TiO2-Integrated Carbon Prepared via Pyrolysis of Ti-Loaded Metal

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TiO2-Integrated Carbon Prepared via Pyrolysis of TiLoaded Metal-Organic Frameworks for Redox Catalysis Mithun Sarker, Biswa Nath Bhadra, Subin Shin, and Sung Hwa Jhung ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01841 • Publication Date (Web): 21 Dec 2018 Downloaded from http://pubs.acs.org on December 24, 2018

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ACS Applied Nano Materials

TiO2-Integrated Carbon Prepared via Pyrolysis of Ti-Loaded Metal-Organic Frameworks for Redox Catalysis Mithun Sarker, Biswa Nath Bhadra, Subin Shin, and Sung Hwa Jhung* Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu 41566, Republic of Korea. *Corresponding Author: Prof. Sung Hwa Jhung Fax: 82-53-950-6330 E-mail: [email protected]

KEYWORDS: double solvent method; hydrophilicity-hydrophobicity; oxidative desulfurization; reduction of 4-nitrophenol; titania-carbon composite

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ABSTRACT: By suitably selecting metal−organic frameworks (MOFs) and solvents (to dissolve Ti precursors) applied for double solvent method (based on the hydrophilicity or hydrophobicity of both MOF and solvent), the position of loaded Ti precursors could be controlled (inside or outside of MOFs). For example, hydrophobic solution of Ti precursors loaded on hydrophobic MOF (MAF-6) or hydrophilic solution of Ti precursors loaded on hydrophilic MOF (MOF-74) leads to MOFs with Ti precursors selectively inside of the MOFs. On the contrary, Ti precursors loaded outside of MOFs could be obtained by introducing hydrophilic and hydrophobic solution onto hydrophobic and hydrophilic MOF, respectively. TiO2-integrated carbons (TiO2@M-6, TiO2@M-74, M-6@TiO2 and M-74@TiO2) were further prepared by pyrolysis of Ti loaded-MAF6 and MOF-74, where particle size of TiO2 depends on the position of loaded Ti precursor. Or, the TiO2 obtained by pyrolysis of Ti precursor-loaded inside of MOF has smaller particle size compared with the particle prepared similarly from Ti precursor-loaded outside of MOF. The reduced TiO2 size might be due to the residence of Ti-precursor inside of MOFs. In order to estimate the possible applications of the obtained TiO2-integrated carbons in redox catalyses, those materials, together with synthesized TiO2 (rutile and anatase), commercial P-25 and TiO2-loaded activated carbon, were applied as catalyst in oxidative desulfurization (ODS) and reduction of 4nitrophenol (4-NP). The catalytic result shows that TiO2–integrated carbons obtained from pyrolysis of Ti-precursors@MOF have higher catalytic activity than those from pyrolysis of MOF@Ti-precursors (because of smaller TiO2 size and higher porosity). Finally, the TiO2@M-6, prepared from pyrolysis of TiCl4@MAF-6, could be suggested as a highly effective catalyst for ODS and the reduction of 4-NP.


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1. INTRODUCTION Metal-organic frameworks (MOFs),1-3 with or without modification, are an advanced class of porous materials4,5 that have received much attention because of their simple synthesis, excellent porosity, and several potential applications.6-8 Recently, MOFs have been employed as support materials for loading various materials including metallic precursors or nanoparticles.9-11 There are several approaches12 such as chemical vapor deposition,13 solution infiltration or incipient wetness impregnation,14 and solid grinding15 to introduce metal precursors onto MOFs. Recently, double solvent (DS) method16-21 has attracted much attention to introduce metal precursors inside the cavities of MOFs because of the possibility to minimize the aggregation of metal precursors on the external surface,16 which is beneficial for catalysis.16 In contrast, in the typical singlesolvent impregnation process, metal precursors can be deposited on the outer surfaces of MOF after drying, producing aggregated nanoparticles on the external surfaces which may lead to reduction in catalytic activity/durability during a reaction.12, 16 In DS method, hydrophilic and hydrophobic solvent were employed to avoid aggregation of metal precursors on external surfaces of MOF. So far, usually hydrophilic MOFs such as MIIL-10116-18 and UiO-6619,

20

have been

employed in DS method using hydrophilic solvent containing the metal precursor with a volume less than the pore volume of the MOF, which finally can be introduced inside of the pores of hydrophilic MOF by capillary force.16 However, the DS method is an emerging technology that needs more study for various applications. Further research such as the effect of hydrophilicity and hydrophobicity of MOFs (these properties of MOFs might be controlled or selected because of designable and rich chemistry of MOFs) and solvents on the products is, therefore, required to understand details of DS method including mechanism. On the other hand, metallic or metal oxide nanoparticles, especially when dispersed well on nanostructured material, have received much attention because of their well-defined size and 3 ACS Paragon Plus Environment


shape as well as various potential applications.22, 23 Among them, supported TiO2 has been studied extensively because of their excellent chemical stability, non-toxicity and remarkable potential applications especially in catalyses.24, 25 Several materials such as mesoporous silica,26 zeolite,27, 28 activated

carbon,29 graphene oxide30 and MOFs31, 32 have been used as catalyst supports to load

titania nanoparticles. However, it is not easy to control the properties such as shape, size, and dispersity of nanoparticle including TiO2 on the supports,33 especially when loaded via a simple impregnation method. Among the catalyst supports, MOFs attract much interest because of their ordered framework structures, well-defined cavities, and hydrophilic or hydrophobic characteristics with relatively good stability especially during the process of loading of guest materials such as metallic nanoparticles.9,

31

Recently, metal nanoparticles embedded in

carbonaceous material derived from MOFs34, 35 showed high activity in heterogeneous catalysis. Compared with direct carbonization of some specific MOFs (MOFs containing Ni, Co, and other catalytically active metals), carbonizing of MOF−guest precursors (such as Ti-precursor loaded MOFs) is an alternative and more versatile approach to achieve hierarchical carbon-based structures with active metal sites36,

