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Journal of Nanomaterials
Volume 2014, Article ID 275197, 9 pages
http://dx.doi.org/10.1155/2014/275197
Research Article

Platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane Complex as a Pt Source for Pt/SnO2 Catalyst

1Institute of Non-Ferrous Metals Division in Poznan, Central Laboratory of Batteries and Cells, 12 Forteczna Street, 61-362 Poznan, Poland
2Poznan Science and Technology Park of Adam Mickiewicz University Foundation, 46 Rubiez Street, 61-612 Poznan, Poland
3Centre of Advanced Technologies Adam Mickiewicz University, 6 Grunwaldzka Street, 60-780 Poznan, Poland

Received 10 January 2014; Revised 13 March 2014; Accepted 27 March 2014; Published 27 April 2014

Academic Editor: Sylwia Mozia

Copyright © 2014 Agnieszka Martyla et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

This paper presents new preparation method of Pt/SnO2, an important catalytic system. Besides of its application as a heterogenic industrial catalyst, it is also used as a catalyst in electrochemical processes, especially in fuel cells. Platinum is commonly used as an anode catalyst in low temperature fuel cells, fuelled with alcohols of low molecular weight such as methanol. Platinum(0)-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex was used as a precursor of metallic phase. The aim of the research was to obtain a highly active in electrochemical system Pt/SnO2 catalyst with low metal load. Considering small size of Pt crystallites, it should result in high activity of Pt/SnO2 system. The presented method of SnO2 synthesis allows for obtaining support consisting of nanoparticles. The effect of the thermal treatment on activity of Pt/SnO2 gel was demonstrated. The system properties were investigated using TEM, FTIR (ATR), and XRD techniques to describe its thermal structural evolution. The results showed two electrocatalytical activity peaks for drying at a temperature of 430 K and above 650 K.

1. Introduction

The hydrosilylation reaction as practiced industrially employs silicone-soluble, low-valent platinum catalysts called Karstedt’s catalyst which is a platinum divinyl tetramethyl disiloxane complex, typically containing about one-weight percent of platinum in an organic solvent, Figure 1.

275197.fig.001
Figure 1: Karstedt’s catalyst.

It is known that solutions of Karstedt’s catalyst (Pt(dvs)) thermally decompose with deposition of platinum [1]. This property may be used effectively in formation of metallic phase precursors [1] from such molecular complexes when applying CVD [2] or PVD technique [3]. Typically, an organometallic precursor of the desired metal is vaporized and deposited on a surface, hot enough to decompose the precursor, resulting in deposition of the metal and release of the precursor ligands. Many of the CVD precursors are potential candidates for a low temperature metal deposition process. These metal precursors are frequently highly soluble in organic media and decompose at moderate temperatures or even under mild UV irradiation conditions [1].

Formation of metallic phase in a low temperature process could compete with traditional methods, in which a high temperature treatment plays a key role [4]. All platinum complexes decompose under oxidizing conditions at below 473 K. The vinyl complex, the so-called Karstedt’s catalyst, was used to produce small Pt clusters [1, 5].

This report describes our first attempts to deposit platinum on SnO2 support via low temperature processes. Pt/SnO2 has been well-known in catalysis for years and used widely in reactions of reforming, CO, and alcohols oxidation [69]. Oxidation of alcohols is a reaction used in fuel cells that are an alternative form of obtaining energy. Their usage is still limited by deactivation of platinum catalyst [1012] caused by methanol oxidation intermediates.

Alcohol oxidation is a multistage process taking place on an electrode surface [13] ((1)–(6)). Carbon dioxide developing on the platinum and unoxidized intermediates block active sites and cause deactivation of the electrode.

1.1. Dissociative Adsorption

Consider

1.2. Adsorbed Residues Reacts with an O-Containing Species

Consider

Efforts to mitigate the poisoning effects of Pt have been concentrated on the addition of cocatalysts, particularly ruthenium and tin. Another way to improve the catalysts for the oxidation of CO and organic fuels is the production of Pt catalysts on oxide supports, which are highly stable in an oxidative medium of fuel cells [1417].

The purpose of this work was to prepare catalyst Pt/SnO2 systems with various contents of platinum from Karstedt’s catalyst and to investigate the effect of the thermal treatment on the structural properties and electrocatalytic activity in the oxidation of methanol [9].

