Effect of preparation conditions on platinum metal dispersion and turnover frequency of several reactions over platinum-supported on alumina catalysts

Applied Catalysis A: General 272 (2004) 329–338

Effect of preparation conditions on platinum metal dispersion and turnover frequency of several reactions over platinum-supported on alumina catalysts Hiromi Matsuhashi a,∗ , Satoru Nishiyama b , Hiroshi Miura c , Koichi Eguchi d , Koji Hasegawa e , Yasuo Iizuka f , Akira Igarashi g , Naonobu Katada h , Junya Kobayashi i , Takashi Kubota j , Tohru Mori k , Kazuyuki Nakai l , Noriyasu Okazaki m , Masatoshi Sugioka n , Takashi Umeki o , Yoshiteru Yazawa p , Daling Lu q a

Department of Science Hokkaido University of Education, 1-2 Hachiman-cho, Hakodate 040-8567, Japan Department of Applied Chemistry, Faculty of Engineering, Kobe University, Nada-ku, Kobe 657-8501, Japan c Department of Applied Chemistry, Faculty of Engineering, Saitama University, Saitama 338-8570, Japan d Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan e N.E. Chemcat Corporation, Technical Center of Chemical Catalysis Manufacturing Department, Numazu Office, 678 Ipponmatsu, Numazu 410-0314, Japan f Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan g Department of Environmental Chemical Engineering, Faculty of Engineering, Kogakuin University, 2665-1 Nakano-machi, Hachioji 192-0015, Japan h Department of Materials Science, Faculty of Engineering, Tottori University, 4-101 Koyama-cho Minami, Tottori 680-8552, Japan i Hakodate National College of Technology, 14-1 Tokura, Hakodate 042-8501, Japan j Department of Material Science, Interdisciplinary Faculty of Science and Engineering, Shimane University, 1060 Nishikazuwa-cho, Matsue, Shimane 690-8504, Japan k Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, Yokohama 226-8503, Japan l Nippon-Bel, 11-27, 2-Chome Shinkitano, Yodogawa-ku, Osaka 532-0025, Japan m Department of Applied and Environmental Chemistry, Kitami Institute of Technology,165 Koencho, Kitami, 090-8507, Japan n Department of Applied Chemistry, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran 050-8585, Japan o Central Research Laboratories, Idemitsu Kosan Co., Ltd., Kamiizumi 1280, Sodegaura 299-0293, Japan p Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan q Japan Science and Technology Agency, Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Yokohama 226-8503, Japan b

Received in revised form 17 May 2004; accepted 4 June 2004 Available online 20 July 2004

Abstract To obtain a broad understanding of the effect of preparation conditions on the platinum metal dispersion and the effect of dispersion on turnover frequencies (TF) of several representative reactions, we prepared platinum-supported alumina catalysts by impregnation methods in which preparation conditions were fixed or not fixed using certain alumina. Platinum dispersions on the alumina catalysts were determined by several methods. Catalytic activities for combustion of propane, oxidation of CO, hydrogenations of ethylene and naphthalene, hydrodechlorination of 1,1,1-trichloroethane, hydrodesulfurization of thiophene, and hydrocracking of hexane were compared based on the TF of these reactions. The following results were obtained: (i) high dispersion of platinum was achieved without extra modification for the catalyst preparation method; (ii) an attentive pretreatment brought high reproducible results in platinum dispersions; (iii) the catalytic activities for combustion of propane, hydrodesulfurization of thiophene and hydrogenation of naphthalene were affected by starting materials of platinum; (iv) the relation between the TF and platinum dispersion was negative in all reactions tested except hydrocracking of hexane. © 2004 Elsevier B.V. All rights reserved. Keywords: Platinum; Alumina; Preparation conditions; Dispersion; Turnover frequencies

∗

Corresponding author. Tel.: +81 138 444 325; fax: +81 138 444 325. E-mail address: [email protected] (H. Matsuhashi).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.06.005

