Study of electronic effects in a catalytic diode: DFT calculations and MEIRAS experiments
The role of electron transfer between metal catalyst and its oxide support in modifying the binding of the molecules adsorbed on the catalyst surface is investigated using Pt/TiO2 catalytic diode as a novel model catalyst system. The effect of electron transfer is interpreted and demonstrated as that of creating an electric field at the metal-support interface. The effect of such electric fields is studied by using Density Functional Theory (DFT) calculations to simulate adsorption of different molecules under a uniform external electric field. Experimentally, the charge transfer at metal support junction and the resulting electric field is controlled by applying an external bias voltage to the catalytic diode. The effect of controlling the charge transfer on CO molecules used as probe adsorbates is studied using a multilayer enhanced reflection adsorption spectroscopy (MEIRAS) technique developed in this work.
DFT calculations show that the sensitivity of adsorption energy of a molecule on a metal surface to the external electric field depends on its dipole moment and polarizability. The dipole moment varies significantly from one molecule to another, and changes with the surface adsorbate coverage due to the electrostatic interaction from neighboring adsorbates on the surface. Vibrational frequency of molecules shifting linearly with external electric field is shown to be a key experimental observable useful in the study of these effects.
MEIRAS is a new form of the Fourier Transform Infrared (FTIR) spectroscopy performed in reflection mode, in which, the multilayer structure of the sample and its interaction with the incident infrared causes a large enhancement in the sensitivity. The mechanism of sensitivity enhancement is elucidated through experimental measurement of wavelength dependent reflectance of multilayer structures and theoretical optical analysis of infrared reflection on such structures. An experimental setup is developed for MEIRAS measurements, and its application for study of CO adsorption on low area Pt thin-film and nanowire catalysts under in-situ and CO oxidation reaction conditions is demonstrated.
The effect of external voltage applied to induce charge transfer between metal-support junctions in a multilayer thin-film Pt/TiO2 catalytic diode on adsorbed CO molecules is studied using MEIRAS. The bias voltage causes a reversible shift in the vibrational frequency of CO adsorbed near Pt/TiO 2 interfaces exposed through the cracks in the Pt film. These results provide a direct demonstration of the electronic effects associated with metal-support junctions that have been extensively discussed and debated in the catalysis literature.
Nanofabrication techniques are used to prepare regular arrays of uniform and parallel Pt nanowires on TiO2 in order to control the fraction of Pt/TiO2 interfaces while maintaining electrical continuity in the Pt layer. Such nanostructures incorporated in a catalytic diode can provide a better control of electron-transfer effects due to larger fractions of exposed Pt/TiO2 interfaces and potentially lead to tuning of catalytic activity in supported catalysts using bias voltages.