2. Preparation

2.1. Preparation of Pt/SnO2 by the Sol-Gel Technique

The precursor of the tin(IV) oxide was tin(IV) acetate. To a mixture consisting of 48 cm3 of isopropanol and 8 cm3 of methanol 1.6 g of tin (IV) acetate was added and immersed in an ultrasonic bath at 323 K until tin (IV) acetate dissolved. Increasing amounts of xylene Pt(dvs) catalyst solution were added to the tin acetate alcohol solutions, corresponding to 13 mg and 26 mg of Pt to obtain samples containing 1.87 wt% (series A) and 3.74 wt% (series B) of Pt. After Pt(dvs) dosing, the systems were dispersed in an ultrasonic bath at 323 K for 24 hour. In these conditions, with no water added, a SnO2 gel is formed by subsequent processes of transesterification (7), condensation (8), hydrolysis (9), and esterification (10) [1820]:

Next, the resulting gel was dried under vacuum for 48 h at a temperature of 293 K.

Series of samples containing 1.87% Pt (A series) and 3.74% Pt (B series) were heated under air atmosphere at temperatures: 373 K, 473 K, 573 K, 673 K, and 773 K using a temperature gradient 20 K/min, for 4 hours.

2.2. Physicochemical Characteristics

The phase identification and the influence of the thermal treatment on the SnO2 phase were performed using a X-ray diffraction (XRD) powder diffractometer (Philips, PW 1050) using CuK lamp radiation and a Ni filter. X-ray spectra were recorded in the angular range of 5 to 80 (2). The Scherrer equation (11) was used to estimate the SnO2 crystallite size. The equation shows that the crystallite size is inversely proportional to the peak width [21, 22]: where is an average crystallite size, is the X-ray wavelength for Cu radiation ( Å), is the peak width of the diffraction peak profile at half maximum height resulting from a small crystallite, is the Bragg diffraction angle, and is a constant related to the crystallite shape.

The surface imaging was performed with Transmission Electron Microscopy (JOEL JEM 1200 EX).

The surface area of Pt/SnO2 calcined at various temperatures was measured by N2 adsorption using BET, with Micromeritics Instrument Corporation Model ASAP 2010. The sample, which was only dried, was outgassed at 293 K for 48 hours under vacuum. After calcinations, the samples were thoroughly outgassed at preparation temperatures for 24 hours.

The thermal analysis of SnO2 gel was performed using a TA Instruments Gravimetric Analyzer (TA50) under a nitrogen and air atmosphere, 20 cm3/min flow, at heat rate of 20 K/min in both cases.

Surface studies were carried out using a FT-IR spectrometer (Bruker, TENSOR 27) with ATR accessory (SPECAC). Measurements resolution was 4 cm−1.

The cyclic voltammetry (CV) experiment was performed in a two-electrode Swagelok type cell with the application of ECLAB V10.12 VMP model 0.3 potentiostat/galvanostat by Bio-Logic in the potential range of −0.5 to 1.0 V. The scan rate of 0.05 mV/s was applied.

The catalyst powders were suspended in a solution of PVdF-HFP in acetone with graphite and applied onto a stainless steel electrode Steel-Pt/SnO2  separator Steel. The electrolyte contained 1 mol/dm3 of H2SO4 and 2 mol/dm3 of CH3OH.

3. Results and Discussion

Structural and morphological properties of the materials were performed to study the effect of the temperature and the concentration of platinum on their catalytic activity. The studies of surface area show that the temperature has a significant effect on the change in surface area of received systems. Samples dried at room temperature have small surface area (Table 1) which proves the nonporous structure. The thermal treatment at 373 K results in a rapid increase in surface area which is characteristic for most systems obtained by a sol-gel method [23]. For high-temperature treatment, we observe a gradual decrease in surface area, up to about 60 m2/g (after treatment at 773 K). It should be noted that the specific surface area values are significantly higher than for the other porous tin(IV) oxides [24].

tab1
Table 1: Surface area of Pt/SnO2 systems.

The first XRD pattern for all systems is typical for the as-synthesized air-dried gel product (Figure 2). This treatment results in a predominantly amorphous structure (Table 2). After heating (in air atmosphere, temperatures from 373 K to 773 K), according to the X-ray diffraction data, the Pt/SnO2 samples consist of single phase, corresponding to the rutile-like structure with a tetragonal crystalline lattice. At higher temperatures, as a result of nanocrystalline SnO2 formation, the diffraction peaks become progressively more intense and sharp. A typical tendency of crystallites growth with the temperature was observed (Table 2). The XRD patterns for B series are omitted since they represent analogical progression.

tab2
Table 2: Crystallite size of SnO2 calculated from XRD and TEM technique.
275197.fig.002
Figure 2: XRD patterns of 1.87% Pt/SnO2 systems.

