Surface chemistry imposes selective reduction of CO2 to CO over Ta3N5/LaTiO2N photocatalyst

Article abstract of DOI:10.1039/c8ta05339a

Understanding the mechanism underlying photocatalytic product selectivity will provide valuable guidance in developing novel catalysts and reaction pathways. In this article, the role of surface chemistry in product generation is well-demonstrated using L

Full text of DOI:10.1039/c8ta05339a

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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Surface chemistry imposing selective reduction of CO2 to CO over
Ta3N5/LaTiO2N photocatalyst†
Received 00th January 20xx,
Accepted 00th January 20xx
Lei Lu,‡a Shaomang Wang,‡b,c Chenguang Zhou,‡a Zhan Shi,b Heng Zhu,a Zhenyu Xin,a Xiaohui
Wang,a Shicheng Yan,*a and Zhigang Zoua,b
DOI: 10.1039/x0xx00000x
Revealing the mechanism of photocatalytic product selectivity will provide valuable guidance on developing novel catalysts
and reaction pathways. Here, the roles of surface chemistry on product generation are well demonstrated over LaTiO2N,
Ta3N5, and their hybrids, respectivelywith dominant (002) and (020) facet. The photocatalytic test results show that the
CO2 is reduced to CH4 and CO over LaTiO2N and Ta3N5 photocatalysts, respectively. The CO2 reduction over LaTiO2N and
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*
Ta3N5 follows the possible reaction pathway via CO2 → COOH* → CO → CHx → CH4, as proposed by the detected
intermediates. The product selectivity is dependent on the ability of capturing CO onto photocatalyst surface, probably
related to electronegativity of the adsorption sites. The strongly adsorbed CO on LaTiO2N surface favors the subsequent
hydrogenation reaction to generate CH4, while the weak CO adsorption on Ta3N5 induces the CO as a main product of CO2
reduction, as well demonstrated by theoretical calculation. Such product selectivity is mainly related to the surface
chemistry and is independent on the surface charge concentration of photocatalyst. Our results provide a progressive
understanding of the selective product formation during the photocatalytic CO2 reduction.
CH4 formation was observed over ZnGa2O4 and Zn2GeO4.[8,9] Surface
modification including introducing basic sites or loading cocatalysts
could induce the CH4 as main product over TiO2 or C3N4.[10-14]
Meanwhile, exposing the (110) facet of Zn2GeO4 produced the CO
as main product.[15] The similar facet dependence of reduced
products was also demonstrated on TiO2 and BiVO4.[16-18] These
results indicate that the product selectivity of CO2 reduction is
strongly related to surface chemistry of photocatalyst, since the two
half-reactions of H2O oxidation and CO2 reduction both take place
on the photocatalyst surface.
Introduction
Reduction of CO2 over semiconductor photocatalyst is recognized to
be a potential solution to achieve the recycling of carbon source.[1-3]
Under irradiation, driven by photogenerated electrons of semi-
conductor, CO2 can be reduced into various value-added fuels, such
as CO, HCO2H, HCOH, CH3OH, or CH4, in the presence of protons
that are commonly provided from H2O splitting by photogenerated
hole oxidation.[4] To date, much concern has been focused on
improving the efficiency of the solar-driven CO2 conversion.
However, the product selectivity is still not well understood.
In the previous reports, accompanied by the increase of
electron concentration on the surface of photocatalysts by loading
cocatalysts, effectively promoted CH4 formation was observed.
Therefore, the product selectivity was considered to be dependent
In theory, the formation of reduced products depends on the
number of transferring electrons and protons, typically two-
electron process for CO and eight-electron requirement for CH4.[5]
Actually, among the reported gaseous CO2 reductions, the CO and
CH4 are usually detected as the main product over different
photocatalysts.[6-9] For instance, the TiO2 or C3N4 without additional
decoration presented excellent CO selectivity,[6,7] while the selective
on the charge concentration. Such
a standpoint was well
demonstrated by forming heterojunction or metal-semiconductor
junction to enhance the charge separation efficiency.[10,11,19-22]
Typically, the Pt modification on TiO2 accelerated the formation of
CH4.[10,11] However, different reduced products could be detected
when using the different electrocatalysts. Correspondingly, the Ag
and Au are beneficial for CO formation,[23,24] while Cu commonly
shows a combination of hydrocarbons.[25] This fact means that, in
addition to electron concentration, the surface chemistry of
catalysts may also play an important role on the product selectivity.
Indeed, the difference in CO2 activation on the surface of various
metals or oxides, which determines the reaction pathway and
product selectivity, has been investigated in thermocatalysis of CO2
conversion.[26] Actually, for explaining the product selectivity of
photocatalytic CO2 reduction, theoretical calculations that using the
TiO2 as a material model have proposed two reaction pathways,
a.Eco-materials and Renewable Energy Research Center (ERERC),
National Laboratory of Solid State Microstructures, Collaborative Innovation Center
of Advanced Microstructures, College of Engineering and Applied Sciences, Nanjing
University, 210093 (P. R. China)
b.Jiangsu Province Key Laboratory for Nanotechnology, School of Physics, Nanjing
University, Nanjing, Jiangsu 210093 (P. R. China)
c.School of Environment and Safety Engineering, Changzhou University, Changzhou
213164 (P. R. China)
E-mail: yscfei@nju.edu.cn
†
Electronic
10.1039/x0xx00000x
These authors contributed equally to this work.
Supplementary
Information
(ESI)
available.
See
DOI:
‡
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respectively calling fast-hydrogenation (FH) pathway (CO2
→
obtain the heterojunction of Ta3N5/LaTiO2N, the La2Ti2O7 and KTaO3
HCOOH/CO → CH2O → CH3OH → CH4) and fast deoxygenation (FdO) precursors were mixed intensively, and the same nitridation
*
pathway (CO2 → CO → C* → CH3 → CH4).[5,27] However, it is process was carried out. For comparison, Ta3N5/LaTiO2N samples
necessary to further understand the effect of surface chemistry on with different Ta3N5 ratio were prepared, and the ratio of Ta3N5 to
product selectivity, due to that the molecule activation, reaction LaTiO2N was achieved by changing the amount of KTaO3. All the
pathway, and product formation are highly sensitive to the surface obtained products were washed with deionized water, and dried in
properties of catalysts.
a vacuum oven.
