Introduction
Syngas (CO, H2) with different ratio (1:1 to 1:3), as the main raw in C1 chemistry, plays a vital role in the synthesis of hydrocarbons, alcohols or fine chemicals through Fischer-Tropsch process, etc. In industry, the desirable ratio of syngas generally was produced from gasification of fossil fuels combination with water gas shift reaction (CO+ H2O= CO2 + H2) under harsh conditions, forming abundant of CO2 as by-product. Photocatalysis provide an alternative and green approach for replacing the conventional thermal catalysis, mainly ascribe to the abundance of solar energy on Earth and the high selectivity to desirable products under mild reaction conditions. Photocatalyst for photoreduction of CO2 (CO2PR) into syngas in water with precise control of selectivity have attracted various attention for the utilization of renewable and clean energy [1‒3]. In the past decades, various typical photocatalysts have been reported for efficient CO2PR to syngas, such as C3N4 [4‒6], TiO2 [7‒10], CeO2 [11‒13], CuInSx [2], CdS [14] etc., ascribed to their stability and environmental benignity. Although the performance of them have been fully investigated in the synthesis of CO from CO2PR, the efficiency of CO2PR still suffers from the weak absorption in the visible light region and relatively poor charge transfer in photocatalytic process, which has restricted those photocatalysts further industry application [15‒17]. Therefore, achieving cost-effective catalysts with high performance in syngas synthesis, wider light absorption area and superior stability for CO2PR remains a big challenge.
Amongst the reported photocatalysts, layered double hydroxides (LDHs) represent a typical inorganic two-dimensional (2D) materials with a formula [M1−x2+ Mx3+(OH)2]x+(An−)x/n·mH2O, where M2+ is the divalent cation, M3+ is trivalent cation in the laminate layers and An− is the interlaminated anion [18‒23]. Owing to the special lamellar structure with highly dispersed metal elements and remarkable tunable light absorption capacity, LDHs are regarded as a promising photocatalysts for H2O splitting [24], CO2 reduction [25‒29], N2 fixation [30] and selective oxidation of benzene to phenol, etc [31‒33]. Recently, several strategies (e.g., tuning the composition of the LDHs host layers [34], loading Pd on the surface of LDHs [35‒37], integrating with traditional catalysts (e.g., g-C3N4/NiAl-LDH and CoAl-LDH@TiO2-NT) [25,26], intercalation [38], and defects engineering [39,40]) have been investigated to enhance the performance of LDHs-based photocatalysts. Nevertheless, due to poor charge mobility, serious recombination of photoinduced charge carriers is inevitable for traditional LDHs-based photocatalysts. Therefore, efforts for the efficient separation of photo-induced carriers and thus facilitating the interfacial kinetics are highly necessary for LDHs-based photocatalysts.
Cerium as one of typical rare earth elements (REEs) has been widely used for the catalytic conversion of automobile gasoline engine exhausts, CO oxidation, and some photocatalytic reactions, as mainly due to the abundance of oxygen vacancies with related to the easily passing from Ce3+ and Ce4+ [41‒43]. Heterostructured photocatalysts that composing with two semiconductors could facilitate charge transfer, providing a promising strategy for the improvement of photocatalytic performance (e.g., Pd/Au/CeO2 [44], Au/CeO2 [45]). Compared with pristine CeO2, the heterostructured CeO2 structure exhibits superior performance in CO2PR under visible light irradiation by narrowing the bandgap of CeO2 [46,47]. Therefore, heterostructured photocatalysts designed reasonably have considerable prospects for the efficient CO2PR for syngas synthesis.
