Abstract Different approaches to combine electrochemical peroxydisulfate production with efficient leaching of cathode material from lithium-ion batteries are demonstrated. Leaching of LCO, NCM, and black mass was investigated with well-established batch leaching as well as less common semi-continuous leaching. For this purpose, peroxydisulfate is converted to peroxymonosulfate and hydrogen peroxide via temperature-controlled hydrolysis reactions to improve the leaching outcome. Each leaching method is matched with a suitable peroxydisulfate production method (multi-pass or single-pass) and a heat conversion step. The conversion of peroxydisulfate to a mixture of peroxymonosulfate and/or hydrogen peroxide at 65 °C will enhance the leaching of LCO and NCM significantly for batch (s/l = 50 g L–1) and semi-continuous leaching. While batch leaching of NCM results in complete conversion of all metals after 1 h, semi-continuous leaching yields 30% of the transition metals and 50% of lithium with the potential for complete conversion by further addition of reagents, whereas using black mass (s/l = 200 g L–1) results in complete leaching of all metals with both approaches without applying an external heating.

In this study, anthraquinone derivatives were explored as cocatalysts on titanium dioxide nanoparticles (P25) for the photocatalytic production of hydrogen peroxide from oxygen. The screening of anthraquinone derivatives revealed that 1-hydroxyanthraquinone is the most effective cocatalyst on P25 and increases the reaction rate greatly, by a factor of 30. In addition, the impact of various reaction parameters on the kinetics of hydrogen peroxide formation such as light intensity, wavelength, the amount of cocatalyst on P25, oxygen flux and the pH value, were investigated. Using this data allowed optimization of the reaction with respect to high reaction rate (1.9 mM min -1 ), quantum yield (26.7%) or turnover number of the AQ co-catalyst (2300). In particular, the extremely high formation rate represents a new benchmark for this reaction.

Peroxides such as peroxydisulfate (PDS), peroxymonosulfate (PMS) and hydrogen peroxide (H 2 O 2 ) are important bulk oxidants used in wastewater treatment, bleaching or leaching. In many of these processes and during electrosynthesis, they often coexist in complex mixtures. Selective quantification of each peroxide constitutes a fundamental step towards understanding and optimizing these processes. However, achieving selectivity poses a major analytical challenge since most quantification methods rely on the oxidative properties towards indicators and interference through coexisting peroxides is common. In this study, the different iodide oxidation kinetics of PDS, PMS, and H 2 O 2 were utilized to differentiate between them. Time-resolved spectrophotometry at 350 nm under optimized assay conditions was established as a prerequisite for achieving sufficient selectivity and reliable quantification. Employing low iodide concentrations and short reaction times, enable selective PMS quantification. By adding molybdate catalyst, the determination of H 2 O 2 is realized. Finally, the PDS concentration is derived from the total peroxide content obtained under optimized assay conditions by subtracting the H 2 O 2 and PMS concentration. The assay sensitivities reach 7.62 × 10 3 M −1 (PMS), 7.68 × 10 3 M −1 (H 2 O 2 ) and 4.1 × 10 3 M −1 (PDS) while the limit of quantification is as low as 6.6 µM (PMS), 3.9 µM (H 2 O 2 ) and 17 µM (PDS). The degree of interference caused by coexisting peroxides was thorougly investigated and the proposed quantification procedure was successfully applied to real electrolysis samples. The simplicity and reliability of the developed method enables high-throughput screening of electrodes, electrolytes or reaction conditions and allows quality control of processes.

TiO2-based photocatalytic coatings on substrates often require thermal annealing to achieve sufficient long-term stability and resistance to abrasion. This is difficult to achieve when using polymer substrates and/or other components in the system which are not stable enough for high temperature annealing. Herein, we present a method for annealing in those cases which relies on localized heating using a laser. This allows the coating the thermally anneal while at the same time protecting the substrate from higher temperatures. This method was then successfully adopted for more complex geometries, namely (half-)spherical substrates used for the so-called wireless light emitter technology.

