Understanding of precipitated Cu,Zn-based precursors linked to the properties and activities of their final catalyst systems

Abstract

Co-precipitation has been widely used for the preparation of multi-component catalyst systems composed of non-reducible and/or reducible metal oxides. Among a great number of industrial catalysts, Cu/ZnO/Al2O3 for methanol synthesis has been prepared by co-precipitation for many decades. Previous studies on this conventional catalyst examined consistently that the phase of co-precipitated Cu,Zn,Al precursors strongly affected the activity of their final catalysts (known as so-called “chemical memory”) and varied by adjusting several variables in the stages of precipitation and ageing. In this thesis, various synthesis approaches were explored in order to understand the characteristics of precipitated Cu,Zn-based precursors linked to the properties and activities of their catalyst systems. The first approach was to vary pH in a different way by normal precipitation (pH increase) or reverse precipitation (pH decrease). The outstanding difference in two types of Cu,Zn,Al precipitates was the amount of carbonate species present in the precursor material as well as in the calcined sample. This carbonate species affected the sintering of copper oxide and metallic copper happening in calcination and H2 reduction, respectively. Thus, all precipitates were prepared by reverse precipitation throughout this thesis. Next, the effect of Zr4+ addition to Cu,Zn precipitate was investigated. Zr ion was not incorporated into the lattice of zincian malachite upon precipitation and subsequent ageing, hence hydroxy-rich Zr aggregates being produced independent of mixed Cu,Zn particles. This type of Zr species was transformed by calcination into amorphous ZrO2 serving as a nano-spacer to make CuO/ZnO particles smaller. Even after H2 reduction, the size of Cu0 particles were small owing to the spacer role of ZrO2. The effect of organic (tetraethylammonium, TEA+) precipitation agent was also studied because NaHCO3 has been used conventionally for Cu/ZnO-based catalysts. Irrespective of the washing efficiency, all TEA+-based precipitates were composed of typical zincian malachite with the similar Zn2+ replacement and carbonate presence. The activities of TEA+-based catalysts were similar as well. These were very different from the results observed in Na+-based precipitates and catalysts. All results suggested that a metal-free precipitation agent be of advantage in simplifying the washing step. The ratio of Cu2+ to Zn2+ in Cu,Zn co-precipitate to affect the property and activity of calcined CuO/ZnO was then investigated because the Cu:Zn ratio was fixed at 7/3 in the above two works. When tested in the decomposition of dimethylhexane-1,6-dicarbamate, co-precipitated catalysts showed the peculiar activity curve with the Cu:Zn ratio that was different from the physical mixtures of CuO and ZnO. The activity enhancement in Cu-rich oxides resulted from intimate intergrowth of nano-sized CuO and ZnO, whereas the inhibition effect in Zn-rich oxides was due to the interfacial contact between individual CuO and ZnO nanoparticles. This indicates that so-called “chemical memory” is effective in CuO/ZnO catalysts. Based upon the lessons gathered through co-precipitation studies, a new precipitation method was suggested, which is called “sequential precipitation” in which Al3+ was precipitated onto the fully aged Cu,Zn precipitate of zincian malachite. This method could prevent the formation of hydrotalcite phase at high Al contents. The resulting Al2O3/Cu/ZnO catalysts with the Al content of 13–40% enabled the direct production of dimethyl ether from syngas with yields that were 2–3-fold higher than those obtained with their co-precipitated counterparts. This was achieved by preparing the fully developed Cu,Zn precursor of a specific structure with surface-decorated Al species. In a continuous work, the Cu,Zn,Al precursor was prepared by precipitation of Al3+ onto primitive, amorphous Cu,Zn precipitate. This precursor turned out to be a phase mixture of zincian malachite and hydrotalcite in which the latter phase was less abundant compared to the co-precipitated precursor. The final catalyst derived from this precursor exhibited a little higher copper surface area and methanol synthesis activity than the co-precipitated counterpart. This thesis addresses that it is of prime importance to tune the phases of precipitated Cu,Zn-based precursors in an adequate manner for acquiring better performance of their final catalysts. Aside from the attempts made herein, a variety of approaches will need to be studied in the future. In particular, conventional co-precipitation has much room to be modified in order to develop a new, unknown Cu,Zn-based precursor phase, which will contribute to development of new catalytic materials for a great number of applications operated with Cu/ZnO-based catalysts at present. Finally, I hope that the provided lessons will be utilized as a useful guideline to whoever desire to design and manipulate a precipitated catalyst system for a specific reaction or purpose.