Particle size effects in the acetylene cyclotrimerization on planar model Al2O3 thin film supported Pd clusters

Article abstract of DOI:10.1021/jp971318p

The low-temperature cyclotrimerization of acetylene to benzene on alumina thin film supported Pd particles of controlled size has been studied by temperature-programmed desorption. The reactively formed benzene is observed to desorb in three distinctly separate features with kinetics resembling those observed for acetylene cyclotrimerization on Pd single crystals. An enhanced activity for low-temperature benzene desorption is observed for the larger Pd particles, consistent with single-crystal results that reveal that the Pd(111) surface favors low-temperature desorption of the reactively formed benzene relative to more open surfaces. Additionally, a pronounced drop-off in benzene production is observed for the smallest particles studied, suggestive of an ensemble size requirement for the reaction.

Full text of DOI:10.1021/jp971318p

J. Phys. Chem. B 1997, 101, 8883-8886
8883
Particle Size Effects in the Acetylene Cyclotrimerization on Planar Model Al2O3 Thin Film
Supported Pd Clusters
P. M. Holmblad, D. R. Rainer, and D. W. Goodman*
Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843-3255
ReceiVed: April 16, 1997; In Final Form: August 24, 1997X
The low-temperature cyclotrimerization of acetylene to benzene on alumina thin film supported Pd particles
of controlled size has been studied by temperature-programmed desorption. The reactively formed benzene
is observed to desorb in three distinctly separate features with kinetics resembling those observed for acetylene
cyclotrimerization on Pd single crystals. An enhanced activity for low-temperature benzene desorption is
observed for the larger Pd particles, consistent with single-crystal results that reveal that the Pd(111) surface
favors low-temperature desorption of the reactively formed benzene relative to more open surfaces.
Additionally, a pronounced drop-off in benzene production is observed for the smallest particles studied,
suggestive of an ensemble size requirement for the reaction.
1. Introduction
addressed using Au/Pd(111) and Pd/Au(111) surface alloys and
Au/Pd colloid particles.20-22 In this work, it has been argued
that an ensemble of seven Pd atoms or six Pd atoms with a
central Au atom is required for benzene formation. Addition-
ally, it was also concluded that a Pd ensemble involving a Au
atom is more effective than a pure Pd ensemble, possibly by
reducing decomposition as opposed to desorption of the
reactively formed benzene. This ensemble model has also been
applied to rationalize experimental results for cyclotrimerization
of acetylene on Pd(111) with coadsorbed oxygen.23
The cyclotrimerization of acetylene to benzene on low-index
single-crystal Pd surfaces has received considerable attention
since its discovery by Tysoe et al.1,2 and Sesselmann et al.3 This
reaction has been shown to be a structure sensitive reaction with
the Pd(111) facet being the most active.4,5 In a temperature-
programmed desorption (TPD) experiment, benzene evolution
resulting from low-temperature (<200 K) adsorption of C2H2
on Pd(111) occurs in a low-temperature feature at ∼250 K and
a high-temperature feature at ∼500 K. An additional intermedi-
ate feature is present for Pd(100) just below 400 K. In one
case, a similar desorption feature at ∼400 K has been reported
for Pd(111).6 It is generally agreed that the high-temperature
desorption feature arises from benzene lying flat on the surface,
while the low-temperature desorption feature results from
reactively formed benzene on an initially crowded surface that
forces the benzene molecules into a weaker binding configu-
ration with the molecular plane tilted with respect to the surface.
The origin of the intermediate desorption feature at ∼400 K
has alternately been explained as desorption from a close-packed
flat-bonded layer of benzene7 or from adsorption sites at defect
sites.8
The mechanism of benzene formation is not completely
understood, but is believed to involve the consecutive addition
of acetylene molecules without C-C bond scission as inferred
from isotopic labeling experiments.9 A pentadiene metallocycle
(PdC4H4) has been proposed as an intermediate species;10,11 this
model is further supported by the observation of small amounts
of furan12 and thiophene13 reactively formed from C2H2 on
Pd(111) in the presence of oxygen and sulfur, respectively. The
adsorption of acetylene and its thermal evolution have also been
studied by vibrational spectroscopies, and CCH, CCH2 CCH3,
and CH surface species have been detected.14-17 Acetylene
adsorbs in the 3-fold hollow site on the Pd(111) surface,17,18
and the conversion to benzene from this flat-lying acetylene
species proceeds at temperatures as low as 140 K.6 There exists
a critical threshold acetylene coverage of 0.33 ML (monolayers)
for benzene formation to occur,2,19 and this threshold can be
reduced by inert spectator molecules.19
Finally, the cyclotrimerization of acetylene has been studied
at elevated pressures on Pd single crystals and supported Pd
particles, and it has been demonstrated that the reaction can be
operated as a genuine catalytic process.5,24
The morphology of Pd planar model catalysts has previously
been addressed by scanning tunneling microscopy (STM),
transmission electron microscopy (TEM), and infrared reflection
adsorption spectroscopic (IRAS) studies of adsorbed CO.25,26
The number density and size distribution of these Pd particles
obtained from vapor deposition onto thin film oxide substrates
can be controlled by the amount of Pd deposited and subsequent
annealing. The IRAS results for adsorbed CO exhibit absorption
features indicating that the particles are primarily comprised of
(111) and (100) facets with a higher proportion of edge/defect
sites for the smaller particles.
Due to the structure sensitivity of the acetylene cyclotrim-
erization, it is expected that the reaction will be sensitive to the
Pd particle size. The results reported here do in fact unambigu-
ously reveal a pronounced particle size effect.
2. Experimental Section
The experiments were carried out in a stainless steel ultrahigh-
vacuum chamber with a base pressure less than 10-9 Torr. The
apparatus was equipped with Auger electron spectroscopy
(AES), a quadrupole mass spectrometer (QMS) for TPD, and
IRAS.
The alumina thin films were grown on a Mo(110) substrate
initially cleaned by several cycles of oxidation and electron beam
heating to above 2000 K. Cleanliness was verified by AES.
The alumina films were prepared by evaporating aluminum onto
the Mo(110) in 2.5 × 10-6 Torr ambient oxygen at 900 K with
a subsequent annealing to 1200 K. The thickness of the films
(∼30 Å ) was estimated from the deposition time and the ratio
of the Al3+(∼54 eV) to the Mo(∼180 eV) AES signals.27
It has been suggested that a critical ensemble size is required
for acetylene trimerization to benzene; this possibility has been
* To whom correspondence should be addressed.
X Abstract published in AdVance ACS Abstracts, October 1, 1997.
S1089-5647(97)01318-7 CCC: $14.00 © 1997 American Chemical Society

