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Toluene hydrogenation over Pd and Pt catalysts as a model hydrogen storage process using low grade hydrogen containing catalyst inhibitors
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Toluene hydrogenation over Pd and Pt catalysts as a model hydrogen storage process using low grade hydrogen containing catalyst inhibitors Masanori Gonda a, Masaeaki Ohshima b, Hideki Kurokawa b, Hiroshi Miura b,* a
Fuji Oil Company, Sodegaura Refinery, 1 Kitasode, Sodegauraecity, Chiba 299e0266, Japan Graduate School of Science and Engineering, Saitama University, 255 Shimoeokubo, Sakuraeku, Saitama 338e8570, Japan
b
article info
abstract
Article history:
The effects of CO and H2S as catalyst inhibitors on the rate of toluene hydrogenation were
Received 9 April 2014
studied as a means of hydrogen storage using low-grade hydrogen. Pd/SiO2 suffered
Received in revised form
serious negative effects from catalyst inhibitors; however, Pd/TiO2eSiO2 exhibited high CO
11 July 2014
and H2S tolerance because the acidic support decreased the electron density of the Pd
Accepted 26 July 2014
metal particles, which, in turn, decreased the interaction between the Pd surface and CO
Available online 27 August 2014
(or H2S). The TiO2eSiO2-supported Pd catalyst exhibited activity greater than that of the
Keywords:
in presence of H2S. Catalyst characterization after sulfidation with H2S revealed that Pd
Hydrogen storage
particles were fully sulfided, whereas Pt particles were sulfided only on their surface. We
Catalyst inhibitor
concluded that Pd catalysts supported on acidic oxides exhibit excellent activity toward
Toluene hydrogenation
toluene hydrogenation in the presence of CO and that Pt catalysts exhibit excellent activity
Supported Pd catalyst
in the presence of H2S.
Titania-silica support
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
TiO2eSiO2-supported Pt catalyst in the presence of CO; however, it exhibited lower activity
reserved.
Introduction Hydrogen is expected to be an energy medium for fuel cells in vehicles and power generators. Hydrogen storage and transportation methods need to be developed to distribute hydrogen to wide range of applications and to thereby satisfy the need for a safe, environmentally friendly, economically viable energy source with high energy density. The use of
organic hydrides is one of the most promising methods of hydrogen storage [1e3]. When hydrogen reacts with aromatic hydrocarbons such as toluene or naphthalene, it is stored as a stable organic hydride, such as methyl cyclohexane or decalin. The stored hydrogen is recovered through the dehydrogenation of organic hydrides. These organic hydrides have properties similar to gasoline and diesel fuel, such as boiling point and flammability, and are easy to handle even at room temperature and atmospheric pressure. In addition, their
* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Miura). http://dx.doi.org/10.1016/j.ijhydene.2014.07.158 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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density of hydrogen storage is high enough as compared with other methods [2]. A substantial amount of research related to organic hydrides has been reported in the literature; however, most of the previous reports have been focused on the dehydrogenation step [2e7]. Okada et al. [2] reported that a K-promoted Pt/ Al2O3 catalyst exhibited high stability for a prolonged period with sufficient activity and selectivity. On the other side, hydrogenation of aromatic hydrocarbons is considered as a well-established process because benzene hydrogenation has been industrially performed as a part of nylon production [8]. However several researches have been reported on the hydrogenation of toluene as a hydrogenation step of organic hydride method [1,8e13]. For example Lindfors [8] tried toluene hydrogenation over Ni/ SiO2 using a differential reactor and found that the reaction mechanism obeyed the LangmuireHinshelwood model. Hiyoshi et al. [13] investigated the hydrogenation of naphthalene as the hydrogen storage step; they attempted the reaction in supercritical CO2 as a solvent and observed greater catalytic activity in supercritical CO2 than in an organic solvent. They also reported that Ru/C was the most active catalyst. With respect to the hydrogen storage step, most research has been performed using high-purity hydrogen. However, a great deal of low-grade hydrogen is produced in the petroleum-refining and steel industries. For example, 1010 m3/year of coke oven gas is generated in Japan, but it is not utilized effectively. If such hydrogen could be stored in an organic hydride, both storage and purification would be simultaneously achieved because high-purity hydrogen is recovered from the dehydrogenation of organic hydrides. However, such low-grade hydrogen gases contain impurities [14,15] such as CO (2e5%) and H2S (0.1e1%) that are typical poisons for metal catalysts used in hydrogenation reactions; thus, the development of CO- and sulfur-tolerant catalysts is required. We reported our attempts to hydrogenate naphthalene and tetralin in the presence of 2% CO over various supported metal catalysts [9,16e19]. Rh and Ru are highly active catalysts in pure H2; however, they are inactive in the presence of CO [16]. Ni is active even in the presence of CO; however, it is also active toward CO hydrogenation, and significant fraction of H2 is lost by methanation [17]. In contrast, Pd and Pt were active only for naphthalene hydrogenation without promoting CO hydrogenation. We attempted to improve the CO and H2S tolerance of Pd by using the acidic supports SiO2eAl2O3 [18], TiO2eSiO2 [19], and TiO2eAl2O3 [20]. As revealed by FT-IR spectroscopy of the adsorbed CO, the acidic supports decreased the electron density of Pd particles, thereby weakening the PdeCO bond; thus, the coverage of CO on the Pd surface decreased and a Pd catalyst with improved CO tolerance was attained. Although the naphthaleneedecalin system is advantageous with respect to hydrogen storage density, its handling poses a problem because naphthalene is solid at room temperature. In this work, in the presence of CO and H2S, we attempted to hydrogenate toluene instead of naphthalene. We used an acidic support, TiO2eSiO2, and also investigated its effects on CO and H2S tolerance.
Experimental Preparation of the support materials We prepared TiO2, SiO2, and the binary oxide TiO2eSiO2 using a solegel method. Ti(OCH(CH3)2)4, Si(OC2H5)4 (Kanto Chemical Co, Japan), or their mixtures were dissolved in 2-propanol. While the solution was agitated at 150 rpm, aqueous ammonia was added dropwise at a rate of 3 mL/min. In the case of SiO2, pure water was added instead of aqueous ammonia. After 10 min of agitation, the slurry was maintained at 60 C for 24 h. The precipitate was dried in a rotary evaporator and subsequently calcined in air at 500 C for 6 h. Granules smaller than 100 mesh were used for catalyst preparation. The composition of the TiO2eSiO2 binary oxide is expressed as TiO2eSiO2(X wt %), where X represents the TiO2 content calculated from the amount of Ti and Si used for preparation.
Preparation of supported metal catalysts Supported Pt and Pd catalysts were prepared by a conventional impregnation method using aqueous solutions of H2PtCl6 and PdCl2 (Kanto Chemical Co., Japan) as the starting materials. The support materials (i.e., TiO2, SiO2, and TiO2eSiO2) were dispersed in the metal solutions containing calculated amounts of metal precursors and stirred for 1 h; the water was subsequently evaporated under reduced pressure. The samples were dried at 130 C overnight, calcined in air at 400 C for 3 h, and reduced under flowing H2 at 300 C for 5 h. The metal loading was 187 mmol g1 (Pd: 2.0 wt%, Pt: 3.6 wt%). The composition of the catalysts was expressed according to the amount of metal precursors used for catalyst preparation. We used another series of catalysts for structural characterization using non-acidic, high purity of silica as a support material, because TiO2eSiO2 supported catalysts exhibited serious disturbance due to background signals in XRD and FTIR measurements, especially after sulfidation. Pt/SiO2(AERO) and Pd/SiO2(AERO), in which AEROSIL-380 (Japan Aerosil Co.) were used as a support, were prepared using a procedure similar to that above described. In these catalysts, metal loadings were 256 mmol g1 (Pt: 5 wt%, Pd: 2.7 wt%).
