Angewandte
Communications
Chemie
The Fe-Ni/SiC catalyst shows an extremely high hydrogen
response to incident microwave irradiation. These properties
selectivity of ca. 98 vol.% and CO formation was zero. We
have been carefully characterized by cavity perturbation
measurements under both room temperature and high
temperature conditions (Table S3 and Figure S6). Although
no simple dependency has been found between the so-called
loss tangent (tand) and the product distributions, microwave
irradiation clearly gives higher product selectivity as com-
pared to conventional thermal reactions (Figure 2D). How-
ever, the dielectric properties of the catalysts are also
temperature dependent, which could alter any trend observed
at a single temperature. We find that the dielectric property of
metal catalysts showed a silimar trend with increasing
temperature. A higher loss tangent was obtained at higher
temperatures, but no clear differences were observed for the
dielectric constant (e’). Unlike carbon based catalyst materi-
als which can be highly efficient absorbers of electromagnetic
energy, for metal/SiC catalysts, various polarization and
dissipation mechanisms clearly contribute to drive the
reactions under microwave irradiation which lead to the
high hydrogen selectivity.
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found that the presence of nickel improves the dispersion of
metal particles on the SiC surface and appears to suppress the
formation of iron carbides, implicated in the gradual reduc-
tion in catalytic activity.
For comparison with our microwave-promoted experi-
ments, a set of thermal dehydrogenation reactions were
investigated through two different procedures (Figure 2D).
Labelled Thermal-1, an oil pre-loaded Fe/SiC sample was
subjected to a pre-heated furnace (5508C), while in the
Thermal-2 procedure, the Fe/SiC catalyst (without fuel) was
pre-heated in the furnace to 5508C and the hexadecane was
then carefully introduced to the hot catalyst bed by a syringe.
The lack of H2 formation over the oil pre-loaded Fe/SiC
catalyst under conventional thermal reaction conditions
(
Thermal-1), but very high yield of H2 under microwave
irradiation illustrates that the high H selectivity obtained
2
over metal/SiC catalyst is strongly dependent on the use of
microwaves. The reactions may be catalyzed by microwaves at
the temperature below the (bulk) boiling point of the fuel,
while the thermal process necessarily gives rise to fuel
vaporization before any dehydrogenation on the catalyst
surface. A second hypothesis could be that the rapid heating
of microwave energy converts part of the hydrocarbons
before losing the liquid by vaporization, suggesting that two
main processes are taking place simultaneously: vaporization
and chemical reactions. Vaporization predominates in the
thermal experiment because of slow heating of the catalyst
particle, whilst the microwave-promoted catalysis proceeds
rapidly, probably as a consequence of the rapid heating of the
particles by microwave radiation. We note that SiC supported
catalysts have very low surface areas, typically less than
In relation to microwave field effects, the applied micro-
wave electric field causes highly non-uniform field distribu-
tions in the catalyst bed, leading to regions of high electric
field on the surfaces of metal particles on the SiC, conse-
quently, both an (induced) electron-rich and strong polar
medium can be created that could enhance the polarization
of, for example, a CÀH bond and subsequently the hydrogen
formation. The complete cleavage of CÀH via two mecha-
nisms of oxidative addition and electrophilic substitution are
accelerated by microwave irradiation which leads to the high
[11]
hydrogen selectivity.
The high thermal conductivity and
mechanical strength of SiC, together with a clearly reduced
tendency to yield oxygenated products, are also considered
important. These factors contribute by restricting the forma-
tion of by-products under microwave irradiation by avoiding
any thermal degradation to the support during the reactions
2
À1
2
.0 m g . Although, the Fe/SiC catalyst can clearly catalyse
the dehydrogenation of hexadecane under pre-heated con-
ditions (Thermal-2), the hydrogen selectivity is noticeably
different.
[12]
and quickly achieving a thermal balance in the system.
Unlike conventional thermal fluid catalytic cracking
The catalyst particles were examined by X-ray diffraction
and high resolution transmission electron microscopy both
before and after microwave treatment. Metallic iron, nickel
and iron-nickel alloy particles, having typical particle diam-
eters less than 150 nm, have been identified in fresh samples
(Figure 3A and Figure S3).
Following the microwave-promoted catalytic decomposi-
tion of hexadecane, multi-walled carbon nanotubes that
encapsulate some of the metal nanoparticles were found
(Figure 3C and 3E). Iron carbide was observed in spent Fe/
SiC catalyst but not in FeNi/SiC catalyst, which suggests—
importantly—that the presence of Ni can suppress the
formation of iron carbide (Figure 3D and 3F). These results
are consistent with data obtained from XRD (Figure S3 and
S4).
(
FCC), our results show that the cleavage of CÀH bonds is
strongly preferred upon microwave irradiation or promotion.
Our previous studies suggested that the scission of CÀH
bonds during dehydrogenation could take place through
[
6]
a consecutive mechanism. Dehydrogenation steps occur on
the metal catalyst surface to give adsorbed intermediates
before the cleavage of CÀH. Adsorbed species with metalÀC
bonds and subsequent cleavage of CÀH leads to the formation
of H and carbon. Meanwhile, the minor products of small
2
hydrocarbons may be generated through the hydrogenation
of developing carbon residues or directly via a cracking
[
8]
process. Clearly, further studies are needed to ascertain the
precise catalytic processes.
The presence of high-frequency electromagnetic fields
makes microwave systems more complex and challenging to
Thus, the key challenge of rapid production of, and high
[
9,10]
understand.
We believe that the extremely high hydrogen
selectivity towards, H production has been demonstrated
2
selectivity obtained in these studies may be a direct result of
the intrinsic response of the metal catalysts under microwave
irradiation.
The intrinsic dielectric properties of individual metal
particle catalysts are clearly a critical factor affecting its
using resonant microwave-electric field irradiation applied in
a laboratory device, typically achieving > 98% H2 purity.
Such a rapid and selective production of hydrogen from liquid
hydrocarbon alkanes reveals a powerful new platform for
hydrogen production. We believe these findings represent
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers!
Jie, Xiangyu
