解説

先進セラミックス研究センター年報（2021）. Vol. 10, 6‑15

Hydroxyapatite as Green Catalyst for Environmental Cleaning
Yunzi Xin*, Takashi Shirai *, ***Advanced Ceramics Research Center, Nagoya Institute of Technology
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, Japan**Department of Life Science and Applied Chemistry,
Graduate School of Engineering, Nagoya Institute of Technology
Gokiso-cho, Showa-ku, Nagoya, Aichi 466-8555, Japan
Hydroxyapatite, one of the most well-known calcium phosphate compounds, has been extensively applied as a bioceramic for
artificial bone tissue crafting, ion-exchange fillers for heavy metal ions, and adsorbents of proteins owing to its unique chemicaland physical properties. The application of hydroxyapatite as a green catalyst for the oxidative decomposition of volatile organic
compounds (VOCs) opens a new avenue for the design and development of functional ceramic materials. This review summarizes
the catalytic decomposition of VOCs on hydroxyapatites under both thermal- and photo-excited conditions, followed by adetailed summary of the correlated chemical structure and surface characteristics, as well as elucidation of the catalytic
mechanism.
Keywords: Catalyst, hydroxyapatite, oxidative decomposition, volatile organic compound

Ca 10(PO 4) 6(OH) 2), a well-known calcium phosphate
compound, has been extensively applied in artificial
bone tissue crafting, ion-exchange fillers for heavy metal
ions, and adsorbents of proteins owing to its uniquechemical and physical properties. Both stoichiometricand non-stoichiometric hydroxyapatites with altered Ca/
P ratios, controlled crystal morphology, and different
particle sizes can be easily synthesized via various
strategies, such as liquid-phase precipitation,hydrothermal approach, microwave-assisted method,and high-temperature hydrolysis. [12] In the 1990s,
Nishikawa et al. started to study the thermal/
photocatalytic activities of hydroxyapatites in theoxidative decomposition of VOCs. [13] Investigations oft h e t h er m al - / p h o t o - ex ci t ed r ad i cal g en e ra t i on
phenomenon on hydroxyapatites have progressed in thenext several decades. [14-17] However, further practicalapplications for VOC elimination have rarely persisted
because of the lack of knowledge and deep
understanding of the catalytic mechanism of thehydroxyapatite surface. In 2020, our group
unprecedently reported the thermal-catalytic mechanismand succeeded in achieving complete decomposition of
VOCs on hydroxyapatites, which unlocks a newprospect for the development of noble metal-freecatalysts for VOC purification and environmental
cleaning owing to their facile preparation, costefficiency, and non-toxicity. [18] Furthermore, we

The emission of VOCs has become one of the mostserious global environmental issues owing to rapid
urbanization and industrialization. VOCs emitted fromvarious sources, such as automobiles, chemical
manufacturing, and printing, become the most prevalentpollutants in air and water and precursors forphotochemical smog and secondary aerosols. Owing to
the toxic and carcinogenic nature of VOCs, they alsopose short- and long-term harmful impacts on humanbeings. [1-3] Thermal-induced catalytic decomposition of
VOCs on porous ceramic-carried noble metal
nanoparticles has attracted worldwide attention because
this process can oxidize VOCs into CO2 and H2O underrelatively low temperatures. [4-6] However, its high cost,heavy loading amount, and stringent requirement forgood dispersibility limit its application. Recently, noble
metal-free catalysts, such as metal-doped ceramiccomplexes (Mn-SBA15, CuCexZr1-xOy/ZSM-5), metaloxides (CoO x-CeO 2, MnO x-CeO 2), and perovskites(LaFeO 3 , LaNiO 3 , LaFe 1-y Ni y O 3 ) have emerged as
efficient catalysts for the oxidative decomposition of
VOCs. [7-11] Nevertheless, the complex synthetic routeand difficulties in precisely controlling the chemicalcompositions are the main drawbacks of these catalysts.
Therefore, the development of alternative noble metalfree catalysts is urgently required to meet sustainable
development goals.
Hydroxyapatite (stoichiometric formula:―6―

