Selective low temperature NH3 oxidation to N2 on copper-based catalysts

Article abstract of DOI:10.1006/jcat.1999.2524

TPD, TPR, UV-visible spectroscopy, and high-resolution electron microscopy (HREM) have been used to characterize the state and reactivity of alumina-supported copper-based catalysts for the oxidation of ammonia to nitrogen. The results of HREM and UV spectra show that a CuAl₂O₄-like phase is more active than a CuO phase for the ammonia oxidation reaction. Both surface oxygen and copper lattice oxygen can react with NH₃ to produce N₂ but surface oxygen is much more active than lattice oxygen at low temperature. For copper zeolite catalysts [Cu-O-Cu]²⁺-like species or small copper oxygen aggregates are the likely forms of the catalytically active centers at low temperature. The activity of CuY was increased by treating the sample with NaOH. This treatment presumably increases the amount of low temperature active centers.

Full text of DOI:10.1006/jcat.1999.2524

Journal of Catalysis 186, 100–109 (1999)
Article ID jcat.1999.2524, available online at http://www.idealibrary.com on
Selective Low Temperature NH3 Oxidation to N2
on Copper-Based Catalysts
Lu Gang, J. van Grondelle, B. G. Anderson, and R. A. van Santen
Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology,
P.O. Box 513, 5600 MB Eindhoven, The Netherlands
Received December 23, 1998; revised March 18, 1999; accepted April 14, 1999
oxidation of ammonia in off-gas at temperatures between
200 and 500 C. The catalysts used were silica-supported
Cu, Co, and Ni oxides doped with small amounts of noble
metals (100 to 2000 ppm). They claimed that the selectivity
and activity of the catalysts was improved when the cata-
lysts were sulfated either during or prior to contact with the
ammonia-containing gas. Their data show that the selectiv-
ity can be improved from 53 to 99% by sulfation. Wollner
reported that high ammonia conversion (80–100% ) was ob-
tained over mixed copper–manganese oxides supported on
titania catalysts at temperatures greater than about 300 C
(9). However, the selectivity was not reported clearly. The
results of Hodnett (10) show that Cu/Al2O3, which is a very
active SCR catalyst, is also very active and selective for am-
monia oxidation at a temperature of about 325 C. A pulse
reaction study of various transition metal-exchanged zeo-
lite Y catalysts revealed CuY as the most active catalyst for
ammonia oxidation at low temperature (11).
TPD, TPR, UV–visible spectroscopy, and high-resolution elec-
tron microscopy (HREM) have been used to characterize the state
and reactivity of alumina-supported copper-based catalysts for the
oxidation of ammonia to nitrogen. The results of HREM and UV
spectra show that a CuAl2O4-like phase is more active than a CuO
phase for the ammonia oxidation reaction. Both surface oxygen
and copper lattice oxygen can react with NH3 to produce N2 but
surface oxygen is much more active than lattice oxygen at low tem-
perature. For copper zeolite catalysts [Cu–O–Cu]2+-like species or
small copper oxygen aggregates are the likely forms of the catalyt-
ically active centers at low temperature. The activity of CuY was
increased by treating the sample with NaOH. This treatment pre-
sumably increases the amount of low temperature active centers.
c
1999 Academic Press
Key Words: ammonia; oxidation; copper oxides; copper zeolite;
copper catalysts.
1. INTRODUCTION
The aforementioned studiesshowthat copper oxidessup-
ported on alumina, SiO2, TiO2, or zeolites are all active am-
monia oxidation catalysts. The operating temperature for
these catalysts is still rather high (greater than 300 C). Little
is known about the relationship between catalyst prepara-
tion and the active phase for these copper-based ammonia
oxidation catalysts. In the present study various kinds of
copper oxides supported on alumina and on zeolite Y have
been prepared and tested as catalysts for the ammonia ox-
idation reaction. TPD, TPR, UV–visible spectroscopy, and
high-resolution electron microscopy (HREM) were used
to characterize these catalysts in an attempt to shed light
on the optimal preparation for active and selective low-
temperature ammonia oxidation catalysts.
Gas phase ammonia oxidation to nitrogen may find ap-
plication in the treatment of ammonia containing indus-
trial flue gases, in the SCR de-NOx process for ammonia
slipstream treatment, in the purification of reformates for
fuel-cell systems, in the deodorization of ammonia/amine
containing gas, and for the small scale production of pure
nitrogen as a safety gas. Previous studies show that noble
metals such as Pt and Ir are very active for this reaction
but are less selective. Significant amounts of nitrous oxide
or nitric oxide are produced on these catalysts (1–3). Sup-
ported molybdenum and vanadium catalysts show higher
selectivity but the reaction temperature needed is too high
for most of the industrial applications (4–6).
Copper oxide is one of the most active catalysts and has
been considered as a potential substitute for noble metal-
based emission control catalysts. Sazonova et al. (7) in-
vestigated the activity of various catalysts for ammonia
oxidation at temperatures between 250 and 400 C. They
2. EXPERIMENTAL
2.1. Catalyst Preparation
The starting zeolite was a commercial NaY zeolite from
concluded that the most active and selective catalysts are Akzo Co. with a Si/Al ratio of 2.4 (Na56Al56Si136O384
V/TiO2, Cu/TiO2, and Cu-ZSM-5. Topsøe of Denmark (8) 249H2O). NaY was ion-exchanged in an aqueous solu-
recently patented a process for catalytic low temperature tion of copper nitrate at room temperature for 24 h. After
100
0021-9517/99 $30.00
c
Copyright
1999 by Academic Press
All rights of reproduction in any form reserved.