330

H. Matsuhashi et al. / Applied Catalysis A: General 272 (2004) 329–338

1. Introduction Platinum-supported on alumina is most popular as a metal catalyst. This catalyst has been applied to many kinds of reactions, such as oxidation, reduction, and decomposition. In general, catalytic activities for these reactions are strongly-dependent on the metal dispersion of the prepared catalysts. The dispersion of a supported metal is affected by preparation methods, conditions, and procedures. A relation between the turnover frequencies (TF) and the platinum metal dispersion is expected to be different for each reaction. Therefore, it is important to obtain a common understanding about the effect of preparation conditions on the platinum metal dispersion and the effect of the dispersion on turnover frequencies of the selected typical reactions. For this purpose, our study was carried out with the cooperation of many research groups. Several kinds of platinum-supported alumina catalysts were prepared by impregnation methods in which preparation conditions were fixed or not fixed using certain alumina (JRC-ALO-6, supplied by the Reference Catalyst Committee of the Catalysis Society of Japan) and three kinds of platinum sources (H2 PtCl6 ·6H2 O, [Pt(NO2 )2 (NH3 )2 ], and [Pt(NH3 )4 ]Cl2 ). Platinum dispersions on the alumina catalysts were also determined by several methods. Catalytic activities for reactions of oxidation, reduction, and decomposition were compared based on the TF of these reactions. The following results were obtained: (i) high dispersion of platinum was achieved without extra modification for the catalyst preparation method; (ii) an attentive pretreatment brought high reproducible results in platinum dispersion measurement; (iii) the catalytic activities for combustion of propane, hydrodesulfurization of thiophene and hydrogenation of

naphthalene were affected by starting materials of platinum; (iv) the relation between the TF and platinum dispersion was negative in all reactions tested except hydrocracking of hexane.

2. Experimental 2.1. Catalyst preparation The alumina JRC-ALO-6 (1.6 mm spherical form) used as the support was a reference catalyst supplied by the Reference Catalyst Committee of the Catalysis Society of Japan. Among the alumina reference catalysts, JRC-ALO-6 has quite low impurity content. High (4.8 wt.%) and low (1 wt.%) contents of platinum samples were prepared considering their suitability for spectroscopic analysis and general use. Preparation conditions of platinum-supported alumina catalysts are presented in Table 1. The catalysts of loaded with 4.8 wt.% of platinum alumina were prepared by impregnating Al2 O3 with an aqueous solution of platinum source, except Entry 7 (incipient wetness). Three kinds of platinum compounds: (H2 PtCl6 ·6H2 O (Entries. 1–5), Pt(NO2 )2 (NH3 )2 (Entries 6–9), and [Pt(NH3 )4 ]Cl2 (Entry 10)), were used as the aqueous solution or as the diluted nitric acid solution (Entry 6). The pH of H2 PtCl6 solution was controlled by adding NaOH (Entries 2 and 3) to clarify the effect of pH of impregnating solution and the adsorption phenomenon of anions including platinum. Excess water was removed with a rotary evaporator (Entries 1–3, 5, 6, 8 and 9) or by heating on a hot plate (Entries 4

Table 1 Preparation conditions of platinum-supported alumina catalysts Entry

Sample

Pt source

Loaded Pt (wt.%)

Form

Calcination temperature (◦ C) (condition)

Reduction temperature (◦ C) (condition)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Hakodate-0 Hakodate-8 Hakodate-11 Kyoto Kitami Kobe Ichikawa Nagoya-1 Nagoya-2 Hachioji HC300A HC400A HC500A HC300F HC400F HC500F DN300F DN400F DN500F

H2 PtCl6 ·6H2 O H2 PtCl6 ·6H2 O H2 PtCl6 ·6H2 O H2 PtCl6 ·6H2 O H2 PtCl6 ·6H2 O Pt(NO2 )2 (NH3 )2 Pt(NO2 )2 (NH3 )2 Pt(NO2 )2 (NH3 )2 Pt(NO2 )2 (NH3 )2 [Pt(NH3 )4 ]Cl2 H2 PtCl6 ·6H2 O H2 PtCl6 ·6H2 O H2 PtCl6 ·6H2 O H2 PtCl6 ·6H2 O H2 PtCl6 ·6H2 O H2 PtCl6 ·6H2 O Pt(NO2 )2 (NH3 )2 Pt(NO2 )2 (NH3 )2 Pt(NO2 )2 (NH3 )2

5.24 4.36 3.54 4.84 4.90 5.21 5.05 5.12 5.30 4.61 0.95 0.95 0.97 0.91 0.99 0.91 0.86 0.98 0.90

24–50 mesh 24–50 mesh 24–50 mesh 60–200 mesh Fine powder Spherical (1.6 mm) Spherical (1.6 mm) 25–50 mesh 25–50 mesh Spherical (1.6 mm) Spherical (1.6 mm) Spherical (1.6 mm) Spherical (1.6 mm) Spherical (1.6 mm) Spherical (1.6 mm) Spherical (1.6 mm) Spherical (1.6 mm) Spherical (1.6 mm) Spherical (1.6 mm)

