Reactivity of CO on stepped and non-stepped surfaces of transition metals

Abstract

Results of extended Huckel calculations show that coordination of CO to Group VIII transition-metal surfaces depends on a subtle balance of the interaction with the CO 5cr orbital, that tends to direct the CO molecule to the atop position and the CO 2 7 ~" orbital, that directs the molecule to higher coordination sites. In the atop position, the changes in bonding to different surfaces of the same metal can be mainly attributed to differences in the interaction with the CO SCT molecular orbital. The favoured dissociation path is such that carbon and oxygen atoms end in high coordination sites, sharing as few surface metal atoms as possible. The CO bond is activated by the metal atoms that are crossed upon dissociation.

Quantum Chemistry of Surface Chemical Reactivity

“…We argued in section 2 that changes of the interaction of the CO 5u, a.nd the surface metal d orbitals dominate coordination of CO to Pt. The analysis presented here not only agrees with this, it also predicts that CO binds most strongly atop to the (111) surface (7). Only at lower d-valence electron occupation coordination to the more open surfaces becomes favoured.…”

“…Figures 1 and 2 show 'lrij 's computed according to the Extended Htickel method for a CO molecule atop adsorbed to a 29 atom cluster of Rh atoms simulating the Rh ( 111) surface. For details we refer to (7). Figures 1 present the BOOPD 1fsu,d 2 , 7rsu,s and 7rsu,p· These are the only non-zero BOOPD's of s symmetry.…”

“…The bi·oaclening of the adsorbate orbitals will be found to relate to the local density of states of the metal surfate orbitals close to the maximum of the adsorbate orbital density. Extending HOMO-LUMO theory to metalsurfaces locai density of states around the Fermi level ( 5 ), the theoretical results of this section wiil be illustrated by a discussion of 02 chemisorption to the silversurface (6) and the CO bond strength on different transition metalsurfaces (7). In the final section the concepts presented in the earlier two sections are applied to changes in chemical reactivity clue to coadsorption of promoters.…”

C–O bond scission on “defect-rich and perfect” Pd(111)?

“…molecular orbital of CO into a free d orbital of the metal and an electron back-donation from an occupied d orbital of the metal into the unoccupied 2p à antibonding orbital of CO. As discussed by Broden et al [18], the activation energy for dissociation may be related to the degree of population of the 2p à antibonding CO orbital (which weakens the C-O bond). CO dissociation seems to be also facilitated if CO bends towards the surface [19][20][21]. For CO dissociation on rhodium, deKoster et al [19] reported a tilting of the C-O bond, with an angle of about 70°between the C-O axis and the surface normal.…”

Field-dependent chemisorption of carbon monoxide and nitric oxide on platinum-group (111) surfaces: Quantum chemical calculations compared with infrared spectroscopy at electrochemical and vacuum-based interfaces

“…Indeed, theoretical examinations of the marked variations in CO coordination geometry observed experimentally between these surfaces, even at zero external field, are relatively sparse. 21 To this end, the present paper reports DFT calculations for CO and NO chemisorption on the four Pt-group ͑111͒ surfacesplatinum, iridium, palladium, and rhodium, as well as on the related hexagonal Ru͑0001͒ surface-for which potentialdependent IRAS data, along with detailed results for the UHV-based interfaces, are available. We undertake a broadbased comparison of the DFT results with the experimental findings, focusing attention on the quantum-chemical factors responsible for the observed variations in the preferred binding site and the vibrational frequencies with the metal surface, chemisorbate, and electrostatic field.…”