Performance upgrade of reactor for ethylene photocatalytic degradation with pulsed direct current DC square-wave

Abstract: In the research, we designed a new-style reactor that included a digital signal controller as the core of voltage source for ethylene photocatalytic degradation, which could carefully control the parameters (voltage, duty cycle and frequency). Area external standard method was used to detect the ethylene content with the gas chromatograph flame ionization detector (GC-FID) every 30 min during the whole research. The photoelectrode film material, i.e. (silver+titanium dioxide)/activated carbon fiber ((Ag+TiO2)/ACF) was made by Ag particles adding, ultrasonication and 60Co-γ irradiation and so on. The (Ag+TiO2)/ACF photoelectrode under photoelectrocatalysis in the pulsed direct current square-wave (PDCSW) or direct current (DC) after 24 h were characterized by means of the energy diffraction spectrum (EDS). The characterization results showed that the Ag/Ti ratio sharply decreased from 1% to 0.24% after 24 hours' DC photoelectrocatalysis, while the ratio was 0.83% after 24 hours' PDCSW photoelectrocatalysis, and the latter exhibited higher activity than the former. The reason for the phenomenon was that the intermittent electric current could keep Ag particles close to the surface of TiO2 for a catalytic effective distance so as to provide a carrier accept electrons. The study compared the photocatalytic degradation efficiency of ethylene under 2 different conditions of PDCSW and DC photoelectrocatalysis, and the ethylene degradation efficiency was evaluated by the K value as the main index. The results showed that the K value of PDCSW was 2.296×10-4 min-1, which was obviously higher than that of DC, so the application of PDCSW to ethylene degradation efficiency would be of great significance. We tried to observe the variation tendency of the K value by adjusting these parameters (voltage, duty cycle and frequency) of pulse electric field in the PDC reactor system, and employed response surface methodology to analyze 3 single-factor experiments to find out the optimum assembly. The research adopted the method of quadratic rotation combination experiment and regression fitting to build up the quadratic multinomial mathematical model of the K value and the parameters, and verified the validity of the model. The 3 single-factor experiments showed that the K value increased at first and then decreased, and reached the maximum when the voltage was 20 V, the duty cycle was 20% and the frequency was 300 Hz. The degradation efficiency increased with the voltage changing from 0 to 20 V and decreased from 20 to 30 V, increased with the duty cycle changing from 0 to 20% and decreased from 20% to 80%, and increased with the frequency changing from 200 to 300 Hz and decreased from 300 to 400 Hz under the condition that the 2 other factors remained unchanged. Response surface analysis showed that the optimum assembly of the K value would achieve the highest efficiency when the voltage was 25.5 V, the duty cycle was 23% and the frequency was 310 Hz. The research can provide a basis for the application of pulsed direct current square-wave photocatalytic technology in the degradation of ethylene of fruit and vegetable in cold storage.