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The Modification of Pd and Mn on the Surface of TiO with Enhanced Photocatalytic Activity for Photoreduction of CO into CH 2

4

Chi Cao, Yabin Yan, Yanlong Yu, Xiaodan Yang, Weisheng Liu, and Yaan Cao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08921 • Publication Date (Web): 14 Dec 2016 Downloaded from http://pubs.acs.org on December 15, 2016

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The Journal of Physical Chemistry

The Modification of Pd and Mn on the Surface of TiO2 with Enhanced Photocatalytic Activity for Photoreduction of CO2 into CH4 Chi Caoa, Yabin Yanb Yanlong Yub Xiaodan Yangb, Weisheng Liua and Yaan Caob* a College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China b Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education, TEDA Applied Physics Institute and School of Physics, Nankai University, Tianjin 300457, China

Abstract The novel Pd and Mn co-modified TiO2 photocatalyst (TiO2-Pd-Mn) was prepared via simple sol-gel method. The introduced Pd and Mn existed as the -O-Pd-O- and -O-Mn-O- species on the surface of the photocatalyst. The band structure and density of states are studied by the theoretical calculation, which is demonstrated by the experimental results. The modification with Pd and Mn ions results in the strong visible response and efficient separation of photogenerated carriers. Thus, the TiO2-Pd-Mn exhibit improved photocatalytic activity compared with pure TiO2, TiO2-Pd and TiO2-Mn for photoreduction of CO2 and H2O into CH4. It is an effective method on developing the highly active TiO2-based materials by modification with double elements on surface.

ACS Paragon Plus Environment


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1. Introduction Photoreduction of CO2 into CH4 is regarded as an effective route to resolve the energy crysis1-4, which has been studied extensively, in the recent years. TiO2 has drawn lots of attention, owing to its good stability and high photocatalytic performance5-9. However, the high recombination rate of photogenerated carriers and poor visible response retard the practical application of TiO2. Hence it is necessary to improve the photocatalytic activity of TiO2. TiO2 with metal elements modification has been considered as one of the most promising ways to improve the photocatalytic activity on photoreduction of CO2 into CH4. Yabin Yan et al. noted that the introduction of Cu could enhance the photocatalytic activity for CO2 photoreduction10. Shichao Feng et al. reported that the Ag-TiO2 hollow sphere exhibited enhanced photocatalytic activity for reduction of H2O and CO2 into CH411. Wei-Ning Wang et al reported that the Pt-TiO2 film exhibits extremely high CO2 photoreduction efficiency with selective formation of methane12. However, the photocatalytic performance for the metal elements modified TiO2 is still inefficient for practical application. Therefore, further increasing the photocatalytic activity of TiO2 based photocatalysts is still a big challenge. The palladium-doped TiO2 (TiO2-Pd) is widely investigated13-15, which exhibits improved photocatalytic activity. Tatsuto Yui et al reported that Pd/TiO2 would decrease generated CO and increase the amount of CH413. Tengfeng Xie et al. reported that the Pd/TiO2 shows an improved catalytic activity for photoreduction of CO214. It has been demonstrated by our previous work16 that the TiO2 with Pd modification would enhance the visible light absorption and facilitate separation of photogenerated charge carriers, improving the photocatalytic activity effectively. Moreover, the TiO2 with Mn modification is also found to enhance the photocatalytic activity of TiO217-18. We would like to combine the advantage of Pd and Mn modification, to further improve the photocatalytic activity of TiO2 based photocatalysts. Herein, Pd and Mn co-modified TiO2 photocatalyst (TiO2-Pd-Mn) was prepared by a sol-gel method. It can be deduced from the theory calculation that the TiO2-Pd-Mn photocatalyst exhibits an enhanced photocatalytic activity on


