The effect of CO2 and H2O on the kinetics of NO reduction by CH4 over a La2O3/γ-Al 2O3 catalyst

Article abstract of DOI:10.1016/S0021-9517(02)00092-1

NO reduction by CH₄ over a 40% La₂O ₃/γ-Al₂O₃ catalyst in the absence and presence of O₂ in the feed was studied. The addition of either CO₂ or H₂O to the feed produced a reversible inhibitory effect on the rate similar to that observed with unsupported La ₂O₃; however, the extent of rate inhibition was considerably smaller than on unsupported La₂O₃. At 973 K, either CO₂ (9%) or H₂O (2%) in the feed decreased activity by about 35% in the absence of O₂ and by only 20% with excess O₂ in the feed. In the absence of O₂, a reaction mechanism previously proposed for La₂O₃ was altered to include competitive CO₂ and H₂O adsorption and to give the following rate expression for N₂ formation: r _N2=k′P_NOP_CH4(1+K_NOP _NO+K_CH4P_CH4+K_CO2P _CO2+K_H2OP_H2O)². This equation fit the data well, had apparent activation energies of 14-25 kcal/mol, and gave thermodynamically consistent enthalpies and entropies of adsorption. Stable rates at 973 K with O₂ and either CO₂ or H₂O in the feed were between 0.94 and 0.99 μmol N₂/s/g catalyst. In the presence of excess O₂, after CO₂ and H₂O adsorption were again included, a rate equation proposed earlier for La ₂O₃ again provided a good fit to the data with H ₂O in the feed as well as thermodynamically consistent parameters determined under integral reaction operation. However, with both CO₂ and excess O₂ in the feed, this rate expression could not provide thermodynamically meaningful parameters from the fitting constants even though it fit the data well. This was attributed to a major contribution from the alumina to the overall rate, because CO₂ had no significant effect on NO reduction on alumina, but it inhibited this reaction on La ₂O₃. A reaction model was proposed for γ-Al ₂O₃ that gave the rate expression for total CH ₄ disappearance due to both combustion and NO reduction over γ-Al₂O₃ (r_CH4)T=k′ _comP_CH4P_O2^0.5+k′ _NOP_NOP_CH4P_O2 ^0.5(1+K′_NO2P_NOP_O2 ^0.5+K_CH4P_CH4+K_O2^0.5P _O2^0.5+K_CO2P_CO2+K_H2OP _H2O)², which gave a satisfactory fit to the data along with thermodynamically consistent parameters. The second term in this equation, which represents the rate of N₂ formation, was then combined with the rate equation for N₂ formation on pure La₂O ₃ in the presence of O₂ to describe overall catalyst performance, and the data were fit well, assuming that La₂O ₃ composed 6.8% of the total surface area, a value close to that of 6.1% obtained from XRD line-broadening calculations.

Full text of DOI:10.1016/S0021-9517(02)00092-1

Journal of Catalysis 214 (2003) 292–307
www.elsevier.com/locate/jcat
The effect of CO2 and H2O on the kinetics of NO reduction by CH4
over a La2O3/γ -Al2O3 catalyst
Todd J. Toops, Arden B. Walters, and M. Albert Vannice ∗
Department of Chemical Engineering, The Pennsylvania State University, University Park, PA 16802-4400, USA
Received 13 August 2002; revised 22 October 2002; accepted 4 November 2002
Abstract
NO reduction by CH over a 40% La O /γ -Al O catalyst in the absence and presence of O in the feed was studied. The addition
4
2
3
2
3
2
of either CO or H O to the feed produced a reversible inhibitory effect on the rate similar to that observed with unsupported La O ;
2
2
2 3
however, the extent of rate inhibition was considerably smaller than on unsupported La O . At 973 K, either CO (9%) or H O (2%) in the
2
3
2
2
feed decreased activity by about 35% in the absence of O and by only 20% with excess O in the feed. In the absence of O , a reaction
2
2
2
mechanism previously proposed for La O was altered to include competitive CO and H O adsorption and to give the following rate
2
3
2
2
expression for N formation:
2
ꢀ
k P
P
NO CH4
r
N2
=
.
2
)
H2O H2O
(1 + K
P
+ K
P
+ K
P
+ K
P
NO NO
CH4 CH4
CO2 CO2
This equation fit the data well, had apparent activation energies of 14–25 kcal/mol, and gave thermodynamically consistent enthalpies and
entropies of adsorption. Stable rates at 973 K with O and either CO or H O in the feed were between 0.94 and 0.99 µmol N /s/g catalyst.
2
2
2
2
In the presence of excess O , after CO and H O adsorption were again included, a rate equation proposed earlier for La O again provided
2
2
2
2 3
a good fit to the data with H O in the feed as well as thermodynamically consistent parameters determined under integral reaction operation.
2
However, with both CO and excess O in the feed, this rate expression could not provide thermodynamically meaningful parameters from
2
2
the fitting constants even though it fit the data well. This was attributed to a major contribution from the alumina to the overall rate, because
CO had no significant effect on NO reduction on alumina, but it inhibited this reaction on La O . A reaction model was proposed for
2
2 3
γ -Al O that gave the rate expression for total CH disappearance due to both combustion and NO reduction over γ -Al O
2
3
4
2
3
ꢀ
0.5
ꢀ
0.5
P
NO CH4
k
P
P
+ k
P
P
CH4
com
O2
NO
0.5 0.5
O2
(r
CH4
) =
T
,
ꢀ
0.5
O2
2
)
H2O H2O
(1 + K
P
NO
P
+ K
P
+ K
P
+ K
P
+ K
P
CH4 CH4
CO2 CO2
NO2
O2 O2
which gave a satisfactory fit to the data along with thermodynamically consistent parameters. The second term in this equation, which
represents the rate of N formation, was then combined with the rate equation for N formation on pure La O in the presence of O to
2
2
2
3
2
describe overall catalyst performance, and the data were fit well, assuming that La O composed 6.8% of the total surface area, a value close
2
3
to that of 6.1% obtained from XRD line-broadening calculations.
2003 Elsevier Science (USA). All rights reserved.
Keywords: NO reduction; CH ; SCR; Kinetics; La O /Al O
2 3
4
2
3
1. Introduction
results in low rates and limits any industrial applications.
Dispersing La2O3 on Al2O3 enhances its surface area and in-
creases reaction rates (g−1), and a 40 wt% La2O3/γ -Al2O3
catalyst was the most active at higher temperatures, with
rates comparable to those on zeolite catalysts [5,6]. Flue gas
components such as CO2 and H2O can adversely affect SCR
of NO on these systems, including La2O3 [7], and steam
is known to facilitate sintering in γ -Al2O3 [8–11]; there-
fore, it is of interest to determine the impact of CO2 and
La2O3 has one of the most active surfaces above 773 K
for the selective catalytic reduction (SCR) of NO by CH4
[1–4]; however, its surface area is typically very low, which
*
Corresponding author.
E-mail address: mavche@engr.psu.edu (M.A. Vannice).
0021-9517/03/$ – see front matter
doi:10.1016/S0021-9517(02)00092-1
2003 Elsevier Science (USA). All rights reserved.

