Enhanced Photocatalytic Reduction of CO₂ on Rutile TiO₂/MgAl Layered Double Oxides with H₂O Under Ambient Temperature

Abstract

Photocatalytic reduction of CO₂ with H₂O is a fascinating approach to convert CO₂ into available fuels using solar energy as driven force. However, it still suffers from low efficiency owing to the instinct stability of CO₂. In this work, a hybrid photocatalyst of rutile TiO₂ nanorods supported on MgAl layered double oxides (TiO₂/MgAl-LDO) were designed and facilely fabricated via an in situ growth followed by a high temperature calcination process. The resulted TiO₂/MgAl-LDO demonstrated significantly enhanced photocatalytic reduction of CO₂ with the optimal CO and CH₄ evolution of 0.65 and 1.60 μmol in 8 h reaction under ambient temperature, which is ca. 4.6 and 48 times that of TiO₂ respectively. X-ray photoelectron spectroscopy revealed a strong electron interaction between TiO₂ and MgAl-LDO, as well as electrochemical characterization showed enhanced photocurrent, suggesting a promoted charge separation in photocatalytic process. CO₂-temperature-programmed desorption (CO₂-TPD) unveiled the relatively active bicarbonate, bidentate carbonate and monodentate carbonate species were formed on MgAl-LDO, which could boost the CO₂ reduction half-reaction. Meanwhile, NH₃-TPD revealed acidic sites existed in TiO₂/MgAl-LDO, which could act as active sites for H₂O adsorption and activation and thus promote the H₂O oxidation half-reaction. The strategy of simultaneous promotion on the reduction and the oxidation half-reactions will open a new vane to fabricate highly efficient catalysts toward photocatalytic reduction of CO₂ with H₂O.

Graphical Abstract

A hybrid photocatalyst of MgAl layered double oxides supported rutile TiO₂ with abundant acidic and/or basic sites exhibit significant enhancement on the photocatalytic reduction of CO₂ with H₂O under ambient temperature.

1 Introduction

The photocatalytic reduction of CO₂ with H₂O to hydrocarbon over light excited semiconductors, which was inspired from natural photosynthesis, offers a fascinating strategy to reduce CO₂ emission and solve energy crisis [1,2,3,4,5]. Since the pioneering discovery of photocatalytic conversion of CO₂ into CH₃OH, CH₄, HCHO and HCOOH over semiconductors such as SiC, GaP and TiO₂ suspension by Inoue et al. [6], various efficient and ecofriendly semiconductor-based photocatalysts (including TiO₂ [7, 8], C₃N₄ [9,10,11] and ZnO [12], etc.) have been developed to realize photocatalytic reduction of CO₂ with H₂O. Among these photocatalysts, TiO₂ has received great attractions due to it high physicochemical stability, elemental abundance, ease availability and nontoxicity [7]. However, the photocatalytic reduction of CO₂ over TiO₂ still suffers from low efficiency. To improve the photocatalytic performance, many efforts have been devoted to optimizing the separation of photogenerated electrons and holes pairs, such as the construction of facet-based homojunctions [13] and crystalline phase-based homojunctions [14], or the decoration with noble metals (e.g., Pt) [15], transition metal (e.g., Cu) [16] and alloy (e.g. PtRu) [17]. In spite of these progresses, apparent quantum efficiencies (AQE) for photocatalytic reduction of CO₂ with H₂O over TiO₂ remains lower than 1% and the reaction rates are typically in the range of μmol·g⁻¹·h⁻¹ [18], which is severely insufficient for any practical applications.

