Alumina ternary catalyst

The low-pressure methanol synthesis process utilizes ternary catalysts based on copper, zinc oxide, and another oxide, such as alumina or chromia, prepared by coprecipitation. Cu-Zn0-Al203 and Cu-Zn0-Cr203 are usually the most important industrial catalysts. A significant advance was made when a two-stage precipitation was suggested in which ZnAl2C>4, a crystalline zinc aluminate spinel, was prepared prior to the main precipitation of copper-zinc species.372 This alteration resulted in an increase in catalyst stability for long-term performance with respect to deactivation. Catalyst lifetimes industrially are typically about 2 years. 

Up to 48 ternary catalyst mixtures were prepared simultaneously in less than 1 h. Hence the sputtering procedure is much faster than the wet chemical route and in fact one of the fastest syntheses available. This advantage is gained at the expense of low layer porosity. Thus, sputtered catalysts are new artificial catalysts and not directly comparable to catalysts prepared by wet-chemical procedures. These catalysts offer the advantage of quick preparation and characterization compared with alumina-based catalysts. They can also be used for obtaining so-called intrinsic kinetics because there is no influence of diffusion. 

Another type of preparation of the Cu/ZnO/A120 ( catalyst involves coprecipitation of the three components and the resulting catalyst contains neither spinel nor crystalline alumina. Fischer et al. (76) reported that a ternary catalyst of the nominal composition Cu/Zn0/Al203 = 60/35/5 mol% contained only crystalline copper and zinc oxide, and all the alumina... 

Conversely, interaction with the poison H s causes disruption of the Cu particles leaving the Cu-0 structure apparently unperturbed. There is a significant effect of the third component (alumina) on the reduction temperature required, suggesting some proximity of copper and aluminium in the calcined ternary catalyst. Reduction is then effected at the higher temperature of 533K. Again this appears to form small fee copper particles in addition to a component of copper with Cu-0 bonds. After extended use as a... 

In recent years, the role of methanol as a basic chemical has strongly increased, and therefore further development of the ternary Cu/Zn0/Al203 catalyst for methanol synthesis has become more important. It is widely accepted that ZnO acts both as an electronic and stractmal promoter and exhibits a major influence on the catalytic activity, while alumina mainly increases the long-term stability of the ternary catalyst system. Thus, the interest in the binary Zn/Al as well as the ternary Cu0/Zn0/Al203 system as catalytic materials is very high. 

Topsee and coworkers—in situ XRD synchrotron studies indicate well-dispersed metallic Cu particles upon activation ZnO observed to strain Cu particles by EELS. Topsoe and coworkers,264 utilizing in situ XRD with synchrotron radiation, demonstrated that the Cu phase transforms primarily to a crystalline metallic Cu phase from CuO precursor during activation. Smaller particles were detected when the ternary A1203 component was present (9.5 nm versus 14 nm for the binary Cu/Zn catalyst), indicating that alumina acts primarily as a structural stabilizer, a spacer for well-dispersed Cu particles, which assists in minimizing sintering. 

Copper chloride is universally applied as catalyst. - Known as the modified Deacon catalyst, CuCl2 is supported on alumina and contains KC1. Under operating conditions a CUCI2-CU2CI2-KCI ternary mixture, possibly in the molten state, is... 

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... 

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. 

Mossbauer spectroscopy is a technique which is particularly sensitive to the local chemical environment. If the chemical environment is modified locally by the presence of heteroatoms such as dispersed alumina arising from the addition of textural promoters, this would affect many iron atoms within the sample and the method should detect differences in the spectral parameters, i.e., linewidths of the iron metal in the catalyst would change. If the iron contained larger inclusions of hercynite, the spectra should exhibit a quadrupole split component, with a typical ferrous ion shift characteristic of the ternary aluminum iron oxide. Three groups have studied these problems and their results are reviewed critically in Ref. 14. 

The broadening of the spectrum upon treatment at 790 K is similar to observations made when calcium carbonate single crystals are subject to argon ion bombardment. The defects induced in the calcium oxide component of the catalyst cannot be caused by an initial reduction of the oxide, because calcium oxide is stable to hydrogen at 790 K. The defects may be indicative of the formation of a ternary calcium iron oxide since there is no observed increase in the dispersion of calcium, which would be associated with a disintegration of the CaO crystals. It should be pointed out that the other structural promoter element, aluminum, exhibits the same spectral changes in its A12 emission. The dispersion of the alumina increases, however, with reduction of the catalyst, particularly when the wet reduction method is applied (see Table 2.1). 

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