Effect of the Oxidation Degree of a Nickel Foil Surface on Its Catalytic Activity in the Reaction of Ethylene Oxidation

Article abstract of DOI:10.1134/S0023158420040023

The effect of the oxidation degree of a nickel foil surface on the rate of catalytic oxidation of ethylene was studied by a pulse method at 600 and 700°C. It was shown that a reduced metallic surface demonstrated a high activity in partial ethylene oxidation, whereas a partially oxidized surface with an oxidation degree of ~24 formal O₂ monolayers, in the total oxidation of C₂H₄. A SEM investigation has revealed that the oxidized surface was partially coated with nickel oxide nanocrystals. A further increase in the surface oxidation degree led to a continuous coverage of the Ni surface with oxide crystals and a dramatic decrease of catalytic activity. In addition, a low maximum of total ethylene oxidation was observed at 700°C in the range of surface oxidation degree of 95–135 O₂ monolayers.

Full text of DOI:10.1134/S0023158420040023

ISSN 0023-1584, Kinetics and Catalysis, 2020, Vol. 61, No. 4, pp. 631–636. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Kinetika i Kataliz, 2020, Vol. 61, No. 4, pp. 561–567.
Effect of the Oxidation Degree of a Nickel Foil Surface on Its
Catalytic Activity in the Reaction of Ethylene Oxidation
a,
a
a
a
V. Yu. Bychkov *, Yu. P. Tulenin , A. Ya. Gorenberg , and V. N. Korchak
a
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 119991 Russia
е-mail: bychkov@chph.ras.ru
Received December 24, 2019; revised January 28, 2020; accepted March 11, 2020
*
Abstract—The effect of the oxidation degree of a nickel foil surface on the rate of catalytic oxidation of eth-
ylene was studied by a pulse method at 600 and 700°C. It was shown that a reduced metallic surface demon-
strated a high activity in partial ethylene oxidation, whereas a partially oxidized surface with an oxidation
degree of ~24 formal O monolayers, in the total oxidation of C H . A SEM investigation has revealed that
2
2
4
the oxidized surface was partially coated with nickel oxide nanocrystals. A further increase in the surface oxi-
dation degree led to a continuous coverage of the Ni surface with oxide crystals and a dramatic decrease of
catalytic activity. In addition, a low maximum of total ethylene oxidation was observed at 700°C in the range
of surface oxidation degree of 95–135 O monolayers.
2
Keywords: nickel catalysts, ethylene oxidation
DOI: 10.1134/S0023158420040023
INTRODUCTION
rate of methane oxidation on nickel is the acceleration
of this reaction on the surface of NiO, which contains
cationic or anionic vacancies, i.e., is partially reduced
or oxidized.
Nickel-based catalysts are widely used in applied
and basic research. It is well known that the catalytic
activity of nickel depends on its oxidation state. In
reducing media, nickel occurs in the form of a metal.
In this state, it catalyzes a water-gas shift reaction [1],
partial oxidation of hydrocarbons to synthesis gas [2,
The effect of the partial oxidation of metallic nickel
on its catalytic activity was studied. Seo [19] noted that
the carbon dioxide conversion of methane was cata-
lyzed by metallic nickel, and its oxidation led to cata-
lyst deactivation. Mutz et al. [20] monitored the state
3
], carbon dioxide conversion of hydrocarbons [4–6],
carbon dioxide methanation [7], etc. In oxidizing
media, nickel is usually present in the form of nickel
of Ni in the reaction of CO methanation using
2
oxide NiO, which catalyzes mainly the deep oxidation operando XAS and EXAFS methods. The oxidation
of hydrocarbons [8].
state of Ni was determined as a linear combination of
the completely oxidized and reduced states (at 400°C).
They found that the partial oxidation of Ni deactivated
the catalyst, and they did not try to construct the
dependence of catalytic activity on the degree of oxi-
dation. At the same time, the data reported indicate
that, at a ~50% degree of nickel oxidation, a ~50%
decrease in the catalytic activity was observed in com-
parison with a completely reduced sample. Bi et al.
Modern studies suggest that the dependence of the
catalytic activity of Ni on the degree of oxidation is not
limited to data on the activity of metallic Ni and NiO.
