The reduction of 15N14NO by CO and by H2 over Rh/SiO2: A test of a mechanistic proposal

Article abstract of DOI:10.1016/j.jcat.2010.11.005

A literature proposal that the reduction of nitrous oxide by carbon monoxide over rhodium catalysts proceeds by cleavage of the NN bond has been tested through the use of ¹⁵N¹⁴NO as the reactant. The results disprove the suggestion in that ¹⁴N¹⁵N is the dominant product at temperatures from 336 °C to 356 °C with nitrous oxide conversions from 26% to >99%. Little, if any, ¹⁴N ₂ and ¹⁵N₂ is formed, in contrast with the 25% of each expected for the model. Results for the corresponding reaction of ¹⁵N¹⁴NO with H₂ are even more clear-cut in demonstrating the absence of NN bond cleavage. The activity of the Rh/SiO ₂ used here for the N₂O/CO system fell within the rather wide of values reported in the literature for other Rh catalysts. However, activity for the reduction of N₂O by H₂ was approximately five times higher than the only previous result in the literature, that for a Rh/Al₂O₃ catalyst.

Full text of DOI:10.1016/j.jcat.2010.11.005

Journal of Catalysis 278 (2011) 162–166
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Research Note
The reduction of 15N14NO by CO and by H2 over Rh/SiO2: A test
of a mechanistic proposal
⇑
Noel W. Cant , Dean C. Chambers, Irene O.Y. Liu
Department of Chemistry and Biomolecular Sciences, Macquarie University, NSW 2109, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
A literature proposal that the reduction of nitrous oxide by carbon monoxide over rhodium catalysts pro-
ceeds by cleavage of the NAN bond has been tested through the use of 15N14NO as the reactant. The
results disprove the suggestion in that 14N15N is the dominant product at temperatures from 336 °C to
356 °C with nitrous oxide conversions from 26% to >99%. Little, if any, 14N2 and 15N2 is formed, in contrast
with the 25% of each expected for the model. Results for the corresponding reaction of 15N14NO with H2
are even more clear-cut in demonstrating the absence of NAN bond cleavage. The activity of the Rh/SiO2
used here for the N2O/CO system fell within the rather wide of values reported in the literature for other
Rh catalysts. However, activity for the reduction of N2O by H2 was approximately five times higher than
the only previous result in the literature, that for a Rh/Al2O3 catalyst.
Received 1 September 2010
Revised 4 November 2010
Accepted 10 November 2010
Available online 28 December 2010
Keywords:
Rh/SiO2, N2O + CO reaction
N2O + H2 reaction
15N14NO reduction mechanism
Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction
dissociation on a largely CO-covered surface without participation
of a vacant site (ꢂ), i.e.
The formation of N2O, a potent greenhouse gas, is an undesir-
able feature of the reduction of NO in automotive catalytic convert-
ers containing Pt, Pd or Rh. In tests with simulated exhaust
mixtures [1], Rh is the most active metal with N2O production
commencing at temperatures below 200 °C due to the reaction
COðgÞ þ ꢂ $ ꢂCO
ð3Þ
ð4Þ
ð5Þ
ð6Þ
N2OðgÞ þ ꢂ $ ꢂN2O
ꢂN2O ! N2ðgÞ þ ꢂO
ꢂCO þ ꢂO ! CO2ðgÞ þ 2ꢂ:
2NO þ CO ! N2O þ CO2:
ð1Þ
It starts to disappear above ꢀ350 °C due to the advent of N2O
Under the assumptions made, this model requires an integral order
of +1 for N2O if the order in CO is ꢁ1. McCabe and Wong [3] spec-
ulated that the discrepancy between +1 and the +0.65 order they
observed experimentally ‘‘reflects more complicated precursor
kinetics than the simple treatment in our kinetic model’’.
The non-integral order led Uner [12] to postulate a model in
which N2O, weakly adsorbed according to Eq. (3), dissociated by
reversible nitrogenAnitrogen bond cleavage with use of an empty
site
reduction
N2O þ CO ! N2 þ CO2:
ð2Þ
There have been a number of studies of the kinetics of the latter
reaction over rhodium in various forms [2–8]. All agree that the
kinetic order in CO is strongly negative and that this is largely com-
pensated by a positive order in N2O such that the overall order
becomes close to zero for stoichiometric mixtures. However, there
are some differences between the individual orders obtained and,
more particularly, in their interpretation.
