Metal Nanoparticles Supported on Al-MCM-41 via In Situ Aqueous Synthesis

MCM-41 have been used to custom synthesize catalysts in because of the controllable properties, such as pore size, active phase incorporation, crystal size, and morphology, among others. In this paper, a simple and versatile method for the incorporation of platinum, ruthenium, and palladium onto Al-MCM-41 mesoporous silica by direct inclusion of various precursors was studied. M/Al-MCM-41 structure, textural properties, morphology, and elemental composition were analyzed. The results obtained indicate that the Al-MCM-41 mesoporous-ordered structure was not affected by metallic particle incorporation. Highsurface areas were obtained (1131 m2/g). Metallic nanoparticles dispersion on Al-MCM-41 was homogeneous for all samples and its particles sizes were between 6 nm to 20 nm. Microscopy results show round shape particles in platinum and palladium samples; however, ruthenium catalysts exhibit a spherical and rod shapes. Electrochemical testing for Pt/Al-MCM-41 showed electrocatalytic activity for H2 oxidation which indicates that these materials can be used as a catalyst in electrochemical devices.


Introduction
Mesoporous materials have played an important role in catalytic applications [1][2][3].Despite the availability of natural mesoporous materials, their use is limited in catalytic applications due to the presence of undesired phases.The chemical composition of natural mesoporous materials changes from one mineral deposit to another, even from one stratum to another in the same mine.Unfortunately, this makes it difficult to optimize their properties for tailor applications.Thus, custom synthesizing mesoporous materials by incorporating metallic particles is an important issue for the development of catalytic applications [4].
Kresge et al. discovered a series of mesoporous materials named as the M41S family [5].The M41S family has three members: MCM-41 with a hexagonal array of unidirectional pores, MCM-48 with a three-dimensional cubic pore structure, and MCM-50 with an unstable lamellar structure.These materials have exceptionally high-surface areas (>1000 m 2 /g) and narrow pore size distributions [6].Among the three members of the M41S family, MCM-41 became the most popular mesoporous molecular sieve.MCM-41 has been used in many applications such as production of intermediates/fine chemicals, petrochemical reactions [7,8], optical applications [9], development of sensors [10], and different industrial reactions (e.g., oxidation, condensation, and enzymatic reactions) [11][12][13][14][15]. MCM-41 is better option than microporous zeolites for some applications with diffusional problems, especially when large reacting molecules are involved.
Preparation methods to support metals on mesoporous materials are complicated and involve several steps in order to produce nanoparticles.Different synthesis methods have been developed for depositing metallic particles into mesoporous supports.The incipient wetness impregnation method has the advantage of technical simplicity and reproducible metal loading [16][17][18].However, metal distribution may be affected upon subsequent drying.Yao et al. [19] introduced a technique called vacuum evaporation impregnation to prepare Pt into MCM-41.Through the use of this impregnation method, it was possible to obtain material with a high specific surface area (900 m 2 /g) and 1.26% wt. of platinum as maximum loading.Junges et al. [20] compared three synthesis methods for preparing Pt/MCM-41: (i) direct platinum incorporation during the MCM-41 synthesis, (ii) incipient wetness, and (iii) ion exchange.The best performance was achieved over samples prepared by means of incipient wetness method.According to the authors, the nanometric size of the Pt-particles (around 2 nm) promoted high conversions in CO oxidation.Other mesoporous materials such as MSU (Michigan State University material) have been evaluated in some catalytic applications.Aramendía et al. [21] supported Pt metallic particles in MSU prepared by direct synthesis.Platinum particle size was 6 nm and specific surface area between 600-850 m 2 /g.
Often, the incorporation of the active phase is carried out after support synthesis and has been reported in many works [22][23][24][25][26][27][28][29].A new technique which uses supercritical fluids has been developed for incorporating platinum into FSM-16 [22,23].The use of CO 2 in the supercritical state improves active phase dispersion.Yamamoto et al. [24,25] prepared platinum carbonyl clusters in FSM-16 by a "ship-in-thebottle" technique.The technique consists of encapsulating [Pt 3 (CO) 6 ] 5 −2 clusters in the FSM-16 hexagonal channels.However, a disadvantage of this technique is that the carbonyl groups must be carefully evacuated in order to conserve the mesoporous support structure.
Different metallic particles have been supported in MCM-41 for several catalytic applications.Ruthenium [26] and bimetallic particles such as Ru-Pt [27] and Ru-Cu [28] have been supported after MCM-41 synthesis.Palladium in MCM-41 has been reported [29] and can be used for polluted groundwater treatment [30] and Heck reactions [31,32].In all cases, the objective of catalyst synthesis is to have a homogeneous and high dispersion of the active phase with a very small metallic particle size.
This work presents a simple and versatile synthesis method for metallic nanoparticles (i.e., Pt, Ru, and Pd) supported on mesoporous silica.Al-MCM-41 was used as support because has been demonstrated that Al can be incorporated in silica lattice substituting a Si atom [33].With aluminum inclusion is expected to modify the surface acidity of mesoporous support creating anchorage sites for the metallic nanoparticles.The physical, chemical, and electrochemical properties of the synthesized materials are also discussed.

