Photoreduction of Carbon Dioxide Over NaNbO₃ Nanostructured Photocatalysts

Abstract

NaNbO₃ had been successfully developed as a new photocatalyst for CO₂ reduction. The catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and ultraviolet–visible spectroscopy (UV–Vis). The DFT calculations revealed that the top of VB consisted of the hybridized O 2p orbital, while the bottom of CB was constructed by Nb 3d orbital, respectively. In addition, the photocatalytic activities of the NaNbO₃ samples for reduction of CO₂ into methanol under UV light irradiation were investigated systematically. Compared with the bulk NaNbO₃ prepared by a solid state reaction method, the present NaNbO₃ nanowires exhibited a much higher photocatalytic activity for CH₄ production. This is the first example that CO₂ conversion into CH₄ proceeded on the semiconductor nanowire photocatalyst.

Graphical Abstract

NaNbO₃ had been successfully developed as a new photocatalyst for CO₂ reduction. It was noted that NaNbO₃ nanowires showed a much higher activity for CH₄ production compared with bulk counterpart (SSR NNO).

1 Introduction

The photocatalytic conversion of carbon oxide (CO₂, green house gas) into valuable chemicals (such as methanol or methane) is of importance in view of preventing the continuous rise in global temperature and providing the alternative fuels [1–4]. In particular, since the first report that the photocatalytic reduction of CO₂ into organic compounds over suspending semiconductor particles in water [1], many efforts have been expanded to develop efficient photocatalysts [5–8]. To date, a large number of reports have focused on the photoreduction of CO₂ over TiO₂-based photocatalysts [9–15]. Recently, some mixed metal oxides such as FeCaO₄ [16], InTaO₄ [17], and BiVO₄ [18] have been developed as photocatalysts for conversion of CO₂ into carbon products. It should be pointed out that the development of new photocatalyst could enhance our understanding of searching for a highly efficient photocatalyst. Thus, developing novel active photocatalysts for CO₂ reduction is still of great interest and urgency.

Environmentally friendly sodium niobate (NaNbO₃) is known as a promising lead-free piezoelectric material. More recently, NaNbO₃ was developed as an efficient photocatalyst for hydrogen generation [19]. However, to the best of our knowledge, there is no report about the photocatalytic reduction of CO₂ properties of NaNbO₃. In addition, the morphology of a photocatalyst strongly affects its photocatalytic activity since the photoreduction reaction occurs on the catalyst surface [20, 21]. Hence, in this study, NaNbO₃ with different morphologies was tested as a photocatalyst for photoreduction of carbon dioxide with water under light irradiation. The physical characteristics of samples were examined by the techniques, such as XRD, BET measurement, and UV–vis diffuse reflectance spectroscopy. The electronic structures of NaNbO₃ were investigated using plane-wave based density functional theory calculations.

2 Experimental Section

2.1 Catalyst Synthesis

NaNbO₃ bulk powders (SSR) were prepared by a conventional solid-state reaction method according to a previous report [22]. As for the preparation of NaNbO₃ nanowires, we first successfully synthesized Na₂Nb₂O₆·H₂O nanowires via a facile hydrothermal route and then converted the precursors into NaNbO₃ with similar nanostructures by a heating treatment. In a typical case, 1 g of P123 (EO₂₀PO₇₀EO₂₀, BASF, USA) was firstly added to 25 ml of distilled water under continuous stirring at 40 °C for 2 h, followed by addition of 5 g of Nb(OC₂H₅)₅ (Aldrich, USA). Then NaOH (Wako, Japan) solution (20 mol L⁻¹, 10 ml) was added drop by drop into the above solution. After being stirred for 1 h at 40 °C, the so-obtained suspension was transferred into a Teflon-lined autoclave and thermally treated at 200 °C for 24 h. The so-obtained white precipitate was washed with distilled water and ethanol, and subsequently dried in an oven at 70 °C over night. The so-obtained precursors were heated at 550 °C for 4 h to synthesis the NaNbO₃ orthorhombic phase. Platinum-loaded NaNbO₃ samples (Pt-NaNbO₃) were prepared by a photodeposition method. In a typical case, an aqueous methanol (20 vol.%) solution containing the NaNbO₃ powders and hexachloroplatinic acid (0.5% H₂PtCl₆·6H₂O) was irradiated by a 400 W high-pressure Hg lamp. After 1 h of irradiation, the suspension was filter and dried for 12 h.

