Abstract
Improvements in the efficiency of combustion within a vehicle can lead to reductions in the emission of harmful pollutants and increased fuel efficiency. Gas sensors have a role to play in this process, since they can provide real time feedback to vehicular fuel and emissions management systems as well as reducing the discrepancy between emissions observed in factory tests and 'real world' scenarios. In this review we survey the current state-of-the-art in using porous materials for sensing the gases relevant to automotive emissions. Two broad classes of porous material - zeolites and metal-organic frameworks (MOFs) - are introduced, and their potential for gas sensing is discussed. The adsorptive, spectroscopic and electronic techniques for sensing gases using porous materials are summarised. Examples of the use of zeolites and MOFs in the sensing of water vapour, oxygen, NOx, carbon monoxide and carbon dioxide, hydrocarbons and volatile organic compounds, ammonia, hydrogen sulfide, sulfur dioxide and hydrogen are then detailed. Both types of porous material (zeolites and MOFs) reveal great promise for the fabrication of sensors for exhaust gases and vapours due to high selectivity and sensitivity. The size and shape selectivity of the zeolite and MOF materials are controlled by variation of pore dimensions, chemical composition (hydrophilicity/hydrophobicity), crystal size and orientation, thus enabling detection and differentiation between different gases and vapours.
Highlights
Introduction a Department of ChemicalEngineering, University of Bath, Claverton Down, Bath, BA2 7AY, UKClimate change and the need to combat its associated negative effects constitutes one of the key global challenges of the modern age
The results suggested that whilst legislation imposed on factory-tested nitrogen oxides (NOx) emissions had achieved a reduction from 0.5 g kmÀ1 to 0.2 g kmÀ1 since the year 2000, real world emissions had stagnated at around 1 g kmÀ1.12 Another review of real world fuel consumption by Mock et al summarising customer reviews of observed mileage, on-board emissions measurement and laboratory tests, showed that whilst in 2002 the average vehicle had 7% higher fuel consumption on the road compared to the NEDC, in 2012 the difference had increased to 25%
It is clear that porous materials, exemplified by zeolites and metal–organic frameworks (MOFs), offer tremendous flexibility and versatility in terms of selectivity and sensitivity and demonstrate great promise for the fabrication of sensors for exhaust gases and vapours
Summary
The major motivations for the use of sensors to monitor automotive emissions are the legislative requirements to reduce harmful emissions that have negative effects on health and air quality and contribute to global climate change. Carslaw et al used road-side remote sensing equipment to measure real world vehicle NOx emissions from a large number of vehicles.[12] The results suggested that whilst legislation imposed on factory-tested NOx emissions had achieved a reduction from 0.5 g kmÀ1 to 0.2 g kmÀ1 since the year 2000 (with all new vehicles sold meeting this standard), real world emissions had stagnated at around 1 g kmÀ1.12 Another review of real world fuel consumption by Mock et al summarising customer reviews of observed mileage, on-board emissions measurement and laboratory tests, showed that whilst in 2002 the average vehicle had 7% higher fuel consumption on the road compared to the NEDC, in 2012 the difference had increased to 25%.13 Such studies are motivating the automotive industry to move away from current legislative testing on idealised drive cycles and replace this with more dynamic and representative test cycles.[14] This change presents significant opportunities for emissions sensing. In both of these cases, the different applications will require sensors with different characteristics in terms of the intrusiveness of instrumentation (size, ability to install on vehicles), absolute measurement accuracy, sensor/measurement chain dynamics, feedback and cost
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