Synthesis of Palladium-Platinum Bimetallic Nanoparticles and their Catalytic Activity towards the Hydrogenation Reaction of Palm Olein

Bimetallic Pd-Pt catalyst was successfully prepared via conventional heating (CH), microwave (MW), and ultrasonic irradiation (US) methods. Bimetallic Pd-Pt nanoparticles stabilized with polyvinylpyrrolidone (PVP) were prepared with molar ratio of PVP to metal, 40:1. Smaller particles sizes with narrow distribution obtained after 30min and 10min with US and MW methods respectively. All the particles are spherical or near spherical in shape.


Introduction
The partial hydrogenation of vegetables is an industrial process that use to obtain a more stable product (which no oxidation on storage), together with a suitable texture and melting-temperature range at human mouth conditions for use as margarine, edible shortenings and baking applications [1,2]. During the hydrogenation process, the carbon double bonds are partially or fully saturated. Unfortunately, the catalytic isomerization of naturally occurring cis isomer to trans isomer fatty acids takes place [3]. In recent years, however it has been published that the intake of trans fatty acids (TFA) adversely affects blood lipid levels. Metabolic and epidemiologic studies confirm the potential role of TFA in increasing the risk of coronary heart disease. Furthermore, on each gram basis the negative health impact of TFA appears to be stronger than that of saturated fatty acids [4,5]. A numbers of alternative have been studied to figure out the ways for substantial reduction of inevitable TFA content in the hydrogenated edible oils such as the electro-catalytic hydrogenation, supercritical fluid hydrogenation, membrane reactor technology, catalytic transfer hydrogenation, and modification including supported precious metals [3].The investigated catalysts offer various advantages but also have considerable shortcomings.
In this part of work, such modification of precious metals included the use of microwave and ultrasonic irradiation selected and the effects of the catalyst on hydrogenation of palm olein were studied. Obviously, the use of microwave energy to heat chemical reactions has attracted a considerable amount of attention, due to its numerous successful applications in organic synthesis, polymer chemistry, material sciences, nanotechnology, and biochemical processes [6][7][8][9][10]. A huge number of paper published, had discovered that by using microwave technology can synthesis nanoparticles successfully. The motivation for the use of microwave energy has mainly been to design faster, cleaner and economically more viable methods for synthesis. The super rapid heating and sometimes extreme temperatures observable in microwave chemistry generally lead to faster processes. The transformations that normally required several hours when performed in a solvent at reflux temperature in an oil bath however may reach completion in a few minutes or even seconds using superheated solvent in a seal vessel or microwave reactor [11]. These unique features explained that this heating method has gained its popularity in many different field of chemistry. Mispa and co workers in their previous research in 2010 determined that the electric field applies a force on charged particles, resulted the charged particles started to migrate or rotate. Due to the movement of charged particles, further polarization of polar particles takes place. The concerted force applied by the electric and magnetic components of microwaves are rapidly changing in direction, which creates friction and collisions of the molecules, claimed effects of the microwave irradiation include thermal and non-thermal effects [12]. Many papers published about microwave compared with conventional method, shows that microwave synthesis has the advantages of short reaction time, small particles size and high purity [8,9,11,13,14].
On the other hand, the ultrasonic irradiation in particular the acoustic techniques is one of the most useful approaches for acceleration of chemical processes. Acoustic waves with frequencies of more than 20 kHz, can cause structural changes and accelerate chemical reactions [15]. The initiation of most of the sonochemical reactions in aqueous solution applying acoustic vibration was caused by cavitation. Acoustic cavitation is a disturbance of the continuity of a liquid, which connected to the creation, growth, oscillation, and collapse of steamto-gas bubbles inside the liquid [16,17]. The development of cavitation bubbles follows the sound field in a liquid during the cycles of compression and expansion and stimulated by time-varying pressure. Bubbles either oscillate around their equilibrium position over several expansion or compression cycles or can grow over one or more acoustic cycles to double their initial size and then finally collapse violently [16][17][18]. Radziuk D et al. [18] report that the tremendous heating or cooling rates of bubbles and transient of high temperature inside hotspot in heterogeneous medium which contained metal nanoparticles, caused it not only to accelerate catalytic activity of metal nanoparticles but also produced smaller size particles that are highly dispersed. Another comprehensive study was on the sonochemical synthesis of colloidal bimetallic Pd-Sn nanoparticles and conducted by Kim et al. [19]. Metals salt of Pd (NH 4 ) 2 Cl 4 and SnCl 2 dissolved in an aqueous ethanol solution containing citric acid, which acts as the stabilizing agent for the bimetallic nanoparticles. Ultrasonic irradiation performed with collimated 20 kHz bean from a ceramic transducer with a titanium amplifying horn directly immersed in the solution and operated with an input power of 42 Wcm-2 for 2 hours. The bimetallic nanoparticles shape and size were characterized with TEM. Highly dispersed Pd-Sn nanoparticles with sizes of 3-5 nm in average were obtained. Thus, implies that the ultrasonic irradiation method is an effective to form nanoparticles.
The aim of the present work is to synthesized bimetallic nanoparticles of Pd-Pt under ultrasonic irradiation and microwave heating and compared with the conventional heating method. As a lot papers published about the interesting characteristic of bimetallic catalyst compared to monometallic catalyst, it is not only helps to enhance catalytic activity but also increasing the yield of desired product [20,21]. Thus, Pd-Pt catalysts were used in hydrogenation reaction of palm olein to determine the effect of Pd-Pt catalyst especially on catalytic activity and the formation of elaidate (trans fatty acid).

