# Enhanced Photoreduction Activity of Carbon Dioxide over Co3O4/CeO2 Catalysts under Visible Light Irradiation

Authors: Ying Huang; Chang-Feng Yan; Chang-Qing Guo; Shi-Lin Huang
Journal: International Journal of Photoenergy
 (2015)

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## Abstract

A series of new two semiconductor catalysts, Co3O4/CeO2, were prepared by glycine-nitrate combustion method for photocatalytic reduction of carbon dioxide to produce methanol and ethanol under visible light (λ
>
400 nm) irradiation. The catalysts were characterized by BET, UV-vis spectra, XRD, SEM, PL, and XPS and the results indicated that the catalyst with 5 wt.% of Co3O4 has the highest yield among all kinds of tests with the methanol yield of 1.52 μmol·g−1·h−1 and the ethanol yield of 4.75 μmol·g−1·h−1, which are about 2.34 and 1.71 times as large as those of CeO2. However, methanol and ethanol can hardly be detected for Co3O4 under the same condition because of its too narrow band gap. The improvement of the photoreduction activity of Co3O4 doped CeO2 was caused by the separation of electron-hole pairs of Co3O4/CeO2 and charge transfer between Co3O4 and CeO2, mimicking the Z-scheme in photosynthesis.


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## Body

## 1. Introduction

Carbon dioxide, one of the major greenhouse gases which is largely formed by fossil fuels consumption, can be conversed back to fuel with water or hydrogen by thermochemical or photocatalytic approach [1]. Photocatalytic reduction of carbon dioxide to methanol or other organic fuels, which is a process to both reduce carbon dioxide emissions and resolve energy crisis, has been proved to be a prospective way to convert solar energy by using semiconductor catalysts, like TiO2 [2–4], CdS [5], SiC [6], g-C3N4 [7, 8], and graphene oxide [9, 10]. Due to shortages of narrow band gap of single semiconductor catalyst, two-semiconductor ones such as Co3O4/CeO2 are expected to be explored in order to improve photocatalytic properties and to widen the responsive wavelength range for visible light, thus enhancing the total efficiency of photocatalysis.Among various semiconductor photocatalysts, CeO2 has some properties similar to titania catalyst such as wide band gap, nontoxicity, and high stability [11]. However, the main light absorption region of CeO2 lies in the near ultraviolet and its low quantum efficiency and high recombination rate of electrons holes also constrain its development [12]. Due to the special f and d electron orbital structure of cerium atom, some metal oxide semiconductors like TiO2 [13], Bi2O3 [14], and Cu2O [11] can be easily doped with pure CeO2 to enhance visible light absorption and quantum efficiency. Cobalt-based catalysts have been recently reported as photocatalysts for water oxidation [15–18], hydrogen production [19], and organic pollutants degradation [20–22]. Co is significantly less abundant than other transition metals like Fe, Mn, and Ni, but it is emerging as a potential metal for catalytic processes because of its light harvesting and electron mediating properties [19]. Cobalt oxides, for example, CoO and Co3O4, are p-type semiconductors with interesting electronic and magnetic properties [23]. The p-n heterojunction semiconductor structure combined with cobalt oxide and other n-type semiconductors such as BiVO4 [23] or WO3 [20] has been investigated as an effective way to improve the photocatalytic activities under visible light irradiation. In this case, CeO2 as an n-type semiconductor with wide band gap may possibly bring out a similar heterojunction structure with Co3O4 to enhance the efficiency of visible light absorption by shortening the band gap, in which such p-n heterojunction structure can hardly work to reduce carbon dioxide according to the band theory because the conduction band potential of Co3O4 is too positive to start the reduction. However, CeO2 is also a good reducing agent because of its relatively more negative reductive potential (around −0.5 eV) and Co3O4 doped CeO2 with shorter band gap (2.07 eV) can act as a good cocatalyst, sensitizer, and oxidation agent of the composite with high positive potential (+2.44 eV) in the Z-scheme reaction mimicking photosynthesis. Recent studies on Z-scheme without mediators or so-called direct Z-scheme were carried out to explain charge separation at the interface between two semiconductors in a hybrid photocatalyst [24–27]. Further application of Co3O4/CeO2 photocatalytic reduction of carbon dioxide to methanol and ethanol in the Z-scheme model has not been reported, which could enhance photocatalytic activity.In this paper, a series of Co3O4/CeO2 catalysts were prepared by glycine-nitrate combustion method for photocatalytic reduction of carbon dioxide to methanol and ethanol under visible light (λ
>
400 nm) irradiation. The structures of catalysts were characterized by XRD, UV-vis absorption spectra, BET analysis, and SEM testing. And a series of measurements, such as XPS and PL, were carried out for the mechanism of improvement of the photoreduction activity of Co3O4 doped CeO2.


## 2. Experimental

### 2.1. Preparation of Co3O4/CeO2 Catalysts

The Co(NO3)2·6H2O and Ce(NO3)3·6H2O were mixed in the appropriate molar ratios with a minimum volume of distilled water to form a transparent solution. Then the glycine solution was slowly dropped to the metal nitrate aqueous solution during stirring. The glycine to nitrate ratio was set to 0.3 as the literature described [28]. The resulting solution was then kept stirring for an hour in order to gain transparent sol. Afterwards, the crucible was dried at 75°C in an oven for more than six hours until a transparent moisture-sensitive glassy material was obtained. This glassy material with fine mesh was heated in a muffle furnace up to around 180°C, and then it generated spontaneous combustion with large amounts of gases evolution and a foamy voluminous powder. Then the powder was calcined at 300°C or 400°C or 500°C or 600°C for an hour to burn off any carbonaceous residues in the catalyst. In addition, CeO2 was prepared by the same method without adding Co(NO3)2·6H2O.


### 2.2. Catalyst Characterization

Specific surface area, average pore diameter, total pore volume, and pore size distribution of the samples were determined from the adsorption and desorption isotherms of nitrogen at 77 K after outgassing procedure under vacuum at 250°C for 10 h, using a Quantachrome Autosorb-1 instrument. X-ray diffraction (XRD) analysis was performed with the PANalytical X’Pert diffractometer (X’Pert PRO MPD, PW3040/60) within the 2-θ ranging from 5° to 80° by a speed of 6° per minute with Cu-Kα (λ = 0.154060 nm) radiation (40 kV, 40 mA). The crystallite size was calculated by the Scherrer equation from the XRD spectra. The surface morphology was observed by the field emission-scanning electron microscope (FE-SEM, S-4800). UV-vis spectroscopy in the 300 nm–1000 nm was measured with a Lambda 750 Diffuse Reflectance Spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a thermo ESCALAB 250XI multifunctional imaging electron spectrometer (Thermo Fisher Scientific Inc.) equipped with an Al Kα radiation source. Fluorescence spectra were obtained using a photoluminescence (PL) spectrometer (Perkin Elmer, LS-55). The concentration of methanol and ethanol was detected by a gas chromatograph (GC-7890II), equipped a flame ionization detector and a stainless steel packed column (PorapakQ, 2 mm × 1 m).


