Plasmon Photocatalysis for Sunlight-Powered Reduction of CO₂ to CO—The Impact of Photothermal Heating of the Catalyst Bed on Reactor and Process Design

To transition from the current fossil-based chemical industry to a climate neutral one, both the use of renewable energy and sustainable feedstock are essential. Here, we present the use of CO₂ as sustainable feedstock and sunlight as renewable energy source for the photocatalytic conversion of CO₂ to CO. Combined with green hydrogen, this provides access to renewable syngas, which is pivotal for the decarbonisation of industry.¹<br/>Various technologies are explored for CO₂ conversion to CO, including approaches driven by electrical or thermal energy. We selected photochemical conversion using sunlight, since it provides following advantages: (i) high energy efficiency with minimized conversion and transportation losses, (ii) high process selectivity with minimized need for energy and cost intensive downstream processing, (iii) ease of scaling up and down (numbering up) making a good fit with small, medium and large sized CO₂ sources, (iv) steep learning curves and fast cost reductions expected based on technology modularity, (v) decentralized and potentially off-grid production, and (vi) low carbon footprint for direct use of sunlight.²<br/>The catalyst we developed for sunlight-powered production of CO is plasmonic Au/TiO₂. This was prepared by deposition-precipitation of Au on anatase, and we achieved a Au loading of 3.12% w/w (ICP-AES).^3,4 The average size of the Au nanoparticles was 1.6 nm (lognormal distribution), and their (111) lattice planes were perfectly alligned with the (101) TiO₂ planes. The catalyst displayed a strong plasmonic absorption in the visible, with a maximum in the wavelength range between 500 nm and 600 nm. We demonstrated that under illumination with sunlight (irradiance up to 14.4 kW/m²), CO was produced at high selectivity of 98% without external heating of the reactor. In comparative dark experiments at similar catalyst bed temperature (~200^○C), only CH₄ was produced. Under optimized conditions, we achieved a CO production rate of 429 mmol/g_Auper hour, and reached an apparent quantum efficiency of 4.7%.³ When studying the CO production rate as function of solar irradiance, we obtained an exponential relationship indicating the presence of a photothermal contributor.^3,4 To quantitatively distinguish between photothermal heating and non-thermal contributors, we performed experiments using a tailored fiber Bragg grating-based fiber optic sensor (FBG-FOS) for <i>in operando</i> temperature monitoring. We previously demonstrated this concept for off-line measurements only.⁵The FBG-FOS enabled us to obtain temperature data during catalysis at various depths inside the catalyst bed: at the surface (0 mm), and at 0.15 mm, 0.30 mm and 0.50 mm depth. We observed substantial temperature gradients inside the catalyst bed during catalysis, with the highest temperature achieved at the surface of the catalyst bed. These temperature gradients are caused by the fact that more than 90% of the light was absorbed in the top 100 µm of the catalyst bed, as determined by UV-vis-NIR spectrophotometry. The temperature gradient remained intact due to the low thermal conductivity of the catalyst powder bed.<br/>To complete photochemical reaction systems for sunlight-powered chemical production, additional components such as reactors, solar concentrators and sensors for on-line process monitoring are required.⁶ We designed a complete system for sunlight-powered reduction of CO₂ to CO, and validated it using natural sunlight as energy source. We will particularly highlight the impact of photothermal heating on reactor and process design.<br/><br/>[1] <i>ChemSusChem</i> <b>2024</b>, e202400059.<br/>[2] <i>Sustainable Energy Technologies and Assessments</i><b> </b><b>2024</b>,<i> 65</i>, 103768.<br/>[3] <i>ChemCatChem</i> <b>2021</b>, <i>13</i>, 4507.<br/>[4] <i>Chem. Asian J. </i><b>2023</b>, <i>18</i>, e202300405.<br/>[5] <i>ChemPhotoChem</i> <b>2022</b>, <i>6</i>, e202100289.<br/>[6] <i>ChemSusChem </i><b>2024</b>, <i>17</i>, e202301405.