Palladium oxide-supported metal catalysts

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

Supported palladium oxide catalysts present the best performance for methane combustion in lean conditions. Consequently, the interactions between methane and palladium oxide or metallic palladium supported on AI2O3, Zr02 and BN at 673 K have been studied by microcalorimetry. At this temperature, methane reduced the palladium oxide, and the heat of reduction of palladium oxide was shown to depend on the dispersion and the support. The lowest heats of reduction corresponded to the highest rates of methane combustion [75]. 

The reduction of palladium oxide, rhodium oxide or ruthenium oxide gives the corresponding metal blacks generated by in situ hydrogenation in the reaction mixture. At present the use of these oxides, as well as Adam s catalyst, is not common because of the cost of the materials and the relatively large amounts which are required. These materials have been replaced by the more reactive and less expensive supported metal catalysts described in Chapter 13. 

Supported catalysts in which a metal oxide is supported on a different metal oxide find tremendous use in commercial chemical production, petroleum refining, and environmental remediation [7], Table 2.1 lists a number of important industrial reactions that are catalyzed by supported metal oxides. The active phase consists of single metal oxides, metal oxide mixtures, or complex metal oxides. Some supported metal catalysts can, technically, be considered supported metal oxide catalysts if the metal is partially oxidized under reaction conditions, such as alumina-supported silver epoxidation catalysts and palladium-on-stabilized alumina combustion catalysts. Clearly, supported metal oxide compositions are diverse, especially considering that the range of relative amounts of the active phase to the support is large, and that additional metal oxides and modifiers can be introduced. 

Palladium catalysts have been prepared by fusion of palladium chloride in sodium nitrate to give palladium oxide by reduction of palladium salts by alkaline formaldehyde or sodium formate, by hydrazine and by the reduction of palladium salts with hydrogen.The metal has been prepared in the form of palladium black, and in colloidal form in water containing a protective material, as well as upon supports. The supports commonly used are asbestos, barium carbonate, ... 

Methanol is a major bulk chemical, and its global annual production exceeds 37 million tons. It is mainly used for the production of formaldehyde and methyl 6butyl ether (MTBE). Especially, formaldehyde is dominantly used for producing resins. At present, methanol and its decomposed derivatives can be oxidized to CO2 and H2O by the proper selection of supported noble metal catalysts such as palladium, platinum, and gold. 

In the case of palladium particles supported on magnesium oxide, Heiz and his colleagues have shown,29 in an elegant study, a correlation between the number of palladium atoms in a cluster and the selectivity for the conversion of acetylene to benzene, butadiene and butane, whereas in the industrially significant area of catalytic hydrodesulfurisation, the Aarhus group,33 with support from theory, have pinpointed by STM metallic edge states as the active sites in the MoS2 catalysts. 

It is usually difficult to discuss unambiguously on the role of the formation of sulphate, which may explain the deactivation. Their formation can equally occur on the support and on the noble metals. The poisoning effect of S02 has been reported by Qi el al. on Pd/Ti02/Al203 [112], However, in the presence of water, the stabilisation of hydroxyl groups could inhibit the adsorption of S02 [113], Burch also suggested a possible redispersion of palladium oxide promoted by the formation of hydroxyl species [114], Such tentative interpretations could correctly explain the tendencies that we observed irrespective to the nature of the supports, which indicate an improvement in the conversion of NO into N2 at high temperature. Nevertheless, the accentuation of those tendencies particularly on prereduced perovskite-based catalysts could be in connection with structural modifications associated with the reconstruction of the rhombohedral structure of... 

Electrophilic catalysis may play an important role in the case of the similar benzylic carbon, too. For an O-benzyl system, it was found in a 1997 experiment that palladium oxide is a much more effective catalyst than palladium metal when the catalyst has been prereduced with chemical reducing agents. This finding shows very clearly that the electrophilic character of the unreduced metal ions plays an important role in the hydrogenolysis of the benzyl C—O bonds. Additional support for this mechanism is the fact that a small amount of butylamine can inhibit the hydrogenolysis of the benzyl C—O bond. 

It was found in the case of O-benzyl systems that palladium oxide is much more effective than palladium metal. No such effect was observed with the N-benzyl system.8 It is possible that the N-compounds can poison the electrophile metal ions, and the hydrogenolysis of the N-benzyl bond can take place only by the hydrogenolytic cleavage instead of the insertion mechanism. This is supported by the experimental finding that the product amine can inhibit the catalyst, and this can be minimized by buffering at a pH less than 4. 

Both uncalcined and calcined LDHs have also been shown to be effective supports for noble metal catalysts [18-25]. For example, palladium supported on Cu/Mg/Al LDHs has been used in the liquid phase oxidation of limonene [24], and on calcined Mg/Al LDHs for the one-pot synthesis of 4-methyl-2-pentanone (methyl isobutyl ketone) from acetone and hydrogen at atmospheric pressure [25]. In the latter case, the performance depends on the interplay between the acid-base and hydrogenation properties. More recently. 

Pure decarbonylation typically employs noble metal catalysts. Carbon supported palladium, in particular, is highly elfective for furan and CO formation.Typically, alkali carbonates are added as promoters for the palladium catalyst.The decarbonylation reaction can be carried out at reflux conditions in pure furfural (165 °C), which achieves continuous removal of CO and furan from the reactor. However, a continuous flow system at 159-162 °C gave the highest activity of 36 kg furan per gram of palladium with potassium carbonate added as promoter. In oxidative decarbonylation, gaseous furfural and steam is passed over a catalyst at high temperatures (300 00 °C). Typical catalysts are zinc-iron chromite or zinc-manganese chromite catalyst and furfural can be obtained in yields of... 

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