Interactions of Plasma Species on Nickel Catalyst Surfaces as a Function of
Temperature: a Reactive Molecular Dynamics Study
W. Somers, A. Bogaerts, S. Huygh, E.C. Neyts
Research group PLASMANT, Department of Chemistry, University of Antwerp, Antwerp, Belgium
Abstract: In this contribution, we present a reactive molecular dynamics study on the
interactions of CHx (x={1,2,3}) plasma species on different nickel catalyst surfaces, with
the main focus on the H2 formation after impact. The simulations cover a temperature range
of 400 – 1600 K, and substantial H2 formation is observed at temperatures of 1400 K.
Furthermore, the influence of the different surfaces on the H2 formation is highlighted.
Keywords: Molecular Dynamics simulation, Methane, H2 formation, Temperature effect.
1.
Introduction
Among the possible clean energy sources, a
considerable amount of attention goes towards the use of
H2, which is typically formed by Ni-catalyzed reforming
of methane. In order to warrant the economic viability of
the steam methane reforming (SMR), the efficiency of
this process needs to be improved. As an effective
approach to improve the SMR process, plasma technology
has gained a growing interest, since the highly reactive
plasma can lead to enhanced dissociation of methane.
Furthermore, the combination of plasma technology with
catalysts, referred to as plasma-catalysis, can lead to
additional beneficial synergistic effects.
As described by Chen et al., plasma catalysis can
enhance the energy efficiency and selectivity by both
effects of the plasma on the catalysts, and effects of the
catalyst on the plasma [1]. However, this combination of
effect leads to numerous complex mechanisms that need
to be understood to make a breakthrough in
environmental applications. In the extension of
experimental research, simulation techniques can provide
insight in the fundamental mechanisms of this technique.
In this work, we investigate the interactions of CH x
(x={1,2,3}) plasma species on six different nickel catalyst
surfaces in a temperature range of 400 – 1600 K by
performing single and consecutive impacts with reactive
molecular dynamics (MD) simulations. The main focus is
to highlight to what extent the H2 formation is influenced
by the temperature and type of nickel surface. These
simulations were carried out with the Reactive Force
Field (ReaxFF) potential [2]. The selected temperatures
are typical for dielectric barrier discharges (400 K) and
warm plasmas (1000 – 2000 K) [3]. The latter type of
plasmas are included because in, for instance the
translational regime of a gliding arc discharge, the nonequilibrium properties of low-temperature plasmas are
combined with an increased gas temperature. This leads
to an increase in the number of excited species, which
results in an enhanced dissociation. Additionally, the
increased gas temperature might also improve the
reactivity of the catalyst, and thus create a synergistic
effect.
2. Computational details
2.1. Reactive Molecular Dynamics simulations
During MD simulations, the movement of all atoms in
the system is followed through time and space by
integrating the equations of motion. For this purpose, it is
essential to know the forces between the atoms, and
therefore, a suitable interatomic potential to derive these
forces is required. In this work, we used the Reactive
Force Field (ReaxFF) potential, with parameters
developed by Mueller and coworkers [2,4]. A detailed
description of the force field and the development of the
parameter set can be found elsewhere [4,5].
2.2. Nickel surfaces
In this study, six different nickel catalyst surfaces are
selected: Ni(111), which is the most stable and abundant
Ni surface, Ni(100), which has a higher surface energy,
two different step-edged Ni(111) surfaces, denoted as
sNi1 and sNi2, an amorphous surface (aNi) and a
polycrystalline surface (pNi). These last two surfaces are
included since it is essential to keep in mind that the
plasma might alter the reactivity of the catalyst by
changing the crystallinity. The addition of these two
surfaces allows us to estimate the effect of such a change
in crystallinity. The polycrystalline surface is equally
divided in a (111) and (100) structure, in order to study
the interactions at the grain boundary. The four flat
surfaces each consist of 300 nickel atoms, equally divided
over 6 layers, except for aNi, where the 300 atoms are
randomly connected so separate layers are difficult to
distinguish. Comparison of the results on the four flat
surfaces can elucidate the influence of the connectivity of
the nickel atoms on the reactivity after impact of the
radicals. Furthermore, by comparing the results of
Ni(111) and the stepped surfaces, whose atoms have the
same connectivity, the effect of small curvatures or
defects can be investigated. Finally, the two step-edged
surfaces have the same number of steps, but the steps
have a different size, in order to determine the influence
of this deviation in the surface. The structures of Ni(111),
Ni(100) and the stepped surfaces are illustrated in our
previous work [6].