In fact, as CO₂ is an extremely stable molecule with a higher dissociation energy of C=O bond (~ 750 kJ mol⁻¹), the low adsorption and the poor activation of CO₂ over the surface of photocatalyst are the biggest challenge for photocatalytic reduction of CO₂. Literature reported the first step in photocatalytic CO₂ reduction was the transformation from CO₂ to CO₂·⁻ by a single electron transfer from the catalyst. However, such transformation required to overcome an energy barrier as high as − 1.90 V versus NHE [19, 20]. The surface chemistry of CO₂ on oxides verified the energy barrier could be significantly decreased by enhancing the interaction between CO₂ and oxides surface, which induced various adsorption model of CO₂ on oxides surface [21, 22]. Inspired by such consideration, CO₂ adsorbents, such as NaOH [23] and MgO [24,25,26,27,28], have been widely employed as promoters to construct TiO₂-based photocatalysts, due to their superior adsorption capacity for CO₂. For example, NaOH could enhance the chemisorption of CO₂ by forming carbonate and/or CO₂·⁻, while MgO was beneficial for CO₂ adsorption to form carbonate. As results, TiO₂ modified with NaOH and MgO exhibited enhanced performance for the photocatalytic reduction of CO₂, which was mainly attributed to the accelerated CO₂ reduction half-reaction. However, owing to the strong basic character, these compounds inevitably suffered from the carbonation and subsequent deactivation, resulting in instability of the photocatalysts. On the other hand, the photocatalytic reduction of CO₂ with H₂O was also inevitably accompanied with the H₂O oxidation half-reaction, which was an energetically uphill reaction involved a four-electron transfer process. The lower oxidation of H₂O would lead to the accumulation of charges and cause recombination of separated charges. Thus, the oxidation of H₂O was also a challenge during the photocatalytic CO₂ reduction.

In view of the above considerations, both CO₂ reduction and H₂O oxidation half-reactions are important in the process of photocatalytic reduction of CO₂ with H₂O. The modifiers, which could not only facilitate the CO₂ reduction half-reaction but also promote the H₂O oxidation half-reaction, would significantly enhance the photocatalytic reduction of CO₂ with H₂O [29]. MgAl layered double oxides (MgAl-LDOs), which derived from MgAl layered double hydroxides (MgAl-LDHs), have recently been found with great potential to enhance the photocatalytic reduction of CO₂ with H₂O on TiO₂ nanorods [30], where MgAl-LDO exhibited excellent performance that enhanced the adsorption/activation of CO₂ and the adsorption/dissociation of H₂O (namely, the half-reactions of CO₂ reduction and water oxidation). However, the efficiency for the photocatalytic reduction of CO₂ remains low and requires further investigations to achieve highly efficient photocatalysts based on TiO₂ and MgAl-LDO. Therefore, in the present work, rutile TiO₂ nanorods were supported on MgAl-LDO substrate to fabricated TiO₂/MgAl-LDO hybrid photocatalyst. Interestingly, TiO₂/MgAl-LDO exhibits a higher performance for the photocatalytic reduction of CO₂ into CH₄ under ambient temperature, which might be attributed to the enhanced the adsorption/activation of CO₂ and the adsorption/dissociation of H₂O at the interface of TiO₂ and MgAl-LDO. Comparing with the previous MgAl-LDO/TiO₂, the present work also provides a feasible in situ growth followed by a high temperature calcination process to achieve highly efficient photocatalysts, as well as a depth understanding on the photocatalytic reduction of CO₂ with H₂O over TiO₂.

2 Experimental Section

2.1 Preparation of TiO₂/MgAl-LDO Samples

TiO₂/MgAl-LDO were fabricated via an in situ growth of MgAl-LDH on TiO₂ nanorods followed by a high temperature calcination as Fig. 1. Typically, TiO₂ nanorods were first prepared by using a hydrothermal method. 6 mL HCl was dropped into 15 mL titanium isopropoxide in a 100 mL Teflon-lined autoclave at room temperature, and then the mixture was sealed and kept at 180 °C for 36 h. The resulting white precipitates were filtered out and washed with diluent NaOH solution followed by water until the filtrate became neutral. The products (TiO₂) were finally dried overnight in vacuum at 80 °C. Secondly, the desired amount of as-prepared TiO₂ nanorods were dispersed into a 200 mL solution containing 2.10 mmol L⁻¹ Mg(NO₃)₂, 1.05 mmol L⁻¹ Al(NO₃)₃ and 0.25 mol L⁻¹ urea, followed by the mixture was continuously stirred at 100 °C for 12 h to prepare TiO₂/MgAl-LDH. The obtained TiO₂/MgAl-LDH was finally calcined at 550 °C for 3 h to prepare TiO₂/MgAl-LDO. According to the dosages of TiO₂ with 0.1, 0.05, 0.02 and 0.01 g, TiO₂/MgAl-LDH and TiO₂/MgAl-LDO were correspondingly denoted as TiO₂/MgAl-LDH-n and TiO₂/MgAl-LDO-n (n = 1, 2, 3 and 4).