Delocalized internal irradiation is a promising technique for intensification and scale-up of photochemical processes as it avoids the otherwise inevitable light concentration. In particular, wireless light emitters (WLEs) powered by resonant inductive coupling showed promise in previous studies. However, the achieved photon flux was still relatively. Herein, we show that through systematic optimization, the WLE technique can reach extremely high photon flux densities and energy efficiency, surpassing conventional photochemical reaction systems. Moreover, the intensification potential is demonstrated on the basis of a case study: the photocatalytic reduction of nitrobenzene in isopropanol using a TiO2 photocatalyst. The results also reveal a significant advantage of the WLE system. Due to the decentralized light emitters, light concentration is minimized which prevents or reduces efficiency losses due to kinetic limitations at high light intensity.

This study demonstrates the successful fabrication of 3D-printed, photoactive reactor components utilizing a TiO2-polypropylene composite filament. The reactor components were employed in the photocatalytic reduction of nitrobenzene and the photocatalyzed synthesis of quinaldine. The reactor inserts were subjected to hydrodynamic and reactive characterization. Furthermore, the viability of employing a solid acid as an alternative to a homogeneous acid catalyst was demonstrated for the photocatalytic synthesis of quinaldine. It was determined that the immobilization of the solid acid not only affects the activity but also the selectivity of the reaction. The immobilization of catalysts obviates the necessity for downstream separation and allows for the modification of reactor designs to achieve optimal reaction performance. The selected rapid prototyping approach facilitated the acceleration of development cycles, and the use of multiple parts comprising different chemically active components enabled the tailoring of (photo)chemical reactors, ultimately paving the way for the development of multi-functional reactors for cascade reactions with high performance and selectivities.

In recent years, the electrochemical synthesis of peroxides has attracted renewed interest as a potential environmentally friendly production compared to the established anthraquinone process. In addition, it is possible to produce the peroxides directly on site, eliminating the need for expensive and hazardous transportation and storage. Cathodic production of hydrogen peroxide from oxygen is already quite well developed. Anodic production from water, on the other hand, is still facing significant challenges, despite its historic pioneering role. In this manuscript we show that anodic and cathodic synthesis of peroxides can even be combined to achieve greater than 100% current efficiency (CE) due to the combined effect of both half-reactions. So far, similar devices have always employed different electrolytes for each, which necessitated the use of a membrane and posed contamination risk. However, herein we show that both half-reactions can also employ the same electrolyte. This enables even an undivided cell, omitting the need for the expensive membranes. Despite its simplicity, this setup yielded an outstanding performance with a combined CE of 144%.

Access to clean water is one of the UN’s strategic development goals. Though often seen to target mostly the developing counties or rural regions with low infrastructure, this is an important topic even in modern industrialized countries. Here, drinking water is certainly clean by most standards, yet it is often still contaminated with micropollutants with known adverse effects even at minute concentrations which are difficult to mitigate with conventional technology. Promising technologies to tackle this challenge include electrochemical or photoelectrochemical wastewater treatment, which can effectively completely mineralize organic pollutants using in-situ generated reactive oxygen species (ROS) like hydroxyl radicals. Photoelectrochemical wastewater treatment is an emerging technology for the removal of organic and inorganic pollutants from wastewater using sunlight as an additional or sole energy source and thereby saving on electricity costs. Since the water quality should not be subject to the intermittency of solar energy, we employ an additional conventional “dark” electrode which will operate with supplied external current in the absence of sufficient light energy. In particular, we use a BDD (boron-doped diamond) as anode in combination with a bismuth vanadate (BiVO4) based photoanode. In case of light illumination, this photoanode generates a photovoltage and current on its own without an external power supply. In both cases, ROS like hydroxyl radicals and hydrogen peroxide are formed from water oxidation. These two anodes are used in conjunction with a carbon-based gas diffusion electrode (GDE) as cathode. Here, atmospheric oxygen is reduced to form hydrogen peroxide also serving as oxidant. In addition to the described reactions, the electrolyte ions may also play an important role. These may form other peroxo-species at the electrodes themselves or indirectly via hydrogen peroxide and hydroxyl radicals. Recently, we could show that bicarbonate in particular plays a very important role, forming the highly potent peroxomonocarbonate which was shown about 200 times as active as hydrogen peroxide itself.[1] Overall, we show that the photoelectrochemical process can efficiently remove a wide range of recalcitrant pollutants from wastewater, including organic dyes, pesticides and pharmaceuticals. It has the advantage of being a sustainable and energy-efficient technology, as it uses sunlight as the primary energy source, meaning that a smaller amount of electricity needs to be fed in. We will further discuss the influence of the reaction parameters (light intensity, voltage, current density, pollutant concentration, flow rate) as well as the role the supporting electrolyte plays. [1] Schanz, T.; Burek, B. O.; Bloh, J. Z. ACS Energy Lett. 2023, 8, 1463–1467. Figure 1

