No reduction by urea under lean conditions over single-step sol-gel Cu/alumina catalyst

Article abstract of DOI:10.1006/jcat.2002.3572

We present the catalytic activity of 1% Cu on alumina catalyst prepared using a single-step sol-gel process (designated 1% Cu-SG) as a function of oxygen, water, and SO₂ in the feed for the reduction of NO with an aqueous urea solution. Our results show that NO_x conversion activity of 1% Cu-SG is dependent on oxygen in the feed. The maximum conversion increased from 91 to 99% when the amount of oxygen in the feed was changed from 2 to 14%. Also, the location of maximum conversion temperature decreased from 400 to 377°C. Short-term and long-term exposure of the catalyst to a gas mixture containing 25 ppm SO₂ did not result in activity loss at any temperature as long as aqueous urea was present in the feed. However, temporary interruption of aqueous urea solution resulted in permanent activity loss. Our attempt to reactivate the catalyst with propene failed to recover the original activity.

Full text of DOI:10.1006/jcat.2002.3572

Journal of Catalysis 208, 15–20 (2002)
doi:10.1006/jcat.2002.3572, available online at http://www.idealibrary.com on
NO Reduction by Urea under Lean Conditions over Single-Step
Sol–Gel Cu/Alumina Catalyst
∗
∗
Erol Seker, Nail Yasyerli, Erdogan Gulari,∗,1 Christine Lambert,† and Robert H. Hammerle†
∗
Chemical Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136; and †Ford Scientific Research
Laboratories, Dearborn, Michigan 48124
Received June 28, 2001; revised February 13, 2002; accepted February 13, 2002
decomposed in situ to generate ammonia.
We present the catalytic activity of 1% Cu on alumina catalyst
prepared using a single-step sol–gel process (designated 1% Cu–SG)
as a function of oxygen, water, and SO2 in the feed for the reduc-
tion of NO with an aqueous urea solution. Our results show that
NOx conversion activity of 1% Cu–SG is dependent on oxygen
in the feed. The maximum conversion increased from 91 to 99%
when the amount of oxygen in the feed was changed from 2 to
14%. Also, the location of maximum conversion temperature de-
creased from 400 to 377◦C. Short-term and long-term exposure of
the catalyst to a gas mixture containing 25 ppm SO2 did not re-
sult in activity loss at any temperature as long as aqueous urea was
present in the feed. However, temporary interruption of aqueous
urea solution resulted in permanent activity loss. Our attempt to
reactivate the catalyst with propene failed to recover the original
c
CO(NH2)2 ⇒ NH3 + HNCO
HNCO + H2O ⇒ NH3 + CO2
[1]
[2]
It seems that urea, as ammonia source, is the best choice
for such applications because urea is not toxic and also
can be easily transported as a high-concentration aqueous
solution. As a result, NOx can be reduced with not only
ammonia but also the urea itself and its decomposition by-
product, HNCO, as shown in reactions [3]–[5].
2CO(NH2)2 + 6NO ⇒ 5N2 + 2CO2 + 4H2O
4HNCO + 6NO ⇒ 5N2 + 4CO2 + 2H2O
4NH3 + 4NO + O2 ⇒ 4N2 + 6H2O
[3]
[4]
[5]
activity.
ꢀ 2002 Elsevier Science (USA)
Key Words: selective NO reduction; copper/alumina catalyst; sol–
gel preparation; urea; SO2.
Even though the use of urea in the reduction of NOx from
the flue gas streams of power plants is a well-established
method (3), there have not been many studies on the use of
urea as a reductant in treatment of the exhaust of lean-burn
engines.
