Chemists from the US Department of Energy’s Brookhaven National Laboratory and their collaborators have definitively identified the active sites of a catalyst commonly used for making methanol from CO2. The results, published in the journal Science, resolve a longstanding debate about exactly which catalytic components take part in the chemical reactions—and thus which should be the focus of efforts to boost performance.
The hydrogenation of carbon dioxide is a key step in the production of methanol; catalysts made from copper (Cu) and zinc oxide (ZnO) on alumina supports are often used.
This catalyst—made of copper, zinc oxide, and aluminum oxide—is used in industry, but it’s not very efficient or selective. We want to improve it, and get it to operate at lower temperatures and lower pressures, which would save energy.—Brookhaven chemist Ping Liu, lead author
There has been a debate over the actual active sites for the reaction on the catalyst. Different groups of scientists had proposed two different active sites for the catalyst—a portion of the system with just copper and zinc atoms, or a portion with copper zinc oxide.
To determine which part of the molecular structure binds and breaks and makes bonds to convert reactants to product—and how it does that—co-author Jose Rodriguez performed a series of laboratory experiments 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, he used an energetic x-ray beam to zap the samples, and measured the properties of electrons emitted. These electronic “signatures” contain information about the oxidation state of the atoms the electrons came from—whether zinc or zinc oxide.
Meanwhile Liu, Jingguang Chen of Brookhaven Lab and Columbia University, and Shyam Kattel, the first author of the paper and a postdoctoral fellow co-advised by Liu and Chen, used computational resources at Brookhaven’s Center for Functional Nanomaterials (CFN) and the National Energy Research Scientific Computing Center (NERSC)—two DOE Office of Science User Facilities—to model 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.
We found that copper zinc oxide should give the best results, and that copper zinc is not even stable under reaction conditions. In fact, it reacts with oxygen and transforms to copper zinc oxide.—Ping Liu
Those predictions matched what Rodriguez observed in the laboratory. All the sites participating in these reactions were copper zinc oxide.
In our simulations, all the reaction intermediates—the chemicals that form on the pathway from CO2 to methanol—bind at both the copper and zinc oxide. So there’s a synergy between the copper and zinc oxide that accelerates the chemical transformation. You need both the copper and the zinc oxide.—Shyam Kattel
Optimizing the copper/zinc oxide interface will become the driving principal for designing a new catalyst, the scientists say.
This work clearly demonstrates the synergy from combining theoretical and experimental efforts for studying catalytic systems of industrial importance. We will continue to utilize the same combined approaches in future studies.—Jingguang Chen
Rodriguez said that the team will try different configurations of the atoms at the copper/zinc oxide interface to see how that affects the reaction rate. Additionally, they will move from studying the model system to systems that would be more practical for use by industry.
An essential tool for this next step will be Brookhaven’s National Synchrotron Light Source II (NSLS-II), another Office of Science User Facility. NSLS-II produces extremely bright beams of x-rays—about 10,000 times brighter than the broad-beam laboratory x-ray source used in this study. Those intense x-ray beams will allow the scientists to take high-resolution snapshots that reveal both structural and chemical information about the catalyst, the reactants, and the chemical intermediates that form as the reaction occurs.
An additional co-author, Pedro Ramírez of Universidad Central de Venezuela, made important contributions to this study by helping to test the activity of the copper zinc and copper zinc oxide catalysts.
This research was supported by the DOE Office of Science.
Shyam Kattel, Pedro J. Ramírez, Jingguang G. Chen, José A. Rodriguez, Ping Liu (2017) “Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts” Science Vol. 355, Issue 6331, pp. 1296-1299 doi: 10.1126/science.aal3573