Anodic electrochemical peroxide generation is promising for AOP applications. This study evaluates the electrochemical efficiency and oxidizing power of the resulting peroxide mixtures, depending on the electrolyte composition.

Nitrates are essential compounds for the chemical industry and in particular fertilizer production and are therefore critical for our global food production. They are nowadays almost exclusively produced in centralized plants by burning ammonia, which presents a major energetic detour in which the majority of ammonia’s high energy content is lost as heat. It would be much more efficient to synthesize nitrates directly from the elements (i.e., air) instead. It has been shown already that air can be oxidized using heterogenous photocatalysis in aqueous suspensions to yield nitrate. However, the low nitrogen solubility and the fact that nitrate will also be available as electron scavenger (back-reaction) limit the overall efficiency and performance. We have recently demonstrated that this reaction is also possible in the gas phase. [1] Air can be directly oxidized over illuminated TiO2 surfaces to nitrogen oxides, predominantly NO2, which can then subsequently be adsorbed into water to form nitric acid. [1] This approach circumvents the limitations of the aqueous phase as nitrogen and water content can be more freely controlled and nitrates are only formed on the subsequent step, suppressing the back-reaction. Overall, this presents and interesting new synthesis approach to nitrates in a decentralized manner, which could then be the basis for sustainable (solar) fertilizers which are produced directly in the areas in which they are needed. [2] This circumvents the energetic retour via ammonia and also saves on storage and transportation costs and emissions. In this talk we will present results of our first studies on this reaction, in particular which reaction parameters (such as humidity and light intensity) are critical for the reaction and what influence they have. This reveals first insights into possible reaction mechanisms which will then be discussed. Based on our screening experiments, we will also show which catalyst materials and potential co-catalysts are suited and discuss why that may be the case, further elaborating about the reaction mechanism and potential active sites. This information is then used to predict ideal catalyst materials in a knowledge-based catalyst design approach. References: [1] Pashkova, A.; Burek, B. O.; Bloh, J. Z. Sustainable Nitrate Production out of Thin Air: The Photocatalytic Oxidation of Molecular Nitrogen. Catal. Sci. Technol. 2022 , 12 (9), 2755–2760, http://dx.doi.org/10.1039/D2CY00350C [2] Comer, B. M.; Fuentes, P.; Dimkpa, C. O.; Liu, Y. H.; Fernandez, C. A.; Arora, P.; Realff, M.; Singh, U.; Hatzell, M. C.; Medford, A. J. Prospects and Challenges for Solar Fertilizers. Joule 2019 , 3 (7), 1578–1605, http://dx.doi.org/10.1016/j.joule.2019.05.001