1. INTRODUCTION
The removal of nitrogen oxides from the exhaust of
Held et al. (4) was the first to demonstrate that a Cu/ZSM-
a diesel vehicle is still a very challenging problem even 5-coated monolith catalyst was active in the reduction of
though there have been many studies. Hydrocarbons and NOx by urea under laboratory conditions. However, cata-
oxygenated hydrocarbons seemed to be possible reducing lysts based on zeolites proved to be unstable in the hy-
agents in the reduction of nitrogen oxides (1, 2). However, it drothermal environment of engine exhaust. In addition,
was soon discovered that they had low-to-moderate reduc- V2O5-based catalysts were studied for the reduction of
ing efficiency and resulted in undesired by-products. Am- NOx by urea in real diesel exhaust gas (5–7). These stud-
monia, on the other hand, has been used as a reducing agent ies raised questions as to urea dosage control and, for au-
to remove NOx from the flue gas streams of power plants. tomotive applications, the appropriateness of using highly
Using ammonia as a reductant for the removal of NOx from toxic vanadia-based catalysts. Even though there have been
automobile exhaust gas may not be commercially viable be- many studies on the selective catalytic reduction of NO with
causetherearedifficultiesinthestoringandhandlingofam- urea under oxidizing conditions, there are limited reports
monia and a robust controller is required to accommodate on the hydrolysis of HNCO and urea. Koebel and Elsener
the rapidly changing load conditions of automobile exhaust (8) studied the decomposition/hydrolysis products of urea
gas with little ammonia slip. To overcome the difficulties as- over a SCR catalyst and also evaluated urea as a possible re-
sociated with pure ammonia, urea can be hydrolyzed and ducing agent for the catalytic reduction of NO. They found
low emissions of higher molecular mass compounds, such
as melamine, formed during the hydrolysis and decompo-
sition of urea. They also pointed out that the emission of
urea, isocyanic acid, and NH3 was possible under nonideal
1 To whom correspondence should be addressed. Fax: 1-734-763-0459.
E-mail: gulari@engin.umich.edu.
15
0021-9517/02 $35.00
c
ꢀ 2002 Elsevier Science (USA)
All rights reserved.

16
SEKER ET AL.
operating conditions of the catalyst. Dumpelmann et al. the reactor was higher, especially at high temperatures, due
(9) reported that the hydrolysis of HNCO over alu- to the receding interface of gas and liquid in the urea feed-
mina reached completion but the hydrolysis mechanism of ing tube, resulting in an increased rate of urea injection. For
HNCO over alumina was unclear. Kleemann et al. (10) also this reason, we report the urea concentration as ∼300 ppm,
studied the hydrolysis of HNCO over SCR catalysts and re- which was found to be the maximum and stayed constant
ported that a typical vanadia on titania catalyst was highly above 300◦C, determined through CO2 measurement with
active for HNCO hydrolysis.
In this paper, we report the catalytic activity of 1%
FTIR.
The reactor outlet stream was analyzed by using a
Cu on alumina catalyst, prepared by a single-step sol–gel Thermo Environmental 42CHL NOx chemiluminescence
method, for NOx reduction with aqueous urea solutions analyzer to determine unreacted NOx and also a FTIR with
under oxidizing conditions in the absence and presence a 10-cm path length gas cell (Galaxy 7020 Spectroscopy by
of SO2.
Mattson, Inc., and WinFirst Software version 3.61 for data
analysis) for quantitative determination of CO2, CO, and
N2O at each temperature. The activity measurements were
reproducible within 2% and the N2O selectivity measure-
ments were accurate to ∼5 ppm, resulting in an error bar
of 2% in N2 selectivity at 50% NOx conversion.
2. EXPERIMENTAL DETAILS
2.1. Catalyst Preparations
We prepared the 1% Cu on alumina catalyst using a sol–
gel method. Aluminum tri-sec-butoxide, 95% pure (from
Alfa Aesar, Inc.), was used to synthesize the catalyst in
one step at room temperature. Aluminum tri-sec-butoxide
was first mixed with the water–ethanol solution (35 mol of
ethanol/1 mol of Al and 8 mol of water/1 mol of Al) and
then the necessary amount of cupric nitrate, 99%+ (from
Aldrich, Inc.), was added to the sol solution to obtain 1%
Cu loading. Details are given in Ref. (11). The gel was dried
in air at 100◦C for 12 h to remove the solvent and water.
The dry gel was put in a furnace and its temperature was
increased from room temperature to 600◦C with an 8◦C/min
heating rate. Once 600◦C was reached, it was kept at this
temperature in air for 24 h and was ground and sieved to
80–120 mesh size.
2.3. Catalyst Characterization
Approximate crystallite size and phases present in the
catalystweredeterminedbyX-raydiffraction(Rigakupow-
der diffractometer, operated at 40 kV and 100 mA). BET
surface area was measured with a Micromeritics 2010 in-
strument. Prior to analysis, the sample was degassed under
vacuum at 300◦C until the vacuum inside the sample tube
stayedconstantataround5 µmHg. AstandardMicromerit-
ics program was employed to calculate both BET surface
area and BJH pore size distribution (using the desorption
isotherm).
