At. Gee et Be. Hayden, The dynamics of O-2 adsorption on Pt(533): Step mediated molecular chemisorption and dissociation, J CHEM PHYS, 113(22), 2000, pp. 10333-10343
The dissociative adsorption of oxygen on the stepped Pt(533) surface has be
en investigated using supersonic molecular beams by measuring the initial d
issociative sticking probability S-0 as a function of incident kinetic ener
gy E-i, angle Phi, surface temperature T-S, and S as a function of coverage
Theta. By comparison with dynamical data available on the Pt(111) surface
we have been able to establish that step sites dominate the dissociative ad
sorption process. S-0(E-i) for oxygen on Pt(533) at T-S=200 in the energy r
ange 52 meV-1.4 eV shows a similar functional dependence to results on Pt(1
11), however, the magnitude of S-0 on Pt(533) is significantly greater at a
ll energies. The measurement of S-0(Phi), scattering in a plane perpendicul
ar to the step direction at E-i=1.18 eV at T-S=350 K, reveals a strong and
asymmetric angular dependence which contains contributions associated with
activated adsorption and dissociation of the chemisorbed precursor on the (
111) terraces, and a second contribution associated with activated dissocia
tion through a similar channel at the step sites. The latter exhibits a max
imum in S-0(Phi) at 35 degrees, near the angle corresponding to the normal
of the (100) step plane. S-0(T-S) at E-i=1.18 eV and Phi =0 degrees reveals
a much smaller temperature dependence in the range 150 >T-S(K)> 800 on Pt(
533) than on Pt(111). At E-i=1.18 eV and Phi =0 degrees ca. 15%-25% of diss
ociation takes place through molecules impinging directly at step sites. Th
e remaining fraction dissociate through activated adsorption of the chemiso
rbed species on the (111) terrace and subsequent partition between desorpti
on, and dissociation at step sites. Dissociation of the chemisorbed precurs
or on the (111) terrace appears highly activated, a result which is consist
ent with theory. The rapid decrease in S-0(E-i) observed below 0.15 eV on P
t(533), observed also on Pt(111), is consistent with a trapping mechanism w
here the need to dissipate energy limits the probability of adsorption, and
subsequent dissociation, of the physisorbed precursor. Kinetic modelling o
f this partition on Pt(533), between the conversion of the physisorbed prec
ursor to the chemisorbed species, and desorption yields DeltaE=120 meV and
v(d)/v(pc)=80. We conclude that the effective barrier to conversion of the
physisorbed to chemisorbed species on Pt(533) is effectively zero. We concl
ude that defects will tend to dominate this conversion process on the close
packed surface. In addition to this channel, at E-i=0.05 meV ca. 50% of mo
lecules dissociate through the same channel operating at higher energy on P
t(533). (C) 2000 American Institute of Physics. [S0021-9606(00)71046-4].