Effect of catalyst pre-reduction temperature on the reaction of 1,2-dichloroethane and H2 catalyzed by SiO2-supported PtCu bimetallics

Article abstract of DOI:10.1016/j.apcata.2011.12.002

A combination of reaction kinetics experiments, temperature-programmed investigations and FTIR CO adsorption studies were used to understand the effect of catalyst pretreatment on the reaction of 1,2-dichloroethane and H ₂ catalyzed by SiO₂ supported PtCu bimetallics prepared from metal chloride precursors. Higher initial and steady-state selectivities towards ethylene were obtained for catalysts with a Cu to Pt atomic ratio of 1, 2, and 3 after pre-reduction at 220 °C than after pre-reduction at 500 °C. For catalysts with a Cu to Pt atomic ratio of 4 and 5, the initial ethylene selectivity was higher after the 220 °C pre-reduction, but the steady-state ethylene selectivities were essentially the same. Based on the catalyst characterization results, the low temperature pre-reduction yields supported particles with Pt-rich cores covered with Cu-rich layers that are still well-chlorinated, whereas the high temperature pre-reduction yields particles both more metallic and with surfaces richer in Pt. The factors governing the evolution of the catalysts during the two different pretreatments are discussed as well as the impact catalyst pretreatment has on the macroscopic performance behavior.

Full text of DOI:10.1016/j.apcata.2011.12.002

Applied Catalysis A: General 415–416 (2012) 59–69
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Effect of catalyst pre-reduction temperature on the reaction of
1
,2-dichloroethane and H catalyzed by SiO -supported PtCu bimetallics
2 2
∗
Lindney N. Akonwie, D.V. Kazachkin, D.R. Luebke, Julie L. d’Itri
Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA 15261, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 July 2011
Received in revised form 1 December 2011
Accepted 3 December 2011
Available online 9 December 2011
A combination of reaction kinetics experiments, temperature-programmed investigations and FTIR CO
adsorption studies were used to understand the effect of catalyst pretreatment on the reaction of 1,2-
dichloroethane and H2 catalyzed by SiO2 supported PtCu bimetallics prepared from metal chloride
precursors. Higher initial and steady-state selectivities towards ethylene were obtained for catalysts
◦
◦
with a Cu to Pt atomic ratio of 1, 2, and 3 after pre-reduction at 220 C than after pre-reduction at 500 C.
For catalysts with a Cu to Pt atomic ratio of 4 and 5, the initial ethylene selectivity was higher after
Keywords:
Bimetallics
PtCu bimetallics
Adsorbate-induced surface enrichment
Dual site mechanism
◦
the 220 C pre-reduction, but the steady-state ethylene selectivities were essentially the same. Based on
the catalyst characterization results, the low temperature pre-reduction yields supported particles with
Pt-rich cores covered with Cu-rich layers that are still well-chlorinated, whereas the high temperature
pre-reduction yields particles both more metallic and with surfaces richer in Pt. The factors governing
the evolution of the catalysts during the two different pretreatments are discussed as well as the impact
catalyst pretreatment has on the macroscopic performance behavior.
1
,2-Dichloroethane dechlorination
Catalyst pretreatment
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
Unlike on Pt, the ethyl species on Cu is unstable and readily rear-
ranges to ethylene. And again unlike on Pt, the interaction strength
of ethylene with Cu is very weak and thus the ethylene desorbs
without undergoing further reaction. In short, by forcing the 1,2-
dichloroethane to react on Cu, the strongly adsorbed alkyl species
on Pt that ultimately yield ethane are prevented from forming.
This investigation centers on the effect of catalyst pretreat-
ment on the reaction of 1,2-dichloroethane and H2 catalyzed by
Catalysis by bimetallics continues to be an active field of
research. Combinations of a Group VIII metal, such as Pt, and a
Group IB metal, such as Cu, have been extensively studied using
reactions such as reforming, hydrogenation, and hydrogenolysis
[
1–4]. The role of the Group IB metal in altering the catalytic per-
formance has been attributed to an electronic effect that modifies
the Group VIII metal reaction sites [5–7], to an alteration in the
geometric size of the Group VIII metal reaction sites [4–7], and to
participation as an active site for one or more elementary reaction
steps of the overall reaction [6,8,9].
Pt–Cu/SiO . Of particular interest is the relationship between the
2
pre-reduction temperature and the genesis of catalysts with vary-
ing Cu to Pt atomic ratio. Temperature-programmed desorption
and reduction experiments were conducted to puzzle out the evo-
lution of the catalysts from impregnation to reaction start, and FTIR
spectroscopy of adsorbed CO was used to probe the surface compo-
sition of the metal particles. The impact catalyst pretreatment on
reaction performance was ascertained through chemical reaction
kinetics investigations.
For the reaction of 1,2-dichloroethane and H catalyzed by PtCu
2
bimetallics, it has been suggested that Cu participants as an active
site for the formation of ethylene, as the selectivity towards ethy-
lene increases at the expense of ethane when the concentration of
Cu atoms is increased [10–15]. In mechanistic terms, it is proposed
that at sufficiently high surface concentrations of Cu, the ensembles
of Pt are smaller than the size required to catalyze the dissociative
adsorption of the 1,2-dichloroethane, but still large enough to dis-
2. Experimental
sociate H and provide by spillover the H atoms that react with Cl to
2
2.1. Catalysts preparation
regenerate the Cu sites [10]. The dissociative adsorption of the 1,2-
dichloroethane then occurs on the large surface ensembles of Cu
atoms, forming an ethyl surface species and two surface Cl species.
2
Silica powder (Aldrich, 99+%, 60–100 mesh, 300 m /g, 150 A)
˚
was used as the catalyst support. The silica was calcined in air at
00 C for 12 h prior to the impregnation of the metal precursors
◦
5
using a 0.1 N HCl aqueous solution of H PtCl ·6H O (Alfa, 99.9%)
2
6
2
∗ Corresponding author. Tel.: +1 412 624 9634; fax: +1 412 624 9639.
E-mail address: jditri@pitt.edu (J.L. d’Itri).
and/or a 0.1 N HCl aqueous solution CuCl ·2H O (MCB Manufac-
2
2
turing Chemist, 99.5%). The concentrations of the metals in the
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.12.002

6
0
L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
impregnating solutions were adjusted to obtain the desired loading
of each metal. The Pt content was same (2.7 wt%) for all catalysts
that contained it, and the amount of Cu was varied from 0 up to
a Pt to Cu atomic ratio of 1:5. As well, a 0.5 wt% Cu catalyst was
prepared.
catalyst was loaded into reactor and pretreated as specified before
reaction.
