In situ observation of a surface chemical reaction by fast X-ray photoelectron spectroscopy [20]

Full text of DOI:10.1021/ja991858v

J. Am. Chem. Soc. 1999, 121, 7969-7970
7969
a photon energy of 400 eV and energy resolution of ∼100 meV.
Temperature-programmed reaction data were acquired by appli-
cation of a linear heating ramp (∼0.4 Ks-1) to the ethyne-covered
sample.
In Situ Observation of a Surface Chemical Reaction
by Fast X-Ray Photoelectron Spectroscopy
Adam F. Lee,† Karen Wilson,‡ Ruth L. Middleton,§
Alessandro Baraldi,| Andrea Goldoni,| Giorgio Paolucci,| and
Richard M. Lambert*,§
Ethyne trimerization occurs at low temperatures (<180 K).
Therefore, adsorption was carried out with the sample held at
100 K so as to avoid immediate benzene formation. Figure 1
shows a sequence of C 1s XP spectra as a function of ethyne
coverage. These data were obtained during continuous exposure
of the initially clean Pd(111) sample over a time interval of ∼850
s, each spectrum being recorded in ∼10 s. All of the background-
subtracted, low temperature C 1s spectra can be readily fitted
using a single peak centered at 283.88 eV modeled by a Doniach-
Sunjic function convoluted with a Gaussian. Derived peak para-
meters are given in Table 1 and the resulting C 1s integrated int-
ensities are shown in the inset to Figure 1. This shows the presence
just a single ethyne adsorption state at all coverages. We also
obtained near-edge X-ray absorption spectra (NEXAFS, not
shown) which show that the C-C axis of the reactant molecule
remained parallel to the metal surface under all conditions. The
surface coverage of ethyne increases linearly with exposure up
to the saturation value of θ ) 0.43 monolayers (1 ML ) 1 ×
10-15 molecules cm-2), indicative of precursor-mediated adsorp-
tion kinetics, consistent with previous studies.5 Ethyne adsorption
is accompanied by significant attenuation of the clean Pd(111)
surface-state emission at 335.34 eV, relative to the emission from
bulk Pd at 335.94 eV (Figure 2). The surface-state emission is
associated with the topmost layer of Pd atoms,6 and its quenching
signifies Pd-ethyne bond formation.
Temperature-programmed C 1s XP spectra resulting from
heating a saturated C2H2 overlayer are shown in Figure 3. Large
temperature-dependent binding energy (BE) shifts and variations
in peak width are apparent, due to changes in both the chemical
state and number density of adsorbed hydrocarbon species. The
initial peak shift to ∼1 eV higher BE is accompanied by little
change in the overall C 1s intensity. However subsequent temper-
ature rises induce a monotonic drop in C surface coverage and a
gradual shift back towards lower BE. Using comparative C 1s
spectra obtained during the low temperature chemisorption of
benzene overlayers on Pd(111), all spectra in Figure 3 can be
fitted by three components centered at 283.88, 284.54, and 285.01
eV, possessing a common lineshape. The fitted peak parameters
are given in Table 1. The sequence in which these components
are populated allows their assignment as unreacted ethyne,
strongly chemisorbed flat-lying benzene, and weakly-bound tilted
benzene, respectively. The temperature-dependent intensity varia-
tions of these components, and their sum (after correction for
relative photon excitation cross-sections) are shown in Figure 4.
The interpretation we offer below is fully in accord with all of
the available experimental data,7 especially the isotope tracing
and temperature-programmed reaction data.8 The results show
clearly that the primary chemical process is formation of flat-
lying (η6) benzene which commences at ∼150 K. The associated
activation energy (obtained by analysis of the leading edge of
the η6 intensities shown in Figure 4) is ∼5 ( 1 kJ mol-1. At
∼280 K, where ∼50% of the ethyne has reacted and a substantial
amount of η6 benzene is present, tilted (η1) benzene appears on
the surface. The formation of this species, either via direct reaction
from ethyne and/or by tilting of η6 benzene, reflects changing
intermolecular interactions in the adsorbed layer. By ∼300 K most
Department of Chemistry, UniVersity of Hull
Hull HU6 7RX, U.K.
