Ethylene, a key commodity chemical and starting material for ∼60% of all plastics, is commonly obtained by steam cracking together with acetylene impurity (poisonous to polymerization catalysts) that is converted to ethylene to obtain polymer-grade ethylene. Traditional thermocatalytic routes for acetylene hydrogenation to ethylene, despite progress, still suffer from high temperatures (100-300 °C), H2 consumption and low selectivity for ethylene over ethane, and the need of a noble metal (Pd) catalysts (Figure ). This is a prime example of a system with ample room for improvement in terms of economics, sustainability, activity, and selectivity. Alternative routes to acetylene hydrogenation could also replace altogether the petroleum-based ethylene production route, since acetylene can be produced commercially either from methane (from sustainable sources such as biogas) or calcium carbide. Recent efforts, including ours, demonstrated acetylene hydrogenation catalysts powered either by electricity or light. Especially copper surfaces are used in the electrochemical field to reduce acetylene to ethylene. Light-powered catalysts are, at least, equally important since they can use directly sustainable and abundant solar energy. We have contributed to this area of research reporting the first photocatalytic system for the semi-hydrogenation of acetylene to ethylene that operates under visible light, uses water as proton source and a non-noble metal homogeneous cocatalyst (cobalt, in the form of cobalt porphyrin). Besides homogeneous, cobalt-based heterogeneous catalysts in the form of metal-organic frameworks (MOFs) or covalent-organic frameworks (COFs) were also found to selectively reduce acetylene to ethylene when powered by light. In this context, we have reported heterogenous catalysts that incorporate unsaturated cobalt atoms into a porphyrin-based or a cobaloxime-based MOF. These cobalt heterogeneous catalysts were found to selectively reduce acetylene to ethylene and, as opposed to the homogeneous catalysts, could be recycled and reused several times. However, all these cobalt catalysts act only as cocatalysts and need a photosensitizer (e.g. tris (2,2′-bipyridyl)ruthenium(II)) to be active in photocatalysis (Figure ). They were also found to undergo hydrolysis and degradation in aqueous solvents (and, therefore, the need to use acetonitrile and an organic as additional proton source). Having shown that mononuclear metals are beneficial in heterogeneous catalytic hydrogenation, we sought to explore a cobalt single-atom catalyst supported on carbon nitride (Co-CN) as an integrated heterogeneous photocatalyst for the selective semi-hydrogenation of acetylene to ethylene in water (Figure ).