Search for: All records

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. 

Light element alloying in iron is required to explain density deficit and seismic wave velocities in Earth’s core. However, the light element composition of the Earth’s core seems hard to constrain as nearly all light element alloying would reduce the density and sound velocity (elastic moduli). The alloying light elements include oxidizing elements like oxygen and sulfur and reducing elements like hydrogen and carbon, yet their chemical effects in the alloy system are less discussed. Moreover, Fe-X-ray Absorption Near Edge Structure (Fe-XANES) fingerprints have been studied for silicate materials with ferrous and ferric ions, while not many X-ray absorption spectroscopy (XAS) studies have focused on iron alloys, especially at high pressures. To investigate the bonding nature of iron alloys in planetary interiors, we presented X-ray absorption spectroscopy of iron–nitrogen and iron–carbon alloys at high pressures up to 50 GPa. Together with existing literature on iron–carbon, –hydrogen alloys, we analyzed their edge positions and found no significant difference in the degree of oxidation among these alloys. Pressure effects on edge positions were also found negligible. Our theoretical simulation of the valence state of iron, alloyed with S, C, O, N, and P also showed nearly unchanged behavior under pressures up to 300 GPa. This finding indicates that the high pressure bonding of iron alloyed with light elements closely resembles bonding at the ambient conditions. We suggest that the chemical properties of light elements constrain which ones can coexist within iron alloys. 

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.