Fig. 1

Schematic illustration of the synthesis process of TiO₂/MgAl-LDO composite

2.2 Deposition of Pt Cocatalyst

Before the use of photocatalysts, 1 wt% Pt were loaded as cocatalysts by an in situ photodeposition method, using H₂PtCl₆·6H₂O as precursors. Taking bare TiO₂ as an example, the deposition of Pt was carried out as following procedure. In typical, 0.1 g TiO₂ was suspended in 25 mL 0.01 mol L⁻¹ methanol/H₂O solution containing 1 mg Pt with H₂PtCl₆ (1 mL 1 mg/mL Pt) as precursor. Methanol here was used as hole scavenger. The suspension was irradiated with a 300 W Xe lamp for 1 h, and then the photocatalysts were recovered by filtration, washing with water and dry at 80 °C overnight.

2.3 Characterizations

The crystalline phases were characterized by X-ray powder diffraction (XRD) on a Bruker D8 Advance powder diffractometer using Cu Ka radiation (operating voltage: 40 kV, operating current: 20 mA, scan rate: 5 ^o/min). Fourier transformed infrared spectroscopy (FT-IR) were recorded on a Bruker VERTEX 70 spectrometer in 4000–400 cm⁻¹ using pressed KBr pellet method. UV–visible diffuse reflectance spectra (UV–vis DRS) were recorded on a UV–vis 2600 spectrophotometer (SHIMADZA) and calibrated with Kubelka–Munk method. The morphology was examined by a JSM-7610F scanning electron microscopy (SEM) at an accelerating voltage of 15 kV. Elemental analysis was collected by energy dispersive spectroscopy (EDS, OXFORD X-act) on the SEM (accelerating voltage: 20 kV, probe current: 0.02 mA, Time: 300 s). Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were performed on a JEOL JEM-2010 TEM (Japan) with the accelerating voltage of 200 kV. The chemical states were characterized on a VG ESCALab 220i XL X-ray photoelectron spectroscopy (XPS) and the binding energies were calibrated with respect to C1 s (284.8 eV). PEC measurements were performed on a CHI 760D electrochemical workstation (Shanghai) with a three-electrode configuration using Hg/HgCl₂ (in sat. KCl) as reference electrode, Pt wire as counter electrode and the prepared films with an active area ca. 0.15 cm² as working electrodes. The light source was a 300 W Xe lamp (LS-SXE300CUV, Perfectlight Technology Co. Ltd., Beijing). The electrolyte was 0.5 mol L⁻¹ Na₂SO₄ solution. For preparation of working electrodes, the catalysts were dispersed in ethanol to form a homogeneous slurry, and then the slurry was directly casted on an F-doped SnO₂-coated glass (FTO, 1 cm × 2 cm). The film was dried at 353 K for 1 h and then thermally treated at 573 K for 2 h. The transient photocurrent was also conducted at the potential of 0.23 V (vs. SCE).

CO₂ and NH₃ temperature-programmed desorption (CO₂-TPD and NH₃-TPD) experiments were carried out using a Micromeritics AutoChem II 2920 Automated Catalyst Characterization System. Typically, 0.12 g of sample was placed in a quartz reactor for each test. Before introducing CO₂ or NH₃ by impulse injections, the sample was pretreated in a He flow (50 mL min⁻¹) at 300 °C for 2 h and then cooled down to 50 °C in He flow. After saturation adsorption of CO₂ or NH₃, the sample was heated in He from 50 to 550 °C at a heating rate of 10 °C min⁻¹ and the TPD profile was recorded with a thermal conductivity detector. The areas under the TPD peaks with the normalized base line were integrated to determine the amount of desorbed CO₂ or NH₃ during TPD.

