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Synergy of Single Atoms Pd and Oxygen Vacancies
on In2O3 for Highly Selective C1 Oxygenates
Production from Methane under Visible Light
Lei Luo
Northwest University
Lei Fu
Northwest University
Huifen Liu
Northwest University
Youxun Xu
University College London
Jialiang Xing
Northwest University
Junwang Tang ( junwang.tang@ucl.ac.uk )
University College London https://orcid.org/0000-0002-2323-5510
Article
Keywords: methane conversion, single atom catalyst, methanol, defect engineering, photocatalysis
Posted Date: September 30th, 2021
DOI: https://doi.org/10.21203/rs.3.rs-942037/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
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Abstract
Methane (CH4) oxidation to high value chemicals under mild conditions through photocatalysis is a
sustainable and appealing pathway, nevertheless confronting the critical issues on both conversion and
selectivity. Herein, under visible irradiation (420 nm), the synergy of palladium (Pd) atom cocatalyst and
oxygen vacancies (OVs) on In2O3 nanorods enabled superior photocatalytic CH4 activation by O2. The
optimised catalyst reached ca. 100 µmol·h− 1 of C1 oxygenates, with a selectivity of primary products
(CH3OH and CH3OOH) up to 82.5 %. Mechanism investigation elucidated that such superior
photocatalysis was induced by the dedicated function of Pd single atoms and oxygen vacancies on
boosting hole and electron transfer pathway, respectively. O2 was proven to be the only oxygen source for
CH3OH production, while H2O acted as the promoter for ecient CH4 activation through ·OH production
and facilitated product desorption as indicated by DFT modelling. This work thus provides new
understandings on simultaneous regulation of activity and selectivity by the signicant synergy of single
atom cocatalysts and oxygen vacancies.
Introduction
As the predominant constituent of natural gas, methane hydrate and shale gas resources, selective
methane (CH4) oxidation to value-added chemicals holds considerable nancial and environmental
prospective [1–5]. However, the inert symmetrical tetrahedral structure of CH4 makes it rather dicult for
the dissociation of the rst C-H bond, which is the most important step for activation of methane. [6–8]
Industrial multistep route via steam reforming and subsequent Fischer-Tropsch synthesis could eciently
activate CH4, while it requires harsh experimental conditions (eg. > 700 oC temperature and/or high
pressure), causing huge energy-consumption and safety issues [9–13]. In parallel, it is relatively dicult
to achieve high selectivity due to the more reactive characteristics of the desired oxygenates against both
the reactant CH4 and stable product CO2. [14–17] Therefore, selective CH4 conversion to value-added
chemicals under mild conditions other than CO2 is highly attractive, while confronting considerable
challenges.
Photocatalysis offers an appealing alternative to drive many tough redox reactions under mild conditions
including CO2 conversion [18, 19], N2 reduction[20] and selective CH4 oxidation [8]. Recently, various
value-added chemicals such as methanol [1, 21–23], formaldehyde [24, 25], ethanol [26, 27], ethane and
ethylene [28–33] were produced by photocatalysis. For example, we found that up to 90 % selectivity with
a yield of 3.5 µmol·h− 1 methanol could be achieved over the optimized FeOx/TiO2 photocatalyst under
ambient condition using H2O2 as an oxidant [22]. Recently a high yield of liquid oxygenates including
CH3OH, CH3OOH and HCHO were produced under full arc irradiation over Au supported ZnO, together with
the good selectivity of primary products (CH3OH and CH3OOH) (< 70 %) [1]. Very recent, the yields of 18.7
µmol·h− 1 HCHO and 3.7 µmol·h− 1 CH3OH were reported on quantum BiVO4 with an excellent selectivity
toward HCHO (87 %) and CH3OH (99 %) under 300–400 nm or 400-780nm irradiation[25]. Given these
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signicant advances in photocatalytic methane conversion, the yield and/or selectivity to high value
chemicals are still quite moderate, in particular it is very challenging to achieve methane activation under
visible light irradiation instead of a full arc spectrum due to a narrowed bandgap with mitigated reduction
or oxidation potentials
To realize visible driven methane oxidation by O2 gas on narrow bandgap photocatalysts, cocatalyst is
crucial that does not only promote charge separation, more importantly manipulates the activation
energy of the methane conversion and the selectivity [34–39]. Furthermore rationally regulating the
production of reactive oxygen species (ROS) through cocatalyst modication is necessary as ·OH
radicals have been widely regarded as the main species that induced CH4 activation and over-oxidation
[40, 41]. When CH3OH served as the desired products, over-oxidation to HCHO or CO2 would be
suppressed by lowering the oxidative potential of photogenerated hole through cocatalyst modication,
thus improving the selectivity. Stimulated by molecular catalysis, single atom cocatalysts promise an
extremely high eciency, where atomic dispersed species with unsaturated coordination environment
could improve the catalytic performances based on the unique electronic structure [42–44]. Meanwhile,
high atom utilization eciency could be achieved [45, 46]. On the other hand, since CH4 exhibited low
electron and proton anity, moderate decoration of defective sites could enhance the chemical-
adsorption of non-polar molecular, then promoting the activation of CH4 [47]. Therefore, the integration of
both defects and single atom cocatalyst decoration could boost charge separation, weaken oxidative
potential and enhance CH4 activation on a photocatalyst.