Evaluation of the catalytic activities toward the hydrogenation of toluene and naphthalene The hydrogenation of toluene and naphthalene was examined in a 100-mL, stainless-steel autoclave. A sample of 0.1e0.5 g of catalyst was pre-reduced under flowing hydrogen (100 mL min1) at 300 C for 1 h and then transferred into the reactor without being exposed to air. The detail of the procedure is reported in the literature [21]. Toluene (7.8 mmol, Reagent grade, >99.0%, Kanto Chemical Co.) was dissolved in 40 mL of n-tridecane and added to the reactor, which was then purged by reactant gas, i.e., pure H2 (Suzuki Shokan Co.), 2% CO/H2 (Takachiho Chemical Co.), or 1% H2S/H2 (TaiyoeNissan Co.), at normal pressure. After the reactor was heated to reaction temperature, the reactant gas was introduced at a pressure of 1 MP and the reaction was initiated with vigorous stirring at 1000 rpm. Under pure H2, 2% CO/H2, or 1% H2S/H2,
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Table 1 e Composition of the oxide supports and the surface characteristics of the supported Pd catalysts. Catalyst
Pd/SiO2 Pd/TiO2 Pd/TiO2eSiO2 Pd/TiO2eSiO2 Pd/TiO2eSiO2
TiO2 content [wt%]
BET surface area [m2 g1]
CO uptake [mmol g1]
CO/Pd [%]
0 100 80 50 33
468 39 389 408 524
92.4 40.1 93.5 58.9 55.5
49.2 21.3 49.8 31.3 29.5
toluene hydrogenation was performed at 70 C, 200 C, or 270 C, respectively. The reaction products were analyzed using a GC-18A gas chromatograph (Shimadzu Co.) equipped with a flame ionization detector and a 30-m DB-17 capillary column. We used the reactor as a differential reactor. When the reaction time was short enough and conversion was kept small, we found that conversion was proportional to reaction time and we could get a constant value of reaction rate (initial reaction rate) [16,18]. We determined the reaction rate within this range, selecting the conditions carefully. Rate of reaction ¼ ðmoles of hydrogen consumed for reactionÞ= ðmass of catalystÞðreaction timeÞ The turnover frequency (TOF) was calculated based on the reaction rate and the number of active sites. TOF ¼ ðrate of reactionÞ=ðnumber of active sitesÞ The number of active sites was determined from the amount of CO adsorption at room temperature using the stoichiometry of CO/Pt ¼ 1 and CO/Pd ¼ 1. The parameter TOFH indicates the TOF value in pure hydrogen, whereas the parameter TOFI indicates the TOF value in the presence of inhibitors.
Characterization of catalysts The surface area of the support materials was measured using an SA-6200 (Horiba, Japan) flow type N2 adsorption apparatus; the samples were immersed in a liquid nitrogen bath after
Fig. 2 e Reaction rate and TOFI of toluene hydrogenation over Pd/TiO2eSiO2 in presence of CO as a function of the TiO2 content (C): Hydrogenation rate, (B): TOFI. Reaction conditions: temperature 200 C, pressure 1 MPa (2% CO/H2).
being pretreated under flowing N2 at 300 C for 0.5 h. The dispersion of the supported metals was determined by CO chemisorption at room temperature using a BP-1 (Ohkura Riken Co., Japan) pulse adsorption instrument. The samples were placed under flowing He at room temperature, pretreated in H2 at 300 C for 1 h, and then purged under heat at 300 C for 0.5 h. FT-IR spectra of adsorbed CO were obtained using an FT/IR4100 (JASCO Co., Japan) spectrometer equipped with an MCT (HgeCdeTe) detector. A 10-mm-diameter thin disk of catalyst sample was placed in a Pyrex transmittance cell with CaF2 windows. The sample was reduced at 300 C for 1 h, evacuated (<104 Torr) at 300 C for 1 h, and cooled to room temperature. CO at 20 Torr was introduced into the cell, and the sample was exposed to CO at room temperature for 20 min, followed by evacuation for 30 min at 27 C. FT-IR spectra were recorded at a resolution of 2 cm1 after the accumulation of 320 scans. XRD patterns were collected on a RINTeUltima III/Bu (Rigaku Co., Japan) diffractometer equipped with a Cu-Ka radiation source operated at a working voltage of 40 kV and a current of 40 mA. The scanning step was 0.001 or 0.02 , and the scanning speed was 0.1 or 4 min1.
Sulfidation and re-reduction Characterization of the catalysts was performed for fresh, sulfided and re-reduced catalysts. The prepared catalysts were heated in a flowing H2 (100 mL min1) to 300 C and kept
Table 2 e Reaction rate and TOFI of toluene hydrogenation in presence of H2S over Pd catalysts. Catalyst
Fig. 1 e XRD patterns of TiO2, SiO2, and TiO2eSiO2 mixedoxide supports. (A): Anatase (TiO2).
Pd/SiO2 Pd/TiO2eSiO2 (80 wt%)
CO uptake [mmol g1]
Hydrogenation rate [mmol g1 h1]
TOFI [102 s1]
92.4 93.5
0.113 0.392
0.0340 0.116
Reaction conditions: temperature 270 C, pressure 1 MPa (1% H2S/H2).
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in Table 1. The surface area of TiO2 was small (39 m2/g); however, it was substantially increased by the addition of SiO2. The surface area of the binary oxide was ten times greater than that of TiO2. Accordingly, a greater dispersion of Pd (CO/Pd) was achieved in the Pd/TiO2eSiO2 catalysts than in the Pd/TiO2 catalysts. XRD patterns of the TiO2eSiO2 supports are shown in Fig. 1. Although the XRD pattern of the TiO2 suggested that the sample consisted of well-crystallized anatase, all the TiO2eSiO2 binary oxides were amorphous. SiO2 retarded the agglomeration and crystallization of TiO2, consistent with the results of the surface-area measurements. Similar results have been reported by Yang [22,23] in the case of TiO2eAl2O3. We concluded that TiO2 and SiO2 mixed with each other at the atomic level.
Effect of the composition of the supports for Pd/TiO2eSiO2 catalysts
Fig. 3 e Deconvolution of the FT-IR spectra of CO adsorbed onto Pd catalysts: (a) Pd/SiO2 and (b) Pd/TiO2eSiO2 (80 wt%). Pretreatment: reduction at 300 C for 1 h and evacuation for 1 h. CO adsorption: 20 Torr of CO at 27 C for 20 min, then evacuated for 30 min.
at 300 C for 1 h (fresh sample). After H2 reduction the sample was treated in 1%H2S/H2 (50 mL min1) at 270 C for 1 h and then cooled down to room temperature in a He flow (sulfide sample). The sulfided sample was reduced again in flowing H2 at 300 C for 1 h (re-reduced sample).
The reaction rate and TOFI of toluene hydrogenation in the presence of CO over Pd/TiO2eSiO2 catalysts with different support compositions are shown in Fig. 2. Pd catalysts supported on TiO2 and TiO2eSiO2 exhibited substantially greater TOFI values than Pd/SiO2. The highest TOFI that was observed for Pd/TiO2eSiO2 (50 wt%) was 6.9 102 s1, which is 4.6 times higher than that for Pd/SiO2. TiO2eSiO2 is a well-known acidic support [19] and its acidity promoted the reaction, because acidic support can decrease the electron density of Pd, which in turn weakens the PdeCO bond strength [18,20]. The Pd/TiO2eSiO2 (80 wt%) catalyst exhibited the highest rate of hydrogenation (19.9 mmol g1h1), because of its highly dispersed Pd. Therefore, we used the Pd/TiO2eSiO2 (80 wt%) catalyst in subsequent experiments in which we compared the sulfur tolerance of this catalyst to that of Pd/SiO2. The results of toluene hydrogenation in the presence of H2S over Pd/SiO2 and Pd/TiO2eSiO2 (80 wt%) catalysts are shown in Table 2. Pd/TiO2eSiO2 (80 wt%) exhibited TOFI values greater than those of Pd/SiO2 in the presence of H2S indicating that the acidity of the support positively affected the performance of the catalyst [19].
