Palladium/Zirconia

For illustration, we may consider the preparation of a palladium/zirconia catalyst highly active for the oxidation of CO [4.47,71], the preparation of a copper/zirconia catalyst for the hydrogenation of C02 [4.23], and the preparation of iron/zirconia for ammonia synthesis [4.44]. 

I ig- 4.17. Schematic illustration of the structural properties and the CO oxidation mechanism of palladium, zirconia catalyst derived by in situ activation from glassy PdjjZr, precursor... 

As-prepared catalysts exhibited selectivities up to more than 80% with regard to methanol, when used at temperatures lower than about 470 K. It should be mentioned that, using a similar activation procedure, palladium/ zirconia catalysts were prepared from glassy Pd33Zr67, which were highly active for the hydrogenation of C02 to methane [4.24]. 

Palladium/zirconia catalysts were derived from the glassy PdZr3 precursor by either in situ oxidation in the reactant CO/O2 mixture (PdZr-i) or by oxidation in air (PdZr-a). The conditions of these oxidation pretreatments are described in the experimental part. 

The contribution of Spiney and Gogate (Spivey and Gogate, 1998) in developing heterogeneous catalysts for the condensation of acetone to methyl isobutyl ketone (MIBK) is commendable. The reaction typically requires stoichiometric amounts of base and could also result in considerably overcondensed products. The catalysts tested for the process are nickel/alumina (see Fig. 3.9), palladium, zirconia, nickel, niobium, and ZSM-5 with palladium, which have exhibited various levels of selectivity and a degree of conversion. 

Partial oxidation of methanol is less frequently reported in the open literature. Cubeiro et al. investigated the performance of palladium/zinc oxide, palladium/ zirconia and copper/zinc oxide catalysts for partial oxidation of methanol in the temperature range between 230 and 270 °C (194j. Increasing selectivity towards hydrogen and carbon dioxide was achieved with increasing conversion, while selectivity towards steam and carbon monoxide decreased. The palladium/zinc oxide catalyst showed lower selectivity towards carbon monoxide compared with the palladium/zirconia catalyst. However, the lowest carbon monoxide selectivity was determined for the copper/zinc oxide catalyst. 

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). 

Conventionally, a fixed bed catalyst containing palladium, a promoter metal, and an alkali metal acetate is used. The fixed bed catalyst components are supported on a porous carrier such as silica, zirconia or alumina. 

Although the decomposition of ozone to dioxygen is a thermodynamically favoured process,126 it is thermally stable up to 523 K and catalysts are needed to decompose it at ambient temperature in ventilation systems, in the presence of water vapour and at high space velocity. A limited number of catalysts have been evaluated and active components are mainly metals such as platinum, palladium and rhodium, and metal oxides including those of manganese, cobalt, copper, iron, nickel and silver. Supports that have been used include 7-alumina, silica, zirconia, titania and activated carbon.125,170... 

G. Larsen, E. Lotero, R. D. Parra, L. M. Petkovic, H. S. Silva, and S. Raghavan, Characterization of palladium supported on sulfated zirconia catalysts by DRIFTS, XAS and n - butane isomerzation reaction in the presence of hydrogen, Appl. Catal A 130, 213-226 (1995). 

Although some inorganic membranes such as porous glass and dense palladium membranes have been commercially available for some time, the recent escalated commercial activities of inorganic membranes can be attributed to the availability of large-scale ceramic membranes of consistent quality. As indicated in Chapter 2, commercialization of alumina and zirconia membranes mostly has been the technical and marketing extensions of the development activities in gas diffusion membranes for the nuclear industry. 

The membrane material varies from alumina, zirconia, glass, titania, cordierite, mulUte, carbon to such metals as stainless steel, palladium and silver. The resulting pore diameter ranges from 10pm down to4 nm and the membrane thickness varies from 3 to 10 pm. The membrane porosity depends on the pore size and is 40-55%. 

While dense inorganic membranes such as palladium-based or zirconia membranes provide extremely high-purity gases, their permeabilities are usually low, thus making the process economics unfavorable. Therefore, most of the recent investigations focus on porous inorganic membranes. 

Thermal stability. Thermal stability of several common ceramic and metallic membrane materials has been briefly reviewed in Chapter 4. The materials include alumina, glass, silica, zirconia, titania and palladium. As the reactor temperature increases, phase transition of the membrane material may occur. Even if the temperature has not reached but is approaching the phase transition temperature, the membrane may still undergo some structural change which could result in corresponding permeability and permselectivity changes. These issues for the more common ceramic membranes will be further discussed here. 

Dense palladium-based membranes. Shown in Table 10.1 are modeling studies of packed-bed dense membrane shell-and-tube reactors. All utilized Pd or Pd-alloy membranes except one [Itoh et al., 19931 which used yttria-stabilized zirconia membranes. As mentioned earlier, the permeation term used in Ihe governing equations for the tube and shell sides of the membrane is expressed by Equation (10-51b) with n equal to 0.5 [c.g., Itoh, 1987] or 0.76 [e.g., Uemiya et al., 1991]. 

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... 

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