Synthesis of Highly Dispersed Ruthenium Nanoparticles Supported on

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Synthesis of Highly Dispersed Ruthenium Nanoparticles Supported on Activated Carbon via Supercritical Fluid Deposition Yimin Zhang, Haoxi Jiang, Yanhui Wang, and Minhua Zhang* Key Laboratory for Green Chemical Technology of Ministry of Education, R&D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, China ABSTRACT: Highly dispersed ruthenium (Ru) nanoparticles supported on activated carbon (AC) were controllably synthesized by supercritical fluid deposition (SFD). The Ru nanoparticles prepared by SFD presented a smaller mean particle size than particles prepared by the ethanol impregnation method. The effects of temperature, pressure, and Ru loading on catalyst preparation were systematically investigated. As any one of above parameters increased, Ru mean particle sizes always decreased first and then increased. The smallest mean particle size (1.6 nm) was obtained at Ru loading of 2%, 45 °C, and 10 MPa. On the scale of Ru mean particle size from 1.6 to 3.9 nm, the catalysts with smaller Ru mean particle size presented higher catalytic activity. The interactions between the Ru precursor and AC were characterized by TG−MS, TPR, and FTIR spectroscopy. The results obtained have revealed the reduction of the carbonyl (CO) in Ru precursors after depositing on AC with the aid of SCCO2.

1. INTRODUCTION Transition metal nanoparticles (NPs), which possess unique chemical and physical properties, have been widely employed as catalysts in various reactions because of their high activities and selectivities which result from their small particle sizes and large surface areas.1−3 Unfortunately, metal NPs are unstable because they tend to agglomerate, which results in the loss of their catalytic activity.4,5 It has been proven that the presence of a support can prevent metal NPs from mobilizing and aggregating, especially when there are strong interactions between the NPs and the support.6 Ru NPs have attracted a great deal of attention because even under mild conditions they are one of the most active catalysts among the noble metals.7−15 In recent years, Ru NPs including both unsupported Ru NPs9,16,17 and Ru NPs supported on carbon nanotubes,18−21 silica,22 aluminum oxide,23 silica nanotubes,24 or other matrixes6,25 have been synthesized and investigated. Supported metal NPs have been prepared via various pathways such as impregnation,23,24 coprecipitation,26 ion exchange,27 and chemical vapor deposition.19 However, obtaining a good dispersion of metal NPs is still a serious challenge. Thus, the development of new methods to obtain well-dispersed metal NPs on supports is of great interest. In 1995, Watkins et al.28 reported supercritical fluid deposition (SFD) as a method to prepare metal NPs. This method is proposed on the basis of the chemical vapor deposition method. Supercritical fluids (SCFs) have unique properties such as gas-like diffusivities, liquid-like densities and dissolving capacities, low viscosities, near-zero surface tensions, and tunability.29 Among all supercritical fluids, supercritical carbon dioxide (SCCO2) is the most promising because it exhibits many favorable properties such as abundance, low cost, nonflammability, nontoxicity, and, in particular, relatively moderate critical pressure and temperature values (Tc = 31.1 °C, Pc = 7.38 MPa). Its high diffusivity and low viscosity make it easy to diffuse on the surface and in the pores of porous © 2014 American Chemical Society

