FT-IR spectroscopic studies of thiophene adsorption and reactions on MO2N/γ-Al2O3 catalysts

Article abstract of DOI:10.1021/jp011577l

Transition metal carbide and nitride catalysts have attracted considerable attention due to their potential use in HDN and HDS processes. The adsorption and reactions of thiophene on reduced passivated and nitrided Mo₂N/γ-Al₂O₃ catalysts were investigated using FTIR spectroscopy. Thiophene mainly adsorbed on the cus Mo sites of both reduced and nitrided Mo₂N/γAl₂O₃ catalysts. Thiophene alone showed no reaction on the two catalysts even at 673 K. In the presence of H₂, thiophene was more reactive on nitrided Mo catalyst than on the reduced passivated one, producing a surface σ-bonded butene species that was confirmed by the adsorption of 1-butene, cis- and trans-2-butenes, and 1,3-butadiene on the catalysts. CO probing showed that catalyst surfaces were completely sulfided with a treatment of thiophene/H₂ at > 573 K. The surface sulfidation and the σ-bonded C₄ species could affect the deactivation of the nitride catalysts for the HDS of thiophene. The high reactivity of nitrided Mo₂N/γAl₂O₃ catalyst to thiophene compared to reduced passivated Mo nitride and Mo sulfide was attributed to the higher density of active sites and unique electronic property of nitrided Mo₂N/γAl₂O₃ catalyst for H₂ and thiophene activation.

Full text of DOI:10.1021/jp011577l

J. Phys. Chem. B 2002, 106, 979-987
979
FT-IR Spectroscopic Studies of Thiophene Adsorption and Reactions on Mo2N/γ-Al2O3
Catalysts
Zili Wu, Can Li,* Zhaobin Wei, Pinliang Ying, and Qin Xin*
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,
P.O. Box 110, Dalian 116023, China
ReceiVed: April 26, 2001; In Final Form: September 6, 2001
The adsorption and reactions behaviors of thiophene on reduced passivated and nitrided Mo2N/γ-Al2O3 catalysts
have been studied by in situ FT-IR spectroscopy at temperatures from room temperature to 673 K. Thiophene
mainly adsorbs on the cus Mo sites of the two catalysts surfaces, and its adsorption mode is determined to
be η1(S). Thiophene alone shows no reaction on the two catalysts even at 673 K, while in the presence of H2,
it becomes reactive initially at 573 and 373 K on reduced passivated and nitrided Mo catalysts, respectively,
producing a surface σ-bonded butene species that is confirmed by the adsorption of 1-butene, cis- and trans-
2-butenes, and 1,3-butadiene on the catalysts. Adsorption of CO clearly shows that the catalyst surfaces become
sulfided completely with a treatment of thiophene/H2 at temperatures higher than 573 K. The surface sulfidation
together with the σ-bonded C4 species may play a role in the deactivation of the nitride catalysts for the HDS
of thiophene. The high reactivity of thiophene toward nitrided Mo catalyst in the initial stage of thiophene
HDS is ascribed to the high density of surface sites and unique electronic property of nitrided Mo for the
activation of both H2 and thiophene.
1. Introduction
of studies have been published for the adsorption of thiophene
on sulfided Mo catalysts,15-20 but no such experimental result
In recent years, transition metal carbide and nitride catalysts
have attracted much attention because they show strong potential
for use in both the hydrodenitrogenation (HDN)1-4 and hydro-
desulfurization (HDS)1,3,5-12 processes. There are a number of
results reporting the better HDS activity of molybdenum nitride
over molybdenum sulfide catalysts although the working surface
of Mo nitride catalyst is supposed to be a thin layer of sulfide
under HDS conditions.5,6 Sajkowski et al.7 found unsupported
Mo2N to be nearly twice as active as the commercial Ni-Mo/
Al2O3 catalyst on the basis of Mo atoms measured by CO
chemisorption for the HDS of coal-derived feed. Bussell and
co-workers5,6 and Nagai et al.10,11 also reported that nitrided
Mo/Al2O3 catalysts are more active for the HDS of thiophene
than sulfided Mo/Al2O3 catalysts. These studies suggest that
Mo nitride catalysts have the potential advantages over Mo
sulfide catalysts in industrial HDS processes.
Thiophene, a classical model organosulfur compound, is the
most extensively studied molecule in the HDS reactions over
nitride catalysts as well as sulfide catalysts. Most studies have
focused on the reactivities of HDS reactions. Yet quite little
work has been done on the fundamental understanding the
surface chemistry of the HDS process on nitride catalysts.13
Chen and Xin,13 using ab initio effective potential calculation
on the Mo2N-thiophene systems, gave some important points
involving steps of the HDS reaction mechanism.
is reported for nitride catalysts up to now.
Because the surface of nitride catalyst becomes sulfided under
HDS conditions,5,6,21,22 the study of thiophene HDS on nitride
catalyst should cover two different surfaces, a nitride surface
in the initial stage of HDS and a thin layer of sulfide surface
over nitride in the steady state of HDS process. As the adsorption
and reactions of thiophene on sulfided Mo catalyst have been
well and effectively studied by Bussell and co-workers using
IR spectroscopy,15,19,20 the aim of this study is to learn the
adsorption and reactions of thiophene on Mo nitride, also the
effect of thiophene on the property of surface sites, to get a
better understanding of the initial stage of HDS process over
nitride catalysts. In situ FT-IR spectroscopy is employed to
investigate the surface chemistry of thiophene on Mo2N/γ-Al2O3
catalyst that was renitrided from passivated one by NH3.21-25
The adsorption mode of thiophene on the nitrided Mo catalyst
was found possibly to be η1(S). Furthermore, the reactions of
thiophene on nitrided Mo2N/γ-Al2O3 catalyst in the presence
of H2 were observed even at 373 K by IR spectroscopy, resulting
in the surface sulfidation as probed by CO adsorption. Similar
cases for reduced passivated on Mo2N/γ-Al2O3 catalyst were
also studied.
