Catalyst Cu/ZnO

The dynamics of particle morphology can be used to an advantage, to counteract the effect of sintering ofthe copper particles. As Fig. 8.13 shows, a Cu/ZnO catalyst slowly loses activity, which is attributed to sintering. Exposing the catalyst for a short time to a highly reducing mixture of C02-free synthesis gas restores the activity. 

Several previous studies have demonstrated the power of AEH in various catalyst systems (1-11). Often AEM can provide reasons for variations in activity and selectivity during catalyst aging by providing information about the location of the elements involved in the active catalyst, promoter, or poison. In some cases, direct quantitative correlations of AEM analysis and catalyst performance can be made. This paper first reviews some of the techniques for AEM analysis of catalysts and then provides some descriptions of applications to bismuth molybdates, Pd on carbon, zeolites, and Cu/ZnO catalysts. 

Figure 7. Electron energy loss spectroscopy (EELS) of a Cu/ZnO catalyst a) bright-field STEM image showing a 20nm copper oxide particle and a small 2nm Cu metal particle on ZnO, b) and c)...

As discussed below in the section on Cu catalysts, it has been noted that Cu/ZnO catalysts have low activities and deactivate quickly in the decomposition reaction. However, the inclusion of nickel, introduced in co-precipitation preparation, has been reported to enhance initial activity and retard deactivation.29 The role of nickel has been ascribed to an increase in dispersion of the active Cu species and also the prevention of deactivation via the formation of Cu-Zn alloys. 

Klier and coworkers—Role of ZnO in stabilizing Cu in Cu+ oxidation state, proposed to be the active site. Klier and coworkers235 241 provided a different explanation for the role of zinc in promoting the activity of Cu/ZnO catalysts. They suggested that zinc stabilizes the Cu in the Cu1 + oxidation state, and that it is the Cu ions in the 1 + oxidation state that serve as the active sites. 

Ronning, Holmen, and coworkers—Ce doping of Cu/Zn/Al catalysts improves stability. Ronning et al,339 explored the impact of ceria addition to Cu/ZnO catalysts. Catalysts were prepared by co-precipitation of Cu, Zn, and Al from their corresponding nitrates. Ceria was incorporated into the catalyst by impregnation of cerium nitrate either before or after calcination (6 hours at 350 °C or 400 °C). The chemical compositions of the resulting catalysts are reported in Table 62. 

For the chemical reactor, the researchers used a nanoparticle catalyst deposited on metallic micro-structured foils. They tested Cu/ZnO and Pd/ZnO catalysts deposited on the microstructured foils. The Cu/ZnO catalyst was more active than the Pd/ZnO catalyst and had a lower selectivity to undesired carbon monoxide. However, because the Pd/ZnO catalyst was more stable, it was selected for use in their fuel processor. The Pd/ZnO carbon monoxide selectivity of the powder catalyst pressed into a pellet was lower than that of the nanoparticle catalyst deposited on the microstructured foils. This effect was attributed to contact phases between the catalyst and the metal foils. ... 

This reaction takes place either at high temperatures T >500°C) over an Fe or Ni catalyst or at lower temperatures (-250°C) over a Cu/ZnO catalyst. [Why is it much easier to recover pure H2 by separating a mixture of H2 and CO2 than a mixture of H2 and CO ]... 

This process has many similarities to NH3 synthesis. The pressure is not as high for acceptable conversions, and modem methanol plants operate at -250°C at 30-100 atm and produce nearly equilibrium conversions using Cu/ZnO catalysts with unreacted CO and H2 recycled back into the reactor. 

Figiu 3-18 Plot of equilibrium conversion Xq versus temperatiwe for methanol synthesis starting witii stoichiomeh ic feed. While die equilibriimi is favorable at ambient temperature, die conversion rapidly decreases at higher temperature, and industrial reactors operate with a Cu/ZnO catalyst at pressures as high as 100 atm. 

In contrast to the Fisher Tropsch synthesis of higher alkanes from CO and H2, Ni catalysts produce primarily methane, and Cu/ZnO catalysts produce mainly methanol. Sketch the mechanistic steps that favor these products rather than polymerization ... 

EXAFS has been very useful in the study of catalysts, especially in investigating the nature of metal clusters on surfaces of the supported metal catalysts (Kulkarni et al, 1989 Sinfelt et al, 1984). A variety of systems has been examined already and there is still considerable scope for investigation in this area. Since EXAFS gives bond distances and coordination numbers and is absorber-selective, it is possible to study one metal at a time (Fig. 2.12). Thus, an EXAFS investigation of sulphided Co—Mo— Al20j and related catalysts has shown the nature of the reactive surface species (Kulkarni Rao, 1991). Cu/ZnO catalysts have revealed certain unusual features suggesting the complex nature of the species involved in methanol synthesis (Arunarkavalli et al, 1993). Time-resolved EXAFS is of considerable value for the study of catalysts (Sankar et al, 1992). 