2.4 Photocatalytic Reduction of CO₂ with H₂O

The photocatalytic reduction of CO₂ with H₂O was carried out using gas–solid mode in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. The light source was a top irradiated 300 W Xe lamp (LS-SXE300 CUV, Beijing Perfectlight Technology Co. Ltd). Typically, 20 mg of catalyst was evenly dispersed onto a glass-sheet (1.5 cm × 2 cm), and then was placed on the holder in the upper region of the reactor, in which 40 mL water was charged on the bottom in advance. The reactor was sealed and degassed for 30 min, and then CO₂ was bubbled slowly till the pressure up to 1 atm. Finally, the reaction was conducted under light irradiation at 20 °C. The generated CO and CH₄, as well as H₂ and O₂, were determined by an on-line GC 7900 gas chromatograph (Shanghai Tianmei) equipped with methanator, flame ionization detector (FID) and thermal conductivity detector (TCD), and the relative standard deviations for the amounts of H₂, CO and CH₄ formed were < 5%.

2.5 Calculation of Apparent Quantum efficiency (AQE)

The experimental results indicated CH₄ and CO were the main reduction products for CO₂, thus AQE was defined as the total extracted electron in CH₄ and CO divided by the number of incident photons (Eq. 1).

$$AQE = \frac{{8 \times N_{{{\text{CH}}4}} + 2 \times N_{\text{CO}} }}{{N_{\text{I}} }} \times 100\% ,$$

(1)

where N_CH4, N_CO and N_I represent the numbers of evolved CH₄, evolved CO and incident photons. 8 and 2 are the numbers of the required electrons for the conversion of CO₂ to CH₄ and CO. The experiment was performed with Xe lamp equipped with band-pass filters of 310, 330, 350 and 370 nm, respectively.

3 Results and Discussion

3.1 XRD Anallysis

Figure 2a exhibits XRD patterns of TiO₂ and TiO₂/MgAl-LDH. The reflections in TiO₂ matches well with that of rutile TiO₂ [31]. All TiO₂/MgAl-LDH show additional peaks at 2θ = 11.8^o, 23.6^o, 35.9^o and 47.2^o, corresponding to (003) (006), (012) and (018) planes of hexagonal MgAl-LDH [32]. In addition, with the decrease of TiO₂ content, the diffraction peaks of MgAl-LDH intensify gradually at the expense of TiO₂ peaks, reflecting their respective contents in the TiO₂/MgAl-LDH composites. After being calcined, all the calcined TiO₂/MgAl-LDH composites show a high crystallinity of TiO₂ rutile phase with complete disappearance of the LDH-related reflections (Fig. 2b). The absence of typical reflection peaks of MgAl-LDO suggested that calcination at 550 °C destroyed the crystal structure of LDH. No other new peaks related to mixed oxides of magnesium and aluminum appeared, indicating that LDH was transformed to a nearly amorphous LDO phase during the calcination process.

Fig. 2

XRD patterns of the synthesized a TiO₂/MgAl-LDH-n and b TiO₂/MgAl-LDO-n

3.2 FT-IR and UV–Vis DRS Analysis

FT-IR spectra confirmed the difference of bonding types about TiO₂/MgAl-LDH and TiO₂/MgAl-LDO. As shown in Fig. 3a, the broad band at 3432 cm⁻¹ was corresponded to the stretching mode of −OH (ν(OH)) of the interlayer H₂O molecules and hydroxyl groups in the brucite-like layers and the weak peak at 1640 cm⁻¹ was attributed to the bending vibration of (H–O–H) of interlayer H₂O molecules in TiO₂/MgAl-LDH, which these band become less intense when TiO₂/MgAl-LDH nanocomposite was calcined to TiO₂/MgAl-LDH. The characteristic vibration at 1360 and 1380 cm⁻¹ associated with the interlayer anions (CO₃²⁻ and NO₃⁻) in the FTIR spectra of TiO₂/MgAl-LDH become very weak in TiO₂/MgAl-LDO nanocomposite [33,34,35,36]. These results indicated that the interlayer anions of LDH were destroyed via dehydration, dehydroxylation, decarbonation and denitration, leading to the formation of LDO during the calcination process. The bands in the range of 500–800 cm⁻¹ were attributed to the lattice vibrations of M–O lattice and M–O–M (M = Mg, Al and Ti) [37, 38] and the similar peaks were also observed in the FT-IR spectrum of TiO₂/MgAl-LDH, indicating that the basic bonding types of TiO₂/MgAl-LDO is maintained after calcination treatment. UV–vis DRS (Fig. 3b) displays the absorption onset of TiO₂ extends to approximately 390 nm, corresponding to the band gap of 3.2 eV. While all TiO₂/MgAl-LDO exhibit slightly negative shift, that is slight enlarged bandgap, indicating MgAl-LDO causes negligible effects on the optical property of TiO₂. In addition, the absorption intensity of TiO₂/MgAl-LDO samples was decreased with the increase of MgAl-LDO contents, which might be attributed to the decrease of TiO₂ content in the samples.

