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Cu2O under UV-Visible light to solar fuels
Materials Chemistry and Physics 227 (2019) 90–97
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CO2 adsorption and photocatalytic reduction over Mg(OH)2/CuO/Cu2O under UV-Visible light to solar fuels
T
M. Flores-Floresa, E. Luévano-Hipólitob, Leticia M. Torres-Martíneza,∗, Trong-On Doc a
Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P. 66455, San Nicolás de los Garza, NL, Mexico b CONACYT - Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P. 66455, San Nicolás de los Garza, NL, Mexico c Department of Chemical Engineering, Laval University, Québec, G1 V 0A6, Canada
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
materials were applied • Bifunctional for CO capture and photocatalytic 2
• • • •
conversion. Mg(OH)2 were effectively coupled with CuO/Cu2O by microwave-hydrothermal method. Mg(OH)2/CuO/Cu2O was used as photocatalyst to convert CO2 to CH3OH, HCOH, CO and CH4. The coupling of materials promoted an efficient charge transfer in the reaction. The presence of Cu2O promoted the selectivity to CH3OH generation in liquid phase.
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 capture Mg(OH)2 Solar fuels CO2 utilization Cu2O CuO Copper oxides
Powders of Mg(OH)2, CuO, and Cu2O were effectively coupled by microwave-hydrothermal method, in order to proposed bifunctional materials for both capture and photocatalytic conversion of CO2 under UV-visible light irradiation. This was done to take advantage of the high CO2 adsorption capacity of Mg(OH)2 and the good photocatalytic performance of CuO/Cu2O for CO2 reduction to solar fuels in liquid (CH3OH and HCOH) and gas phase (CH4 and CO). In order to establish correlations between the physical properties and the photocatalytic activity, the composites were characterized by XRD, XPS, UV-Vis DRS, SEM, HRTEM, N2 physisorption and photoelectrochemical techniques. According to the characterization, the synthesis method employed allowed the adequate interaction between Mg(OH)2 with CuO and Cu2O, which not inhibited the ability of Mg(OH)2 for gas adsorption. The best yield to obtain liquid fuels such as CH3OH (6 μmol g−1 h−1) and HCOH (9 μmol g−1 h−1) was obtained using 10% of CuO in the composite. The improved photocatalytic activity in liquid phase was assigned to a high CO2 adsorption and a more negative potential of conduction band. It was found that the presence of Cu2O favored the selectivity towards CH3OH production; as higher Cu+ concentration better selectivity. A reaction mechanism is discussed on the basis of combined CO2 adsorption and photocatalytic activity of the materials involved. Furthermore, it was study the photocatalytic activity in gas phase, and it was determined the presence of CO and CH4 in low concentrations (< 0.4 μmol g−1 h−1).
∗
Corresponding author. E-mail address: [email protected] (L.M. Torres-Martínez).
https://doi.org/10.1016/j.matchemphys.2019.01.062 Received 23 July 2018; Received in revised form 15 January 2019; Accepted 25 January 2019 Available online 29 January 2019 0254-0584/ © 2019 Elsevier B.V. All rights reserved.
Materials Chemistry and Physics 227 (2019) 90–97
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1. Introduction
synthesized by one-pot microwave-hydrothermal method. In this method, a 2M solution of magnesium nitrate (Mg(NO3)2·6H2O) (Aldrich, 99%) was dissolved in 50 mL of distilled water. Then, different amounts of copper acetate (Cu(CH3COO)2·H2O) (Aldrich, 99%) was added into the magnesium solution to reach different loads of copper oxide (0.5, 5, 10, 15, and 20% by weight). Then, NaOH was added to promote the formation of the hydroxide phase under vigorous stirring for 10 min. The resulting solution was exposed at microwave radiation using a programmable microwave MARS-6, CEM at 600 W and 100 °C for 30 min. The samples were washed with deionized water and ethanol and dried at 100 °C before their use. Hereafter, the composites will be identified as MgCux, where x refers to the weight percentage of copper oxides in the samples.
The rising in CO2 emissions due to the accelerated population growth and the industrial sector has resulted in an increase of the greenhouse effect and consequently, a climate change. One of the potential solutions to reduce CO2 in the atmosphere is related to its capture and utilization [1,2]. In recent years, the capture of CO2 and photocatalytic conversion (CCPC) has gained more attention since it is possible to obtain solar fuels from CO2, water, and photocatalysts [3]. In regard to CO2 capture, in last years several technologies had been integrated in post-combustion plants, including physical and chemical absorption (solvents), adsorption (solid sorbents), and membranes [4]. In particular, the use of solid sorbents has received an increased interest due to the high CO2 capture efficiencies reached. This has been related to its thermodynamically feasibility (ΔG < 0), low-cost, and because they are easy to scale compared to membranes [5]. One of these solid sorbents is the magnesium hydroxide (Mg(OH)2) with a hexagonal (brucite) structure, which it is widely abundant in minerals such as olivine ((Mg,Fe)2SiO4), orthopyroxene (Mg2Si2O6–Fe2Si2O6), clinopyroxene (CaMgSi2O6–CaFeSi2O6), serpentine ((Mg,Fe)3Si2O5(OH)4), among others [6]. Another interesting development as part of CCPC research is the use of captured CO2 as chemical feedstock. CO2 conversion to gas and liquid fuels (i.e., syngas, methane, methanol, formaldehyde, formic acid) via chemical, electrochemical or photochemical reactions has been proposed and significant efforts are being carried out in this field [7]. Among the different methods suggested so far, the photocatalytic conversion of CO2 into value added compounds (solar fuels) enables the utilization of clean energy (i.e. solar energy) to produce sustainable sources of energy [8–11]. In these processes, it has been reported that the selectivity of the reaction depends of the nature, physical properties of the photocatalysts, and also it is highly dependent of the reaction medium in which it is evaluated (liquid and gas). In gas phase, the CO2 is reduced to CO and to radicals •CO− that in contact with H+ promote the evolution towards CH4 and CH3OH. In liquid phase, the consecutive reactions of CO2 reduction end in the evolution of alcohols (CH3OH, C2H5OH), carboxyl groups (HCOOH), and aldehyde compounds (HCOH) [12–14]. Semiconductor oxides are the most investigated materials that had been proposed as photocatalysts. Among them, copper oxides (CuO and Cu2O) are related to high stability, low cost, narrow band gap energies that make possible to absorb solar light, and appropriate conduction and valence band potentials, which make them ideal photocatalysts for CO2 reduction [15]. The coupling of Cu2O and CuO with other semiconductor materials has been proposed as an alternative to improve the efficiency of the reaction for other photocatalytic reactions, i.e. degradation of methylene blue [16], and in recent years have proposed the formation of heterostructures of CuxO with TiO2 for CH4 formation from CO2 reduction [17]. In particular, the enhancement in the photocatalytic activity by adding CuO and Cu2O to other semiconductors such as SiC has been related to an improvement in the charge transfer due to the favorable position (more negative) of the conduction band (CB) of CuO and Cu2O. This effect has been resulted in a higher methanol production from CO2 photoreduction [18]. On the other hand, there are a few reports related to the use of Mg(OH)2 as photocatalyst for CO2 photoconversion [19]. To the best of our knowledge, there are no reports related to the coupling of adsorbents such as Mg(OH)2 with photocatalysts such as copper oxides to produce solar fuels from CO2 photoreduction. Thus, in this work is proposed a methodology to coupling Mg(OH)2 with CuO and Cu2O to capture and photoconvert CO2 under UV-visible light irradiation to liquid and gas fuels for first time.
2.2. Characterization The structural characterization of the samples was carried out by Xray powder diffraction using a Bruker D8 Advance diffractometer with Cu Kα radiation (40 kV, 30 mA). A typical run was made with a 0.05° of step size and a dwell time of 0.5 s. From this data, it was obtained the crystallite size using the Debye's-Scherrer equation. Previous works have reported that some cations such as Cu2+, Cr+2, Mn3+, and Ni3+ show the Jahn-Teller effect, which is defined as a distortion caused by the incorporation of these cations in the structure [20]. Thus, in order to study the distortion occasioned by the addition of copper on the Mg(OH)2 structure, it was evaluated the lattice strain using equation (1):
ε=
βhkl 4 tan θ
(1)
where ε is the lattice strain and βhkl is the instrument broadening, which it was calculated using equation (2). In this case, we used the Al2O3 corundum as reference.
βhkl = [(βhkl )2measured − (βhkl )2instrumental]1/2
(2)
The optical properties of samples were analyzed between 200 and 1500 nm using a UV-Vis NIR spectrophotometer (Cary 5000) coupled with an integration sphere for diffuse reflectance measurements. The BET surface area measurements were carried out by N2 adsorptiondesorption isotherms using a Bel-Japan Minisorp II surface area and pore size analyzer. The morphology of the samples was analyzed by scanning electron microscopy using a JEOL 6490 LV. The surface groups in the samples were study by FTIR using a Perkin Elmer FTIR/ FIR Frontier with ATR accessory in a range of 400–4000 cm−1. The identification of crystal planes in the brucite structure was performed by high-resolution transmission electron microscopy using a FEI Titan G2 80-300 microscopes with an accelerating voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) data were collected by a K-Alpha-Xray photoelectron spectrometer under ultrahigh vacuum 10−9 Torr, using an Al Kα, 1486.6 eV monochromated X-ray source operated at 3 mA corresponding to a spot size of 400 mm. Survey scans were acquired at a pass energy of 200 eV and 50 eV for the acquisition of highresolution windows over the binding energy range 1200-0 eV. The C 1s signal at 284.8 eV was used as reference. The flat band potentials were determined by electrochemical impedance spectroscopy (EIS) by using the Mott-Schottky plots. For this purpose, it was using a three electrode cell using NaSO4 (0.5 M) as electrolyte, Ag/AgCl as reference electrode, and Pt as counter electrode. The electrochemical experiments were carried out using a potentiostat AUTOLAB PGSTAT 302N connected to a computer running the software NOVA for data acquisition. The samples were fixed in ITO glass of 1 × 1 cm2. In this method, a plot of 1/C2 against V should yield a straight line from which the potential of the flat band can be determined from the intercept on the V axis [21]. The experimental energy band diagrams were constructed from the estimation of the CB by
2. Materials and methods 2.1. Synthesis of composites The composite materials of Mg(OH)2, CuO, and Cu2O were 91
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3. Results and discussion
this technique, and the VB potential was determined by adding to CB the band gap value obtained from the UV-Vis diffuse reflectance spectra.
