Catalytic activity and surface properties of nitrided molybdena-alumina for carbazole hydrodenitrogenation

Article abstract of DOI:10.1006/jcat.1999.2788

The catalytic activity and surface properties of nitrided 12.5% Mo/Al₂O₃ catalysts were studied using TPR and diffuse reflectance FTIR and XPS spectroscopy for the HDN of carbazole. The catalytic activity of the 43, 58, 77.3, and 97.1% MoO₃/Al₂O₃ catalysts was compared to that of nitrided 12.5% Mo/Al₂O₃ catalysts. The MoO₃/Al₂O₃ precursors with various molybdenum loadings were nitrided by the TPR with ammonia, showing that the nitrided Mo/Al₂O₃ catalysts had a broad peak, which was deconvoluted to 6 nitrogen peaks. The IR spectra of ammonia showed that the 1173 K nitrided catalyst was less acidic than the 773 K nitrided catalyst, but its Lewis/Broensted acidity ratio was 25 times higher. The 1173 K nitrided 12.5% Mo/Al₂O₃ catalyst had the highest TOF for the HDN of carbazole. The XPS measurement showed that metallic Mo and Mo²⁺ were predominant in the 12.5% Mo/Al₂O₃ catalysts and led to the hydrogenation in HDN of carbazole.

Full text of DOI:10.1006/jcat.1999.2788

Journal of Catalysis 191, 128–137 (2000)
doi:10.1006/jcat.1999.2788, available online at http://www.idealibrary.com on
Catalytic Activity and Surface Properties of Nitrided Molybdena–Alumina
for Carbazole Hydrodenitrogenation
Masatoshi Nagai,1 Yosuke Goto,2 Atsushi Irisawa,3 and Shinzo Omi
Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology,
2-24 Nakamachi, Koganei, Tokyo 184-8588, Japan
Received August 20, 1999; revised December 13, 1999; accepted December 13, 1999
selectivity for C–N bond cleavage than hydrogenation and
to have HDN activity similar to or higher than that of com-
mercial sulfided Ni–Mo catalysts on the basis of a CO or O2.
There are a few studies on the surface properties and reac-
tivity of unsupported molybdenum nitrides (7–20). Boudart
et al. (7) studied nitrogen adsorption on molybdenum metal
polycrystalline and reported that the surface was predom-
inantly covered with adsorbed nitrogen and some dissoci-
ated species (N, NH, and NH2). Haddix et al. (10) stud-
ied the dynamics of ammonia adsorbed on unsupported
-Mo2N using a proton NMR technique. The NMR mea-
surement indicated that adsorbed ammonia was dehydro-
genated to produce NH2 and NH groups as well as N and H
atoms above 573 K. Moreover, Thompson and co-workers
(13) found that the reaction rate during pyridine HDN in-
creased linearly with the amount of NH3 chemisorbed in
the NH3–TPD experiment, suggesting that HDN and pyri-
dine adsorption occurred on sites that adsorbed NH3. The
relationship between the surface molybdenum and the ad-
sorbed nitrogen species and the catalytic activity of the ni-
trided alumina-supported catalysts during the HDN of car-
bazole have, however, received less attention. There are
very few reports on the molybdenum oxidation state of ac-
tive molybdenum species of the nitrided catalysts. Further-
more, it is not fully understood whether or not molybdenum
nitride is formed on the surface of the nitrided molybdena–
alumina.
The catalytic activity and surface properties of nitrided 12.5%
Mo/Al2O3 catalysts were studied using temperature-programmed
reduction and diffuse reflectance FTIR and XPS spectroscopy. The
activity of the nitrided Mo/Al2O3 catalysts was tested for the hy-
drodenitrogenation of carbazole. The catalytic activity of the 43.0,
58.0, 77.3, and 97.1% MoO3/Al2O3 catalysts was also determined for
comparison. The MoO3/Al2O3 precursors with various molybdenum
loadings were nitrided by the temperature-programmed reaction
with ammonia. The temperature-programmed reduction showed
that the nitrided Mo/Al2O3 catalysts had a broad peak which was de-
convoluted to six nitrogen peaks: four peaks due to nitrogen species
adsorbed on the surface molybdenum species (MoO2, -Mo2N, and
Mo metal) and alumina and two peaks due to nitrogen release from
the bulk molybdenum nitride. The infrared spectra of ammonia
showed that the 1173 K nitrided catalyst was less acidic than the
773 K nitrided catalyst but that its Lewis to Brønsted acidity ratio
was 25 times higher. The 1173 K nitrided 12.5% Mo/Al2O3 catalyst
had the highest TOF for the hydrodenitrogenation of carbazole.
The XPS measurement showed that metallic Mo and Mo2+ were
predominant in the 12.5% Mo/Al2O3 catalysts and led to the hydro-
c
genation in the hydrodenitrogenation of carbazole.
2000 Academic
Press
Key Words: carbazole; diffuse reflectance FTIR; molybdenum
nitride; hydrodenitrogenation; molybdenum oxidation state; sup-
ported catalyst; temperature-programmed reduction; XPS.
INTRODUCTION
For characterization, temperature-programmed reduc-
tion (TPR) with hydrogen can be used to determine the
reactivity of individual molybdenum species of nitrided
molybdena–alumina during the reaction of adsorbed ni-
trogen species associated with molybdenum species with
hydrogen to release nitrogen and ammonia gases (2, 16, 19,
21). The desorption of these gases during TPR can identify
the molybdenum species associated with surface and struc-
tural molybdenum compounds of the nitrided catalysts. The
surface composition and molybdenum oxidation state of
the Mo/Al2O3 catalysts nitrided at several temperatures
are determined by XPS spectroscopy. Therefore, in this
study, the surface molybdenum species were determined
Molybdenum oxides, nitrided with ammonia supported
on a high surface area alumina, recently exhibited high ac-
tivity for hydrodenitrogenation (HDN) of nitrogen com-
pounds (1–6). These catalysts are reported to have better
1
To whom correspondence should be addressed. E-mail: mnagai@
cc.tuat.ac.jp.