Fig. 3

a FT-IR spectra of TiO₂, TiO₂/MgAl-LDH-2 and TiO₂/MgAl-LDO-2; b UV–vis DRS of TiO₂ and TiO₂/MgAl-LDO-n. The inset shows slight changes on the bandgap between TiO₂ and TiO₂/MgAl LDO-n

3.3 SEM and TEM Analysis

The morphologies of as-obtained TiO₂/MgAl-LDH and TiO₂/MgAl-LDO heterostructures were characterized by SEM (Figure S1). As shown, TiO₂ nanorods were randomly grafted on the surface of MgAl-LDH hexagonal nanoplatelets in the coprecipitation process. Interestingly, after calcination treatment, the morphology TiO₂/MgAl-LDO was similar to those of TiO₂/MgAl-LDH precursor, where the morphologies of MgAl-LDH hexagonal platelets and TiO₂ nanorods were integrally preserved. As shown in Fig. 4a–h, the MgAl-LDO consists of relatively uniform hexagonal platelets with a lateral size in the range of 4–5 μm and a thickness of ca. 160 nm. Moreover, with the decrease of TiO₂ content, the densities of TiO₂ on MgAl-LDO hexagonal platelets were decreased. The SEM image combined with the EDX elemental mappings of representative TiO₂/MgAl-LDO-2 (Figure S2) showed the distribution of four elements (Mg, Al, Ti and O) within the heterostructures, which reveal that TiO₂ supported on MgAl-LDO platelets. The morphology and structural information of the TiO₂/MgAl-LDO loaded with Pt nanoparticles were further revealed by TEM. TiO₂/MgAl-LDO heterostructures display that numerous TiO₂ nanorods are well assembled on MgAl-LDO platelets, confirming the coexistence of TiO₂ and LDO (Fig. 4i–k). The HRTEM image of shows that the lattice spacing of 0.320 nm corresponds to (110) plane of rutile TiO₂ and no obvious lattice fringe is ascribed to MgAl-LDO, indicating LDO might exist in amorphous phase [30]. Besides that, TEM images also exhibit Pt nanoparticles with a diameter of ca. 4 nm are in intimate contact with TiO₂. These results clearly demonstrated that both Pt and LDO are indeed intimately contacted with TiO₂, rather than existing as separate aggregates in the Pt/TiO₂/MgAl-LDO heterostructures.

Fig. 4

SEM images of a and e TiO₂/MgAl-LDO-1, b and f TiO₂/MgAl-LDO-2, c and g TiO₂/MgAl-LDO-3, d and h TiO₂/MgAl-LDO-4. TEM and HRTEM images i–k of Pt/TiO₂/MgAl-LDO-2 photocatalyst