Preparation of PVP-stabilized bimetallic Pd-Pt colloidal nanoparticles
PVP-stabilized Pd-Pt bimetal colloids was prepared where 0.0222g PdCl 2 (1.25×10 −4 mol) were converted to H 2 PdCl 4 ·nH 2 O by adding 100μl of concentrated HCl, then 0.065g H 2 PtCl 6 ·6H 2 O (1.25×10 −4 mol) and 1.11g PVP (1.00×10 −2 mol) was added to the mixture of methanol (130 ml) -distilled water (150 ml) which act as solvent. The solution was mixed in the 500 mL 3-neck round bottom flask. Then, 20 ml of 0.1 M NaOH in methanolic was added drop wise under vigorous stirring. Then the solution was divided into three portions of 100 ml. Each portion was synthesized under different method to produced colloidal metal nanoparticles. Brief descriptions of the different method condition were shown in the Table 1 below. Finally, the final colloidal solution resulted in a dark brown color will be stored in a dark bottle at 4°C [18].

Hydrogenation of palm oil
A constant gas flow rate of 36-37 ml/min of hydrogen gas was used for the whole experiments. The gas flow was first stabilized for 30 min before calibrated thrice. The hydrogenation reaction was performed at ambient temperature and atmospheric pressure. Colloidal catalyst of 0.5 ml (4.17 x 10 -7 mol) and 48 ml of butan-1-ol was fed into the reactor. Next, hydrogen gas flown inside the reaction vessel continuously and the catalyst was activated for 60 min. Finally, palm olein (0.01mol) was injected into the reaction vessel and the reaction started immediately for 180 min. The hydrogenation reactions were done under conventional stirring at the speed of 200 rpm. Partially hydrogenated palm olein were sampled at selected interval; 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 90 min, 120 min, 150 min, and 180 min.

Analysis method
UV-Visible spectrophotometer (UV-Vis): UV-Vis analysis was using Hitachi U-1800 UV-Visible spectrophotometer. All spectra were taken in the range of 200-700 nm with scan speed of 400 nm/min.

Transmission electron microscopy (TEM):
Characterization of the particles size and dispersion were done by using the TEM with LaB 6 source Tecnai G 2 20 S-TWIN. Sample for TEM were prepared by placing a drop of a colloidal catalyst onto a perforated carbon copper grid, then the solvent was evaporated for at least 3 min. The average particle size and standard deviation was determined based on that of 250-300 particles, from enlarged photographs via Gatan micrograph software Choo et al. [22]. X-ray diffractometer (XRD): X-Ray diffraction is a strong method to investigate the solid structure ofmetal nanoparticles. The X-ray diffraction (XRD) patterns of bimetallic Pd-Pt nanoparticles were

Type of method Synthesized process condition
Conventional heating, Pd-Pt(CH) This method was done by using oil bath to maintain the refluxing process condition was heated at reflux temperature for 3hr [22,23].
Microwave irradiation, Pd-Pt(MW) Power of 100 watts was used. Time for the heating was dependent to the color of colloidal solution until it is started to change to dark brown.

