Pumice-supported Cu-Pd catalysts: Influence of copper on the activity and selectivity of palladium in the hydrogenation of phenylacetylene and but-1-ene

Article abstract of DOI:10.1006/jcat.1998.2344

Pumice-supported copper-palladium catalysts prepared from organometallic precursor have been tested in the hydrogenation of phenylacetylene and in the hydrogenation/isomerization of the but-1-ene. The structure and catalytic behaviour of the bimetallic catalysts depended on the different temperatures of reduction. The presence of CuO or Cu metal in an alloyed state with Pd influenced the two reactions. The system containing CuO is the most active and selective towards the formation of the monoalkene in the hydrogenation of the highly unsaturated hydrocarbon. The system containing Cu partially alloyed with Pd is more active and selective towards the isomerization of the but-1-ene. The performance in the two reactions is discussed in terms of the electronic effect prevailing when CuO is present and in terms of the geometric ensemble size variation prevailing with the Cu-Pd system.

Full text of DOI:10.1006/jcat.1998.2344

Journal of Catalysis 182, 456–462 (1999)
Article ID jcat.1998.2344, available online at http://www.idealibrary.com on
Pumice-Supported Cu–Pd Catalysts: Influence of Copper on the
Activity and Selectivity of Palladium in the Hydrogenation
of Phenylacetylene and But-1-ene
�
�
�
,
,1
L. Guczi, Z. Schay, Gy. Stefler, L. F. Liotta,† G. Deganello,† ‡ and A. M. Venezia†
�
Surface Science and Catalysis Laboratory, Institute of Isotope and Surface Chemistry, Chemical Research Center, HAS, P.O. Box 77,
H-1525 Budapest, Hungary; †ICTPN-CNR, via Ugo La Malfa 153, I-90146 Palermo, Italy; and ‡Dipartimento
di Chimica Inorganica, Universit a` di Palermo, viale delle Scienze, I-90128 Palermo, Italy
Received July 30, 1998; revised October 20, 1998; accepted November 4, 1998
ther higher (9) or lower (10) on highly dispersed particles.
Pumice-supported copper–palladium catalysts prepared from A study on the hydrogenation of acetylene on alumina-
organometallic precursor have been tested in the hydrogenation supported Pd catalystsunder industrialconditionswith high
of phenylacetylene and in the hydrogenation/isomerization of the
but-1-ene. The structure and catalytic behaviour of the bimetallic
catalysts depended on the different temperatures of reduction. The
presence of CuO or Cu metal in an alloyed state with Pd influenced
the two reactions. The system containing CuO is the most active
and selective towards the formation of the monoalkene in the hy-
drogenation of the highly unsaturated hydrocarbon. The system
containing Cu partially alloyed with Pd is more active and selective
H2/C2H2 ratio has detected a decrease of specific activity
and an increase in the selectivity to ethylene with increasing
metal dispersion (11). In a recent study on the same reac-
tion under the same conditions, but usingpumice-supported
Pd catalyst, a decrease in activity and a constant selectivity
upon varying the dispersion have been found (12).
Addition of a small amount of a second metal is a way
to increase the selectivity of the system to alkenes (13).
towards the isomerization of the but-1-ene. The performance in the
The consequent changes in activity and selectivity are ex-
two reactions is discussed in terms of the electronic effect prevailing
when CuO is present and in terms of the geometric ensemble size plained in termsofdifferent ensemble requirementsfor par-
variation prevailing with the Cu –Pd system.
allel/consecutive reaction paths and also in terms of the so-
Key Words: selective hydrogenation of alkyne derivatives; Pd– called ligand effect (14). The “ensemble size effect” refers
�c 1999 Academic Press
Cu/pumice catalysts; structure/activity relationship.
