Nitrogen oxides (NOx) -mostly caused by the combustion of fossil fuels- contribute to the formation of ozon, smog and acid rain, so lowering NOx emissions to the atmosphere is desired. [1] Phosphotungstic acid is a selective NOx adsorbent with high adsorption capacity that could be used to capture NOxmolecules from exhaust gases and later release them for processing. [2]A variety of experimental and computational approaches - including ex-situ and in-situ X-ray diffraction, neutron diffraction, NMR and DFT calculations - determined the nature of the chemisorbed NOxspecies and elucidated the reaction behavior.Experiments show that at high temperature, NOx adsorption occurs, while at low temperature, the reverse reaction takes place. DFT calculations could assign this behavior to the entropy change during the reaction. Upon chemisorption of 3 NO molecules and 3 NO2molecules, 15 water molecules are released from the phosphotungstic acid hexahydrate (See Figure). This gives rise to a positive reaction entropy whereas the reaction energy is also positive. The free energy is negative -and thus chemisorption occurs- when the temperature are sufficiently high.The insights from this study will allow optimization of the adsorbent material for application in after-treatment systems for NOxelimination.[1] TEDxGhent talk: http://youtu.be/SvhLtiBDJ3s[2] Heylen, S., Joos, L., Parac-Vogt, T. N., Van Speybroeck, V., Kirschhock, C. E. A. and Martens, J. A., Angew. Chem. Int. Ed. 2012 , 51 (44), 11010-11013

Entropy-driven chemisorption of NO<sub>x</sub> on phosphotungstic acid

248th National Meeting (2014)

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Entropy-driven chemisorption of NOx on phosphotungstic acid

Entropy-driven chemisorption of NO_x on phosphotungstic acid