Role of NH3 as an intermediate in reduction of NO with CH4 over sol-gel Pd catalysts on TiO2

Article abstract of DOI:10.1016/S1381-1169(02)00217-0

The use of NH₃ as the reducing agent for NO reduction on the Gd-Pd/TiO₂ catalyst was investigated under steady-state conditions. NH₃ was effective in NO reduction over the reduced catalyst. However, in the presence of O₂, NH₃ oxidation, which had a higher activation energy, became more dominant at higher temperatures compared to NO reduction and the observed NO conversion decreased. The NH₃ experiments indicated that the reversible NH₃ adsorption capacity was higher on the reduced catalyst compared to the oxidized catalyst. There was a strong evidence provided by DRIFTS that adsorbed NH₃ was transformed into monodentate nitrate species on the oxidized catalyst.

Full text of DOI:10.1016/S1381-1169(02)00217-0

Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
Role of NH as an intermediate in reduction of NO with CH
3
4
over sol–gel Pd catalysts on TiO2
∗
Junko M. Watson, Umit S. Ozkan
Department of Chemical Engineering, The Ohio State University, 140 W. 19th Avenue, Columbus, OH 43210, USA
Received 7 February 2002; received in revised form 15 April 2002; accepted 1 May 2002
Abstract
Our previous studies [Catal. Commun., in press; J. Catal., in press] on sol–gel-prepared Pd catalysts for reduction of NO with
CH4 suggested that one of the key surface species for the NO–CH4–O2 reaction involved NHx species. In this study, the use of
NH3 as the reducing agent for NO reduction on the Gd–Pd/TiO2 catalyst was investigated under steady-state conditions. Also,
temperature-programmed desorption using mass spectrometry and diffuse reflectance infrared Fourier transform spectroscopy
(
DRIFTS) were used to study the adsorption/desorption characteristics of NH3 on Gd–Pd/TiO2. It was found that NH3 was
◦
effective in reduction of NO on the reduced catalyst in the absence of O2 at 300 C (100% NO conversion). This result
was consistent with our earlier suggestion that NHx species formed through the interaction of methane and NO adspecies
could act as a reducing agent for Pd–NO species. In the presence of O2, however, NH3 oxidation takes place faster than NO
reduction and NO conversion decreased. Our NH3-TPD experiments indicated that the reversible NH3 adsorption capacity
was higher on the reduced catalyst compared to the oxidized catalyst. Also, there was a strong evidence obtained by DRIFTS
that adsorbed NH3 is transformed into monodentate nitrate species on the oxidized catalyst.
©
2002 Elsevier Science B.V. All rights reserved.
Keywords: Temperature-programmed desorption; DRIFTS; Gd–Pd/TiO2 catalyst; No reduction with CH4
1
. Introduction
mechanism where strongly adsorbed NH3 and gaseous
NO react [6]. However, there are different proposals
for the type of ammonia activation sites and the nature
of adsorbed species. In elucidation of reaction mecha-
nisms, spectroscopic characterization such as isotopic
labeling, in situ FTIR, TPD, and ESR have been used
in recent years [2,7–11]. In the SCR reactions, direct
oxidation of ammonia has been found to play an im-
portant role [8,12] and the activity and selectivity of
the NO reduction reaction cannot be considered alone
without investigating the role of ammonia oxidation.
More recently, the use of hydrocarbons, particularly
CH4, has been investigated as a reducing agent for
NO reduction reaction in place of NH3, use of which
has lingering environmental and economic concerns.
The selective catalytic reduction (SCR) using NH3
is known to be the most widely applied technology to
reduce NOx in the presence of O2 for stationary com-
bustion sources. The industrial catalyst systems, which
are based on V2O –TiO2 (anatase), have been inves-
5
tigated extensively due to their high effectiveness in
NO reduction by NH3 and their high resistance to SO2
poisoning [1–5]. One of the mechanisms proposed
earlier for the NH3-SCR reactions is an Eley–Rideal
∗
Corresponding author. Tel.: +1-614-292-6623;
fax: +1-614-292-3769.
E-mail address: ozkan.1@osu.edu (U.S. Ozkan).
1
381-1169/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(02)00217-0