3. RESULTS
2.2. Catalyst Testing
3.1. Catalyst Characterization
In all experiments, 0.1 g of catalyst was held between
two quartz wool plugs in a quartz U-tube (I.D., 3 mm) flow
reactor and tested under a total flow rate of 176 ml/min at
1 atm and room temperature. The reactant gas mixture was
blended by using four independent mass flow controllers
to give 300 ppm NO (plus ∼10 ppm NO2 impurity in the
NO cylinder), 2–14% O2, 2–8% water, 25 ppm SO2 (when
used), and He as balance.
Water and aqueous urea solution were used to get the
desired water and urea concentration in the feed. The nec-
essary amount of urea (0.6–1.2 g) was first dissolved in 50 ml
of water and then injected into the feed stream with a peri-
staltic tube pump (Cole Palmer) set to the desired flow rate
XRD spectrum of the fresh 1% Cu–SG catalyst showed
alumina peaks (γ and η) at 2θ of ∼39, ∼45.5, and ∼66.8◦.
However, we did not observe any peak corresponding to
Cu, CuO, or Cu2O. The sol–gel synthesis and calcination
treatment that we used produced 1% Cu–SG catalyst with
BET surface area of ∼297 m2/g and a narrow pore size dis-
˚
tribution centered at D ∼ 55 A. In contrast, when alumina
is synthesized alone there is no change in surface area but
˚
the average pore size becomes ∼77 A.
3.2. Oxygen and Water Effect on the Activity
Figure 1 shows the catalytic activity of 1% Cu–SG cata-
to obtain 2–8% water and corresponding urea concentra- lyst as a function of oxygen concentration. Catalyst (0.1 g)
tion in the gas phase. The aqueous solution mixed with the was tested under 176 ml/min (1 atm and room tempera-
feed gas stream was heated in the entrance section of the ture) of a reactant gas mixture containing 300 ppm NO, 4%
U-tube reactor (the residence time in this heating section water, 300 ppm urea (based on CO2 amount measured with
was around 0.2 s) before entering the catalyst bed. The tem- FTIR), 2–14% oxygen, and He as balance. For all oxygen
perature of this section was the same as that of the reactor concentrations, the conversion versus temperature had an S
bed. Even though the pump was set to a flow rate to obtain shape. At 2% oxygen in the feed, the catalyst showed ∼5%
150 ppm urea in the gas phase, we found that the urea fed to conversion at 250◦C and ∼60% at 350◦C. The maximum

SELECTIVE REDUCTION OF NO WITH UREA OVER Cu/Al2O3
17
3.3. Comparison of the Activity Obtained with Urea
to that Obtained with NH3
Figure 2 shows the activity of 1% Cu–SG as a function
of NH3 and urea in the feed. A mixture of 300 ppm NO,
600 ppm NH3 or 300 ppm urea, 7% oxygen, 4% water, and
helium as balance was passed over 0.1 g of the catalyst. In
this case, NH3 was introduced as a gas and the necessary
amount of water was injected into the hot section of the
reactor. When urea was used instead of NH3, it was first
dissolved in 50 ml of water and then aqueous urea solution
was injected into the heated section of the reactor entrance.
As seen in the figure, when ammonia in the feed is 600 ppm,
the conversion versus temperature curve up to 400◦C is sim-
ilar to that obtained with 300 ppm urea in the feed. Con-
version (∼15%) at 250◦C increased to ∼83% at 350◦C and
the maximum conversion, ∼97%, was reached at 400◦C.
In contrast, the constant conversion of ∼98% between 450
and 500◦C obtained with aqueous urea dropped to ∼81% at
450◦C and to ∼31% at 500◦C when there was 600 ppm NH3
in the feed. In addition, we found that as seen in Fig. 3,
the oxidation of NH3 to N2 in the absence of NO over
1% Cu–SG was highly selective to N2. Conversion (∼10%)
to N2 occurred at 350◦C and ∼77% conversion to N2 was
reached at 450◦C and stayed at this level until 500◦C. The
only by-product was NO and we did not observe any N2O or
NO2. The conversion to NO reached a maximum of 2% at
500◦C.
FIG. 1. The activity of 1% Cu–SG as a function of oxygen. Reaction
conditions: 300 ppm NO, the above oxygen concentrations, 4% water,
300 ppm urea, and He as balance. Also, 0.1 g of catalyst and 176 ml/min
(1 atm and room temperature) of flow rate.
conversion, ∼91%, was reached at 400◦C and conversion
stayed at this level until 500◦C. When oxygen in the feed is
increased to 7%, ∼15% NOx conversion occurs at 250◦C
and the conversion reaches ∼86% at 350◦C. The maximum
∼98% conversion is obtained at 400◦C and stays at 98% un-
til 500◦C. Increasing the oxygen concentration to 14% did
not change the activity of 1% Cu–SG above 350◦C within
our experimental error even though we observed a slight
enhancement in the activity between 250 and 300◦C. Also,
at all temperatures and oxygen concentrations we did not
observe any N2O formation in the feed. The only product
was N2.