Department of Chemistry, UniVersity of York
York YO1 5DD, U.K.
Department of Chemistry, UniVersity of Cambridge
Cambridge CB2 1EW, U.K.
ReceiVed June 4, 1999
Despite their immense socioeconomic importance the discovery
and optimization of new heterogeneous catalysts remains ineffi-
cient. This reflects the empirical methodologies often adopted and,
in most cases, the absence of a microscopic understanding of cat-
alyst behavior. By studying reactions on well-defined single-crys-
tal surfaces, in conjunction with measurements on the correspon-
ding practical dispersed catalysts, it is possible to obtain fundam-
ental insight into reaction pathways and to develop predictive
capabilities.1,2 However, to further the rational design of catalyti-
cally active materials, new in situ analytical techniques are re-
quired. In particular, it would be valuable to obtain time-resolved
information about the temperature-dependent evolution of the re-
acting adsorbed layer. Here, we demonstrate the first use of time-
resolved fast X-ray photoelectron spectroscopy (XPS) as a chemi-
cally specific, quantitative probe in a study of the trimerization
of ethyne to benzene over a catalytically active Pd(111) surface.
We have measured the threshold temperature and activation barrier
for trimerization, elucidated details of the reaction pathway, and
identified configurational changes in the adsorbed layer.
The potential of fast XPS as a probe in studies of molecular
desorption processes was recently demonstrated for the CO/Rh-
(110) system.3 The success of this technique hinges on the high
photon flux and high resolution available at current third-gener-
ation synchrotrons. These attributes permit rapid acquisition of
time-resolved XP spectra while ramping the sample temperature.
Here, we extend the technique to follow a surface-catalyzed reac-
tion: the trimerization of ethyne to benzene on a Pd(111) single-
crystal surface. This reaction may be regarded as the prototypi-
cal metal-catalyzed alkyne coupling reaction. These processes are
of interest because they constitute a versatile network of relative-
ly complex reactions which, depending on the conditions, can
yield a variety of products including linear and cyclic hydrocar-
bons and even heterocycles. In addition, they can be operated ov-
er a wide range of pressure ranging from ultra high vacuum to 1
bar using both single-crystal samples and dispersed catalysts. They
therefore provide a valuable testing ground for many concepts
that are central to catalytic science (see ref 4 and refs therein).4
Experiments were performed at the SuperESCA beamline of
the ELETTRA (Trieste) synchrotron radiation source using on a
Pd(111) single-crystal substrate prepared by standard procedures
and maintained under ultra-high vacuum (system pressure ∼1 ×
10-10 Torr). Quoted exposures are given in langmuirs (1 langmuir
) 1 × 10-6 Torr s-1). Carbon 1s XP spectra were acquired with
* Corresponding author. E-mail: rml1@cam.ac.uk. Telephone: +44 1223
336467. Fax: +44 1223 336362.
† University of Hull.
‡ University of York.
§ University of Cambridge.
| Sincrotrone Trieste, Trieste, Italy.
(5) Tysoe, W. T.; Nyberg, G. L.; Lambert, R. M. J. Chem. Soc. Chem.
Comm. 1983, 623.
(1) Goodman, D. W. Surf. ReV. Lett. 1995, 2, 9.
(2) Lambert, R. M.; Ormerod, R. M. Mater. Chem. Phys. 1991, 29, 105.
(3) Baraldi, A.; Comelli, G.; Lizzit, S.; Cocco, D.; Paolucci, G.; Rosei, R.
Surf. Sci. 1996, 367, L67.
(6) Andersen, J. N.; Hennig, D.; Lundgren, E.; Methfessel, M.; Nyholm,
R.; Scheffler, M. Phys. ReV. B 1994, 50, 17525.