8
0
J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
Methane is considered to be an attractive replacement
for NH3 because it is the most abundant and inex-
pensive form of hydrocarbon in natural gas; however,
it is also the most stable molecule among alkanes,
thus most difficult to activate. Noble metals have been
investigated heavily for catalytic combustion of hy-
drocarbons [13,14]. Among noble metals, palladium
and platinum are widely used in oxidation applica-
tions such as three-way catalysts and methane com-
bustion. The type of catalysts that are most studied
for the CH4–NO reaction is metal-exchanged zeolite
catalysts [15–17].
Use of FTIR has been demonstrated to be a power-
ful tool for elucidation of possible mechanisms for the
NO–CH4–O2 reaction in recent years. Among the pro-
posed mechanisms for the reaction, common reaction
steps proposed to be involved over metal-exchanged
zeolitic catalysts are formation of adsorbed NO2, ac-
tivation of CH4 by adsorbed NO2, and formation of
nitromethane (CH3NO2) [17–20].
At lower temperatures, bridged/bidentate nitrate and
monodentate nitrate are formed on the support and
are seen to be decomposed or transformed to other
compounds at higher temperatures. Subsequently, lin-
ear NO is formed on Pd sites. There was a strong
evidence that CH4 and NO competitively adsorb on
the surface even at room temperature [25].
An interesting feature that emerged through the
DRIFTS experiments was the formation of surface
ammonia species. When the reduced catalyst sur-
face was exposed to NO and CH4 under conditions
that gave complete NO conversion, bands that cor-
respond to NH3 coordinated to Lewis acid sites
−
1
(∼1180/1200 cm ) were observed. These bands
were accompanied by features in the –NH stretching
−
1
region (3242–3150 cm ). Interestingly these bands
were never present over the oxidized catalyst, or un-
der conditions where the catalyst was inactive for
NO reduction. Also, in comparison to the Gd-free
catalyst, the NH3 formation seemed to be much more
prominent over the Gd–Pd. Another interesting find-
ing from our earlier study was the apparent interaction
between the surface coordinated NH3 species and
linear Pd–NO species. DRIFTS experiments show-
ed that disappearance of the Pd–NO species always
coincided with the formation of surface ammonia
species. These observations led us to suggest that
adsorbed NH3 could be an important player in the
NO reduction scheme. In this paper, we present
the findings from our steady-state reaction experi-
ments of the NO–NH3–O2 reaction, NH3-TPD, and
NH3-DRIFTS studies, which were performed to exam-
ine the role of NH3 in the NO–CH4–O2 reaction over
Gd–Pd/TiO2.
Previously we have reported titania-supported Pd
catalysts to be effective for the reduction of NO to
N2 with high selectivity to N2 [21–23]. The incorpo-
ration of lanthanide elements such as Gd was found
to improve the oxygen tolerance of the catalysts [23].
The mechanistic aspects of NO–CH4 reactions were
studied using isotopic labeling techniques under both
steady-state and transient conditions [21]. From a
series of unsteady-state and steady-state isotopic la-
beling studies that used various labeled species such
1
5
16
15 18
13
18
N O, N O, CH4, O2, it was concluded that
N2 was formed through direct participation of CH4
possibly through a methyl nitrosyl type intermediate,
whereas N2O formation was mainly a result of NO
decomposition reaction [21]. Also, steady-state oscil-
lations were used as a probe to gain insight into the
reaction network and it was found that the oxidation
state of the active metal determined the extent of the
three different reaction steps, namely, NO reduction
with CH4, direct CH4 oxidation, and NO decomposi-
tion [22].
2. Methods
2.1. Catalyst preparation
Catalysts were synthesized using a modified sol–gel
technique in which co-precipitation and sol–gel meth-
ods are incorporated as described elsewhere [23]. The
metal loadings of the catalysts were 2%Pd/TiO2 and
1%Gd–2%Pd/TiO2 by weight percentage. The BET
surface area measurement by N2 adsorption resulted
More recently, we have used in situ diffuse re-
flectance infrared Fourier transform spectroscopy
(DRIFTS) and TPD to investigate possible mecha-
nisms for the NO–CH4–O2 reaction over sol–gel-
prepared Pd catalysts at lower temperatures [24,25].
◦
2
The major surface species under NO flow at 300 C
in the specific surface areas of 80 and 79 m /g for
was identified as linear NO adsorbed on palladium.
2%Pd/TiO2 and 1%Gd–2%Pd/TiO2, respectively.

