A material-based study of hot carrier / heat synergy in plasmonic CO2 reduction with water in the gas phase – TOGETHER-FOR-CO2

CO2 photoreduction with water is an attractive photosynthesis-like reaction which could lead to the direct, selective, on-site recycling of CO2 industrial emission sources into synthetic natural gas (methane), provided that the reaction can be carried out selectively and in the continuous flow, gas phase. However, the competing reduction of water severely inhibits viability of this still hypothetic process. Metal oxide semi-conductor (MOS)-based photocatalysts in particular are inherently limited by both a low selectivity and by their poor stability with time-on-stream, which hinders application of these materials. Recently it was found that illumination of Au nanoparticles (NPs) in contact with a flowing mixture of CO2 and water vapor yielded methane selectively and steadily over extended period of times. This reaction was attributed to absorption of light by Au NPs in the visible range via their localized surface plasmon resonance . These promising plasmonic materials could be an interesting alternative to MOS for large scale recycling of CO2 into methane. However they so far exhibit a low activity and will thus need to be optimized. One major hurdle towards that goal is that no consensual description of the underlying mechanism has been reached to date, making any potential development of the field dependent on inefficient, time-consuming, trial and error strategies. In particular, the nature of the energy transfer controlling light-to-chemical conversion in the plasmon-induced reaction is highly debated. Both hot carriers and heat are generated in the process at very short time interval. Distinguishing their respective role in the process is a challenge.
TOGETHER-FOR-CO2 has the ambition to undertake a paradigm shift in the optimization of plasmonic catalysts and set-up a rational mechanistic-based optimization approach to plasmonic catalyst design, in order to take the development of the plasmon-induced continuous flow, gas phase CO2-to-CH4 reduction with water to the next level. In order to achieve that, TOGETHER-FOR-CO2 intends to understand the roles of both hot carriers and heat in the plasmon-induced reaction. TOGETHER-FOR-CO2 will use the fact that both phenomena are dependent on the intrinsic properties of the metals and on the geometric characteristics of plasmonic NPs assemblies to undertake a systematic, experimental, material-based study aimed at unraveling the respective roles of hot carriers and heat in the plasmon-induced CO2-to-CH4 reduction with water. Assuming that both phenomena likely contribute to the plasmon-induced catalytic performance, TOGETHER-FOR-CO2 will further optimize their synergy to boost plasmon-induced methane production rates. This requires (1) smart control of plasmonic substrates configuration to allow fine tuning of the hot carriers vs. heat phenomena (2) design of a unique photoreactor to evaluate plasmon-induced catalytic performances under strict temperature control (3) in-depth characterization and simulations of optical and photothermal properties. Hence, by combining expertise in (nano)material synthesis, (photo)catalysis and thermoplasmonics, the TOGETHER-FOR-CO2 team will (1) synthesize a large variety of 2D plasmonic substrates with well-defined configurations using well-controlled organometallic routes applicable to metal, alloy and oxide NP synthesis (2) implement a pioneering combination of temperature metrologies, including nanothermometry with lanthanide particles, to accurately measure the temperature of the working substrate (3) use photothermal characterization and simulation to validate the initial assumptions, (4) evaluate the impact of structural parameters of the plasmonic substrate and of temperature on the plasmon-induced catalytic performances, and ultimately (5) optimize plasmonic catalysts on the basis of structure-activity relationships, with special focus on combining high activity with both full selectivity towards methane and long-term stability.