3.1. X-ray powder diffraction XRD patterns of the MgCux composite materials are shown in Fig. 2a. In the reference sample (Mg(OH)2) all the reflections were assigned in accordance to the JCDPS card 07-0239. Magnesium hydroxide crystallized in the hexagonal system with the linear parameters: a = 3.147 Å, c = 4.769 Å, and angular parameters: α = β = 90°, and γ = 120°. After the introduction of copper oxides in the base material, the main intensity reflection of Mg(OH)2 (101) was decreased, and only in the sample with 10% of copper (MgCu10) appears a reflection at 2θ = 36° associated to monoclinic CuO (JCPDS card 01-089-5895), as it can be seen in the zoom-in from 34 to 40° (Fig. 2b). However, this reflection was not observed in samples with higher copper content such as MgCu15 and MgCu20, which could be related to a decrement in its crystallite size (Table 1). Previous reports had determined that higher loads of Cu (> 4.5%) in semiconductor oxides, promoted the formation of nanometric clusters that cannot be detected by XRD [23,24]. In addition, the diminishment in the intensity of reflections might be attributed to distortions in the brucite structure generated by the substitution of Cu2+ for Mg2+. This fact is associated to that Cu2+ has a higher ionic radius than Mg2+, which could promote the Jahn-Teller effect. To corroborate this, it was calculated the distortion of lattice strain (by equation (1)) from XRD data, which results are shown in Table 1. As it can be seen, a higher lattice strain was observed as long as the load of copper increase in the composites.
2.3. CO2 capture experiments The CO2 uptake experiments were carried out in slurry-phase in a cylindrical batch reactor of 200 mL at 20 °C and 2 psi. The mass of the sorbent was fixed to 0.1 g and, it was dispersed in 100 mL of deionized water under vigorous stirring. The CO2 concentration was monitored by gas chromatography using a Thermo Scientific equipped with TCD, using Argon as diluent.
2.4. Photocatalytic CO2 reduction 2.4.1. Liquid phase The reactions in liquid phase were carried out in the same cylindrical batch reactor that it was used for the CO2 uptake experiments, under the same experimental conditions with the addition of visible light provided by two halogen lamps of 20 W each one emitting between 300 and 900 nm in order to simulate the UV-visible lamp of the solar spectrum, which spectra is shown in Fig. 1. As it can be seen, the main emission comes from the visible part (84%) and the rest comes from UV (5%) and IR (11%) parts. In the beginning of the experiment, the suspension was purged with CO2 for 15 min to saturate the medium and remove the oxygen. Then, the suspension was irradiated from the outside of the reactor. The reaction products in liquid phase were analyzed by a gas chromatograph Thermo Scientific equipped with a FID detector (for CH3OH) and with an UV-Vis NIR spectrophotometer Cary 5000 using the complexation of formaldehyde with chromotropic acid [22].
3.2. Scanning electron microscopy SEM images of Mg(OH)2 and MgCux composites are shown in Fig. 3. The morphology of pure Mg(OH)2 presented flake-like morphology of an average particle size of 111 nm (Fig. 3a). This type of morphology prevailed when 0.5–5% of copper oxides was introduced in the reference sample. Nevertheless, it was observed an increase in the size and agglomeration degree in its particles (Fig. 3 a–c). From 10% of copper oxide, the particles tended to agglomerate with each other to form bigger clusters with small particles deposited on them (Fig. 3d–f). Since it was not observed different morphologies in the reference and composite materials, supplementary analysis of EDS was performed to analyze the distribution of copper in Mg(OH)2 (Supplementary Fig. S1). From these data, it was observed that copper is distributed homogeneously in the surface of the base-material, which confirms the advantage of the synthesis method proposed.
2.4.2. Gas phase The photocatalytic experiments in gas phase were carried out in a stainless steel cylindrical reactor of 200 mL. The reactor at the top has a quartz window. The sample was irradiated from the outside of the reactor with a solar simulator using a 150 W Xe lamp (AM 1.5 G 100 mW cm−2) for 3 h. The mass of the photocatalyst was fixed to 0.1 g. The system was purged with N2 for 5 min and then saturated with CO2 with a 3 mL h−1 flow for 30 min. The gas products (CO and CH4) were analyzed with a gas chromatograph Agilent Technologies 7820 A equipped with TCD and FID detector.
3.3. N2 physisorption The textural properties of the composites were investigated by means of N2 isotherms (Supplementary Fig. S2), since one of the key aspects of Mg(OH)2 in the composites was to promote CO2 capture. Table 2 summarizes the BET surface area of the MgCux composites. The loading of copper oxides on Mg(OH)2 had a positive effect in the specific surface area, which increase almost 2.5 times: from 54 m2 g−1 (Mg (OH)2) to 142 m2 g−1 MgCu15. This could be beneficial because a growth in the surface area is related to an increase of the active sites available for the adsorption of CO2. Additionally, the profile of N2 isotherms showed the same trend that the reference sample (type-III), which is characteristic of materials related to low energy of adsorption. In addition, a small hysteresis was developed in the range of relative pressure from 0.85 to 0.99 in MgCu15 and MgCu20 samples, related to the formation of small pores in the particles. 3.4. UV-Vis spectroscopy Fig. 1. Comparison between the spectra of the sun and the halogen lamp.
The band gap energy of the composites was study by UV-Vis 92
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Fig. 2. X-ray powder diffraction patterns of the MgCux composites, b. zoom-in of reflections between 2θ = 34–40°. Table 1 Crystallite size and lattice strain of the MgCux composites.
Table 2 Band gap energy and specific surface area of the MgCux composites.
Sample
Crystallite size (nm)
Lattice strain (%)
Sample
Eg (eV)
sBET (m2 g−1)
Mg(OH)2 MgCu0.5 MgCu5 MgCu10 MgCu15 MgCu20
97 97 29 30 16 16
0.107 0.109 0.358 0.358 0.668 0.667
Mg(OH)2 MgCu0.5 MgCu5 MgCu10 MgCu15 MgCu20
4.9 4.4 3.8 3.5 2.9 2.7
54 70 117 96 142 136
the synergy effect between the p-type CuO and n-type Mg(OH)2 [25]. An adequate coupling of semiconductors p and n-type can promote an improving in the charge transfer. Also, it should be note a broad absorption band in wavelengths from 600 to 800 nm in samples with the highest load of copper (MgCu15, and MgCu20), which might be related to the plasmonic effect of Cu2O [26,27]. This effect is affected by particle size, thus it seems that in MgCu15 and MgCu20 the presence of nanoclusters of copper (previously discussed in XRD section) promote the plasmonic effect that could affect the photocatalytic activity.
spectroscopy, which spectra are shown in Fig. 4. The band gap energy of the reference material was 4.9 eV, which is indicative that could present low photocatalytic activity under visible irradiation. However, when copper oxides were introduced in the composites, the band gap energies tended to lower values (the lowest was 2.7 eV for MgCu20) that make possible the absorption of solar light (Table 2). The analysis in the absorption spectra of the composites showed two intervals of absorption in wavelengths: i) 200–350, and ii) 400–800 nm. The second absorption (400–800 nm) is attributed to the presence of higher amounts of CuO (particularly in MgCu15 and MgCu20), which favored a higher visible light absorption. The increase in the light absorption could be associated to a coupling between the semiconductors due to
Fig. 3. SEM micrographs of the samples a) Mg(OH)2, b) MgCu0.5, c) MgCu5, d) MgCu10, and e), f) MgCu20. 93
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performed in the reference and MgCu5 sample, which results are shown in Fig. 6. The HRTEM image of the reference sample shows the presence of crystal planes (101) and (102) of Mg(OH)2, which is in agreement with the XRD results (Fig. 6a). On the other hand, a carefully analysis of the MgCu5 sample confirmed the presence of both CuO and Cu2O in the composite (Fig. 6b). In this sample, it was found additional d spacings, which were 0.231 nm and 0.246 nm, indexed as CuO (111) and Cu2O (111), respectively. Thus, according to these results, it was confirmed the effective coupling in the composite of Mg(OH)2 with CuO, and Cu2O, which junction could benefit the photocatalytic reaction. 3.7. Photo(electro)chemical characterization The potential of the conduction band (CB) of some representative samples (Mg(OH)2, MgCu5, and MgCu10) was estimated by means of potentiodynamic electrochemical impedance spectroscopy (Fig. 7a), using the conditions described in the experimental section. The MottSchottky plots obtained from these measurements show a positive slope in the linear region of the C−2 vs potential (Fig. 7a), confirming that the materials showed are n-type semiconductors. The flat band potential values of the materials were determined from the interception point between the extrapolated slope and the x-axis. These Efb values were assumed as CB in the schematic energy level diagram, because the materials showed a n-behavior. According to the schematic energy diagram, the CB potential of the base material (Mg(OH)2) tended to more positive potentials after the introduction of copper oxides. Nevertheless, the composited analyzed had the required thermodynamic potential to reduce CO2 to CO, HCHO, CH3OH and CH4. In particular, the CB potential of MgCu10 was slightly more negative than MgCu5, which could be related to differences in their chemical composition. MgCu10 composite had a higher amount of p-type (CuO), which under adequate coupling could move up its CB, while the Fermi energy of the n-type (Mg(OH)2) might move down until a state of equilibrium, favoring more negative potentials and also, a more efficient charge transfer [33]. To corroborate if the charge transfer in the composite was improved after the coupling of Mg(OH)2 with CuO and Cu2O, an analysis in the Nyquist plot under light and dark conditions was performed (Fig. 8). In general, a smaller arc radius in the plot indicates a smaller electron transfer resistance at the surface of the photocatalysts, which normally leads to more effective separation of the electron and hole pair [34]. MgCu5 sample showed a lower semicircle under dark and light conditions than MgCu10 sample, which indicates a better separation of the electron-hole pair and faster interfacial charge transfer under UV-visible light irradiation. This fact could benefit the reduction of CO2.
