Hierarchical NiCo2O4 hollow nanocages for photoreduction of diluted CO2: Adsorption and active sites engineering

Applied Catalysis B: Environmental 260 (2020) 118208

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Hierarchical NiCo2O4 hollow nanocages for photoreduction of diluted CO2: Adsorption and active sites engineering

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Bin Hana,b, Jianing Songa,b, Shujie Lianga,b, Weiyi Chena,b, Hong Denga,b, , Xinwen Oua,b, ⁎⁎ Yi-Jun Xuc, , Zhang Lina,b a

School of Environment and Energy, Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), South China University of Technology, Guangzhou, 510006, PR China b Guangdong Engineering and Technology Research Center for Environmental Nanomaterials, South China University of Technology, Guangzhou, 510006, PR China c State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350116, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Diluted CO2 photoreduction Spinel metal oxides Hierarchical hollow nanocages Adsorption Activation

CO2 adsorption is a critical step for CO2 photoreduction, especially in diluted CO2, whereas an in-depth understanding of CO2 adsorption effects is still lacking. Herein, isostructural NiCo2O4 and MgCo2O4 hierarchical hollow nanocages (NiCo2O4 HCs and MgCo2O4 HCs) have been ingeniously constructed for diluted CO2 photoreduction. NiCo2O4 HCs exerts an apparent quantum yield of 1.56% with CO selectivity of 89%, exceeding most previous inorganic catalysts in pure CO2. Nevertheless, MgCo2O4 HCs is almost inert, even though it shows higher CO2 uptake ability. DFT calculations results indicate that CO2 molecule adsorbed on Ni sites can be readily reduced to CO, while the CO2 on Mg sites cannot participate in this reaction. Therefore, only adsorbed CO2 which can participate in reduction reactions, named active adsorption, can accelerate the whole reactions. This work unearths atomic-level insights into the relation between CO2 adsorption and reduction, providing fundamental guidance to improve photocatalytic performance.

1. Introduction The depletion of fossil fuels induces excessive anthropogenic CO2 emission, which has substantially contributed to climate change, such as global warming and glaciers melting [1,2]. Photocatalytic conversion of CO2 to solar fuel is emerging a global research hotspot for its potential to simultaneously relieve energy shortage and global warming [3–5]. To date, however, CO2 photoreduction with high efficiency and selectivity still remains a grand challenge due to the high thermodynamic stability of C = O (806 kJ/mol) and fast recombination of photoinduced charge pairs during photocatalysis [6–9]. In order to acquire high catalytic performance, most of the researches on CO2 photoreduction are conducted in pure CO2 [10–13]. Nevertheless, the CO2 concentration in anthropogenic CO2 emission, such as firepower plants, is relatively low (ca. 10%) [14,15]. In this regard, a highly energy-consuming process for the purification of CO2 is prerequisite, which further restricts the practical implementation of artificial CO2 photoreduction [16,17]. As such, directly converting diluted CO2 into desired products meanwhile suppressing other competing reactions is of

strategic significance yet rarely reported. Considering the high thermodynamic stability of CO2 linear molecule, the adsorption of CO2 molecule onto the surface of the catalyst is of prerequisite significance since non-linear CO2 molecule on the catalysts is more destabilized than the linear one [18–20]. Moreover, selectively adsorbing CO2 rather than other foreign gas from diluted CO2 plays a significantly important role in the photocatalytic conversion of diluted CO2 [21,22]. Zhang and his co-workers have recently demonstrated efficient photoreduction of diluted CO2 by intruding OH− onto the Co based MOFs and confirming the significant roles of high CO2 binding affinity, which is able to facilitate the stabilization of the initial Co−CO2 adduct and promote the CO2 reduction [23]. More recently, our previous work also confirmed the crucial role of CO2 uptake ability in diluted CO2 photoreduction [24]. Therefore, enhancing CO2 capture ability of catalysts has become an effective strategy for improving CO2 photoreduction property, especially in low concentration of CO2 [25]. However, does more adsorption lead to better performance? Which kind of adsorption is effective? Obviously, the intrinsic relationship between CO2 uptake ability and the photoreduction performance,

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Corresponding author at: School of Environment and Energy, Key Laboratory of Pollution Control and Ecosystem Restoration in Industry Clusters (Ministry of Education), South China University of Technology, Guangzhou, 510006, PR China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (H. Deng), [email protected] (Y.-J. Xu). https://doi.org/10.1016/j.apcatb.2019.118208 Received 4 July 2019; Received in revised form 12 September 2019; Accepted 16 September 2019 Available online 20 September 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.

