Stability, magnetic, energetic, and reactivity properties of icosahedral M@Pd12 (M = Fe, Co, Ni, and Cu) core-shell nanoparticles supported on pyridinic N3-doped graphene
Graphical abstract
Description: Icosahedral M@Pd12 (M = Fe, Co, Ni, and Cu) core-shell nanoparticles supported on pyridinic N3-doped graphene.
Introduction
The study of transition metal nanoparticles represents a very important research area, because their physical-chemical properties depending strongly on their composition, shape, and size [[1], [2], [3], [4]]. Additionally, they are used for many applications such as in catalysis, electronics, biomedicine, nanotechnology, among others [[5], [6], [7], [8]]. To the date, there are several studies of Pd nanoparticles as catalysts, some interesting examples are oxygen reduction in acid media, formic acid oxidation, and Suzuki reaction [[9], [10], [11]]. However, some of the drawbacks of the use of Pd nanoparticles as catalyst are its high cost and the long-term unavailability. To overcome these disadvantages and to optimize catalytic performance of Pd nanoparticles, the use of bimetallic Pd-based nanoparticles has been proposed because they have demonstrated equal or better catalytic activity than the monometallic Pd nanoparticles [[12], [13], [14], [15]].
During the last decade, numerous attempts have been made to propose transition metal alloy catalysts because they present superior catalytic activities than pure metals due to the synergistic effect between both metals [16]. In this context, bimetallic clusters core-shell-type have been designed and studied extensively, since this type of structures, composed of a metal located in the center of the nanoparticle (core) and another one distributed on the outer layer (shell), offers several advantages. Firstly, the interaction between the core and shell can improve the catalytic properties and stability of the nanoparticles. Secondly, the efficient use of noble metals because they are usually used in the shell of the nanoparticles, among other advantages [[17], [18], [19]].
Nevertheless, nanoparticles tend to agglomerate owing to their high surface energy [20]. Therefore, the use of support materials that presents a good interaction with the metallic nanoparticles is required to avoid the agglomeration of the nanoparticles [[21], [22], [23], [24], [25], [26], [27]]. In addition, the catalytic activity of nanoparticles depends strongly on the nature of the support material [28]. In this context, the versatility of graphene has been demonstrated in several works. Particularly, properties such as the large specific surface area, excellent electrical conductivity, resistance to corrosion, and good chemical stability make graphene an interesting support material for metal nanoparticles in catalysts [29,30]. Pristine graphene is a sp2-bonded carbon allotrope and is relatively inert [31]. However, it has been established that point defects on graphene, such as vacancies, doping or functionalization can modulate the interaction of support with metal nanoparticles [[32], [33], [34]]. This modifies the electronic properties, generally enhancing the catalytic activity of the nanoparticles [35]. When these types of defects appear in graphene, sp2-bonds are absent and carbon atoms surrounding defects are no longer flat, presenting a different electronic structure, and being activated for further chemical reactivity [36]. For these reasons, theoretical and experimental studies about the stability and reactivity of nanoparticles supported on modified graphene are of great relevance in catalysis.
From theoretical point of view, Song et al. investigated the stability of monometallic Fen and Nin nanoclusters (n = 13, 38, and 55) over pristine, defective (monovacancy and divacancy) and strained graphene using self-consistent charge density-functional tight binding. They demonstrated that the interaction and the charge transfer increase when the defect in graphene is introduced, enhancing the stability of the nanoclusters [37]. Similar conclusions were obtained by Sahoo et al. when they studied Fe, Co, and Ni nanoparticles supported on pristine and defective graphene (sheets and flakes) employing the density functional theory (DFT) [34]. It was concluded that it is necessary a defect in the sheets or flakes for the stabilization of the nanoparticles. Referent to another kind of graphene modification, to dope graphene with N, B, P, I, and S atoms has demonstrated to be an excellent strategy to tune the electron-donor properties and to optimize the catalytic activity of graphene [38].
