Recently, the demands for catalysts capable of
reducing NOx in net oxidizing atmosphere has been
increasing, which promotes the research on catalysts
using hydrocarbons to reduce NOx. Cu-ZSM-5 is
one of the catalysts which reduce NOx with hydrocarbons
in net oxidizing atmosphere. We investigated the
relationship between the states of copper in Cu-ZSM-5
and its reactivity, and also the change in the
state of Cu with thermal deactivation.
The results of infrared absorption spectroscopy
of adsorbed NH3 and electron spin resonance
(ESR) of Cu2+ show that the states
of copper in Cu-ZSM-5 vary with Cu loading. It
was clarified by comparison between the states
of copper and the activity of NOx reduction of
Cu-ZSM-5 that the active sites for NOx reduction
were Cu ions at the ion exchange sites in zeolite
and that Brφnsted acid
sites did not contribute to the NOx reduction
directly.
From solid state nuclear magnetic resonance and
ESR results, the thermal deactivation was found
to occur by the migration of copper ions which
was induced by the dealumination of zeolite, not
by the aggregation of copper ion. Ab initio molecular
orbital calculation on the model of zeolites suggested
that the dealumination was caused by the Brφnsted
acid sites.
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