Herein, a series of heterostructured CeO2/LDHs with different loading of CeO2 (denoted as, Ce-x, x = 0.05, 0.10, 0.15, 0.20, 0.30 and 0.40) were prepared via hydrothermal method. The selectivity and productivity of product syngas (CO and H2) in CO2PR under visible light performed a distinct “volcano-like” trend in conjunction with Ru-complex photosensitizer, with the tunable CO/H2 ratio from 1/7.7 to 1/1.3 and the highest CO evolution rate of 85 µmol·g‒1·h‒1 at Ce-0.15 photocatalysts. Compared with pristine LDH, the Ce-0.15 photocatalyst displayed moderate photo-induced charge separation/transformation efficiency as confirmed by photoluminescence and electrochemical measurements. Furthermore, the Ce-0.15 can even drive CO2PR to syngas under 600 nm irradiation. In all, this work reports a sustainable route for the synthesis of syngas with tunable ratio using heterostructured CeO2/LDHs photocatalysts under visible light irradiation (Scheme 1).
Experimental
Materials
AlCl3·6H2O, CeCl3·6H2O, MgCl2·6H2O, KOH, and KCl, Na2SO4, Ru(bpy)3Cl2·6H2O, triethanolamine (TEOA) and acetonitrile were purchased from Sigma-Aldrich Co. and used directly without any purification. Pure CO2 gas (99.999%) was purchased from Beijing Beiwen Gas Co. and used directly as a substrate for photocatalytic reduction. The water used in the synthesis of the materials was deionized.
Methods
Synthesis of CeO2 modified Mg6Al2-xCex-LDH (denoted as Ce-x)
A series of CeO2 modified MgAl-LDH were prepared by the hydrothermal method using chloride salts of Mg, Al and Ce where 0≤x≤0.40 [48]. Typically, aqueous solutions containing MgCl2·6H2O, AlCl3·6H2O and CeCl3·6H2O in the ratios of (i) 24:8:0, (ii) 24:7.8:0.2, (iii) 24:1.6:0.40, (iv) 24:7.4:0.6, (v) 24:7.2:0.8, (vi) 24:6.8:1.2 and (vii) 24:6.4:1.6 together with 5.899 g of KCl were prepared in 80 mL of deionized H2O. The mixed solution was vigorously stirred about 10 min and then adjusted to pH= 10 with drop-wise addition of 2 mol∙L−1 KOH solution. The slurry was further stirred for 20 min, then transferred to 100 mL Teflon-lined stainless steel autoclave, and treated at 65 °C about 18 h. The resulting milky white to pale yellow product was centrifuged and washed to pH= 7. The product was oven-dried at 50 °C about 12 h.
Characterization
X-ray diffraction (XRD) patterns were characterized from Rigaku XRD. Infrared measurements were examined by Bruker 22 Fourier transform infrared spectroscopy equipment. Scanning electron microscopy (SEM) images were obtained on Zeiss Supra 55 SEM. High-resolution transmission electron microscopy (HRTEM) images were recorded on a JEOL JEM-2010. Brunauer-Emmett-Teller (BET) surface area was obtained on Quantachrome Autosorb-1C analyzer. UV-visible spectra were collected on a Beijing PGENERAL TU-1901. X-ray photoelectron spectroscopy (XPS) spectra were calibrated against C 1s (284.8 eV). The photoluminescence (PL) spectra were collected on Shimadzu RF-6000 at room temperature.
Electrochemical impedance spectroscopy (EIS) spectra were performed on CHI760A electrochemical workstation (Shanghai Chenhua, China). The Ce-x modified glassy carbon electrode was employed as a working electrode, Pt electrode was regarded as counter electrode and saturated calomel electrode was used as reference electrode. 1 cm2 area of glassy carbon electrodes, 130 µL of homogeneous Nafion-treated Ce-x sample suspension was applied. The sample suspension was dispersed by adding 50 µL of Nafion and 2 mg of Ce-x in 1 mL ethanol.
Photocatalytic CO2 reduction
Photocatalytic CO2 reduction was explored in a 50 mL closed stainless reactor. At first, 10 mg Ce-x was dispersed in the 10 mL solution (H2O:CH3CN:TEOA= 1:3:1 (v:v:v)), next 3.3 mg [Ru(bpy)3]Cl2·6H2O was added into the mixed system, and then the reactor was evaculated and refilled with pure CO2. Finally, 0.18 MPa CO2 was filled into the reactor. The performance of each photocatalyst was checked at 30 °C with 300 W Xe lamp under visible light. The products were analyzed by Shimadzu GC-2014 chromatography with flame ionization detector and thermal conductivity detector. The isotopic experiment was carried out under the same condition filling 13CO2 (Linde Gas Comp. 99%) using gas chromatography-mass spectrometry (GC-MS, QP2020) to check the products.