2.3. Simulation method
Both non-consecutive, i.e. single impacts, and
consecutive impacts were performed. The single impacts
provide information concerning the number of C-H bonds
that are broken after adsorption, while the consecutive
impacts mimic experimental conditions, and describe the
formation of new species as a consequence of bond
breaking and recombining reactions.
Prior to each impact, the surface is equilibrated at the
desired temperature, in the range of 400 – 1600 K,
employing the Bussi thermostat [7] with a coupling
constant of 100 ps. Periodic boundary conditions are
applied in the {x,y} directions, to simulate a semi-infinite
surface. The impacting CHx radicals are added to the
system at a z position of 10 Å above the nickel surface,
while the {x,y} coordinates are randomized, just as the
initial velocity vector.
Each single impact occurs on a pristine nickel surface,
and is followed for 4 ps. After impact, the radical can be
adsorbed, decomposed, or reflected. Each impact of each
case is repeated for 500 times, to obtain statistically
reasonable results.
In the case of the consecutive impacts, each impact is
performed with the Bussi thermostat, for a time of 6.25
ps. Again, the impacting radical is adsorbed, decomposed
or reflected. In the latter case, the radical is removed from
the system. The resulting surface is subsequently used as
the input configuration for the next impacts. This
procedure is repeated 150 times for the CH3 impacts, and
250 times for the CH2 and CH impacts. The higher
number of simulated CH2 and CH impacts is due the
higher reactivity after adsorption, and therefore, more
impacts are required to obtain clear trends.
3. Results
3.1. Reactivity at 400 K
From the three impacting radicals, CH3 has the lowest
reactivity after adsorption at a temperature of 400 K, with
only in a few occasions the breaking of a C-H bond. This
bond breaking is sometimes followed by the formation of
methane through the recombination of the H-atom with an
impinging CH3 radical. Such a recombination reaction
occurs more on the (111) surfaces than on Ni(100),
indicating the influence of the nickel surface on the
reactivity. This influence is more explicit in case of the
CH2 and CH radicals, as illustrated in Fig. 1. On each of
the surfaces, a large number of CHx (x={3,4}) species is
formed after 250 consecutive CH2 impacts. The
amorphous and stepped surfaces have the most of these
species formed, through subtraction of a H-atom of an
adsorbed radical by an incoming one. On Ni(100), an
additional mechanism for the formation of CHx species is
observed, i.e. the H-atoms adsorbed on this nickel surface
also react with the incoming radicals. Obviously, on pNi,
both mechanism also occur, since it has both a (111) and
(100) structure. Remarkably, after the CH impacts, the
CHx species are formed with both mechanisms on each
surface. Besides the formation of CHx species, a low
number of H2 and C2Hx molecules is formed after the CH2
and CH impacts, with only small differences between the
surfaces (detailed figures can be found in our previous
work [6]).
Fig. 1 Average number of formed species after 250
consecutive CH2 impacts at 400 K on the different nickel
surfaces.
While the influence of steps in a surface is clearly seen
in Fig. 1, the size of the steps seems to have little effect.
This is further illustrated in Fig. 2, where the C-H bond
breaking probability of one adsorbed CH radical is
compared for the three (111) surfaces. Again, the higher
reactivity of the stepped surfaces is observed, with around
80 % of the adsorptions followed by the bond breaking.
However, the difference between both stepped surfaces is
minimal. The C-H bond breaking probability after
adsorption of one CH3 radical is also similar for both
stepped surfaces [6]. We can thus conclude that the size
of the steps only has a small influence, and for this
reason, we did not include sNi2 in our further study.
Fig. 2 C-H bond breaking probabilities after CH
adsorption on Ni(111), sNi1 and sNi2 at 400 K.
