This focus group consists of Hans Fischer Senior Fellow Prof. Ib Chorkendorff (Technical University of Denmark) and his hosts Prof. Ulrich Heiz (TUM Department of Chemistry) and Prof. Ian Sharp (TUM Department of Physics). It seeks for new and efficient catalysts for photoinduced hydrogen and oxygen evolution reactions as well as the hydrogenation of CO2 by accessing the non-scalable size regime, i.e. the cluster regime and the transition regime (cluster-nanoparticle transition regime). In most catalytic applications, we are used to working with materials that are scalable: consider a particle that occupies a certain volume, then we would expect that if we double the number of atoms in the particle then the volume also doubles. We could expect similar effects for a catalytically active surface: Doubling the area, doubles the number of molecules turned over, should all other parameters be constant. Nonetheless, this is not the case when the nanoparticles become smaller. Below 2 nm (non-scalable size regime), they also begin to behave more molecular-like, with discrete electronic levels. This has strong implications for the reactivity of the individual surface atoms, drastically influencing the overall activity and thus opens possibilities for tuning reactivity by size.