Ultrasonic irradiation, Pd-Pt(US)
Ultrasonic bath (ELMA D-78224, type S60H) with frequency of 37kHz was used for this experiment. Temperature of 80°C as it was the maximum temperature for this equipment. Time for the heating was same with the microwave method.

X-Ray photoelectron spectroscopy (XPS):
The XPS measurements of the ITO (indium-tin-oxide) substrates were carried out in a VG ESCALAB MK II spectrometer, using a monochromatic Al Kα 1486.60 eV as X-ray source. The vacuum in the analysis chamber was maintained at approximately 10 -8 Pa or lower. All binding energies were referenced to the binding energy of the carbon C 1s peak at 284.0 eV.

Gas Chromatography with FID detector (GC-FID):
The hydrogenated palm olein was methylated before analyzed with Agilent-6890 gas chromatograph (GC) equipped with FID detector and a HP-88 capillary column (L, 100m; i.d., 0.25 mm; thickness of film, 0.25 μm). The column temperature was programmed from 185 to 210°C at 2°C.min -1 and held at 210°C for 2 min and then raised to 230°C at 3°C.min -1 and held for 3 min with helium as carrier gas. Both the injector and detector temperatures were set at 250°C. Methyl oleate and methyl elaidate served as standard for determination of cis-and trans-isomers. Below are the formulas used for calculating linoleate conversion, trans selectivity and iodine value (IV):

Calculations for trans selectivity
The trans (elaidate) selectivity was determined directly from the GC fatty acid composition using the following formula (D.

Calculations for Iodine Values
The iodine value for oil samples was determined directly from the GC fatty acid composition using the following formula (Xiao, 2007):

Results and Discussion
In this part, Pd-Pt bimetallic nanoparticles were synthesized using chemical reduction method as mentioned in the literature. Then, all the metal nanoparticles were reduced by conventional heating (CH), microwave (MW) and ultrasonic irradiation (US). Refluxing-reduction method was done by repeating previous work that has been done before by Yu and Liu [23]. The same method uses to reproduce the previous work and compared with the new preparation method of microwave and ultrasonic irradiation. Comparison was not only based on times required to reduce metal nanoparticles but also the effect on the size and particles distribution, but also its activity towards hydrogenation reaction of palm olein.  [22,23]. On the other hand, ultrasonic and microwave method does not required long time to produce a homogeneous colloidal nanoparticles solution. The result in the Table 2 below shows that bimetallic nanoparticles Pd-Pt took only 10 min and 30 min for microwave and ultrasonic irradiation method respectively. It show that the microwave resulted faster reduction to produced colloidal bimetallic Pd-Pt nanoparticles, followed by ultrasonic irradiation and then the classical heating method.

UV-Vis analysis
Even though a large number of researches have been reported on the synthesizing bimetallic nanoparticles [11,19,[24][25][26], but the preparation of Pd-Pt stabilized by PVP via microwave and ultrasonic irradiation method has never been reported yet. The UV-Vis spectrums of Pd-Pt (CH), Pd-Pt (MW) and Pd-Pt (US) under different reduction process method shows in Figure 1. All samples including before and after reduction were taken for analysis to determine the changes of reduction processes for both Pd and Pt ions in the solution. A very broad peak was observed before the reduction occurred which indicated that an abundance of metal ions contained in the solution [27]. After the reduction process was completed where it can be determined via visual color changed, the broad peak totally disappeared. It is because of the synthesis of this bimetallic are using the co-reduction method. Where both metal salt of PdCl 2 and H 2 PtCl 6 ·6H 2 O, stabilizer and reducing agent are added at the same time and started the reduction process. Besides that, Pd-Pt bimetallic nanoparticles dispersion shows that at shorter wavelengths, the absorbance are the highest, which was described that the formation of smaller size and dispersed bimetallic colloidal nanoparticles [28,29].

