Metal Oxides in Water

In a paper published recently in the journal Nature Communications, Manos Mavrikakis, professor of chemical and biological engineering at the University of Wisconsin-Madison, and his collaborators report fundamental discoveries about how water reacts with metal oxides. The paper opens doors for greater understanding and control of chemical reactions in fields ranging from catalysis to geochemistry and atmospheric chemistry.

Manos Mavrikakis

“These metal oxide materials are everywhere, and water is everywhere, ” Mavrikakis says. “It would be nice to see how something so abundant as water interacts with materials that are accelerating chemical reactions.”

These reactions play a huge role in the catalysis-driven creation of common chemical platforms such as methanol, which is produced on the order of 10 million tons per year as raw material for chemicals production and for uses like fuel. “Ninety percent of all catalytic processes use metal oxides as a support, ” Mavrikakis says. “Therefore, all of the reactions including water as an impurity or reactant or product would be affected by the insights developed.”

Chemists understand how water interacts with many non-oxide metals, which are very homogeneous. Metal oxides are trickier: an occasional oxygen atom is missing, causing what Mavrikakis calls “oxygen defects.” When water meets with one of those defects, it forms two adjacent hydroxyls — a stable compound comprised of one oxygen atom and one hydrogen atom.

Mavrikakis, assistant scientist Guowen Peng and Ph.D. student Carrie Farberow, along with researchers at Aarhus University in Denmark and Lund University in Sweden, investigated how hydroxyls affect water molecules around them, and how that differs from water molecules contacting a pristine metal oxide surface.

The Aarhus researchers generated data on the reactions using scanning tunneling microscopy (STM). The Wisconsin researchers then subjected the STM images to quantum mechanical analysis that decoded the resulting chemical structures, defining which atom is which. “If you don’t have the component of the work that we provided, there is no way that you can tell from STM alone what the atomic-scale structure of the water is when absorbed on various surfaces” Mavrikakis says.