(3aj) Control of Electronic Properties of Supported Catalysts and Design of Novel Multilayer Catalytic and Photocatalytic Materials

Design of new catalysts with a precise control over structure and function is extremely important as it can help meet future energy needs and reduce environmental impact of several industrial processes.  We have carried out research to design novel catalysts in which the electronic properties can be tuned by applying external voltage to the metal-support catalytic junctions and thus control catalytic activity.  This new approach to gas-phase heterogeneous catalysis involves use of nanofabrication techniques to prepare supported catalysts with controlled structure, development of new spectroscopy technique and multiscale simulations. 

Density Functional Theory (DFT) based molecular simulations were used to study the effect of electrostatic interactions resulting from support induced electron transfer and from co-adsorbed dipoles on a range of adsorbed molecules on catalysts surface.  These studies revealed that such interactions can significantly affect the metal-adsobate bond as indicated by shifts in vibration frequencies of adsorbate and corresponding changes in adsorption energies. 

For experimental demonstration of such effect on supported catalysts, a new multilayer enhanced infrared spectroscopy (MEIRAS) technique, capable of sensing small vibrational signals from adsorbed molecules on low area model catalysts was developed.  Theoretical electromagnetic simulations show that the metal-dielectric-metal multilayer substrate enhances the surface intensity of incident infrared by a constructive interference, which is optimized by matching the dielectric thickness to infrared wavelength.  This enhanced surface intensity and diminished reflectance of the substrate, were shown to cause the observed large signal enhancement.  We have prepared thin film and nanofabricated nanowire platinum catalysts and conducted polarization dependent MEIRAS studies of CO adsorption.  These experiments combined with theoretical simulations have demonstrated several potential advantages of this simple new technique in catalysis including the capability to selectively probe adsorbates near metal-support interfaces of supported catalysts.

Furthermore, we designed a Pt-TiO₂-Au multilayer catalytic diode in which an electron transfer to CO molecules adsorbed on Pt-TiO₂ junction can be induced by applying external bias voltage, and its effect can be studied using MEIRAS technique.  These experiments have led to a first direct demonstration of the effect of tunable external voltage induced electron transfer in supported gas-phase catalysis on the adsorbed reactants and opened up several new areas of investigation in this field.

The techniques and approaches developed in this work, combined with novel material synthesis methods and insights from computational studies can lead to novel advances in materials for energy needs.  Current multilayer structures used for infrared wavelength can be redesigned to match visible wavelengths and select desirable frequency windows and concentrate its intensity on the top surface by interference effects.  External voltage effects combined with optical interference effects and plasmonic effects by the top metal layer of well defined size and structure can have extremely important applications to enhance activity and sensitivity in catalysis, photocatalysis and sensors applications.  A research plan including application based materials with fundamental experimental investigation and theoretical studies will be presented.