SELECTIVE LOW TEMPERATURE NH3 OXIDATION
101
TABLE 1
filtration, it was washed with deionized water three times,
dried at 110 C, and then calcined at 400 C for 2 h. The cop-
per contents were 3.7 and 8.4 wt% , respectively. The sam-
ples will be referred to as CuY-3.7 and CuY-8.4 in the sub-
sequent discussion.
Three Cu/Al2O3 samples with Cu loadings of 5, 10,
and 15 wt% , respectively, were prepared by incipient
wet impregnation. The precursor for these catalysts was
Cu(NO3)2 3H2O distributed over the Al2O3 support. The
samples were calcined in air at 600 C for 24 h before testing.
These samples will be referred to as Cu–Al-5, Cu–Al-10,
and Cu–Al-15.
Ammonia Oxidation over Various Copper Catalysts
Temperature
( C)
NH3 conversion
(% )
N2 selectivity
(% )
Catalyst
Cu–Al-5
250
300
350
0
21
75
—
97
96
Cu–Al-10
Cu–Al-15
CuY-3.7
CuY-8.4
250
300
350
12
90
100
96
97
90
250
300
350
9
46
100
97
96
94
2.2. Catalyst Testing
250
300
350
16
54
100
97
98
97
Catalytic activity measurements were carried out in a
quartz, fixed-bed reactor (4 mm internal diameter). The
amount of catalyst used was about 0.2 g (250–425 m par-
ticles). Ammonia, oxygen, and helium flow rates were con-
trolled by mass flow meters. Water vapor was introduced
by passing the helium flow through a water saturator at
elevated temperature. The inlet and outlet gas composi-
tions were analyzed using an on-line gas chromatograph
equipped with a thermal conductivity detector. A quadru-
ple mass spectrometer was also used to monitor the differ-
ent products.
250
300
350
23
88
100
97
98
98
CuY-3.7 (With
aftertreatment)
200
250
300
21
56
100
97
98
98
CuY-8.4 (With
aftertreatment)
200
250
300
35
68
100
95
97
98
Note. Reaction condition: NH3, 1.14% ; O2, 8.21% ; flow rate, 74.7 N ml/
min; cat. weight, 0.2 g.
2.3. Catalyst Characterization
High-resolution transmission electron microscopy mea-
surements were done on a Philips CM30-T electron mi-
croscope at the National Centre for HREM at the Delft
University of Technology. Diffuse reflectance UV–visible
spectra were recorded at room temperature using a Shi-
madzu UV-2401PC spectrometer in the 190- to 900-nm
wavelength range. The Kubelka–Munk function F(R) was
plotted against the wavelength (in nm).
NH3 and O2 TPD experiments were performed using a
fixed-bed flow reactor system equipped with a computer in-
terfaced quadrupole mass spectrometer. After adsorption
of NH3 or O2 at room temperature the TPD data were
recorded by mass spectrometer while the temperature was
increased from 50 to 500 C at a heating rate of 5 C/min. The
TPR data were obtained on a typical computer-controlled
temperature programmed reduction apparatus. The hydro-
gen consumption was measured using a thermal conductiv-
ity detector. The temperature was ramped from 25 to 800 C
at a rate of 5 C/min.
temperature, the copper zeolite catalysts become superior.
An optimal copper loading exists for the copper alumina
catalysts. For CuY catalysts, increased loading led to higher
activity.
Ione et al. (12) and Schoonheydt et al. (13) reported that
the polynuclear nickel or copper ions were formed in the
framework of zeolite Y when NaY was ion exchanged with
an aqueous Ni(NO3)2 or Cu(NO3)2 solution at pH 6–7.
These polynuclear cations gave a higher activity for CO
oxidation than the mononuclear state. Suzuki et al. (14, 15)
successfully prepared a highly dispersed nickel oxide cata-
lyst, which was also a very good CO oxidation catalyst, by
hydrolyzing the Ni2+ exchanged zeolite Y with an aqueous
solution of NaOH or ammonia at various pH values. In this
research the effect of NaOH aftertreatment has also been
examined. We have used the procedure reported by Suzuki
et al. (14, 15) to study the effect of NaOH aftertreatment
on CuY catalysts. The results are included in Table 1.
As can be seen in Table 1, the activity of the two copper
zeolite catalysts for ammonia oxidation increased signifi-
cantly following NaOH treatment. This significantly higher
activity was probably induced by the formation of small
metal oxides in the zeolite. It should be noted that the CuY
3. RESULTS
3.1. Catalytic Oxidation of NH3 on Copper Based Catalysts
The results of Table 1 show that the performance of the catalysts are active at temperatures as low as 200 C. Tem-
copper alumina catalysts is comparable to that of the copper peratures of at least 250 C are necessary for reaction on Cu
zeolite catalysts at high temperature (>300 C). At lower alumina catalysts.