### 2.3. Photocatalytic Reaction Testing

The photocatalytic reaction was performed in a continuous-flow reactor system as shown in Figure1. A 300W Xe lamp located in the center of the quartz cool trap was the light source and the UV light wavelength below 400 nm was filtered by a 2.0 M sodium nitrite solution layer between light source and reacting liquid [4]. Prior to experiment, sodium carbonate (3.18 g) and sodium sulfite (3.78 g) were dissolved in 300 mL deionized water. In this paper, sodium sulfite and sodium sulfide play a role as the sacrifice agent to scavenge photogenerated holes. Then the solution was injected into the reactor. Before irradiation, ultrapure carbon dioxide was bubbled through the solution in the reactor for at least 30 minutes to ensure that all dissolved oxygen was eliminated while adsorption and desorption of carbon dioxide in the solution and on the photocatalyst reached the equilibrium. 300 mg of catalyst powder was then added into above solution in the reactor for a five-hour irradiation. The solution of about 1 mL in the reactor was withdrawn as a sample each hour.Figure 1
Schematic view of reactor system for photocatalytic reduction of carbon dioxide.


## 2.1. Preparation of Co3O4/CeO2 Catalysts

The Co(NO3)2·6H2O and Ce(NO3)3·6H2O were mixed in the appropriate molar ratios with a minimum volume of distilled water to form a transparent solution. Then the glycine solution was slowly dropped to the metal nitrate aqueous solution during stirring. The glycine to nitrate ratio was set to 0.3 as the literature described [28]. The resulting solution was then kept stirring for an hour in order to gain transparent sol. Afterwards, the crucible was dried at 75°C in an oven for more than six hours until a transparent moisture-sensitive glassy material was obtained. This glassy material with fine mesh was heated in a muffle furnace up to around 180°C, and then it generated spontaneous combustion with large amounts of gases evolution and a foamy voluminous powder. Then the powder was calcined at 300°C or 400°C or 500°C or 600°C for an hour to burn off any carbonaceous residues in the catalyst. In addition, CeO2 was prepared by the same method without adding Co(NO3)2·6H2O.


## 2.2. Catalyst Characterization

Specific surface area, average pore diameter, total pore volume, and pore size distribution of the samples were determined from the adsorption and desorption isotherms of nitrogen at 77 K after outgassing procedure under vacuum at 250°C for 10 h, using a Quantachrome Autosorb-1 instrument. X-ray diffraction (XRD) analysis was performed with the PANalytical X’Pert diffractometer (X’Pert PRO MPD, PW3040/60) within the 2-θ ranging from 5° to 80° by a speed of 6° per minute with Cu-Kα (λ = 0.154060 nm) radiation (40 kV, 40 mA). The crystallite size was calculated by the Scherrer equation from the XRD spectra. The surface morphology was observed by the field emission-scanning electron microscope (FE-SEM, S-4800). UV-vis spectroscopy in the 300 nm–1000 nm was measured with a Lambda 750 Diffuse Reflectance Spectroscopy. X-ray photoelectron spectroscopy (XPS) analysis was conducted on a thermo ESCALAB 250XI multifunctional imaging electron spectrometer (Thermo Fisher Scientific Inc.) equipped with an Al Kα radiation source. Fluorescence spectra were obtained using a photoluminescence (PL) spectrometer (Perkin Elmer, LS-55). The concentration of methanol and ethanol was detected by a gas chromatograph (GC-7890II), equipped a flame ionization detector and a stainless steel packed column (PorapakQ, 2 mm × 1 m).


## 2.3. Photocatalytic Reaction Testing

The photocatalytic reaction was performed in a continuous-flow reactor system as shown in Figure1. A 300W Xe lamp located in the center of the quartz cool trap was the light source and the UV light wavelength below 400 nm was filtered by a 2.0 M sodium nitrite solution layer between light source and reacting liquid [4]. Prior to experiment, sodium carbonate (3.18 g) and sodium sulfite (3.78 g) were dissolved in 300 mL deionized water. In this paper, sodium sulfite and sodium sulfide play a role as the sacrifice agent to scavenge photogenerated holes. Then the solution was injected into the reactor. Before irradiation, ultrapure carbon dioxide was bubbled through the solution in the reactor for at least 30 minutes to ensure that all dissolved oxygen was eliminated while adsorption and desorption of carbon dioxide in the solution and on the photocatalyst reached the equilibrium. 300 mg of catalyst powder was then added into above solution in the reactor for a five-hour irradiation. The solution of about 1 mL in the reactor was withdrawn as a sample each hour.Figure 1
Schematic view of reactor system for photocatalytic reduction of carbon dioxide.