37

However, it is difficult to avoid uncontrollable particle

sintering or agglomeration of metal oxides or metal nanoparticles embedded in carbonaceous material due to high pyrolysis temperature.38 Therefore, the selection of the support and the method of loading are very crucial in the design of heterogeneous catalysts that can be helpful to control the properties and enhance the activity of catalysts. On the other hand, hazardous materials from different sources such as burning of fossil fuels and industrial wastewater are considered as one of the most serious environmental concerns of these days.39 Sulfur-containing compounds (SCCs) in fuel can cause air pollution (when burned for energy), by releasing SOx which create health hazards and mainly responsible for acid rain.40


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Several organics including nitroaromatic compounds are responsible for water pollution.41 Among different types of nitroaromatics, nitrophenols are considered as one of the most harmful species to human, plant and aquatic ecosystem.42 Therefore, removing such harmful materials especially SCCs (including dibenzothiophenes or DBT) and nitroaromatic compounds (including 4nitrophenol or 4-NP) from both fuel and water, respectively, is essential for our safe environment and health. Currently, oxidative desulfurization (ODS) has drawn much interest to the researchers because of its simple processing and high efficiency to remove stubborn SCCs.43 Various catalysts especially titania-carbon composites showed excellent performance in ODS.44-47 Additionally, reduction of 4-NP to p-aminophenol (4-AP) over metal oxide@carbon composite has attracted much attention because of applications of 4-AP in pharmaceuticals production.48, 49 Therefore, further study is required to understand catalytic behavior (considering both oxidation and reduction) of TiO2-loaded carbons. Herein, Ti precursors were loaded on zinc-based hydrophobic MOF (MAF-6(Zn),50 Zn(eim)2, chemical formulae: C10H16N4Zn) and hydrophilic MOF (MOF-74(Zn),51,

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Zn2(DHBDC)(DMF)2‚(H2O)2, chemical formulae: C14H20N2O10Zn2) via DS method (where, eim, DHBDC

and

DMF

mean

2-ethylimidazole,

2,6-dihdroxybenzenedicarboxylate

and

dimethylformamide, respectively). Moreover, selected solvents, based also on hydrophobicity and hydrophilicity, were also applied to load Ti precursors onto the hydrophobic and hydrophilic MOFs. We expected the possibility to control the loaded position of Ti precursors (inside or outside of pore of MOFs) depending on the hydrophilicity or hydrophobicity of MOFs and solvents (as shown in Scheme 1). Moreover, various kinds of titania-loaded carbons derived from MOFs (TCMs such as TiO2@M-6, TiO2@M-74, M-6@TiO2 and M-74@TiO2; the notations will be explained in next section and Supporting Information) might also be expected by pyrolysis of Ti



precursor-loaded MAF-6 and MOF-74 (shown in Scheme 1), depending on the properties of MOFs/solvents. The produced composites were characterized thoroughly and applied as catalyst in ODS and reduction of 4-NP; and showed the importance of loaded position of precursors (or, the hydrophilicity/hydrophobicity of MOFs and solvents). Among titania-loaded carbons, TiO2@M-6 showed excellent performances in both of the studied catalytic reactions. Therefore, selection of MOFs, Ti precursors and solvents (based on hydrophilicity and hydrophobicity) is one way to control the loading position of Ti precursors and particle size of active species of products (and finally the performances of the materials as catalyst) obtained via pyrolysis of MOFs with loaded precursors. 2. EXPERIMENTAL 2.1. Materials Zinc nitrate hexahydrate (ZnNO3·6H2O, 99.0%), 2-ethylimidazole (98%), 2,5dihydroxyterephthalic acid (C8H6O6, 97%), anhydrous N,N-dimethylformamide (DMF, 99.8%) and diethanolamine (NH(C2H4OH)2, 99%) were acquired from Alfa Aesar. Titanium (IV) chloride (TiCl4, 99%), titanium (IV) butoxide (Ti(OC4H9)4, 97%), commercial P-25 (TiO2, 99.5%), DBT (C12H8S, 99%), anhydrous n-hexane (95%) were purchased from Sigma-Aldrich. Titanium (IV) sulfate (Ti(SO4)2, 24%) was obtained from Kanto chemical Co., Inc. Hydrochloric acid (HCl, 36%), ammonium hydroxide solution (25%), methanol (99.6%), acetonitrile (99%) absolute ethanol (99.9%), and tetrahydrofuan (THF, 99.5%) were purchased from OCI Chemicals. nOctane (C8H18, 97%) and hydrogen peroxide (H2O2, 35%) were acquired from Yakuri and Junsei Chemical Co., Ltd, respectively. Granular AC (2–3 mm) was acquired from Daejung Chemicals & Metal Co., Ltd and cyclohexane (99%) was bought from Duksan Pure Chemical Co. Ltd. All acquired chemicals were applied without purification. Air and moisture-sensitive reagents were


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handled using a nitrogen-filled glove box. 2.2. Preparation of Catalysts MAF-6 and MOF-74 were synthesized following a previously reported procedure.50, TiCl4 and Ti(SO4)2-loaded MAF-6 and MOF-74 were prepared via the DS method.16,