A study of the self-oscillations of the oxidation rates of
lower alkanes [9–13], alkenes [14], and CO [15] on Ni
and attempts to mathematically simulate these self-
oscillations [16–18] indicated that their occurrence
cannot be explained without taking into account a
more detailed dependence of the rates of heteroge-
neous reactions (that are responsible for the mecha-
nism of hydrocarbon oxidation) on the oxidation state
of the nickel surface. For example, Gladkii et al. [13]
believed that a reason for the self-oscillations of the
[
21] studied the state of Ni in the above reaction using
AP-XPS and in situ DRIFTS. They found that both
reduced and oxidized nickel was present under the
conditions of catalysis and, in contrast to Mutz et al.
[
20], concluded that oxidized nickel did not inhibit the
reaction but, on the contrary, accelerated it because
Abbreviations: XAS, X-ray absorption spectroscopy; EXAFS, the activation of CO proceeded on oxidized nickel.
2
extended X-ray absorption fine structure; AP-XPS, ambient
Heine et al. [22] used XPS to demonstrate that the oxi-
pressure X-ray photoelectron spectroscopy; DRIFTS, diffuse
dation state of Ni significantly changed depending on
reflectance infrared Fourier transform spectroscopy; SEM,
the composition of a mixture and the temperature:
scanning electron microscopy; TPR-H , temperature-pro-
2
grammed reduction with hydrogen.
CO caused the oxidation of Ni to NiO on the surface,
2
631

6
32
BYCHKOV et al.
Table 1. Degree of oxidation of Ni foil in the course of pulse oxidation at 600 and 700°C
Temperature, °C Absorption of O , μmol “Monolayer” absorption of O at 100°С, μmol Number of “monolayers”
2
2
6
7
00
00
1.0
2.1
80
170
0.0125
and H led to the reduction of NiO. Unfortunately, an mass spectrometer (Pfeiffer, Germany). The mass
2
attempt at constructing the dependence of the cata- spectrometer continuously recorded signals with
lytic activity of nickel on its oxidation state has not m/z = 2 (Н ), 18 (Н О), 27 (С Н ), 28 (СО, СО , and
2 2 2 4 2
been made in the above publications.
С Н ), 32 (О ), 40 (Ar), and 44 (CO ). To determine
2 4 2 2
It is well known that the use of pulsed methods the concentration of CO in the products, the values
helps to distinguish between the catalytic activities of corresponding to the contributions of СО and С Н
2 2 4
pre-reduced or oxidized catalysts. The interaction of were subtracted from the signal with m/z = 28. The
CH /O pulses with oxidized catalysts Ni/La O [23], concentrations of components in the reaction pulses
4
2
2
3
Ni/SiO [24], and Ni/TiO [25] was studied, and it after contact with a catalyst were calculated using the
2
2
signal from Ar as an internal standard.
was shown that the deep oxidation of methane
occurred on oxidized samples, whereas partial oxida-
The monolayer surface oxidation of these metal
tion to CO and H occurred on reduced samples. samples was found using a SETSYS EVOLUTION
2
Unfortunately, the authors of the cited publications 16/18 thermobalance (Setaram, France) connected to
did not attempt to determine the catalytic activity on an OmniStar GSD 301 mass spectrometer. The above
partially oxidized/reduced samples with a measured Ni foil sample was placed in a cell of the thermobal-
oxidation state. The interaction of nickel catalysts with ance, reduced in a flow of H at 500°C, and then oxi-
2
the pulses of pure methane was studied [5, 26–32] in dized in a flow of a 1% O –He mixture at 100°C for
2
order to elucidate the role of lattice oxygen in the cat-
alytic oxidation of methane. The measurement of cat-
alytic activity depending on the degree of oxidation of
the sample was also beyond the scope of these publi-
cations.
2
0 min. Then, the oxidized sample was heated in a
flow of H (30 mL/min) at a rate of 10 K/min to
2
5
00°C, and a decrease in the weight of the sample in a
range of 150–250°C due to the reduction of nickel
oxide was determined.