## 3. Results and Discussion

### 3.1. Characterization of Samples with Different Cobalt Loading

Figure2 shows the X-ray diffraction patterns of Co3O4/CeO2 photocatalysts prepared by glycine-nitride combustion method and calcined at 400°C, along with CeO2 and Co3O4 which were prepared at the same conditions. Diffraction peaks corresponding to the hexagonal phases of CeO2 are present for all Co3O4/CeO2 photocatalysts with different mass faction of Co3O4 loading. Six main reflections belong to a typical fcc fluorite structure of CeO2 (ICSD-169029), corresponding, respectively, to the [
111
], [
200
], [
220
], [
311
], [
222
], and [
400
] planes. And ten main reflections belong to Co3O4 (ICSD-069366) as shown in Figure 2. With the increase of the Co3O4 from 1 wt.% to 20 wt.%, the diffraction peaks of CeO2 become weaker, which indicates a continuous decrease in the crystallization degree of CeO2. The diffraction peaks of Co3O4 are hard to be detected in all Co3O4/CeO2 catalysts. The reasons why the strong Co oxides peaks absence had been discussed in our previous work [29], which may be attributed to (1) highly dispersed Co3O4 on the surface of ceria, (2) the formation of Co-Ce-O solid solution, or (3) a combination of two points stated above. However, some weak diffraction peaks for Co3O4 can still be noted, which suggests the existence of Co3O4 on the surface of particles. Either high dispersion of Co3O4 on the surface of CeO2 or formation of Co-Ce-O solid solution indicates CeO2 doped with Co3O4 instead of mixed oxides formed.Figure 2
XRD pattern of Co3O4/CeO2 photocatalysts with different mass faction of Co3O4 doping.The UV-vis spectra of the catalysts are shown in Figure3. The light absorption of all catalysts covers the visible region. The band gap of CeO2 is estimated to be 2.93 eV from the plot of the (
α
h
ν
)
2 versus photon energy (h
ν) [4, 6] as shown in the inset of Figure 3(a), which is 0.27 eV narrower than that of the commercial CeO2 with the band gap of 3.2 eV. It has been reported that CeO2, as an n-type semiconductor, has band gap varying from 2.7 to 3.4 eV depending on the preparation method [30]. The red shift of the CeO2 sample might be attributed to trivalent ionic cerium compounds. Similarly, the band gap of Co3O4 is estimated to be 2.0 eV, which is matched with the reported band gap (2.07 eV) [20]. In contrast to CeO2, the Co3O4/CeO2 have strong absorption in the visible region (λ
>
400
n
m) and the visible light absorbance increases along with the addition of Co3O4 (Figure 3(c)).Figure 3
UV-vis absorption spectra of CeO2 and commercial CeO2 catalyst (a), Co3O4 (b), and the Co3O4/CeO2 photocatalysts (c).
(a)
(b)
(c)In order to investigate the electronic environment and oxidation state of the CeO2 sample, XPS analysis was performed. Eight peaks for Ce 3d of CeO2 sample are shown in Figure 4(a), which are different from those of the reported CeO2 with six peaks at binding energies 882.5, 888.7, 898.2, 900.7, 907.6, and 916.5 (±0.2) eV [31, 32]. Other authors have proved that the shape of Ce 3d peaks changes after normalizing the intensity of Ce 3d peaks for Ce2O3 and CeO2 [32] when Ce2O3 is mixed or doped with CeO2. And the peaks can be divided into more than six peaks, which may be attributed to the presence of Ce3+ [33]. Among all of these ten peaks in Figure 4(a), v, v
′
′, v
′
′
′, u, u
′
′, and u
′
′
′ are attributed to the presence of Ce(IV), and v
0, v
′, u
0, and u
′ are attributed to the presence of Ce(III) [34]. Three peaks for O 1s of CeO2 sample are shown in Figure 4(b). Two peaks at 529.7 and 531.3 eV are attributed to O-Ce(IV) bond and O-Ce(III) bond, respectively [34], which indicates the existence of both CeO2 and Ce2O3 [35]. In this case, Ce2O3 with much shorter band gap of 2.4 eV can shorten the band gap of CeO2 and enhance the visible light absorption, which is in line with the UV-vis spectra.Figure 4
XPS spectra of the Ce 3d region (a) and the O 1s region (b) for the CeO2 sample.
(a)
(b)Table1 displays the specific surface area, the pore volume, the total pore volume of pure CeO2 and Co3O4, and the catalysts with Co3O4 loading of 2 wt.%, 5 wt.%, and 8 wt.%. As the Co3O4 loading increases from 0 wt.% to 100 wt.%, the surface areas decrease from 100.14 to 24.47 m2/g, and the total pore volume decreases as well. It can be attributed to the mutual filling of the pore structure during the forming of Co3O4 and CeO2 which can be proved by the pore diameter distribution in Figure 5. And pore size distribution of Co3O4/CeO2 catalysts that smaller than 10 nm decreases sharply compared to that of Co3O4 or CeO2 and mainly is distributed in mesopore region from 10 to 50 nm. When the Co3O4 loading increased from 2 wt.% to 8 wt.%, there is no distinct difference in specific surface area, average pore diameter, and total pore volume, which indicates that addition of Co3O4 in the Co3O4 loading range has little effect on the corresponding structure of Co3O4/CeO2 catalysts. The sample with 5 wt.% Co3O4 loading has biggest specific surface area and uniform pore diameter distribution among all the tested Co3O4/CeO2 catalysts. In addition, larger specific surface area, smaller pore volume, and total pore volume compared to those of commercial CeO2 suggest that CeO2 prepared by glycine-nitrate combustion method is a more potential carrier for catalysis.Table 1
Characteristic of Co3O4/CeO2 catalysts with different mass fraction of Co3O4.
Catalyst mass fraction of Co3O4 doping (%)
BET surface area (m2/g)
Average pore diameter (nm)
Total pore volume (cm3g−1)
0 (CeO2)
100.14
4.68
0.12
0 (commercial CeO2)
71.41
15.22
0.27
2.0
34.69
11.41
0.11
5.0
38.28
10.19
0.10
8.0
33.20
11.65
0.10
100 (Co3O4)
25.47
13.28
0.08Figure 5
Pore diameter distribution of catalysts with different mass faction of Co3O4 doping.The average methanol and ethanol yields, denoted as the average production rate of methanol and ethanol for each hour over the photocatalysts, are shown in Table2. The photocatalytic activity of CeO2 can be well improved via loading Co3O4 on CeO2 by glycine-nitrate combustion method. No methanol and ethanol conversion was detected for Co3O4 catalyst under visible light irradiation because of its conduction band maximum being too positive to reduce carbon dioxide to methanol or ethanol. The sample with 5 wt.% Co3O4 has the best performance to produce both methanol and ethanol, which can be attributed to its better specific surface area, the proper pore volume, and pore size (Table 1). Increasing the Co3O4 loading from 0 to 5 wt.%, the methanol yield increased from 0.68 to 1.52 μmol·g−1·h−1 and the ethanol yield increased from 2.77 to 4.75 μmol·g−1·h−1. Further increasing the Co3O4 loading (>5 wt.%) leads to the decrease of the methanol and ethanol yields because of the significant decrease of the crystallization degree of CeO2 shown in Figure 2. Photocatalytic activities of the sample with 5 wt.% Co3O4 were tested in different solutions. It is obvious that solution containing 0.1 M Na2CO3 and 0.1 M Na2SO3 has the best performance among (0.1 M Na2CO3 + 0.1 M Na2SO3), (0.1 M NaOH + 0.1 M Na2SO3), and (0.1 M Na2CO3 + 0.1M Na2S), which indicates that extra carbon source (C
O
3
2
-) and better sacrifice reagent (S
O
3
2
-) can greatly enhance the photocatalytic activities for methanol and ethanol generation as shown in Table 2.Table 2
Mean methanol and ethanol yield on Co3O4/CeO2 catalysts under visible light irradiation (λ > 400 nm).
Catalyst mass fraction of Co3O4 doping (%)
Mean yield of methanolμmol·h−1
g
cat
-
1
Mean yield of ethanolμmol·h−1
g
cat
-
1
0 (CeO2)(a)
0.68
2.77
1.0(a)
1.51
2.87
2.0(a)
1.5
3.95
8.0(a)
1.22
3.15
10.0(a)
1.18
2.43
100 (Co3O4)(a)
0
0
5.0(a)
1.52
4.75
5.0(b)
0.91
2.22
5.0(c)
0.75
2.16
0.30 g catalysts reacted in 300 mL solution of (a) 0.1 M NaOH + 0.1 M Na2SO3, (b) 0.1 M Na2CO3 + 0.1 M Na2S, and (c) 0.1 M Na2CO3 + 0.1 M Na2SO3. A 300 W Xenon lamp (λ > 400 nm) irradiated for 5 hours.