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2.3. Fabrication of NiCo2O4 HCs

which relates to taking care of the activity and selectivity challenges [26–28], still remains largely ambiguous. To answer these fundamental questions needs the rational design of catalysts. Recently, spinel ternary transition metal oxides, an intriguing type of functional materials being isomorphic to the Co3O4 crystal structure with the replacement of Co2+ by other bivalent atoms, are emerging and promising catalysts for CO2 photoreduction due to their stable crystal structures, rich redox chemistry, and excellent electromagnetic properties [29,30]. The transition metal ions in the spinel oxides with multiple redox states are favourable for building electron transport chains for CO2 conversion, efficiently restraining the generation of undesired intermediates, thereby facilitating the multi-electron progress of CO2 photoreduction [31]. Especially, spinel oxides with hierarchical hollow arrangements featuring rich cavity and thin wall favour the separation of photogenerated charge pairs by shortening the diffusion distance and enriching reactive sites on both the interior and exterior shells [32–35]. Accordingly, hierarchical spinel metal oxides hollow nanocages are advantageous for elevating the diluted CO2 photoreduction performance, providing ideal models to unveil the manners in which the CO2 photoreduction performance is impacted by CO2 adsorption. Herein, on account of the potential of Ni species in diluted CO2 photoreduction and the good CO2 affinity of Mg2+, two kinds of welldefined isostructural hierarchical spinel metal oxides hollow nanocages (i.e., NiCo2O4 HCs and MgCo2O4 HCs) have been deliberately fabricated to unravel the intrinsic relationships between CO2 adsorption ability and CO2 photoreduction performance with [Ru(bpy)3]2+ and triethanolamine as the photosensitizer and sacrificial agent, respectively. In diluted, the as-prepared NiCo2O4 HCs exerts an optimal apparent quantum yield of 1.56% with CO selectivity of 89%, which is superior to most inorganic catalysts in pure CO2. However, its isostructural MgCo2O4 HCs is almost inactive in diluted CO2, even though it shows boosted CO2 uptake ability. DFT calculations results reveal that the adsorbed CO2 on Ni sites can be readily reduced to CO, while the CO2 on Mg sites cannot take part in the following reduction reactions due to its very high energy increase for COOH* formation. That is, Mg species are only the adsorption sites rather than catalytic sites. This work reveals that only active adsorption, whose adsorbed CO2 can take part in the following reduction, can accelerate the whole reactions, especially in diluted CO2, opening new frontiers for the rational design of multimetal compounds towards solar-to-fuels conversion.

First, NiCo Layered Double Hydroxide (LDHs) was constructed similar to a previous synthesis strategy [37]. Typically, 0.3 g nickel nitrate hexahydrate was dissolved in 100 mL of ethanol, followed by adding 0.2 g resultant ZIF-67 powders. Then, the above solution was stirred at room temperature for 1 h. Subsequently, the dark green precipitate was collected by centrifugation, washed with deionized water and ethanol for 5 times, respectively, followed by dring at 60 °C overnight. Finally, NiCo2O4 HCs was prepared by a calcining process at 350 °C (2 h, 1 °C/min) in the air with of NiCo LDHs as the self-sacrificing template. Its isostructural MgCo2O4 was fabricated in a similar way except for using 1.8 g Mg(NO3)2·6H2O instead of 0.2 g Ni(NO3)2·6H2O as the etching agent. For comparison, bulk NiCo2O4 was fabricated via a traditionary solvothermal method according to the previous report [38], followed by a calcining process (350 °C, 2 h, 1 °C/min) in the air.

2.4. Characterizations The powder X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance Powder X-ray diffractometer using Cu Kα radiation at a scan rate of 0.02°s−1. Field-emission scanning electron microscopy (FE-SEM) was conducted on a ZEISS Merlin spectrophotometer. Transmission electron microscopy (TEM) was conducted on a JEOL model JEM 2100F instrument. X-ray photoelectron spectroscopy (XPS) was obtained on a Thermo Scientific ESCA Lab250 spectrometer and C 1s peak at 284.6 eV as a signal-calibrating standard for all the binding energies. Brunauer-Emmett-Teller (BET) surface areas and the CO2 adsorption property were carried out on a Micromeritics ASAP2020 equipment (Micromeritics Instrument Corp., USA) at -80 °C and 25 °C, respectively. CO2 temperature-programmed desorption (CO2 TPD) was implemented at a Micromeritics AutoChem II 2920 instrument. Photoluminescence (PL) spectra and fluorescence emission decay spectra were performed on Edinburgh Analytical Instruments FL/ FSTCSPC920 at room temperature. The measurements were carried out in a solution system similar to the CO2 photoreduction reaction. The electrochemical impedance spectroscopy (EIS) measurements and Mott − Schottky plots were performed on an electrochemical workstation (CHI 660E, China) in the presence of 5.0 mM solution of K3[Fe (CN)6]/K4[Fe(CN)6] and phosphate buffer saline (100 mM, pH = 7.4), respectively. The working electrode was prepared on fluorine-doped tin oxide (FTO) glass, similar to previous work [39].