3.4. SO2 Effect on the Activity
Figure 4 shows the comparison of the catalytic activities
obtained under steady state condition with 25 ppm SO2
CO2 analysis of the reactor outlet with FTIR showed
that 12 ppm CO2 formed at 150◦C and increased to 54 ppm
at 200◦C. CO2 formation further increased to 190 ppm at
250◦C and 300 ppm at 300◦C. Then it stayed at this level
until 500◦C. Our preliminary NH3 analysis of the reactor
outlet revealed fewer than 100 ppm NH3 at temperatures
below 300◦C but above 350◦C there was no NH3 at the
outlet. This was also confirmed with the determination of
N2 production, measured by the GC, at all temperatures. In
fact, we found that the amount of N2 produced above 300◦C
was ∼47% higher than the amount expected through the
selective reduction reaction.
We also tested the activity of 1% Cu–SG catalyst as a
function of water concentration in the feed. We found that
the conversion-versus-temperature curve up to 450◦C did
not change when water concentration was changed from
2 to 8% (data not shown). For all water concentrations,
∼15% conversion occurred at ∼250◦C and the conversion
reached ∼98% at 400◦C and stayed there until 450◦C within
our experimental error.
FIG. 2. The activity of 1% Cu–SG as a function of urea and ammonia.
Reaction conditions: 300 ppm NO, 7% oxygen, 4% water, the above urea
and NH3 concentrations, and He as balance. Also, 0.1 g of catalyst and
176 ml/min (1 atm and room temperature) of flow rate.

18
SEKER ET AL.
Then we retested the catalyst with the gas mixture contain-
ing 300 ppm NO, 300 ppm urea, 4% water, 7% oxygen, and
helium as balance. As seen in Fig. 4, the activity of 1% Cu–
SG did not change at any temperature. In both cases, ∼17%
conversion occurred at 250◦C and the conversion reached
98% at 400◦C and stayed until 450◦C was reached. We ob-
served a slight decrease in conversion, ∼96%, at 500◦C.
4. DISCUSSION
To our knowledge, this is the first report ever published
in the literature on the activity of Cu on alumina catalyst
for NO reduction with aqueous urea solution under oxidiz-
ing conditions. On the other hand, there are many studies
on NO reduction with NH3 and also on the oxidation of
NH3 to N2 over Cu on alumina. It is difficult to make a
sound comparison between different laboratories because
of different reaction conditions, preparation methods, and
copper loading. However, we will try to explain why the ob-
served activity and N2 selectivity of our 1% Cu–SG changed
as a function of oxygen, water and SO2 in the feed.
FIG. 3. The NH3 oxidation to N2 activity of 1% Cu–SG as a function
of temperature. Reaction conditions: 600 ppm NH3, 7% oxygen, 4% water,
and He as balance. Also, 0.1 g of catalyst and 176 ml/min (1 atm and room
temperature) of flow rate.
4.1. Oxygen and Water Effect
in the feed to that obtained after keeping the catalyst un-
der the gas mixture containing 25 ppm SO2 in the feed for
2 days on stream. For a steady state test, we used a reac-
tant gas mixture containing 300 ppm NO, 7% oxygen, 4%
water, 25 ppm SO2, 300 ppm urea, and He as balance. For
the long-term effect of SO2 on the activity, we kept a 1%
Cu–SG catalyst at 300◦C for 2 days under the reaction gas
mixture containing SO2. At the end of this period, the cata-
lyst was treated with 5% oxygen in helium at 500◦C for 20 h.
Our results showed that water concentration in the feed
didnotchangetheNOconversionactivityofthe1%Cu–SG
catalyst. This may be due to the constant amount of ammo-
nia formation regardless of water concentration in the feed.
Indeed, we found that the decomposition/hydrolysis of urea
during the NO reduction yielded similar CO2 amounts for
all water concentrations in the feed within our experimen-
tal error. This indicates that the decomposition of urea
seems to be the rate-determining step and also strongly de-
pendsonthetemperature, whereasthehydrolysisofHNCO
seems to be very fast and reaches completion and, hence, is
independent of water concentration for the constant urea
in the feed.
