Thermodynamic Stability of Molybdenum Oxycarbides Formed from Orthorhombic Mo₂C in Oxygen-Rich Environments

Abstract

Molybdenum carbide (Mo2C) nanoparticles and thin films are particularly suitable catalysts for catalytic fast pyrolysis (CFP) as they are effective for deoxygenation and can catalyze certain reactions that typically occur on noble metals. Oxygen deposited during deoxygenation reactions may alter the carbide structure leading to the formation of oxycarbides, which can determine changes in catalytic activity or selectivity. Despite emerging spectroscopic evidence of bulk oxycarbides, so far there have been no reports of their precise atomic structure or their relative stability with respect to orthorhombic Mo2C. This knowledge is essential for assessing the catalytic properties of molybdenum (oxy)carbides for CFP. In this article, we use density functional theory (DFT) calculations to (a) describe the thermodynamic stability of surface and subsurface configurations of oxygen and carbon atoms for a commonly studied Mo-terminated surface of orthorhombic Mo2C, and (b) determine atomic structures for oxycarbides with a Mo:C ratio of 2:1. The surface calculations suggest that oxygen atoms are not stable under the top Mo layer of the Mo2C(100) surface. Coupling DFT calculations with a polymorph sampling method, we determine (Mo2C)xOy oxycarbide structures for a wide range of oxygen compositions. Oxycarbides with lower oxygen content ( / ≤ 2) adopt layered structures reminiscent of the parent carbide phase, with flat Mo layers separated by layers of oxygen and carbon; for higher oxygen content, our results suggest the formation of amorphous phases, as the atomic layers lose their planarity with increasing oxygen content. We † These authors made equal contributions to the results in this work * To whom correspondence may be addressed: carrie.farberow@nrel.gov (C. A. Farberow), david.robichaud@nrel.gov (D. J. Robichaud), and cciobanu@mines.edu (C.V. Ciobanu) characterize the oxidation states of Mo in the oxycarbide structures determined computationally, and simulate their X-ray diffraction (XRD) patterns in order to facilitate comparisons with experiments. Our study may provide a platform for large-scale investigations of the catalytic properties of oxycarbides and their surfaces, and for tailoring the catalytic properties for different desired reactions.

Uncovering the Surface and Phase Effect of Molybdenum Carbides on Hydrogen Evolution: A First-Principles Study

“…The five MoC phases are δ-MoC, , α-MoC, , G-MoC, η-MoC, ,, and TiP-MoC , ­(it is denoted γ′ in refs and ), as shown in Figure c–g, respectively. The two Mo 2 C phases are β-Mo 2 C ,,− and α-Mo 2 C, ,, as shown in Figure a,b, respectively. The structural parameters of different Mo x C ( x = 1, 2) phases, including space group, number of formula units, lattice parameters, cohesive energy, as well as theoretical and experimental data from previous literature, are listed in Table S1.…”

Supported Molybdenum Carbide Nanoparticles as an Excellent Catalyst for CO2 Hydrogenation

“…Postreaction characterization with XPS showed that O adatoms were always the dominant species on this surface with a very small amount (<0.05 ML) of a carbonate. The hydrogen atoms generated by H 2 dissociation on this system are not reactive enough to remove the O adatoms yielded by CO 2 decomposition, the catalyst is metastable, and the carbide NPs transform into a highly stable oxycarbide which also has been detected on surfaces and powders of bulk Mo 2 C. 14,16,36,37 The product distribution seen for the hydrogenation of CO 2 on MoC 0.6 /Au(111) is similar to that reported for NPs of Mo 2 C which are mainly methanation or Fischer−Tropsch catalysts. 13 On the other hand, Figure 3 summarizes the results for the hydrogenation of CO 2 on MoC 1.1 /Au(111).…”

Exploiting two-dimensional morphology of molybdenum oxycarbide to enable efficient catalytic dry reforming of methane

“…3 ). The stability of the orthorhombic Mo 2 C and Mo 2 CO x phases has been previously evaluated by DFT calculations 48 , 49 . We derived 2D-Mo 2 C surface from the crystal structure of Mo 2 Ga 2 C, slicing it along the 001 direction, since it is the most convenient approach to obtain a 2D system with the desired stoichiometry (Supplementary Fig.…”