supports for metal precursors dissolved in SCCO2. Compared to water and organic solvents, SCCO2 with near-zero surface tension can wet the outer and inner surfaces of supports more easily. Consequently, compared with traditional impregnation methods using water or organic solvents, SFD is better at transferring the metal precursors into the support pores and dispersing them on support surfaces. Small and highly dispersed metal particles can form more easily with the aid of SCCO2. In recent years, the preparation of metal NPs using SFD has been widely investigated.21,30−35 Liu et al.21 prepared Ru nanoparticles supported on carbon nanotubes (CNTs) in supercritical water and investigated the effect of the loading of metal precursor and the preparation temperature on the particle size. When the ratio of RuCl3·3H2O to CNTs was 1:1, the average particle size was 5 nm; when this ratio was higher than 1:1, the mean particle size increased to dozens of nanometers. In addition, Ru particles fabricated at 450 °C were larger than the particles prepared at 400 °C. Popovska et al.19 obtained supported metal catalysts using SFD and obtained higher dispersions and activities than those catalysts prepared by wet impregnation. Antonetti et al.20 prepared Ru NPs supported on both functionalized and untreated CNTs by SFD. The particle sizes of the Ru NPs on CNTs treated with HNO3 and HCl were nearly identical, but the former ones had a narrower particle size distribution. In this work, highly dispersed Ru NPs supported on activated carbon (AC) were controllably synthesized by a SFD method. For understanding the importance of SCCO2 in catalyst preparation, some catalysts for comparison were prepared by the ethanol impregnation method and characterized. In the process of SFD, we intend to control effectually the size and Received: Revised: Accepted: Published: 6380

July 18, 2013 February 27, 2014 March 28, 2014 March 28, 2014 dx.doi.org/10.1021/ie404155u | Ind. Eng. Chem. Res. 2014, 53, 6380−6387

Industrial & Engineering Chemistry Research

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(Parr, 4575). The feedstock simulated a reaction mixture of industrial synthesis of ethyl acetate using ethanol. It included acetaldehyde (0.36 mol %), butanone (0.23 mol %), H2O (1.67 mol %), ethanol (74.09 mol %), and ethyl acetate (23.65 mol %). The reaction mixture was periodically sampled and analyzed by gas chromatography−mass spectrometry (GC− MS; Agilent 5973-6890).

dispersion of Ru particles by changing the preparation conditions. Consequently, the influence of the SFD preparation conditions including Ru loading, temperature, and pressure on particle size and dispersion was systematically investigated. In addition, the catalytic activities of the synthesized materials were tested for the hydrogenation of butanone. Moreover, in order to understand the advantage of SCCO2 in preparation of small size and highly dispersed metal nanoparticles better, the interactions between the Ru precursors and AC were studied.

3. RESULTS AND DISCUSSION 3.1. Effect of Different Preparation Methods and Metal Precursors on Ru NP Size and Catalytic Activity. A series of catalysts using different precursors were prepared by different methods and characterized. The average particle size and the conversion of butanone as the catalytic activity are shown in Table 1, and the morphology and particle size

