Hydrogen Diffusion on a Solid Surface Accelerated by Water

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The presence of water had a remarkable effect on the reaction. Photo by Jimi Oke.

Novel Study with Important Results

A study in this field was performed by Professor Manos Mavrikakis, Professor Flemming Besenbacher and their coworkers.  The research was conducted at the Department of Chemical and Biological Engineering of University of Wisconsin Madison (US), and at the Interdisciplinary Nanoscience Centre of Aarhus University (Denmark).

The research team studied hydrogen diffusion on the surface of iron oxide (FeO) thin films, deposited on a platinum sheet. The investigation was particularly focused on the effect that water may have on the diffusion of H atoms; they chose this aspect because of its importance. Their results demonstrated that water’s presence is much more important and effective than was believed.

The research determined that even very small amounts of water, such as the water present in ultrahigh vacuum conditions (approximately 10-5 mbar), hugely affected the hydrogen diffusion rate – at room temperature, the diffusion was 16 orders of magnitude (10 000 000 000 000 000 times) faster than in absence of it.

Hydrogen Diffusion: Experiments and Theory

Hydrogen migration was followed using a very advanced microscope, a fast Scanning Tunneling Microscopy (STM). This instrument also has a very high spatial resolution: atomic level; this means that it is possible to detect the hydrogen atoms, and distinguish them from the atoms of the solid oxide, in the image.

To complement their data, Prof. Mavrikakis and coworkers also performed theoretical calculations, using Density Functional Theory (DFT) – a model based on the electron distribution in atoms, and particularly suited to describe this type of system. These calculations allowed them to determine the mechanism of atomic hydrogen diffusion; in this case, H is moving through the formation of the ionic intermediate H3O+.

Important Findings

Commenting their results, Professor Mavrikakis told Decoded Science that, “We suspected that the presence of water was important, but we did not realize how important it was. The difference in the diffusion rate between the system with even a tiny amount of water and no water at all is amazing. This has very important ramifications for numerous catalytic reactions where hydrogen plays a role as a reactant or product.”

Sources

Merte L.R. et al. Water-mediated proton hopping on an iron oxide surface. (2012). Science, 336, 889. Accessed May 20, 2012.

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