(766e) First Direct Demonstration of the Schwab Effect Via a Novel External Voltage Induced Control of Electron Transfer to Surface-CO Bond On Pt/TiO2/Au Multilayer Catalytic Nanodiode

Electronic effects are known to play an important role in catalysis since first proposed by Schwab [1].  Charge transfer is often postulated to play a role in metal-titania [2] and other reducible oxide supported catalysts (sometimes referred as SMSI effect).  However these electronic effects are often coupled with geometric factors such as decoration effects or higher concentration of low coordination sites on small metal particles, which are difficult to decouple.  Furthermore, in dispersed systems, there is no electrical connectivity so no direct demonstration of electronic effects in catalytic junctions has been possible. An early attempt in this direction was made in our group about 15 years ago by microfabricating Pt particles on a flat silica support for butadiene hydrogenation reactions [3].  While external voltage effects were not detected, the concept of controlling the surface chemical activity by a bias voltage was reported in a related patent [4].  Recently, Somorjai and coworkers prepared a catalytic nanodiode with similar characteristics as described in this work and reported a chemicurrent during CO oxidation [5].

In this work we present results on a nanofabricated Pt/TiO₂ catalytic diode modified with a multilayer Pt/TiO₂/Au structure. The latter was introduced to obtain multilayer enhanced infrared reflection spectroscopy (MEIRAS) results of CO adsorbed on Pt, a novel technique [6] recently developed in our laboratory. Using this technique, we directly demonstrate for the first time that Pt-CO bond can be modified by controlling electron transfer at the metal-support junction through an external bias voltage.  The results are consistent with DFT simulations [7] of the effect of electric fields on adsorbed CO which predicted variations in the IR frequency due to polarization of the adsorbate.  Pt/TiO₂/Au diodes consisting of Pt thin films as well as nanofabricated Pt nanowires have been studied.  By studying the effect on infrared polarization, we have demonstrated that using MEIRAS technique and we can selectively probe adsorbates on edges of nanowire which are closer to the Pt/TiO₂ junction.  This effect of proximity to the junctions on observed voltage dependent frequency shifts will be analyzed using incident infrared polarized along the nanowires and normal to the nanowires.

References: [1] GM Schwab Adv. Catal. 27, 1 (1978).  [2] SJ Tauster et al. JACS 100, 170 (1978); DW Goodman Catal Let. 99, 1 (2005).  [3] AC Krauth et al. Catal. Lett. 27, 43 (1994); [4] EE Wolf, US Patent 5,688,474 (1997).  [4] XZ Ji et al. Nano Lett. 5, 763 (2005).  [5] P Deshlahra et al. Surf. Sci. 604, 2 (2010); P Deshlahra et al. Appl. Catal. A 391, 22 (2011); P Deshlahra and EE Wolf J. Phys. Chem. C 114, 16505 (2010).  [6] P Deshlahra et al. J. Phys. Chem. A 113, 4125 (2009).