Gas-Phase Reduction of Oxides of Nitrogen with CO Catalyzed by Atomic Transition-Metal Cations

Article abstract of DOI:10.1002/anie.200351628

A novel catalytic role for transition-metal cations in the homogeneous conversion of the nitrogen oxides NO₂, NO, or N₂O and CO into N₂ and CO₂ is demonstrated by thermodynamic and kinetic studies of gas-phase reactions of 29 transition-metal cations and some of their oxides (see scheme). Fe⁺, Os⁺, and Ir ⁺ ions are shown to be most effective. The chemistry identified may have practical applications in new designs of catalytic converters.

Full text of DOI:10.1002/anie.200351628

Angewandte
Chemie
Catalyzed Reduction of N Oxides
Gas-Phase Reduction of Oxides of Nitrogen with
CO Catalyzed by Atomic Transition-Metal
Cations**
Voislav Blagojevic, Michael J. Y. Jarvis, Eric Flaim,
Gregory K. Koyanagi, Vitali V. Lavrov, and
Diethard K. Bohme*
Catalytic conversion of harmful gases, such as the oxides of
nitrogen produced in fossil-fuel combustion into nitrogen and
carbon dioxide, is of utmost importance, both environmen-
tally and economically. CO was one of the first gases
investigated for eliminating NO from automobile exhaust
gas: the reaction of NO with CO [Eq. (1)] is one of the most
1
NO þ CO ! = N2 þ CO2
ð1Þ
2
important reactions occurring in automotive catalytic con-
verters where both reactants are undesirable pollutants.[1]
Base-metal oxides, mixed metal-oxide compounds such as
perovskites, supported metal catalysts, metal zeolites, and
alloys have all been investigated as heterogeneous catalysts
for this reaction, which does not occur directly in the gas
phase at room temperature to any measurable extent.[1] For
example, copper in the + 2 oxidation state has been found to
be active both as a base-metal-oxide catalyst and in a metal-
supported or perovskite form.
We report here the first example of homogeneous
catalysis by the reaction described in Equation (1), which
occurs in the gas phase with atomic transition-metal cations
serving as catalysts. The catalysis occurs in two steps in which
NO is first reduced to N2O. An analogous three-step catalytic
reduction of NO2, in which NO2 is first reduced to NO, was
also discovered.
The overall catalytic scheme that was established in the
study reported here consists of the three catalytic cycles
shown in Figure 1. These three cycles were characterized with
laboratory measurements of reactions of each of the three
nitrogen oxides NO2, NO, and N2O with up to 29 different
transition-metal cations M+ [Eqs. (2)–(4)]:
[*] Prof. D. K. Bohme, V. Blagojevic, M. J. Y. Jarvis, E. Flaim,
G. K. Koyanagi, V. V. Lavrov
Department of Chemistry
York University
Toronto, ON, M3J 1P3 (Canada)
Fax: (+1)416-736-5936
E-mail: dkbohme@yorku.ca
[**] Support for this research by the Natural Sciences and Engineering
Research Council of Canada is greatly appreciated. Also, we
acknowledge support from the National Research Council, the
Natural Science and Engineering Research Council, and MDS SCIEX
in the form of a Research Partnership grant. As holder of a Canada
Research Chair in Physical Chemistry, D.K.B. thanks the contribu-
tions of the Canada Research Chair Program to this research. We
also thank Dr. I. Kretzschmar for helpful discussions.
Angew. Chem. Int. Ed. 2003, 42, 4923 –4927
DOI: 10.1002/anie.200351628
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Fe+, Co+, Ni+, Mo+, Ru+, Re+, Os+ and Ir+. The reaction given
in Equation (6) is endothermic for these latter ions (see
À
Table 1) but that in Equation (3), driven by N NO bond
formation, is exothermic since OA (M+) > OA (N)ÀD (N-
NO) = 36 kcalmolÀ1.[2] Figure 2 (top left) presents experi-
mental data obtained for the second-order (in NO) reduction
of NO by Fe+. With V+, Cr+, Mn+ and Mo+, this reaction
occurs in competition with the second-order ionization of NO
[Eq. (7)], driven by the formation of the stable metal nitrosyl
molecule MNO.