Fig. 4. UV-Vis absorption spectra obtained of the MgCux composites.
3.5. X-ray photoelectron spectroscopy In order to study the composition of the composite surface, an XPS analysis was performed in selected samples (Mg(OH)2, MgCu5, and MgCu10). In the survey spectrum, the characteristic peaks of Mg2p and Mg2s were observed and the presence of Cu2p1/2 and Cu2p3/2 was confirmed in MgCu5 and MgCu10 samples (Supplementary Fig. S3). Fig. 5 shows the Cu2p XPS spectra of the MgCu5 and MgCu10 samples. The characteristic peaks of Cu 2p3/2 and Cu2p1/2 were detected for both samples accompanied by a series of peaks (satellites) in 940.3 and 942.9 eV, respectively. In both XPS spectra of MgCu5 and MgCu10, the Cu2p3/2 peak can be deconvoluted in two peaks at 932.0 and 933.9 eV that correspond to Cu+ and Cu2+, respectively [28–32]. Based on these results, it was corroborated the presence of Cu2O and CuO in both MgCu5 and MgCu10 samples, with some differences between them. For a more precise analysis, the peak area associated to Cu2O and CuO was quantified in order to estimate the relation between these oxides. The relation of Cu2O/CuO was higher in the sample MgCu5, which it suggests a higher amount of Cu2O. According to the literature, CuO is preferentially formed at lower temperatures, and it comes from the decomposition of the acetate anion to CH3COOH and OH−, which reacts with Cu+2 to produce CuO. At higher temperatures, the CH3COOH is converted to HCOOH, reacting with CuO to produce Cu2O. However, in this work it was used the same temperature for all the experiments, which it suggests that when the amount of copper was 5%, a more efficient heating was enhance inside the reactor that produce a higher localized temperature in the composite surface, which could promote the reduction of small parts of CuO to Cu2O, as it was detected by XPS.
3.8. CO2 capture 3.6. High resolution transmission electron microscopy As the effective coupling of composites was confirmed, the amount of CO2 capture was investigated, which results are shown in Table 3. The MgCux composites presented lower adsorption capacities (ranging
In order to confirm the presence of both CuO and Cu2O in the composite and their coupling with Mg(OH)2, an HRTEM analysis was
Fig. 5. XPS spectra of Cu 2p 3/2 of a. MgCu5 and b. MgCu10 samples. 94
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Fig. 6. HRTEM of: a. Mg(OH)2 and b. MgCu5 sample.
Fig. 7. a. Mott-Schottky plots obtained through potentiodynamic EIS at 1000 Hz and b. Flat band potential of CB and VB of reference and MgCu5 and MgCu10 samples.
32 to 149 mgCO2 g−1) in comparison with pure Mg(OH)2 (175 mgCO2 g−1). Hence, the adsorption of CO2 in the composites was attributed mainly to the presence of Mg(OH)2. These results indicated that the incorporation of copper oxides in the composites did not inhibit the complete CO2 uptake of Mg(OH)2. Regarding to the MgCux composites, the best result was reached with the MgCu10 sample (148 mgCO2 g−1), which could favor a high photocatalytic performance in CO2 photoconversion. Conversely, it seems that the presence of Cu2O not favors the gas adsorption, which is in agreement with its higher enthalpy (ΔH = +141 kJ mol−1) in comparison with CuO (ΔH = −45 kJ mol−1) [35].
3.9. Photocatalytic CO2 reduction 3.9.1. Liquid phase The reference material Mg(OH)2 and the composites were subsequently tested for the photocatalytic reduction of CO2 in liquid phase for methanol and formaldehyde generation, which results are shown in Fig. 9. The methanol generation was favored by increasing the copper oxides from 0.5 (2.4 μmol g−1 h−1) up to 10% (6.5 μmol g−1 h−1). After this point, CH3OH concentration tended to lower values (< 2 μmol g−1 h−1). On the other hand, formaldehyde was only detected in MgCu10 sample, which also showed the highest activity for methanol generation. Thus, according to previous characterization, the highest photocatalytic activity of MgCu10 can be related mainly with two factors: i) a higher CO2 adsorption and ii) to its CB potential. At higher loads of copper oxides (15 and 20%), the composites did not present good photocatalytic performance probably due to an excess of CuO/Cu2O on its surface that could promote a photo screening effect. This effect is produced by an excess of suspended particles, resulting in
Fig. 8. Nyquist plot of MgCu5 and MgCu10 under dark and light conditions.
Table 3 CO2 adsorption capacities of Mg(OH)2 and MgCux composites at 2 psi and 25 °C. Sample
CO2 adsorption (mgCO2 gads-1)
Mg(OH)2 MgCu0.5 MgCu5 MgCu10 MgCu15 MgCu20
175 101 64 149 59 32
95
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Fig. 9. Solar fuels generated in liquid phase (Methanol and Formaldehyde) after 3 h of reaction using MgCux composites as photocatalyst (0.1 g). ND refers to no detected.
Fig. 11. X-ray diffraction of the MgCu10 sample before and after the photocatalyst test.
an inefficient absorption of light by the photocatalyst, decreasing the reaction yield [36]. In addition, it can be noted that the presence of localized surface plasmon resonance did not have a significant impact on the photocatalytic activity since the yields obtained with MgCu15 and MgCu20 were low. Thus, it is proposing the photocatalytic reaction mechanism schematized in Fig. 10. In a beginning, CO2 is captured on Mg(OH)2 and then, when the composite is photoexcited, an electron from VB is promoted to CB, leaving a hole in the first band. The CB potential of copper oxides (CuO and Cu2O) are more positive in relation to Mg(OH)2 (as presented in Fig. 7b), which favor a higher efficiency in the electron transfer from Mg(OH)2 to Cu2O and CuO, since copper oxides act as electron traps. In the photocatalytic mechanism, the holes generated in the VB of Mg(OH)2 had the required thermodynamic potential to oxidize the H2O molecule to provide protons, which plays a very important role in the reaction because they are necessary to hydrogenate the CO2 molecule and form the solar fuels expected (CH3OH and HCOH). On the other hand, in the MgCu5 sample was identified the presence of Cu2O in higher proportion, which it promoted the selectivity towards CH3OH production, probably because a lower charge transfer resistance as it was observed in Fig. 8. This is attributed to the ability of Cu2O to selectively adsorb CO (one product of the photocatalytic CO2 reduction), which is subsequently hydrogenated until the formation of HCO [37]. This reaction is promoted due to the presence of unsaturated oxygen on Cu2O. Thus, once the electrons start to transfer from CB of Mg(OH)2 until Cu2O, the reaction starts reducing CO2 to CO until the formation of CH3OH.
3.9.2. Gas phase In order to study the selectivity of the composites (MgCu5, and MgCu10) and the reference Mg(OH)2, photocatalytic experiments were performed in gas phase. During the reaction in gas phase, only MgCu5 showed photocatalytic activity, since it was detected the apparition of both CO and CH4 by gas chromatography. At the final of the photocatalytic reaction, it was possible to quantified the yields generated for CH4 (0.07 μmol g−1 h−1) and CO (0.40 μmol g−1 h−1). According to CB potentials (Fig. 7b), all the samples had the required thermodynamic potentials to produce CH4 and CO. However, the yields obtained were lower than the corresponding efficiencies obtained in liquid phase. Thus, it seems that the water amount is a critical parameter in order to provide more protons that will be available for the CO2 reduction. Finally, after the photocatalytic experiments, the crystal structure of the MgCu10 composite was characterized by XRD in order to analyzed the stability of the copper oxides during the reaction (Fig. 11). As it can be seen, the reflection associated with CuO appears before and after the photocatalytic reaction, which suggest the high stability of the composite developed. Also, this results indicated that the CO2 is adsorbed on the surface by physisorption, which can promote its utilization in the photocatalytic reaction. 4. Conclusions Bifunctional composite materials based on Mg(OH)2, CuO, and Cu2O were synthesized in-situ by microwave-hydrothermal method that allows an effective coupling between the materials. The presence of
Fig. 10. Schematic reaction mechanism proposed for the photocatalytic CO2 reduction using the composite of Mg(OH)2, Cu2O and CuO. 96
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copper oxides in the composites favored both good performance for CO2 capture (148 mgCO2 g−1) and photocatalytic conversion to liquid (CH3OH and HCOH) and gas (CO, CH4) solar fuels. The composite with 10% of copper oxides showed the highest photocatalytic efficiency (6 μmol g−1 h−1 CH3OH and 9 μmol g−1 h−1 HCOH) for liquid fuels generation (CH3OH and HCOH) that can be related with a high CO2 adsorption, and a more negative potential of its CB. In addition, it was found that a higher proportion of Cu2O in the composite promoted the selectivity towards the formation of CH3OH. On the other hand, in gas phase the composites did not show significant photocatalytic activity; however, the generation of low concentrations of CH4 and CO was detected. The stability of copper oxides in the composite was confirmed by XRD after the photocatalytic reaction.