Here, to identify the importance of surface chemistry in
product formation of the photocatalytic CO2 reduction, photo-
Photoreduction CO2 testing: The gas phase photocatalytic
catalysts of porous LaTiO2N plates with exposure of (002) facet, reduction of CO2 was tested in a glass reactor with an area of 4.2
Ta3N5 nanorods with exposure of (020) facet, and the corresponding cm2 using H2O as reducing agent, and photocatalyst (40 mg) was
Ta3N5/LaTiO2N composite were prepared by respectively nitriding uniformly dispersed. The light source was a 300 W Xenon arc lamp
La2Ti2O7 plates, KTaO3 microcubes, and their mixtures under flowing fitted with a cutoff filter (λ＞420 nm). The volume of the reaction
NH3. The photocatalytic CO2 reduction tests showed that the CO2 chamber was about 230 mL. Before the irradiation, the reaction
was, respectively, reduced into CH4 and CO over LaTiO2N and Ta3N5. system was vacuumed several times, and then high-purity gas of
Proposed by the detected intermediates, the CO2 reduction over compressed CO2 (purity 99.999%) was introduced into the reaction
LaTiO2N and Ta3N5 possibly followed the reaction pathway of CO2 → chamber to achieve an ambient pressure. Subsequently, deionized
*
COOH* → CO → CHx → CH4. Theoretical calculation showed that water (0.4 mL) was injected into the chamber as reactant. Prior to
the product selectivity was dependent on the ability of capturing irradiation, the adsorption process was held for 2 h. During the
CO onto photocatalyst surface, probably resulting from the reaction, 1 mL gas was extracted by a sampling needle from the
different electronegativity of adsorption sites. The strongly chamber at given intervals for subsequent concentration analysis.
adsorbed CO on LaTiO2N surface favored the subsequent The carbon-based products were quantified by
a
gas
hydrogenation reaction to generate CH4. However, the weak CO chromatography (GC-2014, Shimadzu Corp., Japan) with flame
adsorption on Ta3N5 induced the CO as a main product of CO2 ionization detector (FID).
reduction. Indeed, when using CO as carbon source, the LaTiO2N
could rapidly convert the CO into CH4, while the Ta3N5 showed a
Materials characterizations: The crystal phases were
much lower conversion efficiency. Moreover, all the Ta3N5/LaTiO2N determined by powder X-ray diffraction (XRD, Rigaku Ultima III,
samples, where CO2 reduction took place on Ta3N5 surface, evolved Japan) operated at 20 kV and 40 mA with Cu-Kα radiation. The
CO as main product during the CO2 conversion. These results surface morphology was observed by scanning electron microscopy
confirmed that such product selectivity was mainly related to the (SEM, FEI Nova Nano SEM 230, USA). High-solution lattice images
surface chemistry and was independent on the surface charge and selected area electron diffraction (SAED) patterns was obtained
concentration of photocatalyst, providing us a progressive under- by transmission electron microscopy (TEM, FEI Tecnai G2 F30 S-
standing of the selective product formation for the photocatalytic Twin, USA) operated at 200 kV. Diffused reflectance spectrum was
CO2 reduction.
scanned by a UV-vis spectrophotometer (UV-2500, Shimadzu Co.,
Japan) and transformed into absorption spectrum with Kubelka-
Munk relationship. The specific surface area was measured from
Experimental Section
nitrogen (N2) adsorption-desorption isotherm at 77
K by an
automatic surface area analyzer (Micromeritics Tristar-3000, USA)
after the samples had been dehydrated at 423 K for 2 h in the
flowing N2. The CO2 adsorption was measured by the applied BET
method at 273 K. The surface chemical species was investigated by
X-ray photoelectron spectroscopy (XPS) on PHI5000 Versa Probe
(ULVAC-PHI, Japan) with monochromatized Al Kα X-ray radiation
(1486.6 eV). The energy resolution of the electrons analyzed by the
hemi-spherical mirror analyzer is about 0.2 eV. The C 1s core level
at 284.6 eV was taken as an internal reference to correct the shift of
the binding energies. The room temperature photoluminescence
(PL) spectra were acquired using an objective-scanning confocal
microscope system, in which the samples were excited through an
oil-immersion objective lens (Olympus, SR-ASZ-0103, 150) using a
40 mW pulsed laser (488 nm). The flat band positions of the Ta3N5
and LaTiO2N were estimated by Mott-Schottky measurement,
which was carried out using a Princeton Applied Research PARSTAT
2273, using 0.5 M Na2SO4 aqueous solution as electrolyte with a pH
value of 7. The frequency was 500, 1000, and 2000 Hz.
Synthesis of La2Ti2O7 and KTaO3 precursors: The solid La2Ti2O7
powder was synthesized by a typical flux-assisted growth method.
Simply, a stoichiometric mixture of La2O3 and TiO2 was thoroughly
mixed with NaCl and KCl compounds (NaCl: KCl=1:1), in which the
La2Ti2O7 product accounts for 5% mole ratio. The resulting mixtures
were heated to 1273 K with a raise rate of 100 K/h and held at this
temperature for 10 h. After a cooling process to 973 K at a rate of
50 K/h, it was cooled to room temperature naturally. The KTaO3
microcubes were prepared by a hydrothermal reaction. Typically,
Ta2O5 powder (0.4 g) was added to KOH solution (30 mL, 1 M). After
ultrasonic dispersion, the resultant suspension was poured into a 50
mL Teflon vessel and heated to 433 K for 12 h. Notably, both the
obtained white powders of La2Ti2O7 and KTaO3 were separated by
deionized water washing and dried in an oven at 353 K overnight.