2. Experimental section 2.1. Materials

2.5. Photoactivity testing Chemicals including cobalt nitrate hexahydrate (AR), nickel nitrate hexahydrate (AR), 2-methyl imidazole (Z-MIM, AR), methanol absolute (AR, 99.5%), ethanol absolute (AR, 99.7%), [Ru(bpy)3]Cl2·6H2O (AR), triethanolamine (TEOA, GC, > 99.9%), acetonitrile (MeCN, GC, > 99.9%), potassium ferricyanide (AR) and potassium ferrocyanide (AR) were obtained from Shanghai Aladdin Reagent Co. Ltd, without further purification. Deionized (DI) water used in all the experiments.

The CO2 photoreduction experiments were conducted in a 60 mL quartz reactor at ambient temperature and atmospheric pressure [31]. In a typical reduction reaction, 1 mg as-prepared catalyst, 7.5 mg [Ru (bpy)3]Cl2·6H2O (bpy = 2′2-bipyridine), 2 mL water, 3 mL acetonitrile (MeCN) and 1 mL triethanolamine (TEOA) were added in a gas-closed quartz reactor. This system was thoroughly degassed and then backfilled with CO2 (99.9999% or 10%) for repeated three times (CO2 flow above the liquids), respectively. Finally, the reactor was backfilled with 1 atm of gas (pure CO2 or 10% CO2, as shown in Fig. S1). Then the quartz reactor was put in a reaction system (Fig. S1, PCX50B, Beijing Perfect Light Co., Ltd.) under stirring with a 5 W LED light (400–800 nm). After reaction for a specific time interval, the products were analysed on gas chromatography (GC-7890B, Agilent) equipped with a high-sensitivity thermal conductivity detector (TCD) detector and flame ionization detector (FID) detector for quantifying the amounts of H2 and CO/CH4, respectively. The selectivity for CO is calculated using the equation below, in which R (μmol·h−1) refers to the generating rate of the product.

2.2. Construction of ZIF-67 ZIF-67 nanocrystals were fabricated according to the previous report [36]. Typically, 3.3 g 2-MIM and 2.9 g of cobalt nitrate hexahydrate were dissolved in 200 mL methanol, respectively. Then, the 2MIM methanol solution was poured into cobalt nitrate one and mixed under vigorous stirring for 30 min, and then aged for 24 h at room temperature. The precipitate, i.e., ZIF-67, was washed with methanol for three times and vacuum dried at 80 °C.

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Selectivity for CO=

2R (CO) × 100% 2R (H2 )+2R (CO)