2
Present address: Denki Chemical Kogyo Co., 6 Goi-minami-Kaigan,
Ichihara, Chiba 290-8588, Japan.
3
Present address: San-ei Chemical Co., 1-31, Horifune, Kita, Tokyo
114-0004, Japan.
128
0021-9517/00 $35.00
c
Copyright
2000 by Academic Press
All rights of reproduction in any form reserved.

NITRIDED Mo/Al2O3 AND HDN
129
according to the desorption of nitrogen and ammonia gases FTIR measurement, the nitrided catalyst was purged with
during TPR. A positive relationship between the molyb- helium at 973 K for 1 h and cooled to room temperature in a
denum oxidation state of the nitrided catalysts and the stream of helium in order to prevent interference by excess
catalytic activity for carbazole HDN was determined by ammonia. To determine the activity of the 12.5% Mo/Al2O3
XPS analysis. Furthermore, diffuse reflectance FTIR spec- catalysts, the helium-cooled catalysts were also used for
troscopy can determine the nature of the acid sites on the ni- comparison of the ammonia-cooled catalyst. The nitrided
trided 12.5% Mo/Al2O3 catalysts. The relationship between catalysts were passivated in 1.0% O2 in helium at room
the highly active molybdenum species and the activity of the temperature for more than 12 h. The TPR experiment was
catalysts for carbazole HDN is also discussed.
carried out in situ, in order to desorb the water during TPR
that interferes with the nitrogen and ammonia spectra. The
space velocity of ammonia was 5096 h 1 for the ammonia
flow rate of 49.6 mol s 1. The following terms denote the
catalysts: 12MTN denotes 12.5% MoO3/Al2O3 nitrided in a
stream of ammonia at 973 K for 3 h. 43HTN denotes 43.0%
MoO3/Al2O3 nitrided in a stream of ammonia at 1173 K
for 3 h.
EXPERIMENTAL
Catalyst Preparation and Nitriding
Hydrogen and helium (99.9999% ) were dried by passing
them through a Deoxo unit (SUPELCO, Oxysorb) and
a Linde 13X molecular sieve trap prior to use. Ammonia
(Tomo-e, 99.99% ), 1.0% O2 in helium, carbazole (Tokyo
Kasei, 99.9% ), and xylene (mixed o-, m-, and p-xylene,
extra pure; specific gravity 0.86) were used without further
Characterization
The surface area of the catalysts was measured at 78 K
purification. MoO3/ -Al2O3 catalysts with various loadings by nitrogen adsorption using a BET apparatus (Coulter
(12.5, 43.0, 58.0, 77.3, and 97.1% MoO3; Nikki Chemical Co., Omnisorp 100CX) after evacuation at 473 K and
Co.) were prepared as follows: Ammonium heptamolyb- 1 Pa for 2 h. The nitrided Mo/Al2O3 catalysts were char-
date was dissolved in an aqueous ammonium solution at acterized in situ after the ammonia treatment by means
308 K. -Alumina xerogel was added to the ammonium of TPR. The microreactor in the TPR apparatus was con-
paramolybdate solution, and the solution was boiled while structed from a quartz tube (12 mm o.d.) with a total
being stirred. The solid product was dried at 473 K for 24 h volume of 2.4 ml. The catalyst was held in place with
and ground to a powder. For the 97.1% MoO3 catalyst, a fretted ceramic disk. The temperature of the catalyst
ammonium paramolybdate was mechanically mixed with (0.2 g) was increased from room temperature to 373 K in
2.9 wt% -alumina hydrogel as a binder. The solid product a stream of helium and was maintained at 373 K for 1 h,
was molded into 3-mm o.d. pellets with a kneader. After after which it was heated to 1263 K at a rate of 0.167 K
1
drying at 393 K for 24 h followed by calcination at 823 K
s
in a hydrogen flow of 11.2 mol s 1. The desorption
for 3 h in air, the pellets of the molybdena–alumina were gases that escaped from the catalysts during TPR were
crushed and sieved to 10 to 20 mesh (0.85 to 1.70 mm) monitored on-line using a quadrupole mass spectrometer
granules for measurement of activity and to powder (ULVAC, MSQ-150A). The spectra of the desorbed gases
(above 100 mesh) for characterization. The MoO3/Al2O3 were obtained by curve-fitting (Microcal Co., ORIGIN)
was supported on a fretted ceramic disk in a tubular quartz the data that were transferred from the quadrupole mass
microreactor in the catalyst pretreatment unit. The mi- spectrometer. The surface composition of the molybde-
croreactor was heated externally by an oven connected to a num atoms in the nitrided catalysts was measured using a
variable temperature programmer with a chromel-alumel Shimadzu ESCA 3200 spectrometer with monochromatic
thermocouple positioned outside the reactor near the cata- MgK exciting radiation (8 kV, 30 mA) at a pressure of
lyst bed to monitor the temperature and to relay it to the
5
10 4 Pa. Argon etching was done for 5 min before the
PID temperature programmer. A separate thermocouple spectra were measured by XPS. The binding energy of the
was placed at the center of the catalyst bed to measure the catalysts was referenced to Al 2p at 74.7 0.2 eV. Curve-
temperature of the catalyst. The catalyst was oxidized at fitting of the Mo 3d peaks was accomplished by using linked
723 K for 24 h in air and cooled to 573 K in a stream of dry doublets of the same full width at half-maximum (fwhm), an
air, followed by a stream of pure ammonia. The tempera- intensityratio of2/3, and a splittingof3.2eV for the Mo 3d3/2
ture of the molybdena–alumina was raised from 573 to 773 and 3d5/2 lines (22–24). The Mo 3d spectra were fitted into
(low temperature nitriding; LTN), 973 (MTN), and 1173 K five sets of Mo doublets (Mo 3d3/2 and 3d5/2) correspond-
1
(HTN) at a rate of 0.0167 K s in a stream of ammonia ing to five molybdenum species (Mo6+, Mo5+, Mo4+, Mo2+
,
at 49.6 mol s 1 and held at the nitriding temperature for and Mo0). The acidity of the nitrided Mo/Al2O3 catalyst
3 h in ammonia. After the ammonia treatment, the catalyst was analyzed according to an FTIR measurement with am-
was cooled to room temperature in a stream of ammonia monia as the probe. The catalyst was packed into an FTIR
for the TPR, XPS, and activity measurements. For the cell (Harrick Co., HVC-5). To remove the oxygen that was

130
NAGAI ET AL.