FT-IR spectra of CO adsorbed onto supported Pd catalysts
Results and discussion Structure of the TiO2eSiO2 supports and the Pd dispersed in Pd/TiO2eSiO2 catalysts The BrunauereEmmetteTeller (BET) surface areas of the TiO2eSiO2 supports and the Pd dispersion (Pdsurface/Pdtotal; determined by CO adsorption measurements) are presented
FT-IR spectra of CO adsorbed onto supported Pd are shown in Fig. 3. We deconvoluted the spectra into HF 1e3 (high-frequency, linear adsorption peaks) and LF 1e3 (low-frequency, bridge, and multi-bonded adsorption peaks) peaks. According to accounts in the literature [24e29], HF1 peaks correspond to CO adsorbed onto Pddþ, whereas HF2 peaks are attributed to CO adsorbed on the edge or corner of a Pd surface and HF3 peaks are attributed to CO adsorbed onto Pd(100) and Pd(111)
Table 3 e Hydrogenation of toluene over TiO2eSiO2-supported Pt and Pd catalysts in presence of inhibitors. Catalyst Pt/TiO2eSiO2 Pd/TiO2eSiO2 Pt/TiO2eSiO2 Pd/TiO2eSiO2
CO uptake [mmol g1]
Inhibitor
Reaction temperature [ C]
Reaction rate [mmol g1 h1]
TOFI [sec1]
81.0 93.5 81.0 93.5
CO CO H 2S H 2S
200 200 270 270
1.70 19.9 0.932 0.392
0.582 5.90 0.324 0.116
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Table 4 e Hydrogenation of toluene and naphthalene on Pt and Pd catalysts in the absence (TOFH) or presence (TOFI) of H2S. Catalyst
Pt/SiO2（AERO） Pd/SiO2（AERO） Pt/SiO2（AERO） Pd/SiO2（AERO）
CO uptake [mmol g1]
Substrate
22.4 22.3 22.4 22.3
Naphthalene Naphthalene Toluene Toluene
Absence of H2S
Presence of H2S
Reaction temperature [ C] TOFH [102 s1] Reaction temperature [ C] TOFI [102 s1] 50 50 70 70
2.48 1.61 9.67 0.424
270 270 270 270
1.12 3.92 0.0645 0.0274
Table 5 e Hydrogenation of toluene on silica-supported Pt and Pd catalysts in presence of inhibitors. Catalyst Pt/SiO2(AERO) Pd/SiO2(AERO) Pt/SiO2(AERO) Pd/SiO2(AERO)
CO uptake [mmol g1]
Inhibitor
Reaction temperature [ C]
TOFI [102 s1]
TOFI/TOFH
22.4 22.3 22.4 22.3
CO CO H2S H2S
200 200 270 270
0.00331 0.935 0.0639 0.0278
0.00134 0.581 0.00660 0.0655
terraces. LF1 peaks are due to bridge type CO adsorbed onto a corner or edge, LF2 peaks are due to bridge type CO adsorbed onto Pd(111), and LF3 peaks are due to presence of multibonded CO. Compared with the spectra of SiO2-supported Pd catalysts (Fig. 3a), the spectra of CO adsorbed onto the Pd/TiO2eSiO2 (80%) catalyst exhibits a greater shift in the wavenumber of the maximum of each deconvoluted peak. Such an effect is expected when the electron density of Pd particles is decreased by their interaction with the acidic sites of a support surface. When the electron density of Pd decreases, the back-donation from Pd to a p*-orbital of CO decreases, and results in a stronger CeO bond (higher shift of IR spectra) and a weaker PdeCO bond. Because CO can easily desorb from Pd/ TiO2eSiO2, the poisoning effect of CO is diminished during the hydrogenation of toluene.
catalysts exhibited hydrogenation activities toward toluene and naphthalene in the presence of H2S, as shown in Table 4. Under pure hydrogen, Pt exhibited greater activity than Pd toward naphthalene and toluene hydrogenation. In the presence of H2S, Pt exhibited greater activity than Pd in the hydrogenation of toluene, but the reverse order was observed in the hydrogenation of naphthalene; i.e., Pd was more active than Pt. These results are consistent with the previous results related to the use of TiO2eSiO2 as well as with our previous results related to the hydrogenation of naphthalene. Similar results have also been reported in the literature. Thomas et al. [31], who used FAUezeolite-supported Pt and Pd catalysts, reported that Pt is more active than Pd toward toluene hydrogenation in the presence of H2S. Matsui et al. [32] used USYezeolite supported catalysts for tetralin hydrogenation in the presence of H2S and observed that the order of activity was Pd > Pt.
Hydrogenation of toluene on TiO2eSiO2 supported Pd and Pt catalysts We compared the CO and H2S tolerance of Pd and Pt during toluene hydrogenation. The results in Table 3 provide detail on the rates of toluene hydrogenation and TOFI values for Pd/ TiO2eSiO2 and Pt/TiO2eSiO2 catalysts in the presence of CO or H2S. In the presence of CO, Pd exhibited a substantially higher rate of hydrogenation and a greater TOFI compared with those of Pt. However, Pt exhibited twofold greater activity than Pd in the presence of H2S. Thus, we concluded that in the case of toluene hydrogenation, Pd > Pt in the presence of CO and Pd < Pt in the presence of H2S. However, this order of activity in the presence of H2S contradicts our previous research results related to the hydrogenation of naphthalene, where we observed that Pd is more active than Pt [30]. Therefore, the effect of H2S appears to depend on the reactant.
CO and H2S tolerance of silica supported Pt and Pd catalysts To clarify the effect of H2S on the catalyst structure, we used another series of catalysts, Pd/SiO2(AERO) and Pt/SiO2(AERO), which are suitable for structural characterization. These
Fig. 4 e XRD patterns of used Pd/SiO2(AERO) and Pt/ SiO2(AERO) catalysts after reaction under various conditions: (a) Pd/SiO2 (fresh), (b) Pd/SiO2 (CO), (c) Pd/SiO2 (H2S), (d) Pt/SiO2 (fresh), (e) Pt/SiO2 (CO), (f) Pt/SiO2 (H2S). (CO): Hydrogenation of toluene in presence of CO; (H2S): Hydrogenation of toluene in presence of H2S. (Δ): Pt(0); (B): Pd(0); (C): Pd4S.
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Fig. 5 e XRD patterns of Pd/SiO2(AERO) and Pt/SiO2(AERO) after various treatments: (a) Pd/SiO2 (fresh), (b) Pd/SiO2 (sulfurization), (c) Pd/SiO2 (sulfurization / reduction), (d) Pt/SiO2 (fresh), (e) Pt/SiO2 (sulfurization), (f) Pt/SiO2 (sulfurization / reduction). (Δ):Pt(0); (B):Pd(0); (C):Pd4S.
Under pure hydrogen, Pt was more active than Pd for both toluene and naphthalene hydrogenation, as shown in Table 4. The TOFH of Pt was 1.5 times greater than that of Pd in the hydrogenation of naphthalene, whereas the TOFH of Pt was 23 times greater than that of Pd in the hydrogenation of toluene. The poisoning effects due to CO and H2S were more severe in the case of Pt than in the case of Pd. The combination of these factors resulted in the reversal of the order of activity between Pt and Pd. The hydrogenation of toluene in the presence of CO or H2S was examined over Pd/SiO2(AERO) and Pt/SiO2(AERO) catalysts. As shown in Table 5, Pd/SiO2(AERO) exhibited toluene hydrogenation activity in the presence of CO that was more than two orders of magnitude greater than that exhibited by Pt/SiO2(AERO). However, Pt/SiO2(AERO) exhibited twice the activity of Pd/SiO2(AERO) in the presence of H2S. These results are consistent with the results for the catalysts supported on TiO2eSiO2. The XRD patterns of the catalysts used for hydrogenation in the presence of CO or H2S were collected; the results are shown in Fig. 4. The XRD peaks caused by metallic Pd (40.10 ) and Pt (39.75 ) [33,34] were observed in catalysts used in the presence of CO. A peak associated with Pt0 was observed in the XRD pattern of the Pt/SiO2 catalyst used for hydrogenation in the presence of H2S; however, no peak of Pd0 was observed in the pattern of the Pd/SiO2. Instead, diffraction peaks associated with Pd4S [35] were observed in the pattern of the Pd/SiO2 used for hydrogenation in the presence of H2S.
Fig. 6 e Deconvolution of the FT-IR spectra of CO adsorbed on sulfided Pd catalysts: (a) fresh Pd/SiO2 and (b) sulfided Pd/SiO2. Pretreatment: reduction at 300 C for 1 h and evacuation for 1 h. CO adsorption: 20 Torr of CO at 27 C for 20 min, then evacuated for 30 min.