Mþ þ NO þ NO ! NOþ þ MNO
ð7Þ
Cu+, Rh+, Pd+, Ag+, Cd+, and Pt+ ions were observed to
react only by second-order ionization of NO (as well as by NO
addition in some cases). Reactions of the metal-oxide MO+
ions with CO [Eq. (5)] are exothermic if OA (M+) < OA
(CO) = 127 kcalmolÀ1.[2] Of those ions with OA (M+) <
127 kcalmolÀ1 (see Table 1) that have been observed to
react measurably with N2O according to Equation (4), only
Fe+, Os+, and Ir+ have been observed to undergo the reaction
given in Equation (5), with k > 2.6 ” 10À11 cm3 moleculeÀ1 sÀ1.
Figure 2 (top right) shows experimental data obtained for the
reduction of FeO+ by CO, which is exothermic by 47 kcal-
molÀ1.
The reactions described by Equations (3) and (5) taken
together in principle constitute the catalytic cycle shown in
Scheme 1 (and the middle of Figure 1), which leads to the
reduction of NO to N2O and requires that 36 < OA (M+) <
127 kcalmolÀ1. Also, the ionization energy of the metal and
metal oxide must be less than that of NO, Ei (M) and Ei
(MO) < Ei (NO) = 9.26 eV,[2] to avoid competition with
Figure 1. Catalytic cycles for the homogeneous reduction of nitrogen
oxides by carbon monoxide, mediated by atomic transition-metal
cations.
Mþ þ NO2 ! MOþ þ NO
Mþ þ NO þ NO ! MOþ þ N2O
Mþ þ N2O ! MOþ þ N2
ð2Þ
ð3Þ
ð4Þ
These reactions lead to the reduction of the corresponding
nitrogen oxide as well as the oxidation of M+ to MO+. Also,
metal-oxide cations, MO+, were reacted with CO. These
reactions lead to the reduction of MO+ to M+ as well as the
oxidation of CO to CO2 according
to Equation (5), where the catalyst
Table 1: O-atom affinities, OA=D0 (M+-O) [kcalmolÀ1],[3] and ionization energies, Ei (M) and Ei (MO)
[eV],[2] for transition-metal cations.
MOþ þ CO ! Mþ þ CO2
ð5Þ
First Row
M+ OA (M+)
Ei
(M) (MO)
Second Row
M+ OA (M+)
Ei
Third Row
M+ OA (M+) Ei
(M)
M+, which reduces the three nitro-
gen oxides, is regenerated:
Ei
Ei
Ei
(MO)
(M) (MO)
Two modes were observed for
the reactions of M+ with NO. M+
ions with a high O-atom affinity,
OA (M+) > OA (N) = 151 kcal-
molÀ1,[2] were found to abstract an
O atom from NO exothermically
according to Equation (6) with high
Sc+ 164.6Æ1.4 6.56
Y+
167.0Æ4.2 6.22 5.85
La+
206Æ4
173Æ5
5.58 4.9
Ti+ 158.6Æ1.6 6.75 6.56
Zr+ 178.9Æ2.5 6.63 6.1
Nb+ 164.4Æ2.5 6.76 6.1
Hf+
6.83 7.55
7.55 7.92
V+
134.9Æ3.5 6.77 7.5
85.8Æ2.8 7.43 7.85
Ta+ 188Æ15
Cr+
Mo+ 116.7Æ0.5 7.09
8
W+ 126Æ10[a] 7.86 9.1
efficiency,
k > 10À10 cm3 mole-
Mn+ 68.0Æ3.0 7.43 8.65
Re+ 115Æ15
Os+ 100Æ12
7.83
8.44
culeÀ1 sÀ1.
