  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 (y/x <=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.