to the number of surface atoms which are needed for the
activation of the organic substrate. The ligand effect gener-
ally refers to the electronic influence exercised by the neigh-
bours of “ligands” of a particular surface atoms. The Cu–Pd
system is an interesting combination from a physical stand-
point since the two metals have similar surface free energies
and theyform a continuousseriesofface-centred-cubicsub-
stitutionalsolid solutionsup to a Cu content ofabout 40at%
1
. INTRODUCTION
In the hydrogenation of highly unsaturated hydrocar-
bons, high selectivity to alkenes is required for making lin-
ear polyolefines (1, 2). Palladium is one of the most fre-
quently used metals in such processes since it has the unique
ability to selectively hydrogenate alkynes and alkadienes
to olefines (3, 4). However, upon ageing, a buildup of car-
bonaceous material on the support is responsible for the
decreased selectivity of alkynes to alkene (5–7). Among the
factors controlling the activity and selectivity, formation of
the �-hydride phase occurring preferentially on large par-
ticles is considered to enhance the activity and decrease the
selectivity to the alkene (8). Controversies exist regarding
the size effect of palladium particles in catalytic hydrogena-
tion. In different studies on the hydrogenation of but-1-yne
opposite results were found: the amount of but-1-ene pro-
duced without further hydrogenation was found to be ei-
(
15). Supported Cu–Pd catalysts have been widely investi-
gated in the hydrogenation of different substrates (5, 16–
8). It has been found that copper addition to Pd catalysts
1
markedly improves the catalytic selectivity for partial hy-
drogenation of alkadienes and also alkynes while decreas-
ing the activity (5, 16). Detailed structural studies have also
found a varietyofsurface and bulk structuresofthe bimetal-
lic catalysts as a function of the preparation procedure and
also of the type of interaction with the support (19, 20).
Recent investigations on palladium catalysts supported
on pumice have shown a beneficial effect of this type of sup-
port in the selective hydrogenation of 1,3-cyclooctadiene
(
21) and phenylacetylene (22). An enhanced activity and
selectivity of the pumice-supported palladium was ob-
served, even at metal dispersion above 30% when usually,
1
To whom correspondence should be addressed.
456
0021-9517/99 $30.00
Copyright �c 1999 by Academic Press
All rights of reproduction in any form reserved.

PUMICE-SUPPORTED Cu–Pd CATALYSTS
457
with other Pd catalysts, a decrease of activity occurs. 2.2. Structural Analyses
Moreover, this type of catalyst did not show any ageing
The XRD measurements performed with a Philips X-ray
diffractometer using Nickel-filtered CuK� radiation allow-
ed us to obtain the crystallite sizes as well as the lattice para-
meters of the metal phase (27). X-ray photoelectron spec-
troscopy measurements performed with a Kratos ES 300
ESCA instrument working in fixed analyser transmission
phenomena during the hydrogenation of acetylene in ethy-
lene feedstocks (12). The peculiar behaviour of the pumice-
supported catalysts was attributed to an increased electron
density on the metal (23, 24). A charge donation from the
support to the palladium was ascribed to the presence of the
alkali ions into the structure of the pumice, as indicated by
X-rayphotoelectron spectroscopy(XPS, 25). Since the elec-
tron enrichment of the metal is larger on smaller particles,
the decrease in activity of the pumice-supported catalysts,
which occurs at metal dispersion above 30% , is attributed
to an insufficient activation of the electron rich substrate in-
(
FAT) mode at pass energy of 40 eV gave information on
the chemical state of the Cu–Pd system and also on segrega-
tion processes. The spectra were generated by AlK� X rays
(
h� = 1486.6 eV, 150 W). Details on the experimental setup
and on the analyses of the data have been described (27).
stead of being assigned to an increased strength of the bond 2.3. Catalytic Reactions
between the substrate and the metal as indeed claimed for
other supported Pd catalysts (26).