2. Experimental Section
As a first step to approach the HDS mechanism, study of the
adsorption of thiophene is of significance. Four different bonding
modes have been observed for thiophene in organometallic
complexes: η1(S)-, η2-, η4-, and η5-coordinated thiophene, and
these coordination structures have been suggested as possible
modes for thiophene adsorption on HDS catalysts.14 A number
2.1. Catalyst Preparation. The preparation of passivated
Mo2N/γ-Al2O3 catalyst is described elsewhere.21-25 MoO3/γ-
Al2O3 sample with Mo loading of 10 wt % was prepared by
incipient wetness impregnation of γ-Al2O3 (SBET ) 172 m2/g)
with an aqueous solution of (NH4)6Mo7O24, followed by drying
at 393 K overnight and calcination at 773 K for 4 h. The nitrided
Mo2N/γ-Al2O3 catalyst, i.e., nitrided catalyst, was prepared by
the temperature-programmed reaction of MoO3/γ-Al2O3 with
* Corresponding authors. Fax: +86 411 4694447. E-mails: canli@
ms.dicp.ac.cn (C.L.) and xinqin@ms.dicp.ac.cn (Q.X.).
10.1021/jp011577l CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/10/2002

980 J. Phys. Chem. B, Vol. 106, No. 5, 2002
Wu et al.
ammonia under the following procedures: the temperature was
increased from room temperature (RT) to 623 K in 1 h and
from 623 to 973 K in 6 h; then the temperature was maintained
at 973 K for another 2 h. The sample was finally cooled to RT
in flowing ammonia. Passivated Mo2N/γ-Al2O3 sample was
prepared from the nitrided sample which was passivated at RT
in a stream of 1% O2/N2 so as to avoid the violent oxidation of
the freshly prepared nitride.
2.2. IR Studies. A passivated sample was pressed into a self-
supporting wafer (ca. 15 mg/cm2) and put into a quartz IR cell
with CaF2 windows to renitride in flowing ammonia. The sample
was heated from RT to 623 K in 30 min, then to 723 K in 100
min, further from 723 to 873 K in 75 min, and finally held at
this temperature for 60 min. The sample renitrided in the IR
cell is called nitrided Mo2N/γ-Al2O3 or nitrided sample, while
the passivated sample treated with H2 at 773 K for 2 h is denoted
as reduced passivated Mo2N/γ-Al2O3. The same IR spectra in
the νCO region were obtained from CO adsorption on the samples
from the nitridation of MoO3/γ-Al2O3 and the renitridation of
passivated Mo2N/γ-Al2O3. It demonstrates that the renitridation
procedure mentioned above can produce fresh nitride from the
passivated nitride.26,27 To save time, we use passivated Mo2N/
γ-Al2O3 instead of MoO3/γ-Al2O3 as the starting sample in the
IR cell because it takes longer time to nitride MoO3/γ-Al2O3
sample. Haddix et al.28 have also reported that air-exposed
γ-Mo2N sample can be renitrided by simply treating it in flowing
ammonia at 973 K, and the procedure did not affect the BET
surface area, the crystal structure, and the H2 uptake charac-
teristics. Previous IR results of CO adsorption on the two
samples, nitrided and reduced passivated Mo2N/γ-Al2O3, indi-
cate that the two samples have quite different surface proper-
ties.26,27
Figure 1. IR spectra of thiophene (5 Torr) adsorbed at RT on (a)
nitrided γ-Al2O3 and (b) Mo2N/γ-Al2O3 catalyst.
C-H stretching region, three IR bands at 3105, 3097, and 3078
cm-1 monotonically increase in intensity with increased thiophene
amount. Focusing in the 1450-1370 cm-1 range, when a small
amount of thiophene (0.02 Torr) is dosed, the IR band at 1434
cm-1 is observed together with other three weak bands at 1418,
1408, and 1400 cm-1. With the increasing of thiophene amount,
the 1434 cm-1 band becomes shoulder and the band at 1408
cm-1 is observed to be dominant. These four bands at 1434,
1418, 1408, and 1400 cm-1 can be assigned to the ν3 mode of
thiophene.15,30 The ν3 mode is a ring vibration that consists
primarily of the symmetric stretching of the CdC bonds of
thiophene31 and will hereafter be referred to as the symmetric
νcc mode of thiophene. The band at 1408 cm-1 is due to weakly/
physically adsorbed thiophene on the nitrided catalyst because
it is also observed in the gas phase of thiophene. So the other
IR bands at 1434, 1418, and 1400 cm-1 could be attributed to
chemically adsorbed thiophene on the nitrided Mo2N/γ-Al2O3
catalyst.
To make clear the assignment of the IR bands observed on
nitrided Mo2N/γ-Al2O3, adsorption of thiophene on nitrided
γ-Al2O3 was conducted, and the IR spectra are shown in Figure
3. Several bands at 3104, 3097, and 3076 cm-1 in the 3200-
2900 cm-1 region and 1418, 1408, and 1400 cm-1 in the 1450-
1370 cm-1 range are observed to increase in intensity with the
increased thiophene dosages. The IR bands at 1418 and 1400
cm-1 are new from those of thiophene in gas phase. According
to the literature,29 the band at 1418 cm-1 could be assigned to
thiophene coordinated via its sulfur atom to coordinately
unsaturated Al3+ sites created by dehydroxylation of the
γ-Al2O3, and the band at 1400 cm-1 is due to thiophene
hydrogen bonded to hydroxyl groups on the alumina surface.