The initial study using a supported catalyst, that of McCabe and
Wong [3] for Rh/Al2O3, reported orders of ꢁ1.0 0.15 and
+0.65 0.1 for CO and N2O, respectively. This was interpreted in
terms of a model, also adopted for Rh(1 1 1) [4], for Rh foil [2–5]
and in Cho’s model for the NO + CO reaction over Rh/Al2O3 [9–
11], in which N2O was adsorbed weakly and then underwent
ꢂN2O þ ꢂ $ ꢂN þ ꢂNO;
ð7Þ
ð8Þ
ð9Þ
followed by dissociation of the adsorbed NO
ꢂNO þ ꢂ ! ꢂN þ ꢂO;
and N2 formation by dimerization of N adatoms
ꢂN þ ꢂN ! N2ðgÞ þ 2ꢂ:
⇑
This model predicts a N2O order of two-thirds, in line with McCabe
and Wong’s experimental value, given the assumptions that NO
Corresponding author. Fax: +61 2 9850 8313.
E-mail address: noel.cant@mq.edu.au (N.W. Cant).
0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.11.005

N.W. Cant et al. / Journal of Catalysis 278 (2011) 162–166
163
does not desorb intact, that its dissociation by Eq. (7) is rate limiting
and that the surface is largely covered by bridge-bonded CO.
The model of Uner [12] does not seem very reasonable in that
the nitrogenAnitrogen bond in gaseous N2O is much stronger than
the nitrogenAoxygen one, 474 kJ/mol versus 161 kJ/mol [13].
Nonetheless, Holles et al. [6] concluded, on the basis of in situ
infrared spectroscopy and reaction order data, that the N2O + CO
reaction ‘‘appeared to involve N2O dissociating into N and NO with
subsequent dissociation of NO’’, as per the model of Uner [12]. In
particular, they observed the accumulation of NCO species, the for-
mation of which necessarily required cleavage of the NAN bond.
Recent DFT calculations have also indicated that cleavage of the
nitrogenAnitrogen bond in N2O is feasible on a Rh(1 1 1) surface,
via an intermediate in which N2O is bound through its terminal
atoms, albeit at a rate predicted to be several orders of magnitude
slower than that of nitrogenAoxygen cleavage [14].
The stream leaving the reactor was analysed on-line in three
ways – by quadrupole mass spectrometry (Balzers Thermostar
model GSD 300T) every 20 s, by gas chromatography (MTI model
M200 with molecular sieve and Poraplot U columns) every 3 min
during isotopic substitution and by FTIR spectroscopy (Nicolet
Magna 550 fitted with a 16 cm flow-through cell) accumulating
64 scans at 0.25 cmꢁ1 resolution every 10 min. The infrared spectra
were processed to obtain outlet concentrations using the program,
MALT, developed by Griffith [16] and, in the case of the isotopo-
mers of N2O, using a database attributable to Toth [17].
The 15N14NO was obtained from Cambridge Isotope Laborato-
ries with the chemical and isotopic purity each stated to be
>98%. Mass spectrometric analysis over the parent ion region
showed 1.0% 14N2O and 0.4% 15N2O, with the balance nitrous oxide
containing a single 15N. FTIR analysis showed that the latter was
>95% 15N14NO with 14N15NO below the detection limit of 5%. Unla-
belled and labelled nitrous oxides were each made up as ꢀ5% mix-
tures in helium and supplied from separate electronic mass
controllers with the two streams interchanged using a low volume
four-port valve (Valco Inst., 1/1600). The streams were run in turn
for 20 min each with the changeover between them completed
by the second subsequent cycle of the mass spectrometer (ꢀ40 s).
