Spontaneous reduction of O2 on PtVFe nanocatalysts

doi:10.1016/j.cattod.2011.01.015

Catalysis Today

Volume 165, Issue 1, 16 May 2011, Pages 150-159

https://doi.org/10.1016/j.cattod.2011.01.015 Get rights and content

Abstract

Reduction of O₂ in the presence of model PtVFe nanocatalysts was studied using the PBE functional with a plane wave basis set. The model catalysts consisted of trimers and a 0.6-nm particle. The results show that among three molecular chemisorption configurations, i.e. Pauling, Griffith, and Yeager configurations, the O₂ bond is weakened the most in the Yeager configuration, then the Griffth configuration, and then the Pauling configuration. A new molecular chemisorption configuration, i.e. 5-atom ring configuration, was also identified. With the O–O distance up to 1.4 Å, a linear correlation was found between the O₂ stretching frequency and the O₂ bond distance regardless of the metal or adsorption site. However, the charge transfer and the adsorption energy are highly dependent on the metal and adsorption site. The alloyed clusters are most effective in transferring electrons to O₂ species and weakening O₂ bond especially when the O atoms are attached to non-Pt atoms. Our results suggest that the superior catalytic activity of PtVFe nanoparticles in the oxygen reduction is due to the effectiveness in charge transfer and the presence of direct (spontaneous) dissociation pathway.

Introduction

One of the most significant challenges for proton exchange membrane (PEM) cell applications is the kinetic limitation of oxygen reduction reaction (ORR) at the cathode at temperatures lower than 100 °C. The traditional catalyst used in the cathode of a PEM cell is platinum. Studies of O₂ reduction on a single Pt atom, clusters, or surfaces have been carried out extensively [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. Great efforts have been made towards developing active, robust, and low-cost electrocatalysts for ORR including Pt-containing multi-metallic nanoparticles [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82], [83], [84], [85], [86], [87], [88], [89], [90], [91], [92], [93], [94], [95], [96], [97], [98]. For instance, the ternary PtVFe catalysts, prepared by pre-synthesized trimetallic nanoparticles, have been shown to exhibit superior electrocatalytic activities for ORR in actual fuel cells [97], [98]. Although many exciting reports can be found on the improved performance of catalysts on ORR, the reaction mechanism is still not well understood, partially due to the complex reaction pathways exhibited by ORR [99], [100], [101], [102], [103]. For instance, most recently, Goddard's group has studied the solvent effect on the ORR and their results suggested that alternative mechanisms must be considered other than the well-known plausible 4-electron or 2-electron mechanisms [101].

Reaction pathways for ORR in the presence of multi-component nanoparticles are even more complicated than the single-element nanoparticles. The recent experimental work by Li et al. suggests that the ORR catalyzed by Co-N/C catalysts is a mixture of 2- and 4-electron transfer pathways, dominated by a 4-electron transfer process [104]. Furthermore, as the results have demonstrated in Goddard's group, the preferable reaction pathways could be altered when solvent is considered [101]. On the other hand, the central theme of the ORR is ultimately the breaking of O–O bond, either in a pure O₂ or an O–OH environment. We, therefore, chose to study breaking O–O bond of O₂ as a representative reaction for the ORR. Moreover, we chose to use trimers and a 0.6-nm particle as the model nanocatalysts in the density functional theory (DFT) study.

Molecular chemisorption of O₂ molecules on Pt catalysts can take place in three distinguished configurations. The first adsorption configuration is denoted as the Pauling model where only one O atom in O₂ is bonded to a metal atom. The second configuration is denoted as the Griffith model where both O atoms are bonded to a single metal atom. The third configuration is denoted as Yeager model where two O atoms are bonded to two metal atoms. In Fig. 1, these three molecular adsorption configurations are illustrated using a Pt₃ trimer as an example. Among the three molecular chemisorption configurations, the Pauling and Yeager configurations have been studied more extensively theoretically. Two states denoted as superoxo-like (O₂^−δ) and peroxo-like (O₂^−2δ) species have been identified [3]. It has been proposed that peroxo-like states are associated with the Yeager configuration and the superoxo-like states to the Pauling configuration [3]. The stretching frequency and bond length of the adsorbed O₂ are also associated closely with the adsorption configurations. For instance, in the study of O₂ adsorption to one Pt atom in the presence of organic molecules, Cramer et al. have shown that the superoxo-like species have a bond length of about 1.2–1.3 Å while the peroxo-like species have a bond length of 1.4–1.5 Å [8]. The reported frequency range associated with two different adsorption states, however, are different. Cramer et al. found the adsorbed O₂ frequencies are 1050–1200 cm⁻¹ and 800–980 cm⁻¹ for the superoxo-like and peroxo-like species, respectively. Siegbahn and Panas calculations gave the frequency range of 870–1020 cm⁻¹ for the superoxo-like species and 610–660 cm⁻¹ for the peroxo-like species [7]. Even earlier studies by Vaska gave the frequency range of 1075–1195 cm⁻¹ for the superoxo-like species and 790–932 cm⁻¹ for the peroxo-like species.

Dissociative chemisorption of O₂ molecules in the presence of Pt catalysts has been studied as well [6], [15], [16], [105], [106], [107], [108], [109], [110]. It was believed the O₂ dissociation takes place mostly through a barrier and the activation energy from the peroxo-like precursor is about 0.29 eV [15]. Nolan et al. has shown that direct (spontaneous) pathways for O₂ dissociation on Pt surfaces are negligible [15]. The dissociation mechanism may be changed by altering the dissociation energy barrier or by stabilizing the precursor and transition states [106] as well as the chemisorbed species in the vicinity of the O₂ that is to be dissociated [16].