(7) Hoffman, H.; Zaera, F.; Ormerod, R. M.; Lambert, R. M.; Wang, L.
P.; Tysoe, W. T. Surf. Sci. 1990, 232, 259.
(4) Lambert, R. M.; Ormerod, R. M. In Surface Reactions, Springer Series
in Surface Sciences; Springer-Verlag: New York, 1994.
(8) Patterson, C. H.; Lambert, R. M. J. Phys. Chem. 1988, 92, 1266.
10.1021/ja991858v CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/17/1999

7970 J. Am. Chem. Soc., Vol. 121, No. 34, 1999
Communications to the Editor
Figure 3. C 1s XP spectra obtained from a C2H2-saturated Pd(111)
surface as a function of temperature.
Figure 1. Main: C 1s XP spectra obtained during C2H2 uptake at 100
K over a clean Pd(111) surface. Inset: fitted, integrated C 1s peak
intensities as a function of C2H2 exposure.
Table 1a
molecular assignment
binding energy, eV
C2H2
283.88
284.54
285.01
η6 C6H6
η1 C6H6
a Lorentzian FWHM ) 0.115 eV, gaussian FWHM ) 0.522 eV,
asymmetry factor ) 0.188.
Figure 4. Fitted, integrated C 1s peak intensities derived from a C2H2-
saturated Pd(111) surface as a function of temperature, showing C2H2)
(s), η6 C6H6 (O), and η1 C6H6 ([) components.
is hard to see how this high temperature species could be ascribed
to conversion of strongly adsorbed η6 benzene to weakly adsorbed
η1 benzene. A much more likely explanation is as follows. The
nearly bare Pd(111) surface is efficient at inducing C-H cleavage
in adsorbed benzene.9 The resulting η1 and η2 phenyl species
would be strongly adsorbed but also tilted with respect to the
surface, resulting in a C 1s binding energy very similar to that of
η1 benzene. Finally, these phenyl species are lost by dispropor-
tionation, yielding gaseous benzene and residual adsorbed carbon.
Our observations provide the first direct evidence that surface
reactions are not rate-limiting for the appearance of the gaseous
benzene product: the desorption step is rate-limiting. They also
demonstrate that important configurational changes occur in the
adsorbed product as the the reaction proceeds. These changes are
critically important in determining the kinetics of the overall
reaction. Kinetic control can be achieved by designing the catalyst
surface (e.g., by alloying Pd with Au)10,11 so as to favour the
formation of η1 (tilted) benzene thus maximizing activity and
optimizing selectivity.
Figure 2. Pd 3d XP spectra from clean (O) and C2H2-saturated (4) Pd-
(111) surfaces showing quenching of clean Pd surface state.
of the ethyne has been removed by desorption or reaction, but
important changes continue within the predominately benzene
containing adsorbed phase. At ∼330 K ethyne is no longer present
and it is clear that tilted η1 tilted benzene starts to convert into η6
flat benzene, presumably due to reduced adsorbate density. Closer
insepection of the η6 and η1 intensities in this temperature regime
shows that η1 depletion exceeds η6 accumulation, implying that
in addition to the tilted f flat conversion, some of the tilted ben-
zene actually desorbs. This desorption acts to deplete the surface
of adsorbate, thus assisting the tilted f flat conversion. At ∼500
K the coverage of η6 (flat) benzene starts to decrease due to its
desorption. At this point the Pd surface is rather bare and, quite
unexpectedly, the η1 population apparently starts to rise again. It
JA991858V
(9) Moyes, R. B.; Baron, K.; Squire, R. C. J. Catal. 1971, 22, 333.
(10) Baddeley, C. J.; Tikhov, M. S.; Hardacre, C.; Lomas, J. R.; Lambert,
R. M. J. Phys. Chem. 1996, 100, 2189.
(11) Lee, A. F.; Baddeley, C. J.; Hardacre, C.; Ormerod, R. M.; Lambert,
R. M.; Schmid, G.; West, H. J. Phys. Chem. 1995, 99, 6096.