Phase Controllable Synthesis of Well-Crystallized Rhodium Sulfides

Jun 19, 2009 - Tel: +81-88-844-8352. Fax: +81-88-844-8362. E-mail: [email protected]., †. Huazhong University of Science & Technology. , ‡. Koc...
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Phase Controllable Synthesis of Well-Crystallized Rhodium Sulfides by the Hydrothermal Method Wuxing Zhang,†,‡ Kazumichi Yanagisawa,*,‡ Sumio Kamiya,§ and Tatsuo Shou‡ Department of Electronic Science & Technology, Huazhong UniVersity of Science & Technology, Wuhan 430074, P. R. China, Research Laboratory of Hydrothermal Chemistry, Kochi UniVersity, Kochi 780-8520, Japan, and Toyota Motor Corporation, Toyota 471-8572, Japan

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 8 3765–3770

ReceiVed April 24, 2009; ReVised Manuscript ReceiVed May 28, 2009

ABSTRACT: We developed a simple hydrothermal method to synthesize crystalline Rh2S3 and Rh17S15 at 400 °C for 5 h with a S/Rh molar ratio of 3 and 0.9, respectively. The formation mechanisms of the rhodium sulfides were proposed by decomposition of the CO ligand and sulfur incorporation during the hydrothermal reaction. The crystallization of the sulfides was closely related to the S/Rh ratio, reaction temperature, and reaction time. The formation of Rh2S3 requires abundant sulfur (S/Rh ) 3), while Rh17S15 can be synthesized with a stoichiometric composition (S/Rh ) 0.9). The thermal analysis shows that the low crystalline products obtained by hydrothermal reactions at 220 °C with S/Rh ) 3 and 0.9 crystallize to Rh2S3 and Rh17S15 at 400 and 310 °C, respectively. However, calcination of Rh2S3 at high temperatures in Ar is accompanied by the release of sulfur, which finally results in the transformation of Rh2S3 into Rh3S4 and Rh17S15 at 700 °C.

1. Introduction Rhodium sulfides are attracting more and more research interest due to their applications in hydrodesulfurization (HDS),1 photochemical decomposition of aqueous sulfide,2 and oxygen reduction reaction.3 The rhodium sulfides system consists of Rh2S3, Rh3S4, and Rh17S15. Among them, Rh2S3 has a unique layered structure formed by face-sharing pairs of distorted [RhS6] octahedra and the layers are loosely bound to each other only by van der Waals forces,4 while Rh3S4 and Rh17S15 have the structure composed of [RhS6] octahedra and Rh-Rh metal bonds which give them properties such as metallic conducting. Rh2S3, Rh3S4, and Rh17S15 single crystals have been synthesized from the metallic Rh powder and S vapor at temperatures above 1000 K.5-7 In preparation of catalyst powders at low temperatures, Cattenot et al.8 synthesized the rhodium sulfide powders by precipitating RhCl3 and Na2S followed by sulfidation at 673 K in H2-H2S atmosphere.8 Revaprasadu et al. prepared nanoRh2S3 particles by pyrolysis of [Rh(S2CNEt2)2] at 280 °C.9 More recently, we presented a simple and phase controllable solvothermal method to synthesize Rh2S3 and Rh17S15 nanoparticles.10 The phase controllable synthesis of the rhodium sulfide powders is important to identify the catalysis mechanisms of different rhodium sulfide phases. Our results show that crystalline Rh2S3 and Rh17S15 can be directly formed at 400 °C, which is much lower than reported 1000 K. In this paper, we synthesized wellcrystallized rhodium sulfides powders (Rh2S3 and Rh17S15) by a hydrothermal method and found different formation mechanisms compared to that in a solvothermal method. On the basis of the work in this paper, the hydrothermal method could be hopefully extended to fabricate rhodium sulfide films with controllable phases because of the unique growth mechanisms of rhodium sulfides under hydrothermal conditions.

Figure 1. XRD patterns of the hydrothermally synthesized rhodium sulfide powders at 220 °C for 10 h with (a) S/Rh ) 0.9, (b) S/Rh ) 1.5, and (c) S/Rh ) 3. Rh2S3, 0.5 g of Rh6(CO)16 and 0.27 g of sulfur powder (S/Rh molar ratio ) 3) are put into a Teflon-lined autoclave (inner volume 30 mL) with water (22 mL) to get a fill ratio of 75%. The hydrothermal reaction

2. Experimental Procedures The starting materials are rhodium carbonyl (Rh6(CO)16) and sulfur powders (Wako Pure Chem. Ind.). For a typical process to synthesize * To whom correspondence should be addressed. Tel: +81-88-844-8352. Fax: +81-88-844-8362. E-mail: [email protected]. † Huazhong University of Science & Technology. ‡ Kochi University. § Toyota Motor Corporation.

