H₂ production via dehydrogenation of formic acid (HCOOH, FA), sodium formate (HCOONa, SF), or their mixtures, at near-ambient conditions, T < 100 °C, P = 1 bar, is intensively pursued, in the context of the most economically and environmentally eligible technologies. Herein we discuss molecular catalysts (ML), consisting of a metal center (M, e.g., Ru, Ir, Fe, Co) and an appropriate ligand (L), which exemplify highly efficient Turnover Numbers (TONs) and Turnover Frequencies (TOFs) in H₂ production from FA/SF. Typically, many of these ML catalysts require the presence of a cofactor that promotes their optimal cycling. Thus, we distinguish the concept of such cofactors in additives vs. co-catalysts: When used at high concentrations, that is stoichiometric amounts vs. the substrate (HCOONa, SF), the cofactors are sacrificial additives. In contrast, co-catalysts are used at much lower concentrations, that is at stoichiometric amount vs. the catalyst. The first part of the present review article discusses the mechanistic key steps and key controversies in the literature, taking into account theoretical modeling data. Then, in the second part, the role of additives and co-catalysts as well as the role of the solvent and the eventual inhibitory role of H₂O are discussed in connection to the main mechanistic steps. For completeness, photons used as activators of ML catalysts are also discussed in the context of co-catalysts. In the third part, we discuss examples of promising hybrid nanocatalysts, consisting of a molecular catalyst ML attached on the surface of a nanoparticle. In the same context, we discuss nanoparticulate co-catalysts and hybrid co-catalysts, consisting of catalyst attached on the surface of a nanoparticle, and their role in the performance of molecular catalysts ML.

A flame spray pyrolysis (FSP) method has been developed, for controlled doping of BiVO₄ nanoparticles with W and Zr in tandem with the oxygen vacancies (Vo) of the BiVO₄ lattice. Based on XPS and Raman data, we show that the nanolattice of W-BiVO₄ and Zr-BiO₄ can be controlled to achieve optimal O₂ evolution from H₂O photocatalysis. A synergistic effect is found between the W- and Zr-doping level in correlation with the Vo-concentration. FSP- made W-BiVO₄ show optimal photocatalytic O₂-production from H₂O, up to 1020 μmol/(g × h) for 5%W-BiVO₄, while the best performing Zr-doped achieved 970 μmol/(g × h) for 5%Zr-BiVO₄. Higher W-or Zr-doping resulted in deterioration in photocatalytic O₂-production from H₂O. Thus, engineering of FSP-made BiVO₄ nanoparticles by precise control of the lattice and doping-level, allows significant enhancement of the photocatalytic O₂-evolution efficiency. Technology-wise, the present work demonstrates that flame spray pyrolysis as an inherently scalable technology, allows precise control of the BiVO₄ nanolattice, to achieve significant improvement of its photocatalytic efficiency.

Palladium is among the most versatile noble-metal atoms that, when dispersed on solid supports, can be stabilized in 0, +1, +2, +3 redox states. Moreover, despite its noble-metal character, Pd shows a considerable degree of chemical reactivity. In Pd Nanoparticles (NPs), atomic {Pdⁿ⁺-X} states, where n = 0, 1, 2, 3, and X = atom or hydride, can play key roles in catalytic processes. Pd-oxygen moieties can be stabilized at nanointerfaces of Pd in contact with metal-oxides. These {Pdⁿ⁺-X}s can be either isolated Pd atoms dispersed on the support, or, more interestingly, atomic states of Pd occurring on the Pd NPs. The present review focuses on the role of such {Pdⁿ⁺-X} states in catalytic processes related to energy storage or energy conversion, with specific focus on photocatalysis, H₂ production reaction (HRR), oxygen reduction reaction (ORR), and water-splitting. Synthesis of atomic {Pdⁿ⁺-X} states and their detection methodology is among the current challenges. Herein, the chemistry of {Pdⁿ⁺-X} states on Pd- [metal oxide] interfaces, methods of detection, and identification are discussed. The implication of {Pdⁿ⁺-X} in transient catalytic intermediates is reviewed. Finally, the role of {Pdⁿ⁺-X} in photo electrocatalytic processes is critically discussed.