Pd/MWNTs Nanocatalysts Toward Formic Acid Oxidation

Operating conditions such as the composition of electrolyte and temperature can greatly influence the performance of formic acid oxidation reaction (FAOR). In order to achieve the best performance at various conditions, we employed an as-synthesized high electrocatalytic palladium decorated multi-walled carbon nanotubes nanocatalysts (Pd/MWNTs) to investigate the effects of formic acid (HCOOH) and sulfuric acid (H₂SO₄) concentrations as well as temperature on FAOR. Electro-characterization techniques such as cyclic voltammetry (CV) and slow linear sweep voltammetry (SLV) were employed to comprehensively investigate the catalyst performance and reaction kinetics, which indicate that the as-synthesized Pd/MWNTs gave the best performance under a condition with balanced adsorptions of HCOOH and H₂SO₄ molecules, and increasing temperature has a positive effect on FAOR. Two well acknowledged reaction pathways of FAOR as dehydrogenation and dehydration were detailed characterized by chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) conducted at various potentials. The dehydrogenation pathway was largely depressed by the increased dehydration pathway, which leads to the increased charge transfer resistance (R_ct) with increasing the relative ratio of HCOOH to H₂SO₄ concentration. Enhanced tolerance stability and reduced R_ctwith increasing temperature is observed due to the enhanced reaction kinetics which simultaneously increased the dehydration process.

 

Experimental

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 were uniformly dispersed and anchored on the MWNTs support due to the promoting function of carboxylic groups on the surface of MWNTs, which is demonstrated by TEM, Raman and XPS spectra. The hydrogen adsorption and the oxidation of poisoning species processes during the potential sweeping are essential for the enhancement of electroactivity. The main oxidation process shifts between dehydrogenation and dehydration with varying the ratio of HCOOH to H₂SO₄ concentration, which is illustrated by the variation of oxidation peak potential and current. Greatly facilitated reaction kinetics are demonstrated by the increased oxidation current and decreased charge transfer resistance with increasing temperature.

Conclusion

Pd/MWNTs as a representative of the Pd-based catalyst were employed to investigate the FAOR mechanism and poisoning effect on Pd/MWNTs electrode in H₂SO₄ solution. It is demonstrated that the hydrogen adsorption in low potential range and the oxidation of poisoning species during the high potential range both contribute to the enhancement of electroactivity of Pd/MWNTs. The dual pathway mechanism are comprehensively studied by CA and EIS which indicate that the oxidation of HCOOH through dehydrogenation pathway only proceeded in conditions with relatively lower HCOOH and H₂SO₄ concentrations. Increasing HCOOH concentration can directly increase the dehydration process proportion and cause the production of CO_ads species. H₂SO₄ as donor of H⁺ can greatly facilitate the on-set oxidation of HCOOH in the beginning process but it will largely depress the HCOOH oxidation with excess amount of H⁺. Finally, increasing temperature can boast the mobility of ions thus enhance the reaction kinetics. Dehydration pathway is also existed at higher temperature due to the increasing produced CO_ads species.

Acknowledgments

The financial supports from University of Tennessee Knoxville are kindly acknowledged.

Figure 1. CV of the formic acid oxidation reaction on the Pd/MWNTs electrode in 0.5 M H₂SO₄ solution containing 0.2 M HCOOH. Background shows the TEM image of the Pd/MWNTs.

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