On the surface steps of a heterogeneous catalytic reaction

Article abstract of DOI:10.1134/S0023158407060080

Based on a study of the properties of intermediate complexes in the reactions of low-temperature ammonia oxidation and nitrogen oxide reduction in the presence and absence of oxygen on various catalysts, it was hypothesized that the sequence of steps in the formation of reaction products on chemically different catalysts can be the same if the initial adsorption forms of reactants are identical. In this case, variations in catalysts or reaction conditions affect the ratio between reaction rates.

Full text of DOI:10.1134/S0023158407060080

ISSN 0023-1584, Kinetics and Catalysis, 2007, Vol. 48, No. 6, pp. 812–817. © MAIK “Nauka /Interperiodica” (Russia), 2007.
Original Russian Text © V.A. Matyshak, 2007, published in Kinetika i Kataliz, 2007, Vol. 48, No. 6, pp. 870–875.
MECHANISMS
OF CATALYTIC REACTIONS
On the Surface Steps of a Heterogeneous Catalytic Reaction
V. A. Matyshak
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, 117977 Russia
E-mail: matyshak@polymer.chph.ras.ru
Received June 19, 2006
Abstract—Based on a study of the properties of intermediate complexes in the reactions of low-temperature
ammonia oxidation and nitrogen oxide reduction in the presence and absence of oxygen on various catalysts, it
was hypothesized that the sequence of steps in the formation of reaction products on chemically different cat-
alysts can be the same if the initial adsorption forms of reactants are identical. In this case, variations in catalysts
or reaction conditions affect the ratio between reaction rates.
DOI: 10.1134/S0023158407060080
INTRODUCTION
such a test [5, 6]. If these rates are equal, it is believed
that an identified surface compound is an intermediate
in the test reaction.
The mechanism of a heterogeneous catalytic reac-
tion is usually considered to mean a sequence of steps;
in each particular step, intermediate complexes (inter-
mediates) are formed and consumed [1]. Advances in
both the theory and practice of heterogeneous catalysis
are based on the knowledge of a detailed catalytic reac-
tion mechanism.
The rates were determined based on spectrokinetic
measurements, in which the spectra of surface com-
pounds and the rates of formation of reaction products
were measured simultaneously. After the identification
of absorption bands in the IR spectra and the quantita-
tive treatment of spectroscopic data (determination of
the concentrations and the rates of conversion of sur-
face compounds), the rates of reaction and conversion
of surface compounds were compared quantitatively.
Data on the structure and reactivity of intermediates
are required for revealing the detailed mechanism of a
catalytic reaction. IR spectroscopy is a suitable tech-
nique for obtaining this information.
Figure 1 shows a schematic diagram of the spectro-
kinetic measurements. The main unit of this diagram is
a spectroscopic cell, which simultaneously serves as a
catalytic reactor [7, 8]. The main special features of this
cell are the small volume and the absence of cold parts.
The reaction mixture was analyzed before and after the
reactor cell. The spectrokinetic measurements can be
By now, a great body of experimental data on the
properties of surface complexes has been accumulated
and the roles of these complexes in various reactions
have been hypothesized [2–4]. However, a test is
required in order to determine the true role of surface
complexes in catalytic conversions. In this paper, a
comparison between the rates of surface compound performed under steady-state and non-steady-state con-
consumption and product formation is proposed as ditions.
Fourier transform IR spectrometer
IR beam
Temperature
programmer
Reactor cell
Gas
preparation
unit
Gas analysis
Fig. 1. Schematic diagram of the setup for spectrokinetic measurements.
812