on the anatase TiO2 (101) surface, ACS Catal. 6 (2016) 2018–2025. [14] Y. Izumi, Recent Advances (2012-2015) in the photocatalytic conversion of carbon dioxide to fuels using solar energy: feasibility for a new energy, ACS Symp. Ser. 1 (2015) 1–46. [15] X. An, K. Li, J. Tang, Cu2O/Reduced graphene oxide composites for the photocatalytic conversion of CO2, ChemSusChem 7 (2014) 1086–1093. [16] L. Xu, Y. Zhou, Z. Wu, G. Zheng, J. He, Y. Zho, Improved photocatalytic activity of nanocrystalline ZnO by coupling with CuO, J. Phys. Chem. Solid. 106 (2017) 29–36. [17] S.-M. Park, A. Razzaq, Y.H. Park, S. Sorcar, Y. Park, C.A. Grimes, Hybrid CuxO−TiO2 heterostructured composites for photocatalytic CO2 reduction into methane using solar irradiation: sunlight into fuel, ACS Omega 1 (2016) 868–875. [18] H. Li, Y. Lei, Y. Huang, Y. Fang, Y. Xu, L. Zhu, X. Li, Photocatalytic reduction of carbon dioxide to methanol by Cu2O/SiC nanocrystallite under visible light irradiation, J. Nat. Gas Chem. 20 (2011) 145–150. [19] E. Luévano-Hipólito, L.M. Torres-Martínez, Mg(OH)2 films prepared by ink-jet printing and their photocatalytic activity in CO2 reduction and H2O conversion, Top. Catal. (2018), https://doi.org/10.1007/s11244-018-0966-6. [20] Z. Wang, Q. Li, L. Wang, W. Shangguan, Simultaneous catalytic removal of NOx and soot particulates over CuMgAl hydrotalcites derived mixed metal oxides, Appl. Clay Sci. 55 (2012) 125–130. [21] K. Gelderman, L. Lee, S.W. Donne, Flat-band potential of a semiconductor: using the mott–Schottky equation, J. Chem. Educ. 84 (2007) 685–688. [22] A. Gigante, M. Gotardo, J. Tognolli, L. Pezza, H. Pezza, Spectrophotometric determination of formaldehyde with chromotropic acid in phosphoric acid medium assisted by microwave oven, Microchem. J. 77 (2004) 47–51. [23] A. Kubacka, M. Muñoz-Batista, M. Fernández-García, S. Obregón, G. Colón, Evolution of H2 photoproduction with Cu content on CuOx-TiO2 composite catalysts prepared by a microemulsion method, Appl. Catal., B 163 (2015) 214–222. [24] S. Obregón, M.J. Muñoz-Batista, M. Fernández-García, A. Kubacka, G. Colón, CuTiO2 systems for the photocatalytic H2 production: influence of structural and surface support features, Appl. Catal., B 179 (2015) 468–478. [25] D. Barreca, G. Carraro, E. Comini, A. Gasparotto, C. Maccato, C. Sada, G. Sberveglieri, E. Tondello, D. Barreca, Novel synthesis and gas sensing performances of CuO-TiO2 nanocomposites functionalized with Au nanoparticles, J. Phys. Chem. C 21 (2011) 10510–10517. [26] T. Ghodselahi, M.A. Vesaghi, Localized surface plasmon resonance of Cu@Cu2O core–shell nanoparticles: absorption, scattering and luminescence, Physica B 406 (2011) 2678–2683. [27] O. Peña-Rodríguez, U. Pal, Effects of surface oxidation on the linear optical properties of Cu nanoparticles, J. Opt. Soc. Am. 28 (2011) 2735–2740. [28] Y. Xie, L. Ma, Z.-Q. Cheng, D.-J. Yang, L. Zhou, Z.-H. Hao, Q.-Q. Wang, Plasmonassisted site-selective growth of Ag nanotriangles and Ag-Cu2O hybrids, Sci. Rep. 7 (2017) 1–9. [29] Q. Pan, K. Huang, S. Ni, F. Yang, S. Lin, D. He, Synthesis of sheaf-like CuO from aqueous solution and their application in lithium-ion batteries, J. Alloy. Comp. 484 (2009) 322–326. [30] A. Sahai, N. Goswami, S.D. Kaushik, S. Tripathi, Cu/Cu2O/CuO nanoparticles: novel synthesis by exploding wire technique and extensive characterization, Appl. Surf. Sci. 390 (2016) 974–983. [31] M. Francisco, V. Mastelaro, P. Nascente, A. Florentino, Activity and characterization by XPS, HR-TEM, Raman spectroscopy, and bet surface area of CuO/CeO2-TiO2 catalysts, J. Phys. Chem. B 105 (2001) 10515–10522. [32] V. Vinod Kumar, A. Dharani, M. Mariappan, S. Anthony, Synthesis of CuO and Cu2O nano/microparticles from a single precursor: effect of temperature on CuO/ Cu2O formation and morphology dependent nitroarene reduction, RSC Adv. 6 (2016) 85083–85090. [33] V. Scuderi, G. Amiard, R. Sanz, S. Boninelli, G. Impellizzeri, V. Privitera, TiO2 coated CuO nanowire array: ultrathin p–n heterojunction to modulate cationic/ anionic dye photo-degradation in water, Appl. Surf. Sci. 416 (2017) 885–890. [34] S. Daviðsdóttir, R. Shabadi, A.C. Galca, I.H. Andersen, K. Dirscherl, R. Ambat, Investigation of DC magnetron-sputtered TiO2 coatings: effect of coating thickness, structure, and morphology on photocatalytic activity, Appl. Surf. Sci. 313 (2014) 677–686. [35] W.N.R.W. Isahak, Z.A.C. Ramli, M.W. Ismail, K. Ismail, R.M. Yusop, M.W.M. Hisham, M.A. Yarmo, Adsorption–desorption of CO2 on different type of copper oxides surfaces: physical and chemical attractions studies, J. CO2 Util. 2 (2013) 8–15. [36] E. Albiter, M.A. Valenzuela, S. Alfaro, G. Valverde-Aguilar, F.M. Martínez-Pallares, Photocatalytic deposition of Ag nanoparticles on TiO2: metal precursor effect on the structural and photoactivity properties, J. Saudi Chem. Soc. 19 (2015) 563–573. [37] M. Le, M. Ren, Z. Zhang, P. Sprunger, R. Kurtz, J. Flake, Electrochemical reduction of CO2 to CH3OH at copper oxide surfaces, J. Electrochem. Soc. 158 (2011) 45–49.
Acknowledgments The authors wish to thank CONACYT for financial support for this research through the following projects: Cátedras CONACYT 1060, Beca mixta No. 740859, Scholarship CONACYT 598840, CONACYT-CB2014-237049, CONACYT-PDCPN-2015-487, CONACYT-NRF-2016278729. In addition, the authors want to thank to Dr. Juan Edgar Carrera Crespo for its valuable help with the photoelectrochemical characterization. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.matchemphys.2019.01.062. References [1] J.A. Herron, J. Kim, A.A. Upadhye, G.W. Hubera, C.T. Maravelias, A general framework for the assessment of solar fuel technologies, Energy Environ. Sci. 8 (2015) 126–157. [2] S.C. Roy, O.K. Varghese, M. Paulose, C.A. Grimes, Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons, ACS Nano 4 (2010) 1259–1278. [3] L. Liu, C. Zhao, J. Xu, Y. Li, Integrated CO2 capture and photocatalytic conversion by a hybrid adsorbent/photocatalyst material, Appl. Catal. B Environ. 179 (2015) 489–499. [4] C.-H. Yu, C.-H. Huang, C.-S. Tan, A review of CO2 capture by absorption and adsorption, Aerosol Air Qual. Res. 12 (2012) 745–769. [5] B. Zhang, Y. Duan, K. Johnson, Density functional theory study of CO2 capture with transition metal oxides and hydroxides, J. Chem. Phys. 136 (2012) 0645161–064516-13. [6] A. Sanna, M. Uibu, G. Caramanna, R. Kuusikb, M.M. Maroto-Valer, A review of mineral carbonation technologies to sequester CO2, Chem. Soc. Rev. 43 (2014) 8049–8080. [7] M. Mikkelsen, M. Jorgensen, F.C. Krebs, The teraton challenge. A review of fixation and transformation of carbon dioxide, Energy Environ. Sci. 3 (2010) 43–81. [8] D. Whipple, P. Kenis, Prospects of CO2 utilization via direct heterogeneous electrochemical reduction, J. Phys. Chem. Lett. 24 (2010) 3451–3458. [9] M. Gattrell, N. Gupta, A. Co, A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper, J. Electroanal. Chem. 594 (2006) 1–19. [10] T. Guaraldo, J. De Brito, D. Wood, M.V.B. Zanoni, A new Si/TiO2/Pt p-n junction semiconductor to demonstrate photoelectrochemical CO2 conversion, Electrochim. Acta 185 (2015) 117–124. [11] J. Cheng, M. Zhang, G. Wu, X. Wang, J. Zhou, K. Cen, Photoelectrocatalytic reduction of CO2 into chemicals using Pt-modified reduced graphene oxide combined with Pt-modified TiO2 nanotubes, Environ. Sci. Technol. 48 (2014) 7076–7084. [12] E. Karamian, S. Sharifnia, On the general mechanism of photocatalytic reduction of CO2, Biochem. Pharmacol. 16 (2016) 194–203. [13] Y. Ji, Y. Luo, Theoretical study on the mechanism of photoreduction of CO2 to CH4
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CO2 adsorption and photocatalytic reduction over Mg(OH)2/CuO/Cu2O under UV-Visible light to solar fuels
T
M. Flores-Floresa, E. Luévano-Hipólitob, Leticia M. Torres-Martíneza,∗, Trong-On Doc a
Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P. 66455, San Nicolás de los Garza, NL, Mexico b CONACYT - Universidad Autónoma de Nuevo León, Facultad de Ingeniería Civil-Departamento de Ecomateriales y Energía, Cd. Universitaria, C.P. 66455, San Nicolás de los Garza, NL, Mexico c Department of Chemical Engineering, Laval University, Québec, G1 V 0A6, Canada
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
materials were applied • Bifunctional for CO capture and photocatalytic 2
• • • •
conversion. Mg(OH)2 were effectively coupled with CuO/Cu2O by microwave-hydrothermal method. Mg(OH)2/CuO/Cu2O was used as photocatalyst to convert CO2 to CH3OH, HCOH, CO and CH4. The coupling of materials promoted an efficient charge transfer in the reaction. The presence of Cu2O promoted the selectivity to CH3OH generation in liquid phase.
