Abstract

Herein, CNT supported Pt and Pt-Au catalysts were carefully synthesized via NaBH₄ reduction method. Pt-Au catalysts were obtained at varying ratios. For all of these CNT supported Pt and Pt-Au catalysts, BET, EDX, and XRD measurements were performed. The atomic ratio of the catalyst was obtained by EDX analysis. Surface area was defined via BET and crystal structure was examined by XRD. For microwave assisted n-hexane reforming measurements, microwave reactor was used to investigate the decomposition of n-hexane on CNT supported bimetallic Pt-Au catalysts. After applying microwave heating during the reaction time, the volume change is defined and read out. As a result, it was observed that Pt-Au catalysts prepared at 50:50 atomic ratio gives the best catalytic activity.

1. Introduction

Energy is important and permanent needs for human kind. Fossil fuels such as oil, coal and natural gas are the currentlu used fuels. Nevertheless, it is predicted that fossil fuel will exhaust n the near future. According to the Hubbert theory, the global production of fossil fuels will start to decrease by 2020 ¹. Thus, the researchers are intensively studying on the alternative energy sources ^{2, 3, 4, 5, 6, 7} In that context, heterogeneous catalysis is also crucial for these studies to provides a promising means to develop alternative energy technologies. In this regard, catalysts having high activity and high selectivity should be produced ⁸.

Gasoline is a mixture of hydrocarbons. It could be used as a fuel in internal combustion engines (ICEs). Due to its high extent consumption in automobile industry, the request for the gasoline is increasing globally ^{9, 10}. The catalytic reforming is an important process. During the catalytic eforming of n-hexane, naphtha was converted into high-octane reformates ¹¹. Hydrocarbons or naphtha have low octane number. Hence, it is not suitable to use them directly for direct gasoline blending. Therefore, catalytic reforming of naphtha and isomerization of light alkanes are required to enhance the gasoline quality (>70). High octane numbers relate to good engine performance ¹² .

Heterogenous catalysts consisting of metal nanoparticles onto supports could be synthesized via various techniques. Furthermore, their catalytic activity could be investigated in a number of catalytic chemical reactions. One could note that activity could be enhanced greatly by varying the size, shape and composition of nanoparticle ¹³. For instance, it was reported that when pure mesoporous silica (MCF-17) was modified with Aluminum, Lewis acid sites were created. This material is not active for the n hexane catalytic reforming. When it is modified with colloidally synthesized platinum nanoparticles, the catalyst showed enhanced activity ^{8, 14, 15}. Ni and Pt-Ni were also used to increase the ativity of catalytic reforming ¹². It was reported that the bimetallic catalysts presented a higher activity in the isomerization of n-hexane compared to the monometallic ones.

N-hexane decomposition is an important parameter for the n-hexzane catalytic reforming process. At present, ne-hexzane decomposition was examined on CNT supported Pt and Pt-Au bimetallic catalysts under microwave irradiation. In our previous, microwave irradiation was also used for different applications ^{16, 17}. CNT supported Pt and Pt-Au catalysts were prepared by NaBH₄ reduction method, characterized via BET, EDX, and XRD.

2. Materials and Methods

In this research, the synthesis weight 0.1 g of Carbon Nano Tube (CNT) support dissolved in 40 ml water and stirrer for 10 min. Then, H₂Cl₆Pt₆.xH₂O precursor was added to the catalyst solution for absorbent CNT to Pt stay in ultrasonic bath about 20 min. Following this, the different amount of NaBH₄ was added to the solution for reduction after 20 min in ultrasonic As a result, the obtained precipitate was filtrated, washed with by-distilled water and dried at oven vacuum 75 °C. By this method, CNT supported Pt catalysts was obtained. The synthesis details of the CNT supported Pt catalysts were given Table 3.1. For the synthesis of CNT supported Pt-Au catalysts, 0.1 g of CNT support was used and dissolved in the water. Then, this mixture was ultrasonicated and stirred for 10 min and H₂Cl₆Pt₆.xH₂O precursor was dissolved in the same kind of solvent for absorbent CNT to Pt. These two solution were mixed, ultrasonicated and stirred for 20 min. Then NaBH₄ was added to reduce this solution and ultrasonicated for reduction after 20 min in ultrasonic bath. Following this, AuCl was added to this solution and further reduction was performed. A precipitate was obtained, filtrated, washed with by-distilled water and dried under 75 °C an oven vacuum. As a result, Pt-Au catalysts was obtained at varying ratios. The synthesis details of the CNT supported Pt-Au catalysts were given Table 1.

The atomic ratio of the CNT supported Pt and Pt-Au catalysts was obtained by EDX analysis using a scanning electron microscope Phillips XL30 with a 20 keV electron beam. X-ray diffraction (XRD) analyses were performed using a Rigaku diffractometer model Miniflex II using Cu Kα radiation source. The diffractograms were recorded from 2θ = 50° to 85° with a step size of 0.05° and a scan time of 2 s per step. All the catalysts and the characterization methods used for these catalysts were given in Table 2.

In this study, a Microwave Anton Paar reactor was used to investigate the decomposition of n-hexane on CNT bimetallic catalysts. 0.05 g catalyst was inserted in 20 ml reactor tube. Furthermore, 10 ml n-hexane was added in to the reactor tube. Then, this mixture was heated to 150 °C and its temperature was kept constant at this temperature for 30 min. At the end of the 30 min, the tube was cooled and removed from the reactor. The volume change is defined and read out.

3. Results and Discussion

Characterization results of these catalysts were given on Table 3. The elemental compositions of these catalysts investigated using EDX spectroscopy were given in Table 3. Considering all CNT supported Pt and Pt-Au catalysts, the nominal weight percent of Pt is 10% and the EDX results show that weight percentage of Pt is around 10% similar to the nominal values. Furthermore, BET surface areas were also given on Table 3. According to these values, BET surface areas decrease by increasing metal loading.

X-ray diffractograms of Pt-Au (50:50) catalyst were given in Figure 1. XRD pattern of this catalyst exhibited a broad peak at about 2θ = 25° associated to the carbon support material. Furthermore, (111), (200), (220) of face centered cubic (fcc) structure of Pt and Pt alloys were presented at XRD pattern.

N hexane decomposition measurements were performed on Pt and Pt-Au catalysts. The volume change was given for every catalyst on the Table 4. It was observed that Pt-Au catalysts at prepared at 50:50 atomic ratio gives the best catalytic activity.

4. Conclusions

In the light of the experimental results, the following conclusion could be drawn:

• CNT supported Pt and Pt-Au catalysts at different atomic ratios were prepared succesfully via NaHB₄ reduction method

• Pt and Pt-Au catalysts were characterized by BET, EDX, and XRD.

• EDX measurements revealed that catalysts were prepared at the desired Pt content.

• BET measurements indicated that Au addition leads to decrease the surface area of bimetallic catalysts.

• XRD pattern of Pt-Au catalyst indicate Pt fcc peaks belonging the Pt-Au structure were observed.

• Best catalytic activity for the n-hexzane decomposition reaction was obtained for Pt-Au (50:50)/CNT catalyst.

Acknowledgements

Authors would like to thank for the financial support for The Scientific and Technological Research Council of Turkey (TUBITAK) project (project no: 113Z249) and TUBITAK project (Project no: 114M156).

References