Photocatalytic Semi-hydrogenation of Acetylene to Ethylene 2.1.1 Non-competitive conditions (pure acetylene, no ethylene co-feed). Our photocatalytic system consists of four components: [Co(dmgH)2pyCl] as catalyst, [Ru(bpy)3] 2+ as photosensitizer (PS), 2,2,2-trifluoroethanol (TFE) as proton source, and 1,3-dimethyl-2-phenyl-2,3-dihydro-1Hbenzo [d]imidazole (BIH) as sacrificial donor (SD). In a typical experiment, we illuminated the catalytic mixture under 1 atm C2H2 (≥99.5 vol.%) using a 450 nm light-emitting diode (LED, 140 mW•cm -2 ); the details of the purging and photocatalytic setups are published elsewhere. Crucially, a typical gas chromatogram and mass spectrum (Figure ) of the reaction mixture, after four hours of irradiation, shows conversion of C2H2 to C2H4 without any appreciable overhydrogenation to C2H6 based on the limits of detection of our instruments (calibration curves are shown in Figure and Figure ). We chose BIH and TFE in an attempt to create mild photocatalytic conditions for our cobaloxime catalyst and due to previous reports touting their excellence as SD and proton source, respectively. A combination of BIH and TFE showed the highest catalytic activity in a screening of various combinations (Figure ). Based on optimization of the system (Figure and Figure ), we chose concentrations for each of our reaction components: 2. [Co(dmgH)2pyCl] produced 63 μmol C2H4 (Figure ) with a quantum yield (QY) of 1.0%, a 10and 50-fold improvement over our recently investigated Co-porphyrin (QY = 0.11%) or Co-PCN-222 system (QY = 0.02%), respectively. In the absence of [Co(dmgH)2pyCl], [Ru(bpy)3] 2+ , TFE, BIH, light, or C2H2, the C2H2 reduction does not occur, which confirms the photocatalytic nature of the reaction (Figure ). Using 1.0 µM [Co(dmgH)2pyCl], we obtain TON >1,000 (TON = 1,148) with selectivity for ethylene over ethane >99.9% after 20 hours of illumination (Figure ). We detected no other gaseous products, except for an extremely small amount of H2 (2 nmol of H2 were generated after four hours). As a result, the overall selectivity for C2H4 vs. C2H6 and H2 combined is >99.9%.