Research on Converting Carbon Dioxide to Methanol

Brookhaven scientists identified how a zinc/copper (Zn/Cu) catalyst transforms carbon dioxide (two red and one grey balls) and hydrogen (two white balls) to methanol (one grey, one red, and four white balls), a potential fuel. Under reaction conditions, Zn/Cu transforms to ZnO/Cu, where the interface between the ZnO and Cu provides the active sites that allow the formation of methanol. Credit: Brookhaven National Laboratory

Pollution and mankind’s dependence on petroleum products can both be reduced by capturing carbon dioxide (CO2) and converting it to methanol and other useful chemicals. Therefore, scientists are extremely interested in catalysts that help in such chemical conversions.

Catalysts, like molecular dealmakers, bring all of the reacting chemicals together in a way that enables them to break and rearrange their chemical bonds very easily. Understanding the details of these molecular interactions could point to strategies that will enhance the catalysts for more energy-efficient reactions.

Keeping this goal in mind, chemists from the U.S. Department of Energy’s Brookhaven National Laboratory and their collaborators recently released results from computational modeling studies and experiments that definitively identify the “active site” of a catalyst usually used for producing methanol from CO2.

The results, featured in Science, resolve a very old debate about exactly which catalytic components participate in the chemical reactions - and should be the focus of efforts to enhance performance.

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.

Ping Liu, Chemist, Brookhaven National Laboratory

Before this study, two different active sites for the catalyst, a portion with copper zinc oxide, or a portion of the system with just copper and zinc atoms, were suggested by different groups of scientists.

“We wanted to know which part of the molecular structure binds and breaks and makes bonds to convert reactants to product—and how it does that,” said co-author Jose Rodriguez, another Brookhaven chemist associated with SBU.

To find out, Rodriguez conducted a series of laboratory experiments using well-defined model catalysts, including one made of zinc oxide nanoparticles on copper, and another made of zinc nanoparticles supported on a copper surface.

To distinguish the two, an energetic x-ray beam was used by Rodriguez to zap the samples. The properties of electrons emitted were then measured. These electronic “signatures” carry information about the oxidation state of the atoms the electrons came from, whether zinc oxide or zinc.

In the mean time, 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, modeled how these two types of catalysts would take part in the CO2-to-methanol transformations by using 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.

These theoretical studies use calculations that take into account the fundamental principles of breaking and making chemical bonds, including the energy required, the reaction conditions, and the electronic states of the atoms, thus allowing scientists to obtain the reaction rates and establish which catalyst will offer 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, Chemist, Brookhaven National Laboratory

Those predictions matched the observations made by Rodriguez in the laboratory. “We found that all the sites participating in these reactions were copper zinc oxide,” he said.

However, the copper should not be forgotten.

“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,” Kattel said. “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.”

The scientists explain that the optimization of the copper/zinc oxide interface will become the key principal for designing a new catalyst.

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, Brookhaven National Laboratory

For example, said Rodriguez, “We’ll try different configurations of the atoms at the copper/zinc oxide interface to see how that affects the reaction rate. Also, we’ll be going from studying the model system to systems that would be more practical for use by industry.”

Brookhaven’s National Synchrotron Light Source II (NSLS-II), another Office of Science User Facility, will be an essential tool for this next step. NSLS-II produces very bright x-ray beams, almost 10,000 times brighter than the broad-beam laboratory x-ray source employed in this study.

With these intense x-ray beams, scientists will be able to take high-resolution snapshots that disclose both chemical and structural information about the reactants, the catalyst, and the chemical intermediates that develop as the reaction takes place.

And we’ll continue to expand the theory. The theory points to the mechanistic details. We want to modify interactions at the copper/zinc oxide interface to see how that affects the activity and efficiency of the catalyst, and we’ll need the theory to move forward with that as well.

Ping Liu, Chemist, Brookhaven National Laboratory

Pedro Ramírez of Universidad Central de Venezuela, an additional co-author, made vital contributions to this study by enabling to test the activity of the copper zinc oxide and copper zinc catalysts.

The DOE Office of Science supported this research.

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