102
GANG ET AL.
FIG. 2. NH3 TPD profiles on copper zeolite catalysts (catalyst pre-
treatment, 400 C in O2 heated for 2 h).
FIG. 1. N2 production profiles on an oxidized Cu–Al-10 catalyst dur-
ing NH3 TPD (obtained during four consective NH3 exposures).
observed on the CuY-8.4 catalyst. The introduction of Cu
into NaY not only creates a more strongly bonded NH3 ad-
sorption center but it also increases the amount of weakly
bonded NH3 centers. After NaOH treatment, a third ammo-
nia adsorption center seemed to be produced, although this
center is not very strong. There was almost no N2 produced
on NaY, while on CuY-8.3 there were two N2 peaks at the
temperature at which ammonia desorption occurred. This
shows that apparently there were two active ammonia oxi-
dation centers created by the introduction of Cu into NaY.
The first active center (at around 150 C) is very interesting
as it indicates that ammonia oxidation at low temperature
on copper catalysts is possible. A third active center (at
around 190 C) for ammonia oxidation is created following
NaOH treatment. Comparison of the N2 production pro-
files measured on the alumina-based catalyst (Cu–Al-10)
and on the zeolite-based catalyst shows that CuY is appar-
ently much more active than Cu–Al2O3 at low temperature
(<200 C).
3.2. TPD and TPR Profiles for Copper Based Catalysts
Four successive cycles consisting of isothermal NH3 ad-
sorption at 50 C followed by TPD up to 500 C were car-
ried out on an oxidized Cu–Al-10 catalyst. The catalyst
was heated in O2/He flow at 500 C for 2 h before the first
cycle.
After a TPD run the sample was cooled in flowing he-
lium to 50 C. NH3 was then readsorbed and the N2 TPD
spectrum was measured. The nitrogen production profiles
for four such successive cycles of NH3 TPD experiments
are shown in Fig. 1. It can be seen that there are two peaks
around 250 and 330 C for the first cycle TPD. In the second
cycle, the first peak almost disappeared while the second
peak shifted to a slightly higher temperature. In the third
and fourth cycles, the peak moved to even higher temper-
atures. As copper is dispersed very well on alumina (see
HREM results) and since no nitrogen is produced during
NH3 TPD on alumina alone, the possibility of different ac-
tive centers for the two peaks can be excluded. We propose
that the first peak is caused by the reaction of adsorbed sur-
face oxygen with NH3 and that the second peak is caused
by reaction of ammonia with subsurface or possibly with
lattice oxygen. After the first TPD cycle the surface oxygen
is almost totally consumed. This explains the disappearance
of the first peak in the latter cycles.
Catalysts used in O2 TPD were all treated in O2 at 400 C
for 2 h and then cooled to room temperature. For H2 TPR
experiments, however, all of the catalysts were pretreated
in He at 400 C for 2 h. The amount of catalyst used for
Figures 2 and 3 compare NH3 and N2 desorption profiles
measured during NH3 TPD experiments on copper zeolite
catalysts that had been subjected to NaOH aftertreatment.
The zeolite catalysts were pretreated in O2 at 400 C for 2 h
prior to the experiments. The amount ofcatalystsissame for
all the samples used in these experiments. Only one cycle
of NH3 TPD data are shown in Figs. 2 and 3 since the suc-
cessive cycles measured on these zeolite catalysts showed
no N2 production. This means that either no oxygen ex-
isted on the copper zeolite catalysts or that the copper was
completely reduced after the first cycle of the NH3 TPD ex-
periment. The desorption of NH3 on NaY showed only one
FIG. 3. N2 production profiles on CuY catalysts during NH3 TPD
peak at 150 C, whereas two peaks (at 150 and 350 C) were (catalyst pretreatment, 400 C in O2 heated for 2 h).