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The

obtained titanium precursors-loaded MOFs (4 samples; TiCl4@MAF-6, MOF-74@TiCl4, Ti(SO4)2@MOF-74, MAF-6@Ti(SO4)2) were pyrolysed at 1000 °C (temperature was increased from room temperature, with a ramping rate of 5 °C/min) for 6 h under continuous flow of N2 (50 mL/min). The preparation procedure of TCMs is illustrated in Scheme 1. The detailed descriptions of synthesis of TCMS with other studied catalysts (synthesized TiO2 (rutile and antase), TiO2/AC) are described in Supporting Information. 2.3. Characterization Methods The physicochemical properties of the studied materials including TCMs were characterized by using various analytical methods. The crystal phases of studied materials were analyzed with X-ray powder diffraction (XRD, D2 Phaser, Bruker) incorporated with Cu-Kα radiation. The textural properties were measured by nitrogen adsorption at −196 °C applying a surface area and porosity analyzer (Quantachrome (Autosorb-iQ & Quadrasorb SI) after evacuation of samples at 150 °C for 12 h. The surface areas were estimated using the Brunauer−Emmett−Teller (BET) equation. The morphology, size, and distribution of TiO2 in TCMs were observed with a field-emission transmission electron microscopy (FE-TEM; Titan G2ChemiSTEM Cs probe). Raman spectra (in the range of 200–3000 cm-1) were got at room temperature using a Raman spectrometer (Renishaw inVia Reflex, excitation at 514 nm, and laser spot size = 100 m). X-ray photoelectron spectroscopy (XPS) was carried out using a X-ray photoelectron spectrometer (Quantera SXM, ULVAC-PHI) with a dual beam charge neutralizer. 7 ACS Paragon Plus Environment


Moreover, the chemical compositions (C, O, H and N) of the studied materials were evaluated with an elemental analyzer (Thermo Fisher, Flash-2000) and the Ti and Zn contents were examined using inductively coupled plasma (ICP) optical emission spectrometry (PerkinElmer, optima 7300 DV). 2.4. ODS or oxidation of DBT Model fuel solution containing DBT (1000 mg·L−1) was prepared by dissolving DBT in noctane. For the oxidation of DBT, the model fuel (10 ml), extractive solvent (acetonitrile, 5 mL), oxidant (35% aqueous H2O2, 0.1 mL; oxidant to sulfur molar ratio (O/S) = 15, unless otherwise stated), and dried catalyst (0.02 g) were placed in a 30 mL glass reactor fitted with a condenser. The reaction mixtures were then vigorously stirred at different temperature (60, 70 and 80 °C) for 2 h. After finishing the catalytic reactions, the solution was filtered using a polytetrafluoroethylene syringe filter (hydrophobic, 0.5 µm), and separated into polar (acetonitrile-phase) and non-polar (n-octane) phases. The residual concentration of DBT was determined by analyzing the non-polar phase with UV spectrometric analyzer (UV-1800, Shimadzu, Japan). The product (DBTO2) that formed via oxidation of DBT was extracted into the polar acetonitrile solution and was further affirmed with UV. 2.5. Reduction of 4-NP The TCMs were applied as catalysts for the reduction of aqueous solution of 4-NP to 4AP. For the catalytic reduction of 4-NP, 20 mL aqueous solution of 4-NP (1,000 mg/L) was mixed with the freshly prepared aqueous solution of NaBH4 (0.5 M, 1.0 mL) and then catalyst (0.01 g) was added to the reaction mixture. The catalytic performance of TCMs in the reaction was monitored by UV-Vis measurements.


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3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of TCMs Titanium precursors, such as Ti(SO4)2 and TiCl4 were dissolved in hydrophilic (water) and hydrophobic (n-octane) solvent, respectively, and then solutions were loaded (solution volume less than the pore volume of MOF) into not only hydrophilic (MOF-74) but also hydrophobic (MAF6) MOFs via DS method, as shown in Scheme 1. Titanium precursors were selected based on the solubility in the typical hydrophilic and hydrophobic solvents. So far, hydrophilic MOFs such as MIL-101 and UiO-66 have been applied in DS method, excluding only one hydrophobic MOF or ZIF-8.53 According to the idea of DS method,16-21 Ti(SO4)2 dissolved in water probably goes inside the pore of the hydrophilic MOF-74; and TiCl4 dissolved in hydrophobic (n-octane) solvent might go inside the hydrophobic pore of MAF-6. The powder XRD patterns of TiCl4@MAF-6 and MAF-6@Ti(SO4)2 are shown in Figure S1a together with that of the pristine MAF-6. The XRD patterns are nicely matched with the simulated pattern, confirming that not only the pristine MAF-6 was successfully synthesized but also the crystal structure of MAF-6 remains unchanged after loading of Ti precursors. Similar results were observed in case of Ti(SO4)2@MOF-74 and MOF-74@ TiCl4, as shown in Figure S1b. FE-SEM images of pristine MOFs together with Ti precursors loaded MOFs are shown in Figure S2. SEM images of MOFs did not change even after loading of Ti precursors, re-confirming the MOFs are stable during the precursors loading. In this study, the position of loaded Ti-precursors on the MOFs was analyzed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) elemental mapping with STEM–EDX line scan. Figure S3a shows the HAADF-STEM elemental mapping for the hydrophobic MAF-6 after TiCl4 loading. As shown in Figure S3a, HAADF-STEM elemental mapping confirms the generally homogeneous