To study the effect of the oxidation state of the sur-
face of Ni foil on its catalytic activity, we decided to
apply another pulse method, in which the catalytic
activity of pre-reduced nickel is tested using the pulses
of a reaction mixture and the surface of the catalysts is
stepwise oxidized with small oxygen-containing gas
pulses between these test pulses. In this case, the con-
centrations of reactants in the mixture for testing cata-
lytic activity should be significantly lower than the
The morphology of the catalysts was studied using
a JSM-7001F scanning electron microscope (JEOL,
Japan).
RESULTS AND DISCUSSION
Determination of the Range of Ni Foil Oxidation States
Achieved in the Course of Pulsed Oxidation
concentration of O in the oxidizing pulses in order to
Metallic Ni in the form of a foil was not completely
2
decrease changes in the oxidation state of the sample oxidized in a temperature range to 700°C; however,
due to the catalytic reaction. Based on the results of the depth of surface oxidation was significant and not
trial experiments, the reaction of ethylene oxidation limited to a monolayer of metal oxide. In the first
was chosen as a test reaction.
pulses of a 1% O –1% Ar–He mixture, the pulse oxy-
2
gen was almost completely consumed. Then, the rate
of oxidation gradually decreased, and the increase in
the degree of oxidation almost stopped. Table 1 sum-
EXPERIMENTAL
We studied Ni foil samples 5 × 6 × 0.1 mm in size. marizes the degrees of oxidation (the numbers of arbi-
A sample was placed in a quartz reactor with an inner trary monolayers) of the Ni-foil samples at 600 and
diameter of 6 mm and heated in a flow of He to 600– 700°C. These are the boundary values of ranges in
7
00°C. Before each particular experiment, the sample which we studied the effect of the oxidation state of the
was reduced in a flow of hydrogen (20 mL/min). Ni surface on its catalytic activity. Note that the term
Then, the flow of H was replaced by a flow of He surface oxidation state used in this work refers to the
2
(
20 mL/min) and pulses (0.5 mL) of a 0.2% C H – degree of oxidation of the nickel surface rater than to
2 4
0
.25% O –1% Ar–He test reaction mixture and a 1% the well-known oxidation state of Ni as a chemical ele-
2
ment.
Visual observation of color changes in the surface
O –1% Ar–He oxidation mixture were alternately
injected after 3 min. The time interval between pulses
was 1 min. A portion of the gas flow leaving the reactor of nickel in the course of stepwise oxidation made it
was directed to an OmniStar GSD 301 quadrupole possible to evaluate the uniformity of foil surface oxi-
2
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EFFECT OF THE OXIDATION DEGREE OF A NICKEL FOIL SURFACE
633
–3
dation along the sample in the direction of a gas flow or 10 μmol of an adsorbent based on the assumption
beside the sample and the thickness of the oxidized
layer. The initial foil, having a light gray metallic color,
became yellow, blue, gray-blue, and gray in the course
of pulse oxidation. After each pulse of the oxidizing
mixture, the surface of the entire sample was colored
uniformly in the direction of the gas flow. It is well
known that color changes observed in the surface of a
metal during its oxidation (so-called temper colors)
19
2
that the metal surface contains 1 × 10 atom/m . The
measured value (0.125 μmol of O ) was higher by an
2
order of magnitude than the calculated estimate. This
is likely due to the large roughness of the surface of our
sample, which was confirmed by the data of scanning
electron microscopy (see below).
are caused by the interference of light reflected from Changes in the Catalytic Activity of Nickel in the Course
the upper and lower sides of an oxidized surface layer
of Its Stepwise Oxidation
[
33]. It is generally accepted that first-order interfer-
Alternating pulses of the test mixture of 0.2%
ence colors change with increasing surface film thick-
ness from yellow (~50 nm) to brown (~60 nm) and
then blue (~70 nm). Obviously, the thickness of an
C H –0.25% O –1% Ar–He and the oxidative mix-
2
4
2
ture of 1% O –1% Ar–He were injected onto pre-
2
oxidized film was of the same order of magnitude in reduced Ni foil at reactor temperatures of 600 and
our case.