Characterization of sulfided catalysts The effect of sulfidation on the catalysts' structures was investigated. Sulfidation (treatment by H2S) of the catalysts was performed in a flow reactor at atmospheric pressure. The pretreated catalysts were treated in a flow of 2% CO/H2 at 270 C for 1 h. XRD patterns of the fresh catalysts, sulfided catalysts, and catalysts re-reduced after sulfidation were collected; the results are shown in Fig. 5. After sulfidation, Pd4S was observed in Pd/SiO2, in agreement with the catalyst used for hydrogenation in the presence of H2S. After rereduction of this catalyst, Pd4S was still observed, and the
Table 6 e CO uptake of Pt and Pd catalysts under various treatment conditions. Catalyst
Pt/SiO2(AERO) Pt/SiO2(AERO) Pd/SiO2(AERO) Pd/SiO2(AERO)
Treatment
CO uptake [mmol g1]
Dispersion [%]
Reduction (300 C) Sulfuration (270 C)/Reduction (300 C) Reduction (300 C) Sulfuration (270 C)/Reduction (300 C)
22.4 6.08 22.3 4.52
8.7 2.4 8.7 1.8
Particle size[nm] CO uptake
XRD
17.0 (62.8) 16.9 (82.7)
16.9 16.8 19.0 e
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sulfide was stable even under reducing conditions. The amount of CO that was adsorbed after the catalyst was rereduced after the sulfidation treatment decreased to 20% amount of CO adsorbed by the fresh catalyst (Table 6). In contrast, the Pt in the Pt/SiO2 catalyst remained in the metallic state even after the sulfidation treatment. However, the amount of CO adsorbed onto Pt/SiO2 decreased from 22.3 mmol g1 to 6.08 mmol g1 after the sulfidation treatment (Table 6), suggesting that the surface of the Pt particles was covered by S. The FT-IR spectra of the CO adsorbed onto fresh and sulfided Pd/SiO2 and Pt/SiO2 are shown in Fig. 6 and Fig. 7. After sulfidation, the intensities of the peaks associated with bridging and multibonded CO on Pd/SiO2 decreased substantially, and the peak associated with linearly adsorbed CO molecules was the main peak. Therefore, isolated Pd0 sites remained on the surface after sulfidation. Peaks associated with both linear and bridging types of adsorbed CO were observed in the FT-IR spectrum of fresh Pt/ SiO2. The linear peak was deconvoluted into three separate peaks: HF1 corresponds to CO adsorbed onto a Pt(111) surface, HF2 corresponds to CO adsorbed onto a Pt(100) surface, and HF3 corresponds to CO adsorbed onto steps and edges of Pt
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particles [36,37]. After sulfidation, the IR band at 1700e1900 cm1, which is associated with bridging CO, disappeared, in agreement with the results of Gracia et al. [38]. The intensity of the peaks associated with linear CO decreased substantially after sulfidation, and the wavenumber of each peak shifted to higher frequencies, thereby indicating that the electron density of Pt was decreased by sulfidation. Barbier et al. [39] have reported that the adsorption of sulfur onto the surface of Pt decreases the electron density of Pt. Matsui et al. [40] reported the results of EXAFS measurements of supported Pt and Pd catalysts subjected to sulfurization treatments. Their results indicate that Pd particles were fully sulfurized and that only a portion of the surface remained as Pd0. In contrast, most of Pt atoms in Pt particles remained as Pt0 after the sulfurization treatment; only a portion of the surface Pt atoms were sulfurized. Our present experimental results are in agreement with their results under the similar sulfidation conditions (Matsui; 500 vol. ppm H2SeH2 stream at 553 K for 3 h). Supported Pd catalysts are active toward toluene hydrogenation in the presence of CO; however, Pt catalysts exhibited greater activity than Pd in the presence of H2S. The Pd catalysts suffered more serious retardation effects due to H2S exposure because the Pd particles were fully sulfurized by H2S.
Conclusion We hydrogenated toluene using supported Pd and Pt catalysts in the presence of catalyst inhibitors. The TiO2eSiO2-supported Pd catalysts exhibited greater activity than Pd/SiO2 in the presence of CO because the acidic sites of the TiO2eSiO2 support decreased the electron density of the Pd particles, which in turn weakened the interaction between CO and Pd. In the presence of CO, Pd exhibited greater activity than Pt, but Pt exhibited greater activity than Pd in the presence of H2S. The structures of the catalysts were studied after they were subjected to sulfurization treatments that revealed the deep sulfurization of Pd particles, whereas the Pt particles were sulfurized only on their surface. This difference in affinity toward H2S resulted in more serious inhibition of the activity of Pd by H2S than of the activity of Pt.
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Fig. 7 e Deconvolution of the IR spectra of CO adsorbed on sulfide Pt catalysts: (a) fresh Pt/SiO2 and (b) sulfided Pt/SiO2. Pretreatment: reduction at 300 C for 1 h and evacuation for 1 h. CO adsorption: 20 Torr of CO at 27 C for 20 min, then evacuated for 30 min.
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[5] Hodoshima S, Arai H, Takaiwa S, Saito Y. Catalytic decalin dehydrogenation/naphthalene hydrogenation pair as a hydrogen source for fuel-cell vehicle. Int J Hydrogen Energy 2003;28:1255. [6] Kariya N, Fukuoka A, Ichikawa M. Development of dehydrogenation catalyst for hydrogen generation in organic chemical hydride method. Appl Catal A Gen 2002;233:91e102. [7] Kariya N, Fukuoka A, Utagawa T, Sakuramoto M, Goto Y, Ichikawa M. Efficient hydrogen production using cyclohexane and decalin by pulse-spray mode reactor with Pt catalysts. Appl Catal A Gen 2003;247:247. [8] Lindfors LP, Salmi T. Development of dehydrogenation catalyst for hydrogen generation in organic chemical hydride method. Ind Eng Chem Res 1993;32(1):34e42. [9] Danjo Y, Kikuchi I, Ino Y, Ohshima M, Kurokawa H, Miura H. Development of dehydrogenation catalyst for hydrogen generation in organic chemical hydride method. React Kinet Mech Catal 2012;105:381. [10] Roy S, Datta S. Hydrogenation of toluene on zirconiummodified hexagonal molecular sieve supported platinum and palladium catalysts. Ind Eng Chem Res 2013;52(49):17360e8. € [11] Tonbul Y, Zahmakiran M, Ozkar S. Development of dehydrogenation catalyst for hydrogen generation in organic chemical hydride method. Appl Catal B Environ 2014;148e149:466e72. [12] Stanley J.N.G., Heinroth F., Weber C. C., Masters A.F., Maschmeyer T., “Robust bimetallic PteRu catalysts for the rapid hydrogenation of toluene and tetralin at ambient temperature and pressure”, Appl Catal A Gen 454, 15, 46e52. [13] Hiyoshi N, Mine E, Rode CV, Sato O, Shirai M. Development of dehydrogenation catalyst for hydrogen generation in organic chemical hydride method. Catal Today 2006;115:248. [14] Terada A., Noguchi H., Takegami H., Kamiji Y., Inagaki Y., JAEA(Japan Atomic Energy Agency)-Research 2011; 041, 1e23. [15] Mizutani N, Kishimoto T, Maeda N. Shin Nittetsu Gihou, 391; 2011. p. 201. [16] Suzuki T, Sekine H, Ohshima M, Kurokawa H, Miura H. Hydrogenation of naphthalene and tetralin in the presence of CO over various supported metal catalysts. kagaku kougaku Ronbunshu 2007;33:593. [17] Sekine H, Ohshima M, Kurokawa H, Miura H. Liquid phase hydrogenation of naphthalene in the presence of CO over supported Ni catalyst. React Kinet Catal Lett 2008;95:99. [18] Ezuka K, Ohshima M, Kurokawa H, Miura H. Support effect of palladium catalysts for naphthalene hydrogenation as model hydrogen storage process using low grade hydrogen containing CO. J Jpn Petrol Inst 2009;52:10. [19] Iot K, Tamura J, Ohshima M, Kurokawa H, Miura H. J Jpn Petrol Inst 2003;46:84. [20] Ino Y, Ezuka K, Ohshima M, Kurokawa H, Miura H. Effect of Pd/TiO2eAl2O3 catalyst support on naphthalene hydrogenation in the presence of CO. J Jpn Petrol Inst 2010;53:239. [21] Seki H, Ohshima M, Kurokawa H, Miura H. Influence of trace oxygen on the hydrogenation activity of Ru/Al2O3 catalysts. Reac. Kinet Mech Cat 2010;101:313e20. [22] Yang J, Ferreira JMF. Inhibitory effect of the Al2O3eSiO2 mixed additives on the anataseerutile phase transformation. Mater Lett 1998;36:320. [23] Yang J, Huang YX, Ferreira JMF. Inhibitory effect of alumina additive on the titania phase transformation of a sol-gelderived powder. J Mater Sci Lett 1997;16:1933.
