Pressure-induced dehydrogenative coupling of methane to ethane by platinum-loaded gallium oxide photocatalyst

Article abstract of DOI:10.1039/d0cc01730b

Pt/Ga₂O₃exhibited high activity for dehydrogenative coupling of methane into ethane (2CH₄→ C₂H₆+ H₂) in a fixed-bed flow reactor at 25 °C under 254 nm UV irradiation. The C₂H₆formation was negligible at CH₄pressure of 10 kPa, but it was linearly increased with an increase in the pressure to 300 kPa.

Full text of DOI:10.1039/d0cc01730b

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Pressure-induced Dehydrogenative Coupling of Methane to
Ethane by Platinum-loaded Gallium Oxide Photocatalyst
Fumiaki Amano,*ab Chiho Akamoto,a Mizuki Ishimaru,a Satoshi Inagakib,c and Hisao Yoshidad,e
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Pt/Ga2O3 exhibited high activity for dehydrogenative coupling of
methane into ethane (2CH4 → C2H6 + H2) in a fixed-bed flow
reactor at 25 °C under 254-nm UV irradiation. The C2H6 formation
was negligible at CH4 pressure of 10 kPa, but it was linearly
increased with an increase in the pressure to 300 kPa.
under UV irradiation.13-21 Semiconductor photocatalysts are
also investigated in the absence of oxidant for NOCM.19, 22 The
issue in NOCM is the low quantum efficiency owing to the slow
kinetics. Pt-loaded TiO2−SiO2:Ga (0.2 g) showed a CH4
conversion rate of 0.70 μmol h−1 with 90% selectivity of C2H6,
but the apparent quantum efficiency (AQE) was very low
(1×10−4 % at 350-nm UV irradiation).23 The CH4 conversion rate
over β-Ga2O3 (0.2 g) was 0.05 μmol h−1 for NOCM, and the AQE
was about 0.01% at wavelengths 220–270 nm.19
CH4 is an abundant carbon source from natural gas including
shale gas, methane hydrate, and renewable biogas.1 The
production of liquid fuels and chemicals from petroleum may
be replaced by CH4.2, 3 Therefore, catalytic conversion of CH4
directly into higher hydrocarbons (C2+) is a crucial technology to
cope with the concerns of energy security. However, controlling
the product selectivity in the direct conversion is a big challenge
in catalysis science.4-6 The most severe issue is the sequential
reaction of the desired products, which are more reactive than
CH4, at the high temperatures required to activate the strong
C−H bonds (439 kJ mol−1).7 In this regard, the activation of CH4
at low temperatures, which may improve the selectivity, is the
holy grail of sustainable chemistry.1-3, 6, 8-12
Photocatalytic activation of CH4 proceeds at room
temperature despite the endergonic reactions (ΔrG > 0).1-3
Many heterogeneous photocatalysts have been investigated for
the conversion of CH4 to yield C2H6, which called a non-oxidative
coupling of methane (NOCM), 2CH4 → C2H6 + H2, ΔrG = 68.6 kJ
mol−1 at 25 °C.13-23 The photocatalytic NOCM is induced by
highly dispersed metal oxides supported on SiO2 and zeolites
In this study, we investigated the effect of CH4 pressure,
P(CH4) on the photocatalytic conversion of CH4 in the presence
of H2O vapor. We expected that the high concentration of
methyl radical (•CH3) promotes the carbon-carbon bond
formation to produce C2H6. In order to increase the collision
frequency of CH4 on the photocatalyst surface, P(CH4) was
increased from 10 kPa to 300 kPa. First, we screened transition
metal oxide photocatalysts loaded with a small amount of Pt
cocatalyst, which is the best catalyst minimizing the
overpotential necessary to drive H2 evolution reaction.24
Second, we focused on the highly active Pt/Ga2O3 to prove the
concept that high P(CH4) induce the direct production of C2H6.
We performed photocatalytic conversion of CH4 with H2O
vapor using a fixed-bed flow-type reactor under UV irradiation
at 25 °C (Fig. S1, ESI†). Hexachloroplatinate(IV) (H2PtCl6, 0.1 wt%
as Pt) was loaded on each metal oxide powder by an incipient-
wetness impregnation method. The Pt precursor-loaded
powder (50 mg) was coated on a glass substrate using water and
dried at room temperature. The Pt precursor is quickly reduced
to Pt0 during the initial period of the photocatalytic reaction
promoting H2 evolution.25 The light source was a 40-W low-
pressure mercury lamp (wavelength 254 nm, light intensity 33
or 15 mW cm−2, irradiation area 25 cm2). The gas mixture of
CH4/Ar/H2O was continuously supplied to the reactor (volume 5
× 5 × 0.025 cm3) at a flow rate of 20 mL min−1. The products in
the flow system were analyzed by gas chromatography (GC).