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photo-reduction of CO2 into CH4, which is confirmed by the experiment. The influence of the introduced Pd and Mn ions on the band structures, behavior of the photogenerated carriers and photocatalytic mechanisms is also discussed in detail. 2.Experimental Details 2.1. Catalyst Preparation At room temperature, 1 mL of deionized water was mixed with 40 mL of anhydrous ethanol. The pH value of the mixture was adjusted by adding 1 ml of concentrated HCl (12 mol/L). Then 3 ml of MnCl2 solution (0.0585mol/L) was added into the mixture. After stirring for 20 min, tetrabutyl titanate(12 mL) was added dropwise under vigorous stirring. The mixture was stirred continuously until the formation of TiO2 gel. The resultant precipitate was dried at 100 °C for 12 h after aging at room temperature for 24 h and calcined at 450 °C for 2.5 h. So TiO2-Mn 0.5% sample was got. Pure TiO2, TiO2-Pd 1% and TiO2-Pd 1%-Mn 0.5% samples were prepared with the same procedure, but by replacing the MnCl2 solution (0.0585mol/L) with 3 ml of deionized water, 3 ml of PdCl2 solution (0.0326mol/L) and 1 ml of PdCl2 solution(0.0978mol/L) with 2 ml of MnCl2 solution (0.0878mol/L), respectively. Moreover, a series of manganese-modified TiO2 and palladium-modified TiO2 nanoparticles with different concentrations of manganese was prepared by changing the concentration of MnCl2 solution or PdCl2 solution. The TiO2-Pd 1%, TiO2-Mn 0.5% and TiO2-Pd 1%-Mn 0.5% catalysts were designated as TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn for simplicity, respectively. 2.2. Characterization. X-ray diffraction (XRD) patterns were collected on a Rigaku D/max 2500 X-ray diffraction spectrometer(Cu Kα, λ=1.54056 Å) in the range of 20°~80°. The average crystal size was calculated based on Scherrer equation (D = kλ/Bcosθ, parameter B is the peak half width). The BET surface areas of samples were determined by nitrogen absorption-desorption isotherm measurement at 77K (Micromeritics Automatic Surface Area Analyzer Gemini 2360, Shimadzu). Raman spectra were taken on a Renishaw in Via Raman microscope by using the 785 nm line of Renishaw HPNIR 785 semiconductor laser. X-ray photoelectron spectroscopy (XPS) measurements



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were carried out with an ESCA Lab 220i-XL spectrometer by using an unmonochromated Al Kα(1486.6eV) X-ray source. All of the XPS spectra were calibrated with respect to the binding energy of the adventitious C1s peak at 284.8eV. Diffuse reflectance UV-visible (UV-vis) absorption spectra were recorded on a UV-vis spectrometer (U-4100, Hitachi). The photoluminescence (PL) spectra were measured by fluorescence spectrophotometer (Edinburgh Instruments, FLS920) using the 340 nm line of a Xe light as the excitation source. The experimental setup consists of a gas chromatograph (GC7890F, Shanghai Techcomp Instrument Co., Ltd.), a hydrogen generator (SPH-300A, Beijing BCHP Analytical Technology Institute), an automatic air source (SPB-3, Beijing BCHP Analytical Technology Institute) and a computer for data processing. All the measurements were carried out at room temperature (25±2) °C unless stated otherwise. 2.3. Photocatalytic Activity The photoreduction of CO2 into CH4 was carried out in a 225 mL cylindrical glass reactor with built-in light source of a 32 W UV lamp (Cnlight ZW11D12W-H215), shown in Figure S9. A stirred annular reactor (length 20 cm, width 4 cm) with a suspended catalyst was illuminated by a peak light wavelength at 253.7 nm situated in the center of the quartz tube. The whole system contains solution layer (125 ml) and gas layer (100 ml). First, 125 mg of catalysts was suspended in 125 mL of NaOH solutions (0.2 mol/L) in the reactor and the whole system was continuously inflated by CO2 gas (99.999%) at a flux of 0.3 L min-1 for at least 45 min, which was used to exclude the O2 and other gas in the whole system. The pH value of the solution is estimated to be 6.5. Except for the NaOH solutions in the reactor, the 100 mL of gas layer filled with CO2 is applied for quantitative test of the generated product CH4 and CO. The pressure in the reactor is almost the same as the atmosphere. The cooling water in the outer layer of reactor was used to maintain the temperature balance of the whole system ((25±2) °C). Every 2 h, 0.4 mL gas of gas layer was extracted to measure the produced amount of CH4 and CO by a gas chromatograph (GC7890F, Shanghai Techcomp Instrument Co., Ltd.). The chemical reagents were all of analytical grade in the experiments and water was deionized water (>18.2 MΩ cm).


