ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 6, pp. 858–860. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.D. Makhaev, L.A. Petrova, B.P. Tarasov, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 6, pp. 928–931.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Hydrolysis of Magnesium Hydride in the Presence
of Ammonium Salts
V. D. Makhaev, L. A. Petrova, and B. P. Tarasov
Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia
Received May 4, 2007
Abstract—The influence of the nature of ammonium salts (NH Cl, (NH ) SO , (NH )HSO , [(CH )NH ]Cl)
4
4 2
4
4
4
3
3
and their concentration in aqueous solutions on the hydrolysis of magnesium hydride has been studied. The
highest degree and fastest rate of hydrolysis are observed at an ammonium salt concentration of ~7.5%. The
most efficient activator among the ammonium salts under consideration is (NH )HSO .
4
4
DOI: 10.1134/S0036023608060077
Light metal hydrides contain the largest amount of hydrolysis of MgH as a function of the nature and con-
2
active hydrogen (12.7% in LiH, 4,2% in NaH, 7.66% in centration of ammonium salts.
MgH ) and are thought of as good candidates for pro-
2
duction of hydrogen, which can be released by various
EXPERIMENTAL
methods, including thermolysis or hydrolysis [1]. The
use of alkali-metal hydrides in hydrolytic processes of
A magnesium hydride powder was obtained by
hydrogen generation is hindered by their extremely hydrogenation of magnesium with hydrogen in the
high reactivity [2]. At the same time, the chemical presence of 4 wt % Fe and 4 wt % V O upon mechan-
2
5
activity of magnesium hydride varies in a wide range ical activation in a ball mill. The sample under consid-
depending on the method of preparation [3–5]. Magne- eration contained ~75% MgH and 17% Mg. Commer-
2
sium hydride obtained as a fine powder by pyrolysis of
organomagnesium compounds is an extremely reactive
substance igniting when brought in contact with water
and rapidly decomposing or self-igniting in air [3, 4].
Magnesium hydride obtained by direct hydrogenation
of metallic magnesium at high temperature and pres-
sure is stable in air, is slowly hydrolyzed by water, and
is rapidly hydrolyzed by acids [5]. The hydrolysis of
cially available acids and ammonium salts of chemi-
cally pure and pure for analysis grades were used as
received. Solutions of HCl, H SO , H PO , NH Cl,
2
4
3
4
4
(
NH ) SO , (NH )HSO , and [(CH )NH ]Cl were pre-
4 2 4 4 4 3 3
pared in distilled water.
Hydrolysis of magnesium hydride was carried out in a
setup for volumetric analysis by treating a weighed sam-
ple of MgH (25–30 mg) with a definite amount of a solu-
2
MgH theoretically yields more than 15 wt % of hydro-
2
tion of one of the above reagents (20 mL). The volume of
released hydrogen was determined volumetrically [1].
gen in terms of the magnesium hydride used:
MgH + 2H O = Mg(OH) + 2H .
(1)
2
2
2
2
RESULTS AND DISCUSSION
Magnesium is an inexpensive and readily available
metal, and the synthesis of magnesium hydride from
elements is an efficient and effective method. The low
activity of magnesium hydride synthesized from ele-
ments in air ensures its safe use. Inasmuch as the use of
acids in processes of hydrolytic hydrogen generation is
often undesirable, it is of interest to search for neutral
reagents that promote the hydrolysis of nonactivated
air-stable magnesium hydride obtained from elements.
As shown in [5], a 5% solution of ammonium chloride
rapidly and quantitatively hydrolyses magnesium
hydride obtained from elements. At the same time, it is
worth noting that the hydrolysis parameters have not
been studied as a function of the ammonium chloride
concentration and the nature of ammonium salt [5]. The
Treatment of the sample with a 10% solution of
phosphoric, hydrochloric, or acetic acid led to rapid
release of hydrogen. The conversion (α) in the reactions
of hydrolysis caused by the action of other reagents was
assessed as the ratio of the amount of hydrogen released
in the reaction with an aqueous solution of a reagent to
the above amount of hydrogen. It is worth noting that the
reaction is highly exothermic and that, when MgH is
decomposed by a small excess of a solution (MgH : solu-
tion = 1 : 10, 1 : 20), the reaction mixture is strongly
heated. Therefore, in all experiments described below,
a large excess of an aqueous solution was used: 20 mL
of a solution per ~30 mg of hydride.
2
2
For comparison with earlier results [5], we studied
present work is aimed at studying the rate and degree of the hydrolysis of magnesium hydride by distilled water
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