Hydroxyapatite as Green Catalyst for Environmental Cleaning

probably substituted with OH- due to the fact that anenhanced amount of Cl- was detected in thehydroxyapatite sample after the catalytic reaction, which
led to a gradual reduction of conversion efficiency uponlong-term utilization, as shown in Figure 1b. In
addition, the authors compared the catalytic activities ofDAps prepared via two different methods: liquid-phase
precipitation (DAp-B) and hydrolysis of α-tricalcium
phosphate ( α-TCP, DAp-A). Interestingly, α-TCP
exhibits inactivity toward the catalytic decomposition ofchlorobenzene, as summarized in Figure 1c. Based on
the results of FT-IR (Figure 1d), DAp-A exhibits lower
crystallinity as the split bands of PO4 and OH liberations
at 630 and 610 cm -1, respectively, were also clearlyobserved in DAp-A. These results are consistent withthe powder X-ray diffraction (PXRD) pattern, whichshows that wider peak of (300) in DAp-A (Figure 1e).
The authors also demonstrated that the activity ofcalcium-deficient hydroxyapatite can be attributed to its

reported a novel and facile mechanochemical activation
technique of hydroxyapatite, for perusing the deepoxidation of byproducts with 100% CO2/CO selectivity. [19]In addition, an active porous hydroxyapatite filter was
also successfully developed by utilizing the ceramic
shaping process of gel casting, and the results provide
direct evidence for the practical application ofhydroxyapatite catalysts in manufacturing. [20]This review summarizes the catalytic decompositionof VOCs on hydroxyapatites for both thermal- andphoto-excited conditions and reveals the corresponding
chemical structure and surface characteristic study toelucidate the catalytic mechanism. We believe that thecontents of this review can provide important insights
for not only the fields of catalytic science andtechnology but also for the design and development offunctional ceramic materials.
1. Thermocatalytic decomposition of VOCs
on hydroxyapatites(a)

In 1991, Nishikawa et al. reported the catalyticoxidative decomposition of trichloroethylene (TCE) on
calcium-deficient hydroxyapatite (DAP). [13] The DAP,
with a Ca/P molar ratio of 1.54, was synthesized via theliquid-phase precipitation approach with diammoniumhydrogen phosphate and calcium nitride as precursors.
As shown in Figure 1a, catalytic decomposition of TCE
was observed on the synthesized DAP in the temperature
range of 100–500 °C, while concentration of TCE
remained constant when no catalyst was loaded (blankcase). The maximum composition efficiency of TCE on
DAP was 95% at 500 °C. They also studied the gas
species produced at 430 °C and observed the formationof 1,1-dichloroethylene with a trace amount of TCE
using gas chromatography (GC) equipped with a flame
ionization detector as well as CO 2 /CO using GC
equipped with a thermal conductivity detector.
Regarding the small amount of HCl in the final product,
the authors suggested that the generated Cl - might
substitute OH - in the DAP structure. However, thedetailed reaction mechanism is not clarified in this study.
Nishikawa et al. also reported the oxidative decompositionof chlorobenzene on another calcium-deficienthydroxyapatite with a Ca/P molar ratio of 1.58,
synthesized using different precursors of calcium oxideand phosphoric acids. [21] A decomposition efficiency of
43% at 450 °C and the main products of CO2/CO wereobtained for the catalytic decomposition of chlorobenzene.
Similar to TCE, the authors also presumed that Cl- was

(c)

(b)

(e)

(d)

Figure 1. (a) Catalytic decomposition of TCE on DAp,

compared with blank (Reprinted from 日本化学誌1991, 11,1562. Copyright © 1991 The Chemical Society of Japan.). (b)Chlorobenzene conversion and CO2/CO selectivity using DAp.
(c) Comparison of catalytic activities between DAp-B(synthesized via liquid-phase precipitation) and DAp-A(synthesized through hydrolysis of α-TCP) by (d) Fouriertransform infrared (FT/IR) and (e) PXRD characterizations.
(Reprinted from Bull. Chem. Soc. Jpn., 1994, 67, 2454.
Copyright: © 1994 The Chemical Society of Japan.).
―7―

