Density functional theory study of CO adsorption and dissociation on molybdenum(100)

The adsorption of CO on Mo(100) has been calculated for several adsorption states at four surface coverages using density functional theory (DFT). Dissociation of CO on Mo(100) has been investigated for two surface coverages: 0.25 and 0.5 monolayer (ML). A full analysis of the vibrational frequencies of CO was performed, to determine whether structures are stable adsorption states or transition states. Results show that CO adsorbs molecularly on the Mo(100) surfaces up to coverages of 0.5 ML at 4-fold hollow sites with the molecular axis tilted away from the surface normal by 55-57 and dissociates easily with activation energies ranging from 0.45 to 0.56 eV, leading to energy gains of -1.71 and -0.59 eV at 0.25 and 0.5 ML, after dissociation, respectively. The adsorption energy of the CO molecule at 0.25 ML is -2.64 eV with a C-O stretching vibration of 1062 cm-1. Increasing the CO surface concentration leads to a lower C-O stretching frequency of 958 cm-1, which is remarkable, and it is in conflict with the Blyholder model and previous experimental observations for CO on transition-metal surfaces. Furthermore, calculations reveal that reported CO desorption peaks in literature, thought to be due to recombination of carbon and oxygen, are more likely due to molecular desorption of CO at the 4-fold hollow position with a tilted geometry. This conclusion is supported by the low recombination energies calculated (one-third of that described in literature)

Adsorption and dissociation of CO on body-centered cubic transition metals and alloys: Effect of coverage and scaling relations

The adsorption and dissociation of CO have been calculated on the (100) surfaces of the body-centered cubic transition metals Fe, Mo, Cr, and W and the alloys Fe3Mo and Fe3Cr using density functional theory for two CO coverages, 0.25 and 0.5 ML. A complete analysis of the vibrational frequencies was performed to check whether the calculated structures are stable geometries or transition-state structures. For coverages up to 0.25 ML, carbon monoxide adsorbs molecularly onto all four metals at fourfold hollow sites with tilting angles with respect to the surface normal of 47°, 57°, 57°, and 58° and adsorption energies of -1.53, -2.64, -3.03, and -3.01 eV for Fe, Mo, Cr, and W, respectively. The calculated CO stretching frequencies at this coverage are 1211, 1062, 1037, and 926 cm-1. At higher coverages, CO adsorption does not exhibit significant changes in both adsorption energy and tilting angle on all four metals but leads to blue shifts of the CO frequency for Fe and Cr and red shifts for Mo and W. Furthermore, scaling relations apply to a bent CO species at a surface coverage of 0.25 ML of CO on all four transition metals as well as the metal alloys Fe3Mo and Fe3Cr, in the sense that the heat of adsorption of CO and the activation energy of CO dissociation scale linearly with the heat of adsorption of the carbon as well as both dissociation products. © 2009 American Chemical Society