Application of VOx/Al2O3 and Fe2(MoO4)3–MoO3 catalysts for the selective reaction and detection of ethanol in multi-component hydrocarbon fuel mixtures
Graphical abstract
Research highlights
▶ Selective reaction of ethanol in typical gasoline constituents is possible. ▶ Metal oxides provide chemical specificity for electrothermal ethanol detection. ▶ Catalyst and operating temperature selection are vital for catalytic sensor design.
Introduction
Recent emphasis on the use of ethanol as the primary oxygenate in automotive fuel has energized a demand for microsensors capable of pre- and post-engine analysis of ethanol content in complex volatile organic compound (VOC) mixtures [1]. Current VOC microsensor technologies are capable of only handling nonselective applications and are not adaptable for the analysis of ethanol in complex hydrocarbon mixtures including automotive fuels. Arrays comprised of 100 s of microsensors must be used to generate any degree of specificity in order to handle VOC mixtures of the complexity seen in automotive applications [2], [3], [4]; however, the use of microsensor arrays brings the disadvantages of higher power consumption and complex analytical programs, while still providing undesirable false positive responses [3].
New sensors will require the precision and accuracy of laboratory scale analytical tools, but at the same time must be inexpensive, low power, fast responding, and portable. Numerous classes of micro-gas sensor technologies are available and fabricated using MEMS or microsystem techniques, including semiconductor, dielectric, catalytic, resonance, and electrochemical sensors that transduce resistance, capacitance, temperature, or mass changes, among others [5], [6]. However, all such microsensors are limited by their ability to sufficiently detect a specific analyte in complex multi-component gas mixtures [5], [6], [7], [8]. Chemical specificity is the key bottleneck in the design of micro-gas sensors [5].
The sensing schematic explored in this work centers on addressing the issue of VOC selectivity without utilizing complex sensor arrays to accomplish such a goal. It builds upon the concept of a catalytic sensor by introducing a reactant selective catalyst to uniquely oxidize only the target VOC while allowing all other non-target VOCs to pass by it without generating a response. Commercially available catalytic microsensors use catalysts and conditions where all VOCs are unselectively oxidized [9]. In the new sensor scheme presented (Fig. 1), reaction and quantification of the target VOC will come via the placement of a selective catalyst on top of a microcalorimetric sensor. Temperature changes resulting from an isolated ethanol partial oxidation event can be transduced on chip into a quantitative measurement of ethanol concentration.
The challenge addressed in this work is the choice of a catalyst that oxidizes ethanol selectively in the presence of various classes of hydrocarbons representative of those present in automotive fuel. This can take the form of either selective adsorption and reaction of the target VOC in question or in the case where multiple species are active, by operating at a temperature at which the reaction rate of the target VOC is significantly higher than that of the non-target VOCs. On top of reactant selectivity, it is critical that the sensing catalyst provides the highest ethanol oxidation rate/gram possible as the sensitivity of such a sensor is directly tied to the amount of energy released due to reaction. Catalytic oxidation of ethanol to CO2 is highly exothermic and favorable for maximum sensor sensitivity; however, this reaction typically occurs at temperatures where other hydrocarbons are also active for oxidation reactions [10]. A strong candidate explored in this paper is the partial oxidation of ethanol to acetaldehyde. This reaction pathway provides a significant heat of reaction while occurring at low temperatures over several metal oxide catalysts, thus minimizing the chance of undesirable side reactions involving non-target VOCs.
Metal oxide catalysts, specifically those containing dispersed vanadium and molybdenum oxides have a long history of catalytic reactions involving hydrocarbons [11], [12], [13], [14]. Studies have shown that these oxides provide selectivity for reactions converting methanol to formaldehyde with selectivity as high as 96% at low conversions for temperatures under 573 K [11]. Another candidate is iron molybdate, which is a commercial catalyst for the production of formaldehyde from methanol. This catalyst exhibits formaldehyde yields as high as 95% and conversions approaching 100% at reaction temperatures under 673 K [12], [13], [14], [15].
This work explores the application of vanadium and molybdenum oxides as selective catalysts for the detection of ethanol in complex hydrocarbon mixtures when coupled with a microcalorimetric sensor. A particular focus is on the impact of non-target VOCs on the rates and selectivity of the ethanol oxidative dehydrogenation reaction in order to investigate the impact on ethanol quantification. Gas mixtures studied are limited to binary and ternary mixtures of representative hydrocarbons present in automotive fuel in order to identify classes of compounds against which ethanol can be detected. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) studies and temperature programmed desorption were used to study the surface species present during reaction. These studies highlight that both selective adsorption and temperature can be used to achieve high reactant selectivity for target VOCs.
Section snippets
Catalyst preparation
Supported metal oxide catalysts for this study were prepared by incipient wetness impregnation of γ-Al2O3 (Alcoa, HiQ® 7214F, surface area 151 m2 g−1, pore volume 0.50 cm3 g−1) with aqueous solutions of ammonium metavanadate (NH4VO3, Aldrich). Catalysts with vanadium surface densities ranging from 2 to 8 V-atoms/nm2 were prepared. Oxalic acid at an oxalic acid/metal oxide salt weight ratio up to 1:1 was used to increase precursor solubility at higher metal oxide loadings. Metal atom surface
Structural characterization of VOx/Al2O3 and Fe2(MoO4)3
Edge energies determined from UV–vis absorbance spectra for supported vanadium oxide catalysts on alumina with surface densities ranging from 2 to 8 V/nm2 were characterized (data not shown) and are consistent with trends and values reported in the literature [22], including the emergence of a second edge for catalysts with surface densities of 8 V/nm2 and higher. Absorption edge energies decrease from 3.30 eV to 2.15 eV with increasing surface density from 0.5 to 8 V/nm2 and characterize a
Conclusions
Selective reaction of ethanol in mixtures involving many of the hydrocarbons present in fuel is shown to be possible using either 8VOx/Al2O3 or Fe2(MoO4)3–MoO3 as the active catalyst coupled with a microthermal gas sensor. Both catalysts meet the requirements of promoting the oxidative dehydrogenation of ethanol to acetaldehyde at low temperatures with nearly 100% selectivity. Comparing the two, Fe2(MoO4)3–MoO3 provides a higher acetaldehyde production rate on a per gram basis and thus would
Acknowledgements
This work, including graduate student support for J.E. Gatt, was provided by NSF (CBET Career Award #0644707) and Purdue University. H. Nair was supported by the Bilsland Dissertation Fellowship at Purdue University.
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