Advances in Pd Nanoparticle Size Decoration of Mesoporous Carbon Spheres for Energy Application
© Zielinska et al. 2015
Received: 26 August 2015
Accepted: 9 October 2015
Published: 30 October 2015
Pd nanoparticles with different sizes and diameter distributions were successfully deposited on the surface of disordered mesoporous carbon spheres (DMHCS). The size and diameter distribution of the Pd particles were controlled by the application of different experimental conditions. Two methods of synthesis (reflux and impregnation) and two Pd precursors (palladium (II) acetyloacetonate (Pd(acac) 2) and palladium (II) acetate (Pd(OAc)2)) were investigated and compared for the preparation of Pd-decorated DMHCS. The hydrogen storage properties of the pristine DMHCS and Pd-modified DMHCS at 40 °C and a pressure range of 0–45 bar were studied. The results showed that Pd-supported carbon samples synthesized in the presence of Pd(OAc)2 exhibited enhanced hydrogen storage capacity in respect to the pristine DMHCS. The maximum hydrogen storage of 0.38 wt.% exhibited the sample with the Pd nanoparticle diameter distribution of 2–14 nm and the average Pd crystallite size of 7.6 nm. It was found that the Pd nanoparticle content, size, and diameter distribution have a noticeable influence on H2 storage capacity.
In the past few decades, a variety of energy storage and conversion materials have been applied to high-powered energy devices such as lithium batteries, supercapacitors, fuel cell, and solar energy cells [1–3]. In comparison to the conventional energy materials, carbon nanomaterials have attracted extensive attention because of their unusual size and surface dependent properties useful in enhancing energy conversion and storage performance . In particular, the mesoporous carbons have been extensively studied due to their potential applications in hydrogen storage [1, 4].
There have been many studies of hydrogen adsorption which showed that H2 storage capacity is enhanced by added metal particles (Ag, Pd, Pt, Ri, Fe, Ni, and Ru) to carbon materials [5–18]. This phenomenon is well known for heterogeneous catalysis and called as hydrogen spillover [6, 7]. Hydrogen storage properties of different carbon materials functionalized with metal particles such as activated carbon, carbon nanotubes, carbon nanofibers, and graphene have been extensively studied. For example, B. P. Vinayan et al.  studied the hydrogen storage properties of nitrogen-doped graphene and Pd-decorated nitrogen-doped graphene. The results showed that hydrogen uptake capacity of nitrogen-doped graphene and palladium-decorated nitrogen-doped graphene (32 bar and 25 °C) is 0.42 and 1.25 wt.%, respectively. Moreover, the authors argued that high dispersion of Pd nanoparticles on nitrogen-doped graphene nanoplatelets and strong adhesion between metal and graphene enhanced the spillover mechanism. The effect of nickel distribution and content in Ni-modified carbon nanospheres on hydrogen storage capacity have been investigated . It was found that Ni distribution has a noticeable influence on H2 storage capacity. The samples with the most homogeneous Ni distribution exhibited the highest hydrogen uptake capacity. Moreover, the authors stated that storage properties of Ni-doped carbon spheres were influenced by the amount of nickel. Pd nanoparticles with different sizes and diameter distributions were deposited on the surface of carbon nanotubes and synthesized using a simple in situ technique (sonication and stirring) . The authors revealed that the system composed of carbon nanotubes and Pd nanoparticles has a potential as hydrogen storage medium due to the enhanced H2 adsorption capacity and stability after multiple adsorption–desorption cycles. Moreover, the hydrogen storage properties of Pd supported in hollow carbon spheres (HCS) under room temperature and at partial pressures were investigated . It was found that the Pd-decorated HCS sample with suitable diameters of Pd nanoparticles was more favorable for H2 storage, even when lower mass of Pd was used. The maximum hydrogen uptake capacity (0.36 wt.%) showed the material with the Pd nanoparticle diameter of 11 nm, and the hydrogen storage capacity was enhanced by the factor of two in respect to the pristine HCS.
The aim of this work is to examine the effects of palladium source and synthesis procedure on the optical, morphological, nanoparticles size distribution, and hydrogen adsorption capacity of DMHCS. In the study, carbon spheres were produced via chemical vapor deposition (CVD) method using disordered mesoporous silica templates (SiO2@m-SiO2_C18TMS) and ethylene as a carbon source. Two methods of synthesis such as reflux and impregnation and two Pd precursors (palladium (II) acetyloacetonate and palladium (II) acetate) were selected and compared for the preparation of Pd-modified DMHCS. Finally, the hydrogen storage capacity of the materials was revealed.
Synthesis of SiO2@m-SiO2_C18TMS Template
The disordered mesoporous hollow silica sphere templates (SiO2@m-SiO2_C18TMS) were synthesized by using octadecyltrimethoxysilane (C18TMS) as the surfactant and tetraethyl orthosilicate (TEOS) as the silica source. TEOS (3 cm3) was added to a mixture of ethanol (50 cm3), concentrated ammonia (28 wt %, 4 cm3), and deionized water (4 cm3). The obtained mixture was stirred for 24 h, and then the solution of C18TMS (1 cm3) and TEOS (2.5 cm3) was added dropwise while stirring. The solution was sonicated and stirred for another 6 h. The final product was isolated by filtration, cleaned with ethanol and water several times, and dried in air at 100 °C for 24 h.