### 3.2. Effects of Calcination Temperature on the Photocatalytic Activity

Calcination temperature also affects the surface structures of catalysts. The samples with 5 wt.% of Co3O4 loading were calcined at 300°C, 400°C, and 500°C, respectively. As shown in Table 3, the specific surface area decreases with the increase of calcination temperature. And the pore diameter distribution becomes uneven when catalyst calcined at 500°C. Pore diameter bigger than 984 nm can be found from Figure 6, which indicates that the particle was melted and the pore collapsed.Table 3
Characteristic of Co3O4/CeO2 catalysts with 5 wt.% Co3O4 doping at different calcination temperatures.
Calcination temperature (°C)
BET surface area (m2/g)
Average pore diameter (nm)
Total pore volume (cm3g−1)
The crystallite size of CeO2 (nm)
300
41.16
10.61
0.11
37.5
400
38.28
10.19
0.10
35.4
500
30.88
15.13
0.12
41.8Figure 6
Pore diameter distribution of catalysts under different calcination temperature.Figure7 shows the SEM images of the 5.0 wt.% Co3O4/CeO2 catalysts prepared by calcining the spontaneous combusted precursor at temperatures of 300 to 600°C when CeO2 calcined at 400°C. The shape of CeO2 particle becomes very irregular. However, more pores can be seen on the surface of the CeO2 particles (Figures 7(i) and 7(j)) compared to those of the Co3O4/CeO2 catalysts (Figures 7(a)–7(h)). It can explain why the CeO2 has larger surface area than the other samples. The particle size of the Co3O4/CeO2 catalysts, however, increased greatly with the calcination temperature higher than 400°C, as shown in Figures 7(c) and 7(d). However, the particles of the sample calcined at 300°C are more irregular and the pores on the surface are bigger than those of the sample calcined at 400°C, which is in good correspondence to BET analysis results.Figure 7
SEM images of Co3O4/CeO2 photocatalysts with 5.0 wt.% Co3O4 calcined at 300°C (a and e), 400°C (b and f), 500°C (c and g), 600°C (d and h), and CeO2 (i and j).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)In Figure3, XRD peaks for Co3O4 of Co3O4/CeO2 catalysts are absent, however, the peaks appear when the calcination temperature is higher than 400°C as shown in Figure 8. And the CeO2 crystallite size of catalysts was estimated by using the Scherrer equation, and the order is 500 (41.8 nm) > 300 (37.5 nm) > 400 (35.4 nm) as shown in Table 3. The diffraction peak at 48.6° corresponding to hexagonal Co-Ce-O phase (JCPDS file 65-5917) [36] becomes stronger with increase of calcination temperature, which suggests that the treatment by combustion method strengthens the Co-Ce interaction and the calcination temperature improves the crystallization structure of catalysts. Thus, the CeO2 crystallite size is getting smaller under calcination temperature of 400°C than under 300°C. And the enlarged size of CeO2 crystallite under 500°C is caused by sintering of particles which is in line with the SEM.Figure 8
XRD pattern of Co3O4/CeO2 photocatalysts under different calcination temperature.In order to determine the amount of Ce(III) in cerium oxides, the deconvolution method of the Ce3d features [34, 37] has been used. The amounts of Ce(III) and Ce(IV) were calculated from the components of the Ce 3d core level using the following equations [37]:(1)
C
e
I
I
I
=
v
0
+
v
′
+
u
0
+
u
′
(2)
C
e
I
V
=
v
+
v
′
′
+
v
′
′
′
+
u
+
u
′
′
+
u
′
′
′
(3)
%
C
e
I
I
I
=
C
e
I
I
I
C
e
I
I
I
+
C
e
I
V
×
100
.The calculated result shows that the amount of Ce(III) increases from 3.7% to 6.5%, the amount of Ce(IV) decreases from 32.0% to 25.9%, and the ratio of Ce(III) to Ce(IV) increases with the calcination temperature increase from 300°C to 500°C. These may be attributed to the reduction of carbon species which come from pyrolysis of residual glycine. The ratios of O to Ce atoms in CeO2 and Ce2O3 lattice are 2 and 1.5, respectively. The measured results (Table 4) suggest that the oxygen amount on the surface of particles meets stoichiometric ratio with Ce(IV) and Ce(III) for the CeO2 sample; however, it is far from sufficient for the Co-doped samples, which means that oxygen vacancies can be significantly formed on the surface of CeO2 crystallite after cobalt is doped. Thus, the catalytic activity can be largely enhanced [38]. Among the Co-doped samples, the percentages of cobalt on surface of particle are almost constant, which indicates that the amount of oxygen vacancies decreases with the increase of calcination temperature. It is attributed to reduction of Co species on the surface during the generation of hexagonal Co-Ce-O phase [36].Table 4
Atomic compositions (%) measured from the O 1s, Ce 3d, and Co 3s spectra of CeO2 and 5 wt.% Co3O4/CeO2 samples at different calcination temperatures.
Calcination temperature (°C)
Atomic compositions (%)
Ce(III) : Ce(IV)
O
Ce
Co
Ce(III)
Ce(IV)
400 (CeO2)
65.9
5.5
28.5
—
0.19
300
52.7
3.7
32.0
11.5
0.12
400
54.1
5.6
28.4
11.8
0.20
500
55.8
6.5
25.9
11.7
0.25Figure9(a) shows the XPS spectra for Co 2p in 5 wt.% Co3O4/CeO2 prepared by calcining the spontaneous combusted precursor at temperatures form 300°C to 500°C. The doublet peaks, corresponding to Co 2p3/2 and a satellite peak, for all the samples, were found at 788.0
±
0.3 and 782.6 eV in Co 2p scanning pattern, which indicates the generation of Co3O4 on the surface of catalysts, which is in line with XRD analysis (Figure 8). Other researchers have reported that the satellite peak is absent in the diamagnetic Co3+ compound [39, 40]; however, it grows along with the increase of calcination temperature, which indicates the transformation of Co3+ to Co2+ species [41]. Whereas the binding energies of Co 2p3/2 and the satellite peak are slightly higher than the reported binding energies at ~780.0 and ~786.0 eV [41–43], respectively, this shift toward higher energies may be caused by the conjunction with Ce orbits. Binding energies of Co 3s and Ce 4d have been shown in Figure 9(b) to present the connection between Co and Ce orbits. Binding energy of Co 3s shifts toward higher energy from 110.9 (handbook [44]) to 111.4 or 111.1 eV, while binding energy of Ce 4d shifts toward lower energy from 110.0 (handbook [44]) to 107.7 or 107.5 eV, which suggests the formation of a kind of conjunction structure, and this structure becomes stable after calcining at temperature higher than 400°C.Figure 9
XPS spectra of (a) the Co 2p region and (b) the Co 3s-Ce 4d region for 5 wt.% Co3O4/CeO2 prepared by calcining the spontaneous combusted precursor at temperatures 300°C, 400°C, and 500°C.
(a)
(b)Figure10 shows that catalysts calcined at 400°C have highest methanol and ethanol yield among all, which can be well explained by the BET and SEM analysis. The order of the photocatalytic activities arranged as the calcination temperature is 400
>
500
≈
600
>
300
> CeO2, which is in line with that of XPS quantitative analysis.Figure 10
Photocatalytic activities of 5 wt.% Co3O4/CeO2 under different calcination temperature (the sample without calcination is denoted as “0”).