Preparation of Ta3N5/LaTiO2N photocatalyst: The LaTiO2N and
Ta3N5 were prepared by respectively nitriding the as-prepared
La2Ti2O7 and KTaO3 precursors (about 0.2 g for each run) under 200
mL/min flowing NH3 at 1223 K for 18 h at a rate of 10 K/min,
followed by a natural cooling process to room temperature. To
Computational calculations: The calculations of adsorption
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energy were conducted through the Cambridge Serial Total Energy by directly nitriding their precursors (Fig. S1a and S1b, ESI ). The
†
Package (CSTEP) code. The general gradient approximation with obtained hybrids were denoted as xTa3N5/LaTiO2N, where the x
Perdew-Bueke-Ernzerh (PBE) functional was employed to describe represented the mole ratio between Ta3N5 and LaTiO2N.
the exchange–correlation effects. The attractive energy between Accordingly, with increasing the ratio of KTaO3/La2Ti2O7,
nuclear and electrons was calculated via ultrasoft pseudopotential. continuously enhanced intensity of Ta3N5 diffraction peaks was
The convergence threshold of geometric optimization was set at 2.0 observed.
× 10−5 eV per atom for total energy, 0.05 eV Å−1 for maximum force,
The SEM image of the obtained LaTiO2N showed a porous
0.1 GPa for stress, and 0.002 Å for maximum displacement. The structure with apparent profile and size of La2Ti2O7 precursor (Fig.
crystal structure of Ta3N5 (020) was built including 9 atoms of Ta, 15 1b). However, great morphology change was observed for phase
atoms of N, and lattice parameter was a = 10.3 Å, b = 3.9 Å, c = 17.0 transformation from KTaO3 microcubes to Ta3N5, which was shaped
Å and α = β = γ = 90°. The crystal structure of LaTiO2N (002) in nanorods with particle size of 60-80 nm in width and 0.3-0.5 ꢀm
contained 3 atoms of La, 3 atoms of Ti, 6 atoms of O, and 3 atoms in length (Fig. 1c). This fact indicated that the formation of LaTiO2N
of N, and lattice parameter was a = 5.6 Å, b = 7.9 Å, c = 16.5 Å, α = β driven by nitriding La2Ti2O7 was pseudomorphic and topotactic.[28]
= 90° and γ = 89.9°.
The particles with porous structure were resulted from the volume
shrinkage during replacing the three O atoms by two N atoms.[28] In
contrast, a recrystallization-growth process maybe occur during the
Ta3N5 formation due to the potassium removal from KTaO3, thus
destroying the profile of precursor. For the composite materials,
taken 0.3Ta3N5/LaTiO2N as an example, the typical morphologies of
LaTiO2N porous plates and Ta3N5 nanorods were also observed (Fig.
1d), in which the Ta3N5 nanorods were uniformly distributed around
LaTiO2N plates. This indicated that the one-step nitriding did not
change their morphologies.
Results and Discussion
Fig. 1 (a) XRD patterns for the as-prepared xTa3N5/LaTiO2N (x = 0.1,
0.3, and 0.5) samples. SEM images of (b) LaTiO2N, (c) Ta3N5, and (d)
0.3Ta3N5/LaTiO2N.
To obtain our designed photocatalysts, precursors of La2Ti2O7 and
KTaO3 were first prepared. La2Ti2O7 was prepared by heating
stoichiometric mixture of La2O3 and TiO2 in a molten salt of NaCl
and KCl at 1273 K. KTaO3 was synthesized by a hydrothermal
method at 433 K using Ta2O5 and KOH as reagents. X-ray diffraction
(XRD) patterns showed that the as-prepared La2Ti2O7 (JCPDS No. 70-
1690) and KTaO3 (JCPDS No. 77-0198) were both crystallized in
Fig. 2 TEM images of (a) LaTiO2N and (b) Ta3N5. The inset shows the
corresponding SAED images. HR-TEM images of (c) LaTiO2N and (d)
Ta3N5. (e) and (f) TEM images of 0.3Ta3N5/LaTiO2N. HR-TEM images
of (g) LaTiO2N and (h) Ta3N5 in 0.3Ta3N5/LaTiO2N. Side view of
surface structure model for (i) (002) facet of LaTiO2N and (j) (020)
facet of Ta3N5.
single phase with orthorhombic structure (Fig. S1a and S1b, ESI
†).
