Mechanistic Study of 1,3-Butadiene Formation in Acetylene Hydrogenation over
the Pd-Based Catalysts Using Density Functional Calculations
Bo Yang1, Robbie Burch1, Christopher Hardacre*,1, P. Hu*,1, Philip Hughes2
1
CenTACat, School of Chemistry & Chemical Engineering, The Queen's University of
Belfast, Belfast, BT9 5AG, U. K.
2
Johnson Matthey Catalysts, PO Box 1, Billingham, Teesside, TS23 1LB, U.K.
The corresponding authors: Professor Christopher Hardacre and Professor Peijun Hu,
Email: c.hardacre@qub.ac.uk; p.hu@qub.ac.uk
Abstract
Green oil, which leads to the deactivation of the catalysts used for the selective
hydrogenation of acetylene, has long been observed, but its formation mechanism is
not fully understood. In this work, the formation of 1,3-butadiene, known to be the
precursor of green oil, on both Pd(111) and Pd(211) surfaces is examined using
density functional theory calculations. The pathways containing C2+C2 coupling
reactions as well as the corresponding hydrogenation reactions are studied in detail.
Three pathways for 1,3-butadiene production, namely coupling plus hydrogenation
and further hydrogenation, hydrogenation plus coupling plus hydrogenation and a two
step hydrogenation followed by coupling, are determined. By comparing the effective
barriers, we identify the favoured pathway on both surfaces. A general understanding
towards the deactivation process of the industrial catalysts is also provided. In
addition, the effects of the formation of subsurface carbon atoms as well as the Ag
alloying on the 1,3-butadiene formation on Pd based catalysts are also investigated
and compared with experimental results.
Keywords DFT, 1,3-butadiene formation, selective hydrogenation, acetylene,
coupling
1
1. Introduction
The selective hydrogenation of acetylene to ethylene is an important process in
the petrochemical industry, since acetylene is the major impurity in ethylene feeds
produced from naphtha cracking processes. Acetylene is also a significant poisoning
species to the catalyst used for downstream ethylene polymerization.1,2 As has been
addressed in our previous work and the work from other groups, the major issues in
this process are the over hydrogenation of ethylene to ethane and the oligomerization
of acetylene to higher molecular weight species, such as butadiene and polymers,
known as green oil.3,4 The over hydrogenation results in yield loss and the
accumulation of oligomers on the catalyst surface leads to accelerated catalyst
deactivation.
Extensive experimental and theoretical work has been carried out to improve the
selectivity of acetylene hydrogenation catalysts to produce only ethylene. The critical
factors determining the selectivity have also been identified, namely the adsorption
energy and the hydrogenation barrier of ethylene. Due to the sensitivity of the
ethylene polymerisation catalyst to acetylene and the economic consideration, the
selectivity/activity requirements are very high and, although several catalysts have
been developed and commercialized, new catalyst formulations are still required to
provide maximum ethylene product at low temperature.5-11
In addition to the selectivity/activity of the catalyst, due to the high throughput of
gas and the severe cost implications of downtime in refinery processes, these catalysts
must be robust and typically must maintain their activity for 5+ years. However, much
less attention has been paid to the deactivation of the catalyst compared with the
selectivity in acetylene hydrogenation processes, and the mechanism of the green oil
formation on these catalysts is still unknown despite its importance in catalyst
deactivation.12-24 In fact, the majority of the ethylene purification industries use the
tail-end process where the hydrogen is present in stoichiometric amounts with respect
to acetylene, i.e. H2/acetylene is around 1.1 In this case, the coverage of acetylene, or
its hydrogenation intermediates, may be very high on the catalyst surface and thus it is
likely to increase the probability of green oil formation. This has been observed
experimentally, where the concentration of oligomeric species increases with the
decreasing H2/acetylene ratio.13 As well as deactivating the catalyst via surface
adsorption, green oil can block the catalyst pores, reducing mass transfer processes
such as hydrogen diffusion.12,13,15
2
Asplund and co-workers proposed that 1,3-butadiene may be the precursor of
green oil formation.12,13,15 This has also been observed by Moon and co-workers22 and
they also found that the deactivation of Pd catalyst proceeds in three stages.24 Initially,
the activity of the catalyst is only slightly decreased due to the formation of light
green oil (stage I). Thereafter, further polymerization takes place to form several
heavier species causing increased surface deactivation (stage II). Finally, the heavy
green oil condenses in the pores of the catalyst, leading to pore blocking and reducing
the hydrogen transfer which exacerbates the polymerisation and reduces the activity
of the catalyst dramatically (stage III). The mechanism for the coupling of propyne
and acetylene on Cu, Ni and Pd surfaces was studied by Bridier et al.25,26 and GarciaMota et al.27 previously using density functional theory (DFT) calculations. They
found that on Cu(111) the coupling of propyne was slightly endothermic by 0.28 eV,
while it was endothermic by 0.83 eV on Ni(111). Consequently, the lowest barrier for
propyne coupling was 1.01 eV on Cu while for Ni(111) it was 1.49 eV. On the clean
Pd(111) surface, the barrier for acetylene coupling was calculated to be 1.38 eV.