### 3.3. Separation of Electron-Hole Pairs on Co3O4/CeO2 Catalysts

The separation efficiency of electron-hole pairs of the composite photocatalyst is always the key factor to enhance the photocatalytic activity [26], which is mainly resulting from the addiction of cocatalyst. Ce3+ species formed on the surface of the composite might contribute to the separation of electron-hole pairs thus enhancing the photocatalytic activity [45] and further evidences will be discussed later. And doping Co3O4 can contribute to enhancing the visible absorption and it plays a role as a kind of cocatalyst to increase methanol and ethanol yields of photocatalytic reaction.Two widely used equations for band edge calculation are showed below:(4)
E
C
B
=
χ
-
E
c
-
1
2
E
g
E
V
B
=
E
g
+
E
C
B
.
E
C
B is the conduction band edge of a semiconductor at the point of zero charge. χ is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy. E
c is the energy of free electrons on the hydrogen scale (~4.5 eV). E
g is the band gap of the semiconductor [46, 47]. χ values for Co3O4 and CeO2 are 5.93 and 5.53 eV, respectively. So the E
C
B of Co3O4 and CeO2 are calculated to be +0.43 and −0.44 eV, respectively, and the E
V
B of Co3O4 and CeO2 are estimated to be +2.43 and +2.49 eV, respectively. These results are close to those of the reported references [20, 30]. Figure 11(a) shows an assumption which was made to explain the separation of electron-hole pairs of Co3O4/CeO2 catalysts. Photoelectron on the surface of Co3O4 transfers to the CeO2 to avoid recombining to holes, which may also partly trap in the holes on CeO2 and tend to reduce the recombination of electron-hole pairs on CeO2. It works like a Z-scheme reaction. However, if the separation of electron-hole pairs works as p-n heterojunction, no methanol and ethanol would be produced because the conduction band potential of Co3O4 is too positive to reduce carbon dioxide (Figure 11(b)).Figure 11
The scheme of photocatalytic reduction of carbon dioxide with water to methanol and ethanol.In order to confirm that the enhancement of photocatalytic activity is due to charge transfer between Co3O4 and CeO2, PL measurements were carried out. Figure 12 shows the spectra of CeO2, Co3O4, 5 wt.% Co3O4/CeO2, and mixture of 5 wt.% Co3O4 and CeO2 while the 5 wt.% Co3O4/CeO2 sample was prepared as a reference sample. The excitation wavelength was 425 nm. CeO2 sample has a strong PL emission at about 630 nm, which indicates rapid recombination of electrons and holes. The other samples have a PL peak at the same position with a peak intensity order of CeO2 > mixture > 5 wt.% C
o
3
O
4
/
C
e
O
2
≈
C
o
3
O
4. The weak PL intensity of 5 wt.% Co3O4 and CeO2 mixture could be attributed to the decrease of CeO2 content. Although the sample contains only 5 wt.% of Co3O4, it has similar PL peak intensity and shape to Co3O4. It suggests that Co3O4 can remarkably reduce the recombination of electrons and holes, and the charge transfer between Co3O4 and its carrier CeO2 does exist, in which the result is in line with the XPS measurement (Figure 9(b)).Figure 12
Photoluminescence spectra of CeO2, 5 wt.% Co3O4 and CeO2 mixture, and 5 wt.% Co3O4/CeO2 and Co3O4.