2.3. Temperature programmed desorption (TPD) and reduction
(TPR) investigations
The SiO2 powder was added to the solution containing the
dissolved metal precursors at ambient conditions while stirring
continuously, and then the mixture was stirred for 12 h. The
impregnated SiO2 was isolated from the solution by filtration and
dried at ambient conditions for 24 h. Subsequently, it was dried
An FTIR cell and instrument (described below) were used to
conduct temperature programmed studies with the as prepared
catalysts in order to puzzle out the surface chemistry that occurred
during pretreatment. A wafer of sample was prepared as described
above and then placed in the holder of the cell, which then was
◦
for 12 h at 100 C under a static vacuum (∼25 Torr). The resulting
−
6
materials were stored until use in vessels maintained at the same
evacuated at 10 Torr for 2 h at room temperature before being
sealed off. The wafer temperature was then increased from room
partial pressure of H O to eliminate variations in the properties and
2
◦
◦
performance of the catalysts attributable to aging phenomena [11].
temperature to 130 C at 5 C/min as FTIR spectra of the gas phase
in the cell were collected. The wafer temperature was maintained
◦
2.2. Chemical kinetics study
at 130 C for 1 h before the cell was opened to vacuum and evac-
uated for 0.5 h. Evacuation of the cell was continued as the wafer
was cooled to room temperature in order to begin the temperature
programmed reduction experiment.
Two different series of temperature-programmed reduction
studies were conducted: one to determine type and stability metal
precursor species formed during impregnation and subsequent
treatments and another to determine the relative fraction of Cl and
other types of ligands that remained after the low temperature pre-
treatment procedure used for the kinetics investigations. For the
The kinetics experiments were conducted in a stainless-steel
flow reaction system operated under conditions that made the dif-
ferential reactor assumption valid. The reactor consisted of a quartz
tube (10 mm i.d.) in which the catalyst was supported on a quartz
frit. The pretreatment gases and gaseous reactants were metered
by mass flow controllers (5850E, Brooks) and were mixed prior
entering the reactor. The 1,2-dichloroethane was metered into the
system by flowing He through a saturator containing the liquid
◦
reactant at 0(± 1) C. The saturator temperature was maintained by a
first TPR series, 600 Torr of H (99.99%, PennOxygen) was admitted
2
recirculating cooling system (RTE-111, Neslab). A blank experiment
showed zero conversion for the system without catalyst.
The amount of the catalyst used in an experiment ranged
between 50 and 300 mg so that the steady-state conversion for all
the bimetallic catalysts was comparable (between 1.5 and 3%). The
conversions were sufficiently low and limited to a narrow enough
range that the reported differences in the product selectivity dis-
tributions cannot be attributed to differences in conversion levels
at the points of comparison.
to the evacuated cell containing wafer that underwent the TPD and
subsequent treatment. The wafer was heated from room temper-
◦
◦
ature to 500 C at a rate of 5 C/min while FTIR spectra of the gas
phase in the cell were collected.
For the other series of TPR experiments, a different treatment
was used on the wafer following the TPD and subsequent treat-
◦
ment at 130 C that lasted for 1.5 h. Instead of cooling to room
temperature while continuing to evacuate, a flow of 10% H /Ar was
2
◦
◦
sent through the cell as the wafer was heated at 5 C/min to 220 C
and then maintained at 220 C for 2 h. Next, the temperature was
decreased to 200 C, after which the H /Ar flow was stopped and
◦
◦
The catalyst temperature was controlled to ± 1 C using a
◦
temperature controller (CN2011, Omega) connected to K-type ther-
mocouple (Omega) placed in a quartz pocket that was in direct
contact with the catalyst bed. The reaction products were analyzed
by on-line GC (HP 5890). The GC was equipped with 3 m 60/80
Porapak Q packed column (Supelco) and flame ionization detec-
tor with a detection limit of <0.2 ppm for all chlorocarbons and
hydrocarbons involved in this study.
2
the cell was evacuated for 0.5 h. The cell was filled with 600 Torr
of H2 and the TPR experiment was begun. The spectra of the gas
phase in the cell were collected as the wafer was heated from 200
◦
◦
to 500 C at 5 C/min.
2.4. CO adsorption FTIR spectroscopy characterization
After a catalyst sample was loaded into reactor, it was pretreated
prior to the start of the reaction. To begin, it was heated in He
The infrared spectra were recorded using a Research Series II
◦
(
5
30 mL/min, PraxAir, 99.999%) from room temperature to 130 C at
C/min and then held in the flowing He at 130 C for 1.5 h. Next, the
FTIR spectrometer (Mattson) equipped with a liquid N cooled MCT
2
◦
◦
detector (Judson Technologies). The spectra were measured in the
−
1
−1
flowing gas was switched from the He to 10% H /Ar (30 mL/min).
range of 400 to 4000 cm with resolution of 4 cm . For each spec-
trum 64 scans were collected and averaged. Sample preparation
consisted of pressing a powdered catalyst at 800 atm for 1 min into
2
For the samples identified as low temperature pretreated catalysts,
◦
◦
the temperature was increased at 5 C/min to 220 C, then held at
◦
2
2
20 C for 2 h in the 10% H /Ar flow before cooling to the reac-
a wafer with a density of 20–30 mg/cm , and the collected spec-
2
◦
tion temperature of 200 C where the gas flow was switch to He
for 0.5 h to purge the system prior to starting the reaction. For the
samples identified as high temperature pretreated catalysts, the
temperature was increased from 130 to 500 C at 5 C/min in the
0% H /Ar flow then held at 500 C for 2 h in the 10% H /Ar flow
before cooling to the reaction temperature of 200 C where the gas
flow was switch to He for 0.5 h to purge the system prior to starting
the reaction. Comparable pretreatment routines were used in the
volumetric sorption studies and the FTIR investigations.
tra were normalized on a per unit wafer density basis. The wafer
was balanced in a quartz sample holder in a 0.5 L vacuum cell with
NaCl windows and a 12 cm optical path length. The cell operated in
◦
◦
◦
the temperature range from 20 to 500 C, and it was equipped with
◦
1
grease-free glass stopcocks (Ace Glass) connected to gas inlet/outlet
ports and a vacuum port with a turbo-molecular pump (Leibold,
2000 L/sec).
2
2
◦
The wafer was pretreated in the cell first by evacuation at
−
6
◦
10 Torr while the temperature was increased at 5 C/min from
◦
◦
◦
The reaction was conducted at 200 C and atmospheric pressure.
room temperature to 130 C and then holding at 130 C for 1.5 h.