However, the whole reaction pathway of the precursor formation was not considered.
Moreover, the effect of surface structure on the formation of the precursors was also
not examined.
In this work, we investigate the mechanism of 1,3-butadiene formation with
several pathways on both Pd(111) and Pd(211) surfaces, which are representative of
typical flat and defect Pd surfaces, in order to gain an insight to the effect of catalyst
surface structures on 1,3-butadiene formation. Since much evidence has shown that
the Pd catalyst might be Pd-carbide phase during the reaction, the influence of the
subsurface carbon formation are also examined. In addition to the ‘three stages’
mechanism for the green oil formation on Pd catalyst surface proposed by Moon et al.,
they also found that the drastic decrease of the activity was not observed during
period III on Pd-Ag catalyst, indicating that the lifetime of the Pd catalyst was
extended by Ag promotion.24 Therefore, the influence of alloying with Ag on the 1,3butadiene formation pathways over Pd surface has been studied.
2. Computational Details
In this work, the DFT calculations were performed within a periodic slab model
using the plane wave based VASP package.28-31 The generalized gradient
approximation (GGA) was used with the exchange-correlation functional PW91.32
3
The atomic cores and electrons were described with the projector augmented wave
(PAW) method.33,34 A four layer slab with a (4×4) surface supercell was employed for
the clean Pd(111) surface, the Pd(111) surface with subsurface carbon and surface PdAg alloy, the two uppermost layers were relaxed with the adsorbates while the bottom
two layers were fixed during the optimization. The calculations for Pd(211) used
twelve layers (1×4) slabs with the upmost six layers relaxed, with four Pd atoms in the
direction parallel to the step edge. We tested the adsorption energy of butadiene over
Pd(211) surface with a larger unit cell (2×8) and found that the difference is within
0.01 eV, indicating that the size of the unit cell used in the current work is sufficient
to describe the processes involving butadiene. A vacuum cutoff of more than 12 Å
was set to minimize the interactions between slabs. A 2×2×1 k-point grid was used in
the calculations for the Pd(111) surface as well as the carbide and alloyed surfaces
and that used for the Pd(211) surface was 4×2×1. We used an energy cut-off of 500
eV and the force threshold in structural optimization was set to 0.05 eV/Å. The
transition states were located with a constrained minimization method.35-37 The
adsorption and binding energies were calculated as follows:
Ead  Etotal  ( Eg  Eslab )
where Etotal is the energy of the system after adsorption, Eg is the energy of the gas
phase adsorbent and Eslab is the energy of slab.
3. Results and Discussion
3.1 Schemes of 1,3-butadiene formation
A C2+C2 coupling mechanism was examined to investigate the formation of 1,3butadiene on the surface of Pd catalyst, as shown in Scheme 1. Propagation of this
mechanism may also be applied to study the reactions to form heavier green oil as it
has been reported that the green oil contains an even number of carbon atoms.18
Our recent work showed that the Horiuti-Polanyi mechanism,38-41 which
describes a sequential addition of hydrogen atoms to the substrate in heterogeneous
catalytic hydrogenation reactions, are not favoured in some cases when the catalysts
used are Ag or Au, but it is still the preferred mechanism on many transition metal
(e.g. Pd and Pt) catalyst surfaces.42 In the current work, there are three species on the
Pd surface, which are important in the formation of 1,3-butadiene: H, C2H2 and C2H3.
