Immobilization of Ir(OH) 3 Nanoparticles in Mesospaces of Al-SiO 2 Nanoparticles Assembly to Enhance Stability for Photocatalytic Water Oxidation

: Iridium hydroxide (Ir(OH) 3 ) nanoparticles exhibiting high catalytic activity for water oxidation were immobilized inside mesospaces of a silica-nanoparticles assembly (SiO 2 NPA) to suppress catalytic deactivation due to agglomeration. The Ir(OH) 3 nanoparticles immobilized in SiO 2 NPA (Ir(OH) 3 / SiO 2 NPA) catalyzed water oxidation by visible light irradiation of a solution containing persulfate ion (S 2 O 82 − ) and tris(2,2 (cid:48) -bipyridine)ruthenium(II) ion ([Ru II (bpy) 3 ] 2 + ) as a sacriﬁcial electron acceptor and a photosensitizer, respectively. The yield of oxygen (O 2 ) based on the used amount of S 2 O 82 − was maintained over 80% for four repetitive runs using Ir(OH) 3 / SiO 2 NPA prepared by the co-accumulation method, although the yield decreased for the reaction system using Ir(OH) 3 / SiO 2 NPA prepared by the equilibrium adsorption method or Ir(OH) 3 nanoparticles without SiO 2 NPA support under the same reaction conditions. Immobilization of Ir(OH) 3 nanoparticles in Al 3 + -doped SiO 2 NPA (Al-SiO 2 NPA) results in further enhancement of the catalytic stability with the yield of more than 95% at the fourth run of the repetitive experiments.

Recently, an assembly of aluminated silica (Al-SiO2) nanoparticles in the size of ~20 nm has been reported as a support for a photocatalytic hydrogen evolution system composed of 2-phenyl-4-(1naphthyl)quinolinium ion and platinum nanoparticles (~2 nm in diameter) as a photosensitizer and hydrogen-evolution catalysts, respectively [41]. Photocatalytic activity of the composite catalyst was enhanced compared with that using conventional mesoporous silica as a support, because the discrete mesospaces among Al-SiO2 nanoparticles are suitable for electron transfer to highly dispersed platinum nanoparticles from multiple photosensitizers nearby. The structure was easily fabricated by co-accumulation of Al-SiO2 and platinum nanoparticles with the photosensitizer.
We report herein the immobilization of iridium hydroxide (Ir(OH)3) nanoparticles in mesospaces of an assembly of silica or aluminated silica nanoparticles (SiO2NPA or Al-SiO2NPA, respectively) for the enhancement of catalytic stability during photocatalytic water oxidation. The stability of a series of water oxidation catalysts was evaluated by the repetitive experiments for photocatalytic water oxidation using [Ru II (bpy)3] 2+ and S2O8 2− as a photosensitizer and a sacrificial electron acceptor, respectively. Ir(OH)3/SiO2NPAs prepared by two different methods were compared in terms of catalytic activity and stability for the photocatalytic water oxidation. The first catalyst was prepared by co-accumulation (CA) method, in which a dispersion containing both Ir(OH)3 and SiO2 nanoparticles was used for co-assembly formation (Figure 1b). The second one was Agglomeration of catalytic nanoparticles can be generally suppressed by supporting the nanoparticles on a support with high surface area [29][30][31][32]. Especially, mesoporous supports such as MCM-41 effectively suppress the agglomeration of nanoparticles because of spatial separation [33][34][35]. However, catalytic nanoparticles larger than the aperture size of a mesoporous support are hardly immobilized. Moreover, incorporation of catalytic nanoparticles into a mesoporous support often results in low catalytic activity because of slow diffusion of a photosensitizer to access water oxidation catalysts located inside narrow mesospaces.
Such problems can be solved by using a mesoporous support composed of a bottom-up assembly of spherical nanoparticles with uniform size [36]. The assembly possesses discrete mesospaces among nanoparticles where various molecules and nanoparticles such as organic molecules, enzymes, and metal nanoparticles are stably immobilized by encapsulation [37][38][39][40][41]. Recently, an assembly of aluminated silica (Al-SiO 2 ) nanoparticles in the size of~20 nm has been reported as a support for a photocatalytic hydrogen evolution system composed of 2-phenyl-4-(1-naphthyl)quinolinium ion and platinum nanoparticles (~2 nm in diameter) as a photosensitizer and hydrogen-evolution catalysts, respectively [41]. Photocatalytic activity of the composite catalyst was enhanced compared with that using conventional mesoporous silica as a support, because the discrete mesospaces among Al-SiO 2 nanoparticles are suitable for electron transfer to highly dispersed platinum nanoparticles from multiple photosensitizers nearby. The structure was easily fabricated by co-accumulation of Al-SiO 2 and platinum nanoparticles with the photosensitizer.
We report herein the immobilization of iridium hydroxide (Ir(OH) 3 ) nanoparticles in mesospaces of an assembly of silica or aluminated silica nanoparticles (SiO 2 NPA or Al-SiO 2 NPA, respectively) for Catalysts 2020, 10, 1015 3 of 14 the enhancement of catalytic stability during photocatalytic water oxidation. The stability of a series of water oxidation catalysts was evaluated by the repetitive experiments for photocatalytic water oxidation using [Ru II (bpy) 3 ] 2+ and S 2 O 8 2− as a photosensitizer and a sacrificial electron acceptor, respectively.
Ir(OH) 3 /SiO 2 NPAs prepared by two different methods were compared in terms of catalytic activity and stability for the photocatalytic water oxidation. The first catalyst was prepared by co-accumulation (CA) method, in which a dispersion containing both Ir(OH) 3 and SiO 2 nanoparticles was used for co-assembly formation (Figure 1b). The second one was prepared by equilibrium adsorption (EA) method, in which Ir(OH) 3 nanoparticles were immobilized in preassembled SiO 2 NPA (Figure 1c). Size effect of SiO 2 nanoparticles in SiO 2 NPAs was also scrutinized on the catalysis stability during photocatalytic water oxidation. Then, the effect of surface charge modification by using Al-SiO 2 nanoparticles was investigated on the catalytic stability.