## 3.1. Characterization of Samples with Different Cobalt Loading

Figure2 shows the X-ray diffraction patterns of Co3O4/CeO2 photocatalysts prepared by glycine-nitride combustion method and calcined at 400°C, along with CeO2 and Co3O4 which were prepared at the same conditions. Diffraction peaks corresponding to the hexagonal phases of CeO2 are present for all Co3O4/CeO2 photocatalysts with different mass faction of Co3O4 loading. Six main reflections belong to a typical fcc fluorite structure of CeO2 (ICSD-169029), corresponding, respectively, to the [
111
], [
200
], [
220
], [
311
], [
222
], and [
400
] planes. And ten main reflections belong to Co3O4 (ICSD-069366) as shown in Figure 2. With the increase of the Co3O4 from 1 wt.% to 20 wt.%, the diffraction peaks of CeO2 become weaker, which indicates a continuous decrease in the crystallization degree of CeO2. The diffraction peaks of Co3O4 are hard to be detected in all Co3O4/CeO2 catalysts. The reasons why the strong Co oxides peaks absence had been discussed in our previous work [29], which may be attributed to (1) highly dispersed Co3O4 on the surface of ceria, (2) the formation of Co-Ce-O solid solution, or (3) a combination of two points stated above. However, some weak diffraction peaks for Co3O4 can still be noted, which suggests the existence of Co3O4 on the surface of particles. Either high dispersion of Co3O4 on the surface of CeO2 or formation of Co-Ce-O solid solution indicates CeO2 doped with Co3O4 instead of mixed oxides formed.Figure 2
XRD pattern of Co3O4/CeO2 photocatalysts with different mass faction of Co3O4 doping.The UV-vis spectra of the catalysts are shown in Figure3. The light absorption of all catalysts covers the visible region. The band gap of CeO2 is estimated to be 2.93 eV from the plot of the (
α
h
ν
)
2 versus photon energy (h
ν) [4, 6] as shown in the inset of Figure 3(a), which is 0.27 eV narrower than that of the commercial CeO2 with the band gap of 3.2 eV. It has been reported that CeO2, as an n-type semiconductor, has band gap varying from 2.7 to 3.4 eV depending on the preparation method [30]. The red shift of the CeO2 sample might be attributed to trivalent ionic cerium compounds. Similarly, the band gap of Co3O4 is estimated to be 2.0 eV, which is matched with the reported band gap (2.07 eV) [20]. In contrast to CeO2, the Co3O4/CeO2 have strong absorption in the visible region (λ
>
400
n
m) and the visible light absorbance increases along with the addition of Co3O4 (Figure 3(c)).Figure 3
UV-vis absorption spectra of CeO2 and commercial CeO2 catalyst (a), Co3O4 (b), and the Co3O4/CeO2 photocatalysts (c).
(a)
(b)
(c)In order to investigate the electronic environment and oxidation state of the CeO2 sample, XPS analysis was performed. Eight peaks for Ce 3d of CeO2 sample are shown in Figure 4(a), which are different from those of the reported CeO2 with six peaks at binding energies 882.5, 888.7, 898.2, 900.7, 907.6, and 916.5 (±0.2) eV [31, 32]. Other authors have proved that the shape of Ce 3d peaks changes after normalizing the intensity of Ce 3d peaks for Ce2O3 and CeO2 [32] when Ce2O3 is mixed or doped with CeO2. And the peaks can be divided into more than six peaks, which may be attributed to the presence of Ce3+ [33]. Among all of these ten peaks in Figure 4(a), v, v
′
′, v
′
′
′, u, u
′
′, and u
′
′
′ are attributed to the presence of Ce(IV), and v
0, v
′, u
0, and u
′ are attributed to the presence of Ce(III) [34]. Three peaks for O 1s of CeO2 sample are shown in Figure 4(b). Two peaks at 529.7 and 531.3 eV are attributed to O-Ce(IV) bond and O-Ce(III) bond, respectively [34], which indicates the existence of both CeO2 and Ce2O3 [35]. In this case, Ce2O3 with much shorter band gap of 2.4 eV can shorten the band gap of CeO2 and enhance the visible light absorption, which is in line with the UV-vis spectra.Figure 4
XPS spectra of the Ce 3d region (a) and the O 1s region (b) for the CeO2 sample.
(a)
(b)Table1 displays the specific surface area, the pore volume, the total pore volume of pure CeO2 and Co3O4, and the catalysts with Co3O4 loading of 2 wt.%, 5 wt.%, and 8 wt.%. As the Co3O4 loading increases from 0 wt.% to 100 wt.%, the surface areas decrease from 100.14 to 24.47 m2/g, and the total pore volume decreases as well. It can be attributed to the mutual filling of the pore structure during the forming of Co3O4 and CeO2 which can be proved by the pore diameter distribution in Figure 5. And pore size distribution of Co3O4/CeO2 catalysts that smaller than 10 nm decreases sharply compared to that of Co3O4 or CeO2 and mainly is distributed in mesopore region from 10 to 50 nm. When the Co3O4 loading increased from 2 wt.% to 8 wt.%, there is no distinct difference in specific surface area, average pore diameter, and total pore volume, which indicates that addition of Co3O4 in the Co3O4 loading range has little effect on the corresponding structure of Co3O4/CeO2 catalysts. The sample with 5 wt.% Co3O4 loading has biggest specific surface area and uniform pore diameter distribution among all the tested Co3O4/CeO2 catalysts. In addition, larger specific surface area, smaller pore volume, and total pore volume compared to those of commercial CeO2 suggest that CeO2 prepared by glycine-nitrate combustion method is a more potential carrier for catalysis.Table 1
Characteristic of Co3O4/CeO2 catalysts with different mass fraction of Co3O4.
Catalyst mass fraction of Co3O4 doping (%)
BET surface area (m2/g)
Average pore diameter (nm)
Total pore volume (cm3g−1)
0 (CeO2)
100.14
4.68
0.12
0 (commercial CeO2)
71.41
15.22
0.27
2.0
34.69
11.41
0.11
5.0
38.28
10.19
0.10
8.0
33.20
11.65
0.10
100 (Co3O4)
25.47
13.28
0.08Figure 5
Pore diameter distribution of catalysts with different mass faction of Co3O4 doping.The average methanol and ethanol yields, denoted as the average production rate of methanol and ethanol for each hour over the photocatalysts, are shown in Table2. The photocatalytic activity of CeO2 can be well improved via loading Co3O4 on CeO2 by glycine-nitrate combustion method. No methanol and ethanol conversion was detected for Co3O4 catalyst under visible light irradiation because of its conduction band maximum being too positive to reduce carbon dioxide to methanol or ethanol. The sample with 5 wt.% Co3O4 has the best performance to produce both methanol and ethanol, which can be attributed to its better specific surface area, the proper pore volume, and pore size (Table 1). Increasing the Co3O4 loading from 0 to 5 wt.%, the methanol yield increased from 0.68 to 1.52 μmol·g−1·h−1 and the ethanol yield increased from 2.77 to 4.75 μmol·g−1·h−1. Further increasing the Co3O4 loading (>5 wt.%) leads to the decrease of the methanol and ethanol yields because of the significant decrease of the crystallization degree of CeO2 shown in Figure 2. Photocatalytic activities of the sample with 5 wt.% Co3O4 were tested in different solutions. It is obvious that solution containing 0.1 M Na2CO3 and 0.1 M Na2SO3 has the best performance among (0.1 M Na2CO3 + 0.1 M Na2SO3), (0.1 M NaOH + 0.1 M Na2SO3), and (0.1 M Na2CO3 + 0.1M Na2S), which indicates that extra carbon source (C
O
3
2
-) and better sacrifice reagent (S
O
3
2
-) can greatly enhance the photocatalytic activities for methanol and ethanol generation as shown in Table 2.Table 2
Mean methanol and ethanol yield on Co3O4/CeO2 catalysts under visible light irradiation (λ > 400 nm).
Catalyst mass fraction of Co3O4 doping (%)
Mean yield of methanolμmol·h−1
g
cat
-
1
Mean yield of ethanolμmol·h−1
g
cat
-
1
0 (CeO2)(a)
0.68
2.77
1.0(a)
1.51
2.87
2.0(a)
1.5
3.95
8.0(a)
1.22
3.15
10.0(a)
1.18
2.43
100 (Co3O4)(a)
0
0
5.0(a)
1.52
4.75
5.0(b)
0.91
2.22
5.0(c)
0.75
2.16
0.30 g catalysts reacted in 300 mL solution of (a) 0.1 M NaOH + 0.1 M Na2SO3, (b) 0.1 M Na2CO3 + 0.1 M Na2S, and (c) 0.1 M Na2CO3 + 0.1 M Na2SO3. A 300 W Xenon lamp (λ > 400 nm) irradiated for 5 hours.


