Promotion of photocatalytic steam reforming of methane over Ag0/Ag+-SrTiO3

Article abstract of DOI:10.1016/j.cclet.2019.11.007

Methane (CH₄) is not only used as a fuel but also as a promising clean energy source for hydrogen generation. The steam reforming of CH₄ (SRM) using photocatalysts can realize the production of syngas (CO + H₂) with low energy consumption. In this work, Ag⁰/Ag⁺-loaded SrTiO₃ nanocomposites were successfully prepared through a photodeposition method. When the loading amount of Ag is 0.5 mol%, the atom ratio of Ag⁺ to Ag⁰ was found to be 51:49. In this case, a synergistic effect of Ag⁰ and Ag⁺ was observed, in which Ag⁰ was proposed to improve the adsorption of H₂O to produce hydroxyl radicals and enhance the utilization of light energy as well as the separation of charge carriers. Meanwhile, Ag⁰ was regarded as the reduction reaction site with the function of an electron trapping agent. In addition, Ag⁺ adsorbed the CH₄ molecules and acted as the oxidation reaction sites in the process of photocatalytic SRM to further promote electron-hole separation. As a result, 0.5 mol% Ag-SrTiO₃ exhibited enhancement of photocatalytic activity for SRM with the highest CO production rate of 4.3 μmol g^?1 h^?1, which is ca. 5 times higher than that of pure SrTiO₃. This work provides a facile route to fabricate nanocomposite with cocatalyst featuring different functions in promoting photocatalytic activity for SRM.

Full text of DOI:10.1016/j.cclet.2019.11.007

Journal Pre-proof
Promotion of photocatalytic steam reforming of methane over
Ag0/Ag+-SrTiO3
Bingqing Tan, Yinghao Ye, Zeai Huang, Liqun Ye, Minzhi Ma, Ying
Zhou
PII:
S1001-8417(19)30654-0
https://doi.org/10.1016/j.cclet.2019.11.007
CCLET 5314
DOI:
Reference:
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Chinese Chemical Letters
Received Date:
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Accepted Date:
30 September 2019
29 October 2019
6 November 2019
Please cite this article as: Tan B, Ye Y, Huang Z, Ye L, Ma M, Zhou Y, Promotion of
photocatalytic steam reforming of methane over Ag0/Ag+-SrTiO3, Chinese Chemical Letters
(2019), doi: https://doi.org/10.1016/j.cclet.2019.11.007
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Communication
Promotion of photocatalytic steam reforming of methane over Ag0/Ag+-SrTiO3
Bingqing Tana,b, Yinghao Yea,b, Zeai Huanga,b, Liqun Yec, Minzhi Maa,b, Ying Zhoua,b,

a State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China
b The Center of New Energy Materials and Technology, School of Materials Science and Engineering, Southwest Petroleum University, Chengdu 610500, China
c Engineering Technology Research Center of Henan Province for Solar Catalysis, Collaborative Innovation Center of Water Security for Water Source Region
of Mid-line of South-to-North Diversion Project of Henan Province, Nanyang Normal University, Nanyang 473061, China
 Corresponding author: E-mail address: yzhou@swpu.edu.cn (Y. zhou).
ARTICLE INFO
Article history:
Received 30 September 2019
Received in revised form 29 October 2019
Accepted 1 November 2019
Available online
Graphical Abstract
The synergistic effect of Ag0/Ag+ is achieved to improve the photocatalytic performance. The Ag0 can be regarded as the reduction
reaction sites and significantly promote the adsorption of H2O to produce more •OH radicals. While the Ag+ species can enhance
the adsorption of CH4 and be regarded as the oxidation reaction sites.
