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Cu/ZnO/Al2O3 , Methanol , Synthesis , Steam Reforming
Abstract:
Cu/ZnO/Al2O3 catalysts represent a versatile catalyst system for methanol chemistry, including the synthesis and steam reforming of methanol. Formally, the steam reforming of methanol is the reverse of methanol synthesis from CO2 and H2. In the present work a set of differently prepared Cu/ZnO/Al2O3 catalysts with a fixed composition of Cu/Zn/Al = 60:30:10 were investigated by in situ bulk techniques, X-ray diffraction, and X-ray absorption spectroscopy. Additionally, microscopic and morphological structural characteristics have been examined by electron microscopy (TEM). Temperature programmed techniques (TPR, TPO) were used to study the reducibility of the derived catalysts. The objective was to elucidate structure - activity correlations of Cu/ZnO/Al2O3 catalysts employed for the synthesis and steam reforming of methanol. These correlations provide the basic principles for a rational design of superior Cu/ZnO/Al2O3 catalysts for methanol chemistry.
The activity of Cu/ZnO/Al2O3 catalysts in methanol chemistry was roughly determined by the overall copper surface area. Additionally, it was shown that the abundance of non-equilibrium structures in Cu, such as planar defects, lattice strain, and subsurface oxygen correlated with the catalytic activity in the synthesis as well as steam reforming of methanol. The presence of microstructural imperfections in the bulk accounted for the deviations from a linear Cu surface area - activity correlation observed. Thus, catalytically active copper in the Cu/ZnO/Al2O3 catalysts for both reactions (methanol synthesis, methanol steam reforming) was a highly defective, strained form of the metal originating from an appropriate nanostructuring of Cu and the ZnO/Al2O3 matrix. The latter were a direct consequence of an advanced Cu-ZnO/Al2O3 interface predominately found in those samples exhibiting a pronounced homogeneous microstructure. Conversely, a heterogeneous microstructure with variously sized and textured copper particles was observed for the less active Cu/ZnO/Al2O3 catalysts studied.
Moreover, it was shown that Cu/ZnO catalysts with or without alumina as an additional refractory phase could be microstructurally modified by carbon dioxide calcination. Calcination under proper CO2/O2 partial pressures resulted in incorporation of carbonates into the bulk of the Cu/ZnO/(Al2O3) catalyst leading to more defective microstructure of the copper phase. These structurally modified Cu/ZnO/(Al2O3) catalysts featured a considerably lower selectivity to CO, an undesired consecutive by-product of the methanol steam reforming reaction, even at high methanol conversions. Considering the low CO levels obtained carbon dioxide calcination is a promising alternative for the preparation of highly selective Cu/ZnO/Al2O3 catalysts for methanol steam reforming reaction to be applied for fuel cells. Conversely, activities in methanol steam reforming and methanol synthesis were not improved by carbon dioxide calcination (“principle of microscopic reversibility”).
