Transition metal bonding functions and their applications in catalytic adsorptions and reactions: Part IV. Characterization of CO adsorption - bonding, IR spectra, surface species population and adsorption heats

Abstract

Calculations of metal bonding functions show that the controlling factors for forming end-on monocarbonyl, bridge-on carbonyl and three-fold sites on carbonyl are their corresponding functions AB, A 2B 2 and A 3B 3, respectively. A 2B 2 = 2A sB s cosα. A 3B 3 = A 2B 2 + AB when the terminal CO adsorbs on the sublayer metal atom. A and A s represent the various vacant s and d orbitais of metal M.O. bands which accept electrons from the CO 5σ and CO 1π M.O.s to form end-on and side-on σ bonds, respectively. B and B s represent the occupied d orbitais which back-donate d electrons to that part of the CO 2π M.O. located near the terminal carbon and that part of the CO 2π M.O. located on the side of the CO axis, to form end-on and side-on π bonds, respectively. α is the angle between the CO bridging bond and the CO side-on bond. Using the above metal bonding functions, it is possible to demonstrate the following experimental facts: (1) Among the Group VIII metals, excepting Pd, A s is large than A, and the value of A 3B 3 or A 2B 2 is significantly larger than AB; thus at room temperature the multi-site carbonyls prevail on Pd, whereas the end-on monocarbonyls are predominant on most Group VIII metals except Pd. (2) On Ni, the sequence of bonding functions is: A 3B 3 > AB > A 2B 2. Thus at low CO exposures, the three-fold site carbonyl begins to form on Ni(111), whereas the end-on monocarbonyl begins to form on Ni(100), since Ni(100) has no appropriate sublayer atom for forming the three-fold site bonding. (3) The end-on monocarbonyl prevails on Rh(111) and Pt(111) at low CO exposures, because in the neutral state AB > A 2B 2. But if the values of B/A and B s/A s on Rh and Pt are large, the metal is induced to a positive valence after CO adsorption, resulting in A 2B 2 > AB, which is why the bridge-on species begin to form on Rh and Pt at intermediate coverages. (4) On Ru, AB is significantly larger than A 2B 2 at a valence state between 0 and +0.5; thus Ru is totally different from Pd, Ni, Rh and Pt, and only a single peak of end-on carbonyl is present at all coverages. (5) The metal bonding functions semiquantitatively characterize the populations of various carbonyl structures on the surface and can be used to estimate adsorption heats on Group VIII metals.