3.4 XPS Analysis

To unravel the interaction between TiO₂ and MgAl-LDO, the chemical states of TiO₂/MgAl-LDO were examined by XPS. The survey spectrum indicates all expected elements, such as Ti, O, Mg and Al, exist in TiO₂/MgAl-LDO (Figure S3). High resolution XPS (Fig. 5a) reveals Ti 2p3/2 and Ti 2p1/2 located at ∼ 458.5 and 464.3 eV in TiO₂ are negatively shifted to ∼ 458.1 and 463.8 eV in TiO₂/MgAl-LDO, suggesting a strong electron interaction between TiO₂ and MgAl-LDO. Such interaction will be favorable for the charge separation during the photocatalytic process. To further confirm the enhanced charge separation, the transient photocurrents based on TiO₂ and TiO₂/MgAl-LDO-2 were comparably shown in Figure S4. As seen, comparing with that of TiO₂, the photocurrent density of TiO₂/MgAl-LDO-2 was significantly enhanced, indicating much more effective separation of photogenerated charges over TiO₂/MgAl-LDO-2. Figure 5b displays O 1s spectra for TiO₂ and TiO₂/MgAl-LDO. As shown, the O 1s spectrum of TiO₂ could be well deconvoluted into two peaks at 529.9 eV (O1s^A) and 531.6 eV (O1s^B) [39], which could be ascribed to the lattice oxygen and the oxygen defects/surface oxygen species with low coordination respectively. After being loaded on MgAl-LDO, the binding energy of O1s^B shows negligible change, while that of O1s^A shows a slight negative shift to 529.5 eV. Such negative shift might be ascribed to the lattice oxygen existed in MgAl-LDO. Due to the lower electronegativity of Mg compared with Ti, the electron cloud density of oxygen was increased. The lattice oxygen with higher electron cloud density would act as much more effective basic sites for the adsorption of CO₂. It is also noted, the intensity ratio of O1s^B/O1s^A increases from 0.48 to 0.63 after TiO₂ being loaded on MgAl-LDO, suggesting an increase of the oxygen defects/surface oxygen species on TiO₂/MgAl-LDO. Such oxygen defects/surface oxygen species would also be favorable to the adsorption and activation of CO₂. In a word, MgAl-LDO load has great potential to facilitate the charge separation and the CO₂ adsorption/activation, thus promotes the photocatalytic reduction of CO₂ over TiO₂.

Fig. 5

XPS of a Ti 2p and b O 1s spectra of TiO₂/MgAl-LDO-2

3.5 Photocatalytic CO₂ Reduction Performance

The photocatalytic activities of TiO₂/MgAl-LDO photocatalysts, including individual TiO₂ and MgAl-LDO with Pt as cocatalyst were evaluated under illumination and the results are shown in Fig. 6. CO and CH₄ were found to be the main and direct products of CO₂ reduction reaction and no other carbonaceous products were observed. The controlled experiments in the absence of light irradiation or catalysts were conducted, and no products could be detected, demonstrating that light irradiation and photocatalysts were essential for photocatalytic reduction of CO₂ with H₂O. On the other hand, neither CO or CH₄ was detected by introducing Ar instead of CO₂, indicating CO or CH₄ were originated from the reduction of CO₂ rather than the residual carbon species in photocatalysts. Figure 6a, b illustrate the evolutions of CO and CH₄ as a function of reaction time over all as-prepared photocatalysts. As shown, the amounts of CO and CH₄ increase almost linearly with reaction time, is indicative of excellent stability of photocatalysts. It is also noticeable that all TiO₂/MgAl-LDO exhibit superior activities of CO and CH₄ evolutions to that of TiO₂. In 8 h of reaction time, CO and CH₄ evolutions over TiO₂ show the minimum yields of 0.14 and 0.033 μmol respectively, while that over TiO₂/MgAl-LDO-2 present the maximum yields of 0.65 and 1.6 μmol respectively. The activities over Pt/TiO₂/MgAl-LDO-2 were obviously enhanced by ca. 4.6- and 48-times respect to that obtained on Pt/TiO₂, indicating MgAl-LDO-2 played vital roles on the photocatalytic reduction of CO₂ over TiO₂. The average rates of CO and CH₄ evolution over TiO₂/MgAl-LDO also exhibit a volcano-type activity against the MgAl-LDO content. This would be due to that excessive amount of MgAl-LDO reduces the content of TiO₂ responded for light absorption, and finally decreases the densities of photogenerated electrons and holes.

Fig. 6

Time-dependent a CO, b CH₄ amounts and c the average evolution rate of CO and CH₄ over CO over TiO₂ and TiO₂/MgAl-LDO-n photocatalysts. (Conditions: photocatalyst, 0.02 g; cocatalyst, 1 wt% Pt; CO₂ pressure: 1 atm; light source, 300 W xenon lamp; reaction temperature, 20 °C). d An AQY on account of CO and CH₄ production by Pt/TiO₂/MgAl-LDO-2 photocatalyst along with the UV–vis DRS spectrum of TiO₂/MgAl-LDO-2