Herein, atomically dispersed palladium (Pd) supported on defective In2O3 was prepared and served as the
visible-light responsive photocatalyst for CH4 conversion to high value chemicals. Under 420 nm
irradiation, the optimized production of oxygenates reached up to ca. 300 µmol in 3 h, with a very high
selectivity of 82.5 % of the primary products. In-situ XPS and EPR spectra were conducted to investigate
the charge transfer dynamics. The results indicated the dedicated roles of Pd atoms and oxygen
vacancies (OVs) in promoting the transfer of photo-induced holes and electrons, respectively. DFT
calculation results indicated H2O could also promoted the desorption of the oxygenate products, thus
suppressing over-oxidation and facilitate high selectivity of primary products. The introduction of atomic
Pd and oxygen vacancies further enhanced this effect on suppressing over-oxidation. Isotopic labelled
experiments further proved the methane conversion pathway.
Results And Discussion
Visible-light photocatalytic CH4 oxidation by O2
Atomic Pd cocatalyst was prepared by the in-situ photo-deposition method withK2PdCl4 and (NH4)2PdCl4
as the precursors on the visible driven In2O3 nanorod photocatalyst. Two types of photocatalysts were
synthesized as the defect-rich and defect-lean materials, denoted as Pd-def-In2O3 and Pd-In2O3,
respectively. For comparison, other noble metals including Pt and Au modied photocatalysts were also
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prepared with the same dosage, denoted as M-(def)-In2O3 (M = Pt and Au). With different K2PdCl4
dosage, the as-prepared samples were denoted as Pdx-def-In2O3, where x % represented the weight
percentage of Pd to In2O3. In the following discussion, the best sample Pd-def-In2O3 and the reference Pd-
In2O3 referred to Pd0.1-def-In2O3 and Pd0.1-In2O3 unless otherwise specied.
Typical noble metal cocatalysts (Pt, Pd, Au) loaded onIn2O3 nanorods were rst tested via photocatalytic
CH4 conversion with O2 as the oxidant (Figure 1a). Under 420 nm irradiation, the products including
CH3OH, CH3OOH and HCHO over Pd-In2O3 reaches 13.4, 32.3 and 27.5 μmol in 3 h reaction, respectively.
The selectivity of the primary products (CH3OH and CH3OOH) was 62.1 % and the selectivity to the
overoxidation products (HCHO and CO2) was 37.9 %. In comparison, Au-In2O3 and Pt-In2O3 performed
almost 100 % over-oxidized products (HCHO), with the trace yields of 1.4 and 0.9 μmol HCHO,
respectively. Such differences suggested Pd cocatalyst was more suitable than Pt and Au for CH4
activation to produce these primary products. The yield of oxygenates for Pd- In2O3 was improved further
to 179.7 μmol by the introduction of defective sites to form Pd-def-In2O3, 2.5 times higher than that of Pd-
In2O3 (73.2 μmol). Meanwhile, the selectivity of the primary products was improved from 62.1 % to 80.4
%, suggesting that deep-oxidation to HCHO and CO2 was greatly suppressed under the synergy of Pd
single atoms and oxygen vacancies (OVs). In the case of Au-def-In2O3 and Pt-def-In2O3, defect
modication exhibited the similar phenomenon on promoting CH4 conversion although the yield was
much lower than that achieved on the Pd modied photocatalyst.
The effect of Pd single atoms was explored over Pdx-def-In2O3. As shown inFigure 1b, Pdx-def-In2O3
photocatalysts exhibited much higher oxygenates production than that of the pristine In2O3. With the
raising of K2PdCl4 dosage, the production of the liquid oxygenates exhibited the volcanic trend,
increasing from 48.7 μmol on Pd0.01-def-In2O3 to 179.7 μmol on Pd0.1-def-In2O3. Furthermore, the
increasing K2PdCl4 dosage resulted in the decreased photocatalytic performance. Notably, the selectivity
exhibited slight improvement from 76.5 % to 82.8 %. With a close look at the above results, OVs and Pd
cocatalyst played the synergistic role in optimizing the activity and selectivity for photocatalytic CH4
conversion.