preparation of precursors [45,46]. After annealing, NiCO2O4 HCs and MgCo2O4 HCs feature typical diffraction peaks of spinel structures (JCPDS card: 20-0781 and 021073, respectively) [38,47], as displayed in Fig. 1c, indicating the successful preparation of target spinel metal oxides. Subsequently, the surface chemical states of the as-prepared samples were obtained by XPS analysis. The wide scan XPS spectra (Fig. S2) confirm that Ni (or Mg), Co, and O are the main elements for NiCo2O4 HCs and MgCo2O4 HCs, respectively. The high-resolution XPS spectra of Co (Fig. 1d) show two spin-orbit doublets at 780.3 eV and 795.4 eV, respectively, along with two satellites peaks. For Co element, the peaks at 796.8 eV and 795.1 eV correspond to Co 2p3/2 for Co2+ and Co3+, respectively, while peaks at 781.3 eV and 779.8 eV correspond to Co 2p1/2 for Co2+ and Co3+, respectively [48]. Hence, XPS results indicate that Co2+ and Co3+ are the main forms of Co element in both NiCo2O4 HCs and MgCo2O4 HCs. As shown in Fig.1e, Ni 2p spectrum shows the signals at 855.3 and 872.7 eV, which are associated with Ni 2p3/2 and Ni 2p1/2, respectively and can be divided into four peaks belonging to Ni2+ and Ni3+ [49]. The intense satellite peaks at 861.2 and 879.7 eV indicate the Ni2+ cations abound in the catalyst material [50]. Fig. 1f shows the Mg 1 s region of MgCo2O4 and MgCo2O4@C, in which one distinct peak centre at 1303.4 eV can be observed, corresponding to Mg2+ valence state [38]. The results of ICPOES reveal that ratios of M/Co are 1:1.86 and 1:1.82 for NiCo2O4 HCs (Ni/Co) and MgCo2O4 HCs (Mg/Co), respectively, which are close to the value calculated from the crystal structures. The evolution of the morphology of the resultant samples during the sequential templating synthesis procedure is illuminated by FE-SEM and TEM images. As displayed in Fig. S3a and Fig. 2a, ZIF-67 features uniform rhombic dodecahedral morphology with an average diameter of approximately 400 nm. Unique NiCo LDHs hollow nanocages structure with abundant interior cavities were observed after hydrolysis process with ZIF-67 as the self-sacrificing template, which is displayed in the panoramic SEM image (Fig. S3b). Magnified SEM image (Fig. 2b) and typical TEM images (Fig. S3c-d) provide the detailed information of the as-prepared hollow nanocages, confirming that the nanocages are assembled by continued edge-to-face stacking of hierarchical nanosheet subunits, while the interior remains hollow, inheriting the size of ZIF67 template. After the calcination process, NiCo LDHs transforms into NiCo2O4 HCs, which exhibits apparent hollow nanocage-like morphology assembled by hierarchical nanosheets (Fig. 2c), like its maternal NiCo LDHs template. Notably, its isostructural MgCo2O4 HCs counterpart shares almost identical evolution procedure and ultima morphology, as confirmed by a set of SEM images (Fig. S4). TEM has been conducted to further check the detailed morphology information and microscopic structure of as-prepared samples. After calcining, NiCo2O4 HCs still keeps the hollow structure with ultrathin hierarchical nanosheets, as revealed in Fig. 2d–e, which inherits the morphology of its maternal precursor during the calcining process. Fig. 2f reveals the high-resolution TEM (HRTEM) image of NiCo2O4 HCs with obvious lattice spacings of ca. 0.203 nm, which is assigned to the (400) facet of hexagonal NiCo2O4. As displayed in Fig. 2g–j, scanning transmission electron microscopy and corresponding mapping confirm the hollow structure and the uniform distribution of Ni, Co, and O throughout the whole nanocages. As for MgCo2O4 HCs, it shows a similar hierarchical hollow structure with large exposed (400) facet and even-distributed elements (Fig. S5). Therefore, the joint

A monochromatic LED light (420 nm) was employed to test the apparent quantum yield (A.Q.Y.) of as-prepared samples, which was calculated based on the following equation.

A.Q.Y.(CO) %=

=

number of reacted electrons × 100% number of incident electrons

number of evolved CO molecules×2 × 100% number of incident electrons

In which, radiant power energy meter (Ushio spectroradiometer, USR-40) adopted to collect the number of incident photons. The 13CO2 instead of 12CO2 is employed to conduct the isotope-labeling experiments with chromatography-mass spectrometry (7890A and 5975C, Agilent) as gas analytical equipment. 2.6. Computational methods We theoretically examined the catalytic activity of NiCo2O4 and MgCo2O4 for CO2 reduction via the projector augmented wave (PAW)based density functional theory (DFT) calculations as implemented in Vienna ab initio simulation package (VASP) [40,41]. The PBE (Perdew, Burke and Ernzerhof) functional at the level of generalized gradient approximation (GGA) was used for electron exchange-correlation [42]. The surface slabs of NiCo2O4 (400) and MgCo2O4 (400) with four-layer thickness were constructed (lateral dimensions: a = b = 11.29 Å for NiCo2O4 (400) slab, and a = b = 11.47 Å for MgCo2O4 (400) slab). During the geometry optimizations, only the topmost two layers were allowed to relax, and the vacuum layer was larger than 12 Å along the z axis. The energy cutoff was 450 eV, and a 3 × 3×1 k-point based on Monkhorst-Pack mesh was used to sample the Brillouin zone. The convergence for optimizations was set to be lower than 10−5 eV in total energy and the residue force was less than 0.01 eV/Å. In this work, we considered the photoreduction of CO2 to CO. At standard conditions, we use the model of computational hydrogen electrode to calculate the energy of proton-electron pair, which is equal to half of the energy of free H2 molecule [43]. 3. Results and discussion The target well-defined hierarchical NiCo2O4 and MgCo2O4 hollow nanocages (NiCo2O4 HCs and MgCo2O4 HCs, respectively) have been constructed by a novel sequential templating strategy with MOFs as the self-sacrificing template, as shown in Scheme 1. In the initial step, purple ZIF-67 powders were obtained with 2-methyl imidazole and cobalt nitrate hexahydrate as the precursors. By employing the rhombic dodecahedral ZIF-67 as the self-sacrificing template, peak green NiCo LDHs hollow nanocages are expected to be prepared due to the etching reaction between released H+ and 2-methyl imidazole linkers of ZIF-67 [44]. Finally, the target NiCo2O4 HCs have been constructed by a facile calcination process with NiCo LDHs as the template, which inherits the hollow nanocages of the maternal NiCo LDHs. For comparison, its isostructural MgCo2O4 HCs have been constructed by a similar strategy. Fig. 1a–b show the XRD patterns of ZIF-67 and MCo LDHs (M = Ni and Mg), in which target diffraction peaks are clearly observable without noticeable signals of residues, suggesting the successful