300 300 300 450 600 370 350 400 600 500 300 400 500 300 400 500 300 400 500

– – – 450 – – 180 350 500

(1 h, H2 ) (3 h, H2 ) (6 h, H2 )

400 400 400 400 400 400 400 400 400

(3 h (H2 ) (3 h, H2 ) (3 h, H2 ) (3 h, H2 ) (3 h, H2 ) (3 h, H2 ) (3 h, H2 ) (3 h, H2 ) (3 h, H2 )

(3 h, air) (or 500) (3 h, air) (or 500) (3 h, air) (3 h, O2 ) (4 h, air) (1 h, N2 + O2 ) (5 h, air) (3 h, air flow) (6 h, air flow) (1 h, air) (3 h, air) (3 h, air) (3 h, air) (3 h, air flow) (3 h, air flow) (3 h, air flow) (3 h, air flow) (3 h, air flow) (3 h, air flow)

Remarks

pH = 8.62 by NaOH pH = 11.14 by NaOH (3 h, H2 )

Incipient wetness

H. Matsuhashi et al. / Applied Catalysis A: General 272 (2004) 329–338

and 10), followed by drying at 383 K for 12 h (Entries 1–4), 393 K for 24 h (Entry 5), 483 K for 12 h (Entries 8 and 9) or 373 K for 22 h (Entry 10); this was followed by calcination and reduction in desired conditions as indicated in Table 1. The catalysts of loaded with 1 wt.% of platinum alumina were prepared in similar manners. Platinum sources were H2 PtCl6 ·6H2 O (Entries 11–16) and Pt(NO2 )2 (NH3 )2 (Entries 17–19). These were impregnated with the aqueous solution of H2 PtCl6 ·6H2 O or diluted nitric acid solution of Pt(NO2 )2 (NH3 )2 . After alumina was added in solution, each was allowed to stand for 24 h before removing excess water. The calcination and reduction conditions are also shown in Table 1.

3. Characterization 3.1. Acid property of alumina support The acid–base property of a solid surface is an important factor to understand the adsorption phenomena of ions from an aqueous solution. Temperature-programmed desorption of ammonia (NH3 -TPD) and differential heat of ammonia adsorption were measured to evaluate the acid property of alumina used as support. The TPD experiments were carried out according to a method reported elsewhere [1] after degassing the sample at 773 K for 1 h in a quartz cell, followed by the adsorption of ammonia and water vapor treatment at 373 K. The temperature was increased at the heating rate of 10 K min−1 , and the desorbed materials were analyzed with a mass spectrometer. The amount of desorbed ammonia was determined on the basis of the intensity of the fragment with m/e = 16. The sample amount was 0.1 g, the flowing rate of the carrier helium was 1 cm3 STP s−1 , and the pressure of the sample bed was 13.3 kPa. The differential heat of ammonia adsorption was measured with a microcalorimeter. The sample (0.65 g) was pretreated in vacuum at 873 K for 1 h, heated in O2 (13.3 kPa) and finally evacuated for 12 h. 3.2. Pore structure of alumina and platinum particle distribution Pore structures of alumina and platinum loaded alumina were evaluated by N2 adsorption (BJH and D-H method) and mercury porosimetric analysis. Platinum distribution on alumina particle was observed by EPMA. The sample was held in an adhesive and cut in the diameter direction. 3.3. Amount of loaded Pt, particle size, and consumption of H2 in reduction process Amount of Pt loaded on alumina surface was measured by X-ray fluroscence for 4.8 wt.% Pt loaded samples and by atomic absorption for 1 wt.% Pt loaded samples. Residual Cl contents on 4.8 wt.% Pt samples were determined by EDX. Consumption of H2 of unreduced sample was determined by temperature-programmed reduction (TPR). The peak area

