Capturing carbon dioxide (CO2) and converting it to useful chemicals such as methanol could reduce both pollution and U.S. dependence on petroleum products. Catalysts are used to bring the reacting chemicals together in a way that makes it easier for them to break and rearrange their chemical bonds. Understanding details of these molecular interactions could point to strategies to improve the catalysts for more energy-efficient reactions.
This work identified the “active site” of a catalyst commonly used for making methanol from CO2, describing which catalytic components take part in the chemical reactions — and should be the focus of efforts to boost performance.
The catalyst — made of copper, zinc oxide, and aluminum oxide — is used in industry, but is not very efficient or selective. It does not operate at lower temperatures and lower pressures, which would save energy. Two different active sites for the catalyst had been proposed — a portion of the system with just copper and zinc atoms, or a portion with copper zinc oxide. This work determined which part of the molecular structure binds and breaks, and makes bonds to convert reactants to product.
Laboratory experiments were conducted using well-defined model catalysts, including one made of zinc nanoparticles supported on a copper surface, and another with zinc oxide nanoparticles on copper. To tell the two apart, an energetic x-ray beam was used on the samples, and the properties of electrons emitted were measured. These electronic “signatures” contain information about the oxidation state of the atoms the electrons came from, whether zinc or zinc oxide.
Computational models were made to determine how these two types of catalysts would engage in the CO2-to-methanol transformations. These theoretical studies use calculations that take into account the basic principles of breaking and making chemical bonds, including the energy required, the electronic states of the atoms, and the reaction conditions, allowing scientists to derive the reaction rates and determine which catalyst will give the best rate of conversion.
It was found that copper zinc oxide should provide the best results, and that copper zinc is not even stable under reaction conditions — it reacts with oxygen and transforms to copper zinc oxide. In simulations, all of the reaction intermediates — the chemicals that form on the pathway from CO2 to methanol — bind at both the copper and zinc oxide. The synergy between the copper and zinc oxide accelerates the chemical transformation. Optimizing the copper/zinc oxide interface will become the driving principle for designing a new catalyst. Different configurations of the atoms at the copper/zinc oxide interface will be tested to see how that affects the reaction rate.