Kinetic studies subsequent to Uner’s proposal have examined
other models. Angelides and Tzitzios [7] tested their data for Rh/
Al2O3 for conformity to three models – that of McCabe and Wong
[3], a modified version in which N2O dissociation required an adja-
cent vacant site
ꢂN2O þ ꢂ ! N2ðgÞ þ Oꢂ þ ꢂ;
ð10Þ
and
a standard bimolecular Langmuir–Hinshelwood model in
3. Results and discussion
which the rate limiting step was the reaction between adsorbed
N2O and adsorbed CO
3.1. The reduction of N2O by CO
ꢂN2O þ ꢂCO ! N2ðgÞ þ CO2ðgÞ þ 2ꢂ:
ð11Þ
The lines in Fig. 1 show the course of the 14N2O + CO reaction
over the Rh/SiO2 catalyst when ramped up at 2 °C/min. Conver-
sions (X) were calculated from the concentrations (C) of nitrous
oxide and carbon dioxide using the relationships
The bimolecular model gave the best fit. However, the most recent
study, that of Granger et al. [8], favoured the modified (i.e. adjacent-
site requiring) version of the McCabe and Wong model on the basis
that the kinetic parameters obtained were in better conformity with
those for the NO + CO reaction.
XN2O ¼ 100ꢂ½CN2OðinÞ ꢁ CN2OðoutÞꢃ=CN2OðinÞ
and
ð12Þ
Overall, although the model proposed by Uner [12] does not
seem likely, a direct test, as is possible when nitrous oxide contain-
ing a single 15N atom is used, seems worthwhile. If 15N14NO is re-
acted with CO, then, according to the standard McCabe and Wong
scheme, the modified version of it and also the bimolecular model
of Angelides and Tzitzios [7], 14N15N alone should be produced. On
the other hand, Uner’s scheme implies that product nitrogen arises
through random combination of individual nitrogen atoms and
hence 14N2, 14N15N and 15N2 should be formed in the statistical
ratio 1:2:1. In the present work, we have carried out this experi-
ment and the corresponding one between 15N14NO and H2 where
some of the complicating features of the analysis that are present
when CO is the reductant are absent.
XCO ¼ 100ꢂCCO2ðoutÞ=CCOðinÞ:
ð13Þ
The concentration of nitrous oxide was monitored using the signal
from its fragment ion at m/z = 30 (14N16O+) and that of carbon
2. Experimental
The 0.5 wt% Rh/SiO2 sample was made by impregnation of silica
gel (Davison grade 62, BET surface area of 285 m2/g, particle size
180 to 210 lm), with a solution of RhCl3. The dispersion was 38%
as measured by H2 chemisorption following overnight drying at
140 °C and reduction in 10% H2/He on a ramp (at 1 °C/min) ending
with 2 h at 350 °C.
Catalytic measurements were carried out in a flow system de-
scribed in detail previously [15]. In essence, the reactant stream
(nominally 2000 ppm N2O, 2000 ppm CO or H2, balance He with
total flow rate of 100 cm3 (STP)/min) was passed down-flow over
75 mg of the Rh/SiO2 supported between quartz wool plugs in a
Pyrex reactor with an internal diameter of 6 mm. The sample of
Rh/SiO2 had been used previously in studies involving reactions
of mixtures of NO, CO and H2 [15] during which it exhibited con-
stant activity if flushed with He overnight between runs and the
same procedure was followed here.
Fig. 1. Conversions versus temperature for the reaction of 1927 ppm N2O with
1980 ppm CO over 75 mg of 0.5 wt% Rh/SiO2. The continuous curves are calculated
from the MS data for the reaction of 14N2O when the temperature of the catalyst
was ramped up at 2 °C/min. Symbols are for subsequent reaction at the fixed
temperatures used for the runs with 15N14NO. The inset plot shows an Arrhenius
plot for turnover frequencies over the conversion range from 5% to 20% during the
ramp-up.

164
N.W. Cant et al. / Journal of Catalysis 278 (2011) 162–166
Bypass analyses showed that the 15N14N16O used here frag-
mented to yield ions with m/z = 28, 29, 30 and 31 for which the
intensities were 0.6%, 19%, 42% and 4%, respectively, of the intensity
of the parent ion at m/z = 45. This pattern was qualitatively in good
agreement with the data of Begun and Landau [18] in showing a
minor extent of interchange between 15N and 14N during the course
of fragmentation. However, the extent of fragmentation to the ions
at 29 and 30 in the present work was approximately twice as great
as they found, presumably due mass spectrometer differences
(quadrupole here versus the magnetic sector instrument used in
their study).