T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
293
H2O on La2O3/Al2O3 catalysts. This study has examined
NO reduction by CH4 over 40% La2O3/γ -Al2O3 as well
as over the γ -Al2O3 support between 773 and 973 K in the
presence and absence of O2. The standard concentrations of
the flue components were the same as in earlier studies of
La2O3 and Sr-promoted La2O3 [7,12–14], i.e., 9% CO2 and
2% H2O. SO2 was not investigated here because it poisoned
La2O3 [7]. Reaction orders were determined along with spe-
cific activities and activation energies, and reaction mech-
anisms accounting for the effects of CO2 and H2O in the
absence or presence of O2, as well as the concurrent direct
combustion of CH4, were proposed. The equilibrium adsorp-
tion constants for CH4, NO, O2, CO2, and H2O, which were
obtained from the optimized rate expressions, were further
analyzed to verify their thermodynamic consistency. The
catalysts were characterized before and after the kinetic ex-
periments using X-ray diffraction (XRD), BET surface area
measurements, and NO chemisorption.
along with a chemiluminescence NOx analyzer (Thermo En-
vironmental Instr. Inc., Model 42H) to differentiate between
NO and NO2 in the effluent [7]. The GC temperature pro-
gram, i.e., hold at 228 K for 5 min, ramp at 35 K/min to
423 K, and hold for 5 min, provided good separation of
N2, O2, NO, CO, CH4, CO2, N2O, and H2O, although the
last peak was not quantifiable [7]. Unless otherwise speci-
fied, experiments in the presence of O2 were performed with
25 mg of catalyst and a feed containing 1.4% NO, 0.35%
CH4, and 1.0% O2 in He flowing at 45 cm3 (STP)/min. The
effects of CO2 and H2O were studied separately using stan-
dard concentrations of 9.0 and 2.0%, respectively. Distilled,
deionized water was introduced to the reactor system via He
flowing through a saturator held in a Neslab GP-100 constant
temperature bath maintained between 305 and 325 K. The
stainless steel tubing from the saturator to the reactor and all
stainless steel lines downstream of the reactor were heated
at 323 K to avoid water condensation. Before the reactor ef-
fluent entered the NOx analyzer, it was diluted 10-fold with
He. The gas mixtures used in the experiments were 4.03%
NO in He, 9.80% O2 in He, and 1.0% CH4 in He, and they
were prepared with gases from MG Ind. (99.999%, except
for NO, which was 99.0+%). The CO2 was also from MG
Ind. (99.995%).
The standard catalyst pretreatment involved heating for
1 h at 973 K under 20 cm3 (STP)/min of 10% O2 in He.
Following pretreatment the gas mixture was flowed for
at least 30 min before any effluent stream analyses were
conducted. NO reduction with CH4 was performed between
773 and 973 K to obtain Arrhenius plots and to examine
the effects of CO2 and H2O on apparent activation energies.
A descending, then ascending temperature sequence was
used to check for any deactivation. Activity versus time
on stream was also monitored to determine both long-
term and transient effects of CO2 and H2O in the feed
stream on NO reduction. Partial pressure dependencies of
activity on H2O and CO2 were determined at three or four
different temperatures between 798 and 973 K by varying
the inlet partial pressure of one reactant while keeping all
other reactant concentrations and the total flow rate constant.
The standard feed concentrations were 11 Torr NO (1.4%),
2.7 Torr CH4 (0.35%), and 7.6 Torr O2 (1.0%) in He. When
the partial pressure of any of these three components was
varied with either CO2 or H2O in the feed stream, the former
was held at 68 Torr (9.0%) and the latter at 15 Torr (2.0%).
NO was varied from 1.9 to 15 Torr (0.25–2.2%), CH4 from
0.38 to 3.4 Torr (0.05–0.55%), O2 from 1.9 to 22 Torr (0.35–
1.8%), CO2 from 8 to 130 Torr (1.0–17%), and H2O from
2.3 to 22 Torr (0.3–3.0%).
2. Experimental
The supported catalyst was prepared as described else-
where with La acetate (Molycorp, 99%, 14.64% in H2O)
as the La source and γ -Al2O3 (Englehard, 150 m2/g), us-
ing an incipient wetness technique [13,14]. The catalyst was
dried at 400 K in air, calcined at 1023 K for 4 h under
50 cm3 (STP) O2/min, cooled, and stored in a desiccator.
Surface areas from N2 physisorption, NO chemisorption at
300 K, and XRD patterns of each sample were determined
before and after reaction as described elsewhere [7]; how-
ever, a brief summary is as follows. XRD spectra were ob-
tained ex situ over a 2θ range of 10◦–80◦ at a scan speed of
5.0◦/min and a step size of 0.02◦ using a Philips MPD X-
ray diffractometer with Cu-Kα radiation (40 kV and 20 mA).
The N2 BET isotherms were determined volumetrically in
an adsorption system with a base pressure in the sample cell
ꢀ 1 × 10−5 Torr using a Mensor DPG II Model 15000 pres-
sure transducer with 0.01% full scale accuracy. Fresh sam-
ples (ca. 0.15 g) were treated for 1 h at 400 K in flowing
He, evacuated to the base pressure, cooled to 300 K, and
then submerged in a liquid N2 bath. Chemisorption of NO
was measured in the same volumetric system after a standard
pretreatment of heating from room temperature to 973 K at
11 K/min under 20 cm3 (STP)/min of 10% O2 in He, hold-
ing for 1 h, evacuating for 30 min, and cooling to 300 K.
The sample was evacuated for 1 h at 300 K after the initial
isotherm, and a second isotherm was measured to establish
reversible adsorption. NO chemisorption on the catalyst af-
ter various times on stream was measured after flushing with
He at 973 K, cooling to 300 K and measuring NO isotherms
and BET surface areas, in that order [7].
3. Results
The kinetic studies were performed in a reactor system
similar to that described previously [15], except that a P-E
Sigma 3 gas chromatograph (GC) with a 2.4-m Chromosorb
102 column and a P-E Nelson 1020 integrator were used
As observed with other La2O3-based catalysts [7,12],
CO2 reversibly inhibited the rate of NO reduction by CH4
over supported La2O3 in either the absence or presence of

