The dehydrogenation of isopropanol on a nickel-manganese catalyst subjected to treatment in glow-discharge oxygen, argon, and hydrogen plasmas

Article abstract of DOI:10.1134/S0036024408010068

The dependence of the activity of the (Ni 20 wt %-Mn 4 wt %)/SiO ₂ catalyst on the treatment of its surface in glow-discharge oxygen, argon, and hydrogen plasmas was studied. Catalytic experiments were performed in a flow reactor and under static conditions in a vacuum. The highest activity was observed after catalyst treatment in an argon plasma. Glow-discharge plasma treatment changed the structure and number of active centers, which resulted in a change in the reaction mechanism. Ab initio quantum-chemical calculations were performed using the Hartree-Fock (UHF) method for the Ni₅ cluster. The results substantiated the suggested that the active center contained the hydrogen atom.

Full text of DOI:10.1134/S0036024408010068

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 1, pp. 50–55. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © V.D. Yagodovskii, K.V. Bozhenko, T.V. Yagodovskaya, A.A. Trofimova, I.A. Malyshkina, E.P. Chainikov, 2008, published in Zhurnal Fizicheskoi Khimii,
2008, Vol. 82, No. 1, pp. 59–64.
CHEMICAL KINETICS
AND CATALYSIS
The Dehydrogenation of Isopropanol
on a Nickel–Manganese Catalyst Subjected to Treatment
in Glow-Discharge Oxygen, Argon, and Hydrogen Plasmas
V. D. Yagodovskiia, K. V. Bozhenkoa, T. V. Yagodovskayab, A. A. Trofimovaa,
I. A. Malyshkinaa, and E. P. Chainikova
a Peoples Friendship University, ul. Miklukho-Maklaya 6, Moscow, 117198 Russia
b Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119992 Russia
Received October 10, 2006
Abstract—The dependence of the activity of the (Ni 20 wt %–Mn 4 wt %)/SiO2 catalyst on the treatment of
its surface in glow-discharge oxygen, argon, and hydrogen plasmas was studied. Catalytic experiments were
performed in a flow reactor and under static conditions in a vacuum. The highest activity was observed after
catalyst treatment in an argon plasma. Glow-discharge plasma treatment changed the structure and number of
active centers, which resulted in a change in the reaction mechanism. Ab initio quantum-chemical calculations
were performed using the Hartree–Fock (UHF) method for the Ni5 cluster. The results substantiated the sug-
gested that the active center contained the hydrogen atom.
DOI: 10.1134/S0036024408010068
INTRODUCTION
clusions about the possible composition of the catalyti-
cally active center of this reaction.
Various kinds of plasmas brought in contact with the
surface of solids create point defects on the surface and
in the surface layer, which changes the properties of the
surface substantially [1]. Review [2] contains data that
characterize changes in the catalytic activity of metal
oxides and metals with respect to various reactions
after catalyst treatments in glow-discharge oxygen,
argon, hydrogen, and water plasmas.
Changes in the catalytic parameters of iridium, cop-
per–iridium, copper, and copper–rhenium deposited
catalysts for the oxidation of CO, dehydrocyclization of
n-hexane, and dehydrogenation of isopropanol were
observed in [3, 4] for catalysts treated in a glow-dis-
charge oxygen plasma. It was shown in [5] that the
activity of massive nickel in the dehydrogenation of
isopropanol increased to a substantially greater extent
after treatment in a glow-discharge oxygen plasma than
after treatment in a high-frequency discharge hydrogen
plasma. A similar result was obtained in [6] for the
same reaction and a nickel–rhenium catalyst deposited
on sibunite.
EXPERIMENTAL
We used Silochrom S-120 with a specific surface
area of 80 m2/g as a carrier. A weighed carrier sample
was impregnated with a solution of nickel and manga-
nese chlorides taken in amounts necessary to obtain
20 wt % nickel + 4 wt % manganese catalysts. The sam-
ples were then dried at 313 K and subjected to step
reduction in a flow of hydrogen. The temperature con-
ditions were 373 K for 30 min, 473 K for 30 min, 573 K
for 30 min, and 673 K for 30 min. Catalyst treatment in
glow-discharge oxygen, argon, and hydrogen plasmas
was performed in a flow electric-discharge alternating
current (50 Hz) unit [2]. The flow regime ensured the
removal of gaseous products and their condensation in
a low-temperature trap cooled with liquid nitrogen.
Before switching a discharge on, the samples were
evacuated to a pressure of 10–4 torr. The discharge cur-
rent was 200 mA, voltage on electrodes 1.8 kV, pres-
sure 0.5–1 torr, discharge burning time 15 min, and
temperature 433 K. After treatment, the samples were
cooled to room temperature under evacuation.
In this work, we studied the influence of the treat-
ment of a nickel–manganese catalyst deposited on sil-
ica gel with glow-discharge oxygen, argon, and hydro-
gen plasmas on its activity in the dehydrogenation of
isopropanol.
Catalyst samples, initial and subjected to glow-dis-
charge plasma treatment, were studied using a flow reac-
The purpose of this work was to estimate the role tor with a chromatographic analysis (an LKhM-8MD
played by the plasma type in changes in the activity of chromatograph) of reaction products. A catalyst sample
the catalyst and its influence on the mechanism of the (30 mg) was placed on a Pyrex glass filter in a quartz
reaction. Experimental studies were augmented by ab reactor (diameter 12 mm and height 10 mm). Isopro-
initio quantum-chemical calculations for drawing con- panol of kh. ch. (chemically pure) grade was supplied
50