2. EXPERIMENTAL SECTION 2.1. Materials. Ruthenium trichloride hydrate (RuCl3· 3H2O, 99%) and ruthenium(III) acetylacetonate (Ru(acac)3, 99%) were purchased from Heraeus, Shanghai, and Jiangsu Industrial Additives Co., respectively. AC from Tangshan New Activated Carbon Co., China, was used as the support for the Ru NPs. Prior to use, the AC was dried under vacuum at 100 °C and 10 mbar for 8 h to remove any adsorbents on the surface. Ethanol (99.9%), ethyl acetate (99.9%), and butanone (99.9%) were purchased from Tianjin Jiangtian Chemical Technology Co. Acetaldehyde aqueous solution (40%) was supplied by Tianjin Standard Co., and CO2 (>99.9%) and H2 (99.999%) were provided by Tianjin Liufang Gas Co. With the exception of the AC, all the gases, reagents, and materials were used as received without further treatment. 2.2. Fabrication and Characterization of Supported Ru NPs. 2.2.1. Preparation of Ruthenium Catalyst Precursors. The desired amount of ruthenium precursor was dissolved in ethanol, and then the solution was mixed with AC (∼5 g). The mixture was then placed in an autoclave (Parr 4575), into which CO2 was added until the desired pressure was achieved. The desired temperature was set, and 4 h later the product was removed from the autoclave and dried under vacuum for 4 h. This product was the catalyst precursor. A vessel (Thur) with an adjustable volume was used to investigate the effects of temperature and pressure independently. For the purpose of comparison, Ru/AC catalysts using RuCl3·3H2O and Ru(acac)3 as metal precursors were also prepared using the ethanol impregnation method. 2.2.2. Reduction of Ruthenium Catalyst Precursors. The ruthenium catalyst precursors were reduced in H2−N2 (vol %, 10−90) for 3 h in a fixed bed reactor at 350 °C. The resulting products were stored in a desiccator. 2.2.3. Characterization of Catalysts. The structure and morphology of the synthesized catalysts were examined by high-resolution transmission electron microscopy (HRTEM; FEI, Tecnai G2F20). The average size and size distribution of the Ru NPs were determined from the HRTEM images with statistical analysis, where the total number of Ru NPs in every HRTEM image was 100. The dispersion of the Ru NPs was determined from measuring the consumption of H2,36 which was carried out with an AutoChemII2920 chemisorption analyzer (Micromeritics). Thermogravimetric (TG) profiles and mass spectra were measured with a TGA/DSTA851 (METTLER) equipped with a mass spectrometer (ThermoStar, Balzers) which was used to determine the composition of the effluent gas. Temperature-programmed reduction (TPR) was also carried out with the AutoChemII2920 chemisorption analyzer. A Nicolet 560 FTIR infrared spectrometer was used to collect the Fourier transform infrared (FTIR) spectra. 2.3. Catalytic Activity Measurements. The catalytic activity for hydrogenation was determined in a stirred autoclave

Table 1. Effect of Preparation Methods and Precursors on Particle Size and Catalytic Activitiesa Ru NP series

prep method

metal precursor

av particle size (nm)b

butanone conv (%)

1 2 3 4

impregnation impregnation SFD SFD

RuCl3·3H2O Ru(acac)3 RuCl3·3H2O Ru(acac)3

− 3.8 3.2 2.5

20.1 27.0 29.1 35.4

All catalysts were prepared at 1% Ru loading, 45 °C, and 10 MPa. Particle size of sample 1 could not be determined because of aggregation.

a b

distributions are shown in Figure 1. Samples 1 and 3 were prepared by impregnation and SFD respectively using the same metal precursor RuCl3·3H2O. Both Table 1 and the TEM images in Figure 1a,b show that the Ru NPs prepared by SFD are smaller and have a narrower particle size distribution. The

Figure 1. TEM images of Ru/AC catalysts prepared using SFD and impregnation methods: (a) Ru/AC imp‑R uCl 3 ·3H 2 O ; (b) Ru/ ACSFD‑RuCl3·3H2O; (c) Ru/ACimp‑Ru(acac)3; (d) Ru/ACSFD‑Ru(acac)3. Deposition conditions : initial metal loading 2%, temperature 45 °C, and pressure 10 MPa. 6381

dx.doi.org/10.1021/ie404155u | Ind. Eng. Chem. Res. 2014, 53, 6380−6387


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Table 2. Effect of Ru Loading on Particle Size, Dispersion, and Butanone Conversiona

a

Ru NP series

prep method

metal precursor

init loading (wt %)

av particle size (nm)b

dispersion (%)

butanone conv (%)

5 6 7 8 9

SFD SFD SFD SFD SFD

Ru(acac)3 Ru(acac)3 Ru(acac)3 Ru(acac)3 Ru(acac)3

0.5 1 2 3 4

− 2.5 1.6 3.5 3.9

2.89 12.74 18.84 15.97 15.28

19.9 36.1 76.1 64.8 51.9

All catalysts were prepared at 45 °C and 10 MPa. bParticle size of sample 1 could not be determined because of aggregation.