294
T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
(a)
(a)
(b)
(b)
Fig. 1. Arrhenius plots characterizing the effect of CO on NO reduction
2
Fig. 2. Arrhenius plots for NO reduction by CH over γ -Al O in the
4
2 3
by CH over 40% La O /γ -Al O in (a) the absence of O and (b) the
presence of O and with either (a) CO or (b) H O in the feed. Reaction
4
2
3
2
3
2
2
2
2
presence of O . Reaction conditions: 1.4% NO, 0.35% CH , 0 or 1.0% O ,
conditions: 1.4% NO, 0.35% CH , 1.0% O , 0 or 2.0% H O, balance He;
2
4
2
4
2
2
−1
−1
−1
0 or 9.0% CO , balance He; GHSV = 60,000 h in the absence of O and
GHSV = 87,000 h with CO in the feed and 110,000 h with H O.
2
2
2
2
−1
100,000 h in the presence of O .
2
CH4 in He were flowed over this catalyst at 973 K, the ini-
tial activity increased slightly overnight (point A), as shown
in Fig. 8a. Introducing steam into the feed decreased the ac-
tivity overnight by about 50%, which is behavior consistent
with that for unsupported La2O3 [7], but when steam was
removed from the feed, the activity increased markedly to a
value almost double the initial activity (point B). During the
next 100 h on stream, the activity slowly declined to equal
that during the initial period (point C). This result was in-
vestigated further using a modified reaction chamber so that
any effects on surface area and NO uptakes could be moni-
tored throughout this reaction sequence [7], and the kinetic
sequence with H2O was reproduced in this modified cham-
ber, as shown in Fig. 8b. The corresponding NO isotherms
at 300 K and surface areas measured at the labeled points,
I, A, B, and C, are displayed in Fig. 9 and Table 3, respec-
tively, along with the turnover frequency (TOF), which is
normalized to the sites capable of irreversible NO adsorp-
tion at 300 K, i.e.,
O2; however, the inhibitory effect of CO2 on the perfor-
mance of the γ -Al2O3 alone was significantly smaller, es-
pecially in the presence of O2 where virtually no decrease
in activity was observed. Figs. 1–3 show the Arrhenius plots
for 40% La2O3/Al2O3 and Al2O3, while the rates, specific
activities, and apparent activation energies in the differen-
tial regime, in which all reactant conversions were less than
20% and which is depicted as a solid line in the Arrhenius
plots, are reported in Tables 1 and 2. Each gas hourly space
velocity (GHSV) was calculated using an approximate cata-
lyst density of 1 g/cm3. The effect of CO2 was further ex-
amined by monitoring the activity versus time on stream to
determine both the long term and the transient nature of the
inhibition, as shown in Fig. 4 for La2O3/Al2O3 and Fig. 5
for Al2O3. With H2O in the feed, a similar reversible effect
on NO reduction was observed, as depicted by the Arrhenius
plots in Fig. 6; however, unlike CO2, water also caused con-
siderable reversible inhibition of this reaction over Al2O3.
The apparent activation energies, along with 90% probabil-
ity limits, are reported in Table 1 for La2O3/Al2O3 and Ta-
ble 2 for γ -Al2O3, and the activity vs time on stream re-
sults involving H2O are displayed in Figs. 5, 7, and 8. In the
absence of O2, La2O3/γ -Al2O3 showed surprising behav-
ior during time-on-stream runs when H2O was added to and
deleted from the feed (Fig. 8). While 1.4% NO and 0.35%
rate of N2 formation [mol/sg]
TOF[s−1] =
.
(1)
irreversible NO uptake [mol/g]
The relatively constant TOF values indicate that the change
in activity can be attributed to a variation in the number of
NO chemisorption sites due, most likely, to a change in the
surface area of the La2O3 dispersed on the alumina surface.

T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
295
Table 1
Kinetic behavior with 40% La O /γ -Al O
2
3
2
3
Rate
Specific activity
2
E
a
3
3
(µmol N /s g × 10 ) (µmol N /s m × 10 ) (kcal/mol)
2
2
873 K
973 K
873 K
973 K
9% CO study
2
O
O
absent
Before CO
With CO
After CO
present
Before CO
With CO
2
54
9.3
56
160
110
144
0.57
0.10
0.59
1.6
1.1
1.5
18
18
14
2
2
3
2
2
2
2
2
470
300
480
1300
990
1400
4.9
3.2
5.2
14
10
15
20
17
20
2
2
2
2
(a)
After CO
2
2% H O study
2
O
O
absent
Before H O
2
35
17
40
130
73
180
0.37
0.17
0.42
1.4
0.80
1.9
20
25
24
2
2
2
2
With H O
2
After H O
present
2
2
Before H O
750
416
790
1200
940
1100
7.9
4.4
8.3
13
9.9
12
26
31
24
4
5
4
2
With H O
2
After H O
2
Reaction conditions: 1.4% NO, 0.35% CH , 0 or 1.0% O , 0 or 9.0% CO ,
4
2
2
−1
0 or 2% H O, balance He; GHSV = 60,000 h in the absence of O and
2
2
−1
100,000 h in the presence of O .
2
(b)
Fig. 3. Arrhenius plots for NO reduction by CH over γ -Al O in the
well, as indicated by the broad quartz peak at 2θ = 25◦ in
the pattern for regenerated La2O3 in Fig. 14b. With H2O
in the feed, no bulk phase transformation was observed,
which is consistent with previous observations for La2O3
and Sr/La2O3 [7,12,16,17]. XRD patterns for pure γ -Al2O3
showed no change in the bulk-phase properties.
4
2 3
absence of O and with either (a) CO or (b) H O in the feed. Reaction
2
2
2
conditions: 1.4% NO, 0.35% CH , 0 or 9.0% CO , 0 or 2.0% H O; GHSV:
4
2
2
−1
50,000 h
.
Reaction orders for this supported catalyst were deter-
mined in the absence of O2 at four temperatures between 898
and 973 K following the procedure outlined earlier [7], and
these results are tabulated in Table 4, while the experimental
data are provided in Fig. 10 for CO2 and Fig. 11 for H2O. As
expected with O2 in the feed, direct combustion of CH4 con-
tinued to be significant although NO conversions were kept
below 20%; therefore, data analysis was conducted assum-
ing integral reactor behavior [13,14]. These apparent reac-
tion orders are also listed in Table 4, and the kinetic data are
shown in Figs. 12 and 13.
XRD measurements were used to probe the effects of
CO2 and H2O on the phases present in Al2O3-supported
La2O3. As with unsupported La2O3 and Sr-promoted La2O3
[7,12], under reaction conditions with CO2 in the feed,
bulk transformation from La2O3 to an oxycarbonate (II-
La2O2CO3) phase occurred, as shown by the XRD patterns
in Fig. 14. As with unsupported La2O3, the oxycarbonate
phase disappeared after CO2 was removed from the feed
stream and the La2O3 phase was reestablished in either the
presence or absence of O2; however, a small remnant of the
oxycarbonate phase (at 2θ = 33.5◦) remained in the absence
of O2. After several experiments with the same sample, it be-
came difficult to separate the catalyst from the quartz wool
and a significant amount of quartz was detected by XRD as
Table 2
Kinetic behavior for NO reduction by CH over α-Al O
2 3
4
Rate
Specific activity
2
E
a
3
3
(µmol N /s g × 10 )
(µmol N /s m × 10 ) (kcal/mol)
2
2
873 K
973 K
873 K
973 K
9% CO study
2
O
O
absent
2
No CO
48
37
92
74
0.32
0.25
0.61
0.49
15
16
3
3
2
With CO
present
2
2
2
No CO
330
320
610
600
2.2
2.1
4.1
4.0
8.8
9.5
2
2
2
With CO
2% H O study
2
O
O
absent
2
No H O
48
27
92
60
0.32
0.18
0.61
0.40
15
17
3
2
2
With H O
2
present
2
No H O
247
224
611
507
1.6
1.5
4.1
3.4
13
17
2
4
2
With H O
2
Reaction conditions: 1.4% NO, 0.35% CH , 1.0% O , 0 or 9.0% CO , and
4
2
2
−1
0 or 2% H O, balance He; GHSV = 85,000 h
with CO in feed and
2
2
−1
110,000 h H O.
2