8884 J. Phys. Chem. B, Vol. 101, No. 44, 1997
Holmblad et al.
Figure 1. IRA spectra of CO chemisorbed on alumina thin film
supported Pd particles corresponding to deposition of 1, 4, and 7 ML
of Pd. All spectra are obtained at 110 K in 1.5 × 10-6 Torr CO.
Figure 2. Desorption of benzene (m/e ) 78) following a 70 langmuir
The Pd particles were deposited by vapor deposition of Pd
onto the alumina film at 500 K at a rate of 0.6 ML/min, followed
by annealing to 800 K. This preparation method has been used
in studies of Pd planar model catalysts; the particle sizes here
have been estimated from the TEM and STM images and
particle size distributions reported previously.25,26 Deposition
of 1, 2, 4, and 10 ML of Pd results in particles with average
diameters of approximately 20-25, 40-50, 70-80, and 100-
120 Å, respectively.
acetylene exposure at 150 K for various Pd loadings as indicated.
vibrational spectra of adsorbed CO on the thin film alumina
supported Pd particles shown in Figure 1 are consistent with
those previously reported for this system,25,26 displaying bands
associated with CO adsorbed on Pd(111) and Pd(100). Of
special interest is the feature at 1886 cm-1, indicative of CO
bound in the 3-fold hollow site of the 111 facet. The intensity
of this band is clearly increasing with increasing Pd particle
size, consistent with previous work25 and with expectations
based purely on thermodynamic considerations, with the larger
particles exhibiting a higher incidence of 111 faceting.
Benzene TPD results for the supported particles are very
similar to the analogous low-index single-crystal results. Figure
2 illustrates that three distinct desorption features are apparent
across a range of Pd loadings in planar model supported Pd/
Al2O3/Mo(110) catalysts; a low-temperature feature at 270 K,
an intermediate feature at 370 K, and a high-temperature peak
at 530 K. Comparisons of the TPD results reveal distinct
particle size effects. For the smallest particles (∼15-20 Å dav),
most of the benzene desorption occurs above 500 K, with very
little low-temperature benzene evolution. As the particle size
is increased, the high-temperature peak begins to attenuate, while
the desorption peak at 230 K increases in intensity. For Pd
loadings g4 ML (average particle size g50 Å), the low
temperature desorption pathway is clearly favored. This will
be discussed in detail below.
Calibration of the Pd evaporation rate was accomplished by
several TPD experiments following deposition of Pd onto the
bare Mo(110) sample. The monolayer equivalent referred to
in this paper according to this calibration is 1 ML ) 1.43 ×
1015 cm-2
. The acetylene (Matheson 99.6%) was passed
through a charcoal trap at room temperature and stored in a
glass bulb prior to use. A TPD experiment was carried out by
initially growing a fresh alumina film and then cooling the
sample and manipulator to 90 K prior to evaporation of Pd.
Seventy langmuirs of acetylene was then adsorbed at 150 K
onto the Pd particles; after 20 min a TPD was recorded with a
heating rate of 5 K/s.
3. Results and Discussion
The Pd particles were characterized using IRAS of chemi-
sorbed CO. Figure 1 shows IRA spectra of CO adsorbed onto
three different particle sizes corresponding to Pd depositions
of 1, 4, and 7 ML onto the alumina film. The spectra were all
obtained in a background of 1.5 × 10-6 Torr CO at 110 K.
Several other interesting observations can be made for this
data set. First of all, a substantial decrease in benzene
production is observed between the 1.0 and 0.5 ML Pd
coverages; the TPD peak area decreases by about a factor of 4
going from the higher to the lower Pd coverage, even though
the Pd surface area is decreasing by just under a factor of 2.
This marked drop-off in activity at the extreme lower end of
the particle size range studied (average particle diameter ∼15
Three sharp absorption features at 2108, 1995, and 1886 cm-1
,
are resolved for the particles resulting from 7 ML of Pd, with
the latter two superimposed on a broader band stretching from
2000 cm-1 downward. As the Pd loading is decreased, all bands
attenuate in intensity; for 1 ML of Pd the bands at 1886 and
1995 cm-1 are no longer visible and only the band at 2108 cm-1
and the broad band in the 2000-1900 cm-1 region remain. The

