Homogeneous and heterogeneous CO chemical kinetics in liquid-nitrogen cooled 5% COHe discharges

Abstract

The CO dissociation of a 5% COHe radiofrequency discharge has been analyzed as a function of CO residence time in discharge, in a reactor cooled at liquid-nitrogen temperature. The gas phase products (CO 2, O, C 2) as well as CO 2 trapped and carbonaceous film deposited on the cooled walls of the reactor, were analyzed by emission spectroscopy and mass spectrometry. The composition of carbonaceous deposit was found to be like C 2OC 3O 2, rather than C-like as generally assumed for COHe discharges. CO 2 and C 2OC 3O 2 formation yields increase with residence time, leading to CO loss up to about 80%, at a residence time of a few seconds. The first step of CO dissociation occurs mainly by electron impact, then the hetrogeneous reactions CO+O+wall→(CO 2) w and CO+C w→(C 2O) w effectively enhance the CO loss. In fact an autocatalytic CO dissociation, which arises by the sink of dissociation products on the reactor cooled walls has been measured. The high CO depletion in COHe mixture, as CO residence time is increasing, leads to an increase of electron mean energy which accelerates the electron impact CO dissociation.