296
T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
(a)
(a)
(b)
Fig. 5. Activity vs time on stream in the presence of O during NO reduction
(b)
Fig. 4. Activity vs time on stream in the presence of CO during NO
2
2
by CH over γ -Al O at 973 K (a) in the presence of CO and (b) in
4
2
3
2
reduction by CH over 40% La O /γ -Al O at 973 K (a) in the absence
4
2
3
2 3
the presence of H O. Reaction conditions: 1.4% NO, 0.35% CH , 1.0%
2
4
of O and (b) in the presence of O . Reaction conditions: 1.4% NO, 0.35%
2
2
−1
O , and either 0 or 9.0% CO or 0 or 2% H O, balance He; GHSV =
2
2
2
CH , 0 or 1.0% O , and 0 or 9.0% CO , balance He; GHSV = 60,000 h
4
2
2
−1
−1
85,000 h
.
in the absence of O and 100,000 h in the presence of O .
2
2
The activity of alumina for NO reduction by CH4
has been reported earlier [5], and while some studies
have examined γ -Al2O3 as an NOx reduction catalyst
by itself, its primary utility has been as a support for
more active catalysts. It is an attractive support because
of its strong thermal stability in oxidizing environments
[8–11]; however, as mentioned earlier, steam can facilitate
sintering in transitional aluminas. The temperatures were
low enough in this study so that no significant sintering
was observed with either pure Al2O3 or Al2O3-supported
La2O3. Coincidentally, La2O3 has been used to inhibit
steam-induced sintering of alumina [29–32], so the presence
of La2O3 may stabilize its surface at higher temperatures.
It is clear from the kinetic results provided in Tables 1
and 2 that γ -Al2O3 can make a significant contribution to
NO reduction activity, especially in the presence of CO2 and
H2O, and the results in Fig. 8 suggest that there may be
a synergistic effect as well. The large increase in activity
after removal of H2O from the feed was unique to this
catalyst and was observed only in the absence of O2. From
the chemisorption results reported in Table 3, it is clear
that the surface has a substantially higher concentration of
NO adsorption sites after exposure to H2O; however, this
concentration decreases with time and reaches the original
4. Discussion
The rate inhibition on the 40% La2O3/γ -Al2O3 catalyst
caused by CO2 and H2O was qualitatively the same as that
observed with La2O3 and 4% Sr–La2O3 [7,12]; however,
the degree of inhibition was considerably less with the
supported catalyst. Either CO2 or H2O decreased the activity
of unsupported La2O3 by around 75% in the absence of
O2 and about 50% in the presence of O2, whereas the rate
inhibition was only about 35% and 20%, respectively, for
La2O3/Al2O3. This reduced degree of inhibition, especially
in the presence of O2, is a promising result because this
supported system is the most industrially relevant of the
La2O3-based catalysts investigated. When compared to the
kinetic performance of other NOx reduction catalysts that
have been tested with H2O in the feed [18–28], it is apparent
that this is one of the most active catalysts for SCR of NO
above 773 K with steam present, as indicated in Table 5.
The TOFs for the La2O3/Al2O3 catalyst in this study were
calculated from Eq. (1), but in the other studies, the rate
was normalized to the concentration of metal cations in the
catalyst, when possible. There is very limited information
in the literature about the kinetic effects of CO2 on oxide
catalysts, as discussed more fully elsewhere [7].

T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
297
(a)
(a)
(b)
Fig. 6. Arrhenius plot characterizing the effect of H O on NO reduction
2
by CH over 40% La O /γ -Al O in (a) the absence of O and (b) the
4
2
3
2
3
2
presence of O . Reaction conditions: 1.4% NO, 0.35% CH , 0 or 1.0% O ,
2
4
2
−1
0 or 2.0% H O, balance He; GHSV = 60,000 h in the presence of O
2
2
−1
and 100,000 h in the absence of O .
2
(b)
Fig. 8. Time on stream study of the effect of steam on NO reduction by
level about 50 h after the removal of water from the feed.
This behavior is quite reproducible and is most likely due
to a La2O3 redispersion effect that occurs in the absence
of O2, but not with O2 in the feed. However, another
explanation for this behavior is possible. The results in this
study show that the activity doubles only in the absence of
O2 with H2O present and that under these conditions the
CH over 40% La O /γ -Al O at 973 K (a) in the microreactor with
4
2
3
2 3
−1
GHSV = 56,000 h and (b) in the chemisorption system with GHSV =
−1
36,000 h . Reaction conditions: 1.4% NO, 0.35% CH , 0% O , 0 or 2.0%
4
2
H O, balance He.
2
reaction orders on NO increase to near unity, which suggests
a lower surface coverage of NO. Earlier experiments have
shown that: (a) La2O3 is susceptible to the loss of surface
lattice oxygen [6]; (b) the concentration of surface oxygen
vacancies positively affects the NO adsorption behavior
[6,33]; (c) NO adsorption is aided by surface O–S groups
[6]; and (d) H2O can create OH− species on high surface
area alumina surfaces [10,34]. With these aspects in mind,
it may be that when H2O was removed from the feed, the
high concentration of OH− groups on the Al2O3 surface,
which were equilibrated with gas-phase H2O, decreases due
to dehydroxylation and generates surface oxygen sites, as
shown in the equation
Fig. 7. Activity vs time on stream in the presence of H O during NO
2
2OH–S
O–S + H2O–S,
(2)
reduction by CH 40% La O /γ -Al O at 973 K in the presence of O .
4
2
3
2
3
2
which would increase the concentration of NO adsorption
sites after H2O desorption as shown below (Eq. (8)).
Reaction conditions: 1.4% NO, 0.35% CH , 1.0% O , and 0 or 2.0% H O,
4
2
2
−1
balance He; GHSV = 108,000 h
.