[24] SimonL J, van Ommen JG, Jentys A, Lercher JA. “Sulfurtolerant Pt-supported catalysts for benzene hydrogenation: II. Influence of cation exchange level for Pt/MOR-based catalysts. J Catal 2001;203:434. ski Z, Juszczyk W, Pielaszek J, Kempin ski L, [25] Skotak M, Karpin Kazachkin DV, et al. Characterization and catalytic activity of differently pretreated Pd/Al2O3 catalysts: the role of acid sites and of palladiumealumina interactions. J Catal 2004;227:11. [26] Dulaurent O, Chandes K, Bouly C, Bianchi D. Heat of adsorption of carbon monoxide on a Pd/Al2O3 solid using in situ infrared spectroscopy at high temperatures. J Catal 1999;188:237. [27] Dulaurent O, Chandes K, Bouly C, Bianchi D. Heat of adsorption of carbon monoxide on various Pd-containing solids using in situ infrared spectroscopy at high temperatures. J Catal 2000;192:273. € sch N. CO adsorption [28] Yudanov V, Sahnoun R, Neyman KM, Ro on Pd nanoparticles: density functional and vibrational spectroscopy studies. J Phys Chem 2003;B107:255. [29] Liotta LF, Martin GA, Deganello G. The influence of alkali metal ions in the chemisorption of CO and CO2 on supported palladium catalysts: a fourier transform infrared spectroscopic study. J Catal 1996;164:322. [30] Valden M, Keiski RL, Xiang N, Pere, Aaltonen J, Pessa M, et al. Reactivity of Pd/Al2O3, Pd/La2O3eAl2O3 and Pd/LaAlO3 catalysts for the reduction of NO by CO: CO and NO adsorption. J Catal 1996;161:614. [31] Thomas K, Binet C, Chevreau T, Cornet D, Gilson JP. Hydrogenation of toluene over supported Pt and Pd catalysts: influence of structural factors on the sulfur tolerance. J Catal 2002;212:63. [32] Matsui T, Harada M, Yoshimura Y. Effect of the coexistence of nitrogen compounds on the sulfur tolerance and catalytic activity of Pd and Pt monometallic catalysts supported on high-silica USY zeolite and amorphous silica. Appl Catal A Gen 2005;293:137. [33] Baba K, Miyagawa U, Watanabe K, Sakamoto Y, Flanagan TB. Electrical resistivity changes due to interstitial hydrogen in palladium-rich substitutional alloys. J Mater Sci 1990;25:3910. € nder J. Magnetic susceptibility of [34] Ebert H, Abart J, Voitla AuxPt1x. Z. Phys Chem 1985;144:223. [35] Grønvold F, Røst E. On the sulfides, selenides, and tellurides of palladium. Acta Chem Scand 1956;10:1620. [36] Hoxha F, Schimmoeller B, Cakl Z, Urakawa A, Mallat T, Pratsinis SE, et al. Influence of support acidebase properties on the platinum-catalyzed enantioselective hydrogenation of activated ketones. J Catal 2010;271:115. [37] Pillonel P, Derrouiche S, Bourane A, Gaillard F, Vernoux P, Bianchi D. Impact of the support on the heat of adsorption of the linear CO species on Pt-containing catalysts. Appl Catal A Gen 2005;278:223. [38] Gracia FJ, Guerrero S, Wolf EE, Miller JT, Kropf AJ. Kinetics, operando FTIR, and controlled atmosphere EXAFS study of the effect of sulfur on Pt-supported catalysts during CO oxidation. J Catal 2005;233:372. cot P, Tifonti L. Modifications of the electronic [39] Barbier J, Mare properties of Pt catalysts induced by sulfur adsorption or coke deposition. React Kinet Catal Lett 1986;32:269. [40] Matsui T, Harada M, Ichihashi Y, Bando K, Matsubayashi N, Toba M, et al. Effect of noble metal particle size on the sulfur tolerance of monometallic Pd and Pt catalysts supported on high-silica USY zeolite. Appl Catal A Gen 2005;286:249.
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Toluene hydrogenation over Pd and Pt catalysts as a model hydrogen storage process using low grade hydrogen containing catalyst inhibitors Masanori Gonda a, Masaeaki Ohshima b, Hideki Kurokawa b, Hiroshi Miura b,* a
Fuji Oil Company, Sodegaura Refinery, 1 Kitasode, Sodegauraecity, Chiba 299e0266, Japan Graduate School of Science and Engineering, Saitama University, 255 Shimoeokubo, Sakuraeku, Saitama 338e8570, Japan
b
article info
abstract
Article history:
The effects of CO and H2S as catalyst inhibitors on the rate of toluene hydrogenation were
Received 9 April 2014
studied as a means of hydrogen storage using low-grade hydrogen. Pd/SiO2 suffered
Received in revised form
serious negative effects from catalyst inhibitors; however, Pd/TiO2eSiO2 exhibited high CO
11 July 2014
and H2S tolerance because the acidic support decreased the electron density of the Pd
Accepted 26 July 2014
metal particles, which, in turn, decreased the interaction between the Pd surface and CO
Available online 27 August 2014
(or H2S). The TiO2eSiO2-supported Pd catalyst exhibited activity greater than that of the
Keywords:
in presence of H2S. Catalyst characterization after sulfidation with H2S revealed that Pd
Hydrogen storage
particles were fully sulfided, whereas Pt particles were sulfided only on their surface. We
Catalyst inhibitor
concluded that Pd catalysts supported on acidic oxides exhibit excellent activity toward
Toluene hydrogenation
toluene hydrogenation in the presence of CO and that Pt catalysts exhibit excellent activity
Supported Pd catalyst
in the presence of H2S.
Titania-silica support
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
TiO2eSiO2-supported Pt catalyst in the presence of CO; however, it exhibited lower activity
reserved.
Introduction Hydrogen is expected to be an energy medium for fuel cells in vehicles and power generators. Hydrogen storage and transportation methods need to be developed to distribute hydrogen to wide range of applications and to thereby satisfy the need for a safe, environmentally friendly, economically viable energy source with high energy density. The use of
organic hydrides is one of the most promising methods of hydrogen storage [1e3]. When hydrogen reacts with aromatic hydrocarbons such as toluene or naphthalene, it is stored as a stable organic hydride, such as methyl cyclohexane or decalin. The stored hydrogen is recovered through the dehydrogenation of organic hydrides. These organic hydrides have properties similar to gasoline and diesel fuel, such as boiling point and flammability, and are easy to handle even at room temperature and atmospheric pressure. In addition, their
* Corresponding author. E-mail addresses: [email protected], [email protected] (H. Miura). http://dx.doi.org/10.1016/j.ijhydene.2014.07.158 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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density of hydrogen storage is high enough as compared with other methods [2]. A substantial amount of research related to organic hydrides has been reported in the literature; however, most of the previous reports have been focused on the dehydrogenation step [2e7]. Okada et al. [2] reported that a K-promoted Pt/ Al2O3 catalyst exhibited high stability for a prolonged period with sufficient activity and selectivity. On the other side, hydrogenation of aromatic hydrocarbons is considered as a well-established process because benzene hydrogenation has been industrially performed as a part of nylon production [8]. However several researches have been reported on the hydrogenation of toluene as a hydrogenation step of organic hydride method [1,8e13]. For example Lindfors [8] tried toluene hydrogenation over Ni/ SiO2 using a differential reactor and found that the reaction mechanism obeyed the LangmuireHinshelwood model. Hiyoshi et al. [13] investigated the hydrogenation of naphthalene as the hydrogen storage step; they attempted the reaction in supercritical CO2 as a solvent and observed greater catalytic activity in supercritical CO2 than in an organic solvent. They also reported that Ru/C was the most active catalyst. With respect to the hydrogen storage step, most research has been performed using high-purity hydrogen. However, a great deal of low-grade hydrogen is produced in the petroleum-refining and steel industries. For example, 1010 m3/year of coke oven gas is generated in Japan, but it is not utilized effectively. If such hydrogen could be stored in an organic hydride, both storage and purification would be simultaneously achieved because high-purity hydrogen is recovered from the dehydrogenation of organic hydrides. However, such low-grade hydrogen gases contain impurities [14,15] such as CO (2e5%) and H2S (0.1e1%) that are typical poisons for metal catalysts used in hydrogenation reactions; thus, the development of CO- and sulfur-tolerant catalysts is required. We reported our attempts to hydrogenate naphthalene and tetralin in the presence of 2% CO over various supported metal catalysts [9,16e19]. Rh and Ru are highly active catalysts in pure H2; however, they are inactive in the presence of CO [16]. Ni is active even in the presence of CO; however, it is also active toward CO hydrogenation, and significant fraction of H2 is lost by methanation [17]. In contrast, Pd and Pt were active only for naphthalene hydrogenation without promoting CO hydrogenation. We attempted to improve the CO and H2S tolerance of Pd by using the acidic supports SiO2eAl2O3 [18], TiO2eSiO2 [19], and TiO2eAl2O3 [20]. As revealed by FT-IR spectroscopy of the adsorbed CO, the acidic supports decreased the electron density of Pd particles, thereby weakening the PdeCO bond; thus, the coverage of CO on the Pd surface decreased and a Pd catalyst with improved CO tolerance was attained. Although the naphthaleneedecalin system is advantageous with respect to hydrogen storage density, its handling poses a problem because naphthalene is solid at room temperature. In this work, in the presence of CO and H2S, we attempted to hydrogenate toluene instead of naphthalene. We used an acidic support, TiO2eSiO2, and also investigated its effects on CO and H2S tolerance.
