PNNL

Researchers adding water to the surface of alumina measured some surprising results. They were preparing the surface of an alumina crystal in ultrahigh vacuum, intending to achieve a hydroxylated surface to approximate another form of aluminum, called gibbsite (α-Al(OH)₃). They're interested in gibbsite because it is part of the mix of materials that comprise highly radioactive waste stored in tanks at various former weapons production sites.

The water was supposed to adsorb to the alumina and then immediately dissociate or separate into its components creating hydroxyl or OH components. Surprisingly, the water remained intact, raising important questions regarding the fundamental reactions that govern chemical transformations of aluminum oxides and hydroxides. The new research stems from work at the Interfacial Dynamics in Radioactive Environments and Materials (IDREAM) Energy Frontier Research Center.

The work, led by Pacific Northwest National Laboratory scientists in collaboration with scientists at University of Notre Dame and Georgia Institute of Technology was published in The Journal of Physical Chemistry C in a paper titled, "Molecular Water Adsorption and Reactions on α-Al2O3(0001) and α-Alumina Particles."

Why it matters: Aluminum oxides play an important role in the processing of high-level radioactive waste because they are one of the largest, nonradioactive components of the wastes generated from defense nuclear materials production during the Cold War.

Detailed knowledge of water behavior on aluminum oxide surfaces is key to understanding corrosion and catalytic properties of alumina and other aluminum-based materials in engineering applications.

Summary:Researchers investigated the adsorption and reactions of water on well-characterized alumina under well-controlled experimental conditions: under ultrahigh vacuum (UHV), using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), electron-stimulated desorption (ESD) and other surface science techniques.

They found that water adsorbs on the pristine surface of an alumina crystal α-Al₂O₃(0001), with no evidence of further reaction. Conversely, when water contacts α-alumina particles under ambient conditions it dissociates and does create hydroxyls on the particles. Dissociation is energetically favorable for both surfaces, but it appears to be hindered on the defect-free surface of alumina, which is counter to some findings in the literature.

Using UHV and IR spectroscopy techniques, which is typically only done on metals, researchers observed the vibration of water when adsorbed on the face of an alumina crystal. Although surface hydroxyls from dissociation of the molecule were expected, no evidence was observed by IRAS used to measure the vibration. But when alumina nanoparticles were exposed to water, a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) instrument indicated the presence of surface hydroxyls.

What's next? More research is needed to understand why the experiments and theory indicate such different reactivities for this important surface of alumina and to discover if there’s a barrier for water to dissociate and, if so, how to get over that barrier.

Acknowledgments

This work was supported as part of IDREAM (Interfacial Dynamics in Radioactive Environments and Materials), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences. Pacific Northwest National Laboratory (PNNL) is a multiprogram national laboratory operated for DOE by Battelle. The authors acknowledge the Notre Dame Radiation Laboratory, which is supported by DOE BES through Grant DE-FC02-04ER15533.

Sponsors: This work was supported as part of IDREAM (Interfacial Dynamics in Radioactive Environments and Materials), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences and led by Pacific Northwest National Laboratory (PNNL)

User Facilities: The experiments on α-Al₂O₃(0001) were performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at PNNL.

Research Team: Nikolay G. Petrik and Greg A. Kimmel (Pacific Northwest National Laboratory), Thomas M. Orlando and Patricia L. Huestis (Georgia Institute of Technology), Jay A. LaVerne and Alexandr B. Aleksandrov (University of Notre Dame).

Reference: N.G. Petrik, P.L. Huestis, J.A. LaVerne, A.B. Alekandr, T.M. Orlando, G.A. Kimmel. 2018. “Molecular Water Adsorption and Reactions on α-Al₂O₃(0001) and α-Alumina Particles.” The Journal of Physical Chemistry C2018, 122, 17, 9540-9551 DOI: 10.1021/acs.jpcc.8b01969