Selective CO2reduction to HCOOH on a Pt/In2O3/g-C3N4multifunctional visible-photocatalyst

Article abstract of DOI:10.1039/d0ra03959d

Selective photocatalytic reduction of CO2 has been regarded as one of the most amazing ways for re-using CO2. However, its application is still limited by the low CO2 conversion efficiency. This work developed a novel Pt/In2O3/g-C3N4 multifunctional catalyst, which exhibited high activity and selectivity to HCOOH during photocatalytic CO2 reduction under visible light irradiation owing to the synergistic effect between photocatalyst, thermocatalyst, and heterojunctions. Both In2O3 and g-C3N4 acted as visible photocatalysts, in which porous g-C3N4 facilitated H2 production from water splitting while the In2O3 nanosheets embedded in g-C3N4 pores favored CO2 fixation and H adsorption onto the Lewis acid sites. Besides, the In2O3/g-C3N4 heterojunctions could efficiently inhibit the photoelectron-hole recombination, leading to enhanced quantum efficiency. The Pt could act as a co-catalyst in H2 production from photocatalytic water splitting and also accelerated electron transfer to inhibit electron-hole recombination and generated a plasma effect. More importantly, the Pt could activate H atoms and CO2 molecules toward the formation of HCOOH. At normal pressure and room temperature, the TON of HCOOH in CO2 conversion was 63.1 μmol g-1 h-1 and could reach up to 736.3 μmol g-1 h-1 at 40 atm. This journal is

Full text of DOI:10.1039/d0ra03959d

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PAPER
Selective CO reduction to HCOOH on a Pt/In O /
2
2 3
g-C N multifunctional visible-photocatalyst†
3
4
Cite this: RSC Adv., 2020, 10, 22460
Jiehong He, Pin Lv,
Jian Zhu* and Hexing Li
*
Selective photocatalytic reduction of CO
2
has been regarded as one of the most amazing ways for re-using
conversion efficiency. This work developed
multifunctional catalyst, which exhibited high activity and selectivity to HCOOH
during photocatalytic CO reduction under visible light irradiation owing to the synergistic effect
between photocatalyst, thermocatalyst, and heterojunctions. Both In and g-C acted as visible
photocatalysts, in which porous g-C facilitated H production from water splitting while the In
nanosheets embedded in g-C pores favored CO fixation and H adsorption onto the Lewis acid sites.
Besides, the In /g-C heterojunctions could efficiently inhibit the photoelectron–hole
recombination, leading to enhanced quantum efficiency. The Pt could act as a co-catalyst in H
CO
2
. However, its application is still limited by the low CO
2
a novel Pt/In
2 3 3 4
O /g-C N
2
2
O
3
3 4
N
3
N
4
2
2 3
O
3
N
4
2
2
O
3
3 4
N
2
Received 2nd May 2020
Accepted 2nd June 2020
production from photocatalytic water splitting and also accelerated electron transfer to inhibit electron–
hole recombination and generated a plasma effect. More importantly, the Pt could activate H atoms and
DOI: 10.1039/d0ra03959d
2
CO molecules toward the formation of HCOOH. At normal pressure and room temperature, the TON
À1 À1
À1 À1
rsc.li/rsc-advances
of HCOOH in CO
2
conversion was 63.1 mmol g
h
and could reach up to 736.3 mmol g
h at 40 atm.
14
(
3.2 eV) and thus cannot sufficiently make use of sunlight. In
addition, the radical reaction steps disfavor the selectivity
CO conversion has attracted increasing interest owing to the toward the target product. In can be activated by visible light
consideration to both reduce the green-house effect and recycle and also can easily adsorb/activate CO molecules via Lewis acid
carbon resources. The key problem is the CO2 xation and sites, which greatly benets the photocatalytic CO reduction
exhibits poor activity to
reduction was mainly generate H atoms through photocatalytic water splitting. The g-
driven by metal thermocatalysts, but it was usually performed
is a typical visible photocatalyst for efficient water split-
at high temperature due to the highly stable CO
molecule. ting, but could effectively adsorb and activate CO
Electrocatalytic CO
reduction has also been reported, but In addition, both In and g-C exhibit low activities due to
additional energy is needed and the special reactor limits large the rapid combination between photoelectrons and holes.
1
. Introduction
2 3
O
2
2
2
15,16
activation steps, which play a critical role in determining the toward HCOOH.
But, pure In O
2 3
1,2
activity and selectivity. Previously, CO
2
3–6
C N
3 4
17
2
molecules.
2
2
O
3
3 4
N
2
18
5,6
scale-production. Photocatalytic CO2 reduction represents Construction of heterojunction structures is frequently used in
a promising way for CO conversion due to it being an energy designing photocatalysts with the low photoelectron–hole
2
saving and green process, which uses mild conditions, and has recombination rate and the high photocatalytic activity in
19,20
7
–10
long durability.
very low because of the poor CO
and the easy photoelectron–hole recombination.
such a reaction usually displays low selectivity with the forma- In
However, the photocatalytic efficiency is still pollutant-degradation, water splitting, and CO
reduction.