A comparison of the IR spectra of thiophene adsorption on
γ-Al2O3 with those on nitrided Mo2N/γ-Al2O3 makes it certain
to attribute the IR band at 1434 cm-1 to thiophene adsorbed on
the surface of Mo2N dispersed on the γ-Al2O3 support. As it is
well know that the vibrational frequencies of ring stretching
modes of heterocyclic aromatic molecules are sensitive to their
bonding environment on a catalyst surface, the IR band at 1434
cm-1, the symmetric mode of adsorbed thiophene, can be used
to determine the adsorption mode of thiophene on nitrided
Mo2N/γ-Al2O3 catalyst. Previous vibrational spectroscopic
studies15,19,20,29,32 provide strong evidence that the coordination
of thiophene to surface sites via the sulfur atom (η1(S)) causes
a shift of the νcc band to higher wavenumbers while to lower
wavenumbers for π-bonded thiophene (η4 and η5 coordination).
The as-prepared sample was evacuated at 773 K for 60 min
and subsequently cooled to RT. Three different IR experiments
were prepared as follows: (1) Thiophene was introduced into
the IR cell by dosage, and IR spectra were collected at different
dosages (see figure captions). (2) The sample was treated with
5 Torr thiophene or a thiophene/H2 (5/500 Torr) mixture at
different temperatures (373, 473, 573, and 673 K) for 1 h and
cooled to RT, and IR spectra were acquired. The sample was
then evacuated at 773 K for 20 min. As the sample was cooled
to RT, 10 Torr of CO was introduced. (3) 1-Butene, cis- and
trans-2-butene, and 1,3-butadiene (ca. 1 Torr) were respectively
adsorbed on the sample at RT. After an adsorption time of 30
min, the sample was outgassed, and IR spectra were collected.
All infrared spectra were collected on a Fourier transform
infrared spectrometer (Nicolet Impact 410) with a resolution of
4 cm-1 and 64 scans in the region 4000-1000 cm-1
.
3. Results and Spectra Interpretation
3.1. Thiophene Adsorption at RT. Figure 1 shows the IR
spectra of thiophene adsorbed on nitrided Mo2N/γ-Al2O3 and
nitrided γ-Al2O3 at RT. The assignment of the labeled peaks
can be found elsewhere.29 There is no apparent difference
between the two spectra, indicating that the IR bands of adsorbed
thiophene on the Mo2N surface of nitrided Mo2N/γ-Al2O3 may
be relatively weak. But as studied by Bussell and co-work-
ers,15,19,20 for thiophene adsorption on sulfided Mo/γ-Al2O3 and
γ-Al2O3, some delicate differences exist in the range of 1450-
1370 cm-1 in the IR spectra. So it is possible to get more
information of thiophene adsorption on nitrided Mo2N/γ-Al2O3
catalyst with a more careful study of thiophene adsorption on
nitrided Mo2N/γ-Al2O3 and nitrided γ-Al2O3.
Figure 2 shows the IR spectra of thiophene adsorbed on
nitrided Mo2N/γ-Al2O3 catalyst with different dosages. In the

Thiophene Adsorption and Reactions on Mo2N/γ-Al2O3
J. Phys. Chem. B, Vol. 106, No. 5, 2002 981
Figure 2. IR spectra of thiophene adsorbed at RT on Mo2N/γ-Al2O3 catalyst with increased dosage: (a) 0.005, (b) 0.02, (c) 0.06, (d) 0.2, (e) 0.4,
(f) 0.6, and (g) 0.9 Torr.
Figure 3. IR spectra of thiophene adsorbed at RT on nitrided γ-Al2O3 support with increased dosage: (a) 0.005, (b) 0.02, (c) 0.05, (d) 0.1, (e) 0.18,
(f) 0.4, (g) 0.6, and (h) 0.95 Torr
Using IR spectroscopy and temperature-programmed desorption
(TPD) techniques, Bussell and co-workers15,19,20,29 observed νcc
absorbances at 1431 and 1420 cm-1 for adsorbed thiophene on
sulfided Mo/γ-Al2O3 catalysts and γ-Al2O3, respectively. The
authors concluded from the shift of the νcc band that the
adsorption mode of thiophene is η1(S) on both γ-Al2O3 and
sulfided Mo/γ-Al2O3. Therefore, it is reasonable to assign the
IR band at 1434 cm-1 (Figure 2) to adsorbed thiophene on
nitrided Mo2N/γ-Al2O3 catalyst via its sulfur atom (η1(S)).
Additional information concerning the adsorbed thiophene
species associated with the IR band at 1434 cm-1 can be
obtained from the coadsorption of CO and thiophene on nitrided
Mo2N/γ-Al2O3 catalyst. Figure 4 presents the IR spectra of
thiophene adsorbed on nitrided Mo2N/γ-Al2O3 catalyst pread-
sorbed with CO. As exhibited in Figure 4b, the IR band at 1434
cm-1 is greatly decreased with the presence of preadsorbed CO,
indicating that thiophene competes with CO on the sites
associated with the symmetric νcc absorbance at 1434 cm-1
.
