First-principles calculations of the adsorption, diffusion, and dissociation of a CO molecule on the Fe(100) surface

Abstract

First-principles pseudopotential plane wave calculations based on spin-polarized density functional theory ͑DFT͒ and the generalized gradient approximation ͑GGA͒ have been used to study the adsorption of CO molecules on the Fe͑100͒ surface. Among several possible adsorption configurations considered here, the most stable corresponds to a fourfold state in which a CO molecule is tilted relative to the surface normal by 50°. In this case, the CO bond is elongated to 1.32 Å and has a low vibrational stretching frequency of 1246 cm Ϫ1 to be compared with the experimental gas phase value of 2143 cm Ϫ1 . The adsorption energy for this state is found to vary between 46.7 and 43.8 kcal/mol depending on the choice of exchange-correlation functional used in the DFT. A total of three adsorption sites have been located, and the relative adsorption energies are E(fourfold)ϾE(twofold)ϷE(onefold) at lower surface coverage, and E(fourfold)ϾE(onefold) ϾE(twofold) at higher coverage. A similar analysis performed for the C and O atoms indicates that the adsorption at the fourfold site is the most stable among various configurations, with adsorption energies of 186 and 145 kcal/mol, respectively. Additionally, we have demonstrated the possibility that a C atom embeds into the lattice in a twofold, bridgelike configuration with an adsorption energy of 154 kcal/mol. The minimum energy pathways for the surface diffusion of a CO molecule between selected pairs of local minima indicate that the barriers for these processes are generally quite small with values less than 2 kcal/mol. One exception to this is the diffusion out of the most stable fourfold site, where the barrier is predicted to be around 13 kcal/mol. Finally, the barriers for dissociation of CO bound in a fourfold site have been calculated to have values in the range of 24.5-28.2 kcal/mol, supporting the experimental observation that dissociation of CO bound to the surface seems to compete with CO desorption at 440 K.

Probing C–O bond activation on gas-phase transition metal clusters: Infrared multiple photon dissociation spectroscopy of Fe, Ru, Re, and W cluster CO complexes

“…33 It predicts that two, three, and four-fold coordinated CO species form the most stable binding geometry in Fe 3,4 , Fe 5 , and Fe 6 , respectively. The reported CO stretching frequencies for these species are significantly below our experimental values for CO atop bound to Fe [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] , as would be expected. Unfortunately, we were unable to measure clusters this small to confirm the transition from high coordination binding in small clusters to atop binding in large clusters, that has some parallels in rhodium cluster CO complexes.…”

“…They are also high compared to the values calculated for atop bound CO species on extended iron surfaces at low coverage, which are in the 1880-1920 cm -1 range. [27][28][29] As the bands above 2000 cm -1 appear only at higher CO coverage on the surfaces, it might be that they are not due to isolated CO molecules but related to the presence of geminal carbonyls formed at low coordinated Fe atoms at steps or defects. Slab model calculations studying the mechanism of Fe(CO) 5 formation on Fe(100) find that geminal binding can energetically compete with the formation of isolated CO ligands.…”

Mechanism of dicarbonyl(2,4-pentanedionato)iridium(I) decomposition on iron surface and in gas phase: Complex experimental and theoretical study

“…Thus, CO fragment starts moving from its initial position without any obstacles immediately after the adsorption and migrates along the substrate. Then CO adsorbs on the iron surface elsewhere and its desorption is detected with temperature increasing, which is in agreement with the previously reported data on CO adsorption and migration on the iron surface [39]. This is a possible explanation why no signal from CO molecules was detected in the mass spectrum below 300ºC ( Figure 2) [24].…”

Effects of Alloying on the Chemistry of CO and H2S on Fe Surfaces

“…66,67 We predicted that CO dissociation on Fe(110) has a barrier of 1.5 eV, 16 while we determined the barrier on Fe(100) to be 1.1 eV, which agrees very well with experiment (1.09 eV) 68 and previous DFT work. 69,70 The lower barrier on Fe(100) can be attributed to the highly tilted state of CO on Fe(100), which has a much weakened CO bond already before dissociation. The OT site preference by CO on Fe(110) then requires more rearrangement of the CO molecule to reach the TS, leading to a higher barrier.…”