HAp/TiO2 heterojunction catalyst towards low-temperature thermal oxidation of VOC

Ceramic catalyst without precious metals and rare-earth elements is a promising technology for removing volatile organic compounds (VOC) produced in the manufacturing process to feasibly solve worldwide health and environmental problems. We first investigate the influence of hydroxyapatite (HAp)/TiO2 heterojunction formation on the temperature dependence of VOC catalytic performance. The comprehensive evaluation by XRD, FT-IR, UV–vis, and in situ ESR clarifies that the anisotropic crystal distortion along the c-axis of HAp lattice is caused by hydrolysis and hetero-condensation of TiO2 precursor accompanying with the defective structure in HAp. The structural modified HAp (m-HAp) provides the notable alteration of optical bandgap with the visible-light coloration and the preferential generation of oxygen radicals. Furthermore, we propose a new model that the m-HAp/TiO2 heterojunction should be a possible main factor affecting the more than twice higher catalytic performance in thermal oxidation of ethyl acetate at a lower temperature, as typically shown in HAp-T1.


Introduction
Volatile organic compounds (VOC) are a type of pollutants produced in the manufacturing process using organic solvent for coating and adhesion, putting human beings at risk of various diseases [1][2][3] in addition to the generation of harmful substances responsible for the worldwide environmental issues [4][5][6], such as photochemical oxidant and particulate matter (PM). VOC catalytic oxidation using non-noble metals (Pt, Pd, Au, Ag, etc) has been extensively investigated owing to their excellent performance. However, in addition to cost issues and ease of poisoning [7], precise controlling in morphology, particle size, and dispersibility of metal nanoparticle catalysts on support materials such as Al 2 O 3 , SiO 2 and zeolite are necessary to maximize catalytic activity [8][9][10]. Moreover, small and medium-scale facilities for VOC treatment suffer from high running costs and difficulty recycling, usually utilizing a catalytic combustion method with precious metal nanoparticles. In recent years, metal oxide catalysts, including rare-earth metal or transition metal elements, have been reported as cheaper alternatives to noble metals [7]. As examples of metal oxide catalysts, CeO 2 incorporated with Mn ions as Mn-Ce-O catalytic system is reported to efficiently oxidize ethyl acetate and toluene [11]. Highly dispersed CuCe x Zr 1−x O y species on ZSM-5 support facilitate the complete conversion of ethyl acetate to CO 2 due to excellent reducibility [12]. In addition, LaMO 3−δ (M=Co, Mn) perovskite with La partially substituted by Sr can reduce lower temperature oxidation of ethyl acetate [13]. However, the imperative requirement in the precise adjustment of chemical composition is the technical drawback.
Recently, our group has proposed a novel oxidation catalysis strategy using a thermal ceramic catalyst based on Ca type hydroxyapatite (HAp; Ca 10 (PO 4 ) 6 (OH) 2 ) without precious metals and rare-earth elements for highly efficient removal of VOC [14,15]. HAp, as a promising earth-abundant and low-cost material with biological safety [16][17][18], possesses a dual function as a support and a novel thermal catalyst, providing practical and economic advantages with environmental conservation and effective use of resources. Furthermore, our previous research has discussed that the thermally-induced structural defects in HAp catalyst surface potentially offers generation of oxygen radical species from adsorbed oxygen molecules as electron scavenger, which is the critical component for VOC oxidation catalysis [19]. Furthermore, in photocatalyst, some researchers have reported photo-activated titanium dioxide (TiO 2 ) catalyst with HAp support to enhance the efficiency of catalytic reaction using material design to foster electrons-holes separation for NO removal in the air [19] or degradation of azo-dye pollutants [20].
Here we prepare the HAp based composites through TiO 2 coating on hydroxyl-enriched HAp particle surface by hetero-condensation reaction in the sol-gel synthesis. Moreover, we first demonstrate that the heterojunction structure in the interface between the structural modified HAp and TiO 2 particles facilitates the efficient conversion of ethyl acetate into CO or CO 2 at lower temperatures (300°C) than single-phase HAp or TiO 2 .