Three pathways involving these species can lead to 1,3-butadiene involving three
4
coupling reactions, i.e. C2H2+C2H2, C2H2+C2H3 and C2H3+C2H3. The three possible
pathways shown in Scheme 1 can be generalized as: coupling → hydrogenation →
hydrogenation; hydrogenation → coupling → hydrogenation and hydrogenation →
hydrogenation → coupling. It should be noted that there are some competing steps not
shown in Scheme 1, which are the hydrogenation of the different carbon atoms in
C4H4 and C4H5, e.g. for the hydrogenation of CHCHCHCH* (* denotes the surface
adsorption state), all four carbon atoms might be attacked by hydrogen, depending on
the symmetry of its surface adsorption configurations, forming CH2CHCHCH* or
CHCH2CHCH*. Scheme 1 is used to investigate the formation of 1,3-butadiene on all
the surfaces studied in the current work.
I)
II)
III)
H
H
C2 H 2 +C2 H 2  C4 H 4 
 C4 H5 
 C4 H 6
H
H
C2 H 2 +C2 H 2 
 C2 H 2 +C2 H3  C4 H5 
 C4 H 6
H
H
C2 H 2 +C2 H 2 
 C2 H 2 +C2 H3 
 C2 H3  C2 H3  C4 H 6
Scheme 1 Three possible pathways containing coupling reactions and hydrogenation
reactions to produce 1,3-butadiene on Pd surface.
3.2 Pathways of 1,3-butadiene formation on Pd surfaces
3.2.1 1,3-butadiene formation on Pd(111)
In the current work, we consider the diffusion of acetylene on the Pd surface as
an equilibrated process and will not influence the reaction rate of the whole process,
since experimental results have shown that for the adsorbed acetylene species on
Pd(111) surface, the diffusion occurs at temperatures as low as 70 K.43 The calculated
highest diffusion barrier of acetylene is only 0.36 eV, which is much lower than the
hydrogenation barrier of the adsorbed acetylene (0.96 eV) reported in our previous
work.4
The corresponding transition state structures of coupling reactions, i.e.
C2H2+C2H2, C2H2+C2H3 and C2H3+C2H3, are shown in Figure 1. In addition, the
adsorption geometries of the C4H4, C4H5 and C4H6 on Pd(111) are illustrated in this
figure. The adsorption energy of C4H6 on Pd(111) was calculated to be -1.64 eV. We
should mention that a wide range of different transitions states and adsorption
geometries were calculated and the structures shown in Figure 1 (as well as those
structures shown in the Figures, hereafter) are the most stable adsorption geometries
5
and the transition state structures with the lowest barriers. One can see from Figure 1
that, at the transition states of the coupling reactions, one C2 species (either C2H2 or
C2H3) adsorbs at the fcc site whilst the other C2 species sits at an adjoining hcp site
with the two sites sharing one Pd atom. The arrangement is thought to facilitate the
coupling reactions. One can also see from Figure 1 that C4H4, C4H5 and C4H6 interact
with 6, 5 and 4 Pd atoms, respectively, due to the increasing saturation of the terminal
carbon atoms. It should be mentioned that the favoured adsorption geometry of C4H6
shown in Figure 1, as well as its adsorption energy on Pd(111), is in good agreement
with previously reported DFT results.44
Since both the pathways I and II include the hydrogenation steps of C4H4 and
C4H5 and the product distribution of these steps is diverse, the hydrogenation on each
different carbon atom in C4H4 and C4H5 was studied on the Pd(111) surface. The
corresponding hydrogenation barriers are listed in Table 1. Since C4H4 adsorbs
symmetrically on Pd(111) (see Figure 1), we only located the transition states for the
hydrogenation of the carbon atoms marked as 1 and 2 in Figure 1. The hydrogenation
of C4H5 is more complex since all the carbon atoms are different. From Table 1, it is
clear that the hydrogenation of the terminal CH (carbon 4) in C4H5 is favoured and
thus C4H6 (1,3-butadiene) is formed. Therefore, it is evident that 1,3-butadiene can be
readily formed from C4H4 and C4H6 hydrogenation.