THE DEHYDROGENATION OF ISOPROPANOL
51
from a saturator with a flow of helium. The products
n‡c, mmol/(g h)
were analyzed on a column with 5% polyethylene gly-
col on diatomite (1 m × 3 mm). A series of experiments
was performed with the same sample at different tem-
peratures. Product yields are given in mmol per g of
deposited metal in one h (mmol/(g h)). The mean rela-
tive errors in alcohol and acetone determinations were
3 and 7%, respectively.
500
2
400
300
Prior to measurements, the samples were reduced
with hydrogen at 623 K for 30 min.
The kinetics of dehydrogenation of isopropanol was
also studied under static conditions in a vacuum unit by
measuring the accumulation of hydrogen with time,
because, under the conditions of our experiments, the
contribution of the side reaction of alcohol cracking
with the formation of propane was insignificant. The
initial sample or the sample after plasma treatment was
placed into a quartz reactor, evacuated to a residual
pressure of 10–6 torr, and reduced with hydrogen
(0.1 torr) at 623 K for 40 min. The procedure for kinetic
measurements was described in [5].
3
200
100
RESULTS AND DISCUSSION
1
Experiments in a Flow Reactor
Studies were performed over the temperature range
363–588 K. The reaction products were acetone, hydro-
gen, and a small amount of propane. Water was not
detected, which was in agreement with the data
obtained for nickel–rhenium and nickel catalysts
deposited on sibunite [6]. The temperature depen-
dences of the yield of acetone are shown in Fig. 1 for Ni
(20%)–Mn (4%)/SiO2 samples treated in (1) argon, (2)
oxygen, and (3) hydrogen glow-discharge plasmas. The
temperature dependences of the depth of conversion
(Y, %) and selectivity with respect to acetone (S, %) for
the samples specified are given in Table 1.
It follows from a comparison of the temperatures of
reaction initiation, yields, and conversion depths (Fig. 1
and Table 1) that the sample treated in an argon plasma
had a higher activity than the samples treated in oxygen
and hydrogen plasmas. Treatment in argon and oxygen
plasmas provides a higher selectivity up to higher tem-
peratures compared with treatment in a hydrogen
plasma. A 100% selectivity up to a ~25% depth of con-
version was obtained for all the three samples. We
therefore constructed the temperature dependences of
the yield of acetone in the Arrhenius equation coordi-
nates lnn‡c – T–1 over the Y range specified. The experi-
mental (apparent) activation energies (Ea, kJ/mol) and
logarithms of the preexponential factor (lnn0) for cata-
lyst samples treated with all the three plasma types
were as follows over the temperature range 363–463 K:
0
400
500
600
T, K
Fig. 1. Temperature dependences of the yield of acetone for
(Ni 20 wt %–Mn 4 wt %)/SiO samples treated with
2
(1) hydrogen, (2) argon, and (3) oxygen glow-discharge
plasmas.
The activation energy for the samples treated in
argon and oxygen plasmas was different from that for
the sample treated in a hydrogen plasma. This means
that, in argon and oxygen, more active centers formed
than in hydrogen. The differences in the lnn0 values are
evidence that the number of low-activity centers
formed after treatment in a hydrogen plasma was sub-
stantially larger than after treatment in argon and oxy-
gen plasmas. We calculated the yields of acetone at
393 K, when selectivity was 100%. The ratios between
the yields for samples treated in argon and hydrogen
plasmas and the yield obtained after treatment in an
oxygen plasma were nAr/nO = 3.33 and nH /nO =
2
2
2
1.77. This means that the highest activity is observed
after treatment in an argon plasma because of an
increase in the number of new active centers and a com-
paratively low activation energy.
Plasma type
Oxygen
Argon
Hydrogen
Kinetic Experiments in a Vacuum
Ea
37.2
14.1
3
1
54.9
4
103.3
36.7
5
2
In the vacuum unit, we measured the rate of the
reaction W1 1 min after its beginning and the order of
lnn0
20.7 1.2
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 1 2008