adsorbed by the water that formed on the catalyst, the cata-
RESULTS AND DISCUSSION
Nitrogen Desorption during TPR
lyst was reduced in a stream of hydrogen (11.2 mol s 1) at
1
773 K for 1 h, purged at 773 K with helium (11.2 mol s
)
for 1 h, and then cooled to room temperature in a stream
of helium. Diffuse reflectance FTIR spectra of the ammo-
nia adsorbed by the catalysts were recorded by an FTIR
instrument (Nicolet, Model 740, MCT detector), equipped
with a Harrick diffuse reflectance accessory. The infrared
The desorption of nitrogen for the catalysts nitrided at
773 K during TPR (e.g., 0LTN, 12LTN, and 97LTN) is
shown in Fig. 1. The profile showed only one large des-
orption peak of nitrogen at temperatures of 877 (97LTN),
898 (12LTN), and 900 K (0LTN); 97LTN had an addi-
tional large desorption peak above 1253 K. A small peak of
ammonia was also observed around 700 K. Since the
molybdenum oxide kept NHx (x = 0–3) adsorbed on the
unsupported Mo2N during cooling from the nitriding tem-
perature to room temperature (2, 7, 18, 19), molybdenum
oxides reacted with some of the adsorbed NHx during TPR
to form molybdenum nitrides, and the other adsorbed NHx
species desorbed as nitrogen and ammonia gases at a des-
orption temperature around 877 to 900 K. Thus, treating
molybdena–alumina with ammonia at 773 K did not lead
directly to the formation of molybdenum nitrides but rather
to molybdenum oxides which were nitrided with adsorbed
NHx at temperatures of 877 to 900 K. For the catalysts ni-
trided at 973 K (Fig. 2), the TPR profile for 97MTN shows
three distinct peaks of nitrogen desorption at 897, 1097,
and 1197 K. For 43MTN, two peaks of nitrogen desorp-
tion were observed at low and moderate temperatures but
with a broad peak at 1197 K. These peaks decreased fur-
ther for 12MTN. In a previous paper (4), the TPD study
showed that the desorption of nitrogen was not accompa-
nied by hydrogen above 1000 K, and therefore, the nitrogen
desorption was not due to the products in the decompo-
sition of the surface NHx. The broad peak at 993 K for
12MTN consisted of five peaks which appeared at 985 K
(nitrogen from alumina nitrided at 973 K for 0MTN), 885
and 945 K (from MoO2 and -Mo2N), 1104 K (during
the transformation of -Mo2N to -Mo2N0.78), and 1242 K
1
spectra were measured with a resolution of 4 cm in 100
scans at room temperature. After treatment in flowing he-
lium at 373 K for 1 h, the catalyst was treated in a stream
of 5.06% NH3/He (11.2 mol s 1) at 373 K for 1 h, purged
with helium (11.2 mol s 1) for 1.5 h, and heated at ele-
vated temperature in a stream of helium. To compare the
IR spectra of the catalysts with the spectrum of the re-
duced catalyst, the fresh 12.5% MoO3/Al2O3 was reduced
at 623 K in a stream of hydrogen for 3 h. The amount of
CO chemisorbed on the surface of the catalysts was deter-
mined by a volumetric analyzer after evacuation (Coulter
Co., Omnisorp 100CX). Before measuring the CO uptake,
the catalyst (0.2 g) was pretreated in a stream of helium at
723 K for 2 h and reduced in hydrogen at 653 K for 2 h. After
the reduction, the catalyst was degassed at 10 2 Pa and at
653 K for 1 h and then cooled slowly to room temperature
in a vacuum. The amount of irreversible CO uptake was ob-
tained from the difference between the two CO isotherms
which were extrapolated to zero pressure. The molybde-
num content was analyzed by means of atomic absorption
spectroscopy.
Measurement of Activity
The HDN of carbazole was measured using a fixed-bed
microreactor in a high-pressure flow system as described
elsewhere (25). The microreactor, consisted of a stainless-
steel tube (325 mm long, 17.3 mm wide, 3.2 mm thick), the
top of which was connected to a hydrogen gas cylinder and
a high-pressure feeder pump and the bottom of which was
connected to a high-pressure separator. Catalyst loading
was 2.0 g. The passivated catalysts were reduced in the re-
actor at 773 K with hydrogen for 1 h at 10.1 MPa to remove
water. The liquid feed, consisting of 0.25 wt% carbazole in
xylene, was introduced into the reactor at 20 ml h 1 with a
hydrogen flow of 74.4 mol s 1 at 573 K and a total pressure
of 10.1 MPa. The liquid reaction products were separated
from gaseous products, such as hydrogen and ammonia,
with a boiling point lower than that of the xylene solvent
in the high-pressure separator. The reaction products were
analyzed quantitatively by means of an FID gas chromato-
graph with 2% silicon OV-17 (25). The rate of carbazole
HDN was calculated according to the conversion of car-
bazole at 573 K. The TOF (h 1) is given by
1
TOF (h 1) = HDN rate at 0.5 h ( mol h
g
1)/
FIG. 1. N2 desorption during TPR of (᭺) 0LTN, (᭝) 12LTN, and
(᭿) 97LTN catalysts.
irreversibly adsorbed CO ( mol g 1).