Molar ratio of CH4 to O2 was tuned over Pd-def-In2O3 (Figure 1c). The production of oxygenates
demonstrated the volcanic trend again, with the highest oxygenate production and selectivity achieved at
1 bar O2 pressure. Reducing molar ratio of CH4 to O2 caused the decreased production to 116.0 μmol of
primary products when CH4/O2 = 10/10, mainly ascribed to the decrease of CH4 concentration. In parallel,
the increased concentration of O2 induced over-oxidation and decreased selectivity of the primary
products from 80.4 % to 67.6 %. With the increase of H2O dosage (Figure 1d), the production of
oxygenates gradually increased, reaching the highest value with 100 mL H2O dosage. The highest
oxygenates achieved 299.0 μmol, 2.3 times improvement than that of 25 mL dosage (128.0 μmol) over
Pd-def-In2O3. Moreover, the selectivity of the primary products improved from 74.2 to 82.5 % with H2O
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dosage increasing from 25 to 100 mL, which could be attributed to the enhanced desorption of the
products from the surface of the photocatalyst when more water was used as discussed later. Notably, in
the absence of H2O dosage, CO2 (8.5 μmol) was produced as the only product, suggesting the critical role
of H2O in promoting CH4 activation as well suppressing over-oxidation, probably ascribed to the
production of ·OH radical and promotion desorption of oxygenates by H2O[48]. While increasing the total
pressure of the gaseous reactants, CH4 dissolved increased and the oxygenate production gradually
increased (Figure 1e), e.g. only trace amount of HCHO (4.1 μmol) produced at 1 bar and reaching the
highest yield of 179.7 μmol when the pressure was 20 bar. To investigate the stability of the optimized
photocatalyst, we carried out thecycling test experiment over Pd-def-In2O3 photocatalyst. No obvious
decrease of oxygenates was observed under 15 hours reaction (Figure 1f), demonstrating the good
stability of Pd-def-In2O3.
Structural identication
X-ray diffraction (XRD) patterns were recorded to probe the crystalline structure of the representative
photocatalysts (In2O3, Pd-In2O3 and Pd-def-In2O3) (Figure S1). The diffraction peaks on all three samples
at 30.7o, 35.5o, 51.0o and 60.7o were well matched with the standard phase of In2O3 (PDF#71-2194).
While no Pd and PdOx diffraction peaks were observed onPd-In2O3 and Pd-def-In2O3, indicating the high
dispersion of Pd species. The slightly weakened relative intensity from 100 % of In2O3 to 97 % and 93 %
of Pd-In2O3 and Pd-def-In2O3 could be probably ascribed to the introduction of defects. Raman spectra
(Figure 2a) further supported the well-established In2O3 phase. The typical Raman peaks for In2O3 were
clearly observed at 130.6, 305.1 and 494.8 cm-1 [49]. For Pd-In2O3 and Pd-def-In2O3, the dominant peak
exhibited a slight left-shift from 130.6 to 129.9 cm-1, attributed to the surface stain effect induced by the
Pd cocatalyst deposition[50].
Electron paramagnetic resonance (EPR) spectra were conducted to evaluate the spin-electrons including
oxygen vacancies (Figure 2b). For the pristine In2O3 and Pd-In2O3, a single Lorentz peak at g = 1.882 was
observed, ascribed to the electrons on the conduction band (CB)[51, 52]. In the case of Pd-def-In2O3, the
signal of this peak exhibited much stronger intensity than the others, suggesting the higher electron
density on CB. Meanwhile, an additional Lorentz peak was observed at g = 2.001, which could be
attributed to the free-electrons trapped by the oxygen vacancies[52], thus suggesting the existence of
oxygen vacancies in Pd-def-In2O3. The introduction of oxygen vacancies might contribute to the stronger
EPR peak at g = 1.882.