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The quantum efficiency (QE) is estimated using CH4 and CO yield noting that 8 and 2 electrons are required to reduce CO2 to CH4 and CO, respectively. ΦCH4 (%) =

ΦCO (%) =

× [ × ] [ ]

× [ × ] [ ]

Mole of photon =

[ × ] [ ! × "]

I is light intensity (2.0 mW cm−2 for UV); S is the irradiated area of the reactor (102 cm2); E is the photon energy, (7.8 × 10−19 J at 253.7 nm); NA is the Avogadro number (6.02 × 1023 mol−1). 2.4. Calculation. The calculations were carried out by a first-principle calculation software package

CASTEP.

Generalized

gradient

approximation

(GGA)19-20

based

density-functional theory (DFT) was used to calculate the electronic band structure and density of states (DOS) for pure TiO2, TiO2-Pd and TiO2-Mn. An anatase TiO2 model of 48 atoms with exposed (101) facet is created. The vacuum lamb has almost the same thickness as the anatase slab. For TiO2-Pd and TiO2-Mn, one doping ion is linked with two surface bridge O ions on the (101) facet. The pseudopotential used is Vanderbilt-type ultrasoft pseudopotential with 2s22p4, 3s23p63d24s2, 4d10 and 3d54s2 as the valence-electron configurations for the oxygen, titanium, palladium and manganese atoms, respectively. The plane wave energy cutoffs were taken to be 420 eV. In all the cases, geometry optimizations were carried out first, and convergence was assumed when the forces on atoms were less than 50 meV/Å. Compared with the experimental results, the theoretical calculation usually results in an underestimated band gap, caused by the shortcoming of the exchange-correction functional in describing the excited states21-22.

3. Results and Discussion ACS Paragon Plus Environment


anatase

Intensity(a.u.)

¨

● rutile

Intensity(a.u.)

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¨ ¨ ●

TiO2-Pd-Mn ●

¨

24.0

●

¨

24.5

25.0

25.5

26.0

26.5

27.0

2theta(degree)

TiO2-Pd TiO2-Mn TiO2 10

20

30

40

50

60

70

80

2theta(degree)

Figure 1.XRD patterns of pure TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn. Inset is the enlarged XRD peaks of crystal plane (101). Table 1. Cell Parameters, Crystal Size and Specific Surface Area of pure TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples. Cell parameters Cell

Crystallite

SBET

volume(Å3)

size(nm)

(m2 /g)

(Å)

Sample

WRutile (Phase

Band Gap

Composition)

(eV)

a =b

c

TiO2

3.783

9.507

136.15

18.1

54

10.98%

3.0

TiO2-Pda

3.784

9.509

136.19

10.9

74

5.60%

2.5

TiO2-Mna

3.786

9.484

136.15

11.6

69

8.48%

2.8

TiO2-Pd-Mna

3.792

9.485

136.16

11.2

70

7.91%

2.3

a

the Pd and Mn amount calculated from the XPS is estimated to be about (3.77%，0), (0, 7.47%) and (5.22%,

10.20%) for TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn, respectively.