2
adsorption/activation ability Many efforts have been devoted to improving the photocatalytic
2
11,12
21–24
Moreover, CO
2
reduction efficiency.
and g-C could mainly overcome their individual
reduction. More impor-
displays energy level matching well with the g-
/g-C heterojunctions can be easily estab-
availability, high activity, and excellent stability. However, lished, which can promote the photoelectron–hole separation
Obviously, the combination of
2
O
3
3 4
N
tion of various products like methane, methanol, formalde- shortcomings in photocatalytic CO
hyde, formic acid, etc. TiO has been frequently employed as in tantly, the In
2
2 3
O
2
photocatalysis owing to its environmental friendship, easy
C
3
N
4
. Thus, In
2
O
3
3 4
N
13
TiO
2
can only be activated by UV lights due to its large bandgap to diminish photoelectron–hole recombination, leading to the
high quantum efficiency in photocatalysis, which has applied
into H
Cao et al. reported an In
evolution and CO
2 3
heterojunctions between g-C and In O
2
evolution, CO
2
reduction and pollutants degradation.
/g-C with enhanced activity in
reduction owing to the
, which promoted the
Education Ministry Key and International Joint Lab of Resource Chemistry, Shanghai
Key Laboratory of Rare Earth Functional Materials, Department of Chemistry,
2
O
3
3 4
N
Shanghai Normal University, Shanghai 200234, China. E-mail: hexing-li@shnu.edu. photocatalytic H
cn; jianzhu@shnu.edu.cn; Fax: +86 21 64322272; Tel: +86 21 64322272
2
2
3
N
4
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra03959d
transfer of photo-generated electrons and thus decreased the
1
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25
photoelectron–hole recombination. Non-noble metal like B 10 mL ultra-pure water containing the desired amount of
and S has also been induced in In O /g-C N to promote pho- chloroplatinic acid and 1 mL TEOA (Aladdin). Then, the solu-
2
3
3 4
26,27
tocatalytic efficiency.
Besides, noble metals like Pt and Pd tion was irradiated with LED lamps for 30 min. Aer separation,
have been widely applied in catalytic CO reduction with high the solid product was washed thoroughly with H O and ethyl
2
2
2
8,29
ꢀ
activity and selectivity.
as co-catalysts in photocatalysis to facilitate electron transfer sample was marked as yPt–xINCN, where y is theoretical mass
Meanwhile, they could also be used alcohol, followed by drying at 60 C for 8 h. The as-received
30,31
owing to the excellent electric conductivity
reactant molecules owing to the low work function.
and to activate ratio of Pt to xINCN.
32
2 3 3 4
Herein, we reported for the rst time a Pt/In O /g-C N
2.2 Characterization
nanocomposite and used as a multifunctional catalyst in pho-
The real contents of different components in composites were
determined by using Inductive Coupled Plasma Emission
tocatalytic CO reduction under visible light irradiation. The
2
In O /g-C N was prepared via a solvothermal route, while Pt
2
3
3 4
Spectrometer (ICP, a Jarrel-Asm Atom Scan 2000). The N
2
was loaded through a photo-reduced method. The unique
structural assemble and the strong interaction between nano-
adsorption–desorption isotherms were obtained at 77 K using
a Micromeritics ASAP 2010 instrument in which the specic
particle Pt, nanosheet In
2 3 4
O3, and porous g-C N . As a result, the
surface area (SBET) and pore volume (V ) were calculated by
P
Pt/In /g-C exhibited high activity and selectivity to
2
O
3
3 4
N
applying Brunauer–Emmett–Teller (BET) and Barrett–Joyner–
Halenda (BJH) models. X-ray diffraction (XRD) patterns were
recorded on a Bruker D8 Advance X-ray diffractometer with Cu-
2
HCOOH owing to the synergetic effects. The CO conversion
À1 À1
TON reached up to 736.3 mmol g
h
at 40 atm and the
catalyst exhibited excellent durability, showing good potential
in the practical application.
ꢀ
À1
Ka radiation at a scan rate of 0.02 s . The morphologies were
measured on a eld emission scanning electronic micrograph
(
FESEM, JEOL JSM-6380LV) and transmission electronic
2
. Experimental
micrograph (TEM, JEM-2010). Energy-dispersive X-ray spec-
troscopy (EDS) was also performed by the JSM. The surface
electronic states were investigated by X-ray photoelectron
spectroscopy (XPS; ULVAC-PHI PHI5000 VersaProbe using Al Ka
radiation). UV-Vis diffuse reection spectra (DRS) were obtained
2
.1 Sample preparation
In was prepared through a hydrothermal method. In
a typical run of synthesis, 6 mmol In(NO (Aladdin) was added
2 3
O
3 3
)
into a 40 mL aqueous solution, followed by stirring for 10 min to
get a clear solution. Then, 24 mmol urea (Aladdin) was added
and the solution was continuously stirred for 2 h at room
temperature. Subsequently, the solution was transferred into
a 50 mL Teon lined autoclave and was kept at 140 C for 16 h.
