CO₂Hydrogenation over Copper/ZnO Single‐Atom Catalysts: Water‐Promoted Transient Synthesis of Methanol

Abstract

The hydrogenation of CO2 by renewable power‐generated hydrogen offers a promising approach to a sustainable carbon cycle. However, the role of water during CO2 hydrogenation is still under debate. Herein, we demonstrated that either too low or too high contents of water hampered the methanol synthesis over Cu/ZnO based catalysts. For Cu single atoms on ZnO supports, the optimal content of water was 0.11 vol. % under 30 bar (CO2 : H2=1 : 3) at 170 °C. Upon the introduction of optimal‐content water, the methanol selectivity immediately became 99.1 %, meanwhile the conversion of CO2 underwent a volcano‐type trend with the maximum of 4.9 %. According to mechanistic studies, water acted as a bridge between H atoms and CO2/intermediates, facilitating the transformation of COOH* and CH2O*. The enhanced activity induced the generation of more water to react with CO via water‐gas shift reaction, resulting in the increase in methanol selectivity.

Modulation of Electronic Metal-Support Interaction between Cu and ZnO by Er for Effective Low-Temperature CO2 Hydrogenation to Methanol

“…To date, there is a range of heterogeneous catalysts to convert CO 2 into methanol. For example, CuZnO, − CuCeO 2 , − In 2 O 3 , − ZnZrO x , − MoS 2 , , alloy, − etc., have been widely reported, in which Cu/ZnO-based catalysts are the most promising and have the greatest potential for industrial application due to the advantages of high methanol selectivity and low cost …”

Origin of Autocatalytic Behavior of Water over CuZn Alloy in CO2 Hydrogenation

“…10 However, it is interesting that a moderate amount of water has a positive effect on the reaction, with water acting as a catalyst to accelerate the reaction and promote the formation of the target products, such as methanol (CH 3 OH) and carbon monoxide (CO). [13][14][15][16]22 For instance, Wang et al observed that the selectivity of methanol over Cu-ZnO-ZrO 2 catalyst was enhanced after the addition of a reasonable amount of water. Their experimental results showed that water in the desorbed state directly participates in methanol production by promoting the hydrolysis of *CH 3 O.…”

“…The CH 3 OH selectivity reached about 99% with 0.11 vol % water added, where water is the bridge between the key intermediate and *H atom, accounting for the improve-ment of the catalytic performance. 13 In addition to the promotion to methanol production, Yang et al found that the rates of methanol and carbon monoxide synthesis were both increased over Cu/SiO 2 catalyst due to the assistance of water or water-derived species by introducing a 0.04−0.5% mole fraction of water into the feed gas. 14 Moreover, the test results of Nie et al showed that, under the influence of water, the production rate of CO is faster than that of CH 3 OH over PdCu-based catalyst, implying that water contributes to the formation of CO to a greater extent than CH 3 OH.…”

“…Not coincidentally, Wu et al demonstrated the essential role of water over the Cu/ZnO catalyst. The CH 3 OH selectivity reached about 99% with 0.11 vol % water added, where water is the bridge between the key intermediate and *H atom, accounting for the improvement of the catalytic performance . In addition to the promotion to methanol production, Yang et al found that the rates of methanol and carbon monoxide synthesis were both increased over Cu/SiO 2 catalyst due to the assistance of water or water-derived species by introducing a 0.04–0.5% mole fraction of water into the feed gas .…”

“…The presence of excess water in the system has been reported to lead to the deactivation of catalysts. − Excess water molecules may compete with key reaction intermediates for the adsorption of active sites, resulting in the block or oxidation of active sites. , The accumulation of water may also accelerate the crystallization process and cause the sintering of catalysts . However, it is interesting that a moderate amount of water has a positive effect on the reaction, with water acting as a catalyst to accelerate the reaction and promote the formation of the target products, such as methanol (CH 3 OH) and carbon monoxide (CO). − , For instance, Wang et al observed that the selectivity of methanol over Cu-ZnO-ZrO 2 catalyst was enhanced after the addition of a reasonable amount of water. Their experimental results showed that water in the desorbed state directly participates in methanol production by promoting the hydrolysis of *CH 3 O .…”