Copper catalysts binary

Following the development of sponge-metal nickel catalysts by alkali leaching of Ni-Al alloys by Raney, other alloy systems were considered. These include iron [4], cobalt [5], copper [6], platinum [7], ruthenium [8], and palladium [9]. Small amounts of a third metal such as chromium [10], molybdenum [11], or zinc [12] have been added to the binary alloy to promote catalyst activity. The two most common skeletal metal catalysts currently in use are nickel and copper in unpromoted or promoted forms. Skeletal copper is less active and more selective than skeletal nickel in hydrogenation reactions. It also finds use in the selective hydrolysis of nitriles [13]. This chapter is therefore mainly concerned with the preparation, properties and applications of promoted and unpromoted skeletal nickel and skeletal copper catalysts which are produced by the selective leaching of aluminum from binary or ternary alloys. 

The possible surface contaminations were carefully followed by Auger-XPS analysis. Similarly, as with the copper catalyst described earlier (Section III) the Cu/ZnO binaries were free from alkali metals, iron, chlorine, and sulfur, and contained only small amounts of carbon after the use in catalytic reactor (39). The latter result indicates that reactants, intermediates, and the product are adsorbed with moderate strength, a feature that is desirable for all efficient catalysts. 

Among copper based binary catalyst systems, CuO/ZrOj was proved to be the most reactive toward methanol synthesis. The methanol synthesis activity of the CuO/ZrOj catalyst was greatly affected by the copper dispersion (or copper crystallite size) the smaller the crystallite size, the higher the rate of methanol synthesis (Table 1). When some components of Ce, Cr, Pd, K, V and Zn were added as promoters into CuO/ZrOj, the crystallite size of copper particles changed significantly. CeOj increased the copper crystallite size significantly, while ZnO made the copper crystallite size much smaller than those of the Cu/ZrOj samples. 

In every case for copper catalyst 31, the absolute stereochemistry of the cycloadducts is accounted for by the intervention of the substrate-catalyst complex depicted in Fig. 23, in which the s-cis configured dienophile is bound to the catalyst in the plane of the ligand in a bidentate fashion. The ferf-butyl group shields the top face and cycloaddition occurs from the exposed si enantioface. Support for this model derives from X-ray structures of aquo complexes of catalysts 31a and 31b which show that the complex possesses a distorted square planar geometry EPR spectroscopy on the binary catalyst-dienophile complex indicates that this geometry carries over from the solid state into solution. Calculations at the PM3 level of theory further favor the indicated reactive assembly [85]. 

A series of binary (Cu/ZnO and C11/AI2O3) and ternary (Cu/ZnO/A Oa) copper catalysts employed in methanol synthesis has been investigated by d Alnoncourt et al. [142]. The results obtained using CO adsorption microcalorimetry showed that Cu/ZnO/AlaOa had the lowest initial heat of adsorption of 68 kJ mof and was the most active catalyst for methanol synthesis. Cu/ZnO showed a heat of adsorption of 71 kJ mol" and a lower activity, and CU/AI2O3 had the highest initial heat of adsorption of 81 kJ mof and the lowest activity. The decrease of about 10 kJ mol in the heat of adsorption of CO induced by the presence of ZnO has been attributed to strong metal-support interactions. 

Amenomiya, Y. Methanol S5mthesis from CO2 4- H2. 2. Copper-based binary and ternary catalysts. Appl Catal 1987, 30, 57-68. 

Cu-Mn mixed-oxide binary spinel catalysts (CuxMn3 x04, where x = 0, 0.25, 0.5, 0.75 and 1) prepared through co-precipitation method exhibit phenol methylation activity imder vapor phase conditions [75]. All of the catalysts, irrespective of the compositions, produced only C-methylated phenols. However, a total ortho selectivity of 100% with 2,6-xylenol selectivity of 74% was observed over x = 0.25 compositions at 400°C. This composition was found to be relatively stable under reaction conditions compared with the other compositions studied. The catalysts with high copper content suffered severe reduction under methylation conditions whereas, catalysts with low copper content had a hausmannite phase (Mu304) that sustained... 

Dowden and Reynolds (49,50) in further experimental work on the hydrogenation of benzene and styrene with nickel-copper alloys as catalysts, found a similar dependence. The specific activities of the nickel-copper alloy catalysts decreased with increasing copper content to a negligible value at 60% copper and 30-40% copper for benzene and styrene, respectively. Low-temperature specific heat data indicated a sharp fall (1) in the energy density of electron levels N(E) at the Fermi surface, where the d-band of nickel becomes filled at 60 % copper, and (2) from nickel to the binary alloy 80 nickel -)- 20 iron. Further work by these authors (50) on styrene hydrogenation with nickel-iron alloy... 

Historically, the first important acrolein catalyst was cuprous oxide. Since the discovery of bismuth molybdate as the first specimen Of a group of superior catalysts, however, attention has been primarily focussed on binary and compound metal oxide mixtures in which copper plays no role. 

Alloys are prepared commercially and in the laboratory by melting the active metal and aluminum in a crucible and quenching the resultant melt which is then crushed and screened to the particle size range required for a particular application. The alloy composition is very important as different phases leach quite differently leading to markedly different porosities and crystallite sizes of the active metal. Mondolfo [14] provides an excellent compilation of the binary and ternary phase diagrams for aluminum alloys including those used for the preparation of skeletal metal catalysts. Alloys of a number of compositions are available commercially for activation in the laboratory or plant. They include alloys of aluminum with nickel, copper, cobalt, chromium-nickel, molybdenum-nickel, cobalt-nickel, and iron-nickel. 