Results and Discussion
2.1. Preparation and Water Oxidation Catalysis of Ir(OH) 3 Nanoparticles with and without SiO 2 NPA Ir(OH) 3 nanoparticles were synthesized by a reported procedure with slight modifications [42]. Hydrogen hexachloroiridate(IV) was treated with an aqueous solution of sodium hydroxide to form Ir(OH) 3 nanoparticles at a raised temperature. The nature of Ir(OH) 3 nanoparticles prone to agglomeration was evidenced by dynamic light scattering (DLS) and transmission electron microscopy (TEM). DLS measurements indicated that the size of Ir(OH) 3 nanoparticles is~40 nm ( Figure S1). However, the TEM image shown in Figure 2a suggests that the size of primary particles was several nanometers in diameter, indicating that the particles size of~40 nm observed in DLS measurements was that of secondary particles formed by agglomeration even in dilute dispersions.
Catalysts 2020, 10, x FOR PEER REVIEW 3 of 13 prepared by equilibrium adsorption (EA) method, in which Ir(OH)3 nanoparticles were immobilized in preassembled SiO2NPA (Figure 1c). Size effect of SiO2 nanoparticles in SiO2NPAs was also scrutinized on the catalysis stability during photocatalytic water oxidation. Then, the effect of surface charge modification by using Al-SiO2 nanoparticles was investigated on the catalytic stability.

Preparation and Water Oxidation Catalysis of Ir(OH)3 Nanoparticles With and Without SiO2NPA
Ir(OH)3 nanoparticles were synthesized by a reported procedure with slight modifications [42]. Hydrogen hexachloroiridate(IV) was treated with an aqueous solution of sodium hydroxide to form Ir(OH)3 nanoparticles at a raised temperature. The nature of Ir(OH)3 nanoparticles prone to agglomeration was evidenced by dynamic light scattering (DLS) and transmission electron microscopy (TEM). DLS measurements indicated that the size of Ir(OH)3 nanoparticles is ~40 nm ( Figure S1). However, the TEM image shown in Figure 2a suggests that the size of primary particles was several nanometers in diameter, indicating that the particles size of ~40 nm observed in DLS measurements was that of secondary particles formed by agglomeration even in dilute dispersions.   Table 1. Molar ratios of iridium (Ir) and silicon (Si) of catalysts before and after the photocatalytic water oxidation determined by XRF analyses.
Brunauer−Emmett−Teller (BET) surface areas (S), pore size determined by the Barrett-Joyner-Halenda (BJH) plot (R), and inner surface areas obtained by t-plots (S int ) of a series of catalysts calculated from the nitrogen adsorption−desorption isotherms. a  (20)-EA showed high catalytic activity (>85% based on the used amount of Na 2 S 2 O 8 ) up to the 2nd run compared with Ir(OH) 3 nanoparticles without a support; however, the O 2 yield was as low as 61% and 37% at the 3rd and 4th runs, respectively. High O 2 yield (>80%) even at the 4th run for the reaction system using Ir(OH) 3 /SiO 2 NPA(20)-CA as the catalyst indicated that the CA method is a suitable preparation method for the improvement of catalytic stability compared with the EA method. Photocatalytic water oxidation was examined under visible light irradiation of a phosphate buffer solution (50 mM, 2.0 mL, pH 8.0) containing a catalyst (5.0 mg, [Ir] = 0.072 mM), tris(2,2′bipyridine)ruthenium(II) sulfate (Ru II (bpy)3SO4, 1.0 mM) as a photosensitizer and sodium persulfate (Na2S2O8, 5.0 mM) as a sacrificial electron acceptor, respectively, by a white LED lamp (λ > 400 nm, 33.2 mW cm −2 ) at room temperature. The catalytic activity of Ir(OH)3 nanoparticles without a support gradually decreased in the repetitive runs ( Figure 3). The O2 yield of 78% at the 1st run based on the used amount of Na2S2O8 gradually decreased to 29 % at the 4th run. Ir(OH)3/SiO2NPA(20)-EA showed high catalytic activity (>85% based on the used amount of Na2S2O8) up to the 2nd run compared with Ir(OH)3 nanoparticles without a support; however, the O2 yield was as low as 61% and 37% at the 3rd and 4th runs, respectively. High O2 yield (>80%) even at the 4th run for the reaction system using Ir(OH)3/SiO2NPA(20)-CA as the catalyst indicated that the CA method is a suitable preparation method for the improvement of catalytic stability compared with the EA method.

