Hydroxyapatite decorated TiO2 as efficient photocatalyst for selective reduction of CO2 with H2O into CH4

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Hydroxyapatite decorated TiO2 as efficient photocatalyst for selective reduction of CO2 with H2O into CH4 Ruifeng Chong a, Yangyang Fan a, Yuqing Du a, Ling Liu b, Zhixian Chang a,*, Deliang Li a,** a

Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China b School of Science, Henan University Minsheng College, Kaifeng 475004, China

article info

abstract

Article history:

Efficient catalysts with high selectivity in products are highly desirable for photocatalytic

Received 15 August 2018

CO2 reduction. In this work, hydroxyapatite (HAP) decorated TiO2 (HAP/TiO2) were suc-

Received in revised form

cessfully fabricated via in-situ deposition of Ca(OH)2 on rutile TiO2 followed by a facile

25 September 2018

hydrothermal reaction. Comparing with TiO2, HAP/TiO2 exhibited significant enhancement

Accepted 8 October 2018

(ca. 40 times) toward photocatalytic CO2 reduction in the presence of H2O with a >95%

Available online xxx

selectivity of CH4. The characterizations revealed HAP possessed Lewis basic sites (O2
in 2þ or OH
vacancies), where Lewis basic sites could -PO34 groups) and Lewis acidic sites (Ca

Keywords:

enhance the adsorption/activation of CO2 and Lewis acidic sites facilitated the adsorption/

CO2 reduction

dissociation of H2O respectively, thus promoting the photocatalytic reduction and oxida-

Photocatalysis

tion half-reactions of CO2 and H2O over Pt/TiO2. The formation of much more stable in-

TiO2

termediates over HAP/TiO2 would be responsible for the high selectivity of CH4. Moreover,

Hydroxyapatite

photoelectrochemical and electrochemical characterizations revealed HAP could also

CO2 activation

promote the charge separation of TiO2 and the charge transfer between TiO2 and adsorbed species. The findings demonstrate HAP has a great potential as efficient assistant for photocatalytic CO2 reduction with H2O and will stimulate us to design novel semiconductor-based materials with tuned Lewis acidic and Lewis basic sites to achieve highly efficient photocatalysts. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The current depletion of fossil fuel reserves and the atmospheric levels of carbon dioxide have adverse and irreversible impacts on the future of some energy sources and the global

climate. To address both problems simultaneously, the capture and utilization of CO2 as an alternative carbon feedstock to generate fuels and chemicals have attracted much research attention in recent years. However, the activation and conversion of CO2 is a big challenge because CO2 is an extremely stable small molecule. Because its C]O bond possesses a

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z. Chang), [email protected] (D. Li). https://doi.org/10.1016/j.ijhydene.2018.10.045 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Chong R, et al., Hydroxyapatite decorated TiO2 as efficient photocatalyst for selective reduction of CO2 with H2O into CH4, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.045

