Angewandte
Communications
Chemie
Owing to the strong ionic character of the atomic bonds
with high bond energy up to 8.92 eV/atom, III–V nitrides are
suitable electronically active supports for the photocatalytic
reaction under harsh conditions.[26] Therefore, III–V nitride
semiconductors may provide a more promising alternative for
the NOCM reaction. Furthermore, some recent research
demonstrated that the non-polar facets of GaN or ZnO with
wurtzite structure could significantly promote the polariza-
were detected during the reaction by using a thermal
desorption treatment.
Furthermore, an extremely high CH4 conversion rate of
331 mmolgÀ1 hÀ1 was accessed using the SS-F sample with an
area of 5 cm2, representing a low catalyst requirement, which
is beneficial for scale-up (Figure S7). No reaction occurred in
the absence of catalyst or light. For comparison, we also tested
the catalytic activity of commercial GaN, ZnO, and their
physical mixtures, as well as Ga2O3 and Zn3N2 materials. The
poor photocatalytic activity of these materials clearly dem-
onstrated that the superior activity of SS cannot be attributed
solely to the factors that already exist in these reference
materials. There must be some unique local structures in the
solid solution that are essential for the activation of methane
molecules. Furthermore, the catalytic performances of ZnO,
Ga2O3, and Ga2O3–ZnO mixture treated in argon at 1123 K
were also studied (Figure S8). It is found that the activities of
these samples after the high-temperature treatment without
NH3 were all slightly reduced, indicating that high-temper-
ature treatment does not lead to an increase in catalytic
activity.
[27–29]
À
tion and cleavage of methane C H bonds.
characteristics aroused our great interest in exploring III–V
nitride-based photocatalysts for the NOCM reaction.
These
Here, we present both wurtzite GaN:ZnO solid solution
powders (SS) and its thin-films (SS-F) as noble-metal-free
photocatalysts able to catalyze the NOCM reaction with
extraordinary activity, stability, and coke resistance. The
methane conversion rates of SS and SS-F are 57 mmolgÀ1 hÀ1
and 331 mmolgÀ1 hÀ1, which are 5.8 and 33.7 times higher than
the highest value previously reported, respectively. Further
studies reveal that the high-temperature nitridation treatment
in the catalyst preparation can completely eliminate the acid
sites and enhance the basicity of the catalyst surface, which
could greatly depress the rate of coke formation and improve
the durability of GaN:ZnO solid solution, with no deactiva-
tion for 35 cycles during a continuous 70 hour test.
Due to the intrinsic instability of the catalysts and the easy
formation of coke on the surface, the previously reported
photocatalytic systems for the NOCM reactions are generally
active at the beginning of the reaction, followed by a fast
deactivation after a few hours. So far, few NOCM photo-
catalysts can be used repeatedly. However, distinctively, fresh
The SS samples were obtained by heating a mixture of
Ga2O3 and ZnO powders at 1123 K under NH3 flow
(10 mLminÀ1).[30,31] The molar ratio of GaN to ZnO in the
starting material was 1:1. The SS-F samples were prepared
using sol–gel spin coating techniques on quartz substrates
(0.4 mgcmÀ2). As revealed by powder X-ray diffraction
(PXRD) and transmission electron microscopy (TEM) (Fig-
ures S1 and S2), the as-synthesized SS possesses a single
hexagonal wurtzite phase with high crystallinity. The major
SS gave
a remarkable methane conversion rate of
57 mmolgÀ1 hÀ1 and kept almost unchanged (53 mmolgÀ1 hÀ1)
after 35 cycles during a continuous 70 hour test (Figure 1c
and Table S4). The ethane selectivity remained > 89% in
each cycle. Most notably, only a 30 second evacuation of the
reactor at room temperature is required before each cycle
without any regeneration treatments. After a 70 hour reac-
tion, the structure of SS remained intact. By contrast, GaN
and ZnO completely lost their activity and selectivity after
four cycles under the same conditions.
¯
exposed facet is a (1010) plane, which has lattice parameters
between GaN and ZnO. The Brunauer-Emmett-Teller (BET)
specific surface area of SS is 20.1 m2 gÀ1, which is also between
ZnO and GaN (Table S2). The band gap is estimated to be
2.6 eV, which is significantly smaller than that for either GaN
or ZnO (Figure S3). Energy-dispersive X-ray spectroscopy
(EDS) analysis in a scanning electron microscope (SEM)
clearly indicates a large-scale homogeneous distribution of
Ga, Zn, N, and O elements in SS (Figure S4). From the ICP-
AAS measurements (inductively coupled plasma atomic
absorption spectroscopy), the composition of the as-synthe-
sized SS, as well as SS-F, can be determined to be
Next, we investigated the effects of temperature and light
intensity on the NOCM process. Figure 2a shows the temper-
ature dependence of the NOCM reaction. The dashed line
represents the thermodynamic limitation of using a thermal
system for NOCM, meaning that no thermal catalyst could
convert methane in quantities above this line. In contrast, the
methane conversion achieved by SS under light irradiation
Ga0.75Zn0.25N0.75O0.25
.
The SS catalyst was evaluated for the NOCM reaction in
a batch-type system under 300 W xenon lamp irradiation.
Specifically, 50 mg of photocatalysts were uniformly dispersed
at the bottom of an air-tight quartz reactor, activated in
vacuum (P < 1 Pa) at 3008C for 2 h. After cooling to room
temperature, 300 mmol of methane were introduced into the
reactor (Figure S5). As shown in Figures 1b, S6 and Table S3,
a high methane conversion rate of 57 mmolgÀ1 hÀ1 with nearly
stoichiometric amounts of ethane (98% selectivity) and H2
was achieved over SS under 2 h of light irradiation at 293 K,
whereas the total hydrocarbon selectivity exceeded 99.5%.
No benzene, toluene, naphthalene, or polycyclic aromatics
Figure 2. a) The temperature dependence of NOCM under light irradi-
ation. b) Plots of the methane conversion and apparent quantum
efficiency (AQE) at 325 nm over SS in 2 h as a function of the light
intensity.
2
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Angew. Chem. Int. Ed. 2021, 60, 1 – 6
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