298
T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
Table 4
Reaction orders for the reduction of NO by CH over 40% La O /γ -Al O
4
2
3
2 3
with CO or H O in the feed
2
2
NO
CH
O
CO
2
H O
2
4
2
CO study
2
O
O
absent
2
973 K
948 K
923 K
0.52
0.43
0.44
0.31
0.33
0.30
–
–
–
−0.10
−0.10
−0.10
–
–
–
present
2
923 K
898 K
873 K
0.89
0.82
0.92
0.20
0.23
0.29
0.54
0.53
0.37
−0.07
−0.03
−0.04
–
–
–
H O study
2
O
O
absent
2
973 K
948 K
923 K
898 K
0.97
0.89
0.96
1.07
0.32
–
–
–
–
–
–
–
–
−0.29
−0.19
−0.20
−0.22
0.28
0.26
0.21
present
2
873 K
848 K
823 K
798 K
0.59
0.47
0.45
0.57
0.84
1.14
0.95
0.66
0.08
−0.01
0.06
–
–
–
–
−0.22
−0.19
−0.32
−0.16
0.08
Fig. 9. NO chemisorption at 300 K on 40% La O /γ -Al O after reaction
2
3
2 3
Reaction conditions: 1.4% NO, 0.35% CH , 0 or 1.0% O , 0 or 9.0% CO ,
4
2
2
at points noted in Fig. 7. Reaction conditions: T = 973 K, GHSV =
−1
0 or 2.0% H O, balance He; GHSV = 20,000–50,000 h in the absence
2
−1
36,000 h , and 1.4% NO, 0.35% CH , 0% O , 0 or 2% H O, balance
−1
4
2
2
of O and 90,000–100,000 h in the presence of O .
2
2
He.
+ 2S
(6)
(7)
Despite this behavior, one can begin by assuming that
1/KCO
2
the reaction mechanisms and rate laws proposed earlier for
unsupported La2O3 remain applicable for La2O3 dispersed
on alumina [35]. The effects of CO2 and H2O can be easily
incorporated into the kinetic rate expression by assuming
that these two products compete for adsorption on the active
sites. A detailed reaction mechanism in the absence of O2
has been proposed previously [7], and only a shortened form
containing the kinetically significant steps for NO reduction
by CH4 is shown below,
CO2–S
CO2 + S
1/KH2
O
ꢀ
ꢁ
2 H2O–S
H2O + S
(8)
4NO + CH4 → 2N2 + 2H2O + CO2,
(9)
where reaction (5) is the rate-determining step (rds) and
Ki represent adsorption equilibrium constants. If adsorbed
CH4, NO, and H2O constitute the principal surface interme-
diates, so that the resulting site balance for total active site
concentration, L, is L = [CH4–S] + [NO–S] + [CO2–S] +
[H2O–S] + [S], the following rate expression for N2 forma-
tion is derived [14],
KNO
ꢀ
ꢁ
4 NO + S
NO–S
(3)
KCH
4
CH4 + S
CH4–S
(4)
(5)
k
r
N2
= kNꢀ OPNO
P
CH4
NO–S + CH4–S → HNO–S + CH3–S (rds)
K
× (1 + KNOPNO + KCH PCH + KCO PCO
4
4
2
2
3NO–S + HNO–S + CH3–S
2H2O–S + 2N2 + CO2–S
−2
+ KH OPH O
)
,
(10)
2
2
Table 3
Kinetic parameters at various times on stream noted in Fig. 7b during NO reduction by CH over 40% La O /γ -Al O
4
2
3
2
3
a
Rate
Surface area
(m /g)
Specific activity
(µmol N /s m × 10 )
NO uptake
(µmol NO/g)
TOF
2
2
2
3
−1
3
(µmol N /s g)
(s × 10 )
2
Initial rate(I)
Before H O(A)
0.120
0.100
0.160
0.090
97
93
93
93
1.2
1.1
1.7
1.0
17
15
23
15
7.0
6.7
6.9
6.1
2
H O effect (B)
2
After H O(C)
2
−1
Reaction conditions: 1.4% NO, 0.35% CH , 0 or 2.0% H O, balance He; GHSV = 36,000 h
.
4
2
a
TOF defined as µmol N /s/µmol NO
.
2
ad

T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
299
(a)
(b)
(c)
Fig. 10. Partial pressure dependencies for NO reduction by CH over 40% La O /γ -Al O in the absence of O but with CO in the feed at (a) 973 K,
4
2
3
2
3
2
2
(b) 948 K, and (c) 923 K. Experimental data are represented by symbols and the rate expression given by Eq. (10) is represented by lines. Reaction conditions:
−1
1.4% NO, 0.35% CH , and 9% CO , balance He; GHSV = 20,000 h
.
4
2
where kNꢀ O = LkKNOKCH . As with La2O3, differential re-
tions [7]. The activation energy for the rate constant k in the
rate-determining step to form a methyl group (Eq. (5)) was
25 9 kcal/mol with CO2 in the feed, but it could not be
determined in the H2O study because the KNOP NO term es-
sentially went to zero.
4
actor conditions were maintained throughout the experi-
ments in the absence of O2 and a straightforward optimiza-
tion routine could be used to determine the fitting parameters
[7,13,14]. The capability of this equation to fit the data is sat-
isfactory and is represented as solid lines in Figs. 10 and 11
for CO2 and H2O, respectively, while the optimized fitting
parameters at each temperature are listed in Table 6. The
results indicate that surface coverage of oxygen and water
are negligible with CO2 in the feed, while coverages of NO,
oxygen, and CO2 are very low with H2O in the feed and
can be neglected in the site balance. All but one of these fit-
ting parameters represent equilibrium adsorption constants,
so they were further examined for thermodynamic consis-
tency [14]. The entropies and enthalpies of adsorption de-
rived from these constants are listed in Table 7 along with
90% confidence limits, and they were analyzed based on cri-
teria discussed elsewhere to verify they are thermodynami-
cally meaningful [36,37]. These values are also quite con-
sistent with those reported for La2O3 under similar condi-
As discussed elsewhere for La2O3 and Sr-promoted
La2O3 [7,12], both the surface chemistry and the subsequent
data analysis become much more complex under lean-
burn conditions in the presence of O2, and here it is
even more difficult because Al2O3 can also contribute to
the kinetics of CH4 combustion. Both La2O3 and Al2O3
catalyze the direct oxidation of CH4 with O2, as shown
elsewhere [13]; consequently, differential reactor conditions
could not be maintained for all reactants at all conditions.
Even though NO conversion was always less than 20%,
total methane and oxygen conversions were around 50% at
times; thus rate data were analyzed assuming an integral
reactor design equation to describe the concentration profiles
throughout the catalyst bed [13,14]. The rate expressions for
both the NO reduction reaction and the CH4 combustion

300
T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
(a)
(b)
(c)
(d)
Fig. 11. Partial pressure dependencies for NO reduction by CH over 40% La O /γ -Al O in the absence of O but with H O in the feed at (a) 973 K,
4
2
3
2
3
2
2
(b) 948 K, (c) 923 K, and (d) 898 K. Experimental data are represented by symbols and the rate expression given by Eq. (10) is represented by lines. Reaction
−1
conditions: 1.4% NO, 0.35% CH , and 2.0% H O, balance He; GHSV = 50,000 h
.
4
2
K
reaction were simultaneously utilized to calculate reactant
concentrations throughout the catalyst bed and to optimize
the fitting parameters in these two rate equations [13,14].
It is again assumed that the surface chemistry and reac-
tion mechanism are the same for both supported and unsup-
ported La2O3 and, if so, after the inclusion of molecular CO2
and H2O adsorption the abbreviated mechanism with all ki-
netically significant steps is [7,35]
2CO2–S + 4H2O–S + 2N2 + S
(16)
(17)
(18)
1/KCO
2
ꢀ
ꢁ
2 CO2–S
CO2 + S
H2O + S
1/KH2
O
ꢀ
ꢁ
4 H2O–S
4NO + 2CH4 + O2 → 2N2 + 4H2O + 2CO2.
With reaction (15) as the rds and CH4–S, NO–S, O–S,
CO2–S, and H2O–S as the principal surface species in the
site balance, the derived rate expression for N2 formation is
(19)
KNO
ꢀ
ꢁ
4 NO + S
NO–S
(11)
(12)
(13)
KCH
4
ꢀ
ꢁ
ꢂ
ꢃ
2 CH4 + S
CH4–S
rN = LkKNO KNOKCH KO0.5
P
NOPCH PO0.5
4
2
2
2
4
2
ꢂ
KO2
ꢀ
ꢁ
× 1 + KNOPNO + KCH PCH + KCO PCO
4
4
2
2
2 O2 + 2S
2O–S
ꢃ
−2
+ KH OPH O + K0.5P0.5
.
(20)
KNO
2
2
O2 O2
2
ꢀ
ꢁ
3 NO–S + O–S
NO2–S + S
(14)
(15)
Based on the stoichiometry in Eq. (19), this expression re-
lates directly to CH4 disappearance due to NO reduction be-
ꢀ
ꢁ
k
2 NO2–S + CH4–S → HNO2–S + CH3–S (rds)
2HNO2–S + 2CH3–S + NO–S + NO2–S + O–S
cause rN = (rCH
)
NO = d[N2]/dt = −d[CH4]/dt. This rate
2
4
expression can be combined with that for CH4 disappearance

