Abstract
Cu,Zn,Al layered double hydroxide (LDH) are formed as a by-phase during the coprecipitation step of the well known Cu/ZnO/Al2O3 catalysts for methanol synthesis. In the present work the LDH system was investigated systematically.
First of all, a phase pure Cu,Zn,Al LDH was prepared by co-precipitation, resulting in the composition (Cu0.5Zn0.17)Al0.33(OH)2(CO3) 0.17 •mH2O and showing the typical platelet-like morphology and homogeneous elemental distribution. Upon decomposition in air a carbonate-modified, amorphous mixed oxide was formed. This mixed oxide can be described as well dispersed CuO within ZnAl2O4 still containing stabilizing carbonate and showing a strong interaction of Cu2+ ions with the Zn-Al matrix. After reduction, the main nanostructuring step of ex-LDH catalyst, a Cu based catalyst was obtained with Cu dispersed in a ZnAl2O4 spinel-like matrix. Due to the high embedment of the
small Cu particles (7nm) it exhibits just a small absolute activity in methanol synthesis compared to the conventionally prepared reference catalyst but a high intrinsic one (relative to the Cu surface area) showing the high importance of the interface contact of Cu and its surrounding.
By pH-controlled co-precipitation within microemulsion droplets a Cu-based catalysts was prepared from a phase pure LDH with smaller particles. Thus, smaller Cu/ZnAl2O4 aggregates with less embedment of the resulting Cu particles were obtained and a higher activity in methanol steam reforming was observed. However, a smaller intrinsic activity was observed in methanol steam reforming.
Furthermore, two sample series were prepared using Cr and Ga as substituting elements for Al to investigate their effects onto the ex-LDH catalyst system and to gain understanding of the functionality of Al. After decomposition in air carbonate-modified mixed oxides were obtained for Cr containing samples as well as for Ga contents <15 at%, similar to the Cu,Zn,Al system. Higher Ga contents led to the formation of crystalline Zn(Al,Ga)2O4 or ZnGa2O4 spinel. Both sample series did not show a correlation of Cu surface area and catalytic activity. After all, these sample series show that it is possible to prepare a series of Cu based catalysts with the same preparation history, which are characterized by a homogeneous elemental distribution and a similar microstructure but significant differences in catalytic performance attributed to differences in intrinsic activity in methanol synthesis.
In all prepared sample series, the interaction between Cu and the oxide matrix was identified as an important parameter to control this intrinsic activity.
In summary, interface interactions between Cu and the oxide seem to beneficially affect the activity of the Cu particles and the optimal catalyst requires a compromise of exposed surface and interface. The modification of the interface area (Cr) as well as the type of the interface interaction (Ga) were reported to be a possibility to use these positive interactions and modify the resulting Cu based catalyst.