A R T I C LE I N FO
A B S T R A C T
Keywords: CO2 capture Mg(OH)2 Solar fuels CO2 utilization Cu2O CuO Copper oxides
Powders of Mg(OH)2, CuO, and Cu2O were effectively coupled by microwave-hydrothermal method, in order to proposed bifunctional materials for both capture and photocatalytic conversion of CO2 under UV-visible light irradiation. This was done to take advantage of the high CO2 adsorption capacity of Mg(OH)2 and the good photocatalytic performance of CuO/Cu2O for CO2 reduction to solar fuels in liquid (CH3OH and HCOH) and gas phase (CH4 and CO). In order to establish correlations between the physical properties and the photocatalytic activity, the composites were characterized by XRD, XPS, UV-Vis DRS, SEM, HRTEM, N2 physisorption and photoelectrochemical techniques. According to the characterization, the synthesis method employed allowed the adequate interaction between Mg(OH)2 with CuO and Cu2O, which not inhibited the ability of Mg(OH)2 for gas adsorption. The best yield to obtain liquid fuels such as CH3OH (6 μmol g−1 h−1) and HCOH (9 μmol g−1 h−1) was obtained using 10% of CuO in the composite. The improved photocatalytic activity in liquid phase was assigned to a high CO2 adsorption and a more negative potential of conduction band. It was found that the presence of Cu2O favored the selectivity towards CH3OH production; as higher Cu+ concentration better selectivity. A reaction mechanism is discussed on the basis of combined CO2 adsorption and photocatalytic activity of the materials involved. Furthermore, it was study the photocatalytic activity in gas phase, and it was determined the presence of CO and CH4 in low concentrations (< 0.4 μmol g−1 h−1).
∗
Corresponding author. E-mail address: [email protected] (L.M. Torres-Martínez).
https://doi.org/10.1016/j.matchemphys.2019.01.062 Received 23 July 2018; Received in revised form 15 January 2019; Accepted 25 January 2019 Available online 29 January 2019 0254-0584/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
synthesized by one-pot microwave-hydrothermal method. In this method, a 2M solution of magnesium nitrate (Mg(NO3)2·6H2O) (Aldrich, 99%) was dissolved in 50 mL of distilled water. Then, different amounts of copper acetate (Cu(CH3COO)2·H2O) (Aldrich, 99%) was added into the magnesium solution to reach different loads of copper oxide (0.5, 5, 10, 15, and 20% by weight). Then, NaOH was added to promote the formation of the hydroxide phase under vigorous stirring for 10 min. The resulting solution was exposed at microwave radiation using a programmable microwave MARS-6, CEM at 600 W and 100 °C for 30 min. The samples were washed with deionized water and ethanol and dried at 100 °C before their use. Hereafter, the composites will be identified as MgCux, where x refers to the weight percentage of copper oxides in the samples.
The rising in CO2 emissions due to the accelerated population growth and the industrial sector has resulted in an increase of the greenhouse effect and consequently, a climate change. One of the potential solutions to reduce CO2 in the atmosphere is related to its capture and utilization [1,2]. In recent years, the capture of CO2 and photocatalytic conversion (CCPC) has gained more attention since it is possible to obtain solar fuels from CO2, water, and photocatalysts [3]. In regard to CO2 capture, in last years several technologies had been integrated in post-combustion plants, including physical and chemical absorption (solvents), adsorption (solid sorbents), and membranes [4]. In particular, the use of solid sorbents has received an increased interest due to the high CO2 capture efficiencies reached. This has been related to its thermodynamically feasibility (ΔG < 0), low-cost, and because they are easy to scale compared to membranes [5]. One of these solid sorbents is the magnesium hydroxide (Mg(OH)2) with a hexagonal (brucite) structure, which it is widely abundant in minerals such as olivine ((Mg,Fe)2SiO4), orthopyroxene (Mg2Si2O6–Fe2Si2O6), clinopyroxene (CaMgSi2O6–CaFeSi2O6), serpentine ((Mg,Fe)3Si2O5(OH)4), among others [6]. Another interesting development as part of CCPC research is the use of captured CO2 as chemical feedstock. CO2 conversion to gas and liquid fuels (i.e., syngas, methane, methanol, formaldehyde, formic acid) via chemical, electrochemical or photochemical reactions has been proposed and significant efforts are being carried out in this field [7]. Among the different methods suggested so far, the photocatalytic conversion of CO2 into value added compounds (solar fuels) enables the utilization of clean energy (i.e. solar energy) to produce sustainable sources of energy [8–11]. In these processes, it has been reported that the selectivity of the reaction depends of the nature, physical properties of the photocatalysts, and also it is highly dependent of the reaction medium in which it is evaluated (liquid and gas). In gas phase, the CO2 is reduced to CO and to radicals •CO− that in contact with H+ promote the evolution towards CH4 and CH3OH. In liquid phase, the consecutive reactions of CO2 reduction end in the evolution of alcohols (CH3OH, C2H5OH), carboxyl groups (HCOOH), and aldehyde compounds (HCOH) [12–14]. Semiconductor oxides are the most investigated materials that had been proposed as photocatalysts. Among them, copper oxides (CuO and Cu2O) are related to high stability, low cost, narrow band gap energies that make possible to absorb solar light, and appropriate conduction and valence band potentials, which make them ideal photocatalysts for CO2 reduction [15]. The coupling of Cu2O and CuO with other semiconductor materials has been proposed as an alternative to improve the efficiency of the reaction for other photocatalytic reactions, i.e. degradation of methylene blue [16], and in recent years have proposed the formation of heterostructures of CuxO with TiO2 for CH4 formation from CO2 reduction [17]. In particular, the enhancement in the photocatalytic activity by adding CuO and Cu2O to other semiconductors such as SiC has been related to an improvement in the charge transfer due to the favorable position (more negative) of the conduction band (CB) of CuO and Cu2O. This effect has been resulted in a higher methanol production from CO2 photoreduction [18]. On the other hand, there are a few reports related to the use of Mg(OH)2 as photocatalyst for CO2 photoconversion [19]. To the best of our knowledge, there are no reports related to the coupling of adsorbents such as Mg(OH)2 with photocatalysts such as copper oxides to produce solar fuels from CO2 photoreduction. Thus, in this work is proposed a methodology to coupling Mg(OH)2 with CuO and Cu2O to capture and photoconvert CO2 under UV-visible light irradiation to liquid and gas fuels for first time.
2.2. Characterization The structural characterization of the samples was carried out by Xray powder diffraction using a Bruker D8 Advance diffractometer with Cu Kα radiation (40 kV, 30 mA). A typical run was made with a 0.05° of step size and a dwell time of 0.5 s. From this data, it was obtained the crystallite size using the Debye's-Scherrer equation. Previous works have reported that some cations such as Cu2+, Cr+2, Mn3+, and Ni3+ show the Jahn-Teller effect, which is defined as a distortion caused by the incorporation of these cations in the structure [20]. Thus, in order to study the distortion occasioned by the addition of copper on the Mg(OH)2 structure, it was evaluated the lattice strain using equation (1):
ε=
βhkl 4 tan θ
(1)
where ε is the lattice strain and βhkl is the instrument broadening, which it was calculated using equation (2). In this case, we used the Al2O3 corundum as reference.
βhkl = [(βhkl )2measured − (βhkl )2instrumental]1/2
(2)
The optical properties of samples were analyzed between 200 and 1500 nm using a UV-Vis NIR spectrophotometer (Cary 5000) coupled with an integration sphere for diffuse reflectance measurements. The BET surface area measurements were carried out by N2 adsorptiondesorption isotherms using a Bel-Japan Minisorp II surface area and pore size analyzer. The morphology of the samples was analyzed by scanning electron microscopy using a JEOL 6490 LV. The surface groups in the samples were study by FTIR using a Perkin Elmer FTIR/ FIR Frontier with ATR accessory in a range of 400–4000 cm−1. The identification of crystal planes in the brucite structure was performed by high-resolution transmission electron microscopy using a FEI Titan G2 80-300 microscopes with an accelerating voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) data were collected by a K-Alpha-Xray photoelectron spectrometer under ultrahigh vacuum 10−9 Torr, using an Al Kα, 1486.6 eV monochromated X-ray source operated at 3 mA corresponding to a spot size of 400 mm. Survey scans were acquired at a pass energy of 200 eV and 50 eV for the acquisition of highresolution windows over the binding energy range 1200-0 eV. The C 1s signal at 284.8 eV was used as reference. The flat band potentials were determined by electrochemical impedance spectroscopy (EIS) by using the Mott-Schottky plots. For this purpose, it was using a three electrode cell using NaSO4 (0.5 M) as electrolyte, Ag/AgCl as reference electrode, and Pt as counter electrode. The electrochemical experiments were carried out using a potentiostat AUTOLAB PGSTAT 302N connected to a computer running the software NOVA for data acquisition. The samples were fixed in ITO glass of 1 × 1 cm2. In this method, a plot of 1/C2 against V should yield a straight line from which the potential of the flat band can be determined from the intercept on the V axis [21]. The experimental energy band diagrams were constructed from the estimation of the CB by
2. Materials and methods 2.1. Synthesis of composites The composite materials of Mg(OH)2, CuO, and Cu2O were 91
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3. Results and discussion
this technique, and the VB potential was determined by adding to CB the band gap value obtained from the UV-Vis diffuse reflectance spectra.
