Multi-Walled Carbon Nanotubes Composite Catalysts with Pd Nanoparticles for Methanol Oxidation

Multi-walled carbon nanotubes (MWNTs)-supported palladium (Pd) nanoparticles (NPs) catalyst (Pd/MWNTs) for methanol oxidation was prepared by a novel one-pot bottom-up method, i.e., direct reduction of palladium acetylacetonate (Pd(AcAc)₂) in the refluxing xylene solution in the presence of carboxylic groups functionalized MWNTs without any assistance of reduction agents. Transmission electron microscope (TEM) micrographs revealed that Pd NPs with an average size of 14.0 nm were uniformly grown onto MWNTs surface, confirming a successful synthesis of the catalysts. The loading of Pd was calculated to be 24.1% from the thermal gravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) results. Raman spectroscopy and X-ray diffraction (XRD) results indicate an obvious interaction between Pd NPs and MWNTs. The electrocatalytic activity of the Pd/MWNTs catalyst was evaluated for methanol oxidation in KOH solutions. Parameters including the concentration of methanol and KOH were investigated.

 

Experimental

A facile one-pot solution-based reduction method was developed to synthesize the composite catalyst with Pd NPs decorated on MWNTs. Briefly, 100.0 mg MWNTs was dispersed with 50 mL xylene in a 100 mL beaker under ultra-sonication for 1 hour, after which the MWNTs/xylene mixture was transferred into a 250 mL 3-neck flask. Then, the solution was heated to reflux ( ~140 °C) after 20 min. At the meantime, 304.0 mg Pd(AcAc)₂was dispersed with 20 mL xylene in a 50 mL beaker under magnetic stirring for 10 min, and the mixture was then transferred into the 3-neck flask. The whole solution was then kept refluxing for an additional 3 hours to complete the reaction. After that, the final solution was cooled down to room temperature naturally, filtered under vacuum and rinsed with ethanol and distilled water 3 times, respectively. The final product (black powders) was collected after vacuum drying at 50 °C for 24 hours.

 

Results and Discussion

Pd NPs are successfully synthesized and uniformly dispersed on the MWNTs support with no obvious agglomeration, Pd NPs are strongly insert into the MWNTs, implying an intensively chemical interaction between Pd NPs and MWNTs due to the promoting function of carboxylic groups on the surface of MWNTs.The peak current increase when the KOH concentration increases from 0.2 M to 0.5 M, a further increase of the KOH concentration to 1.0 M leads to an obvious decrease in peak current, the peak potential has a continuous negative – shift with the increase of KOH concentration, the negative-shift in the peak potential suggests that increasing KOH concentration has a favorable effect on the oxidation of methanol, the changes of peak current and peak potential clearly illustrate that the optimum peak current can only be achieved by a balance between the methanol and hydroxyl adsorbates coverage.

Conclusion

In this work, Pd/MWNTs catalyst has been successfully prepared by a facile one-pot bottom-up method. The electrocatalytic activities of the Pd/MWNTs catalyst for methanol oxidation are evaluated by varying the concentration of methanol and alkaline and changing the temperature. The reaction of the adsorbed intermediate species with the adsorbed hydroxyl is found to be the rate-determining step and an optimum peak current can only be achieved by a balance between methanol and hydroxyl adsorbates coverage. A pronounced influence of temperature on the Pd/MWNTs is manifested, implying greater tolerance of the catalyst towards poisoning residues at high temperatures.

Acknowledgments

The financial supports from Seeded Research Enhanced Grant (REG) and College of Engineering at Lamar University are kindly acknowledged.

Figure 1. Cyclic voltammograms of the methanol oxidation reaction on the Pd/MWNTs electrode in solutions containing 3.0 M methanol with various KOH concentrations of (a) 0.2 M, (b) 0.5 M, and (c)1.0 M at a scan rate of 50 mV s^-1at room temperature. Inset shows TEM image of the Pd/MWNTs.

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