Figure 2. XRD patterns of the hydrothermally synthesized rhodium sulfide powders at 400 °C for 5 h with (a) S/Rh ) 0.9, (b) S/Rh ) 1.5, and (c) S/Rh ) 3.

10.1021/cg900454g CCC: $40.75  2009 American Chemical Society Published on Web 06/19/2009

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Figure 3. SEM pictures of the hydrothermally synthesized rhodium sulfide powders. (a) S/Rh ) 0.9, 220 °C for 10 h; (b) S/Rh ) 1.5, 220 °C for 10 h; (c) S/Rh ) 3, 220 °C for 10 h; (d) S/Rh ) 0.9, 400 °C for 5 h; (e) S/Rh ) 1.5, 400 °C for 5 h; (f) S/Rh ) 3, 400 °C for 5 h. is conducted at temperatures from 220 to 400 °C for 0.5 to 10 h. For the reactions at 400 °C, a Hastelloy-lined autoclave (inner volume 10 mL) was used with 1/3 of the above starting materials and 75% filling ratio. After the hydrothermal reactions, the products are washed by acetone, and finally dried at 80 °C for characterization. The calcination is conducted in Ar atmosphere. The morphologies of the products are observed by scanning electron microscopy (SEM, Hitachi S-530). The structures and composition analysis are characterized by X-ray diffraction (XRD, Rigaku RTP300 RC), and transmission electron microscopy equipped with an energy dispersive X-ray spectrometer (TEM/EDX, JEOL JEM-2010). The TGDTA (Seiko TG/DTA 6300) is conducted in N2 atmosphere. Fourier transform infrared spectrometry (FTIR) is recorded between 4000 and 400 cm-1 (Bruker, VERTEX 70).

3. Results and Discussion 3.1. Hydrothermal Synthesis of the Rhodium Sulfides. Figure 1 shows the XRD patterns of rhodium sulfide powders synthesized at 220 and 400 °C for 5 h with a S/Rh ratio of 0.9, 1.5, and 3, respectively. The result shows that the obtained products were poorly crystallized at 220 °C. However, the products were crystallized at 400 °C for 5 h (Figure 2), and the crystallized phases of the products were different depending on the S/Rh ratio. Rh17S15 was synthesized with a S/Rh ratio of 0.9 and Rh2S3 with a S/Rh ratio of 3 (JCPDS No: 35-0736 for

Rh2S3 and 73-1443 for Rh17S15), while both Rh2S3 and Rh17S15 were obtained with a S/Rh ratio of 1.5. These results indicate that the phase composition of rhodium sulfides is closely related to the S/Rh molar ratio in the starting materials. It is interesting that Rh17S15 was synthesized with nearly stoichiometric S/Rh ratio. However, formation of pure Rh2S3 needed excessive sulfur in the starting materials and the stoichiometric S/Rh ratio of 1.5 just gave the mixed phases of Rh2S3 and Rh17S15. Figure 3 shows the SEM photos of rhodium sulfides synthesized at 220 and 400 °C with S/Rh ratios of 0.9, 1.5, and 3. The particle size of rhodium sulfides synthesized at 220 °C ranges from several micrometers to 100 µm and their surface is quite smooth. For the products synthesized at 400 °C for 5 h, a rough surface is observed for Rh17S15 with a S/Rh ratio of 0.9. It is noted that all products synthesized at both 220 and 400 °C have a shape similar to that of the starting Rh6(CO)16 particles as shown in Figure 5a. Figure 4 shows TEM photos of the Rh2S3 and Rh17S15 particles synthesized at 400 °C for 5 h with a S/Rh ratio of 0.9 and 3, respectively. Small particles are observed around a large particle, which suggests that the large particle may be an aggregate of the small particle. The small particle size of Rh17S15 is about 50 nm. This result is consistent with the rough surface of the Rh17S15 particles

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Figure 4. TEM pictures of the hydrothermally synthesized rhodium sulfide powders. (a) S/Rh ) 0.9, 400 °C for 5 h; (b) S/Rh ) 3, 400 °C for 5 h.

