De-coupling thermal and photo-illumination effects induced by hybrid semiconductor-plasmonic catalysts during oxidative and reductive reactions.

Abstract

In recent years, there has been a renewed interest in the harnessing and conversion of solar energy with localised surface plasmon resonance (LSPR) driven photocatalysts. In addition to exceptional photocatalytic activity in the visible light region, plasmonic catalysts can also be designed to have a synergistic thermal-catalysis effect. Herein, we investigate the contributions of plasmonic and bandgap excitation catalytic oxidative and reductive reactions on hybrid semiconductor-plasmonic catalysts by probing the effects of varying excitation wavelengths on catalytic performance and surface intermediate species.

Ethanol oxidation was used as a characteristic probing reaction, wherein Au/TiO2 displayed a considerable photo-thermal-synergism at temperatures >175 oC, with over 50% and 100% increases in catalytic performance compared to neat TiO2 under visible light and UV illumination, respectively. Detailed probing of post-reaction surface carbon species on Au/TiO2 indicated that photo-enhancement under UV illumination was due to congruent roles of the photo- and thermal- catalysis, while photo-enhancement under visible light illumination was driven by plasmon-mediated charge transfer from Au deposits to the TiO2 support. In-situ Diffuse Reflectance Infrared Fourier Transformed Spectroscopy (DRIFTS) revealed that plasmon-mediated electron charge transfer at the Au-TiO2 interfacial perimeter promotes C-C bond cleaving and CO oxidation by Au/TiO2, two critical steps in ethanol oxidation by Au/TiO2.

Furthermore, it was shown by alloying Au with Cu, plasmon-mediated charge transfer can be diverted from the metal-support interface to the metal-metal interface between Au-Cu atoms; thus, enhancing the selectivity of Au-Cu/TiO2 towards the formation of acetaldehyde, with over 800% increase in ethanol conversion under visible light illumination. Lastly, the knowledge was applied on catalytic hydrogenation of CO2 by La-Ni/TiO2, a less known plasmonic bimetallic catalyst which exhibited 5-10 times photo-enhancement in the 200-300 oC temperature range. Photo-thermal-catalytic results and X-ray Photoelectron Spectroscopy suggested that photo-enhancement in La-Ni/TiO2 is driven by photothermal heating rather than plasmon-mediated charge transfer.

Ultimately, the study elucidates the synergistic effect of plasmon enhanced catalytic reactions driven by distinct LSPR effects, including photothermal heating and plasmon mediated charged transfers, which are dependent on the inherent metal-metal or metal-support interactions. Incorporating these findings enable the versatile design of plasmonic based heterogeneous catalysts.