Improvement of Cu-SAPO-34 hydrothermal stability by tuning P/Al ratio for selective catalytic reduction of NO by NH3.

Cu-SAPO-34 is a promising catalyst for abatement of NO via selective catalytic reduction with NH3 (NH3-SCR), but its hydrothermal stability needs to be enhanced. In this work, the Cu-SAPO-34 catalysts with different P/Al ratios of 0.8, 1.0 and 1.2 were prepared, and the temperature window with NO conversion >90% (T90) for all catalysts were similar (160-570 °C). The T90 of Cu-SAPO-34 with P/Al of 0.8 dramatically decreased (220-470 °C) after hydrothermal treatment, and interestingly, the catalysts with high P/Al ratios (1.0 and 1.2) remained high activity. The T90 of the aged catalysts with P/Al of 1.2 was 155-525 °C. The characterizations showed that the increase of P/Al ratio not only enhanced the crystallinity but also enlarged the grain size of catalysts, which were conducive to the zeolite framework stability. Moreover, the Cu-SAPO-34 with large grain size facilitated the conversion of CuO to isolated Cu2+ ions as well as inhibited the aggregation of Cu species. Furthermore, the large grain sized catalysts provided more acid sites, and thus, the catalysts presented excellent hydrothermal stability. In situ DRIFTS analysis confirmed the existence of both Langmuir-Hinshelwood and Eley-Rideal pathway over the catalyst with a P/Al ratio of 1.2. This work provided a facile method to promote the hydrothermal stability of Cu-based zeolite catalysts.

Insight into the effects of Cu2+ ions and CuO species in Cu-SSZ-13 catalysts for selective catalytic reduction of NO by NH3.

Different Cu precursors were adopted to adjust the isolated Cu2+ ions and CuO species in the Cu-SSZ-13 catalysts. The Cu2+ ions were mainly located at eight-member rings in the catalysts with the Cu precursors of Cu(NO3)2 (Fresh-I) and CuSO4 (Fresh-II), while that located at six-member rings in the catalysts with the Cu precursors of CuCl2 (Fresh-III) and Cu(CH3COO)2 (Fresh-IV). All catalysts showed>90% of NO conversion and 100% of N2 selectivity at 200-680 °C. Fresh-I and Fresh-II catalysts showed an unexpected increase of the high-temperature activity after hydrothermal treatment. The characterizations revealed that the hydrothermal treatment promoted the production of CuO species, which accelerated the adsorption of NOx species to enhance the high-temperature SCR activity. Moreover, the SCR reaction pathway changed from Langmuir-Hinshelwood to Eley-Rideal mechanisms for the Fresh-II catalyst after the hydrothermal treatment, which was verified by in situ DRIFTS.