The microscopy results characterizing the Cu/ZnO catalyst are in accord with EXAFS data representing the dynamic morphology changes (39—41), and they also provide an important additional insight On the basis of the lattice-resolved images, the nature of the exposed facets of the projected copper nanoclusters and the epitaxial relationship between the copper and ZnO can be identified. The majority of the copper nanocrystals appear to be in contact with the ZnO support with their (111) facets, as was also observed for copper particles prepared by vapor... 

Fig. 5. Normalized EEL spectra of the Cu/ZnO catalyst Cu L. s-edges. The spectra were acquired from the Cu/ZnO catalyst (A) in 1.5mbar of H2 at 493 K and (B) in a H2 (85%)/CO (15%) mixture at a total pressure of 1.5 mbar at 493 K (after heating to 723 K for 1 h). The hatched areas indicate changes in the ELNES intensity at the Ls-edge by variations in the gas environment. Reprinted in part with permission of the American Chemical Society from Reference 42. ...

Cu/ZnO catalyst under the influence of the gaseous environments stated in the figure, (b) Variation in the apparent Cu-Cu coordination number with changes in the gaseous environment [adapted from Clausen et al. (5S)]. 

New catalysts such as a Raney Cu-based system containing Zr49 and ultrafine CuB with Cr, Zr, and Th50 exhibit good characteristics. The improved catalyst performance observed for a Cu-ZnO catalyst with added Pd is explained by the relative ease of hydrogen dissociation by the incorporated Pd particles and then spillover to the Cu-ZnO.51... 

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... 

Later, Pattekar and Kothare [21] presented a silicon reactor fabricated by deep reactive ion etching (DRIE). It carried seven parallel micro channels of 400 pm depth and 1 000 pm width filled with commercial Cu/ZnO catalyst particles (from Siid-Chemie) trapped by a 20 pm filter, which also was made by DRIE, in the reactor. The reactor was covered by a Pyrex wafer applying anodic bonding. Details of the reactor are shown in Figure 2.3. 

Germani et al. [82] wash-coated Cu/ZnO catalyst on to micro channels and compared their performance with that of conventional monoliths for the low-temperature water-gas shift. Up to six plates could be put into a stack-like reactor heated by cartridges, which had a maximum operation temperature of600 °C (see Figure 2.47). The reactor had capabilities for measuring the inlet and outlet temperature of the gases via thermocouples. 

The reforming reactor was built of copper powder, which could be sintered at temperatures between 500 and 700 °C, being low enough to avoid damage to the catalyst. In the same fabrication stage, the Cu/ZnO catalyst with a particle size between 300 and 500 pm was incorporated into the device. Copper and aluminum powder were used as inert materials for parts such as channels and diffusion layers. 

Schuth, F., High-throughput screening under demanding conditions Cu/ZnO catalysts in high pressure methanol synthesis as an example, J. Catal. 2003, 216, 110-119. 

The preparation of Cu/ZnO catalysts and precursors for the methanol synthesis reaction have been described [87, 88], while others [89] used a mixture of Pt, Ru and a leachable metal such as A1 to prepare catalysts for CO-tolerant catalysts for fuel cells. 

W. Wieldraaijer, et ah, Methanol Synthesis over Cu/ZnO Catalysts prepared by Ball Milling, Catalysis Lett., 1997, 48, 55. 

Oxidation reactions have been the focus of several researchers in recent years. Several pathways are available for the activation of methane with two predominant ones being partial oxidation via oxidative coupling and the other being formation of oxygenates. Sojka, Herman and Klier have recently reported that formaldehyde selectivity can be enhanced by using a doubly promoted (Cu-Fe) doped ZnO with yields of 76 g HCHO (kg cat.) 1h . 31 These reactions were carried out at 750°C at 2.5% conversion. Singly doped Cu-ZnO catalysts yielded CO2 and H2O via deep oxidation whereas singly doped Fe-ZnO primarily yielded HCHO. Doubly doped Cu-Fe-ZnO minimized the formation of C2 products and therefore, the Cu-Fe-ZnO catalyst decreased C2 products and enhanced HCHO formation. 

Grunwaldt J-D, Molenbroek AM, Topsoe N-Y, Topsoe H, Clausen BS. In situ investigations of structural changes in Cu/ZnO catalysts. J Catal. 2000 194 452. 

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