The apparatus to perform in-situ pressure measurement is
schematically drawn in Fig. 2a, it includes the batch reactor
(70 cm3) and three gas reservoirs (each 1 L). The gas reservoirs
are kept at constant temperature (25 C). In the experiments,
NaBO2 was obtained by heating sodium metaborate tetrahydrate
(NaBO2$4H2O 99%, average grain size 10 mm, SigmaeAldrich)
at 350 C for 1 h. The NaBH4 regenerations were
performed at temperature ranging between 558 and 634 C with H2 pressures from 2 to 31 bars. The heating rate was fixed
at 10 C/min. The NaBO2/Mg mixtures (Mg 98% reagent grade,
size distribution 100e200 mm, SigmaeAldrich) were placed
into the reactor and calcined at 350 C under vacuum condition
for 1 h to remove possible adsorbed H2O by NaBO2 during
the mixture preparation Then, H2 gas was introduced and the
reactor was heated to the desired temperature. The results
were typically presented by P-T-t diagram as shown by Fig. 2b.
With respect to the pressure variation at isotherm, the curve
displays a first pressure plateau (a/b) at beginning, followed
by a rapid pressure drop (b / c). A second pressure plateau
(c/d) indicates the equilibrium of the reaction.