The change in the SnO2 crystallites size calculated using Scherrer equation and estimated from micrographs shows similar correlation as a function of temperature.

Miller indexes are indicated on each diffraction peak. The reflection peaks at ~26 (2theta)/, ~33 (2theta)/, ~51 (2theta)/, and ~65 (2theta)/ can be readily assigned to a rutile-type tetragonal structure of SnO2 (PDF 4+ Card File number 04-003-5853).

There are no noticeable reflexes of platinum. We believe that it is related to the small size of platinum crystallites which cannot be detected [25] using XRD method and at those concentrations.

Changes in crystallite morphology and support grains due to the change in temperature were observed with TEM (Figures 3 and 4). Images have dimensions  nm. The data of transmission electron microscopy exhibited a narrow size distribution of particles with an average diameter of 2 (Figure 4(b)) to 6 nm (Figure 3(f)). For sample, after treatment at 373 K (Figures 3(a) and 4(a)), the particle size is very small, which is approximately similar to the microscope image of Karstedt’s complex after the evaporation of solvent form the solution [26]. With the increase in heating temperature, it can be seen that the SnO2 nanoparticles’ size slightly rises (Figures 3(e), 3(f), 4(e), and 4(f)).

fig3
Figure 3: Pt/SnO2 TEM images of A series: 1.87% Pt, after heating: (a) 293 K, (b) 373 K, (c) 473 K, (d) 573 K, (e) 673 K, and (f) 773 K. The visible area size is  nm. SnO2 particles are matched.
fig4
Figure 4: Pt/SnO2 TEM images of B series: 3.74% Pt, after heating: (a) 293 K, (b) 373 K, (c) 473 K, (d) 573 K, (e) 673 K, and (f) 773 K. The visible area size is  nm. SnO2 particles are matched.

On the basis of XRD measurements and microscopy, the presence of Pt crystallites larger than 1 nm was excluded. SnO2 support crystallites of 2 to 10 nm size depending on processing temperature are only observed. In the synthesis method used for preparation, decomposition of metal precursor and formation of nanocrystalline SnO2 occur simultaneously. The use of Pt divinyl complex instead of H2PtCl6 [27] significantly affects the size of metallic Pt clusters. The use of hexachloroplatinic acid and incipient wetness impregnation method results in Pt crystallites larger than 2.5 nm [28, 29] at the same loads.

Pt visualization on a support surface with XRD method is possible only in case of very high metal load, exceeding 10% [3032]. When seeking explanation of that fact, we reminded about the stabilization possibility of small Pt clusters in presence of other oxide components in the metal/support system [33]. Also, the presence of small quantity of silica on a surface of tin dioxide increases activity of supported metal catalysts [34]. Hypothetical illustration of that phenomenon is presented in Figure 5.

275197.fig.005
Figure 5: Hypothetical mechanism of small platinum particles stabilization by SiO2.

Figures 6 and 7 show results of FTIR studies. The biggest changes in spectra occur in a 1000 to 1500 cm−1 range after heating at 373–473 K. They result from removal of organic solvents residue from the bulk (isopropyl alcohol, acetic acid, and the corresponding ester). Fainting of bands d–h is correlated with decomposition of Pt complex at approximately 400 K. At the same time, appearance of a new band at 921 cm−1, related to formation of SiO2 (c) [35], can be observed. The peaks in the regions d—1018 cm−1, e—1087 cm−1 are the complex’s band from the Si–O–Si stretching vibrations and they are observable only in case of higher initial Pt concentration systems.

275197.fig.006
Figure 6: ATR/FT-IR spectra of 1.87% Pt on SnO2 powders (A series).
275197.fig.007
Figure 7: ATR/FT-IR spectra of 3.74% Pt on SnO2 powders (B series).

A broad band between 500 and 772 cm−1 (Figures 6 and 7(a,b)) is due to Sn–OH bond stretching.

The region of f (1255 cm−1), g (1321 cm−1), and h (1389 cm−1) peaks is connected with C–O rocking, O–H bending vibrations, and C–H rocking. We observe a peak associated with the presence of acetic acid [36] (i) at about 1700 cm−1. A broad band at 3400 cm−1 (j) is attributed to the O–H stretching vibrations from residual alcohol, water, and SnO–H bonds.

For the annealed samples, the desorption of organics takes place, and the peaks at 3400 cm−1 appear as a shoulder superimposed on the O–H band. The elimination of organic residues provokes a change in intensity of the peak at 1650 cm−1 (i). The absorption band from 500 to 772 cm−1 decreases with the change in intensity of its components (a, b). The SnO2 modes can be seen, with the Sn–O–Sn mode at 636 cm−1, while a lattice mode of SnO2 appears at 772 cm−1 [37].