The corresponding reaction barriers of the elementary steps in Scheme 1, taking
place on Pd(111), are listed in Table 2 together with the C-C and C-H distances at the
transition states. The hydrogenation barrier of C2H2 and the C-H distance at the
transition state on Pd(111) were obtained from our previous work.4 It should be noted
that the hydrogenation barriers of C4H4 and C4H5 listed in Table 2 are the most
favoured among the hydrogenation reactions of all four carbon atoms. The
corresponding transition state structures of the hydrogenation of C4H4 and C4H5 with
the lowest barriers are shown in Figure 2. It can be seen from Figure 2 that, in all of
the transition states calculated, the hydrogen atoms interact with the same Pd atom to
which the carbon atoms bind at the transition states. This is in agreement with
previously reported transition state structures for hydrogenation reactions.3,4,45-50
Those C-C distances of the reacting carbon atoms are found to increase from the
reactions of C2H2+C2H2, C2H2+C2H3 to C2H3+C2H3, whilst the reaction barriers lie in
the order of C2H2+C2H2 > C2H3+C2H3 > C2H2+C2H3.
6
Using these energetic values, we can obtain the energy profiles of these three
pathways as shown in Figure 1. It should be mentioned that, in Pathway III, the two
hydrogenation reactions (C2H2 hydrogenation) are not sequential and, therefore, only
one transition state (TS1) is represented in the energy profile of this pathway. From
Figure 1, one can see that pathway I shows the lowest energies for almost all the
transition states and intermediate states compared with the other two pathways,
indicating that this is the favoured pathway, which is different with the order of the
reaction barriers of the coupling reactions. Furthermore, on combining all the steps in
the pathway to obtain the effective barriers,4 it is found that the effective barriers of
pathways I, II and III are 0.97 eV, 0.99 eV and 1.19 eV, respectively. Therefore,
pathway I is found to be relatively favoured on Pd(111), but pathway II has a
comparable preference.
3.2.2 1,3-butadiene formation on Pd(211)
An analogous study was undertaken for the formation of 1,3-butadiene on a
stepped Pd surface, Pd(211). Previous studies on the adsorption of acetylene and vinyl
group on Pd(211) surface showed that the favoured adsorption sites of them are the
four fold site under the step edge (B5 site) and the step edge sites, respectively.4 The
corresponding coupling reaction transition state structures for C2H2+C2H2,
C2H2+C2H3 and C2H3+C2H3 and the adsorption geometries of the products C4H4,
C4H5 and C4H6 are shown in Figure 3. The adsorption energy of C4H6 on Pd(211) was
calculated to be -1.94 eV, which is much stronger than on Pd(111) surface (-1.64 eV).
The C-C distances of the reacting carbon atoms are also found to increase following
the order of C2H2+C2H2<C2H2+C2H3< C2H3+C2H3 whilst the reaction barriers follow
the opposite order. This is different from the trends observed on Pd(111) where the
reaction barriers are similar. All the C4 species formed, i.e. C4H4, C4H5 and C4H6, are
found to adsorb at the step edge on Pd(211).
The hydrogenation of C4H4 and C4H5 at the step sites of Pd(211) was also studied.
As the adsorption geometry of C4H4 is no longer symmetric, each of the carbon atoms
is inequivalent and must be considered separately. Therefore, we considered the
hydrogenation of all of the carbon atoms in C4H4, which are marked as 1, 2, 3 and 4 in
Figure 3. The corresponding hydrogenation barriers for C4H4 and C4H5 on Pd(211) are
also summarised in Table 1. It is found that hydrogenation of terminal carbon atom 1
in C4H4 to produce C4H5 is still favoured. For the hydrogenation of C4H5, the
7
hydrogenation of carbon 4 is favoured and 1,3-butadiene is again readily formed from
C4H5 hydrogenation on Pd(211).
The activation barriers of these reactions and C-C and C-H distances at the
transition states for the coupling and hydrogenation reactions, respectively, on Pd(211)
surface are listed in Table 2. The transition states of the hydrogenation of C4H4 and
C4H5 on Pd(211) are shown in Figure 2. Interestingly, we find that the C-C and C-H
distances at the transition states for the coupling and hydrogenation reactions,
respectively, on Pd(211) are similar to those found on Pd(111), although the reaction
barriers and the structures are quite different. The reaction barriers for coupling
reactions follow the order of C2H2+C2H2 > C2H2+C2H3 > C2H3+C2H3 on Pd(211). In
order to compare the energetics of each pathway on Pd(211), the energy profiles of
them are shown in Figure 3, and the effective barriers for pathway I, pathway II and
pathway III are estimated. These are found to be 1.86 eV, 1.80 eV and 0.97 eV,
respectively, indicating that, on Pd(211), pathway III should be favoured for 1,3butadiene formation.
