ABSTRACT
The reduction of carbon dioxide to useful compounds such as fuels and chemical intermediates is inherently difficult, as it involves several steps of both electron and proton transfers. In the absence of activation by an external energy source - electrochemical, photochemical, or radiation induced - CO2 may be activated by reduced compounds such as by hydrocarbons in the reforming reaction or by molecular hydrogen in the hydrogenation reaction. Catalysts are usually required to decrease the activation energy. The heterogeneous catalytic reactions of carbon dioxide were carefully reviewed by Fox,1 Krylov and Mamedov,2 and Wang et a1.3
The direct or indirect hydrogenation of carbon dioxide may lead to several reactions, which are either kinetically or thermodynamically controlled, depending on the reaction conditions:4
6H298K = 247 kJ mol-I (10.1)
2. The exothermic methanation by the Sabatier process:
6H298 K = -128 kJ mol-I (10.4)
316 Greenhouse Gas Carbon Dioxide Mitigation: Science and Technology
4. The reverse water-gas-shift reaction, producing carbon monoxide and water:
5. The hydrogenation of CO2 to formic acid:
CO2 + H2 = HCOOH(l) Mi298K = -31.2 kJ mol-i (10.6)
6. The gasification of carbon by carbon dioxide (the reverse of CO disproportionation):
CO2 + C = 2CO AH298 K = 172.5 kJ mol-i (10.7)
The reforming of natural gas, followed by the hydrogenation of mixtures of carbon monoxide and carbon dioxide to methanol on various transition element catalysts, is one of the most important processes in the huge petrochemical industry. Together with carbon monoxide, carbon dioxide can also serve as the carbon source in the Fischer-Tropsch synthesis of mixtures of various aliphatic and aromatic hydrocarbons, aldehydes, ketones, and carboxylic acids. The mechanisms of these reactions have been clarified with the tools of modem surface science.