observed in the SEM photo. HRTEM photos show clear lattice fringes for both Rh2S3 and Rh17S15 particles, which proves the high crystallinity of the products synthesized at 400 °C (Figure 4). Energy dispersive X-ray analysis shows that the S/Rh ratio of Rh17S15 particles is 0.89 while that of Rh2S3 particles is 1.8. The large S/Rh ratio of Rh2S3 particles indicates that Rh2S3 particles include amorphous sulfur. This result is supported by the large weight loss by calcination of Rh2S3 particles at low temperatures below 350 °C (Figure 10a). 3.2. Formation Mechanism of Rhodium Sulfides. Figure 5 shows FTIR spectra of Rh6(CO)16 and hydrothermally obtained rhodium sulfides. The absorption peaks at 2071 cm-1 and 1797 cm-1 belong to the stretching vibrations of terminal carbonyls and bridging carbonyls, respectively.11 It was found that carbonyl related absorption peaks disappear after hydrothermal reactions at above 220 °C. Furthermore, the hydrothermal treatment of Rh6(CO)16 in pure water at 400 °C for 5 h resulted in formation of Rh metal, and the product did not have the morphology of the original Rh6(CO)16 particles (Figure 6). These results indicate that the hydrothermal treatment decomposes the rhodium carbonyl and destroys the rhodium carbonyl particles. On the other hand, most of the rhodium sulfide products obtained by the hydrothermal reactions well inherited the morphology of the Rh6(CO)16 precursor and had a smooth surface. According to this phenomenon, the formation mechanism of the rhodium sulfide under hydrothermal condition is proposed by the process including rhodium carbonyl decomposition and sulfur incorporation. Water at high temperatures removes the CO ligand from Rh6(CO)16, and ionizes sulfur to HS- or S2because H2S gas was detected after the reactions. The ionized sulfur is then transferred to the particle surface and diffuses

Figure 5. FTIR spectrum of Rh6(CO)16 and hydrothermally obtained rhodium sulfides. (a) Rh6(CO)16; (b) S/Rh ) 3, 220 °C for 10 h; (c) S/Rh ) 3, 400 °C for 5 h.

into the particles. The sulfur incorporation plays a role in keeping the morphology of the particles with a smooth surface. The rough surface of Rh17S15 may be ascribed to a large amount of released CO with deficient sulfur incorporation (Figure 3d). The rhodium sulfide particles obtained by the hydrothermal treatments retain the original morphology of rhodium carbonyl large particles. This result also suggests that the sulfur incorporation may follow a “surface-to-inside” diffusion

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Figure 8. XRD patterns of the products hydrothermally synthesized with S/Rh ) 3 at 400 °C for (a) 0.5 h, (b) 1 h, and (c) 2 h.

Figure 6. SEM pictures of the Rh6(CO)16 raw materials (a) and Rh metal obtained by hydrothermal treatment of Rh6(CO)16 in pure water at 400 °C for 5 h (b).

Figure 7. XRD patterns of the products hydrothermally synthesized at 220 °C for (a) 2 h and (b) 5 h with S/Rh ) 3 followed by calcination at 400 °C for 5 h.

Figure 9. SEM photos of the products hydrothermally synthesized with S/Rh ) 3 at 400 °C for (a) 0.5 h and (b) 1 h.

process. Figure 7 shows the XRD patterns of the rhodium sulfide powders (S/Rh ) 3) synthesized at 220 °C for 2 and 5 h followed by calcinations at 400 °C for 5 h in Ar. The low crystalline products obtained by hydrothermal reactions crystallized to rhodium sulfides by the calcination. After the hydrothermal reaction for 2 h, the calcined product was a mixture of Rh17S15 and Rh2S3 (Figure 7a). On the other hand, pure Rh2S3 was produced after 5 h hydrothermal reaction followed by the calcination (Figure 7b). The above XRD results prove that sulfur content in the products increases with increasing reaction time by the diffusion process, which accordingly affects the phase composition. Formation of Rh2S3 needs abundant sulfur content in the starting materials because of the slow diffusion rate of sulfur. Figure 8 shows XRD patterns of the products hydrothermally synthesized with S/Rh ) 3 at 400 °C for different reaction

intervals. The product obtained by the reaction for 0.5 h is nearly amorphous and mixed phases of Rh17S15 and Rh2S3 are formed by the reaction for 1 h. Pure Rh2S3 is synthesized for 2 h. These results show the diffusion of sulfur continues during hydrothermal reaction even at 400 °C. The release rate of CO in rhodium carbonyl is also related to the reaction temperature. The particle surface of the products obtained at 220 °C (S/Rh ) 3) keeps smooth. However, the amorphous product synthesized at 400 °C for 0.5 h with S/Rh ) 3 first experiences a “rough surface” stage (Figure 9a), and then the rough surface finally changes to be smooth after the reaction for 1 h (Figure 9b). This phenomenon is explained by the difference of the CO decomposition rate and S incorporation rate. The CO decomposition rate is comparable with the S incorporation rate at low reaction temperature but slightly higher than the S incorporation rate at 400 °C.