NITRIDED Mo/Al2O3 AND HDN
131
FIG. 4. Nitrogen desorption deconvoluted for TPR of 12MTN catalyst.
FIG. 2. N2 desorption during TPR of (᭺) Al2O3, (᭝) 12.5% , (ᮀ) 43.0,
and (᭿) 97.1% Mo/Al2O3 nitrided with ammonia at 973 K.
97HTN and resulted in nitrogen desorption from molyb-
denum species. Furthermore, during TPR, the MTN and
HTN catalysts had a high peak due to nitrogen desorption
at temperatures of 1160 to 1210 K, whereas the peak for the
LTN catalyst was negligible.
(during the reduction of -Mo2N0.78 to molybdenum metal
for 97MTN). The desorption of nitrogen at 885 and 945 K
indicates that the ammonia treatment of 12.5% Mo/Al2O3
at 973 K produced molybdenum nitrides and that subse-
quent TPR nitrided the remaining molybdenum oxides and
molybdenum nitrides with adsorbed NHx. Nitrogen desorp-
tion for the 1173 K nitrided Mo/Al2O3 with several molyb-
denum loadings is shown in Fig. 3. The two peaks at 916
and 1033 K were observed for 77HTN, while a broad peak
at 1020 was observed with a shoulder peak at 966 K for
12HTN. The peaks at about 1050 and 1170 K increased for
To clarify the relationship between HDN activity and
the desorption of nitrogen from the catalysts during TPR,
the peaks of nitrogen desorption were deconvoluted to six
components for the nitrided 12.5% Mo/Al2O3, according
to results reported earlier (19) (Fig. 4). Six peaks of nitro-
gen desorption were due to adsorbed NHx on (a) MoO2 at
about 885 K, (b) -Mo2N at about 945 K, (c) alumina at
about 985 K, and (d) molybdenum metal at about 1016 K,
and were due to a structural change in (e) the transfor-
mation of -Mo2N into -Mo2N0.78 at 1104 K and (f) the
reduction to -Mo2N0.78 to molybdenum metal at 1242 K.
The amount of each nitrogen desorption for the nitrided
12.5% Mo/Al2O3 catalysts is shown in Table 1. The peaks
at 945 and 1104 K reached maximum values at the nitriding
temperature of 973 K (12MTN), suggesting that the molyb-
denum species derived from the nitrogen peak at 1104 K
TABLE 1
Deconvoluted Peak Area of N2 Desorption in the TPR Profiles
of Nitrided 12.5% Mo/Al2O3 Catalysts
N2 amount/ mol g 1 (N2 peak/K)
Correlation
Catalyst
12LTN
(a)
(b)
(c)
(d)
(e)
(f)
coefficient
703
71
63
1
2
0
0.999
(884) (945) (985) (1022) (1165)
40 157 40 138 210
12MTN
12HTN
75
0.997
0.993
(885) (945) (985) (1016) (1104) (1242)
43 113 223 60 111
(885) (945) (985) (1016) (1104) (1242)
9
FIG. 3. N2 desorption during TPR of (᭺) 0HTN, (᭝) 12HTN, (ᮀ)
43HTN, (᭹) 77HTN, and (᭿) 97HTN catalysts.

132
NAGAI ET AL.
was -Mo2N. The peak at 898 K reached a maximum due to phase with a lower nitrogen coordination. They suggested
the formation of molybdenum oxide at the nitriding tem- that the Mo 3d5/2 bindingenergyof227.6eV would be added
perature of 773 K (12LTN), but no peaks were observed as Mo0 to the binding energies of the molybdenum species
for 12MTN and 12HTN. Furthermore, the peaks at about to archive a good fit of the curve of the XPS Mo 3d spec-
1100 to 1250 K increased with increasing nitriding tempera- tra. Furthermore, McGarvey and Kasztelan (30) reported
ture due to nitrogen desorption as a result of the structural a reduced molybdenum species with oxidation numbers
transformation of -Mo2N0.78 to molybdenum metal.
lower than 4+ are referred to as Mo + and Mo0. Thus, the
Mo 3d5/2 molybdenum signals at 227.6 and 228.5 eV were at-
tributed to Mo0 and Mo2+, respectively. The percentage of
Mo6+ decreased with increasing nitriding temperature but
remained at 10% distribution even at 1173 K, probably due
to the passivation of the catalysts after nitriding; the per-
centage was different from that of the in situ samples (28).
