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Black plate (246,1) is 0.9 kJ mol 1. Thus, the much larger energy involved in confining H2 in the zeolite than in liquid hydrogen offers promise that zeolites might be used as storage media for hydrogen, the validity of which is discussed in the following section. The value of DH 1⁄4 10:3ð+0:5Þ kJ mol 1 for H2 in Na-ZSM-5 can be compared with its equivalent of DH 1⁄4 6:5ð+0:5Þ kJ mol 1 for H2 in Li-ZSM-560 and DH 1⁄4 9:1ð+0:5Þ kJ mol 1 for H2 in K- ZSM-561. A useful comparison may also be made between the entropy change that occurs for adsorption of H2 in Na-ZSM-5, DS 1⁄4 121ð+10Þ J mol 1 K 1, and the absolute entropy for dihydrogen of 163J mol1yK1 at a temperature of 100K and a pressure of 1 mm Hg, which are close to the prevailing conditions in the zeoliteyH2 measurements. We may deduce therefore that the standard entropy of the adsorbed phase is about 40 J mol 1 K 1, probably due to the conversion of translational modes (gas) to low- lying vibrational modes (of surface ‘‘bonds’’) with the retention of considerable rotational freedom58. 5. Zeolites as hydrogen storage materials In an effort to address the twin-problems of dependence of nations upon imported hydrocarbon fuel and climate change forcingyglobal warming induced by emissions of CO2 from burning fossil fuels, hydrogen is being investigated as a clean, carbon-free fuel that could be made on a national (or regional) basis7. However, hydrogen is not actually a ‘‘fuel’’ but an energy carrier. Hydrogen is not available in its aboriginal form as are oil, coal or gas, (which are known as ‘‘primary fuels’’), but rather it must be ‘‘made’’ by some artificial means. Most of the hydrogen currently used in the world (mainly for chemical purposes, such as the wholesale manufacture of ammonia for fertilisers and hydrocarbon reforming in the petrochemical industry) is produced from natural gas by a process known as ‘‘steam reforming’’ sometimes with the use of a zeolite either as a catalyst or to separate62 the carbon monoxide formed when the carbon is ‘‘extracted’’ from methane by its reaction with steam: CH4 þH2O?COþ3H2; or to separate carbon dioxide when that CO is used to squeeze-out another molecule of H2 by adaptations of the ‘‘water-gas shift reaction’’: 246 Christopher J. Rhodes COþH2O?CO2 þH2:PDF Image | Properties and applications of zeolites
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