Supplementary information
Wax: A benign hydrogen-storage material that rapidly releases H2-rich gases
through microwave-assisted catalytic decomposition
Sergio Gonzalez-Cortes, Daniel R. Slocombe, Tiancun Xiao, Afrah Aldawsari, Benzhen Yao,
Vladimir L. Kuznetsov, Emanuela Liberti, Angus Kirkland, Mohammed S. Alkinani, Hamid A.
Al-Megren, John Meurig Thomas and Peter P. Edwards
Supplementary Fig. S1
Supplementary Fig. S2
Supplementary Fig. S3
Supplementary Fig. S4
Supplementary Table S1
Supplementary Table S2
Supplementary Table S3
Supplementary Table S4
120
Rate (µmol/cat-g.s)
a
H2
100
80
60
40
20
CH4
0
0
b
5
10 15 20 25 30
Time on stream (min)
35
Rate (µmol/cat-g.s)
500
H2
400
300
200
100
CH4
0
0
5
5.0
Rate (µmol/cat-g.s)
c
4.0
10 15 20 25 30
Time on stream (min)
35
H2
3.0
2.0
1.0
CH4
0.0
0 10 20 30 40 50 60 70 80 90 100
Time on stream (min)
Supplementary Fig. S1
Dependence of the hydrogen and methane formation reaction rates with the reaction time. a, 45 wt. % paraffin wax @
activated carbon at absorbed power of 80 W for 30 minutes. b, 35 wt. % paraffin wax @ 5 wt. % Ru/ activated carbon at
absorbed power of ca. 80 W for a period of 32 minutes. c, 35 wt. % paraffin wax @ 5 wt. % Ru/ activated carbon at 20-15
W for a period of 90 minutes.
100
Weight loss (%)
a
Spent 50PW@5Ru/AC
Activated
carbon
(AC)
80
Spent 50PW@AC
60
50PW@AC
40
50PW@5Ru/AC
20
Paraffin wax
0
250
b
450
5.0
650 850 1050 1250
Temperature (K)
Deriv. weight (%/K)
575
4.0
Paraffin wax
* 0.5
Activated carbon (AC)
3.0
768
2.0
50PW@AC
1.0
50PW@5Ru/AC
Spent 50PW@AC
Spent 50PW@5Ru/AC
0.0
250
c
Heat flow (W/g)
20
450
331
650 850 1050 1250
Temperature (K)
580
780
Activated carbon
16
Paraffin wax
50PW@AC
12
Spent 50PW@AC
50PW@5Ru/AC
8
Spent 50PW@5Ru/AC
EXO
4
ENDO
0
250
450
650 850 1050 1250
Temperature (K)
Supplementary Fig. S2
Thermogravimetric analysis, b, the first derivative of the weight losses and c, differential
scanning calorimetry for 50 wt. % paraffin wax @ activated carbon and 50 wt. % paraffin wax
@ 5 wt. % Ru/activated carbon before and after the microwave-assisted catalytic decomposition
of waxes. The tests were made by irradiating the samples with 20-15 W for nearly 30 s, then the power
was continuously increased to 30 W and held for another 30 s. The power was subsequently increased
up to 60 W and held for 60 s. Finally, the delivered power was increased to 170-180 W and held for a
period (30 min) until no further gas generation was detected.
100
mole (%)
80
60
50 % PW @ 5 % Ru/AC (Microwave at 80 W)
50 % PW @ 5 % Ru/AC (Furnance at 823 K)
50 % PW @ AC (Furnance at 823 K)
40
20
0
CH4 C2H6 C2H4 C3H8 C3H6 C4H10 C4H8
Light gases
H2
CO2
CO
Supplementary Fig. S3
Comparisons of microwave and thermal treatment on the resulting evolved gas
mixture composition
Supplementary Fig. S4
High-resolution phase-contrast HRTEM images of typical activated carbon-supported 5 wt. %
ruthenium nanoparticles without paraffin wax impregnation.
Supplementary Table S1.
Maximum temperatures, weight losses and overall weight losses for 50 wt. % paraffin waxes @
activated carbon and 50 wt. % paraffin waxes @ 5 wt. % Ru/activated carbon before and after
the microwave-assisted catalytic decomposition of waxes.
Maximum temperatures Paraffin
and major weight losses
wax
Temperature (K)
Weight loss (%)
Total WL (%)
574
99.0
99.0
Spent 50 wt. %
Activated
50 wt. % PW @ AC
PW @ AC
carbon
333
6.04
6.04
575
768
39.83
11.49
52.72
440
1.87
50 wt. % PW @ 5 wt. Spent 50 wt. % PW
% Ru/AC
@ 5 wt. % Ru/AC
767
9.28
13.15
575
41.77
768
10.98
704
2.03
9.19
53.94
Supplementary Table S2.
Comparisons of the catalytic performances of microwave - activated carbon -supported noble
metal catalysts blended with paraffin wax.
a
Sample (wt. %)
50.0 % PW @ AC
Mass balance bParaffin wax
(wt. %)
conv. (wt. %)
96.2
71.9
c
c
H2 yield
(wt. %)
1.7
Liquid
(wt. %)
3.9
c
CH4 yield cC2-C4 = yield
(wt. %)
(wt. %)
12.7
20.4
c
C2-C5 yield
(wt. %)
30.6
49.9 % PW @ 10 % Pd/C
100.9
53.1
6.2
8.0
13.4
1.3
7.9
50 .0 % PW @ 5 % Pt/C
98.2
62.4
6.6
12.4
10.3
0.6
4.3
51.4 % PW @ 5 % Ru/C
99.0
54.1
7.2
6.6
9.0
2.1
5.4
a.
 m f ( Cat)   m i 
x100
Mass balance  
 mo