TEM analysis
Among the techniques commonly used, transmission electron microscopy (TEM) is indispensable for metal nanoparticles study. Toshima and Yonezawa [30] stated that in their previous work, where noble metal nanoparticles such as Pd and Pt, give high contrast when particles are dispersed on the carbon supported copper grids, thus, the TEM image are much easier to analyze and clearly seen.
Typical micrographs and distribution histograms were shown in Figure 2A-C. The particles are of nanometer sizes with narrow distribution. Microwave-reduction method shows that it has resulted in smallest average particles size than the other methods, followed by ultrasonic irradiation and refluxing method. The average particles size are 1.05 ± 0.21 nm, 1.28 ± 0.29 and 1.23 ± 0.25 nm for MW, US, and CH method respectively. All the TEM images show spherical and nearly spherical in shape and are highly dispersed. Both microwave and ultrasonic irradiation have shown well-improved method in synthesizing metal nanoparticles especially in chemical reduction technique that has been used in this research.

Method Time
Refluxing* Ultrasonic Microwave 3 hr 30 min 10 min *Following methods from literature [22].  particles size compared to the other method. It can be explained by the microwave heating which allows an instantaneous volumetric and therefore a more rapid heating in comparison with the conventional heating process [11]. The MW technique has demonstrated as a better alternative in synthesizing bimetallic nanoparticles.
On the other hand, it was determined that particles are spherical or near spherical in shape and highly dispersed were also produced by ultrasonic irradiation method. Cavitation bubbles produced the shock wave increased the momentum of metal nanoparticles in the solution and caused them to collide with great force [31,32]. The particles are fracture upon collusions, leading to an overall decrease in the average particles sizes [33].

XPS analysis study
All the Pd-Pt (CH), Pd-Pt (MW), and Pd-Pt (US) samples were analyzed by XPS in order to identify the effect of all different method prepared on the surface composition of both Pd and Pt elements in bimetallic nanoparticles. Figure 3 (A-C) indicates the XPS spectrum for both Pd and Pt elements for each method of catalysts preparation. Both Pd and Pt element spectrums have been deconvoluted. Binding energy (BE) for each element and the Pd-Pt surface composition ratio of Pd to Pt was presented in the Table 3 below.
Pd-Pt (CH) sample shows that Pd spectrums have two peaks in Figure 3 (A1-A2). The binding energy for both peaks are, 335.1 and 341.4 eV assigned to electron form Pd3d 5/2 and Pd3d 3/2 orbital respectively. On the other hand,this spectrum can be curve-fitted with a spin orbit-split doublet by having BE at about 342.9 eV (3d 3/2 ) and 337.9 eV (3d 5/2 ) for Pd 2+ species [34]. At the same table, the BEs for Pt element was also presented. The binding energy signals obtained are 70.9 and 74.2 eV. The Pt0 species was found at binding energy signal of 70.9 and 74.2 eV for the 4f 7/2 and 4f 5/2 components respectively [35]. The same XPS spectrum also indicated the existence of Pt 2+ species at BEs of 73.1 (4f 7/2 ) and 76.4eV (4f 5/2 ). Next, the ratio of surface composition of Pd to Pt calculated was 2.7. As the composition of Pd was almost three times higher than Pt, it shows that the Pd enrichment might be to the polymer in anchoring Pt nanoparticles and thus allowing Pd to coalesce onto the Pt nanoparticles. The result shows that this bimetallic catalyst formation described that Pt metal atom had been covered by the Pd metal atom.
The Pd-Pt (MW) sample was presented in single element spectrum of Pd and Pt in Figure 3 (B1-B2). The binding energy of Pd3d5/2 and Pd3d3/2 electrons amounts are 334 and 340.3 eV respectively, that virtually coincides with the data for palladium in zero-valent metal state [36,37]. However Pd 2+ species was also observed at BE of 339.3 eV (3d3/2) [38]. This same XPS spectrum was also indicated the presence of oxidized Pt 2+ species at about Bes of 73.4 and 76.5eV for the (4f 7/2 ) and (4f 5/2 ) component respectively [34,35]. Then the ratio of Pd to Pt surface composition for this sample was 2.5. Like Pd-Pt (CH) samples, it can shows that the Pd has enriched onto the Pt nanoparticles, and resulted higher composition of Pd on the surface of the bimetallic catalyst [30].

