Catalytic Reactions of Highly Conjugated Polymers: the Removal of Nitrogen Monoxide from Gas Streams using Pyrolysed Polyacrylonitrile

Abstract

This work is concerned with the use of pyrolysed polyacrylonitrile,a highly conjugated organic polymer, as a heterogeneous catalyst for the removal of nitrogen monoxide from motor vehicle exhaust gas. Previous work had shown that the polymer was capable of removing nitro-gen monoxide from gas streams and it was intended to optimise the activity of the polymer and to investigate the nature of the reaction. Although attempts were made to maximise the surface area of the polymer by coating it on to high surface area inorganic supports prior to pyrolysis, the bulk of the experimental work was carried out using pyrolysed polyacrylonitrile fibre as the catalyst. BET nitrogen adsorption measurements, infrared spectroscopy, electron spin resonance spectroscopy and electron microscopy were used to characterise the catalyst samples.Kinetic studies showed that the removal of nitrogen monoxide bypyrolysed polyacrylonitrile commenced at about 200° C and increased in rate with increasing temperature. At 5OO° C the initial nitrogen monoxide removal rate by the polymer was 2.4 x 10-5 mol min-1 - 1 g-. The rate of nitrogen monoxide removal decreased with time due to the formation of oxides on the polymer surface. Nitrogen was believed to be the other main product of the reaction. At temperatures above 720º C continuous oxidation of the polymer by nitrogen monoxide took place. Oxygen also reacted readily with the polymer at temperatures above 200°C. Electron spin resonance studies indicated that unpaired electrons in the polymer structure were involved in the reactions with nitrogen monoxide and oxygen. An activation energy of 10.8 kJ mol-1 was measured for the reaction between nitrogen monoxide and the 'clean' polymer surface. A mechanism is proposed for the reaction between nitrogen monoxide and pyrolysed polyacrylonitrile and a critical survey is given of the use of the polymer for the removal of nitrogen monoxide from motor vehicle exhaust gas.