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Title:
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Influence of zeolite structure on the activity and durability of Co-Pd-zeolite catalysts in the reduction of NOx with methane
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Author(s):
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Published by:
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Publication date:
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ECN
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1-12-2002
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ECN report number:
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Document type:
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ECN-RX--02-062
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Article (scientific)
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Number of pages:
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22
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Published in: Accepted for publication in 'Applied Catalysis, 2003' (), , , Vol., p.-.
Abstract:
Selective catalytic reduction of NO with CH4 was studied over ZSM-5,MOR, FER and BEA zeolite-based cobalt (Co) and palladium (Pd) catalysts
in the presence of oxygen and water. As compared to other catalytic
systems reported in literature for CH-4-SCR in the presence of water,
zeolite supported Co-Pd combination catalysts are very active and selective.
The most active catalysts, based on MOR and ZSM-5, are characterised
by well-dispersed Pd ions in the zeolite that activate methane. Wet
ion-exchange is a good method to achieve high dispersion of Pd provided
that it is carried out in a competitive manner. The presence of cobalt
(Co3O4, Co-oxo ions) boosts SCR activity by oxidising NO to NO2. The
activity of the zeolite-based Co-Pd combination catalysts decreases
with prolonged times on stream. The severity of the deactivation was
found to be different for different zeolite topologies. The characterisation
and evaluation of freshly calcined catalysts and spent catalysts show
two things that occur during reaction: 1) zeolite solvated metal cations
disappear in favour of (inactive) metal oxides and presumably larger
metal entities, i.e. loss of dispersion, 2) loss of crystallinity affiliated
with steam-dealumination and the concomitant formation of extra-framework
aluminium in the presence of water. Both phenomena strongly depend on
the (reaction) temperature. The deactivation of Co-Pd-zeolite resembles
the deactivation of Pd-zeolite. Hence, future research could encompass
the stabilisation of Pd (cations) in the zeolite pores by exploring
additives other than cobalt. For this, detailed understanding on the
siting of Pd in zeolites is important.
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