Results and discussion
MgAl-LDH was chosen as support for loading CeO2, since MgAl-LDH was one of the most successful scale-up LDH based products in the industry due to the easy synthesis and cheap raw materials. A series of Ce-x samples were prepared by co-precipitation method and followed by hydrothermal treatment, as the previous report [48]. As presented in Fig. 1, the peaks of (00l), (110) and (113) in XRD patterns and the weakness carbonate feature at 1373 cm‒1 in the FTIR spectra illustrated the successful synthesis of CO32‒ intercalated LDH structure. The modified Ce did not affect the crystal structure of LDHs. More interestingly, the intensity of peak at 28° in XRD patterns ascribed to (111) of CeO2 in cubic fluorite structure [12,47,49], enhanced with increasing the amount of Ce in LDHs structure. Furthermore, the Ce-x structure possessed type IV isotherm adsorption curves and BET surface area of Ce-0.15 was 116.9 m2·g‒1, which was highest among the as-prepared catalyst (Fig. 1(c) and Table S1 (cf. Electronic Supplementary Material, ESM), demonstrated that Ce-0.15 provided the larger reaction area and more active sites for the photocatalysis. Ce-0.15 also exhibited relatively uniform mesopores with a narrow pore diameter distribution (4‒9 nm) (Fig. 1(d)), thus may benifical for the diffusion of reactants, playing a key role in the following photocatalytic efficiency [43], as discussed below.
The SEM (Fig. S1, cf. ESM) and TEM (Fig. 2) images showed nanosheet morphology of LDHs-based substrate. Besides, there were some black dots on the LDHs nanosheet, which was corresponding to the CeO2 nanostructure. The size of ultrathin CeO2 nanoparticles was determined to be ~3.19 nm (Fig. 2(c) inset). Meanwhile, as presented in HRTEM images (Fig. 2(d)), a lattice fringe spacing of 0.30 nm can be determined to be the (111) facet of CeO2 [11]. Moreover, the energy-dispersive X-ray spectroscopy mapping images suggested the uniform distributions of Mg, Al, O and Ce over the entire Ce-0.15 structure (Fig. S2, cf. ESM). Furthermore, the ratio of Mg/Al/Ce in LDH and Ce-x (x = 0.05, 0.10, 0.15, 0.20, 0.30 and 0.40) was measured to be 3.64:1.00:0.00, 3.62:1.00:0.01, 3.63:1.00:0.03, 3.60:1.00:0.06, 3.66:1.00:0.09, 3.68:1.00:0.19 and 3.65:1.00:0.59, respectively, as presented in Fig. S2(f) and Fig. S3 (cf. ESM). It proved that the intensity of CeO2 concentration was enhanced with increasing the amount of Ce in the synthetic process. The surface characteristics and chemical compositions of Ce-x were determined via XPS measurement (Fig. S4, cf. ESM). As shown in Fig. S4(a), the XPS spectra provided complete views of the surface elemental compositions of Ce-x, which all showed the presence of Mg and Al elements. With increasing the Cerium concentration, the enhanced intensity of Ce 3d and the presence of Ce4+ in Ce-x can be ascribed to the modified CeO2 in LDHs nanosheets. Besides, the Mg 1s, Al 2p or O 1s exhibited similar to each other in Ce-x, indicating the modified of CeO2 had little effect on the structure of LDHs.
