Understanding the effects of water on oxide catalyst surfaces

Vanadium oxides are widely researched for their use in catalysts and electronic devices, such as sensors. However, previous studies – which were mainly conducted under low-temperature, ultra-high-vacuum conditions – have not clearly addressed how real-world conditions may impact the materials’ reactivities and electronic properties.

A paper recently published in Journal of Chemical Physics examines the initial interactions between water vapor and vanadium oxide (4+) films to understand how the oxides are wetted, or adhered to by liquid, at ambient conditions. Such insights could help scientists understand how water, a ubiquitous substance, may change the compounds’ electronic and surface structures.

Using ambient pressure X-ray photoelectron spectroscopy, a technique that enables the surface characterization of materials at or near catalytic or ambient conditions, the authors collected electronic spectra of vanadium oxide films that had been exposed to different concentrations of water vapor. They determined the species present on the surfaces of the films at each reaction condition.

The researchers found that water vapor interacts with the films in two ways – by adsorbing as an intact molecule and by dissociating into surface hydroxides. At low humidity, only hydroxides were present on the film surfaces. Starting at around 0.01 percent relative humidity, water molecules began to adsorb, with their concentration increasing as humidity level was further increased.

“These trends are independent of temperature, contamination levels and differences in the surface preparation method,” co-author Dana Goodacre said. “The findings are consistent with the general trends seen on oxide surfaces.”

Goodacre added that these experiments could be extended to different types of vanadium oxides and potentially used to improve existing catalysts.

Source: “Water adsorption on vanadium oxide thin films in ambient relative humidity,” by Dana Goodacre, Monika Blum, Christin Buechner, Harmen Hoek, Sabrina Gericke, Vedran Jovic, Joseph B Franklin, Salinporn Kittiwatanakul, Tilo Söehnel, Hendrik Bluhm, and Kevin E. Smith, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/1.5138959 .