The reaction mixture consisted of 7000 ppm of 1,2-dichloroethane
Afterwards a 30 mL/min flow of 10% H /Ar (Airgas, 99.99%) was
2
(
Fisher Scientific, 99.8 + %), 36,000 ppm H , and the balance He.
sent through the cell, and the temperature was further increased
2
◦ ◦
at 5 C/min from 130 to 220 C. The temperature was maintained at
The total flow-rate was 41 mL/min. The reaction was run for
approximately 40 h, a time sufficient to achieve steady-state
performance. For every kinetics experiment a new sample of
◦
◦
◦
220 C for 2 h before cooling at 5 C/min to 200 C. Then, the flow of
−
6
10% H /Ar was stopped and the cell was evacuated at 10 Torr for
2

L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
61
Table 1
volume of chemisorbed CO was below the detection limit of the
instrument, which is not surprising given that the low temperature
pretreatment only partially reduced the metal precursor species to
lesser chlorinated species. However, the CO adsorption FTIR results
for the bimetallic catalysts pretreated at low temperature do indi-
cate the presence of surface Pt atoms metallic in character, albeit
far fewer in concentration as when the bimetallic catalysts were
reduced at high temperature.
Initial and steady-state selectivity distributions for the reaction of 1,2-
dichloroethane (7000 ppm) and H2 (35,000 ppm) in He at 200 C (total flow rate
◦
◦
4
1 mL/min) catalyzed by SiO2-support PtCu bimetallics pre-reduced at 220 C and
◦
at 500 C.
◦
◦
C
220
C
500
C2H4
0/0
54/55
89/77
98/95
99/96
100/95
100/0
C2H6
C2H5Cl
C2H4
C2H6
C2H5Cl
Pt
93/89
46/45
11/23
2/5
1/4
0/5
7/11
0/0
0/0
0/0
0/0
0/0
0/0
0/0
0/3
90/92
95/96
94/58
80/15
85/2
10/8
5/1
0/0
0/0
0/0
0/0
0/0
PtCu1
PtCu2
PtCu3
PtCu4
PtCu5
Cu
6/42
20/85
15/98
38/96
100/0
3. Results
62/4
0/0
0/0
3.1. Chemical kinetics investigations
The initial and steady-state selectivity behavior for the catalysts
0
2
.5 h. The evacuation was continued as the wafer was cooled from
00 C to room temperature during a time period of 0.5 h.
After pretreating the wafer, a background spectrum was
in the reaction of H2 and 1,2-DCE are summarized in Table 1, both
after low temperature and high temperature pretreatment, and
the time on stream behavior is provided in Figs. 1–7. Of primary
interest is the behavior of the bimetallic catalysts. However, the
results for the Pt and the Cu catalysts have been included as points
of reference.
The Pt catalyst was the most active catalyst, converting approxi-
mately 4.75 ␮mol/s/gcat of reactant regardless of the pre-reduction
treatment, and selectively formed ethane as the major product with
monochloroethane (11% or less) as the only other product (Fig. 1).
The Cu catalyst was the least active catalyst, roughly a factor of 100
less active than the Pt catalyst, and it was 100% selective towards
ethylene at all time on stream after both the low temperature and
the high temperature pretreatments (Fig. 2). For both monometal-
lic catalysts the pre-reduction temperature had little effect on the
product selectivity.
◦
recorded. The 10 Torr of CO was admitted to the cell, and after
5 min a spectrum was recorded. The cell was evacuated for 15 min,
1
and then another spectrum was recorded. The wafer was then sub-
jected to the high temperature pretreatment. Specifically, a flow
of the 10% H /Ar (30 mL/min) was admitted to the cell and the
wafer was heated at 5 C/min from room temperature to 500 C, and
then held at 500 C for 2 h before the temperature was decreased
to 200 C at 5 C/min. The 10% H /Ar flow was then stopped, and
the cell was evacuated for 0.5 h. Afterwards, the evacuation was
continued as the wafer was cooled from 200 C to room tempera-
2
◦
◦
◦
◦
◦
2
◦
ture during a time period of 0.5 h. Finally, the CO adsorption FTIR
experiments were repeated with this high temperature pretreated
catalyst.
For the bimetallic catalysts the steady state activities were all in
the range of 1–2 ␮mol/s/gcat regardless of the pre-reduction tem-
perature. And in contrast to the monometallic catalysts, the product
selectivity was more affected by the pre-reduction temperature. For
the PtCu1 catalyst pretreated at low temperature, the initial ethy-
lene selectivity was 54%, with ethane as the only other product. The
selectivity increased by 1% to 55% during the 40 h reaction (Fig. 3A).
However with the PtCu1 catalyst pretreated at high temperature,
the initial ethylene selectivity was zero and increased with time on
stream to a steady-state level of 3% (Fig. 3B). The ethane selectivity
was initially 95% and increased during the length of the reaction to
96%. Plus, the PtCu1 catalyst pretreated at high temperature was the
only bimetallic catalyst to form monochloroethane (MCE). There
was 5% MCE in the effluent of the first sample taken on-stream and
then it appeared again roughly 10 h later and increased to 1% by the
end of the reaction.
2
.5. Chemisorption measurements
The samples were characterized by CO chemisorptions mea-
surements using a volumetric sorption analyzer (Micromeritics,
ASAP 2010). The results were used to calculate the per cent Pt atoms
exposed for each catalyst, as described elsewhere [12].
Not surprisingly the monometallic catalyst had the highest per-
cent Pt atoms exposed: 27% for both the low temperature and high
temperature pretreated samples. For the bimetallic catalysts pre-
treated at high temperature, increasing the Cu content decreased
the per cent Pt atoms exposed, a trend that has been reported in
earlier published investigations [12,13]. Specifically, the catalysts
with Cu/Pt atomic ratios of 1,2,3,4, and 5 had per cent Pt metal
atoms exposed of 1.95, 1.53, 0.88, 0.64, and 0.58%, respectively. For
the bimetallic catalysts pretreated at low temperature, the total
◦
◦
◦
Fig. 1. Time on stream behavior of Pt/SiO2 pre-reduced at 220 C (A) and 500 C (B) in the reaction of 1,2-dichloroethane (7000 ppm) and H2 (35,000 ppm) in He at 200
total flow rate 41 mL/min): ꢀ – Ethane, ꢁ – Monochloroethane.