distributions of the studied elements, excluding Ti, through a crystal of MAF-6. The distributions of Ti and Zn through the crystal were studied more with STEM–EDX line scan, as shown in Figure 1a. The EDX line scan shows that Ti of TiCl4 exists mainly inside of the MOF crystal. On the contrary, the concentration of Zn (one of main component of the MAF-6) was relatively high in the edge or outside of the crystal. This is agreeable with the existence of TiCl4 mainly inside of the crystal. Therefore, it could be suggested that TiCl4 was loaded mainly inside of the MAF-6 crystal. Even though the EDX line scan was not obtained for each pore of MAF-6 (because of too small pore size of the MOF), it might be concluded that the loaded TiCl4 exists mainly inside of pores of MAF-6 considering the fact that the outside of MAF-6 crystal was enriched with Zn (with low concentration of Ti). Ti(SO4)2 loaded MAF-6 was analyzed similarly with the HAADF-STEM elemental mapping and STEM–EDX line scan. The HAADF-STEM elemental mapping (Figure S3b) again confirms the homogeneous distribution of studied elements, excluding Ti, through the MAF-6 crystal. However, the STEM–EDX line scans for Ti and Zn shown in Figure 1b are quite different from those in Figure 1a. Or, the loaded Ti(SO4)2 might be mainly in the outside of the crystal, which is the expected result based on the hydrophobicity of the studied MAF-6 and loaded hydrophilic solution, as shown in Scheme 1. Therefore, we could control the loaded position (inside or outside of pore) by selecting the MOF and solution based on the hydrophilicity and hydrophobicity. However, EDX-line scan or 3D TEM, especially for each pore, is required to confirm the loaded positions. Even though studies with HAADF-STEM elemental mapping and STEM–EDX line scan were not carried out for MOF-74s loaded with Ti precursors, the similar conclusion can be expected based on the size of TiO2 of pyrolyzed products (vide infra). The Ti-precursors-loaded MOFs were then pyrolyzed at 1000 oC under nitrogen atmosphere to produce TCMs with different sizes or positions of obtained TiO2 (such as TiO2@C


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and C@TiO2) depending on the nature of solvent (in which titanium precursors are dissolved) and MOFs. The obtained TCMs were characterized with various techniques in order to confirm the size or property of produced TiO2 particles via pyrolysis.46 The powder XRD patterns of the prepared TCMs are shown in Figure 2a and the XRD patterns of synthesized TiO2 particles (rutile and anatase), commercial P-25, and TiO2/activated carbon (AC) are illustrated similarly in Figure S4a. The XRD patterns show that synthesized TCMs are composed of rutile-TiO2 phase (JCPDF No.: 021-1276)46 with anatase-TiO2 (JCPDF No.: 0211272)46 in small quantity. Curiously, TCMs obtained from MOF-74 (such as TiO2@M-74 and M74@TiO2) have not only TiO2 but also a small amount of ZnO (in case of M-74@TiO2, Ti2O3 was also observed in small quantity). Moreover, TiO2/AC has rutile-TiO2 with small quantity of anatase-TiO2, which is understandable with the fact that rutile is the stable phase of TiO2 (compared with anatase) and usually obtained at high temperature.54 Importantly, the XRD diffraction of TiO2@M-6 was broader than the other TCMs including M-6@TiO2. The average sizes of TiO2 nanoparticles in various products including TiO2/AC and commercial P-25 were estimated by using the Scherrer equation, and are represented in Table 1. The average size (∼17.4 nm) of TiO2 nanoparticles in TiO2@M-6 was the smallest among any of the studied materials including synthesized and commercial TiO2 particles. Moreover, the size of TiO2 of TiO2@M-74 was smaller than that of M-74@TiO2, which is agreeable with the results observed with MAF-6. The smaller size of TiO2 in TiO2@M-6 and TiO2@M-74 than that in M-6@TiO2 and M-74@TiO2, respectively, could be explained with the position of the loaded Ti precursors on MOFs. Or, it is clear that the pyrolysis of Ti-precursors@MOFs lead to smaller TiO2 particles compared with that obtained from pyrolysis of MOFs@Ti-precursors, because of preventing the gathering of Ti precursors or TiO2 thorough pyrolysis (when precursors are inside of MOFs).



The nitrogen adsorption isotherms of TCMs are shown Figure 2b. Similarly, the nitrogen adsorption isotherms of synthesized TiO2 particles, commercial P-25, and TiO2/AC are shown as Figure S4b. The BET surface areas and pore volumes (micro and total) of all the studied materials were obtained from the isotherms, and are summarized in Table S1. Among them, TiO2@M-6 showed not only the highest surface area (541 m2/g) but also the highest total pore volume. To understand more the size and shape of TiO2 nanoparticles in carbons, TCMs were further investigated by using high-resolution FE-TEM. The FE-TEM images, shown in Figure S5, reveal that TiO2 nanoparticles in TiO2@M-6 are smaller than the others, which is agreeable with the XRD results. Importantly, TiO2 nanoparticles in TiO2@M-6 are dispersed quite uniformly with regular in size and shape, as shown in Figure S5a. The TEM-EDS mappings of TiO2@M-6 (shown in Figure S6) demonstrate that elements including C, O, and Ti atoms in the composite are highly dispersed, which might be helpful for various types of heterogeneous catalysis.55, 56 The TiO2 nanoparticles of TiO2@M-6 were further analyzed by HR-TEM, as shown in Figure 3a. The d-spacings of marked fringes of TiO2 nanoparticles in TiO2@M-6 are 0.247 and 0.326 nm, which are in harmony with those of (101) and (110) planes of rutile TiO2, respectively.57 The bright spots in the SAED patterns (shown in Figure 3b) affirmed the formation of TiO2 with high crystallinity. Moreover, the d-spacing calculated from the SAED patterns also confirmed the (101) and (110) planes of rutile TiO2 which is agreeable with XRD and HR-TEM results. The dspacings of rutile TiO2 in TiO2@M-6, obtained from crystal structure, HR-TEM and SAED pattern, are summarized in Table S2. The values obtained from the three information or methods are completely agreeable with one another, confirming that the major phase of the crystalline product is rutile TiO2. The chemical compositions of the studied materials were then analyzed by elemental and