To investigate the crystal structure of the obtained samples, XRD patterns of pure TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples are shown Figure 1. Figure S1 and Figure S2 show the XRD patterns of TiO2-Pd x% and TiO2-Mn x%. All samples (Figure 1, Figure S1 and Figure S2) exhibit major anatase structure23. Moreover, the weak peaks ascribed to rutile at 27.3°, 36.0°, 41.3° are also observed, respectively24. No other characteristic diffractive peaks of palladium and manganese species, such as


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PdO, MnO, and Mn2O3 etc, were observed in the TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn. The lattice parameters, cell volumes and crystal size for TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples are calculated and summarized in Table 1. The results indicate that the specific surface areas (BET) increase and the crystallite size of the catalysts decreases, meaning that the introduced Pd and/or Mn can inhibit the grain growth of TiO2 remarkably. The diffractive peak (Inset of Figure 1), the lattice parameters and cell volume (Table 1)of TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn are almost the same as TiO2, implying the introduced Pd or/and Mn didn’t enter into the TiO2 lattice. Based on the doping mechanism, the Pd and Mn ions can hardly be doped into the crystal cell of TiO2 in interstitial mode, due to the large ionic radius (Pd2+: 86 pm13 , Mn2+ : 80 pm,25 Ti4+: 68 pm)26. If the Pd or/and Mn was doped in substitutional mode by replacing lattice Ti4+, the increased lattice parameters and cell volume are expected for TiO2-Pd, TiO2-Mn, and TiO2-Pd-Mn. However, the increase of lattice parameters and cell volume and shift of the diffractive peaks are not found in Figure 1 and Table 1 for TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn. Thus, the introduced Pd or/and Mn ions are not doped in TiO2 in substitutional mode. Accordingly, it becomes reasonable to infer that the Pd and Mn could form some species on surface of TiO2 for TiO2-Pd, TiO2-Mn, and TiO2-Pd-Mn samples. TiO2-Pd-Mn TiO2-Mn TiO2-Pd TiO2

Intensity (a.u.)

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200

400

600

Raman Shift (cm-1)

800

Figure 2. Raman spectra of TiO2, TiO2-Pd, TiO2-Mn, and TiO2-Pd-Mn.

To further determine the crystal structure of samples, the Raman patterns of the TiO2, TiO2-Pd, TiO2-Mn, and TiO2-Pd-Mn samples are shown in Figure 2, and the



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Raman patterns of TiO2-Pd x% and TiO2-Mn x% are also shown in Figure S3 and Figure S4, respectively. All samples show the typical characteristic bands at about 142 cm-1, 195 cm-1, 395 cm-1, 515 cm-1 and 637 cm-1, attributed to the Eg, B1g, A1g, B2g and Eg vibrational modes of anatase27, respectively. Compare with pure TiO2, the peak position of Raman peaks remain unchanged for TiO2-Pd x%, TiO2-Mn x% and TiO2-Pd-Mn samples, indicating both Pd and Mn ions don’t enter into TiO2 lattice, and may exist on the surface of TiO2. In addition, a new Raman peak at about 302 cm-1 in TiO2-Pd is observed, ascribed to stretching modes of O-Pd bonds on surface. Another new Raman peak at about 364 cm-1 is observed for TiO2-Mn, ascribed to stretching modes of O-Mn bonds on surface28. Both the Raman peaks at about 302 cm-1 and 364 cm-1 are also observed for TiO2-Pd-Mn samples, suggesting the co-existence of -O-Pd-O- and -O-MnO- species on surface of TiO2-Pd-Mn. The chemical states of Pd and Mn are further investigated by the XPS spectra for TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples. Figure 3a shows the Pd 3d XPS spectra of TiO2-Pd and TiO2-Pd-Mn sample. The double peaks at 342.0 and 336.7 eV was assigned to the Pd 3d3/2 and 3d5/2 of -OPd-O- structure on the surface of TiO2, which locates between Pd (335.2 eV)29 and PdO (337.2 eV)30. It has been demonstrated by our previous work that the Pd2+ ions linked with two unsaturated O2- ions to form the O-Pd-O species16. For Figure 3b, the double peaks at 640.8 and 650.1 eV is ascribed to the Mn 2p3/2 and Mn 2p1/2 of the Mn species on the surface of TiO2-Mn and state

of

Mn

in

this

work

is

+2.