High-resolution transmission electron microscope (HRTEM) images further proved the defective structure
of Pd-def-In2O3 (Figure 2c-d). Pd-def-In2O3 reserved the nanorod morphology with dimension of 203 nm
in diameter and 1450 nm in length (insert ofFigure 2c) as that of the In2O3 substrate and Pd-In2O3
(Figure S2). In addition, a thin amorphous/defective layer of ca. 4 nm was observed on the edge (Figure
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2d). The bulk crystal plane distance of 0.415 nm was indexed to the (211) facet of In2O3. On the contrary,
there is not such defective layer as indicated in Figure S2b. Elemental distribution of the corresponding
area was analysed by the EDS-mapping images (Figure 2e). Obviously, Pd-def-In2O3 exhibited uniform
palladium distribution with indium and oxygen elements. The aberration corrected HAADF-STEM image
inFigure 2f clearly indicated the atomic distribution of Pd, where the weak intensity spots cycled by the
yellow corresponded to Pd atoms.The x-y line scan along the yellow rectangle ofFigure 2f clearly
presents the atomic dispersion of Pd as shown inFigure 2g. Therefore the best sample was composed of
single atom Pd and oxygen vacancies on In2O3 nanorods.
Mechanism investigation
UV-vis diffuse reection spectra (UV-DRS) were conducted to evaluate the photoabsorption ability of the
representative photocatalysts (In2O3, Pd-In2O3 and Pd-def-In2O3) (Figure 3a). All the three photocatalysts
exhibited the similar photoabsorption onset at ca. 450 nm, indicating that the modication of single atom
Pd and OVs has little inuence on bandgap energy and bandgap energy was not the decisive reason that
induced improvement of photocatalysis.
In-situ high-resolution Pd3d X-ray photoelectric spectra (XPS) in dark and under light were conducted to
study the charge transfer direction of Pd-def-In2O3 (Figure 3b and Figure S3). In dark, the Pd3d5/2 XPS
peak could be deconvoluted into two binding peaks at 336.55 and 335.38 eV, which were assigned to the
Pd2+ and Pd0 species, respectively [53]. Under light irradiation, the peak exhibited a left-shift to higher
binding energy (Figure S3). Further deconvoluted results (Figure 3b) suggested that Pd2+ content
increased to 26.3 %, much higher than 6.9 % in dark. Such increased Pd2+ content suggested Pd served
as the hole acceptors upon excitation. In-situ EPR spectra under light were conducted to evaluate the role
of OVs. As shown in Figure 3c, the signal at g = 2.0009 was attributed to the electrons trapped by the
oxygen vacancies, which performed gradually increasing intensity from 100 % to 226 % with the
prolonged irradiation to 360 seconds. This stronger EPR intensity suggested a higher concentration of
spin-electrons and thus demonstrated OVs served as the electron acceptor [54]. Therefore, single atom Pd
and OVs separately acted as the hole and electron acceptors under light irradiation, which would greatly
contribute to the enhanced charge separation.
Photocurrent responses (Figure S4) were tested to evaluate the charge separation eciency. Pristine
In2O3 exhibited a relatively low photocurrent density of 61.8 μA·cm-2. After photo-depositing high
dispersed Pd cocatalyst, Pd-In2O3 nearly doubled photocurrent density to 120.6 μA·cm-2. The
photocurrent density was further improved to 168.1 μA·cm-2 on Pd-def-In2O3, almost 2.7 and 1.4 times
enhancement than that of In2O3 and Pd-In2O3, respectively. Such highest photocurrent density on Pd-def-
In2O3 attributed to the most ecient charge separation, indicating the defects and single atom Pd could
greatly enhance charge transfer, which is consistent with the analysis mentioned above. Steady-state
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uorescence (PL) spectra further evidenced the enhanced charge separation eciency. As shown
inFigure 3d, a relatively strong PL emission peak was observed for the pristine In2O3, attributed to the
severe charge recombination. In comparison, the PL intensity for Pd-In2O3 was greatly weakened,
indicating the suppressed charge recombination. For Pd-def-In2O3 photocatalyst, the most weakened PL
peak were observed, ascribed to the most enhanced charge separation eciency, which was
corresponding with the photocurrent analysis. Time-decay PL spectra were conducted to evaluate the PL
lifetime. As shown in Figure S5, Pd-def-In2O3 photocatalyst exhibited the slowest PL decay kinetics. The
tting results (Table S1) showed that Pd-def-In2O3 exhibited the average PL lifetime at 4.99 ns, longer
than that of In2O3 (3.60 ns) and Pd-In2O3 (4.28 ns), which would be benecial to the ecient utilization of
separated charge carriers.
