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.

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.

Both in design and production of nanoparticles and nanocomposites it is of vital importance to have information about their size and concentration. During the formation of nanoparticles, real-time monitoring of particle size and concentration during bottom-up synthesis in liquids allows for a detailed study of nucleation and growth. This provides valuable insights into the formation of nanoparticles that can be used for process optimization and scale up. In the production of nanoparticles, real-time monitoring enables intervention to minimize the number of off-spec batches. In this paper we will qualify an ultrasound nanoparticle sizer (UNPS) as a real-time monitor for the growth of nanoparticles (or sub-micro particles) in the 100 nm-1 μm range. Nanoparticles affect the speed and attenuation of ultrasonic waves in the dispersion. The size of the change depends, amongst other things, on the size and concentration of the nanoparticles. This dependency is used in the UNPS method. The qualification of the UNPS was undertaken in two successful experiments. The first experiment consisted of static measurements on commercially available silica particles, and the second experiment was real-time monitoring of the size and concentration during the growth of silica nanoparticles in Stöber synthesis in a water-alcohol mixture starting from the molecular precursor tetraethyl orthosilicate. The results of the UNPS were verified by measurements of a dynamic light scattering device and a transmission electron microscope.

Nanoparticles are applied in an increasing number of products (e.g. medicine, food and cosmetics). There is a need to real-time monitor the growth of nanoparticles during production inside the chemical reactor. However, standard particle sizing methods such as dynamic light scattering (DLS) and transmission electron microscopy (TEM) cannot do this. A suitable candidate may be ultrasonic transmission spectroscopy (UTS). Here, the sound speed and attenuation are measured of a dispersion and a physics based model is inverted and fitted to the measurements. The measurements have to be done with great accuracy: e.g. one should be able to measure frequency dependent variations in the speed of sound smaller than 0.05%. The measurements also have to be carried out fast enough compared to the time scale of the reaction. Moreover, the inversion has to be able to deal with the changing properties of the solvent during particle formation. Here, we report on the new ultrasound nanoparticle sizer (UNPS) that has these abilities and therefore could successfully monitor the size and concentration growth of nanoparticles during a reaction inside a reactor in real-time.

Au and Ag nanoshells are of interest for a wide range of applications. The plasmon resonance of such nanoshells is the property of interest and can be tuned in a broad spectral regime, ranging from the ultraviolet to the mid-infrared. To date, a large number of manuscripts have been published on the optics of such nanoshells. Few of these, however, address the effect of particle size distribution and metal shell imperfections on the plasmon resonance. Both are inherent to the chemical synthesis of metal nanoshells and therefore to a large extent unavoidable. It is of vital importance to understand their effect on the plasmon resonance, since this determines the scope and limitations of the technology and may have a direct impact on the application of such particles. Here, we elucidate the effect of particle size distribution and imperfections in the metal shell on the plasmon resonance of Au and Ag nanoshells. The size of the polystyrene core and the thickness of the Au and Ag shells are systematically varied to study their influence on the plasmon resonance, and the results are compared to values obtained through optical simulations using extended Mie theory and finite element method. Discrepancies between theory and practice are studied in detail and discussed extensively. Quantitative information on the minimum thickness of the metal shell, which is required to realize a satisfactory plasmon resonance of a metal nanoshell, is provided for Au and Ag.