3.2. Temperature influence on the H2 formation on
Ni(111)
It is important to point out that hardly any H2
formation was observed at a temperature of 400 K after
the CHx impacts. Hence, it is essential to verify to what
extent varying process parameters, such as the
temperature, will influence the number of H2 molecules
formed. For this purpose, we performed a temperature
study on Ni(111).
Until a temperature of 1200 K, the H2 formation
remains low after the CHx impacts on Ni(111), as shown
in Fig. 3. However, when the temperature increases
further, the formation of H2 molecules becomes more
dominant, especially after the CH2 and CH impacts. The
adsorbed CH3 is kinetically stable on Ni(111) at low
temperatures, and higher temperatures are needed to
overcome the energy barrier of 0.80 eV for the C-H bond
breaking. When the temperature rises until 1200 K, the
CH4 formation increases significantly, induced by the
increased C-H bond breaking, since the barrier is
overcome. However, the recombination of two H-atoms
to H2 still requires too much energy. At 1400 K, the H 2
formation is initiated, and at 1600 K, almost the same
numbers of H2 and CH4 molecules are formed in the case
of CH3 impacts.
Fig. 3 Average number of formed H2 molecules after
consecutive impacts of the CHx radicals on Ni(111) as a
function of temperature.
It was discussed in the previous section that after the
impacts of CH2 and CH radicals at 400 K, a more diverse
set of species was formed compared to the CH3 radicals.
This difference in reactivity is maintained at higher
temperatures, where again a variety of CHx species is
formed. More importantly, at 1400 K, there is a
considerable number of H2 molecules formed (see Fig. 3).
Further increase of temperature even makes the H2
formation the most dominant reaction; over the formation
of the CHx species (see detailed figures in our previous
work [8]). Up to 4%, 16% and 12% of the CH3, CH2 and
CH impacts, respectively, are followed by H2 formation.
At such high temperatures, the surface-bound H-atoms
have enough energy to recombine into H2, while at lower
temperatures the H-atoms are more likely to react with the
impacting radicals. Calculations with the nudged elastic
band (NEB) method confirm the high energy barrier for
the recombination of 2 H-atoms into H2, namely 1.66 eV.
This is higher than the calculated barrier for H-diffusion
over the nickel surface, which we calculated as 0.35 eV.
Thus, at lower temperatures the H-atoms can already
diffuse over the surface, but recombining into H 2 requires
too much energy.
A cautionary remark is that in direct correlation with
the increasing dehydrogenation at elevated temperatures,
an increased surface-to-subsurface C-diffusivity is
observed as well. Up to temperatures of 1000 K, C-atoms
only diffuse to the first subsurface layer. However, at
higher temperatures, more of the adsorbed CHx radicals
are dehydrogenated, and consequently, more C-atoms
diffuse to lower lying Ni-layers (see a detailed figure in
our previous work [8]). The diffusion from the first
subsurface layer to a lower lying Ni-layer requires energy
in the order of 2.76 eV, as we calculated with the NEB
method. This is significantly higher than the 1.16 eV that
is required to diffuse from a µ3 hcp site to an octahedral
site in the first subsurface layer. Thus, as mentioned
above, high temperatures are required to overcome the
energy barrier of diffusion to lower subsurface layers.
Overall, the results imply that an optimal temperature
for an efficient H2 formation on Ni(111) lies around 1400
K. At higher temperatures, the diffusion of C-atoms into
the nickel surface becomes overly dominant, while at
lower temperatures the H2 formation is too limited. Under
experimental conditions, a temperature of 1400 K may
lead to sintering of the catalyst. This increases the size of
the nickel particles, and decreases the activity due to the
reduced catalytic surface area. However, it should be
noted that in our simulations, sintering and thus this loss
of catalytic surface area is not included, since we study
the interactions on a semi-infinite surface. It is indeed
correct that the total catalytic surface area might be
reduced due to the high temperature, but we simulated the
H2 formation processes on surfaces which are already
highly localized small-areas.
