A comparison of the conversion of isopropyl alcohol by non-thermal plasma and thermally-driven catalysis using in-situ FTIR spectroscopy

Abstract

In-situ Non-Thermal Plasma (NTP) InfraRed (IR) transmission and reflectance cells were designed, fabricated, modified and commissioned in order to facilitate the study of the catalysed NTP conversion of IsoPropyl Alcohol (IPA). IPA was chosen as the model compound because of its relevance to domestic air pollution and the significant body of infrared data on the reaction of the compound in thermal and NTP systems. As catalyst selection for NTP systems has often been made based on those materials active in the analogous thermal systems, the NTP experiments were supported by studies employing a Diffuse Reflectance Infrared Fourier Transform system and a commercial environmental chamber capable of being heated up to 600 °C in a controlled atmosphere. The initial studies focussed on Macor, a ceramic comprising predominantly the oxides of Al, Mg and Si as they were intended to furnish benchmark data, as it was assumed that Macor would be inactive, but had a reasonable dielectric constant and was thermally stable. Macor did indeed prove inactive towards IPA on heating up to 600 °C: however, the material was highly active in the NTP process, with IPA reacting in the bulk of the plasma to produce acetone, which then reacted at the Macor to produce a polymer and isophorone. In addition, HCN, methane and cold CO, CO at ca. 115 K, were produced in the plasma bulk: the production of methane and cold CO was interpreted in terms of the fragmentation of acetaldehyde, produced but not directly observed, via a loose transition state. SnO2 and CeO2 were also selected for study: in the former case primarily because of the wealth of IR data on the oxide from previous work in Newcastle, and the CeO2 was selected for its known activity towards IPA and the concomitant IR data in the literature both from thermal and NTP studies. IPA did not react in the plasma over SnO2, but reacted to produce acetone and then CO2 in the analogous thermal experiments. In contrast, IPA did react over CeO2 in the plasma, giving the same products as with Macor and it also reacted at CeO2 in the thermal experiments, to produce acetone and then CO2, the products and their onset temperatures depending strongly on the pretreatment of the CeO2 to remove adventitious adsorbed carbonates and bicarbonates. The NTP-driven process did not appear to be inhibited by these species.