Photothermal catalysis: a viewpoint from exergy

Abstract

The field of photothermal catalysis has almost exclusively focused on understanding the kinetics of hot electrons from catalysts, but some aspects of the thermodynamics remain poorly understood. In this opinion article, we discuss the theoretical limits and thermodynamic mechanisms of photothermal catalysis, and identify prospective research directions, by developing an exergy balance model and taking the methane dry reforming reaction as an example. The balance between the needed and extracted exergies was found to provide a rational explanation for the striking observation that photothermal catalysis is not the simple summation of photo- and thermo-catalysis. As a result, the thermodynamic limiting efficiency and the corresponding conversion rate of the reaction are considerably enhanced at mild temperatures by photothermal catalysis compared with thermocatalysis, which is highly encouraging for the photothermal catalysis community. Furthermore, we analyzed the discrepancies between realistic limiting efficiencies and experimental demonstrations by using parameters that capture the non-idealities in the extraction of photon exergy. The analysis highlights the importance of reactor design for mild-temperature photothermal catalysis to achieve efficiency comparable to that of high-temperature thermocatalysis. This is a much less explored area that we encourage the community to turn their attention toward.