## 3.2. Effects of Calcination Temperature on the Photocatalytic Activity

Calcination temperature also affects the surface structures of catalysts. The samples with 5 wt.% of Co3O4 loading were calcined at 300°C, 400°C, and 500°C, respectively. As shown in Table 3, the specific surface area decreases with the increase of calcination temperature. And the pore diameter distribution becomes uneven when catalyst calcined at 500°C. Pore diameter bigger than 984 nm can be found from Figure 6, which indicates that the particle was melted and the pore collapsed.Table 3
Characteristic of Co3O4/CeO2 catalysts with 5 wt.% Co3O4 doping at different calcination temperatures.
Calcination temperature (°C)
BET surface area (m2/g)
Average pore diameter (nm)
Total pore volume (cm3g−1)
The crystallite size of CeO2 (nm)
300
41.16
10.61
0.11
37.5
400
38.28
10.19
0.10
35.4
500
30.88
15.13
0.12
41.8Figure 6
Pore diameter distribution of catalysts under different calcination temperature.Figure7 shows the SEM images of the 5.0 wt.% Co3O4/CeO2 catalysts prepared by calcining the spontaneous combusted precursor at temperatures of 300 to 600°C when CeO2 calcined at 400°C. The shape of CeO2 particle becomes very irregular. However, more pores can be seen on the surface of the CeO2 particles (Figures 7(i) and 7(j)) compared to those of the Co3O4/CeO2 catalysts (Figures 7(a)–7(h)). It can explain why the CeO2 has larger surface area than the other samples. The particle size of the Co3O4/CeO2 catalysts, however, increased greatly with the calcination temperature higher than 400°C, as shown in Figures 7(c) and 7(d). However, the particles of the sample calcined at 300°C are more irregular and the pores on the surface are bigger than those of the sample calcined at 400°C, which is in good correspondence to BET analysis results.Figure 7
SEM images of Co3O4/CeO2 photocatalysts with 5.0 wt.% Co3O4 calcined at 300°C (a and e), 400°C (b and f), 500°C (c and g), 600°C (d and h), and CeO2 (i and j).
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)In Figure3, XRD peaks for Co3O4 of Co3O4/CeO2 catalysts are absent, however, the peaks appear when the calcination temperature is higher than 400°C as shown in Figure 8. And the CeO2 crystallite size of catalysts was estimated by using the Scherrer equation, and the order is 500 (41.8 nm) > 300 (37.5 nm) > 400 (35.4 nm) as shown in Table 3. The diffraction peak at 48.6° corresponding to hexagonal Co-Ce-O phase (JCPDS file 65-5917) [36] becomes stronger with increase of calcination temperature, which suggests that the treatment by combustion method strengthens the Co-Ce interaction and the calcination temperature improves the crystallization structure of catalysts. Thus, the CeO2 crystallite size is getting smaller under calcination temperature of 400°C than under 300°C. And the enlarged size of CeO2 crystallite under 500°C is caused by sintering of particles which is in line with the SEM.Figure 8
XRD pattern of Co3O4/CeO2 photocatalysts under different calcination temperature.In order to determine the amount of Ce(III) in cerium oxides, the deconvolution method of the Ce3d features [34, 37] has been used. The amounts of Ce(III) and Ce(IV) were calculated from the components of the Ce 3d core level using the following equations [37]:(1)
C
e
I
I
I
=
v
0
+
v
′
+
u
0
+
u
′
(2)
C
e
I
V
=
v
+
v
′
′
+
v
′
′
′
+
u
+
u
′
′
+
u
′
′
′
(3)
%
C
e
I
I
I
=
C
e
I
I
I
C
e
I
I
I
+
C
e
I
V
×
100
.The calculated result shows that the amount of Ce(III) increases from 3.7% to 6.5%, the amount of Ce(IV) decreases from 32.0% to 25.9%, and the ratio of Ce(III) to Ce(IV) increases with the calcination temperature increase from 300°C to 500°C. These may be attributed to the reduction of carbon species which come from pyrolysis of residual glycine. The ratios of O to Ce atoms in CeO2 and Ce2O3 lattice are 2 and 1.5, respectively. The measured results (Table 4) suggest that the oxygen amount on the surface of particles meets stoichiometric ratio with Ce(IV) and Ce(III) for the CeO2 sample; however, it is far from sufficient for the Co-doped samples, which means that oxygen vacancies can be significantly formed on the surface of CeO2 crystallite after cobalt is doped. Thus, the catalytic activity can be largely enhanced [38]. Among the Co-doped samples, the percentages of cobalt on surface of particle are almost constant, which indicates that the amount of oxygen vacancies decreases with the increase of calcination temperature. It is attributed to reduction of Co species on the surface during the generation of hexagonal Co-Ce-O phase [36].Table 4
Atomic compositions (%) measured from the O 1s, Ce 3d, and Co 3s spectra of CeO2 and 5 wt.% Co3O4/CeO2 samples at different calcination temperatures.
Calcination temperature (°C)
Atomic compositions (%)
Ce(III) : Ce(IV)
O
Ce
Co
Ce(III)
Ce(IV)
400 (CeO2)
65.9
5.5
28.5
—
0.19
300
52.7
3.7
32.0
11.5
0.12
400
54.1
5.6
28.4
11.8
0.20
500
55.8
6.5
25.9
11.7
0.25Figure9(a) shows the XPS spectra for Co 2p in 5 wt.% Co3O4/CeO2 prepared by calcining the spontaneous combusted precursor at temperatures form 300°C to 500°C. The doublet peaks, corresponding to Co 2p3/2 and a satellite peak, for all the samples, were found at 788.0
±
0.3 and 782.6 eV in Co 2p scanning pattern, which indicates the generation of Co3O4 on the surface of catalysts, which is in line with XRD analysis (Figure 8). Other researchers have reported that the satellite peak is absent in the diamagnetic Co3+ compound [39, 40]; however, it grows along with the increase of calcination temperature, which indicates the transformation of Co3+ to Co2+ species [41]. Whereas the binding energies of Co 2p3/2 and the satellite peak are slightly higher than the reported binding energies at ~780.0 and ~786.0 eV [41–43], respectively, this shift toward higher energies may be caused by the conjunction with Ce orbits. Binding energies of Co 3s and Ce 4d have been shown in Figure 9(b) to present the connection between Co and Ce orbits. Binding energy of Co 3s shifts toward higher energy from 110.9 (handbook [44]) to 111.4 or 111.1 eV, while binding energy of Ce 4d shifts toward lower energy from 110.0 (handbook [44]) to 107.7 or 107.5 eV, which suggests the formation of a kind of conjunction structure, and this structure becomes stable after calcining at temperature higher than 400°C.Figure 9
XPS spectra of (a) the Co 2p region and (b) the Co 3s-Ce 4d region for 5 wt.% Co3O4/CeO2 prepared by calcining the spontaneous combusted precursor at temperatures 300°C, 400°C, and 500°C.
(a)
(b)Figure10 shows that catalysts calcined at 400°C have highest methanol and ethanol yield among all, which can be well explained by the BET and SEM analysis. The order of the photocatalytic activities arranged as the calcination temperature is 400
>
500
≈
600
>
300
> CeO2, which is in line with that of XPS quantitative analysis.Figure 10
Photocatalytic activities of 5 wt.% Co3O4/CeO2 under different calcination temperature (the sample without calcination is denoted as “0”).