The obtained solid product was washed thoroughly by water
and ethanol, followed by drying at 60 C for 12 h. Finally, it was
calcined at 450 C for 2 h to improve the crystallization degree of
In
4
on a Shimadzu UV-2450 Spectrophotometer using BaSO as
a reference. The steady-state photoluminescence (PL) spectra
were obtained by a Varian Cary-Eclipse 500 uorometer at an
excitation wavelength of 350 nm. Photocurrents were measured
using an electrochemical analyzer (CHI660D Instruments)
ꢀ
containing a standard three-electrode system in 0.5 M Na SO4
ꢀ
2
aqueous solution as the electrolyte. The as-prepared sample was
ꢀ
2
used as a working electrode with an active area of 20 Â 20 mm .
2
O
3
and remove impurities.
The sample was prepared by traditional technology. First, 2 mg
PVDF was added into 2 mL DMF solution under ultrasonic
condition. Then, 20 mg catalyst was added. The suspension was
obtained and treated by ultrasound for another 5 min, followed
g-C was prepared by calcination of melamine (Aladdin) in
3 4
N
ꢀ
ꢀ
a special reactor at 500 C for 2 h, followed by calcining at 520 C
for another 2 h until the formation of faint yellow powder.
For preparing In O /g-C N , a certain amount of g-C N was
dispersed into 20 mL aqueous solution containing a certain
amount of urea and was stirred for 2 h at room temperature.
Meanwhile, a certain amount of indium nitrate was added to
another 20 mL aqueous solution and was stirred until trans-
parent. Then, the solution containing In(NO
added into the solution containing g-C and urea. Aer being
stirred for 2 h, the mixed solution was transferred into a 50 mL
Teon lined autoclave and kept at 140 C for 16 h. The obtained
2
3
3
4
3
4
2
by dropping onto a FTO piece (20 Â 20 mm ), vacuum drying at
ꢀ ꢀ
0 C for 8 h, and calcination at 150 C for 1 h. A Pt piece with an
8
2
area of 20 Â 20 mm was used as a counter electrode and
a saturated calomel electrode was used as a reference electrode.
Each run of the measurements was performed by irradiating
with a 300 W Xe lamp for 30 s at an interval of 30 s. Electro-
chemical impedance spectra (EIS) were obtained under the
open-circuit potential condition, with an amplitude of 10 mV
and frequency span from 10 kHz to 0.01 Hz.
3 3
) was carefully
3 4
N
ꢀ
solid product was washed by water and ethanol, followed by
drying at 60 C for 12 h, and nally, calcining at 450 C for 2 h in
argon atmosphere. The as-received sample was donated as
ꢀ
ꢀ
2
.3 Activity test
xINCN, where x refers to the theoretical mass ratio of In to g- The photocatalytic CO
2
O
3
2
reduction was performed in a self-made
C
3
N
4
, which is adjusted by the added amount of g-C N in 50 mL quartz reactor. Before reaction, 20 mg catalyst was
3 4
mother solution. For comparison, the mechanical mixture of dispersed in 10 mL ultra-pure water under ultrasonication for
In O and g-C N was also prepared and was designated as 30 min. Then, 1 mL TEOA (Aladdin) was introduced as a sacri-
2
3
3 4
INCN-mix.
The Pt-deposited photocatalysts were prepared by photore- quartz reactor with two vents. High purity CO
duction of Pt ions. Briey, 100 mg xINCN was dispersed in introduced into the reactor and bubbled for 0.5 h to evacuate
cial agent. The suspension was transferred onto a 50 mL
2
(99.99%) was
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the air inside the reactor. Then, the reactor was sealed and lled
with CO to 1 atm. For each run of reactions, the reaction system
2
was irradiated for 4 h by four 3 W LED lights with the wave-
ꢀ
length of 420 nm at a constant temperature of 35 C and stirring
rate of 800 rpm. The gas products were analyzed by a gas
chromatography equipped by HP-PLOT Q column connected
with the FID detector and TDX-01 connected with TCD detector.
The liquid products were detected by ion chromatography
(Dionex DX-320) with an analytical column (Dionex IonPac
AS19-4 mm Analytical Column).
