Addressing the Reproducibility of Photocatalytic Carbon Dioxide Reduction

Abstract

Reproducibility of photocatalytic reactions, especially when conducted on small scale for improved turnover numbers with in situ formed catalysts can prove challenging. Herein, we showcase the problematic reproducibility on the example of attractive photocatalytic CO2 reduction utilizing [FeFe] hydrogenase mimics. These Fe complexes, well‐known for their application in proton reduction reactions, were combined with a heteroleptic Cu photosensitizer and produced CO/H2/HCO2H mixtures of variable constitution. However, the reactions indicated a poor reproducibility, even when conducted with well‐defined complexes. Based on our experience, we make suggestions for scientists working in the field of photocatalysis on how to address and report the reproducibility of novel photocatalytic reaction protocols. In addition, we would like to highlight the importance of studying reproducibility of novel reaction protocols, especially in the fields of photocatalytic water splitting and CO2 reduction, where TONs are widely used as the comparable measure for catalytic activity.

Highly Functional Dinuclear CuI-Complex Photosensitizers for Photocatalytic CO2Reduction

“…However, the weak bonding character of the ligand coordination to the d 10 Cu + center often causes inefficient photosensitizing abilities, especially in coordinating solvents, such as CH 3 CN, dimethylformamide (DMF), and water, which are often used as solvents for photocatalytic reactions. Even at room temperature, the detachment of the ligands proceeds from these Cu I diimine bisphosphine complexes in solutions …”

Exploring the Full Potential of Photocatalytic Carbon Dioxide Reduction Using a Dinuclear Re2Cl2 Complex Assisted by Various Photosensitizers

“…[34] Intermolecular photosensitization using environmentally benign photosensitizer (PS), for example, [Cu(xant)(bcp)] + (Cu, see also Scheme 1) has only recently been exploited in the photocatalytic CO 2 to CO reduction. [35][36][37][38][39][40] Exemplarily, Cu in combination with Re(bpy)(CO) 3 Br as catalyst [37] increased the catalytic activity (TON CO ) by a factor of~6 from 40 to 240 after 1 h illumination with hν > 370 nm (optimized conditions:…”

Metal–Organic Framework Catalysts for Solar Fuels: Light-Driven Conversion of Carbon Dioxide into Formic Acid

“…As mentioned in the Introduction, the lack of precise information about the reactor design, the light source used (position from the reactor, constant radiant flux, or irradiance), and the reproducibility of photocatalytic system remains a major challenge for comparing the activity in photochemical CO 2 reduction reaction (CRR) and thus for designing better performing catalysts. As the same problems are also reported in homogeneous photochemical CRR, also the recommendation made by Beller and co-workers for homogeneous catalysis should be considered for scientist working on heterogeneous photochemical CRR alongside recommendations for classical heterogeneous photocatalysis . Beside the detailed description of the reactor setup and the light source, one should consider the following points to ensure the accurate analysis of the catalytic performances obtained: Stating the number of independent experiments conducted. Performing at least two independent measurements on independent samples. Reporting of increased catalytic activity by reducing the catalyst loading and/or solid loading should prerequisitely insure that the reproducibility is also given at the lowest, and thus optimum, catalyst/solid loading. Reporting quantum efficiencies for catalyst loadings within the catalyst loading of the so-called “optimal reaction rate”.…”

“…Indeed the comparison of the catalytic activity per gram of solid material has been stated to allow for direct comparison between different systems and reports . However, as for homogeneous catalysis, , often precise information about the reactor design and the light source (position from the reactor, constant radiant flux, or irradiance) are not reported, making the direct comparison of turnover numbers/frequencies or quantum efficiency, but also productivities between different research laboratories, challenging. Also, the reproducibility of photocatalytic experiments remains scarce, adding another challenge for reproducing and comparison of data.…”