Palladium alloys monoxide

Guryanova, O. S., Y. M. Serov, S. G. Gul yanova and V. M. Gryaznov. 1988. Conversion of carbon monoxide on membrane catalysts of palladium alloys Reaction between CO and H2 on binary palladium alloys with ruthenium and nickel. Kinet. and Catal. 29(4) 728-731. 

Fig. 6. Activation energy of carbon monoxide oxidation over nickel oxide on gold-palladium alloys (24). (Copyright by Akademische Verlags-Gessellschaft. Reprinted with permission.)...

Soma-Noto Y, Sachtler WMH. 1974. Infrared spectra of carbon monoxide adsorbed on supported palladium and palladium-silver alloys, J Catal 32 315. 

Measurements of the infrared spectra of carbon monoxide on supported palladium and Pd-Ag atoms (75a) shed light on the relative importance of the ensemble and ligand effects. Three CO absorption bands were observed on palladium and its alloys at 2060,1960, and 1920 cm-1. 

Compensation effects have been reported for the oxidation of ethylene on Pd-Ru and on Pd-Ag alloys (207, 254, 255) discussion of the activity patterns for these catalysts includes consideration of the influence of hydrogen dissolved in the metal on the occupancy of energy bands. Arrhenius parameters reported (208) for ethylene oxidation on Pd-Au alloys were an appreciable distance from the line calculated for oxidation reactions on palladium and platinum metals (Table III, H). Oxidation of carbon monoxide on Pd-Au alloys also exhibits a compensation effect (256). 

A Mdssbauer investigation of the reduction of iron oxide (0.05 wt % Fe) and iron-oxide-with-palladium (0.05 wt % Fe, 2.2 wt % Pd), carried upon 7 -Al203, reveals that supported ferric ion alone, under hydrogen, yields ferrous ion only at 500—700 °C this reduction takes place at room temperature with the bimetallic catalyst and proceeds to form a PdFe alloy at 500 °C. Similar effects are found in reduction by carbon monoxide, which yields iron-palladium metal clusters at 400 °C. The view is taken that migration over T7-A1203 is not involved but that activated hydrogen transfers only at bridgeheads on the contact line between the metal and iron oxide. 

F. Sakamoto, Effect of carbon monoxide on hydrogen permeation in some palladium-based alloy membranes, Int. J. Hydrogen Energy 1996, 21(11/12), 1017-1024. 

As with all catalysts, palladium and its alloys are susceptible to poisoning [69]. Catalysts must be designed with resistance to poisoning, and proper precautions must be taken to minimize exposure of the membranes to catalyst poisons [69]. Typical poisons for palladium include H2S and other compounds of sulfur such as carbon disulfide (CS2), carbonyl sulfide (COS), aromatic thiophenes and mercap-tans (thiols, R-SH). Palladium is poisoned by the Group VA elements, P, As, Sb and Bi, the halides (Cl, Br, I), and Si, Pb and Hg. Alkenes and unsaturated organic compounds can poison palladium as can elemental carbon deposited from decomposition of carbonaceous materials. Carbon monoxide in high concentrations and at low temperatures can form a monolayer which blocks adsorption and dissociation of molecular hydrogen. Carbon monoxide can also decompose to produce car-... 

Adsorbed carbon monoxide can serve as a useful infrared probe of surface composition in bimetallic colloids if both metals bind CO. This is exemplified in the infrared spectrum of CO on a PVP stabilized colloidal alloy CutgPdjT, [38] Carbon monoxide adsorbs readily onto these PdCu particles (co. 45 A) in dichloro-methane at 25 °C, as shown by the infrared absorption spectrum in Figure 6-28. By comparing this to the IR spectrum of CO on a pure palladium colloid of similar size [34] in Figure 6-27d, it can be clearly seen that CO occupies both palladium and copper sites. Whereas the bands at 2046 cm" and 1936 cm" are in the frequency ranges found for linear and bridged CO on the pure palladium particles, the new band at 2089 cm corresponds to CO on surface copper atoms, thus demonstrating that both metals are present at the surfaces of the particles. 

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. 

Noh J., Yang O.B., Kim D.H., Woo S.I. Characteristics ofthe Pd-only three-way catalysts prepared by sol-gel method. Catal. Today 1999 53 575-582 Noto S.Y., Sachtler W.M.H. Infrared spectra of carbon monoxide adsorbed on supported palladium and palladium-silver alloys. J. Catal. 1974 32 315-324 Ochoa R., van Woet H., Lee W.H., Subramanian R., Kugla- E., Eklund PC. Catalytic degradation of medium density polyethylene over silica-alumina supports. Appl. Catal. A Gen. 1996 49 119-136... 

In order to produce methanol the catalyst should only dissociate the hydrogen but leave the carbon monoxide intact. Metals such as copper (in practice promoted with ZnO) and palladium as well as several alloys based on noble group VIII metals fulfill these requirements. Iron, cobalt, nickel, and ruthenium, on the other hand, are active for the production of hydrocarbons, because in contrast to copper, these metals easily dissociate CO. Nickel is a selective catalyst for methane formation. Carbidic carbon formed on the surface of the catalyst is hydrogenated to methane. The oxygen atoms from dissociated CO react with CO to CO2 or with H-atoms to water. The conversion of CO and H2 to higher hydrocarbons (on Fe, Co, and Ru) is called the Fischer-Tropsch reaction. The Fischer-Tropsch process provides a way to produce liquid fuels from coal or natural gas. 

The sandwich technique has the advantage that it enables the analytical sample to react in a bath which is probably not yet saturated with carbon, and from which the escape of carbon monoxide is thus not yet impeded by needles of graphite or carbide deposited on the surface of the bath, as frequently occurs in the bath procedure. In addition, a further considerable effect on the release of gas, is obtained by the fact that titanium and zirconium form alloys with platinum or palladium, which results in a large evolution of heat. This causes a sudden and considerable increase in temperature (a flash of light ). The sandwich method is however limited to bath metals with very low oxygen contents, which is e.g. not the case with nickel, cobalt or iron. Therefore, the metal foil required is preferably a piece of platinum or palladium foil of about 50 Mm thick. The oxygen blank values of platinum and palladium in compact form are between 1 and 5 Mg/g. They amount to 5 to 15 Mg/g for the corresponding foils. 

Much better catalysts now provide improved operation. Hydrogenation can be controlled by adding traces of carbon monoxide to the hydrogen. Adsorbed carbon monoxide modifies the relative adsorption of acetylene and ethylene on the palladium and minimizes ethylene loss. The catalyst itself can also be made more selective by alloying the palladium with a further metal such as copper or silver. This also affects pallacfium dispersion and the relative adsorption of acetylene and ethylene on the catalyst surface to improve selectivity. To minimize temperature rise catalyst suppliers recommend that one or more catalyst beds with intercoolers be used in each reactor, depending on the acetylene content of the C2 stream ... 

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