ABSTRACT
Methane (CH4) is not only used as a fuel but also as a promising clean energy source for hydrogen generation. The steam reforming of CH4 (SRM)
using photocatalysts can realize the production of syngas (CO + H2) with low energy consumption. In this work, Ag0/Ag+-loaded SrTiO3
nanocomposites were successfully prepared through a photodeposition method. When the loading amount of Ag is 0.5 mol%, the atom ratio of Ag+ to
Ag0 was found to be 51:49. In this case, a synergistic effect of Ag0 and Ag+ was observed, in which Ag0 was proposed to improve the adsorption of
H2O to produce hydroxyl radicals and enhance the utilization of light energy as well as the separation of charge carriers. Meanwhile, Ag0 was regarded
as the reduction reaction site with the function of an electron trapping agent. In addition, Ag+ adsorbed the CH4 molecules and acted as the oxidation
reaction sites in the process of photocatalytic SRM to further promote electron-hole separation. As a result, 0.5 mol% Ag-SrTiO3 exhibited enhancement
of photocatalytic activity for SRM with the highest CO production rate of 4.3 μmol g−1 h−1, which is ca. 5 times higher than that of pure SrTiO3. This
work provides a facile route to fabricate nanocomposite with cocatalyst featuring different functions in promoting photocatalytic activity for SRM.
Keywords:
Photocatalysis; Steam reforming of CH4; Hydrogen; Carbon monoxide; Cocatalyst; Ag0/Ag+-SrTiO3
———
 Corresponding author.
E-mail address: yzhou@swpu.edu.cn (Y. zhou).

Methane (CH4) is widely used as an essential fuel and chemical feedstock [1,2]. The release of CH4 during its storage, transportation,
and use has long been neglected, which makes a significant contribution to its concentration increases in the atmosphere. In recent
decades, various relevant researches pointed out the greenhouse effect of CH4 is twenty times greater than that of the effect of carbon
dioxide [3,4]. Therefore, decreasing the CH4 emission is highly demanded to migrate the worldwide global warming caused by the
greenhouse effect. The studies on effective utilization methods for CH4 conversion have been paid more and more attention [5]. Among
various methods for CH4 conversion, steam reforming of CH4 (SRM: CH4 + H2O → 3H2 + CO) to produce hydrogen and syngas over
heterogenous catalysts showed a promising way to utilization of CH4 [6-9]. However, CH4 as a symmetric molecule with an extremely
high C-H energy (434 kJ/mol) and a non-polar feature is very stable, a high temperature (normally higher than 1073 K) is essential for
SRM. This disadvantage significantly inhibits the development and applications of SRM [10].
Photocatalysis using semiconductors and sunlight has shown great potential in promoting difficult reactions that cannot be occurred
under mild conditions [11-15]. Among them, photocatalytic steam reforming of CH4 (PSRM) is exceptionally challenging. This reaction
not only demands semiconductors featuring high photocatalytic oxidative activity to break the C-H bond of CH4, but also requires that
semiconductors have strong reductive abilities for water splitting. Therefore, it is necessary to explore wide bandgap semiconductors
with strong redox potentials for PSRM [16,17]. In addition, the poor adsorption of reactant gases and the utilization of light energy should
also be taken into considerations in the gas-solid-phase photocatalytic reaction [18-21].
Perovskite oxides (ABO3) with flexible chemical composition, photostability, and low cost are promising candidates for PSRM [17].
For example, strontium titanate (SrTiO3) as one of perovskite oxide photocatalysts with a wide bandgap was reported to show moderate
photocatalytic activity for CH4 oxidation in the gas phase [22]. However, the efficiency of PSRM over SrTiO3 remains relatively low
due to the inadequate utilization of light energy and its poor adsorption of CH4. Cocatalysts such as noble metals and carbon dots are
widely used for improving the photocatalytic efficiency of PSRM. Au, Pt, Rh, and Pd showed well photocatalytic activity for PSRM
[23]. For the case of carbon dots and other carbon contain cocatalysts, it might be not well to use in this stage. Because the recent
formation of products is extremely low for the PSRM (a few nmol/h levels), if carbon-contain materials are used, it is difficult to confirm
the origin of products without using isotopic labeling experiments. Some noble-metal loaded perovskite oxides such as Rh-K2Ti6O13
[24,25], Pt-CaTiO3 [26,27], La-NaTaO3 [28] have been developed for PSRM reaction and exhibited improved photocatalytic activity.