Hydrogenation of phenylacetylene was performed in a
mechanically stirred tank reactor as reported previously
22). The reactor was connected to an inert gas-vacuum
line and to a H2 line. A constant atmospheric hydrogen
pressure was used during the reaction. A side arm closed by
a silicon septum allowed the injection of reagents and with-
drawal of samples for GC analyses. Constant temperature
On the basis of the above considerations new pumice-
supported Cu–Pd catalysts were prepared with the aim of
takingadvantage ofthe combination ofthe propertiesofthe
support with those of a bimetallic system (27). The charac-
terisation study, described previously, has shown through
XPS and X-ray diffraction (XRD) techniques that the
preparation methods involving different reduction temper-
aturesproduced significant changesin the surface and struc-
ture of the systems (27). These catalysts have been tested
in the selective liquid phase hydrogenation of phenylacety-
lene considered a “structure insensitive” reaction (22) and
in the gas phase isomerization–hydrogenation of but-1-ene,
where the relative extent of the two processes is “struc-
ture sensitive” (28). The comparison with the catalytic be-
haviour of corresponding monometallic Pd catalysts and
the relationship between activity/selectivity and structural
properties are described in the present study.
(
(
298 � 0.1 K) was ensured by a thermostat. The absence
of any influence of diffusion effects under operating con-
ditions was adequately checked (22). The initial reagent
concentration of the phenylacetylene in THF was always
�
1
2
.5 � 10 M, and the amount of catalyst was always set so
that the initial ratio between the reagent moles and the total
Pd atoms would be equal to 1000. The reaction was start-
ed without preconditioning and also with 1 h precondi-
2
tioning of the system THF—catalyst with hydrogen and/or
phenylacetylene. With all the bimetallic catalysts an induc-
tion period of about 10 min was observed without precondi-
tioning and with phenylacetylene preconditioning. On the
contrary after the H2 preconditioning no induction period
was detected. Kinetic analyses were performed with a Dani
2
. EXPERIMENTAL
3
800HRPTV GC equipped with a Supelco capillarycolumn
SPB1701 and with a DANI68/10 FID detector. This system
was linked to a Shimadzu C-R1B Chromatopac integrator.
The catalytic hydrogenation–isomerization of but-1-ene
was performed in an all glass made grease and mercury free
2
.1. Catalyst Preparation
2
The pumice (5 m /g) supported catalysts were prepared
from organometallic precursors. The adopted procedure
is aimed at reaching a higher copper dispersion (29). The
monometallic palladium catalysts were prepared from pen-
tane solution of Pd(C3H5)2 according to a previous method
batch circulation reactor at 298 K using 20 Torr (1 Torr =
� 2
1
33.3 Nm ) of H2 and 10 Torr of but-1-ene. The analysis
was performed by a gas chromatograph Varian Aerograph
series 1400 equipped with a Porapac-Q column which was
operated at 313 K. The absence of any influence of diffusion
effects under operating conditions was adequately checked.
Prior to each catalytic test the catalysts were reduced in
situ under hydrogen flow for 1 h at the desired temperature
(
30); then they were subsequently reduced in a stream of
H2 between 258 and 298 K. Then the obtained Pd cata-
lysts were contacted with the green-brown solution of (E)-
2
-ethyl-1-hexenyl-copper (I) (31) of known concentration
in (C2H5)O under nitrogen at a temperature of 258 K un-
der stirring for 3 h. The anchored copper was reduced un-
der hydrogen between 258 K and room temperature (27).
The chemical composition of the catalysts was determined
by atomic absorption spectroscopy. Henceforth, the figures
preceding the metal in the notation of the catalysts indicate
the corresponding weight percentage.
(
298 or 623 K).
3
. RESULTS AND DISCUSSION
In Table 1 the atomic ratios Cu/Pd from chemical anal-
ysis (CA) and XPS measurements, the Pd and Cu particle

4
58
GUCZI ET AL.
