AIO

Surface Science 165 (1986) 337-344 337 North-Holland, Amsterdam

CAPACITANCE OF JELLIUM S. HOLMSTROM and P. APELL

Institute of Theoretical Physics, Chalmers University of Technology, S-412 96 GOteborg, Sweden

Received 18 June 1985; accepted for publication 4 September 1985

The capacitance of a simple metal modelled by jellium is studied in the presence of an applied static field. An explicitly non-linear calculation gives a differential capacitance that varies with the strength of the applied field in a way that closely resembles what has been measured for some free-electron-like metals in electrochemical cells. This indicates that the metal plays a much more important role than is usually assumed.

Surface Science 165 (1986) 345-354 345 North-Holland, Amsterdam

THE S T R U C T U R E OF THE (v~ × v~)R30 ° S U P E R L A T r l C E P H A S E O N ( l i d a -Cu-16 at% AI; A LEED INTENSITY ANALYSIS R . J . B A I R D

Research Staff, Ford Motor Co., Dearborn, Michigan 48121, USA

and

D.F. OGLETREE, M.A. V A N HOVE and G.A. SOMORJAI

Materials and Molecular Research Division, Lawrence Berkeley Laboratory and Department of Chemistry, University of California, Berkeley, California 94720, USA

Received 21 May 1985; accepted for publication 25 August 1985

A dynamical LEED analysis is presented of the ( f3x~ /3 )R30 ° superlattice on the (111) surface of an a -Cu-Al alloy single crystal with a bulk A1 concentration of 16 at%. This surface ordering contrasts with the lack of long-range order in the bulk. In the ordered surface the AI atoms are found to be substitutionally arranged within the topmost layer with (v~-× v/3)R30 ° symmetry. No evidence for the ordering of A1 in deeper layers is detected by this analysis. The fcc- ( l l l ) stacking sequence is maintained at the surface, and the surface atoms occupy positions within +0.05 ~- of bulk Cu lattice positions, i.e. no appreciable top-layer spacing change and no mixed-layer buckling are detected.

Surface Science 165 (1986) 355-374 355 North-Holland, Amsterdam

THE A D S O R P T I O N OF ACETYLENE ON A GRAPHITE SURFACE C. P E T E R S a n d J . A . M O R R I S O N

Department of Physics and Institute for Materials Research, McMaster University, Hamilton, Ontario, L8S 4M1/ Canada

and

A l l

M.L. KLEIN

Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, KIA OR6, Canada Received 31 May 1985; accepted for publication 27 August 1985

Adsorption isotherms and Henry's Law constants are reported for acetylene adsorbed on exfoliated graphite in the region 109 < T < 184 K. The isotherms yield evidence for two solid phases, and for coexisting l iquid-gas and liquid-solid phases. Possible structures of the solid phases are discussed in the light of the results of Monte Carlo simulations performed with several intermolecular potential models. The Henry's Law constants are used to refine the parameters of the acetylene-graphite potential.

Surface Science 165 (1986) 375-392 375 North-Holland, Amsterdam

A COMPARATIVE STUDY OF THE BEHAVIOUR OF THE PdAg(l l l ) AND Pd( l l l ) SURFACES TOWARDS THE INTERACTION WITH HYDROGEN AND CARBON MONOXIDE A. NOORDERMEER, G.A. K O K and B.E. NIEUWENHUYS * Gorlaeus Laboratories, State University of Leyden, P.O. Box 9502, 2300 RA Leyden, The Netherlands

Received 28 June 1985; accepted for publication 4 September 1985

AES, LEED and TDS studies are reported describing the adsorption behaviour of a Pd67%-Ag33%(111) surface (with surface composition of about 10% Pd, 90% Ag) towards CO and hydrogen. The results are compared to those obtained for a Pd(111) surface. In addition, the effect of preadsorbed hydrogen on subsequent CO dosing and the effect of preadsorbed CO on postdosed hydrogen are described. The amount of CO that can be adsorbed on PdAg(111) per unit surface area at 250 K is only 8 ___ 5% of that on Pd( l l l ) . CO desorption from P d A g ( l l l ) occurs in a single peak with a maximum shifting with exposure from 400 to 375 K (heating rate 30 K/s ) . This should be compared with a desorption peak from P d ( l l l ) with a max imum shifting from 500 to 475 K and with a shoulder around 340 K. The results are explained in terms of size and relative concentration of ensembles of Pd atoms on the surface. It can be concluded that the heat of adsorption of CO adsorbed on the triplet sites of P d ( l l l ) is much larger ( ~ 30 kJ /mol ) than that of CO adsorbed on the singlet sites of Pd( l l l ) . The effect of lateral interaction in the CO adlayer on P d ( l l l ) is about 15 k J / m o l and, hence, much smaller than the effect of the sites. Hydrogen is desorbed from PdAg at a significantly higher temperature than from Pd( l l l ) . Postdosed CO does not modify the amount of desorbing hydrogen at subsequent heating. The presence of adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. The CO desorption spectra are not influenced by pre- or post-exposure of P d A g ( l l l ) to hydrogen. The results suggest that hydrogen is not adsorbed on P d A g ( l l l ) at 230 K but that it is absorbed into the bulk. This process of hydrogen dissolution is much easier than for Pd( l l l ) . We conclude that isolated Pd atoms on the PdAg(111) surface act as trapping and dissociation centres for hydrogen. The heat of adsorption of hydrogen on Pd singlet sites is too low for detection at 220 K.