C
(

6
2
L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
◦
◦
◦
C
Fig. 2. Time on stream behavior of Cu/SiO2 pre-reduced at 220 C (A) and 500 C (B) in the reaction of 1,2-dichloroethane (7000 ppm) and H2 (35,000 ppm) in He at 200
total flow rate 41 mL/min): ꢀ – Ethylene.
(
◦
◦
◦
Fig. 3. Time on stream behavior of PtCu1/SiO2 pre-reduced at 220 C (A) and 500 C (B) in the reaction of 1,2-dichloroethane (7000 ppm) and H2 (35,000 ppm) in He at 200 C
(
total flow rate 41 mL/min): ꢀ – Ethylene, ꢀ - Ethane, ꢁ - Monochloroethane.
For the PtCu2 catalyst pretreated at low temperature, the ini-
For the PtCu3 catalyst pretreated at low temperature, the
initial ethylene selectivity was 98%, and again ethane was the
only other reaction product (Fig. 5A). The ethylene selectiv-
ity decreased during the course of the 40 h reaction to 95%
as the ethane selectivity increased to 5%. However, with the
PtCu3 catalyst pretreated at high temperature, the initial ethy-
lene selectivity was only 20% and the rest of the product
stream was ethane (Fig. 5B). By the second time the efflu-
ent stream was sampled, less than an hour later, the ethylene
selectivity had increased to more than 70% at the expense of
ethane. And thereafter the ethylene selectivity continued to
tial ethylene selectivity was 89%, and ethane was the only other
reaction product (Fig. 4A). During the course of the reaction, the
ethylene selectivity decreased to 77%, which was mirrored by a
corresponding increase in the selectivity towards ethane. However,
after the PtCu2 catalyst was pretreated at high temperature, the ini-
tial ethylene selectivity was only 6%. The other 94% of the product
stream was ethane (Fig. 4B). The ethylene selectivity increased to
2% at the expense of ethane during the first 20 h time on stream
and then stayed essentially constant for the remainder of the 40 h
reaction.
4
◦
◦
◦
Fig. 4. Time on stream behavior of PtCu2/SiO2 pre-reduced at 220 C (A) and 500 C (B) in the reaction of 1,2-dichloroethane (7000 ppm) and H2 (35,000 ppm) in He at 200
total flow rate 41 mL/min): ꢀ -Ethylene, ꢀ – Ethane.
C
(

L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
63
◦
◦
◦
Fig. 5. Time on stream behavior of PtCu3/SiO2 pre-reduced at 220 C (A) and 500 C (B) in the reaction of 1,2-dichloroethane (7000 ppm) and H2 (35,000 ppm) in He at 200
total flow rate 41 mL/min): ꢀ – Ethylene, ꢀ – Ethane.
C
(
◦
◦
◦
Fig. 6. Time on stream behavior of PtCu4/SiO2 pre-reduced at 220 C (A) and 500 C (B) in the reaction of 1,2-dichloroethane (7000 ppm) and H2 (35,000 ppm) in He at 200
total flow rate 41 mL/min): ꢀ – Ethylene, ꢀ – Ethane.
C
(
increase at the expense of ethane to 85% after 40 h time on
stream.
For the PtCu4 catalyst pretreated at low temperature, the ini-
tial ethylene selectivity was 99% and the ethane selectivity was
the second sampling of the effluent stream was made, roughly an
hour later, the ethylene selectivity was 92% and the ethane selec-
tivity was 8%. Thereafter, the ethylene selectivity increased at the
expense of ethane to 98% by the end of the 40 h reaction.
1
% (Fig. 6A). The ethylene selectivity decreased to 96% during the
For the PtCu5 catalyst pretreated at low temperature, the initial
ethylene selectivity was 100% and remained essentially constant
(Fig. 7A). However, with the PtCu5 catalyst pretreated at high tem-
perature, the initial ethylene selectivity was only 38%, with ethane
comprising the other 62% of the product stream (Fig. 7B). By the
course of the 40 h reaction, while the ethane selectivity corre-
spondingly increased to 4%. With the PtCu4 catalyst pretreated at
high temperature, the initial ethylene selectivity was only 15%, the
other 85% of the product stream was ethane (Fig. 6B). By the time
◦
◦
◦
Fig. 7. Time on stream behavior of PtCu5/SiO2 pre-reduced at 220 C (A) and 500 C (B) in the reaction of 1,2-dichloroethane (7000 ppm) and H2 (35,000 ppm) in He at 200
total flow rate 41 mL/min): ꢀ – Ethylene, ꢀ – Ethane.
C
(

6
4
L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
◦
◦
Fig. 8. FTIR spectra of room temperature CO adsorption on Pt/SiO2 pre-reduced at 220 C (A) and 500 C (B).
second sampling of the product stream, roughly an hour later, the
ethylene selectivity had increased to 83% at the expense of ethane.
And thereafter the ethylene selectivity continued to increase to 96%
at the expense of ethane by the end of the 40 h reaction time.
spectra of the five bimetallic catalysts. After both the low tem-
perature and the high temperature pre-reduction, exposure of
the Pt catalyst to 10 Torr of CO resulted in the appearance of
two spectral bands. The positions of these bands were 2077 and
−
1
1
834 cm
and 1768 cm
assignments of these vibrational bands are well established. The
for the low temperature sample (Fig. 8A) and 2075
−
1
3.2. Temperature programmed desorption (TPD) and reduction
for the high temperature sample (Fig. 8B). The
(
TPR) investigations
−
1
high frequency band (2075–2077 cm ) is attributed to CO lin-
early adsorbed on a Pt atom [10,12,15–17], and the low frequency
band (1768–1835 cm ) is attributed to CO with two and three fold
coordination to Pt atoms [15,16]. The evacuation of the gas phase
It must be acknowledged that the rate of species evolution
−
1
measured during temperature-programmed investigations with
powdered catalysts such as the type conducted for this work
depend on many parameters including the geometry of the system,
the mass of sample, and the extent to which the powdered mate-
rial has been compacted during waferization [14]. Thus, the results
presented herein emphasize the qualitative behavior and the rela-
tive trends, as opposed to asserting an undue level of quantitative
detail given the scope of this research investigation.
−
1
for 15 min resulted in band shifts of less than 10 cm
to lower
frequency and a very small decrease in the intensity of the high fre-
quency band, which is indicative of a small change in the coverage
of CO due to the strong CO–Pt interaction.
