Effect of phosphorus-modification of titania supports on the iridium-catalyzed synthesis of benzimidazoles

Article abstract of DOI:10.1016/j.cattod.2020.02.014

Modification of titania supports for iridium catalysts by phosphorus species greatly enhanced the activity for the synthesis of benzimidazoles via hydrogen transfer. Two series of phosphorus-modified titanias were used: The iridium catalysts supported on phosphorus-doped rutile titania prepared by the hydrothermal method showed nearly 5 times higher activity for the reaction of 2-nitroaniline (1a) and benzyl alcohol (2a) to 2-phenylbenzimidazole (3aa) than the catalysts supported on unmodified rutile. XPS depth profile study substantiated that the dopant was present mainly on the surface of the rutile support. Therefore, a facile wet impregnation method was applied to modify the surface of anatase titania with phosphoric acid. The use of thus-modified titania enhanced the activity of the iridium catalysts by more than 2.7 times. The FTIR and XPS studies revealed the presence of bidentate phosphorus species on the surface of titania, and the H₂-TPR study indicated that phosphorus-modification promoted the formation of iridium species reduced at higher temperature, which would be suitable for the present catalysis.

Full text of DOI:10.1016/j.cattod.2020.02.014

Journal Pre-proof
Effect of phosphorus-modification of titania supports on the
iridium-catalyzed synthesis of benzimidazoles
Han Yu (Investigation) (Formal analysis) (Writing - original draft),
Kenji Wada (Conceptualization) (Supervision) (Methodology)
(Validation), Tatsuhiro Fukutake (Investigation), Qi Feng
(Methodology) (Validation) (Resources) (Writing - review and
editing), Shinobu Uemura (Writing - review and editing), Kyosuke
Isoda (Writing - review and editing), Tomomi Hirai (Investigation),
Shinji Iwamoto (Resources) (Writing - review and editing)
PII:
S0920-5861(20)30066-3
DOI:
https://doi.org/10.1016/j.cattod.2020.02.014
CATTOD 12682
Reference:
To appear in:
Catalysis Today
Received Date:
Revised Date:
Accepted Date:
12 October 2019
16 January 2020
12 February 2020
Please cite this article as: Yu H, Wada K, Fukutake T, Feng Q, Uemura S, Isoda K, Hirai T,
Iwamoto S, Effect of phosphorus-modification of titania supports on the iridium-catalyzed
synthesis of benzimidazoles, Catalysis Today (2020),
doi: https://doi.org/10.1016/j.cattod.2020.02.014

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Effect of phosphorus-modification of titania supports on the
iridium-catalyzed synthesis of benzimidazoles
Han Yu,a Kenji Wada,b,* Tatsuhiro Fukutake,a Qi Feng,a Shinobu Uemura,a Kyosuke
Isoda,a Tomomi Hirai,c Shinji Iwamotoc
aDepartment of Advanced Materials Science, Faculty of Engineering and Design,
Kagawa University, Takamatsu 761-0396, Japan
bDepartment of Chemistry for Medicine, Faculty of Medicine,
Kagawa University, 1750-1 Ikenobe, Miki-cho, Kita-gun, Kagawa 761-0793, Japan
Phone +81-87-891-2249, Fax +81-87-891-2249, e-mail: wadaken@med.kagawa-
u.ac.jp
cGraduate School of Science and Technology, Gunma University, Kiryu, Gunma 376-
8515, Japan
Graphical Abstract
By P-modification!