We screened 12 oxide powders for photocatalytic H2
evolution under the stream of 10-kPa CH4 and 3-kPa H2O at 1
atm. The physical properties of the oxides are reported in ESI†
a. Department of Chemical and Environmental Engineering, The University of
Kitakyushu, 1-1 Hibikino, Wakamatsu-ku, Kitakyushu, Fukuoka 808-0135, Japan.
b. Precursory Research for Embryonic Science and Technology (PRESTO), Japan
Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-
0012, Japan.
c. Division of Materials Science and Chemical Engineering, Yokohama National
University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan.
d. Graduate School of Human and Environmental Studies, Kyoto University, Yoshida-
nihonmatsu-cho, Sakyo-ku, Kyoto 606-8501, Japan.
e. Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University,
Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8520, Japan.
†Electronic Supplementary Information (ESI) available: Experimental details,
characterization of the oxide samples, SEM and TEM images of Ga2O3, the effect of
P(CH4) on the time courses of product formation rate, TG–DTA, and ESR study. See
DOI: 10.1039/x0xx00000x
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(Tables S1, S2, Fig. S2–S5). Figure 1 shows the H2 evolution rate, production of CO2 and C2H6 at P(CH4) = 98 kPa was significantly
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DOI: 10.1039/D0CC01730B
r(H2), with 0.1wt% Pt-loaded oxide at 5-min and 65-min higher than that at P(CH4) = 10 kPa (Fig. 2b). The increase in the
photoreaction. Pt/TiO2 samples (P 25 and ST-01) and Pt/Ta2O5 yield of CO2 and C2H6 indicates that the high P(CH4) facilitates
exhibited moderate photocatalytic activity. Pt/ZrO2 was CH4 activation and increases the concentration of activated
deactivated in 60 min during the photoreaction. Pt/Ga2O3 methane species like •CH3. The O2 evolution was initially high
exhibited the highest activity under 254-nm UV irradiation.26-29 but diminished with irradiation time. We further increased the
Since the bandgap of β-Ga2O3 was 4.6 eV, the photoexcitation total pressure to 3 atm using a back-pressure valve and found
required UV irradiation of wavelengths < 270 nm. The loading that C2H6 production was much increased by increasing P(CH4)
of a small amount of Pt cocatalyst significantly enhanced the from 98 kPa to 300 kPa. The steady-state r(C2H6) was 0.58 μmol
r(H2) of Ga2O3 (Fig. S6, ESI†).30 Not only H2PtCl6 but also min−1 at 300 kPa. The ratio of H2 : CO2 : CO : C2H6 was 1 : 0.17 :
tetraammineplatinum(II) chloride (Pt(NH3)4Cl2) could be used 0.024 : 0.19.§ There was no O2 evolution at P(CH4) = 300 kPa.
for the precursor of Pt cocatalyst (Fig. S7, ESI†). The stability and
durability of Pt/Ga2O3 were confirmed in a long-term test and
repeated reactions (Fig. S8 and S9, ESI†).
Figure 1. H2 evolution rate from CH4 and H2O with 0.1 wt% Pt-loaded metal oxides under
UV light irradiation (254 nm, 33 mW cm−2) for 5 min and 65 min. The metal oxides are
Y2O3, TiO2(P 25)a, TiO2(ST-01)b, ZrO2, Nb2O5, Ta2O5, WO3, ZnO, Ga2O3, In2O3, GeO2, and
SnO2. Total pressure (Ptotal) =101 kPa. CH4/H2O/Ar = 10/3/88.
We selected the highly active TiO2 and β-Ga2O3 photocatalysts for
Figure 2. Formation rate of products over (a) Pt/TiO2(ST-01) and (b) Pt/Ga2O3 at Ptotal of
further investigation. Both oxides have been reported to exhibit
101 kPa (CH4/H2O/Ar = 10/3/88). UV light (254 nm, 15 mW cm−2) was irradiated for 3 h
(60–240 min time on stream).