Table 2 shows the raw ion currents observed in the four isotopic
substitution experiments, the corresponding signals after correc-
tion (in parentheses) and the distribution amongst the nitrogen spe-
cies derived from the latter. As may be seen, the magnitude of the
corrections to the signals at m/z = 28 and 30 was very large, >95%
of the raw signals in some cases. The uncertainties shown for the
distributions are calculated on the basis of an estimated 3% error
in each of the systematic corrections applied to the raw signals.
The estimate for 14N2 includes an additional uncertainty, allowed
for to the extent shown in the footnote, to account for a noticeable
drift in the background during the course of the measurements.
The model of Uner predicts that the nitrogen produced by the
reduction of 15N14NO by CO should comprise 14N2, 14N15N and
15N2 in the ratio 1:2:1 given the random combination of nitrogen
adatoms implied by reaction Eq. (8) Despite the magnitude of the
corrections required, it is clear from the data in Table 2 that the
amounts of 14N2 and 15N2 formed, if any, were very small relative
to that of 14N15N. In particular, the amount of 15N2 formed was cer-
tainly <1% based on the most accurate measurement, that at 356 °C
when the conversion of nitrous oxide was >99% and the correction
for its fragmentation very small. Thus, reaction according to the
model proposed by Uner [12] is ruled out. On the other hand, the
exclusive production of 14N15N is in accord with the other mecha-
nistic models in the literature.
dioxide from the signal at m/z = 22 attributable to the doubly
charged COþ2 þ ion alone. The signals of the parent ions could not
be used as the resolution of the mass spectrometer was insufficient
to resolve 14N2 from 12C16O (both at m/z = 28) or 14N216O from
12C16O2 (both at m/z = 44).
Table 1 compares the activity of the Rh/SiO2 catalyst with liter-
ature data for other catalysts in terms of turnover frequency (TOF)
expressed as molecules N2O per surface rhodium atom per second.
The range of experimental conditions in the studies was quite large
and the activities reported by the authors have been converted to a
mid-point temperature (340 °C) and the concentrations used here
using the values given for activation energies and kinetic orders.
The present results are reasonably close to those in the original
study by Wong and McCabe [3] in terms of both activity and acti-
vation energy. However, the activity was much below that of
Belton and Schmieg [4] for a Rh(1 1 1) surface and also that of
Granger et al. [8] for a very highly dispersed Rh/Al2O3 catalyst.
Experiments using 15N14NO in place of 14N2O were carried out
subsequent to completion of the ramp at four temperatures in the
sequence 1–4 as shown in Fig. 1. The conversions determined by
gas chromatography (gc) and FTIR spectroscopy (FT) were in good
agreement and close to those measured by mass spectrometry
during the initial ramp-up with 14N2O. FTIR spectra of the unre-
acted nitrous oxide from the experiments at the three lowest
temperatures (not shown) did not show any 14N15NO (i.e. <5%
as in the starting material), thus excluding isomerization of
15N14NO.
The calculation of the distribution between the three isotopic
nitrogen product species from the mass spectrometer signals with
m/z = 28, 29 and 30 required accurate correction for the contribu-
tions from several interfering species. The most significant correc-
tions were the contributions from fragmentation of unreacted
15N14NO to the signals at m/z = 29 and 30 and for the combined
contributions of unreacted carbon monoxide and fragmentation
of carbon dioxide (yielding 12C16O+) to the signal at m/z = 28. The
allowance for unreacted carbon monoxide included correction for
the contribution of natural abundance 13C16O to the signal at
m/z = 29 and of natural abundance 12C18O to m/z = 30. The former
correction was always insignificant but the latter became signifi-
cant when carbon monoxide conversions were low.
3.2. The reduction of nitrous oxide by hydrogen
A similar set of experiments was carried out for the reduction of
nitrous oxide by hydrogen (Fig. 2). Here, it was possible to follow,
Table 1
Activation energies and turnover frequencies for the reaction of N2O with CO.
