By Ertl G., Knozinger H., Weitkamp J. (ed.)

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12]. Copper, having the highest melting point, is melted and then aluminum is added. ± Zn(OH2) + 2NaOH (7) It is the reprecipitation of Zn(OH)2 in the porous skeletal copper which provides promotion in methanol synthesis, water gas shift, and other reactions. The highly dispersed reprecipitated Zn(OH)2 decomposes at around 400 K to form ZnO which is an active promoter of copper catalysts. When leaching aluminum alloys it is generally desirable to use high concentrations of sodium hydroxide in order to remove the Al as NaAlO2 and avoid reprecipitation of Al as bayerite (Al2Oj 3H2O).

He prepared his catalysts from Devarda' s alloy which contained 45wt% Cu, 50wt% Al, and 5wt% Zn. Marsden and co-workers [12] were the first to use skeletal catalysts prepared from CuZn-Al alloys for methanol synthesis. Alloys containing 50wt% Al and 0-50 wt% Cu with the balance Zn were employed. Optimum activity for low temperature methanol synthesis was found for catalysts prepared by leaching alloys containing 50wt% Al, 33-43 wt% Cu and 7-17 wt% Zn. Fricdrich et al. [26] showed that catalysts prepared from alloys containing approximately 50wt% Al, 30-36 wt% Cu and 14-20wt% Zn had greatest activity for methanol synthesis.

Often the composition of the precipitate will reflect the solution concentrations, as was shown for CuO/ZnO catalysts for methanol synthesis [18], but this is not necessarily the case. For alminum phosphates it was found that at low P:A1 ratios the precipitate composition is identical to the solution composition, but if the P:A1 ratio in the solution comes close to and exceeds unity, the precipitate composition asymptotically approaches a P:A1 ratio of 1 [19]. Deviations from solution composition in coprecipitation processes will generally occur if solubilities of the different compounds differ strongly and precipitation is not complete or, if in addition to stoichiometric compounds, only one component forms an insoluble precipitate; this the case for the aluminum phosphate.

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Handbook of Heterogeneous Catalysis by Ertl G., Knozinger H., Weitkamp J. (ed.)


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