A New Method for Synthesizing Co-precipitated Cu–ZnO Catalyst and Its Activity for Methanol Decomposition at High Temperature

Global warming caused by increased emissions of greenhouse gases, especially CO2, is a pressing global problem. Atmospheric CO2 concentration rose above 400 ppm1) in 2015, and continues to increase. Therefore, CO2 emissions must be drastically reduced or eliminated entirely to prevent an increase in global temperature of more than 2 K by 21002). One solution is to recycle CO2 into fuel using hydrogen produced from renewable energy sources. Previous studies have examined the synthesis of hydrocarbons such as methane, methanol, gasoline, and diesel fuel from CO2. Methane and methanol can be effectively synthesized from CO2; but the methanol yield is strongly limited by the thermodynamic equilibrium of the reaction. For example, the yield of methanol from synthesis gas at 4 MPa is only 30 % at 473 K, and 13 % at 523 K. Conversion of methanol to dimethyl ether (DME) may increase the yield of methanol and DME3). The highest yield of methanol and DME was 24 % at 543 K, achieved by combining a methanol synthesis catalyst with γ-Al2O3. Similar possibilities can be applied to the conversion of CO2 to hydrocarbons, which are currently a more important fuel. Other studies have reported hydrocarbon synthesis from syngas4)~6). Use of a combination of CuZnO and ultra-stable Y-type zeolite catalyst at 623 K and 2.1 MPa obtained a yield of hydrocarbons of 30 %, about 75 % of which were C3 and C4 paraffins, or liquefied petroleum gas (LPG). These techniques can be applied to hydrocarbon synthesis from CO2; but methanol synthesis catalysts such as CuZnOAl2O3 have low thermal stabilities at the conditions under which the zeolite catalyst actively forms hydrocarbons from methanol/dimethyl ether. Therefore, development of a new methanol synthesis catalyst with high thermal stability is required. Fine bubbles smaller than 100 μm show unique behavior. Fine bubbles have a large specific surface area, so gas molecules inside the bubble will easily and rapidly contact with the surrounding liquid. Here we describe a new method for the preparation of precipitated catalyst by passing fine bubbles through a solution. Mixed metal salts solution will quickly react with ammonia in fine bubbles but the resulting metal hydroxide is unlikely to aggregate into large particles because the precipitate is generated only at the interface of the liquid and fine bubbles. Consequently, the particle diameter of the precipitate prepared by this fine bubble method is thought to be very fine with a large specific surface area. A CuZnOAl2O3 catalyst, prepared with [Regular Paper]