This phenomenon is also observed in the case of thiophene
coadsorbed with CO on sulfided Mo/γ-Al2O3 catalysts.15
Previous studies26,27 show that there are two kinds of surface
sites, namely, Moδ+ (0 < δ < 2) and N sites, for CO adsorption

982 J. Phys. Chem. B, Vol. 106, No. 5, 2002
Wu et al.
dosage becomes a shoulder of the band at 1408 cm-1 with a
saturated thiophene dosage. Similar to the case of thiophene
adsorption on nitrided Mo2N/γ-Al2O3 catalyst, the IR band at
1435 cm-1 can be ascribed to the symmetric νcc absorbance of
thiophene adsorbed on reduced passivated Mo2N surface of the
supported catalyst. According to our previous study,21 it is
indicated that thiophene is also adsorbed via its sulfur atom (η1-
(S)) on the surface Mo sites of reduced passivated Mo2N/γ-
Al2O3 catalyst.
3.2. Thiophene Adsorption at High Temperatures (373-
673 K) in the Presence or Absence of H2. HDS reactions are
performed at higher temperatures, usually between 573 and 673
K. So it is of great interest to investigate the adsorption behaviors
of thiophene at high temperatures. Since H2 is a necessary
reactant involved in the HDS process, a study of its influence
on thiophene adsorption behaviors is desired. Also a research
of the effect of thiophene and thiophene/H2 on the surface sites
of nitride catalysts at high temperatures is expected since nitrides
are found sensitive to sulfur species.5,21,22
Figure 4. IR spectra in the νCO region for thiophene adsorbed at RT
on different catalysts (Pthiophene ) 0.6 Torr): (a) nitrided γ-Al2O3; (b)
Mo2N/γ-Al2O3 catalyst preadsorbed CO; and (c) Mo2N/γ-Al2O3 catalyst.
on nitrided Mo2N/γ-Al2O3 catalyst. Our recent work21 also
suggests that thiophene adsorbs mainly on the surface Mo sites
on nitrided Mo2N/γ-Al2O3 catalyst. So it can be concluded that
the IR band at 1434 cm-1 is mainly due to adsorbed thiophene
on cus Moδ+ (0 < δ < 2) sites of Mo2N surface of the nitrided
Mo2N/γ-Al2O3 catalyst.
To avoid violent oxidation of the freshly prepared nitride, a
passivation procedure is usually employed in the preparation.
But it is known that the passivation procedure causes a dramatic
change in the nitride surface, i.e., from nitride to oxygen-covered
nitride. A study of thiophene adsorption on reduced passivated
Mo2N/γ-Al2O3 catalyst is also conducted to compare with the
nitrided catalyst.
Figure 6 shows the IR spectra of thiophene adsorbed on
nitrided Mo2N/γ-Al2O3 catalyst at different temperatures, from
RT to 673 K. At each temperature, the sample was kept heating
in the presence of thiophene for 1 h. Except for the gradual
reduction in intensity, no new IR bands are observed for
thiophene adsorption in the νCH, νCC, and δCH regions at elevated
temperatures even to 673 K. An evacuation of the sample after
it was treated with thiophene at 673 K shows no obvious IR
absorbance, indicating no adsorbed species left on the catalyst
surface. This demonstrates that thiophene alone is not decom-
posed/reactive on the nitrided Mo2N/γ-Al2O3 catalyst even at
temperatures as high as 673 K.
Figure 7 exhibits the IR spectra of CO adsorbed on nitrided
Mo2N/γ-Al2O3 sample treated with thiophene at different
temperatures. Shown in Figure 7a, adsorbed CO on nitrided
Figure 5 shows the IR spectra of thiophene adsorbed on
reduced passivated Mo2N/γ-Al2O3 catalyst with gradually
increased amount. The bands at 3105, 3097, and 3078 cm-1 in
the C-H stretching region increase in intensity simply with
increasing the thiophene amount. In the 1450-1370 cm-1
region, the main band at 1435 cm-1 with the initial thiophene
Mo2N/γ-Al2O3 gives two IR bands at 2045 and 2200 cm-1
,
corresponding to the adsorbed CO on the surface Mo and N
sites, respectively, forming linearly adsorbed CO and NCO
Figure 5. IR spectra of thiophene adsorbed at RT on reduced passivated Mo2N/γ-Al2O3 catalyst with increased dosage: (a) 0.005, (b) 0.01, (c)
0.05, (d) 0.12, (e) 0.3, (f) 0.7, and (g) 1.1 Torr.

Thiophene Adsorption and Reactions on Mo2N/γ-Al2O3
J. Phys. Chem. B, Vol. 106, No. 5, 2002 983
Figure 6. IR spectra of thiophene (5 Torr) adsorbed on Mo2N/γ-Al2O3 catalyst for 1 h at (a) RT, (b) 373 K, (c) 473 K, (d) 573 K, (e) 673 K, and
(f) evacuation of (e) at RT.
Figure 8. IR spectra of a thiophene/H2 (5/500 Torr) mixture adsorbed
on Mo2N/γ-Al2O3 catalyst for 1 h at (a) RT, (b) 373 K, (c) 473 K, (d)
573 K, (e) 673 K, and (f) evacuation of (e) at RT.
Figure 7. IR spectra CO adsorbed at RT (a) on Mo2N/γ-Al2O3 catalyst
and (b) on the same catalyst after it was treated with thiophene (5 Torr)
catalyst at RT does not show any new feature in the IR spectrum
(Figure 8a), indicating that thiophene is not reactive on the
at 373, (c) 473, (d) 573, and (e) 673 K for 1 h.
species.26,27 When the sample was treated with thiophene at 373
K, IR band at 2200 cm-1 shifts slightly to 2195 cm-1, while
the band at 2045 cm-1 shifts to 2060 cm-1. Meanwhile, a small
shoulder at 2100 cm-1 is also observed, indicating the partial
sulfidation of the nitride surface because this is a characteristic
band of CO adsorbed on sulfided Mo catalyst and has been
assigned to linearly adsorbed CO on cus Mo2+ sites.33 With
the elevated treatment temperatures of thiophene, the three bands
at 2195, 2100, and 2060 cm-1 of adsorbed CO are still observed
with decreased intensities. As the treatment temperature is 673
K (Figure 7e), the 2100 cm-1 band still appears as a shoulder.