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ICP analyses, and the obtained results are summarized in Table S3. The compositions show that synthesized rutile, anatase and commercial P-25 contain higher quantities of Ti (51.8, 52.5 and 51.2 wt%, respectively, and the results are generally agreeable with the stoichiometry of TiO2) than TiO2/AC and TCMs. As mentioned earlier, TiO2@M-6 and M-6@TiO2 contain a negligible amount (less than 0.1%) of Zn species. On the contrary, TiO2@M-74 and M-74@TiO2 have appreciable amount (even though less than 2.0%) of Zn. This difference in remaining Zn in the products might be because of the oxygen and carbon contents in the used MOFs that pyrolyzed at 1000 °C. When MOFs composed of Zn are pyrolyzed, Zn might be removed effectively as metallic Zn (because of reduction of ZnO with carbons) considering relatively low boiling point of Zn (907 °C).58,

59

MOF-74, different from MAF-6, has oxygen species in large quantity because of

carboxylate group that linked to Zn (and two –OH groups on benzene ring). Therefore, the removal of Zn via evaporation during pyrolysis is less efficient in MOF-74 than in MAF-6. Similar result was observed in case of pyrolysis of Zn-MOF.60 However, effects of loaded salts (such as Ti(SO4)2 and TiCl4) and amount of MOFs (1 g was applied in the present study) on the remained Zn in TiO2@M-74 and M-74@TiO2 cannot be ruled out. Moreover, TiO2@M-6 and M-6@TiO2 are carbon and nitrogen-rich compared with TiO2@M-74 and M-74@TiO2; however, TiO2@M-74 and M-74@TiO2 have higher O-contents than TiO2@M-6 and M-6@TiO2. The reason of this observation can be similarly explained with the linker materials used for the synthesis of the two MOFs. Or, 2-ethylimidazole used for MAF-6 is N-rich; 2,5-dihydroxyterephthalic acid applied for MOF-74 is O-rich. Under the pyrolysis condition, if oxygen species are present in MOFs, carbonaceous species of MOFs can be effectively eliminated via formation of CO2 and CO; and oxygen can be naturally high even though oxygen can be decreased because of oxidation reactions to form CO, CO2, and ZnO, etc. On the other hand, the MAF-6 does not have O species (and



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naturally have high C and N contents), excluding the crystalline water, considering the composition of 2-ethylimidazole that does not contain oxygen. TiO2@M-6 and M-6@TiO2 were further investigated by using Raman spectroscopy, as shown in Figure 4a. Three Raman bands (in Figure 4a) observed from 200 to 700 cm-1 are ascribed to the rutile TiO2 phase,46 which is comply with the XRD and HRTEM analysis, which are shown in Figure 2a and 3a, respectively. TiO2@M-6 has Raman bands at 256, 430, and 610 cm-1 which exist in higher energy region compared with those bands at 250, 416, and 602 cm-1, originated from M-6@TiO2. This might be due to the presence of smaller TiO2 nanoparticles in TiO2@M-6 than [email protected] In addition, the two stretching bands (D and G bands at ∼1340 and 1586 cm-1, respectively) are because of disordered carbon materials.46,

61

Moreover, the ID/IG values of

TiO2@M-6 and M-6@TiO2 (0.87 and 0.84, respectively) suggest the formation of more defects in TiO2@M-6 than in M-6@TiO2, in agreement with the higher surface area and pore volumes (as shown in Table S1) in TiO2@M-6. TiO2@M-6 might be effective in adsorption and catalysis based on the beneficial effect of defects on such applications.61 Furthermore, the XPS survey spectra of TiO2@M-6 and M-6@TiO2 (shown in Figure 4b) indicate the presence of C, N, O and Ti species. The high resolution XPS spectra of TiO2@M-6 in C1s, N1s, O1s and Ti2p regions are presented in Figure S7. The C1s spectrum (Figure S7a) showed an unsymmetrical broad peak that can be deconvoluted into three peaks corresponding to C=C or sp2 carbons (284.5), C-OH or defect-containing sp2 carbons (285.3), and C=O (288.4 eV) bonds.62 The Ti2p XPS spectra (Figure S7b) in the binding energies of 464.8 and 458.9 eV correspond to the Ti 2p1/2 and 2p3/2 energy states, respectively, of the TiO2 phase (TiIV) of [email protected] The O1s XPS spectrum (Figure S7c) of the TiO2@M-6 demonstrates that three types of oxygen are present with binding energies of 530.2, 530.6, and 531.8 eV, which might be due to the Ti-O-Ti (TiO2)


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lattice, C=O and C−O (or Ti−OH), respectively.46 Moreover, the N1s spectrum (Figure S7d) shows that three nitrogen species are evident on the surface of TiO2@M-6 with binding energy of 398.2, 399.9, and 400.7 eV, which might be assigned to pyridinic (N-6), pyrrolic (N-5), and quaternarygraphitic (N-Q) nitrogen, respectively.61, 63 Therefore, the position and size of TiO2 particles in TiO2-carbon composites can be controlled as shown in Scheme 1, at least in part, by selection of MOFs and solvents for precursors loading (before pyrolysis). For example, loading of titanium precursor dissolved in hydrophobic solvent to hydrophobic MOF (MAF-6) in polar solvent may be helpful to load precursor selectively inside of pore of MAF-6. Similarly, aqueous (or polar) solution of titanium precursor might be loaded selectively inside of MOF-74 (hydrophilic MOF) dispersed in hydrophobic solvent. Moreover, simple pyrolysis of precursor-loaded inside of MOF will lead to smaller TiO2 nanoparticles incorporated in mesoporous carbon, because of well-dispersed or confined precursors that are located inside of MOF pores. The surface area and pore volumes of those products were relatively high, partly because of low possibility of blocking aperture of porous carbon with TiO2 which can be obtained via pyrolysis of precursors outside of pores. The obtained TCMs were applied in both oxidation (ODS) and reduction of 4-NP in order to understand the effect of porosity, particle size, and loading position (inside or outside) of TiO2 on redox catalyses. 3.2. Catalytic oxidation of DBT To understand the catalytic performance of TCMs, those materials were first applied in ODS (for the removal of DBT from model fuel) since Ti-based materials