The average rates of CO and CH₄ evolution (Fig. 6c) show a clear inversion of CH₄/CO between TiO₂ and TiO₂/MgAl-LDO-n, suggesting MgAl-LDO is beneficial to the selective photocatalytic reduction of CO₂ with H₂O to CH₄ rather than CO. The high selectivity of CH₄ over TiO₂/MgAl-LDO might be resulted from the basic sites provided by MgAl-LDO, which is favorable for the preferential stabilization of CO₂ into chemisorbed species and the hydrogenation of CO₂. Comparing with MgAl-LDO/TiO₂ photocatalyst [30], in which MgAl-LDO was used as a modifier of TiO₂, the average rates of CO and CH₄ evolution over TiO₂/MgAl-LDO-2 are about 1.7 and 3.2 times higher than these over MgAl-LDO/TiO₂, respectively. In addition, the amount of O₂ generated form the photocatalytic reduction of CO₂ over Pt/TiO₂/MgAl-LDO-2 was also quantified (Figure S5). The molar ratio of $\left( {n_{CO} + 4n_{{CH_{4} }} } \right)/n_{{O_{2} }}$ was calculated to be 1.91, which was slightly lower than the theoretical value of 2. The deviation might be attributed to the minor undetectable H₂ or the incomplete vacuum degassing. No matter what, it suggests CO and CH₄ are the main reduction products from the photocatalytic reduction of CO₂. As further investigation, apparent quantum efficiency (AQE), related to the light energy-chemical energy conversion over TiO₂/MgAl-LDO-2, was also calculated. As seen in Fig. 6d, AQE is highly dependent on the incident wavelength, where AQE decreases with the increase of incident wavelength in 300–400 nm of the optical absorption spectrum of TiO₂/MgAl-LDO-2. AQE for TiO₂/MgAl-LDO-2 at 350 nm is shown as ca. 1.58%, which is ca. 22 times higher than that of TiO₂ (0.07%). While AQEs for TiO₂/MgAl-LDO-1, TiO₂/MgAl-LDO-3 and TiO₂/MgAl-LDO-4 at 350 nm are 1.20%, 1.22% and 0.38%, respectively. It is further confirmed that the introduce of MgAl-LDO platelets into Pt/TiO₂ photocatalyst is beneficial to improve the utilization efficiency of solar irradiation. Finally, the stability for the photocatalysts was examined by using Pt/TiO₂/MgAl-LDO-2. Before the cycle, the system was degassed with vacuum and then bubbled with CO₂ to atmospheric pressure. As shown in Figure S6, similar activities, without significant decrease of the amounts for CO and CH₄ formation, are observed in the second runs, suggesting the prepared photocatalyst possesses excellent stability.

3.6 CO₂- and NH₃- TPD Characterization

Apparently, MgAl-LDO has greatly promote the photocatalytic reduction of CO₂ over TiO₂; that is, CO and CH₄ evolution, the selectivity of CH₄ and AQE have been boosted after TiO₂ being loaded on MgAl-LDO. To understand the mechanism that MgAl-LDO enhanced CO₂ reduction over TiO₂, CO₂- and NH₃- temperature-programmed desorption (CO₂-TPD and NH₃-TPD) were carried out based on the as-prepared samples (Fig. 7). As shown in Fig. 7a, CO₂-TPD for TiO₂/MgAl-LDO exhibit similar profiles. The profiles could be well deconvoluted into four desorption peaks at low temperature (140 °C), moderate temperature (210 and 280 °C) and high temperature (380 °C), corresponding to the bicarbonate binding on surface OH, bidentate carbonate of chemisorbed CO₂ on Mg²⁺-O²⁻ pairs and monodentate carbonate associated with low-coordination O²⁻ anions [40,41,42]. The intensities CO₂-TPD for TiO₂/MgAl-LDO are also observed to be increased with the increase content of MgAl-LDO, suggesting MgAl-LDO plays a crucial role that enhance CO₂ adsorption. By taking the acidic character of CO₂, MgAl-LDO obviously possesses much more and stronger basic sites than that in TiO₂ for CO₂ adsorption [32, 43]. The absorbed CO₂ interacted with such basic sites and were activated by forming bicarbonate, bidentate carbonate and monodentate carbonate. It then deduced MgAl-LDO could promote the photocatalytic reduction of CO₂ over TiO₂ by improving the adsorption and activation of CO₂ molecule, thus facilitating the reduction half-reaction of CO₂ with H₂O.