SELECTIVE LOW TEMPERATURE NH3 OXIDATION
103
due to the formation of these CuO particles on the alumina
surface. By contrast the catalysts with lower copper load-
ings (Cu–Al-5 and Cu–Al-10), on which no particles could
be detected, were green in color.
No differences in the HREM images were observed on
samples of the catalyst CuY-3.7 before and after NaOH
treatment. However, a very small amount of particles was
discovered on the outer surface of the zeolite crystals for
CuY-8.4 (see Figs. 8 and 9). Again there was no big differ-
ence observed before and after NaOH treatment.
3.4. UV–Visible Spectroscopy
The Cu2+ ion has a 3d9 electronic structure. In the pres-
ence of a crystal field generated by ligands or oxygen ions,
d–d transitions appear in the visible or near-infrared range.
For an octahedral environment, transitions appear between
600 and 800 nm depending on the crystal field strength.
Table 2 shows the summary of our assignments of the UV
spectral bands measured on different kind of catalysts.
Figure 10 shows the UV spectra measured on copper–
alumina catalysts with different copper loadings. It can be
seen that there is a weak absorption band at about 740 nm
for Cu–Al-5 and Cu–Al-10 catalysts. This band shifts to
650 nm for the Cu–Al-15 catalyst. According to previous
studies (18) the band at 650 nm corresponds to a CuO
phase. A UV spectrum measured on CuO showed indeed
that a broad band around 650 nm exists (see Fig. 11). The
shift of this band to higher wavelength indicates that the
distance between Cu2+ and the ligands or oxygen ions in-
creases; i.e., the Cu2+–O2 bond should exhibit greater ionic
character.
FIG. 4. O2 desorption on CuY catalyst with and without aftertreat-
ment.
all O2 TPD and H2 TPR experiments is the same, so the
peak areas of different catalysts can be compared directly
with each other. It can be seen from Fig. 4. that two peaks
exist in the O2 TPD profiles on the CuY-8.4 catalyst. Since
no O2 desorption was observed on NaY, these two O2 des-
orption peaks must be caused by the Cu in the zeolite. Af-
ter the NaOH treatment the amount of O2 desorbed was
greatly increased. Many investigations (16, 17) on the com-
plete oxidation of CO and hydrocarbons have shown that
their reactivities directly depend on the capacity of oxygen
adsorption. Here it also appears that oxygen adsorption
relates to the rate of ammonia oxidation. Figure 5 shows
the TPR profiles of the CuY catalyst with and without the
aftertreatment. Integration of the peaks revelas that the
amount of reducible copper ions increased by a factor of
1.4 when the CuY catalyst was treated with NaOH.
It can be seen from Fig. 12 that the absorption band ap-
pears at about 840 nm for CuY samples without NaOH af-
tertreatment. This indicates the existence of isolated Cu2+
ions of square pyramid symmetry (19–21). After NaOH
treatment, this band shifts to about 750 nm, similar to the
bandsobserved on the alumina-based copper catalysts(Cu–
Al-5 and Cu–Al-10).
CuY-3.7 and CuY-3.7 (with and without aftertreatment)
were reduced in H2/He at 500 C for 2 h before measur-
ing the UV spectra (see Fig. 13). The appearance of an
3.3. HREM Results
No metal oxide particles were observed on the alumina
surface of the Cu–Al-5 and the Cu–Al-10 catalysts (see
Fig. 6). Metal particles with diameters ranging between 5
and 10 nm were observed on the Cu–Al-15 catalyst (see
Fig. 7). The Cu–Al-15 catalyst was black in color, probably
TABLE 2
Summary of the Assignments of UV Spectral Bands Measured on
Different Copper Catalysts
Absorption
wavelength
Copper species
Cu2+ ions
Support
Zeolite Y
840 nm
740–750 nm
CuAl2O4-like phase or
[Cu–O–Cu]2+ species
CuO phase
Al2O3, Zeolite Y
650 nm
560 nm
No support or Al2O3
No support or Zeolite Y
Cu0 metal phase
FIG. 5. TPR by H2 profiles for CuY with and without aftertreatment.

