Lifting of Ir{100} reconstruction by CO adsorption: An ab initio study

Abstract

The adsorption of CO on unreconstructed and reconstructed Ir{100} has been studied, using a combination of density functional theory and thermodynamics, to determine the relative stability of the two phases as a function of CO coverage, temperature, and pressure. We obtain good agreement with experimental data. At zero temperature, the (5 x 1) reconstruction becomes less stable than the unreconstructed (1 x 1) surface when the CO coverage exceeds a critical value of 0.09 ML. The interaction between CO molecules is found to be weakly repulsive on the reconstructed surface but attractive on the unreconstructed, explaining the experimental observation of high CO coverage on growing (1 x 1) islands. At all temperatures and pressures, we find only two possible stable states: 0.5 ML CO c(2 x 2) overlayer on the (1 x 1) substrate and the clean (5 x 1) reconstructed surface.

Catalytic oxidation of CO on Ir(100)

“…Two structures with low energies are obtained. One structure has O a on a bridge site and CO on a top site, which are the same stable sites obtained for O a in Ir(100)–O and CO in Ir(100)–CO pure systems, which is consistent with previous studies 41, 44–47. The other structure has both O a and CO on the bridge sites.…”

Adsorption and Activation of CO on Co3O4(111) Thin Films

“…From a simple Redhead analysis, assuming a constant heating rate of 15 K·s –1 , first order desorption, and a pre-exponential factor of 10 13 s –1 , this value corresponds to desorption activation energies of 36–43 kJ·mol –1 for the more strongly bound CO and to an upper limit of 21–26 kJ·mol –1 for the weakly bound species (for which desorption starts already at 90 K). We note that the total amount of CO desorbing from the fully saturated Co 3 O 4 surface is about twice as high as that from the Ir(100)-c(2 × 1)CO structure that was used for comparison . This would roughly correspond to four CO molecules per Co 3 O 4 (111) unit cell if the film were perfectly flat and single crystalline.…”

“…The TPD spectra were taken with a quadrupole mass spectrometer (QMS) equipped with Feulner cup, the entrance of the nozzle was placed ∼0.5 mm in front of the sample surface, and only gases desorbing from the area of the Ir crystal could enter the QMS. To calibrate the amount of CO desorbed from the sample, the desorption of CO from the well-known Ir(100)-c(2 × 2)-CO structure was measured using exactly the same geometry. Due to the small heating power needed at temperatures below 100 K, the employed temperature ramp was nonlinear.…”

Benzene and Its Dissociation Products on Ir{100}

“…The structure and energetics of c(2 × 2) H on Ir{100} were calculated for several geometries at 0.5 ML adatom coverage, and the bridge site was found to be the most favorable option, with a dissociative adsorption energy of E H 2 ad = 1.46 eV per original hydrogen molecule (141 kJ mol −1 of H 2 ). The adsorption energy has been defined according to the following equation − 1 2 E H 2 ad = E Ir/H c ( 2 × 2 ) − true( E Ir normalc false( 2 × 2 false) + 1 2 E normalH 2 normalg true) + 2 E R where E Ir c(2×2) and E Ir/H c(2×2) represent the total energies per supercell of clean-unreconstructed and H-adsorbed Ir surfaces, as calculated within a supercell of cross section equivalent to a c(2 × 2) primitive unit cell; E H 2 g is the gas phase energy of a hydrogen molecule (calculated as −31.749 eV per molecule), and E R is the reconstruction energy per (1 × 1) cell for the (1 × 5) → (1 × 1) phase transition of the clean surface (calculated by Ge et al to be 0.06 eV and by Ghosh et al to be 0.05 eV; here, we have adopted the value of 0.06 eV, as the present calculational parameters are closer to those of the earlier work). The dissociative adsorption energy thus includes the energy required to lift the (1 × 5) reconstruction (over an area four times that of a (1 × 1) cell) as well as the dissociation energy of the molecule.…”