A Systematic Density Functional Study of Molecular Oxygen Adsorption and Dissociation on the (001) Surface of Group IV−VI Transition Metal Carbides

A systematic density functional study of the adsorption and dissociation of O 2 on the (001) surface of several transition metal carbides (TCMs; TM = Ti, Zr, Hf, V, Nb, Ta, Mo) is presented. It is found that O 2 may adsorb molecularly on two different sites with similar adsorption energy. At these sites, either O 2 bridges two surface metal (M) atoms or it is placed directly on top of a M surface atom. A case apart is δ-MoC, where O 2 adsorption on top of surface Mo atoms is far up in energy with respect to bridging two surface Mo atoms. The relative stability of O 2 on these TMCs is dominated by the electron back-donation between the surface and O 2 and the stabilization of the resulting partially charged molecule by the surface metal sites. Three reaction paths leading to O 2 dissociation have been considered. The first reaction pathway starts from M−M bridge molecular adsorption and lead to O atoms on top of surface M atoms (TS M ) and the second one (TS C ) starts from on top molecular adsorption and lead to final states where O atoms are adsorbed on 3-fold hollow sites neighboring two M and one C surface atoms, while the third pathway (TS BC ) starts from O on the M−M bridge and leads to TS C products. For each reaction path, transition state structures have been located and the corresponding energy barriers obtained. At low temperatures, O 2 dissociation on group IV TMCs can only occur via the TS BC pathway whereas at high temperatures it may also take place starting through TS C . For the rest of the carbides, only TS C and TS M paths are possible. The calculated transition state theory rate constants reveal that TMCs of groups IV and V are easy to oxidize whereas this is especially difficult for δ-MoC. The rate constant trends follow the calculated energy barriers and explain the oxygen preference for carbon on group IV TMCs and δ-MoC, as well as the preference for metal atoms on group V TMCs.