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Cu ZnO ; Katalysator ; Methanolsynthese ; Cu ZnO ; Catalyst ; Methanol Synthesis ; Chemistry and allied sciences
Abstract:
Methanol is one of the most important industrial base chemicals and also a starting material for many organic syntheses. A steadily rising amount is used as fuel or fuel additive because methanol can serve as liquid hydrogen carrier. In the chemical industry, methanol is produced from synthesis gas (H2, CO and CO2) over Cu/ZnO/Al2O3 catalysts at temperatures of 250-350 C and pressures up to 100 bars. It is known that Cu should be present finely dispersed to obtain a high Cu surface area and a resultung high activity. ZnO not only acts as geometrical spacer for Cu but also induces a synergetic effect. Al2O3 contributes to thermal and long term stability. In this work correlations between microstructure and activity of Cu/ZnO based catalysts were investigated. Because the catalyst is prepared by a multi step synthesis (co-precipitation, calcination, reduction), already in early stages of the preparation structural characteristics (e.g. of the precursor phase zincian malachite) can provide indications for the resulting activity of the final catalyst. Precursors, calcined and reduced samples were investigated with different methods (XRD, XRF, TG-MS, N2 physisorption, TPR, N2O-RFC, UV-Vis, XAS, XPS, SEM, TEM) and selected samples tested in methanol synthesis. An important part of the work was the refinement of X-ray diffraction patterns to gain information about phase composition and structural parameters. In some preparations Al2O3 was replaced by Ga2O3 to gain better access to X-ray spectroscopic methods (XAS). It was shown that Ga3+, similar to Al3+, exhibited a promoting effect on Cu/ZnO methanol synthesis catalysts. This influence was most efficient, when applying small Ga contents of around 3 mol-% during catalyst preparation. Higher contents led to Ga segregation (formation of ZnGa2O4 spinel) and finally to an inhomogeneous microstructure and lower catalytic activity. Part of the Ga was introduced into the zincian malachite precursor phase and increased the activity of the final catalyst in methanol synthesis. In general, the promoting effect can consist of two contributions: I) The geometric effect leads to an increased Cu surface area. Here, Cu2+ in zincian malachite is partly replaced by Zn2+ and Ga3+ and results in smaller Cu particles. II) The synergetic effect can increase the intrinsic activity (activity related to the Cu surface area) and can be explained by electronically modified ZnO. Ga only induces the geometric effect which is strongly dependent on the homogeneous distribution of the Ga in the samples. Introduction of Ga in the zincian malachite precursor phase was found to be advantageous. This was confirmed by a linear correlation between introduction of (Zn + Ga) in the malachite precursor phase and the catalytic activity. Further results in the course of the dissertation: Comparison of Cu/MgO and Cu/ZnO systems with similar Cu surface areas showed high activities in methanol synthesis from synthesis gas only in the presence of ZnO. MgO can act as geometrical spacer but not as synergetic component. Usage of different pH-values during co-precipitation of the precursors revealed an effect on the precursor phase composition and finally on the resulting Cu surface areas.
