text: In the 1980s, Lehn observed that Co(I) species were produced in solutions containing CoCl, 2,2'-bipyridine (bpy), a tertiary amine, and a Ru(bpy)Cl photosensitizer. The high affinity of CO to cobalt centers led both him and Ziessel to study cobalt centers as electrocatalysts for reduction. In 1982, they reported CO and H as products from the irradiation of a solution containing 700ml of CO, Ru(bpy) and Co(bpy).
  Since the work of Lehn and Ziessel, several catalysts have been paired with the Ru(bpy) photosensitizer.
  When paired with methylviologen, cobalt, and nickel-based catalysts, carbon monoxide and hydrogen gas are observed as products. 
  Paired with rhenium catalysts, carbon monoxide is observed as the major product, and with ruthenium catalysts formic acid is observed. Some product selection is attainable through tuning of the reaction environment. Other photosensitizers have also been employed as catalysts. They include FeTPP (TPP=5,10,15,20-tetraphenyl-21H,23H-porphine) and CoTPP, both of which produce CO while the latter produces formate also. Non-metal photocatalysts include pyridine and N-heterocyclic carbenes.  
  In August 2022, it was developed a photocatalyst based on lead–sulfur (Pb–S) bonds, with promising results.