This suggests that the surface of nitrided Mo2N/γ-Al2O3 catalyst
is only slightly sulfided even when thiophene alone is adsorbed
at 673 K.
nitrided sample at RT in the presence of H2. But some changes
are observed in the IR spectra when the adsorption temperature
is raised slightly higher than RT. A treatment of the nitrided
sample with the mixture of thiophene/H2 at 373 K, shown in
Figure 8b, produces four new bands at 2958, 2936, 2910, and
2873 cm-1 in the νCH region and two new bands at 1460 and
1443 cm-1 in the δCH region. The appearance of these new bands
indicates that thiophene becomes reactive with hydrogen on the
nitrided catalyst at 373 K. An increase in intensity of the 1605
cm-1 band is also observed. A further treatment to 673 K results
in the increase in intensities of these bands. After an evacuation
at RT of the sample treated at 673 K (Figure 8f), the absorbances
at 2958, 2936, 2910, 1605, 1586 (shoulder), 1460, 1443, 1393,
and 1380 cm-1 still remain. These IR bands can be assigned to
the vibrations of CH2 and CH3 groups of adsorbed C4 hydro-
carbon species.34,35
Figure 8 presents the IR spectra of adsorbed thiophene on
nitrided Mo2N/γ-Al2O3 catalyst in the presence of H2 at different
temperatures. Similar to the case of thiophene adsorption alone,
adsorption of thiophene with H2 on the nitrided Mo2N/γ-Al2O3
Figure 9 exhibits the IR spectra of CO adsorbed on nitrided
Mo2N/γ-Al2O3 sample treated with a thiophene/H2 mixture at

984 J. Phys. Chem. B, Vol. 106, No. 5, 2002
Wu et al.
Figure 11. IR spectra of different adsorbates on Mo2N/γ-Al2O3
catalyst: (a) 1-butene; (b) cis-2-butene; (c) trans-2-butene; (d) 1,
3-butadiene; and (e) thiophene/H2 adsorbed at 673 K and outgassed at
RT.
Figure 9. IR spectra of CO adsorbed at RT (a) on Mo2N/γ-Al2O3
catalyst and (b) on the same catalyst after it was treated with a thiophene
/H2 (5/500 Torr) mixture at 373, (c) 473, (d) 573, and (e) 673 K for 1
h.
up to 573 K and above. An increase of adsorption temperature
leads to the increase of the intensities of these new absorbances.
After an evacuation at RT, the IR bands at 2960, 2936, 2912,
2868, 1652, 1620, 1563, 1465, 1438, and 1400 cm-1 are still
persisted (Figure 10f), and these bands can be also ascribed to
the adsorbed C4 hydrocarbon species which are derived from
the reactions of thiophene and hydrogen.
3.3. Adsorption of 1,3-Butadiene, 1-Butene, cis- and trans-
2-Butenes. To aid in the interpretation of the IR spectra of the
surface species formed from the adsorption of thiophene and
thiophene/H2 mixture on the two catalysts, adsorption of 1,3-
butadiene, 1-butene, and cis- and trans-2-butenes were con-
ducted on both nitrided and reduced passivated Mo2N/γ-Al2O3
catalysts.
Figure 11a-d shows the IR spectra of 1-butene, cis-2-butene,
trans-2-butene, and 1,3-butadien adsorbed on nitrided Mo2N/
γ-Al2O3 catalyst at RT, respectively. The contours of the IR
spectra of the three butenes are quite similar to each other,
exhibiting IR bands at 3010, 2965, 2922, 2860, 1619, 1612,
1452, 1408, and 1381 cm-1, indicating similar surface species
formed from the adsorption of these butenes on nitrided Mo2N/
γ-Al2O3 catalyst. The surface species have been attributed to
π- and σ-bonded 2-butenes in our previous studies.23-25 Adsorp-
tion of 1,3-butadiene gives IR bands at 3088, 3005, 2920, 2850,
1616, 1565, 1490, 1457, 1417, and 1378 cm-1, which are
assigned to π-adsorbed butadiene (πs and πd) and σ-bonded and
dehydrogenated butadiene species on the nitrided Mo catalyst.23
Figure 11e presents the IR spectrum of the adsorbed species
on the nitrided Mo2N/γ-Al2O3 catalyst after a treatment with a
thiophene/H2 mixture at 673 K.
The intensities of IR bands in the νCH region in Figure 11e
are quite different from those in the other four spectra (Figure
11a-11d). For butenes and 1,3-butadiene adsorption on the
nitrided Mo2N/γ-Al2O3 catalyst, the νas(CH2) band at 2920 cm-1
dominates, while for thiophene/H2 mixture adsorption on the
nitrided catalyst (Figure 11e), the νas(CH3) band at 2958 cm-1
and νs(CH3) band at 2873 cm-1 are more prominent. These
differences in spectra can be explained by two possibilities:
different surface species or different properties of surface sites.