The adsorption of CO on Cu (not shown) has also been stud-
ied by infrared spectroscopy. The band positions (excluding zeolite
systems) have been found to fall in different regions for each
For all of the as prepared catalysts, both H O and HCl evolved
2
0
−1
1+
during the temperature-programmed desorption experiment. The
metal oxidation state: Cu – 2110 cm
and lower, Cu – 2110
◦
−1
2+
−1
desorption of H O started at roughly 70 C, whereas the start of
to 2140 cm , and Cu – 2145 cm and above [18]. These ranges
are considered typical for Cu species of oxides and on oxide sup-
2
◦
HCl desorption was roughly 100–110 C for the Pt and bimetallic
samples and 120 C for the monometallic Cu sample.
◦
ports. The interaction strength of CO with Cu0 and with Cu is
very weak, to the extent that such adsorption complexes are unsta-
ble at room temperature in the absence of a CO partial pressure.
The CO–Cu1+ interaction is comparably stronger, and thus more
robust evacuation conditions are required for the CO to desorbs.
Most importantly, the lack of stability of CO adsorbed on Cu species
as compared to CO adsorbed on Pt allows the adsorption bands for
the bimetallic catalysts to be readily classified by metal type, as
described below.
2+
Gas-phase H O and HCl also formed with all of the catalysts dur-
2
◦
◦
ing the room temperature to 500 C TPR after the 130 C in vacuum
treatment. The formation of H O occurred over the entire tempera-
2
ture range with all of the catalysts. The formation of HCl occurred at
◦
the lowest temperature with the Pt catalyst, roughly 85 C, and was
◦
essentially complete before 150 C. It formed at the highest temper-
◦
ature with the Cu catalyst, starting at roughly 180 C and evolving
at one rate to the temperature of 225 C and then continuing from
25 C to 280 C at a different rate. The results for the bimetallic
◦
◦
◦
2
catalysts were in between those of Pt and Cu. The formation of HCl
started roughly at 130 C at one rate to the temperature of 160 C
3.3.2. Bimetallic catalysts
◦
◦
For the PtCu1 catalyst pre-reduced at low temperature, expo-
sure to 10 Torr of CO resulted in the appearance of two vibrational
◦
◦
and then continued at a different rate from 160 C to 225 C. Carbon
monoxide also formed during the TPR, most likely from the decom-
−
1
−1
bands: one at 2137 cm and the other at 2036 cm (Fig. 9A). The
◦
−1
position of surface carbonate species, at roughly 250 C with Pt and
the bimetallics and at 280 C with the Cu catalyst.
For the 200–500 C TPR experiments conducted after reduction
at 220 C in 10% H /Ar, the formation of HCl occurred with all of the
intensity of the band at 2137 cm decreased by 70% after 0.5 min
of evacuation and to the baseline after evacuation for 15 min. The
intensity of the 2036 cm
◦
◦
−1
band changed little during the evac-
◦
2
uation, based on peak height, but the band maximum shifted to
◦
−1
catalysts. The onset temperature was roughly 300 C or more and
2044 cm . After pre-reduction at high temperature, exposure to
the amount of HCl formed was approximately 6 times greater with
the bimetallics and Cu catalyst than with the Pt catalyst. Water and
CO were also formed during the TPR.
10 Torr of CO resulted in the appearance of three spectral fea-
tures: a band at 2125 cm , a convoluted band at 2052 cm , and a
−
1
−1
−
1
−1
broad band at 1753 cm (Fig. 9B). The intensity of the 2125 cm
band decreased by 4% after 0.5 min evacuation, whereas the band
−
1
3.3. FTIR CO adsorption spectroscopy investigations
at 2052 cm increased 7% in intensity. After 15 min evacuation,
the intensity of the 2125 cm had decreased to 30% of its origi-
nal size and the intensity of the 2052 cm had increased to 133%
of its original size. It should also be noted that the shape of the
−
1
−
1
3.3.1. Monometallic catalysts
The spectra for CO adsorption on the Pt catalyst are shown
in Fig. 8 and are provided as a basis of comparison for the
−
1
band at 2052 cm
became more symmetric after evacuation of

L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
65
◦
◦
Fig. 9. FTIR spectra of room temperature CO adsorption on PtCu1/SiO2 pre-reduced at 220 C (A) and 500 C (B).
◦
◦
Fig. 10. FTIR spectra of room temperature CO adsorption on PtCu2/SiO2 pre-reduced at 220 C (A) and 500 C (B).
the gas phase. The broad band at 1753 cm−1 remained essentially
unchanged during the evacuation procedure.
For the PtCu2 catalyst pre-reduced at low temperature, expo-
sure to 10 Torr of CO resulted in the appearance of two bands: 2137
evacuation, whereas the band at 2050 cm increased by 17%. After
−1
−
1
15 min evacuation, the intensity of the 2125 cm had decreased
−
1
to 16% of its original size and the intensity of the 2050 cm had
increased to146% of its original size. The prominence of the low
frequency shoulder feature also decreased with evacuation time.
For the PtCu3 catalyst pretreated at low temperature, exposure
to 10 Torr of CO resulted in the appearance of two bands: 2139 and
−
1
−1
and 2033 cm (Fig. 10A). The intensity of the band at 2137 cm
decreased by 74% after 0.5 min of evacuation and to the base-
−
1
line after evacuation for 15 min. The intensity of the 2033 cm
band changed little during the evacuation, but the band maximum
shifted 2038 cm . After pre-reduction at high temperature, expo-
sure to 10 Torr CO resulted in a band at 2129 cm and a band at
050 cm that had a shoulder feature at 2018 cm (Fig. 10B). The
−
1
2029 cm (Fig. 11A). After evacuation for 0.5 min, the intensity of
the high frequency band decreased by 66% and the resulting band
−1
−
1
−1
showed maxima at 2150 and 2135 cm . After 15 min of evacua-
−1
−1
2
tion the band intensity had decreased to the baseline. However,
the low frequency band intensity was stable to evacuation, based
−1
intensity of the 2129 cm
band decreased by 35% after 0.5 min
◦
◦
Fig. 11. FTIR spectra of room temperature CO adsorption on PtCu3/SiO2 pre-reduced at 220 C (A) and 500 C (B).

6
6
L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
◦
◦
Fig. 12. FTIR spectra of room temperature CO adsorption on PtCu4/SiO2 pre-reduced at 220 C (A) and 500 C (B).
◦
◦
Fig. 13. FTIR spectra of room temperature CO adsorption on PtCu5/SiO2 pre-reduced at 220 C (A) and 500 C (B).