good performance for photocatalytic H2 evolution reactions,25-31
since they have suitable band structures. Figure 2 shows the time
course of product formation rates from CH4 and H2O vapor with
Pt/TiO2(ST-01) and Pt/Ga2O3. ST-01 is an anatase TiO2 photocatalyst
with a large BET specific surface area (SBET). The SBET of ST-01 and β-
Ga2O3 was 302 m2 g−1 and 10 m2 g−1. A continuous H2 evolution (0.21
μmol min−1) was observed for Pt/TiO2. The O2 evolution indicates the
progress of water splitting (2H2O → O2 + 2H2) even in the presence
of CH4. The CO2 generation is explained by photocatalytic steam
reforming of methane (photo-SRM, CH4 + 2H2O → CO2 + 4H2)30-
32 or oxidation by the evolved O2 (CH4 + 2O2 → CO2 + 2H2O). The ratio
of H2 : O2 : CO2 was 1 : 0.26 : 0.075 for Pt/TiO2. In the case of Pt/Ga2O3,
the evolution of H2 and O2 was much higher than that of Pt/TiO2 but
gradually decreased with time. After 3-h photoreaction, the r(H2) was
1.98 μmol min−1, and the ratio of H2 : O2 : CO2 was 1 : 0.30 : 0.065.
The CO2 production was constant during photoreaction. However,
there was no production of C2H6 and other oxidized products under
the condition of P(CH4) = 10 kPa.‡
Figure 3. Formation rate of products over Pt/Ga2O3 at Ptotal of (a) 101 kPa (CH4/H2O =
98/3) and (b) 303 kPa (CH4/H2O = 300/3) under 254-nm UV irradiation (15 mW cm−2, 60–
240 min time on stream).
The adsorption of CH4 on the solid surface is not stable since
the symmetrical tetrahedral geometry has no dipole moment
and small polarizability.5 Thus, we increased the P(CH4) to
improve the collision frequency of CH4 molecule on the surface
of Pt/Ga2O3. Figure 3 shows the time course of the
photocatalytic CH4 conversion using Pt/Ga2O3 under high P(CH4).
We have succeeded in the detection of C2H6 at P(CH4) = 98 kPa.
We also detected CO formation. At P(CH4) = 98 kPa, the rate of
C2H6 production, r(C2H6), was slowly increased with time and
reached to 0.18 μmol min−1 after 3-h photoreaction. The
Figure 4a shows the effect of P(CH4) on the formation rate
over Pt/Ga2O3 after 3-h photoirradiation. The effect of P(CH4)
on the time courses of H2, C2H6, and CO2 are shown in Fig. S10,
ESI†. The C2H6 formation was gradually increased with time on
stream. The r(C2H6) after 3-h photoreaction was monotonically
increased with an increase in P(CH4) from 10 kPa to 300 kPa. In
the absence of CH4, water splitting was induced as proved by
2 | J. Name., 2012, 00, 1-3
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that the H2/O2 ratio is 2. The increase in P(CH4) increased CO2 photocatalytic system. However, this evaluation isVnieowtAarticccleuOrnalitnee
production in contrary to the decrease of O2 production. H2 because the reaction rate is not alwayDsOpI:r1o0p.1o0r3t9i/oDn0aClCt0o173th0Be
production was also increased with P(CH4) owing to the photocatalyst weight.35 When we tentatively calculated it from
progress of photo-SRM.30 However, the production rate of H2 the formation rate of carbon-contained products, the CH4
and CO2 was almost constant at P(CH4) ≥ 100 kPa. The conversion rate was 2090 μmol g−1 h−1 (1.74 μmol min−1 over 50-
monotonous dependence on P(CH4) was characteristic of C2H6 mg Pt/Ga2O3), which was much higher than the reported
production.
values.23
We measured electron spin resonance (ESR) spectra of the Ga2O3
powder at −196 °C. The Ga2O3 evacuated at room temperature for 30
min generated a sharp signal at around 326 mT (g = 2.003) by UV
irradiation (Fig. S14, ESI†). The presence of CH4 (10 kPa) changed the
intensity and the shape of the ESR signal. The interaction with CH4
suggests that the radical species are related to the photogenerated
+
holes trapped in the surface states (htr ). The UV-induced signal was
not observed for the dehydrated Ga2O3, which was pre-evacuated at
200 °C to remove the adsorbed water species (Fig. S15, ESI†).
Therefore, the hole trapping site interacts with the hydroxyl group or
the water molecules adsorbed on the surface. The h+ trapped in the
surface hydroxyl group (htr(OH)+) would be like a surface hydroxyl
radical (•OHS). We could not detect the ESR signal of •CH3 in the gas
phase,36 but the C2H6 is considered to form via the coupling of •CH3
on the surface. The proposed reaction mechanism can be expressed
as follows.
―
+
+
⟶
+
#(1)
Ga2O3 hυ
e
htr(OH)
+
+ #(2)#
H
푘
+
+
CH4 htr(OH) •CH3
Figure 4. Effect of P(CH4) on (a) the product formation rate and (b) the carbon-based
selectivity in the photocatalytic reaction on Pt/Ga2O3. The values were taken from the
time course in the flow-type reactor after 3-h photoirradiation. The GC system could not
analyze CO when P(CH4) < 98 kPa due to the interference with Ar.