TABLE 1
Cu/Pd Atomic Ratios from Chemical Analysis (CA) and XPS Analysis, Particle Sizes, d, Lattice
Parameters, a, as Determined by XRD and Estimated Alloy Composition (27)
Alloy composition
Catalyst
Cu/PdCA
Cu/PdXPS
dPd (Å)
aPd (� 0.005 Å)
(Cu content at% )
1
0
0
0
0
0
.5Pd
(298 K)
623 K)
(298 K)
623 K)
.025Cu–1.5Pd (298 K)
623 K)
.05Cu–1.5Pd (298 K)
623 K)
.025Cu–0.6Pd (298 K)
623 K)
.05Cu–0.6Pd (298 K)
623 K)
110
200
3.899
3.885
(
.6Pd
63
200
3.883
3.881
(
0.03
0.05
0.07
0.14
nda
80
316
3.901
3.886
(
0.2
0.1
nda
78
214
3.901
3.871
(
6
6
83
185
3.895
3.876
(
0.3
0.2
76
200
3.883
3.866
(
10
Note. Bulk values: aPd = 3.890 Å; aCu = 3.615 Å.
a
Not determined due to the poor quality of the Cu 2p signal.
sizes, and the lattice parameters for the monometallic and CuO with only a small amount of metallic Cu. H2 treatment
bimetallic Cu–Pd catalysts are listed (27). Values relative at 623 K achieves the complete reduction of copper. As far
to the samples reduced at 298 and 623 K are reported. The as Pd is concerned, it is in a metallic state; however, it is in-
metal dispersion in the catalysts obtained from the particle teresting to notice that in the samples containing CuO, the
dimensions range between 3 and 18% . Some H2 adsorption Pd 3d5/2 binding energy does not reflect the interaction with
experiment were also performed; however; the low con- the support (24, 25), and the proper value for the Pd cata-
centration of the metals did not yield reliable results. As lysts supported on pumice was found only after H2 treat-
already mentioned, the addition of copper does not pro- ment at 623 K. No chemical shifts are observed between
duce significant change in the Pd crystallite sizes, contrary the monometallic and bimetallic catalysts.
to what is observed on silica-supported catalysts (18). All
the bimetallic catalysts reduced at 298 K have a lattice pa- 3.1. Hydrogenation of Phenylacetylene
rameter typical of the palladium metal lattice. After reduc-
During the activity measurements performed in excess
tion at higher temperature the parameters changed signifi-
of hydrogen, the concentration of the styrene produced
cantly with increasing copper content, and copper particles
changed linearlywith time. The rate calculated wasconstant
were also visible in the X-ray pattern, although the quality
and denoted by k. The turnover frequencies (TOF) calcu-
lated from such a rate constant normalised by the number of
of the Cu (111) reflection signal did not allow the attain-
ment of the corresponding lattice parameter. It is worth
noting that the surface atomic ratio of the bimetallic cata-
lysts, as derived from the XPS measurements, are larger
than the overall values determined by chemical analysis.
TABLE 2
Pd 3d5/2 and Cu 2p3/2 Binding Energies after Different
Moreover, the increased reduction temperature decreases
the Cu/Pd atomic surface ratio which has already been ex-
Thermal Treatments
plained with an inward diffusion of copper concomitant
Catalyst
Treatment
Pd 3d5/2 (eV)
Cu 2p3/2 (eV)
with the formation of alloy (27). The last column of Table 1
reports the estimated alloy compositions obtained from the
variation of the lattice parameter computed from the Pd-
related diffraction peaks, following the results of Burch and
Buss (32).