Phosphorus-modification of titania supports for iridium catalysts greatly enhanced the
activity for the synthesis of benzimidazoles via hydrogen transfer.
Highlights
 Phosphorus-modified titanias were prepared by the hydrothermal and
impregnation methods.
 Iridium catalysts supported on the thus-modified titanias showed excellent
activity for the synthesis of benzimidazoles via hydrogen transfer.
 Phosphorus-modification would promote the formation of iridium species
suitable for the present catalysis.
Abstract
Modification of titania supports for iridium catalysts by phosphorus species greatly
enhanced the activity for the synthesis of benzimidazoles via hydrogen transfer. Two
series of phosphorus-modified titanias were used: The iridium catalysts supported on
phosphorus-doped rutile titania prepared by the hydrothermal method showed nearly
5 times higher activity for the reaction of 2-nitroaniline (1a) and benzyl alcohol (2a)
to 2-phenylbenzimidazole (3aa) than the catalysts supported on unmodified rutile.
XPS depth profile study substantiated that the dopant was present mainly on the
surface of the rutile support. Therefore, a facile wet impregnation method was applied
to modify the surface of anatase titania with phosphoric acid. The use of thus-
modified titania enhanced the activity of the iridium catalysts by more than 2.7 times.
The FTIR and XPS studies revealed the presence of bidentate phosphorus species on
the surface of titania, and the H2-TPR study indicated that phosphorus-modification
promoted the formation of iridium species reduced at higher temperature, which
would be suitable for the present catalysis.
Keywords: Phosphorus-modification, Supported catalyst, Iridium, Titania,
Benzimidazole

1. Introduction
Benzimidazoles are important building blocks in pharmaceutical and agrichemical
industries because of excellent biological, physiological, and/or therapeutical
activities of themselves or their derivatives. A series of benzimidazoles derivatives
have been developed and applied in the antiviral, anti-cancer, anti-inflammatory, anti-
hypertension treatments, and so on [1]. Recently the heterogeneous catalysis for
synthesis of benzimidazole derivatives has attracted great attention as one of the most
promising approaches because of their advantages in reusability, scalability, and
environmental friendliness [2], which is different from the traditional routes using
homogeneous catalysts [3,4]. Supported gold, palladium, platinum and iridium
catalysts have been reported to be effective for the synthesis of benzimidazoles via
dehydrogenative or hydrogen transfer pathways [5], in which titania supported
iridium (Ir/TiO2) catalysts are most attractive due to their reusabilities, high
selectivities, and high yield under significantly mild conditions [6].
Furthermore, the optimization of supports has been reported to improve the
performance of the supported catalyst. Remarkably, the dehydrogenative synthesis of
benzimidazoles from 1,2-phenylenediamine and primary alcohols was significantly
promoted in the presence of the iridium catalysts supported on rutile titania, namely
JRC-TIO-6 (JRC: Japan Reference Catalyst) [6b]. On the other hand, anatase titania
enclosed by {010} and {101} facets was found to be an excellent support for the
iridium-catalyzed synthesis of benzimidazoles from 2-nitroaniline and primary
alcohols via hydrogen transfer [7]. Meanwhile, the enhanced catalytic activities of
iridium catalysts by adding other elements have been reported [8]. For example,
Tomishige et al. reported that supported rhenium-modified iridium catalysts show
excellent activities for various reactions including hydrogenolysis of glycerol [8a] and
cyclic ethers [8b, d], and transformation of hemicellulose [8c].

Herein, a significant promotional effect of the modification of titania supports by
phosphorus species on the activities of iridium catalysts is reported. The phosphorus-
modified titania supports significantly improved the activities of the iridium catalysts.
Roles of the phosphorus species are discussed on the bases of the characterization of
the catalysts using nitrogen adsorption, transmission electron microscopy (TEM), H2
temperature-programmed reduction (H2-TPR), Fourier-transform infrared
spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS).
2. Experiment Section
2.1 General information
All reactions were carried out under an argon atmosphere in Schlenk tubes.
Tris(acetylacetonate)iridium(III) (Ir(acac)3, C15H21O6Ir), mesitylene, methanol,
tetrahydrofuran (THF), biphenyl, and benzyl alcohol were delivered from Wako and
used as received. Phosphoric acid, 2-nitroaniline, and butan-1-ol were delivered from
TCI Cooperation and used as received. JRC (Japan Reference Catalyst) titanias (JRC-
TIO-7 and 10, further designated as TIO-7 and -10, respectively) were delivered by
the Catalysis Society of Japan.
2.2 Preparation of phosphorus-doped rutile titania by a hydrothermal method
The phosphorus-doped rutile titanias (TiO2, designated as Rutile (HT)) were
prepared by a hydrothermal process as described in a previous report by one of the
present authors [9]. A white precipitate was obtained by adding titanium
tetraisopropoxide (TTIP; 24.2 g) to 450 mL of deionized water. The white precipitate
collected by filtration was added to 340 mL of 0.5 M hydrochloric acid aqueous
solution, stirred at room temperature for 4 h to obtain a transparent solution. After
aging at 40 °C for 48 h in an incubator, the solution was centrifuged and washed
repeatedly with deionized water, and then dried at 100 °C for 12 h. Subsequently, 3.0
g of the obtained powder was added to 10 mL of deionized water, and a desired
amount of phosphoric acid (P/Ti molar ratio, 0 to 0.06) was added to the suspension.