Size Effect of SiO2 Nanoparticles in Ir(OH)3/SiO2NPA-CAs on Catalytic Stability for Water Oxidation Catalysis
The size effect of SiO2 nanoparticles in Ir(OH)3/SiO2NPA-CAs was investigated on catalytic activity and stability. SiO2 nanoparticles in the size of 10, 20, 50, or 100 nm were used for the preparation of Ir(OH)3/SiO2NPA-CAs. Immobilization of Ir(OH)3 nanoparticles was evidenced by XRF spectroscopy (Table 1). TEM observation of Ir(OH)3/SiO2NPA(10)-CA and Ir(OH)3/SiO2NPA(50)-CA indicated that Ir(OH)3 nanoparticles were highly dispersed in SiO2NPAs as well as Ir(OH)3/SiO2NPA(20)-CA (Figure 5a,b). On the other hand, some agglomerations of Ir(OH)3 nanoparticles were observed on the surface of Ir(OH)3/SiO2NPA(100)-CA, suggesting that the larger pores formed in SiO2NPA(100)-CA may not be suitable for the immobilization of primary particles of Ir(OH)3 (Figure 5c).

Suppression of Leaching of Ir(OH)3 Nanoparticles from SiO2NPA by Surface Modification with Al 3+
Surface modification of SiO2NPA with Al 3+ was examined to suppress the leaching of Ir(OH)3 by lessening electrostatic repulsion. Al 3+ -modified SiO2 nanoparticles were synthesized by the surface alumination of SiO2 nanoparticles (20 nm in diameter) with sodium aluminate in an aqueous dispersion. Ir(OH)3 nanoparticles were immobilized in an Al 3+ -modified SiO2 nanoparticles assembly (Al-SiO2NPA) via the co-accumulation method to obtain Ir(OH)3/Al-SiO2NPA (20)

Suppression of Leaching of Ir(OH) 3 Nanoparticles from SiO 2 NPA by Surface Modification with Al 3+
Surface modification of SiO 2 NPA with Al 3+ was examined to suppress the leaching of Ir(OH) 3 by lessening electrostatic repulsion. Al 3+ -modified SiO 2 nanoparticles were synthesized by the  Figure 7a (white arrows). Moreover, elemental mapping images obtained by an X-ray energy dispersive spectrometer indicated that Al 3+ ions added on SiO 2 nanoparticles were highly dispersed through the entire body of Ir(OH) 3 /Al-SiO 2 NPA(20)-CA, although the signal from Ir species was too weak to be detected (Figure 7b-d). The amount of Ir(OH) 3 immobilized in Ir(OH) 3 /Al-SiO 2 NPA(20)-CA was comparable to that in Ir(OH) 3 /SiO 2 NPA(20)-CA as determined by XRF measurements ( Table 1). The sizes (R), surface areas (S), and inner surface areas (S int ) of the mesopores of Ir(OH) 3 /Al-SiO 2 NPA(20)-CA were also almost comparable to those of Ir(OH) 3 /SiO 2 NPA(20)-CA (Table 2).