The noble metals were reported to efficiently promote photogenerated carriers transfer and light adsorption. For instance, Iwashina et al.
have demonstrated that Rh doping could enhance the visible-light absorption of SrTiO3, resulting in the improvement of photocatalytic
activity [29]. Moreover, various studies have shown that noble metals can improve the adsorption of gas due to their unique electronic
structures [30,31]. Among these noble-metals, Ag was found to be active for CH4 activation in both thermocatalysis and photocatalysis.
The silver cationic species (Ag+) can promote the dissociation of CH4 to form Ag-H bonds [3,32,33]. Nevertheless, the instability of Ag+
under light irradiation significantly limits its photocatalytic application. Coexisting with metallic silver (Ag0) has been proved to be an
effective solution to this issue [33].
In this work, we fabricated Ag0/Ag+ decorated SrTiO3 catalysts through a photodeposition method and used for PSRM. TiO2 was
synthesized via a hydrothermal method according to the previous works [34,35]. Then, 199.7 mg (0.25 mmol) of the as-prepared anatase
TiO2 and 664.4 mg (0.25 mmol) Sr(OH)2·8H2O were added into a 15 mL solvent mixture (13 mL of deionized water and 2 mL of ethylene
glycol (EG)). After the pH was adjusted to 14.0 with a 12 mol/L NaOH aqueous solution, the precursor mixture was added to a 50 mL
Teflon-lined vessel and maintained at 160 °C for 24 h, afterward cooled down to ambient temperature. Finally, the obtained samples
were filtrated and washed by 0.2 mol/L HNO3 solution and deionized water and dried at 60 °C for 24 h to obtain SrTiO3 powder.
Ag loaded SrTiO3 was fabricated using a photodeposition method. 200 mg of SrTiO3 was dispersed in 50 mL water by ultrasonic
treatment in a round-bottom flask. Then AgNO3 solution (2 mg/mL) and 3 mL isopropanol were added into the suspension. After
deoxygenated with Ar gas for 30 min, the flask was illuminated by a Xe lamp (300 W, Perfect light PLS-SXE 300/300UV) for 2 h. The
precipitation was centrifugally washed to obtain the solid composites. The catalysts were named as X% Ag-STO, where X represents the
molar ratio of Ag in products.
X-ray diffraction (XRD) was carried out on a X’pert diffractometer (PANalytical) with Cu Kα radiation (40 kV, 40 mA). The
morphology and microstructure were recorded by transmission electron microscope (TEM) and high resolution transmission electron
microscope (HRTEM) (FEI Tecnai G220). X-ray photoelectron spectroscopy (XPS) spectra of catalysts were obtained by X-ray
photoelectron analyzer (Thermo ESCALAB 250Xi) and the binding energies were corrected according to the C 1s peak at 284.6 eV. The
optical performances were characterized through a UV-vis spectrometer. The N2 adsorption-desorption isotherms were obtained via
Quadrasorb SI. Photoluminescence (PL) spectrums were determined on the fluorescence spectrometer (Hitachi F-7000). Surface
photovoltage (SPV) tests were proceeded by using a monochromatic-light (Omni-λ 3007) and a SR830 lock-in amplifier. Electron spin
resonance (ESR) spectra were observed with a JESFA200 spectrometer.
The reaction tests were conducted in a fixed-bed photoreactor with 380 mL capacity under room temperature (25 oC), the schematic
photocatalytic reactor as shown in Fig. S1 (Supporting information). In a typical process, 10 mL deionized water and 100 mg catalyst
diffused uniformly on a glass culture dish were added into the stainless-steel beaker. Then, the photoreactor was flowed with 99.9% Ar
gas repeatedly 3 times to remove O2 and CO2. Subsequently, 1 mL CH4 was injected into the photoreactor via a micro-syringe. Finally,
the 300W Xe lamp as the light resource was opened up for the PSRM experiment. 1 mL gas was taken out every hour and analyzed using
a Techcomp GC7900. The GC was equipped with a TDX-01 column, flame ionization detector (FID), thermal conductivity detector
(TCD), and a CH4 conversion oven. Additionally, in order to verify that the oxidation products are indeed derived from the PSRM

reaction, the isotope gas of 13CH4 (m/z = 17) was used as the reaction gas to replace 12CH4 (m/z = 16) with the detection of products using
a mass spectrometer. Photoelectrochemical measurements (PEC) were tested by a CHI 660 workstation. The density functional theory
(DFT) calculations were performed using the DMol3 procedure of Materials Studio software [36-38], details of the calculation parameters
were shown in the Supporting information.