McGarvey and Kasztelan (30) also reported that Mo6+ and
Mo4+ were present even at reduction temperatures higher
than 973 K for the 10.2% Mo/Al2O3 catalyst based on XPS
spectroscopy. The relative abundance of Mo5+ and Mo4+
reached a maximum at the nitriding temperature of 973 K
and decreased at high temperatures. Mo2+ and Mo0, both
of which appeared at 973 and 1073 K, increased to 21.6 and
16.2% , respectively, at 1173 K. Thus, nitriding a catalyst
with ammonia at higher temperatures is necessary to sig-
nificantly reduce the oxidation states to Mo2+ and Mo0. On
the other hand, there were several peaks of N 1s and Mo 3p
binding energies in the nitrided 12.5% Mo/Al2O3 catalysts
in the region of XPS N 1s from 390 to 410 eV. The XPS N 1s
spectra are reported to be the binding energies at 398.1 eV
(15), 397.4 eV (2), and 397.2 eV (31) (XPS N 1s binding en-
ergy for Mo–N). The peaks higher than 398.8 eV were also
assigned to the N 1s levels of NH3 adsorbed on the surface
Lewis acid site of the Al-ZSM-5 zeolite (32). The Mo 3p3/2
spectra in the range of 399.4 to 393.6 eV overlapped with
the N 1s spectra (33–35).
Surface Species by XPS
The XPS binding energies of the Mo 3d5/2 and 3d3/2 lines
for the 12.5% Mo/Al2O3 catalysts, nitrided at various tem-
peratures before the reaction, were determined. The XPS
Mo 3d spectra were broad for the nitrided 12.5% Mo/Al2O3
catalysts. The XPS data indicate that the molybdenum
species of the catalysts were widely distributed from Mo6+
to Mo0. These spectra ofthe bindingenergyofMo 3d5/2 were
deconvoluted into five doublets, with Mo 3d5/2 binding ener-
gies lower than 232.7 eV for Mo6+ and higher than 227.6 eV
for Mo0, i.e., the molybdenum oxidation statesofthe molyb-
denum species between the binding energies for Mo6+
(binding energy, 232.7 eV; fwhm, 2.07 eV), Mo5+ (231.4,
1.98 eV; 231.6 eV (24)), Mo4+ (229.6, 1.87 eV; 229.7 eV
(24)), Mo2+ (228.5, 1.6 eV), and Mo0 (227.6, 1.4 eV; 227.4 eV
(22), 227.6 eV (23), 227.8 eV (26, 27)). These molybde-
num species were assigned on the basis of the binding en-
ergy reported elsewhere (22, 23, 26, 27). The distribution
of the molybdenum oxidation state for the nitrided 12.5%
M/Al2O3 catalysts is shown in Table 2. The Mo 3d5/2 spectra
of 227.6 and 228.5 eV have yet to be identified. Thompson
and co-workers (29) assigned the Mo 3d5/2 binding energy
of the 227.8 and 228.5 eV levels in the XPS spectra of the
molybdenum nitride samples to molybdenum metal and
nitride ( +), respectively, indicating that Mo + is a molyb-
denum atom of molybdenum nitride ( +, probably 2+).
Ozkan et al. (15) reported that the peak at 228.5 eV was as-
signed to Mo + with < 4, corresponding to a molybdenum
FTIR Spectra of NH3 on Mo/Al2O3
The IR spectra of ammonia adsorbed on the nitrided
Mo/Al2O3 catalysts after dosing with several pulses of am-
monia in a stream of helium at 573 K (Fig. 5) were used to
determine the nature of the acid sites present on the cata-
lysts. The nitrided catalysts exhibited a strong spectrum at
TABLE 2
The Distribution of Molybdenum Oxidation State According to
XPS Analysis of the 12.5% Mo/Al2O3 Catalysts Nitrided at Several
Temperatures
1
about 1264 cm in addition to small spectra at 1481 and
1616 cm 1. The band at 1264 cm 1 was reported to be the
band at 1267 cm 1 attributed to the chemisorbed ammonia
on the sulfided Mo/Al2O3 (36, 37). The band for 12MTN and
12HTN shifted by6cm 1 from 1264to 1258cm 1. Thisresult
indicates that the electron-rich molybdenum surface has a
low frequency because of the formation of molybdenum ni-
tride. The bands from 1258 to 1264 cm 1 are ascribed to the
presence of the surface Lewis acid sites (surface-bonded
species of NH3) (38–40). Although a very weak band at
about 1481 cm 1 was observed for Mo/Al2O3, the IR band
Distributiona (% )
Nitriding
temperature
(K)
Mo6+
Mo5+
Mo4+
Mo2+
Mo0
773
873
973
1073
1173
33.2
22.1
16.1
13.3
11.7
33.5
35.0
36.0
30.5
18.3
26.8
40.4
45.0
38.2
26.6
0
0
3.0
7.5
21.6
0
0
0
5.0
16.2
a
The relative abundance of each molybdenum oxidation state was
1
1
at 1481 cm was close to the band at 1468 to 1480 cm
calculated by dividing the Mo (3d2/3 and 3d5/3)/Al 2p atomic ratio for a
given molybdenum oxidation state by the total area of the Mo (3d2/3 and
3d5/3)/Al 2p atomic ratio.
(oxided and reduced Mo/Al2O3 catalysts). The IR bands
1
at 1450 to 1485 cm ( -Al2O3) (39, 40) were due to the

NITRIDED Mo/Al2O3 AND HDN
133
FIG. 6. Surface (᭹) Lewis acidity and (᭿) Brønsted acidity of the
12.5% Mo/Al2O3 catalyst as a function of nitriding temperature.
thermore, molybdenum nitride would attract a hydrogen
atom of ammonia to compensate for the lack of nitrogen
atoms of the Mo2N structure (e.g., -Mo2N easily releases
nitrogen to form -Mo2N0.78) and to polarize the hydrogen
atom to a proton on molybdenum nitrides. These Brønsted
acid sites might be the molybdenum oxynitrides reported
by Dje´ga-Mariadassou et al. (17).