52
YAGODOVSKII et al.
Table 1. Temperature dependences of the depth of conver-
sion (Y) and selectivity (S) for catalysts treated in glow-dis-
charge argon, oxygen, and hydrogen plasmas
The kinetic curves obtained at 393 and 383 K for the
reaction on the catalyst treated in a glow-discharge
argon plasma are by way of example shown in Fig. 2 in
the coordinates pH –t and ln pH –lnt.
T, K
Y, %
S, %
2
2
The order of the reaction was determined from the
dependence of the rate W1 on the initial isopropanol
pressure palc. The rate W1 was independent of pressure
Hydrogen plasma
378
413
478
3.4
100
60.5
93.3
76.3
over the palc range from 2.6 × 10–2 to 7.0 × 10–2 torr for
the sample treated in a glow-discharge hydrogen
plasma; that is, the order of the reaction was zero. After
treatment in a glow-discharge argon plasma, the depen-
dence of W1 on palc was linear over the pressure range
94
Argon plasma
413
433
478
508
24.8
100
100
100
84.6
38.6
0–4.6 × 10–2 torr; that is, the reaction was first-order.
81.1
Catalyst treatment in an oxygen plasma showed that
the dependence of W1 on palc was concave toward the
W1 axis (see Fig. 3). This curve was described by the
empirical equation
98
Oxygen plasma
413
433
463
488
508
558
0.9
9.9
100
100
100
91.4
91.8
81.8
W1 = (K'p2alc)/(1 + K''palc).
(4)
23.3
57.9
71.1
84.9
The corresponding dependence is shown in the lin-
ear coordinates of Eq. (4) in Fig. 4b,
p2alc/W1 = 1/K' + K''/K'palc.
(4‡)
Equation (4a) can be rewritten as W1 = Kpnalc , where
n = 1.7.
The temperature dependences of the reaction rate
were measured over the temperature range 367–473 K
for all samples. The temperature dependence of the ini-
tial rate was represented in the Arrhenius equation
coordinates (at the initial pressure palc = 2.0 × 10–2 torr
at all temperatures).
The initial rates W1 at 453 K and initial pressure
palc = 2.0 × 10–2 torr, the experimental activation ener-
gies Ea, and the logarithms of the preexponential factor
lnW0 are compared in Table 2.
We see that the highest activity is achieved after
treatment with a glow-discharge Ar plasma; this is
caused by the high lnW0 value, that is, the large number
of active centers (Table 2).After hydrogen plasma treat-
ment, the number of active centers is still larger, but
their activities are substantially lower. The most active
centers are formed after treatment in an oxygen plasma
(this gives the lowest Ea value), but the number of active
centers is much smaller than after treatment in argon
and hydrogen.
Table 2. Isopropanol dehydrogenation rates (W1, torr/min)
on Ni 20 wt %–Mn 4 wt % catalyst samples treated in glow-
discharge Ar, O2, and H2 plasmas at 453 K and palc = 2.0 ×
10–2 torr, activation energies (Ea, kJ/mol), and logarithms of
preexponential factors (lnW0)
W1
Ea
lnW0
Argon plasma
1.70 × 10–1
6.3 × 10–3
1.1 × 10–3
120.8
Hydrogen plasma
201.3
Oxygen plasma
113.1
5
30.5
48.2
23.1
7
7
the reaction and its activation energy. The dependence
of the pressure of hydrogen formed pH on time t is
2
described by the empirical equation
pH = atn.
(1)
2
The kinetic patterns observed after catalyst treat-
ments by plasmas of different types (argon, oxygen,
and hydrogen) are evidence of different reaction mech-
anisms (reaction orders are different).
It was of interest to analyze possible step mecha-
nisms of isopropanol dehydrogenation on the catalyst
samples studied. For this purpose, we used one of the
linear algebra methods, the Horiuti–Temkin method [7,
8], based on Bodenstein’s steady state principle for
multistage reactions.
The differentiation of this equation yields the time
dependence of the reaction rate,
d pH /dt = antn – 1
,
(2)
2
and W1 = an at t = 1 min.
(3)
The a and n constants were determined from the
dependence ln pH = lna + nlnt.
2
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 1 2008

