Effect of Sm Doping on CO2-to-Methanol Hydrogenation of Cu/Amorphous-ZrO2 Catalysts

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 g(cat)(-1) h(-1), which was ca. 20% greater than that with no Sm dopant (3.1 mmol g(cat)(-1) h(-1)), at 1.0 MPa and 230 degrees C with a space velocity = 6 L(STP) g(cat)(-1) h(-1). When we took into consideration the results of temperature-programmed reduction by H-2, 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 CO,. 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.

Automated summary

This study demonstrates that doping Sm into Cu/a-ZrO2 can increase the number of surface-dispersed Cu2+ species, enhance the dispersion of Cu nanoparticles, and increase the number of active sites, promoting CO2 adsorption and CO activation. However, Sm doping decreases methanol selectivity and leads to Cu sintering, weakening the interaction between Cu and a-ZrO2.