same trend is observed for the samples prepared with the Ru(acac)3 precursor (samples 2 and 4, Figure 1c,d), although the differences are smaller. This is because, compared to the polar ethanol solvent, nonpolar SCCO2 possesses many unique properties such as high diffusivity like gas, high-dissolving capacity like liquid, low viscosity, near-zero surface tension, and hydrophobicity. Its high diffusivity and low viscosity decrease the mass transfer resistance and then make it easy to diffuse on the surface and in the pores of porous supports for Ru precursor solution. Compared to ethanol solvent, SCCO2 with near-zero surface tension and hydrophobicity can wet the outer and inner hydrophobic surfaces of AC more easily. The effect of the metal precursor on the particle sizes in the samples prepared by SFD can be seen in Figure 1b,d. The Ru NPs prepared with Ru(acac)3 as the precursor (sample 4) are more uniformly distributed than those prepared using RuCl3· 3H2O (sample 3). In addition, as shown in Table 1, the Ru NPs in sample 4 have a mean diameter of 2.5 nm, which is slightly smaller than the mean diameter of NPs in sample 3, which is 3.2 nm. This result could be related to the fact that organic metal salts have a higher solubility than the inorganic metal salts in SCCO2.37 In the same conditions, more Ru(acac)3 than RuCl3·3H2O dissolves in SCCO2 and then disperses on the surface and in the pores of AC with the aid of SCCO2 with unique properties. Another reason may be related to water crystals in RuCl3·3H2O. Before the deposition, RuCl3·3H2O was dissociated in ethanol to form a solution consisting of Ru3+, Cl−, and H2O. With the CO2 introduced, H2O reacted with CO2 to generate the ions of CO32− and HCO3−.38 During the deposition process, the free groups of CO32−, together with H2O molecules, could coordinate directly to Ru3+ by bridged oxygen to form a solid compound. Because the coordination ability of CO32− with metal ions was stronger than the coordination ability of Cl−, the compound could form stably and precipitate from SCCO2−ethanol homogeneous fluid on the surface of the AC. The strong coordination ability of CO32− resulted from CO2 with Ru3+ can inhibit the Ru3+ from diffusing in SCCO2 and finally lead to the poor dispersion of Ru particles. The catalytic activity of the catalyst for the hydrogenation of butanone was also tested. The feedstock simulated an reaction mixture of industrial synthesis of ethyl acetate using ethanol. It included acetaldehyde, butanone, water, ethanol, and ethyl acetate. According to the results of GC−MS, the hydrogenation products included ethanol, butanone, ethyl acetate, butanol, and water. In the process of hydrogenation, there are only two chemical reactions happening, which are hydrogenation of acetaldehyde to ethanol and hydrogenation of butanone to butanol. For the hydrogenation of the key component butanone, there are no other byproducts except the desired product butanol. Consequently, the butanol selectivity is 100% for all catalytic reactions in this work. As shown in Table 1, the catalytic activity increased with the decrease in Ru average

particle size. This coincides with previous results that an increase in the metal particle size results in a loss of catalytic activity.4,5,39 The best explanation of this phenomenon is that smaller size and better dispersion of metal particles can enhance the exposed metal surface area followed by the increase of catalytic activity. 3.2. Effect of Ru Loading on Ru NP Dispersion and Catalytic Activity. Table 2 and Figure 2 show the particle

Figure 2. TEM images of catalysts with different Ru loadings. Metal loading (wt %): (a) 0.5, (b) 1, (c) 2, (d) 3, and (e) 4. Deposition conditions: metal precursor Ru(acac)3, temperature 45 °C, and pressure 10 MPa.

sizes and dispersion for catalysts prepared with different Ru loadings. Figure 3 shows a graphical representation of the effect of Ru loading on the particle size, the dispersion, and the butanone conversion. Obviously all these parameters changed considerably as the Ru loading increased. The particle size of Ru showed a significant difference with the increase in the amount of Ru(acac)3, which can be recognized from the TEM images and the inset particle size distributions shown in Figure 2. The Ru particles tended to aggregate at 0.5% Ru loading, which could be understood from the perspective of catalyst preparation process. First, Ru(acac)3 was dissolved in ethanol and then the solution was mixed with 6382