ON THE SURFACE STEPS OF A HETEROGENEOUS CATALYTIC REACTION
813
ln(D0/D)
An example of the quantitative comparison of the
D
rate of reaction is calculated based on spectroscopic
measurements with the rate measured experimentally
under conditions of NO reduction with propane in an
excess of oxygen on Cu/ZSM-5 [8]. Under reaction
conditions, an absorption band at 1630 cm–1 was
detected in the transmission spectra. According to pub-
lished data [9], this absorption band was attributed to
antisymmetric vibrations in the Cu–O–N=O complex.
On heating the sample under thermal desorption condi-
tions with the analysis of the amount of released NOx
molecules, the absorptivity (ε) was determined to be
equal to 3.4 × 10−18 cm2/molecule.
0.25
2.0
1.5
1.0
0.5
0
0.20
0.15
0.10
0.05
1
2
Under reaction conditions at 300°ë, the absorbance
0
5
10
15
20
(D) of a band at 1630 cm–1 was equal to 0.05. By this is
t, s
meant that NNO = DS/ε = 3 × 1016 complexes occurred
on the sample surface. If NOx is converted through this
x
–1
Fig. 2. (1) The time dependence of absorbance at 1630 cm
after the replacement of a flow of NO + O with a flow of
2
C H + O and (2) the result of its treatment in the coordi-
complex, the rate of reaction is w = kNNO = 1.8 ×
3
8
2
x
nates of a first-order equation.
1017 molecule/min.
The value of k was determined from data in Fig. 2.
Curve 1 characterizes absorbance changes at the
absorption band of the nitrite complex with time upon
replacing a reaction mixture flow with a flow contain-
ing propane. This procedure was oriented to terminate
the formation of the nitrite complex in order to measure
the rate of its consumption. The treatment of curve 1 in
the coordinates of a first-order equation gave k =
6 min−1 (see straight line 2).
The experimentally measured rate of NOx conver-
sion on this sample was w = 1.9 × 1017 molecule/min. A
satisfactory agreement between the rates demonstrates
that the observed complex is an intermediate in the
reduction of NOx with propane.
With the use of steady-state and non-steady-state
spectrokinetic measurements based on transmission
and diffuse-reflectance IR spectroscopy, it was found
that NH3ads and N2Oδ– are reaction intermediates on
ZnO and Fe2O3 and NH3ads, N2Oδ+, and NH2ads are reac-
tion intermediates on Cr2O3.
Assuming that all of the detected intermediates par-
ticipate in the reaction, the following reaction scheme
can be proposed to describe the low-temperature oxida-
tion of ammonia on the test metal oxides:
(1) NH3gas + Z
NH3Z
(ZnO, Cr2O3, Fe2O3—1620 cm–1),
(2) NH3Z
(3) O2 + Z
(4) O2Z
(5) O2Z + Z
(6) NH3Z + OZ
(Cr2O3—1560 cm–1),
NH3gas + Z,
O2Z,
O2 + Z,
The above example shows that the spectrokinetic
method can be successfully applied to studying the sur-
face steps of heterogeneous catalytic reactions.
2OZ (Cr2O3—1010, 990 cm–1),
NH2Z + OHZ
The starting reagent for subsequent reactions results
from the adsorption of molecules on catalyst surfaces.
The same surface complexes (the same reagents) can be
formed on different catalysts. In this case, the reaction
occurs by the same mechanism on these catalysts.
However, if chemically different reagents are formed
upon the adsorption of the same molecules on a group
of catalysts, different reactions occur on these catalysts.
To test this hypothesis, the spectrokinetic method was
applied to study the same reaction on a wide range of
catalysts.
(7) 2NH2Z
N2H4Z + Z,
(8) N2H4Z + O2Z
(9) NH2Z + O2Z
(Cr2O3—2080 cm–1),
(10) NOZ + OZ
(11) NH2Z + NO2Z
N2gas + 2H2O + 2Z,
NOZ + H2O + Z
NO2Z + Z,
N2OZ + Z
(ZnO, Fe2O3—2200 cm–1),
(12) N2OZ N2Ogas + Z,
(13) OHZ + OHZ H2O + OZ + Z.
Low-Temperature Ammonia Oxidation
Here, Z is a free site of the oxide surface; oxides on
which the given surface compounds were observed
experimentally are specified in parentheses.
The low-temperature catalytic oxidation of ammo-
nia to nitrogen is an important process for decreasing
the concentration of ammonia in industrial waste gases.
Reaction intermediates in ammonia oxidation [10]
Based on the above reaction scheme, product con-
were studied on oxide catalysts characterized by Lewis centrations and surface coverages with intermediate
(Cr2O3, Fe2O3, and ZnO) or Brönsted acidity (Cr2O3 compounds at various reaction mixture compositions
and åÓé3) and on bulk metals (Pt and Fe) [11].
were calculated. The following reaction constants
KINETICS AND CATALYSIS Vol. 48 No. 6 2007

