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T.T.T. Nguyen et al. / Journal of Alloys and Compounds 645 (2015) S295–S298
2.4. Rehydrogenation
These results suggest the reaction sequence given in Eqs. (2)–
(4). ZnCl2 and LiNH2 may react in a 1:2 ratio to give Zn3N2 and LiCl
with the release of ammonia (Eq. (2)). If further LiNH2 is present a
subsequent reaction with Zn3N2 is possible yielding LiZnN with the
evolution of ammonia (Eq. (3)). These two reactions can be
combined to give Eq. (4).
Samples were ground and sealed in a reaction vessel in an argon-atmosphere
glovebox and then transferred to the hydrogenation apparatus. Hydrogenation of
the samples was attempted at 200 and 300 °C under 90 bar H2 for up to 24 h.
3. Results and discussion
3ZnCl2 þ 6LiNH2 ! Zn3N2 þ 6LiCl þ 4NH3
ð2Þ
ð3Þ
3.1. Zinc chloride and lithium amide
Zn3N2 þ 3LiNH2 ! 3LiZnN þ 2NH3
Overall reaction:
The reactions between ZnCl2 and LiNH2 were studied at molar
ratios of 1:2, 1:3, 1:4, 1:5, and 1:6 in the temperature range
150–600 °C. The heating rate was 2 °C/min and the reactions were
kept at the set temperature for 12 h.
ZnCl2 þ 3LiNH2 ! LiZnN þ 2LiCl þ 2NH3
ð4Þ
The evolution of ammonia was confirmed by TPD–MS, where a
broad peak at 280 °C corresponding to ammonia gas was observed.
A lesser amount of hydrogen gas was also observed beginning
above 300 °C and peaking at the higher temperature of about
400 °C (Fig. 2a). The appearance of a small amount of hydrogen
here may be from the decomposition of NH3 [18]. It should be
noted that although the 1:2 ratio should in principle yield Zn3N2
stoichiometrically according to Eq. (2), both nitrides were observed
under virtually all the reaction conditions and ratios studied, indi-
cating that, once formed, Zn3N2 competes with ZnCl2 to react with
LiNH2 via Eq. (3).
For samples prepared at 150 °C, along with ZnCl2 and LiNH2,
LiCl was observed in the reaction products. The presence of LiCl
is believed to result from a salt metathesis reaction such as that
in Eq. (1). Although Zn(NH2)2 was not observed in the XRD
patterns, this may be present in finely divided or amorphous form,
or may have reacted further to give another finely divided or amor-
phous product.
ZnCl2 þ 2LiNH2 ! 2LiCl þ ZnðNH2Þ2
ð1Þ
The reaction products from the samples prepared at 200 °C
contained a mixture of LiNH2, Li2NH, LiCl and ZnCl2 (Fig. S1). A sub-
stantial proportion of LiNH2 had decomposed into Li2NH at 200 °C.
Previous research [15] reported that at this temperature LiNH2
decomposed only to a limited extent. This improvement may be
caused by the effect of the Clꢀ ion, which is in agreement with pre-
vious work [12,16].
3.2. Zinc chloride and lithium amide in the presence of lithium hydride
The reaction between ammonia and lithium hydride to yield
lithium amide and hydrogen (Eq. (5)) has been demonstrated
previously [18]
At all reactant ratios the products after reaction at 250 °C con-
tained small amounts Zn3N2 and LiZnN. After heating to 300 °C,
no lithium amide or imide were observed at the ratio of 1:2, how-
ever, some remained at higher ratios of LiNH2; no remaining ZnCl2
was observed. The proportion of LiZnN present increased quickly
when the temperature was raised to 400 °C while that of Zn3N2
decreased. Except for the ratio of 1:2, at all other ratios the reac-
tions seemed to be complete at 500 °C, as the remaining Zn3N2
was converted into LiZnN (Fig. 1). At the ratio of 1:2, a considerable
amount of Zn3N2 (38 wt%) remained at 500 °C. It is believed that
the 1:2 sample is deficient in Li (vide infra) preventing complete
transformation of Zn3N2 into LiZnN. At 600 °C, Zn3N2 was not
observed. A small amount of LiZnN had been decomposed into LiZn
and Zn as reported previously by Toyoura et al. [17].
2LiH þ 2NH3 ! 2LiNH2 þ 2H2
Lithium hydride was therefore added to the ZnCl2–nLiNH2
system in an attempt to change the overall reaction pathway to
that given in Eq. (6), which is the sum of Eqs. (4) and (5):
ð5Þ
ZnCl2 þ LiNH2 þ 2LiH ! LiZnN þ LiCl þ 2H2
ð6Þ
In practice, the addition of LiH resulted in not only the desired
products of LiZnN and LiCl but also Zn (from 150 °C) and LiZn (from
500 °C). TPD–MS confirmed the main gaseous product changed
from NH3 to H2 with an onset temperature of around 90 °C
(Fig. 2b). Hydrogen release at such a low temperature could be
due to a metathesis reaction between ZnCl2 and LiH to form LiCl
and, transiently, ZnH2, which is unstable above 90 °C decomposing
to form Zn and H2 [19]. The decomposition of LiZnN to form LiZn
and Zn was observed at the lower temperature of 500 °C.
3.3. Zinc nitride and lithium amide
Zn3N2 (Alfa–Aesar, 99%) and LiNH2 were mixed in a molar ratio
of 1:3, in the temperature range 300–500 °C for 1–24 h, in order to
produce pure LiZnN, via Eq. (3), without the presence of LiCl. The
reaction occurred slowly at 300 °C with a small amount of LiZnN
obtained and then faster at 400 °C. At 500 °C LiZnN (ꢁ92 wt%)
was achieved. A longer reaction time (24 h) did not change the
remaining Zn3N2 into LiZnN with a small amount of Zn3N2
(ꢁ6 wt%) remaining.
In order to achieve pure LiZnN, excess LiNH2 was added to the
reactants, which were heated at 500 °C for up to 12 h. Pure LiZnN
could be obtained after a 1 h reaction between Zn3N2 and LiNH2,
in a molar ratio of 1:4.2.
The hydrogen desorption properties of the reaction between
Zn3N2 and LiNH2 were tested using TPD–MS. At about 360 °C,
ammonia release was observed, indicating the expected reaction
(3) occurred (Fig. 3a).
Fig. 1. Powder XRD patterns of the products of reactions between ZnCl2 and LiNH2
in a ratio of (a) 1:2, (b) 1:3, (c) 1:4, (d) 1:5, (e) 1:6, at 500 °C for 12 h.