Experimental
2.1.Metals/Al-MCM41 Synthesis.The metal/Al-MCM-41 (M/Al-MCM-41) materials were prepared as follows: a fixed amount of sodium aluminate (NaAlO 2 , 99.95%, Riedel-de Haën) was dissolved in 6 M ammonium hydroxide (NH 4 OH, 30%, Aldrich) solution (molar ratio of Si/Al = 20).Then, each metal precursor was dissolved in this alkaline solution.The solution was combined with cetyl trimethyl ammonium bromide (CTAB, 99%, Alfa Aesar) as organic template (CTAB/Si molar ratio was 0.12).Finally, the mixture was stirred in order to ensure a clear homogeneous solution; then tetraethyl orthosilicate (TEOS, 98%, Aldrich) was added as silica source.Solution was stirred for 24 h at room temperature and atmospheric pressure.Metal precursor was calculated in order to have 5% wt of metal supported in Al-MCM-41.Ammonium hexachloroplatinate ((NH 4 ) 2 PtCl 6 , 99.9%, Alfa Aesar), ammonium hexachloropaladate ((NH 4 ) 2 PdCl 6 , 99.9%, Alfa Aesar), and ruthenium chloride (RuCl 3 , Aldrich) were used as metal precursors for Pt, Pd, and Ru, respectively.In all cases, the resulting synthesized products were recovered by filtration and washed with deionized water.The products were calcined in air at 823 K during 4 hours.The M/Al-MCM-41 materials were reduced in hydrogen atmosphere at 673 K during 4 hours.Additionally, an Al-MCM-41 sample control was synthesized.

Physical and Chemical
Characterization.X-Ray diffraction (XRD) was used to determine crystalline structure and crystallite size in all samples.An analytical X'PertPRO diffractometer (CuKα-1 radiation (40 kV, 30 mA), step size = 0.016 • , 24.13 s per step) was used with an X'Celerator accessory at room temperature.
Nitrogen adsorption analyses were performed in order to evaluate textural properties of samples such as surface area, pore size distribution, and pore shape.Nitrogen adsorption isotherms were determined by a Quantachrome Autosorb1 gas sorption analyzer using high-purity nitrogen as adsorbate.Prior to adsorption isotherm determination, the samples were outgassed for 5 h at 573 K; surface area was obtained by multipoint BET method.
In order to have an approximation of the elemental composition and metal loading in M/Al-MCM-41 samples, energy dispersive spectroscopy (EDS) was carried out with EDAX Prime equipment coupled to a JEOL 5800 LV Scanning Electron Microscope; the analyses were randomly taken in several sample zones to have a representative value of the elemental composition at low magnification.Morphological characterization was carried out using a Philips CM-200 (200 KV, 25 pA) transmission electron microscope (TEM) and JEOL7410 (5.0 KV) Field Emission Scanning Electron Microscope (FESEM).TEM and SEM specimens were prepared by dispersing the sample in ethanol with ultrasound for 5 minutes.A drop of the suspension was placed into a holey carbon Cu grid and was allowed to dry.

