The interaction of alkanes with low-index surfaces of iridium at low temperatures and pressures has been studied in our laboratory by thermal desorption mass spectrometry

and low-energy electron diffraction. While the reconstructed (110)-(1 × 2) surface of iridium initiates dissociative chemisorption of ethane and all higher alkanes at 125 K in the low coverage limit, similar reactions with the close-packed (111) surface occur readily only at defect sites. Thus, surface geometry influences strongly the activation barriers to C-H bond scission in alkanes on iridium surfaces

Engstrom, J. R.

Szuromi, P. D.

Weinberg, W. H.

Wittrig, T. S.

Caltech Authors - Main

Banholzer et al : Summary Abstract: A comparison of nitric oxide 
there is an enormous variation in rate. 
We have attempted to model the variations. The model is 
based on the previous work of Woodward and Hoffmann, 5 
who showed that one can identify especially active reaction 
pathways in reactions of organic and simple inorganic mole-
cules by examining the symmetry of the orbitals available for 
reaction. In general, it has been found that a reaction is fast, 
if the orbitals of the right symmetry are present, while it is 
slow if they are absent. 
Banholzer et aU examined the reaction N01adJ---*NiadJ 
+ 0 1adJ in detail and noted that the availability of the anti-
bonding orbitals needed to break the 1T bond in the NO varies 
significantly from face to face. Figure 1 also shows the site 
average orbital availability calculated as a function of crystal 
face. One observes that on Pt( 111) there are no orbitals of the 
right symmetry available for reaction while on Pt(410) all the 
necessary orbitals are present. There is a continuous vari-
ation in the orbital availability for faces in between. 
A comparison of the data and the calculations shows that 
the orbital availability is a good indicator of the catalytic 
activity. The rate is low when the orbital availability is low 
and high when the orbital availability is high. The magni-
tude of the effect is enormous. The changes in orbital avail-
ability explain why Pt(210) is less active than Pt(410) and 
why Pt(511) is less active than Pt(100). Neither of these ob-
servations is easily explained by previous models. Clearly, 
the orbital availability is not the only effect; Pt(210), for in-
stance, is slightly more active than Pt(lOO) even though the 
orbital availabilities of the two faces are the same. However, 
the effects of changes in the orbital availability explains 
much of the data. These effects need to be more extensively 
explored. 
Acknowledgments: This work was supported by the Na-
tional Science Foundation under Grant No. CPE83-11791. 
This work made use of the University of Illinois Center for 
Microanalysis of Materials which is supported under Grant 
No. DMR83-16981. 
1R. J. Gorte, L. D. Schmidt, and J. L. Gland, Surf. Sci. 109, 367 (1981). 
2H. P. Bonze!, G. Broden, and G. Pirog, J. Catal. 53, 96 (1978). 
3W. F. Banholzer, Y. 0. Park, K. M. Mak, and R. I. Masel, Surf. Sci. 128 
176 (1983). 
4Y. 0. Park, W. F. Banholzer, and R.I. Masel, Appl. Surf. Sci. 19, 145 
( 1984), and references therein. 
5R. B. Woodward and R. Hoffmann, The Conservation of Orbital Symmetry 
(Chemie, Weinheim, 1969). 
Summary Abstract: Comparative studies of alkane activation by low-index 
surfaces of iridium and platinuma> 
P. D. Szuromi, J. A. Engstrom, T. S. Wittrig,b> and W. H. Weinberg 
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 
91125 
(Received 29 October 1984; accepted 4 December 1984) 
The interaction of alkanes with low-index surfaces of iri-
dium at low temperatures and pressures has been studied in 
our laboratory by thermal desorption mass spectrometry 
and low-energy electron diffraction. 1•2 While the recon-
structed ( 110)-(1 X 2) surface of iridium initiates dissociative 
chemisorption of ethane and all higher alkanes at 125 K in 
