H. Zou et al. / Journal of Alloys and Compounds 578 (2013) 380–384
381
achieved when water entering in the reactor was vaporized par-
tially and liquid water, respectively [9].
Aluminum cans are one of the most common package items and
more than 80% of Al in the municipal solid waste is Al cans [10].
Producing hydrogen with used aluminum cans will save natural re-
sources and reduce greenhouse gas emissions. Hiraki et al. have
proposed that the energy requirement of waste aluminum-based
hydrogen generation is only 2% and the amount of CO2 emission
is 4% of the conventional method [11]. But the used aluminum cans
are usually coated with a colorful and thick paint film, which inhib-
its the contact between the reactants and delays hydrogen
evolution.
In this study, the concentrations of sodium hydroxide solutions
and the pretreated methods of the used cans, such as mechanical
rubbing, dissolution extraction and heat treatment, are
investigated.
2. Experimental
Fig. 1. The curves of hydrogen generation of Al scrap-1 in different NaOH solutions.
2.1. Materials
Aluminum powder (99.0% purity), aluminum foil (99.9% purity) and sodium
borohydride (98.0% purity) were supplied by Tianjin kermel chemical reagent Co.
Ltd. Sodium hydroxide solutions with different concentrations were prepared with
distilled water. Al scraps were obtained from used beverage cans of Sprite, a bever-
age brand of the Coca-Cola Company. The used aluminum can was first cut off both
ends and the rest was then cut into small slices. The un-pretreated Al slice was de-
noted as Al scrap-1. To remove the lacquer and coated plastics, the Al scraps were
treated with different methods and denoted as Al scrap-2 to Al scrap-4. The Al
scrap-2 was calcined at 500 °C for 20 min and the Al scrap-3 was mechanically pol-
ished with abrasive papers. The Al scrap-4 was dipped into the concentrated sulfu-
ric acid for two minutes to eliminate the coating layer. The used aluminum cans of
other popular soft drink brands, such as Fanta, 7-up and pineapple beer, were inves-
tigated too, but all these Al scraps were pretreated with the same pretreatment
method as the Al scrap-4 mentioned above.
2.2. Apparatus and measurements
The given weighted Al foil or Al scrap is placed into a 250 mL round-bottomed
flask and 100 mL NaOH aqueous solution is fed into the flask quickly. The produced
hydrogen is collected by an upside down burette cylinder filled with water. The
cumulative hydrogen amounts are measured by the volumes of water displaced
from the cylinder. The absolute measurement error of the hydrogen volume is
0.5 mL. The hydrogen generation rate refers to the volume of hydrogen generated
in unit minute per unit mass of the reactant.
Fig. 2. Effects of pretreatment of used Al cans on the hydrogen generation.
The effects of different pretreatment methods on the hydrolysis
of Al scraps are shown in Fig. 2. It is found that the hydrogen vol-
ume of un-pretreated Al scrap-1 at 5 min is only 20 mL gꢀ1 in 1.0 M
NaOH solution, while the amounts of Al scrap-2, Al scrap-3 and Al
scrap-4 can reach 302, 344, 354 mL gꢀ1, respectively. The hydrogen
evolution rates of pretreated Al scraps are much higher than those
of un-pretreated Al scrap. The Al scrap treated with mechanical
method (Al scrap-3) releases hydrogen more quickly than those
pretreated with other two methods. The Al scrap immersed into
the concentrated H2SO4 (Al scrap-4) has a higher initial hydrolysis
rate than the calcined Al scrap (Al scrap-2) in the first eight min-
utes. The total volume of hydrogen generated of Al scrap-2 is more
than those of other Al scraps. It was cited that annealing of the alu-
minum-based metal composites at 450 °C led to the redistribution
of the components in the solid phase[13], which may be promote
the complete hydrolysis of Al scrap-2. Increasing the concentration
of NaOH solution enhances the hydrogen generation of pretreated
Al scraps, too. The tendency of hydrogen evolution for the Al scraps
pretreated with various methods in 1.0 M NaOH solution is same
as that in 1.5 M NaOH solution.
X-ray diffraction analysis was performed on the XD-3 diffractometer with Cu
K
a radiation, operating at 30 kV and 20 mA. SEM analysis was carried out on the
ISM-6360LA scanning electron microscope at the voltage of 10 kV.
3. Results and discussions
3.1. Hydrolysis of waste aluminum
Fig. 1 shows a comparison of hydrogen generation in different
NaOH solutions. It is seen that the hydrolysis reaction rates of
un-pretreated Al scraps are very slow. The total volumes of hydro-
gen evolved in different NaOH solutions are much lower than the
theoretical values due to the presence of coating layer. Higher
hydrogen evolution rates and yields are obtained at higher NaOH
solution concentrations. With increasing the concentration of
NaOH solution from 1.0 M to 5.0 M, the amount of hydrogen gen-
erated increases from 117.1 mL gꢀ1 to 360.8 mL gꢀ1 after 1 h
hydrolysis, corresponding to the hydrogen generation rates of
1.9 mL minꢀ1 gꢀ1 and 6.0 mL minꢀ1 gꢀ1, respectively. It is noticed
that there are some effervescence regions on the surface of alumi-
num scraps during the course of hydrolysis. As Andersen said [12],
sodium hydroxide, acting as the catalyst for the hydrolysis of alu-
minum, promotes the evolution of hydrogen and the generated
bubbles prevent the precipitation from attaching on the reaction
zone. Thus higher NaOH concentration causes an increase of the
effervescence regions on the aluminum matrix.
Chemical hydride, such as MgH2, NaBH4, can be used as a stor-
age medium for hydrogen, recently. NaBH4 and different aluminum
materials, i.e. Al powder, Al foil and Al scraps, are applied into the
hydrolysis and the results are present in Fig. 3. It shows that Al
powder releases hydrogen most quickly, followed by Al foil. The
hydrogen volume and the hydrolysis reaction rate of NaBH4 are