Adsorption and reduction of CO2 on 12-tungstophosphoric and 12-molybdophosphoric acids in the absence of added reductants

12-tungstophosphoric and 12-molybdophosphoric acids are capable of sorbing and reducing CO2 without the presence of added reductants. The adsorbed CO2 (physisorbed as CO2) desorbs from the acids and salts at low temperatures (50-200 degrees C). Carbon dioxide also desorbs only from the acids at high temperatures (500-600 degrees C), suggesting the direct or indirect involvement of the protons in the process and the importance of the Keggin unit as well. Raman and DRIFTS results show that the species responsible for the evolution of CO2 (at high temperatures) are carbonaceous in nature. Carbon deposits extract the anionic oxygen atoms from the Keggin units during the high-temperature desorption process. The extraction of anionic oxygen atoms by carbon is confirmed using labeled carbon dioxide. The spectroscopic data imply that carbon exists on the surface as such but, additionally, as either carbon or CO2 bound to the terminal oxygen atoms of the Keggin anion.