H. Wilmer

Deactivation of Supported Copper Catalysts for Methanol Synthesis

Binary Cu/ZnO and Cu/Al2O3 as well as ternary Cu/ZnO/Al2O3 catalysts were investigated with respect to their catalytic activity and stability in methanol synthesis. In a rapid aging test, activity measurements were carried out in combination with the determination of the specific Cu surface area. A close correlation between the loss of catalytic activity and the decrease in specific Cu surface area was found due to sintering of the Cu particles. Differences in the deactivation behavior and the area-activity relationship of each catalyst system imply that the catalysts should be grouped in different classes.

Methanol synthesis over ZnO: A structure-sensitive reaction?

In order to identify active sites on ZnO powdered catalysts in methanol synthesis a total of five ZnO samples with different degrees of crystallinity were characterized by means of N2 physisorption, XRD, TEM, and EXAFS. With respect to catalysis, high-pressure methanol synthesis was performed as a test reaction. A linear area-activity relationship for the highly crystalline materials was obtained, but at high BET surface areas a strong deviation from linearity was found. The observed phenomena provide evidence for a structure-sensitivity, suggesting that a specific active site is favored for methanol formation. Based on earlier work with polycrystalline ZnO powder and with respect to the current theoretical and experimental work with ZnO single crystals, the polar ZnO faces are assumed to be highly relevant for the catalytic activity under methanol synthesis conditions.

The interaction of hydrogen with alumina-supported copper catalysts: a temperature-programmed adsorption/temperature-programmed desorption/isotopic exchange reaction study

The interaction of hydrogen with a series of copper catalysts (Cu/Al2O3, Cu/ZnO, and Cu/ZnO/Al2O3) was studied by combining temperature-programmed (TP) techniques and the isotopic exchange reaction of H2 and D2 with microkinetic modeling. Various TP experiments (TP desorption, TP adsorption) were carried out, resulting in a set of kinetic parameters for a quantitative description. Only small differences in the kinetics of the ZnO-containing Cu catalysts and Cu/Al2O3 were observed, suggesting that the interaction of H2 with the Cu surface is therefore only slightly influenced by the presence of zinc oxide, and alumina seems to act only as a structural promoter. Significant changes in the results were found when the treatment prior to the actual experiments was altered. From these observations and further supporting experiments it was deduced that a change in the morphology of the metallic Cu particles and surface alloying occur under more severe reducing conditions. These dynamical changes seem to be highly relevant for methanol synthesis.

High-throughput screening under demanding conditions: Cu/ZnO catalysts in high pressure methanol synthesis as an example

High-throughput experimentation is by now an established technology for the synthesis and evaluation of catalysts. However, so far most of the systems described in the literature for the study of gas-phase reactions have been restricted to less demanding reaction conditions, i.e., atmospheric pressure. We have developed a 49-channel parallel flow reactor for use under elevated pressures up to 5 MPa and used this system to screen methanol synthesis catalysts based on the Cu/ZnO system. The catalysts have been prepared by co-precipitation under various preparation conditions. Catalysts obtained from the same precursors, but showing vastly different performance, were then selected for a more detailed study. Differences in performance could be traced back to differences in phase composition and reduction behavior. This study demonstrates that high-throughput experimentation not only is a suitable tool to screen catalysts, giving little scientific insight—as it is often perceived—but can also be used as a first step to obtaining more fundamental insight by rapidly identifying those compositions which are most suitable for detailed study.

Dynamical changes in Cu/ZnO/Al2O3 catalysts

A combination of various transient and steady-state kinetic experiments was used to provide evidence for dynamical changes in a Cu/ZnO/Al2O3 catalyst of industrial interest. From these it can be deduced that the reversible structural alterations strongly depend on the reaction conditions as well as on the pretreatment. The pretreatment was found to induce changes in the morphology of the metallic Cu particles to some extent, and surface alloying under more severe reducing conditions.