Lampton, V.

Merrill, R. P.

Monroe, D. R.

Weinberg, V.

Weinberg, W. H.

English

Caltech Authors - Main

Abstract: Interaction of H2 and 0 2 on platinum (111) 
W. H. Weinberg 
California Institute of Technology, Pasadena, California 91625 
D. R. Monroe, V. Lampton (V. Weinberg), and R.P. Merrill 
University of California, Berkeley, California 94720 
(Received 26 August 1976) 
PACS numbers: 82.65.My, 73.30. +y, 61.14.Hg, 82.65.Jv 
The interaction of 0 2 and H2 on Pt(lll) surfaces has been 
examined using atomic- and molecular-beam scattering, AES, 
LEED, and thermal-desorption mass spectrometry. The 
surfaces are prepared in situ by thermal annealing in oxygen. 
Careful scanning of the LEED to reveal the possible existence 
of ordered steps and the Debye-Waller extrapolation of elastic 
helium scattering1 has been used to establish that the surface 
contains approximately 10% defects, most of which appear 
to be disordered. 
Molecular oxygen was adsorbed onto such surfaces from 
a thermal-energy molecular beam. Sticking coefficients were 
found to be ,.,10-2 with an activation energy for adsorption 
of "'2 kcal/mole.2 Note that adsorption on the defects only 
with such an activation energy would rationalize the absolute 
value of the sticking probability. Adsorption of 02 from an 
isotropic source at 300 K shows similar values for the sticking 
coefficient, first order desorption kinetics, and produces a 2 
X 2 LEED pattern often observed by others. 3 At an elevated 
surface temperature (above 700 K), an epitaxial oxide is 
formed which has a (v3 X v3) R-30c structure superposed 
on a weak (3 X 15) pattern. On Pt(llO) the (100) face of PtO 
rationalizes a (6 X 12) coincidence lattice which has been 
found for the high-temperature oxidation.4 The work function 
of the epitaxial oxide is I V less than the clean surface, while 
Spicer-5 has. shown that chemisorbed oxygen increases the work 
function by 0.8 V. 
Hydrogen adsorbs on this surface through a precursor state6 
evidenced by a decrease in the sticking coefficient from 0.1 
to 0.01 as the surface temperature increases from 300 to 400 
K. There is evidence of two adsorbed states, one with a 
binding energy of 15-20 kcal/mole, and one with a binding 
energy of 30-32 kcal/mole. There is strong kinetic mixing 
between these two states during the flash desorption, and so 
a detailed interpretation is not possible. The gross features of 
444 J. Vac. Sci. Technol., Vol. 14, No. 1, Jan./Feb. 1977 
the adsorption-desorption and inter-state transfer are re-
produced in both the flash-desorption experiments and con-
stant-temperature desorption experiments using elastic helium 
scattering to measure the amount of hydrogen adsorbed. 
Using CSFO-BEBO model calculations,7 the low-energy 
binding state is tentatively identified as molecular hydrogen 
and the more strongly bound state as atomic hydrogen. 
The reaction of hydrogen with chemisorbed oxygen, as-
sumed to be dissociated, proceeds with a cross section of 35 
A 2 up to a coverage of 2.9 X 1014 cm-2, above which it is in-
dependent of oxygen with a reaction probability of unity. 
These kinetics are consistent with neither the Langmuir-
Hinshelwood (surface-surface) nor the simple Rideal-Eley 
(gas-surface) mechanism, but suggest a kinetic precursor 
which is different from the precursor associated with H2 
chemisorption. These results are compared with previously 
published CFSO-BEBO calculations7•8 which predicted the 
following: (1) a small activation energy (2-4 kcal/mole) for 
0 2 adsorption; (2) an activation energy for the Rideal-Eley 
mechanism which is less than that for the Langmuir-Hin-
selwood mechanism; and (3) the correct adsorption energies 
for both the molecular and atomic adsorption states. 
1 A. G. Stoll, Jr., J. J. Ehrhardt. and R. P. :\1errill, J. Chern. Phys. 64, 34 
(1976). 
2A. G. Stoll, Jr., Ph.D. thesis (Department of Chemical Engineering, Gni-
versity of California at Berkeley, 1973) (unpublished). 
'
1H. P. Bonze) and H. Ku, Surf. Sci. 40, &5 (1973). 
4R. Ducros and R. P. :\1errill, Surf. Sci. 55, 389 ( 1976). 
50. M. Collins, J. B. Lee, and W. E. Spicer, Surf. Sci. 55,389 (1976). 
6V. Lampton (V. Weinberg), M.S. thesis (Department of Chemical Engi-
neering, Oniversity of California at Berkeley, 1971) (unpublished). 
7W. H. Weinberg and H. P. Merrill, Surf. Sei. 33,493 (1972). 
8W. H. Weinberg and H. P. Merrill,]. Catal. 40, 268 ( 1975). 
Copyrlghl © 1977 by the American Vacuum Soclely 444 