3. Results and discussion
3
.1 Structural characteristics
As shown in Fig. 1, the XRD patterns revealed that all the
samples were present in well crystalline states of In
. The pure g-C displayed two peaks around 13.1 and
7.4 characteristic of (100) and (002) facets. The In was
present in cubic structure (JCPDS card no. 06-0416) with the
Fig. 2 SEM images of (a) In
O and g- images of (d) g-C N , (e) In O , (f) 10INCN, and (g) 2Pt–10INCN. (h)
2 3
3 4 2 3
2
O
3
, (b) g-C
3
N
4
and (c) 10INCN. TEM
ꢀ
HRTEM image of 2Pt–10INCN. (i) Mapping images of 2Pt–10INCN.
C
2
3
N
4
ꢀ
3 4
N
2 3
O
ꢀ
ꢀ
ꢀ
ꢀ
characteristic peaks at 30 , 35 , 51 , and 60 , corresponding and g-C
3
N
4
were present in well-dened crystals, in good
(
111), (100), (110), (622) facets, respectively. All the Pt/In O /g- accordance with the XRD patterns. The heterojunction was
2
3
C
C
3
N
4
showed typical peaks indicative of individual In
, indicating no chemical bonding formed between In
. No signicant diffraction peaks indicative of Pt with the uniform distribution of Pt nanoparticles.
were observed in Pt/In , Pt/g-C , or Pt–INCN owing to the
The actual In -contents and Pt-loading in the composite
low Pt-loading with high dispersion of Pt nanoparticles.
photocatalysts were determined by ICP and expressed in mass
2
O
3
and g- formed between In
2
O
3
and g-C
3
N
4
. The mapping images clari-
N in 2Pt–10INCN, together
3 4
N
4
2
O
3
ed the existence of In
2
O
3
and g-C
3
3 4
and g-C N
2
O
3
3
N
4
O
2 3
The SEM and TEM images in Fig. 2 demonstrated that the percentage (Table S1†), which generally matched with the
In O was present in nanosheets with an average length of 1–2 theoretical contents in the mother solution. All samples dis-
2
3
mm and widths of 50–100 nm, while the g-C N was present in played type-IV
N
2
adsorption–desorption isotherms with
a lamina structure with uniform porous structure. The 10INCN a hysteresis loop of type H3 characteristic of porous structure
composite displayed well dispersion of In
nanosheets onto (Fig. S1†). Based on the N adsorption–desorption isotherms,
the g-C
. From 2Pt–10INCN, we could see very tiny Pt nano- the surface area, pore size, and pore volume were calculated
particles uniformly distributed onto the In /g-C
support. (Table S2†). Although the similar pore volume, the In
Interestingly, most of Pt nanoparticles were deposited onto the exhibited higher surface area and bigger pore size than the g-
In O nanosheets. This could be easily understood since Pt ions
owing to the porous structure constructed layer-by-layer
from In nanosheets. The INCN exhibited decreased surface
owing to the presence of Lewis acid sites which favored the area and pore size due to the incorporation of In nanosheets
, leading to the close package of In
conrmed that the Pt nanoparticles were embedded on the nanosheets. The pore volume of INCN greatly increased in
interface between In and g-C in 2Pt–10INCN. Both In
comparison with both In and g-C , possibly owing to the
formation of new micropores during the co-assembling process
of In and g-C . All the Pt–INCN displayed lower surface
3
4
2
O
3
2
3 4
N
2
O
3
3
N
4
O
2 3
3 4
C N
2
3
would be more easily reduced on the In O than on the g-C N
3 4
2 3
O
2
3
2 3
O
2
À
adsorption of PtCl
6
ions. The HRTEM image further into the porous g-C
3
N
4
2 3
O
2
O
3
3
N
4
2
O
3
2
O
3
N
3 4
2
O
3
3 4
N
area and larger pore size as well as smaller pore volume, sug-
gesting most Pt nanoparticles were embedded into the pore
channels constructed layer-by-layer from In O nanosheets.
2 3
Some micropores were blocked by those Pt nanoparticles, which
could account for the increase of average pore size and abrupt
decrease of pore volume.
The XPS spectra in Fig. 3 revealed that all the In species were
3
+
present in In state, corresponding to the binding energies (BE)
33
of 445.8 eV and 453.4 eV at In 3d5/2 and 3d3/2 levels. The
negative BE shi suggested the strong interaction between
In
C N
3 4
2
O
3
and g-C
3
N
4
, in which partial electrons transferred from g-
to In due to the d–p feedback. Meanwhile, all the Pt
species on In O , g-C N , and xINCN exhibited two principal
3
+
2
3
3 4
peaks around BE of 71.4 eV and 74.5 eV in Pt 4f7/2 and Pt 4f5/2
Fig. 1 XRD patterns of pure In
2 3 3 4
O , pure g-C N and their corre-
sponding composites with or without Pt-loading.
energy levels (also see Fig. S2†), indicating they were all present
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Fig. 3 XPS spectra in (a) C 1s, (b) N 1s, (c) O 1s and (d) In 3d and (e) Pt 4f levels.