Scanning electron microscope (SEM, Fig. S1c and S1d, ESI ) images
†
showed the plate-like La2Ti2O7 (0.4-0.6 μm in thickness, 1.5-1.7 μm
in width, and 3.5-4 μm in length) and KTaO3 microcubes (0.2-0.3 μm
in side length). After heating the mixtures of La2Ti2O7 and KTaO3
under flowing NH3 at 1223 K, the XRD peaks (Fig. 1a) of the
products were well indexed to mixtures of Ta3N5 (JCPDS No. 79-
1533) and LaTiO2N (JCPDS No. 48-1230). This evidence indicated
that both La2Ti2O7 and KTaO3 could be completely converted into
LaTiO2N and Ta3N5. Indeed, pure LaTiO2N and Ta3N5 were obtained
Observations of the transmission electron microscope (TEM)
further confirmed the typical morphologies of LaTiO2N porous plate
and Ta3N5 nanorod (Fig. 2a and 2b). The corresponding selected
area electron diffraction (SAED) patterns well demonstrated the
single-crystal nature of the porous LaTiO2N sheets and Ta3N5
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nanorods. As shown in high-resolution TEM (HR-TEM) lattice images the light absorption process of defect states.[30-33] It has been widely
(Fig. 2c and 2d), the measured lattice spacing of 0.394 and 0.512 reported that some defects, such as reduced Ta species, and N or O
nm were, respectively, assigned to (002) and (020) plane of the vacancies,[34-36] could be introduced into the crystal lattice during
orthorhombic LaTiO2N and Ta3N5. Apparently, both the (002) facet high-temperature heating process. To exclude the possibility of
in LaTiO2N and (020) facet in Ta3N5 were highly exposed. The element doping, Ta-doped LaTiO2N was prepared by nitriding Ta-
representative TEM images of 0.3Ta3N5/LaTiO2N were shown in Fig. doped La2Ti2O7. XRD analysis showed that (112) plane of LaTiO2N
2e and 2f. Due to the significant difference in morphology and shifted to smaller angle (Fig. S2a, ESI ), due to the slight larger ionic
†
shape size, LaTiO2N and Ta3N5 could be easily identified. Intimate radius of Ta5+ (64 pm) than Ti4+ (61 pm), thus confirming that the Ta
contact at the interface was found, well confirming the formation of was successfully incorporated into the lattice of LaTiO2N. Compared
Ta3N5/LaTiO2N heterojunction. Moreover, the (002) facet of with the LaTiO2N, about 10 nm blue-shift of the UV-vis absorption
LaTiO2N and (020) facet of Ta3N5 were observed in the composite of curve and absent absorption trail were found for Ta-doped LaTiO2N
LaTiO2N and Ta3N5 (Fig. 2g and 2h), indicating that they followed (Fig. S2b, ESI ). This evidence meant that, during formation of
†
the same growth mechanism during the two synthetic processes. Ta3N5/LaTiO2N composite, no doping effects were involved. Indeed,
The calculated surface structure model showed that the (002) facet XRD peak without position shift was observed in the Ta3N5/LaTiO2N
of LaTiO2N was composed of equal anions and cations, respectively composite (Fig. S2c, ESI
involving one La atom, two Ti atoms, one N atom, and two O atoms doping occurred. None absorption trail was also observed in the
(Fig. 2i). In contrast, the (020) facet in Ta3N5 was dominated by N physically mixed Ta3N5+LaTiO2N sample (Fig. S3a, ESI ), suggesting
†), further confirming that no element
†
atoms, and Ta atoms located at the sublayer (Fig. 2j). Commonly, that the increased defect absorption may relate to the strong
the metal sites in catalysts are demonstrated to be active sites for interaction between Ta3N5 and LaTiO2N due to their compact
CO2 adsorption.[29] This result suggested that the exposed (020) contact.
facet of Ta3N5 was possibly unfavorable for capturing CO2 or the
intermediates.
To check the interactions between Ta3N5 and LaTiO2N, X-ray
photoelectron spectroscopy (XPS) analysis was carried out. As can
be seen, N 1s XPS for the 0.3Ta3N5/LaTiO2N photocatalyst can be
deconvoluted into two peaks at 395.6 and 394.9 eV (Fig. 3b),
respectively assigned to lattice N in Ta3N5 and LaTiO2N,[19,30] well
demonstrating the composite materials. The binding energy
difference was resulted from slightly smaller electronegativity for Ti
(3.45 eV) than Ta (4.11 eV). For the single LaTiO2N, Ti 2p XPS
spectrum was fitted for Ti4+ at 457.8 eV and for Ti3+ at 456.7 eV (Fig.
3c).[36,37] The Ti3+ species in the LaTiO2N was resulted from the
reduction of Ti4+ by NH3 during the nitridation process.[30] The
amount of Ti3+ species obviously decreased with the formation of
Ta3N5/LaTiO2N composite. This fact suggested that the compact
contact between Ta3N5 and LaTiO2N could stabilize the lattice Ti4+
under NH3, thus decreasing the Ti3+ defect.
For charge balance, the oxygen vacancy and Ti3+ species were
usually coexisted in the LaTiO2N or TiO2.[30,38-41] Usually, the O 1s
XPS peak at 531.2 eV was attributed to lattice oxygen atoms near
the oxygen vacancy.[30,42] Compared with the LaTiO2N, the
0.3Ta3N5/LaTiO2N showed lower oxygen vacancy concentration.
This fact further suggested that the compactly contacting Ta3N5
with LaTiO2N was beneficial to stabilize the Ti4+ in the lattice of
LaTiO2N. The XPS peaks at 529.2 and 532.2 eV were respectively
assigned to Ti-O-Ti lattice oxygen and surface adsorbed oxygen
species (Fig. 3d).[30,43] Notably, for 0.3Ta3N5/LaTiO2N, one additional
peak at 530.2 eV was assigned to TaOx species in Ta3N5.[44] The Ta 4f
XPS spectra for both Ta3N5 and 0.3Ta3N5/LaTiO2N sample were
deconvoluted into four peaks: 24.2 eV for Ta 4f 7/2 and 26.0 eV for
Ta 4f 5/2 in Ta3N5, and 25.4 eV for Ta 4f 7/2 and 27.2 eV for Ta 4f
5/2 in TaOx species (Fig. 3e).[44,45] Obviously, the 0.3Ta3N5/LaTiO2N
showed higher amount of TaOx species (39.4%) than the Ta3N5
(13.1%). No obvious binding energy difference was observed in La
3d XPS spectra for both the LaTiO2N and 0.3Ta3N5/LaTiO2N (Fig. S3b,
Fig. 3 (a) UV-vis absorption spectra for LaTiO2N, Ta3N5, and
xTa3N5/LaTiO2N (x = 0.1, 0.3, and 0.5) samples. (b) N 1s XPS spectra
for LaTiO2N, Ta3N5, and 0.3Ta3N5/LaTiO2N. (c) Ti 2p and (d) O 1s XPS
spectra for LaTiO2N and 0.3Ta3N5/LaTiO2N. (e) Ta 4f XPS spectra for
Ta3N5 and 0.3Ta3N5/LaTiO2N.
