Plasmonic CO₂ methanation using γ-Al₂O₃-supported Ru nanorods was carried out under continuous-flow conditions without conventional heating, using mildly concentrated sunlight as the sole and sustainable energy source (AM 1.5, irradiance 5.5–14.4 kW·m⁻² = 5.5–14.4 suns). Under 12.5 suns, a CO₂ conversion exceeding 97% was achieved with complete selectivity towards CH₄ and a stable production rate (261.9 mmol·g^{−1 Ru·h−1}) for at least 12 h. The CH₄ production rate showed an exponential increase with increasing light intensity, suggesting that the process was mainly promoted by photothermal heating. This was confirmed by the apparent activation energy of 64.3 kJ·mol⁻¹, which is very similar to the activation energy obtained for reference experiments in dark (67.3 kJ·mol⁻¹). The flow rate influence was studied under 14.4 suns, achieving a CH₄ production plateau of 264 µmol min⁻¹ (792 mmol·g^{−1 Ru·h−1}) with a constant catalyst bed temperature of approximately 204^◦C.

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Distinguishing between photothermal and non-thermal contributions is essential in plasmon catalysis. Use of a tailored optical temperature sensor based on fiber Bragg gratings enabled us to obtain an accurate temperature map of an illuminated plasmonic catalyst bed with high spatiotemporal resolution. Its importance for quantification of the photothermal and non-thermal contributions to plasmon catalysis is demonstrated using a Ru/Al₂O₃ catalyst. Upon illumination with LEDs, we measured temperature differences exceeding 50 °C in the top 0.5 mm of the catalyst bed. Furthermore, we discovered differences between the surface temperature and the temperature obtained via conventional thermocouple measurements underneath the catalyst bed exceeding 200 °C at 2.6 W cm⁻² light intensity. This demonstrates that accurate multi-point temperature measurements are a prerequisite for a correct interpretation of catalysis results of light-powered chemical reactions obtained with plasmonic catalysts.

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Sunlight-powered reduction of CO₂ to fuels and chemicals is a promising strategy to close the carbon loop and facilitate the energy transition. In this research, we demonstrate that Au nanoparticles supported on TiO₂ are an efficient plasmonic catalyst for the sunlight-powered reverse water-gas shift (rWGS) reaction. A maximum CO production rate of 429 mmol ⋅ g_Au⁻¹ ⋅ h⁻¹ with a selectivity of 98 % and an apparent quantum efficiency of 4.7 % were achieved using mildly concentrated sunlight (1.44 W ⋅ cm⁻² equals 14.4 sun). The CO production rate showed an exponential increase with increasing light intensity, suggesting that the process is mainly promoted by a photothermal effect. Thermal reference experiments with the same catalysts promoted CH₄ formation, dropping the CO selectivity to 70 %. Thus, mildly concentrated sunlight can efficiently and selectively enhance the promotion of the rWGS reaction without using external heating.

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The CO2 that comes from the use of fossil fuels accounts for about 65% of the global greenhouse gases emissions, and it plays a critical role in global climate changes. Among the different strategies that have been considered to address the storage and reutilization of CO2, the transformation of CO2 into chemicals or fuels with a high added-value has been considered a winning approach. This transformation is able to reduce the carbon emissions and induce a “fuel switching” that exploits renewable energy sources. The aim of this brief review is to gather and critically analyse the main efforts that have been made and achievements that have been reached in the electrochemical reduction of CO2 for the production of CO. The main focus is on the prospective of exploiting the intrinsic nature of the electrolysis process, in which CO2 reduction and H2 evolution reactions can be combined, into a competitive approach, to produce syngas. Several well-stablished processes already exist for the generation of fuels and fine-chemicals from H2/CO mixtures of different ratios. Hence, the different kinds of electrocatalysts and electrochemical reactors that have been used for the CO and H2 evolution reactions have been analysed, as well as the main factors that influence the performance of the system from the thermodynamic, kinetic and mass transport points of view.@en