Chemical manufacturers often use toxic solvents such as alcohol and benzene to make products such as pharmaceuticals and plastics. Researchers are researching chemical phenomena that are pre-existing and not understood in the chemical reactions used to make these products. These findings provide a new basic understanding of catalytic chemistry and stepping stones for practical applications that sometimes make chemistry less wasteful and more environmentally friendly.
The study, led by University of Illinois Urbana-Champaign David Flaherty, University of Minnesota, twin city researcher Matthew Neurock and Virginia Tech researcher Ayman Karim, is published in the journal Science.
Combining solvents with metal nanoparticles accelerates chemical reactions and helps maximize yields and profit margins for the chemical industry. However, many solvents are toxic and difficult to dispose of safely, researchers say. Water can also be used, but it is not nearly as efficient or reliable as organic solvents. The reason for the difference is thought to be the limited solubility of some reactants in water. However, some inaccuracies in the experimental data have caused the team to realize the reason for this difference is not fully understood.
To better understand the process, the team conducted experiments to analyze the reduction of oxygen to hydrogen peroxide – one set with water, the other with methanol, and the other with a mixture of water and methanol. All experiments with palladium nanoparticles.
“In experiments with methanol, we observed the spontaneous decomposition of solvents that leave organic residues, or dirt, on the surface of nanoparticles,” said Flaherty, professor of chemical and biomolecular engineering in Illinois. “In some cases, such residues stick to nanoparticles and increase the reaction rate and the amount of hydrogen peroxide formed rather than inhibit the reaction. These observations make us wonder how it can help.”
The team found that residues, or surface redox mediators, of oxygen-containing species, are key components of hydroxyimetric. They accumulate on the palladium nanoparticles’ surface and open new chemical reaction pathways, the research report.
“Once formed, the residue becomes part of the catalytic cycle and is likely to be responsible for several different efficiencies among solvents that have been reported for 40 years working on this reaction,” Flaherty said. “Our work provides strong evidence that these surface redox mediators form in alcohol solvents and that they can explain many of the mysteries of the past for these chemicals.”
By working with several types of computational experiments and simulations, the team learned that these redox mediators effectively transfer protons and electrons to reactants, whereas reactions in pure water transfer protons are easy, but not electrons. These mediators also alter the surface of nanoparticles in a way that lowers the energy barrier to overcome for protons and electron transfer, research reports.
“We suggest that alcohol solvents as well as organic additives can react to form metal-bound surface mediators that act in much the same way that enzymatic cofactors in our body do in catalyzing oxidation and reduction reactions,” Neurock said.
In addition, this work is thought to have implications for reducing the number of solvents used and the waste generated in the chemical industry.
“Our research shows that for some conditions, chemical producers can build surface redox mediators by adding a small amount of additives to pure water instead of pumping thousands of gallons of organic solvent through these reactors,” Flaherty said.
References: “Solvent molecules form surface redox mediators in situ with cocatalyze oxygen reduction in Pd” by Jason S. Adams, Ashwin Chemburkar, Pranjali Priyadarshini, Tomas Ricciardulli, Yubing Lu, Vineet Maliekkal, Abinaya Sampath, Stuart Winikoff, Ayman M. Karim, Matthew Neurock and David W. Flaherty, February 5, 2020, Science.
DOI: 10.1126 / science.abc1339
The Institute of Energy and Biosciences through the EBI-Shell program and the National Science Foundation supported this research.