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higher dissociation energy of ~750 kJ mol than those of many other chemical bonds such as CeH (~430 kJ$mol
1) and CeC (~336 kJ, which indicates the conversion of CO2 requires a high external energy. As a promising approach for realizing sustainable development, photocatalytic reduction of CO2 into hydrocarbon has been persistently drawing attention due to the use of an inexhaustible solar energy as energy source [1,2]. Since the pioneering work for the photocatalytic reduction of CO2 in H2O reported by Inoue in 1979 [3], various semiconductors, such as TiO2 [4,5], Ga2O3 [6] and ZnGa2O4 [7] etc., have demonstrated outstanding performance for the reduction CO2 with H2O to CO, CH4 and other hydrocarbons. Among these semiconductors, TiO2 has become a favored photocatalyst due to its availability, stability, low-cost and nontoxicity. However, the catalytic performance of TiO2 is far from satisfaction due to the slow separation of photogenerated electron-hole pairs, slow kinetics and poor product selectivity [1,2,8]. Significant achievements have been made in optimizing the former limitation as the same issues in photocatalytic water splitting and CO2 reduction systems, including constructing facet-based homojunctions [9] and crystalline phase-based homojunctions [10] and decorating with noble metals (e.g., Pt, Ag, Pt-Ru and Cu) [11e15]. For instance, noble metals loaded on the surface of TiO2 could serve as a reservoir of photoelectrons, which could suppress the recombination of photo-generated electrons and holes and improve the quantum efficiency of photocatalytic reactions. However, limited attention has been focused on the latter, which is also a key factor that affected the photocatalytic reduction of CO2. The activation of CO2 is the primary obstacle for in chemistry since a single electron reduction of CO2 to CO2$- occurs at
1.90 V vs. NHE, which is too high to proceed [1,2]. In this process, a high reorganization energy arising from the significant structural change exists due to the transformation from linear CO2 to bent CO2$-. The surface chemistry of CO2 on oxides indicated the D∞h geometry of linear CO2 could be significantly distorted to form species [16]. Spectroscopic analysis have adsorbed COd$2 revealed COd$2 was a kind of OeCeO species with lower bond angle [17,18]. In view of this, the introduction of functional basic sites on semiconductor for enhancing CO2 adsorption has been verified as an efficient approach to improve the activity of photocatalytic CO2 reduction. For example, the basic promoters, NaOH, MgO and CeO2, have exhibited positive effects on photocatalytic CO2 reduction over TiO2 [8]. NaOH could enhance the chemisorption of CO2 by forming carbonate and/or CO
2 [19], while MgO loaded on TiO2 is beneficial for CO2 adsorption to form carbonate [20e24], and Ce3þ in CeO2 on TiO2 would strengthen the bonding of CO2 and facilitate the separation of photogenerated charges [25]. Moreover, in CO2 molecule, the carbon atom is in its highest oxidation state, and the reduction reaction can proceed through the exchange of two, four, six, and eight electrons per C atom through different pathways to produce a wide variety of products with different oxidation states to CH4 [26,27]. Previous works demonranging from C2O2
4 strated the products depended strongly on the surface structure of the catalysts [28e30]. Besides that, photocatalytic CO2 reduction with H2O was inevitably coupled with the halfreaction of H2O oxidation. As water oxidation reaction

(2H2O/4HþþO2þ4e
) was energetically uphill reaction involved a four-electron transfer process, it was also considered as a challenge in photocatalytic CO2 reduction [31,32]. Based on the above discussion, to achieve high efficiency of photocatalytic CO2 reduction, it is very necessary to mediate the adsorption/activation of CO2, as well as the water oxidation half-reaction. As a thermodynamically stable calcium phosphate salt with hexagonal lattice symmetry (P63/m), hydroxyapatite (Ca10(PO4)6(OH)2, HAP) presents both Lewis acidic and Lewis basic sites in a single crystal lattice [33e35], and has been widely used for various catalytic reactions [36,37]. Recent
studies indicated Lewis basic sites (O2
in PO3
4 and OH ) in HAP could adsorb and activate CO2, meanwhile Lewis acid sites (Ca2þ ions or oxygen vacancy) and H-bonding to basic O atoms could promote H2O dissociation [38,39]. It is thus expected the modification of TiO2 with HAP would provide an effective way to enhance the efficiency of photocatalytic CO2 reduction. In fact, comparing with bare TiO2, HAP supported TiO2 has demonstrated superior photocatalytic degeneration of pollutants, due to its excellent adsorption and electron transfer performance [40e42]. However, few works on photocatalytic CO2 reduction over HAP-decorated TiO2, especially the role of HAP, have been addressed. In this paper, we report that HAP modified rutile TiO2 nanorods (HAP/TiO2) show a significantly enhanced activity and an ultra-high selectivity of photocatalytic reduction of CO2 to CH4 with H2O as both the reductant and electron donor and Pt nanoparticles as co-catalyst, comparing with the catalysts of pure TiO2. The experimental results from photocurrent tests and electrochemical impedance spectroscopy (EIS) demonstrated a higher separation efficiency and faster transfer of the photogenerated charges in HAP/TiO2 composite. The adsorbed species and intermediates of CO2 reduction detected by in-situ infrared spectroscopy (IR) and the reactive oxygen species (ROS) collected by confocal laser scanning microscope (CLSM) further revealed the mechanism of HAP promoted the activity and selectivity of photocatalytic reduction of CO2 to CH4.