3.2.3 General discussion on the surface structure effect
The above results provide a basis for a full description of 1,3-butadiene and
heavier green oil formation on Pd surfaces. Firstly, as H2/acetylene ratio is rather low
in tail-end systems (commonly around 1) compared with the high ratio (commonly
higher than 10) in front end process, the surface coverage of acetylene is high and
thus increases the probability of C2H2+C2H2 coupling. The effective barrier of
ethylene formation (1.07 eV) is higher than that of 1,3-butadiene formation (0.97 eV)
on Pd(111), whilst on Pd(211), the two effective barriers are the same (both are 0.97
eV), suggesting that butadiene formation will be much easier at the flat sites than at
the step sites. Subsequent polymerization of C4H6 may also take place on Pd(111)
surface due to its strong adsorption (-1.64 eV). In addition to the formation of the
coupled products, hydrogenation of C2H2 will still occur at the same time at the
stepped sites. However, the formed green oil will first block flat sites and then diffuse
to the step sites of Pd catalyst and the support, when a large amount of green oil are
accumulated on the catalyst. As we showed previously that the flat sites prefer to
selectively produce ethylene whilst the step sites prefer to produce ethane,4 the
undesired over-hydrogenation product, it can be reasoned that the selectivity of
ethylene on the Pd catalyst will decrease during the formation of green oil, which is
8
consistent with the experimental work of Asplund and co-workers.12,13 Eventually, the
pores of catalyst will be blocked once the amount of the heavier green oil formed is
too much, leading to the reaction becoming mass transfer limited.
3.3 Effect of the formation of subsurface carbon
The Pd(111) surface with subsurface carbon coverage of 0.25 monolayer (ML),
which is denoted as Pd(111)-0.25C, is considered in the current work due to the
following reasons. Firstly, the formation of butadiene has been concluded to be
favoured on Pd(111) over Pd(211). Secondly, the subsurface coverage of carbon on
the Pd catalyst under the reaction condition was determined to be ~0.15 ML
experimentally,51-53 which is close to the coverage studied herein. Furthermore, in our
previous work we calculated the thermodynamics of the carbon adsorption process at
the subsurface sites of Pd(111) and found that the decomposition of acetylene to form
carbon atoms and hydrogen is exothermic until the subsurface carbon coverage is
~0.50 ML, indicating that the subsurface carbon coverage cannot be higher than 0.50
ML. The corresponding transition state structures for the C2H2+C2H2, C2H2+C2H3 and
C2H3+C2H3 coupling reactions for the formation of 1,3-butadiene on Pd(111)-0.25C
are shown in Figure 4. The adsorption geometries for the products C4H4, C4H5 and
C4H6 are also shown in Figure 4. From Figure 4, one can find that C2H2 and C2H3
adsorb at two adjacent fcc and hcp sites without any carbon atoms beneath the surface.
A comparison with Figure 1 shows that the adsorption geometry of C4H4, C4H5 and
C4H6 on Pd(111)-0.25C are also similar to those on Pd(111).
The barriers of hydrogenation reactions on different carbon atoms of C4H4 and
C4H5 on Pd(111)-0.25C surface are listed in Table 1. It should be mentioned that the
adsorption of C4H4 is asymmetric on Pd(111)-0.25C, due to the presence of
subsurface carbon atoms (see Figure 4), and thus it is necessary to calculate the
barriers of hydrogenation on all four carbon atoms. From Table 1, it is clear that the
hydrogenation of C1 in C4H4 is favoured over Pd(111)-0.25C surface, whilst the
hydrogenation of the terminal CH (C4) in C4H5 is favoured, leading to 1,3-butadiene
formation. The adsorption energy of C4H6 on Pd(111)-0.25C was found to be -1.11
eV, which is much lower than on the Pd(111) and Pd(211) surfaces.
The reaction barriers for the coupling reactions and hydrogenation reactions on
Pd(111)-0.25C, according to Scheme 1, are listed in Table 2. The corresponding C-C
and C-H distances at the transition states are also listed (the hydrogenation barrier of
9
C2H2 and the C-H distance at the transition state on Pd(111)-0.25C surface was
obtained from our previous work4). The favoured transition state structures of the
hydrogenation of C4H4 and C4H5 are shown in Figure 2 and the energy profiles of
these three pathways on Pd(111)-0.25C are shown in Figure 4. The effective barriers
of pathway I, pathway II and pathway III were calculated to be 1.10 eV, 1.00 eV and
1.00 eV, respectively, indicating that pathway II and III have the same priority for
1,3-butadien formation on Pd(111)-0.25C.