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Figure 3. Effect of metal loading on particle size, dispersion, and conversion of butanone. Pressure 4 MPa, temperature 45 °C, time 3 h, and mass ratio of catalyst to feed 1%.

by the increase of catalytic activity. However, it is reported that the optimum Ru particle size largely depends on the molecular size of the reactant. Consequently, the Ru size effect is not the same for different reactants. When the Ru particle size is coincident with the molecular size of the reactant, the catalytic activity can be the highest. For butanone hydrogenation reaction, the molecular size of butanone is less than 1 nm. As a consequence, the catalytic activity for butanone conversion increased with the decreasing of Ru particle size from 3.9 to 1.6 nm. 3.3. Effects of Temperature and Pressure on Size of Ru NPs. The particle size data and TEM images for samples prepared at different pressures and temperature are shown in Table 3 and Figure 4, respectively. Actually, the CO2 phase

the AC. Subsequently, the mixture was placed in an autoclave, into which CO2 was added. Before the addition of CO2, the low concentration solution could be introduced into the micropores by capillary attraction which increased with the decrease in diameter of pores. For the low concentration of precursor solution and the large micropore volume, most Ru(acac)3 was introduced into micropores and formed an aggregation. After the addition of CO2, Ru(acac)3 in micropores could not dissolve in SCCO2 for the strong capillary force. Consequently, the SCCO2 had a slight influence on the particle deposition at 0.5% Ru loading, which led to the serious aggregation of particles. When Ru loading reached 1%, some micropores could not be filled with Ru(acac)3 solution and also be blocked by Ru(acac)3 before the addition of CO2 for the increase in precursor concentration in ethanol. Precursors out of micropores could dissolve in SCCO2−ethanol homogeneous fluid and disperse well, taking advantage of the high diffusivity and low viscosity of SCCO2. As a consequence, the particles did not form a serious aggregation in contrast to particles prepared at 0.5% Ru loading. The particle size reached a minimum value at 2% Ru loading. At this condition, most of the Ru(acac)3 solution could not penetrate into the mirocropores before the addition of CO2 for the high precursor concentration in ethanol. Most Ru(acac)3 would dissolve in SCCO2−ethanol homogeneous fluid, then enter the micropores, and finally form a good dispersion for the high diffusivity and low viscosity of SCCO2. When the Ru loading further increased, the particle size increased. The reason may be that when the Ru loading exceeded the optimum value (2%), more Ru precursors entered the macropores and mesopores, which increased the Ru particle size. The dispersion of the particles shows a different trend from that of the particle size. It first increases with loading and then decreases after reaching a maximum at 2% Ru loading. Dispersion is defined as the number of metal atoms exposed divided by the total number of metal atoms in the bulk, and this number theoretically increases as the particle size decreases. The catalytic activity for butanone conversion has a trend similar to the dispersion trend and contrary to the particle size trend. This is in agreement with other reported results that larger metal particle sizes can be accompanied by a loss in catalytic activity.4,5,39 Smaller size and better dispersion of metal particles can enhance the exposed metal surface area followed

Table 3. Effects of Temperature and Pressure on Particle Sizea Ru NP series

temp (°C)

press. (MPa)

av particle size (nm)

10 11 12 13 14 15 16 7 17 18 19

25 31.1 50 60 45 45 45 45 45 45 45

10 10 10 10 7.38 8 9 10 13 16 20

3.5 1.8 3.1 3.3 5.5 2.8 1.9 1.6 1.5 4.2 4.4

a

All catalysts were prepared at metal loading of 2% with Ru(acac)3 used as metal precursor.

state can be changed by tuning the temperature and pressure. In order to understand the influence of SCCO2 on particle deposition better, the experiments were designed to investigate the effect of different CO2 phase states on the size of Ru NPs. For example, sample 10 was prepared in liquid CO2 (25 °C and 10 MPa), whereas sample 11 was prepared at the critical temperature (31.1 °C) and 10 MPa and sample 14 was prepared at 45 °C and the critical pressure (7.38 MPa). The others samples were prepared in SCCO2. 6383