44, 45, 64

such as TiO2-

loaded carbons and TiO2-carbon composites45-47 were reported as potential catalyst for ODS. The ODS was firstly carried out with TiO2@M-6 and M-6@TiO2 under wide range of stirring speed in order to find the minimum agitation speed that can prevent diffusion limitation in the oxidation 15 ACS Paragon Plus Environment


reaction. As shown in Figure S8, the agitation speed of 500 RPM was enough to compare the catalytic activities of various materials; therefore, next reactions were carried out under the selected condition or 500 RPM. Figure 5a shows the conversion of DBT with time over various catalysts, and the obtained results are summarized in Table 1. Importantly, TiO2@M-6 showed the highest removal efficiency (97.4% conversion in 1 h) compared with any other materials, even though Ti-content is relatively a bit low in TiO2@M-6 (Table S3). Since DBT can be removed by adsorption (considering porous structures of the catalysts), removal of DBT via adsorption (in the absence of oxidant H2O2) was also checked over TiO2@M-6 and M-6@TiO2 at 80 °C for up to 120 min. As shown in Figure S9a, the removal by adsorption and oxidation (combined with adsorption) over TiO2@M-6 and M-6@TiO2 are compared. The removal of DBT by adsorption over TiO2@M-6 and M-6@TiO2 is nearly similar to each other; however, the efficiency of removal with adsorption was much lower than that with oxidation or ODS. Similar scenario has been observed in case of TiO2@M-74 and M-74@TiO2 (shown in Figure S9b). Therefore, oxidation (catalysis) played a major role to remove DBT from model fuel with a little contribution of adsorption. The catalytic oxidation of DBT was confirmed by analyzing the produced dibenzothiophene oxide (DBTO2) with UV-visible spectroscopy, as shown in Figure S10. The UVvisible spectrum and the wavelengths of maximum absorbance (λmax) of the product from DBT oxidation are completely matched with those of commercial DBTO2, confirming the successful conversion of DBT into DBTO2. The kinetics for ODS reactions was investigated further to understand the reaction rate properly. Figure 5b presents the pseudo-first-order kinetic plots of DBT decrease over studied materials, and the rate constants (k), correlation coefficients (R2) and turnover frequencies (TOF)


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for the studied catalysts are summarized in Table 1. The high correlation coefficients (~0.998 to 0.999) confirm that the pseudo-first-order kinetic model can be used effectively to evaluate the experimental results. The highest kinetic constant over TiO2@M-6, or the steepest kinetic plot for DBT oxidation (Figure 5b), suggests that TiO2@M-6 is the most efficient catalyst among the studied materials, which is in harmony with the highest conversion of DBT at a fixed time (Figure 5a). Moreover, as summarized in Table 1, TiO2@M-6 showed the highest TOF, among the prepared catalysts and also was competitive against other reported Ti-containing catalysts, as summarized by Leng et al.27, 28 Similar to relative performances of TiO2@M-6 and M-6@TiO2 in oxidative catalysis (based on relative kinetic constants and TOF values, Table 1), ODS performance of TiO2@M-74 was better than M-74@TiO2 (also compared in Table 1). This might be because of high porosity and small size of TiO2 that located inside the carbon matrix of TiO2@M-74, similar to the observation with TiO2@M-6. Further investigation was carried out at other temperatures (60 oC and 70 oC) to compare more the activities of TCMs, and the results are shown in Figure S11. The kinetic plots for the four TCMs at 60 oC and 70 oC were compared in Figure S12a and S12b, respectively. The kinetic constants (k) at different temperature (60 oC, 70 oC and 80 oC) for the four TiO2-loaded carbons were calculated from the kinetic plots and are summarized in Table 2 (for TiO2@M-6 and M6@TiO2) and S4 (for TiO2@M-74 and M-74@TiO2). Moreover, the apparent activation energy (Ea) for DBT oxidation over the four TCMs was calculated from the Arrhenius plots shown in Figure S13; and Ea values for TiO2@M-6 and M-6@TiO2 are 23.3 and 26.6 KJ·mol−1, respectively. Similarly, the Ea values for TiO2@M-74 and M-74@TiO2 are 27.0 and 32.4 KJ·mol−1, respectively. Usually, the effectiveness of catalysts increases with decreasing Ea of the



reaction.65 The Ea value over TiO2@M-6 was very competitive or less than other titanium-based catalysts, as represented in Table S5. Therefore, TiO2@M-6 showed the highest efficiency in ODS (with the highest activity and lowest Ea), compared with all of the studied catalysts, confirming that TiO2 nanoparticles with smaller size (partly due to the loading of Ti precursor inside of MOF, before pyrolysis) was more effective than nanoparticles with larger size. Even though the catalytic reactions were carried out under 500 RPM (selected based on the results in Figure S8) in order to prevent diffusion limitation, oxidation reactions with catalysts having different particle size will be required to confirm the activation energies. Finally, the remarkable efficiency of TiO2@M-6 in ODS might be also because of porous carbons with higher porosity. 3.3. Reduction of 4-NP TiO2-intergrated catalyst (including TiO2-carbon composite) is not only active in oxidation reaction but also shows possible application in reduction reaction, especially in 4-NP reduction.48, 66-68

Therefore, obtained TiO2-loaded carbons (TCMs, derived from MOFs) were applied in 4-NP

reduction reaction to investigate their performance as reduction catalyst. 4-NP exhibited UV-Vis absorbance at 317 nm, however, the band shifted to 400 nm, as shown in Figure S14a, and the color of the solution changed from pale yellow to bright yellow, after just addition of NaBH4. This is due to the formation of 4-nitrophenolate ions with increasing pH by addition of NaBH4,48 even though the solution was quite stable (without change in band position and intensity) for a couple of days in the absence of catalysts.69 The catalytic reduction of 4-NP to 4-AP was carried out over TCMs at 25 oC up to 10 min. Figure S14b shows the UV-Vis spectra after 5 min (as a representative time) of reaction over TCMs to illustrate the reduction of 4-NP to 4-AP. The reduction could be confirmed by the rise of a new absorbance at 298 nm (which is because of 4-AP)66 and the decrease in the absorbance at 18 ACS Paragon Plus Environment