The XPS binding energies of the Pd 3d5/2 and Cu 2p3/2
of selected bimetallic and monometallic samples are pre-
sented in Table 2. As already reported (27), the samples
reduced at 298 K contain most of the copper in the form of
0
0
1
0
.05Cu–1.5Pd
H2 (298 K)
H2 (623 K)
335.7
334.8
931.7, 933.9
931.9
.05Cu–0.6Pd
.5Pd
H2 (298 K)
H2 (623 K)
335.7
334.8
931.7, 933.4
931.7
H2 (298 K)
H2 (623 K)
334.8
334.8
.5Cu
H2 (298 K)
H2 (623 K)
933.8, 932.4
932.4

PUMICE-SUPPORTED Cu–Pd CATALYSTS
459
TABLE 3
Rate (k), Turnover Frequency (TOF), and Selectivity to Styrene
TABLE 4
Rate (k), Turnover Frequency (TOF), and Selectivity to Styrene
(Ssty) Calculated at Total Conversion of 95%
(Ssty) Calculated at Total Conversion of 95%
Catalyst
298 K)
k
TOF
Ssty
(% )
Catalyst
(623 K)
k
TOF
Ssty
(% )
�
1
� 1
� 1
� 1 � 1
� 1
(
(molstyrene s
g
)
(s
)
(molstyrene s
g
)
(s
)
Pd
Pd
2
3
3
2
3
3
� 3
0
0
0
1
0
0
.6Pd
1.01 � 10�
6.0
80
89
88
81
89
90
0.6Pd
3.57 � 10
2.63 � 10
3.76 � 10
4.32 � 10
1.88 � 10
1.50 � 10
6.4
81
92
83
80
89
85
�
� 3
� 4
� 3
� 3
� 3
.025Cu–0.6Pd
.05Cu–0.6Pd
.5Pd
.025Cu–1.5Pd
.05Cu–1.5Pd
8.93 � 10
7.3
4.7
7.7
7.5
7.3
0.025Cu–0.6Pd
0.05Cu–0.6Pd
1.5Pd
0.025–Cu1.5Pd
0.05Cu–1.5Pd
4.7
0.7
7.7
5.0
3.2
�
6.20 � 10
�
1.22 � 10
�
9.87 � 10
9.60 � 10�
palladium sites and the selectivity to styrene, calculated as supported palladium catalysts (22) detected formation of
ratio of styrene concentration over styrene + ethylbenzene polymeric species at the initial stage of the reaction, produc-
concentration at a total conversion of 95% , are reported in ing a flattening effect on the catalytic activities and there-
Table 3 for the catalyst reduced at 298 K.
fore making the reaction “structure insensitive” (17). As il-
Analogous data are reported in Table 4 for the series of lustrated in Table 1, the XPS Pd 3d binding energy shifts of
catalysts reduced at 623 K. Since the calculation of the pal- the bimetallicCu–Pd catalystsreduced at 298K with respect
ladium sites was based on the dispersion obtained from the to the monometallic catalyst indicate that CuO decreases
X-ray diffraction data, the derived TOF values may not take the electron density of palladium. However, according to
into account the real number of the active surface sites spe- the previous finding on the“structure insensitivity” of the
cially in bimetallic systems. For this reason both quantities, considered reaction, the variation of the electron density
k and TOF, are considered. In Figs. 1 and 2 the selectivity of the palladium does not affect the hydrogenation of the
to styrene versus the conversion percentage are plotted for alkyne (22). Consequently, the invariance in the rate as well
the samples reduced at 298 K. As appears from Tables 3 as in the TOF observed in the presence of CuO suggests that
and 4, it is remarkable to note that the presence of copper the number of the active sites is not modified by CuO that
as CuO does not modify the catalytic activity, except for is, therefore, located on the support rather than the pal-
the 0.05Cu–0.6Pd sample for which a decrease is observed, ladium particles. The increase of the styrene selectivity at
whereas the selectivity towards styrene increases. Once the very large phenylacetylene conversion may indeed be due
copper is totally reduced at 623 K, the rate and the turnover to a structure sensitivity of the subsequent hydrogenation
frequencies of the bimetallic decrease with increasing cop- of styrene to ethylbenzene, inhibited by more electron de-
per content while the selectivity trend remains essentially ficient palladium particles (21–25). After the H2 treatment
the same as observed for the samples reduced at 298 K. of the same bimetallic catalysts at 623 K, formation of the
Previous catalytic study of the same reaction on pumice- metallic copper and alloyed Cu–Pd particles determines
FIG. 1. Styrene selectivity versus phenylacetylene conversion on the copper-promoted 0.6Pd/pumice-supported samples prepared by organo-
metallic route and reduced at 298 K.

