Driving Surface Redox Reactions in Heterogeneous Photocatalysis:The Active State of Illuminated Semiconductor-Supported Nanoparticles during Overall Water-Splitting

Materials used for photocatalytic overall water splitting (POWS) are typically composed of light-absorbing semiconductor crystals, functionalized with so-called cocatalytic nanoparticles to improve the kinetics of the hydrogen and/or oxygen evolution reactions. While function, quantity, and protection of such metal(oxide) nanoparticles have been addressed in the literature of photocatalysis, the stability and transients in the active oxidation-state upon illumination have received relatively little attention. In this Perspective, the latest insights in the active state of frequently applied cocatalysts systems, including Pt, Rh/Cr2O3, or Ni/NiOx, will be presented. While the initial morphology and oxidation state of such nanoparticles is a strong function of the applied preparation procedure, significant changes in these properties can occur during water splitting. We discuss these changes in relation to the nature of the cocatalyst/semiconductor interface. We also show how know-how of other disciplines such as heterogeneous catalysis or electro-catalysis and recent advances in analytical methodology can help to determine the active state of cocatalytic nanoparticles in photocatalytic applications.</p

Preface

Click on the DOI link to access the article (may not be free).Energy and environmental issues are of great concerns for the public and will keep increasing in the next few decades. The demand for clean energy sources in our current society also increases with large-scale economic developments and population growth. It is crucial to build clean energy systems

CuBO2:A Potential Alternative for NiO as a Hole Acceptor Layer

P-type metal oxides, and in particular NiO, are typically used as hole accepting layers in dye-sensitized photocathodes. Delafossites (CuMO 2) with M=B, Al, Cr or Ga have recently been proposed as attractive substitutes for NiO, with theoretically a higher hole mobility than NiO, therefore allowing a higher efficiency when the photocathode is applied in solar to fuel devices. We have experimentally validated the photoelectrochemical performance of photocathodes consisting of nanoporous CuBO 2 (CBO) on Fluorine-doped Tin Oxide substrates, photosensitized with a light absorbing P1 dye. Femtosecond transient absorption and time-resolved photoluminescence studies show that light-induced hole injection occurs from the P1 dye into the CBO in a few ps, comparable to the time constant observed for NiO-based photocathodes. Importantly, the CBO-based photocathode shows significantly slower charge recombination than the NiO-based analogue. These results illustrate the promise of CBO as a p-type semiconductor in solar energy conversion devices.</p

The Influence of Alkali Cations on the Performance of Pt Electrodes in Kolbe Electrolysis of Acetic Acid

Electrochemical decarboxylation of carboxylic acids is considered a sustainable method to improve the quality of pyrolysis oil. In this study, we assess the effect of monovalent alkali cations (of the acetates) on the performance of Pt electrodes in acid decarboxylation and the competing OER, using various electrochemical methods. We reveal a strong cation dependence generally following the trend Li +&lt;Na +&lt;K +~Cs + within a large pH range. Using rotating ring disc electrode measurements, we highlight the strong contribution of the oxygen evolution reaction particularly for electrolytes containing Li + and Na + which decreases the selectivity for Kolbe oxidation. In addition, the faradaic efficiency (FE) towards methanol ranges between 16 % (for Li +) and 29 % (for Cs +) at high solution pH (9 or 12). The observed trends are generally explained by a cation-dependent interfacial pH and surface coverage of acetate, both lowest for Li. This is evident from differences in charge transfer resistance determined by impedance measurements and local pH measurements. Additionally, enhanced dissolution of Pt by Li + is also proposed. This work highlights that K + and Cs + cations favor FE for electrochemical Kolbe oxidation, at relatively low current densities.</p

Performance of Transition Metals in Imidazolium-Assisted CO₂ Reduction in Acetonitrile

This study explores the electrochemical reduction of CO2 in dry acetonitrile containing 1,3-dimethyl imidazolium cations, utilizing late-transition metals (Au, Ag, Zn, Cu, and Ni). All metals exhibit remarkable selectivity, nearing 100 %, for CO formation. Particularly noteworthy is Au, which manifests the lowest (−2.37 V vs. Ag/Ag+) overpotential in chronopotentiometry experiments. We propose that, for metals with lower CO binding energies compared to Au (Ag and Zn electrodes) – calculated by DFT, the rate-determining step is the adsorption of CO2. This distinction in CO2 adsorption is reinforced by the examination of partial charge transfer from negatively charged slabs to CO2 (−0.241 a.u with the Au electrode and +0.002 a.u with the Zn electrode). Conversely, the greater CO binding energy calculated for Cu and Ni likely diminishes electrocatalytic activity relative to the Au electrode. Our results unveil a volcano trend in catalyst activity, albeit with smaller performance disparities between the late-transition metals and Au than previously observed in aqueous conditions, possibly due to the co-catalytic influence of imidazolium cations. This study suggests that metals unsuitable for aqueous environments hold promise for cost-effective and viable electrochemical conversion of CO2 to CO in non-aqueous media containing imidazolium compounds.</p

The influence of water vapour on the photocatalytic oxidation of cyclohexane in an internally illuminated monolith reactor

This paper discusses effects of humidity on photocatalytic cyclohexane oxidation performed in an internally illuminated monolith reactor equipped with an immobilised layer of approximately 3 μm titania (Hombikat uv100). Using dry nitrogen containing 10–20% of oxygen, cyclohexanone is produced with high selectivity (>90%) over cyclohexanol. The photocatalytic monolith deactivates within 80 min of operation. Regeneration of activity of such deactivated monolith is possible by air treatment at 450 °C. When the applied nitrogen/oxygen gas is humidified, stable ketone production rates are obtained around 5 × 10−6 mol h−1 at an optimised relative humidity of 65%. Ketone over alcohol selectivity is lower in humidified conditions, the ratio of the cyclohexanol/cyclohexanone production rates increases from 0.4 to 1.0 as a function of increasing humidity from 30% to 90%. Rapid transients in water vapour content lead to relatively slow changes in concentration of in situ produced cyclohexanone and cyclohexanol. The observation of these changes is used to explain the effect of humidity on reactor performance

Photophysical Study on the Effect of the External Potential on NiO-Based Photocathodes

In the present study, we investigate the effects of the applied external potential on a dye-sensitized NiO photocathode by time-resolved photoluminescence and femtosecond transient absorption spectroscopy under operating conditions. Instead of the anticipated acceleration of photoinduced hole injection from dye into NiO at a more negative applied potential, we observe that both hole injection and charge recombination are slowed down. We cautiously assign this effect to a variation in OH– ion concentration in the inner Helmholtz plane of the electrochemical double layer with applied potential, warranting further investigation for the realization of efficient solar fuel devices