Threefold-coordinated hollow adsorption site for Ni(111)-c(4×2)-CO: A surface-extended x-ray-absorption fine-structure study

Abstract

Surface-extended x-ray-absorption fine-structure studies on the Ni(111)-c(4X2)-CO system show that CO adsorbs in threefold-coordinated hollow sites. This result is in conAict with the adsorption-site determination via molecular vibrational frequencies, which for this system led to an assignment of a bridge site.During the 1970s and 1980s a large number of surface structures were solved by analysis of low-energy electron-diffraction (LEED) intensities. As a result information about adsorbate-substrate bond lengths and adsorption sites became available. ' Relatively few results were obtained for molecular adsorbates, however, for which the LEED analyses are especially demanding. The advent of synchrotron-radiation-based structural methods such as surface-extended x-ray-absorption fine structure (SEXAFS) and energy-scan photoelectron diffraction (PED) in the 1980s did not significantly change the situation regarding these adsorbates. Other surface-sensitive techniques which probe surface structure less directly have therefore been used to gather structural information on adsorbed molecular layers. Thus vibrational spectroscopy, in the form of both infrared refiection-absorption spectroscopy (IRAS) and high-resolution electronenergy-loss spectroscopy (HREELS), has proved particularly successful in assigning adsorption sites for adsorbed diatomic molecules. By comparison with measured vibrational frequencies in inorganic clusters of known structure, the accepted picture seems to be that adsorption on a specific site on a given crystal face should give rise to a typical vibrational frequency falling in a defined and rather narrow range and, furthermore, that the ranges for adsorption on di6'erent sites on the same surface should not seriously overlap.However, only a few adsorption-site assignments based on vibrational spectroscopy have been tested to date by LEED, SEXAFS, ' or PED. "' Whereas the results of the latter methods are in agreement with the results of vibrational spectroscopy for all the CO adsorption structures studied, recent SEXAFS (Ref. 10) and PED (Ref. 12) studies of Ni(111)-c(4X2)-NO have shown that the assignment of adsorption site based on vibrational frequencies for this system is in error. Here we present the results of a SEXAFS study of Ni(111)-c(4X2)-CO which are also in disagreement with the interpretation of vibrational studies, ' ' and thus cast doubt on the general reliability of structural assignments based on vibrational frequencies alone.

Aqueous-phase reforming of oxygenated hydrocarbons over Sn-modified Ni catalysts

“…Adsorption of CO on Ni(111) is known to be favored at threefold-hollow sites [27]. Our observation that addition of Sn to R-Ni causes a decrease in the CO uptake is consistent with the report that adsorption of CO is strongly suppressed on a buckled ( (111) surface which has no threefold Ni hollow sites [28].…”

Raney Ni‐Sn Catalyst for H2 Production from Biomass‐Derived Hydrocarbons.

“…The modest decrease in the extent of CO chemisorption by the catalyst after reaction is consistent with a slight decrease in the initial activity of the catalyst. Adsorption of CO on Ni(111) is favored at threefold hollow sites (21). Our observation that addition of Sn to Raney-Ni causes a decrease in the CO uptake is consistent with the report that adsorption of CO is strongly suppressed on a buckled ( ͌ 3x ͌ 3 ) R30°Sn/ Ni(111) surface, which has no threefold Ni hollow sites (22).…”

The Surface Geometry of Carbon Monoxide and Oxygen Co-adsorbed on Ni{111}

“…The adsorption of carbon monoxide on Ni{111} is among the best studied systems in Surface Science [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. Despite this fact, a clear picture of the adsorption geometry of this system emerged only about ten years ago.…”