Experimental Preparation of the support materials We prepared TiO2, SiO2, and the binary oxide TiO2eSiO2 using a solegel method. Ti(OCH(CH3)2)4, Si(OC2H5)4 (Kanto Chemical Co, Japan), or their mixtures were dissolved in 2-propanol. While the solution was agitated at 150 rpm, aqueous ammonia was added dropwise at a rate of 3 mL/min. In the case of SiO2, pure water was added instead of aqueous ammonia. After 10 min of agitation, the slurry was maintained at 60 C for 24 h. The precipitate was dried in a rotary evaporator and subsequently calcined in air at 500 C for 6 h. Granules smaller than 100 mesh were used for catalyst preparation. The composition of the TiO2eSiO2 binary oxide is expressed as TiO2eSiO2(X wt %), where X represents the TiO2 content calculated from the amount of Ti and Si used for preparation.
Preparation of supported metal catalysts Supported Pt and Pd catalysts were prepared by a conventional impregnation method using aqueous solutions of H2PtCl6 and PdCl2 (Kanto Chemical Co., Japan) as the starting materials. The support materials (i.e., TiO2, SiO2, and TiO2eSiO2) were dispersed in the metal solutions containing calculated amounts of metal precursors and stirred for 1 h; the water was subsequently evaporated under reduced pressure. The samples were dried at 130 C overnight, calcined in air at 400 C for 3 h, and reduced under flowing H2 at 300 C for 5 h. The metal loading was 187 mmol g1 (Pd: 2.0 wt%, Pt: 3.6 wt%). The composition of the catalysts was expressed according to the amount of metal precursors used for catalyst preparation. We used another series of catalysts for structural characterization using non-acidic, high purity of silica as a support material, because TiO2eSiO2 supported catalysts exhibited serious disturbance due to background signals in XRD and FTIR measurements, especially after sulfidation. Pt/SiO2(AERO) and Pd/SiO2(AERO), in which AEROSIL-380 (Japan Aerosil Co.) were used as a support, were prepared using a procedure similar to that above described. In these catalysts, metal loadings were 256 mmol g1 (Pt: 5 wt%, Pd: 2.7 wt%).
Evaluation of the catalytic activities toward the hydrogenation of toluene and naphthalene The hydrogenation of toluene and naphthalene was examined in a 100-mL, stainless-steel autoclave. A sample of 0.1e0.5 g of catalyst was pre-reduced under flowing hydrogen (100 mL min1) at 300 C for 1 h and then transferred into the reactor without being exposed to air. The detail of the procedure is reported in the literature [21]. Toluene (7.8 mmol, Reagent grade, >99.0%, Kanto Chemical Co.) was dissolved in 40 mL of n-tridecane and added to the reactor, which was then purged by reactant gas, i.e., pure H2 (Suzuki Shokan Co.), 2% CO/H2 (Takachiho Chemical Co.), or 1% H2S/H2 (TaiyoeNissan Co.), at normal pressure. After the reactor was heated to reaction temperature, the reactant gas was introduced at a pressure of 1 MP and the reaction was initiated with vigorous stirring at 1000 rpm. Under pure H2, 2% CO/H2, or 1% H2S/H2,
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Table 1 e Composition of the oxide supports and the surface characteristics of the supported Pd catalysts. Catalyst
Pd/SiO2 Pd/TiO2 Pd/TiO2eSiO2 Pd/TiO2eSiO2 Pd/TiO2eSiO2
TiO2 content [wt%]
BET surface area [m2 g1]
CO uptake [mmol g1]
CO/Pd [%]
0 100 80 50 33
468 39 389 408 524
92.4 40.1 93.5 58.9 55.5
49.2 21.3 49.8 31.3 29.5
toluene hydrogenation was performed at 70 C, 200 C, or 270 C, respectively. The reaction products were analyzed using a GC-18A gas chromatograph (Shimadzu Co.) equipped with a flame ionization detector and a 30-m DB-17 capillary column. We used the reactor as a differential reactor. When the reaction time was short enough and conversion was kept small, we found that conversion was proportional to reaction time and we could get a constant value of reaction rate (initial reaction rate) [16,18]. We determined the reaction rate within this range, selecting the conditions carefully. Rate of reaction ¼ ðmoles of hydrogen consumed for reactionÞ= ðmass of catalystÞðreaction timeÞ The turnover frequency (TOF) was calculated based on the reaction rate and the number of active sites. TOF ¼ ðrate of reactionÞ=ðnumber of active sitesÞ The number of active sites was determined from the amount of CO adsorption at room temperature using the stoichiometry of CO/Pt ¼ 1 and CO/Pd ¼ 1. The parameter TOFH indicates the TOF value in pure hydrogen, whereas the parameter TOFI indicates the TOF value in the presence of inhibitors.
Characterization of catalysts The surface area of the support materials was measured using an SA-6200 (Horiba, Japan) flow type N2 adsorption apparatus; the samples were immersed in a liquid nitrogen bath after
Fig. 2 e Reaction rate and TOFI of toluene hydrogenation over Pd/TiO2eSiO2 in presence of CO as a function of the TiO2 content (C): Hydrogenation rate, (B): TOFI. Reaction conditions: temperature 200 C, pressure 1 MPa (2% CO/H2).
being pretreated under flowing N2 at 300 C for 0.5 h. The dispersion of the supported metals was determined by CO chemisorption at room temperature using a BP-1 (Ohkura Riken Co., Japan) pulse adsorption instrument. The samples were placed under flowing He at room temperature, pretreated in H2 at 300 C for 1 h, and then purged under heat at 300 C for 0.5 h. FT-IR spectra of adsorbed CO were obtained using an FT/IR4100 (JASCO Co., Japan) spectrometer equipped with an MCT (HgeCdeTe) detector. A 10-mm-diameter thin disk of catalyst sample was placed in a Pyrex transmittance cell with CaF2 windows. The sample was reduced at 300 C for 1 h, evacuated (<104 Torr) at 300 C for 1 h, and cooled to room temperature. CO at 20 Torr was introduced into the cell, and the sample was exposed to CO at room temperature for 20 min, followed by evacuation for 30 min at 27 C. FT-IR spectra were recorded at a resolution of 2 cm1 after the accumulation of 320 scans. XRD patterns were collected on a RINTeUltima III/Bu (Rigaku Co., Japan) diffractometer equipped with a Cu-Ka radiation source operated at a working voltage of 40 kV and a current of 40 mA. The scanning step was 0.001 or 0.02 , and the scanning speed was 0.1 or 4 min1.
Sulfidation and re-reduction Characterization of the catalysts was performed for fresh, sulfided and re-reduced catalysts. The prepared catalysts were heated in a flowing H2 (100 mL min1) to 300 C and kept
Table 2 e Reaction rate and TOFI of toluene hydrogenation in presence of H2S over Pd catalysts. Catalyst
Fig. 1 e XRD patterns of TiO2, SiO2, and TiO2eSiO2 mixedoxide supports. (A): Anatase (TiO2).
Pd/SiO2 Pd/TiO2eSiO2 (80 wt%)
CO uptake [mmol g1]
Hydrogenation rate [mmol g1 h1]
TOFI [102 s1]
92.4 93.5
0.113 0.392
0.0340 0.116
Reaction conditions: temperature 270 C, pressure 1 MPa (1% H2S/H2).