3.4 Effect of the formation of surface Pd-Ag alloy
Similar calculations were performed on a Ag alloyed Pd(111) surface with low
coverage of Ag (0.25 ML), which is denoted as Pd-0.25Ag/Pd(111). Figure 5
illustrates the transition state structures of the coupling reactions of C2H2+C2H2,
C2H2+C2H3 and C2H3+C2H3, and the adsorption geometries of the products from the
coupling reactions on Pd-0.25Ag/Pd(111). All the species, including C2H2, C2H3,
C4H4, C4H5 and C4H6, adsorb at the Pd sites either at the coupling transition states of
C2+C2 or the adsorption states of C4 species, and without direct interaction with the
Ag atoms. From Figure 5, one can see that the 1,3-butadiene adsorbs at three Pd
atoms in a row on Pd-0.25Ag/Pd(111) with an adsorption energy of -1.40 eV. When
the adsorption energies of 1,3-butadiene on Pd-0.25Ag/Pd(111), Pd(111)-0.25C (-1.11
eV) and Pd(111) (-1.64 eV) surfaces are compared, it can be found that the adsorption
on Pd(111) is the strongest. This can be understood from the down-shift of the d-band
of the surface Pd atoms away from the Fermi level.4,54 The hydrogenation barriers of
C4H4 and C4H5 are listed in Table 1, and the corresponding structures are shown in
Figure 2. Only the hydrogenation barriers of C1 and C2 in C4H4 were calculated, due
to the symmetric adsorption geometry of C4H4 on Pd-0.25Ag/Pd(111) surface. As
found on the other Pd surfaces studied, 1,3-butadiene is readily formed from the
hydrogenation of C4H4 and C4H5 on Pd-0.25Ag/Pd(111).
The reaction barriers and the C-C and C-H distances at the transition states of
corresponding coupling and hydrogenation reactions, respectively, on Pd0.25Ag/Pd(111) surface are also listed in Table 2. It is found that the C-C and C-H
distances on Pd-0.25Ag/Pd(111) are similar to those found on Pd(111). The coupling
barriers of C2H2+C2H2, C2H2+C2H3 and C2H3+C2H3 decrease, following the order of
C2H2+C2H2>C2H2+C2H3>C2H3+C2H3. With these calculated reaction barriers, the
energy profiles of the three pathways shown in Scheme 1 from the adsorption state of
10
2C2H2+2H to the adsorption state of C4H6 on Pd-0.25Ag/Pd(111) surface were
obtained and are shown in Figure 5. From these energy profiles, we can determine
that the effective barriers of pathways I, II and III to be 1.13 eV, 1.23 eV and 1.30 eV,
respectively. Therefore, pathway I should be relatively favoured which is also found
on unmodified Pd(111).
3.5 General discussion
It is worth discussing the deactivation by green oil formation with respect to the
ethylene formation. To this end, the effective barriers for the formation of 1,3butadiene must be compared with those for the formation of ethylene from the
hydrogenation of acetylene. The effective barriers of ethylene formation on Pd(111),
Pd(211) and Pd(111)-0.25C can be obtained from our previous work.4 For the
hydrogenation of acetylene on Pd-0.25Ag/Pd(111), the adsorption energy of C2H2 is
found to be -1.96 eV whilst the hydrogenation barriers of C2H2 and C2H3 are 0.98 eV
and 0.82 eV, respectively. Therefore, from the energy profile of acetylene
hydrogenation on Pd-0.25Ag/Pd(111) shown in Figure S1, the effective barrier of
ethylene formation is estimated to be 1.14 eV.