2.2 Sample Characterization

The crystal structure of NaNbO₃ was determined by an X-ray diffractometer (Utilma III., Japan) with Cu–Kα radiation (λ = 1.54056 Å) in the 2θ range of 20–80°. The UV–visible diffuse reflectance spectra of the sample were measured in the range of 250–800 nm using an UV–vis spectrophotometer (UV-2550, Shimadzu., Japan) equipped with an integrating sphere attachment. Morphologies of NaNbO₃ samples were characterized by a field-emission scanning electron microscope (FE-SEM; JSM-6700, JEOL Co., Japan). The specific surface area was deduced according to Brunauer–Emmett–Teller method by a nitrogen adsorption apparatus (TriStar-3000, Micrometrics., USA) at 77 K.

2.3 Evaluation of Photocatalytic Activity

In this study, we applied the photoreduction of CO₂ into CH₄ to evaluate the property of Pt-NaNbO₃ catalyst. Photocatalytic reaction was performed in a Pyrex glass vessel. The NaNbO₃ powder sample (0.03 g nanowires; 0.1 g particles) was uniformly and evenly dispersed on the bottom of a small glass cell that was located in the bottom of a Pyrex glass cell, which was connected with a closed system. After replacing the system air into CO₂, 3 ml of H₂O was added into the reactor by a liquid syringe. Then the reactor was stored in the dark condition for 2 h until an adsorption–desorption equilibrium was reached. Finally, the light was irradiated from a 300-W Xe lamp (ILC Technology, CERMAX LX-300). Sample was periodically extracted from the reaction cell to analyze the concentration of CH₄ using a gas chromatograph (GC-14B, Shimadzu., Japan).

2.4 Calculation of Electronic Structure

The ab initio calculations described here were performed with a CASTEP program package based on the density functional theory [23]. A plane wave basis set was used to describe the electronic wave functions with a kinetic energy cutoff of 800 eV. The interactions between ionic cores and valence electrons were represented by ultrasoft pseudopotentials. Exchange–correlation potentials were described by generalized gradient approximations (GGA-PBE). The Brillouin-zone (BZ) integrations of total energy were calculated using the special 5 × 3 × 2 k point generated by the Monkhorst–Pack scheme.

3 Results and Discussion

Figure 1a and b displays the ball-and-stick model and the polyhedron model of NaNbO₃, respectively. It was found that NaNbO₃ was a typical ABO₃-type pervoskite structure, with an orthorhombic crystal structure (space group, Pbcm). The crystal structure of NaNbO₃ (Fig. 1b) consisted of NbO₆ octahedron sharing their vertex. As shown in Fig. 1c, It was evident that the NaNbO₃ products prepared by a hydrothermal route consisted of straight, smooth, and homogeneous nanowires with a uniform diameter of about 100 nm and length of up to several tens micrometers. The SEM picture of NaNbO₃ prepared by a solid-state reaction method (Fig. 1d) revealed that it mainly contained a number of micron-sized particles in addition to some big particles. Figure 2 shows a comparison of the XRD patterns of NaNbO₃ samples. All diffraction peaks could be indexed to an orthorhombic structured NaNbO₃, in good agreement with those in the JCPDS Card (No. 01-072-7753).

Fig. 1

a The ball-and-stick model, b the polyhedron model of NaNbO₃, c SEM image of NaNbO₃ prepared by a hydrothermal method, and d SEM image of NaNbO₃ SSR sample

Fig. 2

X-ray diffraction patterns of NaNbO₃ samples. As a reference, the standard pattern for NaNbO₃ was shown at the bottom

Figure 3 shows the UV–vis diffuse reflectance spectrum of the sample. The optical band gap E_g of a semiconductor could be deduced according to the following equation

$$ (\alpha h\nu )^{n} = A\left( {h\nu - E_{g} } \right) $$

(1)

where α is the absorption coefficient, hν is the incident photo energy, the value of the index n depends on the electronic transition of the semiconductor (n _direct = 2; n _indirect = 1/2), A is a proportionality constant related to the material, and E _g is the band gap energy of the semiconductor, respectively. The band gap energy was obtained from the intercept of the tangent line in the plot of (αhν)² versus energy, and the value was determined to be ~3.4 eV for NaNbO₃.