FIG. 5. Diffuse reflectance FT-IR spectra of NH3 adsorption on
12.5% Mo/Al2O3 at 553 K. B, Brønsted acidity; L, Lewis acidity.
deformational mode of NH4+ formed by the interaction of
ammonia with the Brønsted acid sites. The treatment of the
12.5% Mo/Al2O3 with ammonia demonstrated that Lewis
acid sites predominate on the catalysts. The 12HTN cata-
HDN of Carbazole on Nitrided Mo/Al2O3
1
lyst maintained the band at 1264 cm without the broad
The time dependence of the rate of the HDN of car-
bazole on the 973 K nitrided catalysts at 573 K and a to-
tal pressure of 10.1 MPa is shown in Fig. 7. The nitrided
catalysts deactivated rapidly during the first hour of the re-
action. The HDN rate for the nitrided 12.5% Mo/Al2O3
band at 1481 cm 1. The spectra at approximately 1264 and
1481 cm 1 were attributed to coordinated ammonia species
on the Lewisacid sitesand to NH+4 ionson the Brønsted acid
sites, respectively. The IR spectra corresponding to Lewis
1
acidity (ca. 1264 cm 1) and Brønsted acidity (ca. 1480 cm
)
for the 12HTN, 12MTN, and 12HTN catalysts are shown
in Fig. 6. Lewis and Brønsted acidity decreased at higher
temperatures. The 773 K nitrided 12.5% Mo/Al2O3 had
25 times more Lewis acid sites than Brønsted acid sites.
The treatment with ammonia at 1173 K resulted in more
Lewis acid sites but caused a slight increase in the num-
ber of Brønsted acid sites compared to the treatment at
973K. In a previous paper (41), the FTIR spectroscopy of
chemisorbed pyridine on the catalysts at 573 K indicated
that Lewis acidity (1264 cm 1) was predominant on the
molybdenum atom of the catalysts in the absence of surface
Brønsted acidity (1480 cm 1) for the nitrided and reduced
12.5% Mo/Al2O3 catalyst. In the present study, however,
some Brønsted acidity was observed even at the nitriding
temperature of 1173 K. This can probably be explained by
the same factor that was responsible for NHx or nitrogen
species remaining in the catalyst even though the catalyst
was purged at 973 K after nitriding at 773 to 1173 K. This ad-
sorbed or dissolved nitrogen species reacted with ammonia
FIG. 7. The HDN of carbazole on the various nitrided catalysts at
as a probe gas to regenerate the Brønsted acid sites. Fur- 573 K and 10.1 MPa.

134
NAGAI ET AL.
TABLE 3
Product Distribution in the HDN of Carbazole on the Nitrided Mo/Al2O3 Catalysts at 573 K and a Total Pressure of 10.1 MPa
Catalyst a,b
Reaction producte (% )
12LTN
12MTN
12HTN
43MTN
58MTN
77MTN
97MTN
12LTRc
Bicyclohexyl
6.1 (6.5)
7.9 (3.1)
0.2 (0.9)
0.9 (3.2)
0.4 (2.3)
8.8 (2.8)
9.3 (1.1)
1.0 (2.4)
0.4 (2.1)
0.3 (1.9)
22.2 (8.8)
58.0 (80.9)e
5.9 (3.6)
9.2 (1.5)
19.8 (3.1)
5.8 (4.2)
8.5 (2.5)
15.3 (8.6)
35.5 (76.5)e
10.9
2.7
3.5
3.6
1.6
17.2
4.9
5.0
6.7
2.7
2.4
0.5
2.2
1.7
1.9
6.7
0.7
0.5
2.1
2.7
1.8
2.5
1.4
2.2
0.5
Cyclohexylhexened
Perhydrocarbazole
Decahydrocarbazole
Hexahydrocarbazole
Tetrahydrocarbazole
Carbazole
23.5 (18.3)
61.0 (65.7)e
17.9
59.9
43.6
19.9
12.3
79.0
28.1
59.2
16.5
75.1
Conversion (% )
39.0 (34.3)
42.0 (19.1)
64.5 (23.5)
40.1
80.1
21.0
40.8
24.9
a Cooled with NH3: The nitrided catalysts were cooled from the nitriding temperature to room temperature in a stream of ammonia.
b (Parentheses) Cooled with He: The nitrided catalysts were purged with helium for 1 h at 973 K and cooled from the nitriding temperature to room
temperature in flowing helium.
c 12.5% Mo/Al2O3 was reduced in a hydrogen flow of 11.2 mol s 1 for 3 h at 673 K.
d Cyclohexylhexene and cyclohexylbenzene.
e Reaction products with a boiling point lower than that of xylene were not calculated.