J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
81
2
.2. Reaction studies
surface temperature or by reduction under 30% hy-
◦
drogen in balance helium at 200 C surface tempera-
◦
The steady-state reaction experiments were per-
ture for 30 min. For sequential adsorption at 300 C
formed using a fixed-bed flow reactor (1/4 in. OD)
made of stainless steel. The amount of catalyst
packed was 69 mg. The feed composition con-
sisted of NO (500–1780 ppm), NH3 (500–1780 ppm),
surface temperature, background spectra were taken
at 300 C under helium before adsorption gases were
◦
introduced. For adsorption spectra obtained at several
different temperature levels, background spectra were
taken at each temperature level under helium prior
to the introduction of adsorption gas. Each spectrum
was averaged over 1000 scans in the mid-IR range
and O2 (0.1–2%), in balance He at a flow rate of
3
4
5 cm (STP)/min. The reaction temperature was var-
◦
ied between 125 and 400 C and was controlled by a
PID temperature controller (Omega). In each reaction,
the sample was pre-reduced using a mixture of 33%
−
1
−1
(400–4000 cm ) to a nominal 2 cm resolution.
◦
H2 in He at 200 C for 30 min. The feed and effluent
3. Results and discussion
were analyzed on-line using a combination of a gas
chromatograph (5890A HP), a chemiluminescence
NO–NO2–NOx analyzer (Thermo Environmental In-
struments, Model 42H), and an IR ammonia analyzer
3.1. Catalytic activity
The effect of temperature on the NO–NH3 reac-
(
Siemens Ultramat 5F). The gas chromatograph was
tion on reduced 1%Gd–2%Pd/TiO2 catalyst is shown
in Fig. 1. Equal concentrations of 1800 ppm NO and
NH3 were introduced to the fixed-bed reactor at 200,
ꢀ
equipped with a 10 × 1/8 in. Porapaq Q column and
ꢀ
an 8 × 1/8 in. molecular sieve column to quantify
N2, O2, and N2O. In all experiments, nitrogen and
carbon balances closed within a 5% margin.
◦
◦
2
3
50, 300, and 400 C. At 200 C, NO conversion was
◦
0% with 16% NH3 conversion. At 250 C, 100% NO
conversion was achieved with 75% NH3 conversion.
2
.3. Catalyst characterization
The N-containing products were N2 and N2O in which
◦
8
5% N2 selectivity was achieved at 250 C. As the re-
Temperature-programmed desorption (TPD) exper-
action temperature was increased, we observed an in-
iments were conducted using a TPR/TPD flow system
equipped with a thermal conductivity detector (TCD)
described elsewhere [21]. The desorption species were
monitored by an HP5890GC-MS. A sample (75 mg)
was loaded to the U-shaped quartz reactor and calcined
in situ in 10% oxygen in balance helium at 500 C for
1
the sample was reduced in 33% hydrogen in balance
helium at 200 C for 30 min followed by flushing for
◦
crease in N2 selectivity, reaching ∼100% at 300 C.
The conversion of NH3, however, was unaffected by
increasing temperature. This seems to suggest that the
stoichiometric reaction 4NH3 +6NO → 5N2 +6H2O
is in control throughout the temperature range studied.
N2O formation, on the other hand, is a possible side
reaction at lower temperatures. Here N2 selectivity is
defined as a fraction of N2 produced over total N2 and
N2O produced.
◦
h followed by flushing in helium for 30 min. Next,
◦
3
0 min in helium. Then, the catalyst was cooled down
The effect of O2 concentration in NO–NH3 reac-
tion was investigated on reduced 1%Gd–2%Pd/TiO2
to room temperature under helium flow. Adsorption of
gas was done at room temperature for 1 h followed by
helium flushing for 1 h to remove physically adsorbed
gas.
◦
at 200 C as shown in Table 1. NO and NH3 feed
concentrations were kept constant at 1800 ppm while
DRIFTS experiments were performed using a
Bruker IFS66 equipped with a DTGS detector and
a KBr beamsplitter. Catalysts were placed in a sam-
ple cup inside a Spectratech diffuse reflectance cell
equipped with KBr windows and a thermocouple
mount that allowed direct measurement of the surface
temperature. Each catalyst was pre-treated in situ
Table 1
Effect of O2 on NO–NH3 reaction (equal concentration of
1800 ppm) on reduced 1%Gd–2%Pd/TiO2 catalyst
Inlet O2 (ppm)
0
30
16
0.53
68
2000
65
77
1.2
61
5000
68
86
1.3
63
8000
62
82
1.3
48
NO conversion (%)
NH3 conversion (%)
NH3/NO conversion ratio
N2 selectivity (%)
◦
either by calcination under 10% oxygen at 400 C