Results and discussion Structural characterizations HAP/TiO2 were fabricated via a two-step of an in-situ deposition of Ca(OH)2 on rutile TiO2 followed by a facile hydrothermal reaction with (NH4)2HPO4. By altering the deposition time for Ca(OH)2, various HAP/TiO2 with different loading amounts of HAP were obtained (see details in Experimental section, Supplementary material). Notes that HAP/TiO2-2 was selected as a representative for the following discussion, unless otherwise specified. Fig. 1a shows XRD patterns of TiO2, Ca(OH)2/TiO2 and HAP/TiO2, as well as that of HAP. Obviously, the crystalline phase for TiO2, without any changes in Ca(OH)2/TiO2 and HAP/TiO2, could be indexed to the tetragonal rutile (JCPDS No. 88-1175). After being coated with Ca(OH)2, several new peaks appeared at 2q ¼ 18.0 , 28.6 , 34.0 , 47.0 and 50.8 , which matches well with (001), (100), (011), (012) and (110) planes of hexagonal Ca(OH)2 (JCPDS No. 040733). The intensities for such peaks are increased with the

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Fig. 1 e (a) XRD patterns of HAP, TiO2, Ca(OH)2/TiO2 and HAP/TiO2; and, (b）IR spectra of TiO2 and HAP/TiO2.

increase of the immersion time in saturated Ca(OH)2 (Fig. S1a), indicates the dependence of the deposited amount of Ca(OH)2 on TiO2 vs. the immersion time. A minor peak at ~29 (2q) could be assigned to (104) plane of rhombohedral calcite (CaCO3), which might be attributed to the inevitable carbonation of Ca(OH)2 in air. XRD pattern of HAP/TiO2 exhibits the addition of the reflections from TiO2 and HAP. A set of peaks at 2q ¼ 32.3 , 33.3 and 34.3 match well with (211), (300) and (202) planes of hexagonal HAP (JCPDS No. 09-0432). No other peaks related to calcite or calcium-phosphate species indicate a pure phase of HAP on TiO2. Also, the peak intensities for HAP in HAP/TiO2 samples increased with the increasing loading Ca(OH)2 on TiO2 (Fig. S1b), suggesting the increased amount of HAP on TiO2. As a further investigation, IR spectra of TiO2 and HAP/TiO2 are comparably illustrated in Fig. 1b. The bands at 1090/1040, 964 and 604/567 cm
1 in HAP/TiO2 could be assigned to v3, v1 and v4 vibration mode of PO3
4 in HAP [36]. The weak bands at 1452 and 1416 cm
1 in both TiO2 and HAP could be attributed to the carbonate species, which might be originated from the adsorbed CO2 [38]. The characteristic adsorption of OH appears at 3572 cm
1 (nOH) and 629 cm
1 (dOH), indicating OH is arranged to form column-like channels in HAP/TiO2 [33]. In addition, the broad bands at 3440 cm
1 could be assigned to the hydrogen-bonding between OH species [39], and the band at 1640 cm
1 would be indexed to the absorbed H2O. According to the observations, it could deduce OH
in HAP has been slightly substituted by CO2
3 impurities. Based on ICP-AES, Ca/P in HAP/TiO2 is calculated to be close to

