VOC Gases Research Articles

Preparation of total VOC sensor with sensor-response stability for humidity by noble metal addition to SnO2

Noble metals addition to SnO2 was examined in order to develop high sensitive total volatile organic compounds (T-VOC) sensors with high stability for humidity. The noble metals loaded SnO2 thick films were prepared, and their sensor responses were measured for various kinds of VOC gases and a T-VOC test gas. The results of the measurements for various kinds of VOC gases showed that simultaneous addition of Pt, Pd and Au or simultaneous addition of Pt and Au were useful for sensitization for VOC gases giving a small response for the unloaded SnO2 thick films. These sensors could detect 0.056 ppm of the T-VOC test gas, and the sensor responses increased with increasing the concentration of the T-VOC test gas. The series of additives with Pd was effective to improve the stability for humidity. The sensor loaded with Pt, Pd and Au simultaneously satisfied both high sensitivity to the T-VOC test gas and sensor-response stability for humidity. The results of X-ray photoelectron spectroscopy of the sensor showed that most of loaded palladium exists as PdO. In addition, it was suggested by the shift of binding energy of a Sn3d orbit and an O1s orbit that the electronic sensitization was dominant in the sensor. It was supposed that the electronic sensitization was effective for the sensor response stability for humidity. From these results, the combination of the chemical sensitization by Pt-Au addition and the electronic sensitization by Pd addition gives high sensitivity to the T-VOC test gas and stability for humidity to the Pt-Pd-Au loaded SnO2 thick films, and it is expected that the SnO2 thick film with Pt, Pd and Au will be applicable to T-VOC gas sensor.

Photochemical Reaction of VOC Gases on the Visible Light Photocatalyst Surfaces

We report the photochemical reaction of volatile organic compounds (VOC's), such as acetaldehyde and toluene, over nitrogen doped TiO2(TiO-N) under visible light irradiation, using liquid chromatography (LC), gas chromatography (GC) and gas chromatograph mass spectrometer (GC-MS) for gases analysis, and diffuse reflectance fourier transform infrared spectroscopy (DRIFTS) and ion chromatography (IC) for adsorbed organics. We evaluated the photo degradation rates and intermediates for the above gases. As a result, the degradation rates of acetaldehyde gas on TiO-N were 1.2-3 times greater than those on TiO2, and those of toluene gas on TiO-N were 4-49 times greater than those on TiO2. Low concentrations of acetone and acetic acid were produced during the photo degradation of acetaldehyde by the TiO-N. From toluene, very low concentrations of formaldehyde, acetaldehyde, acetone and benzaldehyde gases were produced. We also revealed the decomposition processes of toluene. Results obtained indicate that toluene, weakly adsorbed on the photocatalyst surface, is initially photooxidized to benzaldehyde which adsorbs onto the TiO-N surface more strongly, leading to the formation of ring-opening products such as carboxylic acids and aldehydes. These intermediate products were gradually photodegraded to CO2 and H2O under visible light irradiation.

Layered Hybrid Thin Film of Molybdenum Trioxide with Poly(2,5-dimethylaniline) for Gas Sensor Sensitive to VOC Gases in ppm Level

Abstract A layered organic/inorganic hybrid thin film consisting of MoO3 with a double-methylated polyaniline derivative, poly(2,5-dimethylaniline), showed a distinctly increasing resistivity as a response to ppm levels of aldehydic gases, and its sensitivity to acetaldehyde is higher than that to formaldehyde, which can be explained by solubility parameter theory.

Biodegradation of High Concentration Isopropanol Vapour in a Biofilter Inoculated with a Solvent-Tolerant Microbial Consortium

The aerobic biodegradation of high vapour-phase concentrations 2-propanol (IPA) by a previously enriched solvent-tolerant bacterial consortium within a 1.91 biofilter was investigated. Successful biofiltration of solvent vapour concentrations of up to 10,500ppm v was implemented. The aforementioned solvent-tolerant bacterial cells were immobilized onto porous glass cylinders as a means of bioprocess intensification. Biofilter start-up and acclimation were studied and acetone concentration tracked as an indicator of IPA utilization, as the sole carbon source within a minimal salts medium (MSM). Removal efficiencies of up to 100% for both IPA and acetone were successfully demonstrated by this biofiltration system during steady state operating conditions. IPA vapour feed concentrations were varied between 6500 and 10,500ppmv during biofilter start-up and acclimation, where the lowest removal efficiency obtained was 47% during the initial 48 h of operation. It is also proposed that metabolic lag within the biofilm occurs during the acclimation period, due to the presence of the dual substrate system of IPA and acetone and also mass transfer limitations. Analysis of the biofilm throughout the bed demonstrated that most dense growth occurred at the top of the biofilter, with minimal growth present in close proximity to the inlet. The results demonstrate the potential of a previously enriched solvent-tolerant bacterial consortium in biofilter systems, for the aerobic treatment of concentrated VOC gases at levels up to 2 orders of magnitude higher than other studies dealing with gas phase IPA biofiltration.

Transient Diffusion, Adsorption, and Emission of Volatile Organic Vapors in Soils with Fluctuating Low Water Contents

AbstractLaboratory experiments were conducted on large packed soil columns (15 cm diam., 40 cm length) under unsaturated conditions. Trichloroethene (TCE) was introduced at the bottom of the columns. The emissions of TCE from the soil surface were monitored. The soil‐water content at the soil surface was changed by alternating dry and water‐saturated air sweeping over the column surface, thus changing the vapor sorption capacity of the soil. The retardation of VOC gas transport, due to the change in adsorption capacity with change in soil‐water content, was determined by sampling the soil‐air and the sweep air flushing the column headspace. Soil‐water content was monitored by the time‐domain reflectometry (TDR) technique. The transport of VOC gases from experiment initiation until steady‐state was simulated using a numerical simulation model, in which diffusion and equilibrium partition theory were incorporated. Measured and simulated concentration profiles as well as volatilization fluxes agreed very well. The decrease in soil‐water content, caused by sweeping dry air over the column surface, resulted in an increase in the soil adsorption capacity up to several orders of magnitude (mainly in the top few centimeters of soil). Both increase and decrease in soil‐water content affected the volatilization flux manifested in large rapid peaks of TCE volatilizing from the surface. This behavior was likewise simulated using a numerical model with changing diffusion coefficients and retardation factors as function of soil‐air content, and good agreement was obtained between model simulations and measured data.