Fiber Bragg based – fiber optic sensors were applied in operando to monitor the temperature of illuminated plasmonic catalysts at various depths inside the catalyst bed during light-driven CO2 hydrogenation. Multipoint temperature measurements showed that single-sided illumination induced a pronounced vertical temperature gradient, which remained stable throughout the reaction. This behaviour was observed in two light driven reactions: the exothermic Sabatier reaction catalysed by Ru/Al2O3 and the endothermic reverse water gas shift reaction catalysed by Au/TiO2. The temperature gradient, attributed to a combination of limited light penetration depth and poor thermal conductivity of the catalyst bed, must be taken into account in kinetic studies. Metal loading and gas composition had a strong influence on the temperature gradient, while gas flow rate and reaction heat had a negligible effect. For catalyst temperatures up to 250˚ C, radiative heat loss accounted for approximately 15 % of the incident light power. Our study demonstrates that accurate in operando temperature monitoring at multiple positions inside the catalyst bed is essential to distinguish between thermal and non-thermal contributors in plasmon catalysis.

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.

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.