3.1. X-ray powder diffraction XRD patterns of the MgCux composite materials are shown in Fig. 2a. In the reference sample (Mg(OH)2) all the reflections were assigned in accordance to the JCDPS card 07-0239. Magnesium hydroxide crystallized in the hexagonal system with the linear parameters: a = 3.147 Å, c = 4.769 Å, and angular parameters: α = β = 90°, and γ = 120°. After the introduction of copper oxides in the base material, the main intensity reflection of Mg(OH)2 (101) was decreased, and only in the sample with 10% of copper (MgCu10) appears a reflection at 2θ = 36° associated to monoclinic CuO (JCPDS card 01-089-5895), as it can be seen in the zoom-in from 34 to 40° (Fig. 2b). However, this reflection was not observed in samples with higher copper content such as MgCu15 and MgCu20, which could be related to a decrement in its crystallite size (Table 1). Previous reports had determined that higher loads of Cu (> 4.5%) in semiconductor oxides, promoted the formation of nanometric clusters that cannot be detected by XRD [23,24]. In addition, the diminishment in the intensity of reflections might be attributed to distortions in the brucite structure generated by the substitution of Cu2+ for Mg2+. This fact is associated to that Cu2+ has a higher ionic radius than Mg2+, which could promote the Jahn-Teller effect. To corroborate this, it was calculated the distortion of lattice strain (by equation (1)) from XRD data, which results are shown in Table 1. As it can be seen, a higher lattice strain was observed as long as the load of copper increase in the composites.
2.3. CO2 capture experiments The CO2 uptake experiments were carried out in slurry-phase in a cylindrical batch reactor of 200 mL at 20 °C and 2 psi. The mass of the sorbent was fixed to 0.1 g and, it was dispersed in 100 mL of deionized water under vigorous stirring. The CO2 concentration was monitored by gas chromatography using a Thermo Scientific equipped with TCD, using Argon as diluent.
2.4. Photocatalytic CO2 reduction 2.4.1. Liquid phase The reactions in liquid phase were carried out in the same cylindrical batch reactor that it was used for the CO2 uptake experiments, under the same experimental conditions with the addition of visible light provided by two halogen lamps of 20 W each one emitting between 300 and 900 nm in order to simulate the UV-visible lamp of the solar spectrum, which spectra is shown in Fig. 1. As it can be seen, the main emission comes from the visible part (84%) and the rest comes from UV (5%) and IR (11%) parts. In the beginning of the experiment, the suspension was purged with CO2 for 15 min to saturate the medium and remove the oxygen. Then, the suspension was irradiated from the outside of the reactor. The reaction products in liquid phase were analyzed by a gas chromatograph Thermo Scientific equipped with a FID detector (for CH3OH) and with an UV-Vis NIR spectrophotometer Cary 5000 using the complexation of formaldehyde with chromotropic acid [22].
3.2. Scanning electron microscopy SEM images of Mg(OH)2 and MgCux composites are shown in Fig. 3. The morphology of pure Mg(OH)2 presented flake-like morphology of an average particle size of 111 nm (Fig. 3a). This type of morphology prevailed when 0.5–5% of copper oxides was introduced in the reference sample. Nevertheless, it was observed an increase in the size and agglomeration degree in its particles (Fig. 3 a–c). From 10% of copper oxide, the particles tended to agglomerate with each other to form bigger clusters with small particles deposited on them (Fig. 3d–f). Since it was not observed different morphologies in the reference and composite materials, supplementary analysis of EDS was performed to analyze the distribution of copper in Mg(OH)2 (Supplementary Fig. S1). From these data, it was observed that copper is distributed homogeneously in the surface of the base-material, which confirms the advantage of the synthesis method proposed.
2.4.2. Gas phase The photocatalytic experiments in gas phase were carried out in a stainless steel cylindrical reactor of 200 mL. The reactor at the top has a quartz window. The sample was irradiated from the outside of the reactor with a solar simulator using a 150 W Xe lamp (AM 1.5 G 100 mW cm−2) for 3 h. The mass of the photocatalyst was fixed to 0.1 g. The system was purged with N2 for 5 min and then saturated with CO2 with a 3 mL h−1 flow for 30 min. The gas products (CO and CH4) were analyzed with a gas chromatograph Agilent Technologies 7820 A equipped with TCD and FID detector.
3.3. N2 physisorption The textural properties of the composites were investigated by means of N2 isotherms (Supplementary Fig. S2), since one of the key aspects of Mg(OH)2 in the composites was to promote CO2 capture. Table 2 summarizes the BET surface area of the MgCux composites. The loading of copper oxides on Mg(OH)2 had a positive effect in the specific surface area, which increase almost 2.5 times: from 54 m2 g−1 (Mg (OH)2) to 142 m2 g−1 MgCu15. This could be beneficial because a growth in the surface area is related to an increase of the active sites available for the adsorption of CO2. Additionally, the profile of N2 isotherms showed the same trend that the reference sample (type-III), which is characteristic of materials related to low energy of adsorption. In addition, a small hysteresis was developed in the range of relative pressure from 0.85 to 0.99 in MgCu15 and MgCu20 samples, related to the formation of small pores in the particles. 3.4. UV-Vis spectroscopy Fig. 1. Comparison between the spectra of the sun and the halogen lamp.
The band gap energy of the composites was study by UV-Vis 92
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Fig. 2. X-ray powder diffraction patterns of the MgCux composites, b. zoom-in of reflections between 2θ = 34–40°. Table 1 Crystallite size and lattice strain of the MgCux composites.
Table 2 Band gap energy and specific surface area of the MgCux composites.
Sample
Crystallite size (nm)
Lattice strain (%)
Sample
Eg (eV)
sBET (m2 g−1)
Mg(OH)2 MgCu0.5 MgCu5 MgCu10 MgCu15 MgCu20
97 97 29 30 16 16
0.107 0.109 0.358 0.358 0.668 0.667
Mg(OH)2 MgCu0.5 MgCu5 MgCu10 MgCu15 MgCu20
4.9 4.4 3.8 3.5 2.9 2.7
54 70 117 96 142 136
the synergy effect between the p-type CuO and n-type Mg(OH)2 [25]. An adequate coupling of semiconductors p and n-type can promote an improving in the charge transfer. Also, it should be note a broad absorption band in wavelengths from 600 to 800 nm in samples with the highest load of copper (MgCu15, and MgCu20), which might be related to the plasmonic effect of Cu2O [26,27]. This effect is affected by particle size, thus it seems that in MgCu15 and MgCu20 the presence of nanoclusters of copper (previously discussed in XRD section) promote the plasmonic effect that could affect the photocatalytic activity.
spectroscopy, which spectra are shown in Fig. 4. The band gap energy of the reference material was 4.9 eV, which is indicative that could present low photocatalytic activity under visible irradiation. However, when copper oxides were introduced in the composites, the band gap energies tended to lower values (the lowest was 2.7 eV for MgCu20) that make possible the absorption of solar light (Table 2). The analysis in the absorption spectra of the composites showed two intervals of absorption in wavelengths: i) 200–350, and ii) 400–800 nm. The second absorption (400–800 nm) is attributed to the presence of higher amounts of CuO (particularly in MgCu15 and MgCu20), which favored a higher visible light absorption. The increase in the light absorption could be associated to a coupling between the semiconductors due to
Fig. 3. SEM micrographs of the samples a) Mg(OH)2, b) MgCu0.5, c) MgCu5, d) MgCu10, and e), f) MgCu20. 93
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performed in the reference and MgCu5 sample, which results are shown in Fig. 6. The HRTEM image of the reference sample shows the presence of crystal planes (101) and (102) of Mg(OH)2, which is in agreement with the XRD results (Fig. 6a). On the other hand, a carefully analysis of the MgCu5 sample confirmed the presence of both CuO and Cu2O in the composite (Fig. 6b). In this sample, it was found additional d spacings, which were 0.231 nm and 0.246 nm, indexed as CuO (111) and Cu2O (111), respectively. Thus, according to these results, it was confirmed the effective coupling in the composite of Mg(OH)2 with CuO, and Cu2O, which junction could benefit the photocatalytic reaction. 3.7. Photo(electro)chemical characterization The potential of the conduction band (CB) of some representative samples (Mg(OH)2, MgCu5, and MgCu10) was estimated by means of potentiodynamic electrochemical impedance spectroscopy (Fig. 7a), using the conditions described in the experimental section. The MottSchottky plots obtained from these measurements show a positive slope in the linear region of the C−2 vs potential (Fig. 7a), confirming that the materials showed are n-type semiconductors. The flat band potential values of the materials were determined from the interception point between the extrapolated slope and the x-axis. These Efb values were assumed as CB in the schematic energy level diagram, because the materials showed a n-behavior. According to the schematic energy diagram, the CB potential of the base material (Mg(OH)2) tended to more positive potentials after the introduction of copper oxides. Nevertheless, the composited analyzed had the required thermodynamic potential to reduce CO2 to CO, HCHO, CH3OH and CH4. In particular, the CB potential of MgCu10 was slightly more negative than MgCu5, which could be related to differences in their chemical composition. MgCu10 composite had a higher amount of p-type (CuO), which under adequate coupling could move up its CB, while the Fermi energy of the n-type (Mg(OH)2) might move down until a state of equilibrium, favoring more negative potentials and also, a more efficient charge transfer [33]. To corroborate if the charge transfer in the composite was improved after the coupling of Mg(OH)2 with CuO and Cu2O, an analysis in the Nyquist plot under light and dark conditions was performed (Fig. 8). In general, a smaller arc radius in the plot indicates a smaller electron transfer resistance at the surface of the photocatalysts, which normally leads to more effective separation of the electron and hole pair [34]. MgCu5 sample showed a lower semicircle under dark and light conditions than MgCu10 sample, which indicates a better separation of the electron-hole pair and faster interfacial charge transfer under UV-visible light irradiation. This fact could benefit the reduction of CO2.