8
2
J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
Fig. 1. Effect of temperature on NO–NH3 reaction (equal concentration of 1800 ppm) on reduced 1%Gd–2%Pd/TiO2 catalyst.
O2 concentration was varied from 0 to 8000 ppm.
There was a sharp increase in NH3 conversion from
which decreased in the presence of oxygen. At higher
O2 concentration levels (8000 ppm), N2O selectivity
became larger than N2 selectivity.
1
6 to 77% when 2000 ppm O2 added to the feed.
Concurrently, the NO conversion also increased from
0 to 65%. It clearly shows that the presence of O2
Fig. 2 shows the effect of reaction temperature
◦
3
(120–400 C) on NO–NH3–O2 reaction on reduced
helps improve the catalytic activity of NO reduction
by NH3 at 200 C. When O2 level was increased
1%Gd–2%Pd/TiO2. The inlet concentrations of NO,
NH3, and O2 are 1800, 1800, and 4000 ppm, respec-
tively. The NH3 conversion increased from 19 to near
◦
from 2000 up to 8000 ppm, the change in NO or NH3
conversion was relatively small. Table 1 also shows
that the ratio of NH3 conversion to NO conversion
increased significantly when O2 was added to the
feed stream, suggesting that, in the presence of oxy-
gen, direct NH3 oxidation, may also be playing an
important role. It should be noted, however, that the
◦
100% between 125 and 200 C. Also, NO conversion
◦
reached a maximum value at 200 C. The O2 con-
◦
version remained very low below 5% up to 200 C;
however, it started increasing above 200 C. The in-
◦
crease in NH3 conversion is partly due to ammonia
◦
oxidation reaction above 200 C.
“
observed” NO reduction may appear to be lower be-
When the extent of direct ammonia oxidation was
examined using 1600 ppm NH3 and 2000 ppm O2,
we obtained near complete conversion of ammonia
cause direct oxidation of NH3 to NO may be boosting
the NO concentration in the product stream. The ef-
fect of NH3 oxidation was also seen in N2 selectivity,
◦
even at temperatures as low as 200 C. The product

J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
83
Table 2
NO–NH3 reaction over reduced and oxidized Gd–Pd/TiO2 at 200
◦
a
and 250 C
NH3
NO
(200 C)
NH3
(250 C)
NO
(250 C)
◦
◦
◦
◦
(200 C)
Reduced (%)
Oxidized (%)
98
12
74
8
97
13
70
9
a
NO: 1800 ppm, NH3: 1800 ppm, 4900 ppm on Gd–Pd/TiO2.
distribution showed an increase in NO formation with
◦
increasing temperature, where at 400 C, the NO yield
was around 13%. The only other N-containing prod-
uct besides NO was N2O. This result shows that the
sharp decline observed in the NO conversion with in-
creasing temperature was not only due to the fact that
higher temperatures favor direct NH3 oxidation over
NO reduction, but also due to formation of NO from
direct conversion of NH3 to NO.
In another set of experiments, reduced and oxidized
catalysts were compared in NO reduction reaction
◦
with NH3 in the presence of O2 at 200 and 250 C. The
NO and NH3 concentrations were 1800 ppm each and
the O2 concentration was 4900 ppm (Table 2). Over
Fig. 2. Effect of temperature on NO–NH3–O2 reaction on reduced
%Gd–2%Pd/TiO2 (NO, NH3, and O2: 1800, 1800, and 4000 ppm,
respectively).
1
the oxidized catalyst, both NO and NH conversion
3
Fig. 3. N2O-, N2-, and H2O-TPD profiles after NH3 was adsorbed at room temperature on: (a) oxidized 1%Gd–2%Pd/TiO2 and (b) reduced
%Gd–2%Pd/TiO2.
1