Electrochemical Experiment.
Electrochemical characterization was carried out by cyclic voltammetry (CV) in a conventional three-electrode cell.A glassy carbon disk electrode with a surface area of 0.07 cm 2 was used as working electrode.Catalyst suspension was prepared with M/Al-MCM-41 and Vulcan XC72 (Cabot Corporation) at a 1 : 1 weight ratio dispersed in deionized water.Catalyst suspension which contained 10 mg/mL was spread on the disk electrode from the ultrasonicated aliquots.Catalyst film was fixed on the disk electrode using 5 μL of a deionized water : Nafion solution (20 : 1 ratio) and dried at room temperature.Electrochemical measurements were performed

Results and Discussion
3.1.X-Ray Diffraction.Figure 1 shows the XRD patterns for the M/Al-MCM-41 materials.Typically, MCM-41 silica exhibited three peaks at small angles (Figure 1(a)).These peaks are characteristics of hexagonal ordered structures [34] and correspond to (100), (110), and (200) planes.M/Al-MCM-41 mesoporous structure was not affected after metal incorporation and thermal treatment.This suggests that the M/Al-MCM-41 was structurally stable.Table 1 summarizes the chemical and physical properties of M/Al-MCM-41 synthesized.
Interplanar distance can be directly related to MCM-41 pore size due to the fact that unit cell is hexagonal [35].
The (100) peak represents the d spacing.The d spacing obtained from Al-MCM-41 sample was 3.89 nm, while the d spacing of M/Al-MCM-41 materials decreases between 0.1 to 0.3 nm respect to pure Al-MCM-41 (Pt/Al-MCM41 ∼3.55 nm, Pd/Al-MCM41 ∼3.62 nm, and Ru/Al-MCM41 ∼3.73 nm).Interplanar distance modification observed in M/Al-MCM-41 samples suggests a possible introduction of Pt, Pd and Ru into to the silica network.
Additionally, peaks at high angles were observed indicating the presence of metallic crystalline phases.The peaks were indexed as platinum, ruthenium, and palladium in metallic state (Figures 1(b), 1(c) and 1(d), resp.).
Crystal sizes for metallic particles supported in Al-MCM-41 were calculated from the Scherrer equation where L is the crystallite size ( Å), λ the wavelength ( Å), β is the line broadening at half the maximum intensity in radians and the θ the angle of the highest peak in radians.The crystallites sizes calculated from Scherrer equation are shown in Table 1.Pt/MCM-41 and Pd/MCM41 crystallites sizes (8 nm and 6 nm, resp.) were smaller than the Ru/MCM-41  sample (14 nm).The results showed above suggest that most metallic particles are supported outside of the Al-MCM-41 pores because of the calculated d spacing is smaller than metallic particle size derived from Scherrer equation.

Textural Properties. Type IV N 2 adsorption-desorption
isotherms from all M/Al-MCM-41 materials were obtained.These isotherms are associated with capillary condensation in mesopores, which are represented by a steep slope at higher pressures.Type IV isotherms associated with mesoporosity usually exhibit hysteresis between adsorption and desorption isotherms [36].In agreement with the IUPAC classification, M/Al-MCM-41 materials showed a small hysteresis loop type H1 commonly attributed to cylindrical pores (Figure 2) [37].The pore-size distribution (PSD) determined by using a BJH analysis showed a narrow PSD between 2.5 nm to 2.7 nm. Since PSD from Al-MCM-41 was at 2.5 nm and M/Al-MCM-41 samples were between 2.6 nm and 2.7 nm, it is possible to conclude that the pore size of M/Al-MCM-41 materials was slightly affected by metal incorporation during the synthesis.
Table 1 also shows the BET surface areas obtained from M/Al-MCM-41 materials.High surface areas were even maintained with the metal incorporation (1131 m 2 /g for Pt/Al-MCM-41 compared to 1272 m 2 /g for Al-MCM-41).Surface area of M/Al-MCM-41 materials decreased between 11%-30% respects to Al-MCM-41 when metallic particles was incorporated.Surface area decrement also shows the influence of the metallic particles on the Al-MCM-41 pore.The agreement between surface area and XRD results indicate that the mesoporous structure was preserved even after metal incorporation.Textural property results indicate that mesoporous materials with cylindrical pores were synthesized in all samples [38].