Fig. 4. UV-Vis absorption spectra obtained of the MgCux composites.
3.5. X-ray photoelectron spectroscopy In order to study the composition of the composite surface, an XPS analysis was performed in selected samples (Mg(OH)2, MgCu5, and MgCu10). In the survey spectrum, the characteristic peaks of Mg2p and Mg2s were observed and the presence of Cu2p1/2 and Cu2p3/2 was confirmed in MgCu5 and MgCu10 samples (Supplementary Fig. S3). Fig. 5 shows the Cu2p XPS spectra of the MgCu5 and MgCu10 samples. The characteristic peaks of Cu 2p3/2 and Cu2p1/2 were detected for both samples accompanied by a series of peaks (satellites) in 940.3 and 942.9 eV, respectively. In both XPS spectra of MgCu5 and MgCu10, the Cu2p3/2 peak can be deconvoluted in two peaks at 932.0 and 933.9 eV that correspond to Cu+ and Cu2+, respectively [28–32]. Based on these results, it was corroborated the presence of Cu2O and CuO in both MgCu5 and MgCu10 samples, with some differences between them. For a more precise analysis, the peak area associated to Cu2O and CuO was quantified in order to estimate the relation between these oxides. The relation of Cu2O/CuO was higher in the sample MgCu5, which it suggests a higher amount of Cu2O. According to the literature, CuO is preferentially formed at lower temperatures, and it comes from the decomposition of the acetate anion to CH3COOH and OH−, which reacts with Cu+2 to produce CuO. At higher temperatures, the CH3COOH is converted to HCOOH, reacting with CuO to produce Cu2O. However, in this work it was used the same temperature for all the experiments, which it suggests that when the amount of copper was 5%, a more efficient heating was enhance inside the reactor that produce a higher localized temperature in the composite surface, which could promote the reduction of small parts of CuO to Cu2O, as it was detected by XPS.
3.8. CO2 capture 3.6. High resolution transmission electron microscopy As the effective coupling of composites was confirmed, the amount of CO2 capture was investigated, which results are shown in Table 3. The MgCux composites presented lower adsorption capacities (ranging
In order to confirm the presence of both CuO and Cu2O in the composite and their coupling with Mg(OH)2, an HRTEM analysis was
Fig. 5. XPS spectra of Cu 2p 3/2 of a. MgCu5 and b. MgCu10 samples. 94
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Fig. 6. HRTEM of: a. Mg(OH)2 and b. MgCu5 sample.
Fig. 7. a. Mott-Schottky plots obtained through potentiodynamic EIS at 1000 Hz and b. Flat band potential of CB and VB of reference and MgCu5 and MgCu10 samples.
32 to 149 mgCO2 g−1) in comparison with pure Mg(OH)2 (175 mgCO2 g−1). Hence, the adsorption of CO2 in the composites was attributed mainly to the presence of Mg(OH)2. These results indicated that the incorporation of copper oxides in the composites did not inhibit the complete CO2 uptake of Mg(OH)2. Regarding to the MgCux composites, the best result was reached with the MgCu10 sample (148 mgCO2 g−1), which could favor a high photocatalytic performance in CO2 photoconversion. Conversely, it seems that the presence of Cu2O not favors the gas adsorption, which is in agreement with its higher enthalpy (ΔH = +141 kJ mol−1) in comparison with CuO (ΔH = −45 kJ mol−1) [35].
3.9. Photocatalytic CO2 reduction 3.9.1. Liquid phase The reference material Mg(OH)2 and the composites were subsequently tested for the photocatalytic reduction of CO2 in liquid phase for methanol and formaldehyde generation, which results are shown in Fig. 9. The methanol generation was favored by increasing the copper oxides from 0.5 (2.4 μmol g−1 h−1) up to 10% (6.5 μmol g−1 h−1). After this point, CH3OH concentration tended to lower values (< 2 μmol g−1 h−1). On the other hand, formaldehyde was only detected in MgCu10 sample, which also showed the highest activity for methanol generation. Thus, according to previous characterization, the highest photocatalytic activity of MgCu10 can be related mainly with two factors: i) a higher CO2 adsorption and ii) to its CB potential. At higher loads of copper oxides (15 and 20%), the composites did not present good photocatalytic performance probably due to an excess of CuO/Cu2O on its surface that could promote a photo screening effect. This effect is produced by an excess of suspended particles, resulting in
Fig. 8. Nyquist plot of MgCu5 and MgCu10 under dark and light conditions.
Table 3 CO2 adsorption capacities of Mg(OH)2 and MgCux composites at 2 psi and 25 °C. Sample
CO2 adsorption (mgCO2 gads-1)
Mg(OH)2 MgCu0.5 MgCu5 MgCu10 MgCu15 MgCu20
175 101 64 149 59 32
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Fig. 9. Solar fuels generated in liquid phase (Methanol and Formaldehyde) after 3 h of reaction using MgCux composites as photocatalyst (0.1 g). ND refers to no detected.
Fig. 11. X-ray diffraction of the MgCu10 sample before and after the photocatalyst test.
an inefficient absorption of light by the photocatalyst, decreasing the reaction yield [36]. In addition, it can be noted that the presence of localized surface plasmon resonance did not have a significant impact on the photocatalytic activity since the yields obtained with MgCu15 and MgCu20 were low. Thus, it is proposing the photocatalytic reaction mechanism schematized in Fig. 10. In a beginning, CO2 is captured on Mg(OH)2 and then, when the composite is photoexcited, an electron from VB is promoted to CB, leaving a hole in the first band. The CB potential of copper oxides (CuO and Cu2O) are more positive in relation to Mg(OH)2 (as presented in Fig. 7b), which favor a higher efficiency in the electron transfer from Mg(OH)2 to Cu2O and CuO, since copper oxides act as electron traps. In the photocatalytic mechanism, the holes generated in the VB of Mg(OH)2 had the required thermodynamic potential to oxidize the H2O molecule to provide protons, which plays a very important role in the reaction because they are necessary to hydrogenate the CO2 molecule and form the solar fuels expected (CH3OH and HCOH). On the other hand, in the MgCu5 sample was identified the presence of Cu2O in higher proportion, which it promoted the selectivity towards CH3OH production, probably because a lower charge transfer resistance as it was observed in Fig. 8. This is attributed to the ability of Cu2O to selectively adsorb CO (one product of the photocatalytic CO2 reduction), which is subsequently hydrogenated until the formation of HCO [37]. This reaction is promoted due to the presence of unsaturated oxygen on Cu2O. Thus, once the electrons start to transfer from CB of Mg(OH)2 until Cu2O, the reaction starts reducing CO2 to CO until the formation of CH3OH.
3.9.2. Gas phase In order to study the selectivity of the composites (MgCu5, and MgCu10) and the reference Mg(OH)2, photocatalytic experiments were performed in gas phase. During the reaction in gas phase, only MgCu5 showed photocatalytic activity, since it was detected the apparition of both CO and CH4 by gas chromatography. At the final of the photocatalytic reaction, it was possible to quantified the yields generated for CH4 (0.07 μmol g−1 h−1) and CO (0.40 μmol g−1 h−1). According to CB potentials (Fig. 7b), all the samples had the required thermodynamic potentials to produce CH4 and CO. However, the yields obtained were lower than the corresponding efficiencies obtained in liquid phase. Thus, it seems that the water amount is a critical parameter in order to provide more protons that will be available for the CO2 reduction. Finally, after the photocatalytic experiments, the crystal structure of the MgCu10 composite was characterized by XRD in order to analyzed the stability of the copper oxides during the reaction (Fig. 11). As it can be seen, the reflection associated with CuO appears before and after the photocatalytic reaction, which suggest the high stability of the composite developed. Also, this results indicated that the CO2 is adsorbed on the surface by physisorption, which can promote its utilization in the photocatalytic reaction. 4. Conclusions Bifunctional composite materials based on Mg(OH)2, CuO, and Cu2O were synthesized in-situ by microwave-hydrothermal method that allows an effective coupling between the materials. The presence of
Fig. 10. Schematic reaction mechanism proposed for the photocatalytic CO2 reduction using the composite of Mg(OH)2, Cu2O and CuO. 96
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copper oxides in the composites favored both good performance for CO2 capture (148 mgCO2 g−1) and photocatalytic conversion to liquid (CH3OH and HCOH) and gas (CO, CH4) solar fuels. The composite with 10% of copper oxides showed the highest photocatalytic efficiency (6 μmol g−1 h−1 CH3OH and 9 μmol g−1 h−1 HCOH) for liquid fuels generation (CH3OH and HCOH) that can be related with a high CO2 adsorption, and a more negative potential of its CB. In addition, it was found that a higher proportion of Cu2O in the composite promoted the selectivity towards the formation of CH3OH. On the other hand, in gas phase the composites did not show significant photocatalytic activity; however, the generation of low concentrations of CH4 and CO was detected. The stability of copper oxides in the composite was confirmed by XRD after the photocatalytic reaction.