THE DEHYDROGENATION OF ISOPROPANOL
53
pH2 × 102, torr
2
W1 × 103, torr/min
8
(a)
(a)
6
1
4
2
1
0
0.02
0.04
0.06
0.08
palc, torr
2
(p2/W1) × 102, torr min
(b)
6
0
5
10
12
t, min
lnt
3.0
4
2
0
0.5
1.0
1.5
2.0
2.5
–1
–2
–3
(b)
1
2
–4
–5
0.02
0.04
0.06
0.08
palc, torr
–6
–7
ln pH
Fig. 3. Dependences of the initial reaction rate W on the
1
initial pressure of isopropanol p on a catalyst treated with
alc
2
a glow-discharge oxygen plasma (a) in the coordinates W –
1
Fig. 2. Kinetic curves of isopropanol dehydrogenation on a
p
and (b) in the coordinates of the linear form of Eq. (4a),
catalyst treated in a glow-discharge argon plasma at 393 and
alc
2
383 K in the coordinates (a) pH –t and (b) ln pH –lnt.
p /W – p
.
alc
1
2
2
three-stage scheme. The main stages of the two-stage
scheme are
We analyzed nine variants of reaction mechanisms
comprising two to six one-path stages resulting in the
formation of acetone and hydrogen. It was found in [4]
that the active centers of copper and copper–rhenium
catalysts of isopropanol dehydrogenation contained
hydrogen atoms. In [5, 6], this assumption was
extended to nickel and nickel–rhenium systems. In this
work, all the samples were preliminarily reduced in
hydrogen. This assumption can therefore also be
extended to the nickel–manganese catalysts under con-
sideration.
(1) [(CH3)2 – CH – OH(„)] + Z
k1
⇔ (CH3)2 – CH – OH•Z,
k–1
(2) (CH3)2 – CH – OH•Z + Z(‡)
k2
(CH3)2=CO(g) + Z + Z(‡) + H2,
where Z is the site of isopropanol adsorption, Z(a) is the
site of H atom localization, and K = k1/k–1. Let θZ be
the degree of surface coverage by isopropanol and θç,
the fraction of hydrogen in active centers. If stage 2 is
the rate-determining stage, the rate of the reaction can
be written as
Our calculations showed that multistage schemes
(each comprising more than four stages) did not allow
us to obtain kinetic equations describing the experi-
mental dependences at various simplifications of the
equations obtained. The observation of the first and
zeroth reaction orders (treatments in argon and hydro-
gen plasmas) can be explained by a two-stage scheme,
and the reaction after oxygen plasma treatment, by a
'
W = k2θZθH = k2 θZ,
k2 = k2θH, because θH = const
'
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 1 2008