Fig. 3a showed the ultraviolet-visible (UV-vis) absorption
spectra. All the samples exhibited a similar visible light absorption
up to 600 nm. Compared with the LaTiO2N, gradual increase in
absorbance was found for the Ta3N5/LaTiO2N composite with
increasing the content of Ta3N5. This fact should be a result of
higher absorption ability for Ta3N5 than LaTiO2N in the wavelength
range from UV region to 600 nm. Interestingly, for the composite
samples, obvious upward absorption trail in the wavelength above
600 nm could be observed, especially for the 0.5Ta3N5/LaTiO2N
sample. Usually, the absorption trail was strongly associated with
ESI ), indicating their almost identical surface chemistry for La
†
element. After forming Ta3N5/LaTiO2N composite, the decreased
Ti3+ species in LaTiO2N and the increased TaOx species in Ta3N5
maybe suggest that the surface chemistry environment change was
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a result of the strong interaction between Ta and Ti. This was electrons and holes. As expected, the content of Ta3N5 played
comprehensible that after the compact contact, Ti would donate important effects on the CO2 reduction performance for the
electrons to Ta due to the lower electronegativity for Ti than Ta, Ta3N5/LaTiO2N systems. Typically, with increasing the content of
thus causing the decreased content of Ti3+ and the increased Ta3N5, volcano-type distributions of the CH4 and CO yields were
content of TaOx species. The increased TaOx defects may be revealed (Fig. 4d). And, the 0.3Ta3N5/LaTiO2N sample exhibited the
responsible for the increased trail absorption of the Ta3N5/LaTiO2N highest CH4 and CO generation rates to be 2.32 and 11.53 ꢀmol/g,
composite (Fig. 3a).
respectively. For the xTa3N5/LaTiO2N composite, when x ˂ 0.3, the
gradually increased CO2 conversion rate can be attributed to the
increased space charge region by forming heterojunction between
Ta3N5 and LaTiO2N, thus promoting the charge separation and
improving the CO2 reduction. When x > 0.3, the decreased CO2
conversion rate was probably due to the increased TaOx species on
Ta3N5, as demonstrated by UV-vis and XPS analysis. The earlier
theoretical calculations showed that the Ta4+ defect in Ta3N5 was
the recombination center of electron-hole pairs, thus decreasing
the photocatalytic efficiency.[47] All the Ta3N5/LaTiO2N samples
showed evidently faster CO2 reduction rate than the LaTiO2N or
Ta3N5, confirming the contribution of Ta3N5/LaTiO2N heterojunction
structure.
Fig. 4 (a) CO and (b) CH4 generation, and (c) PL spectra for LaTiO2N,
0.3Ta3N5/LaTiO2N, and Ta3N5. (d) Product generation for LaTiO2N,
Ta3N5, and x Ta3N5/LaTiO2N (x = 0.1, 0.3, and 0.5). The inset in Fig.
4a shows the enlarged view of CO yields for LaTiO2N and Ta3N5.
The photocatalytic CO2 reduction tests were carried out over
the as-prepared LaTiO2N porous plates, Ta3N5 nanorods, and
Ta3N5/LaTiO2N samples under irradiation of visible light (λ ≥ 420 nm)
using H2O as reducing agent. The typical time-course curves of
photocatalytic CO2 reduction were shown in Fig. 4a and 4b. The CO
and CH4 were evolved only under irradiation and their yields
increased with prolonging the irradiation time. This result well
demonstrated the formation of CO and CH4 originating from CO2
photoreduction. Compared with the LaTiO2N or Ta3N5, the
0.3Ta3N5/LaTiO2N sample showed much higher CH4 and CO
evolution. As observed by TEM, a compact contact may induce the
formation of heterojunction of LaTiO2N and Ta3N5. The Mott-
Schottky measurement and UV-vis absorption spectra revealed that
the relative conduction band (CB, -0.218 eV) and valence band (VB,
+1.902 eV) levels of Ta3N5 were, respectively, slightly positive than
the CB (-0.332 eV) and VB (+1.768 eV) of LaTiO2N. This fact meant
that a typical type- heterojunction was formed between LaTiO2N
Fig. 5 (a) CO2 adsorption isotherms for LaTiO2N, Ta3N5, and
0.3Ta3N5/LaTiO2N samples. (b). FT-IR spectra for LaTiO2N and Ta3N5
after CO2 adsorption and reduction. C 1s XPS analysis of (c) LaTiO2N
and (d) Ta3N5 after CO2 adsorption and photocatalytic reaction.
After irradiation for 8 h (Fig. 4d), the total evolved CH4 and CO
were 1.61 and 0.28 μmol/g for LaTiO2N and 0.20 and 0.49 μmol/g
for Ta3N5, respectively. This result indicated that CH4 and CO were
the main product over LaTiO2N and Ta3N5, respectively. The CH4/CO
mole ratio was 5.75 for LaTiO2N and 0.41 for Ta3N5, confirming their
different product selectivity for CO2 reduction. In general, the
product selectivity was controlled by the reagent concentration and
molecule activation. Indeed, for the CO2 conversion, it was
proposed that CO generation occurred at a low CO2 reaction rate
resulting from the insufficient electrons.[10,11] In our case, the Ta3N5
with CO selectivity indeed showed obviously lower CO2 reduction
efficiency than LaTiO2N. However, a lower charge recombination for
Ta3N5 than LaTiO2N was verified by PL results (Fig. 4c). This fact
probably suggested that electron concentration was not mainly
responsible for the CO selectivity of Ta3N5. As demonstrated in XPS
and Ta3N5 (Fig. S4, ESI ). The space-charge region electric field,
†
which would contribute to the increased performance in CO2
reduction, was built at their interface as a result of charge carries
diffusion.[46] Under irradiation, this electric field can direct the flow
of electrons and holes to different semiconductors, thus resulting in
efficient charge separation and low recombination. Indeed, much
lower photoluminescence (PL) intensity was observed for the
sample of 0.3Ta3N5/LaTiO2N by comparing with the LaTiO2N or
Ta3N5 (Fig. 4c), well demonstrating the decreased recombination of
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results, the oxygen vacancies were inevitably generated in the selectivity could not be completely attributed to the CO2 molecule
LaTiO2N, which usually served as an important electron transfer adsorption.
route to improve the charge separation efficiency.[30,43] Passivating
Semi-qualitative Fourier transform infrared (FT-IR) spectro-
oxygen vacancies by heating the LaTiO2N at 353 K under air, the PL scopy analysis was carried out on the LaTiO2N and Ta3N5. After 2 h
intensity evidently increased when prolonged the heat time from 12 CO2 adsorption-desorption equilibrium under dark condition, the
h to 24 h (Fig. S5a, ESI ), indicating that the oxygen vacancies were LaTiO2N and Ta3N5 exhibited the nearly similar FT-IR bands (Fig. 5b).