Reactive oxygen species including ·OOH and ·OH radicals were widely regarded as the main active
species for CH4 activation[55]and monitored by in-situ EPR spectra with 5, 5-dimethyl-1-pyrroline N-oxide
(DMPO) as the spin-electron trapping agents. As shown inFigure 4a, the DMPO-OOH adduct was
detected under light over different photocatalysts and ascribed to the presence of ·OOH, which came from
the reduction of O2 molecule with photo-induced electrons and H+.Astronger intensity of DMPO-OOH
was observed forPd-def-In2O3,suggesting the production of ·OOH radical was enhanced by the
integration of single atom Pd and OVs.On the other hand, in-situ EPR spectra under light was used to
monitor the generation of ·OH radical with DMPO as the trapping agent in H2O. The 1:2:2:1 quartet
signals wereobserved and assigned to the DMPO-OH adduct, suggesting the generation of ·OH radical
(Figure 4b). It was obvious thatPd-def-In2O3 produced much more ·OH under identical conditions than
Pd-In2O3 and In2O3was the worse. It is believed that ·OHinitially activates CH4 to methyl radical (·CH3),
thus Pd-def-In2O3 performed CH4 activation best followed byPd-In2O3, which is consistent with the step
by step enhanced photocatalytic performances by Pd and then both Pd and oxygen vacancies, indicating
that oxygen vacancies could promote charge separation and also facilitate water oxidation reaction on
Pd. Coumarin was used as the probe for ·OH radical detection due to the easy reaction between coumarin
and ·OH to produce 7-hydroxycoumain (7-HC) that could be detected by UV-vis spectra at 412 nm (Figure
4c). The results further supported that Pd-def-In2O3 held the strongest ability for ·OH production, which
facilitated CH4 activation. Therefore, single atom Pd worked as the hole acceptor, which then catalysed
·OH radical production from water oxidation. Simultaneously, OVs acted as the electron acceptor, which
then catalysed O2 reduction to generate ·OOH radical.
The reaction pathway was investigated by isotopic labelled experiments, including using H218O and 18O2.
In the presence of 3 mL H218O, 1 bar 16O2 and 19 bar CH4, no isotopic labelled CH318OH (m/z = 33 and
34) was detected by GCMS (Figure 4d), suggesting H2O was not the oxygen source that directly
participated the formation of oxygenates. In parallel, when using 3 mL H216O, 1 bar 18O2 and 19 bar CH4,
the signals at m/z = 34 and 33 were attributed to the isotopic labelled CH318OH and its fragment (Figure
4d), suggesting O2 was the only oxygen source for CH3OH formation. Carbon source for methanol
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production were also studied in the presence of 5 bar isotopic labelled 13CH4 (Figure 4e), where the signal
of mass spectra (MS) at m/z = 33 was ascribed to 13CH3OH, demonstrating that CH4 was the carbon
source for oxygenates production.
DFT calculations (Figure 5) were conducted to explain the improved selectivity of primary products. It
should be noted that timely desorption of the primary products on the active sites could eciently avoid
its deep-oxidation to HCHO and CO2. As ·OH radical was regarded as the main species that induced
oxidation on single atom Pd cocatalyst, it was accordingly considered that the ecient desorption of
primary products like CH3OH on Pd is critical to suppress further oxidation. Thus, the adsorption energies
of H2O and CH3OH were calculated since the stronger adsorption of H2O might promote the desorption of
CH3OH. H2O and CH3OH adsorption on In2O3 and on Pd/ In2O3 were modelled and optimized by the
density functional theory (DFT) calculations. As shown in Figure 5a and b, the adsorption energies of H2O
on In2O3, Pd-In2O3 and Pd-def-In2O3 were -0.76, -1.57 and -2.14 eV, respectively, much larger than the
CH3OH adsorption energy of -0.47, -1.38 and -1.50 eV on the specied model. Such larger adsorption
energies indicate CH3OH could be easily replaced by H2O on In2O3 or Pd atoms, promoting the desorption
of CH3OH. Moreover, adsorption energies further increased with the introduction of both Pd atom and
oxygen vacancy, demonstrating such co-modication of Pd atoms and OVs could promote the adsorption
of H2O most eciently, which is consistent with the increased production of ·OH radicals as analyzed by
the in-situ EPR and coumarin experiments. Though the adsorption of CH3OH was also enhanced due to
the introduction of Pd atoms and OVs, H2O adsorption energy was enhanced much more, thus water
could facilitate the desorption of primary products and avoid overoxidation as indicated in Figure 1d.