Scheme 1. Schematic illustration of the synthesis procedure of MCo2O4 HCs (M = Ni/ Mg) by the sequential templating strategy.

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Fig. 1. XRD pattern of (a) ZIF-67, (b) NiCo LDHs and MgCo LDHs, (c)NiCo2O4 HCs and MgCo2O4 HCs; (d) Co 2p XPS spectra of NiCo2O4 HCs and MgCo2O4 HCs; (e) Ni 2p XPS spectrum of NiCo2O4 HCs; and (f) Mg 1 s XPS spectrum of MgCo2O4 HCs.

been carefully studied. As displayed in Fig. S6a, when 0.5 mg of NiCo2O4 HCs was adopted, the CO production rate and selectivity obviously increased, which is almost 26 times and 3.8 times relative to that of the blank system without catalyst, respectively, indicating that NiCo2O4 HCs shows a conspicuous effect on enhancing the CO production rate and selectivity. With the increase of MgCo2O4 HCs’ amount to 1 mg, the CO production rate reaches its maximum value. However, further increasing the input quantity of NiCo2O4 HCs catalyst results in

characterizations clearly suggest the successful fabrication of NiCo2O4 HCs and its isostructural MgCo2O4 HCs counterpart. Then, pure CO2 photoreduction has been conducted to test the catalytic performance of NiCo2O4 HCs and its isostructural MgCo2O4 HCs in acetonitrile and H2O mixed solution with [Ru(bpy)3]Cl2∙6H2O and TEOS as the photosensitizer and electron donor, respectively. CO and H2 are the main products, which is in line with previous similar systems [29]. Firstly, the effect of the feeding amount of catalysts has

Fig. 2. Typical SEM images of as-prepared samples: (a) ZIF-67; (b) NiCo LDHs; and (c) NiCo2O4 HCs, and (d-e) TEM images, (f) HRTEM image, (g) scanning transmission electron microscopy of as-prepared NiCo2O4 HCs, and (h-j) examination of the corresponding elemental mappings of O, Co, and Ni, respectively. 4

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Fig. 3. (a) CO2 photoreduction performance under various reaction conditions; (b) comparison of CO2 photoreduction performance over NiCo2O4 HCs and MgCo2O4 HCs in pure CO2 and diluted CO2; (c) photoluminescence spectra; (d) electrochemical impedance spectroscopy (EIS) Nyquist plots; (e) N2 adsorption-desorption isotherms and BET surface, (f) CO2 adsorption isotherm of the NiCo2O4 HCs and MgCo2O4 HCs.