331

in TPR spectrum gave the amount of H2 consumed. The amount of H2 consumed was shown as the percentage of that against the amount of H2 for the reduction of all of Pt(IV) to Pt(0). Particle sizes of Pt dispersed on samples were determined by use of TEM images and EXAFS. The numbers of Pt particles were counted from TEM images. The average size of Pt particles for each sample was calculated by Xi/Ni where Xi is the size of Pt particles and Ni is the number of particles. The particle size of Pt on each sample was also calculated from coordination numbers obtained by curve-fitting of Pt L3 -edge EXAFS oscillation results. Dispersions of Pt metal on alumina surface were analyzed by CO or H2 adsorption [2,3] and H2 –O2 titration, as shown in Table 3. Pretreatment of the sample and measurement were carried out according to slightly modified conditions from the standardized ones proposed by the Reference Catalyst Committee. The standardized conditions in CO pulse method was as follows. A weighted sample was heated in flowing air from room temperature to 673 K and kept at that temperature for 30 min. Air was purged by flowing He (20–40 ml min−1 ) for 15 min at the same temperature. The sample was then treated by H2 (20–40 ml min−1 ) for 15 min and cooled to room temperature. After that, 50–200 ␮l of CO was injected repeatedly at 2 or 3 min intervals.

4. Reaction procedure and TF measurement of various reactions 4.1. Propane combustion Propane combustion was carried out in a flow reaction system [4]. The catalyst (0.03 or 0.05 g) was packed in a reaction tube with 1–1.5 g of quartz granules and pretreated at 823 K for 2 h under flowing N2 + H2 (N2 :H2 = 4:1). The reaction was carried out at 423–823 K. The reaction gas mixture (0.25% propane, 1.25% O2 , He balance) was passed through the catalyst bed at the flow rate of 100 ml min−1 . 4.2. Ethylene hydrogenation Ethylene hydrogenation was carried out in a flow reaction system. The catalyst (1–10 mg) was packed in a reaction tube and reduced at room temperature for 0.5 h under flowing H2 . The reaction was carried out at 273 K for 2 h. Ethylene (8%) and H2 (12%) diluted with He were passed through the catalyst bed at the flow rate of 25 ml min−1 . Products were analyzed by GLC using a jointed column of Porapac-Q (2m) and Porapac-R (2m). 4.3. Naphthalene hydrogenation For naphthalene hydrogenation, the catalyst was crushed and the 100–200 mesh (150–75 ␮m) portion was used. The catalyst (200 mg) was treated in vacuum at 408 K for 15 min and then reduced in H2 flow (20 ml min−1 ) at 673 K for 1 h

H. Matsuhashi et al. / Applied Catalysis A: General 272 (2004) 329–338

Hydrodechlorination of 1,1,1-trichloroethane was carried out in a conventional flow system at 353 K [5]. The catalyst was crushed and packed in a reaction tube with ␣-Al2 O3 . The catalyst was pretreated in H2 flow at 673 K for 0.5 h. Gas composition was substrate:H2 :Ar = 2:19:39. The amount of catalyst was controlled to obtain the substrate conversion less than 5% (46–72 mg). 4.5. Hydrocracking of hexane Hydrocracking of hexane was carried out in a conventional flow system. The catalyst (100 mg) was placed in a reaction tube and heated from room temperature to 573 K with a increasing rate of 10 K min−1 in flowing H2 (20 ml min−1 ). When the temperature reached 573 K, the flowing gas was changed to H2 containing hexane. Hexane and H2 were mixed in a saturator cooled at 273 K. The partial pressure of hexane was 5.8 kPa and the amount supplied was 3.48 mmol h−1 . 4.6. Hydrodesulfrization of thiophene Hydrodesulfrization of thiophene was carried out using both conventional fixed-bed flow reactor and closed circulation system. The reaction procedure of the flow system was as follows [6]. After the temperature of catalyst bed reached 623 K in H2 stream, thiophene was introduced into the reactor with flowing hydrogen (30 ml min−1 ) saturated by thiophene at 273 K. The weight of catalyst packed was 0.1 g. Conversion and selectivity data were obtained at 2 h. In the closed circulation system, crushed catalyst was packed in a reaction tube and heated in vacuum for 5 min. The catalyst was presulfided by a mixed gas of H2 and H2 S (9:1, 100 ml min−1 ) at 673 K for 1 h after heating in vacuum for 5 min. The reaction was carried out at 623 K with the reaction gas mixture of H2 (26 kPa) and thiophene (2.8 kPa). 4.7. Oxidation of CO Oxidation of CO with O2 was carried out in a closed circulation system after the reduction with H2 at 673 K. A CO and O2 mixture of stoichiometric composition was introduced into the reaction system in which the catalyst bed was kept at 333 K; CO2 produced was collected in a liquid N2 trap.

