| Abstract
| - A systematic density functional study of the adsorption and dissociation of O2 on the (001) surface of severaltransition metal carbides (TCMs; TM = Ti, Zr, Hf, V, Nb, Ta, Mo) is presented. It is found that O2 mayadsorb molecularly on two different sites with similar adsorption energy. At these sites, either O2 bridges twosurface metal (M) atoms or it is placed directly on top of a M surface atom. A case apart is δ-MoC, whereO2 adsorption on top of surface Mo atoms is far up in energy with respect to bridging two surface Mo atoms.The relative stability of O2 on these TMCs is dominated by the electron back-donation between the surfaceand O2 and the stabilization of the resulting partially charged molecule by the surface metal sites. Threereaction paths leading to O2 dissociation have been considered. The first reaction pathway starts from M−Mbridge molecular adsorption and lead to O atoms on top of surface M atoms (TSM) and the second one (TSC)starts from on top molecular adsorption and lead to final states where O atoms are adsorbed on 3-fold hollowsites neighboring two M and one C surface atoms, while the third pathway (TSBC) starts from O on the M−Mbridge and leads to TSC products. For each reaction path, transition state structures have been located and thecorresponding energy barriers obtained. At low temperatures, O2 dissociation on group IV TMCs can onlyoccur via the TSBC pathway whereas at high temperatures it may also take place starting through TSC. For therest of the carbides, only TSC and TSM paths are possible. The calculated transition state theory rate constantsreveal that TMCs of groups IV and V are easy to oxidize whereas this is especially difficult for δ-MoC. Therate constant trends follow the calculated energy barriers and explain the oxygen preference for carbon ongroup IV TMCs and δ-MoC, as well as the preference for metal atoms on group V TMCs.
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