Temperature-Programmed Desorption Investigation of the Adsorption and Reaction of
Butene Isomers on Pt(111) and Ordered Pt-Sn Surface Alloys
Yi-Li Tsai and Bruce E. Koel*
Department of Chemistry, UniVersity of Southern California, Los Angeles, California 90089-0482
ReceiVed: NoVember 15, 1996; In Final Form: January 6, 1997
X
The influence of alloyed Sn on the chemistry of C
4
butene isomers, including 1-butene, cis-2-butene, and
isobutene, chemisorbed on Pt(111) was investigated by temperature-programmed desorption (TPD), Auger
electron spectroscopy (AES), and low-energy electron diffraction (LEED). Pt-Sn alloy chemistry was probed
by investigation of two ordered surface alloys formed when Sn atoms were incorporated within the topmost
layer on a Pt(111) substrate to form a (2 × 2) Sn/Pt(111) alloy with Θ
Sn
) 0.25 and a (3 ×3)R30°
Sn/Pt(111) alloy with Θ
Sn
) 0.33. Low-coverage states of chemisorbed 1-butene, cis-2-butene, and isobutene
on Pt(111) have desorption activation energies of 17.5, 17, and 17 kcal/mol, respectively. These energies are
reduced to 16, 15.5, and 15 kcal/mol on the (2 × 2) alloy and 13.5, 12, and 11 kcal/mol on the (3 ×
3)R30° alloy. Changing the surface Sn concentration from Θ
Sn
) 0.25 to Θ
Sn
) 0.33 causes a relatively
larger decrease in the chemisorption bond strength of these alkenes, and we associate this with the importance
of a pure Pt 3-fold site for strong alkene bonding. All three butenes undergo decomposition on Pt(111)
during TPD which accounts for 50-60% of the chemisorbed monolayer. Alloying Sn into the surface causes
a large reduction in the reactivity of the surface, and the fraction of the chemisorbed layer which decomposes
is decreased to 3-7% on the (2 × 2) alloy, and no decomposition occurs on the (3 ×3)R30° alloy. The
strong reduction of decomposition on these two surface alloys may be due to the elimination of adjacent pure
Pt 3-fold hollow sites. No large changes occur in the coverage of the chemisorbed monolayer of butenes in
the presence of up to 33% of a monolayer of alloyed Sn, showing that the adsorption ensemble requirement
for chemisorption of these alkenes on Pt(111) and the two Sn/Pt(111) alloys is at most a few Pt atoms. To
the extent that alloying or direct Pt-Sn interactions occur in supported, bimetallic Pt-Sn catalysts, the chemistry
reported here would lead to increased isobutene yields and decreased coking of the catalyst.
1. Introduction
We are investigating the chemistry of a series of hydrocarbons
on ordered Pt-Sn surface alloys that can serve as models of
supported bimetallic Pt-Sn heterogeneous catalysts. Generally,
the addition of Sn to Pt catalysts for hydrocarbon conversion
decreases the catalytic activity but increases the selectivity for
unsaturated hydrocarbon products and reduces coking, thus
prolonging the lifetime of the catalyst. The role of Pt-Sn alloy
phases in this chemistry is a subject of debate, and there is
minimal fundamental underpinning of this discussion from
surface science since so little is known about the chemisorption
and reaction of hydrocarbons on well-defined Pt-Sn alloy
surfaces. In addition, the catalytic dehydrogenation of butane
and isobutane to isobutene is of particular interest since
isobutene is important as a source of MTBE, a gasoline additive.
In order to provide some fundamental information on this
reaction over metal surfaces, we have investigated the adsorption
and reaction of butane and isobutane,
1
H
2
,
2
and now butene
isomers on the Pt(111) surface and on two ordered Pt-Sn
surface alloys.
Extensive work on olefinic molecule adsorption has been
carried out previously on transition metals, especially on
Pt(111). Ethylene,
3-11
propylene,
4,8,12-15
and all of the butene
(C
4
) isomers
4,8,12,16,17
are di-σ-bonded to Pt(111) at low tem-
peratures (95-260 K). The molecular adsorbates desorb or
transform into alkylidyne surface species near room temperature.
In particular, Cassuto and Tourillon
16
showed using near-edge
X-ray adsorption fine structure (NEXAFS) and ultraviolet
photoelectron spectroscopy (UPS) that all four butene isomers
on Pt(111) had identical adsorption geometries, i.e., di-σ-bonded
with the chemisorbed C-C bond parallel to the Pt surface plane,
and similar chemisorption bond strengths (peak desorption
temperatures of 260-280 K).
In this paper, TPD is primarily used to extend information
on the adsorption, desorption, and dehydrogenation of sev-
eral butene isomers (1-butene, cis-2-butene, and isobutene) on
Pt(111) and to provide the first results for butenes adsorbed on
two well-defined Pt-Sn surface alloyssthe (2 × 2) and (3
×3)R30° Sn/Pt(111) alloys. In particular, we determine the
influence of alloyed Sn on the chemistry of these molecules. In
this paper, we have not studied the adsorption and reaction of
trans-2-butene, since the results for cis- and trans-2-butene on
Pt(111) were reported to be very similar using TPD measure-
ments.
4
2. Experimental Methods
These experiments were performed in a stainless steel UHV
chamber pumped by a 220 L/s ion pump, Ti sublimation pump,
and 170 L/s turbomolecular pump. The base pressure was kept
at 1 × 10
-10
Torr during the experiments. The chamber was
equipped with a double-pass cylindrical mirror analyzer (CMA)
for AES, a four-grid electron optics for LEED, a UTI 100 C
quadrupole mass spectrometer (QMS) for TPD, an ion gun for
Ar
+
ion sputtering, and directed beam microcapillary dosers for
gas exposures.
The Pt(111) single-crystal sample (Atomergic, 5 N purity)
was 10 mm in diameter and 1 mm thick. A detailed description
of the sample holder including the heating and cooling setup
was reported previously.
18
We were able to achieve a substrate
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Abstract published in AdVance ACS Abstracts, February 15, 1997.
2895 J. Phys. Chem. B 1997, 101, 2895-2906
S1089-5647(96)03824-2 CCC: $14.00 © 1997 American Chemical Society