+
⟶
#(3)
•CH3 •CH3 C2H6
Pt cocatalyst
2H +
e
―
+ 2
#(4)
H2
Figure 4b shows the effect of P(CH4) on the product selectivity
on a per carbon basis (C-%). The selectivity of C2H6 is defined as
the ratio of carbon atoms of the produced C2H6 to that of all
products containing carbon. The selectivity of C2H6 was
gradually increased with P(CH4) and almost saturated at P(CH4)
= 300 kPa. The selectivity of C2H6 was 67 C-%, which
corresponds to two-thirds of CH4 molecules becoming ethane
and the rest to COX. The AQE of the Pt/Ga2O3 was calculated
from the r(H2), assuming that one H2 molecule requires two
electrons. The AQE was gradually increased and saturated to be
13% at P(CH4) > 100 kPa (Fig. S11, ESI†). Recently, Li et al.
reported H2 evolution from aqueous H2O under CH4 using TiO2
photocatalysts.33, 34 The AQE (13%) of the Pt/Ga2O3 in this study
was higher than that of the gas-solid-liquid Pt/TiO2 system (4.7%
at 254-nm irradiation with C2H6 selectivity of 62 C-%). The high
efficiency is provided by the intrinsic activity of Ga2O3, the
loading of Pt cocatalyst, and the photocatalytic reaction
conditions. Noted that the error in material balance was less
than 10%, assuming that C2H6, O2, CO, and CO2 come from
dehydrogenative reactions (Fig. S12, ESI†). The small amount of
missing was related to the formation of carbon species on the
The valence band maximum of Ga2O3 (Figure S4, ESI†) can
thermodynamically induce one-electron oxidation of CH4 (CH4
→ •CH3 + H+ + e−, E = 2.06 V vs. SHE).9 When the oxidation of
CH4 by the trapped htr(OH)+ expressed by equation (2) is the rate-
determining step, the r(C2H6) is proportional to the P(CH4) and
+
the concentration of htr(OH)
.
+
2
6
푟
0.5푘[
]푃( 4)#(5)
htr(OH) CH
(C H ) =
As shown in Figure 4a, the C2H6 formation over Pt/Ga2O3 was
monotonically increased with an increase in P(CH4). Figure 5
shows the kinetic analysis of the dependence of r(C2H6) on
P(CH4). The reaction order of C2H6 formation was 1.04 with
respect to the CH4 concentration. The first-order kinetics is
consistent with the rate law derived from the proposed reaction
mechanism (Eq. 5). The coupling between •CH3 molecules
should be much faster than the C−H bond cleavage. If •CH3 does
not react with another •CH3 due to the low concentration, the
+
htr(OH) promotes further overoxidation. The r(CO2) was
saturated at least P(CH4) = 50 kPa (Figure 4a), suggesting that
the rate-determining step for CO2 generation includes the
reaction of the adsorbed C1 species. The r(H2) was also
saturated at high P(CH4), implying that the rate-determining
step of H2 evolution is related to the C–H bond cleavage of the
adsorbed C1 species. We confirmed that the product formations
were dependent on the light intensity, but r(C2H6) was
saturated at high light intensity (Fig. S16, ESI†). This suggests
photocatalyst
surface,
which
was
confirmed
by
thermogravimetry–differential thermal analysis (TG–DTA, Fig.
S13, ESI†).
The rate of CH4 conversion per photocatalyst weight was
often reported to compare the performance of the
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7. Y.-R. Luo, Comprehensive Handbook of Chemical BVoinewdAErnticelergOinelsin,e
that high photon flux accelerates overoxidation of CH4 rather
than selective oxidation to C2H6. The role of Pt was found to
promote proton reduction since the r(H2) was significantly
increased by the loading of a small amount of Pt (Fig. S6 and S7,
ESI†). The formation of Pt metal particles was confirmed for the
Pt/Ga2O3 after photocatalytic reaction by UV-Visible
spectroscopy (Fig. S17, ESI†). Transmission electron microscopy
revealed that Pt nanoparticles were highly dispersed on the
Ga2O3 surface, and the particle size is less than at least 2 nm
(Fig. S18, ESI†).
CRC Press, Boca Raton, 2007.
DOI: 10.1039/D0CC01730B
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Table of contents entry
Pt/Ga2O3 induced photocatalytic dehydrogenative coupling of CH4 to yield C2H6 under high
CH4 pressure.