Adsorption, desorption and dissociation of CO on Tungsten (100), a DFT study

Density functional theory (DFT) calculations have been performed to investigate the adsorption of CO on W(100) for several adsorption sites at four different surface coverages. Dissociation of CO on W(100) has been explored for two surface concentrations: 0.25 and 0.5 monolayer (ML). To establish whether the calculated structures are stable adsorption states or transition states, a complete analysis of the vibrational frequencies of CO was carried out. For coverages up to 0.5 ML, the CO adsorbs molecularly on the W(100) surface at fourfold hollow sites with the molecular axis tilted away from the surface normal by 58 with an adsorption energy of -3.00 eV. The calculated C-O stretching frequency at 0.25 ML CO coverage is 926 cm-1, in good agreement with experiments. Increasing CO coverage to 0.5 ML leads to a lower C-O stretching frequency of 847 cm-1, which is extraordinary, since it is not expected from previous experimental observations. The CO is able to dissociate very easily with activation energies of only 0.25-0.31 eV, which results in an energy gain of 1.72 and 0.69 eV at 0.25 and 0.5 ML, after dissociation, respectively. In addition, calculations reveal that reported CO desorption peaks in literature are due to molecular desorption of tilted CO at the fourfold hollow sites and are not limited by C + O recombination, as was concluded experimentally.