The photocatalytic activity of as-prepared photocatalysts (Ce-x) was investigated under visible light in conjunction with Ru-complex and TEOA served as a photosensitizer and sacrificial agent in CO2 atmosphere as previous reports [50,51]. Based on the 1H-NMR result (Figure S5), no detectable liquid chemicals such as HCHO, CH3OH and HCOOH were formed. As shown in Fig. 3(a), LDH, Ce-x and also CeO2 gave the only products of syngas (CO and H2), in detail, the selectivity of CO gave 11.4% for LDH, through decorating CeO2 on the surface of LDH, the CO selectivity further increased to 42.1% for Ce-0.15, and further increasing the loading amount of CeO2, the CO selectivity decreased to 16.4% (Ce-0.40), nearly the same as that of referenced CeO2 (12.9%). The ratio of syngas (CO/H2) can be optimized from 1/7.1 (LDH) to 1/1.30 (Ce-0.15) with the highest CO selectivity of 42.1% for Ce-0.15. This tunable syngas is much beneficial for the methanol synthesis and Fischer-Tropsch process in the industry. As shown in Fig. 3(b), the productivity of CO can be also optimized to 0.85 mmol (Ce-0.15), 4.7 and 9.4 times higher than that of LDH (0.18 mmol) and CeO2 (0.09 mmol), respectively. In all, the selectivity and productivity trend of CO performed an obvious “volcano-like” trend, with the highest point at Ce-0.15 (42.1% selectivity to CO with a rate of 85 mmol·g‒1·h‒1). The reason for Ce-0.15 performed outstanding selectivity and activity in CO2PR will be further discussed.
Furthermore, GC-MS was employed to verify the origin of as-produced CO. As shown in Fig. 3(c), the 12CO2 or 13CO2 was used as a reactant and the corresponding signal of 12CO or 13CO was m/z = 28 or 29, respectively, indicating the produced CO originated from the photoreduction of CO2 gas source (Fig. 3(c)). As shown in Fig. 3(d) and Fig. S6 (cf. ESM), ultrahigh selectivity of H2 in Ar atmosphere (100%, 0.01 mmol) or without adding Ce-0.15 (97.1%, 1.53 mmol) in CO2PR and no detectable products were generated without adding Ru(bpy)3Cl2 and in dark experimental condition. These control experiments indicated the Ru(bpy)3Cl2 together with Ce-x played an important role in CO2PR. In addition, the Ce-0.15 photocatalyst can be recycled at least four times, the selectivity (Fig. 3(e)) and productivity (Fig. 3(f)) nearly maintained as the first cycle, proving the stability of the as-synthesized photocatalyst. The XRD pattern (Fig. S7) and TEM images (Fig. S8, cf. ESM) of recycled Ce-0.15 also proved its good stability.
Fig.4 The (a) selectivity, (b) productivity of LDH and Ce-x (x = 0.05, 0.10, 0.15, 0.20, 0.30 and 0.40, respectively) and CeO2 in CO2PR under visible light irradiation; (c) isotope trace analysis GC-MS spectra using Ce-0.15 photocatalysts; (d) the selectivity of catalyst under control experimental reaction conditions (1. Ar atmosphere; 2. Without Ce-0.15; 3. Without Ru(bpy)3Cl2; 4. In dark); (e) selectivity and (f) productivity of recycle Ce-0.15. |
The UV-vis spectra of Ce-x were collected to gauge their light absorption ability to explore the influence factor of their photocatalytic activity. As shown in Fig. 4(a), CeO2 showed two absorption peaks at 280 and 320 nm, which was attributed to the absorption of Ce3+ and Ce4+, respectively [43]. The MgAl-LDH only absorbed UV light and the absorbance range was improved to a visible light area with modifying CeO2 on the surface of LDH. Most interestingly, the Ce-0.15 photocatalyst exhibited the highest absorbance intensity in the visible range, which may be ascribed to the optimal interaction effect between CeO2 and LDH [47]. Accordingly, we investigated the performance of Ce-0.15 and LDH in CO2PR under different cut-off filter light irradiation (Fig. S9, cf. ESM) to reveal the effect of CeO2 modification on LDH, especially with the presence of photosensitizer Ru(bpy)3Cl2·6H2O since its much wider visible light absorbance from ~600 to 200 nm (Fig. 4(b)). As shown in Figs. 4(c,d) and Fig. S10 (cf. ESM), the performance of both LDH and CeO2 decreased with increasing the irradiation wavelength from 405 to 600 nm, and LDH did not exhibit any photoactivity under irradiation 600 nm, mainly due to the limited light absorbance ability. Especially, under 600 nm irradiation, we found the productivity/selectivity of CO was 0.05 mmol, 8.7% and the productivity/selectivity of H2 was 0.52 µmol, 91.3% by Ce-0.15, respectively. The external quantum efficiency of CO still retain 0.05% (Ce-0.15) under irradiation at 600 nm (Table S2) compared with 0% of LDH. From previous reports (Table S3, cf. ESM), Ce-x achieved precise control of the products syngas (CO/H2) ratio under>400 nm and were able to convert CO2 to syngas under 600 nm irradiation, this 600 nm induced syngas synthesis from CO2 can be well understood due to the heterostructure CeO2/LDH.