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400 nm. Figure 6a shows the conversion of 4-NP over the four selected catalysts as a function of reaction time. TiO2@M-6 was the most effective in the reduction or to show very high conversion of 4-NP (~99.4%) in 5 min. The plots of the pseudo-first-order kinetics for 4-NP reduction (Figure 6b) reconfirm that TiO2@M-6 is the most efficient catalyst among the studied materials. The catalytic performances of the four catalysts including conversions, k values and correlation coefficients (R2) are summarized in Table S6. Based on the conversion and k, it could be confirmed that TiO2@M-6 again showed the best performance among the studied catalysts. Moreover, TiO2@M-74 similarly exhibit higher catalytic performance than M-74@TiO2. Therefore, the above results observed in reduction (4-NP reduction to 4-AP) are agreeable with those in oxidation or ODS of DBT. Accordingly, the highest efficiency of TiO2@M-6 in 4-NP reduction might be explained similar to the ODS performances or by the smallest size of TiO2 nanoparticles which are dispersed well onto the inside of porous carbons having high surface area. Moreover, there might be a synergistic effect of nitrogen (together with TiO2) present in TiO2@M-670 since it has been reported that N-rich carbons act as an efficient catalyst for 4-NP reduction.49, 68 3.4. Reusability of TiO2@M-6 Reusability of a catalyst is considered as an important issue for the commercial applications of the catalyst. The reusability of the TiO2@M-6 in ODS of DBT was evaluated by running several oxidation cycles. After each run, the used catalyst was regenerated by washing with acetone under ultrasound irradiation.47 As shown in Figure 7, the efficiency of the recycled TiO2@M-6 in the ODS did not diminish noticeably with increasing number of recycles. Moreover, the XRD pattern (Figure S15a) and N2-adsorption isotherm (Figure S15b) of regenerated TiO2@M-6 are nearly similar to those of the fresh TiO2@M-6. The average size of TiO2 nanoparticles (estimated by using the Scherrer equation) also remains unchanged (∼16.8 nm, which is similar to 17.4 nm of 19 ACS Paragon Plus Environment


the fresh catalyst) after regeneration of TiO2@M-6. Therefore, TiO2@M-6 can be suggested as a reusable catalyst for the ODS. The reusability of the TiO2@M-6 in reduction reactions under mild conditions including reduction of 4-NP might be not problematic since TiO2 is well-dispersed on carbonaceous materials and obtained via pyrolysis after loading precursor inside of MOF (which might be different from the supported catalyst obtained via simple impregnation).

4. CONCLUSIONS By selecting MOFs and solvents, based on hydrophobic and hydrophilic properties, we could control the position of loaded Ti precursor (inside or outside of MOF pores) in DS method. Additionally, TiO2-loaded carbons could be obtained by pyrolyzing the Ti precursor-loaded MOFs. The size of TiO2 on carbon, produced from pyrolysis of Ti precursor@MOF, was also smaller than the one produced from MOF@Ti precursor. The obtained TiO2-carbon composites were applied as redox catalysts for ODS and 4-NP reduction, and showed the importance of the size of TiO2 (and position of precursors loaded on MOFs). The TiO2@carbons especially TiO2@M-6 showed remarkable activity for the two reactions compared not only with studied materials but also with other TiO2-containing catalysts, due to smaller size of TiO2 nanoparticles which dispersed homogeneously inside of the carbon matrix. Therefore, the TiO2@carbon, derived from TiCl4@MAF-6, can be suggested as a potential catalyst for redox catalyses (such as ODS and 4-NP reduction) based on high activity, low activation energy (for ODS) and ready recyclability.

Funding source: This work was supported by the National Research Foundation of Korea (NRF)


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grant funded by the Korea government (MSIP) (grant number: 2017R1A2B2008774).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:10.1021/xxx.xxx. Synthesis of catalysts; textural properties of studied catalysts; characterization of catalysts; results of catalytic reactions; XRD; FE-SEM; HAADF-STEM; TEM-EDS; XPS of studied materials (pdf). AUTHOR INFORMATION Corresponding Author *Sung Hwa Jhung. Phone: 82-53- 950-5341; Fax: 82-53-950-6330; E-mail: [email protected] ORCID Sung Hwa Jhung: 0000-0002-6941-1583 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ABBREVIATIONS AC, activated carbon; BET, Brunauer−Emmett−Teller; DBT, dibenzothiophene;



EDS, energy dispersive X-ray spectroscopy; HAADF-STEM, high-angle annular darkfield scanning transmission electron microscopy; MOF, metal-organic-framework; MAF, metal-azolate-framework; 4-NP, p-nitrophenol; ODS, oxidative desulfurization; TCMs, titania-loaded carbons derived from MOFs; TEM, transmission electron microscopy


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Table 1. Catalytic Performances of Synthesized TiO2 or TiO2-loaded Carbons in the Oxidation of DBT at 80 oC. The Particle Sizes of TiO2 are also Shown. Average particle

Conversionb

Rate constantb

Relative

TOFd

Relative

sizea (nm)

(%)

k (min-1)

ratio of kc

(h-1)

ratio of TOFc

Synth.TiO2 (Rutile)

64.1

65.1

1.6×10-2

1.0

0.999

0.2

1.0

Synth.TiO2 (Anatase)

52.2

53.2

1.2×10-2

0.8

0.998

0.1

0.5

Comm. P-25

33.5

58.5

1.4×10-2

0.9

0.998

0.2

1.0

TiO2/AC

46.4

63.0

1.5×10-2

0.9

0.999

0.4

2.0

TiO2@M-6

17.4

97.4

5.0×10-2

3.0

0.999

2.4

12.0

M-6@TiO2

29.7

89.2

3.4×10-2

2.1

0.999

2.0

10.0

TiO2@M-74

35.2

84.1

3.2×10-2

2.0

0.999

1.8

9.0

M-74@TiO2

44.6

72.2

2.1×10-2

1.3

0.998

1.5

7.5

Catalysts

a Particle

size of TiO2 calculated with Scherrer equation (however, the actual size may be quite wide, based on the TEM images shown

in Figure S5). b

Calculated with the first order kinetics (reaction time: 0-60 min).

c Relative d

R2

ratio was calculated based on the synthesized TiO2(Rutile) as a reference.