Fig. 7

a CO₂-TPD and b NH₃-TPD profiles of TiO₂/MgAl-LDO samples

The acidic properties of these samples were probed by TPD of NH₃ preadsorbed at room temperature. NH₃-TPD for TiO₂ in Fig. 7b shows a weak desorption peak at 130–280 °C, indicating a little amount of acidity sites on TiO₂ surface. According to literature, the acidic sites on the catalyst are defined in three types according to the different temperature range of NH₃ desorption: i.e. weak acidic sites at 150–250 °C; intermediate acidic sites at 250–450 °C and strong acidic sites at 450–540 °C [44, 45]. As for TiO₂/MgAl-LDO-n samples, four desorption peaks at around 130, 225, 375 and 450 °C were obtained by fitting NH₃-TPD plots, meaning that the weak, intermediate and strong acidic sites coexist in TiO₂/MgAl-LDO. While only the weak and intermediate exist in TiO₂. Moreover, the intensities for NH₃ desorption peaks gradually increase with the increase of MgAl-LDO content in TiO₂/MgAl-LDO. In agreement with literatures, the peaks at 130, 225 and 375 °C could be Lewis acid sites, while the peak at 450 °C could identified to Brønsted acid sites. Lewis acid sites in TiO₂/MgAl-LDO are located on the cations of Al-O-Mg species in MgAl-LDO due to the substitution of Mg²⁺ by Al³⁺ MgO lattice, while Brønsted acid sites are derived from surface OH groups [32, 46]. It should be noted that the peak related to Brønsted acid sites is quietly weak compared with the peaks associated with Lewis acid sites, indicating that Lewis acidity is the predominant acidity in TiO₂/MgAl-LDO. Such acidic sites could act as active sites for water adsorption and activation, due to the feasible coordination of H₂O to Lewis acidic sites, thus promoting the H₂O oxidation half-reaction.

3.7 Possible Mechanism

Based upon above discussion, it could conclude MgAl-LDO plays a curial role that supplies abundant acidic and basic sites for the adsorption/activation of H₂O and CO₂, subsequently facilitates the half-reactions of H₂O oxidation and CO₂ reduction [47, 48]. A possible mechanism for the photocatalytic reduction of CO₂ with H₂O over Pt/TiO₂/MgAl-LDO was schematically illustrated in Fig. 8. In this photocatalytic reaction system, rutile TiO₂ nanorods are the fountain of photogenerated electron–hole (e⁻–h⁺) pairs, while the loaded Pt played a key role to gather photogenerated electrons (e⁻) from the conduction band of rutile TiO₂ nanorods and serve as reduction active sites. CO₂ and H₂O were first absorbed and activated to form CO_2(ad.) and H₂O_(ad.) at the basic and acidic sites of TiO₂/MgAl-LDO respectively. Under light irradiation, the electrons (e⁻) in valence band (VB) of TiO₂ were excited and transferred to conductive band (CB), and then migrated to Pt, nearby which CO_2(ad.) accepted e⁻ to proceed the reduction half-reaction. On the other hand, the holes (h⁺) left on the surface of TiO₂ reacted with the near H₂O_(ad.) to complete the water oxidation half-reaction. By assistance of the protons from H₂O dissociation, CO_2(ad.) were reduced into CO and CH₄ [49,50,51]. Meanwhile, H₂O_(ad.) were oxidized to O₂.

Fig. 8

Schematic illustration of the proposed mechanism for photocatalytic reduction of CO₂ with H₂O over TiO₂/MgAl-LDO photocatalyst

4 Conclusion

In summary, TiO₂/MgAl-LDO photocatalysts were successfully fabricated and exhibited enhanced photocatalytic CO₂ reduction performance with H₂O. The enhanced performance was benefited from the abundant acidic and basic sites supplied by MgAl-LDO. The optimal TiO₂/MgAl-LDO showed the CO and CH₄ yields of 0.65 and 1.6 μmol in 8 h reaction, which were ca. 4.6 and 48 times with respect to that for TiO₂. The work indicates the same importance of acidic and basic sites for photocatalytic CO₂ reduction with H₂O. It is expected modifiers or supporters with abundant acidic and basic sites will be beneficial to the photocatalytic CO₂ reduction with H₂O over various semiconductors. It will also stimulate us to explore much more efficient photocatalysts for CO₂ reduction with H₂O.