m
o 
( Cat )

mo(Cat) and mf(Cat) are the weight of the catalyst (Cat) before and after the catalytic reaction,
respectively. mi is the weight of the gases at standard conditions for temperature and pressure (273 K;
100 kPa) and the liquid product and mo corresponds to the initial weight of the paraffin waxes.
b. Paraffin
wax (PW) conversion corresponds to the amount of transformed wax upon the
microwave treatment.
 m  mf
PW conv .   o
 mo

x100

where mo or mf corresponds to the initial or final weight of the paraffin waxes, respectively.
c.
 mi
Pi yield  
 mo  mf

x100

where Pi is product(s) i (i.e. H2, CH4, liquids, C2-C4= and C2-C5), mi is the weight of the gases at
standard conditions for temperature and pressure (273 K; 100 kPa) or the corresponding liquid product.
Supplementary Table S3.
Influence of the heating sources (microwave vs conventional thermal furnace) over the catalytic
decomposition of paraffin wax. The microwave-assisted experiment was carried out at 80 W for 30
minutes of reaction time, whilst the thermally treated experiments were made in a furnace set at 823 K
for 20 minutes until no further gas generation was detected.
Sample (wt. %)
Heating source
Mass balance Paraffin wax H2 yield Liquid CH4 yield C2-C4 = yield C2-C5 yield
(wt. %)
conv. (wt. %) (wt. %) (wt. %) (wt. %)
(wt. %)
(wt. %)
50 % PW@5 % Ru/AC Microwave at 80 W
95.3
37.3
7.9
6.5
10.2
2.2
5.0
50 % PW@AC
Furnace at 823 K
87.5
12.8
3.5
20.3
15.6
7.0
12.3
50 % PW@5 % Ru/AC
Furnace at 823 K
86.7
25.4
5.8
14.7
7.7
0.9
3.5
Supplementary Table S4.
The d-spacings of pure ruthenium metal (space group P63/mmc) along with the values measured
from the diffractograms in Fig. 4E for 58 wt. % of paraffin wax @ 5 wt. % Ru/AC after the
microwave-assisted catalytic decomposition of paraffin wax. The error bar for the sampling in
reciprocal space is 0.04 nm-1.
Diffracting planes (hkl)
d(nm-1) for Ru
d(nm-1) experimental
110
4.27
4.25
011
4.87
4.84
002
4.67
4.78
101
4.87
4.81
Supplementary Discussion
The evolution of the reaction rate for the production of hydrogen and methane as major gases generated
upon the microwave-assisted catalytic decomposition of paraffin waxes with the reaction time is
illustrated in Supplementary Fig. S1. The hydrogen formation rates are clearly higher than those
associated with the methane production whatever the presence (or absent) of metal catalyst and the
adsorbed power. Note that the presence of Ru functionality enhances markedly the hydrogen
production rate compared to the Ru-free formulation (Supplementary Figs. S1a and S1b) at similar
adsorbed power. Furthermore, the diminution of the absorbed power to 20-15 W markedly decrease the
production rates of hydrogen and methane despite the presence of Ru catalyst (Supplementary Fig.
S1c).
We investigated a series of representative samples by both thermogravimetric analysis (TGA) and
differential scanning calorimetry (DSC) under oxygen-free nitrogen flow to determine the influence of
the activated carbon and 5 wt. % Ru on the activated carbon on the pyrolytic degradation of paraffin
waxes. Two major weight losses are displayed in Supplementary Fig. S2a. The first one between 450
and 625 K with a maximum rate of weight loss change at 575 K (Supplementary Fig. S2b) is clearly
associated to the evaporation of paraffin waxes since nearly 100 wt. % of this material is lost upon
thermal treatment in the pure paraffin waxes TGA profile. The second weight loss between 650 and
825 K with a maximum derivative of weight loss at 768 K is tentatively attributed to the degradation
(or evaporation) of paraffinic-type carbonaceous material (CxHy) produced by the interaction of
activated carbon with melted paraffin waxes. Note that this last weight loss markedly decreased in the
spent 50 wt. % paraffin wax @ 5 wt. % Ru/AC in sharp contrast to the Ru-free sample, suggesting that
the Ru metal catalyses the decomposition of this refractory paraffinic-derived material. The DSC
profiles for these samples show not only the pertinent endothermic peaks associated to these major
weight losses (or decomposition processes) but also a strong endothermic peak at 331 K attributed to
the melting process of the paraffin waxes (Supplementary Fig. S2c), since it is not observed any weight