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the stoichiometric ratio of 1.67 (1.65, 1.65 and 1.64 for HAP/ TiO2-1, HAP/TiO2-2 and HAP/TiO2-3, respectively), which is indicative of the negligible impurities in HAP. TEM images show TiO2 possesses a rod like-shape with the size of 50e100 nm in width and 200-400 nm in length (Fig. 2a and S2a). After being immersed in saturated Ca(OH)2 for 60 min, a uniform Ca(OH)2 film with the thickness of ~5 nm was coated on TiO2 surface (Fig. 2b). Elemental mappings for Ca, O and Ti of Ca(OH)2/TiO2 further confirm Ca(OH)2 is deposited on TiO2 uniformly (Fig. S3). In contrast, the morphology of HAP/TiO2 exhibits irregular HAP nanosheets deposited on TiO2 (Fig. 2c and S2b), which is far distinct from that of Ca(OH)2/TiO2. These results suggest the formation of HAP undergoes a dissolution-reprecipitation process. The lattice fringes with the interplanar spacing of 0.344 nm could be ascribed to (002) plane of hexagonal HAP, while the lattice spacing of 0.320 nm corresponds to (110) plane of rutile TiO2 (Fig. 2d). Fig. 2eei display the elemental mappings for HAP/ TiO2. As seen, Ti is located in the central part of the nanorods, while Ca and P are randomly distributed throughout the whole nanorods. Fig. S4 demonstrates SEM images of TiO2 and Ca(OH)2/TiO2 with smooth surface and HAP/TiO2 with rough surface, which is consistent with TEM results. The roughness of HAP/TiO2 was observed to be enhanced with the increase of the immersion time that for the deposition of Ca(OH)2 (Fig. S5). For further comparison, the morphology of HAP/TiO2(N), which was prepared by direct hydrothermal method without pre-deposition of Ca(OH)2, was also characterized by SEM and TEM. As shown in Fig. S6, irregular HAP with serious agglomerations scattered on TiO2 suggests the necessary predeposition of Ca(OH)2 for the well dispersed HAP. UVevis DRS indicates the loaded HAP hardly influence the optical properties of TiO2 (Fig. S7). And BET characterization shows the specific surface areas of TiO2, HAP/TiO2-1, HAP/TiO2-2 and HAP/TiO2-3 are of slight changes as 28.9, 29.5, 30.2 and 31.5 m2$g
1, respectively. The chemical states of Ca, P, O and Ti were further identified by XPS. Fig. 3a clearly shows the binding energies of Ca 2p and P 2p in HAP/TiO2, indicating the existence of Ca and P species. The high-resolution XPS spectra for O 1s, Ca 2p, P 2p and Ti 2p in TiO2 and HAP/TiO2 are comparably shown in Fig. 3bee. By using Lorentzian-Gaussian fitting, O 1s in TiO2 were well fitted with two peaks at 531.6 and 529.5 eV, which could be assigned to surface hydroxyl (TieOH) and lattice oxygen (TieOeTi) [43]. While O 1s for HAP/TiO2 were deconvoluted into three peaks at 532.9, 531.6 and 529.9 eV, corresponding to O in hydroxyl groups (CaeOH)/chemisorbed water (H2O), termination oxygen (-PO34 ) and lattice oxygen (TieOeTi) respectively [44]. The presence of the termination oxygen indicates the presence of Lewis basic sites in HAP/TiO2. Highresolution XPS spectra of Ca 2p at 347.0 eV (Ca 2p3/2) and 350.6 eV (Ca 2p1/2) and P 2p at 133.2 eV, corresponding to Ca and P in HAP, indicates HAP has been successfully decorated on TiO2. In addition, Ca/P in surface was estimated to be about 1.55 by XPS, which was slightly less than that of the bulk (1.65 by ICP-AES). This finding indicated Ca species were less exposed and excess phosphate ions were located on the surface layer, at least within ca. 10 nm depth limit of XPS technique. It means that the surface HAP is nonstoichiometric with Ca deficiency, which would be improve the electronic

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Fig. 2 e TEM images of (a) TiO2, (b) Ca(OH)2/TiO2 and (c) HAP/TiO2; (d）HRTEM image of spectra of HAP/TiO2; and, (eei) Elemental mappings of Ti, O, Ca and P on HAP/TiO2.

Fig. 3 e (a) Survey XPS spectra and high-resolution XPS spectra of (b) O 1s, (c) Ca 2p, (d) P 2p and (e) Ti 2p of TiO2 and HAP/ TiO2.

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levels of HAP [45,46]. It also reported that the deficiency of Ca could be compensated by the removal of OH
ions, thus generating OH
vacancies (dþ) [36]. The presence of such dþ indicates Lewis acidic sites also exist in HAP/TiO2. On the other hand, adjacent dþ to TiO2 surface would also be beneficial to the transfer and the separation of photogenerated charges. It is also observed that the binding energy of Ti 2p3/2 in HAP/TiO2 exhibits a positive shift comparing with that in TiO2, indicating good contact between HAP and TiO2. XPS analysis reveals HAP on TiO2 could offer Lewis acidic and Lewis basic sites, which have potential to benefit the adsorption/activation of CO2 and the adsorption/dissociation H2O in photocatalytic CO2 reduction. Also, the good contact could facilitate the transfer and the separation of photogenerated charges.

