No abstract

A series of 14 wt % Cu/Sm2O3/a-ZrO2 (a-: amorphous) catalysts for CO2-to-methanol hydrogenation were prepared by a coimpregnation method. When the Sm loading was 5–6 mol %, the CO2 conversion reached the maximum value (10.1% for 5 mol % catalyst and 9.4% for 6 mol % catalyst). In contrast, methanol selectivity decreased monotonically from 79% to 67% as the Sm loading increased from 0 to 7 mol %. Among the prepared catalysts, the 5–6 mol % Sm-doped Cu/a-ZrO2 catalyst exhibited the highest methanol production rate of 3.7 mmol gcat –1 h–1, which was ca. 20% greater than that with no Sm dopant (3.1 mmol gcat –1 h–1), at 1.0 MPa and 230 °C with a space velocity = 6 L­(STP) gcat –1 h–1. When we took into consideration the results of temperature-programmed reduction by H2, X-ray diffraction, and X-ray photoelectron spectroscopy, doping Sm species into Cu/a-ZrO2 increased the number of surface-dispersed Cu2+, resulting in the high dispersion of Cu nanoparticles, as well as an increase in the number of the active sites (interfacial sites between Cu and a-ZrO2). Furthermore, according to the temperature-programmed desorption of CO2, Sm doping promoted CO2 adsorption on the catalysts and simultaneously activated CO2. The negative effect of Sm doping is a drop in methanol selectivity. In other words, it results in an improvement in methanol decomposition to CO. An excess amount of Sm led to Cu sintering. The main active sites (Cu-ZrO2 interface) are expected to be destroyed by sintering the Cu particles, in other words, losing the interaction between Cu and a-ZrO2. Therefore, since the above-mentioned positive effect and negative effect coexist, there is an optimum value for the amount of Sm doping.

The storage of fluctuating renewable energy is critical to increasing its utilization. In this study, we investigate an energy conversion and storage system with high energy density, called the chemical looping solid oxide cell (CL-SOC) system, from the integrated perspectives of redox kinetics and system design. The proposed system generates electricity, reproduces hydrogen, and stores it via metal oxide redox reactions in combination with a standard pressure fluidized bed reactor and a reversible solid oxide cell (SOC). We conducted redox kinetic analyses of Fe supported on an Fe-doped calcium titanate carrier in redox reaction using the modified shrinking core model and determined the scale of a fluidized bed reactor by developing the numerical Kunii–Levenspiel reactor model. Furthermore, the SOC redox system was modeled to estimate the round-trip efficiency and the system cost. The CL-SOC system achieved a stable hydrogen charge and discharge rate operation (i.e., constant redox reaction rate) in the fluidized bed reactor. It also achieved the reduction of system cost compared to the conventional high-pressure hydrogen storage system. In addition, the levelized cost of storage, including electricity costs, was calculated, and the advantage was also discussed. In this way, this study describes the integrated method of the CL-SOC system evaluation, which will provide a guide for material and system design.