Engineering Non‐sintered, Metal‐Terminated Tungsten Carbide Nanoparticles for Catalysis

Abstract

Transition-metal carbides (TMCs) exhibit catalytic activities similar to platinum group metals (PGMs), yet TMCs are orders of magnitude more abundant and less expensive. However, current TMC synthesis methods lead to sintering, support degradation, and surface impurity deposition, ultimately precluding their wide-scale use as catalysts. A method is presented for the production of metal-terminated TMC nanoparticles in the 1-4 nm range with tunable size, composition, and crystal phase. Carbon-supported tungsten carbide (WC) and molybdenum tungsten carbide (Mo x W 1Àx C) nanoparticles are highly active and stable electrocatalysts. Specifically, activities and capacitances about 100-fold higher than commercial WC and within an order of magnitude of platinumbased catalysts are achieved for the hydrogen evolution and methanol electrooxidation reactions. This method opens an attractive avenue to replace PGMs in high energy density applications such as fuel cells and electrolyzers.Early transition-metal carbides (TMCs) are earth-abundant materials with established commercial use in a variety of applications owing to their favorable optical, electronic, mechanical, and chemical properties. [1][2][3] Tungsten carbide (WC) has garnered much attention for its catalytic similarities to the platinum group metals [4] (PGMs) in several thermoand electrocatalytic reactions, such as biomass conversion, [2,5,6] hydrogen evolution and oxygen reduction, [7][8][9][10][11] and alcohol electrooxidation. [12,13] The reactivity of WC has been partially attributed to the intercalation of C to the W lattice, which gives rise to a "Pt-like" d-band electronic density states (DOS). [14] Peppernick et al. demonstrated that WC À ions exhibit isoelectronic correspondence with Pt À ions.[15] Because WC exhibits high thermal and electrochemical stability while resisting common catalyst poisons such as carbon monoxide and sulfur, [1][2][3] it has been identified as a suitable candidate to replace PGM catalysts in emerging renewable energy technologies, such as fuel cells and electrolyzers. [7][8][9][10][11][16][17][18] While there are many methods to synthesize WC nanoparticles (NPs), none of the current methods can simultaneously prevent sintering of the WC nanoparticles while also mitigating surface impurity deposition. Thus, despite the promising catalytic properties of model WC surfaces, the lack of synthesis methods to produce metal-terminated WC NPs of controlled sizes has prevented its wide-spread use as an earthabundant catalyst. [1,2,7,19] The high carburization temperatures (> ca. 700 8C) required to overcome the thermodynamic and kinetic barriers for carbon incorporation into the metal lattice induce uncontrollable particle sintering, generating particles with exceedingly low surface areas that are not suitable for commercial applications (Supporting Information , Figure S1). [1,2] Although alternative synthesis methods with unconventional heating [20][21][22][23] and carbon sources [11,17,19,24,25] have been developed to mitiga...

Hydrogen Evolution Reaction of γ-Mo0.5W0.5 C Achieved by High Pressure High Temperature Synthesis

“…Catalysts 2016, 6, 208 2 of 9 electrocatalytic activity and stability [19]. Wan et al synthetized four phases of molybdenum carbide (α-MoC1−x, β-Mo2C, η-MoC, and γ-MoC), studied and compared the catalytic performances for multiple phases of molybdenum carbide [20,21].…”

Advances in Ceramic Supports for Polymer Electrolyte Fuel Cells

“…Others have shown studies on the catalytic activity of molybdenum and tungsten carbides as catalysts for methane reforming [18]. The attractiveness of these ceramic materials as catalysts was attributed to their high activity [19][20][21], lower price when compared to precious metals [22], unique structure [23], and stability [24] in acidic and alkaline mediums. In order for these materials to be good supports for electro-catalysts, they need to have high surface area (equivalent to 300 m 2 /g of carbon; with ceramic supports, this surface area may be much lower due to the atomic weight of the metals and comparisons should be made carefully) and good electrical conductivity (circa 4 S/cm) [25].…”

“…They showed enhanced corrosion resistance when compared with Vulcan XC-72. Roman-Leshkov et al [23] developed a removable ceramic coating method for the synthesis of WC. Using this technique, they were able to produce high-surface-area, electronically conductive WC, which also exhibited superior stability on the anode when compared to Vulcan XC-72.…”

Core–Shell-Structured Low-Platinum Electrocatalysts for Fuel Cell Applications

“…And although review articles in the electrochemical field have focused on the potential of TMCs as replacements for precious metal catalysts through a more solid understanding of their catalytic activity and stability [506][507][508], the electrocatalytic activities of TMC catalysts are not high enough for practical applications as electrocatalysts [509][510][511]. In addition, most TMCs oxidize at low potentials in acidic media to produce TMC oxides that are inactive for catalytic reactions.…”