The mixture was hydrothermally treated at 220 °C for 4 h, and the product was
centrifuged and washed with deionized water, and then dried at 100 °C for 12 h. The
products thus-obtained are designated as P(x)-Rutile (HT), where x is the P/Ti
changed molar ratio.
2.3 Preparation of phosphorus-modified TiO2 by an impregnation method
The phosphorus-modified TiO2 was prepared using an impregnation method. One
gram of TiO2 was put in a glass evaporation dish, and a desired amount of phosphoric
acid (P/Ti molar ratio, 0 to 0.06) aqueous solution was added. The mixture was dried
on a hot plate at 70 oC under stirring conditions. The obtained powder was dried at 80
oC for 3 h.
2.4 Preparation of supported iridium catalyst
TiO2-supported iridium catalysts were prepared by an impregnation method as
shown in Scheme 1. Ir(acac)3 (0.10 mmol as Ir) was dissolved in 10 mL of methanol
under stirring, then 1.0 g of titania was added in air at 80 oC. Subsequently, the
obtained sample was dried overnight in air at 80 oC. The resulting powder was
calcined in air at 400 oC for 30 min in a box furnace (rising rate; 10 oC/min), and then
reduced in a H2(2 vol.%)/Ar flow (40 mL/min) at 500 oC (rising rate; 10 oC /min) for
30 min by using a conventional flow reactor to give Ir/TiO2 catalysts. After purged by
N2, the catalysts were passivated in O2(0.5 vol.%)/N2 flow (40 mL/min) at 50 oC for 5
min.

Scheme 1. Preparation procedure of titania-supported iridium catalysts.
2.5 General procedure for the catalytic runs
Catalytic reactions were performed in a glass Schlenk tube reactor (20 mL)
equipped with hot stirrer and cooling block to reflux the solution. A typical reaction
procedure for the synthesis of 2-phenylbenzimidazole is as follows: 2-nitroaniline,
benzyl alcohol and mesitylene solvent were added to the Schlenk tube containing the
Ir/TiO2 catalyst. The reaction mixture was stirred at a desired temperature for 18 h
under an argon atmosphere. Then the solid catalyst was removed by passing the
mixture through a polytetrafluoroethylene (PTFE) filter (Millipore Millex LH, 0.45
m), and the yield of products were quantified by a gas-liquid chromatography using
biphenyl as an internal standard.
2.6 Hot filtration tests
The Schlenk tube (20 mL) was charged with 2-nitroaniline (2.0 mmol), benzyl
alcohol (8.0 mmol), and Ir/TiO2 catalyst (0.010 mmol as Ir) in mesitylene (2 mL)
solvent together with an internal standard (hexadecane, 0.1 mL) under an argon
atmosphere. After the reaction was allowed to proceed at 120 °C for 6 h, the mixture
was filtered through the PTFE filter into another preheated Schlenk tube reactor and
then was heated at 120 °C for 24 h under stirring conditions.

