Plasma Eases Carbon Dioxide Conversion

Carbon dioxide conversion through catalysis normally requires a great deal of energy with multiple steps, high temperature (roughly 400–750°F) and high pressure (150–600 lb/in²). Now, a team of researchers have developed a one-step, plasma-enabled catalytic process to convert CO₂ into higher hydrocarbons. It operates at low temperature (75°F) and pressure (15 lb/in²) using a dielectric barrier discharge (DBD) packed-bed plasma reactor. In addition, tests of the process demonstrated a higher hydrocarbons selectivity of 46% at a CO₂ conversion rate of 74%, they report. The researchers used a conventional aluminum oxide-supported Fisher-Tropsch catalyst for the process.

The researchers — from Pennsylvania State University (Penn State), University Park, Penn.; the Chinese University of Hong Kong; and Sichuan University, Chengdu, Sichuan, China — published their findings in a recent article in Green Chemistry.

High-energy electrons within the plasma phase activate both CO₂ and H₂ molecules in the gas phase through excitation and dissociation. This happens without surface adsorption and activation as required for traditional thermal catalysis, so the reaction operates at low temperature, notes Xiaoxing Wang, associate research professor at the Penn State EMS Energy Institute.

fig-1-Plasma-Eases-Carbon-Dioxide-Conversion

Plasma Catalyst Process

Figure 1. Conversion method operates at low temperature and pressure, and can be powered with renewable energy.

“Besides, catalysts, catalyst-bed configuration, and reaction conditions such as gas space velocity, CO₂/H₂ ratio, plasma power, etc., can also greatly influence the final selectivity of C₂₊ hydrocarbons. Our target is to attain over 80% of selectivity for C₂₊ hydrocarbons,” Wang adds.

“The electricity generated from renewable energy easily can be adapted to the plasma-catalysis process. The catalyst deactivation due to high temperature sintering observed in conventional thermal catalysis largely can be avoided in the plasma-catalysis process. Furthermore, plasma can enhance catalyst dispersion and suppress metal agglomeration,” claims Wang.

Compared with the conventional process, the plasma-enabled catalysis offers more flexibility in operating parameters and feed gases, greater energy efficiency, and less sensitivity to poisoning, say the researchers.

“The plasma discharge zone can expand to cover the entire volume of the reactor for the reaction. In addition, the process doesn’t need sophisticated or costly devices and materials, especially for the DBD plasma. Those advantages make the process promising for scaling up,” believes Wang. “To scale the plasma-catalytic reactor up to a size that competes with the well-established large-scale commercial processes would require a significant advancement in plasma-catalysis science and technology,” he admits.

“By changing the catalyst-bed configuration, we successfully managed to obtain a high yield of C₂₊ hydrocarbons and found the developed process likely involves two tandem conversions including plasma-enhanced CO₂ hydrogenation to CH₄ over Co catalyst, followed by plasma-induced methane coupling to C₂₊ hydrocarbons. Both reactions determine the final selectivity of C₂₊ hydrocarbons. Recently, we examined other catalysts including iron and nickel catalysts. Interestingly, we only obtained >99% CO over the Fe catalyst, while CH₄ was the main product over the Ni catalyst. Without CH₄ formation, C₂₊ hydrocarbons barely formed. The Ni catalyst is even more effective than the Co catalyst for CH₄ formation. Now, the key is to find a catalyst for the second conversion, the plasma-induced methane coupling reaction, to further improve C₂₊ production and to control the distribution of C₂₊ hydrocarbons such as C₂-C₄ olefins, C₅-C₁₂ gasoline, C₁₂-C₂₀ diesel,” explains Wang.

“The plasma-activated catalytic process is promising for CO₂ conversion,” emphasizes Wang. “It can be applied to other CO₂ conversion reactions such as CO₂ splitting, dry methane reforming, etc. The investment and operating costs are relatively low. It [also] can be applied in a modular setting and scaled up linearly with the plant output. It can be easily combined with renewable electricity, thus is suitable for converting intermittent renewable energy into base chemicals and/or fuels for peak shaving and grid stabilization,” he says.