the low coverage limit, similar reactions with the close-
packed ( 111) surface occur readily only at defect sites. 1 Thus, 
surface geometry influences strongly the activation barriers 
to C-H bond scission in alkanes on iridium surfaces. 
Likewise on the Pt(111) surface, normal alkanes which 
desorb molecularly below 250 K do not adsorb dissociatively 
under similar conditions. 3 On the other hand, both n-butane 
and n-pentane are observed to adsorb dissociatively on the 
Pt( 110)-( 1 X 2) surface. This demonstrates that the surface 
structure sensitivity of alkane activation, which was ob-
served on iridium surfaces, occurs also on platinum surfaces. 
Low exposures ( < 0.5 L) of n-butane and n-pentane ad-
sorb dissociatively on Pt(110)-(1 X2) with a threshold tem-
perature of approximately 200 K. The hydrocarbon frag-
ments which form evolve hydrogen which is manifest as 
thermal desorption features at approximately 300, 400, and 
550 K. At higher exposures, molecular desorption of the 
parent alkanes is observed at 200 K for n-butane and 230 K 
for n-pentane. No hydrocarbons other than the parent al-
kane which was adsorbed are observed in the thermal de-
sorption spectra. The hydrogen feature at 300 K appears to 
arise from the desorption of hydrogen bound to the platinum 
surface, while the features at approximately 400 and 550 K 
are a consequence of the decompositon of hydrocarbon frag-
ments adsorbed on the surface. Saturation coverages of the 
dissociatively adsorbed alkanes, after adsorption at 150 K, in 
units of 1013 molecules em - 2, are 4.0 ± 0.4 for n-butane and 
5.0 ± 0.5 for n-pentane, based on the known coverage of 
hydrogen at saturation reported by Jackman et a/.,4 namely 
1.1 X 1015 atoms em - 2 • During these thermal desorption 
measurements, no loss of intensity of the half-order LEED 
beams is observed, demonstrating that the reconstruction of 
Pt( 110) is not relaxed by alkane adsorption. 
The dissociative adsorption of n-butane can be inhibited 
by preadsorption of hydrogen. The extent of inhibition is 
linear in precoverage of hydrogen, and is complete after a 
preexposure of 0.5 L of hydrogen. Furthermore, if 0.5 L of 
hydrogen is exposed to the surface after the adsorption of n-
butane at 150 K, then the dissociative adsorption of then-
butane is inhibited completely. However, if the adlayer of n-
1560 J. Vac. Sci. Technol. A 3 (3), May/Jun 1985 0734·2101/85/031560·02$01.00 © 1985 American Vacuum Society 1560 
Szuroml et 111. : Summary Abstract: Comparative studies of alkane activation 
butane is annealed to 200 K, then exposure of hydrogen has 
no inhibiting effect on dissociative adsorption. This thresh-
old temperature for dissociative adsorption of200 K implies 
an activation energy of approximately 12 kcal/mol. This is 
consistent with the observed absence of dissociative adsorp-
tion for propane, for which only molecular desorption is ob-
served at 170 K. 
The Pt(l10)-(1X2) surface activates alkanes at low tem-
peratures (- 200 K) and pressures, although this activation 
is not so effective as that observed on the same surface of 
iridium. On iridium the threshold temperature for dissocia-
tive adsorption is only 125 K, and somewhat higher satura-
tion coverages of dissociatively adsorbed overlayers are ob-
served. While differences in the details of the reaction 
chemistry are manifestations of electronic structural differ-
ences between the metals, for both metals the availability of 
high coordination adsites appears to lower the kinetic bar-
riers that must be overcome to activate carbon-hydrogen 
bonds in alkanes. 
•I Supported by the National Science Foundation under Grant. No. CHE82· 
06487. 
biCurrent address: Amoco Oil Co., P. 0. Box 400, Naperville, Illinois 
60566. 
1W. H. Weinberg, Surv. Prog. Chern. 10, 1 (1983). 
2P. D. Szuromi and W. H. Weinberg, Surf. Sci. 149, 226 (1985). 
3L. E. Firment and G. A. Somorjai, J. Chern. Phys. 66, 2901 (1977); L. E. 
