Vibrational spectra of ethylene and acetylene on metal surfaces - an electron energy loss study of ethylene adsorbed on Ni(110) and its carbided surface, and the use of metal-cluster analogies

Abstract

An analysis has been made of on- and off-specular electron energy loss spectra (EELS) from C 2H 4 and C 2D 4 adsorbed on a clean Ni(110) and also a carbided Ni(110) surface. The carbided surface was prepared by heating the clean Ni surface in ethylene to 573 K or above. EELS spectra were obtained using a Leybold-Heraeus spectrometer at a beam energy of 3.0 eV and with a resolution of ca. 6.5 meV (ca. 50 cm −1). The loss spectrum from ethylene at low temperatures (110 K) showed principal features at 3000 (w), 1468 (w), 1162 (s), 879 (w) and 403 cm −1 (s) (C 2D 4 adsorption) and 2186 (w), 1258 (ms), 944 (ms), 645 (w) and 400 cm −1 (s) (C 2D 4 adsorption). The overall pattern of wavenumbers and intensifies of the C 2H 4/C 2D 4 loss peaks is very similar in form (although systematically different in positions) to those previously observed on Ni(111) (ref.1) and Pt(111) (ref.2) surfaces at low temperatures. Like these earlier spectra,the EELS results for C 2H 4/C 2D 4 adsorbed on clean Ni(110) can be well interpreted in terms of a MCH 2CH 2M/MCD 2CD 2M species (M = metal) with the CC bond parallel to the surface. After adsorption on the carbided Ni(110) surfaces at 125 K,the main loss features occur at 3065 (m), 2992 (m), 1524 (ms), 1250 (s), 895 (s), and 314 cm −1 (vs) (C 2H 4 adsorption) and 2339 (m), 2242 (m), 1395 (s), 968 (s), 661 (m) and 314 cm −1 (vs). With the exceptions of reduced intensities of the bands at 895 cm −1 (C 2H 4) and 661 cm −1 (C 2D 4) this pattern of losses - particularly the 1550-1200 cm −1 features which can be assigned to coupled νCC and δCH 2/δCD 2 modes - is well related to similar results on Cu(100) (ref.3) and Pd(111) (ref.4) which have been interpreted convincingly in terms of the presence of π-bonded species, (C 2H 4)M or (C 2D 4)M on the surface. This structural assignment is supported by comparison with the vibrational spectra of Zeise's salt, K[PtCl 3(C 2H 4)].H 2O (refs.5&6). Spectral changes occur on warming C 2H 4 on the clean Ni(110) surface with a growth of a feature near 895 cm −1 at 200 K. At 300 K a rather poorly-defined spectrum occurs, which differs substantially from those found on (111) surfaces of Pt (ref.2), Rh (ref.7) or Pd (ref.8) at room temperature. These latter have been attributed to the ethylidyne, CH 3.CM 3, surface species (ref.9). For adsorption on Ni(110) there is clearly a mixture of species at room temperature. The analysis of the vibrational spectra of selected metal-cluster compounds of known structure with selected hydrocarbon ligands has helped substantially to assign the spectra of surface species in terms of bonding structures of the adsorbed species, as in the cases of the identification of (C 2H 4)M π-adsorbed (refs.5&6) and the ethylidyne CH 3.CM 3 species (ref.9). We have recently analysed the infrared and Raman spectra of the cluster compound (C 2H 2)Os 3(CO) 10 and its deuterium-containing analogue. The infrared frequency and intensity pattern for the A′ modes (C S symmetry) of the two isotopomers bears a remarkable resemblance to EELS spectra previously obtained at low temperature for C 2H 2/C 2D 2 adsorbed on Pt(111) (ref.2) and (after taking into account systematic frequency shifts) for Pd(111) (ref.4). There is good evidence for believing that the structure of the hydrocarbon ligand interacting with the osmium complex takes the form ▪ where the arrow denotes a π-bond to the third metal atom. This strongly confirms the structure for the low-temperature acetylene species on Pt(111) as proposed by Ibach and Lehwald (ref.2). Finally the room-temperature spectra for ethylene adsorbed on finely-divided silica-supported Pt and Pd catalysts have previously been interpreted in terms of the presence of MCH 2CH 2M (ref.10) and π-bonded (C 2H 4)M species (ref.11). However comparisons with the more recent EELS spectra from ethylene on Pt(111) at room temperature (ref.2) now leads to a reassignment of the 2880 cm −1 band, on Pt, previously assigned to MCH 2CH 2M, together with a new, related,band at 1340 cm −1 (ref.12), to the ethylidyne species CH 3CPt 3 found on the single crystal surface. More detailed analyses of the spectra reported here will be published later. Acknowledgement is given to substantial assistance for this programme of research from the Science and Engineering Research Council.