814
MATYSHAK
C
product, volume fraction
D2200
0.20
θN O
2
0.0015
2
0.0014
(a)
1
2
(‡)
0.0010
0.0005
0
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
1
0.15
0.10
0.05
0
3, 4
0.05 0.10 0.15 0.20 0.25 0.30
C
NH , volume fraction
3
0.0016
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
(b)
2
1
0
50
100
t, s
150
200
D2080
θN O
3, 4
2
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.045
0.040
0.035
0.030
0.025
0.020
0.015
0.010
0.005
0
(b)
0
0.10
0.20
0.30
0.40
C , volume fraction
O2
Fig. 3. Dependence of (solid line) experimental and (dashed
line) calculated concentrations of the reaction products (1,
2) N and (3, 4) N O for Fe O on the concentrations of (a)
2
2
2 3
ammonia and (b) oxygen in the reaction mixture.
0
100 200 300 400 500 600
obtained from non-steady-state spectrokinetic mea-
surements were used as reference values: k12 (0.012 s–1
on ZnO and 0.020 s–1 on Fe2O3), k6 (1 × 10–3 s–1), and
k10 (2 × 10–3 s–1) on Cr2O3 (the dimensions of constants
depend on the fact that surface coverages are expressed
as monolayer fractions, and reagent and product con-
centrations in the gas phase are expressed in volume
fractions, that is, dimensionless units).
t, s
θN O
D2200
0.07
2
0.0035
0.0030
0.0025
0.0020
0.0015
0.0010
0.0005
(c)
0.06
0.05
0.04
0.03
0.02
0.01
0
Figure 3 shows the dependence of reaction product
concentrations (both experimentally measured and cal-
culated using the reaction scheme proposed) on the
concentrations of NH3 and O2. Figure 4 compares the
experimental and calculated surface coverages and
their changes in the course of reaction on ZnO, Cr2O3,
and Fe2O3 after the termination of ammonia supply to
the reaction mixture for the oxides examined. With con-
sideration for the assumptions made in the reaction
scheme, the calculated and experimental functions are
in good agreement.
0
50
100
t, s
150
200
Fig. 4. (1) Experimental and (2) calculated curves illustrat-
ing the decline of absorption at (a) 2200 cm for ZnO,
–1
–1
–1
(b) 2080 cm for Cr O , and (c) 2200 cm for Fe O upon
2
3
2 3
ammonia shutoff.
The set of the above facts suggests that the reaction
mechanism is the same on the test oxides. Note that the
adsorption species of ammonia and oxygen on these
oxides are the same. In this context, it is believed that
the same mechanism of the given reaction occurs in the
presence of catalysts on which the initial adsorption
species of reactants are the same.
and N2. Consequently, the mechanism of ammonia oxi-
dation on these samples is the same. At the same time,
it is fundamentally different from the above mechanism
for Lewis acidic oxides. The results given provide sup-
port for the above hypothesis.
The results of spectrokinetic measurements on
Lewis acidic oxides were discussed above. The NH4+
surface complex is formed upon ammonia adsorption
on oxides with Brönsted acidity (MoO3 and Cr2O3).
The spectrokinetic measurements demonstrated that
this complex is an intermediate in the formation of N2O
The oxidation of ammonia on bulk metals [11] is
another example that supports our hypothesis. Accord-
ing to in situ emission IR-spectroscopic data, the sur-
face NH2 complex is the form of methane activation in
this case and nitrogen oxide was detected in the reac-
tion products.
KINETICS AND CATALYSIS Vol. 48 No. 6 2007