3
4
in the metallic state. The Pt/In
2
O
3
displayed negative BE shi plot was used to estimate band-gap energy of them. From UV-
3
+
1/2
of Pt and positive BE shi of In , which further conrmed that Vis DRS spectra and their corresponding plots of (ahn) vs.
most Pt nanoparticles deposited onto In O nanosheets, in photon energy (hn) (Fig. 4a and S3†), together with the calcu-
which partial electrons transferred from In to Pt owing to the lated energy bandgaps and light absorbance edge wavelength
strong metal–support interaction, taking into account of the (Table S3†), we could see that both In and g-C could be
higher electronegativity of Pt (2.28) than that of In (1.78). All activated by visible lights. The 5INCN displayed a broader
the O species were present in O state. The peak at 531.4 eV adsorption area with an obvious red-shi in comparison with
was assigned to lattice-oxygen and the peak at 532.5 eV was the pure In and g-C , which could be understood by
ascribed to chemisorbed oxygen. Aer Pt-loading, the BE of considering the appearance of some intermediate energy bands
lattice-oxygen showed a negative shi owing to the acceptance owing to the formation of heterojunctions between In and g-
of partial electrons from metallic Pt. Besides, the well-dispersed C N . Further increase in the ratio of In O to g-C N4 (from
2
3
3
+
2
O
3
3 4
N
3
+
32
2
À
2
O
3
3 4
N
35
2 3
O
3
4
2
3
3
Pt nanoparticles on the In O surface promoted the chemi- 5INCN to 20INCN) had no signicant inuence on the light
2
3
sorption of oxygen, leading to the enhanced intensity of the absorbance edge, suggesting no more heterojunctions formed.
3
6
peak around 532.5 eV. The C species in 10INCN were mainly Meanwhile, both Pt/In
presented in C–N–C coordination, corresponding to the BE at comparing to pure In
88.2 eV in C 1s level. The weak shoulder peak at 284.5 eV was plasma effect from Pt nanoparticles, which could also account
2
O
3
and Pt/g-C
3
N
4
exhibited red-shi
2
O
3
and g-C
3 4
N , obviously owing to
2
2
37
ascribed to the sp C–C bond. The N species displayed various for the red-shi of light absorbance by loading Pt onto INCN.
states, corresponding to the different binding energies in the N However, the change of Pt-loading from 1 wt% to 2 wt% had
1s level. The main signal around 398.6 eV corresponded to the N a very limited effect on the light adsorption edge, suggesting the
in C–N–C bond, while the shoulder peak at 400.3 eV was similar size of Pt nanoparticles, and thus, the increased Pt-
3
38
ascribed to the N in the N–(C) group. No signicant BE shi of loading could only enhance the light adsorption intensity.
either the C or the N was observed, possibly due to the However, 2.5Pt–10INCN displayed a blue-shi of light absor-
symmetric structure in C–N–C bond.
Due to the In and g-C are both indirect band semi- which reduced the plasma effect. All the increasing ratio of
It is reasonable that (ahn) vs. photon energy In
bance wavelength due to the gathering of Pt nanoparticles,
2
O
3
3 4
N
39–42
0.5
conductors.
2
O
3
to g-C
3
N
4
had almost on effect on the light absorbance
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metallic Pt. The 2Pt–10INCN displayed the smallest arc radius,
corresponding to the lowest electric resistance. Further increase
in the Pt-loading caused an enlarged arc radius corresponding
to the enhanced electric resistance, which could attribute to the
gathering of Pt nanoparticles.
The photocurrent response spectra (Fig. 4d and S6†) clearly
demonstrated that the INCN exhibited higher photocurrent
2 3 3 4
than either the pure In O or the pure g-C N , which could
attribute to the enhanced light absorbance and decreased
photoelectron–hole recombination rate as discussed above.
Increasing the In O /g-C N ratio from 5 to 10 wt% resulted in
2
3
3 4
the enhanced photocurrent intensity. A possible reason was the
rapid electron transfer from g-C to In since In was
present in well-dened crystals with better electric conductivity
than g-C in poor crystallization, which might decrease the
photoelectron–hole recombination. Further increasing the
In /g-C ratio to 15 wt% caused a decrease in photocurrent
intensity due to the presence of more free In O nanosheets.
3
N
4
2
O
3
2 3
O
3 4
N
2
O
3
3 4
N
2
3
Those free In O displayed higher photoelectron–hole recom-
2
3
bination rate than the In O /g-C N nanocomposite due to the
2
3
3 4
43
absence of heterojunctions (Fig. S4a†). Meanwhile, the Pt–
INCN showed a higher photocurrent than the INCN owing to
the enhanced light absorbance and decreased photoelectron–
hole recombination rate as discussed above. The highest
photocurrent was observed on the 2Pt–10INCN, corresponding
to the highest light absorbance and the lowest photoelectron–
hole recombination rate.