T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
301
(a)
(b)
(c)
(d)
Fig. 12. Partial pressure dependencies for NO reduction by CH over 40% La O /γ -Al O with O and CO in the feed: (a) CO , (b) CH , (c) NO, and
4
2
3
2
3
2
2
2
4
(d) O . Experimental data are represented by symbols and the rate expression given by Eq. (24) is represented by lines. Reaction conditions: 1.4% NO, 0.35%
2
−1
CH , 1.0% O , and 9% CO , balance He; GHSV = 90,000 h
.
4
2
2
due to direct oxidation [13] to give the rate of total methane
disappearance,
calculate the activation energy for the rds described by
Eq. (15). With CO2 in the feed this rate expression could
fit the experimental data satisfactorily but, in contrast, some
of the enthalpies and entropies were thermodynamically
inconsistent [14,36,37], so this model had to be rejected and
other alternatives considered. Because CO2 had little effect
on the SCR of NO over γ -Al2O3 with O2 present, as shown
in Fig. 2, it is hypothesized that the contribution from Al2O3
became increasingly significant as CO2 suppressed the rate
on La2O3, thus explaining the inconsistent thermodynamic
behavior. This inhibition by CO2 may well be due to the
formation of a bulk lanthanum oxycarbonate, La2O2CO3,
which can form if CO2 concentrations are high enough [7].
Fig. 14 verifies that La2O2CO3 was indeed present.
A partial pressure study was then conducted with γ -
Al2O3 to determine a rate law describing the kinetic be-
havior of NO reduction by CH4, and the experimental data
are displayed in Fig. 15 while reaction orders are listed in
Table 8. Eq. (21) proved to be unsuccessful both in terms
of quality of fit and in thermodynamic consistency, and the
ꢂ
ꢃ
(rCH )T = kꢀ PCH PO0.5 + kNꢀ OPNOPCH PO0.5
cꢂom
4
4
4
2
2
× 1 + KNOPNO + KCH PCH + K0.5PO0.5
4
4 ꢃ
O2
2
−2
+ KCO PCO + KH OPH O
,
(21)
2
2
2
2
where kcꢀom = LkꢀKCH K0.5 and kNꢀ O = LkKNO KNO
×
4
2
O2
KCH K0.5 [7,14]. The partial pressure of each component
4
O2
can be quantitatively related to PCH , and a commercial opti-
4
mization program (Scientist 2.01, Micromath Sci. Software)
was used to obtain the fitting parameters in this complex
rate expression, assuming integral reactor behavior to cal-
culate concentration gradients and evaluate performance, as
described elsewhere [7,13,14].
In the presence of H2O this rate law provided a very
good fit, as shown in Fig. 13, and the optimized rate
constants are listed in Table 6, while the thermodynamic
parameters are given in Table 7. Due to the inclusion of
KNO in the lumped rate constant, it was not possible to
2

302
T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
(a)
(b)
(c)
(d)
Fig. 13. Partial pressure dependencies for NO reduction by CH over 40% La O /γ -Al O with O and H O in the feed: (a) H O, (b) CH , (c) NO, and
4
2
3
2
3
2
2
2
4
(d) O . Experimental data are represented by symbols and the rate expression given by Eq. (23) is represented by lines. Reaction conditions: 1.4% NO, 0.35%
2
−1
CH , 1.0% O , and 2.0% H O, balance He; GHSV = 100,000 h
.
4
2
2
ꢃ
−2
fit was especially poor with respect to the NO partial pres-
sure, suggesting that a different N-containing intermediate
may predominate on the surface of γ -Al2O3. Studies of
NO on alumina have suggested that NO2 species may have
strong interactions with the γ -Al2O3 surface [6,38]; thus,
one might propose that adsorbed NO2, rather than NO, is the
most abundant N-containing species. With this and the same
earlier assumptions about other surface species, the rate law
can be derived for N2 formation on γ -Al2O3 [14],
+ KCO PCO + KH OPH O
,
(23)
KNO
2
2
2
2
where kcꢀom = LkꢀK K0.5 and kNꢀ O = LkK
×
CH4
NO2
O2
KCH4 K0.5. This rate law provided a good fit for the data
O2
with alumina, as depicted by the lines in Fig. 15, and the
optimized rate constants are listed in Table 6 along with their
units. Tests for thermodynamic consistency were limited to
just the CH4 equilibrium adsorption constant because the
optimization routine returned zero values for KO P O and
KCO P CO (KH OP H O was not varied), and this indicated
a somewhat higher heat of adsorption for CH4 on alumina of
32 kcal/mol.
At this point it is perhaps appropriate to assess the impli-
cations of the values listed in Table 7 and to determine what
can be said about the nature of the active sites on a lanthana
surface. The heats of adsorption (and entropies) obtained
from the fitting parameters for NO, O2, CO2, and H2O on
a lanthana surface are reasonable, although no experimental
values exist for comparison, but the heats of adsorption in-
dicated for CH4 are far too large for nondissociative adsorp-
2
2
ꢂ
ꢃ
rN = LkKNO KNOKCH K0.5
P
NOPCH P0.5
4
O2
2
2
2
2
2
2
4
O2
ꢂ
× 1 + KNꢀ O PNOP0.5 + KCH PCH + KCO PCO
O2
4
4
2
2
2
ꢃ
−2
+ KH OPH O + K0.5PO0.5
,
(22)
2
2
O2
2
where KNꢀ O = KNO KNO
K
0.5. This can be combined with
2
O2
2
the combustion rate expression [13] to yield the rate equation
for total CH4 disappearance over just the Al2O3 surface,
ꢂ
ꢃ
(rCH )T = kꢀ PCH PO0.5 + kNꢀ OPNOPCH PO0.5
cꢂom
4
4
4
2
2
× 1 + KNꢀ O PNOP0.5 + KCH PCH + KO0.5PO0.5
4
4
O2
2
2
2

