Heptazine‐based polymeric carbon nitrides (PCN) are promising photocatalysts for light‐driven redox transformations. However, their activity is hampered by low surface area resulting in low concentration of accessible active sites. Herein, we report a bottom‐up preparation of PCN nanoparticles with a narrow size distribution (ca. 10±3 nm), which are fully soluble in water showing no gelation or precipitation over several months. They allow photocatalysis to be carried out under quasi‐homogeneous conditions. The superior performance of water‐soluble PCN, compared to conventional solid PCN, is shown in photocatalytic H2O2 production via reduction of oxygen accompanied by highly selective photooxidation of 4‐methoxybenzyl alcohol and benzyl alcohol or lignocellulose‐derived feedstock (ethanol, glycerol, glucose). The dissolved photocatalyst can be easily recovered and re‐dissolved by simple modulation of the ionic strength of the medium, without any loss of activity and selectivity.
Improved understanding of the fundamental processes leading to degradation of platinum nanoparticle electrocatalysts is essential to the continued advancement of their catalytic activity and stability. To this end, the oxidation of platinum nanoparticles is simulated using a ReaxFF reactive force field within a grand-canonical Monte Carlo scheme. 2-4 nm cuboctahedral particles serve as model systems, for which electrochemical potential-dependent phase diagrams are constructed from the thermodynamically most stable oxide structures, including solvation and thermochemical contributions. Calculations in this study suggest that surface oxide structures should become thermodynamically stable at voltages around 0.80-0.85 V versus standard hydrogen electrode, which corresponds to typical fuel cell operating conditions. The potential presence of a surface oxide during catalysis is usually not accounted for in theoretical studies of Pt electrocatalysts. Beyond 1.1 V, fragmentation of the catalyst particles into [Pt 6 O 8 ] 4− clusters is observed. Density functional theory calculations confirm that [Pt 6 O 8 ] 4− is indeed stable and hydrophilic. These results suggest that the formation of [Pt 6 O 8 ] 4− may play an important role in platinum catalyst degradation as well as the electromotoric transport of Pt 2+/4+ ions in fuel cells.
Combining cationic ruthenium photosensitizers (RuPS) with anionic polyoxometalate water oxidation catalysts (POM‐WOCs) is the standard approach for light‐driven POM‐based water oxidation. Here, we show that colloid formation by electrostatic aggregation of a molecular photosensitizer {e.g., [Ru(bpy)3]2+ (bpy = 2,2′‐bipyridine)} and a POM‐WOC {e.g., [Co4(H2O)2(α‐PW9O34)2]10–} significantly affects catalytically relevant system parameters. A facile, quantitative procedure for colloid detection using syringe filtration and UV/Vis spectroscopy is presented, and we illustrate that photosensitizer‐POM colloid formation is a general phenomenon under typical WOC conditions and is observed for a range of photosensitizers and POMs. It is further demonstrated that for some systems an increase in the ionic strength of the solution prevents colloid formation. Significant changes in the electronic interactions between RuPS and POM‐WOC under colloidal and homogeneous conditions are reported, thus highlighting the need for fast and reliable colloid identification. In addition, the study raises awareness about colloid formation in homogeneous solar‐energy conversion schemes driven by two or more ionic species.
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