TiO2-Pd-Mn, respectively. The As the binding

valence

energy of Mn 2p3/2 is

between that of Mn (638.9 eV)31 and MnO (641.4 eV)32 , imply the Mn ion for TiO2-Mn and TiO2-Pd-Mn linked with less O atoms than that for MnO. The XPS spectra of TiO2-Pd x% and TiO2-Mn x% samples are also tested and shown in Figure S5. According to the discussion above, it can be deduced that the introduced Mn atom are linked with two unsaturated oxygen atom to form -O-Mn-O- species on surface of TiO2-Mn and TiO2-Pd-Mn. Therefore, the introduced Pd and Mn ions exist as surface species (-O-M-O-) on the surface of TiO2.


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(a)

336.7 eV

(b)

342.0eV

650.1eV

640.8 eV

Intensity(a.u.)

TiO2-Pd

Intensity(a.u.)

TiO2-Pd-Mn

TiO2-Mn

TiO2-Pd-Mn

335

340

345

635

640

645

650

655

Binding Energy(eV)

Binding Energy(eV)

Figure 3. (a) XPS Pd 3d spectra of TiO2-Pd and TiO2-Pd-Mn; (b) XPS Mn 2p spectra of TiO2-Mn and TiO2-Pd-Mn.

Figure 4. TEM images of TiO2-Pd-Mn. TEM image of TiO2-Pd-Mn is shown in Figure 4, to observe the morphology of TiO2-Pd-Mn nanoparticles directly. The fringe spacing (d) for (101) plane is estimated to be 3.52 Å, which is almost the same as the pure TiO2 samples10. It is found that the TiO2-Pd-Mn samples consist of TiO2 nanoparticles with an average diameter of about 12 nm, which is consistent with XRD results in Table 1. Moreover, there is no other phase, such as PdO or MnOx observed in the TEM image. These TEM results are in good agreement with the discussion above. (a)

(a)

4

Sum O 2p Ti 3d

3

DOS(electrons/eV)

2

Energy(eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 0 -1 -2 -3 -4

G

F

Q

Z

G

-4

-2

0

Energy(eV)


2

4


(b)

(b) Sum O 2p Ti 3d Pd 4d

DOS(electrons/eV)

Energy(eV)

2

0

-2

-4

G

F

Q

Z

G

-6

-4

-2

0

2

Energy(eV)

(c)

3

(c) Sum O 2p Ti 3d Mn 3d

2 1

DOS(electrons/eV)

Energy(eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 -1 -2 -3 -4

G

F

Q

Z

G

-6

-4

-2

0

2

Energy(eV)

Figure 5. Theoretically calculated band structure and density of states for TiO2 (a), TiO2-Pd (b) and TiO2-Mn (c). According to the experimental results above, the models of pure TiO2, TiO2-Pd and TiO2-Mn are designed and DFT calculation was performed. Band structure and density of states for pure TiO2, TiO2-Pd and TiO2-Mn are shown in Figure 5. For pure TiO2 (Figure 5a), TiO2 exhibited an indirect band gap of 2.820 eV, because the conduction band bottom and valence band top were at the different K point. The zero energy level, corresponding to highest state level that electrons occupied, located at the valence band top. And the valence band was dominated by O 2p orbitals with a small fraction of Ti 3d, and the conduction band mainly consisted of the Ti 3d orbitals, hybridized with small amount of O 2p. After the introduction of Pd or Mn, the density of states for valence band and the conduction band for TiO2-Pd and TiO2-Mn almost remain unchanged, compared with pure TiO2. However, For TiO2-Pd (Figure 5b), some new energy levels occurs above the valence band and below the conduction band respectively, owing to the introduced Pd ions. Moreover, it is found from the DOS that a small hump occurs in the middle of band gap, which is attributed to the mixed Pd 4d state with the Ti 3d and O 2p state. For TiO2-Mn (Figure 5c), a new energy level is observed below the conduction band, due to the Mn 3d state hybridized with Ti 3d and O 2p states. These results imply the modification of Pd or/and Mn on TiO2 surface might improve the absorption in visible light region and facilitate the separation of charge carriers. The energy level of the


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introduced Pd and Mn species matches the redox potential of photo-reduction CO2 into CH4, benefiting the photocatalytic activity. To verify the calculation predications, the absorption spectra, XPS valence band spectra as well as PL spectra were carried out as follows.