CO2 photoreduction. As displayed in Fig. 3b, MgCo2O4 exhibits a CO generation rate of 6.3 μmol/h with a selectivity of 62.3% in pure CO2. Evidently, the catalytic performance of NiCo2O4 HCs in diluted CO2 is much better than that of MgCo2O4 HCs in pure CO2. However, when the experiments were conducted in diluted CO2, the CO production rate of MgCo2O4 HCs significantly drops to 0.58 μmol/h with a selectivity of 12.1%, indicating that MgCo2O4 HCs is almost inactive in the diluted CO2. To elucidate the origination of the boosted catalytic performance over NiCo2O4 HCs than its isostructural MgCo2O4 counterpart, comparative characterizations on the above two samples have been systematically conducted. First, charge transfers during the CO2 reduction process have been investigated by PL spectra. As shown in Fig. 3c, the presence of NiCo2O4 HCs or MgCo2O4 HCs catalyst can obviously reduce the PL intensity than the blank system, and NiCo2O4 HCs shows lower PL intensity as compared with MgCo2O4 HCs. Generally, the lower PL emission intensity indicating the more efficient separation of free charge carriers [52,53]. Moreover, the fluorescence emission decay spectra (Fig. S10) reveal that NiCo2O4 HCs contained system shows the lower lifetime (341.5 ns) than MgCo2O4 HCs contained system (354.3 ns) and blank system (360.3 ns), indicating the better electron separation of NiCo2O4 HCs-Ru photosensitizer [53]. To further study the transfer of the charge carriers, electrochemical impedance spectra (EIS) of NiCo2O4 HCs and MgCo2O4 HCs have been comparatively investigated. As revealed in Fig. 3d, the Nyquist plot of NiCo2O4 HCs exhibits an obviously depressed semicircle, indicating a more effective transfer of charge carriers [54]. Therefore, the above observations clearly evidence that NiCo2O4 HCs is able to efficiently accelerate charges migration from Ru photosensitizer and use them for CO2 photoreduction [55]. Then, the surface areas and CO2 uptake ability, which heavily relate to the catalytic performance, have been studied. As shown in Fig. 3e, the two samples feature typical distinct H3 hysteresis loop. Notably, NiCo2O4 HCs exhibits a BET surface area of 164.6 m2/g, which is almost identical to that of MgCo2O4 HCs (163.5 m2/g). The same BET surface areas may be stemmed from their nearly identical crystalline structure and morphology. However, MgCo2O4 HCs features a maximum CO2 uptake of ca. 17 cm3 g−1 at 298 K under 1 atm, which is distinctly higher than that of NiCo2O4 HCs (ca. 12 cm3 g−1), as shown in Fig. 3f. The enhanced CO2 uptake ability of MgCo2O4 may originate from the chemical affinity of Lewis-base-like Mg2+ for the Lewis-acid-like CO2.

a gradually decreased CO evolution rate, which might be caused by the detrimental electron-transfer kinetics in a high concentration of catalyst [51]. As for MgCo2O4 HCs, 1 mg is also the optimal amount for this reaction, as shown in Fig. S6b. Therefore, 1 mg of the catalyst was used in subsequent experiments. As shown in Fig. 3a, under the optimal condition, NiCo2O4 HCs shows a CO generation rate of 10.5 μmol/h with a selectivity of 93.4%, which is ca. 42-times relative to that of the blank system without catalyst (column 3 of Fig. 3a). As for bulk NiCo2O4 (Fig. S7a-b), a similar CO selectivity of 92.3% can be observed, whereas the production rate of both CO (6.9 μmol/h) and H2 (0.58 μmol/h) is substantially decreased as compared with NiCo2O4 HCs, as displayed in Fig. S7c. Therefore, the results clearly confirm the superiority of the hierarchical hollow dodecahedral nanocages than its bulk counterparts. The catalytic performance over NiCo2O4 HCs is significantly higher than that of its precursors (including ZIF-67 and NiCo LDHs), as displayed in Fig. S7d, indicating the validity of converting the precursors to NiCo2O4 HCs. When a 420 nm monochromatic LED light is employed as a light resource, the as-prepared NiCo2O4 HCs shows a high A.Q.Y. of ca. 1.86%. When the CO2 photoreduction reaction was carried out in diluted CO2 (10% CO2), a slightly decreased CO selectivity of 89% can be observed with spectacular A.Q.Y. of 1.56, which is superior to most reported spinel metal oxides and other typical inorganic catalysts in pure CO2 (Table S1). The stability of the as-prepared NiCo2O4 HCs in this reaction system has been confirmed by stable cycle experiments and barely changed XRD patterns before and after CO2 photoreduction (Fig. S8). The control experiment using ultrapure Ar (99.9999%) instead of CO2 as the feedstock gas under otherwise identical conditions demonstrates that almost no CO (column 4 of Fig. 3a). Isotopic experiment performed by using 13CO2 as substrate was conducted to further identify the source of CO. As shown in Fig. S9, mass spectrum signal of 13CO (m/z = 29) has been clearly observed. These above observations clearly indicate that CO indeed originates from CO2 photoreduction. When the reaction is conducted in the absence of sensitizer or light irradiation, as displayed in column 5 ˜ 6 of Fig. 3a, respectively, no products can be detected, suggesting that CO2 reduction is driven by visible light. The inactivation of reaction without the addition of TEOA (column 7 of Fig. 3a) reveals the key effects of the sacrificial agent on the performance of this photocatalytic reaction. Then, control experiments have been conducted to investigate the difference between NiCo2O4 HCs and MgCo2O4 HCs on accelerating 5

