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Abstract

Currently, gas detectors are liable to fail unpredictably when working in the field. For the obvious reason of safety the development of reliable detectors of flammable gases at both low and potentially explosive concentrations is essential. The purpose of this investigation has been to study the mechanism by which one poison, tetramethyl lead, deactivates catalytic pellistor type gas detectors. The following objectives have been partially or fully achieved:
(i) An elucidation of the behaviour of catalytic pellistor detectors of more than one
type.
(ii) An understanding of the modes by which they are poisoned.
(iii) The prediction of their behaviour in a hostile environment.
(iv) The development of a more reliable gas detector, incorporating zeolites.

The detectors studied were basically two types : the VQ1, which consists of an alumina bead supporting a platinum/palladium catalyst mixture, and the VQ3, which is similar in design but comprises a thoria supported palladium catalyst. The detectors were poisoned in a specially designed rig with which the catalytic activity could be monitored continuously. A number of physicochemical techniques were employed to examine the interactions of lead with catalyst on the surface.

Of the principal components of a typical gasoline mixture, tetramethyl lead was shown to be the cause of detector deactivation. Modified detector beads coated with alumina did not show a significantly improved resistance to poisoning. However, zeolite-coated detector beads caused a very marked improvement in poison resistance. Surface studies using techniques such as Scanning Electron Microscopy and Energy Dispersive Analysis of X-rays revealed that the poison was evenly dispersed over the outer surface, but penetration into the pore structure was small. Furthermore, nitrogen adsorption studies revealed that there is little correlation (undetectable in the present data) between deactivation levels and surface area changes. More work has been recommended in this area.

In the Discussion section, the significance of the experi- mental results are considered in detail. Lead which is finally present in the oxide form deactivates the catalyst by adsorption on the outer surface without deep penetration into the porous structure, and it appears that the pore mouth poisoning model is an adequate description of the process. Incorporation of synthetic 'large-port' dealuminated mordenite in particular causes the greatest improvement in poison resistance

Publication Type:	Thesis (Doctoral)
Subjects:	Q Science Q Science > QD Chemistry
Departments:	School of Science & Technology School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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