Hard Template-Assisted Trans-Crystallization Synthesis of Hierarchically Porous Cu-SSZ-13 with Enhanced NH3-SCR Performance

Abstract

Small porous Cu-SSZ-13 catalysts have recently been commercialized for the selective catalytic reduction of NOx with ammonia (NH3-SCR) on diesel vehicles. Unfortunately, the conventional Cu-SSZ-13 catalyst still confronts the challenge of diffusion limitations, which represent a major obstacle that reduces the catalyst’s SCR performance. Herein, a hierarchically porous SSZ-13 zeolite was synthesized via a trans-crystallization method assisted by the use of carbon black as a hard template in a short synthetic period, and the corresponding Cu-SSZ-13 catalysts with mesopores exhibited improved low-temperature activity and hydrothermal stability when compared with their microporous counterpart. A series of characterizations revealed that the mesopores are conducive to an increase in Cu loading while helping to stabilize the CHA structure and maintain the Cu species in their ionic form. More importantly, intra-particle diffusion limitations are reduced via the introduction of the mesopores owing to the shortened diffusion path inside the SSZ-13 zeolite, thus not only enhancing the active sites’ accessibility but also promoting the diffusion of the reactants and products. This work contributes to the design and synthesis of a high-performance Cu-SSZ-13 zeolite SCR catalyst for the removal of NOx emitted from diesel vehicles.

Enhanced Low-Temperature Activity and Hydrothermal Stability of Ce-Mn Oxide-Modified Cu-SSZ-39 Catalysts for NH3-SCR of NOx

“…In order to reveal the stabilizing effect of CeMnOx on the zeolite framework of Cu-SSZ-39, 29 Si, and 27 Al, MAS NMR spectra were measured to determine the local Si and Al Compared to the fresh catalysts, the desorption temperatures of the peaks over the aged ones shifted towards lower values and the NH 3 desorption amount significantly decreased (Figure 6b and Table 3), suggesting that both the strength and amount of acid sites are affected by the hydrothermal aging treatment. The decrease in Peaks α and γ can be attributed to the collapse of the zeolite framework and the loss of active sites; in contrast, the increased intensity in Peak β indicates that the additional Lewis acid sites arise from the transformation of Cu 2+ ions and the aggregation of oxides during hydrothermal aging [4]. Interestingly, CeO 2 /Cu-SSZ-39-A, CeMnO x /Cu-SSZ-39-A, and MnO x /Cu-SSZ-39-A catalysts exhibited a slightly lower decrease in Peaks α and γ compared to Cu-SSZ-39-A.…”

“…Currently, selective catalytic reduction (SCR) of NO x with ammonia (NH 3 ) has been proven to be one of the most efficient technologies for NO x purification [2]. Cu-based small-pore zeolite SCR catalysts (especially Cu-SSZ-13 zeolite with a CHA structure) have been widely adopted in diesel engines owing to their wide active temperature window and hydrothermal stability [3][4][5]. In practical applications, the periodically increased exhaust temperature required to stimulate diesel particulate filter (DPF) regeneration necessitates SCR catalysts with high hydrothermal stability [6].…”