Acetylene Cyclotrimerization
J. Phys. Chem. B, Vol. 101, No. 44, 1997 8885
Figure 4. Benzene (m/e ) 78) desorption following (a) initial and (b)
repeated adsorption of 70 langmuirs of C2H2 at 150 K on Pd particles
(∼50 Å).
Figure 3. Mass fragments m/e ) 2, 28, 41, 56, and 78 detected in
TPD following a 70 langmuir acetylene exposure at 150 K on particles
corresponding to 3 ML of Pd (∼50 Å).
initial TPD experiment preferentially chemisorbs at edges and
defects, leading to suppression of benzene interacting with these
sites in subsequent runs and thus to the attenuation of the
desorption feature at 370 K. The total yield of benzene is not
significantly affected in the TPD following the second adsorp-
tion of acetylene (Figure 4b), but the distinct desorption features
of (a) are broadened in (b). The low-temperature and the high-
temperature desorption features are still resolved and slightly
shifted in peak temperature, whereas the intermediate feature
around 370 K is strongly suppressed. Also (not shown), the
evolution of other hydrocarbons in the temperature range 230-
320 K is completely absent in the second TPD.
Intuitively then, if the feature at 370 K is in fact related to
benzene adsorbed at defect sites, one might expect to see the
ratio of this feature to the other desorption features in the TPD
traces increase with decreasing particle size. The behavior
actually observed, however, is quite different. A relatively small
contribution from the 370 K peak compared to the high-
temperature peak is observed for the smallest particles. The
ratio increases as the contribution from the high-temperature
peak diminishes with increasing particle size up to a Pd coverage
of ∼3.5 ML, where a maximum is reached. Above this
coverage, the ratio starts to decline again as the low-temperature
peak gains in intensity, and the 370 K peak attenuates.
Apparently, then, the relative contribution from this feature is
only partly influenced by the density of surface defects. Perhaps
both of the explanations advanced to explain this feature for
single crystals (discussed above) are valid, and there is a
convolution of the two effects. Alternatively, perhaps the
smaller particles favor the decomposition reaction at the lower
temperature, deactivating the defect sites for benzene adsorption.
This is an interesting issue that warrants further study.
Å, with a significant fraction of even smaller particles in the
lower wing of the distribution) suggests the manifestation of
the anticipated ensemble effect.
Another point of interest involves the desorption peak at 370
K; the presence of this feature on the supported particles
suggests that it is probably related to benzene adsorbed at defect
sites. In fact, this feature is suppressed to a greater degree than
the other desorption maxima upon iterative reaction runs,
consistent with having preferential buildup of inactive carbon
on reactive defects, as discussed in more detail below. That
surface decomposition is one (or one group) of the reaction
pathways available for the benzene formed at low temperatures
is demonstrated by the evolution of smaller hydrocarbons
between 230 and 320 K observed for every Pd loading studied;
this is illustrated by the TPD spectra for the m/e ) 41 and 56
fragments shown in Figure 3. These species also yielded
fragments at m/e ) 55, 43, 42, 40, and 39. The total desorption
yield associated with this feature was essentially constant with
Pd loading above 1 ML of Pd. Figure 3 also shows the
desorption of hydrogen (m/e ) 2) between the low-temperature
benzene and the smaller hydrocarbon desorption. Simultaneous
with this hydrogen desorption, a fragment at m/e ) 28 is also
seen which cannot be attributed to CO, since no m/e ) 12
fragment was associated with this peak. Both CO and H2 desorb
at higher temperatures in rather broad features.
This interpretation is further supported by the effect of
sequential reactions on the intensity of the 370 K benzene peak.
In Figure 4, the TPD of benzene following a second adsorption
of acetylene onto Pd particles with an average diameter of ∼50
Å (3 ML of Pd) clearly reveals that the 370 K benzene feature
is strongly suppressed in a subsequent TPD run. Again, this is
is suggestive that carbon deposited by decomposition in the
Returning to the observation that the larger particles favor
the low-temperature reaction pathway, there are a couple of ways

