A chemical sensor based on a coated long-period grating has been prepared and characterized. Designer coatings based on polydimethylsiloxane were prepared by the incorporation of diphenylsiloxane and titanium cross-linker in order to provide enhanced sensitivity for a variety of key environmental pollutants and optimal refractive index of the coating. Upon microextraction of the analyte into the polymer matrix, an increase in the refractive index of the coating resulted in a change in the attenuation spectrum of the long-period grating. The grating was interrogated using ring-down detection as a means to amplify the optical loss and to gain stability against misalignment and power fluctuations. Chemical differentiation of cyclohexane and xylene was achieved and a detection limit of 300 ppm of xylene vapour was realized.
a b s t r a c tAn etched long-period grating was used as a refractive index sensor for vapours of four volatile organic compounds, i.e. m-xylene, cyclohexane, trichloroethylene and commercial gasoline. The sensitivity to the vapours was further increased by solid-phase microextraction into a coating made of polydimethylsiloxane (PDMS)/polymethyl-octylsiloxane (PMOS) co-polymer. By further amplification of the optical loss in an optical cavity made of two identical fiber-Bragg gratings and interrogation by phase-shift cavity ring-down spectroscopy we could detect and distinguish xylene (detection limit: 134 ppm) from trichloroethylene (3300 ppm), cyclohexane (1850 ppm) and gasoline (10,500 ppm).
Catalytic processes that employ Ru catalysts in supercritical water have been shown to be capable of converting organics, such as wood waste, into synthetic natural gas (CH4) with high efficiencies at relatively moderate temperatures of around 400 °C. However, the exact roles of the catalyst and the descriptors that would enable the search for other catalysts with high conversions have not been determined. In the current work, we use electronic structure calculations coupled with batch experiments to understand the interaction of methane (CH4) and water (H2O) with a common catalyst material, ruthenium, to understand the final steps of the methanation reaction. The calculations predict that when CH4 and H2O react with the Ru surface, the species will undergo rapid scrambling; interchanging most of the hydrogens with the surface before escaping as CH4 and H2O once again. We conducted experiments using CH4 as a feedstock in supercritical D2O (deuterated water) in the presence of a commercially available carbon‐supported Ru catalyst, and found this mechanism to be confirmed: nearly all reacted CH4 was converted to the fully substituted CD4 or the 3/4‐substituted CHD3 isotopomers, with less significant production of the 1/4‐ or 1/2‐substituted species CH3D and CH2D2. The experiment was repeated with an in‐house impregnated RuO2‐on‐carbon catalyst, with similar results. Although other criteria such as the ability to cleave CC and CO bonds and resistance to poisoning will also prove important, this study suggests that a characteristic of an effective catalyst for supercritical water gasification to methane is its ability to promote rapid equilibria through scrambling mechanisms.
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