4
60
GUCZI ET AL.
and formation of the products were extrapolated to zero
time, and the initial rates were calculated. The turnover fre-
quencies of the hydrogenation and isomerization, TOFhyd
and TOFisom, refer to the initial state. The corresponding
selectivities listed in Table 4 have been calculated as the
following ratio:
[TOFisom/(TOFhyd + TOFisom)] � 100.
In Figs. 3a and 3b the plots of selectivity of the reaction
versus time measured on the 0.025Cu–0.6Pd/pumice cata-
lyst reduced at 298 and 623 K, respectively, are reported.
As previously observed on Pd/�-Al2O3, the catalyst exhibits
high isomerization activity (28), producing more trans-but-
FIG. 2. Styrene selectivity versus phenylacetylene conversion of
the copper-promoted 1.5Pd/pumice-supported samples prepared by 2-ene than cis-but-2-ene (16). As reported in Table 5, a
organometallic route and reduced at 298 K.
decrease of the hydrogenation activity over the bimetallic
catalysts reduced at 298 K and an increase of the selectivity
to but-2-eneswith respect to the correspondingmonometal-
the decrease in conversion rate likely due to the dilution
lic Pd catalyst are observed. Reduction at 623 K restores
of the palladium active sites by copper. The larger selectiv-
the catalyst hydrogenation activity with a slight decrease
ity with respect to monometallic samples can be explained
in but-2-ene selectivity. The different catalytic behaviours
with the variation of the ensemble size rather than elec-
exhibited by the various samples may be rationalised in
tronic effects. As already suggested (19), and also indicated
terms of the catalyst structure. The catalysts reduced at
here by the absence of chemical shifts between monometal-
low temperature contain CuO, which affects the Pd 3d5/2
lic and bimetallic catalysts, the addition of metallic copper
binding energy, shifting it by 0.9 eV with respect to palla-
does not modify the electronic properties of palladium. In-
dium on pumice. This corresponds to a substantial decrease
deed, the decrease in the ensemble size needed for multiply
in the palladium electron density (24, 27). The diminished
adsorbed alkylidene species and formation of oligomers in-
hydrogenation rate with respect to the monometallic cata-
hibits the further hydrogenation of styrene to ethylbenzene
lyst reflects the structure sensitivity of this type of reac-
(
5). According to these results, the preparation procedure
tion in accord with a recent study (28). The presence of
CuO produced the same electronic effect attributed to a de-
crease in the Pd particle size. In addition, a decrease in the
amount of hydrogen dissociated on Pd results in lower hy-
drogenation activity. The combination ofthe two effectswill
favour the isomerization for which no external hydrogen
is needed, with a consequent increase of the selectivity to
from organometallic precursor, in which CuO is produced
at low temperature reduction, yields catalysts as active as
the corresponding monometallic samples, but more selec-
tive towards the production of styrene.
3
.2. Hydrogenation of But-1-ene
The catalytic results of the hydrogenation of but-1-ene but-2-ene.
are summarised in Table 5. The rate of but-1-ene conversion
The complete reduction of CuO to metallic copper
achieved at 623 K determines a change of the electronic
properties of the palladium as revealed by the shift of the
Pd 3d binding energy towards the value typical of palladium
on pumice. The increased electron density on the palladium
atoms (21, 26), along with a possible enhanced hydrogen
availability, will favour the hydrogenation as compared to
the isomerization, explainingthe decreased but-2-ene selec-
tivity with respect to the same catalysts reduced at 298 K.
