Solvent molecules catalyze surface reactions

Solvents are known to play key roles in solution-phase catalytic chemistry. For example, a good solvent can boost reaction rates by increasing the solubility and mass transfer of reagents relative to a different solvent. Much less is known, however, about the role solvents play in reactions at the solid-liquid interface.

In a new study focusing on that scenario, researchers find that organic solvent molecules can bind to the surfaces of metal nanoparticles and spontaneously form species that mediate redox reactions (Science 2021, DOI: 10.1126/science.abc1339). The findings may lead to ways of increasing reaction rates and product selectivities and to reducing the volume of organic-solvent waste.

The work grew from efforts aimed at understanding recent observations showing that molecular hydrogen and oxygen react in methanol and other solutions at palladium nanoparticle surfaces to form hydrogen peroxide. Manufacturers make some 5 million metric tons of H₂O₂ annually, mainly via an energy-intensive process with anthraquinone as the starting material. Directly reacting hydrogen and oxygen gases could save energy, but the reaction is tough to control and leads mainly to water, which is thermodynamically favored relative to H₂O₂.

To figure out what’s going on, a team led by David W. Flaherty of the University of Illinois at Urbana-Champaign and Matthew Neurock of the University of Minnesota examined the solid-liquid interface by combining kinetic isotope measurements, spectroscopy, and computational techniques.

The group found that liquid-phase methanol molecules bind to Pd, forming stable hydroxymethyl intermediates. These species readily transfer electrons and protons to adsorbed oxygen, forming H₂O₂ and formaldehyde. Formaldehyde then oxidizes hydrogen, regenerating the hydroxymethyl species, which form more H₂O₂. Pure water doesn’t drive the reaction because water molecules don’t transfer electrons efficiently. However, the team showed that this catalysis does happen when small amounts of methanol or formaldehyde are dissolved in water, suggesting a strategy for reducing industrial organic-solvent waste.

According to catalysis specialist Lars C. Grabow of the University of Houston, surface chemists typically think of solvents in terms of how they stabilize surface intermediates. This team takes the concept a step further, he says, showing that the solvent can serve as a cocatalyst. Grabow notes that the study reveals how these species form on surfaces and that characterizing reactions catalyzed in this way will create opportunities for researchers to further improve reaction rates and selectivity.