1
/2
Fig. 4 (a) UV-Vis DRS spectra and their corresponding plots of (ahn)
vs. photon energy, (b) steady-state PL spectra at lex of 350 nm, (c)
electrochemical impedances, and (d) photocurrent response spectra
obtained under irradiation of a 300 W Xe lamp.
wavelength in INCN, it caused remarkable red-shi in 2Pt–
xINCN. This could be ascribed to the enhanced plasma effect 3.2 Catalytic performances
from Pt nanoparticles at high-content In since the above
TEM images demonstrated that the Pt ions were preferentially
reduced and deposited onto In rather than g-C
2 3
O
Controlled experiments (Fig. S7†) demonstrated that only trace
HCOOH and CH obtained during CO reduction on 2Pt–
0INCN without light irradiation or CO , suggesting the CO
reduction was mainly driven by photocatalysis. The addition of
Cr as an electron capture agent resulted in an abrupt
decrease in CO conversion efficiency, showing the photoelec-
trons played a key role in photocatalytic CO reduction. Mean-
4
2
2
O
3
3 4
N .
1
2
2
The steady-state PL spectra obtained at an excited wave-
length of 350 nm (Fig. 4b and S4†) displayed an emission peak
around 450 nm indicative of the recombination between
photoelectrons and holes. The 10INCN exhibited lower intensity
K
2
2 7
O
2
2
compared to In
tron–hole combination rate, which could be attributed to the
rapid charge transfer through the In –C heterojunctions.
With the increase of the In /C ratio up to 10INCN, the PL
2 3 3 4
O and g-C N , implying the lowest photoelec-
while, it was also found that the HCOOH yield greatly decreased
without adding TEOA in the reaction system. TEOA was
frequently used as a hole capturing agent in photocatalysis,
which could consume photo-induced holes from the photo-
catalyst through its oxidation (see Scheme S1†) and thus could
inhibit their recombination with photoelectrons. Therefore,
TEOA was a necessary sacrice agent in the present photo-
2
O
3
3 4
N
44
2
O
3
3 4
N
peak intensity decreased gradually owing to the formation of
more In O –C N heterojunctions, corresponding to the
2
3
3 4
reduced photoelectron–hole combination rate. However,
further increase in the In O /C N ratio resulted in the
2
3
3 4
2
catalytic CO reduction.
increasing PL peak intensity due to the presentence of more free
In nanosheets, leading to the enhanced photoelectron–hole
Under UV light irradiation, the 2Pt–10INCN exhibited higher
2 3
O
HCOOH and CH yields than that under visible light irradiation
4
combination rate. Loading Pt could greatly decrease the PL peak
intensity, corresponding to the efficient inhibition of photo-
electron–hole combination, obviously owing to the rapid elec-
tron transfer through Pt nanoparticles to separate from holes.
The 2Pt–10INCN showed the lowest photoelectron–hole
combination rate. This could be further by the electrochemical
impedance spectra (Fig. 4c and S5†). The loading of Pt nano-
particles onto In O , C N , and INCN resulted in a signicant
2 3 3 4
decrease of arc radius, corresponding to the reduced electric
resistance owing to the excellent electric conductivity of
(
Fig. S8a†) since the UV light could generate more photoelec-
trons and holes. Taking into account that the sunlight con-
tained only less than 5% UV lights, we still employed the visible
light irradiation for driving the photocatalysis to simulate the
natural photosynthesis. The 2Pt–10INCN exhibited extremely
high selectivity to HCOOH in photocatalytic CO . Only very little
2
CH could be obtained and no other side-products were detec-
ted. With the increase of reaction time to 4 h, the HCOOH yield
4
increased rapidly and an only slight increase in CH yield was
observed. However, further increase in the reaction time had no
4
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signicant inuence on the CO
2
reduction efficiency (Fig. S8b†), photocatalytic water splitting owing to its low CB potential
indicating the reaction arrived equilibrium point. So, 4 h was (À1.1 V vs. NHE). However, its adsorption for CO molecules
2
used as the reaction time in the following activity tests. Mean- was very poor due to the lack of active sites together with the low
while, the increase of CO pressure could also greatly promote surface area, leading to the poor activity of photocatalytic CO2
2
photocatalytic CO
2
reduction toward HCOOH (see Fig. S8c†). reduction. The INCN exhibited higher activity than either the
À1 À1
The HCOOH yield could reach up to 736.3 mmol g
.0 MPa, which represented a high level in CO
photosynthesis. However, the increasing CO pressure resulted while, the INCN contained narrow energy bandgap owing to the
in a rapid increase of the CH yield (see Fig. S8d†), the high cost, presence of intermediate energy bands and thus could be more
h
at In
2
O
3
or g-C
3
N
4
owing to the cooperation between In
2 3
O and g-
4
2
conversion in
C
3
N
4
to adsorb CO
2
molecules and produce H atoms. Mean-
2
4
and the difficult operation. Thus, we employed the normal CO2 easily and effectively activated by visible lights. More impor-
pressure (1 atm) in the following tests to meet the requirement tantly, the INCN contained enriched heterojunctions between
of industrial production.
