Catalyst palladium-zinc oxide

Palladium is the precious metal most frequently apphed for methanol steam reforming [176-178]. Despite its higher price compared with the copper-based systems, it is an attractive alternative owing to the potential for higher activity and greater robustness, which are key features for small scale reformers. The combination of palladium and zinc showed superior performance and soon the formation of a palladium-zinc alloy was identified as a critical issue for optimum catalyst performance [179]. Besides palladium/zinc oxide, palladium/ceria/zinc oxide may well be another favourable catalyst formulation [177]. However, precious metal based catalysts have a tendency to show higher carbon monoxide selectivity than copper-zinc oxide catalysts, because it is a primary product of the reforming reaction over precious metals. 

Palo et al. investigated methanol steam reforming at a 375 °C reaction temperature, applying proprietary catalyst formulations [187], which minimised the carbon monoxide concentration to 1.2 vol.%. This value was significantly lower compared with the 3 vol.% found for a copper/zinc oxide catalyst [188]. It could well be assumed that the proprietary catalyst was also a palladium/zinc oxide formulation. 

Liu et al. observed significant activity gains up to a palladium content of 10 wt.% for their palladium/zinc oxide catalysts [189]. 

Chen et al. prepared a hybrid copper/zinc oxide/alumina/palladium/zinc oxide catalyst by wash-coating a copper/zinc oxide catalyst supported by alumina into microchannels [192]. Palladium/zinc oxide powder was then coated onto this catalyst. 

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. 

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... 

In a later study, Pfeifer et al. [30] prepared Pd/Zn catalysts by both pre- and postimpregnation of wash-coated zinc oxide particles with palladium and compared their performance in methanol steam reforming. The catalytic performance of the samples was tested at a 250 °C reaction temperature, 3 bar pressure, a S/C ratio of two and 250 ms residence time. The WHSV amounted as 0.3 Ndm3 (min gcat) 1. The thickness of the coatings was calculated to 20 pm. The formation of the PdZn alloy was proven to occur at temperatures exceeding 200 °C by XRD measurements. 

The equilibrium is more favorable to acetone at higher temperatures. At 325°C 97% conversion is theoretically possible. The kinetics of the reaction has been studied (23). A large number of catalysts have been investigated, including copper, silver, platinum, and palladium metals, as well as sulfides of transition metals of groups 4, 5, and 6 of the periodic table. These catalysts are made with inert supports and are used at 400 —600°C (24). Lower temperature reactions (315—482°C) have been successfully conducted using zinc oxide-zirconium oxide combinations (25), and combinations of copper-chromium oxide and of copper and silicon dioxide (26). 

Precipitation-deposition can be used to produce catalysts with a variety of supports, not only those that are formed from coprecipitated precursors. It has been employed to prepare nickel deposited on silica, alumina, magnesia, titania, thoria, ceria, zinc oxide and chromium oxide.36 It has also been used to make supported precious metal catalysts. For example, palladium hydroxide was precipitated onto carbon by the addition of lithium hydroxide to a suspension of... 

Metal oxide semiconductive sensors are not limited to tin oxide only. Many other metal oxides, such as zinc oxide, tungsten oxide, and others can also be used for chemical and gas sensing. It is understandable that an incorporation of a selective catalyst or a dopant may enhance the selectivity of the MOS sensors. Palladium, platinum, and others have been used as catalytic dopants for these sensors. The processes... 

Because the catalysts, copper, palladium, and zinc oxide do not dissociate CO, the hydrogenation of molecular CO is one of the likely mechanisms for methanol formation ... 

There are few reports of alkene-deuterium reactions on bimetallic catalysts, but those few contain some points of interest. On very dilute solutions of nickel in copper (as foil), the only product of the reaction with ethene was ethene-di it is not clear whether the scarcity of deuterium atoms close to the presumably isolated nickels inhibits ethane formation, so that alkyl reversal is the only option, or whether (as with nickel film, see above) the exchange occurs by dissociative adsorption of the ethene. Problems also arise in the use of bimetallic powders containing copper plus either nickel, palladium or platinum. Activation energies for the exchange of propene were similar to those for the pure metals (33-43 kJ mol ) and rates were faster than for copper, but the distribution of deuterium atoms in the propene-di clearly resembled that shown by copper. It was suggested that the active centre comprised atoms of both kinds. On Cu/ZnO, the reaction of ethene with deuterium gave only ethane-d2. as hydrogens in the hydroxylated zinc oxide surface did not participate by reverse spillover. ... 

An exception in terms of catalysts is the catalytic partial oxidation or OSR of methanol due to the low reaction temperature required. Copper [25, 32-36] and palladium-zinc ahoy [36-38] have been proven to give high selectivities and space-time yields. For the latter system, the palladium forms an alloy with the zinc oxide support under reducing conditions above 300 °C and is stable under the reaction conditions of methanol steam reforming [39]. However, the stability of the ahoy under CPO has not been proven so far by X-ray diflraction after exposure to reaction conditions. 

Basile et al. performed steam reforming and autothermal reforming of methanol in a fixed-bed of copper/zinc oxide catalyst, which was positioned below a palladium/ silver membrane containing 20 wt.% silver which had athickness of 60 pm [525]. The pressure on the reaction side was 1.2 bar, while the permeate side was under... 

Figure 7.17 Methanol conversion versus system reforming corresponds to the conversion pressure for methanol steam reforming in achieved with a fixed copper/zinc oxide catalyst membrane reactors (MR) with two different bed without membrane the dashed line is the palladium membranes PdCu was a 25-pm thick equilibrium conversion space velocity...

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