700°C. Figure 1 shows the experimental results. In
Fig. 1a, it can be seen that reduced Ni had a very high
catalytic activity: an almost complete conversion of
To assess the depth of an oxidized layer on the Ni-
foil surface, we also measured the degree of oxidation
of the test samples at a temperature of 100°C. It is gen-
erally accepted [34] that the chemisorption of gases
such as H , O , and CO on platinum group metals and
ethylene (XC2H4 = 96.8%) was observed; the main oxi-
dation products were CO and H ; the amount of CO
2
2
was much smaller than that of CO, and H O was
2
2
2
almost absent. During the first pulse of the 1% O –1%
2
nickel in a temperature range of 20–100°C leads to an
Ar–He mixture, the sample absorbed ~0.3 μmol of O
2
approximately monolayer coating of the metal surface.
(
~24 O “monolayers”). The activity of the sample in
Holloway [35] found that, on the adsorption of O on
2
2
Ni(001) in a range of 20–100°C, saturation occurred this state decreased only slightly (XC H = 92.7%), but
2
4
at about a monolayer coverage. Mitchell et al. [36] the reaction selectivity changed greatly. Now, the main
reported the formation of two layers of NiO. Due to products were CO and H O, and the amount of CO
2
2
such discrepancies, we will write the “monolayer” in and H sharply decreased. After the second portion of
2
quotation marks to emphasize the approximateness of
this measurement in our conditions. The Ni-foil sam-
ple after the experiments described below was placed
in a cell of the SETSYS EVOLUTION 16/18 thermo-
oxidation, the conversion of ethylene sharply
decreased to ~12% and remained approximately at this
level even upon further oxidation of the nickel surface
(
0.5–1 μmol of O , 40–80 O “monolayers”). Only
2 2
balance, reduced in a flow of H at 500°C, cooled, and
2
carbon oxides were observed in the products. After the
then oxidized at 100°C in a flow of a 1% O –1% Ar–
2
pulsed experiment, the Ni-foil sample was oxidized
He mixture for 20 min. After that, the sample was with a flow of the 1% O –1% Ar–He mixture for
2
cooled in a vacuum to 30°C and heated in a flow of H
2
5 min; then, the catalytic activity was measured once
at a rate of 10 K/min to 500°C (TPR-H ). In the again in the reaction with a pulse of the 0.2% C H –
2
2
4
course of TPR-H , a stepwise decrease in the sample 0.25% O –1% Ar–He mixture. These results are
2
2
weight was observed, which was accompanied by the shown in Fig. 1a to the right after a gap on the abscissa.
release of H O. It was possible to determine the oxygen It can be seen that the activity of the Ni foil after pro-
2
content of the Ni sample based on the weight change. longed oxidation in a flow of the oxygen-containing
In the test sample of Ni, the amount of oxygen upon mixture at 600°C was as low as at the end of the pulse
“
monolayer” surface oxidation was 0.0125 μmol of O , experiment.
2
which is much smaller than the amount of O totally
2
A similar behavior was observed in a similar exper-
absorbed in the course of pulse oxidation (see Table 1).
This means that, under the test conditions of stepwise
oxidation, the measured degrees of surface oxidation
indicated the formation of multiple atomic layers of
nickel oxide. Table 1 also summarizes the numbers of
iment, which was performed at a temperature of 700°C
(
Fig. 1b): high catalytic activity of pre-reduced Ni with
the predominance of partial oxidation, high activity of
partially oxidized Ni (~0.3 μmol of O , ~24 O
2
2
“
monolayers”) with the predominance of deep oxida-
“
monolayers” calculated for temperatures of 600 and
tion, and relatively low activity (XC2H4 = ~10%) in the
oxidation of Ni with more than 0.8 μmol of O . In the
7
00°C.
2
It is also reasonable to compare the measured value absorption range of 0.8–2.1 μmol of O (65–170 O
2 2
of “monolayer” oxidation with the calculated value of “monolayers”), where the sample exhibited relatively
atomic coverage expected for the specified geometry of a low activity in ethylene oxidation, one can see a
2
Ni-foil sample. For a total foil surface area of ~60 mm , slightly pronounced extended maximum of CO
for-
2
14
the surface monolayer should contain 6 × 10 atoms mation in a range of 1.2–1.7 μmol of O
(95–135 O
2
2
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

6
34
BYCHKOV et al.