Reference
Sample
DRh (%)
% in feed
N2O
Ea (kJ/mol)
Kinetic order
N2O
TOF (sꢁ1) at 340 °Ca
CO
CO
McCabe and Wong [3]
Belton and Schmieg [4]
Holles et al. [6]
Granger et al. [8]
This work
0.5% Rh/Al2O3
Rh(111)
1.85% Rh/Al2O3
0.2% Rh/Al2O3
0.5% Rh/SiO2
33
–
40
93
38
1.0
0.5
4.0
0.6
0.2
1.1
0.5
4.0
0.6
0.2
169
167
109
137
175
0.65
1.14
0.72
0.75
nd
ꢁ1
0.022
0.40
0.061
0.50
ꢁ1.18
ꢁ0.86
ꢁ0.54
nd
0.030
nd: not determined.
a
Interpolated at, or extrapolated to, 340 °C and the feed composition used in this work based on the activation energies and the kinetic orders reported by the authors.
Table 2
Distribution of isotopic N2 species for the reaction of 15N14NO with CO over Rh/SiO2.
Temperature (°C)
N2O (CO) conversions (%)
1010 ꢄ Ion currenta (amps)
Distributionb (%)
14N2
28
29
30
14N15
N
15N2
356
346
336
350
99.2 (96.1)
52 (49)
26 (23)
752 (ꢁ16)
3657 (ꢁ91)
5204 (ꢁ121)
2931 (10)
5534 (5526)
2982 (2760)
1664 (1328)
3618 (3447)
52 (32)
ꢁ0.3 0.6
Balance
Balance
Balance
Balance
0.6 0.1
1.0 0.5
1.4 1.5
0.8 0.3
532 (28)
779 (19)
416 (29)
ꢁ3
4
ꢁ9 12
0.3
64 (62)
3
a
Raw signals with signals after adjustment for contributions from other species in parenthesis.
b
The errors shown correspond to an estimated uncertainty of 3% in each adjustment for other species and, in the case on 14N2, an absolute uncertainty of 10 ꢄ 10ꢁ10 amps
in the background signal at m/z = 28 (see text).

N.W. Cant et al. / Journal of Catalysis 278 (2011) 162–166
165
N2OðgÞ þ ꢂ ! N2ðgÞ þ Oꢂ
ð14Þ
H2ðgÞ þ Oꢂ ! H2OðgÞ þ ꢂ:
ð15Þ
The rate expression for this model is
Rate ¼ k1k2CN2O H2=ðk1CN2O þ k2CH2Þ:
C
ð16Þ
The dashed line in Fig. 2 showing the conversions predicted by
this model was obtained assuming integral reactor behaviour. Eq.
(16) was integrated exactly and the resultant expression solved
at a closely spaced set of temperatures using a simple iterative
method.
With CN2O@CH2 (@Ci), as close to true in the experiments here,
Eq. (16) reduces to the first order expression
Rate ¼ kappCi where kapp ¼ k1k2=ðk1 þ k2Þ:
ð17Þ
The dashed line in the inset plot on Fig. 2 shows an Arrhenius
plot for kapp calculated directly from the k1 and k2 given by Miyam-
oto et al. [19]. The points in the inset plot were calculated from our
experimental data using an equation with the standard form for
integral first order behaviour
Fig. 2. Conversions versus temperature for the reaction of nominally 2036 ppm N2O
with 2076 ppm H2 over 75 mg of 0.5 wt% Rh/SiO2. The continuous curves are
calculated from MS data for the reaction of 14N2O when the temperature of the
catalysts was ramped up at 2 °C/min. Symbols are for subsequent reaction at the
fixed temperatures used for the runs with 15N14NO. The inset plot shows an
Arrhenius plot for the apparent first order rate constant (see text) over the
conversion range 7–80% during the ramp-up.
kappF=A ¼ ꢁlnð1 ꢁ XÞ
ð18Þ
where X is the fraction conversion and F (the gas flow rate) and A
(the surface area of Rh) bring the units for kapp in line with the
cm/s units used by Miyamoto et al. [19]. The conversion range is
7–80%, with higher conversion data excluded to avoid a potential
departure from first order behaviour as the assumption that H2
and N2O concentrations are exactly equal starts to break down.