†
indeed responsible for charge separation. Obviously, passivating the A band at 2347 cm-1 was assigned to linearly adsorbed CO2.[30] The
oxygen vacancies induced the decrease in CO2 reduction activity of band from 1850 cm-1 to 1610 cm-1 was attributed to the asymmetric
LaTiO2N, due to the increased charge recombination (Fig. S5b, ESI ). vibration of some chemisorbed species, such as OH groups at 1618
†
-
And, the CH4/CO mole ratio decreased from 5.75 for fresh LaTiO2N, cm-1, COOH species at 1623 cm-1 or HCO3 species at 1658 cm-1
to 2.2 for passivating LaTiO2N for 12 h, and to 1.8 for passivating resulting from the reaction of CO2 with the surface OH groups.[12,30]
LaTiO2N for 24 h. Although the CO yield was increased, the CH4 was The wide band from 1590 cm-1 to 1290 cm-1 was possibly assigned
still the main product, implying that the product selectivity was not to stretching vibration of C=O or asymmetric stretching of O-C=O
completely attributed to the charge concentration change. Hence, belonging to chemisorbed carbonate species (CO32-).[12] After
for the LaTiO2N or Ta3N5, a possible conclusion could be drawn that irradiation for 8 h, it was found that the intensities of chemisorbed
the surface chemistry of the photocatalyst played a decisive role on species increased on the Ta3N5 while slightly decreased on the
the selective product generation rather than the surface electron LaTiO2N. This fact may imply that the chemisorbed species were
density. The increased CH4 and CO yields verified that the charge important intermediates during CO2 photoreduction. Notably, these
2-
separation efficiency was evidently improved in the Ta3N5/LaTiO2N intermediates, such as CO3 species, were usually the result of CO2
heterojunction. Significantly increased CO product selectivity with activation, thus boosting the CO2 reduction kinetics. That is, the
CH4/CO mole ratio of 0.2 was achieved by the 0.3Ta3N5/LaTiO2N faster/slower consumption of the chemisorbed species would
heterojunction, further demonstrating that evolved products were contribute to the higher/lower CO2 reduction efficiency on LaTiO2N
possibly dependent on the surface chemistry, in spite of the and Ta3N5, respectively. The new bands at 2920 and 2851 cm-1
*
improved photocatalytic activity due to the promoted charge belonging to CHx species,[49] serving as an important intermediate
separation. Similarly, the Z-scheme α-Fe2O3/g-C3N4 with increased for reduction of CO2 to CH4,[27] were only detected by FT-IR on
electron concentration only enhanced the CO2 reduction LaTiO2N during the CO2 conversion, further verifying that the high
performance, still presenting the CO product selectivity as the g- CH4 selectivity on LaTiO2N was possibly a result of efficient CO2
*
2-
C3N4.[7]
reduction via CHx intermediates. The CO3 species was also
It was reported that changing the surface chemistry of TiO2 by observed on Ta3N5 after the irradiation. And to promote the
doping could change the main product from CO to CH4.[48] formation of CO32- species by the modification of KOH (0.5 wt.%) on
Increasing the NH3 flow amount from 200 to 500 mL/min, the Ta3N5 surface, the CO2 reduction efficiency (0.71 μmol/g for CO and
content of TaOx species in Ta3N5 was decreased from the 13% to 0.30 μmol/g for CH4) enhanced significantly, while the CO selectivity
9.3%, as demonstrated by XPS analysis (Fig. S6a, ESI†). (CH4/CO=0.42) kept unchanged (Fig. S6d, ESI†). Similar results were
Correspondingly, the CH4/CO mole ratio slightly decreased from observed for the MgO modified TiO2 and KOH modified C3N4.[10,50]
0.41 to 0.34, although the CO2 reduction activity increased These evidences indicated that the product selectivity on both of
2-
approximately two fold due to the decrease of TaOx as Ta3N5 and LaTiO2N was not dependent on the formation of CO3
recombination centers (Fig. S6b, ESI
†). This evidence indicated that species.
the TaOx species played little effects on the product selectivity of
XPS analysis revealed that the -COOH at 288.4 eV and C=O
Ta3N5. CO2 adsorption and activation have been demonstrated to species at 286.3 eV existed on the LaTiO2N and Ta3N5 surfaces (Fig.