Synthesis of DMHCS
The procedure for the preparation of the mesoporous hollow carbon spheres was in accordance with the reported method by Wenelska et al. . In a typical synthesis, SiO2@m-SiO2_C18TMS template was placed in an alumina boat and set in a tube furnace. Argon and ethylene were introduced in a flow rate of 100 and 30 sccm, respectively. The temperature was raised to 800 °C, and the chemical vapor deposition reaction time was 3 h. Afterwards, the furnace was cooled to room temperature in Ar. Next, the produced SiO2@m-SiO2_C spheres were treated with hydrofluoric acid. Here, the HF treatment step was repeated twice to remove all silica templates from carbon spheres. After this step, the obtained mixture was poured into water (5 dm3). Finally, the sediment was recovered by filtration, washed several times with distilled water, and dried in air at 100 °C for 24 h. The resulting product, prepared in the abovementioned way, is labeled as DMHCS.
Synthesis of Pd-Modified DMHCS
Two palladium sources (palladium (II) acetyloacetonate Pd(acac) 2 and palladium (II) acetate Pd(OAc)2) and two synthesis procedures (reflux and impregnation) were used for the preparation of Pd nanoparticle-decorated DMHCS. In the procedure with reflux, the mixture of DMHCS (50 mg), palladium (II) acetyloacetonate or palladium (II) acetate (50 mg), and ethanol (150 cm3) was placed in a flask fitted with a condenser and refluxed at 110 °C for 24 h. In the case of an impregnation technique, 50 mg of DMHCS and 50 mg of palladium (II) acetyloacetonate or palladium (II) acetate were dispersed in 150 cm3 of ethanol and stirred at the room temperature for 12 h. Finally, each sample was filtered, washed with ethanol and distilled water, and dried in air at 100 °C for 24 h. The samples synthesized in the abovementioned ways are labeled as Pd-R1, Pd-R2, Pd-I1, and Pd-I2 (R reflux, I impregnation, 1 palladium (II) acetyloacetonate, and 2 palladium (II) acetate).
where K is the shape factor (0.9 , λ is the X-ray wavelength (1.54 Å), B is the broadening of the diffraction line measured at half maximum intensity (FWHM), b is the line width originating solely from instrumental broadening, and θ is the Bragg angle corresponding to the (111) plane. Raman scattering was conducted on a Renishaw micro Raman spectrometer (λ = 785 nm). Thermogravimetric analysis (TGA) measurement was carried out in air using the DTA-Q600 SDT TA Instrument with a temperature scan rate of 10 °C per minute. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method via Micromeritics ASAP 2010 M instrument. Moreover, hydrogen adsorption capacity was investigated using a Sievert-type volumetric apparatus (IMI Series-Hiden Isochema), which fully automatically measures adsorption and desorption isotherms at 40 °C and up to pressure of 4.5 MPa using ultra high purity hydrogen gas (99.999 %).
Results and Discussion
The Raman spectra of (a) DMHCS, (b) Pd-I1, (c) Pd-R1, (d) Pd-I2, and (e) Pd-R2 are presented in Fig. 3 (bottom panel). The Raman responses of all studied samples show D and G peaks at about 1311 and 1603 cm−1, respectively. The D band corresponds to the presence of amorphous carbon due to the surface defects of carbon nanomaterials and graphitic carbon while the G band is associated with the C–C stretching vibration in graphitic materials [20–22]. The intensity ratio of the D and G bands (I D/I G) is commonly known as a parameter to characterize the quality of carbon materials. A high intensity ratio indicates a high degree of disorder in the carbon samples [22, 23]. The calculated values of I D/I G ratio which provide insight regarding the quality of the pristine and modified DMHCS are 1.258, 1.269, 1.283, 1.294, and 1.308, respectively. Comparing the ratios of I D/I G for each sample, it can be argued that all Pd-decorated DMHCS samples have more structural defects than the pristine carbon spheres.
Characteristic of the pristine DMHCS and Pd-decorated DMHCS
Average Pd crystallite size (nm)
Pd diameter distribution (nm)
BET surface area (m2/g)
Total pore volume (cm3/g)
From the above described results, it can be concluded that selecting a suitable palladium precursor and preparation route are crucial to obtain Pd nanoparticle-deposited carbon sorbents with high hydrogen capacity. The results indicate that reflux doping method and Pd(OAc)2 as a palladium source are more favorable for the preparation of Pd nanoparticle-deposited disordered mesoporous hollow carbon spheres. Moreover, optimization of the Pd nanoparticle content, size, and diameter distribution is essential to improve the hydrogen uptake of carbon sorbents.
In summary, the hydrogen storage capacity at 40 °C and in the pressure range of 0–45 bar of the pristine and Pd-decorated DMHCS was studied. For the synthesis of modified carbon spheres, the two preparation routes (reflux and impregnation) and the two Pd sources were investigated and compared. The obtained results showed that the synthesis method and Pd nanoparticles precursor had a significant impact on the morphological, optical, thermal, and hydrogen storage properties of palladium-modified DMHCS. Moreover, Pd nanoparticles size, content, and diameter distribution have a significant influence on hydrogen storage uptake. It was found that Pd-deposited DMHCS samples (Pd-I2 and Pd-R2) produced in the presence of palladium (II) acetate exhibited enhanced hydrogen capacity in comparison to the pristine DMHCS. The maximum H2 storage (0.38 wt.%) showed Pd-R2 sample obtained via reflux technique.
This research was funded by National Science Centre (DEC-2012/07/B/ST5/01925).
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