The effective barriers of ethylene and 1,3-butadien formation on Pd(111),
Pd(211), Pd(111)-0.25C and Pd-0.25Ag/Pd(111) surfaces are shown in Figure 6. In
the presence of subsurface carbon on Pd(111), the effective barrier of 1,3-butadiene
formation slightly increases whilst the effective barrier of ethylene formation slightly
decreases indicating that the presence of subsurface carbon will, to some extent,
inhibit the formation of 1,3-butadiene and improve the selectivity to ethylene. On
alloying Pd with Ag, both the effective barriers of ethylene and 1,3-butadiene
formation are found to be slightly increased, indicating that both reaction channels are
hindered, leading to a lower active catalyst, which is in accordance with the
experimental results reported previously.24 When the effective barrier difference are
compared for the same type of reactions, we find that the effective barrier of ethylene
formation is increased by 0.07 eV whilst that of 1,3-butadiene formation is increased
by 0.16 eV upon Ag doping, indicating that the presence of Ag atom on Pd(111) will
to a larger extent reduce the reaction rate of 1,3-butadiene formation. Furthermore, the
adsorption of 1,3-butadiene on Pd(111)-0.25C and Pd-0.25Ag/Pd(111) surfaces are
much weaker than that on Pd(111), which will further prevent the formation of green
oil by reducing the coverage of 1,3-butadiene and thus decreasing the possibility of
11
coupling of the 1,3-butadien. These results are found to be consistent with the
experimental results reported before,24 where it was reported that the deactivation of
Pd catalyst was significantly retarded when Ag was added.
4. Conclusions
This paper provides a first mechanistic understanding on the long standing
problem of green oil formation in the petrochemical industry from first principles
calculations. In summary, we have investigated the formation mechanism of 1,3butadiene in the selective hydrogenation of acetylene on both the flat Pd(111) and
stepped Pd(211) surfaces using DFT calculations in this work. Among many possible
pathways, we have found that C4H6 can readily form through the pathway of
C2H2+C2H2+2H → C4H4+2H → C4H5+H → C4H6 on Pd(111) surface while the
favourite pathway on Pd(211) surface is the one of C2H2+C2H2+2H → C2H2+C2H3+H
→ C2H3+C2H3→ C4H6. Furthermore, we have also investigated the formation of 1,3butadiene at different sites by comparing the effective barriers. It has been found that
the favourable site for C4H6 formation is the site on the flat surface.
The formation of 1,3-butadiene on Pd-C and Pd-Ag surfaces, i.e. Pd(111)-0.25C
and Pd-0.25Ag/Pd(111), have also been studied in order to gain an insight into the
influence of subsurface carbon and Ag alloying on Pd. We have found that the
presence of subsurface carbon on Pd(111) will to some extent prevent the formation
of 1,3-butadiene from happening and the formation rate of 1,3-butadiene on Pd0.25Ag/Pd(111) surface is slightly lower than that on Pd(111), which is consistent
with the experimental results.
Acknowledgements
This work is financially supported by EPSRC and Johnson Matthey through the
CASTech programme. The authors would like to thank The Queen’s University of
Belfast for computing time. B. Y. also acknowledges the financial support of the
Dorothy Hodgkin Postgraduate Award (DHPA) studentship jointly funded by EPSRC
and Johnson Matthey.
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12
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17
Table 1 Reaction barriers (Ea) of hydrogenation reactions of C4H4 and C4H5 on
Pd(111), Pd(211), Pd(111)-0.25C and Pd-0.25Ag/Pd(111) surfaces. The definitions of
C1, C2, C3 and C4 are shown in the Figures 1, 3, 4 and 5. The unit is eV.
C4H4
C4H5
Pd(111)
Pd(211)
Pd(111)-0.25C
Pd-0.25Ag/Pd(111)
C1
0.67
0.56
0.51
0.78
C2
0.91
0.90
1.00
1.60
C3
/
0.79
0.78
/
C4
/
0.60
0.53
/
C1
1.04
0.91
0.75
1.13
C2
1.24
1.34
1.22
1.35
C3
1.13
1.15
1.01
1.30
C4
0.89
0.76
0.55
0.79
18
Table 2 Reaction barriers (Ea) and the C-C (coupling reactions) and C-H
(hydrogenation reactions) distances at the transition states on Pd(111), Pd(211),
Pd(111)-0.25C and Pd-0.25Ag/Pd(111) surfaces, according to Scheme 1.