3.3. Temperature influence on the H2 formation on
the other surfaces
The results of the CHx impacts at 400 K illustrated the
influence of the surface on the reactivity. Now that we
have verified the dependence of the H2 formation on
temperature, the next question is: can the type of nickel
surface induce additional H2 formation at high
temperatures? The answer is seen in Fig. 4, where the
number of formed H2 molecules after 150 consecutive
CH3 impacts is clearly dependent on the surface. While
there is no difference at 800 K, i.e. hardly any H2
molecules are formed on each surface, the stepped surface
shows higher reactivity at 1200 K and 1600 K. The
amorphous and polycrystalline surface show a similar
influence at 1600 K as Ni(111), while Ni(100) has the
lowest selectivity towards H2. The H2 formation after CH3
impacts is determined by two factors: the C-H bond
breaking probability after adsorption of the CH3 radical
and the diffusivity of surface-bound H-atoms into the
nickel surface. Let us first consider the C-H bond
breaking probability after adsorption of the radical. At a
temperature of 1600 K, for which the difference between
the surfaces is the greatest, the C-H bond breaking
probability is the highest for Ni(100). This is followed by
sNi1 and finally the three other surfaces, which have
similar bond breaking probabilities.
Fig. 4 Average number of formed H2 molecules after 150
consecutive CH3 impacts on the different nickel surfaces
as a function of temperature.
Despite of the highest C-H bond breaking probability
after the adsorption of CH3 on Ni(100), this surface has
the lowest selectivity towards H2. This is explained when
taking the diffusivity of surface-bound H-atoms into
account. This value is also the highest for Ni(100), while
the other surfaces have similar values. The H-atoms on
the Ni(100) surface tend to diffuse into the surface,
instead of recombining to H2. Since the Ni(111) and sNi1
have a similar H-diffusivity, the difference in the H2
formation is mainly determined by the C-H bond breaking
probability. As mentioned above, this is higher for sNi1,
where most of the C-H bonds are broken near the stepedges. This results in more H-atoms on the surface, which
are available to form H2. Both the C-H bond breaking
probability as the H-diffusivity are similar for Ni(111),
aNi and pNi, hence the small difference in formed H2
molecules.
Remarkably, the results of the CH2 and CH impacts do
not resemble that of CH3. After the CH2 impacts, Ni(111)
has the highest selectivity towards H2, although the
differences between the surfaces are small. Furthermore,
after the CH impacts, equal numbers of H2 molecules are
formed on Ni(111) and Ni(100). For these impacts, sNi1
has the lowest H2 selectivity. This illustrates that the H2
formation is not only dependent on the temperature and
the surface, but also on the type of adsorbed species.
Therefore, in future work, we will investigate this aspect.
4.
Conclusions
The interactions of CHx species on different nickel
surfaces were investigated as a function of temperature
with reactive molecular dynamics (MD) simulations, with
a specific emphasis on H2 formation.
First, it is shown that, even at low temperatures, the
type of nickel surface influences the reactivity after
adsorption of the radical. The presence of steps in the
surface increases the reactivity, due to a higher number of
broken C-H bonds. However, the size of the steps hardly
affects the reactivity.
Second, H2 formation can occur on nickel surfaces, if
the temperature is sufficiently high. On Ni(111),
substantial H2 formation is obtained at temperatures of
1400 K and above. At 1600 K, the H2 formation is even
the most dominant reaction after CH2 and CH impacts.
However, such high temperatures also induce the
diffusion of C-atoms into the subsurface region of the
catalyst. Considering this effect, an optimal temperature
for H2 formation on Ni(111) lies around 1400 K.
Finally, it is illustrated that the H2 formation is also
influenced by the nickel surface and the type of adsorbed
radical. After CH3 impacts, the highest number of H2
molecules are formed on sNi1, since it has a high C-H
bond breaking probability and a low H-diffusivity.
However, after CH2 and CH impacts, other surfaces have
higher selectivities. This aspect will be investigated in
future work.
Acknowledgements
This work was carried out using the Turing HPC
infrastructure at the CalcUA core facility of the
Universiteit Antwerpen (UA), a division of the Flemish
Supercomputer Center VSC, funded by the Hercules
Foundation, the Flemish Government (department EWI)
and the UA. The authors also thank dr. Adri C.T. van
Duin of Penn State University for the use of the ReaxFF
MD code.
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