Capturing carbon dioxide (CO2) and converting it to useful chemicals such as methanol could reduce both pollution and our dependence on petroleum products. So scientists are intensely interested in the catalysts that facilitate such chemical conversions. Like molecular dealmakers, catalysts 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.
With that goal in mind, chemists from the U.S. Department of Energy's Brookhaven National Laboratory and their collaborators just released results from experiments and computational modeling studies that definitively identify the "active site" 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 should be the focus of efforts to boost performance.
"This catalyst-made of copper, zinc oxide, and aluminum oxide-is used in industry, but it's not very efficient or selective," said Brookhaven chemist Ping Liu, the study's lead author, who also holds an adjunct position at nearby Stony Brook University (SBU). "We want to improve it, and get it to operate at lower temperatures and lower pressures, which would save energy," she said.
But prior to this study, 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.
"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 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 [https://www.bnl.gov/cfn/] (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.