8
4
J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
levels were much lower than those obtained over the
reduced catalysts. These results suggest that reduced
Pd sites are needed for NO–NH3 reaction to proceed
over the Gd–Pd/TiO2 catalysts. One possible explana-
tion for the low activity over the oxidized catalyst may
be related to NH3 activation, which, analogous to CH4
activation, could require reduced Pd sites. Another ex-
planation may be related to the formation of Pd–NO
species, which have been suggested to be the primary
species that react with the NHx species. Our previ-
ous studies with in situ DRIFTS have shown Pd–NO
species to form primarily over the reduced sites
of Pd.
In our previous XPS studies over oxidized Gd–Pd/
TiO2, XPS Pd 3d spectrum showed that nearly all
Pd was in +2 oxidation state after the catalyst was
◦
treated with O2 at 500 C [25]. It is possible that
NH3 is adsorbed on the oxide surface and oxidized
by the surface oxygen, leaving the surface with oxy-
gen vacancies. Additional evidence, which supports
involvement of surface oxygen, is the desorption fea-
ture of water whose temperature maximum coincided
with that of N2 desorption. Although, our previous
studies [27] have shown that oxygen in the TiO2 lat-
tice/surface becomes more accessible when a precious
metal is supported on the TiO2 surface, we believe
reduction of PdO to Pd takes place at significantly
lower temperatures than TiO2 reduction, therefore the
3
.2. NH3-TPD
◦
water formation around 300 C is the result of the
NH3-TPD experiment was performed on oxidized
PdO conversion to Pd.
and reduced 1%Gd–2%Pd/TiO2 catalysts. Fig. 3a
shows the N2O and N2 desorption profiles (m/e = 44
and 28, respectively) following NH3 adsorption on
the oxidized sample. The catalyst was calcined in situ
Desorption profiles from NH3-TPD on the reduced
Gd–Pd catalyst are presented in Fig. 3b. The catalyst
was calcined in situ in 10% O2 in He for 30 min at
◦
500 C followed by in situ reduction in 33% H2 in He
◦
◦
in 10% O2 in He for 30 min at 500 C and cooled to
for 30 min at 200 C before NH3 adsorption at room
room temperature for NH3 adsorption for 1 h. Then,
the catalyst was flushed in He for 1 h before heating
at a rate of 10 C/min.
temperature. After our standard reduction step, major-
ity of PdO is converted to Pd in zero oxidation state
as previously shown through XPS data [23]. There-
fore, we expected desorption of N2 and N2O to be
much smaller compared to the oxidized sample since
both reactions involved use of oxygen from the cat-
alyst lattice. As expected, the N2 and N2O features
◦
Over the oxidized catalyst (Fig. 3a), there was a
◦
large N2 desorption peak at around 320 C indicating
a strong interaction of NH3 with the catalyst surface.
The N2O desorption peaks were observed at 133 and
◦
◦
3
20 C. The latter peak coincided with the N2 desorp-
observed on the oxidized sample at 320 C were not
tion peak. There were a broad low-temperature water
present on the reduced sample. However, the same
low-temperature feature was observed on the reduced
sample. The broad water desorption feature is likely to
be due to water left on the surface from the reduction
step.
◦
desorption feature (between 60 and 250 C) and a
◦
much sharper high temperature peak (325 C). The
latter feature coincided with desorption of N2 peak,
indicating that the formation of nitrogen was cou-
pled with the reduction of the surface by NH3. Over
CuO–TiO2-based catalysts, the catalytic oxidation of
ammonia to N2 by oxygen has been proposed to occur
through formation of hydrazine (N2H4) after molec-
ularly adsorbed NH3 is activated through hydrogen
abstraction [3]. However, Burch and co-workers [26]
recently showed that bidentate nitrate species, with
Fig. 4 shows desorption profiles of NH3 (m/e =
15) on both reduced and oxidized samples. Since the
parental ion for ammonia of 17 is also a fragment of
water ion, we cannot distinguish desorption features
of ammonia and water by using m/e = 17. There-
fore, we used m/e = 15 which does not overlap with
any other ions. We can see that NH3 adsorbs both re-
versibly and irreversibly on both samples. The main
difference between the two profiles was that the high
−
1
IR bands at 1546/1280 cm , could be an intermedi-
ate for N2 formation during temperature-programmed
oxidation of NH3. They also studied N2H4 as a probe
molecule using DRIFTS and found that neither the
thermal stability nor the IR bands correspond to
the studied reaction [26].
◦
temperature feature (320 C) was observed on the re-
duced sample, but it was absent on the oxidized sam-
ple. The temperature at which N2 peak was observed
on the oxidized sample matched exactly with the sharp