Platinum has traditionally been the metal of choice, but other alternatives are being explored due partly to the high cost and modest activity of platinum with respect to the ORR. Since the late 90s, bimetallic nanoparticles, such as PtAu, [111] PtNi, [10], [112], [113] PtCo, [10], [112], [113], [114] PtCr, [113] PtMn, and PtFe, [114] have been found not only to provide some further flexibility of tuning catalytic activities in oxygen reduction reactions but also to have substantially higher activity. In the study of reaction barriers, Anderson et al. have found that the reaction barriers are not very different in the presence of pure Pt with respect to PtNi or PtCo alloys [10]. Using the transition state theory, they further attributed the fast O₂ reduction reaction using the bimetallic alloys to the large prefactor.

Ternary Pt_mV_nFe_k nanoparticles have been shown experimentally to exhibit superior catalytic activities for O₂ reduction over the pure Pt or bimetallic PtNi and PtFe nanoparticles [96], [97], [98]. The Pt_mV_nFe_k nanoparticles were found to be four times as active as the commercially available Pt catalysts and two times as active as the PtFe nanoparticles. Except for the high catalytic activities of the ternary Pt_mV_nFe_k catalysts, no other characterizations of detailed mechanistic aspects, such as the adsorption site, the electronic state of the adsorbed O₂ species, and the O₂ stretching frequency, were available. To better understand the reaction mechanism of the ternary Pt_mV_nFe_k nanoparticle and to provide further insight into searching for more active and sustainable O₂ reduction reaction catalysts, it is critical to investigate the ternary Pt_mV_nFe_k nanoparticles both experimentally and theoretically.

Therefore, we chose to study the O₂ molecular and dissociative adsorptions on a model Pt_mV_nFe_k catalyst and report here the first results obtained for these systems. In this work, the PtVFe trimer and a 0.6-nm PtVFe particle were chosen as the model catalysts. For comparison purposes, the catalytic activities of Pt₃ trimer and PtFe₂ trimer on the O₂ adsorption were also studied. To select model catalysts, we also studied bare metal dimers and trimers with all possible compositions. Density functional theory calculations were performed on these model systems. Detailed description of the computational techniques is given in Section 2. The O₂ molecular adsorption was investigated with all three adsorption configurations, i.e. the Pauling, the Griffith, and the Yeager model. Adsorption energy, charge transfer to the O atoms, and the O₂ stretching frequency were obtained from the DFT calculations. These results are given in Section 3 together with discussion and comparisons with other pure Pt or bimetallic PtFe systems. Finally, the conclusion is drawn in Section 4.

Section snippets

Computational details

The calculations were carried out using spin-polarized DFT method that is implemented in Vienna Ab-initio Simulation Package (VASP) [115], [116], [117]. The electron–ion interactions were described by Projector Augmented Waves (PAW) method [118]. The exchange and correlation energies were calculated using the Perdew–Burke–Ernzerhof (PBE) functional [119]. A plane wave basis set was used with a cutoff energy of 400 eV. Only the Γ point is needed for finite systems and therefore was used in this

Results and discussion

In order to study O₂ chemisorptions on model ternary Pt_mV_nFe_k catalysts, we studied the dimers and trimers with all possible compositions so that the most stable bare clusters can be used in adsorption calculations. In the first part of this section, we present the DFT results in bare cluster studies followed by presentations of the results on the O₂ molecular and dissociative adsorption, respectively. We will also present the results on the O₂ adsorption on a PtVFe particle of 0.6 nm to verify

Conclusions

DFT studies of O₂ molecular and dissociative chemisorptions on the model ternary PtVFe catalysts, PtVFe, Pt₃, PtFe₂, and Pt₄V₂Fe₇ were carried out. In order to choose model catalysts and to investigate the homogeneity of surface composition of alloy catalysts, more than 40 bare alloy dimers and trimers with all possible compositions were studied.

Sixteen O₂–Pt_mV_nFe_k molecular chemisorption complexes were studied with four adsorption configurations. Among the three configurations being studied,

Acknowledgement

We acknowledge the National Science Foundation for support of this research under grant CBET-0709113.

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Spontaneous reduction of O2 on PtVFe nanocatalysts

Abstract

Introduction

Section snippets

Computational details

Results and discussion

Conclusions

Acknowledgement

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Science

J. Power Sources

Electrochem. Commun.

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J. Alloys Comp.

Electrochim. Acta

Electrochem. Commun.

J. Electroanal. Chem.

J. Power Sources

Electrochem. Commun.

J. Power Sources

Electrochem. Commun.

J. Colloid Interface Sci.

Thin Solid Films

J. Colloid Interface Sci.

Appl. Catal. B: Environ.

Int. J. Hydrogen Energy

Catal. Commun.

J. Power Sources

Surf. Sci.

Electrochem. Commun.

Electrochim. Acta

Electrochim. Acta

J. Power Sources

J. Electroanal. Chem.

Electrochem. Commun.

J. Alloys Comp.

Int. J. Hydrogen Energy

Int. J. Hydrogen Energy

Electrochim. Acta

Int. J. Hydrogen Energy

Electrochim. Acta

Electrochem. Commun.

Electrochim. Acta

Electrochem. Commun.

Electrochim. Acta.

Surf. Sci. Rep.

J. Phys. Chem. B

J. Phys. Chem. B

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J. Phys. Chem. B

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Spontaneous reduction of O₂ on PtVFe nanocatalysts