The application of photocatalysis for the utilisation of sunlight energy is intensely investigated in present times, particularly in prospect of generating solar fuels by hydrogen production or CO2 reduction processes as tools for societies aiming to relief their thirst for fossil resources. From the perspective of sustainability, the rational use of biomass-derived feedstocks for photocatalytic H2 production is a feasible, proven and highly efficient process. In this review, in addition to delving into physico-chemical fundamentals of photocatalytic processes on semiconductors, the research activity on this topic related to design of revolutionary semiconductor-based materials, generally including metallic nanoparticles or complexes as hydrogen-evolving co-catalysts, is outlined and critically evaluated. Moreover, the use of sunlight and renewable feedstocks for the generation of hydrogen, as a compelling opportunity for the energy sector, is emphasised. Special focus is also set on the valorisation of biorefinery products, agricultural residues and industrial or municipal waste.

Identification of active sites in heterogeneous metal catalysts is critical for understanding the reaction mechanism at the molecular level and for designing more efficient catalysts. Because of their structural flexibility, subnanometric metal catalysts, including single atoms and clusters with a few atoms, can exhibit dynamic structural evolution when interacting with substrate molecules, making it difficult to determine the catalytically active sites. In this work, Pt catalysts containing selected types of Pt entities (from single atoms to clusters and nanoparticles) have been prepared, and their evolution has been followed, while they were reacting in a variety of heterogeneous catalytic reactions, including selective hydrogenation reactions, CO oxidation, dehydrogenation of propane, and photocatalytic H2 evolution reaction. By in situ X-ray absorption spectroscopy, in situ IR spectroscopy, and high-resolution electron microscopy techniques, we will show that some characterization techniques carried out in an inadequate way can introduce confusion on the interpretation of coordination environment of highly dispersed Pt species. Finally, the combination of catalytic reactivity and in situ characterization techniques shows that, depending on the catalyst–reactant interaction and metal–support interaction, singly dispersed metal atoms can rapidly evolve into metal clusters or nanoparticles, being the working active sites for those abovementioned heterogeneous reactions.