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 N-2, 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 N-2 selectivity for NH3 selective catalytic oxidation at relatively low temperatures (250 degrees 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 degrees C) was found to scale proportionally with the concentration of Cu(II) superoxo species measured by CO temperature-programmed reduction and O-2 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.