Thermal decomposition of the starting gels in oxidizing atmosphere is presented in Figure 8. For both systems, the process occurred in several steps. Mass loss of about 5% at 330 K evidenced by the TG curve can be attributed to the removal of certain amount of organic solvents. Next steps are observed at 553 K and 610 K and they are linked with the further process of removing the organic solvents from the gel network [38]. The elimination of organic species was also observed in FTIR studies. For temperatures exceeding 620 K, no significant mass loss is visible. One can conclude that removal of solvents from the gel structure is accomplished. TG curve shows no distinct mass loss corresponding to Pt complex decomposition which should be localized in a 390–420 K range as one can expect from FTIR measurements and reference data [26]. This is due to a fact that mass loss linked to complex decomposition is a small fraction of the investigated system.

275197.fig.008
Figure 8: TG/DTG curves measured on Pt/SnO2 powders vacuum dried at 293 K.

Cyclic voltammetry analysis is a good technique to obtain information about the functional properties of synthesized Pt/SnO2 [17]. Electrocatalytic activity of Pt/SnO2 in the methanol oxidation reaction is shown in Figure 9 which gives the cyclic voltammograms measured on 1.87% Pt/SnO2 and 3.74% Pt/SnO2.

275197.fig.009
Figure 9: Cyclic voltammetry of the Pt/SnO2 systems: (A) 1.87% Pt, (B) 3.74% Pt, after heating: 1: 293 K, 2: 373 K, 3: 473 K, 4: 573 K, 5: 673 K, and 6: 773 K.

Systems dried at room temperature show relatively low electrocatalytic activity (Figure 9, A1, B1). No peaks from methanol oxidation are observed. They appear in the next sample series only after heating at 373 K (Figure 9, A2, B2). For these systems, the lowest potential value was also noted. It can be seen that the potentials of methanol oxidation peaks for Pt/SnO2 are shifted to the anodic region for both catalysts with the thermal treatment temperature. This may be associated with the crystalline form in which the main catalyst component, platinum, occurs.

According to references, the crystallites with orientation [39] exhibit the highest activity in methanol oxidation and they also exhibit characteristic responses [37].

Concluding from voltammetric curves, in this case the active crystallographic plane is . The decrease in the SnO2 grain size is accompanied by an increase in currents of both processes that occur in the fuel cell.

Above 400 K, Karstedt’s complex decomposes forming silica and platinum. It allows to assume that small Pt clusters are stabilized by the formation of SiO2. After thermal treatment at 393 K, the electrochemical activity of the system is related to Karstedt’s complex in surrounding of SnO2 gel. At 493 K, decomposition of the complex with formation of metallic Pt surrounded by the SiO2 from the decomposed organometallic compound occurs.

To simplify that a parameter has been introduced. Its value is equal to the sum of an oxidation current and an absolute value of reduction current (12):

Plots (Figure 10) present values of of all obtained systems as a function of temperature. Independently from platinum content, both systems show comparable activity in methanol oxidation reaction. First, the maximal electrocatalytic activity for 430 K seems to be related to decomposition of Karstedt’s complex, which occurs at about 400 K in oxidative atmosphere (Figure 8, see also Figure 10). As a result of complex decomposition, small clusters of Pt are formed and they are visible at TEM micrographs. It seems that the decrease in electrocatalytic activity observed above 460 K is related to changes in SnO2 support, not to change in Pt dispersion. In many electrocatalytic processes, the role of structural factors such as nanoparticles orientation in catalytic activity was noticed [27]. The minimal electrocatalytic activity observed at circa 580 K corresponds to the maximal mass loss from DTG curves (Figure 8). This temperature range overlaps with the end of gel support formation (end of clear mass loss). The observed increase in activity can be caused by formation of a system in which the support phase starts to play an important role in electrocatalytic activity by increasing the electronic conductivity of the catalytic layer [27].

275197.fig.0010
Figure 10: The sum of an oxidation current and an absolute value of reduction current as a function of temperature.

4. Conclusions

Siloxyl platinum complex (Pt(dvs)) allows obtaining tin(IV) oxide with high surface area and highly dispersed Pt/SnO2 system. The presence of small amounts of SiO2, formed after decomposition of the complex, may result in stabilization of platinum nanoparticles in SnO2 gel structure. The metallic phase of systems does not tend to sinter and therefore maintain high electrochemical activity even after thermal treatment at 773 K.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

The study was financially supported by statutory activity of IMN Division in Poznan CLAiO no. 7001.01/BM/2013.

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