Plasmon-Enhanced Catalysis: Distinguishing Thermal and Nonthermal Effects

Abstract

In plasmon-enhanced heterogeneous catalysis, illumination accelerates reaction rates by generating hot carriers and hot surfaces in the constituent nanostructured metals. In order to understand how photogenerated carriers enhance the nonthermal reaction rate, the effects of photothermal heating and thermal gradients in the catalyst bed must be confidently and quantitatively characterized. This is a challenging task considering the conflating effects of light absorption, heat transport, and reaction energetics. Here, we introduce a methodology to distinguish the thermal and nonthermal contributions from plasmon-enhanced catalysts, demonstrated by illuminated rhodium nanoparticles on oxide supports to catalyze the CO methanation reaction. By simultaneously measuring the total reaction rate and the temperature gradient of the catalyst bed, the effective thermal reaction rate may be extracted. The residual nonthermal rate of the plasmon-enhanced reaction is found to grow with a superlinear dependence on illumination intensity, and its apparent quantum efficiency reaches ∼46% on a Rh/TiO catalyst at a surface temperature of 350 °C. Heat and light are shown to work synergistically in these reactions: the higher the temperature, the higher the overall nonthermal efficiency in plasmon-enhanced catalysis.

Thermal effects – an alternative mechanism for plasmon-assisted photocatalysis

“…In papers I-III, the origin of the discrepancy between the measured temperature and the actual reactor temperature is simple to understand; its origin is in the fact that a thermometer was placed away from the reactor pellet, thus discarding any temperature gradients which appear in the reactor and beyond it (as was recently discussed in Refs. [13,33,34]).…”

“…However, as was demonstrated in Refs. [13,33,34] (and later acknowledged in IV), the temperatures can vary substantially inside the chemical reactor and they decay rapidly away from it; specifically, the temperature of the reactor can be very different (by 100s of degrees K) from the temperature measured by a thermocouple placed a few mm away. As shown below, indeed, the temperature measurements in I and II underestimated the reactor temperature, hence, led the authors to incorrect conclusions.…”

“Hot” in Plasmonics: Temperature‐Related Concepts and Applications of Metal Nanostructures

“…Therefore, it is apparent that both heat and light can play a role in these reactions, as also illustrated in ref. .…”

Nanocatalosomes as Plasmonic Bilayer Shells with Interlayer Catalytic Nanospaces for Solar‐Light‐Induced Reactions

“…Reaction rates depended on the different structures of Au‐1–4‐NCat , showing highest reactivity in the case of Au‐4‐NCat and similar to the Au‐multilayer NCat ( Au‐M‐NCat ), validating the influence of bilayer/multilayer‐confined plasmonic effect on reaction rates (Figure b and Figure S19). In the bilayer structure of nanocatalosomes, closely located plasmonic units with narrow interparticle gaps and cavities can form extensively coupled plasmonic hot‐spots which can concentrate high electromagnetic fields and other plasmon‐induced effects such as: instantaneous production/transfer of hot charge‐carriers and their equilibration with lattice through an electron‐phonon coupling resulting the photothermal increase in localized temperature; all these factors together are responsible for solar‐light induced high catalytic activities of nanocatalosomes. In order to investigate the individual role of photothermal effect, we tested the folding/unfolding of proteins in the presence of Au‐4‐NCat under light irradiation.…”