Egelhoff, W. F., Jr.

Weinberg, W. H.

Withrow, S. P.

English

Caltech Authors - Main

Abstract: Study of the dynamics of oxygen adsorption on 
lr(111)* 
W.H. Weinberg,t S.P. Withrow.~ and W.F. Egelhoff, Jr.~ 
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 
(Received 25 August 1976) 
PACS numbers: 82.65.My, 64.60.Cn, 64.70.Kb, 66.30.Jt 
Under ultra-high-vacuum conditions, the chemisorption 
of oxygen on Ir( Ill) has been found to proceed via a mobile 
precursor state. This state appears to be physically adsorbed 
oxygen molecules acting as a two-dimensional gas. Auger 
spectroscopy, contact potential difference measurements and 
thermal-desorption mass spectrometry suggest that the 
sticking probability into the chemisorbed state is constant until 
saturation coverage is very nearly reached. The sticking 
probability at 325 K is 0.09, but it increases continuously with 
decreasing temperature until a value of 0.18 is found at 225 
K (the lowest temperature investigated). The accuracy of this 
observed trend is considerably greater than the accuracy as-
sociated with each individual measurement of which the latter 
is on the order of 50%. 
A temperature-dependent temporal delay was noted with 
LEED for the formation of three independent, ordered do-
mains of (2 X 1) superstructure, each rotated 120° with re-
spect to one another. Although the apparent dimensionality 
of the LEED superstructre was (2 X 2), the corresponding 
absolute coverage was measured by x-ray photoelectron 
spectroscopy (XPS) to be 7.8 X 1014 0 atoms/cm2. This is 
indicative of half-monolayer surface coverage at saturation, 
which is consistent with the (2 X 1) domain structure. The 
measured delay, or induction periods (as a function of surface 
temperature) indicate that the activation energy for surface 
diffusion leading to the ordering is between 16 and 19 kcal/ 
mole. The LEED also shows that the chemisorbed oxygen 
atoms undergo an order-disorder transition on the surface 
with an energy difference between the ordered and disor-
dered states of approximately 1 kcal/mole. The data are 
consistent with a two-dimensional Ising model with a critical 
482 J. Vac. Sci. Technol., Vol. 14, No. 1, Jan./Feb. 1977 
temperature for disorder of approximately 460 K. 
Mass spectrometry results show that the heat of adsorption 
of the chemisorbed atomic oxygen is (65 - 100) kcal/mole 
where (} is the fraction of saturation coverage (not absolute 
fractional coverage). The oxygen desorbs according to sec-
ond-order kinetics verifying that the chemisorbed state is 
dissociated oxygen. 
At T ~ 700 K, the chemisorbed oxygen is incorporated into 
the Ir forming a chemically inert nonstoichiometric near-
surface oxide state. In particular, this oxide state is not reactive 
toward either CO or H2 in a temperature regime where the 
chemisorbed oxygen overlayer reacts rapidly. The oxide state 
decomposes, with the evolution of molecular oxygen, at ap-
proximately 1270 K with a kinetic order of unity. The con-
version of the oxygen overlayer into the near-surface oxide 
may be observed directly by means of XPS measurements. 
There is a chemical shift in the binding energy of the O(ls) 
level of the chemisorbed oxygen overlayer (529.8 eV) and the 
near-surface oxide (531.5 eV). Further details concerning the 
interaction of oxygen with the (lll) surface of Ir may be 
found in Refs. 1-3. 
*Supported by NSF Grant Number GP-41807 
t Alfred P. Sloan Foundation Fellow 
*NSF Energy-Related Postdoctoral Fellow 
* Present address: Physics Department, General Motors Research Labora-
tories, Warren, Michigan 48090 
1 V. P. Ivanov, G. K. Boreskov, V. I. Savchenko, W. F. Egelhoff, Jr., and W. 
H. Weinberg, Surf. Sci. (accepted, 1976). 
2 P. A. Zhdan, G. K. Boreskov, A. I. Boronin, W. F. Egelhoff, Jr., and W. H. 
Weinberg, Surf. Sci. (accepted. 1976). 
3 S. P. Withrow and W. H. Weinberg (to be published). 
Copyright© 1977 by the American Vacuum Society 482 


Study of the dynamics of oxygen adsorption on

Ir(111)

https://authors.library.caltech.edu/32878/1/WEIjvst77a.pdf

Study of the dynamics of oxygen adsorption on Ir(111)

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main