а)
(
0.40
0.35
0.30
0.25
0.20
1
2
3
0
.15
.10
0
4
0.05
5
0
0.2
0.4
0.6
0.8
1.0
Total absorption of О , µmol
2
(b)
0.35
0.30
0.25
0.20
3
0
.15
.10 4
2
0
1
0.05
5
0
0.5
1.0
1.5
2.0
Total absorption of О , µmol
2
Fig. 1. Concentrations of (1) Н , (2) СО, (3) СО , (4) Н О, and (5) С Н after the interaction of the pulses of a mixture of 0.2%
2
2
2
2
4
C H –0.25% O –1% Ar–He with a Ni foil at (a) 600 and (b) 700°C depending on the total absorption of O in oxidizing pulses.
2
4
2
2
“
monolayers”). The catalytic activity was minimal at almost completely covered by NiO crystals. The size of
the end of pulse oxidation, and it further decreased oxide crystals was 50–100 nm, which is in good agree-
after prolonged oxidation in a flow of the 1% O –1% ment with the estimated thickness of the oxide layer
2
Ar–He mixture for 5 min (see individual points at the based on its color due to reflected light interference
end of the abscissa axis in Fig. 1b).
(see above).
The set of pulsed kinetic data and SEM results
allowed us to understand a rather complex oxidation
behavior of nickel surface under conditions of the cat-
Microscopic Examination of the Surface of Ni
Changes in the surface morphology of Ni foil in the alytic oxidation of hydrocarbons at 600–700°C. In a
course of its pulsed oxidation were studied using scan- reaction of the first oxidation pulse with reduced Ni
ning electron microscopy. Figure 2a shows the surface foil, the rate of oxygen absorption was so high that the
of a fresh Ni-foil sample after preliminary reduction amount of absorbed oxygen was several times higher
with hydrogen at 700°C and cooling to 20°C. Samples than the amount of oxygen absorbed by the same sam-
in Figs. 2b and 2c were also subjected to preliminary ple at 100°C under conditions of surface saturation
reduction and contact with one or ten pulses of a mix- with oxygen from a flow of the oxygen-containing
ture of 1% O –1% Ar–He, respectively, at 700°C. The mixture (monolayer coating). The formally achieved
2
crystallites of metallic Ni with a relatively smooth sur- degree of surface oxidation corresponded to 24
face are visible in Fig. 2a. In the sample shown in “monolayers” of O , but the sample surface was actu-
2
Fig. 2b, a significant part of this surface is covered with ally a complex structure containing nickel oxide nano-
nickel oxide crystals. The sample surface in Fig. 2c was crystals. At the above degree of oxidation, these crys-
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2020

EFFECT OF THE OXIDATION DEGREE OF A NICKEL FOIL SURFACE
tals did not form a regular layer on the surface of
635
metallic nickel (see Fig. 2b). That is, the uniform tem-
per colors of a metal surface do not indicate the uni-
formity of the resulting oxidized layer at the micro-
scopic level. In general, this “partially oxidized” sur-
face state can be described as a combination of areas
with a ~100-nm thick nickel oxide layer (average oxide
crystal size according to SEM data) and areas that do
not contain such oxide crystals but are, probably,
coated with a real monolayer of chemisorbed oxygen.
The subsequent pulse oxidation at 600–700°C led to
the formation of additional oxide crystals, which did
not increase in size (see Fig. 2c), but more fully occu-
pied the surface of metal particles. The full coating
actually stopped the metal oxidation process under the
conditions of our experiment. Thus, in the test tem-
perature range, a change in the oxidation state of the
nickel surface mainly means a change in the fraction of
the surface coated with a layer of nickel oxide nano-
crystals.
100 nm
(а)
The results of testing the catalytic activity suggest
that nickel foil is highly active in the oxidation of eth-
ylene not only in a reduced state, which is well known,
but also in a partially oxidized state, the oxidation
degree of which formally corresponds to the formation
of 24 “monolayers” of O . The following partial oxida-
2
tion reactions of ethylene proceed on a completely
reduced surface:
С Н + О = 2СО + 2Н ,
(I)
2
4
2
2
100 nm
(b)
С Н + 2О = 2СО + 2Н .
