Fully Copper‐Exchanged High‐Silica LTA Zeolites as Unrivaled Hydrothermally Stable NH₃‐SCR Catalysts

Abstract

Tighter CO2 emission standards from mobile sources are being legislated globally in order to address the concerns regarding anthropogenic climate change. Hence, automotive manufacturers have developed a variety of new propulsion systems, including battery electric, plug-in hybrid, and even fuel cell electric vehicles.[1] However, there is a general consensus that internal combustion engines will continue to dominate the market for the foreseeable future. [1,2] As such, fuel-efficient combustion technologies like diesel engines offer superior green-house gas reduction potential. One technical obstacle to broader diesel implementation is the required lean NOx aftertreatment system, especially to meet upcoming strict emission regulations. NOx is extremely difficult to reduce under an oxygen-rich environment.[3] 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 (~ 200 °C) of urea into NH3 and by SCR catalyst deactivation at temperatures higher than 750 °C.[4]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. Metal-exchanged zeolites have drawn much attention as diesel vehicle SCR catalysts, and with copper-exchanged ZSM-5 and SSZ-13, which are medium-and small-pore zeolites with MFI and CHA topologies, respectively, [5] have been most widely studied for this reaction.[4] Cu-SSZ-13 has recently been implemented as the current standard catalyst in the mobile SCR technology because of its superior thermal durability compared to already known catalysts. When aged at 850 °C, however, even this catalyst, whose fresh form achieves greater than 90% NOx conversion at 250 -400 °C in steady state, loses its CHA structure and forms copper oxide (CuOx) species, leading to severe activity loss. [6] Although zeolite A (framework type LTA) is the first synthetic zeolite to be prepared, [7] 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). However, a recent success in the benzylimidazolium-mediated synthesis of its unprecedented high-silica (Si/Al > 8) form provides a key advantage in terms of structural stability with tunable loading of catalytically-active metal centers.[8] Here 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, i.e., t...

Zeolitic Materials for DeNOx Selective Catalytic Reduction

“…Recently, Ryu et al. reported Cu‐exchanged high silica LTA zeolites, which exhibit excellent activity for NH 3 ‐SCR even after hydrothermal aging at 900 °C . The significance lies in the fact that Cu 2+ cations are only located in the center of single 6‐MR, acting not only as a catalytically active center, but also as a dealumination suppressor.…”

A Cationic Oligomer as an Organic Template for Direct Synthesis of Aluminosilicate ITH Zeolite

“…Possibly, the rearrangement of silicon species with It is well-known that high hydrothermal stability is a key factor for many industrial applications of zeolites. [47,48] Therefore, it is very crucial to investigate the hydrothermal stability of Al-ITH zeolite in this work. After steaming treatment at 800 8C for 5 h with 10 % water, the crystallinity, surface area, and pore volume of aged H-Al-ITH zeolite are very close to those of the fresh one (Supporting Information, Figure S13, Tables S5,S6).…”

In situ Growth Synthesis of CuO@Cu‐MOFs Core‐shell Materials as Novel Low‐temperature NH3‐SCR Catalysts

“…Selective catalytic reduction of NO x by NH 3 (NH 3 ‐SCR) is one of the most popular methods to eliminate NO x from stationary sources . V 2 O 5 ‐WO 3 /TiO 2 materials with high catalytic activity in temperature range of 300–400 °C have been successfully used in coal‐fired power plants for decades.…”