Photocatalysis is a promising strategy for the conversion of the greenhouse gas carbon dioxide (CO₂) to added-value chemicals. However, realizing photocatalysts with high reaction rates has proven to be challenging, motivating researchers to look for alternative methodologies to achieve industrially relevant rates. Recently, photothermal catalysis has emerged as a highly promising strategy for the reduction of CO₂ into added-value products [1-3]. Here, the strengths of photocatalysis and thermocatalysis are combined to achieve high reaction rates, and offers to open new chemical pathways. Of specific interest is the methanization of CO₂, as methane (CH₄) is an important clean energy fuel with low carbon emissions. Typically, photothermal catalysts can be realized through the deposition of metal nanoparticles on a support material, the latter often being a semiconductor. Such metal nanoparticles are able to absorb a broad part of the solar spectrum, including the visible range. Absorbed photon energy can be converted into energetic charge carriers through plasmonic or non-plasmonic effects. Their relaxation leads to localized heat, resulting in enhanced (photo)catalytic performance. For even further enhanced performance, external heating can be applied as well.

In this work we have designed very promising photothermal catalysts by depositing ruthenium (Ru) nanoparticles on titanium dioxide (TiO₂). We demonstrate that such Ru/TiO₂ particles are well suited for the conversion of CO₂ and H₂ into CH₄ (i.e., the Sabatier reaction) under illumination at relatively mild temperatures. The light sensitivity towards selective CH₄ production is most apparent in the temperature range of 180 – 200 ^oC. This behavior is not only observed under UV/vis illumination, but also when only visible light is used. At higher temperatures, the selectivity shifts towards CO production. To fundamentally understand the underlying mechanisms, we first perform DRIFT studies to elucidate the surface chemistry occurring on the Ru/TiO₂ nanostructure. Secondly, we highlight through time-resolved photoluminescence (PL) studies the role charge carriers play under 267 nm UV illumination (photoexciting predominantly the TiO₂ support) and under 532 nm visible light illumination (photoexciting predominantly the Ru particles) [4]. We propose a model underlying the physical and chemical nature of the photothermal methanization of CO₂ utilizing Ru/TiO₂ nanostructures. Finally, we will discuss the opportunities this work offers for future applications.