The XRD characterized the composition and crystal structure of X% Ag-STO (X = 0, 0.5 and 1) catalysts. As shown in Fig. S2
(Supporting information), the peaks of 22.8°, 32.4°, 39.9°, 46.5°, 52.6°, 57.8°, and 67.8° are indexed as the (100), (110), (111), (200),
(210), (211), and (220) planes of cubic SrTiO3 (JCPDS file No. 35-0734) [39]. Compared with pristine SrTiO3, no peak shift was found
in Ag-STO, suggesting that Ag was not doped into the lattice structure of SrTiO3 [40]. In addition, no diffraction peak of Ag was
observed, which might be due to the small amount and highly dispersed of Ag elements [41-43]. The morphology characteristics of Ag-
STO catalysts were studied using TEM and HRTEM (Fig. 1). The nanoparticle distributions of catalysts are similar before and after Ag
loading as shown in Figs. 1a and c. The image in Fig. 1b exhibits continuous and well-resolved lattice fringes, with d-spacing of ca. 0.27
nm corresponding to the (110) planes of SrTiO3 [44]. The lattice spacing of 0.239 nm matches well with the (111) crystal planes of Ag
(Fig. 1d). These results indicated that the loading of Ag showed no obvious effect on the morphology, shape, size of SrTiO3 catalysts
and Ag had been decorated onto the surface of SrTiO3 successfully.
Fig. 1. TEM and HRTEM images of pristine STO (a, b) and 0.5% Ag-STO (c, d).
To confirm the state of Ag and other elements on the surface of SrTiO3, the chemical compositions and states of the as-prepared
catalysts were revealed by the results of XPS. Fig. S3 (Supporting information) gives the survey spectra of SrTiO3, 0.5% and 1% Ag-
STO catalysts. It is difficult to find the peaks of the Ag element in the full-spectra of Ag-STO due to the tiny content of Ag, which is in
good agreement with XRD results. However, the high-resolution XPS results clearly showed the existence of Ag element. Each Ag 3d
could be further fitted at the binding energy of 373.7/373.1 eV and 367.7/367.2 eV, respectively (Fig. 2a). The peaks at 373.7 and 367.7
eV are assigned to be Ag+ species, and 373.1 and 367.2 eV are assigned to be metallic Ag0 [45]. The atom ratio of Ag+ species to metallic
Ag0 on the surface of 0.5% Ag-STO nanocomposites is 51:49. However, when the molar ratio amount of Ag increased to 1%, the atom
ratio of Ag+ to Ag0 was changed to 11:89. Fig. 2b shows Sr 3d XPS spectra, the peaks are attributed to Sr2+ state [46]. Ti4+ state is also
assigned by 457.6/463.3eV peaks (Fig. 2c). Moreover, the high-resolution O 1s spectra reveal two peaks, the stronger peak at 528.8 eV
is fitted as O2– ions, while another peak is assigned to the surface adsorbed oxygen and hydroxyl groups such as Ti-OH species (Fig. 2d)
[47]. Compared to the pristine SrTiO3, the peaks of Sr 3d, Ti 2p, O 1s of Ag-STO catalysts all slightly shifted to higher binding energies,
this might be due to the strong interactions between Ag and SrTiO3, led to alternate charge transfer in the composite. The XPS result
suggested that Ag was successfully loaded onto SrTiO3 and the chemical state of Ag could be changed with the change of Ag content.
