Aldehydes and ketones are two of the most versatile functional groups in organic synthesis, and the development of new reagents and protocols to install them are always warranted. Recently, the reemergence of radical chemistry has provided new opportunities to introduce these important carbonyl motifs. As such, acetal radicals can be employed as synthetic radical equivalents for acyl radicals that can also circumvent known stability challenges. This review aims to summarize the advancements and known uses of acetal radicals, as well as explore acetal radical formation and synthetic applications.

Fuel cells have emerged as a promising clean energy technology with a great potential in various sectors, including transportation and power generation. However, the high cost and scarcity of the noble metals currently used to synthesise electrocatalysts for low-temperature fuel cells has hindered their widespread commercialisation. In recent decades, the development of non-precious metal electrocatalysts for the cathodic oxygen reduction reaction (ORR) have gained significant attention. Among those, electrocatalysts with atomically dispersed active sites, referred to as single-atom catalysts (SACs), are gaining more interest. Nanocarbon materials containing single transition metal atoms coordinated to nitrogen atoms are active electrocatalysts for the ORR in both acidic and alkaline conditions and thus have a great promise to be utilised as non-precious metal cathode electrocatalysts in low-temperature fuel cells. This review article provides an overview of the recent advancements in the utilisation of transition metal-based SACs in proton exchange membrane fuel cells (PEMFCs) and anion exchange membrane fuel cells (AEMFCs). We highlight the main strategies and synthetic approaches for tailoring the properties of SACs to enhance their ORR activity and durability. Based on the already achieved results, it is evident that SACs indeed could be suitable for the cathode of the low-temperature fuel cells.

Demand for technologies using water electrolysis to produce green hydrogen is increasing, although recycling research on membrane electrode assemblies, which contain various precious and highly critical metals, is still limited. This study therefore aims at exploiting the feasibility of fine particle separation processes based on the difference in hydrophobicity of the ultrafine materials used as catalysts in polymer electrolyte membrane electrolyzers and at providing a fundamental study with representative materials of carbon black and TiO2. Since the cathode materials including carbon black are hydrophobic and the anode materials as well as TiO2 are hydrophilic, the characterizations of their various surface properties such as zeta potentials, dispersion characteristics, and bubble coverage angle tests have been investigated. In addition, using liquid-liquid particle extraction in a mixture model, 99 % of carbon black is recovered in the organic phase and 97 % of TiO2 is selectively separated in the aqueous phase with the help of the dispersant, sodium hexametaphosphate.

We herein report the first use of thiosquaramides as polymerization catalysts, which are shown to be effective for the controlled ROP of lactide in the presence of an alcohol source and NEt3. Comparison of their catalytic performances with the less acidic squaramides are also discussed. The observed catalytic activity of variously N-substituted thiosquaramides suggest that a balanced NH Brønsted acidity is required for optimal performance. Most interestingly and rather unexpectedly, DFT-supported calculations on thiosquaramide-mediated lactide ROP catalysis suggest that secondary interactions between the thiosquaramide N-substituents and the incoming lactide (presently of type C-H…p-arene) are crucial for catalytic activity. Though this type of interactions is quite common in organo-catalysis, it has rarely been evidenced to play a key role in the area of organo-catalyzed polymerizations. Such catalyst substituents/substrate interactions may well play a significant role in the catalytic performances of various other systems.

Pd-decorated titania is a versatile heterogeneous photocatalyst capable of driving consecutive reactions by adjusting excitation conditions. Poisoning species generated in the first reaction step can be easily separated by alumina plugs, allowing only the alkyne product to reach the second reaction step and to undergo through the transfer hydrogenation path.

Abstract

Here, we discovered that Pd decorated TiO₂ (Pd@TiO₂) enables consecutive photocatalytic Sonogashira C−C coupling and hydrogenation steps by simply adjusting the excitation conditions of the reaction. We demonstrated that by-products containing iodine species generated in the first reaction step can inhibit subsequent photocatalytic processes, but they can be easily removed from solution to enable a compatible synthetic sequence for new C−C bond formation under mild reaction conditions. This work incorporates heterogeneous photocatalysts into consecutive transformations, promoting elegant reactions while meeting the demands of green chemistry.

Three interstitial structures of palladium which can form under reactive conditions, PdN_x, PdCx, and PdH_x, have been characterised using in situ X-ray absorption spectroscopy. The thermal stability of the carbide and nitride under both inert and reducing conditions were assessed. Solid-state NMR and DFT measurements were performed for palladium nitride to further unravel its structure and stability.