Fe+
Co+
Ni+
Cu+
Zn+
80.0Æ1.4 7.9
8.9
Ru+
Rh+
Pd+
Ag+
Cd+
87.9Æ1.2 7.36 8.7
69.6Æ1.4 7.46 9.3
33.7Æ2.5 8.34 9.1
28.4Æ1.2 7.58
8.99
Mþ þ NO ! MOþ þ N
ð6Þ
74.9Æ1.2 7.88 8.9
63.2Æ1.2 7.64 9.5
37.4Æ3.5 7.72
Ir+
59[b] 8.97 10.1
The bimolecular reaction shown
in Equation (6) was observed with
M+ = Sc+, Ti+, Y+, Zr+, Nb+, La+,
Hf+, Ta+, and W+ (see Table 1 for
values of OA (M+)). Second-order
NO reactions of the type shown in
Equation (3) were observed at high
Pt+
Au+
Hg+
77
8.96 10.1
9.23
38.5Æ1.2 9.39
10.4
[a] This value for OA (W+) is too low according to our observation of the fast reaction W+ + NO!WO+
+ N, k=5.0”10À10 cm3 moleculeÀ1 sÀ1. [b] This value for OA (Ir+) is also too low according to our
NO flows for M+ = V+, Cr+, Mn+, observationof the reactionIr + NO2!IrO+ + NO (see Table 2).
+
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Figure 2. Top left: Experimental data for the second-order (in NO) reduction of NO by Fe+ ions. The product FeO+ reacts further inthis case with
NO by electrontransfer. Top right : Data for the reductionof FeO + by CO. The decay of FeO+ (solid line) is slowed down due to the reoxidation of
Fe+ by N2O added upstream into the flow tube. Bottom left: Data for the reductionof N 2O by Fe+ ions. The FeO+ product adds to N2O sequen-
tially at high flows of N2O. Bottom right: Data for the reductionof NO 2 by Fe+ ions. The FeO+ product reacts further to produce NO+ + FeO2.
IS=Ionsignal.
(N2) = 40 kcalmolÀ1.[2] We have surveyed the transition-metal
cations in the periodic table and, of the M+ ions observed to
undergo the reaction in Equation (3), we have observed the
reaction described in Equation (4) with k > 1 ” 10À12 cm3 mo-
leculeÀ1 sÀ1 for M+ = V+, Fe+, Co+, Os+, and Ir+. Experimental
Scheme 1. Catalytic cycle for the homogeneous reduction of nitric
oxide by carbon monoxide, mediated by atomic transition-metal
cations.
data obtained for the reduction of N2O by Fe+ ions are shown
in Figure 2 (bottom left). FeO+ is seen to add N2O at high
bimolecular electron transfer, which we have observed to
predominate in exothermic reactions.
concentrations of N2O in sequential N2O addition reactions.
The reactions described by Equations (4) and (5) taken
together in principle constitute the catalytic cycle shown in
Scheme 2 (and the bottom of Figure 1), which leads to the
The further reduction of N2O to N2 [Eq. (4)] can be
achieved with transition-metal ions with OA (M+) > OA
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reduction of N2O to N2 and requires that 40 <
Schemes 1, 2, and
3
taken
OA (M+) < 127 kcalmolÀ1 and Ei (M) and Ei
(MO) < Ei (N2O) = 12.886 eV.[3] Figure 2 (top
right) shows catalytic data for M = Fe.
together lead to the conversion of
NO2 and CO to N2 and CO2 accord-
ing to Equation (11):
Scheme 2. Catalytic cycle
for the homogeneous
reductionof nitrous
oxide by carbonmonox-
ide, mediated by atomic
transition-metal cations.
Scheme 3. Catalytic cycle
for the homogeneous
reductionof nitrogen
dioxide by carbonmon-
oxide, mediated by
atomic transition-metal
cations.
