Formation of Highly Active Superoxide Sites on CuO Nanoclusters Encapsulated in SAPO-34 for Catalytic Selective Ammonia Oxidation

Generation of surface active sites with tailor-made structure is a promising way to enhance catalytic properties of inexpensive metal oxides, as a replacement to noble metals. In the abatement of NH3 emissions through selective oxidation to N2, the nature of active sites over Cu-based catalysts plays a decisive role in determining activity and avoiding formation of NOx from excessive oxidation. In the present work, CuO nanoclusters are homogeneously confined in small pore zeolitic SAPO-34 crystals by a Trojan Horse approach, i.e, through combined use of Cu2+ containing complex and morpholine as structure-directing agents in the hydrothermal synthesis stage and a sequential Cu2+ cation impregnation followed by calcination, is presented. Nitrogen activation and reoxidation treatment lead to the formation of encapsulated CuO@SAPO-34 structure that showed promoted activities and N2 selectivity for NH3 selective catalytic oxidation at relatively low temperatures (250 °C), with respect to catalysts obtained from ion-exchange or simple impregnation routes. The structure of catalytically active sites was unveiled to be Cu(II) superoxo species by a panoply of characterization techniques, including in situ Raman spectra, in situ DRIFT, as well as X-ray absorption spectroscopy. The catalytic activity at low temperatures (165–175 °C) was found to scale proportionally with the concentration of Cu(II) superoxo species measured by CO temperature-programmed reduction and O2 temperature-programmed desorption. The reaction mechanism for ammonia catalytic oxidation on Cu(II) superoxo species has also been discussed on the basis of in situ IR and temperature-programmed surface reaction studies. The tailored synthesis and identification of active sites lay the basis for the understanding of the structure–catalysis relationship and future catalyst design for NH3 elimination through selective oxidation.