Catalyst 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). 

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... 

The aim of this contribution is to present data on the preparation of catalysts containing as embedding species a large family of eolloids such as colloids of ruthenium, platinum, or palladium-gold alloys and triflate derivatives such as lanthanum and silver triflate or tert-butyldimethylsilyltrifluoromethanesulfonate (BMSTM). Silica, zirconia and tantalum oxides were used as carrier. All these preparations considered the polymeric sol-gel route using as starting materials silicon, zirconium or tantalum alcoxides. 

It is known that supported palladium catalysts are the most active for the total oxidation of methane [3], and there are many studies focusing on the alumina supported ones [4 and references cited therein] However, alumina is not stable at the temperatures commonly used for methane oxidation. To avoid this problem, other authors [5] have suggested the use of zirconia-based supports, which are considered as more thermally stable. In this way, these supports were found to present very different properties, depending on the synthesis method and the presence of additives. 

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. 

In the present work, the performance of different palladium catalysts supported on cerium-modified zirconia has been studied for the oxidation of methaiK in air, alone and in the presence of SO2. This conqwund has been chosen because it is very commonly present in gaseous emissions. On the other hand, in previous works it was found the effect of other reduced sulphur compounds on Pd catalysts was very similar [6]. 

As it can be observed in Fig. 4, when only methane is added to the feed, catalyst A shows the best behaviour, followed by catalysts B and U. According to this, the addition of cerium to the zirconium hydroxide increases the activity of the zirconia-supported palladium catafysts. Comparing the performance of the cerium-containing catalysts, it is remarkable that catalyst B presents a poorer performance, than catalyst A (slightly lower initial conversion and foster deactivation). This result suggests that the interaction between Pd and Ce, revealed in the TPR experiments, does not enhance the activity of the active phase (Pd). In contrast, the interaction Ce-Zr in catalysts A increases the thermal stability, considered as the main foctor for preventing catalyst deactivation in these reactions [9]. 

On the other hand, in the experiments carried out in presence of SO2, catalyst A shows a higher resistance to deactivation and higher conversions than catalyst B. However, both perform worse than the unmodified zirconia catalyst (Fig. 4). So, it can be concluded that the thioresistance of zirconia-supported palladium catalysts is not increased by the addition of cerium. 

Fig. 4.14. Comparison of CO oxidation activities of palladium-on-zirconia catalysts prepared by in situ activation from amorphous Pd33Zr(>7 ( ) and by conventional impregnation of zir-conia with a palladium salt (O). Arrhenius plots of the turnover frequencies are plotted. Conditions reactant gas mixture. 1700 ppm of CO, and 1700 ppm 03 in nitrogen flow rate. 150 ml (STP) min amount of catalyst, 0.37 g O 1.24 g...

This new single-step synthesis unites the simplicity of preparation and lower production costs, with the outstanding properties of the final catalysts. By the single-step procedure proposed here, deposition of dispersed nanoparticles of noble metals on ceramic supports with customised textural properties and shape was achieved. Noble metals including platinum, palladium, rhodium, ruthenium, iridium, etc. and metal oxides including copper, iron, nickel, chromimn, cerium oxides, etc on sepiolite or its mixtures with alumina, titania, zirconia or other refractory oxides have been also studied. 

A NOVEL THRIFTED PALLADIUM-ZINC CATALYST SUPPORTED ON CERIA STABILISED ZIRCONIA FOR USE IN THREE-WAY VEHICLE EXHAUST CATALYSIS... 

This paper describes the development and performance of a 16 mol% ceria stabilised zirconia supported palladium-zinc catalyst in vehicle exhaust after treatment. The main aspects of the novel catalytic system are i) the low temperature preparation of a thermally stable solid solution of tetragonal phase ceria stabilised zirconia, ii) the role of zinc in the dispersion of the palladium component and its synergetic interaction with palladium in an oxygen ion transfer mechanism, iii) the in situ perfomance of the catalyst under close-coupled engine conditions in producing... 

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