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steps of proton-electron reduction to form COOH* and CO* intermediates [56]. Fig. 4a shows the energy pathway for CO2 reduction at the Co and Mg site of MgCo2O4 (400), starting from the chemisorption of CO2 (Fig. S12), and the structures of the COOH* and CO* intermediates are presented in Fig. S14. Noteworthy, since the two O atoms of chemisorbed CO2 are coordinated and strongly bonded to Mg, the proton addition to the coordinated O atom becomes energetically unfavourable (the formation of the new −OH bond is energetically counterbalanced by the breaking of Mg-O bond), more so for Mg than that for Co. Particularly, the significantly very high energy increase for COOH* formation at Mg sites (2.05 eV) indicates that Mg2+ is not the catalytic centre (red pathway), and CO2 reduction on MgCo2O4 (400) is more likely to occur at the Co site (black pathway). In this process, the formation of COOH* and CO* at the Co site needs to overcome uphill energy of around 0.16 eV and 0.90 eV, respectively. On the other hand, the structures of the COOH* and CO* intermediates on NiCo2O4 HCs (400) surface are presented in Fig. S15. DFT calculation results indicate that CO2 photoreduction at the NiCo2O4 (400) surface is thermodynamically very favorable (Fig. 4b), with the Ni site (black pathway) exhibiting better activity than the Co site (red pathway). The formation of COOH* and CO* at the Ni site is energetically both downhill, with exothermic energy of -0.35 eV and -0.30 eV, respectively. In comparison, the COOH* formation at the Co site is slightly endothermic by 0.01 eV, while the CO* formation is energetically exothermic by 0.20 eV. Therefore, Ni species are both adsorption and reaction sites for CO2-to-CO conversion on NiCo2O4 HCs. Based on the above analysis, a possible photocatalytic reaction mechanism for diluted CO2 photoreduction over NiCo2O4 HCs was proposed (Fig. 4c). Upon visible light excitation, the photosensitizer [Ru(bpy)3]2+ is promoted to [Ru(bpy)3]2+*, which is then quenched by sacrificial electron donor (TEOA), generating the reduced forms ([Ru (bpy)3]+). Subsequently, photogenerated electron transfer [Ru (bpy)3]+ to the NiCo2O4 HCs, which then participates in CO2 photoreduction reactions. The appropriate flat-band potentials of NiCo2O4 HCs and MgCo2O4 HCs (−0.72 V and -0.74 V vs. NHE, pH 7.0, respectively, as shown in Fig. S16), which locates between the redox potential of E (CO2/CO) = −0.53 V (vs. NHE) and E (Ru(bpy)32+*/ Ru

Generally, the significantly enhanced CO2 uptake ability of catalysts would be expected to facilitate the stabilization of initial metal CO2 adducts, thereby boosting the catalytic performances for CO2 photoreduction [23,24]. In this case, the CO2 uptake ability and catalytic performance show inconsistent trends between NiCo2O4 HCs and MgCo2O4 HCs. In order to probe into the origination of the above inconsistent trends and uncover atomic-level insights into the relation between CO2 adsorption and reduction performance, the whole reaction process has been simulated by density functional theory (DFT) calculations based on the mainly exposed (400) facets. Fig. S11 shows the structures of clean MgCo2O4 (400) and NiCo2O4 (400) surfaces. One can see that they have very similar surface structures, where the surface Mg and Ni are two-fold coordinated to O, while the surface Co is five-fold coordinated to O. We then investigated the interaction of CO2 with MgCo2O4 (400) and NiCo2O4 (400). The relaxed adsorption geometry and the corresponding binding energy are demonstrated in Fig. S12. Particularly, it is worth noting that very strong chemisorption of CO2 on MgCo2O4 (400) with the large binding energy of around -1.81 eV can be observed. In this structure, the C atom of the chemisorbed CO2 is bonded to the surface Co, and the two O atoms of CO2 are bonded to the surface Mg (Fig. 12a). In the case of NiCo2O4 (400), however, the binding for CO2 becomes relatively much weaker. The much stronger binding of CO2 on MgCo2O4 HCs has further been confirmed by the results of CO2 TPD over NiCo2O4 HCs and MgCo2O4 HCs, as shown in Fig. S13. The CO2 adsorption properties on both Ni and Co sites have been then compared and found that CO2 prefers to locate at the Ni sites with one of the CeO bonds bonded to Ni (binding energy: -0.54 eV, Fig. S12b). By contrast, CO2 is only physically adsorbed above Co with the weak binding energy of -0.23 eV (Fig. S12c). The above findings thus indicate that the synergism between Mg and Co (Mg-Co-Mg) serves as the active centre for CO2 capture on MgCo2O4 (400), while Ni species are the active sites for CO2 binding on NiCo2O4 (400). To further understand the catalytic process, the thermodynamic energy pathway for CO formation on the different catalytic sites of MgCo2O4 (400) and NiCo2O4 (400) surfaces have been further calculated. Generally, CO2 photoreduction to CO involves two consecutive