T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
303
(a)
(b)
Fig. 14. XRD patterns characterizing the effects of CO on the phases present in 40% La O /γ -Al O (a) in the absence of O and (b) in the presence of O .
2
2
3
2
3
2
2
Lines represent unique peaks for La O (···) and La O CO (–). Symbols represent characteristic peaks for La O (ꢁ,ꢂ) and II-La O CO (ꢃ,♦), with
2
3
2
2
3
2
3
2
2
3
filled symbols representing principal peaks.
tion. Consequently, dissociative adsorption is implied, either
homolytic or heterolytic, but it occurs at a single “site” to be
consistent with our model. Theoretical calculations of het-
erolytic CH4 adsorption on La2O3 and Al2O3 by Capitan et
al. have indicated heats of adsorption of 18–21 kcal/mol on
a lanthana surface and 22–28 kcal/mol on an alumina sur-
face [39]. Not only are these values quite close to those in
Table 7, but that calculated for alumina is also larger, again
consistent with Table 7. Earlier calculations by Ito et al. esti-
mated a heat of adsorption for CH4 on MgO of 23 kcal/mol
for heterolytic adsorption on a coordinatively unsaturated
(cus) Mg2+–O2− site pair [40]. A detailed discussion of the
sites that can exist on a lanthana surface has recently been
provided and various types of oxygen vacancies were identi-
fied [41]. An oxygen vacancy would constitute a cus site,
and the possibility that this could be an active site, espe-
cially since NO and O2 adsorption can also occur on such
a site, has been considered elsewhere [6]. Thus, one reason-
able modification of the original models shown in steps (3)–
(9) and steps (11)–(19) that would reconcile the heat of CH4
adsorption, yet retain the rate expression, is to alter steps (4)
and (5) to
KCH
4
CH4 + S
CH3ꢁ–S–H⊕,
(4a)
(5a)
k
NO–S + CH3ꢁ–S–H⊕ → HNO–S–H + CH3–S (rds);
and steps (12) and (15) to
KCH
4
ꢀ
⊕ꢁ
,
2 CH4 + S
CH3ꢁ–S–H
(12a)

304
T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
Table 5
Comparison of NO reduction catalysts in the presence of O and H O
x
2
2
a
Catalyst
Temp
(K)
Reductant
H O NO
O
Bal
N
Rate
N
2
Activity
2
TOF
GHSV
α
Reference
2
2
2
3
3
−1
3
−1
%
%
%
%
µmol/s g×10
µmol/s m × 10
s
× 10
17
h
%
La O /Al O
923
923
923
CH
CH
C H
3
0.35 2.0
0.40 2.0
0.06 20
1.4
0.40
0.07
1.0
0.40
3.0
He
He
N
2
770
60
75
8.1
0.50
0.46
100,000
30,000
21,000
33
12
This study
[18]
2
3
2
3
3
4
4
e
Y O
2
n/a
n/a
3
e
f
Co/Al O
n/a
[19]
2
3
6
La O /Al O
873
873
873
873
873
873
873
873
873
CH
CH
CH
CH
CH
CH
CH
0.35 2.0
c
1.4
1.0
2.0 He
2.0
2.5
10
2.5
2.5
3.0
3.0
He
420
4.4
9.1
100,000
40,000
40,000
7,500
30,000
30,000
30,000
21,000
21,000
39
14
21
31
This study
[20]
2
3
2
4
d
b
e
Co/ZSM-5
0.10
0.10
0
0
0.10
0.10
0.50
0.10
0.09
0.09
0.07
0.07
67
n/a
n/a
n/a
4
4
4
4
4
4
c
d
b
e
Co–La/ZSM-5
Ce–Ag/ZSM-5
In/ZSM-5
Co/ZSM-5
Co/Ferrierite
He
He
He
He
He
104
n/a
[20]
[21]
[22]
[23]
[23]
[19]
[19]
d
b
0.50 8.3
0.10 10
0.10 2.0
0.10 2.0
0.06 20
0.06 20
60
n/a
2.4
d
b
e
f
15
n/a
n/a
n/a
9
d
b
70
n/a
0.21
0.30
d
b
101
n/a
14
e
f
Co/Al O
C H
N
N
98
84
0.60
0.51
n/a
n/a
n/a
2
3
3
6
6
2
2
e
f
Sn–Co/Al O
C H
3
n/a
2
3
La O /Al O
Co/Ferrierite
823
823
823
823
823
823
CH
CH
0.35 2.0
0.10 2.0
0.20 5.0
0.06 20
0.06 20
0.10 3.0
1.4
0.40
0.2
0.07
0.07
0.10
1.0
2.5
5.0
3.0
3.0
5.0
He
He
He
142
60
56
75
88
1.5
3.1
2.7
0.30
100,000
30,000
15,000
21,000
21,000
40,000
39
17
14
This study
[24]
2
3
2
3
4
b
n/a
n/a
4
b
d
Cu/Al O
C H
[25]
[19]
[19]
[26]
2
3
3
3
6
6
6
6
e
f
Co/Al O
C H
N
N
N
0.46
0.54
1.3
n/a
n/a
n/a
2
3
2
2
2
e
f
Sn–Co/Al O
C H
n/a
2
3
3
e
Ag/Al O
C H
3
250
n/a
25
2
3
La O /Al O
Fe/ZSM-5
Fe/ZSM-5
773
773 C H
773 C H
773
773
CH
4
0.35 2.0
0.50 10
0.20 20
0.06 20
0.06 20
1.4
1.0
3.0
3.0
3.0
3.0
He
He
He
37
290
990
19
0.39
0.81
0.60
2.5
100,000
42,000
42,000
21,000
21,000
37
14
This study
[27]
2
3
2
3
b
0.20
0.20
0.07
0.07
n/a
n/a
4
10
b
> 9
[28]
[19]
[19]
4
10
e
f
Co/Al O
C H
C H
3
N
N
0.12
0.46
n/a
e
n/a
2
3
3
6
6
2
2
f
Sn–Co/Al O
75
n/a
n/a
2
3
a
Defined in Eq. (1).
b
c
d
e
f
Specific surface area not reported.
Steamed for 24 h in 114 Torr of H O at 1073 K.
Assumed catalyst density of 1 g/cm .
Neither NO uptake nor reactive sites reported.
2
3
CH conversion data not reported.
4
ꢀ
ꢁ
consistent with XRD results because line-broadening analy-
ses were conducted on the XRD patterns using the Scherrer
equation to calculate the crystallite size of La2O3 and Al2O3
in the supported sample [14,42]. The average crystallite size
for γ -Al2O3 was 9.2 nm, which translates into 168 m2/g and
is consistent with the BET surface area of 150 m2/g, while
the average crystallite size for La2O3 was 56 nm, which indi-
cates a surface area of 16.4 m2/g. Based on these two values
and a 40 wt% La2O3 loading, the estimated fraction of sur-
face area comprosed of La2O3 is 0.061, which is in good
agreement with the X value from optimization.
k
2 NO2–S + CH3ꢁ–S–H⊕ → HNO2–S + CH3–S (rds),
(15a)
where S represents an oxygen vacancy with its neighboring
atoms.
Finally, with satisfactory rate expressions for NO reduc-
tion in the presence of O2 and CO2 over both La2O3 [7] and
γ -Al2O3, one should be able to fit the rate data in Fig. 12
using the rate constants from the two independent optimiza-
tion routines if the appropriate contribution from each sur-
face is known. Since the rates are presented on an areal ba-
sis, the two rate expressions must be multiplied by weighting
factors corresponding to the fractional surface area of each
oxide such that the overall rate for CH4 disappearance over
40% La2O3/γ -Al2O3 is represented by
5. Summary
rLa O /Al O = XrLa O + (1 − X)rAl O
,
(24)
2
3
2
3
2
3
2
3
The reversible inhibitory effect on the rate of NO reduc-
tion by CH4 over 40% La2O3/γ -Al2O3 due to either CO2
or H2O is similar to that observed with unsupported La2O3;
however, the extent of rate inhibition is considerably smaller
compared to the unsupported catalyst. With no O2 in the
feed, a previous reaction model for La2O3 was able to de-
scribe the kinetic behavior very well for all components af-
ter competitive CO2 and H2O adsorption was incorporated
where rLa O is defined by Eq. (21), rAl O is defined by
2
3
2
3
Eq. (23), and X is the area fraction of La2O3 in the catalyst.
Using Eq. (24), X was optimized to fit the data, as shown
in Fig. 12, this yielded a value of 0.068 for the area frac-
tion of La2O3. This estimate of the La2O3 area fraction indi-
cates that even though a 40 wt% loading of La2O3 represents
112 theoretical monolayers, there is considerable agglomera-
tion, which exposes a large portion of the γ -Al2O3. This is