(b)

(a)

TiO2 TiO2-Pd

TiO2-Pd-Mn

TiO2-Pd

TiO2-Mn

Ef 0

TiO2-Mn TiO2-Pd-Mn

Intensity (a.u.)

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO2 2

4

6

Bingding Energy(eV)

8

300

400

500

600

700

Wavelength (nm)

Figure 6. (a) XPS valence band spectra and (b) Diffuse reflectance UV-Vis spectra for pure TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn.

The XPS valence band spectra of pure TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn are shown in Figure 6a. The energy levels were in alignment with the work function of the XPS instrument (4.10 eV, Fermi level). The binding energy of the onset edge revealed the energy gap between the Fermi level and valence band top.33 The valence band top was at 2.75 eV (2.35 V, vs NHE) for TiO2. However, it is found from Figure 6a that the onset edge for TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn is almost the same as that of pure TiO2, indicating the introduced Pd or/and Mn on surface have no effect on the valence band of TiO2. Moreover, compared with pure TiO2 a small hump occurs close to the valence band for TiO2-Pd, ascribed to the energy level of -O-Pd-Ospecies. There is no hump observed for the valence band spectra for TiO2-Mn, implying the energy level of -O-Mn-O- species exist next to the conduction band of TiO2. For TiO2-Pd-Mn, a small hump is also found, attributed to the energy level of -O-Pd-O- species. These XPS valence band results are in good agreement with the theory calculation above. Diffuse reflectance UV−vis absorption spectra are shown in Figure 6b for pure TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples. For pure TiO2, the strong absorption around 340 nm was caused by band-band transition. The band gap is 3.0 eV, as onset edge is about 412.9 nm34. Hence, the conduction band bottom for TiO2



was about -0.65 V (vs NHE). Compared with TiO2, onset edge of TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples remain almost unchanged, indicating the introduction of the metal ions (Pd or/and Mn) into surface has no influence on the band gap of TiO2. However, the strong absorption in the visible region is enhanced greatly for TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples, due to the introduction of the metal ions (Pd or/and Mn) on TiO2 surface. According to theoretically calculation and XPS valence band spectra, for TiO2-Pd, the strong peak at around 496 nm tailing the visible-light region is

ascribed

to the electrons transition

from energy levels of

O-Pd-O species to the conduction band or from the valence band of TiO2 to the O-Pd-O

energy

level

next

to

the

band. The energy levels of O-Pd-O species located

conduction at about

0.5 eV above the valence band. For TiO2-Mn, the strong absorption around 435 nm was ascribed to electrons transition from the valence band to the energy levels of O-Mn-O species which is determined at around 0.2 eV below the conduction band. Moreover, TiO2-Pd-Mn shows the strongest absorption in visible region, which benefits for the photocatalytic activity, owing the synergetic effect of -O-Pd-O- and -O-Mn-O- species on surface. The absorption spectra of TiO2 modified with different amounts of metal ions (Pd or Mn) are given in Figure S6. The absorption in visible region is further enhanced with the increase of Pd or Mn amount. According to the discussion above, the band structure of TiO2-Pd-Mn photocatalyst can be determined, shown in Figure 9.

TiO2 TiO2-Pd TiO2-Mn TiO2-Pd-Mn

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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400

450

500

550

600

Wavelength(nm)

Figure 7. Photoluminescence spectra of pure TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples.


Page 13 of 21

The behaviors of photogenerated charge carriers for pure TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples are investigated by the PL spectra, shown in Figure 7. The PL peaks can be attributed to the transition related to the oxygen vacancies (defects) to the valence band35-38. The excited electrons on the conduction band of TiO2 first transferred to the defect levels via non-radiative transition, and recombined with the holes in valence band then, leading to the emission of ﬂuorescence39. Hence, the decreased PL intensities indicate the recombination of the carriers was inhibited. As shown in Figure 6, the PL intensities of TiO2-Pd and TiO2-Mn are weakened in comparison with pure TiO2, as the charge carriers are captured by the -O-Pd-O- and -O-Mn-O- surface species, respectively. The PL intensity of TiO2-Pd-Mn further decreases because of the synergetic effect of -O-Pd-O- species and -O-Mn-O- species. Therefore, It can be concluded that the introduction of Pd or/and Mn can inhibit the recombination of charge carriers, benefiting the photocatalytic activity. 6