In O and g-C N , which facilitated the electron transfer from
2 3 3 4
Fig. 5 demonstrated that both In
2
O
3
and g-C
contained suffi- electron–hole recombination rate. Although the pure In
molecules, it showed very exhibited very poor activity in photocatalytic CO reduction, the
low activity due to its high CB potential (À0.62 V vs. NHE) which Pt/In exhibited remarkable enhancement on the activity
could not efficiently generate H atoms by photocatalytic water since the metallic Pt could act as a co-catalyst to produce H
3 4 3 4 2 3
N could be the CB of g-C N to that of In O , leading to the lower photo-
activated by visible lights. Although the In
cient Lewis acid sites to absorb CO
2
O
3
2 3
O
2
2
2 3
O
splitting. The g-C N was active to produce
H
atoms atoms by collecting electrons from In O semiconductor and
2 3
3
4
45
plasma during photocatalytic water splitting. More impor-
tantly, the Pt/In O displayed much higher selectivity to
2
3
HCOOH than the pure In
on a plausible reaction pathway in photocatalytic CO
Scheme S1†). The noble-metal Pt was a typical metal catalyst
with low work function to activate H atoms while the In
could efficiently adsorb CO molecules owing to the presence of
2
O
3
, which could be understood based
2
reduction
(
2 3
O
2
46,47
enriched Lewis acid sites.
The Lewis acid sites also enhanced
the interaction between Pt and In O , which induced the unique
2
3
bridged co-adsorption model of the CO molecule on Pt and
2
In
with active H atoms to HCOO*,
diate in determining the selectivity to HCOOH. Besides, the
metallic Pt nanoparticles also promoted the activation of In
2 3 2
O . Such co-adsorbed CO molecules favored the reaction
48–50
which was a key interme-
16
2 3
O
by visible lights owing to the plasma effect and facilitated the
electron transfer to diminish photoelectron–hole recombina-
tion, which could further promote the photocatalytic CO2
reduction. Similarly, the Pt/g-C N also exhibited higher activity
3
4
3 4
and selectivity than the g-C N , which could also be explained
based on the above discussions. It was noted that the Pt-loading
exhibited much weaker promoting effect on the HCOOH
production in g-C
3
N
4
than that in In
2
O
3
. A possible reason was
molecules due
that the g-C could not effectively adsorb CO
3
N
4
2
to the absence of Lewis acid sites. Although the presence of Pt
could greatly enhance the production of H atoms, no enough
adsorbed CO molecules could be activated and subsequently
2
reacted with those H atoms. Meanwhile, the Pt/g-C
poor interaction between Pt and g-C due to the absence of
Lewis acid sites, leading to the low number of bridge-co-
adsorbed CO molecules on both Pt and g-C , which could
3 4
N displayed
3 4
N
2
3 4
N
account for the lower selectivity to HCOOH in comparison with
the Pt/In O .
2
3
As expected, the Pt-loading onto INCN also greatly enhanced
the activity and selectivity in photocatalytic CO2 reduction,
which could also be accounted by considering various Pt-
functions discussed above. Interestingly, the Pt–INCN exhibi-
ted a much stronger promoting effect than either the Pt/In
2 3
O or
Fig.
5
HCOOH and CH
4
yields on different catalysts. Reaction
Pt/g-C possibly owing to the enhanced role of hetero-
3 4
N
conditions: Four 3 W LED (420 nm), 1 atm CO
2
, 20 mg catalyst, 10 mL
ꢀ
2
H O, 1 mL TEOA, 35 C, 4 h.
junctions. The presence of metallic Pt nanoparticles facilitated
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By xing the In
2
O
3
-content at 10 wt%, the Pt–10INCN
exhibited increasing HCOOH yield and CH4 yield with the
increase of Pt-loading up to 2 wt% since more Pt nanoparticles
would enhance the plasma effect, the activity for activating H
atoms and CO molecules, the electron transfer ability to inhibit
2
photoelectron–hole recombination. However, further increase
in the Pt-loading caused slight decrease in either the HCOOH
4
yield or the CH yield due to the gathering of Pt nanoparticles to
reduce above promoting effects. Besides, the large Pt nano-
particles may also block the pore channels, leading to an abrupt
decrease in surface area (see Table S2†). The 2Pt–10INCN
À1 À1
exhibited the highest HCOOH yield (63.1 mmol g
normal pressure of CO
h ) at the
2
and room temperature. In addition, the
Pt–10INCN exhibited much higher activity than the 2Pt–
0INCN-mix obtained by mechanical mixing In and g-C
2
1
2
O
3
3 4
N
(see Fig. S7†), obviously due to the absence of heterojunctions
and the synergetic effect between In and g-C
2
O
3
3 4
N .