Interaction of H_2 and O_2 on platinum (111)

Abstract: Interaction of H2 and 0 2 on platinum (111) 
W. H. Weinberg 
California Institute of Technology, Pasadena, California 91625 
D. R. Monroe, V. Lampton (V. Weinberg), and R.P. Merrill 
University of California, Berkeley, California 94720 
(Received 26 August 1976) 
PACS numbers: 82.65.My, 73.30. +y, 61.14.Hg, 82.65.Jv 
The interaction of 0 2 and H2 on Pt(lll) surfaces has been 
examined using atomic- and molecular-beam scattering, AES, 
LEED, and thermal-desorption mass spectrometry. The 
surfaces are prepared in situ by thermal annealing in oxygen. 
Careful scanning of the LEED to reveal the possible existence 
of ordered steps and the Debye-Waller extrapolation of elastic 
helium scattering1 has been used to establish that the surface 
contains approximately 10% defects, most of which appear 
to be disordered. 
Molecular oxygen was adsorbed onto such surfaces from 
a thermal-energy molecular beam. Sticking coefficients were 
found to be ,.,10-2 with an activation energy for adsorption 
of "'2 kcal/mole.2 Note that adsorption on the defects only 
with such an activation energy would rationalize the absolute 
value of the sticking probability. Adsorption of 02 from an 
isotropic source at 300 K shows similar values for the sticking 
coefficient, first order desorption kinetics, and produces a 2 
X 2 LEED pattern often observed by others. 3 At an elevated 
surface temperature (above 700 K), an epitaxial oxide is 
formed which has a (v3 X v3) R-30c structure superposed 
on a weak (3 X 15) pattern. On Pt(llO) the (100) face of PtO 
rationalizes a (6 X 12) coincidence lattice which has been 
found for the high-temperature oxidation.4 The work function 
of the epitaxial oxide is I V less than the clean surface, while 
Spicer-5 has. shown that chemisorbed oxygen increases the work 
function by 0.8 V. 
Hydrogen adsorbs on this surface through a precursor state6 
evidenced by a decrease in the sticking coefficient from 0.1 
to 0.01 as the surface temperature increases from 300 to 400 
K. There is evidence of two adsorbed states, one with a 
binding energy of 15-20 kcal/mole, and one with a binding 
energy of 30-32 kcal/mole. There is strong kinetic mixing 
between these two states during the flash desorption, and so 
a detailed interpretation is not possible. The gross features of 
444 J. Vac. Sci. Technol., Vol. 14, No. 1, Jan./Feb. 1977 
the adsorption-desorption and inter-state transfer are re-
produced in both the flash-desorption experiments and con-
stant-temperature desorption experiments using elastic helium 
scattering to measure the amount of hydrogen adsorbed. 
Using CSFO-BEBO model calculations,7 the low-energy 
binding state is tentatively identified as molecular hydrogen 
and the more strongly bound state as atomic hydrogen. 
The reaction of hydrogen with chemisorbed oxygen, as-
sumed to be dissociated, proceeds with a cross section of 35 
A 2 up to a coverage of 2.9 X 1014 cm-2, above which it is in-
dependent of oxygen with a reaction probability of unity. 
These kinetics are consistent with neither the Langmuir-
Hinshelwood (surface-surface) nor the simple Rideal-Eley 
(gas-surface) mechanism, but suggest a kinetic precursor 
which is different from the precursor associated with H2 
chemisorption. These results are compared with previously 
published CFSO-BEBO calculations7•8 which predicted the 
following: (1) a small activation energy (2-4 kcal/mole) for 
0 2 adsorption; (2) an activation energy for the Rideal-Eley 
mechanism which is less than that for the Langmuir-Hin-
selwood mechanism; and (3) the correct adsorption energies 
for both the molecular and atomic adsorption states. 
1 A. G. Stoll, Jr., J. J. Ehrhardt. and R. P. :\1errill, J. Chern. Phys. 64, 34 
(1976). 
2A. G. Stoll, Jr., Ph.D. thesis (Department of Chemical Engineering, Gni-
versity of California at Berkeley, 1973) (unpublished). 
'1H. P. Bonze) and H. Ku, Surf. Sci. 40, &5 (1973). 
4R. Ducros and R. P. :\1errill, Surf. Sci. 55, 389 ( 1976). 
50. M. Collins, J. B. Lee, and W. E. Spicer, Surf. Sci. 55,389 (1976). 
6V. Lampton (V. Weinberg), M.S. thesis (Department of Chemical Engi-
neering, Oniversity of California at Berkeley, 1971) (unpublished). 
7W. H. Weinberg and H. P. Merrill, Surf. Sei. 33,493 (1972). 
8W. H. Weinberg and H. P. Merrill,]. Catal. 40, 268 ( 1975). 
Copyrlghl © 1977 by the American Vacuum Soclely 444 


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Interaction of H_2 and O_2 on platinum (111)

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