Rising atmospheric concentration of CO2 is forecasted to have potentially disastrous effects on the global climate due to its role in global warming and ocean acidification. A catalytic process that utilizes CO2 as a feedstock to make carbon monoxide, methanol, and methane, is potentially more desirable than sequestration. CO is the most desired product because it can be integrated into down-stream processes to produce synthetic fuels via Fischer-Tropsch.
The reduction of CO2 by hydrogen has been conducted on supported catalysts in batch and flow reactors. Catalysts synthesized on a reducible support (CeO2) showed higher activity than on an irreducible support (γ-Al2O3). The active metal also played an important role in controlling the selective reduction of CO2 to CO instead of CH4. Extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) confirmed the formation of uniform, bimetallic particles. Among the monometallic and bimetallic catalysts evaluated in a batch reactor, PdNi/CeO2 was found to be the most active bimetallic catalyst, but formed the greatest amount of CH4. PtCo/CeO2 showed the highest selectivity to CO with very low selectivity to CH4. The selectivity of each catalyst correlated with the d-band center value of each bimetallic surface. Among the catalysts investigated, bimetallics with values of d-band center farther from the Fermi level produced more CO and less CH4 than catalysts with values closer to the Fermi level.
Batch reactor experiments were verified in a flow reactor under corresponding conditions. Results in the flow reactor were consistent with the batch reactor, with Pt–Co showing the highest selectivity to CO and Pd–Ni the lowest. Establishing a selectivity trend for CO2 activation provides a facile means to choose effective catalysts from the large database of d-band center values for a variety of monometallic and bimetallic catalysts.