Found some support suggesting that the cathodic reaction I detailed previously above proceeds per the Aluminum path cited above with Magnesium,
namely:
H2O + e- → OH- + .H
along with the corresponding reaction of the created monatomic hydrogen radical with Magnesium to form a hydride:
Mg + 2 .H → MgH2
My support is from a reported observation published long ago from "Chemical News and Journal of Physical Science, Volumes 87-88, p. 312, link: https://books.google.com/books?id=jvjmAAAAMAAJ&pg=RA1-PA... , to quote:
"Solutions of the chlorides of barium, strontium, and calcium were acted upon but feebly by magnesium, but ammonium chloride solution was attacked at
a lively rate. "
Also, on page 314, to quote:
"It is especially interesting to note that the alcohol solution IS much more vigorous in its action on magnesium than is pure water. The urea solution
is relatively vigorously attacked, though, as has been stated, ammonia is also formed in this case."
In my opinion, with respect to the above observation of the reaction between Mg metal and aqueous NH4Cl, it is consistent with the reaction of aqueous
NH4+ with e-, creating the hydrogen atom radical:
NH4+ + e- = NH3 + .H
where the above reaction could be viewed as flowing from the better known reaction:
H+ + e- = .H
as upon adding NH3 to both sides of the above: NH3 + H+ (= NH4+) + e- = NH3 + .H as claimed above. Another way of viewing this is that NH4+ is a
very weak acid:
NH4+ = NH3 + H+ (see https://www.google.com/url?sa=t&source=web&rct=j&... )
So, upon adding e- to each side:
NH4+ + e- = NH3 + (H+ + e-) = NH3 + .H
as required. In my opinion, the above reaction could provide an added avenue for the creation the monoatomic hydrogen radical that, especially in near
neutral water as per the reported experimental observation above, could enhance the reaction rate.
With respect to the observation of increased reactivity with alcohol over water, this supports the idea of an anodic half cell reaction (likely
consuming MgH2) forming solvated electrons, as ethanol is a cited preferred medium over water. Further, the chemical breakdown of Urea to NH3 supports
a possible underlying electron transfer mechanism (like via solvated electrons).
Also note, the cited Magnesium employed in the reference above is said to contain a small amount of a transition metal (Iron) and Aluminum oxide. Per
a source (page 127 of a Phd thesis by Anna Grzech, "Hydrogen Storage in Porous Materials and Magnesium Hydrides", available online as a 17.1MB pdf) to
quote:
"some of the transition metals [13], transition metal oxides [14,15] or hydrides, transition metal halides (NbF5, TiF3, FeF3)[16-18] are
widely investigated as additives. These are believed to act as a catalyst for the chemisorption of hydrogen and transport into the
magnesium phase. [2] Among of these TiF3 additive appears to be a particularly effective catalyst. [10,19,20] "
Abstract of the cited reference [13] "It has been revealed that ball-milling of MgH2 powders with small amounts of selected 3d-transition metals M
such as Ti, Nb, … or oxides of 3d-metals (e.g. Cr2O3) leads to marked improvements of the hydrogen absorption/desorption kinetics"
So, I would expect that select transition metals (or their oxide) impurities, or present as alloys, could increase reaction rate.
[Edited on 25-5-2017 by AJKOER] |
