Controlling the Hydrogenation Activity of Pt/TiO2 by Defect Engineering of the TiO2 support
The electronic properties of semi-conducting supports such as band gap, carrier concentration and Fermi level provide a potential lever to tune the activity and selectivity of supported metal catalysts , as anticipated by Schwab . Controlling the carrier concentration however has been challenging, limiting progress towards this objective. Recently, Sellers and Seebauer  have shown that the carrier concentration in polycrystalline TiO2 films grown by Atomic Layer Deposition (ALD) can be controlled by the film thickness, since the film thickness determines the concentration of electrically active donor defects at grain boundaries. Using CO oxidation as a probe reaction, Chua et al.  demonstrated that an order of magnitude variation in the carrier TiO2 concentration changes the oxidation activity of sub-1 nm Pt particles by about 50%. The effect for CO oxidation is rather limited because the balance between donation and backdonation limits the effect of the Pt charge density on the CO adsorption energy. DFT calculations indicate that ethylene adsorption is a much more sensitive probe for the Pt charge, in part because of the larger HOMO-LUMO gap. Ethylene hydrogenation experiments over a series of model Pt/TiO2 catalysts confirm the enhanced sensitivity. At 30 °C, under excess ethylene conditions where the surface is covered with ethylene and ethylidyne, the hydrogenation turnover frequency can be increased by 300% with the TiO2 carrier concentration. This demonstrates that the Schwab effect provides a general and controllable handle to tune catalyst activity. An even larger effect can be anticipated for Pt clusters with 1 to 10 Pt atoms, and for catalyst selectivity.
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 M.C.K. Sellers, E.G. Seebauer, Thin Solid Films 519 (2011) 2103.
 Y.P.G. Chua, G.T.K.K. Gunasooriya, M. Saeys, E.G. Seebauer, J. Catal. 311 (2014) 306.