Photoelectrochemical and electrochemical characterizations Fig. 4a exhibits the transient photocurrents of HAP, TiO2, HAP/ TiO2(N) and HAP/TiO2 under light irradiation. No photocurrent was observed on pure HAP, indicating no photocatalytic activity of HAP itself was present in the CO2 reduction process. Comparing with that of TiO2 and HAP/TiO2(N), the photocurrent density of HAP/TiO2 was significantly enhanced, indicating much more effective separation of photogenerated charges over HAP/TiO2. The photocurrent is mainly produced by the diffusion of photogenerated electrons, while the photogenerated holes are captured by hole acceptors in electrolyte. Thus, the enhanced photocurrent density could be attributed to (i) the improved electrical conductivity and charge migration caused by hydroxyl channels in hexagonal HAP and (ii) a high separation efficiency of electron-hole pairs caused by OH
vacancies (dþ) in Ca-deficient HAP [42,47,48]. The charge transfer was also characterized by electrochemical impedance spectroscopy (EIS). The diameter of the semicircle in EIS equals a charge transfer resistance, which reflects the electron transfer kinetics at the interface of electrode/electrolyte. The typical EIS curves for HAP, TiO2, HAP/TiO2 and HAP/TiO2(N) in 0.5 mol$L
1 Na2SO4 were shown in Fig. 4b. The diameter of the semicircle for HAP/TiO2 was apparently smaller than those of HAP, TiO2 and HAP/TiO2(N), indicating an efficient charge transfer at the interface of HAP/TiO2 electrode and electrolyte.

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Photocatalytic CO2 reduction performance Photocatalytic activities of CO2 reduction in the presence of H2O over Pt/TiO2 and Pt/HAP/TiO2 were evaluated by a gassolid system. To enhance the separation of photogenerated electron-hole pairs, Pt was loaded as cocatalyst via a photodeposition method. TEM and HRTEM images of on Pt/HAP/ TiO2 (Fig. S8) demonstrate Pt nanoparticles with a diameter of ca. 4 nm are well dispersed on HAP/TiO2. Four control experiments were conducted under the conditions of (i) Pt/HAP/ TiO2, CO2 and H2O without light irradiation, (ii) CO2, H2O and light irradiation without Pt/HAP/TiO2, (iii) CO2, H2O, light irradiation and HAP/TiO2 without Pt and (iv) H2O, light irradiation and Pt/HAP/TiO2 without CO2. All control experiments were observed without detectable CO and CH4, implying all factors that the presence of CO2, H2O, light irradiation, cocatalyst and HAP/TiO2 or TiO2 were essential to the photocatalytic reaction. Under the selected conditions, CO, CH4 and O2 were found to be the major products. However, a trance of H2 generated from the H2O (vapor) splitting was also detected. Fig. 5a displays CO and CH4 evolution over various photocatalysts with Pt as cocatalyst (not marked in the figure). As seen, the average evolution rates of CO and CH4 over Pt/TiO2 were ca. 0.06 and 0.12 mmol$g
1$h
1, respectively. As for all Pt/ HAP/TiO2, CH4 was found to be the major product with minor CO. The maximum CH4 evolution rate was achieved as 4.64 mmol$g
1$h
1 over Pt/HAP/TiO2-2, which was ca. 40 times as high as that over Pt/TiO2. More importantly, the selectivity of CH4 are >95% over all Pt/HAP/TiO2, which is far higher than that over Pt/TiO2 (Fig. 5b). Such higher selectivity of CH4 might be attributed to the basic sites in HAP, which was favorable for the preferential stabilization and the hydrogenation of CO2. Overall, HAP has significantly enhanced the activity and the CH4 selectivity of Pt/TiO2. Fig. 5a also exhibits a typical volcano for the yield of CH4 vs. the loading amount of HAP; namely, as the HAP amount increases, the CH4 formation rate increases first and then decreases remarkably. On the other hand, the selectivity for CH4 product shows slight decrease with the increase of HAP amount. The similar phenomenon has also been observed in previous literature [49,50]. It might be attributed to the excess HAP could cause detrimental to the charge transfer from TiO2 to the active sites and make Pt cocatalysts with low density of photogenerated electrons.

Fig. 4 e (a) Transient photocurrent densities versus time; and, (b) EIS measurements of HAP, TiO2, HAP/TiO2 and HAP/ TiO2(N). Please cite this article in press as: Chong R, et al., Hydroxyapatite decorated TiO2 as efficient photocatalyst for selective reduction of CO2 with H2O into CH4, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.045

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Fig. 5 e (a) The average evolution rates and (b) the selectivity of CO and CH4 over TiO2 and various HAP/TiO2 photocatalysts loaded with 1 wt% Pt as cocatalyst for 8 h irradiation; and, (c) Three consecutive 8 h cycles of CH4 evolution over Pt/HAP/TiO2 for photocatalytic CO2 reduction in the presence of H2O.