2.7 Physical and analytical measurements
The resulting organic products of the catalytic reactions were analyzed by a gas-
liquid chromatography (Shimadzu GC-14A; Zebron ZB-1 capillary column, i.d. 0.25
mm, length 30 m, at 50250 °C). Solid catalysts were analyzed by the X-ray
photoelectron spectroscopy (ULVAC-PHI 5500MT) equipped with Mg K radiation
(1254 eV) generated by an X-ray tube operating at 15 kV, 400 W. Binding energies
were referenced to the C 1s level of residual graphitic carbon [10]. The structures of
samples were investigated by using a powder X-ray diffractometer (XRD-6100,
Shimadzu, Japan) with Cu Kα (λ = 0.15418 nm) radiation. Transmission electron
microscopy (TEM) observation was performed on a JEOL Model JEM-3010 system
at 300 kV, and the powder sample was supported on a Cu microgrid. Brunauer-
Emmett-Teller (BET) surface areas were determined by a computer-controlled
automatic gas sorption system (Quantachrome NOVA 4200e), and samples were
degassed at 300 °C for 2 h just before the measurements. Temperature-programmed
reduction (TPR) was performed with a gas flow system, where hydrogen (2 vol.% in
Ar; atmospheric pressure of 40 mL/min) was fed in a quartz tube containing the
catalyst. The tube was heated in an electric furnace at rising rate of 10 °C/min and the
amount of H2 consumed was monitored with a TC detector on a Shimadzu 8AIT gas
chromatograph. Zeta potential was measured by a Zeta-potential & particle size
analyzer (Photal ELSZ-2KG) in distilled water at pH 6.8.
3. Results and discussion
3.1 Effects of phosphorus-doped rutile prepared by the hydrothermal method
One of the present authors has reported the hydrothermal preparation of
nanocrystalline rutile titanium dioxides having larger surface areas by the addition of
small amounts of phosphoric acid to the precursor solution [9]. Therefore, in the
present study the P-doping effects of hydrothermally-prepared rutile (see Figure S1 of
the Supporting Information) as a support of the iridium catalysts on the activity were

examined. The reactions (1) and (2) were performed at 120 oC for 18 h using 25 mg of
the catalysts unless otherwise noted. As shown in entries 1 to 3 of Table 1, the yields
of 2-phenylbenzimidazole (3aa) from 2-nitroaniline (1a) and benzyl alcohol (2a) (eq.
1) gradually increased with an increase in the amount of phosphorus dopant. This
trend was also true for the solvent-free synthesis of 2-propylbenzimidazole (3ab)
using a large excess of butan-1-ol (2b) by eq. 2. The desired product 3aa was obtained
in the highest yield up to 93% in the presence of 100 mg of the iridium catalyst
supported on Rutile (HT) with P/Ti molar ratio of 0.06 (Table 1, entry 7), while the
iridium catalyst supported on undoped JRC-TIO-4 that contains 80% anatase and 20%
rutile of TiO2 phases similar to P25 showed a significantly lower activity (Table 1,
entry 8). These results demonstrate the promotional effect of phosphorus-doping on
rutile TiO2 (HT).
Table 1. P-doping effects of rutile (HT) as a support on the iridium-catalyzed
synthesis of benzimidazoles
BET S.A.
(m2/g)
Entry
Alcohol
TiO2 support
Yield of 3 (%)
1
2
3
P0-Rutile (HT)
P0.02-Rutile (HT)
P0.06-Rutile (HT)
32
6
9
2a
2a
2a
82
94
29

4
5
P0-Rutile (HT)
P0.02-Rutile (HT)
P0.06-Rutile (HT)
P0.06-Rutile (HT)
JRC-TIO-4
32
82
94
94
51
6
2b
2b
2b
2a
2a
11
28
93
15
6
7a
8a
Reaction conditions; at 120 oC for 18 h, Ir loading level 2.0 wt%.
For other conditions, see eqs. (1) and (2).
aCatalysts amounts are 100 mg for Ir 2.0 wt% loading.
The removal of the solid catalyst by the hot filtration using a PTFE filter (pore size
0.45 µm) after the reaction for 6 h completely stopped further progress of the
formation of 3aa by reaction (1) as shown in Figure 1. This result substantiates that
the catalysis of reaction (1) is essentially dependent on the presence of the solid
catalyst and proceeds heterogeneously [11].