Yunzi Xin, Takashi Shiraic-plane promotes both the adsorption of ethyl acetate
and the generation of active radicals under thermal
treatment, resulting in enhanced catalytic activity.
Recently, we reported the oxidative decomposition of
VOCs such as ethyl acetate, 2-propanol, and acetone at
400 °C using hydroxyapatites with Ca/P molar ratios of
1.70, 1.67, 1.57, and 1.37. As shown in Figures 4a,c,e,
the chemical composition of hydroxyapatite plays animportant role in catalytic activity. Hydroxyapatite witha Ca/P ratio of 1.67 exhibited superior catalytic activity,
and the hydroxyapatite resulted in the complete
decomposition of ethyl acetate and isopropanol. [18] The
chemical structures of the gaseous products generated
through the catalytic reaction of VOCs weresystemically identified for the first time. As shown in
Figures 4b,d,f, the gaseous products of decomposed
ethyl acetate and isopropanol were obtained as ethylene/
acetaldehyde/ethanol and propylene/acetone, respectively,
whereas no organic compounds were observed foracetone. Based on these results and a detailed
investigation on the chemical and physical properties ofhydroxyapatites with different Ca/P ratios, the catalysis
mechanism was unprecedently established in this work,
which opens a new prospect for hydroxyapatite as anoble metal-free catalyst for environmental cleaning.
Further details are discussed in Section 3. Furthermore,
the influence of Ca and P precursors, utilized in the
preparation of hydroxyapatite (via liquid-phase
precipitation), on the catalytic oxidative decomposition

much-disordered crystal structure compared with
α-TCP.
We previously reported the oxidative decompositionof toluene, ethyl acetate, and isopropanol on thermally
excited hydroxyapatites at 400–500 °C. [22] We usedgranule-type commercial hydroxyapatites in the size
range of 1–2 mm. By systematically comparing with Cadeficient hydroxyapatite, we found that thestoichiometric hydroxyapatite has the potential toconvert VOCs into CO2/CO owing to the intensified
radical generation on the surface, as shown in Figure 2.
Our group also reported the catalytic decomposition of
ethyl acetate on hydroxyapatites with crystal structures
oriented along the a- or c-axis. [23] In this study, needlelike (HAp-N) and plate-like (HAp-P) hydroxyapatites(Figure 3a) were synthesized via air–liquid interface
precipitation. The PXRD patterns (Figure 3e demonstratethat HAp-N exhibits a preferred crystal growth ofa-plane (obvious (300) plane) along c-axis, whereas
HAp-P exhibits a harvested c-plane oriented to a(b)-axis
with only the (002) peak appear. The decomposition
rates for HAp-P and HAp-N were evaluated to be 97.8%
and 48.4%, respectively. It was demonstrated that
HAp-P with a large number of surface OH groups on the
(a)

(b)

(c)

(d)

Figure 2. Conversion of toluene, ethyl acetate, and isopropanolon the (a) stoichiometric and (b) Ca-deficient hydroxyapatites.

Figure 3. SEM images of (a,b) HAp-N and (c,d) HAp-P. (e)Electron spin resonance (ESR) spectra of the (c) stoichiometric

PXRD patterns and (f) VOC decomposition rates of HAp-N andand (d) Ca-deficient hydroxyapatites at different temperatures.

HAp-P. (Reprinted from Journal of the Ceramic Society of(Reprinted from Appl. Surf. Sci. 2012, 258, 5370. Copyright ©

Japan 2019, 127, 263. Copyright © 2019 The Ceramic Society

2012 Elsevier B. V. All rights reserved.).

of Japan.).
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Hydroxyapatite as Green Catalyst for Environmental Cleaning

Figure 4. Temperature-dependent conversion efficiencies and GC spectra of gaseous products for various VOCs:(a,b) ethyl acetate (c,d) isopropanol, and (e,f) acetone. (Reprinted from Catalysis Science & Technology 2020, 10,5453. Copyright © The Royal Society of Chemistry 2020.).

of ethyl acetate was also reported by our group. [24] As
shown in Figure 5a, four types of hydroxyapatite (HAP1/2/3/4) with different particle sizes and morphologies
were synthesized. It is worth noting that the Ca/P molar
ratios for these hydroxyapatites were similar (1.67). It
was found that a more efficient CO2/CO conversion of
80%–100% can be achieved at 400–500 °C by usinghydroxyapatites synthesized from the chemical reaction
between calcium hydroxide and phosphoric acid (HAP-1
and HAP-2 in Figures 5b and 5c, respectively). In
contrast, the catalytic activity of hydroxyapatite wasinhibited by the increased acidity induced by thesubstitution of CO32- when calcium carbonate was usedas the precursor (HAP-3 and HAP-4 in Figures 5d and5e, respectively).
In a recent study, we observed 100% CO 2 /CO
conversion from ethyl acetate on surface-activatedstoichiometric hydroxyapatite using a facile
mechanochemical treatment. [19] Hydroxyapatites treatedby high-energy planetary ball-milling-induced
mechanochemical processes show better catalytic
activity than raw powder, indicating that the size of theball utilized in ball milling plays a major role (Figures6a–d). The predominant defect/oxygen vacancy
generation at the PO 43- site and enhanced basic site
population achieved by the selective activation of thec-plane through the planetary ball-milling process are

attributed to the highly efficient catalytic activity withexcellent cyclic ability, as shown in Figure 6e.
Using a ceramic shaping technique, porous
(a)