104
GANG ET AL.
FIG. 6. An HREM image of the Cu–Al-10 catalyst showing the absence of any detectable metal oxide particles.
absorption maximum at 560 nm indicates the creation of a
metallic copper phase (21). The absorption band at 840 nm
measured on CuY-3.7 means that some Cu 2+ ion cannot be
4. DISCUSSION
The most extensive investigation of Cu/Al2O3 cata-
reduced. The intensity of the absorption band at 560 nm lysts has been performed by Friedman et al. (22). By
(Cu0) did increase at the expense of the 840-nm band applying several techniques and combining results with
(Cu2+), indicating that some reduction of copper is possible previous work, a coherent model of the surface struc-
following NaOH treatment.
ture was developed, resulting in the following conclusions

SELECTIVE LOW TEMPERATURE NH3 OXIDATION
105
FIG. 7. An HREM image of the Cu–Al-15 catalyst showing the presence of metal oxide particles with diameters between 5 and 10 nm.
(22, 23): (i) At low metal loadings and at calcination segregation of bulk-like CuO may also occur. (iii) Cal-
temperature below 500 C the formation of a surface cination at 900 C leads to formation of bulk CuAl2O4. Our
spinel (which resembles CuAl2O4) predominates. Most HREM and UV measurements also show that no CuO par-
of the Cu2+ ions are in a distored octahedral geome- ticles form on the alumina surface for Cu–Al-5 and Cu–
try, unlike bulk CuAl2O4, where about 60% are tetrahe- Al-10 catalysts but that particles are present on the Cu–
dral and 40% octahedral. (ii) At higher metal loadings, Al-15 sample. The fact that the performance of Cu–Al-10

106
GANG ET AL.
FIG. 8. An HREM image of the CuY-8.4 catalyst (without NaOH aftertreatment) showing the presence of very small particles (diameter <5 nm)
on the outer surface of the zeolite Y crystal.
is better than that of the Cu–Al-15 catalyst supports the or with an extraframework O or OH ligand. These species
conclusion that a CuAl2O4-like phase is more active than show a different reactivity (24). (ii) Polymeric chains or
CuO phase in the ammonia oxidation reaction.
multinuclear species (often called small copper–oxygen ag-
At least three types of copper species in CuY have been gregates), for instance [Cu–O–Cu]2+ inside the zeolite su-
described in the literature: (i) Isolated ions interacting with percage (25–27). (iii) CuO particles on the external surface
the framework Al, either without an extraframework ligand of zeolite crystals (28). Although at high loading the HREM

SELECTIVE LOW TEMPERATURE NH3 OXIDATION
107
FIG. 9. An HREM image of the CuY-8.4 catalyst (following NaOH aftertreatment) shows that NaOH treatment produces no visible effects on
the size of the small particles present on the zeolite outer surface.
results show some CuO particles on the outer surface of for ammonia oxidation it can be concluded that a copper
the zeolite, the UV spectra show no apparent CuO adsorp- phase occluded in the micropores of the zeolite, different
tion band intensity at 650 nm. This means that only a small from CuO, forms the main active centers.
amount of copper is on the surface of the zeolite crystals
Since the diameter of an ammonia molecule is larger
˚
as CuO as compared with the amount of copper inside the than the diameter (2.2 A) of entrance windows into the so-
pores of zeolite. Since the CuO phase is not very active dalite cages in CuY zeolites, Cu2+ ions that are accessible to