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Figure 11. XRD patterns of low crystalline product (S/Rh ) 3) hydrothermally synthesized at 220 °C for 10 h followed by calcination. (a) 700 °C for 1 h; (b) 700 °C for 5 h.

Figure 10. TG-DTA curves of the low crystalline products hydrothermally synthesized at 220 °C for 10 h. (a) S/Rh ) 3; (b) S/Rh ) 0.9.

3.3. Thermal Analysis of the Rhodium Sulfides. Figure 10 shows the thermogravimetric-differential thermal analysis (TG-DTA) results of the low crystalline products synthesized at 220 °C for 10 h. For the product with S/Rh ) 3, one exothermic peak was observed at 400 °C (Figure 10a), which was proven to be the crystallization of Rh2S3 because the XRD results show that the low crystalline product crystallized to Rh2S3 by calcinations at 400 °C in Ar. For the product with S/Rh ) 0.9, one exothermic peak was observed at 311 °C (Figure 10b) which corresponds to the formation of the Rh17S15 phase because the XRD results show the crystallization of Rh17S15 occurred by calcinations at 330 °C in Ar. However, the weight loss of the low crystalline product with S/Rh ) 3 is 19.5%, which is much larger than 3.5% of the low crystalline product with S/Rh ) 0.9. Besides the residual CO and water, most of the weight loss from the low crystalline product with S/Rh ) 3 seems to be caused by the release of sulfur because of its high volatility with a boiling point at 444.6 °C. After calcination of the low crystalline product with S/Rh ) 3 for 1 h at 700 °C, mixed Rh3S4 and Rh17S15 phases were obtained in the product (Figure 11a, JCPDS No. 70-5129 for Rh3S4) and the calcination for 5 h at 700 °C gave pure Rh17S15 phase (Figure 11b). These results indicate that the Rh2S3 is transformed to Rh3S4 and Rh17S15 by loss of sulfur during heating in opening condition. Thus, in order to obtain pure Rh2S3, the temperature and time should be carefully controlled during thermal treatment in opening condition. It should be noted that although Rh3S4 and Rh17S15 phase were formed during calcination of the low crystalline

product with S/Rh ) 3, any exothermic peaks corresponding to crystallization of Rh3S4 and Rh17S15 phase were not observed in DTA curves except for Rh2S3 crystallization. This may be explained by the slow release of sulfur from the products. The above results show that crystalline Rh2S3 and Rh17S15 can be synthesized by both hydrothermal reactions and thermal treatments in Ar. However, Rh2S3 is not stable at high temperatures in opening conditions because of the release of sulfur during thermal treatments, which can explain that crystalline Rh2S3, Rh3S4, and Rh17S15 were synthesized in sealed conditions in the previous reports. The key factor to control the phase compositions of rhodium sulfides is to control the sulfur content during the process, because sulfur diffusion and sulfur evaporation should be considered for hydrothermal reactions and thermal treatments, respectively.

4. Conclusions Well-crystallized rhodium sulfides including Rh2S3 and Rh17S15 can be synthesized by hydrothermal treatment at 400 °C for 5 h with a S/Rh ratio of 3 and 0.9, respectively. The growth mechanism is proposed by decomposition of the CO ligand and sulfur incorporation. The sulfur content in the products increases with increasing hydrothermal reaction time, reaction temperature, and S/Rh ratio in the starting materials, which accordingly affects the phase compositions of the products obtained by hydrothermal reactions and heat treatments of the hydrothermally obtained products. From thermal analysis, the low crystalline product synthesized at 220 °C with S/Rh ) 3 can be crystallized to Rh2S3 at 400 °C by calcination in Ar, while the low crystalline product synthesized at 220 °C with S/Rh ) 0.9 can be crystallized to Rh17S15 at about 310 °C. Sulfur release is observed during thermal treatment and Rh2S3 is further transformed into Rh17S15 by calcination at 750 °C for 5 h in Ar. In order to synthesize pure and crystalline Rh2S3 and Rh17S15, the process conditions, such as the starting composition, temperature and time, should be carefully controlled both for hydrothermal reactions and calcinations. Furthermore, the hydrothermal approach could also provide a potential route to fabricate rhodium sulfide films with controllable phases by deposition rhodium carbonyl or rhodium metal on substrates followed by hydrothermal sulfuration, because the deposited rhodium precursor could retain their morphology due to the surface to inside diffusion growth mechanisms.

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Acknowledgment. The authors are very grateful for the support of the Youth Chengguang Project of Science and Technology of Wuhan City of China under Grant No. 200850731354, and for the help from Analytical and Testing Center in Huazhong University of Science & Technology.

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