CO adsorption (Figure 9) showed that the surface of the nitrided
Mo2N/γ-Al2O3 catalyst is sulfided by a treatment with a mixture
of thiophene/H2 at 673 K. According to the HDS schemes of
thiophene,36,37 butane, butenes, and 1,3-butadiene are the most
frequent species formed in the HDS process. So the differences
Figure 10. IR spectra of a thiophene/H2 (5/500 Torr) mixture adsorbed
on reduced passivated Mo2N/γ-Al2O3 catalyst for 1 h at (a) RT, (b)
373 K, (c) 473 K, (d) 573 K, (e) 673 K, and (f) evacuation of (e)
at RT.
different temperatures. The 2200 cm-1 band shifts to 2195 cm-1
,
and its intensity decreases gradually with the elevated treatment
temperatures and disappears when the temperature is higher than
573 K. The band at 2045 cm-1 shifts to higher frequencies when
the treatment temperature is increased, e.g., to 2080 cm-1 at
373 K, to 2090 cm-1 at 473 K, to 2102 cm-1 at 573 K, and to
2105 cm-1 above 573 K. The results clearly show that the
surface of the nitrided Mo2N/γ-Al2O3 catalyst is slowly sulfided
when treated with the thiophene/H2 mixture at temperatures
above RT.
Thiophene adsorption on reduced passivated Mo2N/γ-Al2O3
catalyst at high temperatures (373-673 K) in the absence/
presence of H2 was also studied. Similar to the case of nitrided
Mo2N/γ-Al2O3 catalyst, no reaction of thiophene is observed
on the reduced passivated catalyst even at 673 K in the absence
H2 (Spectra are similar to those in Figure 6 and not shown here).
But when H2 is present, thiophene is found to behave differently
on the two catalysts. As shown in Figure 10, similar IR
absorbances at 2960, 2936, 2912, and 2868 cm-1 in the νCH
region and 1465 and 1438 cm-1 in the δCH region are observed
only when the adsorption temperature of the thiophene/H2 is

Thiophene Adsorption and Reactions on Mo2N/γ-Al2O3
J. Phys. Chem. B, Vol. 106, No. 5, 2002 985
catalysts according to the similarities in νCdC range in the five
spectra in Figure 12. The differences in the olefinic CdC
stretching of the IR spectra from thiophene/H2 adsorption on
reduced passivated and nitrided catalysts indicate the different
ratios of σ-bonded C4H7 species on the two catalysts.
4. Discussion
Thiophene adsorption on reduced passivated and nitrided
Mo2N/γ-Al2O3 catalysts show similar νCdC absorbance at ca.
1434 cm-1. Bussell and co-workers15,19 carefully studied
thiophene adsorption on sulfided Mo and Rh/γ-Al2O3 catalysts
using IR spectroscopy combined with TPD. A νCdC band at
1429 cm-1 was also observed for thiophene adsorption at 140
and 300 K on the sulfided Mo and Rh/γ-Al2O3 catalysts. On
the basis of this band, the authors concluded that thiophene is
η1(S)-bonded on the cus sites of these sulfide catalysts. Their
recent study 20 provides a definitive evidence that the adsorption
mode of adsorbed thiophene on sulfided Mo/γ-Al2O3 catalysts
is η1(S) by a comparison of the vibrational spectra of organo-
metallic complexes containing thiophene ligands with the IR
spectra of adsorbed thiophene. Similarly, from our IR results,
it is suggested that thiophene is adsorbed via its sulfur atom
(η1(S)) on the cus Mo sites of both reduced passivated and
nitrided Mo2N/γ-Al2O3 catalysts. As probed by CO adsorption,26
the surface Mo sites are in low valence state (0-2) on nitrided
Mo catalyst while in high valence state (+4) on reduced
passivated catalyst. Coadsorption of CO with thiophene on the
two catalysts 21 indicates that thiophene adsorbs mainly on the
surface cus Mo sites. So thiophene is η1(S)-adsorbed on the
two catalysts despite their different states of surface Mo sites.
It is interesting that the adsorption mode of thiophene is the
same although the two catalysts show different surface proper-
ties. It seems that the oxidation state of Mo sites has little
influence on the adsorption mode of thiophene.
Thiophene also behaves similarly on the nitrided and reduced
passivated Mo2N/γ-Al2O3 catalysts at high-temperature adsorp-
tion. It shows no reactivity even at 673 K, as evidenced by the
preserved spectra (Figure 6). But this is not the case when H2
is present. New IR features in the νCH range are initially
observed at 373 K for thiophene/H2 adsorption on nitrided
Mo2N/γ-Al2O3 catalyst while at 573 K for reduced passivated
catalyst, which shows the reactivity of thiophene on the two
catalysts in the presence of H2. The observation strongly
suggests that H2 is absolutely necessary for the activation and
further reactions of thiophene in the HDS process.
The changes of the surface properties of the nitrided Mo
catalyst by treatment of thiophene and thiophene/H2 at high
temperatures are probed by CO adsorption. When thiophene
alone adsorbed on nitrided Mo2N/γ-Al2O3 sample, only a
shoulder band at 2100 cm-1 is observed in the treatment
temperature range of 373-673 K (Figure 7). This is possibly
due to the decomposition of a very small amount of adsorbed
thiophene although there is no new IR band observed in Figure
6. The decomposition of thiophene results in a slight sulfidation
of the nitride surface. Since the surface sulfidation is limited in
the temperature range of 373-673 K, it is suggested that most
thiophene keeps intact at high temperatures on nitrided Mo
catalyst.