T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
305
2
Table 6
Optimized rate parameters from the data-fitting routine for Eq. (10) in the absence of O (either CO or H O) and Eqs. (20) and (23) in the presence of O
2
2
2
ꢀ
a
ꢀ
b
k
k
K
K
K
K
K
H O
2
com
NO
NO
−1
CH
O
CO
4
2
2
−1
−1
−1
−1
(Torr )
(Torr
)
(Torr
)
(Torr
)
(Torr
)
CO study
2
O
O
absent
973 K
948 K
923 K
present
2
2
0.00074
0.0010
0.0018
–
–
–
0.15
0.25
0.36
0.91
1.0
1.9
–
–
–
0.0030
0.0074
0.013
–
–
–
La O [from Ref. [7]]
2
3
923 K
898 K
873 K
0.014
0.023
0.025
0.21
0.46
0.054
0.042
0.15
0.16
0.55
0.60
0.91
0.37
0.59
0.65
0.0042
0.010
0.017
–
–
–
ꢀ
c
γ -Al O
[K
]
2
3
NO2
923 K
898 K
873 K
848 K
0.00041
0.00064
0.0011
0.0033
0.0034
0.0032
0.0049
0.012
0.057
0.059
0.085
0.18
0.76
1.1
1.6
3.8
–
–
–
–
–
–
–
–
–
–
–
–
H O study
2
O
O
absent
973 K
948 K
923 K
898 K
present
873 K
848 K
823 K
798 K
2
2
0.0087
0.0003
0.0011
0.0012
–
–
–
–
–
–
–
–
1.1
0.72
1.9
–
–
–
–
–
–
–
–
0.058
0.031
0.086
0.15
2.5
0.042
0.086
0.13
0.85
2.3
1.4
2.4
0.55
1.1
1.9
2.6
0.88
0.65
1.5
15
92
140
180
–
–
–
–
0.26
0.35
1.1
0.090
3.7
0.91
a
ꢀ
ꢀ
2
2 −1
2
2.5 −1
in the presence of O .
2
Units for k
Units for k
are µmol N (s m Torr ) in the absence of O and µmol N (s m Torr
)
NO
2
2
2
b
c
2
1.5 −1
are µmol CH (s m Torr ) .
(defined in Eq. (22)) are Torr
com
ꢀ
4
−1.5
and apply only to γ -Al O .
Units for K
NO2
2 3
into the model to give the following rate expression:
Table 7
Enthalpies and entropies of adsorption based on the equilibrium adsorption
r
= kꢀPNO
P
CH4
N2
constants from Table 6 and a previous study in the absence of O with no
2
ꢂ
a
CO or H O in the feed
2
2
× 1 + KNOPNO + KCH PCH + KCO PCO
4
4
2
2
b
b
b
b
b
NO
CH
O
CO
2
H O
2
ꢃ
4
2
−2
+ KH OPH O
.
2
2
Huang et al. [4]
O
absent
2
This equation fit the data well and gave thermodynamically
consistent parameters.
◦
ꢀH
−28
−23
−20
−9
–
–
–
–
–
–
ad
◦
ꢀS
ad
With excess O2 in the feed, when H2O was also added,
a rate equation proposed earlier for La2O3 was capable of
providing a good fit of the data with thermodynamically
consistent parameters under integral reaction operation.
However, with both CO2 and excess O2 in the feed, this rate
expression for La2O3 could not provide thermodynamically
meaningful parameters from the fitting constants. This was
attributed to a major contribution from the alumina because
CO2 had no significant effect on the reaction over alumina,
but it inhibited that on La2O3. For Al2O3, the reaction model
was altered by assuming that NO2, rather than NO, was the
principal N-containing intermediate on the alumina surface.
This led to the rate expression for total CH4 disappearance
due to both NO reduction and combustion on γ -Al2O3
CO study
2
O
O
absent
2
2
◦
ꢀH
−31
−22
3
3
−25 10
–
–
−52
−52
8
8
–
–
ad
◦
ꢀS
−13
9
ad
present
La O [7]
2
ꢀH
3
◦
−43 20 −21
−40 20 −11
7
7
−35 15 −44
−28 15 −45
7
7
–
–
ad
◦
ꢀS
ad
γ -Al O
2
ꢀH
3
◦
−32
4
–
−23
–
–
–
–
ad
◦
ꢀS
–
5
–
ad
H O study
2
O
O
absent
2
2
◦
ꢀH
–
–
−24 13
−12 12
–
–
–
–
−28 18
−22 16
ad
◦
ꢀS
ad
present
ꢂ
ꢃ
◦
ꢀH
ad
−28
4
4
−29 11 −45 17
−21 13 −32 20
–
–
−27 10
−20 12
(rCH )T = kꢀ PCH P0.5 + kNꢀ OPNOPCH P0.5
4
4
4
cꢂom
O2
O2
◦
ꢀS
−20
ad
× 1 + KNꢀ O PNOP0.5 + KCH PCH + KO0.5PO0.5
4
4 ꢃ
O2
a
◦
◦
ad
2
2
2
Values of ꢀH are in kcal/mol, values of ꢀS —cal/mol K.
Confidence limits of 90%.
ad
−2
b
+ KCO PCO + KH OPH O
,
2
2
2
2

306
T.J. Toops et al. / Journal of Catalysis 214 (2003) 292–307
(a)
(b)
(c)
(d)
Fig. 15. Partial pressure dependencies in excess CO for NO reduction by CH over γ -Al O with O in the feed for (a) CO , (b) CH , (c) NO, and (d) O .
2
4
2
3
2
2
4
2
Experimental data are represented by symbols and the expression given by Eq. (23) is represented by lines. Reaction conditions: 1.4% NO, 0.35% CH , 1.0%
4
−1
O , and 9% CO , balance He; GHSV = 85,000 h
.
2
2
were fit well, assuming that La2O3 occupied 6.8% of the to-
tal surface area, a value in good agreement with that of 6.1%
obtained from XRD line-broadening calculations.
which gave satisfactory fits to the data and thermodynami-
cally consistent parameters, assuming that dissociative CH4
adsorption occurred on a site which presumably consists of
an oxygen vacancy and its neighboring atoms. This equation
was then combined with the rate equation describing NO re-
duction on pure La2O3 in the presence of O2, and the data
Acknowledgment
This study was supported by the National Science Foun-
dation under Grant CTS-9633752.
Table 8
Reaction orders for NO reduction by CH over γ -Al O in the presence of
4
2 3
O
with CO in the feed
2
2
References
NO
CH
O
CO
2
H O
2
4
2
CO study
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