Firment, Ph. D. thesis, University of California, Berkeley, 1976. 
4T. E. Jackman, J. A. Davies, D. P. Jackson, W. N. Unertl, and P. R. 
Norton, Surf. Sci. 120, 389 (1982). 
Summary Abstract: A molecular beam study of the dissociative adsorption of 
C02 on Ni(100) 
G. E. Gdowski, A. V. Hamza, M. P. D'Evelyn, and R. J. Madix 
Department of Chemical Engineering, Stanford University, Stanford, California 94305 
(Received 17 October 1984; accepted 14 November 1984) 
A supersonic molecular beam system was used to study the 
dynamical characteristics of the dissociative adsorption of 
C02 on Ni(100). C02 dissociated into adsorbed CO and 0. 
The dissociative sticking probability s0 was increased by two 
orders of magnitude by increasing the translational kinetic 
energy of the C02 by a factor of slightly more than 12. The 
addition of vibrational energy also increased s0 • Small 
amounts of oxygen completely poisoned the dissociation. 
The results also indicated that in addition to a direct activat-
ed route C02 could dissociate through a molecularly ad-
sorbed state. 
A brief description of the reactive scattering apparatus 
will be given; for a more detailed description see Ref. 1. The 
reactive scattering apparatus was equipped with low energy 
electron diffraction (LEED)I Auger electron spectroscopy 
30f-
>l! 0 
.!: 
15 
(j')o 
0 
• TN=IOOOK 
eTN=298K 
• 
I 
6 
• 
• ·~ • 
• 
• • 
• • 
I 
12 18 
Kinetic Energy of C02 in kcal/gmol 
-f-
• 
• 
I 
24 
FIG. I. The initial dissociative sticking probability for C02 on Ni(100) as a 
function of the C02 translational kinetic energy for nozzle temperatures of 
298 K (circles) and 1000 K (squares). 
(AES) optics, an ion gun for Ar sputtering, and two mass 
spectrometers. The mass spectrometers were used to deter-
mine beam energies and fluxes and for temperature pro-
grammed desorption (TPD) studies. The scattering chamber 
was pumped by a 1500 1/s turbomolecular pump, a 220 1/s 
vac-ion pump, and a titanium sublimation pump. 
The supersonic molecular beam source could deliver up to 
1015 molecules/cm2/s to the sample. The duration of the 
beam dose was controlled by an electronically operated shut-
ter, which intercepted the beam outside the scattering 
chamber. The kinetic energies of the C02 beam ranged from 
1.8 to 30 kcal/mol. The beam energies were obtained by 
varying the temperature of the nozzle and by seeding He and 
H2 beams with C02 (the lighter molecules accelerate the hea-
vier C02 molecules). 2 
Subsequent to exposure of the surface to C02 at a surface 
temperature of 400 K, CO was desorbed by TPD in the range 
of 450 to 470 K (Bco <0.03 monolayer) and oxygen re-
mained on the surface as determined by AES. This allowed 
the use of CO TPD to measure the number of C02 molecules 
dissociating. TPD data were calibrated with a saturation 
dose of CO at room temperature (Osat = 0.55 monolayer). 3 
To test this procedure the initial sticking probability of CO 
on Ni(100) was determined to be approximately 0.9 at two 
kinetic energies. The value is in good agreement with a pre-
vious determination of approximately unity.4 
The data (Fig. 1) for the room temperature nozzle showed 
a 300-fold increase in s0 with the C02 beam energy. At low 
beam energiess0 was 10-4 , it then increased slowly to0.02 at 
a beam energy of ::::::9 kcal/mol and finally rose moderately 
to 0.13 at 25 kcal!mol. In the energy range studies here s0 did 
not appear to saturate with increasing incident energy. 
1561 J. Vac. Sci. Techno!. A 3 (3), May/Jun 1985 0734-2101/85/031561-o2$01.00 @ 1985 American Vacuum Society 1561 


Comparative studies of alkane activation by low-index surfaces of iridium and platinum

https://authors.library.caltech.edu/32173/1/SZUjvsta85.pdf

Comparative studies of alkane activation by low-index surfaces of iridium and platinum

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main