catalyst was 0.736 mol h 1 m 2 0.5 h after the start of the sis of perhydrocarbazole after the consecutive hydrogena-
run and decreased to 0.34 mol h 1 m 2 after 3 h (almost tion ofcarbazole to perhydrocarbazole. Cyclohexylbenzene
the same concentration at 9 h). The product distribution of and cyclohexylcyclohexene were probably formed from the
carbazole HDN on the nitrided Mo/Al2O3 catalysts from C–N bond scission of hexahydrocarbazole and decahydro-
7 to 9 h is shown in Table 3. The major reaction products carbazole, respectively, although decahydrocarbazole was
were bicyclohexyl and tetrahydrocarbazole. Hydrogenated not detected in the reaction products. Since the carbazole
carbazole compounds, such as hexahydrocarbazole and oc- HDN over the nitrided Mo/Al2O3 catalysts contained small
tahydrocarbazole, were observed in small amounts, in con- amounts of these compounds, the nitrided catalysts effec-
trast to tetrahydrocarbazole. Relatively large amounts of tively decreased the nitrogen content which promoted se-
perhydrocarbazole were formed for the 12HTN catalyst, lective C–N hydrogenolysis of partially hydrogenated com-
indicating a high selectivity for hydrogenation compared pounds with reduced hydrogen consumption. In a previous
to that of 12MTN. The 12HTN catalyst, which contained paper (3, 42), the data on the HDN of carbazole on 12.5%
molybdenum metal, was most active during the hydrogena- Mo/Al2O3 nitrided at 773 K were collected in two separate
tion of carbazole. The 12LTN catalyst was much less ac- experiments using the nitrided catalyst with two average
tive than the other nitrided catalysts for carbazole HDN. particle sizes (di) of 1.275 (0.85 to 1.70) and 0.605 (0.5 to
Accordingly, a plausible reaction scheme for the HDN of 0.71) mm at temperatures of 553 to 633 K and a constant
carbazole is shown in Fig. 8. Carbazole was successively hy- partial pressure of carbazole and hydrogen. When the re-
drogenated to perhydrocarbazole on the nitrided catalyst action was limited by interpolate mass transfer, the C–N
as well as on the reduced and sulfided catalysts (25, 42). hydrogenolysis rate (ri) varied inversely with particle size;
Bicyclohexyl was produced during the C–N hydrogenoly- ri ∝ 1/di (43). The ratios of r1/r2 were independent for the
FIG. 8. Reaction scheme of the HDN of carbazole on the nitrided Mo/Al2O3 catalysts.

NITRIDED Mo/Al2O3 AND HDN
135
TABLE 4
Hydrodenitrogenation of Carbazole on the Nitrided Mo/Al2O3 Catalyst at 573 K
CO adsorbed
Irreversible
Surface area
Total
mol g
HDN ratea
mol m 2 h
TOFb
1
1
1
2
1
1
Catalyst
(m2 g
)
(
)
(
mol g
)
1012/cm
(
)
(h
)
12LTN
226c (223)d
195 (183)
138 (132)
64
91
39
33
250
38.8c (23.6)d
18.2 (8.2)
10.5 (23.0)
52.0
44.6
7.90
6.99c (5.43)d
3.18 (1.59)
1.28 (4.8)
28.1
20.3
6.50
43.8
4.60
1.87c
0.988
5.67
26.4
13.4
10.0
79.9
1.11
0.654c (0.478)d
0.736 (0.622)
0.971 (0.864)
1.79
1.01
2.49
21.1c (19.6)d
45.1 (71.6)
104.7 (23.8)
4.08
4.53
14.9
12MTN
12HTN
43MTN
58MTN
77MTN
97MTN
12LTRe
84.4
7.80
2.88
0.539
2.17
29.3
a HDN rate is based on the rate of disappearance of carbazole.
b TOF is the HDN rate at 0.5 h ( mol g 1 h 1) divided by the irriversibly adsorbed CO ( mol g 1).
c,d,e See the respective footnotes (a,b,c) in Table 3.
c The surface areas of Al2O3 nitrided at 773, 973, and 1173 K are 155, 131, and 108 m2 g 1, respectively.
catalyst particle sizes at the reaction temperatures of 573 is 5.5 times greater than that (2.17 h 1) in our experiment
to 633 K. From these results, the effect of external mass on carbazole HDN on the 97MTN catalyst at 573 K. Thus,
transport was expected to be negligible.
the TOF of the nitrided Mo/Al2O3 catalyst was approxi-
1
In a series of catalysts nitrided at 973K, the 12MTN cata- mately 2 to 100 h for the HDN of carbazole at 573 K
lyst showed the lowest HDN rate while the 97MTN cata- and a total pressure of 10.1 MPa. Table 4 shows that the
lyst showed the highest HDN rate. The 12HTN catalyst in ammonia-cooled LTN and HTN catalysts were more active
the nitrided 12.5% Mo/Al2O3 catalysts showed a high HDN than the helium-cooled catalysts (Bracket). Before the ni-
rate. The CO adsorption of the catalysts is shown in Table 4. trided catalysts were passivated with 1% O2 in He for 12 h,
The ratios of the total amount of CO adsorption to the ir- the catalysts were cooled from the nitriding temperature to
reversible amount for the 12.5% Mo/Al2O3 catalysts are room temperature in a stream of ammonia. Adsorbed NHx
greater than those for the 43.0 to 97.1% MTN catalysts. and dissolved nitrogen formed. In this post treatment, the
The amount of irreversible CO uptake for the 12MTN cata- NHx species was associated with oxygen atoms in the cata-
lyst was 1.0 1012 molecules cm 2, which was half that of lysts and probably led to the generation of the Brønsted
the 12LTN catalyst and 0.2 that of the 12HTN catalyst. acid sites and to high activity for the HDN reaction. On
According to Monte Carlo simulation, CO adsorbed on the contrary, when the 12MTN catalyst was purged with
molybdenum atoms and nitrogen-deficient sites (molybde- helium and cooled to room temperature in a stream of he-
num atoms at the second layer) on the top layer of (111) lium, adsorbed NHx and dissolved nitrogen in the cata-
phase -Mo2N (44). Furthermore, previous studies (25, 42) lyst were removed. For helium-cooled 97MTN, -Mo2N
showed that the hydrogenation of carbazole to tetrahydro- was transformed to -Mo2N0.78 and was less active than
carbazole was not fully equilibrated at 573 K in the HDN of
-Mo2N0.78 for the hydrogenation in the HDN of carbazole
carbazole and that the C–N hydrogenolysis rate (the forma- (19). Thus, the helium-cooled 12MTN catalyst was more ac-
tion of bicyclohexyl) was a determining step about at least tive than the ammonium-cooled catalyst. Furthermore, the
593 K. The TOF was calculated by dividing the HDN rate 973 K nitrided catalysts with higher molybdenum loadings
(disappearance rate) by the amount of CO uptake, that were less active than the 12MTN catalyst. The 97MTN cat-
is, the activity of each active site of the nitrided catalysts alyst had the lowest TOF activity. Since the molecular size
representing the hydrogenation of carbazole. The order of of carbazole is larger than that of CO despite a large num-
the TOF of the nitrided 12.5% Mo/Al2O3 catalysts was the ber of irreversible CO adsorbed on the 97MTN catalyst
same as that of the HDN rate as based on the surface area (Table 4), all the adsorption sites of CO were not used
of the catalyst (Table 4). The TOF (45.1 h 1) of the 12MTN for the adsorption of carbazole involving carbazole HDN.