The spectra of adsorbed CO on nitrided Mo2N/γ-Al2O3
sample treated with thiophene/H2 clearly show that the nitride
surface is gradually sulfided with the elevated treatment
temperature (Figure 9). This is in good accordance with the
observation of the increasing reactivity of thiophene on the
nitrided sample in the presence of H2 at elevated temperatures
Figure 12. IR spectra of different adsorbates adsorbed at RT on
reduced passivated Mo2N/γ-Al2O3 catalyst: (a) 1-butene; (b) trans-2-
butene; (c) cis-2-butene; (d) 1, 3-butadiene; (e) thiophene/H2 adsorbed
at 673 K and outgassed at RT; and (f) thiophene/H2 adsorbed on Mo2N/
γ-Al2O3 catalyst at 673 K and outgassed at RT.
in the IR spectra could be mainly due to the differences in
surface sites. Surface species resulted from the adsorption of
butenes and 1,3-butadiene are formed on a nitrided catalyst,
while those from thiophene/H2 are actually on a sulfided surface
layer on nitride catalyst. The IR spectrum collected on sulfided
Mo/γ-Al2O3 catalysts after a treatment with thiophene/H2 at 693
K15,19 gives absorbances similar to that of the nitrided Mo
catalyst at the similar treatment with thiophene/H2 at 693 K
(Figure 11e), which supports the reasoning that the differences
in the IR spectra from the adsorption of butenes, 1,3-butadiene,
and thiophene/H2 are due to totally different surface sites.
Panels a-d of Figure 12 present the IR spectra of 1-butene,
cis-2-butene, trans-2-butene, and 1,3-butadien adsorbed on
reduced passivated Mo2N/γ-Al2O3 catalyst at RT, respectively.
Different from the case for nitrided catalyst, the adsorption of
1-butene shows IR features different from those of the two
2-butenes on reduced passivated nitride. The main surface
species from 1-butene are π- and σ-bonded 1-butene, while π-
and σ-bonded 2-butenes are the surface species from cis- and
trans-2-butenes.24,25 The contour of the IR spectra of 1,3-
butadiene adsorbed on reduced passivated sample mostly
resembles that on nitrided sample (Figure 12d and Figure 11d,
respectively), indicating that similar surface species are formed.
IR spectra of the species from thiophene/H2 adsorption at 673
K on reduced passivated and nitrided sample (Figure 12e,f) show
similar profile in the νCH and δCH regions. Some differences
between the two spectra can be found in the νCdC range: three
bands at 1652, 1620, and 1563 cm-1 for the species on reduced
passivated sample and one broad band centered at 1605 cm-1
for nitrided sample.
It is evident that the IR spectra of the surface species formed
from thiophene/H2 adsorption on reduced passivated and nitrided
Mo2N/γ-Al2O3 catalysts (Figure 12e,f) resemble that from
1-butene adsorption on reduced passivated catalyst (Figure 12a).
Especially in the νCH region, the νas(CH3) and νs(CH3) bands
are more prominent in these three spectra than the other spectra
for cis- and trans-2-butenes and 1,3-butandiene adsorption on
reduced passivated sample (Figure 12b-d). Therefore, it can
be concluded that the surface species from thiophene/H2
adsorption on both reduced passivated and nitrided Mo2N/γ-
Al2O3 catalysts are mainly σ-bonded 1-butene species, namely,
the C4H7 fragment, similar to the case of sulfided catalysts.15,19
σ-Bonded 2-butene species could also be formed on the two

986 J. Phys. Chem. B, Vol. 106, No. 5, 2002
Wu et al.
in Figure 8. When the sample is treated with thiophene/H2 at
673 K, the band at 2200 cm-1 disappears, and the band at 2045
cm-1 totally shifts to 2100 cm-1, suggesting the complete
sulfidation of the surface of nitrided Mo2N/γ-Al2O3. This is
similar to the case of reduced passivated Mo2N/γ-Al2O3 catalyst
whose surface was found to be sulfided at HDS conditions.5
The sulfidation of the nitride surface can be reasonably due to
the incorporation of sulfur species that are produced from the
hydrodesulfurization of thiophene at elevated temperatures.5,21,22
Therefore, it is suggested that the surface of nitrided Mo catalyst
is sulfided as it catalyses the hydrodesulfurization of thiophene
in the initial stage of HDS process.
surface is modified by oxygen sites; thus, the electronic property
would less resemble that of a real nitride. Nagai et al. 10 studied
the HDS of thiophene on Mo/γ-Al2O3 catalysts nitrided at
different temperatures. They found that the distribution of Mo
oxidation states of the nitrided Mo/γ-Al2O3 catalysts is related
to the HDS activity and that metallic Mo and Mo2+ are the
most active species for the HDS of dibenzothiophene. So the
passivated catalyst show lower activity for thiophene HDS than
the nitrided catalyst because of the great differences in the
oxidation states of surface Mo sites of the two catalysts.26
Thus, the high density of surface sites and unique electronic
properties of nitrided Mo2N/γ-Al2O3 catalyst are the possible
factors that results in the good reactivity of nitrided Mo to
thiophene in the initial stage of HDS process. However, as
shown in this study and the literature 5,21,22, the surfaces of the
Mo nitride catalysts become sulfided under HDS conditions.
So the working surfaces of nitride are thus in similar property
to those of sulfide. But activity tests still show that Mo nitrides
are more active in HDS of thiophene than sulfided Mo
catalyst.5-7,10,11 This can be also attributed to the higher density
of surface sites of sulfided Mo2N/γ-Al2O3 than of sulfided Mo/
γ-Al2O3. Bussell and co-workers 5,6 raised two possible explana-
tions for this reason. The first one relates to the structures of
the different catalysts. Mo nitrides is in fcc structure that can
expose Mo atoms on all crystal faces while Mo sulfide is a
layered dichalcogenide material on which Mo atoms are exposed
only on edge planes. As the working surface is a sulfide layer
over Mo nitride, the second explanation locates at the mechan-
ical properties of nitride and sulfide. Due to the high melting
point, hardness, and strength, Mo nitride could serve as rigid
substrate for the sulfide layer exposing large numbers of cus
Mo sites.
