Cu/ZnO binary catalyst

The Cu-ZnO binary catalyst has been known to exhibit high activity in methanol formation at low temperature, but fast deactivation prevented commercialization.373 The addition of a third component, however, results in stable catalyst activity. The... 

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/... 

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. 

In comparison to binary catalysts, ternary Cu/ZnO/AfiOa catalysts were found to be more active. Figure 15.8 shows the activity versus specific copper area for binary and ternary Cu-based catalysts (21 ]. A linear correlation between specific copper area and activity (given in methanol produced per hour and per mass of catalyst) was found for the binary as well as for the ternary catalysts. Deactivation was found to be slower for ternary catalysts. 

In a new study of a series of binary Cu-ZnO catalysts a correlation was found between methanol synthesis activity and strain in the Cu metal phase.619 Structural defects of Cu resulting from ZnO dissolved in Cu, incomplete reduction, or epitaxial orientation to ZnO are believed to bring about strain, which modifies the Cu surface and, consequently, affects the catalytic activity. The higher amount of water formed in methanol synthesis from a C02-rich feed compared to a CO-rich feed brings about significant catalyst deactivation by inducing crystallization of both Cu and ZnO.620... 

Gunter MM, et al. Implication of the microstructure of binary Cu/ZnO catalysts for their catalytic activity in methanol synthesis. Catal Lett. 2001 71 (1—2) 37 44. 

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. 12. Carbon monoxide chemisorption isotherms at 25°C on the binary Cu/ZnO catalysts. The labels at the individual isotherms denote the molar composition Cu/ZnO (43).

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. 

Klier et al. (5<5) presented a kinetic model based on previous observations of the physical and chemical characteristics of the binary Cu/ZnO catalysts and put forward equations that quantitatively account for the C02/C0 dependence of the synthesis rates. Their model was required to accommodate the following observations ... 

Ternary compositions Cu/Zn0/Al203 and Cu/Zn0/Cr203 are currently the most important industrial catalysts. The solid-state chemistry of these composites is often complex, but there is no evidence that addition of alumina or chromia to the binary Cu/ZnO systems causes significant synergic... 

A1203 catalysts originates from interactions other than those existing in the binary system Cu/ZnO. The structural promotion by alumina is a very significant factor in the formulation of industrial catalysts, however, as it imparts chemical and mechanical stability required for a long-lived catalyst in large-scale reactors. 

The chemisorption studies of Parris and Klier (43) using the Cu/ZnO catalyst have been mentioned earlier. Carbon monoxide was irreversibly bonded at room temperature to the surface of the binary catalysts that were also active in methanol synthesis however, this irreversible adsorbate could be desorbed as CO, which indicates that it was not a surface carbonate but rather a strongly bonded carbonyl-type CO. Infrared studies of this chemi-sorbate are lacking and it would be very desirable to determine the structure of this surface species. 

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. 

Fig. 3. Yield of methanol as a function of cesium loading of the binary (Cu/ZnO = 30/70) catalyst obtained at 250°C and 75 atm with H2/CO = 2.33 synthesis gas at GHSV = 6120 (STP)/kg catal/hr.

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