STO
Sr 3d
O 1s
Ti 2p (d) STO
(a)
(b)
Ag 3d
STO
(c) STO
528.8 eV
134.0 eV
457.6 eV
132.2 eV
463.3eV
530.2 eV
0.5% Ag-STO
373.7 eV
367.7 eV
368 eV
0.5% Ag-STO
134.2 eV
0.5% Ag-STO
463.6eV
0.5% Ag-STO
530.4 eV
367.2 eV
367.4 eV
529.1 eV
132.5 eV
132.5 eV
373.1 eV
457.9eV
1% Ag-STO
374 eV
1% Ag-STO
134.3 eV
1% Ag-STO
463.6eV
1% Ag-STO
530.6 eV
529.0 eV
373.3 eV
457.9 eV
459
136
134
132
130
376
374
372
370
368
366
364
534
532
530
528
526
465
462
456
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Fig. 2. (a) Ag 3d, (b) Sr 3d, (c) Ti 2p, and (d) O 1s XPS spectra of X% Ag-STO.
The photocatalytic performances of PSRM over the as-prepared catalysts were estimated under full-spectrum irradiation. The blank
experimental results are shown in Fig. S4 (Supporting information). No substance could be observed in the absence of light irradiation
or photocatalyst. The PSRM data were calibrated according to the blank test results. The photocatalytic performance of Ag-STO (0.5%)
exceeded all other catalysts with a maximum H2 evolution rate of 21.8 μmol g−1 h−1, and a CO production rate of 4.3 μmol g−1 h−1 which
is 4.7 times higher than that of SrTiO3 (Figs. 3a and b). Fig. S5 (Supporting information) shows the changing trend of CO2 formation.
The trend of CO2 formation is consistent with that of H2 and CO, whereas 0.5% Ag-STO sample showed the highest formation of CO2
with a mount of 10 μmol/g after 4 h irradiation. Besides, the generation of H2 and CO had distinctly different variation trend, the
photoirradiated side of catalyst turned brown. These phenomena could be attributed to the formation of medium products and/or carbon

deposition in a close system. Yoshida et al. proposed [CH2O]n,ad might be one of the possible expressions as a composition formula, and
the formation of [CH2O]n,ad can be depicted as follows [16].
n CH4 + n H2O → [CH2O]n,ad + 2n H2
(1)
With the reaction going in the close system, the inactive organic surface species were formed [16,25], the rate of H2 production
decreased while the active reaction intermediates could be persistently decomposed into water and/or carbonaceous compound such as
CO and CO2. In order to investigate the stability of Ag, the XPS spectrum of 0.5% Ag-STO after reaction was conducted (Fig. S6 in
Supporting information). The result confirms that Ag0 and Ag+ can exist stably in the present reaction process. In the presence of a gas
mixture of H2O and CH4, the H2 production was the highest, indicating that H2 can be derived from both CH4 and H2O (Fig. 3c). Fig. 3d
shows the result of l3C isotope experiment, it is clear that the oxidation products of CO (m/z = 29) and CO2 (m/z = 45) contain l3C.
Although mass spectrometry is a semi-quantitative analysis method, it can be seen from the signals of l3CO and l3CO2 that l3CO has bigger
relative amount, indicating that the 0.5% Ag-STO has a better selectivity for CO production. These results verified the photocatalytic
activity of SrTiO3 can be significantly improved through coupling with Ag.
120
90
60
30
0
20
15
10
5
100
75
50
25
0
(a)
STO
0.5% Ag-STO
1% Ag-STO
(b)
STO
0.5% Ag-STO
1% Ag-STO
(c)
13CH m/z = 17
(d)
2 h
4 h
6 h
4
H
m/z = 2
2
13CO m/z = 29
13CO2 m/z = 45
0
0
10
20
30
40
50
0
1
2
3
4
0
1
2
3
4
H2O
CH4
H2O+CH4
Time (h)
Time (h)
m/z
Fig. 3. (a, b) The photocatalytic activities of samples. (c) H2 generation in the presence of only CH4, only H2O and the mixture of CH4 and H2O and (d) 13CH4
isotope calibration results of 0.5% Ag-STO.