Ensemble size variation resulting in a decrease of contigu-
ous sites may be the reason for the increased isomerization
selectivity measured on the bimetallic catalysts reduced at
high temperature with respect to the monometallic sam-
ple (28). The competition between the hydrogenation and
isomerization may be explained in terms of the different
number of adjacent surface atoms of the active metal re-
quired for forming the respective chemisorption complexes
(33). The reaction involving the largest ensemble of these
TABLE 5
Turnover Frequencies for But-1-ene Hydrogenation and Isomer-
ization, TOFhyd and TOFisom, Respectively, and Selectivity, S%, to
But-2-ene for the Catalysts Reduced at 298 K
TOFhyd
TOFisom
S%
But-2-ene
�
1
� 1
Catalyst
(s
)
(s
)
0
0
0
0
1
0
0
0
.6Pd
0.25
0.06
0.31
0.02
0.16
0.08
0.25
0.02
1.69
1.44
5.63
0.37
1.10
1.22
1.86
0.26
87
96
95
95
87
94
88
93
.025Cu–0.6Pd
.025Cu–0.6Pd (623 K)
.05Cu–0.6Pd
.5Pd
.025Cu–1.5Pd
.025Cu–1.5Pd (623 K)
.05Cu–1.5Pd
Note. The values for two catalysts reduced at 623 K are also given.

PUMICE-SUPPORTED Cu–Pd CATALYSTS
461
FIG. 3. Selectivity plot for the but-1-ene hydrogenation and isomerization over the 0.025Cu–0.6Pd reduced (a) at 298 K and (b) at 623 K.
atoms will be the most sensitive to alloying with a second
metal unable to form such chemisorption bonds. While
the hydrogenation of an olefin generally requires adsorp-
4. CONCLUSION
The chemical state of copper added to palladium cata-
tion on multiple centres according to the Horiuti–Polanyi lysts supported on pumice has an important role in control-
HP) mechanism (28), the isomerization of an olefin may ling the catalytic behaviour of palladium in the liquid phase
(
need only one metal centre according to the intramolec- hydrogenation of phenylacetylene and in the gas phase hy-
ular shift mechanism (IM) (28). This would explain the drogenation of but-1-ene.
increased but-2-ene selectivity of the bimetallic catalysts
For the first reaction, the presence of CuO leaves the
reduced at 623 K as compared to the monometallic one. conversion rate unchanged but increases the selectivity to
The so-called “ligand effect” due to the electronic interac- styrene. On the other hand, the presence of metallic Cu
tion of the Pd with the Cu following alloying is not consid- decreases the conversion rate while still increasing the se-
ered important in the comparison of the monometallic with lectivity. This behaviour is explained by the variation of
the bimetallic catalysts, since as shown by XPS, the binding the electronic properties of palladium induced by the CuO,
energy of the Pd 3d5/2 is not modified by the addition of which would affect the hydrogenation of styrene to ethyl-
copper.
benzene, but not the hydrogenation of phenylacetylene to

4
62
GUCZI ET AL.
styrene. In the case of Cu metal, dilution of the palla- 10. Boitiaux, J. P., Cosyns, J., and Vasudevan, S., Appl. Catal. 6, 41 (1983).
1
1
1
1. Gigola, C. E., Aduriz, H. R., and Bodnariuk, P., Appl. Catal. 27, 133
dium surface atoms is claimed to be responsible for the
decreased activity and the ensemble size variation deter-
mines the increased selectivity to styrene with respect to
the monometallic catalyst.
(
1986).
2. Duca, D., Frusteri, F., Parmaliana, A., and Deganello, G., Appl. Catal.
A 146, 269 (1996).
3. Guczi, L., J. Mol. Catal. 25, 13 (1984).
During the hydrogenation and isomerization of but-1- 14. Sachtler, W. M. H., J. Mol. Catal. 25, 1 (1984).
1
1
1
1
1
5. Moss, R. L., Pope, D., Davis, B. J., and Edwards, D. H., J. Catal. 58,
ene, addition of copper as oxide produces a consistent low-
ering of the hydrogenation activity and an increase in iso-
merization selectivity, whereas metallic copper, free and in
an alloyed state with Pd, restores the hydrogenation ac-
tivity and still maintains the higher selectivity with respect
to a monometallic catalyst. Electronic effects determined
by the CuO and ensemble size variations are considered
responsible for the two behaviours.
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ACKNOWLEDGMENTS
The authors acknowledge CNR (Consiglio Nazionale delle Ricerche)
and MTA (Hungarian Academy of Science) for the bilateral program
grant. The CNR is also acknowledged for the funds provided by the
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