The progressing shift in resources used in the petrochemical industry from crude oil to natural gas increases the importance of methanol as a basic chemical for the production of synthetic fuels and polymers. In modern MegaMethanol plants that produce more than 5000 tons of methanol per day, supported copper nanoparticles are employed as highly active heterogeneous catalysts. For the efficient use of natural resources the design of more active and selective catalysts is required. However, structure-activity relationships are often deduced from model systems exhibiting structural characteristics very different from those of industrial catalysts. Here we describe the microstructure [1] of copper nanoparticles on zinc oxide prepared similar to industrially used copper catalysts and exhibiting comparable catalytic activities.
Supported copper nanoparticles are active catalysts for methanol synthesis, methanol oxidation, and methanol steam reforming. Structure-activity relationships for copper catalysts have been studied extensively with a variety of materials ranging from single-crystal model systems to multiple-component industrial catalysts with copper concentrations of more than 50 %. [2] Several suggestions were made as to the nature of the "real" structure [3] of the copper phase under reaction conditions, including the simplifying assumption that the activity increases linearly with the copper surface area. [4] Conversely, copper nanoparticles were prepared recently with surfaces displaying different catalytic activities. In addition to the surface area, bulk structural parameters like microstrain in the copper particles were identified to correlate with the catalytic activity of these catalysts. [5][6][7] Only few microscopic studies of the "real" structure of active copper catalysts have been reported. [8][9][10] High-resolution transmission electron microscopy (TEM) and in situ TEM studies have been performed on copper particles supported on ZnO with a copper concentration of less than 10 %. [11,12] These studies showed faceted well-ordered Cu particles on a large and well-defined ZnO support. Changes in the morphology of the Cu particles, the ratio of the copper facets, and the surface area upon varying the reduction potential of the gas phase were impressively illustrated by in situ TEM. However, these copper model systems were prepared by impregnation of ZnO crystals with a solution of copper acetate. This preparation procedure deviates considerably from the preparation of industrial copper catalysts, [13] and hence, the well-defined model systems studied probably exhibit a microstructure different from that of a catalyst prepared for industrial application. Industrial Cu/ZnO/Al 2 O 3 catalysts are commonly prepared by precipitating copper zinc hydroxycarbonates from metal nitrate solutions. In addition to the precipitation method, each step in the subsequent treatment of the precipitate and the resulting oxide precursor (i.e. ageing, washing, drying, calcination, and reduction) affects the microstructure of the active copper phase (chemical memory).
Precipitate ageing strongly influences the activity of the resulting Cu/ZnO catalysts. [14][15][16][17] The superior activity of Cu/ ZnO catalysts obtained from precipitates aged for more than 30 min was shown to correlate with the increasing microstrain in the copper nanoparticles. [18] Apparently, structure-activity relationships revealed for idealized copper model systems should not be extrapolated to the microstructure and catalytic properties of industrial catalysts.
Here we report HRTEM investigations of the microstructure of reduced Cu/ZnO catalysts obtained from differ-
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