Catalytic partial oxidation ceria

Catalytic partial oxidation (CPOX) reduces the temperature to the order of 800-900 °C. Catalysts for CPOX include platinum group metals (PGMs), i.e. Pt and Ru, NiO-MgO, nickel modified hexa-aluminates on titania- or ceria-containing supports (Ghendu, 2002). The subject has been extensively reviewed by Bharadwaj and Schmidt (1995). 

The selective oxidation or preferential oxidation of CO in hydrogen-rich stream is another important object for ceria based catalysts. The gas mixture from steam reforming/partial oxidation of alcohols or hydrocarbons, followed by the WGS reaction contains mainly FI2, CO2 and a small portion of CO, H2O, and N2. When such gaseous stream would be taken as input for hydrogen fuel cells, the CO has to be removed to avoid poisoning of the anode electrocatalysts. Ceria based nanomaterials, such as ceria/gold, ceria/copper oxide catalysts exhibit suitable catalytic activities and selectivities for CO PROX process. 

Villegas et cd. performed autothermal reforming of isooctane over a 2wt.% platinum catalyst on a ceria/zirconia mixed support (Ceo.67Zro.33O2) [71]. The tests were performed at a gas hourly space velocity of 150000h. Full conversion of isooctane was achieved at 710 °C under conditions of steam supported partial oxidation (O/C = 1 S/C = 2). The optimum hydrogen content in the reformate was achieved at a lower O/C ratio of 0.75 and S/C ratio of 1.5. The carbon dioxide content in the reformate was always higher than the carbon monoxide content, which contradicted the thermodynamic calculations performed by the same workers [71], see Section 3.3. This supports the assumption that platinum preferably catalyses catalytic combustion. 

Gasolines contain a small amount of sulfur which is emitted with the exhaust gas mainly as sulfur dioxide. On passing through the catalyst, the sulfur dioxide in exhaust gas is partially converted to sulfur trioxide which may react with the water vapor to form sulfuric acid (1,2) or with the support oxide to form aluminum sulfate and cerium sulfate (3-6). However, sulfur storage can also occur by the direct interaction of SO2 with both alumina and ceria (4,7). Studies of the oxidation of SO2 over supported noble metal catalysts indicate that Pt catalytically oxidizes more SO2 to SO3 than Rh (8,9) and that this reaction diminishes with increasing Rh content for Pt-Rh catalysts (10). Moreover, it was shown that heating platinum and rhodium catalysts in a SO2 and O2 mixture produces sulfate on the metals (11). 

Hardacre el al. (7 75, 174) investigated the properties, structure, and composition of cerium oxide films prepared by cerium deposition on Pt(lll), finding that the activity for CO oxidation is enhanced on Pt(lll) that is partially covered by ceria. It was suggested that new sites at the Pt-oxide interface become available for reaction. A remarkable observation is the high activity for CO oxidation when the Pt(lll) sample is fully encapsulated by ceria (Pt was undetectable by XPS and AES). It was proposed that an ultrathin, disordered ceria film becomes the active catalyst. It was also demonstrated by XPS and AES that Pt dramatically increases the reducibility of cerium oxide that is in intimate contact with Pt. This result suggests that intimate contact between the noble metal and oxide phases is indeed crucial to facile oxygen release from ceria. High-resolution electron microscopy demonstrated the presence of direct contact between ceria and noble metal for supported Pt-Rh catalysts (775). Hardacre et al. (173,174) related the catalytic activity of the ceria phase to partially reduced cerium oxide. 

On the contrary, for propane combustion on Pd/AljOj modified by ceria addition, Shyu and co-workers observed that ceria allows to maintain Pd in a more stable oxidised state less prompt to react with propane [78], Catalytic oxidation which occurred from 200°C to 350°C is now delayed to higher temperature by 50°C to 100°C from 250°C to 450°C. More dramatically is the decrease of the conversion at 350°C from 100% to 20% as the partial pressure of oxygen increases beyond the stoichiometric ratio up to 8 times. They concluded from their studies that ceria and palladium are in close interaction, may be in a Pd-O-O-Ce-0 model via a O2 species. 

Cerium-zirconium mixed metal oxides are used in conjunction with platinum group metals to reduce and eliminate pollutants in automotive emissions control catalyst systems. The ceria-zirconia promoter materials regulate the partial pressure of oxygen near the catalyst surface, thereby facilitating catalytic oxidation and reduction of gas phase pollutants. However, ceria-zirconia is particularly susceptible to chemical and physical deactivation through sulfur dioxide adsorption. The interaction of sulfur dioxide with ceria-zirconia model catalysts has been studied with Auger spectroscopy to develop fundamental information regarding the sulfur dioxide deactivation mechanism. 

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