DFT Study On Dispersion And Stability Of Pd Supported On The Polyaniline Modified Carbon

Formic acid is a promise alternative candidate as fuel for portable movable power due to low crosslink through membrane, higher energy density, low cost and easy transportation and storage compared to methanol in fuel cell system. Usually, formic acid is oxidized mainly through two pathways. One is direct way, formic acid dissociates into CO₂ and H⁺, as well as e-. The other is indirect way, first resulting into CO and H2O, then CO and H2O reacting into CO₂, H⁺ and electron. Intermediate CO is strongly adsorbed onto catalyst surface, inhibiting sequent adsorption of formic acid onto catalyst surface, so it deactivates catalyst. The way for oxidation of formic acid depends on the type of catalyst. Compared to Pt catalyst, Pd can induce HCOOH to dissociate in the direct way.Experimental results show that Pd particles are well dispersed on the poly-aniline modified carbon support. Enhancement of catalytic activity and longer lifetime is ascribed for increase in ability to anti-agglomeration of Pd/PANI/C catalyst during the operation process of fuel cell. To give deep insight into these macro properties, we employ density functional calculation to explore the exact reasons for well dispersion and high stability of Pd/PANI/C, and the ability of anti-agglomeration.Firstly, we obtained the configuration of Pd cluster adsorbed on the sites near defect and away defect by geometry optimization. Calculated results show that the binding energy for Pd near defect is much higher than that for Pd away defect, which indicate that Pd clusters are more inclined to nucleate and grow near the defect, while there is little difference in strength of adsorption between Pd and Poly-aniline modified carbon. And it indicates that PANI eliminates difference in binding energy induced by defects, and therefore PANI is benefit for dispersion of Pd particles. Also, dispersed Pd particles are more difficult to agglomerate due to larger inter-distance among Pd atoms. This hypothesis is also evidenced by the fact that probability of Pd deposition from Pd ion in solution to Pd atom at any point of PANI/C support is identical.Secondly, in order to explore the formation mechanism of Pd particles with different radius, difference in stability of Pd catalyst deposited onto pure carbon support and PANI-modified carbon support and electronic structure for corresponding system, we calculate binding energy, bond length, total density of states (TDOS) and partial density of states (PDOS). For PANI-modified carbon support, Pd particles are strongly anchored at PANI-modified carbon surface. And therefore, stability of Pd catalyst is enhanced due to strong interaction.Finally, according to the experimental fact that PANI and Pd have synergistic effect on the oxidation of formic acid, we compare bond length, Mulliken net charge, binding energy for Pd/C and Pd/PANI/C system.