Photocatalytic synthesis of organic compounds from CO and water: Involvement of surfaces in the formation and stabilization of products

Summary ¹⁴C-Formic acid and other¹⁴C-organic compounds are formed on surface materials when mixtures of¹⁴CO,¹²CO₂ or N₂ and water vapor are irradiated with ultraviolet light (UV) of > 250 nm. The rate of organic formation is roughly proportional to the quantity of substratum irradiated. The available evidence suggests that¹⁴CO adsorbed to or in contact with the substratum is excited by the long wavelength UV and reacts with adsorbed H₂O or surface hydroxyl groups yielding the organic products. Photodestruction of the¹⁴C-organics yields¹⁴CO₂ and¹⁴CO. A steady state is attained when organic products reach a concentration such that the rate of photodestruction is equal to the rate of synthesis. The product accumulation is greater and the photodestruction is slower when N₂ is used as diluent gas.Differences in the rates of synthesis, rates of photodestruction and amounts of product accumulation are observed with different silica and alumina substrata. The substrata with large surface areas are most effective for synthesis while maximum photoprotection of organics is afforded by substrata containing high concentrations of surface hydroxyl groups.The observation of the synthesis on a variety of substrata using realistic simulations of atmospheres and solar energies strengthens previous proposals that this process may occur on Mars and may have been important on the primitive Earth.