catalyst was about 0.4 times more active than the 12HTN In addition, the 97MTN catalyst may have created some
catalyst and about twice as active as the 12LTN catalyst. molybdenum atoms with high crystallinity on the surface,
Thompson et al. (6) reported the TOF of 11.9 h 1 for quino- different from the molybdenum species interacted with alu-
line HDN at 663 K for 100% Mo2N nitrided at 973 K based mina on the surface of the 12MTN catalyst. Therefore, the
on the titration of sites with CO. They obtained a TOF that TOF of the 97MTN was lower than that of the 12MTN.

136
NAGAI ET AL.
Active Molybdenum Species in Carbazole HDN
The TOF for the HDN of carbazole is plotted versus
the distribution of the molybdenum oxidation state of the
12.5% Mo/Al2O3 catalysts obtained from the XPS analysis
(Fig. 9). The TOF for carbazole HDN (hydrogenation) was
linearly correlated with the distribution of both Mo2+ and
Mo0 on the catalysts. Based on the XPS study, the 1173 K
nitrided catalyst had more oxidation states of Mo2+ and
Mo0 compared to the other catalysts nitrided at the lower
temperatures. Hydrogenation activity increases with a de-
creasingdistribution ofthe fractionsofMo4+ and Mo3+. The
distribution of Mo6+ ions decreased from 33 to 12% , cor-
1
responding to an increase in TOF from 21.1 to 104.7 h
,
respectively. This result indicates that Mo2+ and Mo0 are
active centers for hydrogenation in carbazole HDN on the
nitrided Mo/Al2O3 catalyst. Bussell et al. (45) studied the
infrared spectroscopy of adsorbed CO on a molybdenum
nitride catalyst and deduced that Mo2+ siteswere present on
the surface of the molybdenum nitride. Sullivan and Ekerdt
(46) also reported that Mo2+ was the most active molybde-
num species of the sulfided silica-supported molybdenum
catalysts for thiophene HDS. Furthermore, Yamada et al.
(27) reported that both Mo2+ and molybdenum metal were
active during the hydrogenation of benzene with the former
being more active. Molybdenum metal is probably partially
formed as patches of molybdenum metal on the molybde-
num nitrides or on the reduced molybdenum oxide. There-
fore, the 12HTN catalyst had patches of molybdenum metal
on its surface which were highly active for the hydrogena-
tion in the HDN of carbazole. When the catalyst was placed
in a stream of ammonia and cooled from the nitriding tem-
perature to room temperature after nitriding, excess ad-
FIG. 10. The relationship between the TOF for carbazole HDN and
nitrogen desorption ((d) at 1016–1022 K) during TPR.
sorbed ammonia and NHx reacted with hydrogen during
TPR to form nitrogen. The TPR studyofN2 desorption gave
more acceptable results than the study on ammonia adsorp-
tion as far as the nature of the acid sites on the nitrided cata-
lysts for carbazole HDN is concerned. Figure 10 is the TOF
of the nitrided catalyst as a function of nitrogen desorption
during TPR. The TOF for the carbazole HDN was related
to nitrogen desorption at 1016 K. The nitrogen desorption
at 1016 K is due to the nitrogen release from the surface
NHx on molybdenum metal. According to the TPR study,
therefore, the molybdenum metal on the surface of 12HTN
and 12MTN had the highest TOF for the hydrogenation
in the HDN of carbazole. Furthermore, the TOFs of both
the helium- and the ammonia-cooled catalysts in the HDN
(hydrogenation) of carbazole increased at nitriding tem-
peratures from 773 to 1173 K (Table 4). However, the peak
area of the IR spectra of Lewis and Brønsted acidity clearly
decreased from the nitriding temperature of 773 to 1173 K,
even though the catalysts were purged with helium to elim-
inate excess nitrogen species for the infrared spectroscopy.
Therefore, the TOF of the helium-cooled 12.5% Mo/Al2O3
did not coincide with the number of Lewis and Brønsted
acid sites, except for the increased ratio of Lewis/Brønsted
acidity. The activity of the nitrided catalysts for the hydro-
genation in the carbazole HDN in this study is not related to
surface acidity but rather to the reduced molybdenum ions
Mo2+ and Mo0 on the surface of the molybdenum nitride
catalysts.
CONCLUSIONS
1. This paper revealed mainly the active sites for the hy-
drogenation in the hydrodenitrogenation of carbazole on a
basis on TOF, TPR, XPS, and IR.
FIG. 9. The relationship between the TOF for carbazole HDN and
the molybdenum oxidation state of the nitrided catalysts obtained from
the XPS analysis.

NITRIDED Mo/Al2O3 AND HDN
137
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sorbed on the active sites such as molybdenum atoms and
nitrogen-deficient sites on the top layer surface.
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is created on the surface of the catalysts.
6. XPS analysis showed that TOF is correlated to the
distribution of Mo2+ and Mo0 in the nitrided catalysts, sug-
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genation in the HDN of carbazole.
7. The activity of the nitrided catalysts for the hydro-
genation in carbazole HDN is not related to surface acidity
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