Figure 1. Time-course of the yield of 3aa over Ir/P0.06-Rutile (HT) (a) without and (b)
with the removal of the catalyst by hot filtration after 6 h. Reaction conditions are
shown in the Experimental section.
As reported previously, P-doping on to Rutile (HT) significantly increases its
surface area [9]. To examine the reason of the promotional effects of P-doping, the
dependences of the yield of the desired products on the surface area and P/Ti molar
ratio were plotted as shown in Figure 2(a) and (b), respectively. The results show that
there is a non-linear increasing relationship between the surface area of supports and
the yield of products, suggesting that although the surface area affects catalytic
activities, it is not the dominant factor of the catalytic activities. Meanwhile, a near
linear increasing relationship between the amount of doped phosphorus atoms and the
yield of products was found. This result suggests that the presence of phosphorus
species is responsible for the enhanced activities.

Figure 2. The dependences of the yields of the products on (a) surface area of the
supported catalysts and (b) P/Ti molar ratio and the yield of the product. Numbers in
the graphs show the yields of corresponding benzimidazoles.
Effect of loading level of iridium of the Ir/P0.06-Rutile catalysts (25 mg) was
examined, and the results are shown in Figure S2 in the Supporting Information. The
yield of the desired products increased in proportion to the iridium loading level. This
result implies that the activity per iridium atom is not greatly affected by the iridium
loading level from 0.5 to 4%.
Figure 3 shows HR-TEM images of Rutile (HT)-supported iridium catalysts with
different loading levels. Regardless of the iridium loading level, nanoparticles located
on the surface of supports have a uniform particle size in a range of 1.0 to 2.4 nm.
This result is consistent with the result that the activity of the catalyst is just
dependent on the amount of iridium species on supports surface (see Figure S2). The
similarity of the lattice parameter of IrO2 and rutile titania would be one reason: The
close Ir–O (0.1976 and 0.1978 nm) and Ti–O (0.1962 and 0.1963 nm) bond lengths
and crystalline structure would restrict the growth of Ir particles [12]. Besides, from
these TEM photographs, changes of the particle morphology and size distribution of
iridium nanoparticles on supports surface by the P-doping could not be confirmed.

Figure 3. HR-TEM images and the particle size distribution histograms of (a) Ir(2
wt%)/P0-Rutile (HT), (b) Ir(2 wt%)/P0.06-Rutile (HT), and (c) Ir(4 wt%)/P0.06-Rutile
(HT).
The XPS depth profile (Table S1) clearly shows uneven distribution of phosphorus
atoms. For example, the surface compositional ratio of phosphorus atom was 1.86%
on the surface of P0.02-Rutile (HT), and decreased dramatically to 0.72% after ion
sputtering for 1 min. As the sputtering carried on for 15 minutes, there was only
0.18% of phosphorus atom left. These results reveal that the phosphorus species
mainly present on the surface layer of the P-doped rutile particles.
3.2 Effects of phosphorus-modification of titania by the impregnation method
Following the results using the Ir/Px-Rutile (HT) catalysts shown above, in which
phosphorus species located on the surface layer play a significant promotional role,
the effect of phosphorus-modification by an impregnation method was examined.
Note that the modification of titania surface by phosphoric acid (H3PO4) has been
reported to enhance the adsorption capacity for organic compounds and accelerate the
photocatalytic degradation of these adsorbed organic species [13]. Our preliminary
experiments evidenced that the adsorption capacity of Ir/TIO-7 catalyst towards