*
be pivotal factors to affect photocatalytic CO2 reduction.[7,8,30] 5c and 5d).[23,30] The absent CHx species in XPS was highly possible
Brunauer-Emmett-Teller (BET) tests revealed the nearly same BET for its low amount due to its fast reaction kinetics, as confirmed by
surface areas for the as-prepared LaTiO2N (26.49 m2/g), Ta3N5 the FT-IR analysis. During the irradiation, the formation of -COOH
(23.56 m2/g), and 0.3Ta3N5/LaTiO2N (20.81m2/g) (Fig. S6c, ESI
). species on LaTiO2N is evidently faster than that on the Ta3N5. It was
†
Interestingly, the CO2 adsorption tests (Fig. 5a) showed that the well known that the CO resulted mainly from dehydroxylation of -
maximal adsorption amount of CO2 on the surface of Ta3N5 (3.58 COOH species with the assitance of the photogenerated electrons
mg/g) was obviously lower than LaTiO2N (6.08 mg/g) and and protons.[5,23] This evidence suggested that the the reaction
0.3Ta3N5/LaTiO2N (5.51 mg/g), indicating a stronger CO2 capture kinetics for converting CO2 to -COOH species were mainly
ability for the LaTiO2N-contianing samples than Ta3N5. A similar responsible for the CO generation on Ta3N5 surface. The much
result was obtained by normalizing their CO2 adsorption quantity lower amount of C=O species on LaTiO2N surface under irradiation,
with BET surface area to be 0.23 mg/m2 for LaTiO2N, 0.15 mg/m2 for as indicated by the decreased peak intensity, demonstrated that
Ta3N5, and 0.27 mg/m2 for 0.3Ta3N5/LaTiO2N, respectively. The the C=O species on LaTiO2N surface was rapidly consumed by the
higher CO2 adsorption ability for LaTiO2N-contianing samples was photoelectrons for the CH4 formation. Indeed, the Ta3N5 with low
possibly attributed to the porous structure that helped to enhance CH4 yields showed a little difference in concentration of C=O species
the molecule adsorption. The 0.3Ta3N5/LaTiO2N with strong before and after the irradiation. Hence, the possible reaction
adsorption of CO2 exhibited the higher CO selectivity, similar to the pathway of CO2 reduction was proposed as CO2 → COOH* → CO →
Ta3N5 with weak adsorption of CO2, suggesting that the product CHx* → CH4. Our previous work demonstrated that the La as a basic
site on porous LaTiO2N particles without exposure of specific facet
6 | J. Name., 2012, 00, 1-3
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ARTICLE
contributed to the CO2 activation and the CH4 selectivity.[30] The and energy of an adsorbate, respectively. The calculated results
similar CH4 selectivity on the LaTiO2N with dominant (002) surface were illustrated in Fig. 6 and Table 1. The calculated results showed
probably meant that the CO2 activation and product selectivity on that the Eads for CO2 on Ta3N5 (020) surface was -2.1 eV, which was
LaTiO2N did not depend on the specific facets but still were about 0.8 eV lower than that on LaTiO2N (002) facet (-2.9 eV). As
dominated by La element. However, the performed CO2 reduction can be seen, on the Ta3N5 (020) surface, the CO2 was absorbed at N
experiment over the Ta3N5 without specific exposed crystal facet, sites via interactions between C and N atoms because the stable
which was prepared by nitriding Ta2O5 powders at 1123 K for 8 h terminal facet was consisted of N atom layer. In general, the CO2
under 500 mL/min NH3, exhibited the main product of CH4 with a was inclined to absorb onto the metal atom sites because of their
CH4/CO mole ratio of 1.95 (Fig. S7a, ESI
), confirming that the high low electronegativity.[4,29] Similarly, on the surface of LaTiO2N, the
†
CO selectivity was highly dependent on the exposed (020) facet of CO2 with a tilted structure was absorbed at La sites by contacting
Ta3N5. Judging by the FT-IR and C 1s XPS analysis, the high CO with O atoms of CO2. As well demonstrated in our previous report,
selectivity of Ta3N5 with exposed (020) facet was probably related the La sites were served as basic sites for effectively catalyzing CO2
to the difficulty in hydrogenation of CO for generating CH4 on the N- into CH4.[30] Similarly, the adsorbed CO2 on LaTiO2N was also
rich facet.
activated, as demonstrated by the increased C-O bonding length
(from 1.151 Å to 1.179/1.188 Å) and decreased O-C-O bonding
angle (from 180° to 178.2°). In contrast, the CO2 on Ta3N5 surface
was adsorbed with slightly shorter C-O bonding length (1.178 Å) and
wider O-C-O bonding angle (178.8°). This may induce the faster and
lower formation rate for -COOH species by CO2 activation on
LaTiO2N and Ta3N5 surface, respectively.
Fig. 6 The energies for CO2 or CO adsorbed on LaTiO2N (002) and
Ta3N5 (020) facets. The calculated energies for LaTiO2N (002) and
Ta3N5 (020) facets were -12440.2 and -5297 eV, respectively.
Table 1. Adsorption energy, geometric parameter change before
and after adsorption of CO2 and CO over the surface of LaTiO2N
(002) and Ta3N5 (020) facet.
Fig. 7 (a) The CH4 yields over LaTiO2N, Ta3N5, and 0.3Ta3N5/LaTiO2N
using CO as carbon source under visible light. b) The possible CO2
reduction mechanism for Ta3N5/LaTiO2N.
CO2 adsorption
CO2
Ta3N5 (020)
LaTiO2N (002)
Obviously, the calculated results showed that the Eads for CO
on Ta3N5 (020) surface (-0.33 eV) was about 0.5 eV lower than that
on LaTiO2N (002) facet (-0.83 eV). The strong CO adsorption on
LaTiO2N may provide the possibility to covert the CO into CH4 by
subsequent hydrogenation. In another word, the desorption of CO
on Ta3N5 (020) facet was much easier than that on LaTiO2N surface.
As expected, the (023) surface model of Ta3N5 was built, an exposed
facet for the Ta3N5 prepared from direct nitridation of Ta2O5
particles, as observed by TEM. It was found that the (023) facet with
relatively rich Ta atoms showed a much higher CO capture ability (-
2.7 eV) and the CO was indeed adsorbed at Ta site (Fig. S7a, ESI†).