Hydrogenation Reaction of Palm Olein
It is well established that the double bonds in vegetables oils are naturally in cis-conformation. Upon hydrogenation reaction the trans isomer that is thermodynamically more stable are produced. The trans isomer of the fatty acid is unhealthy for consumption and hence their formation in edible fats during hydrogenation must be suppressed as far as possible [2]. Thus, the hydrogenation reaction has been carried out at ambient condition using a direct flow of hydrogen gas (99.5% purity) at a constant flow rate of 36 ml/min through the reaction vessel. This Pd 2+ (3d3/2) Pt 0 (4f 5/2 ) component respectively. Whereas unreduced Pt2+ ions was seen at 73.1 eV for Pt4f7/2 and 76.4 eV for Pt4f 5/2 components [35]. Unlike Pd-Pt (CH) and Pd-Pt (MW) results, the ratio of Pd to Pt surface composition in this sample was 1.2. The surface composition ratio is almost identical; it suggested the alloy formation of bimetallic Pd-Pt (US). The alloy structure formation might be because due to the high intensity of the ultrasonic irradiation that caused the destruction of the bimetallic particles and thus rearrangement of both Pd and Pt atom to form an alloy structure [18,38].

Hydrogenation reaction of palm olein
It is well established that the double bonds in vegetables oils are naturally in cis-conformation. Upon hydrogenation reaction the trans isomer that is thermodynamically more stable are produced. The trans isomer of the fatty acid is unhealthy for consumption and hence their formation in edible fats during hydrogenation must be suppressed as far as possible [2]. Thus, the hydrogenation reaction has been carried out at ambient condition using a direct flow of hydrogen gas (99.5% purity) at a constant flow rate of 36 ml/min through the reaction vessel. This reaction has been done to demonstrate the effect of the Pd-Pt bimetallic catalyst on the composition of partially hydrogenated palm olein. Figure  On the other hand, it can be seen from the TEM image that although both PdPt (CH) and PdPt (US) catalysts have comparable particles size but PdPt (US) had shows lower linoleate composition compared to PdPt (CH). However, the conversions of linoleate between PdPt (CH) and PdPt (US) from the chart are significantly different. This result occurred maybe due to the well-dispersed of PdPt (US) nanoparticles which in contrary with PdPt (CH) nanoparticles formation (refer to Figure 4C for TEM study). This aggregated PdPt (CH) nanoparticles caused it to have lower in activity and thus less amount of linoleate hydrogenated [23].   Pd shows to be highly active and selective towards monoenes formation rather than Pt catalyst. However, Pd also exhibits higher selectivity towards TFA production than Pt [39]. Thus, in the context of no-TFA production, Pt was viewed as much better for hydrogenation of vegetable oils, as a consequences Pt are favor to produce non-desirable of saturated fatty acid. As a result, the combination of Pd and Pt bimetallic catalyst had shows an interesting result in this part of work. As we know that previously, Pd are highly active on hydrogenation reaction of linoleate, but it is also produced higher trans isomer [23]. However, Pt has the opposite characteristic with Pd catalyst. Thus, the combination of these two noble metals resulted higher activity but lower selectivity of trans isomer. Figure 4B shows the IV versus time of reaction for all catalyst, Pd-Pt (CH), Pd-Pt (MW) and Pd-Pt (US). The IV for palm olein before hydrogenation reaction was 57. When the reactions proceed, the IV started to decrease slowly as the increasing numbers of double bond were hydrogenated. Based on the Figure 4B A summary of the reaction pathway with the bimetallic PdPt is described in scheme 4.1 below. The PdPt catalyst favors the hydrogenation for both dienes and monoenes, which was describes by the dash line arrow in the scheme 4.1. Besides that, hydrogenation and isomerization of C18:1 is slightly favorable.

Conclusion
PVP stabilized Pd-Pt bimetallic nanoparticles with average particle sizes range at 1-2 nm were successfully prepared. Synthesis of bimetallic Pd-Pt with microwave and ultrasonic irradiation shows better alternative as a lot of time can be save and small and highly dispersed particles obtained. Interestingly, almost similar nanoparticles characterization achieved compared to classical heating method. The study of catalytic reaction of bimetallic Pd-Pt shows an impressive result. The combination of two noble metals, improved the compositions of partially hydrogenated palm olein, where higher conversion of C18:2 achieved in short time and in addition the elaidate selectivity are also very low. The IV is highly decreased as the reaction proceeds. Further work will be continued to determine the effect of ultrasonic irradiation on the hydrogenation reaction to compare with the conventional stirring.