HAP-1

HAP-2

0.5ȝm

0.5ȝm

HAP-3

HAP-4

0.5ȝm

0.5ȝm

(b)

(c)

(d)

(e)

Figure 5. (a) SEM images and (b–e) conversion efficiencies ofdifferent hydroxyapatites. (Reprinted from Journal of the
Ceramic Society of Japan 2021, 129, 601. Copyright © 2021
The Ceramic Society of Japan.).
―9―

Yunzi Xin, Takashi Shiraihydroxyapatite ceramic filters with altered pore structures(Figure 7a) were also successfully fabricated via a facile
gel-casting process. [20] The porous hydroxyapatite filterswith controlled pore structures exhibited comparable
catalytic activity with that of powder in the thermaloxidative decomposition of ethyl acetate (see details in
Figures 7b–e). The correlation between the gas
permeability of the established pore structure and thecatalytic performance of the filter was studied inadvance. Without a load of noble metal nanoparticles,
the catalytic activity achieved using the hydroxyapatite
filter by itself opens a new path for future VOC
purification techniques.
2. Photocatalytic decomposition of VOCs
using hydroxyapatites

Figure 6. CO 2/CO conversion of ethyl acetate at different

temperatures over (a) raw and mechanochemically activatedhydroxyapatites using (b) 3 mm, (c) 10 mm, and (d) 15 mm
milling balls. (e) Cyclic test results of hydroxyapatite activatedusing 3 mm milling ball. (Reprinted from Scientific Reports
2021, 11, 7512. Copyright © The Authors(s) 2021.).

The photocatalytic decomposition of methyl
mercaptan (MM) on stoichiometric hydroxyapatite
under UV irradiation was reported by Nishikawa et al. [25]Hydroxyapatite was synthesized by the precipitation ofdiammonium hydrogen phosphate and calcium nitride
precursors. The authors compared the catalytic activities
of hydroxyapatites heated at 200 °C (HAp200) and
1150 °C (HAp11500) and found that only HAp200
exhibited catalytic activity towards the decomposition of
MM when irradiated with UV light of 254 nm
wavelength (Figure 8). The authors presumed that thecatalytic activity of HAp200 could be attributed to the

Figure 7. (a) Pore structure of fabricated hydroxyapatite filters

Figure 8. Comparison of the catalytic decompositions of MM

observed by X-CT. (b–e) Catalytic decomposition of ethyl

using two types of hydroxyapatite under UV light irradiation.

acetate using the selected filters in (a). (Reprinted from Scientific(Reprinted from Journal of Molecular Catalysis A: Chemical

Reports 2021, 11, 7512. Copyright © The Authors(s) 2021.).

2002, 179, 193. Copyright © 2002 Elsevier Science B. V.).
― 10 ―

Hydroxyapatite as Green Catalyst for Environmental Cleaning

generation of O2•- radical generated on the surface. The
inhibitory activity of HAp1150 was not investigated intheir study.
Nishikawa also reported satisfactory photocatalytic
decomposition of dimethyl sulfide (DMS) using
commercial Ca-rich hydroxyapatite. [26] In comparison
with stoichiometric hydroxyapatite (74%–78%) and
α-TCP (45%), Ca-rich hydroxyapatite shows a superior
photocatalytic activity towards the decomposition of
DMS and conversion efficiency of up to 98%–100%(Figure 9). It was assumed that the excellent catalytic
performance of Ca-rich hydroxyapatite might be
attributed to its high crystallinity towards the a-axis.
Although the products of the photocatalytic decompositionof DMS were identified as SO2/CO2/H2O, the detailed
catalytic mechanism has not been clarified.
3. Structural study of hydroxyapatite andelucidation of the catalytic mechanism

For a better understanding of the hydroxyapatite
catalytic mechanism, much effort has been devoted tochemical structure studies. Nishikawa systematicallyinvestigated the change in the chemical structure of