(a)

TiO2-Pd-Mn TiO2-Pd

5

-6 Amount of CH4(10 mol)

TiO2-Mn TiO2

4

Blank

3 2 1 0

(b)

TiO2-Pd-Mn TiO2-Pd

24

Amount of CO(10-6mol)

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20

TiO2-Mn TiO2

16

Blank

12 8 4 0

0

2

4

6

8

10

12

0

2

4

6

8

10

12

Time(h)

Time(h)

Figure 8. Photocatalytic results of photo-reduction of CO2 into CH4 (a) and CO (b) of TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn samples.

The photo-reduction of CO2 into methane under UV irradiation is applied to evaluate photocatalytic activity of the obtained photocatalysts. In this experiment, the intermediate product is carbonic monoxide and the final product is methane40. The results of TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn photocatalysts were plotted in Figure 8 and Table 2. After 12 hours’ irradiation, 1.52 µmol of CH4 is produced for pure TiO2 and 1.13 µmol of CH4 is generated without any catalyst (Blank experiment). In comparison with pure TiO2, the TiO2-Pd and TiO2-Mn samples show enhanced photocatalytic performance, and 4.43 and 3.25 µmol of CH4 was generated.



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However, it is observed that the TiO2-Pd-Mn sample exhibits the highest photocatalytic activity, whose the generation amount of CH4 (5.51 µmol) is 3.6 times as that for pure TiO2. The photo-reduction results of modified TiO2 with different amount of Pd or Mn are shown in Figure S7 and Figure S8. The above results suggest that the Pd and Mn co-modified TiO2 is in favor of improvement of the photocatalytic activity than single modified TiO2 for photoreduction of CO2 into CH4. The enhanced mechanism will be discussed in the following sections. Table 2. Photocatalytic activity of pure TiO2, TiO2-Pd, TiO2-Mn and TiO2-Pd-Mn under UV light irradiation for 12 h sample

CH4

Specific

Quantum

CO

generation

photocatalytic

efficiency

amount(10-6mol)

amount

activity a

of CH4

-6

(10 mol)

-6

-1

-1

(10 mol·g ·h )

generation

specific

Quantum

photocatalytic

efficiency

activity b

of CO

-6

(%)

-1

-1

(10 mol·g ·h )

(%)

blank

1.13±0.05

0.75±0.03

0.38

12.53±0.37

8.35±0.27

1.60

TiO2

1.52±0.06

1.01±0.04

0.51

10.43±0.46

6.95±0.31

1.33

TiO2-Pd

4.43±0.17

2.96±0.11

1.51

14.73±1.36

9.82±0.95

1.88

TiO2-Mn

3.25±0.18

2.17±0.12

1.11

22.46±1.83

14.97±1.22

2.87

TiO2-Pd-Mn

5.51±0.13

3.67±0.09

1.88

17.90±0.80

11.94±0.53

2.29

a

specific photocatalytic activity of CH4, CH4 generation amount per unit mass catalyst per hour.

b

specific photocatalytic activity of CO, CO generation amount per unit mass catalyst per hour.

The mechanism of photocatalytic reduction of CO2 into CH4 in the presence of water can be shown in equation (1)-(3)41-45. Under UV irradiation (hν), the electrons and holes are excited (1); The H2O molecules absorbed on photocatalyst’s surface are oxidized by photogenerated holes (h+) to form O2 and H+ (2). Electrons reacted with the surface absorbed CO2 to form CO and O2. Electrons transferred from the conduction band of TiO2 would reduce the CO to form carbon radicals (•C) which further react with H+ to produce CH4 (3) ( -0.24V ,vs NHE). It is noted that some of the product may inhibit the photocatalytic reaction by blocking the surface active sites. Part of the product could also recombine in some kind of reverse reaction process43. For example, when CO and O2 are generated during reaction and are not involved in photo-reduction of CH4, they msy react with each other back into CO2. Catalyst + hν → e- + h+