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Figure 5. Lattice image and electron diffraction pattern of Ru nanoparticle. Deposition conditions: metal precursor Ru(acac)3, metal loading 2%, pressure 10 MPa, and temperature 45 °C.

selected area in the inset of Figure 5 reveals clear diffraction spots, implying the appearance of metal Ru crystal.21 As shown in Table 3, the Ru NPs in sample 14 which were prepared at the critical pressure were large, and the size decreased considerably as the pressure increased to 13 MPa, which is regarded as a near-supercritical area. The reason may be related to the solubility of Ru(acac)3 in SCCO2. It has been reported that the increase of pressure is followed by the enhancement of solubility of Ru(acac)3 in SCCO2.42,43 The Ru precursors added just dissolved in SCCO2 completely, where the Ru NPs had a minimum average size. The particles became larger at pressures higher than 13 MPa, because higher pressures inhibit diffusion of a SCF through the pores of a support and weaken the dispersion of the metal NPs. 3.4. Interactions between Metal Precursor and AC. In order to understand the small size and good dispersion of Ru particles supported on the AC prepared by SFD, TG−MS, TPR, and FTIR spectroscopy were employed to investigate the interactions between precursors and the AC. 3.4.1. TG−MS Characterization. The TG and MS profiles of the AC, Ru(acac)3, and Ru(acac)3/AC prepared by SFD are shown in Figure 6. The AC had a continuous mass loss over the entire temperature range which according to the mass fragments (18, 17) in the MS profile of AC corresponds to absorbed water. According to the MS profiles of Ru(acac)3 and Ru(acac)3/AC, the decomposition of Ru(acac)3 occurred in the range 150−300 °C, whereas decomposition of Ru(acac)3/AC was ca. 80−610 °C. The wider decomposition range may be from the different states of Ru(acac)3 distributed on the AC surface. Acetylacetone had mass fragments of 15, 43, 85, and 100, and these fragments all appeared at the same temperature during the decomposition of the unsupported Ru(acac)3. However, the Ru(acac)3 loaded on the AC decomposed in a more complicated manner. In the mass spectrum of Ru(acac)3/ AC only two mass fragments of 15 (CH3) and 43 (may be CHCHOH) appeared, and these occurred at different temperatures. Moreover, the weight of Ru(acac)3/AC declined

Figure 4. TEM images of catalysts prepared at different temperatures (°C): (a) 25, (b) 31.1, (c) 45, (d) 50, and (e) 60. (f) Electron diffraction pattern of catalyst at 45 °C. Deposition conditions: metal precursor Ru(acac)3, initial metal loading 2%, and pressure 10 MPa.

The results show that the phase of CO2 greatly influenced the size of the Ru particles. The particles in sample 10 were seriously agglomerated and much larger than those in samples 7 and 11, which were prepared at or above the critical temperature. CO2 was in the gas state and not a SCF during the preparation of sample 10. Consequently, the preparation method of sample 10 was similar to the wet impregnation with ethanol. In a SFD process, the temperature affects both the diffusion of the metal precursor into the support pores and the adsorption of the precursor onto the support. On one hand, an increase in temperature improves the dispersion of the Ru precursor since the diffusion coefficient of SCCO2 increases with temperature, which enables the Ru precursors to diffuse into the AC pores more efficiently. On the other hand, an increase in temperature weakens the adsorption of the Ru precursor on the surface of the pores, which is detrimental to a good dispersion of the Ru precursor in the pores. Consequently, there is an optimal temperature where these two phenomena form an balance. From Table 3, the optimum temperature is 45 °C. Figure 5 shows the lattice image and the electron diffraction pattern of nanoparticles via HRTEM, which provides further insight into the morphologies and microstructure of Ru−AC composites. The lattice image presents the crystalline structure of the nanoparticles.40 The aligned lattice fringes of the Ru nanoparticles are clearly illustrated in the image with adjacent fringe spacing of approximately 0.21 nm. This value is consistent with the separation between (002) plates in Ru crystal.41 The electron diffraction pattern of the corresponding 6384



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Figure 6. TG−MS profiles of AC, Ru(acac)3, and Ru(acac)3/AC during a reduction process.

