconia catalyst at 773 K, reported in Figure I l.l, with data for the unpoisoned catalyst shown on left and data for the poisoned catalyst on the right. 

Under catalytic reaction conditions, one should not necessarily expect species to proceed to the thermodynamic final state. An additional complication comes from the fact that the redox properties of catalytically active ceria and of ceria-zirconia mixed oxides appear to be quite different from the bulk thermodynamic values for ceria [37,38]. For example, ceria films calcined above 1270 K no longer promote the WGS [22] or steam-reforming reactions [20] and are much more difficult to reduce upon heating in vacuum [39]. These observations appear to be explained by calorimetric studies, which have shown that the heat of reoxidation for reduced Pd/ceria and Pd/ceria-zirconia catalysts is much lower than bulk thermodynamics would suggest [38]. Therefore, bulk thermodynamic information may not be entirely relevant for describing the nature of sulfur-containing species on catalytically active materials. 

On the other hand, cerium has been shown to be an effective oxygen reservoir, enhancing the activity of many oxidation catalysts. Due to this property, cerium oxide is also considered to potentially enhance the thioresistance of the catatysts. This aspect is of great practical importance, since the use of palladium catalysts is hindered by the poisonous effect of sulphur compoimds, often present in off gases. Most works dealing with ceria-zirconia catalysts have been carried out with catalysts prepared by coprecipitation methods, whereas in this work an ahemative procedure, based on the incipient wetness technique is used to incorporate ceria to the zirconia support. The aim is to maintain the advantages of zirconia supports, especially the thermal stability. 

A complete range of metastable cerium-zirconium mixed metal oxide powders (CexZr(i.x)Oy, 0 < X < 1) were prepared through a similar hydroxide precipitation technique reported by Hori, et al. [11]. Cerium (IV) ammonium nitrate and zirconium oxynitrate precursors are completely dissolved in de-ionized water with mild heat and precipitated through the addition of excess ammonium hydroxide (-100 vol%). The ceria-zirconia is thoroughly washed with excess distilled water and allowed to evaporate to dryness overnight. The ceria-zirconia system is calcined in atmosphere for 1 hour at 773 K and subsequently milled into a fine powder. The model ceria-zirconia catalysts are prepared from the ground cerium-zirconium oxide powders using a 13 mm diameter pellet die and hydraulic press. 

This study emphasises the special behaviour of ceria-zirconia catalysts and related compounds exhibiting very high oxygen diffusivity. The use of refined mathematical models and computer-assisted calculations are required for processing the data of isotopic exchange with the highest efficiency and accuracy. 

Hoekman et al. [40] studied CO2 methanation reaction over Haldor Topspe commercially available methanation catalysts consisting of Ni and NiO on an alumina substrate with total nickel loading of 20-25% and an operating temperature range of 190-450 C in an extruded ring-shaped catalyst. Approximately 60% conversion of CO2 was observed at r= 300-350°C and stoichiometric CO2/H2 ratio. Aldana et al. [41] found that Ni over ceria-zirconia (prepared by sol-gel synthesis) shows an initial COj activity of almost 80%, with a CH4 selectivity of 97.3%, decreasing down to 84.7% after 90 hours of reaction. By IR operando analysis, they found that for Ni-ceria-zirconia catalysts the main mechanism for CO2 methanation does not require CO as reaction intermediate and the mechanism is based on CO2 adsorption on weak basic sites of the support. 

Choung et al. [313] reported significant activity and stability gains for their platinum/ceria/zirconia catalysts, which were supported on a mixture of 46 atom-ic% of ceria and 54 atomic% of zirconia when adding 1-2 wt.% rhenium to the catalyst formulation. 

Atribak, L, Bueno-Lopez, A. and Garcfa-Garcfa A. (2009). Role of Yttrium Loading in the Physico-chemical Properties and Soot Combustion Activity of Ceria and Ceria-zirconia Catalysts, J. Mol. Catal. A Chemical, 300, pp. 103-110. 

Conditions and Redox Behaviour in High-Temperature Aged Ceria-Zirconia Catalysts... 

L. Yang, O. Kresnawahjuesa, R.J. Gorte, A calorimetric study of oxygen-storage in Pd/ceria and Pd/ceria-zirconia catalysts. Catal. Lett. 72, 33-37 (2001)... 

Zaytseva, Y.A., Panchenko, V.N., Simonov, M.N., Shutilov, A.A., Zenkovets, G.A., Renz, M., Simakova, I.L., Parmon, V.N., 2013. Effect of gas atmosphere on catal3hic behavior of zirconia, ceria, and ceria-zirconia catalysts in valeric acid ketonization. Topics in Catalysis 56, 846-855. 

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