One-pot hydrothermal synthesis of CuBi co-doped mesoporous zeolite Beta for the removal of NOx by selective catalytic reduction with ammonia

A series of CuBi co-doped mesoporous zeolite Beta (Cu_xBi_y-mBeta) were prepared by a facile one-pot hydrothermal treatment approach and were characterized by XRD, N₂ adsorption-desorption, TEM/SEM, XPS, H₂-TPR, NH₃-TPD and in situ DRIFTS. The catalysts Cu_xBi_y-mBeta were applied to the removal of NO_x by selective catalytic reduction with ammonia (NH₃-SCR), especially the optimized Cu₁Bi₁-mBeta achieved the high efficiency for the removal of NO_x and N₂ selectivity, superior water and sulfur resistance as well as good durability. The excellent catalytic performance could be attributed to the acid sites of the support and the synergistic effect between copper and bismuth species. Moreover, in situ DRIFTS results showed that amides NH₂ and NH₄⁺ generated from NH₃ adsorption could be responsible for the high selective catalytic reduction of NO_x to N₂. In addition, a possible catalytic reaction mechanism on Cu₁Bi₁-mBeta for the removal of NO_x by NH₃-SCR was proposed for explaining this catalytic process.

Nowadays, it is still of great challenges for the effectively catalytic purification of diesel exhausts, especially for the NO_x from diesel engine, since the conventional three-way catalysts are no longer effective in selective reducing NO_x¹. The commercial selective catalytic reduction with ammonia (NH₃-SCR) catalyst for the removal of NO_x, i.e., V₂O₅-WO₃/TiO₂, only shows high catalytic efficiency in a narrow temperature window of 300–400 °C², besides, the poor water and sulfur resistance as well as the toxicity of V₂O₅ also greatly prohibit the popularity of vanadium-based composite oxides. Therefore, researchers have devoted to develop a new kind of non-vanadia catalysts to overcome the disadvantages of vanadium-based composite oxides³. It was reported that compared to the traditional V₂O₅-WO₃/TiO₂, non-vanadia catalyst not only presents the wider NH₃-SCR temperature windows with high N₂ selectivity, but also owns good durability and strong resistance against H₂O and SO₂.

Very recently, zeolite-based catalysts with high surface areas and pore volume, abundant acidity sites, outstanding thermal and hydrothermal stability, as good catalyst supports, have attracted much research attention in selective catalytic reduction NO_x by ammonia^4,5. However, small microporous channels of zeolite greatly prevented the diffusion and transport of some large molecules, resulting in the low catalytic performance in a great majority of traditional catalytic reactions⁶. Therefore, a novel zeolite with mesoporous structure has been developed, which combines the advantages of conventional crystalline zeolite and mesoporus material, to enable the quick access for the diffusion and transport attributed to the hierarchically porous structure. It is generally believed that mesoporous zeolites possess remarkably higher catalytic activity and longer catalytic lifetime than conventional zeolites owing to the crystalline framework and the hierarchically porous structure⁷. Thereinto, the mesoporous Beta zeolite (mBeta) with unique three-dimensional network of large pores (12MR) exhibits much high surface area and excellent hydrothermal stability, which is widely applied in fine chemistry⁸. In the past decades, a number of synthetic approaches of mBeta have been explored and some encouraging results have been obtained^8,9,10. In addition, many metal oxides were reported to have good performance in the removal of NO_x by NH₃-SCR, e.g., Cu-loaded zeolite beta exhibited a good activity and hydrothermal stability in the NH₃-SCR of NO_x^11,12. It is also reported that the introduction of Bi₂O₃ can improve the SO₂ resistance¹³.

On the basis of our previous work¹⁴, a novel CuBi co-doped mesoporous zeolite Beta (Cu_xBi_y-mBeta) has been synthesized by one-pot hydrothermal treatment approach, by which the copper and bismuth species can be well dispersed into the framework of mesoporous zeolite Beta. Therein, the optimized prepared catalyst Cu₁Bi₁-mBeta exhibits very high catalytic activity for the selective catalytic reduction of NO_x with NH₃. In addition, the N₂ selectivity, water vapor and sulfur resistance and durability of the Cu₁Bi₁-mBeta were also investigated.