NO_x is extremely difficult to reduce under an oxygen-rich environment. Although its selective catalytic reduction by urea (urea-SCR) has recently been commercialized, the operation window of this technology is severely limited by the decomposition temperature (ca. 200 ˚C) of urea into NH₃ and by SCR catalyst deactivation at temperatures higher than 750 ˚C. This prohibits closer placement of the catalyst to the engine, requiring an aggressive warm-up with extra fuel burning during the cold-start. Furthermore, when integrating a mandatory particulate filter in the modern diesel aftertreatment system to mitigate soot and ash, the frequent regeneration of diesel particulate filters is required before a certain accumulation of soot, resulting in large temperature spikes. Improvement of the thermal durability of the SCR catalysts would therefore be the key to maximizing the fuel efficiency, as well as to producing clean emissions from diesel engines.

… Although zeolite A (framework type LTA) is the first synthetic zeolite to be prepared, its catalytic applications have long been severely restricted due to its poor thermal stability originating from the high framework Al content (Si/ Al = 1.0). … Herein we report that when the copper ion exchange level increases to 100% (Cu/Al=0.50), the high-silica (Si/Al = 16–23) Cu-LTA catalysts hydrothermally aged at 900 ˚C, that is, the current regeneration temperature which the commercial Cu-SSZ-13 catalyst cannot withstand, show remarkable NO reduction activities.

—Ryu et al.

a)–c) NO conversion as a function of temperature over the (left) fresh and (right) 900 ˚C-aged forms of Cu-LTA-16-10.14 (green ■), Cu-LTA-16-0.32 (brown ●), Cu-LTA-16-0.48 (red ▲), Cu-LTA-16-0.65 (purple ◆), Cu-LTA-11-0.48 (light green ►), Cu-LTA-23-0.50 (blue ◀), Cu-SSZ-13-16-0.15 (green □), Cu- SSZ-13-16-0.34 (brown ○), and Cu-SSZ-13-16-0.49 (red △). The feed contains 500 ppm NH3, 500 ppm NO, 5% O2, 10% H2O balanced with N2 at 100,000 h-1 gas hourly space velocity. Hydrothermal aging was performed under flowing air containing 10% H2O at 900 ˚C for 12 h. The last two values of the catalyst identification correspond to the Si/Al and Cu/Al ratios of the catalysts, respectively. d) Effects of 20 ppm SO2 exposure as a function of time at 270 ˚C on the NO conversion over fresh Cu-LTA-16-0.48 (red ▲) and Cu-SSZ-13-16-0.49 (red △). The feed composition is the same as that described above. Source: Ryu et al.Click to enlarge.

The Cu-LTA catalysts open up a new research direction for lean NO_x catalysts—even for gasoline- powered vehicles—and eliminates one of the barriers to expanded use of fuel-efficient diesel engines, the researchers said.

The team is applying for patents worldwide, and plans to continue work on commercialization of the technology.

Resources

• Ryu, T., Ahn, N. H., Seo, S., Cho, J., Kim, H., Jo, D., Park, G. T., Kim, P. S., Kim, C. H., Bruce, E. L., Wright, P. A., Nam, I.-S. and Hong, S. B. (2017), “Fully Copper-Exchanged High-Silica LTA Zeolites as Unrivaled Hydrothermally Stable NH3-SCR Catalysts,” Angew. Chem. Int. Ed.. doi: 10.1002/anie.201610547