Transition Metal Carbides
Early transition metal carbides (for example Mo2C, WC, TiC, VC, NbC and TaC) are hard, refractory compounds in which carbon occupies interstitial sites of a metal lattice. They have been studied as catalysts since reports of “platinum-like” activity, and a central question is how carbon modifies the electronic structure and reactivity of the parent metal surface, which depends strongly on surface termination.
DFT trends on Mo2C and rocksalt carbide surfaces (2005)
Kitchin, Nørskov, Barteau and Chen used PW91 DFT to study β-Mo2C(0001) and the (111) surfaces of TiC, VC, NbC and TaC. Bulk lattice constants and bulk moduli agreed well with experiment, and first-layer contraction on metal-terminated surfaces was reproduced. Carbon did not change the d-electron count of surface Mo (6.45 to 6.69 versus 6.47 for Mo(110)), so it does not make Mo Pt-like by electron donation. Metal-terminated carbide surfaces bound atomic H more strongly than the parent metals (except VC), attributed to tensile strain from the expanded metal-metal distances, while carbon-terminated surfaces bound H more weakly at some sites and formed very stable C-H species (3.92 eV on TaC(111)). The authors suggest surface carbon passivates the metal and that defect chemistry and carbon participation in reactions matter (Kitchin et al. 2005). Summary: kitchin2005.
Metal-modified Mo2N for selective isopropanol bond scission (2024)
Porter and coworkers extended the metal-modified carbide approach to the nitride analogue Mo2N, using isopropanol as a probe of C−O, C−H and C−C scission. Unmodified Mo2N favored dehydration to propylene (about 59% selectivity in TPD, 60% on powders at 413 K), whereas Fe and Pt overlayers suppressed dehydration; Pt/Mo2N powders were 93.4% selective to acetone. DFT on N-terminated γ-Mo2N(111) traced the dehydration preference to the strong oxygen affinity of surface Mo (O binding energy −3.79 eV), which stabilizes an O-anchored isopropoxy that then loses its oxygen, while a Pt monolayer on Mo2N binds O at only −1.31 eV and behaves like Pt(111) (Porter et al. 2024). Summary: porter2024.