ACS Catalysis
Research Article
observed. This result indicated that heat was responsible for
enhancing the catalytic activity, while the addition of light
irradiation played a crucial role in augmenting the percentage
of H2 in the products for DRM.
During the photothermal DRM catalysis, the H2/CO ratio
improved with the increase of temperature, while the H2/CO
ratio showed a decreasing trend during thermal catalysis
(Figure 2b). To be specific, the H2/CO ratio was only 0.55 at
664 K in thermal catalysis, while this value reached 0.94 in
photothermal catalysis, indicating that the H2/CO ratio could
achieve close to 1.0. As aforementioned, this is crucial for the
application of the DRM reaction to other chemical processes,
such as direct FT synthesis.23,24 The Arrhenius plots of ln(rH )
2
vs 1/T were performed based on the rH values of Ni/Ga2O3
2
(Figure S9a). Both thermal catalysis and photothermal
catalysis showed good linear relationships between ln(rH )
2
and 1/T. The apparent activation energy value of H2 over Ni/
Ga2O3 was calculated to be 83.4 kJ mol−1 under thermal
catalysis according to the Arrhenius equation (k = A e−E /RT),
a
consistent with the previous reported nickel-based cata-
lysts.41,42 For the photothermal catalysis, the apparent
activation energy value was decreased to 72.2 kJ mol−1. This
exhibited that the addition of light irradiation lowered the
activation energy of the DRM reaction. Thus, the higher H2
formation rate in the products and lower activation energy
obtained upon light irradiation for the DRM process implied a
unique pathway for methane activation as compared to that of
thermal DRM. Besides, the mass spectrometry (MS) results of
the reactants and products also confirmed good stability to
promote catalytic activity during five cycles of light-on and
In Situ DRIFTS. The in situ DRIFTS technology was
carried out to disclose the origin of the improvement toward
H2 in the products and to understand the effect of light on the
reactants, intermediates, and products over Ni/Ga2O3. With
the increase of reaction temperature, the adsorption peaks of
gaseous CH4 (3016 and 1302 cm−1) and gaseous CO2 (2340
and 2358 cm−1) gradually decreased as a result of the CH4 and
CO2 conversion (Figures 3a and S10).43−45 The broad peaks
at 1341 and 1354 cm−1 after the introduction of CH4 and CO2,
assigned to asymmetric and symmetric deformation vibrations
of adsorbed −CH3 groups,46,47 respectively, indicated that
CH4 might be cleaved to form the adsorbed −CH3 and H
species. This result was consistent with the previously reported
work that the adsorbed CH3 species could be generated and
activated under low temperatures.48,49 In our case, the
intensities of the adsorption peaks of methane and methyl
groups significantly reduced at 616 K under light irradiation
after 120 min (Figure 3b), indicating that light could promote
the cracking of methane and accelerate the further cracking of
the methyl group. Moreover, the reduction of the methyl
group during thermal catalysis was not as significant as that
during photothermal catalysis. Thus, the introduction of light
could not only promote the pyrolysis of methane but also
facilitate the continuous cracking of methyl groups.
Figure 3. In situ DRIFTS of the Ni/Ga2O3 catalyst during the DRM
reaction at 528 K and 616 K for 120 min and under UV−vis light
irradiation at 616 K for 30, 60, and 120 min in ranges of (a) 2525−
2200 cm−1, (b) 1400−1200 cm−1, (c) 2225−1950 cm−1, and (d)
3800−3700 cm−1, respectively.
increasing trend after reaction for only 30 min and then
decreased with the increase of light irradiation time at 616 K.
This result demonstrated that light irradiation inhibited the
formation of gaseous CO.
The presence of several broad peaks in the 3586−3734 cm−1
region could be assigned to the stretching modes of isolated
hydroxyl groups due to the adsorbed hydroxyl groups of H2O
on the surface of Ga2O3 (Figure 3d),52 which was usually
involved in the RWGS reaction.53,54 The peaks of hydroxyl
groups in this range still existed, but the intensities were
changed due to their instability at high temperatures.55
Interestingly, with the increase of temperature during thermal
DRM, a new peak was generated at 3734 cm−1 and was
attributed to the asymmetric free OH of H2O.56 Moreover, the
intensity of this peak after light irradiation for 120 min was
significantly lower than that under thermal conditions alone.
Such a kind of phenomenon, related to the formation of H2O,
was detected by mass spectrometry (MS) as well (Figure S11).
This implied that the addition of light inhibited the production
of H2O in the products compared with the thermal DRM.
To clarify the effect of light on this side reaction during
DRM, the RWGS reaction over thermal and photothermal
catalysis was performed. The consumption of H2 during
thermal catalysis was obviously higher than that during
photothermal catalysis with the increase of the reaction
temperature (Figure S12). Also, a nearly 3-fold decrease in
the yield of CO product was recorded under photothermal
catalysis as compared to that under thermal catalysis at the
On the other hand, two new broad adsorption bands at 2174
and 2113 cm−1 corresponding to adsorbed gaseous CO50,51
(Figure 3c) could be detected, ascribed to the decomposition
of adsorbed carbonate species. During the thermal catalytic
reaction without light irradiation, the gaseous CO showed an
increasing trend after the reaction temperature increased to
616 K at 120 min. Interestingly, the gaseous CO showed an
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ACS Catal. 2021, 11, 4730−4738