The reducibility of the AC, Ru(acac)3/AC, and Ru(acac)3 were investigated by TPR, and the results are shown in Figure 8. In the case of the AC, the reduction curve has a characteristic

less abruptly than that of Ru(acac)3, which reflects the desorption of the product from the AC. In the derivative thermogravimetric analysis (DTG) curves, Ru(acac)3 has three peaks, whereas Ru(acac)3/AC only has two. These results indicated that the AC changes the decomposition pathway of Ru(acac)3. In other words, there is an interaction between the AC and Ru(acac)3. 3.4.2. FTIR and TPR Analysis. The FTIR spectra of AC, Ru/ AC, Ru(acac)3, and Ru(acac)3/AC are presented in Figure 7. The Ru(acac)3 spectrum has a characteristic absorption for carbonyl groups at 1517 cm−1, whereas the Ru(acac)3/AC does not have this peak. This may be due to the conversion of CO groups in Ru(acac)3 into C−O groups by the AC.

Figure 8. H2-TPR profiles of AC, Ru(acac)3, and Ru(acac)3/AC.

peak for the hydrogen consumption by oxygen containing groups on the surface of the AC at 630 °C. For Ru(acac)3/AC, the first two peaks at 208 and 224 °C may be due to the reduction of the highly dispersed metal precursors on the supports, i.e., the breaking of the Ru−O bonds. The Ru−O bond in Ru(acac)3/AC broke at a lower temperature than that in Ru(acac)3 (256 °C), which may be because the Ru−O bond is weakened in the presence of the AC, which results in a decrease in the reduction temperature. The structure of Ru(acac)3 is shown in Figure 9, and it contains three functional groups (CC, CO, Ru−O) that can consume hydrogen. The FTIR results showed that the C

Figure 7. FTIR transmission spectra of AC, Ru/AC, Ru(acac)3/AC, and Ru(acac)3 samples. 6385



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Figure 9. Structure of Ru(acac)3.

O groups in Ru(acac)3were no longer present after interacting with the AC. In other words, in contrast to Ru(acac)3, there was no hydrogen consumption by CO in Ru(acac)3/AC. Therefore, in the TPR profile of Ru(acac)3, the hydrogen consumption peak at 374 °C is ascribed to CO. The hydrogen consumption peaks at 453 °C in Ru(acac)3 and 455 °C in Ru(acac)3/AC belong to CC. The hydrogen consumption at ca. 470−770 °C in Ru(acac)3/AC may be from oxygen containing groups on the AC surface.

4. CONCLUSIONS The preparation method, metal precursor, ruthenium loading, temperature, and pressure all have a meaningful influence on the controllable synthesis of metal ruthenium particles. The particle sizes always decrease first and then increase with increasing ruthenium loading, temperature, and pressure. The use of a supercritical fluid is effective to produce small and highly dispersed particles. An investigation of the interaction between the metal precursor and the support showed that the CO bonds in Ru(acac)3 were broken, which may be caused by interactions with the AC. Results obtained in this paper have emphasized the capability of SFD for porous supports and hydrophobic materials, offering promises of further applications besides catalyst preparation. In the future, further studies of the influencing factors and the metal precursor−support interaction mechanisms may have a positive influence on the decrease of consumption efficiency and the increase of adsorption efficiency of noble metals.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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Synthesis of Highly Dispersed Ruthenium Nanoparticles Supported on

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