methylene blue in water increased by the phosphorus-modification (data not shown).
On the other hand, phosphorus-modification only slightly changed its surface Zeta
potential under neutral conditions (see Table S2).
In the present study, an anatase titania sample with a high surface area, namely
JRC-TIO-7, was impregnated in dilute phosphoric acid (H3PO4) solution, followed by
drying. The iridium catalysts were prepared using the thus-prepared P-impregnated
JRC-TIO-7 (further designated as Px-TIO-7 (IMP)) (see Figure S1). Figure 4 shows
the effect of the atomic ratio of phosphorus to titanium on the solvent-free synthesis
of 2-propylbenzimidazole (3ab) from 2-nitroaniline (1a) and butan-1-ol (2b) (eq. 2).
While unmodified JRC-TIO-7 gave 3ab in the yield of 25%, the highest yield of 69%
was achieved in the presence of Ir/P0.02-TIO-7 (IMP), as shown in Figure 4. As the
molar ratio of phosphorus further increased, the yield decreased correspondingly.
Figure 4. Effect of P/Ti molar ratio (x) of the Ir/Px-TIO-7 (IMP) catalyst on the yield
of 3ab. Reaction conditions are shown in eq. 2.

To examine whether the Ir/P0.02-TIO-7 (IMP) catalyst was still active or not after
the reaction for 18 h, we added another 1.0 mmol of 2-nitroaniline through a syringe
and allowed the reaction for another 24 h. The amount of 3ab formed was increased
from 0.69 mmol (69%) to 0.96 mmol, indicating that the phosphorus-modified
catalysts kept their activity after the initial run for 18 h. On the other hand, when the
used catalyst was recovered from the mixture by centrifugation in an air atmosphere
followed by the calcination at 400 oC and the reduction at 500 oC for 30 min, the thus-
recovered catalyst showed poor activities: the yield of 3ab was only 11%. The reasons
of the lower activity of used catalysts are discussed below.
Meanwhile, the titanium phosphate (TiP2O7, see Figure S3) prepared by a liquid-
assisted synthesis method [14] was also utilized to compare the effect of phosphorus
on the catalytic activity. As shown in Table S3, the use of TiP2O7 resulted in poor
catalytic activity.
As can be seen in Figure 5, the HR-TEM image shows that small iridium particles
are irregularly distributed on P0.02-TIO-7 (IMP). Particle size statistics suggest that
most of the iridium particles are smaller than 1 nm. These small particles would be
responsible for the high catalytic activity, as discussed in our previous report [7]. The
fast Fourier transform (FFT) pattern evidence the dark small particles dispersed in the
lattice fringe are metallic iridium phase. Again, changes of the morphology and size
of surface iridium particles by the phosphorus-modification could not be confirmed,
since TEM images of Ir/TIO-7 were very similar to Ir/P0.02-TIO-7 (IMP) (mean
diameter of iridium nanoparticles; 0.7 nm [6b, 7]). Whereas the isolated iridium atoms
formed on the support along with nanoparticles were often reported [15], it cannot be
identified in the lattice fringe probably due to that the small particles are an
amorphous phase.

Figure 5. HR-TEM images, corresponding FFT pattern, and particle size distribution
histograms of Ir/P0.02-TIO-7 (IMP) catalyst.
Changes of surface functional groups by phosphorus-modification on JRC-TIO-7
and the Ir/Px-TIO-7 (IMP) catalysts were examined by DRIFT (diffuse-reflectance
infrared Fourier transform) spectra, as shown in Figure S4. For all the samples, a
broad band at around 3400 cm−1 as well as a narrow peak at around 1634 cm−1 due to
the stretching and bending vibrations of the surface hydroxyl group, respectively,
were observed [16]. Note that the phosphorus-modification significantly enhanced the
intensity of band due to the O-H stretching vibrations, suggesting an increase of
surface hydroxyl groups as discussed in the following XPS study, which would be one
reason of the enhanced adsorption capacity towards organic substrates of phosphorus-
modified catalysts (see above). An intense peak present at 750 cm−1 is assignable to
the Ti-OTi stretching vibration octahedral coordination [17]. The peaks of Ir/P0.02
-
TIO-7 (IMP) after impregnation with Ir(acac)3 around 1205−1558 cm−1 would be
responsible for organic ligands of Ir(acac)3 [18]. Two shoulders at around 1120 cm−1
and 1020 cm−1 were observed on the P-modified samples, which are assigned to the
stretching vibration of P−O bonds of bidentate adsorbed phosphorus species [19].
These peaks were observable even after the reduction of the catalyst. However, the
stretching vibration of P=O bond of phosphoric acid cannot be observed around
1200−1250 cm−1 probably due to the coordination of the phosphoryl oxygens to