In a similar study of thiophene/H2 adsorption on sulfided Mo
catalyst,19 thiophene is begun to be reactive at 500 K. Thus,
our IR results show that nitrided Mo catalyst is more reactive
to thiophene than both reduced passivated Mo nitride and
sulfided Mo catalysts in the initial stage of HDS process. This
can be related to the better activation of the two reactants, H2
and thiophene, on the nitrided Mo2N/Al2O3 catalyst in the HDS
reaction.
γ-Mo2N catalyst is reported to have a large amount of
irreversibly adsorbed hydrogen at 298 K in comparison with
reversibly adsorbed hydrogen and the surface hydrogen is
strongly bound to the catalyst.26 The hydrogen adsorption
behavior on Mo2N,38 i.e., irreversible hydrogen uptake always
occurs on Mo2N in the temperature range of 308-623 K, is
quite different from that on MoS2,39 where only reversible
hydrogen uptake was found at 473 and 573 K. These literature
results indicate that hydrogen is much strongly adsorbed on the
Mo nitride than on Mo sulfide catalystsnamely, hydrogen can
be better activated on Mo nitride catalyst than on Mo sulfide.
On supported Mo nitride and sulfide catalysts, nitrided Mo2N/
Al2O3 is found to have a much higher capacity for CO
adsorption than both passivated Mo2N/Al2O3 and sulfided Mo/
Al2O3, i.e., 535.5 µmol g-1 for Mo2N/Al2O3 renitrided at 873
K, 64.5 µmol g-1 for Mo2N/Al2O3 reduced at 873 K,26 and 37
µmol g-1 for freshly sulfided Mo/Al2O3.5 This can be explained
by two factors. First, the nitriding procedure can cause a
significant reduction in crystallite and a considerable increasing
in surface area of Mo2N.40 Second, for the alumina-supported
Mo nitride, some Mo2N could exist as two-dimensional, raftlike
domains and the two-dimensional domains possess a high
percentage of exposed Mo atoms.41 These result in a definite
higher density of active sites on nitrided Mo catalyst than
passivated Mo nitride and sulfided Mo catalysts, which indicates
a higher possibility for hydrogen adsorption and activation on
nitrided Mo2N/Al2O3. Therefore, it is possible to deduce that
more hydrogen can be adsorbed and better activated on Mo
nitride than on Mo sulfide. This contributes partly to the better
thiophene reactivity on Mo nitride catalysts than on Mo sulfide
as observed by IR spectroscopy.
The adsorption of 1-butene, 2-butenes, and 1,3-butadiene on
reduced passivated and nitrided Mo2N/γ-Al2O3 catalysts helps
to conclude that the surface species are mainly σ-bonded butene
species formed from thiophene/H2 adsorption on the two
catalysts at high temperatures. The σ-bonded species are usually
considered to play a role in side reactions and the catalyst
deactivation.35 These surface species can be further dehydro-
genated to form coke species. This could be also responsible
for the activity loss in thiophene HDS reaction on nitride
catalysts in addition to the sulfidation of the surfaces.5,21,22
As the surface property, the adsorption mode of thiophene
and the surface species from the HDS of thiophene are similar
for both Mo nitride and sulfide catalysts during HDS process,
it is presumably indicated that the reactions pathways for the
HDS of thiophene on nitride catalysts resemble those on sulfide
catalysts. On sulfide catalysts, a number of different reaction
schemes have been proposed for thiophene HDS: internal
transfer of thiophene’s â-hydrogens to the sulfur atom to give
diacetylene and H2S,44 direct hydrogenolysis of C-S bonds to
give 1,3-butadiene and adsorbed sulfur,45 and hydrogenation of
thiophene to 2,3-dihydrothiophene and 2,5-dihydrothiophene
followed by C-S bond cleavage to give 1,3-butadiene and
adsorbed sulfur.46,47 As adsorption of thiophene alone on the
Mo nitride catalysts does not show activity even at 673 K
(Figure 6), it can be excluded the possibility of the operation
of internal transfer mechanism on Mo nitride catalysts. Normal
mode calculations of η1(S)-coordinated thiophene in organo-
metallic compounds indicate that the C-C bonds in the C4
hydrocarbon backbone are slightly strengthened while the C-S
bonds are significantly weakened relative tot free thiophene.20
So similar changes of the C-C and C-S bonds in η1(S)-
adsorbed thiophene on Mo nitrides are expected. Thus, some
The high reactivity of nitrided Mo2N/γ-Al2O3 catalyst to
thiophene could be also attributed to its unique electronic
property for interaction with thiophene. It is generally accepted
that the formation of carbide and nitride broadens the unfilled-
portion of the metal d-band.42,43 Thiophene is bonded via its
sulfur atom to the surface cus Mo sites, which involves the
donation of the electron pair of the C-S bond to the vacant
orbital on the Mo sites. The availability of the broad, unoccupied
orbitals on the nitride surface should therefore enhance the
probability for the activation of C-S bonds of thiophene. This
electronic property of nitride would inevitably lead to the better
activation of thiophene on nitrided Mo2N/γ-Al2O3 catalyst than
on sulfided Mo catalyst. For the passivated Mo nitride, the

Thiophene Adsorption and Reactions on Mo2N/γ-Al2O3
J. Phys. Chem. B, Vol. 106, No. 5, 2002 987
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5. Conclusions
Using IR spectroscopy, it is found that thiophene is adsorbed
via its sulfur atom on the cus Mo sites of both reduced
passivated and nitrided Mo2N/γ-Al2O3 catalysts. Thiophene
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temperature as high as 673 K, while in the presence of H2,
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activated on a nitrided Mo2N/γ-Al2O3 catalyst that possesses a
higher density of active sites and unique electronic property.
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Acknowledgment. This work was supported financially by
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