Catalytic synthesis of methanol from [formula omitted]: II. Electron microscopy (TEM, STEM, microdiffraction, and energy dispersive analysis) of the [formula omitted] and Cu/ZnO/Cr 2O 3 catalysts

Abstract

Transmission electron microscopy of the Cu ZnO and Cu/ZnO/Cr 2O 3 methanol synthesis catalysts was used to characterize the dispersions of CuO, ZnO, and Cr 2O 3 in the calcined catalysts and Cu metal, ZnO, and Cr 2O 3 in the reduced and used catalysts. All components were found to have morphologies of their independently existing dispersions except ZnO, which appeared as a network of thin crystallites at Cu concentrations of 30% and less and as welldeveloped individual hexagonal platelets at higher Cu concentrations. The hexagonal platelet morphology was probably induced by epitaxial growth of ZnO on Cu 2(OH) 3NO 3 during calcination. Microdiffraction in the scanning transmission electron microscope and selected area diffraction in the transmission electron microscope showed that the network of ZnO crystallites had the hexagonal crystal axis parallel to their major surface plane and the platelet ZnO particles had the hexagonal crystal axis perpendicular to the platelet plane. Quantitative elemental analyses of the individual particles in the composite catalysts were carried out by characteristic X-ray emission in the energy dispersive mode. It was found that both ZnO morphologies in the reduced Cu ZnO catalysts contained up to 12% copper and in the Cu/ ZnO/Cr 2O 3 catalysts up to 16% copper accompanied by Cr(III). These results were interpreted as a dissolution of copper in ZnO in the form of Cu(I) that was assisted by the presence of a trivalent ion. The Cu(I) ZnO solution, which was earlier found to have characteristic optical absorption in the near infrared, was proposed to be the active component of the low pressure methanol synthesis catalysts.