Hydrogen peroxide is a powerful and green oxidant that allows for the oxidation of a wide span of organic and inorganic substrates in liquid media under mild reaction conditions, and forms only molecular water and oxygen as end products. Hydrogen peroxide is therefore used in a wide range of applications, for which the well-documented and established anthraquinone autoxidation process is by far the dominating production method at the industrial scale. As this method is highly energy consuming and environmentally costly, the search for more sustainable synthesis methods is of high interest. To this end, the article reviews the basis and the recent development of the photocatalytic synthesis of hydrogen peroxide. Different oxygen reduction and water oxidation mechanisms are discussed, as well as several kinetic models, and the influence of the main key reaction parameters is itemized. A large range of photocatalytic materials is reviewed, with emphasis on titania-based photocatalysts and on high-prospect graphitic carbon nitride-based systems that take advantage of advanced bulk and surface synthetic approaches. Strategies for enhancing the performances of solar-driven photocatalysts are reported, and the search for new, alternative, photocatalytic materials is detailed. Finally, the promise of in situ photocatalytic synthesis of hydrogen peroxide for water treatment and organic synthesis is described, as well as its coupling with enzymes and the direct in situ synthesis of other technical peroxides.

For the electrochemical and photoelectrochemical synthesis of hydrogen peroxide, aqueous bicarbonate electrolytes have been reported with much higher efficiency compared to other alternatives. It was proposed that this is due to efficient oxidation of the bicarbonate to peroxymonocarbonate (PMC) with subsequently hydrolyzes to hydrogen peroxide. However, as we show herein, PMC forms stable concentrations and does not hydrolyze completely. Due to its much better oxidation kinetics this may influence the sensitivity of the employed peroxide quantification methods. Particularly commercial test strips are susceptible to this, giving rise to uncertainty about the validity of the high peroxide formation efficiency in bicarbonate electrolytes. On the other side, PMC’s superior oxidation kinetics may mean that the resulting solutions are even more potent oxidants than previously suspected.

Unspecific peroxygenases have attracted interest in synthetic chemistry, especially for the oxidative activation of C-H bonds, as they only require hydrogen peroxide (H2O2) instead of a cofactor. Due to their instability in even small amounts of H2O2, different strategies like enzyme immobilization or in situ H2O2 production have been developed to improve the stability of these enzymes. While most strategies have been studied separately, a combination of photocatalysis with immobilized enzymes was only recently reported. To show the advantages and limiting factors of immobilized enzyme in a photobiocatalytic reaction, a comparison is made between free and immobilized enzymes. Adjustment of critical parameters such as (i) enzyme and substrate concentration, (ii) illumination wavelength and (iii) light intensity results in significantly increased enzyme stabilities of the immobilized variant. Moreover, under optimized conditions a turnover number of 334,500 was reached.

Surface modification of heterogeneous photocatalysts with single-atom catalysts (SACs) represents an attractive approach towards enhancing the photocatalytic performance. However, our knowledge on the mechanism of photocatalysis enhancement at SAC-modified photocatalysts is still rather limited, which makes the rational design of high-performance photocatalysts based on SACs challenging. Herein, a series of photocatalysts for aerobic degradation of pollutants based on anatase TiO2 modified with various low-cost, non-noble SACs (vanadate, Cu and Fe ions) is reported. The most active SAC-modified photocatalysts outperform not only TiO2 modified with corresponding metal oxide nanoparticles, but also the state-of-the-art benchmark photocatalysts, such as platinized TiO2 or commercial P25 powders. A combination of in-situ EPR spectroscopy and theoretical calculations revealed that the best-performing photocatalysts modified with Cu(II) and vanadate SACs exhibit significant differences in the mechanism of activity enhancement, in particular with respect to the rate of catalysis of oxygen reduction. The superior performance of vanadate SAC-modified TiO2 is found to be related to the shallow character of the SAC-induced intragap states, which allows for both effective extraction of photogenerated electrons and fast catalytic turnover in reduction of dioxygen, and translates directly into diminished recombination. These results provide essential design guidelines for the development of efficient SAC-based photocatalysts.

