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Transition metal carbides are attractive, low-cost alternatives to Pt group metals, exhibiting multifunctional acidic, basic, and metallic sites for catalysis. Their widespread applications are often impeded by a high surface affinity for oxygen, which blocks catalytic sites. However, recent reports indicate that the α-MoC phase is a stable and effective cocatalyst for reactions in oxidative or aqueous environments. In this work, we elucidate the factors affecting the stability and catalytic activity of α-MoC under mild electrooxidation conditions (0–0.8 V SHE) using density functional theory calculations, kinetics-informed surface Pourbaix diagram analysis, electronic structure analysis, and cyclic voltammetry. Both computational and experimental data indicate that α-MoC is significantly more resistant to electrooxidation by H2O than β-Mo2C. This higher stability is attributed to structural and kinetic factors, as the Mo-terminated α-MoC surface disfavors substitutional oxidation of partially exposed, less oxophilic C* atoms by hindering CO/CO2 removal. The α-MoC surface exposes H2O-protected [MoC2O2] and [MoC(CO)O2] oxycarbidic motifs available for catalysis in a wide potential window. At higher potentials, they convert to unstable [Mo(CO)2O2], resulting in material degradation. Using formic acid as a probe molecule, we obtain evidence for Pt-like O*-mediated O–H and C–H bond activation pathways. The largest kinetic barrier, observed for the C–H bond activation, correlates with the hydrogen affinity of the site in the order O*/Mofcc > O*/Ctop > O*/Motop. To mitigate the site-blocking effect of surface-bound H2O and bidentate formate, doping with Pt was investigated computationally to make the surface less oxophilic and more carbophilic, indicating a possible design strategy toward more active and selective carbide electrocatalysts. 

PerspectiveOn the Surface Compositions of Molybdenum Carbide Nanoparticles for Electrocatalytic ApplicationsSiying Yu and Hong Yang *Department of Chemical and Biomolecular Engineering, University of Illinois Urbana-Champaign, 600 S. Mathews, Urbana, IL 61801, USA* Correspondence: hy66@illinois.eduReceived: 28 November 2024; Accepted: 2 December 2024; Published: 6 December 2024 Abstract: Molybdenum carbide has attracted much research attention for its precious metal-like catalytic properties, especially in hydrogen-involved reactions. It possesses rich crystal and surface structures leading to different activity and product selectivity. With advances in nanoengineering and new understanding of their surfaces and interfaces, one can control the transition between different phases and surface structures for molybdenum carbide nanoparticles. In this context, it is essential to understand their surface compositions and structures under operating conditions in addition to their intrinsic ones under ambient conditions without external cues. The necessity of surface study also comes from the mild oxidation brought by passivation in carbide nanoparticles. made using the bottom-up synthesis or solid-gas phase temperature-programmed reduction. In this perspective, we first introduce the relevant crystal structures of molybdenum carbides and highlight the features of the three types of chemical bonding within. We then briefly review the studies of thermodynamically favored surface components and nanostructures for partially oxidized molybdenum carbide nanoparticles based on both experimental and theoretical data. An electrochemical oxidation method is used to illustrate the feasibility in controlling and understanding the surface oxidation. Finally, structure-property relationship is discussed with several recent examples, focusing on the effect of phase dependency on the adsorption energy of reaction intermediates. 

The surface oxidation of molybdenum carbide nanoparticles was controlled by the electrochemical method. The impact of surface oxidation on catalytic properties was studied by both spectroscopic and computational methods. 

As the development of polymer electrolyte membrane fuel cells (PEMFCs) has sped up in recent years, producing active and durable electrocatalysts has become an increasingly important technical challenge. Platinum–cobalt (Pt–Co) alloy electrocatalyst has been commercially applied to hydrogen-powered fuel cell vehicles, and their intermetallic forms promise better durability, which is crucial to satisfy the 8000 h lifetime target of heavy-duty vehicles and other transportation options. In this feature article, we first present the atomically ordered structures of Pt–Co intermetallic, then discuss the thermodynamic and kinetic driving forces for making the PtCo-based intermetallic nanoparticles with desired structural attributes, followed by recent examples to illustrate how to achieve better control in composition, size, and shape. Discussion on the relationship between the key structural features and catalytic performance is focused on the application of Pt–Co intermetallic nanostructures as oxygen reduction reaction (ORR) electrocatalysts for hydrogen-powered PEMFCs. We emphasize specifically the importance of intermetallic structures for enhancing the durability and summarize the characterizations of their electrocatalytic performance in both three-electrode system and full cell studies. Finally, we provide our perspectives on the design, synthesis, characterization, and property studies of Pt–Co intermetallic nanoparticles as ORR electrocatalysts. This article should provide a new understanding on the design of ORR electrocatalytic applications using this class of intermetallics.