Based on the above results, aerobic photocatalyticCH4 conversion mechanism over Pd-def-In2O3 was
proposed (Scheme 1). Generally, under 420 nm irradiation, electrons (e-) were excited to the conduction
band of In2O3 nanorod photocatalyst, while leaving holes (h+) on the valence band. Then the photo-
induced electrons were trapped by the oxygen vacancies, activating O2 with H+ to produced ·OOH radicals
as detected by the in-situ EPR spectra. In parallel, Pd atoms served as the hole acceptors (Pd + h+ →
Pdδ+), and then reacted with the adsorbed H2O to produced ·OH (Pdδ+ + H2O → Pd0 + ·OH + H+). CH4
molecules were next activated by the as-produced ·OH to ·CH3. The coupling reaction between ·CH3 and
·OOH then generated the primary products (CH3OOH), and subsequently transferred to CH3OH as
indicated in Scheme 1b. Compared with the pristine In2O3, the incorporation of Pd single atoms
signicantly promoted charge separation and facilitated the generation of reactive species, thus
promoting CH4 conversion to oxygenates. Pd atoms loading also moderated the oxidation ability of
photoinduced holes on In2O3 as indicated in Scheme 1a, reducing the overoxidation of the primary
products. Further decoration of oxygen vacancies could strengthen the promoted charge separation
eciency, which eventually resulted in the superior photocatalytic CH4 conversion activity and selectivity.
In order to suppress over-oxidation, it was also critical to enhance the desorption of primary oxygenate
products by H2O, as supported by the DFT calculation.
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In summary, visible-light-driven CH4 conversion at ambient temperature was reported over the In2O3
nanorod photocatalyst with loading of Pd single atoms cocatalysts and oxygen vacancies. Under 420 nm
irradiation, superior yield (99.7 μmol·h-1) and selectivity (82.5 %) of the primary products were achieved
on Pd-def-In2O3 photocatalyst under optimized reaction conditions. In-situ XPS and EPR spectra under
visible light irradiation indicated that Pd and oxygen vacancies acted as the hole and electron acceptors,
respectively, thus synergistically boosted charge separation and transfer. Isotopic labelled experiment
proved that O2 was the only oxygen source for oxygenates production, while H2O was the promoter of
CH4 activation through the production of ·OH radical as monitored by the in-situ EPR spectra with DMPO
as the spin-trapping agent. DFT calculation results suggested that H2O performed much larger adsorption
energies than CH3OH on either In2O3,def-In2O3 orPd-def-In2O3, suggesting the stronger adsorption of
H2O than CH3OH, which was benecial to the timely desorption of the produced CH3OH, thus avoiding
further over-oxidation. The introduction of Pd and oxygen vacancies could further improve the selectivity
of primary oxygenates mainly through the greatly enhanced adsorption of H2O and the reduced oxidation
potential of photoinduced holes. This work provided an useful avenue on co-modication by oxidative
single atom cocatalyst and oxygen vacancies for simultaneous regulation of both activity and selectivity
through enhancing charge separation, moderated photohole oxidation ability and timely promoted
desorption of primary products by a solvent.
Methods
Synthesis of In2O3 nanorods
In2O3 nanorods were prepared according to the previous study[56]. Typically, 12.0 g urea and 1.5 g InCl3
were dissolved in 135 g H2O, followed by stirring at 80 oC for 14 h. After naturally cooling down, the
reactant was centrifuged and washed with H2O for several times. The white powder was then dried in
vacuum at 60 oC overnight. After thermal-treating in air at 700 oC for 5 h at a ramping rate of 5 oC·min-1,
yellow powder was obtained and denoted as In2O3.
Synthesis of Pdx-In2O3 and Pdx-def-In2O3 nanorods
Pdx-In2O3 and Pdx-def-In2O3 nanorods were prepared through photo-deposition with ammonium
tetrachloropalladate(II) ((NH4)2PdCl4) and potassium tetrachloropalladate(II) (K2PdCl4) as the precursors,
respectively. The synthesis was conducted in the multichannel reactor (
Beijing Perfectlight Technology
Co., Ltd
). For Pdx-def-In2O3 preparation, 200 mg In2O3 was rst dispersed through sonication with the
aqueous solution containing 10 vol.% methanol. Then certain amount of K2PdCl4 solution was added.
After sealing and purging with ultrapure argon (99.999 vol.%) for 30 min, the reactor was bottom-
irradiated for 3 h to facilitate Pd photo-deposition. The suspension was then centrifuged, washed with
H2O for several times and dried under vacuum at 60 oC overnight. The as-prepared samples were denoted
as Pdx-def-In2O3, where x % represented the mass percentage of palladium to In2O3 substrates. Pdx-In2O3
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with no oxygen vacancies was prepared under identical conditions except the usage of (NH4)2PdCl4
instead of K2PdCl4.