ON THE SURFACE STEPS OF A HETEROGENEOUS CATALYTIC REACTION
815
Interaction of CO and NO
carbon monoxide and nitrogen oxide were activated as
carbonyl and nitrite complexes, respectively. A reaction
between them resulted in the formation of N2O and
CO2. By this is meant that both the form of reactant
activation and the mechanism of the reaction changed.
This process is important from the standpoint of
gasoline vehicle emission control. The main catalysts
are supported platinum group metals. In this context,
the process was studied on Pt, Pd, Rh, Ru, and, for com-
parison, Cu0 supported on aluminum oxide. Under the
reaction conditions, isocyanate and carbonate com-
plexes were identified as intermediates on catalyst sur-
faces. In addition, carbonyl complexes were observed
at relatively low temperatures, whereas nitrosyl com-
plexes were detected at relatively high concentrations
of NO. Krylov and Matyshak [12], who performed a
spectrokinetic study with wide variations in the reac-
tion atmosphere composition and temperature, found
that the isocyanate complex is an intermediate in the
formation of nitrogen. The NCO complex is formed by
the interaction of adsorbed the nitrogen atom and CO
molecule; it decomposes in a reaction with an adsorbed
NO molecule. With consideration for the above, the fol-
lowing mechanism was proposed for the interaction
between CO and NO:
Reduction of Nitrogen Oxides with Hydrocarbons
The most interesting data were obtained in a study
of the selective catalytic reduction of nitrogen oxides
with hydrocarbons (HC-SCR of NOx) in the presence of
oxygen. This reaction has received much attention in
the literature because of the necessity of diesel vehicle
emission control. The HC-SCR of NOx occurs most
effectively on oxide catalysts [15–18]; this is likely due
to the fact that the presence of oxygen facilitates the
activation of NOx on these catalysts with the formation
of nitrate complexes rather than prevents this activation
(this is characteristic of supported metals).
Spectrokinetic studies [19–31] allowed us to dem-
onstrate that nitrate complexes [20–28] are key reaction
intermediates in the HC-SCR of NOx on various cata-
lysts—copper- and cobalt-substituted zeolites with the
ZSM-5 structure; various modifications of zirconium
dioxide; pillared clays, in which nanosized ZrO2 parti-
cles are fixed between the aluminosilicate layers of nat-
ural montmorillonite clays (both pure pillared clays and
clays with supported active components: copper or
cobalt cations, in particular, promoted with platinum or
silver); and on nickel–chromium oxide, iron–chro-
mium oxide, and copper–zinc–alumina systems
(schemes (I), (II)). The next step is the interaction of
(1) CO + [M]
(2) NO + [M]
[M–CO],
[M–NO],
(3) [M] + [M–NO]
[M–N] + [M–O],
[M–NCO] + [M],
(4) [M–N] + [M–CO]
(5) [M–NCO] + [M–NO]
(6) [M–CO] + [M–O]
[M–CO2] + N2 + [M],
[M–CO2] + [M],
N2O + 2[M],
(7) [M–CO2]
[M] + CO2,
(8) [M–N] + [M–NO]
(9) O2 + 2[M]
2[M–O].
Based on this mechanism, the dependence of the
rate of reaction and selectivity on the reaction atmo-
sphere composition was quantitatively described for all
of the test catalysts (with various sets of constants).
Note that the initial adsorption states of reactants in this
process were the same for the test catalysts: the reaction
mechanism was the same.
NO–3 with a hydrocarbon with the formation of nitroor-
ganic complexes [29–31]. Note that the structure of the
nitroorganic complexes depends on the form of hydro-
carbon activation. In particular, the interaction of a mix-
ture of propylene and oxygen with the surface of bulk
ZrO2 results in the formation of only an acetate com-
Matyshak et al. [13, 14] studied this reaction on bulk plex. Finally, the acetate and the monodentate nitrate
copper–cobalt spinel and copper–cobalt spinel and cop- complexes form a surface complex structurally similar
per oxide supported on aluminum oxide and found that to the adsorbed nitromethane (scheme (I)):
NO2gas
(I)
O2
NO3–
CH3COO–
NOx, T, °C
NO
NO2ads
O2N–CH3
N2 + COx + H2O
The isopropoxide complex is the main form of propylene activation on the ZrO2 pillars of pillared clay. In this case,
the isopropoxide and nitrate complexes form a complex structurally similar to the adsorbed dinitropropane
(scheme (II)):
NO2gas
(II)
O2
NO3–
C3H7O–
NOx, T, °C
NO
NO2ads
O2N–C3H7
N2 + COx + H2O
KINETICS AND CATALYSIS Vol. 48 No. 6 2007

816
MATYSHAK
3. Davydov, A.A., IK-spektroskopiya v khimii poverkhnosti
The dinitropropane complex is consumed in the
interaction with surface NO2 complexes formed by the
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okislov (IR Spectroscopy Applied to the Chemistry of
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tial adsorption state of reactants is responsible for the
sequence of steps in the HC-SCR of NOx: if NO and a
hydrocarbon form nitrate and acetate, respectively,
upon activation, the reaction proceeds through the
nitromethane complex; if NO forms nitrate upon acti-
vation and a hydrocarbon forms isopropoxide, the
nitropropane complex is an intermediate. This state-
ment is true for all of the test systems.
The above facts fall within the framework of the
hypothesis that the mechanism of the given reaction as
a sequence of intermediate transformations on a set of
catalysts is retained if the initial adsorption forms of
reactants on this set are the same. Indirect evidence for
this hypothesis can be found in the literature. In partic-
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acrolein on oxide catalysts, found that the mild oxida-
tion reaction of propylene to acrolein occurs if a π com-
plex of propylene is formed upon propylene adsorption,
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occurs on the formation of a σ complex of propylene.
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KINETICS AND CATALYSIS Vol. 48 No. 6 2007