The reaction shown in Equation (4) has
commonly been used in the gas phase to
produce transition-metal oxide cations[4] and
previous measurements using ion cyclotron
resonance (ICR) spectroscopy actually have
demonstrated that Fe+ ions will catalyze the
2 NO2 þ 4 CO ! N2 þ 4 CO2
ð11Þ
The M+ catalyst must satisfy the
thermodynamic requirements that
73.5 < OA(M+) < 127 kcalmolÀ1 and
Ei (M) and Ei (MO) < Ei (NO) =
9.26 eV. Of the 29 transition-metal
oxidation of CO to CO2 by N2O under ICR conditions
according to Scheme 2 (and that the five transition-metal
cations Ti+, Zr+, V+, Nb+, and Cr+ fail to do so).[5]
Scheme 1 and Scheme 2 taken together lead to the
conversion of NO and CO to N2 and CO2 according to the
overall reaction [Eq. (8)], which is the equivalent of Equa-
tion (1):
ions that were surveyed only Fe, Os, and Ir[2] meet this
requirement (see Table 1) and have been observed to undergo
all the reactions in Equations (2)–(5), with k > 1 ”
10À11 cm3 moleculeÀ1 sÀ1 (see Table 2).
Os+ and Ir+ ions produce higher oxides in sequential
reactions with NO2 and N2O (but not NO) according to the
reactions given in Equations (12a,b) and (13) up to n = 3 and
2 NO þ 2 CO ! N2 þ 2 CO2
ð8Þ
The M+ catalyst must satisfy the thermodynamic require-
ments that 40 < OA (M+) < 127 kcalmolÀ1 and Ei (M) and Ei
(MO) < Ei (NO) = 9.26 eV. This requirement is met (see
Table 1) by the transition metals Fe, Os, and Ir, all of which
have been observed to undergo the reactions given in
Equations (3), (4), and (5) with k > 1 ” 10À11 cm3 mole-
culeÀ1 sÀ1 (see Table 2). Mo+ and Ru+ ions meet this require-
ment, but their reaction with N2O gives only addition (no
MO+ formation). Co+ and Cr+ ions also meet this require-
ment but the reaction in Equation (4) was too slow to produce
enough MO+ species to measure the results of the following
reaction [Eq. (5)].
Finally, NO2 has also been observed to be reduced by
certain transition-metal ions according to Equation (2), often
in competition with formation of NO+ (see Table 2). Figure 2
(bottom right) shows experimental data for the reduction of
NO2 by Fe+ [Eq. (9)], followed by Equation (10):
MOþn þ NO2 ! MOnþþ1 þ NO ðn ¼ 0 À 3Þ
! NOþ þ MOnþ1 ðn ¼ 0Þ
ð12aÞ
ð12bÞ
ð13Þ
MOþn þ N2O ! MOnþþ1 þ N2 ðn ¼ 0 À 3Þ
n = 2 for M = Os and Ir, respectively. Figure 3 shows results
for Os+ in which NO+ is produced primarily by the reaction of
OsO+ with NO2 in competition with OsO2+ formation.
Os+ and Ir+ ions react sequentially with N2O according to
Equation (13). The bare metal cation is recovered by
sequential reactions with CO molecules according to Equa-
tion (14) as shown in Figure 3 for OsOn
+
:
MOþn þ CO ! MOnþÀ1 þ CO2 ðn ¼ 1 À 4Þ
ð14Þ
This demonstrates that the catalytic cycle will not be
inhibited if higher oxides are formed, that is, the transition-
metal oxide cations MOn+ formed in each step of the catalytic
cycle are reverted back to bare metal cations by reaction with
CO.
Feþ þ NO2 ! FeOþ þ NO
FeOþ þ NO2 ! NOþ þ FeO2
ð9Þ
ð10Þ
In conclusion, we have reported a detailed and compre-
hensive thermodynamic and kinetic investigation of a novel
role for transition-metal cations in the homogeneous catalytic
conversion of nitrogen oxides and carbon monoxide to
nitrogen and carbon dioxide. Fe+, Os+, and Ir+ ions are
The reactions described in Equations (2) and (5) taken
together in principle constitute the catalytic cycle shown in
Scheme 3 (and the top of Figure 1), which leads to the
reduction of NO2 to NO.