Characterizations
XRD were measured to obtain the crystalline structure on the
D8 ADVANCE
diffractometer (
Bruker Co.,
Ltd
).Palladium and potassium contents were measured through inductively coupled plasma atomic
emission spectrometry (ICP-AES) on the
Agilent 7900 ICP-MS
instrument. Raman spectra were collected
on the
DXR 2DXR2
instrument (
Thermo Fisher Scientic, Co., Ltd
). HRTEM images were captured on
the
Talos F200X
instrument (
FEI Co., Ltd
). UV-DRS spectra were measured on the UV-3600 plus
spectrophotometer (
ShimadzuCo., Ltd
). Photocurrent test was conducted on the electronic workstation
(
CHI660E
) on thethree-electrode system.Ag/AgCl electrode, platinum sheet electrode and Na2SO4
solution (0.1 M) were used as the reference electrode, counter electrode and electrolyte, respectively. The
mixtures of photocatalyst, ethanol and Nion solution (
Shanghai Adamas Reagent Co., Ltd
) were
suspended and sonicated to prepare the working electrode.In-situ XPS in dark and under light were
measured on the
Thermo ESCALAB 250Xi
instrument with an Al Kα radiation source. In-situ solid-state
EPR spectra in dark and under light were measured with 20 mg photocatalyst on the
ELEXSYS II
EPR
instrument.
PhotocatalyticCH4conversion
Photocatalytic CH4 conversion was conducted in a top-irradiated high-pressure reactor with 200 mL
volume. LED lamp (420 nm,
PLS-LED100C
,
Beijing Perfectlight Technology Co., Ltd
) was used as the light
source. Typically, 20 mg photocatalyst was dispersed in 50 mL distilled water. After sealing and purging
with ultrapure O2 (99.999 vol.%) for 20 min, 1 bar O2 and 19 bar CH4 (99.999 vol.%) were owed into the
reactor. The temperature of the reactor was maintained at 25 oC by the cold-water bath. After reacting for
3 h, the gaseous and liquid products like methanol were measured by the gas chromatography
(
GC2014
,
ShimadzuCo., Ltd
) equipped with thermal conductivity detector (TCD) and ame ionization
detector (FID). CH3OOH and CH3OH were measured through 1H nuclear magnetic resonance
(NMR)(
AVANCE III
,
JEOL Ltd
). As CH3OOH and CH3OH have the same number of methyl, the area ratio of
CH3OOH to CH3OH in 1H NMR should be the molar ratio of CH3OOH to CH3OH. Thus, CH3OOH could be
quantied. HCHO was measuredthrough the colorimetric method[57]on the
UV-3600 Plus
spectrometer
(
Shimadzu Co., Ltd
).
Isotope labelling experiments
For carbon source investigation: 20 mg Pd-def-In2O3 photocatalyst was dispersed in 3 mL H2O. After the
reactor being degassed for 30 min, 1 bar O2 and 5 bar 13CH4 were injected into the reactor. After reacting
for 6 h, the suspension was ltered and then the solvent was analysed by GC-MS (QP2010,
ShimadzuCo.,
Ltd
) equipped withthe Cap WAX column.
Page 11/22
For oxygen source investigation: 20 mg Pd-def-In2O3 photocatalyst was dispersed in 3 mL H216O or
H218O. After the reactor being degassed for 30 min, 1 bar 18O2 or 16O2 and 5 bar CH4 were injected into
the reactor. After reacting for 6 h, the suspension was ltered and then the solvent was analysed by GC-
MS (QP2010,
ShimadzuCo., Ltd
).
Monitor of the reactive species
DMPO was used as the spin-trapping agent for monitor of the reactive species including ·OOH and ·OH
radicals. For ·OOH radical detection, 10 mg Pd-def-In2O3 photocatalyst was dispersed into 5 mL
DMPO/methanol solution. After purging with ultrapure O2 (99.999 vol.%) for 20 min, in-situ EPR spectra in
dark and under light irradiation were collected. For ·OH radical detection, 10 mg Pd-def-In2O3
photocatalyst was dispersed in 5 mL aqueous DMPO solution. After purging with ultrapure O2 (99.999
vol.%) for 20 min, in-situ EPR spectra in dark and under light were collected.
