It is noteworthy that, while the catalysts are highly selective and active only at high temperatures, plasma is reactive even at low temperatures, albeit not selective. Hence, the complementary function of plasma on a catalyst can provide a solution for the low-temperature activation of a catalyst, and previous studies have shown the synergistic effect of using a catalyst−plasma combination in removal of volatile organic compounds (VOCs), automobile exhaust treatment, carbon dioxide reduction, and methane reforming on a lab scale.

Especially in the automobile industry, the plasma−catalyst combination can suggest a solution for the treatment of exhaust emissions in next-generation diesel engines. The trends in diesel engine development are toward low-temperature combustion (LTC), such as homogeneous charge compression ignition (HCCI) engines. Newly arising critical issues are that low-temperature exhaust gas emissions contain more hydrocarbons (HCs).

First, the highly fuel-efficient engine produces lower temperature exhaust gas; this cannot sufficiently activate the exhaust catalyst, which might therefore affect the catalytic decomposition of exhaust gases. In addition, this can exacerbate the problem of cold starts, during which most emissions are released, while the catalyst is insufficiently hot to be activated. The second issue is that the composition of exhaust gas differs in a LTC-based diesel engine. Although LTC emits less NO_x and particulate matter (PM) than previous engines, the emission of HC becomes severe.

—Kang et al.

The most common plasma-catalyst configuration—a plasma reactor with bead- shaped catalyst—is problematic for automobile applications because the bead-shaped catalysts affect pressure drop in a reactor and are vulnerable to mechanical vibrations of a system, the authors note.

Newer studies have used industrially proven honeycomb-structured catalysts, such as three-way catalysts (TWCs) or diesel-oxidation catalysts (DOCs), that are already widely used in automobile exhaust treatment. While shown to be feasible, this approach to combining a honeycomb catalyst with plasma generation still requires excessive applied voltage with high energy consumption.

The KIMM team instead proposed an effective way to generate plasma over a honeycomb-structured catalyst using a dielectric and the conductive nature of the catalyst. This dielectric and conductive nature enabled the plasma to be effectively produced over the catalyst by the application of a moderate voltage with low consuming power.

The hybrid reactor consisted a platinum-coated honeycomb monolith catalyst (Al₂O₃ + 0.507% Pt, with a catalyst thickness of 20 mm and diameter of 40 mm) between high-voltage and ground perforated electrodes. The spacing between the high-voltage electrode and the catalyst was 2 mm, while the ground electrode was in contact with the bottom of the catalyst.

The plasma was produced within the air gap between the high-voltage electrode and the catalyst via moderate levels of applied voltage. For plasma generation, external power was introduced into the reactor at the voltage of 8−12 kV at the frequency of 2 kHz by a high-voltage amplifier (20/20C, TREK, Inc.) synchronized with a function generator.

In their experiments, they used a simulant hydrocarbon-containing (HC) exhaust gas from an LTC diesel engine, comprising a mixture of 900 part per million carbon (ppmC) of propylene (C₃H₆), 5 standard liter per minute (slpm) of nitrogen, and 500 standard cubic centimeters per minute (sccm) of oxygen. The simulant gas was preheated to a desired temperature in a furnace and was subsequently allowed to flow into the reactor through the perforated electrode at the high-voltage side. The gas flowed through the air gap and the catalyst serially; i.e., the flowing gas reacted with the plasma within the air gap first and then reacted with the catalyst. The space velocity was about 1200 h⁻¹.

Temporal variations of the THC and catalyst temperature. Credit: ACS, Kang et al. Click to enlarge.

Their testing procedure was to first switch on heated N₂/O₂; then switch on the HC supply; then operate the plasma. Following that, the plasma was switched off, and then the HC supply was switched off. (See chart at right.)

Among their findings were that the synergistic mechanism between the catalyst and plasma influencing total HC variations is driven by the generated reactive species that affects both the volumetric reaction (oxidation) by plasma and surface reaction (adsorption/desorption) by the catalyst. In this hybrid reaction, the applied voltage and ambient temperature determined the balance of the two reactions: volumetric and surface reactions.

The plasma−catalyst synergistic reaction was more effective at low temperatures. The hybrid reaction reduced the temperature required to achieve the same level of DRE when compared to the temperature of the reaction under the influence of the catalyst alone. On the basis of this work, we verified that the combined plasma−catalyst technology is feasible to control exhaust emissions from next-generation LTC-based diesel engines. Future research should focus on the scale-up study of the developed reactor concept at a commercial level for controlling actual automobile exhaust gas.

—Kang et al.

Resources

• Woo Seok Kang, Dae Hoon Lee, Jae-Ok Lee, Min Hur, and Young-Hoon Song Environmental Science & Technology (2013) Combination of Plasma with a Honeycomb-Structured Catalyst for Automobile Exhaust Treatment. Environmental Science & Technology doi: 10.1021/es402477a