J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
85
flow. We observed two strong IR characteristic bands
for NH3 adsorption on Lewis acid sites at 1587 and a
−
1
doublet at 1176 and 1220 cm (Fig. 5a) These bands
correspond to ␦as(NH3) and ␦sy(NH3) vibrations of
coordinatively adsorbed ammonia, respectively [9,28].
The latter band is located on ␤ Lewis acid sites which
4
+
are five-coordinated Ti sites [28]. The feature at
−
1
1
220 cm signaled the splitting of the symmetric de-
formation mode and is reported to be most sensitive to
the acidic strength of the surface sites [9]. The strong
presence of this band on the reduced Gd–Pd/TiO2
catalyst, therefore, indicated that NH3 was strongly
adsorbed on the surface. There was a weak signal
for NH3 adsorption on Brönsted acid sites that were
−
1
indicated by the band at 1421 cm . This band rep-
resents asymmetric deformation of ammonium ion
+
(
NH4 ) [3,7,28]. It is known that pure anatase does
not possess Brönsted acidity; however, the presence
of impurities such as silica or sulfates, could enhance
Brönsted acidity [7]. The features observed at 3390,
−
1
3
340, 3250, and 3140 cm (Fig. 5b) characterize the
N–H stretching region that are caused by the splitting
of the symmetric deformation band [3]. Also, we ob-
−
1
served the 3572 cm band which arose due to N–H
+
stretching of NH species [28]. When the surface
was heated under He following NH3 adsorption, the
intensity of all features decreased. However, we did
not observe any additional features developing which
would indicate a transformation of adsorbed ammo-
nia to other surface species. This observation seems
to be consistent with our NH3-TPD experiment on
reduced Gd–Pd/TiO2 where we did not observe any
major nitrogen-containing products except for a small
N2O feature at low temperatures.
Fig. 4. NH3-TPD profiles after NH3 adsorption at room tempera-
ture on reduced and oxidized 1%Gd–2%Pd/TiO2.
drop in the NH3 feature on the same sample. On the
reduced sample, the NH3 adsorption was mostly re-
versible, with ammonia desorbing without being trans-
formed to other compounds.
The same experiment was performed on the oxi-
dized Gd–Pd/TiO2. The spectra are presented in Fig. 6.
In addition to the species observed on the reduced cat-
−
1
3
.3. NH3 adsorption with DRIFTS
alyst, we observed a broad peak at 1673 cm , which
+
correspond to symmetric deformation of NH4 on
Surface species formed as a result of NH3 ad-
Brönsted acid sites. Compared to the reduced sample
in Fig. 5a, the intensity of the bands corresponding to
sorption were examined using DRIFTS. Background
spectra were taken at each temperature level that sam-
ple spectra were taken after pre-treatment in either
H2/He or O2/He mixtures. Adsorption of ammonia
−
1
NH3 on Lewis acid sites (1150 and 1592 cm ) was
−1
lower on the oxidized surface. The band (1220 cm
)
signaling the splitting of the symmetric deforma-
tion mode was not observed on the oxidized cata-
lyst. This indicated that the adsorption of molecular
NH3 was much weaker on the oxidized Gd–Pd/TiO2
than it was over the reduced catalyst. Moreover, the
(
3000 ppm in He) was carried out at room tempera-
ture for 30 min followed by He flushing. Fig. 5 shows
spectra taken over the reduced Gd–Pd/TiO2 catalyst
under NH3 flow and during the subsequent TPD in He