To understand the enhanced PSRM efficiency of Ag-STO, the following three characteristics of catalysts were analyzed: reactant gas
adsorption, solar light absorption and charge carriers transfer. The adsorption of reactants is the first step in a typical gas-solid-phase
photocatalytic process, which has an important influence on photocatalytic activity. It is well known that the adsorption performance is
closely related to the surface area and pore structure of catalysts. Fig. S7 (Supporting information) and Table S1 (Supporting information)
show the CH4 adsorption-desorption capacity, BET surface area and pore structure of X% Ag-STO, the obtained 0.5% Ag-STO possesses
the highest CH4 adsorption capacity and the changing trend of CH4 desorption curves over samples is consistent with that of CH4
adsorption. The BET surface area and pore structure results revealed that there is no significant influence of the BET and pore structure
on the CH4 adsorption-desorption capacity of the photocatalyst. Therefore, the addition of Ag elements should be responsible for the
enhancement of the CH4 adsorption-desorption.
Then the light absorption properties of X% Ag-STO were measured. As shown in Fig. S8 (Supporting information), an additional
absorption peak around 400-600 nm was observed for Ag-STO, attributed to the characteristic surface plasmon resonance (SPR) effect
of metallic Ag [41,42,48]. The deposition of Ag could improve light absorption, resulting in more efficiently photogenerated charge
carriers [49,50]. However, the increase of catalyst absorbance did not lead to enhanced PSRM activity. Therefore, the characteristics of
photogenerated charge carriers should be taken into consideration. As shown in Figs. 4a and b, the Ag-STO catalysts showed higher
photocurrents and smaller electrochemical impedance spectroscopy (EIS) curves than that of bare STO, However, when the Ag content
increased to 1.0 mol%, the photocurrent peak decreases and the diameter of EIS curves increases. The results indicated that Ag can
improve the charge carrier transfer, whereas the promoting effect becomes weaker at high loading ratio due to the fact that Ag also could
be the hole-electron recombination sites [50,51]. The PL intensity and SPV patterns of the catalysts are shown in Figs. 4c and d. 1% Ag-
STO and 0.5% Ag-STO possess a lower PL intensity and stronger photovoltage as compared to that of bare SrTiO3, which further
confirms that the coexistence of a suitable amount of Ag0 and Ag+ can synergistically promote charge separation and lead to higher
charge mobility [41,52,53].
10-7
(a)6
(b)1500
1200
(d) 800
STO
0.5% Ag-STO
1% Ag-STO
STO
0.5% Ag-STO
1% Ag-STO
STO
0.5% Ag-STO
1% Ag-STO
STO
0.5% Ag-STO
1% Ag-STO
(c)
600
900
600
300
0
4
2
0
400
200
0
100 200 300 400 500 600
Time (s)
0
100
200
Z' (KW)
300
400
500
300
320
340
360
380
400
350
400
450
500
550
wavelength (nm)
wavelength (nm)
Fig. 4. (a) Transient photocurrent response, (b) EIS, (c) PL and (d) SPV patterns of the prepared samples. (The photoelectrochemical measurements over the
obtained samples at open circuit potential were tested under UV-vis light irradiation, the 0.5 mol/L Na2SO3 solution as electrolyte).
To further determine whether Ag0 and/or Ag+ affects the activity of PSRM, a theoretical calculation was performed. Firstly, the CH4
adsorption capacity of the photocatalyst was studied and shown in Fig. S9 (Supporting information). The Eads of CH4 adsorbed on the
surface of pure SrTiO3 and Ag0-STO are −0.37 eV and −0.27 eV, respectively. Given the little difference of adsorption energy between
those two materials, it indicates that not Ag0 but Ag+ species might promote the adsorption of CH4. The HOMO orbitals of the Ag-STO

catalyst are mainly aggregate on Ag+ species rather than lattice O atoms of SrTiO3 (Fig. S10 in Supporting information), indicating that
the electron donors could be Ag+ species, so it is possible that the electrons transition happened around Ag+ after light irradiation. Thus,
Ag+ could play as the role to active CH4 molecules during the process of photocatalytic reactions. Furthermore, the reaction of H2O on
the catalyst surface is also described by DFT. Since the reduction reaction occurs on the surface of Ag0 particles, Ag atom is placed on
the computational model. The Eads of H2O on the surface are −0.21 eV and −1.82 eV for the SrTiO3 and Ag-STO, respectively (Figs. S11
in Supporting information). Therefore, the existence of Ag0 could promote the adsorption of H2O. Interestingly, the H atom of H2O
directly transfers to the O atom which is nearby the loading Ag0. Then, the O−H bond and dissociative hydroxyl group form. The hydroxyl
groups can react with photoinduced holes to generate •OH radicals which play a vital role to break the C−H bonds of CH4 [54,55]. To
confirm the formation of •OH, ESR spectra were performed on 0.5% Ag-STO and SrTiO3 catalysts as shown in Fig. S12 (Supporting
¯
information). The signals of •O2 are also measured due to its highly oxidative. According to the previous researches, •O2¯ can promote
peroxide CH4 and produce CO2 [56,57]. Besides, both the 0.5% Ag-STO and SrTiO3 catalysts were detected with the unique four-signal
peak of •OH. Even though there is a certain disparity between ESR results and the specific process of PSRM experiments and the •OH
signal intensity of samples are relatively similar, but the changing trend is identical. Therefore, the stronger signal peaks of the 0.5% Ag-
STO could indicate that the promoted photocatalytic activity might be related to •OH participation.