At 4% water, an S-shape conversion versus temperature
was obtained regardless of oxygen content in the feed, as
shown in Fig. 1. When oxygen in the feed was increased
from 2 to 14%, the activity of the catalyst roughly doubled
and the maximum conversion temperature dropped to 377
from 400◦C while the conversion increased from ∼91 to
∼99%. In contrast, Held et al. (4) reported that the activity
of Cu/ZSM-5 monolith catalyst did not change as a function
of oxygen in the feed when an aqueous urea solution was
the reductant. Also, they showed that the NOx conversion
reached ∼100% between 300 and 350◦C and then dropped
to ∼90% at 400◦C. Similarly, Morimune et al. (7) found that
over TiO2/V2O5 monolith catalyst, the maximum NO con-
version, ∼80%, occurred at 400◦C when the ratio of NH3
(generated by hydrolysis of urea) to NO was 1.5, and above
400◦C, the conversion decreased and the formation of N2O
increased. However, over 1% Cu–SG catalyst under similar
reaction conditions, we observed ∼99% NOx conversion
FIG. 4. Long- and short-term activity of 1% Cu–SG under the gas
mixture containing 25 ppm SO2. Reaction conditions: 300 ppm NO, 7%
oxygen, 4% water, 300 ppm urea, and He as balance. Also, 0.1 g of catalyst
and 176 ml/min (1 atm and room temperature) of flow rate.

SELECTIVE REDUCTION OF NO WITH UREA OVER Cu/Al2O3
19
between 377 and 450◦C and 96% conversion at 500◦C. Also, urea. In addition, we found that 1% Cu–SG was highly
at no temperature did we observe N2O formation and N2 active and selective to N2 in the oxidation of NH3, as shown
was the only product seen. Even though the concentra- in Fig. 4. As a result, NO reduction with NH3 under the
tion of urea in the feed was higher than the stoichiometric oxidizing condition above 400◦C decreases sharply because
amount (Nreducing/NOx = 2), our preliminary NH3 analysis of the increased rate of the oxidation of NH3 to N2.
revealed that approximately 100 ppm NH3 at the exit of the
As compared to that of 1% Cu on alumina and 3%
reactor was present below 300◦C. However, above 350◦C, Cu/ZSM-5 catalysts reported in Refs. (19) and (20) under
we did not observe the formation of NH3. The reason is similar reaction conditions, the oxidation of NH3 and also
most likely oxidation of the excess amount of N-containing NO reduction with NH3 under oxidizing conditions over
compounds, such as NH3 or HNCO, to N2 or NO during our 1% Cu–SG catalyst is highly active and selective to N2.
the reduction of NO. It is plausible that the excess amount This may be due to the stabilized small CuOx crystallites
of urea is decomposed and hydrolyzed to NH3 and HNCO (diameter less than 5 nm based on XRD measurements) in
and that these N-containing compounds are oxidized to N2 the Al2O3 network induced during the sol–gel preparation
above 350◦C during the reduction of NO. In fact, we found used in this study. Because Centi et al. (19) and Ramis et al.
that the amount of N2 measured using the GC was ∼47% (13) showed that the copper oxide was the active phase for
higher than the amount of N2 that one could calculate for NO reduction with NH3 and the oxidation of NH3 to N2
the reduction of NO with NH3, HNCO, or urea at temper- under the oxidizing conditions, respectively.
atures above 350◦C. However, N2 balance calculation still
shows less than 30 ppm NH3 slip above 350◦C. This differ-
4.2. SO2 Effect
ence between NH3 analysis and N2 balance seems to be due
Centi et al. (18) reported that above 325◦C, a deep sul-
to either adsorption of NH3 on the catalyst or losses during
fation of the alumina support in addition to sulfation of
the NH3 analysis. This is plausible because it is known that
copper oxide was favored in the presence of SO2 and oxy-
NH3 adsorbs on alumina. At this time, we do not have the
gen, resulting in a detrimental effect on the regenerability
necessary analytical instrumentation for a detailed analysis
and the stability of the alumina. As shown in Fig. 4, under
of the product stream. The difference between this study
steady state conditions (1-h reaction time for each temper-
and others may be due to the different Cu loading, the na-
ature) or 2 days of exposure to the reaction gas mixture,
ture of support, and the preparation technique. We will re-
the presence of 25 ppm SO2 did not hinder the NOx con-
port the effect of Cu loading and the preparation method
version activity at any temperature. This seems to indicate
on the activity of Cu on alumina catalyst in the near future.