Experimental procedures
2.1. Synthesis of HAp/TiO 2 composite catalyst A commercial HAp powder with an average particle size of 15 μm (Taihei Chemical Industrial Co., Ltd, Spherical-HAp; 3Ca 3 (PO 4 ) 2 -Ca(OH) 2 ) was used as a raw material. The reagents such as titanium tetrabutoxide (TBOT; Ti(OC 4 H 9 ) 4 ), 1-butanol (C 4 H 9 OH), and acetic acid (CH 3 COOH) were purchased from FUJIFILM Wako Pure Chemical Corporation. For selective coating of TiO 2 on the HAp particle surface, the hydrolysis/ condensation rate in the sol-gel reaction of TBOT was carefully controlled by the use of acetic acid as a chelating agent for metal alkoxide [21,22] and the two-step dropping water technique [23,24]. Firstly, a certain amount of HAp powder was added in 13.2 ml of 1-butanol solution. Following stirring/sonification at R.T. for 15 min, the solution was mixed with TBOT (5.0 ml), 1-butanol (6.7 ml), and acetic acid (1.2 ml). As the next step, 0.13 ml of distilled water was added to this solution with stirring for 30 min at R.T. Finally, the solution with additional distilled water (0.26 ml) was stirred again for 30 min at 50°C. The precipitated gel-like solids were obtained by centrifugation and washing with ethanol and then calcinated at 550°C for 3 h for crystallization, following drying at 130°C for 1 h. The detailed information for the loading amount of HAp powder and sample labels is shown in table 1. Here, a converted weight fraction is defined as follows: where, M TiO2 and M TBOT is molar mass of TiO 2 (= 79.987 g mol −1 ) and TBOT (= 340.32 g mol −1 ) respectively, c TBOT is concentration of TBOT (mol), m Ti is atomic mass of Ti (= 47.867 g mol −1 ), W HAp is weight of HAp (g).

Characterization methods
The crystal structure of HAp based composite was evaluated by x-ray diffraction (XRD) pattern (UltimaIV, Rigaku) with CuKα radiation. The reference intensity ratio (RIR) was used for quantitative analysis by powder diffraction [25]. The particle morphology and the distribution of elements on composite were analyzed by field emission scanning electron microscopy (FE-SEM) equipped with an EDS analyzer (JSM-7600F, JEOL). The Brunner-Emmett-Teller specific surface area (SSA BET ) was evaluated from N 2 adsorption isotherms on a highprecision gas adsorption measurement instrument (BELSORP-mini2, Microtrac BEL, Japan) at 77 K. The measurement of Fourier transform infrared spectra were conducted by FT-IR spectrometer (FT-IR 6600, JASCO). The optical properties were investigated by UV-vis spectrophotometry (V-7100, Jasco). Optical bandgap (E g ) was determined by applying a modified Kubelka-Munk function as follows: where R is the reflectance, hν is the photon energy. Here, the value 'n = 2' was used since HAp has the indirect bandgap [26]. The evaluation of thermal-induced active oxygen radicals on the HAp surface was confirmed by electron spin resonance (ESR) spectra recorded with a JEOL JES-FA200 spectrometer equipped with an in situ heating system in atmospheric conditions. Mn 2+ was used as an internal standard.

Evaluation of catalytic performance in the oxidative reaction of VOC gas
The experimental setup was referenced in [27]. The flow reaction system is shown in figure 1. Gaseous volatile organic compounds (ethyl acetate (C 4 H 8 O 2 ), 100 ppm)/N 2 and air were mixed in a 1:1 ratio before feeding to the reaction vessel. The flow rate of mixed gas was controlled at 0.125 l min −1 by a flow meter (FCC-3000P-G1,  KOFLOC), and the heating temperature was in the range of 100°C-400°C by tube furnace (KTF030N1, KOYO thermo systems co., Ltd, JAPAN). As gas generation evaluation in VOC oxidation catalysis, the concentration of CO 2 and CO was detected by using CO monitor (UM-300, KOMYO RIKAGAKU KOGYO K.K.) and CO 2 infrared absorption monitor (RI-215D, RIKEN KEIKI co., Ltd). Before the catalytic evaluation, no conversion from ethyl acetate to CO or CO 2 was confirmed in the range of 0°C-500°C due to self-oxidation (Figure is not shown).  (figure 2(c)). Generally, PO 4 and OH sites are highly arranged along c-plane rather than a-plane. The alkoxides hydrolysis is catalyzed by the presence of ceramics particles with the OH group, leading to a fast hetero-condensation reaction [22,28]. Moreover, it is known that OH-PO 4 can work as an active site for generating TiO 2 during hydrolysis of metal-alkoxide precursors [29]. Therefore, it is considered that the deficient structure prefers to be anisotropically formed in not a-plane but c-plane of HAp, and the excess amount of TiO 2 precursor results in the destruction of HAp crystal. Further evidence for the strong interaction between two materials can be confirmed in the lattice structure of anatase TiO 2 . Figure 3 shows the values of lattice parameters (a and c) and unit-cell volume of anatase TiO 2 composited in HAp catalyst. The anisotropic expansion along the c-axis is confirmed by increasing c'W TiO2 compared with the change along the a-axis, increasing unit-cell volume. Such lattice expansion, especially along the c-axis, is often observed in reduced TiO 2 [30] because the nearest-neighbor Ti atoms move away from the vacancy due to the absence of electrostatic attraction [31]. Thus, oxygen-deficient TiO 2 would be formed in the composites through the synthesis with large c'W TiO2 , indicating strong interaction in the interface between two materials through hydrolysis and hetero-condensation on the HAp surface distortion of HAp lattice. Figure 4 shows the N 2 adsorption/desorption isotherm and the summary of BET-specific surface area (SSA BET ) of the specimens. All isotherms could be attributed to Type II solids with a hysteresis loop of Type H3 in the IUPAC classification of physisorption isotherms [32]. The value of SSA BET decrease with the increase of  The chemical structure of HAp was not significantly changed after forming a composite structure in FT-IR spectra (figure 6). The peaks at 1645 and 3470 cm −1 are assigned as the vibration from H 2 O bending and H 2 O stretching. The specific bands of HAp as PO 4 bands (ν 3 : 1095 and 1045 cm −1 , ν 1 : 965 cm −1 , ν 4 : 635, 600 and 565 cm −1 , ν 2 : 460 cm −1 ) and OH− stretching (3565 cm −1 ) [33,34] are also observed. In terms of the chemical structure from HAp, there are no notable changes in the composites to compare with the pristine HAp. The wide broadened band at 400-1000 cm −1 can be assigned to Ti-O-Ti vibrations [35] since it appears a large amount of TiO 2 precursor was added.