108
GANG ET AL.
FIG. 10. UV spectra measured on copper alumina catalysts with dif-
ferent copper loadings (5, 10, and 15% (by weight)).
FIG. 12. UV spectra for CuY-based catalysts.
adsorbed NH3 have to be located in the supercages. Accord-
ing to earlier studies (29), they are localized at SII sites, i.e.,
near the centers of six-membered rings of the sodalite cages.
A different chemical composition of six-membered rings
(Si/Al ratio) could result in the existence of three types of
SII sites in the supercages, since according to Lowenstein’s
rule the number of aluminum atoms in the ring can be ei-
ther one, two, or three. The results of NH3 TPD on CuY-8.4
show only two active centers (S1II and SI2I). However, a third
center (S3II) appears after NaOH treatment. It is suggested
that either the S1II and SI3I are very similar or that the amount
of S3II centers is small. After NaOH treatment, some of the
Cu2+ ions in SI (in the center of the hexagonal prism) mi-
grate to the S3II sites of the supercages. The results of O2
TPD and TPR strongly indicate this migration. The UV
spectra for reduced CuY samples also indicate that more
Cu2+ ions are reduced to Cu0 after NaOH treatment.
Comparison of the UV spectra for CuY samples with and
without aftertreatment shows that there are mainly isolated
copper ions in the zeolite before NaOH treatment. The
shift of the absorption band from 840 nm to 750 nm after
treatment indicates the creation of [Cu–O–Cu]2+ inside the
zeolite supercage or the presence of small copper–oxygen
aggregates (25–27). Perhaps copper ions are first deposited
as Cu(OH)2 by hydroxylation both in the supercages and
in the sodalite cages. Part of the Cu(OH)2 then moves to
the three SII sites in the supercages during this hydroxyla-
tion process. Upon dehydration of Cu(OH)2, [Cu–O–Cu]2+
complexes are produced which are thought to be the active
centers for low temperature ammonia oxidation. Actually
[Cu–O–Cu]2+ already exists in CuY prior to NaOH treat-
ment. According to prior studies (30), [Cu–O–Cu]2+ species
are present even at low ion-exchanged levels in CuY. The
concentration of [Cu–O–Cu]2+ species increases with the
Cu loadings. This is in agreement with the finding that low
temperature active centers for ammonia oxidation exist on
an untreated CuY catalyst.
5. CONCLUSIONS
HREM and UV–visible spectral measurements showed
that no metal oxide particles form on alumina at copper
loadings below 10% (by weight). At higher loadings a
CuO phase was detected. The fact that the performance of
FIG. 11. UV spectra measured using bulk CuO and Cu2O oxides.
FIG. 13. UV spectra for reduced CuY samples.

SELECTIVE LOW TEMPERATURE NH3 OXIDATION
109
10 wt% Cu/Al2O3 was better than that of 15 wt% (Cu–Al-
15) supports the conclusion that a “CuAl2O4”-like surface
phase is more active than a CuO phase in the ammonia ox-
idation reaction. NH3 TPD profiles on Cu–Al-10 indicate
that both surface oxygen and lattice oxygen can react with
NH3 to produce N2. However, surface oxygen is much more
active than lattice oxygen at low temperature.
NH3 TPD on CuY catalysts shows three types of active
centers. Two of these are active at low temperature (below
200 C) and one is active at a higher temperature (above
300 C). The existence of low temperature active centers in-
dicates that ammonia oxidation at low temperature on cop-
per catalysts is possible. According to the UV spectra, the
[Cu–O–Cu]2+-like species or small copper oxygen aggre-
gates are responsible to the low temperature active centers.
However, the amount of low temperature active centers or
the concentration of [Cu–O–Cu]2+ species is small prior to
NaOH treatment. The NaOH treatment of CuY increases
the amount of low temperature active centers.
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has been supplied by the Netherlands Foundation for Chemical Research
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