Fig. 3

UV–vis diffuse reflectance spectra of NaNbO₃ samples, the inset is a plot of (αhν)^1/2 versus energy (eV) for NaNbO₃

Figure 4 represents the evolved CH₄ concentration over Pt-NaNbO₃ catalysts under light irradiation. It was observed that the CH₄ generated and increased with the prolongation of irradiation time over Pt-NaNbO₃ sample, while there was almost no CH₄ to be detected over NaNbO₃ under the same reaction conditions. In addition, there was hardly any CH₄ to be observed either under the dark condition or without any catalyst. This implied that the reaction strongly depended on catalyst and light. Namely, the reaction was a photocatalytic reduction process. It was worthwhile to note that the activity of NaNbO₃ nanowires is higher than that of NaNbO₃ particles, indicating that the NaNbO₃ nanowires catalyst has fairly good activity. In order to investigate the photocatalytic repeatability of the catalyst, the photocatalytic reduction of CO₂ over used NaNbO₃ nanowires sample was performed again. It was revealed that the photocatalytic activity of used sample was similar to that of the fresh sample.

Fig. 4

Photocatalytic methane evolution over Pt-NaNbO₃ samples. Reaction conditions: catalyst, 0.1 g SSR NaNbO₃ (SSR NNO); 0.03 g nanowires NaNbO₃ (NW NNO)

Figure 5 shows a comparison of the XRD patterns of Pt-NaNbO₃ nanowires before and after the photocatalytic reaction. There was no obvious peak change in the XRD patterns of NaNbO₃ before and after the photocatalytic reaction, indicating that the phase of Pt-NaNbO₃ catalyst was a stable photocatalyst without occurrence of structural degradation during the present photocatalytic reaction process.

Fig. 5

X-ray diffraction patterns of NaNbO₃ nanowires samples before (a) and after (b) the photocatalytic reaction

The optical properties and the photocatalytic performances of semiconductor photocatalysts are closely related to their electronic structures. We next applied plane-wave based density functional theory calculations (CASTEP program package) to study the band structures of NaNbO₃. The DOS (Fig. 6b) displayed that the conduction bands of NaNbO₃ are composed of the Nb 3d orbital, while the valence bands are constructed by the hybridized O 2p orbital. The correlation of E and v is known as $v = \Updelta_{k} E(k)/\hbar$, where v is the velocity of electron and E is the energy of electron. If $\Updelta_{k} E(k)$ is large, namely the energy band is dispersive, v will be high. That is, the more dispersive the energy is, the less localized the charge carriers are. As for the present NaNbO₃, both calculated CB was abrupt, namely, v of the holes and the electron was high. Hence, the mobility of both charge carriers are high in NaNbO₃, leading to promote the photocatalytic activity. On the basis of DFT results, one can see that NaNbO₃ has highly mobile charge carriers, which is helpful to carrier transposition in the photocatalysts.

Fig. 6

DFT calculations for NaNbO₃, a energy band dispersion, and b density of states (DOS)

The photocatalytic CO₂ reduction and physical properties of the NaNbO₃ photocatalysts were summarized in Table 1. During the first 1.5 h irradiation, the rate of CH₄ generation of NaNbO₃ nanowires was as high as 653 ppm h⁻¹ g⁻¹ in contrast to that of NaNbO₃ particle (22 ppm h⁻¹ g⁻¹). Namely, the nanowires structured photocatalyst notably enhanced the photocatalytic performance compared with particle photocatalyst. In a previous report, the phenomena that NaNbO₃ nanowires showed the best photocatalytic activity for hydrogen generation among several morphologies photocatalysts was possibly ascribe to its good crystallinity, large surface-to-volume ratio (S/V) and anisotropic aspect [19]. In the present case for CO₂ reduction, nanowires photocatalyst also showed higher activity. It is implied the good crystallinity, large surface-to-volume ratio (S/V) and anisotropic aspect are helpful to improve the photocatalytic activity for CO₂ reduction. All these suggest some useful information to develop the photocatalyst for CO₂ reduction and enhance efficiency of a photocatalytic material.

Table 1 Photocatalytic activities and physical properties of NaNbO₃ photocatalysts

4 Conclusion

In this study, NaNbO₃ had been successfully developed as a photocatalyst for CO₂ reduction. The DFT calculations revealed that the top of VB consisted of O 2p orbital, while the bottom of CB was constructed by Nb 3d orbital, respectively. A band structure calculation showed the charge carrier in the CB was highly mobile. Compared with bulk NaNbO₃ samples, NaNbO₃ nanowires exhibited a much higher photocatalytic activity for CH₄ production. It was possibly due to crystallinity, surface-to-volume ratio and anisotropic aspect. In summary, the present research is positive, and is expected to supply some useful information for enhancing the performance of photocatalysts and developing a new serial of photocatalysts for CO₂ conversion.