Figure 9. Comparison of the catalytic decompositions of DMS

using three types of hydroxyapatite under UV light irradiation.
(Reprinted from Journal of Molecular Catalysis A: Chemical
2004, 207, 149. Copyright © 2004 Elsevier Science B. V.).

hydroxyapatite under thermal treatment at different
temperatures of 100, 550, 900, and 1150 °C. [15] Based on
the sharpness of the PXRD peak observed in thehydroxyapatite samples that were thermally treated at
higher temperatures (Figure 10a), it can be concluded
that the crystallinity was enhanced. Furthermore, the FT/
IR spectra (Figure 10b) revealed that structural water in
the apatitic structure was removed at 900–1150 °C. Theauthor mentioned the coloring phenomenon in thermallytreated samples. As shown in the absorption spectra(Figure 10c), the order of absorption intensity was
1150 > 550 > 900 > 100 °C, and the results show good
agreement with color-changing observations. The
significantly increased UV absorption under 400 nm,
observed in the sample heated at 1150 °C, suggests a
surface structure change in hydroxyapatite. A similar
result can be observed in the photoluminescence spectra
shown in Figure 10d. The hydroxyapatite samples
thermally treated at 900, 550, and 100 °C exhibited
fluorescence centers at 320, 400, and 443 nm,
respectively, whereas the peak intensity at 320 nm
decreased for the sample heated at 1150 °C. The author
assumed that this phenomenon might be attributed to theradical formation and investigated the electron state ofhydroxyapatite by ESR analysis at -196 °C. The ESR
spectra shown in Figure 10e demonstrate that three
types of radicals appeared in the hydroxyapatite samples
heated at 900 °C (g = 2.0089/2.0073/2.0006) and 1150 °C(g = 2.0185/2.0049/1.9864). Such radicals were assigned
as O2- species generated by the reaction between theadsorbed O 2 and electrons. According to the FT/IR
analysis, the difference in the g-values of the samples
heated at 900 and 1150 °C was assumed to be correlated
to the different behaviors of structural water removal.
Radical generation on hydroxyapatite under UV
irradiation was previously studied by Kanai et al. [14] andNishikawa [16]. According to Kanai et al., ESR signals at
g = 2.0599 and 2.0023 were observed for hydroxyapatite
irradiated by an ultrahigh mercury lamp at 77 K (Figure11a). Such radicals were assigned as O- species, whichwere unstable at room temperature. After treatment withoxygen, a further reaction occurred with the O- species
to produce O3-, leading to new ESR signals appearing at
g = 2.002, 2.011, and 2.017. In contrast, hydroxyapatite
irradiated at room temperature generated another new
type of O3- species with ESR peaks at g = 2.006, 2.009,
and 2.0014 (Figure 11b). It has also been demonstrated
that this type of O 3- species decays faster than thatgenerated at a low temperature of 77 K. The fast decay― 11 ―

Yunzi Xin, Takashi Shirai
(a)

(c)

(b)

(d)

(e)

Figure 10. (a) PXRD patterns, (b) FT/IR spectra, (c) UV-vis spectra, and (d) photoluminescence spectra ofhydroxyapatites pre-heated at different temperatures. (No. 1, 2, 3, and 4 represent samples pre-heated at 100, 550, 900,
and 1150 °C, respectively.) (Reprinted from Journal of Molecular Catalysis A: Chemical 2004, 207, 149. Copyright ©
2004 Elsevier Science B. V.).

(a)

(b)

Figure 11. ESR spectra of (a) hydroxyapatite irradiated by an

ultrahigh mercury lamp at 77 K and (b) hydroxyapatite samples
after treated with oxygen at room temperature (solid line) andafter irradiated at room temperature (dashed line). (Reprinted
from Phosphorus research Bulletin 1996, 6, 293. Copyright ©
1996 日本無機リン化学会.)