8
6
J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
−
1
−1
Fig. 5. DRIFT spectra on reduced 1%Gd–2%Pd/TiO2: (a) 1800–1000 cm region and (b) 3800–3100 cm region.
presence of the bands that characterize Brönsted acid
The presence of Gd was found to enhance the cat-
alytic activity for the NO reduction reaction under
more oxidizing conditions compared to the Gd-free
catalyst [23]. To examine if there are any major
differences between the two catalysts or any dis-
tinct features that could be attributed to the pres-
ence of Gd, the same experiment was performed
over the reduced Pd/TiO2 sample. Fig. 7 presents
the spectra taken at different temperature levels un-
der He after NH3 adsorption at room temperature.
−
1
sites (1439 and 1673 cm ; asymmetric and symmet-
ric deformation modes of NH4 , respectively) seemed
to be more pronounced than they were on the reduced
sample. In addition, the presence of peaks at 1560
+
−
1
and 1297 cm were noticed. The frequencies of these
bands seem to correspond to those of monodentate ni-
trate on titania [29]. The formation of nitrate species
indicated that a transformation of adsorbed ammonia
took place on the oxide surface. It is plausible that
PdO sites could participate in the oxidation of ammo-
nia to nitrate species. When the sample was heated
under He, the intensity of all species including the ni-
trate was reduced, suggesting that they desorb easily
with heating. The high-wavenumber regions of the
spectra (Fig. 6b) were similar to those observed over
the reduced catalysts, except for a decrease in intensity
+
The same species (molecular NH3 and NH4 ) that
were found on the reduced Gd–Pd catalyst were
also present on the Gd-free sample. One major
−1
difference was that the feature at 1220 cm
(the
splitting of NH3sy) was significantly weaker on the
reduced Pd/TiO2 catalyst compared to that over
the reduced Gd–Pd/TiO2 catalyst, suggesting that
the NH3 is adsorbed much more strongly on the
−
1
(
e.g., 3337 cm ).

J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
87
−
1
−1
Fig. 6. DRIFT spectra on oxidized 1%Gd–2%Pd/TiO2: (a) 1800–1000 cm region and (b) 3800–3100 cm region.
Gd-containing catalyst. The surface species and their
relative intensities on the reduced Pd/TiO2 catalyst
after NH3 adsorption were similar to those on the
oxidized Gd–Pd/TiO2. Our earlier studies with in situ
DRIFTS under NO–CH4 reaction conditions which
have shown that the formation of coordinated NH3
species was more prominent over the Gd–Pd/TiO2
than the Pd/TiO2 catalyst [25] is consistent with
the finding that NH3 is more strongly held on the
Gd–Pd/TiO2 catalyst. Also, the similarity between
DRIFTS spectra of the calcined Gd–Pd catalyst and
reduced Pd catalyst following NH3 adsorption could
be explained in terms of the electropositivity of the
Gd atoms that could be contributing to the electron
density of the Pd sites and making them more sim-
ilar to those found in a partially reduced Gd-free Pd
matrix.
3.4. Formation of NHx species under reaction
conditions
The formation of NHx species on reduced
Gd–Pd/TiO2 or Pd/TiO2 catalysts was observed dur-
◦
ing NO–CH4 reaction at 300 C. Fig. 8 presents the in
situ DRIFT spectra taken over reduced Gd–Pd/TiO2
under flows of NO, NO + CH4, and NO + CH4 + O2,
sequentially (NO = 1780 ppm, CH4 = 2.13%, O2 =
◦
2900 ppm) at 300 C. The catalyst was exposed to
the gas mixture for 30 min before each spectrum was
−
1
taken. Under NO flow, the band at 1793 cm , cor-
responding to NO linearly adsorbed on Pd, was the
most prominent feature observed. The weaker bands
−
1
located around 1535 cm represents the formation
of nitrate or nitro–nitrito complex [25,29]. Under
NO + CH4 flow, where a complete NO conversion