In addition, the yield of CH4 over Pt/HAP/TiO2(N) is ca. onetwelfth of that over Pt/HAP/TiO2-2, conforming the necessity of the intimate contact between HAP and TiO2. Fig. 5c exhibits the yield of CH4 increases linearly vs. reaction time over Pt/ HAP/TiO2 in three consecutive 8 h cycles. No deactivation observed in the second and the third runs indicates Pt/HAP/ TiO2 possesses excellent stability for the photocatalytic CO2 reduction into CH4. XRD and IR spectra for Pt/HAP/TiO2 after the three 8 h cycles (pretreated at 573 K for 2 h to eliminate the adsorbed species) exhibited negligible changes from that of the fresh (Fig. S9), further indicating the high structural stability of Pt/HAP/TiO2. Given that the oxidation of H2O to O2 is the sole reaction to consume the photogenerated holes, the molar ratio of O2/(COþ4CH4) should be calculated as 0.50 theoretically. The ratio of O2/(COþ4CH4) for our results is calculated as 0.53, which is slightly higher than the theoretical one. Such deviation might be related to the generation of trace H2 as detected by GC. This suggests CO and CH4 are the main reduction products from the photocatalytic reduction of CO2 over our proposed photocatalysts.

Mechanism analysis To insight the mechanism for the photocatalytic reduction of CO2 with H2O, in-situ IR was used to characterize adsorbed

species and intermediates over Pt/HAP/TiO2, as well as that over Pt/TiO2. Fig. 6a displays IR spectra of Pt/HAP/TiO2 with CO2 and H2O before and after light irradiation, using that of original Pt/HAP/TiO2 as reference (the straight line in Fig. 6a). After the adsorption of CO2 and H2O (in dark), the characteristic band of chemisorbed CO2 on the sites (O2
) of basic oxides at 1385 cm
1 was clearly observed [51]. According to the structure of HAP, O2
sites should be assigned to -PO34 groups, thus the corresponding chemisorbed carbon species should be referred to (POx)s-carbonates [38]. Taking CO2 as a probe molecule for basic sites at gas-solid interface, the formation of (POx)s-carbonates reveals the presence of Lewis basic O2
sites on Pt/HAP/TiO2 [51]. The weak peak at 1255 cm
1 could be assigned to the symmetric OCO stretching of carbonate (CO2$-), which was formed during the coadsorption of CO2 and H2O [25]. The bands at 1640 and 3424 cm
1 could be attributed to the adsorbed water and the isolated hydroxyl of Pt/HAP/TiO2, respectively. Such peaks are far higher than those over Pt/TiO2 (in dark, Fig. 6b), suggesting the decoration of HAP could enhance the adsorption of CO2, as well as that of H2O. To reveal the active sites for H2O adsorption on Pt/HAP/TiO2, IR spectra of adsorbed pyridine over TiO2 and HAP/TiO2 in dark were recorded and shown in Fig. 7a. Comparing with that of TiO2, IR spectra of HAP/TiO2 demonstrates obvious n(CCN) absorption at 1607,

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Fig. 6 e In-situ IR spectra for coadsorption of CO2 and H2O on (a) Pt/HAP/TiO2 and (b) Pt/TiO2. The straight line represents the background spectra of Pt/HAP/TiO2 and Pt/TiO2 before the adsorption of CO2 and H2O. Dark corresponds to IR spectra of the samples after being exposed in CO2 and H2O, while L-1 and L-2 means the exposed samples were irradiated with light for 1 and 2 h respectively.

Fig. 7 e (a) In-situ IR spectra of pyridine adsorption on TiO2 and HAP/TiO2; and, (b) typical fluorescence micrographs illustrating the level of reactive oxygen species generated over Pt/TiO2 and Pt/HAP/TiO2.