8886 J. Phys. Chem. B, Vol. 101, No. 44, 1997
Holmblad et al.
to interpret this. Assuming regular polyhedra comprised
exclusively of 111 and 100 facets, the ratio of the former to
the latter increases with increasing particle size. Single-crystal
TPD studies of Pd(111) and (100) have revealed a similar
enhancement for the low-temperature desorption on the (111)
surface.5 Therefore, this effect may well be related to the ratio
of 111 facets on the surface. An alternate explanation invokes
the enhanced curvature of the smaller particles that precludes
the degree of compression in the benzene overlayer that results
in the tilted, weakly bound configuration.
For small particles, benzene evolution is only observed from a
high-temperature state (∼520 K), whereas larger particles desorb
benzene in a high- (∼520 K) and a low-temperature (∼220 K)
state, as observed on Pd single crystals. The results are
consistent with the (111) facet being the most efficient for
trimerization, resulting in an enhanced activity for larger
particles. A pronounced drop-off in benzene yield is observed
beneath a critically small particle size, suggestive of an ensemble
size requirement for the reaction.
These results may also have implications for reaction activity
under equilibrium conditions. The reactively formed benzene
in the tilted configuration in the compressed overlayer has
several reaction pathways available to it, including desorption,
decomposition, and transition to the strongly flat bound species.
While a definitive enhancement for benzene evolution was not
observed for the larger particles in the TPD experiments, the
data suggest an activity dependence nonetheless. Generally,
one would expect an enhanced propensity for low-temperature
desorption to likewise enhance the rate of benzene formation
in a reaction equilibrium situation. Single-crystal studies by
Somorjai et al. that point to benzene desorption as the rate-
determining step lend further support to this viewpoint. For
Pd single crystals at atmospheric pressures under batch reaction
conditions, the (111) surface is more reactive than the (100)
surface if corrections are made for site blocking by inactive
carbon.5 In fact, Lambert et al. have shown that larger Pd
particles in conventional supported catalysts are more active.24
In this interpretation then, the lower activities for the smaller
particles is not due to a critical size ensemble effect (except for
the extremely small particles exhibiting the dramatic intensity
drop-off), but rather to the enhanced curvatures and absence of
large, 111 terraces. These characteristics preclude the forma-
tion of the ordered compressed overlayer of reactively formed
benzene weakly bound in the tilted configuration and favor the
more strongly adsorbed flat bonded benzene. Also, the smaller
particles with their higher densities of defects may favor the
low-temperature decomposition reaction, as suggested by the
attenuation of the benzene peak at 370 K associated with defects.
While these low-temperature adsorption, low-pressure experi-
ments reveal interesting particle size effects, it is difficult (and
perhaps dangerous) to attempt to extrapolate these results to
the high-pressure, high-temperature situation in a conclusive
way. Changes in surface coverages of reactants and poisons
can have profound effects on reaction mechanisms, as shown
by single-crystal work (Somorjai et al.5). Ideally, these experi-
ments must be expanded to include high-temperature, high-
pressure reaction kinetics. Nevertheless, these results provide
yet another example of the excellent utility and scope associated
with these unique model catalysts for investigation of hetero-
geneous catalytic processes.
Acknowledgment. We acknowledge with pleasure the
support of this work by the Department of Energy, Office of
Basic Energy Sciences, Division of Chemical Sciences, and the
Robert A. Welch Foundation.
References and Notes
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4. Conclusion
The cyclotrimerization of acetylene to benzene on alumina
thin film supported Pd particles exhibits a particle size effect.