Development of supported metal oxide catalysts for use in target specific (VOC) gas sensors
Abstract
This work presents design considerations for the selection and development of catalysts enabling selective hydrocarbon detection when coupled with a microcalorimetric sensor. Specifically, the application of VOx/Al2O3 and Fe2(MoO4) 3 catalysts as selective sensor substrates for electrothermal detection of ethanol in automotive fuel is described. Both catalysts promote the oxidative dehydrogenation of ethanol to acetaldehyde at low temperatures (453 K) with nearly 100% selectivity. In addition, common constituents of gasoline such as benzene, toluene, 1-pentene, 1-hexene, 2-methyl butane, and 2-methyl pentane are inactive and do not alter ethanol partial oxidation rates when introduced in binary and ternary mixtures with ethanol at 453 K. A single catalytic sensor can be used to isolate the heat of ethanol partial oxidation in mixtures containing various parafins, olefins, and aromatics. Due to the reactivity and adsorption properties of aldehydes and ethers over these catalysts at 453 K, a single catalytic sensor cannot be used to isolate the ethanol reaction in mixtures containing oxygenates. Methyl tert-butyl ether (MTBE) is active at sensing temperatures and would provide a false positive response, distorting the ethanol electrothermal signal. The decomposition rate of MTBE in an equimolar mixture with ethanol is 12.7 times higher than the partial oxidation rate of ethanol at 453 K. A multiple reaction scheme allows for the sensing scenario to be changed by pre-reacting MTBE to isobutene and methanol at conditions were ethanol is not active. Pre-reaction of MTBE to methanol does provide a significant advantage as the relative rate of ethanol to methanol at comparable concentration ratios is much higher than the relative rate of ethanol to MTBE, allowing for a electrothermal detection of ethanol at 453 K in this perturbed gas mixture with less than 10% of the response contributed from methanol in equimolar ethanol/methanol mixtures. Ratios of adsorption equilibrium constants for various alcohols were determined in order to model alcohol rates at various concentration ratios. By employing a genetic search algorithm paired with a model of the microsensor describing temperature and concentrations vs. position throughout, unknown concentrations of ethanol and methanol can be estimated from a simulated temperature profile.
Degree
Ph.D.
Advisors
Baertsch, Purdue University.
Subject Area
Chemical engineering
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