Unsaturated atoms lend support

Catalytic platinum atoms bind to unsaturated aluminium atoms on the surface of the aluminium oxide catalyst support.

Credit: © 2009 AAAS

The most common type of industrial catalyst consists of a precious metal fixed to the surface of an oxide support. Understanding catalyst–support interactions is key to controlling the dispersion and structure of the catalyst atoms on the support surface and thus crucial to improving catalytic activity. Strong interactions are necessary for high activity under harsh operating conditions and occur at electronic defects on the surface of reducible oxide supports, such as Ti³⁺ sites on the surface of titania. The surface of a non-reducible γ-Al₂O₃ support, however, has no such defects but yet still binds catalytically active precious metals.

Ja Hun Kwak from the Pacific Northwest National Laboratory and co-workers have now shown¹ that platinum atoms bind to unsaturated pentacoordinate Al³⁺ on the support surface. Using ²⁷Al magic-angle spinning NMR spectroscopy they see a decrease in the number of unsaturated pentacoordinate Al³⁺ sites and an increase in the number of saturated sites after platinum loading. Using electron microscopy they also observe that platinum atoms are evenly dispersed at low loading, with one at each Al³⁺ site, but when loading is higher they observe the formation of two-dimensional PtO 'rafts'.

Kwak and colleagues propose that the binding is driven by coordinatively saturating the aluminium sites, rather than electronic interactions between catalyst atoms and reducible oxide supports.