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in Table 1. The surface area of TiO2 was small (39 m2/g); however, it was substantially increased by the addition of SiO2. The surface area of the binary oxide was ten times greater than that of TiO2. Accordingly, a greater dispersion of Pd (CO/Pd) was achieved in the Pd/TiO2eSiO2 catalysts than in the Pd/TiO2 catalysts. XRD patterns of the TiO2eSiO2 supports are shown in Fig. 1. Although the XRD pattern of the TiO2 suggested that the sample consisted of well-crystallized anatase, all the TiO2eSiO2 binary oxides were amorphous. SiO2 retarded the agglomeration and crystallization of TiO2, consistent with the results of the surface-area measurements. Similar results have been reported by Yang [22,23] in the case of TiO2eAl2O3. We concluded that TiO2 and SiO2 mixed with each other at the atomic level.
Effect of the composition of the supports for Pd/TiO2eSiO2 catalysts
Fig. 3 e Deconvolution of the FT-IR spectra of CO adsorbed onto Pd catalysts: (a) Pd/SiO2 and (b) Pd/TiO2eSiO2 (80 wt%). Pretreatment: reduction at 300 C for 1 h and evacuation for 1 h. CO adsorption: 20 Torr of CO at 27 C for 20 min, then evacuated for 30 min.
at 300 C for 1 h (fresh sample). After H2 reduction the sample was treated in 1%H2S/H2 (50 mL min1) at 270 C for 1 h and then cooled down to room temperature in a He flow (sulfide sample). The sulfided sample was reduced again in flowing H2 at 300 C for 1 h (re-reduced sample).
The reaction rate and TOFI of toluene hydrogenation in the presence of CO over Pd/TiO2eSiO2 catalysts with different support compositions are shown in Fig. 2. Pd catalysts supported on TiO2 and TiO2eSiO2 exhibited substantially greater TOFI values than Pd/SiO2. The highest TOFI that was observed for Pd/TiO2eSiO2 (50 wt%) was 6.9 102 s1, which is 4.6 times higher than that for Pd/SiO2. TiO2eSiO2 is a well-known acidic support [19] and its acidity promoted the reaction, because acidic support can decrease the electron density of Pd, which in turn weakens the PdeCO bond strength [18,20]. The Pd/TiO2eSiO2 (80 wt%) catalyst exhibited the highest rate of hydrogenation (19.9 mmol g1h1), because of its highly dispersed Pd. Therefore, we used the Pd/TiO2eSiO2 (80 wt%) catalyst in subsequent experiments in which we compared the sulfur tolerance of this catalyst to that of Pd/SiO2. The results of toluene hydrogenation in the presence of H2S over Pd/SiO2 and Pd/TiO2eSiO2 (80 wt%) catalysts are shown in Table 2. Pd/TiO2eSiO2 (80 wt%) exhibited TOFI values greater than those of Pd/SiO2 in the presence of H2S indicating that the acidity of the support positively affected the performance of the catalyst [19].
FT-IR spectra of CO adsorbed onto supported Pd catalysts
Results and discussion Structure of the TiO2eSiO2 supports and the Pd dispersed in Pd/TiO2eSiO2 catalysts The BrunauereEmmetteTeller (BET) surface areas of the TiO2eSiO2 supports and the Pd dispersion (Pdsurface/Pdtotal; determined by CO adsorption measurements) are presented
FT-IR spectra of CO adsorbed onto supported Pd are shown in Fig. 3. We deconvoluted the spectra into HF 1e3 (high-frequency, linear adsorption peaks) and LF 1e3 (low-frequency, bridge, and multi-bonded adsorption peaks) peaks. According to accounts in the literature [24e29], HF1 peaks correspond to CO adsorbed onto Pddþ, whereas HF2 peaks are attributed to CO adsorbed on the edge or corner of a Pd surface and HF3 peaks are attributed to CO adsorbed onto Pd(100) and Pd(111)
Table 3 e Hydrogenation of toluene over TiO2eSiO2-supported Pt and Pd catalysts in presence of inhibitors. Catalyst Pt/TiO2eSiO2 Pd/TiO2eSiO2 Pt/TiO2eSiO2 Pd/TiO2eSiO2
CO uptake [mmol g1]
Inhibitor
Reaction temperature [ C]
Reaction rate [mmol g1 h1]
TOFI [sec1]
81.0 93.5 81.0 93.5
CO CO H 2S H 2S
200 200 270 270
1.70 19.9 0.932 0.392
0.582 5.90 0.324 0.116
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Table 4 e Hydrogenation of toluene and naphthalene on Pt and Pd catalysts in the absence (TOFH) or presence (TOFI) of H2S. Catalyst
Pt/SiO2（AERO） Pd/SiO2（AERO） Pt/SiO2（AERO） Pd/SiO2（AERO）
CO uptake [mmol g1]
Substrate
22.4 22.3 22.4 22.3
Naphthalene Naphthalene Toluene Toluene
Absence of H2S
Presence of H2S
Reaction temperature [ C] TOFH [102 s1] Reaction temperature [ C] TOFI [102 s1] 50 50 70 70
2.48 1.61 9.67 0.424
270 270 270 270
1.12 3.92 0.0645 0.0274
Table 5 e Hydrogenation of toluene on silica-supported Pt and Pd catalysts in presence of inhibitors. Catalyst Pt/SiO2(AERO) Pd/SiO2(AERO) Pt/SiO2(AERO) Pd/SiO2(AERO)
CO uptake [mmol g1]
Inhibitor
Reaction temperature [ C]
TOFI [102 s1]
TOFI/TOFH
22.4 22.3 22.4 22.3
CO CO H2S H2S
200 200 270 270
0.00331 0.935 0.0639 0.0278
0.00134 0.581 0.00660 0.0655
terraces. LF1 peaks are due to bridge type CO adsorbed onto a corner or edge, LF2 peaks are due to bridge type CO adsorbed onto Pd(111), and LF3 peaks are due to presence of multibonded CO. Compared with the spectra of SiO2-supported Pd catalysts (Fig. 3a), the spectra of CO adsorbed onto the Pd/TiO2eSiO2 (80%) catalyst exhibits a greater shift in the wavenumber of the maximum of each deconvoluted peak. Such an effect is expected when the electron density of Pd particles is decreased by their interaction with the acidic sites of a support surface. When the electron density of Pd decreases, the back-donation from Pd to a p*-orbital of CO decreases, and results in a stronger CeO bond (higher shift of IR spectra) and a weaker PdeCO bond. Because CO can easily desorb from Pd/ TiO2eSiO2, the poisoning effect of CO is diminished during the hydrogenation of toluene.
catalysts exhibited hydrogenation activities toward toluene and naphthalene in the presence of H2S, as shown in Table 4. Under pure hydrogen, Pt exhibited greater activity than Pd toward naphthalene and toluene hydrogenation. In the presence of H2S, Pt exhibited greater activity than Pd in the hydrogenation of toluene, but the reverse order was observed in the hydrogenation of naphthalene; i.e., Pd was more active than Pt. These results are consistent with the previous results related to the use of TiO2eSiO2 as well as with our previous results related to the hydrogenation of naphthalene. Similar results have also been reported in the literature. Thomas et al. [31], who used FAUezeolite-supported Pt and Pd catalysts, reported that Pt is more active than Pd toward toluene hydrogenation in the presence of H2S. Matsui et al. [32] used USYezeolite supported catalysts for tetralin hydrogenation in the presence of H2S and observed that the order of activity was Pd > Pt.
Hydrogenation of toluene on TiO2eSiO2 supported Pd and Pt catalysts We compared the CO and H2S tolerance of Pd and Pt during toluene hydrogenation. The results in Table 3 provide detail on the rates of toluene hydrogenation and TOFI values for Pd/ TiO2eSiO2 and Pt/TiO2eSiO2 catalysts in the presence of CO or H2S. In the presence of CO, Pd exhibited a substantially higher rate of hydrogenation and a greater TOFI compared with those of Pt. However, Pt exhibited twofold greater activity than Pd in the presence of H2S. Thus, we concluded that in the case of toluene hydrogenation, Pd > Pt in the presence of CO and Pd < Pt in the presence of H2S. However, this order of activity in the presence of H2S contradicts our previous research results related to the hydrogenation of naphthalene, where we observed that Pd is more active than Pt [30]. Therefore, the effect of H2S appears to depend on the reactant.