(1)

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H2O + h+→•OH + H+ •OH + H2O + 3h+ → O2 + 3H+ 2H2O + 4h+ → O2+ 4H+

Eoredox = 0.82 V vs NHE

2CO2 + 4e- → 2CO + O2

Eoredox = -0.12V vs NHE

(2)

2CO + 4e- → 2•C + O2 •C + 4e- + 4H+ → CH4

Eoredox = 0.13V vs NHE

CO2 + 8e- + 8H+ → CH4 + 2H2O

Eoredox = -0.24 V vs NHE

(3)

The photocatalytic mechanism of TiO2-Pd-Mn photocatalyst with high photocatalytic activity can be explained with the schematic band structure (Figure 9). For TiO2, the valence band (2.35V, vs NHE) and conduction band (-0.65 V, vs NHE) is more positive than Eoredox (H2O/H+, 0.82 V, vs NHE) and more negative than Eoredox (CO2/CH4, -0.24 V, vs NHE), respectively. So the electrons and holes of TiO2 can precipitate in the catalytic reaction on photoreduction of CO2 into CH4. Pure TiO2 shows a poor photocatalytic activity on reduction of CO2 into CH4, due to its poor visible response and the relatively high recombination rate of the photogenerated charge carriers. However, the photocatalytic activity of TiO2-Pd-Mn is greatly improved compared with TiO2, TiO2-Pd and TiO2-Mn, owing to the synergistic effect of energy levels for -O-Pd-O- and -O-Mn-O- surface species in band gap. For TiO2-Pd-Mn, the strong absorption in visible region is enhanced and more photogenerated charge carriers are induced. In addition, as the energy level of -O-Mn-O- surface species (-0.50 V, vs NHE) and the energy level of -O-Pd-Osurface species (1.85 V, vs NHE) for TiO2-Pd-Mn, are more matching with the redox potential for photoreduction of CO2 into CH4 (Eo=0.82V and -0.24V, vs NHE, respectively) than the valence band and conduction band of TiO2. The photogenerated holes of valence band could move to the surface energy levels of O-Pd-O species, and photogenerated electrons in conduction band could transfer to the energy levels of surface species, leading to a more efficient separation and utilization of photogenerated carriers. As result, more photogenerated electrons and holes enriched in O-Mn-O and O-Pd-O species respectively, which react as reactive sites, can take part in the photocatalytic reaction on surface. Therefore, the TiO2-Pd-Mn



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photocatalyst exhibits a much improved photocatalytic activity in comparison with TiO2, TiO2-Pd and TiO2-Mn for photoreduction of CO2 into CH4. In comparison with TiO2, the photocatalytic activity of TiO2-Pd and TiO2-Mn is slightly enhanced, due to the only existence of the -O-Pd-O- or -O-Mn-O- species on surface of TiO2-Pd and TiO2-Mn, respectively.

Figure 9. Schematic diagram of photocatalytic mechanism for TiO2-Pd-Mn photocatalyst.

4. Conclusion A new type of Pd and Mn co-modified TiO2 photocatalyst (TiO2-Pd-Mn) has been prepared by a simple sol-gel method. According to the theoretical calculation and the experimental results, it was revealed that the -O-Pd-O- and -O-Mn-O- species exist on surface, whose energy levels are next to the conduction band and valence band of TiO2, respectively. The surface O-Pd-O and O-Mn-O species could extend the visible response and separate the charge carriers, enabling more photogenerated electrons and holes take part in the photocatalytic reaction. Thus, the TiO2-Pd-Mn exhibits a higher photocatalytic activity than pure TiO2, TiO2-Pd and TiO2-Mn on photoreduction of CO2 into CH4. Supporting Information. XRD, Raman, XPS, Absorption spectra, Photocatalytic Activity and Spectrum of the UV lamp. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Tel: 86 22 66229419. E-mail: [email protected]. Notes


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