54
YAGODOVSKII et al.
terms of the elementary event constants as
K' = (k''22/k3)K2, K'' = 2K/k3.
H
C
H
H
C
(7‡)
H
H
C
The analysis of the mechanisms of the dehydrogenation
of isopropanol given above presupposed the presence
of the H atom in the active center structure. This sug-
gestion was verified by quantum-chemical calculations
of surface structures. The possibility of isopropanol
dehydrogenation with the participation of copper cata-
lyst centers containing H atoms was evaluated by ab
initio Hartree–Fock (UHF) calculations in [9]. It was
shown that the reaction could occur either on the Cu5H
center containing the hydrogen atom (exothermic effect
–184 kJ/mol) or during the preliminary formation of
Cu5H clusters when alcohol molecules were brought in
contact with the catalyst (–397 kJ/mol).
H
H
O
H
Ni
Ni
Ni
Ni
Ni
Fig. 4. Complex formed by the isopropanol molecule and
Ni H cluster.
5
(H is an active center constituent). This gives the rate in
the form
In this work, similar calculations were performed
for the Ni5 cluster. We calculated the energies of forma-
tion of isopropanol, acetone, and hydrogen molecules
and the energies of the systems formed by the nickel
cluster and hydrogen atom and the system formed by
the Ni5H cluster and isopropanol molecule.
'
W = k2K palc/(1 + K palc),
(5)
'
where K = k1/(k–1 + k2 ).
If Kpalc ꢀ 1, we obtain the first order in isopropanol,
The calculations were performed in the UHF/6-31G
approximation (6-31G is the standard valence split
basis set) using the GAMESS package with the com-
plete optimization of the geometries of all systems
except the Ni5 cluster.
'
W = k2K palc,
(5‡)
and if Kpalc ꢁ 1, the reaction is zeroth-order in isopro-
panol,
The geometric structure of the Ni5 cluster was fixed,
the Ni–Ni distances in it were set equal to the handbook
value 3.52 Å [10]. Several Ni5H systems with different
sites of H localization were calculated. The most favor-
able localization sites were found to be (a) above the
middle of the Ni–Ni bond between the central and “cor-
ner” Ni atoms and (b) above the nickel atom in the cor-
ner of the square.
'
W = k2 .
(6)
Equation (5a) corresponds to the catalyst treated with a
glow-discharge argon plasma, and (6), to the catalyst
treated with a hydrogen plasma. After catalyst treat-
ment in an oxygen plasma, the reaction proceeds in
three stages,
k1
[(CH3)2CH – OH](g) + Z ⇔ (CH3)2CH – OH•Z, (I)
The dehydrogenation of isopropanol in the absence
of the nickel cluster does not occur, because this reac-
tion is endothermic (∆H° = 61.06 kJ/mol). In the pres-
ence of the Ni5H cluster, the reaction
k–1
(CH3)2CH – OH•Z + Z(‡)
k''
2
(CH3)2C=O↑ + ZH + ZH(‡),
(II)
(ëH3)2CHOH + Ni5H(‡)
k
3
ZH(‡) + ZH
Z + Z(‡) + H2.
(III)
(CH3)2=CO + Ni5H(b) + H2
At the second stage, two hydrogen atoms are detached
and transferred to the surface of the catalyst, and ace-
tone is released into the gas phase. The third rate-limit-
ing stage is the recombination desorption of hydrogen.
Under these conditions, the rate of the reaction is
can occur because it is exothermic, ∆H° = –22.21 kJ/mol.
Calculations showed the possibility of the formation of
an intermediate complex, (ëH3)2ëHéH + Ni5H
(ëH3)2ëHOH · Ni5H. Its structure is shown in Fig. 4.
This complex is formed with the participation of the H(a)
atom, and the energy of its formation is –110.82 kJ/mol.
It follows that the suggestion of the presence of the H
atom in the active center made on the basis of the exper-
imental data is substantiated by quantum-chemical cal-
culations.
W = (k''22/k3) × (K2 pa2lc)/(1 + K palc)2 .
Since K 2p2
W = (k''22/k3) × (K2 pa2lc)/(1 + 2K/k3 palc),
where K = k1/k–1.
0, we obtain
(7)
To summarize, we found that the treatment of the
surface of the (Ni 20 wt %–Mn 4 wt %)/SiO2 catalyst
Equation (7) coincides with empirical equation (4), by glow-discharge argon, oxygen, and hydrogen plas-
and the empirical constants of (4) can be written in mas changed the number and structure of active centers
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Vol. 82 No. 1 2008

THE DEHYDROGENATION OF ISOPROPANOL
55
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