Article pubs.acs.org/JPCC
Tungsten Carbide Supports for Single-Atom Platinum-Based FuelCell Catalysts: First-Principles Study on the Metal−Support Interactions and O2 Dissociation on WxC Low-Index Surfaces Chee Kok Poh,†,‡ San Hua Lim,† Jianyi Lin,†,‡ and Yuan Ping Feng*,‡ †
Heterogeneous Catalysis, Institute of Chemical Engineering and Sciences, A*star, 1 Pesek Road, Jurong Island, Singapore 627833 Department of Physics, National University of Singapore, S12-M01, 2 Science Drive 3, Singapore 117551
‡
ABSTRACT: As a first attempt to understand the role of tungsten carbide (WxC) surfaces as a support of single-atom platinum-based catalysts, the adsorption properties of Pt on WxC(100) surfaces and its impact on O2 dissociation are investigated using first-principle DFT calculations. Pt adsorptions on the dominant low-index WxC surfaces are found to be stable on various adsorption sites. The adsorbed Pt atoms are unlikely to diffuse into the bulk and, in general, resistant to bulk-like clustering. A correlation between the adsorption energies and the surface energies is observed; that is, the stronger adsorption of Pt atoms on the surfaces, the lower the surface energies of the Pt/WxC(100) systems. This leads to the further downshift of d-band center of the surface slabs. From the stability of Pt atoms on different WxC low-index surfaces, it is conclusive that WxC(100) surfaces are effective supports for single-atom platinum catalysts. Preliminary DFT results of O2 dissociation on single-atom Pt−WxC(100) system show the generation of new interface sites that thermodynamically favor oxygen dissociation, even at high coverage.
1. INTRODUCTION Electrochemical devices such as fuel cells, metal-air batteries, and electrolyzers are the core technologies for the progression toward a clean energy future.1−4 One of the most important parts for these devices is the catalyst on the electrodes that enhances the rate of the half-cell reactions with high efficiency. Among these electrochemical devices, proton exchange membrane (PEM) fuel cells have drawn increasing attention from consumer electronics and automotive industries due to high-energy density, zero or low emissions, low operating temperature, light weight, and fast start-up time. However, there are several crucial challenges that impede their large-scale commercialization. These include the use of the expensive platinum (Pt) catalyst, carbon monoxide poisoning of the Pt catalyst, and the formation of H2O2 during oxygen reduction reaction (ORR) that may result in polymer electrolyte membrane degradation, severe carbon electrode corrosion, and agglomeration of Pt nanoparticles. Platinum solely accounts for around 30 to 45% of the fuelcell stack cost, and thus the enhancement of the catalytic performance or the reduction of Pt loading will significantly reduce the stack cost.5 Many approaches have been reported in literature to increase the Pt utilization or reduce the Pt loading. For instance, the increase in Pt utilization can be achieved by high dispersion of Pt clusters on 3D aligned carbon nanotubes,6 by tuning the porosity of the catalyst layer,7 or by using wellstructured graphene/Vulcan carbon composite as the Pt catalyst support.8 A variety of nanostructured Pt catalysts, such as core−shell-structured Pt,9 hollow Pt-based nanocatalysts,10 and monolayer (ML) Pt catalysts,11 were also utilized to reduce the loading of Pt. © 2014 American Chemical Society
The idea of Pt ML catalysts is similar to the concept of “single-atom” catalysis proposed by Sir John Meurig Thomas.12 These catalysts are single-site heterogeneous catalysts with individual, isolated atoms anchored to a support. Single-atom catalysts not only have low noble-metal loading but also they may affect the kinetics of the reactions being studied and change the selectivity toward different molecular products. Recently, Kyriakou et al. demonstrated that individual, isolated Pd atoms on a Cu surface as the catalysts for hydrogenation of styrene and acetylene have higher selectivity compared with pure Cu or Pd metal alone.13 Qiao et al. also reported that isolated Pt atoms supported on FeOx exhibited excellent stability and high activity for both CO oxidation and preferential oxidation of CO in H2.14 The performance of an electrocatalyst may be greatly influenced by the properties of the support materials. The effects of the support on catalysis include decorative effect (effective dispersion), electronic effect (charge transfer between the metal and support), new alloy phase formation, and generation of new interface sites. Tungsten carbide was known for its good electrical conductivity and hence has been thought to be a possible support material for electrocatalysis. Levy and Boudart were the first to discover that low-cost tungsten carbide exhibited Pt-like catalytic properties for several reactions that were previously thought to be catalyzed only by Pt-group metals.15 Since then, the catalytic properties of tungsten monocarbide (WC), tungsten semicarbides (W2C), and related transition-metal carbides (TMCs) have been Received: July 16, 2013 Revised: May 25, 2014 Published: May 30, 2014 13525
dx.doi.org/10.1021/jp4070328 | J. Phys. Chem. C 2014, 118, 13525−13538
The Journal of Physical Chemistry C
Article
Table 1. Bulk Properties of WxC Structures phase space group a [Å] b [Å] c [Å] ρ [g/cm3] Ecoh [eV/atom] Eform [eV/atom]
h-WC
c-WC
α-W2C
P6̅m2 2.919 (2.926b)
Fm3̅m 4.374 (4.265b)
P3m ̅ 1 3.057 (3.001b)
2.842 (2.849b) 15.506 (15.395c) −10.60 (−10.64d) −0.145 (−0.106d)
15.541 (15.292c) −10.15 (−10.19d) 0.308 (0.341d)
4.674 (4.728b) 16.671 (16.585c) −10.81 (−10.79d) 0.046 (0.065d)
β-W2C
γ-W2C
Pbcn 4.743 (4.728b) 6.087 (6.009b) 5.219 (5.193b) 16.738 (16.628c) −10.88 (−10.87d) −0.030 (−0.018d)
P1 a 3.047 (3.002b) 6.093 (6.087c) 4.699 (4.75b) 16.743 (16.612c) −10.87 (−10.89d) −0.022 (−0.029d)
ε-W2C P3m ̅ 1 5.237 (5.184b) 4.769 (4.721b) 16.697 (16.585c) −10.86 (−10.86d) −0.014 (−0.003d)
a Bulk γ-W2C has space group of P63/mmc; P1 symmetry was used in the calculation because it was done using the supercell. bExperimental data summarized by Kurlov et al.27 cTheoretical results from Suetin et al.28 dTheoretical results from Li et al.29
minimization of the Hellmann−Feynman forces acting on the atoms to β-W2C > γ-W2C > ε-W2C > αW2C > c-WC, based on the Eform in Table 1. Our stability sequence is different from that of Suetin et al. (i.e., h-WC > εW2C > β-W2C > γ-W2C > α-W2C > c-WC) and Li et al. (h-WC > γ-W2C > β-W2C > ε-W2C > α-W2C > c-WC). This is primarily due to the uncertainty in subtracting the energies of different unit cells, especially when the difference in the energies is
