Synthesis of Cu-Al hydrotalcite precursors with different interlayer anions for application in the NO reduction by CO

Abstract

Cu-Al hydrotalcite materials were obtained by coprecipitation with different interlayer anions: carbonate, terephthalate, and tris-dipicolinate cerium complex. Subsequently, they were calcined and tested for NO reduction by CO. The precursors and catalysts were studied to determine their structural, compositional, textural, morphological and reactivity properties. X-ray diffraction, PDF (pair distribution function) simulation and Fourier transform infrared spectroscopy analyses indicated that the hydrotalcite phase was obtained for all precursors with the desired occupation of interlayer anions. Copper reduction was achieved at lower temperatures with catalysts containing cerium, as indicated by TPR, which is correlated with the oxygen vacancies found by XPS and Raman. Through X-ray diffraction, the crystalline copper species present in the spent catalysts were identified: the catalyst from the carbonate precursor exhibited reduced (Cu⁺ and Cu⁰) and oxidized (Cu²⁺) copper species, while the rest of the catalysts only presented the reduced species. Although the catalyst with carbonate between the layers and without Ce is the most active, this activity is related to its high capacity to form N₂O at low temperatures, which is undesirable. The highest N₂ selectivity at low NO conversions was reached with the catalyst obtained from the terephthalate-precursor and, while at intermediated NO conversions, the highest N₂ selectivity was reached with the catalyst originated from cerium complex-precursor.