Cu-ZSM-5 zeolite catalysts for the selective catalytic reduction of NOx

Connerton, J, 1999. Cu-ZSM-5 zeolite catalysts for the selective catalytic reduction of NOx. PhD, Nottingham Trent University.

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Abstract

The relationship between copper content and activity for the selective catalytic reduction of NOx by propene in the presence of excess oxygen, of Cu-ZSM-5 catalysts was investigated. Turnover numbers were determined for Cu-ZSM-5 catalysts prepared from an aqueous ion exchange solution, with different copper contents. Turnover number was constant at copper contents < 90-100% exchange. Above this degree of exchange turnover increased by a factor of two, then remained constant up to the highest extent of exchange studied. Results suggest that both isolated copper ions and small metal/oxygen clusters, including dimers catalyse the SCR reaction, with the clusters being twice as active per copper ion. Retention of copper (II) acetate in the dimeric form in ethanol, introduces copper with greater unit activity for the SCR reaction per copper ion, than an aqueous ion exchange solution. XPS/XAES and EXAFS shows that copper is well dispersed within the zeolite matrix; in overexchanged Cu-ZSM-5 copper is present as both isolated Cu(II) ions and small Cu-0 clusters. In Cu-ZSM-5 with a low level of ion exchange, copper is present in the form of isolated ions. Entities introduced by overexchange are thus dimers or small metal/oxygen clusters such as [Cu-O-Cu]2+, these species were shown to exhibit the highest turnover number for selective catalytic reduction.

The reactivity of carbonaceous material deliberately deposited on Cu-ZSM-5 catalysts under lean SCR reaction conditions was examined. The coke deposit exhibited significant activity for NOx reduction. Differences in reactivity were noted, with the rate of reaction, for catalysts with higher degrees copper exchange, reaching a maximum at lower reaction temperatures than catalysts with lower levels of copper ion exchange. The total amount of NOx converted was of the same order of magnitude as the amount of deposited coke. A simple kinetic model was formulated in an effort to understand the differences in reactivity. There was very good agreement between calculated and experimental values. The greater activity of coked catalysts with higher levels of copper exchange was mainly due to a lower activation energy, compared with coked catalysts with lower levels of copper ion exchange. These results showed a variation in the stability of the coke deposit towards NO, which was inversely dependent on copper content; suggesting that the residual acidity of the zeolite could be a factor in determining how catalyst activity varies with copper loading. The results suggest an inverse correlation between Bronsted acidity and catalyst activity. FTIR spectroscopy confirmed the presence of a polyaromatic coke deposit. Characteristics of an adsorbed cyanide species were also observed. The presence both of =NH and -NH2 groups was suggested indicating that organic nitro groups may undergo sequential reduction to form cyanide. Powder XRD revealed that Cu-ZSM-5 undergoes a displacive transformation from monoclinic to orthorhombic framework symmetry upon internal coke deposition. This transformation was reversible upon the removal of coke, by reaction with NO and O2.

Item Type:	Thesis
Creators:	Connerton, J.
Date:	1999
ISBN:	9781369313420
Identifiers:	Number Type PQ10183050 Other
Divisions:	Schools > School of Science and Technology
Record created by:	Jeremy Silvester
Date Added:	03 Sep 2020 15:07
Last Modified:	22 Jun 2023 09:58
URI:	https://irep.ntu.ac.uk/id/eprint/40622

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