In-situ characterisation of the interaction of metal-organic frameworks with gases::towards novel exhaust gas sensors

Combustion of fossil fuels in vehicles results in emission of gases such as NOx and CO2; these gases can lead to diminished air quality and contribute to global warming. The environmental impact and introduction of legislation (i.e. EU DIRECTIVE 98/69/EC) have made the regulation of emissions from road vehicle exhausts essential. Research into the sensing of pollutant exhaust gases can result in more efficient fuel use if the measured gas concentrations are fed back into the on-board computer, which can control fuel and air intake levels in modern road vehicles. The aim is to produce suitable exhaust gas sensor technologies and thin film sensor elements from porous materials. The final goal is more sensitive and more selective sensors.Metal-organic frameworks (MOFs) are porous coordination polymers that are comprised of metal secondary building units and organic ligands which possess BET surface areas typically >1500 m2 g-1. The ability to tailor the organic ligands and the metal secondary building units for specific and selective interactions with certain small molecules make metal-organic frameworks an attractive option as gas sensing materials/gas selective filters. The MOFs selected for this work were Cr-MIL101, Cr-MIL101-NH2, Cr-MIL101-SO3H, Fe-MIL101, Fe-MIL101-NH2 and ZIF-8. Cr-MIL101 & Fe-MIL101 possesses good thermal and hydrolytic stability and the derivatives are expected to show increased selectivity to particular gases. Furthermore, in the literature, it has been demonstrated that ZIF-8 possesses high selectivity for CO2. The porous MOFs were used to form thin films deposited onto silicon substrates. The interaction of the MOF thin films with common exhaust gases (CO, NO, NO2 etc.) was investigated using in-situ FT-IR spectroscopy with concurrent gas exposure. The thin films were exposed to different concentrations, at room temperature, (from 1 – 100 ppm) of the individual analyte gases (diluted in argon carrier gas). These FT-IR spectroscopy experiments enabled the key interactions of the different MOF thin films with the different analyte gases to be probed; intensity of the bands and how far the band has been shifted from the free gas wavenumber value both indicate how strongly the gas interacts with the MOF. Thus this enabled selection of the most appropriate MOF for the detection of each analyte gas. In addition this approach showed that measurable changes in the IR spectra of the MOF materials occur at gas concentrations down to 1-3 ppm. Current solid state metal oxide oxygen pump NOx sensors in some vehicles have a lower operating limit of 30 ppm and thus these MOFs show promise for low level concentration exhaust gas sensing.