Analysis of hydroxyl radical (·OH)
Coumarin was used as the probe for the quantication of ·OH via the production of 7-HC[40]. Typically,
20 mg photocatalyst was dispersed in 100 mL aqueous coumarin solution (5×10-4M). After stirring for 30
min in dark, the suspension was irradiated with the LED light source (420 nm,
PLS-LED100C
,
Beijing
Perfectlight Technology Co., Ltd
). Certain amount of suspension was sampled and ltered in the 10 min
intervals. PL intensity of the produced 7-HC in the solution was then measured on the
F4500
spectrouorometer.
DFT calculation of adsorption energies
The rst-principles were employed to perform all the density functional theory (DFT) calculations within
the generalized gradient approximation (GGA) using the PBE formulation. The projected augmented wave
(PAW) potentials have been chosen to describe the ionic cores and take valence electrons into account
using a plane wave basis set with a kinetic energy cutoff of 400 eV. Partial occupancies of the
Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The
electronic energy was considered self-consistent when the energy change was smaller than 10−4 eV. A
geometry optimization was considered convergent when the force change was smaller than 0.05 eV/Å.
Grimme’s DFT-D3 methodology was applied to describe the dispersion interactions. Three models
including In2O3 with (111) facet, def-In2O3 with one oxygen vacancy and Pd-def-In2O3 with both one
oxygen vacancy and single atom Pd modication were conducted. During structural optimizations, the
2×2×1 Monkhorst-Pack k-point grid for Brillouin zone was used for k-point sampling for structures.
Finally, the adsorption energies (
Eads
) were calculated as
Eads
=
Ead/sub
-
Ead
-
Esub
, where
Ead/sub
,
Ead
, and
Esub
were the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure,
and the clean substrate, respectively.
Page 12/22
Data availability
The data that support the ndings of this study are available from the corresponding author upon
reasonable request.
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Declarations
Acknowledgements
L. L., L. F., H. L. and J. X. are grateful for the China Postdoctoral Science Foundation (Grant No.
2019M663802) and the Shannxi Key Research Grant (China, 2020GY-244). Y. X. and J.T. are thankful for
nancial support from the UK EPSRC (EP/S018204/2), Leverhulme Trust (RPG-2017-122), Royal Society
Newton Advanced Fellowship grant (NAF\R1\191163 and NA170422) and Royal Society Leverhulme
Trust Senior Research Fellowship (SRF\R1\21000153).
Author contributions
J.T. conceived and supervised the entire project.L. L., L. F. and H. L. conducted the material synthesis,
characterizations and photocatalytic methane conversion tests.L. L. drafted the manuscript under the
guidance of J. T.. Y. X. helped to discuss the catalytic results and improve the manuscript. All authors
discussed and commented on the manuscript.
Competing intrests
The authors declare no competing interests.
Scheme
Scheme 1 is in the supplementary les section.
Figures
Page 18/22
Figure 1
Photocatalytic CH4 conversion performance under 420 nm irradiation. Investigations on (a) diverse noble
metal species, (b) K2PdCl4 dosage during synthesis, (c) molar ratio of CH4/O2, (d) H2O dosage, (e) total
pressure and (f) cycling tests over the best sample Pd-def-In2O3. Standard reaction conditions: 20 mg
photocatalyst, 50 mL distilled H2O, 19 bar CH4, 1 bar O2, 3 h. For reaction condition investigation, only
the specied parameter was changed.
Page 19/22
Figure 2
(a) Raman and (b) EPR spectra of In2O3, Pd-In2O3 and Pd-def-In2O3. (c, d) HRTEM and (e) HAADF and
EDS-mapping images of Pd-def-In2O3. Blue, red and green colors represent indium, oxygen and
palladium elements, respectively. (f) Aberration corrected HAADF-STEM image of Pd-def-In2O3, where Pd
single atoms with a weak intensity are indicated by yellow circles. (g) Line scan measured along the x-y
rectangle region marked in f.
Page 20/22
Figure 3
(a) UV-DRS spectra of In2O3, Pd-In2O3 and Pd-def-In2O3. (b) In-situ Pd3d XPS spectra and (c) in-situ EPR
spectra of Pd-def-In2O3 in dark and under light. (d) Steady-state PL spectra of In2O3, Pd-In2O3 and Pd-
def-In2O3.
Page 21/22
Figure 4
In-situ EPR spectra of (a) DMPO-OOH and (b) DMPO-OH for the monitor of reactive ·OOH and ·OH radicals
over different photocatalysts. (c) PL intensity of 7-HC versus time over different photocatalysts for the
quantication of ·OH. Isotopic labelled experiments (d) oxygen source and (e) carbon source for
methanol production in the presence of isotopic labelled H218O, 18O2 or 13CH4.