Results and discussion
The pristine HAp shows the poor light-absorbing capacity in the entire region of UV-vis, as shown in figure 7(a), while the significant enhancement in UV and blue light region is confirmed in the synthesized composite even with small c'W TiO2 (HAp-T1). There are two possible reasons for such coloration; involved by (1) oxygen vacancies or deficient structure in a PO 4 group/OH group [36], (2) substitution of Ti element into Ca site in HAp crystal [37]. Moreover, as the characterization of the optical bandgap of the pristine and the synthesized HAp by tauc-plot, the values of optical bandgap are 6.03 and 3.86 eV, respectively ( figure 7(b)). The bandgap of pure HAp, oxygen vacancies in PO 4 group and/or OH group deficient, and Ti-doped HAp are known as >6.0 eV [38], 4.00 eV [36], and 3.45-3.75 eV [38][39][40], respectively. In the tauc-plot for the synthesized TiO 2 , the bandgap (3.14 eV) is slightly lower than pure anatase TiO 2 (3.2 eV) due to oxygen vacancies on the TiO 2 surface (not shown). In our case, oxygen-deficient TiO 2 would form because of the synthesis with a small amount of H 2 O for hydrolysis in sol-gel reaction if there is less influence from the hetero-condensation with the HAp surface.

ESR analysis
ESR spectra are strongly affected by the chemical structure of HAp synthesized by different synthesis methods [41]. For example, in the ESR spectra of the pristine HAp, the signal with g = 2.0028 gradually appears by raising the temperature from 300°C to 400°C (figures 8(a)-(b)). This signal is assigned with oxygen radical [42], observed by the deformation of the P-OH group from the HAp surface. In contrast, HAp-T1 shows an intense signal even at 300°C. Besides, there are no signals attributed by trapped electrons (g = 1.988) and crystalline defects (g = 1.999) from Ti-HAp [43] as well as by oxygen radical in TiO 2 (g=2.003) [44]. Moreover, the typical signal caused by anatase Ti 3+ defects (g = 1.992) [45] does not appear in our synthesized HAp/TiO 2 composites and the synthesized TiO 2 particles without HAp ( figure 8(c)). This case would be directly related to the low-temperature generation of oxygen radicals by the anisotropic distortion of HAp lattice stabilized in the composite interface. Moreover, the amount of TiO 2 in the composite is also a critical factor that preferentially causes the loss in the probability of radical generation because the pristine HAp hardly retains the crystal structure due to strong destruction. From these results, the change in optical bandgap observed from UV-vis spectra (figure 6) should be described by not Ca site defects on Ti-doped HAp but the structural defects in the OH-PO 4 site of HAp. Figure 9 summarized the catalytic performance, which is evaluated by the conversion rate of VOC (ethyl acetate) into CO or CO 2 plotted as a function of the reaction temperature (R.T.-400°C). There is the notable difference that the CO/CO 2 conversion capacity is twice higher in the HAp-based composite catalyst with the small c'W TiO2 (HAp-T1) than the pristine HAp catalyst at a lower temperature region (300°C), despite the deterioration of catalytic activity at higher temperature region (400°C). On the contrary, a significant deterioration is also observed in the composite catalyst with excess anatase TiO 2 (HAp-T3). The synthesized anatase TiO 2 through sol-gel reaction exhibits the thermal activation in VOC decomposition; however, catalytic performance is still lower than HAp catalyst. Although the value of SSA BET of catalyst is regarded as one of the factors on an enhancement of VOC decomposition performance [46], the improvement of catalytic performance in HAp-T1 cannot be described only by SSA BET , and also being not critically affected by the amount of anatase TiO 2 in composite ( figure 10). This fact certainly indicates the synergistic effects of composite catalysts, which enhance the performance at low temperatures by building up the specific heterostructure between two materials. There is a strong correspondence between the VOC catalytic activity at low temperature and the signal intensity from the generated radical species in ESR spectra.

