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Formation of Highly Active Superoxide Sites on CuO Nanoclusters Encapsulated in SAPO-34 for Catalytic Selective Ammonia Oxidation
journal contribution
posted on 2019-10-17, 18:44 authored by Fei Han, Mengqi Yuan, Shinya Mine, Han Sun, Haijun Chen, Takashi Toyao, Masaya Matsuoka, Kake Zhu, Jinlong Zhang, Weichao Wang, Tao XueGeneration
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.
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Keywords
characterization techniquesO 2 temperature-programmed desorptionfuture catalyst designRaman spectrahydrothermal synthesis stageN 2impregnation routesN 2 selectivityActive Superoxide SitesCO temperature-programmed reductionformationsuperoxoNH 3CuO Nanoclusters Encapsulatedreaction mechanismSAPO -34Nitrogen activationbasisTrojan Horse approachCu-based catalystsstructure-directing agentsmetal oxidesreoxidation treatmentNH 3 eliminationsiteX-ray absorption spectroscopyAmmonia Oxidation Generationspeciestemperature-programmed surface reaction studiesDRIFTNH 3 emissionsIRpore zeolitic SAPO -34 crystalsCuO nanoclusters
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