Annealing Effect on As-Synthesized Pd/MWNTs Catalyst for Ethanol Oxidation

The annealing effect on the catalytic performances of multi-walled decorated palladium nanocatalyst (Pd/MWNTs) for ethanol was investigated. The as-synthesized and annealed nanocatalysts were characterized by X-ray diffraction (XRD), which implied similar crystal structure but different average particle sizes. Transmission electron microscope (TEM) revealed different Pd particle sizes, suggesting varying steps: diffusing, contacting and coercing of Pd nanoparticles (NPs) on MWNTs surface with increasing the annealing temperatures. The removal of oxygen-containing groups on the tube wall surface during the annealing process was confirmed by the increased I_D/I_G ratio in raman spectroscopy and thermogravimetric analysis (TGA). Cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) displayed enhanced peak current, increased stability and decreased charge transfer resistance for Pd/MWNTs annealed at 100 °C (Pd/MWNTs-100). The reason might be due to the smallest Pd particle size, increased electrochemically active surface area (ECSA) and decreased oxygen content. Agglomeration of Pd NPs is responsible for the decreased catalytic performance for nanocatalysts annealed at high temperatures. Meantime, it's also found that the electric-catalytic activity towards ethanol oxidation was determined by a delicate cover balance between ethanol and OH_adspecies. Significant fast kinetics at higher temperatures indicates that increasing temperature can greatly activate the ethanol oxidation process, ensuring greater tolerance towards ethanolic residues at higher temperatures.

Experimental

100.0 mg carboxylic groups functionalized MWNTs (MWNTs-COOH) and 304.0 mg palladium acetylacetonate (Pd(acac)₂) were uniformly dispersed in 80 mL xylene using a 250-mL 3-neck flask and kept refluxing for 3 hrs. Pd/MWNTs powder was obtained from filtering the reacted solution. The synthesized Pd/MWNTs was annealed at different temperatures, 100, 300, and 500 °C under hydrogen gas (5 % H₂balanced with 95 % Ar) flow in a quartz tube situated in a tubular furnace. The annealing temperature was increased to a desired temperature at a heating rate of 2 °C/min and was maintained for 2 hrs at the set temperature.

Results and Discussions

Pd NPs with same Pd crystalline planes but different Pd NPs sizes are confirmed in Transmission electron microscope (TEM) and X-ray diffraction (XRD), the Pd NPs of as-synthesized/annealed Pd/MWNTs nanocatalyts varied according to steps as diffusing, contacting and coercing with the increase of annealing temperature. The clean particle surface, the smallest Pd particle size and increased ECSA are responsible for the best electro-catalytic performance of Pd/MWNTs-100, however, agglomeration of Pd NPs at higher annealing temperatures is the main reason causing the decrease of electro-catalytic activity.

Conclusions

In this work, different annealing temperature effect on as-synthesized Pd/MWNTs was investigated, Pd NPs followed steps as diffusing, contacting and coercing on MWNTs surface with the increase of annealing temperature, the particle size was observed to decrease first due to the partially segregated small Pd NPs from agglomerated ones and further aggregated forming size-increased Pd NPs. The smallest Pd NPs size, decreased oxygen content and increased ECSA were responsible for the best performance of Pd/MWNTs-100 ºC demonstrated in CV, CA, and EIS tests, The best electro-catalytic performance of Pd/MWNTs-100 was achieved in 2.0 M KOH containing 4.0 M ethanol which provided balance coverages of OH_adsand ethanol. A pronounced influence of temperature on the reaction kinetics was manifested implying greater tolerance of the catalyst towards ethanol residues at higher temperatures.

Acknowledgments

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

Figure 1(a-c) are TEM images of Pd/MWNTs, Pd/MWNTs-100 and Pd/MWNTs-300, respectively, inset TEMs are corresponding histograms of particle size distribution. Figure (d) is the CVs in 1.0 M KOH containing 1.0 M ethanol at a scan rate of 50 mV s^-1. a, Pd/MWNTs, b, Pd/MWNTs-100, c, Pd/MWNTs-300 and d, Pd/MWNTs-500.

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