An international collaboration of scientists has taken a significant step toward the realization of a nearly “green” zero-net-carbon technology that can efficiently convert CO2 and hydrogen into ethanol.
The study, published in the Journal of the American Chemical Society, lays out a roadmap for successfully navigating this challenging reaction and provides a picture of the full reaction sequence using theoretical modeling and experimental characterization.
The efficient conversion of carbon dioxide, a major air pollutant, into ethanol or higher alcohols is a big challenge in heterogeneous catalysis, generating great interest in both basic scientific research and commercial applications. Here, we report the facilitated methanol synthesis and the enabled ethanol synthesis from carbon dioxide hydrogenation on a catalyst generated by codepositing Cs and Cu on a ZnO(0001̅) substrate.—Wang et al.
Led by the US Department of Energy’s (DOE) Brookhaven National Laboratory (BNL), the group determined that bringing cesium, copper, and zinc oxide together into a close-contact configuration catalyzes a reaction pathway that transforms carbon dioxide into ethanol (C2H6O). They also discovered why this three-part interface is successful.
The study will drive further research into how to develop a practical industrial catalyst for selectively converting CO2 into ethanol. Such processes will lead to technologies that are able to recycle CO2 emitted from combustion and convert it into usable chemicals or fuels.
None of the three components examined in the study is able to individually catalyze the CO2-to-ethanol conversion, nor can they in pairs. But when the trio is brought together in a certain configuration, the region where they meet opens a new route for the carbon-carbon bond formation that makes the conversion of CO2 to ethanol possible. The key to this is the well-tuned interplay between the cesium, copper, and zinc oxide sites.
There has been much work on carbon dioxide conversion to methanol, yet ethanol has many advantages over methanol. As a fuel, ethanol is safer and more potent. But its synthesis is very challenging due to the complexity of the reaction and the difficulty of controlling C-C bond formation. We now know what kind of configuration is necessary to make the transformation, and the roles that each component plays during the reaction. It is a big breakthrough.—Brookhaven chemist Ping Liu, corresponding author
The interface is formed by depositing tiny amounts of copper and cesium onto a surface of zinc oxide. To study the regions where the three materials meet, the group turned to an x-ray technique called in x-ray photoemission spectroscopy, which showed a likely change in the reaction mechanism for CO2 hydrogenation when cesium was added.
More details were revealed using two widely used theoretical approaches: density functional theory calculations and kinetic Monte Carlo simulation.
For this work, the group utilized the computing resources of Brookhaven’s Center for Functional Nanomaterials and Lawrence Berkeley National Laboratory’s National Energy Research Scientific Computing Center, both DOE Office of Science User Facilities.
One of the things they learned from the modeling is that the cesium is a vital component of the active system. Without its presence, ethanol cannot be made. In addition, good coordination with copper and zinc oxide is also important.
There are many challenges to overcome before arriving at an industrial process that can turn carbon dioxide into usable ethanol. For example, there needs to be a clear way to improve the selectivity towards ethanol production. A key issue is to understand the link between the nature of the catalyst and the reaction mechanism; this study is on the front lines of that effort. We are aiming for a fundamental understanding of the process.—Brookhaven chemist José Rodriguez, co-author
Another goal of this area of research is to find an ideal catalyst for CO2 conversion to higher alcohols, which have two or more carbon atoms (ethanol has two) and are, therefore, more useful and desirable for industrial applications and the production of commodity goods. The catalyst studied in this work is advantageous because copper and zinc oxide-based catalysts are already widespread in the chemical industry and utilized in catalytic processes such as methanol synthesis from CO2.
The researchers have planned follow-up studies at Brookhaven’s National Synchrotron Light Source II, also a DOE Office of Science User Facility, which offers a unique suite of tools and techniques for the characterization of catalysts under working conditions. There, they will investigate in more detail the Cu-Cs-ZnO system and catalysts with a different composition.
Xuelong Wang, Pedro J. Ramírez, Wenjie Liao, José A. Rodriguez, and Ping Liu (2021) “Cesium-Induced Active Sites for C–C Coupling and Ethanol Synthesis from CO2 Hydrogenation on Cu/ZnO(0001̅) Surfaces,” Journal of the American Chemical Society 143 (33), 13103-13112 doi: 10.1021/jacs.1c03940