loss in this step. The spent samples did not show evident peak(s) associated to residual paraffin wax
whilst the activated carbons showed a weak endothermic peak at 335 K because of moisture loss.
The gas composition of the 50 wt. % paraffin wax @ 5 wt. % Ru/AC under dielectric heating and
conventional thermal heating also revel a slight increase of the hydrogen production and reduction of
methane in the sample treated under microwave irradiation relative to the equivalent composite treated
in a furnace at 823 K. It is worth remarking that the average temperature for the system under dielectric
heating (ca. 723 K) was indeed significantly lower than that used in the thermal-treated sample (823
K). This finding highlights the remarkable effect of the selective heating of the microwave radiation
and hence the dual effect of the hot and cold spots over the overall gas distribution. Note that Ru
catalyst also enhanced the hydrogen production and decreased the methane formation compared to the
non-metal-containing composite (50 wt. % paraffin wax @ AC) under conventional thermal heating.
The average crystal size of the Ru catalyst on the carbon support without paraffin wax is below 3 nm.
The crystal shaper reflects a projected hexagon. Typical examples are shown in Supplementary Figs.
S3a and S3b.
The series of experiments given in Supplementary Table S2 was carried out by irradiating the samples
with an absorbed microwave power of 20-15 W for nearly 30 s, then the power was continuously
increased to 30 W and held for another 30 s. Subsequently, the power was increased up to 60 W and
held for 60 s. Finally, the delivered power was increased to 170-180 W and held for an extended period
(i.e. 20-35 min) until no further gas generation was detected. The results given in Supplementary Table
S2 represent the various product yields at the end of this sequence.
Based on the hydrogen production and the chemical composition of the major component of the
paraffin wax (i.e. C26H54), we should expect a relatively low carbon yield from the 50.0 wt. %
PW@AC formulation (i.e. 9.75 wt. %), Supplementary Table S2. This would indicate that the
remaining yield (ca. 45 wt. %) corresponds mainly to residual carbonaceous material (CxHy), as was
confirmed by thermo-gravimetric analysis (see Supplementary Figs. S2A, S2B and Table S1), since a
relatively low yield of carbon oxides (CO+CO2) was obtained (i.e. 2.5 wt. %). Note that the generation
of COx is mainly associated to the decomposition of –COO and –CO groups present in activated
carbons. We should also expect significantly higher carbon yields for noble metal-containing
formulations (ca. 35 -45 wt. %) because of the larger production of hydrogen. A mild re-oxidation of
metal catalysts is also envisaged.
The liquid yield for metal particle catalysts, that is, hydrocarbons having a range of composition
between C7 and C17, slightly increased compared to that produced by the 50 wt. % paraffin wax @AC
sample.
In Supplementary Table 3S, the mass balance, the PW conversion and the H2 yield for 50 wt. % PW@5
wt. % Ru/AC are higher, while the methane and C2-C5 yields are lower than those obtained in the
equivalent sample thermally treated in a furnace at 823 K. There is also a positive effect of the 5 wt. %
Ru/AC catalyst over the PW conversion and hydrogen yield compared to the Ru-free sample (i.e. 50
wt. % PW@AC).
A relatively low mass balance for the thermally treated samples is most likely due to the formation of
liquid hydrocarbons with low boiling points (C5-C6) that were not trapped in the cold trap (Fig. 3).
These findings reveal the favourable, important effect of the microwave radiation to effectively release
hydrogen from the paraffin wax despite the relatively low surface temperature of the catalyst bed (i.e.
573-773K) during the irradiation time.
Supplementary Video
Supplementary Video 1 (Edwards _supplementary_Video.mp4)
https://www.dropbox.com/s/zzttvbhgidclngs/Movie%20S1.mp4?dl=0
Supplementary Video Legend
Video illustrating the rapid production of hydrogen from microwave-assisted decomposition of paraffin
wax. The catalyst-wax mixture was exposed to a uniform microwave electric field at a frequency of
2.45 GHz, with an absorbed power of 150W. Hydrogen is generated almost instantaneously as the
microwaves are cycled on and off.