Microwave discharge electrodeless lamps (MDEL)

Abstract

A novel mercury-free Dewar-like (double-walled structure) microwave discharge thermally-insulated electrodeless lamp (MDTIEL) was fabricated and its performance evaluated using the photo-isomerization of trans-urocanic acid (trans-UA) in aqueous media as a test process driven by the emitted UV light when ignited with microwave radiation. The photo-isomerization processes trans-UA → cis-UA and cis-UA → trans-UA were re-visited using light emitted from a conventional high-pressure Hg light source and examined for the influence of UV light irradiance and solution temperature; the temperature dependence of the trans → cis process displayed a negative activation energy, E(a) = -1.3 cal mol(-1). To control the photo-isomerization of urocanic acid from the heat usually dissipated by a microwave discharge electrodeless lamp (single-walled MDEL), it was necessary to suppress the microwave-initiated heat. For comparison, the gas-fill in the MDEL lamp, which typically consists of a mixture of Hg and Ar, was changed to the more eco-friendly N(2) gas in the novel MDTIEL device. The dynamics of the photo-isomerization of urocanic acid driven by the UV wavelengths of the N(2)-MDTIEL light source were compared to those from the more conventional single-walled N(2)-MDEL and Hg/Ar-MDEL light sources, and with those from the Hg lamp used to irradiate, via a fiber optic, the photoreactor located in the wave-guide of the microwave apparatus. The heating efficiency of a solution with the double-walled N(2)-MDTIEL was compared to the efficiency from the single-walled N(2)-MDEL device. Advantages of N(2)-MDTIEL are described from a comparison of the dynamics of the trans-UA → cis-UA process on the basis of unit surface area of the lamp and unit power consumption. The considerably lower temperature on the external surface of the N(2)-MDTIEL light source should make it attractive in carrying out photochemical reactions that may be heat-sensitive such as the photothermochromic urocanic acid system.

Microwaves in Photochemistry and Photocatalysis

“…These EDLs were examined (under the optimized conditions) for using in AOPs (photoassisted or photocatalyzed degradations). A novel Hg-free Dewar-like (double-walled structure) microwave discharge thermally insulated electrodeless lamp (MDTIEL) was fabricated by Horikoshi et al [32]. The gas fill in the lamp was changed to the more eco-friendly N 2 gas.…”

“…Horikoshi and co-workers [32,119] also developed a metallic condensing cone that concentrates MW radiation (equivalent to an optical lens) into an MW-UV photoreactor and used it as a part of the system to activate a quartz Hg-EDL (185 and 254 nm) in the oxidative treatment of wastewater. This approach to self-ignition of an EDL immersed in solution led to considerable energy savings.…”

“…Campanella et al reported a minor but positive enhancement of the photocatalytic degradation efficiency of o-and p-CP aqueous solutions by MW heating [188]. The success of these model chemical systems offered an Photoisomerization of trans-to cis-urocanic acid (aqueous) [32] extension to other environmentally interesting compounds, for example, sodium dodecylbenzenesulfonate or organophosphate pesticides. It has been suggested that MW-assisted photodegradation of pollutants may be of great interest in the future.…”