Synthesis
Synthesis of iridium hydroxide (Ir(OH) 3 ) nanoparticles: Ir(OH) 3 nanoparticles were synthesized according to a reported procedure with slight modification [42]. An aqueous solution of sodium hexachloroiridate(IV) (H 2 IrCl 6 , 10 mM, 25 mL) was dropped into aqueous sodium hydroxide solution (100 mM, 20 mL) using a micropump (1.0 mL/min) with vigorous stirring at 85 • C. After stirring for 1 h, insoluble aggregation appeared were removed by centrifugation. Then, methanol (100 mL) was added into the supernatant to precipitate Ir(OH) 3 nanoparticles. The dark-blue precipitates were collected by centrifugation and washed with ultrapure water for two times. The precipitates were dried in vacuo at room temperature and aged at 60 • C for 10 h.
Synthesis of mesoporous SiO 2 nanoparticles assembly containing Ir(OH) 3 nanoparticles via the co-accumulation method (Ir(OH) 3 /SiO 2 NPA(20)-CA): A dispersion of Ir(OH) 3 nanoparticles ([Ir] = 2.0 mM, 1.5 mL) was added to a suspension of LUDOX ® AS-40 colloidal silica (40 wt%, 0.5 mL, pH 9.5) with magnetic stirring. After 30 min sonication, the dispersion was spread on a glass substrate at room temperature in a clean hood overnight. The obtained powder was washed with ultrapure water for two times and dried in vacuo at room temperature.
Synthesis of silica-alumina (Al-SiO 2 ) nanoparticles: Al-SiO 2 nanoparticles were prepared according to a reported procedure with slight modification [41]. A dispersion of LUDOX ® AS-40 colloidal silica (40 wt%, 1.93 mL) was slowly added to an aqueous solution of sodium aluminate (13 mM, 9.6 mL, 48 mL, or 96 mL for [Al] = 1 wt%, 5 wt%, or 10 wt% samples, respectively) at room temperature. The resulting dispersion was magnetically stirred for 24 h and used for the next procedure without further purification.

Characterization
The atomic ratio of iridium and silicon in each catalyst was determined using a Shimadzu EDX-730 X-ray fluorescence spectrometer. TEM images and elemental mapping images of catalysts were obtained using a JEOL JEM-2100 equipped with a field-emission gun with an accelerating voltage of 200 kV with an X-ray energy dispersive spectrometer. Thin pieces of catalyst were fixed on a Cu-mesh microgrid coated with an amorphous carbon supporting film. Nitrogen (N 2 ) adsorption-desorption isotherms were measured at 77 K using a MicrotracBEL Belsorp-mini II within a relative pressure range from 0.01 to 101.3 kPa. The mass of a sample was as much as ca. 15 mg for adsorption analyses after pretreatment at 150 • C for 1 h. The sample was exposed to a mixed gas of helium and N 2 with a programmed ratio and adsorbed amount of nitrogen was calculated from the change of pressure in a cell after reaching the equilibrium. DLS experiments were conducted at room temperature using a Malvern Zetasizer Nano S90.

Photocatalytic Water Oxidation
A typical procedure for photocatalytic water oxidation is as follows. A phosphate buffer solution (50 mM, 2.0 mL, pH 8.0) containing Ir(OH) 3 /SiO 2 NPA(20)-CA (5.0 mg, [Ir] = 0.072 mM), sodium persulfate (Na 2 S 2 O 8 , 5.0 mM), and tris(2,2 -bipyridine)ruthenium(II) sulfate ([Ru II (bpy) 3 ]SO 4 , 1.0 mM) was flushed with Ar for 15 min in dark. The buffer solution was photoirradiated for a certain time with a RelyOn white LED light (130 mW, λ > 400 nm) positioned perpendicular to a cuvette. The distance between the lamp and sample cell was 3.0 cm, thus the light intensity was 33.2 mW cm −2 . The gas in a headspace was analyzed by using a Shimadzu GC-2014 gas chromatograph with a thermal conductivity detector to determine the amount of O 2 evolved. Recycling performance was evaluated by adding a phosphate buffer solution (50 mM, 2.0 mL, pH 8.0) containing Na 2 S 2 O 8 (5.0 mM) and [Ru II (bpy) 3 ]SO 4 (1.0 mM) to a catalyst taken out from the reaction solution by centrifugation.

Conclusions
Iridium hydroxide (Ir(OH) 3 ) nanoparticles, which act as an active catalyst for photocatalytic water oxidation in the presence of tris(2,2 -bipyridine)ruthenium(II) ion and persulfate ion as a photosensitizer and a sacrificial electron acceptor, respectively, were immobilized in mesopores of a SiO 2 nanoparticles assembly (SiO 2 NPA). The co-accumulation method, in which Ir(OH) 3 nanoparticles were immobilized in the mesospaces during the formation of SiO 2 NPA, is a promising way to fabricate the stable catalyst compared with the conventional equilibrium adsorption method. Further enhancement of catalytic stability was observed by tuning the size of SiO 2 nanoparticles and the surface alumination of SiO 2 NPA. We propose here a new approach to enhance stability of catalytic nanoparticles by using spherical nanoparticles assemblies as porous support.