Following the decomposition of AlH3 to yield free hydrogen, why is the reaction not reversible so H2 can be added back to the aluminum to produce AlH3? As an ideal storage medium one must be able to remove the H2 and then add it back to the Aluminum storage medium to reform AlH3. If AlH3 is stable one thinks that the reaction should be reversible. Is there some fundamental reason why the reaction is not reversible. I believe it has something to do with the high reactivity of Magnesium and reaction sites being blocked or no longer available. I would like if from someone with experience with the Aluminum/Hydrogen system who has more experience can supply more scientific detail other than my vague speculation.
It is a question of the Kinetiks and the entropy. You form 5 molecules out of 2 if you decompose 2 AlH3 molecules, which leads to a beneficial entropy. Also you would need a catalyst or high pressure to rearrange the atoms to regenerate ALH3, otherwise it would be a statistically unfavorable process with to many atoms to react at once.
Maybe this paper will be nice to read for you.
DOI: 10.2172/899889
Sincerely
Indeed AlH3 (also called alane) is a promising hydrogen storage material due to its high theoretical gravimetric capacity for hydrogen (which can be easily calculated). When you dehydrogenate AlH3 (in the desoption reaction), you will get hydrogen gas and Al power (which is also called the spent fuel). In order to reverse this reaction, that is, to form AlH3 from hydrogen gas and Al powder, you will need very high pressure as well as high temperature as the reaction is not thermodynamically favorable at room temperature. Hence, from the hydrogen storage standpoint for light-duty vehicular application, this material is not viewed as a reversible reaction. Current research shows that the high pressure and temperature needed for hydrogenation of aluminum powder can be somewhat lowered by using a catalyst.
Hope this helps!
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