−
1
−1
on peak height, but the band position shifted from 2029 cm to
2029 cm (Fig. 13A). After evacuation for 0.5 min, the intensity of
−1
2
1
042 cm . After pre-reduction at high temperature, exposure to
0 Torr CO resulted in a high frequency band at 2123 cm and a low
frequency band at 2042 cm with shoulder feature at 2015 cm
Fig. 11B). The intensity of the high frequency band decreased
the high frequency band decreased by 77% and the resulting band
showed maxima at 2150 and 2133 cm . After 15 min of evacua-
tion the band intensity had decreased to the baseline. However,
the low frequency band intensity was essentially stable to evacua-
tion, but the band position shifted from 2029 cm to 2036 cm .
−
1
−1
−1
−1
(
−
1
−1
−1
by 22% after 0.5 min evacuation, whereas the band at 2038 cm
increased by 16%. After 15 min evacuation, the intensity of the
After pre-reduction at high temperature, exposure to 10 Torr CO
resulted in a high frequency band at 2125 cm and a low frequency
−1
−1
2
123 cm had decreased to 33% of its original size and the inten-
−1
−1
−1
sity of the 2038 cm had increased to 157% of its original size. The
prominence of the low frequency shoulder feature also decreased
with evacuation time.
band at 2044 cm with shoulder feature at 2015 cm (Fig. 13B).
The intensity of the high frequency band decreased by 28% after
−
1
0.5 min evacuation, whereas the band at 2044 cm increased by
22%. After 15 min evacuation, the high frequency band intensity
had decreased to 16% of its original size and the intensity of the
For the PtCu4 catalyst pretreated at low temperature, exposure
to 10 Torr of CO resulted in the appearance of two bands: 2140 and
−1
−1
2
031 cm (Fig. 12A). After evacuation for 0.5 min, the intensity of
low frequency band (2044 cm ) had increased to 181% of its origi-
the high frequency band decreased by 77% and the resulting band
nal size. The prominence of the low frequency shoulder feature also
decreased with evacuation time.
−1
showed maxima at 2150 and 2135 cm . After 15 min of evacu-
ation the band intensity had decreased to the baseline. The low
frequency band intensity was stable to evacuation, but the band
−
1
−1
position shifted from 2031 cm to 2038 cm . After pre-reduction
at high temperature, exposure to 10 Torr CO resulted in a high fre-
4. Discussion
−1
−1
quency band at 2127 cm and a low frequency band at 2038 cm
The kinetics results of this investigation show that the pre-
reduction temperature affects the performance of the catalysts –
significantly for the bimetallic catalysts with the lowest Cu to Pt
atomic ratio and not so much for those with the highest ratio. The
question of why will be addressed first through the proposal of
descriptive models for the evolution of the catalysts during the
low temperature pretreatment and during the high temperature
pretreatment. Then, the salient trends and features of the reaction
kinetics will be discussed in the context of the catalysts that result
from the two different pretreatments.
−1
with shoulder feature at 2016 cm (Fig. 12B). The intensity of the
high frequency band decreased by 28% after 0.5 min evacuation,
−
1
whereas the band at 2038 cm
increased by 24%. After 15 min
−1
evacuation, the intensity of the 2127 cm had decreased to 20%
−
1
of its original size and the intensity of the 2038 cm had increased
to 183% of its original size. The prominence of the low frequency
shoulder feature also decreased with evacuation time.
For the PtCu5 catalyst pretreated at low temperature, exposure
to 10 Torr of CO resulted in the appearance of two bands: 2142 and

L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
67
4
.1. Evolution of bimetallic catalysts pre-reduced at low
the next evolutionary step. They catalyze the dissociation of H2
and thus serve as stationary sources of H atoms. Mobile species
that diffuse to these site are provide with the H atoms necessary
to hydrogenate their metal–chlorine bonds, and by this process
particle growth occurs one layer after another. Of course, it is
also possible that H atoms formed on the Pt sites spillover to the
support and surface diffuse to react with some metal chloride or
other precursor species. However, regardless of the net direction of
diffusion, the faster rate of platinum-chlorine bond hydrogenation,
as compared to the hydrogenation of copper–chlorine bonds,
drives the growth of particles with Pt-rich cores and outer layers
rich in Cu. Based on the FTIR results, CuCl is present at the outer
most layers.
The above description is consistent a prior investigation of
supported RuCu bimetallics that showed using metal chloride
precursors, rather than metal nitrates, favored the formation of
bimetallic particles having surfaces covered by Cu [28]. While the
aforementioned kinetics pathway involving mobile surface species
most likely controls the assembly process that occurs during the
low temperature reduction, it should also be noted that thermody-
namics favors formation of particles with surfaces enriched in Cu.
Indeed, vacuum conditions alone are sufficient to induce some sur-
face segregation of Cu [29,30], as Cu has a lower solid–vapor surface
free energy than Pt [31]. And more pronounced surface enrichment
in Cu is driven by adsorbed Cl [32–34].
temperature
The temperature-programmed desorption results show that
during the initial drying step both HCl and H O evolve, indicating
2
the formation of chlorides, hydroxychlorides, and oxychlorides of
the metals on the support surface [19]. Based on the temperature-
programmed reduction results, some fraction of these species
most likely contain both types of metals, as the temperature
range in which HCl forms during TPR with the bimetallic catalysts
◦
◦
(
130–225 C) is in between that for the Pt catalyst (85–150 C) and
◦
the Cu catalyst (180–280 C), both of which are in good agreement
with reported literature values [19–23]. As well, the lack of HCl
formation in the 85–130 C range argues against the formation of a
◦
significant separate Pt phase during the drying step [24]. However,
the possibility of Cu only species cannot be excluded, and indeed
seems likely with the catalysts having the highest Cu to Pt atomic
ratios.
From the start of the low temperature pre-reduction to its end,
the metal containing entities converts only from more chlorinated
to less chlorinated, not to zero valent metal. This is evident from
the formation of HCl during the TPR conducted after the low tem-
perature pre-reduction step. As well, the formation of particles
with highly chlorinated surfaces may be inferred from the FTIR
CO adsorption results (Figs. 9A–13A) and the assessment of the
most viable assignment for the high frequency absorption band
at 2137–42 cm . The band is too unstable under evacuation con-
ditions to be attributed to a CO–Pt adsorption complex, and the
band position is too high in frequency to assign to CO adsorbed
on Cu (2110 cm and below [18]). And although the frequency is
in the range for CO adsorbed on Cu of non-zeolitic oxide moi-
However, neither the particle growth process nor the Cl-induced
surface enrichment in Cu results in the complete exclusion of
Pt atoms from the surface of the particles, as evidenced by the
−1
−
1
CO adsorption FTIR spectra that show a band at 2029–36 cm
0
−1
(Figs. 9A–13A). This band is stable to evacuation and is assigned
to CO linearly adsorbed on Pt atoms with nearest neighbor sur-
+1
−1
−1
eties and materials (2110–2140 cm
[18]), the lack of stability
face atoms of Cu [35–38]. The roughly 40 cm lower frequency
of the band in the absence of gas-phase CO is inconsistent with
compared to that of CO linearly adsorbed on the Pt catalyst
(2075–77 cm ) is a results of the decrease in the CO dipole–dipole
+1
−1
the relatively strong interaction between CO and Cu . However,
both the band position and lack of stability under evacuation con-
ditions fit with an assignment of CO adsorbed on CuCl, which has
coupling that occurs when the surface ensembles of Pt are large and
the coverage of CO is high [39,40]. That some Pt atoms are always
at the surface of the bimetallic particles is not surprising given the
presence of H2 in the gas phase and the stronger adsorbate-metal
interaction of hydrogen with Pt [41–43] than Cu [44,45]. Indeed, in
the absence of Cl with H2 present an enrichment of the surface in
Pt would be expected [46].