The best-fit slope to the points corresponds to an activation energy
for kapp of 45 kJ/mol, in close agreement with that expected for the
model of Miyamoto et al. [19] for the same temperature range,
namely 49 kJ/mol at 40 °C falling to 43 kJ/mol at 100 °C.
Table 3 shows the analytical data for the two isotope experi-
ments. In the absence of CO and CO2, corrections to the signal at
m/z = 28 were much smaller than necessary for the data for the
N2O + CO reaction in Table 2. This reduced the uncertainty in the
determination of 14N2 despite a greater drift in the baseline at
m/z = 28. However, the determination of 15N2 was somewhat less
certain as the extent of fragmentation of 15N14N16O to 14N16O+
within the mass spectrometer in this experiment was slightly dif-
ferent from that in the run with CO, 38% versus 42%, and was estab-
lished less accurately.
with varying degrees of accuracy, the concentrations of all four
chemical species in the 14N2O + H2 reaction system by mass
spectrometry. Nitrous oxide could be monitored in two ways
(using the signals due to the parent ion at m/z = 44 and the
fragment ion at 30). Likewise, water could be followed in two
ways (m/z = 17 and 18). The least accurate determinations were
for N2 (due to the drift in the background signal at m/z = 28 arising
from CO produced by reactions involving carbon on the tungsten
source filament) and water (due to adsorption on the silica
support).
All measurements showed that conversions were significant
(ꢀ5%) at 30 °C and rose with temperature to a maximum of 90%
at 130 °C with a slight fall and then rise again at higher tempera-
tures. Two isotope substitution experiments were carried out sub-
sequently, at 114 °C and 72 °C, where the conversions were below
those during the initial ramp-up. Despite the deactivation, the con-
versions observed here were much greater than those expected
from the only previous study of the N2O + H2 reaction over sup-
ported rhodium, that of Miyamoto et al. [19] for a 0.5 wt% Rh/
Al2O3 catalyst with a dispersion of ꢀ49%. In terms of turnover fre-
quency, the Rh/SiO2 here exhibited a value of 0.022 sꢁ1 during the
second isotope experiment at 72 °C versus 0.0044 sꢁ1 calculated
from the parameters of the kinetic model proposed in their work,
i.e.
The results in Table 3 are even more clear-cut than those in
Table 2 for the corresponding reaction using carbon monoxide –
formation of both 14N2 and 15N2 was negligible compared with that
of 14N15N. The most accurate determination of the minor species,
that of 15N2 at 114 °C, is <0.5%. Thus, as with data for the reaction
of 15N14NO with CO, there was no detectable cleavage of the NAN
bond.
Table 3
Distribution of isotopic N2 species for the reaction of 15N14NO with H2 over Rh/SiO2.
Temperature (°C)
N2O (H2) conversion (%)
1010 ꢄ Ion currenta (amps)
Distributionb (%)
14N2
28
29
30
14N15
N
15N2
114
72
72
(71)
117
(76)
3811
(3675)
292
(12)
2
1
Balance
0.3 0.4
23
(21)
55
(6)
1197
(843)
718
(ꢁ6)
1
4
Balance
ꢁ1
3
a
Raw signals with signals after adjustment for contributions from other species in parenthesis.
b
The errors shown correspond to an estimated uncertainty of 5% in each adjustment for the extraneous signals from species other than nitrogen and, in the case of 14N2,
an uncertainty of 40 ꢄ 10ꢁ10 amps in the background signal at m/z = 28.

166
N.W. Cant et al. / Journal of Catalysis 278 (2011) 162–166
4. Conclusions
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The present results show that 14N15N is the exclusive product of
the reduction of 15N14NO by CO over supported rhodium with very
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than previously reported for a Rh/Al2O3 catalyst with similar dis-
persion, but its activity for the N2O + CO reaction was within the
rather wide range of values in the literature.
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Acknowledgment
This work was supported as part of a grant from the Australian
Research Council.