With the recent push towards utilizing ethanol as the primary oxygenate in automotive fuel, there is a demand for sensors capable of on-site control of ethanol/gasoline blending and verification of ethanol content of fuels. Requirements for such technology generally include small size, low power consumption, and selective detection with minimal false responses from mixture components. Current sensor technologies utilize microelectromechanical systems (MEMS) to satisfy size and power constraints; however, selective detection and quantification remains an issue. A new sensing methodology is proposed based on reaction microcalorimetry in which an intrinsically selective catalyst deposited on a thin film temperature sensor is used to selectively oxidize only the target gas analyte.

    For a catalytic sensor to be useful for ethanol detection in multicomponent hydrocarbon mixtures, the catalyst must show high product selectivity towards one exothermic product such as acetaldehyde, be inactive towards background gases at the sensor operating temperature, and exhibit either high rates of ethanol consumption and/or be 1st order in terms of ethanol concentration in order to allow for quantification. Important parameters for designing such a system are operating temperature and catalytic material. Catalysts which exhibit the desired characteristics at low temperatures are desirable in order to limit power consumption of a prospective device and to avoid gas phase oxidation of the more volatile components in gasoline.

    Two promising catalysts for satisfying sensing requirements are VO_x/Al₂O₃ and Fe₂(MoO₄)₃. Acetaldehyde selectivity over VO_x/Al₂O₃ increases with increasing coverage. 84% acetaldehyde selectivity at 180 °C is achieved over a catalyst with 8 V/nm². Ethanol partial oxidation rates over 8 VO_x/Al₂O₃ are zero order in ethanol concentration at 180 °C, with rates equal to 8.8x10^-9 mol ethanol converted/m² s (1.3x10^-6 mol ethanol converted/g catalyst s). In comparison, Fe₂(MoO₄)₃ demonstrates higher selectivity towards acetaldehyde (98%) at 180 °C with ethanol conversion rates zero order in ethanol concentration. However, the rate of ethanol consumption per gram catalyst over Fe₂(MoO₄)₃ is lower than 8 VO_x/Al₂O₃; equal to 8.6x10^-7 mol ethanol converted/g catalyst s. Kinetic investigations involving binary mixtures of ethanol and other hydrocarbons representing key classes of compounds in gasoline have also been conducted at 180 °C. Ethanol partial oxidation rates and acetaldehyde selectivities measured during reactions with equimolar mixtures of benzene or methane with ethanol over 8 VO_x/Al₂O₃ remain unchanged. Neither benzene nor methane show any activity at 180 °C. Reactions involving equimolar mixtures of ethanol and 2-butanol over 4 VO_x/Al₂O₃ exhibit the same selectivity as seen in experiments involving each component individually, but their oxidation rates differ, with ethanol turn over rates dropping from 8.1x10^-4 s^-1 to 2.1x10^-4 s^-1. Both VO_x/Al₂O₃ and Fe₂(MoO₄)₃ are promising catalysts for detection and quantification of ethanol through selective oxidation to acetaldehyde in the presence of less reactive hydrocarbons.