−1
a reported frequency of 2134–36 cm and an adsorption enthalpy
low enough that the adsorption behavior is reversible at 373 K
−
25]. Further, the positions of the two peak maxima (2150 cm
1
[
−1
and 2133–35 cm ) that are clearly identifiable with the three high
Cu content catalysts after 0.5 min evacuation agree surprising well
with assignments reported for CO adsorbed on disordered CuCl
−1
films (2144 and 2125 cm ) [25]. Highly chlorinated particle sur-
faces also serve to explain the negligible volume of CO uptake in
the chemisorption measurements with these catalysts.
4.2. Evolution of bimetallic catalysts pretreated at high
temperature
In putting forth a model describing the series of steps that
In contrast to the low temperature reduction pretreatment step,
the high temperature reduction at 500 C is sufficient to remove the
◦
occur during the low temperature reduction with H , a primary
2
point to consider is the temperature used was low enough that
the reduction of the dried precursor species occurred slowly and
not to completion during the allotted time period. As well, prior
investigations have shown that both platinum and copper chloride
moieties/clusters are mobile on oxide supports in this same tem-
perature range [23,26,27]. Thus, following an initial step in which
majority of the Cl. This is evident from the TPR results of this inves-
tigation and others showing that HCl formation ceases by 300 C,
◦
even for monometallic Cu catalysts [20–22]. It is also reflected in
the FTIR band position for CO adsorbed on the Cu species of the
−
1
bimetallic catalysts (Figs. 9B–13B). At 2123–29 cm , the band is
−
1
approximately 10 cm lower in frequency than the band assigned
to CO adsorbed on CuCl after the low temperature pre-reduction. It
is also more stable under evacuation conditions – albeit certainly
less stable than bands associated with CO adsorbed on Pt species.
Although the frequency range is typical for CO adsorbed on Cu+1, the
fact that the band intensity decreases under evacuation conditions
and the reduction temperature was sufficiently high to reduce Cu+1
species are two reasons cited in the past to preclude an assignment
HCl and H O evolve as the precursor species formed during the dry-
2
ing stage are reduced to metal chloride moieties, surface diffusion
of chlorinated species becomes a factor in the supported bimetallic
particle assembly process.
Platinum–chlorine bonds reduce more readily than
copper–chlorine bonds, based on the TPR results herein and
in the literature, so atoms and/or clusters of Pt with metallic
character should form more readily than their Cu counterparts.
In essence, in this step the support becomes decorated with
metal sites consisting mostly of Pt atoms, as the other chloride
moieties diffuse about surface and any remaining precursor
species undergo the initial partial reduction to form the mobile
intermediates. These newly formed Pt sites are participants in
+1
of CO on Cu [10]. However, attributing the band to CO adsorbed
on Cu0 requires an accounting for a position well about the typ-
−
1
ical range of 2110 cm and below [18]. One viable option is an
electronic modification by neighboring Pt atoms that increases the
CO–Cu0 interaction strength, as XPS and UPS investigations have
shown a Cu core-level binding shift of −0.5 eV for Cu overlayered

6
8
L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
on Pt foil [47]. The FTIR results do provide evidence that the Cu
atoms are in atomic closeness with Pt atoms. First, the shoulder
band at 2015–18 cm is associated with CO bridged between a Pt
and a Cu [48]. And second, the intensity increase in the CO–Pt band
during evacuation is indicative of an energy intensity redistribution
effect [49].
used for the synthesis, the selectivity towards ethylene is increased
at the expense of ethane.
−1
These results support a mechanistic proposal that was put forth
in earlier investigations [10,12,13,53]. Namely, that ethane forms
by a reaction pathway involving sites containing only Pt atoms.
Copper not only inhibits this overall reaction, but is also a site for
the conversion of 1,2-dichloroethane to ethylene. Higher surface
concentrations of Cu yield higher selectivities of ethylene, because
there are fewer Pt sites of sufficient size to catalyze the dissociative
adsorption of 1,2-dichloroethane and subsequent steps in forming
ethane. Yet, the small ensembles of one or a few Pt atoms still avail-
able even at the highest Cu to Pt atomic ratio readily catalyze the
dissociative adsorption of H2 to generate the H atoms necessary to
hydrogenate the Cl atoms deposited on Cu. Thus, using a pretreat-
ment that selectively segregates Cu to the surface and increasing
the amount of Cu relative to Pt both inhibits the total hydrogenation
pathway and promotes the partial hydrogenation catalysis.
A fair question to ask is to what extent the catalyst pretreatment
affects the performance of the catalyst with increasing time on
stream. The proposed models for the transformations the catalysts
undergo as a function of reduction conditions provide a description
of the catalysts at the start of the reaction. With a reactant stream
In putting forth a description of the series of events that occur
during the high temperature reduction, a reasonable starting point
for consideration is that all of the steps described for the low
temperature reduction occur and then some more. The point of
◦
experimental departure occurs at 220 C, where the temperature is
◦
further increased to 500 C and then held there for 2 h. Thus, instead
of 2 h at 220 C during which the hydrogenation rate slows while
◦
remaining metal chloride species move about and assemble, hydro-
genation of Cl continues to near completion as the temperature
◦
advances towards 500 C. Based on the TPR results, the majority of
chlorine has been removed by the time the temperature reaches
◦
3
00 C. Progressively, there is less Cl driving Cu to the surface, and
the effects of an ongoing thermal treatment in a partial pressure of
hydrogen become important. Adsorbed hydrogen induces surface
enrichment in Pt [50–52]. The end result, therefore, is supported
particles that are both more surface rich in Pt and more metallic
than their low temperature reduced counterparts. The difference
in the two is particularly apparent from the spectra of CO adsorp-
tion on PtCu1 (Fig. 9). The characteristic broad band for CO bound
containing both a chlorinated molecule and H the relative tug of Cu
2
by chlorine and pull on Pt by hydrogen is determined by the compo-
sition of the gas phase. Yet, the conversion as a function of time on
stream for all the catalysts is remarkably constant, behavior often
used to infer little change in a catalyst with time on stream. But
with several catalysts the product selectivity changes appreciably
during the first 10 or so hours on stream despite an unremarkable
change in conversion. Most notable in this regard is the behavior of
the PtCu2 catalyst in comparison to the PtCu1 catalyst.