In this context, particularly the performance of pyridinic N3-doped graphene (PNG) as support material for metal nanoparticles in catalytic reactions, has been studied and found to be better than non-defective graphene. This is due to the optimal interaction with the nanoparticles and the ultimate transfer charge, properties that make it a promising support material for catalytic reactions [39,40]. In this direction, several kinds of small clusters supported over PNG have been studied theoretically using the DFT [[39], [40], [41]]. Liu et al. studied the O2 adsorption and dissociation on Pt clusters supported on PNG. They found that the Pt clusters supported on PNG accumulate positive polarized charges, which facilitates the O2 dissociation [39]. In a similar way, it was reported that Ag8 cluster supported on PNG show major catalytic activity toward O2 dissociation than those supported on non-defective graphene [40]. In another study, it was demonstrated that the stability of Nin (n = 1–6) clusters on PNG are stronger than on pristine graphene [41]. These studies provide good evidences on the effect of PNG supports on the stability and reactivity of monometallic clusters supported on PNG. However, DFT studies are still needed to improve the understanding of the effect of PNG support on the stability and catalytic activity of core-shell nanoparticles.
To the best of our knowledge, there is not a computational study about the stability, structural, magnetic, energetic, and reactivity properties of bimetallic Pd-based core-shell nanoparticles supported on PNG. Therefore, a DFT analysis about the stability, magnetic, energetic, and reactivity properties of Pd13 and M@Pd12 (M = Fe, Co, Ni, and Cu) core-shell nanoparticles supported on PNG is developed. The investigated properties of the nanoparticles/PNG composites are compared with the isolated nanoparticles.
Section snippets
Calculations details and models
All electronic structure calculations were performed with the auxiliary density functional theory (ADFT) using deMon2k program [42,43]. A fine grid was used to integrate numerically the exchange correlation potential [44,45]. The variational fitting method proposed by Dunlap and collaborators was used to calculate the Coulomb energy [46,47]. For exchange and correlation functional, the revised PBE functional [48] proposed by Hammer and collaborators was used [49]. The Pd and Cu atoms were
Structural, magnetic, energetic, and reactivity properties of icosahedral Pd13 and M@Pd12 (M = Fe, Co, Ni, and Cu) core-shell nanoparticles
First, the structural, magnetic, energetic, and reactivity properties of icosahedral Pd13 and M@Pd12 core-shell nanoparticles were investigated (see Table 1). The calculated properties of M@Pd12 core-shell nanoparticles are compared with those of pure Pd13 nanoparticles. In Fig. 2, the icosahedral structures of Pd13 and M@Pd12 core-shell nanoparticles are illustrated. To demonstrate that the structures reported in Fig. 1 are minimal, frequency analysis was developed. As can be seen from Table 1
Conclusions
The stability, magnetic, energetic, and reactivity properties of icosahedral Pd and M@Pd12 (M = Fe, Co, Ni, and Cu) core-shell nanoparticles supported on PNG were investigated using the ADFT method. The structural, magnetic, energetic, and reactivity properties of the M@Pd core-shell nanoparticles are notably modified with regards to pure Pd13 nanoparticles. These changes in the properties can be associated with the M atoms positioned in the center of the M@Pd12 core-shell nanoparticles. From
Credit author contribution statement
E. P. Sánchez-Rodríguez: Conceptualization, Methodology, Formal analysis, Writing – original draft, C. N. Vargas-Hernández: Methodology, Investigation, H. Cruz-Martínez: Conceptualization, Formal analysis, Writing – original draft, Writing – review & editing, D. I. Medina: Writing – review & editing, Supervision, Funding acquisition,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors thankfully acknowledge computer resources, technical advice and support provided by Laboratorio Nacional de Supercómputo del Sureste de México (LNS), a member of the CONACYT national laboratories, with project No. 201903079N.
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