To reveal the optoelectronic properties and intrinsic reasons for the efficient syngas synthesis from CO2PR on Ce-0.15, we investigated the PL and EIS spectra. As shown in PL spectra (Fig. 5(a)), a moderate weaker photoluminescence emission of Ru(bpy)3Cl2 on Ce-0.15 than other samples proved efficient electron-hole separation between Ce-0.15 and Ru(bpy)3Cl2 in photocatalysis. In addition, EIS spectra (Fig. 5(b)) confirmed that Ce-0.15 provided relative decreased charge transfer resistance comparing with pristine LDH, which further indicated the efficient separation and transfer of photogenerated electron-hole pairs. The improved conductivity of Ce-0.4 may be ascribed to the enhanced interaction between CeO2 and LDH. Above all, a possible reaction mechanism was proposed as follows (Scheme S1, cf. ESM), under visible light irradiation, the Ru photosensitizer was activated to the excited state ([Ru(bpy)3]2+*) [50], and the photo-induced electrons from the excited state directly transferred to the surface of catalysts (LDH and CeO2). Subsequently, the electrons produced the reduction reaction (splitting water into active hydrogen species (H*)) [40,52], then the absorbed CO2 molecules on the surface of catalyst were further hydrogenated with 2 equivalence mol of active H* for the formation of CO; meantime, the surface-active H* can be easily coupling for the evolution of H2. This competition between hydrogenation and coupling of surface-active H* resulted in the tunable selectivity of syngas (CO and H2). Finally, the oxidized [Ru(bpy)3]3+ can be returned to [Ru(bpy)3]2+ with the assistance of sacrificial agent (TEOA) [37]. For the pure LDH and CeO2 structure, the photoinduced H* on the surface preferred to coupling rather than hydrogenation, leading to the formation of much more favourable H2 evolution rather than the valuable product CO, mainly due to the rapid electron-hole recombination. Desirably, for the heterostructured CeO2/LDH (Ce-0.15), moderate separation efficiency of electron-hole by suppressing their recombination, resulted in an enhanced hydrogenation reaction for the CO evolution around the abundant of interfaces in heterostructured CeO2/LDH, explaining the enhanced CO2PR to CO, and the tunable ratio of syngas.
Conclusions
In summary, a series of Ce-x samples were successfully synthesized by co-precipitation method and followed by hydrothermal treatment. We found that the selectivity and productivity of syngas performed an obvious “volcano-like” trend, with the highest point at Ce-0.15. And the ratio of the products of CO/H2 (syngas) can be tunable from 1/7.1 (LDH) to 1/1.30 (Ce-0.15). More importantly, Ce-0.15 can drove CO2PR to syngas under 600 nm irradiation. UV-vis, PL, and EIS revealed that Ce-0.15 performed a suitable separation and transfer of photogenerated electron-hole in photocatalysis. This work paves the way to fully utilize the rare earth elements and stimulates the development of REEs-based catalysts in the synthesis of syngas under solar irradiation. Other rare-earth element (like Er) doped LDH for photoreduction of CO2 is underway in our lab.