One of the key urban air quality issues is pollution by nitrogen oxides (NOx). To reduce NOx, facade cladding could be provided with photocatalytic properties by incorporating titanium dioxide nanoparticles. For this purpose, a modified phosphoric acid anodizing process (MPAA) was developed for the facade alloy EN AW-5005, in which highly ordered anodized structures with a low degree of arborization and tortuosity were produced. Pore widths between 70 nm and 150 nm and layer thicknesses of about 2–3 μm were obtained. The subsequent impregnation was carried out by dip coating from water-based systems. Depending on the dip-coating parameters and the suspension used, the pores can be filled up to 60% with the TiO2 nanoparticles. Photocatalytic tests according to ISO 22197-1 certify a high photocatalytic activity was obtained with rPCE values > 8 and with rPCE > 2, achieving “photocatalytically active for air purification”. Tests on the corrosion resistance of the anodized coatings with a commercially available aluminum and facade cleaner confirm a protective effect of the anodized coatings when compared with nonanodized aluminum material, as well as with compacted anodized layers.

In our study, we aimed to show how different reaction parameters can affect production rates using photocatalytic hydrogen peroxide formation by different polymeric carbon nitrides (PCN). For this purpose, selected materials were first compared under the same reaction conditions and compared with TiO2 (P25). We also show that different light intensities can have a different influence on seemingly similar materials. Since hydrogen peroxide production in the presence of an electron donor proceeds mainly by reduction of oxygen, we also show an influence of the oxygen flow on the formation rates. Thus, with high oxygen fluxes and high intensities of irradiated light, we were able to achieve an H2O2 concentration of 125 mM after about 25 h. Finally, the two best PCN materials were selected to measure light intensity dependence at different wavelengths up to visible light. It was found that they behaved differently at the different wavelengths and thus it could be shown that an exact specification of the reaction parameters is indispensable for comparisons in the literature. Our study uses the photocatalytic production of hydrogen peroxide by various polymeric carbon nitride materials to demonstrate the importance of precisely adjusting various reaction conditions in the reactor, such as light intensity, oxygen flow, and wavelength. In addition, reaction parameters were chosen to achieve extremely high hydrogen peroxide concentrations.image

Carbon−carbon bond formation by dual Iridium‐Nickel photoredox catalysis has gained a lot of attention for late‐stage cross‐couplings of alkyls with aryls. However, the scalability of such reactions is greatly impeded by the poor light penetration depth into the strongly absorbing reaction media. Wireless internal illumination is a novel technique which circumvents the light penetration issue efficiently and therefore allows to scale photon‐driven reactions by conventional methods. Here we demonstrate that industrially relevant photoredox C−C couplings can be scaled seamlessly to different volumes, achieving qualitatively and quantitatively the same results at all scales. The use of conventional reactor types allows stirred, bubble column or fixed‐bed operation mode and is fully compatible with heterogeneous reaction mixtures. By minimizing reflection losses, we could also show a significant advantage in the reaction rate over an externally illuminated setup at the same light intensity.

The Front Cover shows free-moving wirelessly-powered light emitters, which can be used to internally illuminate photochemical reactions. This enables easier scalability of reactions like the illustrated photoredox C–C coupling via Ir/Ni dual catalysis. More information can be found in the Communication by B. O. Burek, K. Lovis, J. Z. Bloh et al.

Novel processes for the sustainable production of fertilizers are highly sought after to combat climate change. Herein, we demonstrate that by irradiating with strong UVA-light, TiO2 is able to photocatalytically oxidize molecular nitrogen in the gas phase under ambient conditions to NOx and nitrate. The reaction produces predominantly nitrogen dioxide with a high selectivity of up to 93% which could be captured afterwards to produce nitric acid or nitrates and used as sustainable (solar) fertilizer.

To describe and predict the leaching of Ag, Cu and Sn from waste photovoltaic modules with an electrochemical-assisted process kinetic investigations were performed. In this process, peroxydisulfate is generated from sulfuric acid to oxidize metals. It was found that under the reaction conditions peroxymonosulfate is formed as well and has a major contribution to the leaching process. For Ag, autocatalytic decomposition of the leaching reagents is determined to be a limiting step while for Sn passivation influences the process. The leaching is modeled for three different reaction types, a batch reaction, the reaction in a static H-cell with continuous generation of S₂O₈2− and a fed-batch reaction with an electrochemical flow cell for the production of peroxydisulfate.