that neither deep sulfation of alumina nor the formation of
Katona et al. (12) reported that over polycrystalline plat-
CuSO4 occurred during the steady state or long-term tests
inum, NO was reduced by ammonia at a very high rate
when there was urea in the feed. In the literature, there
in the presence of oxygen. Also, Ramis et al. (13) showed
are conflicting reports on the activity of CuSO4 for NO re-
that dissociative adsorption of NH3, leading to adsorbed
duction by NH3. Recently, Centi et al. (19) showed that the
NH2 and H, was the first possible step in the reduction
copper oxide was more active than the copper sulfate. This
of NO with NH3 in the presence of oxygen. Similarly, we
is also in agreement with our findings. In fact, we found that
could speculate that the increased NO conversion activity
the activity of the catalyst decreased if the supply of aque-
as a function of oxygen may be due to the dissociative ad-
ous urea was interrupted temporarily, e.g., for 1 h. In this
sorption of ammonia over copper oxide to adsorbed NH2
case, the maximum conversion irrecoverably dropped from
or N- and H-containing species and that these N- and H-
99 to 89% at 450◦C. Our attempt to reactivate the catalyst
containing species react with NO to yield nitrogen. This is
using propene failed. This may be due to bulk aluminum
plausible because 1% Cu–SG catalyst decomposes and hy-
sulfate formation, leading to plugging of pores. Indeed, the
drolyzes urea starting at 250◦C, thus resulting in NH3 or
measurement of BET and pore size distribution of this used
HNCO. Increased oxygen concentration will accelerate the
catalyst revealed loss of surface area from ∼297 to 199 m2/g
reoxidation of copper patches formed as a result of the dis-
and an increase in the average pore diameter from D ∼ 55
sociative adsorption of NH3 on CuOx particles and move
˚
to ∼83 A.
the NO reduction to lower temperatures, similar to the ef-
fect of oxygen on the oxidation of propene (17).
AsseeninFig. 2, above400◦C, NOx conversiondecreased
5. CONCLUSIONS
sharply when gaseous NH3 was the reductant instead of
urea under similar reaction conditions. This indicates that
NH3 is not the only reductant during the reduction of NO the same reaction conditions.
• Urea is a more efficient reductant than ammonia under
with urea in the presence of oxygen and water over 1%
• The maximum NOx conversion temperature is depen-
Cu–SG catalyst. This is plausible because Koebel et al. (5) dent on oxygen in the feed. Regardless of oxygen concen-
reported that the formation of HNCO and NH3 increased tration, an S-shaped conversion-versus-temperature curve
with temperature during the decomposition/hydrolysis of is observed under our reaction conditions.

20
SEKER ET AL.
2. Kramlich, J. C., and Linak, W. P., Prog. Energy Combust. Sci. 20, 149
• N2 is the only product of NO reduction with urea and
(1994).
also nitrogen selectivity is independent of oxygen in the
feed.
• NOx conversion and decomposition/hydrolysis of urea
was not dependent on water concentration in the feed
above 2% water.
• Exposure of 1% Cu–SG to the reaction gas mixture
containing 25 ppm SO2 either under steady state condition
or after 2 days on stream did not hinder activity at any
temperature.
• Comparisonoftheactivityof1%Cu–SGobtainedwith
300 ppm urea to that obtained with 600 ppm NH3 shows
that the reduction of NO proceeds through not only NH3
but also other N-containing compounds.
3. Hug, H. T., Mayer, A., and Hartenstein, A., SAE Technical Paper
Series, 930363, Detriot, March 1–5, (1993).
4. Held, W., Konig, A., Richter, T., and Puppe, L., SAE Paper, 900496.
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(1996).
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(2001).
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Chem. Res. 39, 4120 (2000).
11. Seker, E., Ph.D. thesis. The University of Michigan, Ann Arbor, MI,
2000.
• The oxidation of NH3 under the oxidizing conditions
in the absence of NO over 1% Cu–SG is highly selective to 12. Katona, T., Guczi, L., and Somorjai, G. A., J. Catal. 135, 434 (1992).
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ACKNOWLEDGMENT
Partial financial support of this research by Ford Motor Company and
17. Seker, E., and Gulari, E., J. Catal. 194, 4 (2000).
18. Centi, G., Passarini, N., Perathoner, S., and Riva, A., Ind. Eng. Chem.
Res. 31, 1947 (1992).
NSF CTS 9985449 is gratefully acknowledged.
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