Elemental Composition.
Metal loading values obtained were 4.28, 3.14, and 3.40% wt. for Pt/Al-MCM-41, Ru/Al-MCM-41, and Pd/Al-MCM-41 samples, respectively (Table 1).The initial loading was calculated at 5% wt for each material, which indicates that with this synthesis method, it is possible to incorporate up to 85% of metal load calculated.Figure 3 shows FESEM images and elemental mapping distribution in each sample.All samples show homogenous dispersion of metallic nanoparticles and no metallic clusters were observed.The agreement among XRD, BET, and EDS analysis indicates that by employing this synthesis method it is possible to obtain high-quality mesoporous materials with homogeneous metal dispersion and high loading of metallic catalyst particles.

Morphology.
Figure 3 shows FESEM images of metallic nanoparticles of Pt (Figure 3(a)), Ru (Figure 3(b)) and Pd (Figure 3(c)) supported on Al-MCM-41 crystals.Images were taken by employing the backscattered electrons technique.Low contrast zones correspond to metallic nanoparticles and high-contrast zones correspond to Al-MCM-41 support.Pt and Pd nanoparticles were observed with similar form and size (around 10 nm) and homogeneously distributed on Al-MCM-41.However, in Ru/Al-MCM-41 sample is possible to observe rods and round shape particles.Metallic ruthenium nanoparticles were larger than platinum and palladium nanoparticles.A statistical analysis was carried out from several images in order to determine the metallic particle size distribution (Figure 4).Crystallite size calculated from Scherrer equation (Table 1) are in agreement with the particle size observed by FESEM.Metallic particle size found and PSD suggests that the most of the nanoparticles are supported on the external surface of the Al-MCM-41 and just a smaller percentage could be inside of the Al-MCM-41 pores.
Figure 4 shows TEM images of Pt, Ru, and Pd nanoparticles deposited on Al-MCM-41 crystals.In Figure 4(a), a hexagonal Al-MCM-41 crystal is observed, platinum nanoparticles around 6 nm to 15 nm are supported outside the support pores.In addition, the crystallite size obtained by the Scherrer equation, FESEM images, and TEM images for Pt, Ru, and Pd are similar (Figures 4(a), and 4(c) resp.).only round shape particles but also rods form; the difference could be attributed to nature of the metallic precursor used in this case (RuCl 3 ).This difference affects the shape and size of ruthenium particles even though the same synthesis method is used.In all samples, metallic nanoparticles were observed blocking some Al-MCM-41 pores, hereby surface area decreases when metals are incorporated to Al-MCM-41.

Cyclic Voltammetry Analysis.
Cyclic Voltammetry (CV) curve in Figure 5 shows Pt/Al-MCM-41 sample and support without platinum for comparison.A peak associated with hydrogen oxidation appears only for Pt/Al-MCM-41 sample near to 0 V versus normal hydrogen electrode (NHE).CV technique was employed to obtain the electrochemical active area (EAA) from hydrogen oxidation peak using the charge associated with hydrogen-adatoms desorption (Q H ) in where Q Ho is the charge required for oxidation of a single molecule of H 2 on a polycrystalline Pt surface of 1 cm 2 (210 mC/cm 2 ) [39,40].G Me is the metal loading in the working electrode.
The EAA obtained from Pt/Al-MCM-41 was 4.46 m 2 /g which suggest that hydrogen oxidation occurred in the proposed material.This type of electrocatalytic activity shows that Pt/Al-MCM41 can be use as electrode in electrochemical devices, however, more extensive studied is needed

Conclusions
Pt, Ru, and Pd incorporation in Al-MCM-41 was carried out by direct synthesis.M/Al-MCM-41 showed high-surface areas (up to 1131 m 2 /g) and the PSD suggests that materials have unidirectional cylindrical pores with diameter between 2.5 nm to 3 nm.Metallic particle size achieved was 8, 14, and 6 nm for Pt, Ru, and Pd, respectively.Metallic precursor nature affected the metallic nanoparticles form.Elemental analyses show high metal nanoparticle loadings (up to 85% of yield).Metal particles were homogeneously dispersed on the Al-MCM-41 crystals.Finally, electrochemical response shows a hydrogen oxidation peak when Pt is incorporated to the Al-MCM-41 sample, therefore Pt/Al-MCM-41 could considered as electrocatalysts in some electrochemical applications.

Table 1 :
Chemical and physical properties of M/Al-MCM-41 materials.