Cobalt-containing oxides display catalytic activity for a wide range of reactions, including many that typically require precious metals.  The activity of Co oxide catalysts is sensitive to particle size, the support, and catalyst preparation conditions.  Therefore, we have been using a combination of macroscopic and microscopic surface science methods to understand the factors that govern the reactivity of Co-containing oxides.  It will be shown that the substrate can play a substantial role in stabilizing CoO nanoparticles under conditions where Co3O4 is normally favored.  On high energy substrates, the inherent bilayer structure of Co3O4 dictates that the oxidation of CoO monolayers increases the exposed area of the substrate, thereby stabilizing CoO against oxidation.  Further, the dewetting required to form Co3O4 increases the occupancy of high energy edge sites, especially for small CoO islands.  As a result, the oxidizability of CoO clusters decreases with size, the opposite of the usual trend.  As CoO is difficult to reduce, this renders small Co oxide islands inactive for CO oxidation while larger clusters that can be cycled between CoO and Co3O4 are reactive towards CO.  It will be shown that specific island edges initiate oxidation of larger CoO islands leading to characteristic defect structures in the resulting Co3O4 nanoparticles.  The nanoparticles expose predominantly CoO and Co3O4(111) surfaces; the reactivity of these nanoparticle surfaces towards CO will be compared with thicker epitaxial Co3O4(111) films and epitaxially grown Co3O4(110) films.  The results reveal how particle size and shape as well as support interactions influence the reactivity of Co oxides.

Reactivity of cobalt oxide nanoparticles and thin films

246th National Meeting (2013)

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