8
8
J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
−
1
−1
Fig. 7. DRIFT spectra on reduced 2%Pd/TiO2: (a) 1800–1000 cm region and (b) 3800–3100 cm region.
was achieved, the formation of coordinated ammo-
nia was observed. This species was characterized by
to the feed stream was changed from NO → CH4 to
CH4 → NO. It was shown that ammonia was formed
and adsorbed on the surface regardless of the order.
One difference is that the bands that correspond to
NH3 are stronger when the catalyst is exposed to CH4
before NO. This is in agreement with our earlier as-
sertion that NH3 formation requires the initial hydro-
gen abstraction from CH4, which takes place on Pd0
sites. Exposing the catalyst to NO flow first may have
−
1
the bands at 1186/1220 and 1598 cm . In the N–H
−
1
stretching region, multiplex at 3369 and 3264 cm
were also observed, indicating the presence of NH3
on Lewis acid sites. Also in the same figure, a com-
−
1
plete disappearance of 1793 cm band was observed
under NO + CH4 flow. When O2 was introduced
to NO + CH4 flow, NHx species seemed to disap-
pear. Concurrently, linear NO species adsorbed on
Pd reappeared and the intensity for nitrate species
0
reduced the number of Pd sites through the partial
oxidation of the surface with the O atoms from NO
decomposition.
−
1
below 1600 cm seemed to grow. When CH4 was
present in the system, in addition to the characteris-
The state of Pd on which linear NO was adsorbed
has been discussed in the previous paper [25]. We are
of the opinion that linear NO under the reaction con-
−
1
tics bands for gas-phase methane (1303/3013 cm ),
−
1
we also observed bands (2333/2356 cm ) signaling
the formation of CO2.
0
ditions could be adsorbed on both metallic (Pd ) and
+
Fig. 9 shows the spectra taken in a similar experi-
ment where the order in which gases are introduced
partially oxidized (Pd ) sites. It was demonstrated
that the vibration frequency of Pd–NO may vary in the

J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
89
Fig. 9. DRIFT spectra taken over reduced Gd–Pd/TiO2 with
sequential addition of the reactants, CH4 → CH4 + NO →
NO+CH4+O2 (NO = 1780 ppm, CH4 = 2.13%, O2 = 2900 ppm,
Fig. 8. DRIFT spectra taken over reduced Gd–Pd/TiO2 with
sequential addition of the reactants, NO → NO + CH4
NO+CH4+O2 (NO = 1780 ppm, CH4 = 2.13%, O2 = 2900 ppm,
→
◦
−1
−1
region and (b) 4000–2000 cm
◦
−1 −1
region and (b) 4000–2000 cm
300 C): (a) 1900–1100 cm
3
00 C): (a) 1900–1100 cm
region.
region.

9
0
J.M. Watson, U.S. Ozkan / Journal of Molecular Catalysis A: Chemical 192 (2003) 79–91
range from 1750 to 1793 cm 1 depending on the sur-
face oxygen concentration. Since oxygen is more elec-
tronegative than Pd, the presence of surface oxygen
reduces the electron density at Pd atoms. Conse-
quently, fewer electrons are available for backbonding
to the antibonding orbital of NO which, in turn, weak-
ens the bond strength between Pd and N-atom of NO
−
is an essential step for ammonia formation, occurs on
metallic Pd sites. In the presence of CH4, NHx species
could be formed through an interaction of CHx with
nitrogen-oxo species such as monodentate nitrate,
nitro-species, or linear NO on Pd. We observed that
NHx and linear NO species were never present on the
surface concurrently. These results strongly suggest
that NO reduction to N2 takes place through reduction
of Pd–NO species by NHx and CHx, both of which
may function as a reducing agent.
[
30,31]. Similar results and conclusions have been
obtained by Almusaiteer and Chuang [32] when they
studied the NO + CO reaction on Pd/Al2O3 using in
situ FTIR.
Acknowledgements
4
. Conclusions
The financial contributions from the national sci-
ence foundation and the Ohio coal development office
are gratefully acknowledged.
Our previous studies [24,25] on sol–gel-prepared Pd
catalysts for reduction of NO with CH4 suggested that
one of the key surface species for the NO–CH4–O2
reaction involved NHx species in which case, NO
reduction could proceed via in situ SCR-NH3 mech-
anism. In this study, the use of NH3 as the reducing
agent for NO reduction on the Gd–Pd/TiO2 catalyst
was investigated under steady-state conditions. It was
found that NH3 was effective in reduction of NO
over the reduced catalyst. This result was consistent
with our earlier suggestion that NHx species formed
through the interaction of methane and the adsorbed
NO species on the surface could act as a reducing agent
for Pd–NO species. In the presence of O2, however,
NH3 oxidation, which has a higher activation energy,
becomes more dominant at higher temperatures com-
pared to NO reduction and the observed NO conver-
sion decreases. Our NH3-TPD experiments indicated
that the reversible NH3 adsorption capacity was higher
on the reduced catalyst compared to the oxidized cat-
alyst. Also, there was a strong evidence provided by
DRIFTS that adsorbed NH3 is transformed into mon-
odentate nitrate species on the oxidized catalyst. The
literature points to the possibility that the key reaction
intermediate for NH3 oxidation to N2 is nitrate species
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