TOF: moles of DBT reacted/moles of Ti present in the catalyst/h (reaction time) 33 ACS Paragon Plus Environment


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Table 2. Pseudo-first-order Rate Constants and Correlation Factors of TiO2@M-6 and M6@TiO2 for the Oxidation of DBT at Different Temperatures. Activation Energies of the Oxidation are also Shown. TiO2@M-6

M-6@TiO2

Temperature (oC)

k (min-1)

60

3.1×10-2

70

3.8×10-2

80

5.0×10-2

Ea (KJ· mol-1)

R2

0.998 0.998

23.3

0.999

k (min-1)

R2

2.0×10-2

0.998

2.6×10-2

0.999

3.4×10-2

0.999

*k: first order rate constant (calculated from the reactions for 0-60 min); Ea: activation energy; R2: correlation coefficient


Ea (KJ· mol-1)

26.6

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Scheme 1. Loading position of Ti-precursor on MOF via double-solvent method. The loading position can be selected by controlling the hydrophobic and hydrophilic properties of MOF and solvent (for loading).



125

(a)

Zn

125

Ti

Counts (a.u.)

75 50

Zn

Ti

75 50 25

25 0

(b)

100

100

Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

200 400 600 Scan position (nm)

0

800

0

200 400 600 Scan position (nm)

Figure 1. STEM–EDX line scans of (a) TiCl4@MAF-6 and (b) MAF-6@Ti(SO4)2.


800

Page 37 of 43

300

)

anatase

rutile

TiO2@M-74 M-6@TiO2 TiO2@M-6

Calc. Ti2O3 Calc. ZnO Calc. TiO2(anatase) Calc. TiO2(rutile)

10

20

(b)

-1

rutile

TiO2@M-6

M-6@TiO2

TiO2@M-74

M-74@TiO2

3

M-74@TiO2

Ti2O3 rutile ZnO

Quantity adsorbed (cm .g

(a) Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 40 2 theta (deg.)

50

60

200

100

0 0.00

0.25 0.50 0.75 Relative pressure (P/P0)

1.00

Figure 2. (a) XRD patterns of TCMs with simulated patterns of TiO2 (rutile and antase), Ti2O3 and ZnO and (b) Nitrogen adsorption isotherms of the synthesized TCMs.



Figure 3. (a) HRTEM image and (b) SAED pattern of TiO2@M-6. The d-spacings of the marked fringes are 0.326 and 0.247 nm which correspond to the (110) and (101) plane, respectively, of rutile TiO2. The SAED pattern can be indexed to the rutile structure with the view of [110] and [101] directions. The calculated d-spacings of rutile in (110) and (101) planes are also shown on the figure.


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TiO2@M-6

D

G

M-6@TiO2

(b)

(a)

O 1s

TiO2 (Rutile)

200

TiO2@M-6

Ti2p C 1s

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


700 1200 1700 -1 Raman shift (cm )

2200

M-6@TiO2

Ti2s N 1s

600

500

400 300 200 100 Binding energy (eV)

Figure 4. (a) Raman spectra and (b) XPS survey spectra of TiO2@M-6 and M-6@TiO2.


Ti3p Ti3s

0


100

TiO2@M-6 M-6@TiO2 TiO2@M-74 M-74@TiO2 P-25 (com.) TiO2/AC Synth.TiO2(rutile)

(a)

Conversion(%)

75 50

Synth.TiO2(anatase)

25 0

0

25

50

75 100 Time (min)

125

150

(b)

0

Synth.TiO2(anatase)

ln(C t/C 0)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Synth.TiO2(rutile) TiO2/AC P-25 (comm.)

-1

M-74@TiO2 TiO2@M-74

-2

M-6@TiO2

-3

TiO2@M-6

0

15

30

45 60 Time (min)

75

90

Figure 5. (a) Effect of time on conversion and (b) pseudo-first-order kinetic plots for the oxidation of DBT (considering both adsorption and catalysis) over synthesized TiO2 (rutile and anatase), commercial P-25, TiO2/AC, and TCMs at 80 oC.


Page 41 of 43

100

0

(a)

75 50

M-6@TiO2 TiO2@M-6

25 0

(b)

-2 ln(Ct/C0)

Reduction(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 Time (min)

9

M-74@TiO2 TiO2@M-74

TiO2@M-74 3

TiO2@M-6 -6

M-74@TiO2 0

M-6@TiO2

-4

12

-8

0

3

6 Time (min)

9

Figure 6. (a) Effect of reaction time on conversion and (b) plots of the pseudo-first-order kinetics of the reduction of 4-NP to 4-AP over M-6@TiO2, TiO2@M-6, M-74@TiO2, and TiO2@M-74 at 25 °C.



100 Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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75 50 25 0

1st

2nd 3rd Number of cycles

4th

Figure 7. Reusability of TiO2@M-6 catalyst for the oxidation of DBT.


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Table of Contents (TOC)


TiO2-Integrated Carbon Prepared via Pyrolysis of Ti-Loaded Metal

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