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Properties of transition metal carbides
By Hector Prats, on 28 September 2021
Bulk structures of several relevant TMCs. Carbon atoms are shown as brown spheres, while metal atoms are shown in other colours.
Transition metal carbides (TMCs) are formed by incorporating C atoms into the interstitial sites of transition metals (TMs), including all 3d, 4d, and 5d elements with the exception of Ru, Rh, Pd, Ir and Pt, which do not form carbide. In general, the metal atoms are arranged in such a way that they form close-packet arrangements of metal layers with a hexagonal (h) or cubic (c) stacking sequence or with a mixture of these, with the C atoms occupying the octahedral interstitial sites. Pure h type TMCs can have a maximum C content of C/TM = 1/2, while c type can have a maximum C content of C/TM = 1.
The bonding in TMCs involves three main contributions: a metallic one arising from the rearrangement of the TM-TM bonds; a covalent one due to the formation of typical chemical bonds between TM and C atoms, and an ionic one arising from the TM-to-C charge transfer. This results in TMCs displaying properties characteristic of three different classes of materials:
- the extreme hardness and brittleness of covalent solids
- the ordered bulk structure and high melting point of ionic solids
- the excellent electrical and thermal conductivities of metals
The band structures and therefore electronic properties of each TMC are, to a large degree, driven by the extent of metallic, covalent and ionic bonding character of the TM-TM and TM-C bonds. The physical properties of TMCs also depend on the surface. For instance, the electronic structure of TiC(001) displays more covalent character, while TiC(111) displays more ionic character. Finally, the position of the TM along the series is also important, with ZrC being considerable more ionic than δ-MoC.
Apart from that, TMCs are also used as catalysts or catalytically active supports due to the following reasons:
- catalytic properties similar to that of Pt-group metals (Ru, Rh, Pd, Ir and Pt) in several types of reactions (e.g. hydrogenation)
- high resistance to C deposition and S poisoning
- relatively low cost
Moreover, the combination of different TMCs and supported metal particles could lead to different catalytic activities towards target reactions. The catalytic performance depends on many factors, such as the even distribution of active metals, the electronic interaction between active metal particles and TMC supports, the carburisation/decarburisation capacity as well as the resistance towards sintering.
Overall, TMCs are a very promising family of materials, especially for catalytic purposes. Certainly, the transition towards a low-carbon future in the context of a circular economy will rely on sustainable catalytic processes and engineered materials with tuneable properties such as TMCs, which can play a vital role in addressing this global challenge.
References:
- F. Viñes et al. A systematic density functional theory study of the electronic structure of bulk and (001) surface of transition-metals carbides, J. Chem. Phys. 2005, 122, 174709
- W. Lengauer, Carbides: Transition metal solid-state chemistry. Encyclopedia of Inorganic and Bioinorganic Chemistry, 2012, John Wiley & Sons, Ltd.
- A. L. Stottlemyer et al. Reactions of oxygen-containing molecules on transition metal carbides: Surface science insight into potential applications in catalysis and electrocatalysis. Surf. Sci. Rep. 2012, 67, 201
- M. G. Quesne et al. Bulk and surface properties of metal carbides: Implications for catalysis. Phys. Chem. Chem. Phys. 2018, 20, 6905
- Q. Zhang et al. Transition metal carbides (TMCs) catalysts for gas phase CO2 upgrading reactions: A comprehensive overview. Catalysts 2020, 10, 955