(II)
2
4
2
2
2
On a partially oxidized surface (~24 formal
“
monolayers” of O ), the following deep oxidation
2
reaction is added:
С Н + 3О = 2СО + 2Н О.
(III)
2
4
2
2
2
The SEM data showed that, in this state, a part of
the surface corresponds in appearance to the initial
large crystallites of metallic nickel. Under the condi-
tions of catalysis, this part of the surface can be (1) a
reduced metal surface or (2) a metal surface coated
with chemisorbed oxygen. If the first version takes
place under the conditions of a catalytic reaction, a
cooperative effect of metallic Ni and NiО can occur in
the oxidation of ethylene, similarly to that described
for the reaction of CO methanation [21, 22]. In this
2
100 nm
(
c)
case, the catalytic reaction may proceed by the sepa-
rate chemisorption of ethylene on metallic Ni and
oxygen on Ni oxide. If the second version takes place
under the conditions of catalysis, the high activity of
the sample can be caused by the catalytic properties of
the free metal surface with adsorbed oxygen, and the
participation of the formed NiO crystals in catalysis is
insignificant.
Fig. 2. SEM images of Ni foil surfaces (a) in a reduced
state and after oxidation with (b) 1 and (c) 10 pulses of a
1
% O –1% Ar–He mixture at 700°C.
2
It is likely that, in the course of the formation of NiO
crystals on interaction with gaseous oxygen, the sur-
The foil became inactive when its entire surface was face of the crystals passed through states containing
coated with nickel oxide nanocrystals. At the same structural defects. These defects can have increased
time, there was a small maximum of catalytic activity catalytic activity in the oxidation of hydrocarbons.
in a range of 95–135 formal O “monolayers” (Fig. 1b). Further oxidation, in particular, with a flow of oxy-
2
KINETICS AND CATALYSIS Vol. 61 No. 4 2020

6
36
BYCHKOV et al.
gen-containing gas, can cause the elimination of these 14. Bychkov, V.Yu., Tulenin, Yu.P., Slinko, M.M., Lo-
monosov, V.I., and Korchak, V.N., Catal. Lett., 2018,
defects and, accordingly, a decrease in the catalytic
activity. Contrary to expectations, we failed to detect
vol. 148, p. 3646.
experimentally a noticeable increase in the catalytic 15. Bychkov, V.Yu., Tulenin, Yu.P., Slinko, M.M., Gordi-
activity upon the “complete” oxidation of the nickel
surface (oxidation in an oxygen-containing gas flow).
Of course, we cannot exclude the occurrence of such a
enko, Yu.A., and Korchak, V.N., Catal. Lett., 2018, vol.
48, p. 653.
1
1
6. Bychkov, V.Yu., Tyulenin, Yu.P., Slinko, M.M., and
Korchak, V.N., Surf. Sci., 2009, vol. 603, p. 1680.
7. Ustyugov, V.V., Kaichev, V.V., Lashina, E.A., Chuma-
kova, N.A., and Bukhtiyarov, V.I., Kinet. Catal., 2016,
vol. 57, no. 1, p. 113.
“
superoxidized” state under conditions with a high
concentration of gaseous oxygen over the oxidized sur-
face.
1
1
8. Makeev, A.G., Peskov, N.V., Semendyaeva, N.L.,
Slinko, M.M., Bychkov, V.Yu., and Korchak, V.N.,
Chem. Eng. Sci., 2019, vol. 207, p. 644.
CONCLUSIONS
As a result of this study, we found quantitative rela-
tionships between the rates of catalytic oxidation of
ethylene on nickel foil at 600 and 700°C and the degree
of oxidation of the nickel surface upon its stepwise oxi-
dation. These relationships can be used, for example,
in the mathematical modeling of self-oscillations of
the rate of ethylene oxidation on nickel, which were
described previously [14].
19. Seo, H.O., Catalysts, 2018, vol. 8, p. 110.
20. Mutz, B., Gänzler, A.M., Nachtegaal, M., Müller, O.,
Frahm, R., Kleist, W., and Grunwaldt, J.-D., Catalysts,
2017, vol. 7, p. 279.
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This work was supported by the Russian Foundation for
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Translated by Valentin Makhlyarchuk
KINETICS AND CATALYSIS
Vol. 61
No. 4
2020