Acetylene Chemisorption on Sn/Pt(100) Alloys
²
Chameli Panja, Najat A. Saliba, and Bruce E. Koel*
UniVersity of Southern California, Department of Chemistry, Los Angeles, California 90089-0482
ReceiVed: September 23, 2000; In Final Form: March 5, 2001
Adsorption and reaction of acetylene on a hexagonally reconstructed (5 × 20)-Pt(100) surface and two ordered
Sn/Pt(100) alloy surfaces were investigated using temperature programmed desorption spectrometry (TPD),
Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and X-ray photoelectron
spectroscopy (XPS). Vapor deposition of Sn onto a Pt(100) single-crystal substrate was used to form two
Pt-Sn alloys, the c(2 × 2) and (32×2)R45° Sn/Pt(100) structures with θ
Sn
) 0.5 and 0.67 ML, respectively,
depending on the initial Sn concentration and annealing temperature. Acetylene nearly completely decomposed
during TPD on Pt(100) in the absence of Sn, forming hydrogen, which then desorbs as H
2
, and surface
carbon. This decomposition, associated with irreversible dissociative adsorption, was strongly suppressed on
the two Pt-Sn alloy surfaces, and a large acetylene desorption peak in TPD was observed. Additionally,
15% of the adsorbed acetylene monolayer was converted to gaseous benzene during TPD on the (32×2)-
R45° Sn/Pt(100) alloy. No such benzene desorption occurred from the c(2 × 2) alloy. Alloyed Sn in the
c(2 × 2) alloy decreased the initial sticking coefficient of acetylene on Pt(100) at 100 K by ∼40%, but
additional Sn in the other alloy had no additional effect. The saturation coverage of C
2
D
2
in the chemisorbed
monolayer at 100 K decreased from that on Pt(100) by 35% on the c(2 × 2) alloy and 50% on the (32×2)-
R45° Sn/Pt(100) alloy. However, the c(2 × 2)-Sn adlayer eliminates acetylene chemisorption, illustrating
that the effectiveness of Sn to “block” sites depends crucially on its location as an adatom or alloyed atom
on Pt surfaces. The acetylene chemisorption bond energy, estimated by the acetylene desorption activation
energy measured in TPD, also decreased (45-65%) as the alloyed Sn concentration increased. Multiple TPD
peaks for C
2
D
2
desorption from both the c(2 × 2) and the (32×2)R45°Sn/Pt(100) alloy surfaces indicate
either several energetically distinguishable adsorption sites for acetylene or the rate-limiting influence of
more complex surface reactions on these surfaces.
1. Introduction
Acetylene (C
2
H
2
) chemisorption and reaction on Pt and other
transition-metal surfaces has been studied many times, mainly
to probe surface reactions related to heterogeneous catalysis.
Pt(111) and Pt(100) surfaces are known to be highly reactive
toward acetylene decomposition, and the formation of hydrogen
and adsorbed carbon on the surface is the only reaction pathway
observed in UHV studies.
1-4
Such a high reactivity, which leads
to nonspecific carbon build-up, is not desired in most industrial
reactions, so commercial hydrocarbon conversion catalysts often
utilize bimetallic Pt-based catalysts containing a second metal
component in order to modify (reduce) the reactivity of Pt.
5,6
Surface science studies of Pt-Sn alloys have also shown
consistent results indicating that hydrocarbon dehydrogenation
rates on Pt-Sn alloys were much slower than on clean Pt.
7-9
As a highly reactive molecule with a C:H stoichiometry of
unity, acetylene can serve as a prototype for reactions of “coke
precursors.” Also, C
2
H
2
can be used to probe C-C bond
coupling reactions under UHV conditions. Cyclotrimerization
of C
2
H
2
to form a benzene desorption peak in TPD is a rather
unique reaction that occurs only on Pd(111)
10-13
and Cu-
(110)
14,15
pure-metal surfaces. Lambert and co-workers
10-12
elucidated that the mechanism for this reaction first involves
dimerization to form a C
4
H
4
metallopentacyclic intermediate,
which then forms benzene with a third C
2
H
2
molecule in an
associative mechanism without cleavage of any C-C or C-H
bonds.
10-12
On Ni(111), there is evidence of benzene formation
on the surface at high C
2
H
2
coverages
16
(or from CH
3
dosing
17
),
but it apparently decomposes during heating to desorb H
2
and
leave carbon on the surface. The clean Pt(111) surface does
not desorb benzene from this reaction.
1-3
Acetylene cyclotrimerization on Pd single-crystal surfaces is
sensitive to both the atomic geometry and the electronic structure
of the metal surface. For example, benzene formation is more
efficient on Pd(111) than on Pd(100) and Pd(110) surfaces.
18
Also, the presence of sulfur enhanced the degree of benzene
formation on Pd surfaces.
18
More recently, this reaction was
found to be effectively carried out on reduced TiO
2
(001)
19
and
several bimetallic alloys.
20-23
We are particularly interested in
these latter results because bimetallic alloys provide the pos-
sibility of investigating the nature of the surface metal atom
ensemble and site requirements for such cyclization activity.
Benzene desorption from cyclotrimerization of acetylene has
been observed on Sn/Pt(111),
20
Au/Pd(111),
21
and Pd/Au-
(111)
22,23
alloy surfaces.
We have previously studied C
2
H
2
adsorption and reaction on
two ordered, Pt-Sn surface alloys formed on Pt(111). These
alloys have (2 × 2) and (3×3)R30° unit cells, with θ
Sn
)
0.25 and 0.33 ML in the outermost layer, respectively.
24,25
Decomposition of acetylene was strongly suppressed by alloying
Sn in the surface layer of Pt(111), and benzene desorption was
observed on both alloys.
20
On the (3×3)R30° alloy, which
desorbed more benzene than the (2 × 2) alloy, 33% of the
²
Part of the special issue “John T. Yates, Jr. Festschrift”.
* To whom correspondence should be addressed.
3786 J. Phys. Chem. B 2001, 105, 3786-3796
10.1021/jp003445i CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/14/2001