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1575, 1491 and 1445 cm
1, indicating the presence of various acidic sites (Lewis or Brønsted acid) on HAP/TiO2. On the other hand, the absence of the characteristic absorption of pyridine bonding on Brønsted acid sites at 1540 cm
1 suggests the acidic sites over HAP/TiO2 are mainly Lewis type, i.e., Ca2þ and OH
vacancies (dþ) [51]. H2O could be adsorbed in a strongly polarized fashion on these Lewis acidic sites and via H bonding to basic O atoms of -PO34 , which is believed to facilitate the dissociation of H2O [52e54]. Overall, HAP has demonstrated multiple active sites for CO2 adsorption/activation and H2O adsorption/dissociation. With continuous light irradiation, the peak intensity for (POx)s-carbonates (at 1385 cm
1) was gradually decreased (Fig. 6a), as well as that for H2O and OH
, indicating the participations of carbonate species and H2O in CO2 reduction reaction. After 1 h irradiation, the peak intensity of CO2$species (at 1255 cm
1) on HAP/TiO2 slightly increased, while that of (POx)s-carbonate species (at 1385 cm
1) decreased, indicating the conversion of (POx)s-carbonate to CO2$- species. In addition, three new bands appeared at 2962, 2922 and 2856 cm
1. As light irradiation prolonging to 2 h, the intensities for such peaks increased, while that for CO2$decreased, indicating the conversion of CO2$- species to the other intermediates. It is difficult to identify what intermediates via the peaks located at 3000e2800 cm
1 in IR spectra. According to the literature, such peaks could not only be assigned to the symmetric and asymmetric stretching of CeH in alkyl fragments, such as the adsorbed CH2O or CH3OH intermediates and the adsorbed CH4 product, but also could be attributed to the combination of nas(CO2) with d(CH), n(CH) and the combination of ns(CO2) with d(CH) of bidentate formate species respectively [55e57]. The conversion of CO2$- species has been proposed with two possible pathways [58], that is, (1) the carbon atom in CO2$binds with a hydrogen atom to form HCOO$ followed by accepting an electron to form HCOO
, and, (2) a hydrogen atom attacks one oxygen atom in CO2$- to form carboxyl radical ($COOH) followed by accepting an electron to form adsorbed (COads). Formic acid and CO could be further reduced by electrons and protonated to form different products, such as CH2O or CH3OH, or the adsorbed CH4. Whichever pathway CO2$- species underwent, an IR peaks at ca. 1585 cm
1, being assigned to the characteristic asymmetric stretching of CeO, was suggested to be observed. Unfortunately, the peak was not detected, likely due to the low abundance, as well as the rapid conversion, of such species. On the other hand, formate-like specie was identified as the key intermediate in water-gas shift reaction over Pt/HAP [57]. According to this result, given CO2$- underwent the later process discussed above, it would be ultimately converted into formate-like specie via the pathway of CO2$-/$COOH/COads/HCOO
. Apparently, such process involving the transfers of multiple electrons and hydrogen atoms was more complicated than that of the direct transfers of a hydrogen atom (CO2$-/HCOO
). It thus deduced CO2$- was converted via an intermediate of formate-like specie (HCOO
), though it has not been detected. And, formate-like specie could be converted quickly into other intermediates containing alkyl fragments. Comparing with that over TiO2 (Fig. 6b), IR peaks at 3000e2800 cm
1 over Pt/

