PNNL

The Science

Developing new materials that have controllable, predictable properties over multiple size scales remains a challenge for scientists. Knowing the precise position of atoms allows detailed studies of how reactions occur during a catalytic cycle. Researchers successfully used a simple, novel method to produce a titanium dioxide (TiO₂) surface covered by an array of tiny molybdenum oxide clusters. They evaporated small (MoO₃)_n polymers, known as oligomers, and deposited them onto a TiO₂ surface from the gas phase. As the oligomers attach to the TiO₂, they begin to fall apart and self-assemble into a layer of single MoO₃ monomer units.

The Impact

A consistent challenge with studying solid catalysts is that they frequently have multiple sites where catalysis occurs. Creating a controlled, hierarchically structured material allows for detailed study at individually known sites. The atomically precise nature of the exposed material surface created in this study provides a single active site where reactions can be studied. Future work will involve detailed studies of catalytic reactions at the molybdenum oxygen, Mo=O, site, using it as a model system for more complex and harder to study catalysts.

Summary

Researchers created an atomically precise, hierarchical oxide material with order present on two different length scales. They devised a simple preparation method to synthesize TiO₂ covered by an array MoO₃ clusters that are identically bonded to TiO₂. The self-assembly occurs despite having starting molybdenum oxide cluster oligomers of different sizes. The resulting material has the individual MoO₃ clusters in an ordered layer. The researchers used a suite of advanced experimental techniques and simulations to determine how the monomers form from the oligomers. A microscope with extremely high magnification allowed the researchers to monitor individual clusters on the TiO_­2 surface. They saw that the oligomers decompose into monomers at room temperature at both high and low concentrations. Theoretical calculations revealed that, on the TiO₂ surface, the monomers are more stable than the oligomers. The MoO₃ clusters bond to the TiO₂ through two Ti-O and two Mo-O bonds between the cluster and the surface. This leaves a single exposed Mo=O site on every cluster that can potentially participate in catalysis.

PNNL Contact

Zdenek Dohnálek, Pacific Northwest National Laboratory, Zdenek.Dohnalek@pnnl.gov

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Catalysis Science. Experiments were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy’s Biological and Environmental Research program and located at Pacific Northwest National Laboratory. Computational resources were provided by a user proposal at the National Energy Research Scientific Computing Center located at Lawrence Berkeley National Laboratory.