Increased consumption of fossil and bio-fuels leads to excessive emissions of CO2, SOx, NOx, toxins and heavy metals, and is a major cause of environmental pollution, poor air quality and global warming. The implementation of increasing numbers of diesel engines and widespread use of bio diesel in transport reduces carbon emissions in operation, but significantly increases the emission of NOx. This is particularly the case in developing countries with heavy and ageing industry and high population densities, where the poor air quality is responsible for over 1 million premature deaths as stated in reports from the World Health Organisation and International Energy Agency. Capture and elimination of toxic gases are, therefore, important targets for a sustainable society.
NH3 is a major chemical commodity as a fertilizer for food production, and is widely regarded a cleaner energy resource owing to its potential zero-carbon-emission at the point of use via conversion to N2 and H2 or within a fuel cell. In comparison to H2, other major advantages of using NH3 as an energy carrier include high volumetric energy density, facile liquefaction for transport, and high octane number. Indeed, it has been proposed that NH3 can act as an excellent H2 store for use in the hydrogen economy. NH3, urea and hydrocarbons can be used to reduce and degrade NOx from combustion engines via the selective catalytic reduction (SCR) based upon metal oxide catalysts, preferably to form N2. However, practical and domestic applications of NH3 are restricted by its corrosive nature, the underdevelopment of economically-viable storage/transportation methods, and potential competition with the food supply chain.
The objectives of the research programme were to synthesise novel metal-organic framework (MOF) materials to address for:
(i) selective capture of toxic gases such as SO2, NO2 and related species
(ii) development of reversible high capacity NH3 stores for portable applications;
(iii) selective catalytic reduction (SCR) of captured NOx within confined nanopores.
MOFs are crystalline porous materials that comprise of metal or metal cluster nodes bridged by polydentate ligands, typically polycarboxylates. The project aimed to develop new porous ultra-stable MOF materials for capture and conversion of toxic and corrosive gases and to generate real understanding of the fundamental chemistry underpinning how these advanced materials operate. It should be emphasised that at the start of this project there were no MOF materials reported that were sufficiently stable or robust to tackle the storage or capture of these highly toxic and corrosive chemicals over multiple cycles, let alone to apply them to any catalytic conversion of NH3 or NO2.