Fig. 4. Potential energy diagram describing the COOH* intermediate from CO2 reduction to CO over (a) Mg/Co sites of MgCo2O4 (400) surface and (b) Ni/Co sites of NiCo2O4 (400) surface; and (c) proposed mechanism for the photocatalytic conversion of diluted CO2 to CO over NiCo2O4 HCs under visible light irradiation with [Ru (bpy)3]2+ as the photosensitizer and triethanolamine (TEOA) as the electron donor. 6

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(bpy)+) = −1.09 V (vs. NHE), ensures electrons transfer from the photosensitizer to drive CO2-to-CO conversion. Meanwhile, part of electrons could also participate in H2 generation. Compared with its MgCo2O4 HCs counterpart, NiCo2O4 features abundant coordinatively unsaturated Nickle active sites on the ultrathin surface and edges, which is in favour of photogenerated charges separation and CO2 uptake/activate, resulting in obviously boosted CO production performance, especially in diluted CO2.

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4. Conclusions In summary, two kinds of isostructural hierarchical spinel oxides hollow dodecahedral nanocages (i.e., NiCo2O4 HCs and MgCo2O4 HCs) have been elaborately prepared by a novel sequential templating strategy to unravel the intrinsic relationships between CO2 adsorption abilities and CO2 photoreduction performance. Compared with its MgCo2O4 HCs, NiCo2O4 HCs features promotional charge transferability, appropriate CO2 up-take property and significantly lower energy barrier for the generation of intermediate (COOH*). Consequently, the NiCo2O4 HCs catalyst exhibits remarkably boosted performance for CO production in pure and diluted CO2, while MgCo2O4 HCs is almost inactive in diluted CO2. The results demonstrate that only active adsorption, whose adsorbed CO2 can participate in the following reduction, can accelerate the whole reactions, especially in diluted CO2. Hopefully, this present study provides a feasible strategy for the fine integration of adsorption and active sites towards solar energy conversion. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The support from the National Natural Science Foundation of China (Grant No. 21777046 and 21836002), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06N569), and the Science and Technology Project of Guangzhou (No. 201803030002) is gratefully acknowledged. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2019.118208. References [1] S.J. Davis, K. Caldeira, H.D. Matthews, Science 329 (2010) 1330–1333. [2] J. Gu, C.-S. Hsu, L. Bai, H.M. Chen, X. Hu, Science 364 (2019) 1091–1094. [3] Y.-L. Men, Y. You, Y.-X. Pan, H. Gao, Y. Xia, D.-G. Cheng, J. Song, D.-X. Cui, N. Wu, Y. Li, S. Xin, J.B. Goodenough, J. Am. Chem. Soc. 140 (2018) 13071–13077. [4] K.-L. Bae, J. Kim, C.K. Lim, K.M. Nam, H. Song, Nat. Commun. 8 (2017) 1156. [5] H. Rao, L.C. Schmidt, J. Bonin, M. Robert, Nature 548 (2017) 74. [6] J. Ran, M. Jaroniec, S.-Z. Qiao, Adv. Mater. 30 (2018) 1704649–1704680. [7] H. Pang, X. Meng, P. Li, K. Chang, W. Zhou, X. Wang, X. Zhang, W. Jevasuwan, N. Fukata, D. Wang, J. Ye, ACS Energy Lett. 4 (2019) 1387–1393. [8] M. Liu, Y.-F. Mu, S. Yao, S. Guo, X.-W. Guo, Z.-M. Zhang, T.-B. Lu, Appl. Catal. B 245 (2019) 496–501. [9] J.-Y. Li, L. Yuan, S.-H. Li, Z.-R. Tang, Y.-J. Xu, J. Mater. Chem. A 7 (2019) 8676–8689.

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