VOC catalytic performance
3.4. Possible mechanism of enhancement of catalytic performance at low temperature in HAp/TiO 2 heterojunction As the well-established mechanism of catalytic activity in photo-activated catalyst [47], the formation of oxygen radicals is a critical intermediate state through the reaction that the excited carrier (electron and hole) by external energy is scavenged by surface oxygen. A similar mechanism of radical generation has been proposed in the case of thermal catalysts [29,30]. Moreover, our group previously reported that the origin of ·O 2− radical species as an accelerator to degrade VOC gas during heating is the electron transfer from the locally disordered structure through the desorption of OH group in HAp crystal [14,15]. Here, we consider that HAp/TiO 2 heterojunction   in the interface between two materials plays a vital role in efficiently transferring thermal-induced electrons, which directly relates to enhancing catalytic performance at low temperatures. Figure 11 is the schematic illustration of the band structure with/without the modified-HAp/TiO 2 heterojunction. The thermally-induced carriers (electrons and holes) on TiO 2 catalysts are also known to involve the oxidative reaction [48,49]. Therefore, the HAp/TiO 2 heterojunction should help reduce recombination of thermal-induced electrons since the localized energy state by OH defects in HAp would have more positive potential than conduction band minimum (CBM) of anatase TiO 2 . On the other hand, the defects such as Ti 3+ and oxygen vacancies on defective TiO 2 (TiO x ) form the impurity level localized at 0.75-1.18 eV below CBM [50]. The impurity level should have more negative potentials than O 2 /·O 2− (−0.33 V, V versus NHE) [51], then the generation of active ·O 2− radicals is prevented due to poor electron transfer efficiency to react with O 2 . One reason that no signal attributed to oxygen radical from TiO 2 is observed in the ESR spectra of HAp catalyst composited with anatase TiO 2 . In the case of the composite catalyst with a large wt% of TiO 2 , the catalytic performance in the oxidative decomposition of ethyl acetate deteriorates due to the preferential generation of TiOx and the substantial destruction of HAp lattice as an inert structure for the oxidative reaction of VOC gas.

Conclusions
We firstly investigate the influence of modified-HAp/TiO 2 heterojunction on the temperature dependence of catalytic performance in VOC oxidative decomposition by HAp based thermal ceramic catalyst without precious and rare-earth metal nanoparticles. The structural-modified HAp catalyst composited with anatase TiO 2 is synthesized through the conventional method using a sol-gel reaction of titanium alkoxide. The comprehensive evaluation by XRD, FT-IR, UV-vis, and in situ ESR clarified that the anisotropic distortion along the c-axis of HAp lattice causes the oxygen vacancies in PO 4 sites and the defective structure of OH group through hydrolysis and hetero-condensation of TiO 2 precursor with HAp surface. Furthermore, it would lead to the notable alteration of optical bandgap with the visible-light coloration from >6.0 eV to 3.61-3.86 eV and the low-temperature generation of oxygen radicals caused by the defective structure HAp lattice. Based on the results, we propose a possible mechanism that the modified-HAp/TiO 2 heterojunction in the composite catalyst would be the main factor to involve the more twice higher enhancement for the efficiency in the oxidative reaction of ethyl acetate gas at a lower temperature (300°C) with the appropriate weight fraction of TiO 2 (typically shown in HAp-Ti1). Therefore, these results provide a guideline for a material design in structure and interface towards creating further efficient VOC oxidation catalysis at low temperatures.