The electronic interaction between the catalyst components is best exemplified by its color and optical spectra. For example, the very active binary catalyst Cu/ZnO = 30/70 has a pitch black color and although it is composed of crystallographically identifiable copper and zinc oxide, its optical spectrum is not a superposition of the spectrum of copper metal and zinc oxide, but rather comprises a very intense continuous absorption band in the visible part of the spectrum that contains no trace of the characteristic... 

Fig. 8. (a) Transmission electron micrograph of a Cu/ZnO = 30/70 binary catalyst (40) 60 A copper spheres are placed on crystalline zinc oxide network, (b) Dark field image of the copper crystallites in the area shown in the bright field image (a) obtained using the [111] reflection of copper. [Adapted with permission from J. Catal. 57, 339 (1979). Copyright (1979) Academic Press, New York.]... 

Fig. 11. A view of copper and zinc oxide crystallites with dispersed copper ions in the binary Cu/ZnO catalyst = 30/70 derived from diffraction and characteristic X-ray emission analysis in TEM and STEM.

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

Fig. 13. The dependence of the carbon monoxide saturation adsorption (total) and irreversible adsorption (irreversible) on the Cu/ZnO ratio in the binary copper-zinc oxide catalysts...

All the binary Cu/ZnO catalysts were found highly selective toward methanol without DME, methane, or higher alcohols and hydrocarbons detected in the product by sensitive gas chromatographic methods (59). Several of the composites were also found to be very active when subjected to a standard test with synthesis gas C0/C02/H2 = 24/6/70 at gas hourly space velocity of 5000 hr- pressure 75 atm, and temperature 250°C. The activities, expressed as carbon conversions and yields, are summarized in Table VIII. The end members of the series, pure copper and pure zinc oxide, were inactive under these testing conditions, and maximum activity was obtained for the composition Cu/ZnO = 30/70. The yields per unit weight, per unit area of the catalyst or the individual components, turnover rates per site titratable by irreversible oxygen and by irreversible carbon monoxide, are graphically... 

To summarize the qualitative findings, the methanol synthesis activity in the binary Cu/ZnO catalysts appears to be linked to sites that also irreversibly chemisorb CO and not to sites that adsorb CO reversibly. Since irreversible adsorption of CO follows linearly the concentration of amorphous copper in zinc oxide, these sites are likely to be that part of the copper solute that is present on the zinc oxide surface. No correlation of the catalyst activity and the copper metal surface area, titrated by reversible form of CO or by oxygen, could be found in the binary Cu/ZnO catalysts (43). In contrast with this result, it has been claimed that the synthesis activity is proportional to copper metal area in copper-chromia (47), copper-zinc aluminate (27), and copper-zinc oxide-alumina (46) catalysts. In these latter communications (27,46,47), the amount of amorphous copper has not been determined, and obviously there is much room for further research to confirm one or another set of results and interpretations. However, in view of the lack of activity of pure copper metal quoted earlier, it is unlikely that the synthesis activity is simply proportional to the copper metal surface area in any of the low-temperature methanol-synthesis catalysts. 

Aside from the recently described Cu/Th02 catalysts, copper on chromia and copper on silica have been reported to catalyze methanol synthesis at low temperatures and pressures in various communications that are neither patents nor refereed publications. It is not feasible to critically review statements unsupported by published data or verifiable examples. However, physical and chemical interactions similar to those documented in the copper-zinc oxide catalysts are possible in several copper-metal oxide systems and the active form of copper may be stabilized by oxides of zinc, thorium, chromium, silicon, and many other elements. At the same time it is doubtful that more active and selective binary copper-based catalysts than... 

In summary, several roles were attributed to alumina in the Cu/Zn0/Al203 catalysts prevention of copper particle sintering through the formation of zinc aluminate, induction of surface defects by endotactic inclusion of alumina clusters in copper, and that of stabilization of highly dispersed Cu/ZnO binary catalyst. It is possible that all these effects take place in varying degrees in the commercial catalysts, but there is no evidence at this time that the specific activity of the present commercial catalysts Cu/ZnO/... 

Binary copper-based catalysts were prepared by coprecipitation method and some components were added as promoters into the binary catalysts. The methanol synthesis reaction was carried out in a continuous flow microreactor operated at 22 atm and at various temperatures. Reaction pathway of the methanol synthesis was investigated through FT-IR spectroscopy. For the catalyst with a copper content over 15wt%, the diffuse reflectance method (DRIFT) was applied, but for fee catalyst wife a copper content of 7wt%, the transmission teclmique was used. For more information about intermediates, TPD of adsorbed methanol was carried out and the products were analyzed using mass spectrometer. 

Since classical Cu/ZnO catalysts exhibited a poor stability while the addition of alumina resulted in much better systems, it was tempting to add alumina to Cu-Ce intermetallic compounds. Jennings et al. (1992a), prepared ternary Cu-Ce-Al alloys of various compositions and also tried a variety of other metals (Ca, Cr, Mn, Pd, Zn). Among these ternary alloys aluminum-containing catalysts were the best. In spite of lower initial activities as compared to binary alloys, they exhibited a much better long-term stability. It is believed that the role of aluminum is to stabilize the disperse copper-ceria phases responsible for methanol synthesis activity, although the mechanism for such a process remains unclear. 

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