CO and H2S tolerance of silica supported Pt and Pd catalysts To clarify the effect of H2S on the catalyst structure, we used another series of catalysts, Pd/SiO2(AERO) and Pt/SiO2(AERO), which are suitable for structural characterization. These
Fig. 4 e XRD patterns of used Pd/SiO2(AERO) and Pt/ SiO2(AERO) catalysts after reaction under various conditions: (a) Pd/SiO2 (fresh), (b) Pd/SiO2 (CO), (c) Pd/SiO2 (H2S), (d) Pt/SiO2 (fresh), (e) Pt/SiO2 (CO), (f) Pt/SiO2 (H2S). (CO): Hydrogenation of toluene in presence of CO; (H2S): Hydrogenation of toluene in presence of H2S. (Δ): Pt(0); (B): Pd(0); (C): Pd4S.
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Fig. 5 e XRD patterns of Pd/SiO2(AERO) and Pt/SiO2(AERO) after various treatments: (a) Pd/SiO2 (fresh), (b) Pd/SiO2 (sulfurization), (c) Pd/SiO2 (sulfurization / reduction), (d) Pt/SiO2 (fresh), (e) Pt/SiO2 (sulfurization), (f) Pt/SiO2 (sulfurization / reduction). (Δ):Pt(0); (B):Pd(0); (C):Pd4S.
Under pure hydrogen, Pt was more active than Pd for both toluene and naphthalene hydrogenation, as shown in Table 4. The TOFH of Pt was 1.5 times greater than that of Pd in the hydrogenation of naphthalene, whereas the TOFH of Pt was 23 times greater than that of Pd in the hydrogenation of toluene. The poisoning effects due to CO and H2S were more severe in the case of Pt than in the case of Pd. The combination of these factors resulted in the reversal of the order of activity between Pt and Pd. The hydrogenation of toluene in the presence of CO or H2S was examined over Pd/SiO2(AERO) and Pt/SiO2(AERO) catalysts. As shown in Table 5, Pd/SiO2(AERO) exhibited toluene hydrogenation activity in the presence of CO that was more than two orders of magnitude greater than that exhibited by Pt/SiO2(AERO). However, Pt/SiO2(AERO) exhibited twice the activity of Pd/SiO2(AERO) in the presence of H2S. These results are consistent with the results for the catalysts supported on TiO2eSiO2. The XRD patterns of the catalysts used for hydrogenation in the presence of CO or H2S were collected; the results are shown in Fig. 4. The XRD peaks caused by metallic Pd (40.10 ) and Pt (39.75 ) [33,34] were observed in catalysts used in the presence of CO. A peak associated with Pt0 was observed in the XRD pattern of the Pt/SiO2 catalyst used for hydrogenation in the presence of H2S; however, no peak of Pd0 was observed in the pattern of the Pd/SiO2. Instead, diffraction peaks associated with Pd4S [35] were observed in the pattern of the Pd/SiO2 used for hydrogenation in the presence of H2S.
Fig. 6 e Deconvolution of the FT-IR spectra of CO adsorbed on sulfided Pd catalysts: (a) fresh Pd/SiO2 and (b) sulfided Pd/SiO2. Pretreatment: reduction at 300 C for 1 h and evacuation for 1 h. CO adsorption: 20 Torr of CO at 27 C for 20 min, then evacuated for 30 min.
Characterization of sulfided catalysts The effect of sulfidation on the catalysts' structures was investigated. Sulfidation (treatment by H2S) of the catalysts was performed in a flow reactor at atmospheric pressure. The pretreated catalysts were treated in a flow of 2% CO/H2 at 270 C for 1 h. XRD patterns of the fresh catalysts, sulfided catalysts, and catalysts re-reduced after sulfidation were collected; the results are shown in Fig. 5. After sulfidation, Pd4S was observed in Pd/SiO2, in agreement with the catalyst used for hydrogenation in the presence of H2S. After rereduction of this catalyst, Pd4S was still observed, and the
Table 6 e CO uptake of Pt and Pd catalysts under various treatment conditions. Catalyst
Pt/SiO2(AERO) Pt/SiO2(AERO) Pd/SiO2(AERO) Pd/SiO2(AERO)
Treatment
CO uptake [mmol g1]
Dispersion [%]
Reduction (300 C) Sulfuration (270 C)/Reduction (300 C) Reduction (300 C) Sulfuration (270 C)/Reduction (300 C)
22.4 6.08 22.3 4.52
8.7 2.4 8.7 1.8
Particle size[nm] CO uptake
XRD
17.0 (62.8) 16.9 (82.7)
16.9 16.8 19.0 e
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sulfide was stable even under reducing conditions. The amount of CO that was adsorbed after the catalyst was rereduced after the sulfidation treatment decreased to 20% amount of CO adsorbed by the fresh catalyst (Table 6). In contrast, the Pt in the Pt/SiO2 catalyst remained in the metallic state even after the sulfidation treatment. However, the amount of CO adsorbed onto Pt/SiO2 decreased from 22.3 mmol g1 to 6.08 mmol g1 after the sulfidation treatment (Table 6), suggesting that the surface of the Pt particles was covered by S. The FT-IR spectra of the CO adsorbed onto fresh and sulfided Pd/SiO2 and Pt/SiO2 are shown in Fig. 6 and Fig. 7. After sulfidation, the intensities of the peaks associated with bridging and multibonded CO on Pd/SiO2 decreased substantially, and the peak associated with linearly adsorbed CO molecules was the main peak. Therefore, isolated Pd0 sites remained on the surface after sulfidation. Peaks associated with both linear and bridging types of adsorbed CO were observed in the FT-IR spectrum of fresh Pt/ SiO2. The linear peak was deconvoluted into three separate peaks: HF1 corresponds to CO adsorbed onto a Pt(111) surface, HF2 corresponds to CO adsorbed onto a Pt(100) surface, and HF3 corresponds to CO adsorbed onto steps and edges of Pt
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particles [36,37]. After sulfidation, the IR band at 1700e1900 cm1, which is associated with bridging CO, disappeared, in agreement with the results of Gracia et al. [38]. The intensity of the peaks associated with linear CO decreased substantially after sulfidation, and the wavenumber of each peak shifted to higher frequencies, thereby indicating that the electron density of Pt was decreased by sulfidation. Barbier et al. [39] have reported that the adsorption of sulfur onto the surface of Pt decreases the electron density of Pt. Matsui et al. [40] reported the results of EXAFS measurements of supported Pt and Pd catalysts subjected to sulfurization treatments. Their results indicate that Pd particles were fully sulfurized and that only a portion of the surface remained as Pd0. In contrast, most of Pt atoms in Pt particles remained as Pt0 after the sulfurization treatment; only a portion of the surface Pt atoms were sulfurized. Our present experimental results are in agreement with their results under the similar sulfidation conditions (Matsui; 500 vol. ppm H2SeH2 stream at 553 K for 3 h). Supported Pd catalysts are active toward toluene hydrogenation in the presence of CO; however, Pt catalysts exhibited greater activity than Pd in the presence of H2S. The Pd catalysts suffered more serious retardation effects due to H2S exposure because the Pd particles were fully sulfurized by H2S.
Conclusion We hydrogenated toluene using supported Pd and Pt catalysts in the presence of catalyst inhibitors. The TiO2eSiO2-supported Pd catalysts exhibited greater activity than Pd/SiO2 in the presence of CO because the acidic sites of the TiO2eSiO2 support decreased the electron density of the Pd particles, which in turn weakened the interaction between CO and Pd. In the presence of CO, Pd exhibited greater activity than Pt, but Pt exhibited greater activity than Pd in the presence of H2S. The structures of the catalysts were studied after they were subjected to sulfurization treatments that revealed the deep sulfurization of Pd particles, whereas the Pt particles were sulfurized only on their surface. This difference in affinity toward H2S resulted in more serious inhibition of the activity of Pd by H2S than of the activity of Pt.
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Fig. 7 e Deconvolution of the IR spectra of CO adsorbed on sulfide Pt catalysts: (a) fresh Pt/SiO2 and (b) sulfided Pt/SiO2. Pretreatment: reduction at 300 C for 1 h and evacuation for 1 h. CO adsorption: 20 Torr of CO at 27 C for 20 min, then evacuated for 30 min.
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