FT-IR Spectroscopic Studies of Thiophene Adsorption and Reactions

cate that the two samples have quite different surface proper- ties.26,27. The as-prepared ..... suggests that H2 is absolutely necessary for the acti...
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J. Phys. Chem. B 2002, 106, 979-987

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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 trans2-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 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 hydrodesulfurization (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. 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 * Corresponding authors. Fax: +86 411 4694447. E-mails: canli@ ms.dicp.ac.cn (C.L.) and [email protected] (Q.X.).

of studies have been published for the adsorption of thiophene on sulfided Mo catalysts,15-20 but no such experimental result 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 2.1. Catalyst Preparation. The preparation of passivated Mo2N/γ-Al2O3 catalyst is described elsewhere.21-25 MoO3/γAl2O3 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

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 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 selfsupporting 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 characteristics. Previous IR results of CO adsorption on the two samples, nitrided and reduced passivated Mo2N/γ-Al2O3, indicate that the two samples have quite different surface properties.26,27 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-workers,15,19,20 for thiophene adsorption on sulfided Mo/γ-Al2O3 and γ-Al2O3, some delicate differences exist in the range of 14501370 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

Wu et al.

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 32002900 cm-1 region and 1418, 1408, and 1400 cm-1 in the 14501370 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).

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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 preadsorbed 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

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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 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

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/γAl2O3 catalyst. 3.2. Thiophene Adsorption at High Temperatures (373673 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 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 decomposed/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 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.

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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 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) 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. 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 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.

catalyst at RT does not show any new feature in the IR spectrum (Figure 8a), indicating that thiophene is not reactive on the 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 hydrocarbon species.34,35 Figure 9 exhibits the IR spectra of CO adsorbed on nitrided Mo2N/γ-Al2O3 sample treated with a thiophene/H2 mixture at

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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.

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

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.

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 trans2-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 1,3butadiene, 1-butene, and cis- and trans-2-butenes were conducted 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 Adsorption 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

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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

Figure 12. IR spectra of different adsorbates adsorbed at RT on reduced passivated Mo2N/γ-Al2O3 catalyst: (a) 1-butene; (b) trans-2butene; (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 πand σ-bonded 2-butenes are the surface species from cis- and trans-2-butenes.24,25 The contour of the IR spectra of 1,3butadiene 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/γAl2O3 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

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 organometallic 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 properties. 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 adsorption. 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

986 J. Phys. Chem. B, Vol. 106, No. 5, 2002 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. 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 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 unfilledportion 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 Mo 2N/γ-Al2O3 catalyst than on sulfided Mo catalyst. For the passivated Mo nitride, the

Wu et al. 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 explanations 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 mechanical 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. 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 dehydrogenated 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 β-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 organometallic 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

Thiophene Adsorption and Reactions on Mo2N/γ-Al2O3 initial steps in the thiophene HDS reaction on Mo nitrides can be proposed as follows: (1) η1(S) bonding of thiophene to the cus Mo sites; (2) cleavage of one of the two weakened C-S bonds, concomitant sulfidation of the nitride surface by adsorbed sulfur species, and formation of σ-bonded butene species. A further exploration is needed for the detailed mechanism of thiophene HDS on Mo nitrides. 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 alone shows no reaction on the two catalysts even at adsorption temperature as high as 673 K, while in the presence of H2, thiophene is more reactive on nitrided Mo catalyst than on reduced passivated one, leaving σ-bonded butene species on the surfaces. CO probing also demonstrates that the majority of thiophene remains intact even at high temperatures adsorption and clearly shows that the surface is sulfided by thiophene adsorption at high temperatures in the presence of H2. The better reactivity of nitrided Mo2N/γ-Al2O3 catalyst to thiophene compared with reduced passivated Mo nitride and Mo sulfide is due to the fact that both hydrogen and thiophene can be better activated on a nitrided Mo2N/γ-Al2O3 catalyst that possesses a higher density of active sites and unique electronic property. Acknowledgment. This work was supported financially by the National Nature Science Foundation of China (NSFC, No. 29625305) and the State Key Project of the Ministry of Science and Technology of China (Grant No. G2000048003). References and Notes (1) Ramanathan, S.; Yu, C. C.; and Oyama, S. T. J. Catal. 1998, 173, 10. (2) Chu, Y.; Wei, Z.; Yang, S.; Li, C.; Xin, Q.; Min, E. Appl. Catal., A: General 1999, 176, 17. (3) Ozkan, U. S.; Zhang, L.; Clark, P. A. J. Catal. 1997, 172, 294. (4) Li, S.; Lee, J. S.; Hyeon, T.; Suslick, K. S. Appl. Catal., A: General 1999, 184, 1. (5) Aegerter, P. A.; Quigley, W. W.; Simpson, G. J.; Ziegler, D. D.; Logan, J. W.; McCrea, K. R.; Glazier, S.; Bussell, M. E. J. Catal. 1996, 164, 109. (6) McCrea, K. R.; Logan, J. W.; Tarbuck, T. L.; Heiser, J. L.; Bussell, M. E. J. Catal. 1997, 171, 255. (7) Sajkowski, D. J.; Oyama, S. T. Appl. Catal., A: General 1996, 134, 339. (8) Zhang, Y.; Wei, Z.; Yan, W.; Ying, Y.; Ji, C.; Li, X.; Zhou, Z.; Sui, X.; Xin, Q. Catal. Today 1996, 30, 135. (9) Zhang, Y.; Xin, Q.; Romos, I. R.; Ruiz, A. G. Appl. Catal., A: General 1999, 180, 145. (10) Nagai, M.; Goto, Y.; Ishii, H.; Omi, S. Appl. Catal., A: General 2000, 192, 189.

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