Pathway I
TS1
TS2
TS3
C2H2+C2H2
C4H4+H
C4H5+H
Ea/eV
DC-C/Ǻ
Ea/eV
DC-H/ Ǻ
Ea/eV
DC-H/ Ǻ
Pd(111)
0.97
1.98
0.67
1.85
0.89
1.82
Pd(211)
1.86
1.97
0.56
1.84
0.76
1.78
Pd(111)-0.25C
1.10
1.97
0.51
1.92
0.55
1.85
Pd-0.25Ag /Pd(111)
1.10
1.96
0.78
1.83
0.79
1.83
Pathway II
C2H2+H
C2H2+C2H3
C4H5+H
Ea/eV
DC-H/Ǻ
Ea/eV
DC-C/Ǻ
Ea/eV
DC-H/Ǻ
Pd(111)
0.96
1.67
0.81
2.05
0.89
1.82
Pd(211)
0.97
1.64
1.59
2.04
0.76
1.78
Pd(111)-0.25C
1.00
1.66
0.75
2.09
0.55
1.85
Pd-0.25Ag /Pd(111)
0.98
1.66
0.91
2.05
0.79
1.83
Pathway III
C2H2+H
C2H2+H
C2H3+C2H3
Ea/eV
DC-H/Ǻ
Ea/eV
DC-H/Ǻ
Ea/eV
DC-C/Ǻ
Pd(111)
0.96
1.67
0.96
1.67
0.83
2.17
Pd(211)
0.97
1.64
0.97
1.64
0.54
2.12
Pd(111)-0.25C
1.00
1.66
1.00
1.66
0.26
2.33
Pd-0.25Ag /Pd(111)
0.98
1.66
0.98
1.66
0.74
2.19
19
TS3
TS2
TS1
2C2H2+2H
C4H6
1
3
2
1
4
2
Figure 1 Top: Energy profiles of the three pathways shown in Scheme 1 from
2C2H2+2H to C4H6 on Pd(111). TS1, TS2 and TS3 are the same as those in Table 1.
Bottom: Transition state structures of coupling reactions of C2H2+C2H2, C2H2+C2H3
and C2H3+C2H3 (from left to right) on Pd(111). The adsorption structures of the C4H4,
C4H5 and C4H6 produced, are also shown. Different carbon atoms are marked as 1, 2,
3 and 4. Blue, grey and white balls are Pd, C and H, respectively. This notation is
used throughout the paper.
20
Pd(111)
Pd(211)
Pd(111)-0.25C
Pd-0.25Ag/Pd(111)
C4H4+H
C4H5+H
Figure 2 Transition state structures of the favourite hydrogenation of C4H4 and C4H5
on the Pd(111), Pd(211), Pd(111)-0.25C and Pd-0.25Ag/Pd(111) surfaces. The light
blue atoms are silver atoms and this notation is used throughout the paper.
21
TS1
TS2
TS3
2C2H2+2H
C4H6
4
2
4
2
1
3
3
1
Figure 3 Top: Energy profiles of the three pathways shown in Scheme 1 from
2C2H2+2H to C4H6 on Pd(211). TS1, TS2 and TS3 are the same as those in Table 1.
Bottom: Transition state structures of coupling reactions of C2H2+C2H2, C2H2+C2H3
and C2H3+C2H3 (from left to right) on Pd(211). The adsorption structures of the C4H4,
C4H5 and C4H6 produced, are also shown. Different carbon atoms are marked as 1, 2,
3 and 4.
22
TS1
TS2
TS3
2C2H2+2H
C4H6
1
4
2
4
3
2
3
1
Figure 4 Top: Energy profiles of the three pathways shown in Scheme 1 from the
adsorption state of 2C2H2+2H to the adsorption state of C4H6 on Pd(111)-0.25C
surface. Bottom: Transition state structures (first column) of the coupling reactions of
C2H2+C2H2, C2H2+C2H3 and C2H3+C2H3 (from left to right) on Pd(111)-0.25C
surface and the adsorption structures (second row) of the C4H4, C4H5 and C4H6
produced. Different carbon atoms are marked as 1, 2, 3 and 4.
23
TS3
TS2
TS1
2C2H2+2H
C4H6
4
4
3
3
2
1
2
1
Figure 5 Top: Energy profiles of the three pathways shown in Scheme 1 from the
adsorption state of 2C2H2+2H to the adsorption state of C4H6 on Pd-0.25Ag/Pd(111)
surface. Bottom: Transition state structures (first column) of the coupling reactions of
C2H2+C2H2, C2H2+C2H3 and C2H3+C2H3 (from left to right) on Pd-0.25Ag/Pd(111)
surface and the adsorption structures (second column) of the C4H4, C4H5 and C4H6
produced. Each of the different carbon atoms are marked as 1, 2, 3 and 4.
24
Figure 6 Comparison of the effective barriers of the formation of ethylene and 1,3butadiene on Pd(111), Pd(211), Pd(111)-0.25C and Pd-0.25Ag/Pd(111) surfaces.
25