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

Density Functional Theory (DFT) is utilized to compute field-dependent bind ing energies and intramolecular vibrational frequencies for carbon monoxide and nitric oxide chemisorbed on five hexagonal Pt-group metal surfaces, Pt , Ir, Pd, Rh, and Ru. The results are compared with corresponding binding g eometries and vibrational frequencies obtained chiefly from infrared spectr oscopy in electrochemical and ultrahigh vacuum environments in order to elu cidate the broad-based quantum-chemical factors responsible for the observe d metal- and potential-dependent surface bonding in these benchmark diatomi c chemisorbate systems. The surfaces are modeled chiefly as 13-atom metal c lusters in a variable external field, enabling examination of potential-dep endent CO and NO bonding at low coverages in atop and threefold-hollow geom etries. The calculated trends in the CO binding-site preferences are in acc ordance with spectral data: Pt and Rh switch from atop to multifold coordin ation at negative fields, whereas Ir and Ru exhibit uniformly atop, and Pd hollow-site binding, throughout the experimentally accessible interfacial f ields. These trends are analyzed with reference to metal d-band parameters by decomposing the field-dependent DFT binding energies into steric (electr ostatic plus Pauli) repulsion, and donation and back-donation orbital compo nents. The increasing tendency towards multifold CO coordination seen at mo re negative fields is due primarily to enhanced back-donation. The decreasi ng propensity for atop vs multifold CO binding seen in moving from the lowe r-left to the upper-right Periodic corner of the Pt-group elements is due t o the combined effects of weaker donation, stronger back-donation, and weak er steric repulsion. The uniformly hollow-site binding seen for NO arises f rom markedly stronger back-donation and weaker donation than for CO. The me tal-dependent zero-field DFT vibrational frequencies are in uniformly good agreement with experiment; a semiquantitative concordance is found between the DFT and experimental frequency-field ("Stark-tuning") slopes. Decomposi tion of the DFT bond frequencies shows that the redshifts observed upon che misorption are due to donation as well as back-donation interactions; the m etal-dependent trends, however, are due to a combination of several factors . While the observed positive Stark-tuning slopes are due predominantly to field-dependent back-donation, their observed sensitivity to the binding si te and metal again reflect the interplay of several interaction components. (C) 2000 American Institute of Physics. [S0021-9606(00)70234-0].