After stable adsorption, the increase of C-O bonding length of CO
(1.129 Å) was obviously larger on the surface of LaTiO2N (1.166 Å)
than Ta3N5 (1.157 Å). This result indicated that CO activation was
easier to occur on the La sites on LaTiO2N by comparing to the N
sites of Ta3N5, thus boosting the CH4 formation. A photocatalytic
test was carried out to check the photocatalytic reaction of CO,
performing under the CO (5% mole)+argon mixed gases. Indeed, in
the presence of H2O, the LaTiO2N could rapidly convert the CO into
CH4 (1.42 μmol/g for 8 h) (Fig. 7a), truly confirming that the CO
could be further converted into CH4. In contrast, a much low
conversion efficiency was observed for the Ta3N5 (0.16 μmol/g for 8
Eads [eV]
dC-O [Å]
-
-2.1
-2.9
1.151
1.178
1.179,1.188
180
CO
178.8
178.2
∠O-C-O [°]
CO adsorption
Ta3N5 (020)
-0.3
LaTiO2N (002)
-0.8
Eads [eV]
dC-O [Å]
-
1.129
1.157
1.166
To further explore the mechanism of the product selectivity,
the surface models of LaTiO2N (002) and Ta3N5 (020) facet, the
highly exposed facets by TEM observations, were built for
calculations of adsorbed energies of CO2 and CO by density
functional theory (DFT). The adsorbed energy of an adsorbate on a
given photocatalyst was calculated by the equation of Eads = Eads/pho
– Epho – Eadsorbate. Where, Eads is adsorption energy. Eads/pho, Epho and
Eadsorbate are energy of adsorption system, energy of a photocatalyst
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5
6
7
8
9
S. N. Habisreutinger, L. Schmidt-Mende and J. K. Stolarczyk,
Angew. Chem. Int. Ed., 2013, 52, 7372.
L. J. Liu, H. L. Zhao, J. M. Andino and Y. Li, ACS Catal., 2012, 2,
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Z. F. Jiang, W. M. Wan, H. M. Li, S. Q. Yuan, H. J. Zhao and P.
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0.3Ta3N5/LaTiO2N, due to the high charge separation, an improved
CH4 yield was also obtained (2.22 μmol/g for 8 h). These results
prove that the product selectivity of CO2 reduction is dependent on
the surface chemistry of Ta3N5 and LaTiO2N.
On the basis of the above results, the product selectivity for
the CO2 reduction over Ta3N5/LaTiO2N heterojunction was proposed
in Fig. 7b. Under the visible light, the lower CB and VB positions of
Ta3N5 induce directional electron flow from CB of LaTiO2N to CB of
Ta3N5 and reversed hole transfer between their VBs. The high CO
selectivity was achieved on (020) surface of Ta3N5 due to the poor
ability in converting -COOH intermediate and the weak CO
adsorption. Meanwhile, the high CH4 selectivity produced on the
LaTiO2N due to the strong CO adsorption and efficiently activating
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*
carbon-based precursors to CHx , probably on the La base metal
sites.
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Conclusions
In summary, using the LaTiO2N with exposure of (002) facet and
Ta3N5 with dominant (020) facet as model photocatalysts, we have
showed that the product selectivity for CO2 reduction on
photocatalyst is strongly associated to the kinetics of molecule
activation and intermediate conversion. The (020) surface of Ta3N5 18 Y. Y. Liu, B. B. Huang, Y. Dai, X. Y. Zhang, X. Y. Qin, M. H. Jiang
and M. Whangbo, Catal. Commun., 2009, 11, 210.
19 S. M. Wang, Y. Guan, L. Lu, Z. Shi, S. C. Yan and Z. G. Zou,
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20 S. B. Wang, B. Y. Guan, Y. Lu, X. Wen and D. Lou, J. Am.
Chem. Soc., 2017, 139, 17305.
terminated by N atoms exhibits the poor ability in converting -
COOH intermediate and the weak CO adsorption, thus inducing the
high CO selectivity. The high CH4 selectivity on the LaTiO2N is not
facet-dependent, more likely to be dominated by La base metal
*
sites to efficiently activate carbon-based precursors to CHx . Our 21 J. G. Hou, S. Y. Cao, Y. Z. Wu, F. Liang, L. Ye, Z. S. Lin and L. C.
Sun, Nano Energy, 2016, 30, 59.
findings indicated that the surface chemistry of catalyst was a
significant factor for the selective product formation of the
photocatalytic CO2 reduction.
22 M. L. Li, L. X. Zhang, M. Y. Wu, Y. Y. Du, X. Q. Fan, M. Wang, L.
L. Zhang, Q. L. Kong and J. L. Shi, Nano Energy, 2016, 19, 145.
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Conflicts of interest
There are no conflicts to declare.
25 D. D. Zhu, J. L. Liu and S. Z. Qiao, Adv. Mater., 2016, 28, 3423.
26 S. Kattel, P. Liu and J. G. Chen, J. Am. Chem. Soc., 2017, 139
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27 Y. F. Ji and Y. Luo, J. Am. Chem. Soc., 2016, 138, 15896.
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Acknowledgements
28 A. E. Maegli, S. Pokrant, T. Hisatomi, M. Trottmann, K.
,
This work was supported primarily by the National Basic Research
Program of China (2013CB632404), the National Natural Science
Foundation of China (51572121, 21603098 and 21633004), the
Natural Science Foundation of Jiangsu Province (BK20151265,
BK20151383 and BK20150580), the Fundamental Research Funds
for the Central Universities (021314380133 and 021314380084),
the Postdoctoral Science Foundation of China (2017M611784), Six
talent peaks project in Jiangsu Province (YY-013) and the program B
for outstanding PhD candidate of Nanjing University (201702B084).
Domen and A. Weidenkaff, J. Phys. Chem. C, 2013, 118
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29 J. C. Matsubu and V. N. Yang, J. Am. Chem. Soc., 2015, 137
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30 L. Lu, B. Wang, S. M. Wang, Z. Shi, S. C. Yan and Z. G. Zou,
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31 R. B. Singh, H. Matsuzaki, Y. Suzuki, K. Seki, T. Minegishi, T.
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32 M. Sluban, P. Umek, Z. Jagličic, J. Buh, P. Smitek, A. Mrzel, C.
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33 Y. Qi, S. S. Chen, M. R. Li, Q. Ding, Z. Li, J. Y. Cui, B. B. Dong, F.
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TOC
The product selection for CO2 reduction depends on the surface
chemistry of Ta3N5 and LaTiO2N photocatalyst.
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