surface Lewis acidic sites [20]. Since earlier studies reported that the phosphorus was
located in the coordinatively unsaturated site of Ti4+ cations [19], it is believed that
the phosphorus exists as bidentate phosphoric acid and anchoring on the surface of
TiO2 through Ti−O−P bonds [21].
The catalysts were characterized by XPS. Figure 6(a) shows the P 2p XP spectra
of Ir/P0.02-TIO-7 (IMP), Ir/P0.02-Rutile (HT), and Ir/P0.06-Rutile (HT). For those
catalysts, the P 2p binding energy appeared at around 133.3 eV, suggesting that the
phosphorus species on the catalyst were in the pentavalent oxidation (P5+) state even
after the hydrogen reduction. This is consistent with the FTIR results, which suggests
the formation of bidentate surface phosphoric species (see above). A peak at around
129 eV, characteristic for P-Ti species was not observed [22]. Figure 6(b-d) shows the
O 1s XP spectra on the surface of phosphorus-modified and unmodified titania-
supported iridium catalysts. The O 1s spectra of phosphorus-modified catalysts were
significantly different from that of unmodified one: There were additional bands at
higher binding energies. These new bands were assignable to oxygen atoms of
phosphorus species and/or those of surface OH groups [23]. As discussed above,
FTIR study revealed that the use of phosphorus-modified titania promotes the
formation of surface OH groups, which is consistent with the present XPS results. On
the other hand, Ir 4f bands of phosphorus-modified catalysts were almost identical to
those of unmodified catalysts as shown in Figure S5. Note that bands at 60.7 eV and
62.0 eV have been assigned to Ir0 and IrIV species, respectively [7], while a band due
to Ti 3s was overlapped with Ir 4f bands.

Figure 6. XP spectra of (a) P 2p of Ir/P0.02-TIO-7 (IMP), Ir/P0.02-Rutile (HT) and
Ir/P0.06-Rutile (HT), (b) O1s of Ir/P0.02-TIO-7 (IMP), (c) Ir/P0-Rutile (HT), and (d)
Ir/P0.06-Rutile (HT).
The H2 temperature-programmed reduction (TPR) study demonstrates the effect of
phosphorus modification on the reducibility of iridium catalysts as shown in Figure 7.
The profile of P0.02-TiO2 (IMP) exhibited a hydrogen consumption peak at around
500 °C which can be ascribed to the reduction of surface titanium species [24], and
there are no significant reduction peaks at below 400 °C. The Ir/TIO-7 catalyst leads
to three reduction peaks at 153 °C and 372 °C together with a weak peak at 220 °C. A
significant shift of the TiO2 reduction temperature to 372 °C would be due to the
spillover effects by supported iridium species [25]. For the Ir/P0.02-TIO-7 (IMP)
catalyst, the reduction peaks of surface Ti4+ species shifted to a lower temperature,
345 °C, while the peak at around 220 °C increased significantly. The intensity of the
peak at 220 °C further increased with Ir/P0.06- TIO-7 (IMP). The first peak at around

150 °C could be assigned to the reduction of iridium particles around 0.5 nm in
diameter or above. The iridium oxide species reduced around 220 °C have been
identified as those having strong interaction with the support, namely well-dispersed
iridium species smaller than 0.5 nm [24]. While the formation of such small iridium
species could not be confirmed by the TEM photographs in the present study, the
iridium species adjacent to phosphorus species would further decrease the reduction
temperature of titania surface to ca. 340 °C. On the other hand, the Ir/P0.06-Rutile
(HT) catalysts do not show a significant reduction peak at 220 °C. Instead, it showed
broad peaks at lower temperatures, which indicates that iridium oxide species on
Rutile (HT) are more easily reducible than those on anatase titania and suggests that
the interaction between iridium species and phosphorus species of P0.06-Rutile (HT) is
weak.
Figure 7. H2-TPR profiles of phosphorus-modified titania and the iridium catalysts.