## 3.3. Separation of Electron-Hole Pairs on Co3O4/CeO2 Catalysts

The separation efficiency of electron-hole pairs of the composite photocatalyst is always the key factor to enhance the photocatalytic activity [26], which is mainly resulting from the addiction of cocatalyst. Ce3+ species formed on the surface of the composite might contribute to the separation of electron-hole pairs thus enhancing the photocatalytic activity [45] and further evidences will be discussed later. And doping Co3O4 can contribute to enhancing the visible absorption and it plays a role as a kind of cocatalyst to increase methanol and ethanol yields of photocatalytic reaction.Two widely used equations for band edge calculation are showed below:(4)
E
C
B
=
χ
-
E
c
-
1
2
E
g
E
V
B
=
E
g
+
E
C
B
.
E
C
B is the conduction band edge of a semiconductor at the point of zero charge. χ is the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy. E
c is the energy of free electrons on the hydrogen scale (~4.5 eV). E
g is the band gap of the semiconductor [46, 47]. χ values for Co3O4 and CeO2 are 5.93 and 5.53 eV, respectively. So the E
C
B of Co3O4 and CeO2 are calculated to be +0.43 and −0.44 eV, respectively, and the E
V
B of Co3O4 and CeO2 are estimated to be +2.43 and +2.49 eV, respectively. These results are close to those of the reported references [20, 30]. Figure 11(a) shows an assumption which was made to explain the separation of electron-hole pairs of Co3O4/CeO2 catalysts. Photoelectron on the surface of Co3O4 transfers to the CeO2 to avoid recombining to holes, which may also partly trap in the holes on CeO2 and tend to reduce the recombination of electron-hole pairs on CeO2. It works like a Z-scheme reaction. However, if the separation of electron-hole pairs works as p-n heterojunction, no methanol and ethanol would be produced because the conduction band potential of Co3O4 is too positive to reduce carbon dioxide (Figure 11(b)).Figure 11
The scheme of photocatalytic reduction of carbon dioxide with water to methanol and ethanol.In order to confirm that the enhancement of photocatalytic activity is due to charge transfer between Co3O4 and CeO2, PL measurements were carried out. Figure 12 shows the spectra of CeO2, Co3O4, 5 wt.% Co3O4/CeO2, and mixture of 5 wt.% Co3O4 and CeO2 while the 5 wt.% Co3O4/CeO2 sample was prepared as a reference sample. The excitation wavelength was 425 nm. CeO2 sample has a strong PL emission at about 630 nm, which indicates rapid recombination of electrons and holes. The other samples have a PL peak at the same position with a peak intensity order of CeO2 > mixture > 5 wt.% C
o
3
O
4
/
C
e
O
2
≈
C
o
3
O
4. The weak PL intensity of 5 wt.% Co3O4 and CeO2 mixture could be attributed to the decrease of CeO2 content. Although the sample contains only 5 wt.% of Co3O4, it has similar PL peak intensity and shape to Co3O4. It suggests that Co3O4 can remarkably reduce the recombination of electrons and holes, and the charge transfer between Co3O4 and its carrier CeO2 does exist, in which the result is in line with the XPS measurement (Figure 9(b)).Figure 12
Photoluminescence spectra of CeO2, 5 wt.% Co3O4 and CeO2 mixture, and 5 wt.% Co3O4/CeO2 and Co3O4.


## 4. Conclusions

The Co3O4/CeO2 photocatalysts with mesoporous structure were synthesized by glycine-nitrate combustion method to improve photocatalytic activity of carbon dioxide reduction. The experimental results indicate that the modification of Co3O4 can not only decrease the specific surface area and pore diameter but also greatly increase the visible light absorbance and the photocatalytic activity of CeO2. The Co3O4/CeO2 photocatalysts with 5 wt.% of Co3O4 loading calcined at 400°C were found to be the best catalysts for carbon dioxide reduction in 0.1 M Na2CO3 + 0.1 M Na2SO3 solution. The largest average yields of methanol and ethanol were 1.52 and 4.75 μmol·g−1·h−1, respectively. Furthermore, doping Co3O4 on the surface of CeO2 can help to promote separation of electron-hole and electron transfer between these two semiconductors. PL spectra revealed that Z-scheme charge transfer, one of the factors to enhance photocatalytic activity, occurred on the surface of Co3O4/CeO2 catalysts under visible light irradiation.



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*Source: 230808-2015-05-21.xml*