Table 2: Rate coefficients, k [cm3 moleculeÀ1 sÀ1], and products observed for reactions with M=Fe, Os, and Ir at room temperature in helium buffer at
0.35 Torr.
ReactionFe
Os
Ir
Products
k[a]
Products
k[a]
Products
k[a]
M+ +NO+NO
M+ +N2O
FeO+ +N2O
FeO+ +N2
FeO+ +NO
1.6”10À11
3.7”10À11
9.1”10À10
OsO+ +N2O
OsO+ +N2
1.5”10À11
5.8”10À11
7.3”10À10
IrO+ +N2O
IrO+ +N2
1.5”10À11
2.9”10À10
7.9”10À10
M+ +NO2
OsO+ +NO (0.8)
NO+ +OsO (0.2)
Os+ +CO2
IrO+ +NO (0.6)
NO+ +IrO (0.4)
Ir+ +CO2
MO+[b] +CO
Fe+ +CO2
>3.7”10À10
>4.6”10À11[c]
>2.6”10À11[c]
[a] Apparent bimolecular rate coefficient with an estimated accuracy of Æ30%. [b] Produced from the reactionof M + with N2O (M=Fe, Os, Ir) and of
MO2+ with CO (M=Os, Ir). [c] Determined from a fit to the production and loss of MO+.
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Figure 3. Experimental data for the sequential oxidation of Os+ ions by NO2 (left) and the reduction of oxides of Os+ by CO (right). The NO+ for-
mation(left) competes with the further oxidationof OsO +. IS=Ionsignal.
oxide cation chemistry, the metal-oxide cation was produced
upstream from the reaction of the metal ion with N2O. CO was
introduced into the reaction region further downstream into the flow
tube.
shown to be most effective. The catalytic conversion has been
demonstrated in principle but it remains to be seen whether
the catalytic cycles shown in Figure 1 have applications in
practical catalysis; they may provide the basis of new designs
of catalytic converters in automobiles through the use, for
example, of cold cathode discharges. For the most part the
reactions measured are clean (single-channel) reactions so
that the specific catalytic cycles that are proposed in principle
have a turnover number of infinity. The two exceptions are
the NO2 cycles involving Ir+ and Os+ ions that show both
MO+ and NO+ production in their reactions with NO2 (60%
and 80% MO+ production for Ir+ and Os+, respectively).
These two cycles have turnover numbers of 1.5 and 4,
respectively. Of course in a practical device that exploits the
catalytic cycles that we have proposed, the turnover numbers
can be further reduced by secondary reactions, competing
reactions with impurities or surfaces, or other losses.
Received: April 10, 2003
Revised: August 4, 2003 [Z51628]
Keywords: cations · homogeneous catalysis · kinetic
.
measurements · nitrogen oxides · transition metals
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Experimental Section
The reactions were investigated in an inductively coupled plasma/
selected-ion flow tube (ICP/SIFT) tandem mass spectrometer.[6] The
transition-metal ions were produced from their metal-salt solutions,
which were sprayed into an argon plasma operating at atmospheric
pressure and ꢀ 5500 K. The ions are then mass selected by a
quadrupole mass filter and injected into a flow tube flushed with He
buffer gas at 0.35 Torr and 294 Æ 3 K. The ions cool by radiation and
collision with argon and helium from their point of origin to the
entrance of the reaction region.[7] The reagent gas (NO, NO2, or N2O)
was added into the reaction region downstream into the flow tube and
the reacting mixture was sampled by a second quadrupole mass
spectrometer. Reaction-rate coefficients were derived from the
measured variation of the ion signal intensities with the reagent
flow with an estimated accuracy of Æ 30%. In the study of metal-
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