Density Functional Theory (DFT) and Experimental Evidences of Metal–Support Interaction in Platinum Nanoparticles Supported on Nitrogen- and Sulfur-Doped Mesoporous Carbons: Synthesis, Activity, and Stability

Abstract

In this paper, we report a comprehensive investigation of Pt nanoparticles (NPs) deposition on nitrogenand sulfur-doped or codoped mesoporous carbons (N-MC, S-MC, and N,S-MC) to develop active and durable oxygen reduction catalysts for fuel cells. N-MC, S-MC, and N,S-MC were prepared by employing mesoporous silica as hard template and suitable organic precursors. Pt NPs were deposited by solidstate reduction of platinum acetylacetonate under N 2 /H 2 flow on the three different supports. Pt NPs resulted to be welldispersed over the doped MC supports with size distributions (from 1.8 nm to 3.5 nm) that are dependent on the type of doping heteroatom (N, S, or N and S). The influence of nitrogen and/or sulfur incorporated into the carbon matrix on the nucleation and growth of Pt NPs was also rationalized based on density functional theory (DFT) simulations. They highlighted that both nitrogen and sulfur increase the interactions between Pt and carbon support, but the interaction decreases as the nitrogen and sulfur functional groups become closer. The effect of sulfur content on the size and activity of Pt NPs was also evaluated. Electrochemical measurements in 0.5 M H 2 SO 4 electrolyte allowed us to investigate the behavior of Pt NPs and to assess the relationship with electrochemical activity and stability. The Pt/S-MC showed mass activity and specific activity comparable with the state-of-the-art commercial standard Pt/C Tanaka (Pt 46% on Vulcan XC72), and the highest catalytic activity, with respect to Pt/N-MC and Pt/N,S-MC, was associated with a stronger interaction between Pt NPs and a thiophenic-like group, as proven by DFT calculations and X-ray photoelectron spectroscopy (XPS) analysis. Pt/S-MC was incorporated in a membrane electrode assembly and tested as cathode material in a PEM fuel cell, while accelerated degradation tests up to 10 000 voltammetric cycles were carried out in 0.5 M H 2 SO 4 : the influence of the doped support on the durability of the catalyst under harsh operational conditions has been highlighted.

Mesoporous Carbon with Different Density of Thiophenic‐Like Functional Groups and Their Effect on Oxygen Reduction

“…This result suggests a cooperative effect among S defects and that the metal–support interaction is more effective when the support contains a high number of sulfur defects, as observed from the XPS analysis. In all cases, the interaction energy can be considered high, based on the coarse scale that we used in our previous work . We recall that, based on the average Pt−C bond energy, we considered the interaction as high, medium, or low if Δ E <−280 kJ mol −1 , between −280 and −200 kJ mol −1 , or >−200 kJ mol −1 , respectively.…”

“…We recall that, based on the average Pt−C bond energy, we considered the interaction as high, medium, or low if Δ E <−280 kJ mol −1 , between −280 and −200 kJ mol −1 , or >−200 kJ mol −1 , respectively. The high interaction regime was expected because, as observed in a previous work, S is able to stabilize Pt on the surface . Moreover, the trend found in the calculations suggests a synergetic effect of the two S atoms on the adsorption.…”

“…It has been extensively demonstrated that nitrogen‐doped carbon exerts a strong interaction with the supported Pt NPs, improving their electronic properties and catalytic activity . Also, sulfur affords a clear metal–support interaction between Pt NPs and the carbon support –. The specific effect of sulfur doping on the electronic properties of Pt was demonstrated by Higgins et al., whereas Hoque et al .…”

“…Although the positive effect of sulfur in a low percentage (2–4 %) is well‐established, to what extent the amount of sulfur can be increased in the carbon support is so far not well understood. For example, in the case of nitrogen and sulfur co‐doping, it was demonstrated that a high density of heteroatoms, that is, closely separated nitrogen and sulfur functional groups, disfavors the formation of small Pt nuclei and their catalytic activity . In this work, sulfur‐doped mesoporous carbons (SMCs) with different densities of sulfur functional groups (0<S wt % <14) were synthetized and modified with Pt NPs, aiming at verifying to what extent heterofunctional groups can modulate the metal–support interaction and the Pt NPs catalytic activity.…”

Understanding Heteroatom-Mediated Metal–Support Interactions in Functionalized Carbons: A Perspective Review

“…When pyridine-like functionalities are introduced on the surface of graphene, highly localized acceptor-like states close to the Fermi level are created, leading to a stronger interaction between graphene and metal atoms, and thus favoring a high metal dispersion. The DFT method has been implemented to determine how far the nucleation and growth of Pt NPs onto a carbon matrix (described by a 4 × 4 supercell of graphene) are affected by the presence of nitrogen (pyridinic, pyrrolic, graphitic nitrogen) and and/or sulfur (thiophenic defects) [74]. For this purpose, the interaction energy of Pt on doped graphene surfaces was evaluated in terms of adsorption energy.…”