Based on these evidences, the photoreaction of Ag-STO is described as the following steps. Firstly, electron and hole pairs were
generated in conductive and valance bands after light irradiation, respectively (Eq. 2), then carriers migrated to the surface of the catalyst.
DFT calculations verified that the Ag+ on the surface of the catalyst was as the electron donor and the lattice Ti was as the receptor of
photogenerated electrons in the process of PSRM reaction. H2O molecules can be easily split to H+ and OH− by Ag nanoparticles (Eq.
3). After that, the OH− ions were transferred to Ag+ species and reacted with photogenerated holes to form •OH radicals (Eq. 4) and
dehydrogenate CH4 to generate the surface organic intermediates [CH2O]n,ad (Eq. 5). Then the reaction intermediates reacted with OH−
ions and photogenerated holes to produce CO and H2O (Eq. 6). The adsorbed CO could be further oxidized to form CO2 (Eq. 7). It is
generally believed that noble metal could accept photoexcited electrons and play a role in photocatalytic reduction reactions.
Consequently, the photoexcited accumulated electrons on the surface of SrTiO3 preferably transferred to Ag0. The protons can be reduced
by these electrons on the surface of Ag0 species to form molecular hydrogen (Eq. 8). The photocatalytic mechanism is proposed in
Scheme S1 (Supporting information).
Ag-STO + h → h+ + e−
H2O + h → [OH−]ad + [H+]ad
[OH−]ad + h+ → •OH
(2)
(3)
(4)
(5)
(6)
(7)
(8)
CH4 +4•OH → [CH2O]ad + 3 H2O
2[CH2O]ad + 4[OH−]ad + 4 h+ → 4H2O + CO↑ + [CO]ad
[CO]ad + 2[OH−]ad + 2 h+ → CO2 ↑ + H2O
2[H+]ad + 2e− → H2 ↑
In summary, PSRM was achieved over Ag-SrTiO3 photocatalysts. The highest activity was acquired over 0.5 mol% Ag-STO prepared
by the photodeposition method, where metallic Ag0 and Ag+ species coexisted on the surface of SrTiO3 with a suitable surface ratio. A
synergistic effect was achieved to improve the photocatalytic performance. The Ag0 can not only extend the light absorption range and
be regarded as the reduction reaction sites during the process of PSRM, but also can significantly promote the adsorption of H2O to
produce •OH radicals. While the Ag+ species can enhance the adsorption of CH4 and be regarded as the oxidation reaction sites. Those
two different valence states of Ag functioned together to promote the separation of electron-holes. The coexisted Ag0 and Ag+ effect with
a suitable ratio was found to improve the photocatalytic activity for SRM. The highest CO production rate of 4.3 μmol g−1 h−1 can be
obtained, which is approximately 5 times higher than that of pure SrTiO3. Finally, the specific and deeper investigation of the PSRM
reaction process should lead to the development of efficient photocatalysts.
Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered as potential
competing interests:
Acknowledgment
We gratefully acknowledge the financial support from the Sichuan Provincial International Cooperation Project (Nos. 2017HH0030,
2019YFH0164), the National Natural Science Foundation of China (No. 21403172). We appreciate the calculation support from the
National Supercomputing Center in Shenzhen.

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