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.

This paper will review the crystal growth mechanism of zeolites and discuss some of the latest findings in this field. A great effort has been dedicated in recent years to improving the fundamental understanding of zeolite crystal growth during the synthesis process, with the overall aim of increasing control over the zeolite properties and structure. The latest studies on zeolite crystallization in colloidal (clear) suspensions, prepared by addition of organic and inorganic structural directing agents, in comparison to traditional gel synthesis and seed-approach will be discussed. Further understanding of the imperative dependence of zeolite crystallization kinetics' on the nature and mode of mixing of the initial reactants, leading to the formation of amorphous particles, intermediates, and final crystalline zeolites will be presented.