of O3- species generated by UV irradiation was attributed
to the location of radical in hydroxyapatite; that is, theradical formed through UV on the surface or near the
inlet of the channel could easily escape. Instead of lowtemperature-treated hydroxyapatite, Nishikawa
investigated the UV irradiation-induced radical onhydroxyapatites that were pre-heated at 200 and 1150 °C.
In the presence of 3,4-Dihydro-2,2-dimethyl-2H-pyrrole
1-oxide (DMPO) /water, •OH radicals were detected on
the photo-induced excitation of hydroxyapatite that waspreheated at 200 °C (Figure 12a). As shown in Figure
12b, the relative intensity of the generated •OH radical
increased as the UV irradiation time was extended. In
contrast, O 2 ⋅- formation was observed in DMPO/
dimethyl sulfoxide (DMSO), and the author also
demonstrated that such radicals can also be efficiently
generated using hydroxyapatite pre-heated at 200 °C(Figures 12d,e). Based on the results (Figure 12c), theinactive oxygen radical of the 1150- °C preheatedhydroxyapatite was assumed to be constrained by thematrix in the hydroxyapatite crystal.
In addition, Nishikawa characterized the chemical― 12 ―

Hydroxyapatite as Green Catalyst for Environmental Cleaning

surface change of hydroxyapatite (pre-heated at 200 °C)
by UV irradiation using FT/IR and X-ray photoelectron
spectroscopy (XPS). [17] Based on the FT/IR spectra of
the hydroxyapatite sample (Figure 13), the peaks at
3570, 1030, and 960 cm-1 can be assigned to the apatiticOH, ν3 of PO43-, and ν1 of PO43-, respectively. The peak
(a)

(c)

(b)

(d)

(e)

Figure 12. (a) ESR spectra of hydroxyapatite pre-heated at

200 °C after UV irradiation for 1 min (DMPO/H2O). (b) Plot of
the relative intensity of •OH signal vs. irradiation time. (c) ESR

intensity of ν3 of PO43- was obviously decreased after
UV irradiation; thus, the structural change of the surface
PO43- probably occurred on the photo-induced excitationof hydroxyapatite. Accordingly, the binding energy
shifts and recoveries of Ca2p, P2p, and O1s to lower energy
levels (0.05–0.10 eV) were also confirmed by XPS
analysis (Figure 14), which demonstrates that the UVirradiated hydroxyapatite is negatively charged due tothe radicals generated on the surface. Additionally,
Nishikawa conducted an ESR analysis on hydroxyapatitepreheated at 1150 °C, and the results are displayed in
Figure 15. Although stable O2⋅- (g = 2.0180/2.0053/
1.9878) and O- (g = 2.0693/2.0651) radicals were detected
for the 1150-°
C pre-heated hydroxyapatite before andafter UV irradiation, respectively, these species were
described as inactive states because hydroxyapatite preheated at 1150 °C showed no activity in the photocatalytic
decomposition of MM. In contrast, for the 200 °C preheated hydroxyapatite, the signal at g = 2.003 appears
by subtracting the ESR spectra before and after UV
irradiation, as shown in Figure 16. This signal was
attributed to the trapped electron in the hydroxyapatitevacancy, probably induced by the surface PO43- site as

C after UV
spectra of hydroxyapatite pre-heated at 1150 °
irradiation for 1 min (DMPO/DMSO). (d) Comparison of the
ESR signals between the hydroxyapatite samples pre-heated at
C after UV irradiation for 1 min (DMPO/
200 and 1150 °DMSO). (Reprinted from Materials Letters 2003, 58, 14.
Copyright © 2003 Elsevier B. V.).

Figure 14. Changes of binding energy summarized from the

XPS measurement of hydroxyapatite pre-heated at 200 °C. (A,
Figure 13. FT/IR spectra of hydroxyapatite pre-heated at 200 °C

before UV irradiation; B, after UV irradiation; C, after standing(a) before UV irradiation, (b) after UV irradiation, and (c) afterfor 1 h after UV irradiation; D, UV re-irradiation; and E, after

further standing for 1 h. (Reprinted from Journal of Molecular

standing for 1 h after UV re-irradiation.) (Reprinted from

Catalysis A: Chemical 2003, 206, 331. Copyright © 2003

Journal of Molecular Catalysis A: Chemical 2003, 206, 331.

Elsevier B. V.).