As noted above, the recovered Ir/P0.02-TIO-7 (IMP) catalyst showed low activity.
The BET surface area of the used catalyst (157 m2 g-1) is slightly smaller than the
fresh catalyst (173 m2 g-1). The TEM image of the used catalyst (see Figure S6)
indicates the absence of large iridium particles. On the other hand, the H2-TPR (see
Figure S7) profile shows a more significant reduction peak at around 220 °C,
suggesting the presence of more strong interaction between titania surface and iridium
species. Such a very strong interaction would be one reason for the lower activity of
the used catalyst. Moreover, the XP spectrum (see Figure S8) reveals the adsorption of
nitrogen species on the surface of the used catalyst. These results indicate that the
recovery procedure of the used catalyst needs further improvement.
As discussed above, the phosphorus-modification of titania support changed the
redox properties of the surface iridium species, whereas the size and morphology
were not significantly affected. The processes of the formation of iridium
nanoparticles on the surface of (IMP)-titanias are discussed based on the above
results. Initially, bidentate phosphorus species would be formed on the surface by the
impregnation of titania with phosphoric acid as supported by the FTIR study. The
bidentate structure would be most stable because of its highest adsorption energies of
257−286 kJ/mol [26]. During the subsequent impregnation with Ir(acac)3 in methanol,
interaction of surface phosphorus species and an iridium complex could be expected.
In the course of the calcination and reduction procedure at a high temperature, these
surface iridium complexes would be converted to small iridium nanoparticles.
Meanwhile, the iridium species adjacent to phosphorus species would play an
important role in the enhancement of the catalytic activity. Our previous study
revealed that relatively high-valent iridium species would be responsible for the
synthesis of benzimidazole via hydrogen transfer [7]. Transition metal species were
often reduced to be zero-valent in the presence of alcohols even under aerobic
conditions [27]. The interaction by phosphorus-modified surface would stabilize the

relatively high-valent state of iridium species during the course of the reactions.
Further characterization of phosphorus-modified catalysts are ongoing.
4. Conclusion
Phosphorus-modified titania supports were prepared in two different ways, namely
by the hydrothermal and wet impregnation methods. In both cases, the iridium
catalysts supported on phosphorus-modified titanias showed the enhanced activity for
the synthesis of benzimidazoles from 2-nitroaniline and primary alcohols via
hydrogen transfer. The hydrothermal P-doping resulted in ca. 5 times promotion of the
catalytic activity. The modification by the wet impregnation of titania with a high
surface area such as JRC-TIO-7 with phosphoric acid also increased the activity of
supported iridium catalysts. The P-modification of both anatase and rutile has a
positive effect on the promotion of the catalytic activity. Especially the catalysts
supported on P-modified anatase with high surface area showed excellent activity.
The hot filtration study indicates that the catalysis proceeded heterogeneously. These
modified catalysts kept their activity even after the additional supply of the substrate,
indicating their fair durability. On the other hand, the lower activity of the recovered
catalysts points out the need for further improvement of the recovery procedure. The
detailed characterization of the catalysts such as the H2-TPR study revealed that
phosphorus-modification facilitated the formation of iridium species reduced at higher
temperature, and would stabilize the state of iridium species suitable for the present
catalysis. Further application of the present catalysts to a wider range of reactions via
hydrogen transfer, as well as the detailed investigation on the effects of phosphorus
and various promoters, are now in progress.
Credit author statement
Han Yu; Investigation, Formal analysis, Writing - Original Draft, Kenji Wada;
Conceptualization, Supervision, Methodology, Validation, Tatsuhiro Fukutake;

Investigation, Qi Feng; Methodology, Validation, Resources, Writing - Review &
Editing, Shinobu Uemura; Writing - Review & Editing, Kyosuke Isoda; Writing -
Review & Editing, Tomomi Hirai; Investigation, Shinji Iwamoto; Resources, Writing
- Review & Editing
Acknowledgements
This research was supported by JSPS KAKENHI grant number 17H03458.

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