−1
to two and three Pt atoms (1750–1860 cm ) is present after the
high temperature pre-reduction – a reflection of the large surface
ensembles of Pt atoms, whereas the same band is absent after the
low temperature pre-reduction. As well, the intensity of the linearly
−1
adsorbed CO–Pt (2038–52 cm ) is larger after the high tempera-
ture pre-reduction than after the low.
Not surprisingly, increasing the amount of Cu relative to Pt at
the impregnation stage places a limit on the amount of Pt that may
segregate to the surface. This is demonstrated by the increase in the
If only to consider the PtCu1 behavior, it would be tempting to
attribute the stability in both selectivity and conversion with time
on stream to bimetallic particles more stable than the proposed
egg and shell type particles, namely ordered PtCu containing alter-
nating layers of Cu and Pt atoms on (1 1 1) planes that is stable
−
1
intensity of the FTIR band for CO on Cu (2123–29 cm ) relative to
−1
the band for CO on Pt (2036–52 cm ) as a function of increasing
Cu to Pt atomic ratio in the catalyst (Figs. 9B–13B).
◦
below 812 C [54]. The preferential formation of particles with Pt
terminated planes during high temperature reduction and parti-
cles with Cu terminated planes during low temperature reduction
would account for the difference in selectivity. Yet with PtCu2,
which does not form an ordered phase, attributing the change in
selectivity with time on stream to reaction gas induced surface
enrichment – in Pt after low temperature reduction and in Cu after
high temperature reduction – is seemingly at odds with the con-
version remaining constant. Indeed, one premise of the dual site
model is the dissociative adsorption of 1,2-dichloroethane is forced
to occur on copper site by making the ensembles of Pt too small
to catalyze the step. In other words, the rate of ethane formation
on Pt is faster than the rate of ethylene formation catalyzed by a
bimetallic. Thus, there must be more to the story.
One possibility is the ethane and ethylene formation pathways
share a common rate-determining step, which would mean the
intrinsic activity of the Pt sites in the bimetallic catalysts for one
or more steps involved in ethane formation are substantially lower
than those of freshly pretreated Pt. In fact, the activity of the Pt
sites in the bimetallics is comparable to the activity of the Pt
catalysts after several tens of hours time on stream (Fig. 1). As treat-
ment in chlorine is actually used to reverse particle sintering with
supported platinum catalysts [26,55,56], the conversion decrease
is probably not due to a loss in metal dispersion. However, the
metal may be gradually becoming more metal chloride in nature,
which is less active for the 1,2-dichloroethane to ethane reaction.
And it would not be surprising that the Pt sites in the bimetallics
would be more rapidly chlorinated as the TPR results show Cu
inhibits the overall chlorine hydrogenation rate in comparison to
pure Pt. Thus, one consistent, albeit speculative, accounting for
4
.3. Impact of catalyst pretreatment on catalyst performance
The kinetics results show that the catalyst pretreatment has the
most impact on the bimetallic catalysts with the least amount of Cu.
With Pt1Cu1, Pt1Cu2, and Pt1Cu3 the ethylene selectivity is higher
after low temperature pre-reduction at all time on stream than after
the high temperature pretreatment. The difference is the largest
with Pt1Cu1 and the smallest with Pt1Cu3, as increasing the Cu
content increases the ethylene selectivity for both the low and high
temperature pretreated catalysts. With the PtCu4 and PtCu5 cata-
lysts, the ethylene selectivity is only higher after low temperature
reduction during the first hour or so of time on stream. Thereafter
the selectivity is approximately the same, in the 90 + % range.
As discussed above, the use of metal chloride precursors and a
low pre-reduction temperature results in supported particles with
a preferential concentration of Pt in the core and Cu at the outer sur-
face, and surface layers better described as a metal chloride than
zero valent metal. In comparison, the supported particles have sur-
faces enriched in Pt after the high temperature pre-reduction. This
difference is most apparent with the PtCu1 catalyst, which has Pt
ensembles large and numerous enough after the high temperature
pre-reduction to exhibit a FTIR absorption band for CO adsorbed
with two-fold and three fold coordination to Pt atoms. This catalyst
exhibited essentially the same product selectivity as monometallic
Pt. Its low temperature pre-reduced counterpart, with less Pt at the
surface, yielded more ethylene than ethane. And by increasing the
amount of Cu at the surface by increasing the Cu to Pt atomic ratio

L.N. Akonwie et al. / Applied Catalysis A: General 415–416 (2012) 59–69
69
the time on stream behavior of the bimetallic catalysts is that the
single site ethane formation pathway and the dual site ethylene
formation pathway share common rate-determining step
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[
[
[
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a
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ad-atoms could be the slow step, or the hydrogenation of chlorine
could be rate-determining – assuming the direct and indirect mixed
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the bimetallic. However, selectivity would be governed by the rel-
ative rates of a step earlier in the mechanisms of the reactions such
as the relative rate of 1,2-dichloroethane dissociative adsorption
on Cu and on Pt sites. Thus, changes in the surface composition of
the particles induced by the reaction gases during time on stream
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5
. Conclusions
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◦
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are still well-chlorinated. The first step in the formation of these
particles is the reduction of Pt containing moieties, which is then
followed by surface diffusion of Cu chloride moieties to the Pt sites
where their reduction, at least partial, takes place. The presence of
Cl plays a critical role in the evolution of egg and shell type particles
as a ligand that facilitates surface mobility of metal moieties and is
more difficult to reduce when coordinated to copper than platinum.
In contrast, particles that are more metallic and have higher surface
concentrations of Pt result from pre-reduction at high tempera-
ture (500 C). Essentially all of the chlorine is hydrogenated before
the temperature reaches 500 C and the continued hydrogen treat-
ment at 500 C drives H-adsorbate induced surface enrichment in
Pt. Increasing the relative amount of copper compared to platinum
offsets the enrichment effects.
3
[
[
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