(414g) Trends in the Catalytic Reduction of CO2 By Hydrogen Over Supported Monometallic and Bimetallic Catalysts

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.