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Titel:
Influence of zeolite structure on the activity and durability of Co-Pd-zeolite catalysts in the reduction of NOx with methane
 
Auteur(s):
 
Gepubliceerd door: Publicatie datum:
ECN SF 1-12-2002
 
ECN publicatienummer: Publicatie type:
ECN-RX--02-062 Artikel wetenschap tijdschrift
 
Aantal pagina's:
22  

Gepubliceerd in: Accepted for publication in 'Applied Catalysis, 2003' (), , , Vol., p.-.

Samenvatting:
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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