on the anatase TiO2 (101) surface, ACS Catal. 6 (2016) 2018–2025. [14] Y. Izumi, Recent Advances (2012-2015) in the photocatalytic conversion of carbon dioxide to fuels using solar energy: feasibility for a new energy, ACS Symp. Ser. 1 (2015) 1–46. [15] X. An, K. Li, J. Tang, Cu2O/Reduced graphene oxide composites for the photocatalytic conversion of CO2, ChemSusChem 7 (2014) 1086–1093. [16] L. Xu, Y. Zhou, Z. Wu, G. Zheng, J. He, Y. Zho, Improved photocatalytic activity of nanocrystalline ZnO by coupling with CuO, J. Phys. Chem. Solid. 106 (2017) 29–36. [17] S.-M. Park, A. Razzaq, Y.H. Park, S. Sorcar, Y. Park, C.A. Grimes, Hybrid CuxO−TiO2 heterostructured composites for photocatalytic CO2 reduction into methane using solar irradiation: sunlight into fuel, ACS Omega 1 (2016) 868–875. [18] H. Li, Y. Lei, Y. Huang, Y. Fang, Y. Xu, L. Zhu, X. Li, Photocatalytic reduction of carbon dioxide to methanol by Cu2O/SiC nanocrystallite under visible light irradiation, J. Nat. Gas Chem. 20 (2011) 145–150. [19] E. Luévano-Hipólito, L.M. Torres-Martínez, Mg(OH)2 films prepared by ink-jet printing and their photocatalytic activity in CO2 reduction and H2O conversion, Top. Catal. (2018), https://doi.org/10.1007/s11244-018-0966-6. [20] Z. Wang, Q. Li, L. Wang, W. Shangguan, Simultaneous catalytic removal of NOx and soot particulates over CuMgAl hydrotalcites derived mixed metal oxides, Appl. Clay Sci. 55 (2012) 125–130. [21] K. Gelderman, L. Lee, S.W. Donne, Flat-band potential of a semiconductor: using the mott–Schottky equation, J. Chem. Educ. 84 (2007) 685–688. [22] A. Gigante, M. Gotardo, J. Tognolli, L. Pezza, H. Pezza, Spectrophotometric determination of formaldehyde with chromotropic acid in phosphoric acid medium assisted by microwave oven, Microchem. J. 77 (2004) 47–51. [23] A. Kubacka, M. Muñoz-Batista, M. Fernández-García, S. Obregón, G. Colón, Evolution of H2 photoproduction with Cu content on CuOx-TiO2 composite catalysts prepared by a microemulsion method, Appl. Catal., B 163 (2015) 214–222. [24] S. Obregón, M.J. Muñoz-Batista, M. Fernández-García, A. Kubacka, G. Colón, CuTiO2 systems for the photocatalytic H2 production: influence of structural and surface support features, Appl. Catal., B 179 (2015) 468–478. [25] D. Barreca, G. Carraro, E. Comini, A. Gasparotto, C. Maccato, C. Sada, G. Sberveglieri, E. Tondello, D. Barreca, Novel synthesis and gas sensing performances of CuO-TiO2 nanocomposites functionalized with Au nanoparticles, J. Phys. Chem. C 21 (2011) 10510–10517. [26] T. Ghodselahi, M.A. Vesaghi, Localized surface plasmon resonance of Cu@Cu2O core–shell nanoparticles: absorption, scattering and luminescence, Physica B 406 (2011) 2678–2683. [27] O. Peña-Rodríguez, U. Pal, Effects of surface oxidation on the linear optical properties of Cu nanoparticles, J. Opt. Soc. Am. 28 (2011) 2735–2740. [28] Y. Xie, L. Ma, Z.-Q. Cheng, D.-J. Yang, L. Zhou, Z.-H. Hao, Q.-Q. Wang, Plasmonassisted site-selective growth of Ag nanotriangles and Ag-Cu2O hybrids, Sci. Rep. 7 (2017) 1–9. [29] Q. Pan, K. Huang, S. Ni, F. Yang, S. Lin, D. He, Synthesis of sheaf-like CuO from aqueous solution and their application in lithium-ion batteries, J. Alloy. Comp. 484 (2009) 322–326. [30] A. Sahai, N. Goswami, S.D. Kaushik, S. Tripathi, Cu/Cu2O/CuO nanoparticles: novel synthesis by exploding wire technique and extensive characterization, Appl. Surf. Sci. 390 (2016) 974–983. [31] M. Francisco, V. Mastelaro, P. Nascente, A. Florentino, Activity and characterization by XPS, HR-TEM, Raman spectroscopy, and bet surface area of CuO/CeO2-TiO2 catalysts, J. Phys. Chem. B 105 (2001) 10515–10522. [32] V. Vinod Kumar, A. Dharani, M. Mariappan, S. Anthony, Synthesis of CuO and Cu2O nano/microparticles from a single precursor: effect of temperature on CuO/ Cu2O formation and morphology dependent nitroarene reduction, RSC Adv. 6 (2016) 85083–85090. [33] V. Scuderi, G. Amiard, R. Sanz, S. Boninelli, G. Impellizzeri, V. Privitera, TiO2 coated CuO nanowire array: ultrathin p–n heterojunction to modulate cationic/ anionic dye photo-degradation in water, Appl. Surf. Sci. 416 (2017) 885–890. [34] S. Daviðsdóttir, R. Shabadi, A.C. Galca, I.H. Andersen, K. Dirscherl, R. Ambat, Investigation of DC magnetron-sputtered TiO2 coatings: effect of coating thickness, structure, and morphology on photocatalytic activity, Appl. Surf. Sci. 313 (2014) 677–686. [35] W.N.R.W. Isahak, Z.A.C. Ramli, M.W. Ismail, K. Ismail, R.M. Yusop, M.W.M. Hisham, M.A. Yarmo, Adsorption–desorption of CO2 on different type of copper oxides surfaces: physical and chemical attractions studies, J. CO2 Util. 2 (2013) 8–15. [36] E. Albiter, M.A. Valenzuela, S. Alfaro, G. Valverde-Aguilar, F.M. Martínez-Pallares, Photocatalytic deposition of Ag nanoparticles on TiO2: metal precursor effect on the structural and photoactivity properties, J. Saudi Chem. Soc. 19 (2015) 563–573. [37] M. Le, M. Ren, Z. Zhang, P. Sprunger, R. Kurtz, J. Flake, Electrochemical reduction of CO2 to CH3OH at copper oxide surfaces, J. Electrochem. Soc. 158 (2011) 45–49.
Acknowledgments The authors wish to thank CONACYT for financial support for this research through the following projects: Cátedras CONACYT 1060, Beca mixta No. 740859, Scholarship CONACYT 598840, CONACYT-CB2014-237049, CONACYT-PDCPN-2015-487, CONACYT-NRF-2016278729. In addition, the authors want to thank to Dr. Juan Edgar Carrera Crespo for its valuable help with the photoelectrochemical characterization. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.matchemphys.2019.01.062. References [1] J.A. Herron, J. Kim, A.A. Upadhye, G.W. Hubera, C.T. Maravelias, A general framework for the assessment of solar fuel technologies, Energy Environ. Sci. 8 (2015) 126–157. [2] S.C. Roy, O.K. Varghese, M. Paulose, C.A. Grimes, Toward solar fuels: photocatalytic conversion of carbon dioxide to hydrocarbons, ACS Nano 4 (2010) 1259–1278. [3] L. Liu, C. Zhao, J. Xu, Y. Li, Integrated CO2 capture and photocatalytic conversion by a hybrid adsorbent/photocatalyst material, Appl. Catal. B Environ. 179 (2015) 489–499. [4] C.-H. Yu, C.-H. Huang, C.-S. Tan, A review of CO2 capture by absorption and adsorption, Aerosol Air Qual. Res. 12 (2012) 745–769. [5] B. Zhang, Y. Duan, K. Johnson, Density functional theory study of CO2 capture with transition metal oxides and hydroxides, J. Chem. Phys. 136 (2012) 0645161–064516-13. [6] A. Sanna, M. Uibu, G. Caramanna, R. Kuusikb, M.M. Maroto-Valer, A review of mineral carbonation technologies to sequester CO2, Chem. Soc. Rev. 43 (2014) 8049–8080. [7] M. Mikkelsen, M. Jorgensen, F.C. Krebs, The teraton challenge. A review of fixation and transformation of carbon dioxide, Energy Environ. Sci. 3 (2010) 43–81. [8] D. Whipple, P. Kenis, Prospects of CO2 utilization via direct heterogeneous electrochemical reduction, J. Phys. Chem. Lett. 24 (2010) 3451–3458. [9] M. Gattrell, N. Gupta, A. Co, A review of the aqueous electrochemical reduction of CO2 to hydrocarbons at copper, J. Electroanal. Chem. 594 (2006) 1–19. [10] T. Guaraldo, J. De Brito, D. Wood, M.V.B. Zanoni, A new Si/TiO2/Pt p-n junction semiconductor to demonstrate photoelectrochemical CO2 conversion, Electrochim. Acta 185 (2015) 117–124. [11] J. Cheng, M. Zhang, G. Wu, X. Wang, J. Zhou, K. Cen, Photoelectrocatalytic reduction of CO2 into chemicals using Pt-modified reduced graphene oxide combined with Pt-modified TiO2 nanotubes, Environ. Sci. Technol. 48 (2014) 7076–7084. [12] E. Karamian, S. Sharifnia, On the general mechanism of photocatalytic reduction of CO2, Biochem. Pharmacol. 16 (2016) 194–203. [13] Y. Ji, Y. Luo, Theoretical study on the mechanism of photoreduction of CO2 to CH4
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