Characterization of the adsorption and decomposition of methanol on Ni(110)

A51 of the bare films increases continuously from 4.25 eV (T,, = 60 K) to 4.72 eV (Ta, = 330 K), Evidence is provided that coldly deposited Ag films are highly porous and that the pores persist up to To, = 170 K. but are irreversibly healed between 170 and 250 K. The m i n i m u m thickness of the evaporated Ag films needed to develop these pores is found to be 50 ,~.. The width of the pores, which are most likely intercrystalline gaps, is estimated to be 5-15 ,~.. Besides the "macroscopic" pores the films contain atomic scale defects, which, in contrast to the pores, are healed continuously with increasing T,,. Films annealed at 330 K are composed of (111) grains with still a few percent of defect sites. The implications of these structural features on the adsorption properties of pyridine as well as on the interpretation of SERS results from such Ag films are dealt with in part I1 of this work.

386

Surface Science 150 (1985) 386-398 North-Holland, Amsterdam

ON THE POROSITY OF COLDLY CONDENSED SERS ACTIVE Ag FILMS !1. Comparison of adsorption and Raman scattering of pyridine E.V. ALBANO *, S. DAISER **, R. MIRANDA *** and K. W A N D E L T Institut ft~r Physikalische Chemie, Unwersttat Mi~nvhen. Sophzenstrasse 11, D- 8000 Mi~nchen 2. Fed. Rep. of Germany Received 15 August 1984; accepted for publication 9 October 1984 The adsorption of pyridine on coldly deposited Ag films annealed at temperatures ranging from 58 to 330 K, the porous surface topography of which has been investigated in part I of this work, has been studied by means of UPS, work function change and thermal desorption measurements. Pyridine induced work function changes have been employed to follow the surface diffusion of pyridine molecles into the pores of these Ag films. The surface diffusion is very slow below 60 K, but readily takes place at 130 K with an estimated activation energy of surface migration of E m = 4 kcal/mol. Preadsorption of Xe into the pores of the films causes inhibition of pyridine diffusion into the pores. The onset of pyridine desorption from porous films is detected at - 200 K while from flat films the desorption begins already at 150 K. The careful analysis of our data on the structure of the coldly deposited Ag films and the adsorption behavior of pyridine on these films as well as a survey of published SERS data lead us to conclude that the SERS active sites of coldly deposited Ag films are within the pores. This conclusion is in agreement with recent theoretical calculations.

Surface Science 150 (1985) 399-418 North-Holland, Amsterdam

399

CHARACTERIZATION OF THE ADSORPTION AND DECOMPOSITION OF METHANOL ON Ni(ll0) Simon R. BARE, Joseph A. STROSCIO and W. HO Laborato.rv of Atomic and Solid State Physics and Materials Science Center, Cornell University, Ithaca, New York 14853, USA Received 6 August 1984; accepted for publication 16 October 1984 The chemisorption, condensation, desorption, and decomposition of methanol, both C H 3 O H and CH3OD, on a clean Ni(110) surface have been characterized using high resolution electron