An infrared study of ethene and CO coadsorption on Pt[111] and a Pt/SiO2 catalyst:: ambiguities in the interpretation of difference spectra

An infrared spectroscopic study shows that the sequential adsorption of ethene and then CO on a finely divided Pt/SiO2 catalyst at 294 K displaces the pi and di-sigma* C2H4 species that occur on non-[111] facets after the initial ethene adsorption, but leaves the ethylidyne species on [111] facets undiminished. Extra absorptions from ethylidyne are revealed by the removal of the other species. The principal absorptions of the ethylidyne species are affected in very different ways, particularly in relative intensities, as a result of the CO post-adsorption. On a single-crystal Pt[111] surface only the the ethylidyne species is present after adsorption at 360 K. This is fractionally removed by CO postadsorption while intensity changes occur to the remaining ethylidyne absorptions that are analogous to those observed with the finely divided catalyst. It is shown that in each case the intensity changes in the ethylidyne spectra are visually exaggerated in the difference spectra. This occurrence is shown be due to the partial overlap of the analogous features in the parent spectra, leading to partial and selective cancellations in the difference spectra. The phenomenon, which is of general significance in difference spectroscopy, is explored systematically by means of computer simulations. The spectrum of CO itself, obtained after post-adsorption on either ethylidyne-covered sample, shows predominantly adsorption on linear ton-top) sites in contrast to the results of analogous earlier experiments involving Rh or Pd surfaces where the CO adsorbs on three-fold sites. (C) 1999 Elsevier Science B.V. All rights reserved.