HAP/TiO2 are obviously enhanced, indicating the stabilizing role of HAP toward the intermediates. Taking the different product selectivity into account, such stabilizing role of HAP is speculated to be responsible for the enhanced selectivity of CH4 in photocatalytic CO2 reduction over Pt/HAP/TiO2. Fig. 6 has demonstrated the lower level of the coadsorption of CO2 and H2O and the intermediates (i.e., CO2$-) over Pt/TiO2 comparing with those over Pt/HAP/TiO2, suggesting the higher activity of CO2 reduction over Pt/HAP/TiO2. For further investigating the promotion of H2O oxidation by HAP, the level of reactive oxygen species (ROS) on Pt/TiO2 and Pt/HAP/TiO2 was assessed by using H2DCF-DA. H2DCFDA could be hydrolyzed to be H2DCF, which could be further oxidized to be the highly fluorescent DCF by reaction with hydroxyl radicals, peroxy radicals and so on [59]. Fig. 7b exhibited the typical fluorescence micrographs over Pt/TiO2 and Pt/HAP/TiO2. Apparently, the fluorescent intensity over Pt/HAP/TiO2 was significantly stronger than that over Pt/ TiO2, suggesting HAP could promote H2O oxidation to ROS over Pt/TiO2. The enhanced H2O oxidation half-reaction would be beneficial to the CO2 reduction half-reaction, thus enhance the photocatalytic activity. Based on the above results, a possible process for the conversion of CO2 to formate-like species is summarized in Scheme 1. First, CO2 is adsorbed on Lewis basic sites (O2
) to form (POx)s-carbonate species, and H2O is adsorbed on Lewis acidic sites (Ca2þ or dþ) and bonds with adjacent O in -PO34 to form hydrogen bonding. Secondly, under light irradiation, H2O is dissociated on Lewis acidic sites to form -Ca-OH and -P-OH species, meanwhile (POx)s-carbonate specie is converted into CO2$- via an one-electron transfer reaction. Thirdly, CO2$- reacts with a nearby H of -P-OH specie and at the same time accept one electron to yield formate-like intermediate. Finally, formate-like intermediate is further reduced to be CH4 via a consecutive one-electron/one-proton transfer reactions, while the dissociated OH species (i.e., Cae OH) is oxidized into O2 by photogenerated holes. Overall, HAP could tune the adsorption/activation of CO2, the adsorption/dissociation of H2O, the stability of main intermediates, the separation of photogenerated charges and the level of ROS, ultimately enhance the selectivity and the activity of Pt/TiO2 toward photocatalytic CO2 reduction in the presence of H2O. A possible mechanism for the improvement on photocatalytic CO2 reduction with H2O over Pt/HAP/TiO2 is shown in Scheme 2. Under light irradiation, TiO2 is excited to generate electrons and holes. The electrons are then transferred to Pt via the oxygen defect level (Ov.s) from the electron state change of -PO34 in HAP [42,60]. This process would supply an efficient separation of hþ and e
in space, thus reduce charge recombination. Meanwhile, CO2 was adsorbed/activated on Lewis basic sites to form (POx)s-carbonates and H2O was adsorbed/dissociated on Lewis acidic sites, in HAP layer. (POx)s-carbonates was then converted to CO2$- species via one-electron transfer. Subsequently, CO2$reacted with a proton and an electron to generate formatelike species, which was finally reduced to CH4 via a successive reaction with multiple electrons/protons. Simultaneously, the adsorbed H2O or dissociated OH was oxidized by photogenerated holes on valence band (VB) of TiO2 to from ROS and finally generate O2.

Please cite this article in press as: Chong R, et al., Hydroxyapatite decorated TiO2 as efficient photocatalyst for selective reduction of CO2 with H2O into CH4, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.10.045

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 1

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Scheme 1 e A possible pathway for the conversion of CO2 to CH4 species over Pt/HAP/TiO2 in the presence of H2O.

Scheme 2 e A proposed mechanism of photocatalytic reduction of CO2 with H2O over Pt/HAP/TiO2.

Conclusions In summary, heterogeneous HAP/TiO2 has been successfully fabricated via a two-step of an in-situ deposition of Ca(OH)2 on rutile TiO2 nanorods followed by a facile hydrothermal reaction with (NH4)2HPO4. With Pt as cocatalyst, HAP/TiO2 exhibited a significant enhancement toward photocatalytic CO2 reduction in the presence of H2O with a high selectivity of CH4 (>95%). Under the optimal conditions, CH4 evolution rate was achieved as 4.64 mmol$g
1$h
1 over Pt/HAP/TiO2, which was ca. 40 times as high as that over Pt/TiO2. In-situ IR analysis revealed the near-neutral surface of TiO2 could be modulated into a Lewis acid-basic bifunctional surface by HAP, wherein 2þ and/or OH
O2
in -PO34 acted as Lewis basic sites and Ca

vacancies (dþ) as Lewis acid sites. Both the adsorption/activation of CO2 on Lewis basic sites and the adsorption/dissociation of H2O on Lewis acid sites would decrease the energy barriers of the reduction of CO2 and the oxidation of H2O, thus accelerating the photocatalytic reduction and oxidation halfreactions of CO2 and H2O over Pt/TiO2. Electrochemical measurements recovered HAP could also promote the charge separation of TiO2 and the charge transfer between TiO2 and electrolyte. Further investigations revealed the formation of much more stable intermediates would be responsible for the high selectivity of CH4 product. This work will stimulate us to design novel materials with Lewis acidic and Lewis basic sites as assistants, as well as other semiconductors (i.e., ZnO, In2O3 and Ga2O3), to explore highly efficient photocatalysts for the photocatalytic CO2 reduction with H2O.

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Acknowledgements [14]

The work was supported by the National Natural Science Foundation of China (51502078), the Major Project of Science and Technology, Education Department of Henan Province (17B610003, 19A150018 and 19A150019), Henan University (YQPY20170013 and 2015YBZR005), and the program for Science & Technology Innovation Team in Universities of Henan Province (19IRTSTHN029).

Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2018.10.045.

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