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.

Using ultra-high vacuum scanning tunneling microscopy (STM) and density functional theory (DFT), we investigated the surface structure of 2D hexagonal boron nitride (hBN) domains on Pd(1 1 1). STM images of polydomain hBN monolayers, grown via dissociative chemisorption of borazine on Pd(1 1 1)/Al2O3(0 0 0 1) thin films, are acquired as a function of tunneling current and bias. The images reveal moiré patterns with four periodicities λ  =  0.6  ±  0.05 nm, 1.8  ±  0.14 nm, 2.7  ±  0.20 nm, and 2.8  ±  0.14 nm, corresponding to different orientations on Pd(1 1 1). We find that the apparent surface corrugation Δz in STM changes little with tunneling current, exhibits an oscillatory dependence on the bias voltage, and increases from Δz  ≈  14 pm for domains with λ  =  0.6 nm to Δz  ≈  200 pm for λ  =  2.8 nm. We attribute the observed tunneling-parameter dependence in Δz to the electronic structure of the hBN/Pd(1 1 1) system. Unlike any other monolayer hBN-on-metal system, we suggest that hBN/Pd can have either mainly geometric or mainly electronic corrugation, depending on the domain orientation. Furthermore, for the largest periodicities, we observe a bifurcation behavior in which some domains are nearly flat, and others develop significant hill-and-valley geometric corrugations. We expect a similar behavior for other substrates for which the interaction energy with hBN is intermediate, i.e. neither mostly chemical nor van der Waals binding: for these substrates, a similar approach can help identify interlayer interactions and electronic structure modifications.