The lifetime is also a key parameter to determine the prac-
tical application of a photocatalyst. As shown in Fig. 6, both 2Pt–
10INCN and 2Pt–In O could be used repetitively for more than
2 3
5
times without a signicant decrease in their activities. The
slight decrease might be caused by the loss of photocatalysts
3 4
during separation and wash processes. However, the 2Pt–g-C N
showed a rapid decrease in photocatalytic activity during the
recycling test. The XRD patterns (Fig. S9†) revealed that, similar
to 2Pt–In O and 2Pt–g-C N , no signicant change in the
2
3
3 4
structure of 2Pt–10INCN was observed, indicating the excellent
stability during the reaction process. Meanwhile, ICP analysis
demonstrated no signicant Pt leaching occurred on either 2Pt–
1
0INCN or 2Pt–In
be attributed to the strong adsorption of Pt nanoparticles onto
In owing to the presence of Lewis acid sites. However, the
Pt–g-C showed Pt leaching during the reaction process,
leading to a decrease in photocatalytic activity.
2 3
O occurred during the reaction, which could
2 3
O
2
3 4
N
2 3 3 4
Fig. 6 Recycling test of 2Pt–In O , 2Pt–g-C N and 2Pt–10INCN.
Reaction conditions are given in Fig. 5.
4. Conclusions
electron-transfer from In O to g-C N and hole-transfer from g-
2
3
3 4
C N to In O , which could greatly diminish photoelectron–hole We developed a Pt/In O /g-C N multifunctional catalyst by
3
4
2
3
2
3
3 4
recombination (see Scheme S1†). By xing the Pt-loading at controlling the assembling method. This catalyst exhibited high
wt%, the Pt–INCN showed a rapid increase in HCOOH yield activity and selectivity to HCOOH during visible-light-driven
while only very slight increase in CH yield with the increasing photocatalytic CO reduction owing to the synergetic effects in
ratio of In /g-C up to 10 wt%. This could be ascribed to the producing H atoms via water splitting, adsorbing/activating
enhanced adsorption of CO molecules by In . Besides, the CO molecules, and inhibiting photoelectron–hole recombina-
increase of heterojunctions played a key role in promoting the tion by accelerating electron transfer and forming hetero-
photocatalytic CO reduction by inhibiting photoelectron–hole
2
4
2
2
O
3
3 4
N
2
2 3
O
2
2
junctions. Besides, it also displayed a long lifetime owing to the
recombination. However, further increase of the In O -content high structural stability and the strong interaction to inhibit Pt
2
3
caused slight decrease in the HCOOH yield due to the appear- leaching. This work supplied a promising way for designing new
ance of free In nanosheets. Such a free In displayed catalysts with multiple functions for photocatalytic CO reduc-
higher photoelectron–hole recombination rate than the In
g-C nanocomposite due to the absence of heterojunctions.
In addition, since the In favored the photocatalytic reduc-
, some of Pt nanoparticles would deposit onto the
free In O nanosheets, leading to the reduced promoting effect
2
O
3
2 3
O
2
2
O
3
/
tion at normal pressure and room temperature with a high yield
of target products.
51
3 4
N
2 3
O
2
+
tion of PtCl
6
2
3
on the role of heterojunctions between In O and g-C N .
Conflicts of interest
2
3
3 4
There are no conicts to declare.
22466 | RSC Adv., 2020, 10, 22460–22467
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2
4 H. Guo, M. Chen, Q. Zhong, Y. Wang, W. Ma and J. Ding, J.
Acknowledgements
CO Util., 2019, 33, 233–241.
2
This work was supported by NSFC (21761142011, 51572174) and 25 S. Cao, X. Liu, Y. Yuan, Z. Zhang, Y. Liao, J. Fang, S. C. J. Loo,
Shanghai
Government
(18JC1412900,
19YF1436600,
T. C. Sum and C. Xue, Appl. Catal., B, 2014, 147, 940–946.
1
9160712900, 17SG44) as well as Shanghai Engineering 26 X. Jin, Q. Guan, T. Tian, H. Li, Y. Han, F. Hao, Y. Cui, W. Li,
Research Center of Green Energy Chemical Engineering.
Y. Zhu and Y. Zhang, Appl. Surf. Sci., 2020, 504, 144241.
7 P. Zhou, X. Meng and T. Sun, Mater. Lett., 2020, 261, 127159.
8 K. Liu, M. Ma, L. Wu, M. Valenti, D. Cardenasmorcoso,
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29 Z. Yin, G. T. R. Palmore and S. Sun, Trends Chem., 2019, 1,
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30 Z. Xiong, Z. Lei, C.-C. Kuang, X. Chen, B. Gong, Y. Zhao,
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