Abstract

The reactions under which interstitial structures of Pd form are profoundly important and prevalent in catalysis; the formation and stability of Pd hydride structures are well understood, however, interstitial structures of the carbide and nitride are relatively under explored. This work reports a systematic study of the formation and stability of PdC_x and PdN_x at elevated temperatures and different atmospheres using in situ Pd L₃ edge XANES spectroscopy. These studies were further complemented by the application of ¹⁴N MAS-NMR experiments and computational DFT investigations. The experiments confirmed that PdC_x was significantly more stable than PdN_x; ¹⁴N MAS-NMR provided direct confirmation on the formation of the nitride, however, the XANES studies evidenced very limited stability under the conditions employed. Moreover, the results suggest that the formation of the nitride imparts some structural changes that are not entirely reversible under the conditions used in these experiments. This work provides important insights into the stability of interstitial structures of Pd and the conditions in which they could be employed for directed catalytic processes.

Silver electrocatalysts offer the possibility to produce CO by converting CO2, enabling the use of a greenhouse gas as chemical building block. Compared to nanoparticles, silver nanowires show an enhanced selectivity towards CO. Recent publications proved that oxide-derived electrocatalysts can exhibit better catalytic performance than the pristine metal phase, but oxide-derived silver nanowires have not been investigated. In this work, we report for the first time the electrocatalytic properties of silver nanowires, synthesized via the polyol method, and pretreated by electrochemical oxidation in basic electrolyte. By increasing the oxidation potential, both the percentage of AgxO and the surface roughness of the catalyst were progressively increased. The most oxidized sample showed a remarkably improved CO selectivity (‑294.2 mA m‑2Ag), producing a 3.3-fold larger CO partial current density than the pristine sample (‑89.4 mA m‑2Ag), normalized by electrochemically active silver surface area. This work demonstrates the beneficial effect of the controlled oxidation treatment even on highly selective nanostructures such as silver nanowires.

Rising CO2 levels are leading to an increase in atmospheric greenhouse gas effect. Hydroxide salts have previously been shown to be promising reagents for capturing CO2. Utilizing a 5%Ru/Al2O3 catalyst, the carbonates obtained through CO2 capture can then be hydrogenated to methane. This conversion occurs at relatively mild temperatures from 200°C to 250°C under 40 to 70 bar H2 with yields of up to 100%. Natural sources of calcium carbonate, like eggshells and seashells, can also be partially converted to methane. The direct air CO2 capture and conversion of CO2 to methane was achieved as well in quantitative yields.

Monoatomic oxygen species is more selective towards C₂-hydrocarbons in the oxidative coupling of methane reaction over MnO_x−Na₂WO₄/support catalysts. They can be formed when using N₂O as an oxidant or by co-fed H₂O. The requirements for support to ensure high selectivity are to have low specific surface area and low mobility of lattice oxygen.

Abstract

The present study of oxidative coupling of methane (OCM) over MnO_x−Na₂WO₄/support catalysts demonstrated that the selectivity to C₂H₆ and C₂H₄ (C₂-hydrocarbons) is affected by the kind of support, co-fed water, and the kind of oxidant (O₂ vs. N₂O). In addition to previous studies with MnO_x−Na₂WO₄/SiO₂, an enhancing water effect was obtained using catalysts based on TiO₂- or ZrO₂-containing supports. However, a negative effect on methane conversion was established for SiO₂−Al₂O₃-supported catalysts. Temporal analysis of products with isotopic tracers suggests that the ability of MnO_x−Na₂WO₄ to generate diatomic adsorbed oxygen species depends on the kind of support and is the key property for the water effect. The strength of the water effect on the activity decreases with an increase in the surface area of working catalysts. The kind of support also affects products selectivity due to its influence on the mobility/releasability of lattice oxygen in supported MnO_x−Na₂WO₄. Among the prepared catalysts, MnO_x−Na₂WO₄/TiO₂ was found to be promising for H₂O-assisted OCM. The use of N₂O instead of O₂ further increases the selectivity to C₂-hydrocarbons to 84 % at 6.8 % CH₄ conversion due to the formation of predominantly monoatomic oxygen species from N₂O that selectively convert CH₄ into C₂H₆.

A series of thirteen 4-arylbut-3-ene-2-amines were prepared and subjected to photosensitization experiments to interrogate their photostationary state (PS) composition of geometrical olefin isomers (E and Z). The amine PS compositions were found to depend on arene structure and temperature, while being largely independent of nitrogen substitution, solvent, or presence of triplet-quenching oxygen. Photonic efficiency of isomerization (ζp) was found to depend on amine structure, solvent choice, and presence of quencher.