The impact that anthropogenic CO 2 is having on the environment has been thoroughly documented over the last 20 years. Many different technologies have been proposed to reduce its impact on global warming such as geological sequestration. However, an interesting and attractive alternative would be the recycling of the gas into energy-rich molecules. Iron rather than cobalt catalysts, based on the Fischer-Tropsch technology, have shown the greatest promise in converting CO 2 to value-added hydrocarbons. The addition of co-catalysts is, however, essential to fine tune the product distribution to the more desired alkene products. The role that both the promoter and support play on the catalyst's activity is reviewed.
The hydrogenation of CO 2 using a traditional Fischer-Tropsch Co-Pt/Al 2 O 3 catalyst for the production of valuable hydrocarbon materials is investigated. The ability to direct product distribution was measured as a function of different feed gas ratios of H 2 and CO 2 (3:1, 2:1, and 1:1) as well as operating pressures (ranging from 450 to 150 psig). As the feed gas ratio was changed from 3:1 to 2:1 and 1:1, the production distribution shifted from methane toward higher chain hydrocarbons. This change in feed gas ratio is believed to lower the methanation ability of Co in favor of chain growth, with possibly two different active sites for methane and C 2 -C 4 products. Furthermore, with decreasing pressure, the methane conversion drops slightly in favor of C 2 -C 4 paraffins. Even though under certain reaction conditions product distribution can be shifted slightly away from the formation of methane, the catalyst studied behaves like a methanation catalyst in the hydrogenation of CO 2 .
Energy-efficient
capture of CO2 from power-plant flue
gas is one of the grand challenges to reduce greenhouse gas (GHG)
emissions. Current CO2-capture technologies are limited
by parasitic energy loss, inefficient capture, and unfavorable process
economics. We present a novel electrochemical method for CO2 capture from coal-fired power-plant flue gas. The method utilizes
in-situ electrochemical pH control with a resin wafer electrodeionization
(RW-EDI) device that continuously shifts the pH of the process fluid
between basic and acidic in sequential chambers (pH swing). This pH
swing enables capture of CO2 from flue gas in the basic
chamber followed by release (recovery) of the captured CO2 (purified) in the acidic chamber of the same device. The approach
is based on the sensitivity of the thermodynamic equilibrium of CO2 hydration/dehydration reactions over a narrow pH range. The
method enables simultaneous absorption (capture) of CO2 from flue gas and desorption (release) at atmospheric pressure without
heating, vacuum, or consumptive chemical usage. In other words, the
method concentrates CO2 from ∼15% in flue gas to
>98% in the recovery stream. To the best of our knowledge, this
is
the first experimental study focusing on simultaneous capture and
release (recovery) of CO2 using an electrochemical method.
We describe the method, the role of operating parameters on CO2 recovery, and advancements in process design and engineering
for improved efficiency. We report on a method to enhance gas/liquid
mixing inside the RW-EDI, which significantly increased CO2 capture rates. We also discuss the importance of using an enzyme/catalyst
in enhancing the reaction kinetics. CO2 capture was observed
to be a strong function of gas and liquid flow rates and applied electrical
field. Up to 80% of the CO2 was captured from a simulated
flue gas stream with >98% purity. The results indicate that a narrow
pH swing from 8 to 6 (near-neutral pH) could offer a viable pathway
for energy-efficient CO2 capture if the reaction kinetics
are enhanced. Carbonic anhydrase enzyme enhances the reaction kinetics
at near-neutral pH; however, the enzyme lost activity due to the instability
at the operating conditions. This observation highlighted the necessity
of robust enzymes/catalysts to enhance kinetics of CO2 recovery
near-neutral pH.
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