Copyright © 2003 Elsevier B. V.).
― 13 ―

Yunzi Xin, Takashi Shirai

indicated by the FT/IR results. Thus, the author
suggested a different photocatalytic mechanism forhydroxyapatite than proposed by Kanai et al.; that is, thetrapped electron in the vacancy of hydroxyapatite led toa further formation of O2⋅- via reaction with adsorbed
oxygen molecules, which oxidize the VOCs.
Owing to the lack of detailed characterization of
products produced through the catalytic reaction of
VOC, the catalysis mechanism for hydroxyapatite has
rarely been clarified. In 2020, a detailed reaction
mechanism was initially derived by our group with theassistance of a systematic investigation of the particle
morphology, crystallinity, surface acidic/basic
properties, thermally activated radical generation
dynamics, surface VOC affinity of hydroxyapatites withaltered Ca/P ratios, and the chemical structure of gaseous
products of VOCs such as ethyl acetate, 2-propanol, andacetone during catalytic decomposition. [18] In addition
to the previously mentioned oxygen radical-induced
oxidation reaction of VOCs, we emphasize the importantrole of the surface VOC adsorption properties as well as
the unique acidity/basicity of the hydroxyapatite surface
in the step-by-step decomposition of VOCs, assummarized in Figure 17. As shown in the ESR spectraof hydroxyapatites with altered Ca/P ratios (Figure 17a),a signal with a g value of 2.003 corresponding to trappedelectron appeared in samples with Ca/P of 1.67 and 1.70,
whose value shifted to 2.002 in calcium-deficient cases.
The ESR signal intensity was further compared (Figure17b), and the results suggest that radical generation
greatly depends on the chemical composition of thehydroxyapatites. Meanwhile, based on the in-situ FT/IR
spectra for the VOC adsorption characterization (Figure17c) and the resultant estimation of adsorption amount(Figure 17d), it can be concluded that the VOC affinity
on the hydroxyapatite surface varied with the Ca/P
ratios. In addition, the results of the quantification of
surface acidic and basic sites (Figure 17e) suggest thatstoichiometric hydroxyapatite possesses the highest
population of basic sites on the surface, whereas many
more acidic sites are present on calcium-rich andcalcium-deficient hydroxyapatites because of the large

Figure 15. ESR spectra of hydroxyapatite pre-heated at 1150 °C

by an ultrahigh mercury lamp at 77 K: (a) before UV
irradiation, (b) after UV irradiation, and (c) after standing for 1
h after UV irradiation. (Reprinted from Journal of Molecular
Catalysis A: Chemical 2003, 206, 331. Copyright © 2003
Elsevier B. V.).

Figure 17. (a) ESR spectra of hydroxyapatites with altered Ca/

P molar ratios. (b) Comparison of the ESR signal intensity for
different hydroxyapatites. (c) In-situ FT/IR spectra ofhydroxyapatites with ethyl acetate flow. (d) Quantification
Figure 16. Substrate ESR spectrum of hydroxyapatite pre-adsorption of ethyl acetate on different hydroxyapatites. (e)heated at 200 °C before and after UV irradiation. (Reprinted

Populations of surface acidic and basic sites. (Reprinted fromfrom Journal of Molecular Catalysis A: Chemical 2003, 206,

Catalysis Science & Technology 2020, 10, 5453. Copyright ©

331. Copyright © 2003 Elsevier B. V.).

The Royal Society of Chemistry 2020.
― 14 ―

Hydroxyapatite as Green Catalyst for Environmental Cleaning

amounts of Ca2+ and HPO42-. Based on the GC results(Figure 4), we propose the catalytic decomposition
mechanism of VOC, for example, ethyl acetate, as
follows. First, ethyl acetate decomposes into
acetaldehyde and ethanol via reactions with thermallyinduced oxygen radicals. Second, the generated ethanoltransforms into ethylene via dehydration on the acidic
sites of Ca2+ (as well as HPO42-), whereas acetaldehyde
can also be produced via the oxidization of ethanol on
the basic site surfaces of PO43- and OH-. Finally, theacetaldehyde generated in the first and second steps
directly oxidizes into CO2 and CO via the superactivityof oxygen radicals. The catalytic mechanism is
illustrated in Scheme 1. The mechanism can also be
adapted for the catalytic decomposition of acetate,alcohol, aldehyde, and ketone.
In conclusion, this review summarizes the applicationof hydroxyapatite as a green catalyst for the thermo/
photocatalytic oxidative decomposition of VOCs.
Following the investigation of the chemical and physicalproperties of hydroxyapatites under thermal/
photoexcitation with respect to radical generation,
surface chemical state, VOC affinity, and surface acidity/
basicity, the catalytic mechanism of hydroxyapatites is
elucidated. We believe that the contents of this review
can provide important information for the future designand development of noble metal-free catalysts for VOC
purification and environmental cleaning.

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