Conventional method without water vapor treatment

0.002 After water vapor treatment

0.001

0 300

500 700 Temperature/K

900

Fig. 1. Ammonia-TPD spectra of alumina (JRC-ALO-6) with and without water vapor treatment.

5. Results and discussion 5.1. Surface acid property of alumina The TPD spectra of alumina treated and not treated with water vapor are shown in Fig. 1. The desorption peak around 500 K was attributed to physically adsorbed ammonia. The TPD spectrum measured after water vapor treatment reflects the acid property of alumina. The adsorption heat of ammonia, which was evaluated based on the temperature of the peak at 578 K, was 152 kJ mol−1 . The value indicates that this alumina has weak acid sites. A similar phenomenon was observed in the measurement of differential heats of adsorption. A high heat of adsorption was obtained when the adsorption amount was small, as shown in Fig. 2. The heat of

300 Differential heat of adsorption/kJ mol

4.4. Hydrodechlorination of 1,1,1-trichloroethane

0.003 Concentration of NH3 in gas phase/mol m-3

before the reaction. Naphthalene hydrogenation was carried out in a autoclave. A tridecene solution of naphthalene (0.195 mol l−1 ) was put into a autoclave with the pretreated catalyst. The reaction system was heated at 473 K and H2 was charged (25 kgf cm−2 ). The reaction was carried out for 2 h.

-1

332

200

100

0 0

0.1

0.2

0.3

0.4

Amount of adsorption/mmol g

0.5 -1

Fig. 2. Differential heat of ammonia adsorption at 473 K on alumina (JRC-ALO-6) evacuated at 873 K.

H. Matsuhashi et al. / Applied Catalysis A: General 272 (2004) 329–338

all pores on alumina calculated by BJH method were micro pores less than 20 nm. The surface area and pore size of the samples of platinum loaded alumina samples are summarized in Table 2. The surface structure was expected to be changed by treating with strong acidic (Entries 1, 4–7) or basic (Entry 3) solution. However, the data in Table 2 shows that changes of both the surface area and pore size (D-H method and mercury porosimetry) by platinum loading treatments were not so large.

80

60 ∆V/ R/a. u.

333

40

5.3. Amount of loaded platinum and distribution in alumina particles

20

0 1

10 Pore size/nm

100

Fig. 3. Pore size distribution of Kyoto sample.

adsorption gradually decreased from ca. 150 to 80 kJ mol−1 . The tendency means that the sample has weak acid sites and a wide acid strength distribution. The acid site density was determined to be 0.19 nm−2 by TPD and 2.12 mmol m−2 (1.28 nm−2 ) by ammonia adsorption. 5.2. Pore size of alumina and platinum-loaded alumina The pore size distribution of platinum-loaded alumina catalyst measured by N2 adsorption is shown in Fig. 3. Almost

The desired amounts of platinum loading were 4.8 and 1 wt.%. Platinum sources were H2 PtCl6 ·6H2 O, Pt(NO2 )2 (NH3 )2 , and [Pt(NH3 )4 ]Cl2 . Ions containing platinum are anion, neutral, and cation in solution, respectively. As shown above, the alumina surface was weakly acidic, meaning it has positive charge in acidic solution and negative charge in basic solution. Consequently, H2 PtCl6 ·6H2 O was adsorbed on the alumina surface by the strong interaction between the positive charge of the surface and negative charge of the anion with high efficiency. In the case of basic solution (Entries 2 and 3, NaOH added), there was a repulsion force between the surface with negative charge and the anion. As a result, the desired amount of platinum could not be loaded. The preparation using [Pt(NH3 )4 ]Cl2 showed high efficiency of loading because the solution of [Pt(NH3 )4 ]Cl2 was neutral and the alumina surface was negative. The cation

Table 2 Physico-chemical properties of alumina and platinum-supported alumina catalysts Entry

Sample

Loaded Pt (wt.%)

Surface area (m2 g−1 )a

Pore size (nm)b

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

JRC-ALO-6 Hakodate-0 Hakodate-8 Hakodate-11 Kyoto Kitami Kobe Ichikawa Nagoya-1 Nagoya-2 Hachioji HC300A HC400A HC500A HC300F HC400F HC500F DN300F DN400F DN500F

5.24c 4.36c 3.54c 4.84c 4.90c 5.21c 5.05c 5.12c 5.30c 4.61c 0.95e 0.95e 0.97e 0.91e 0.99e 0.91e 0.86e 0.98e 0.90e

180 157 158 160 168 164 180 163 164 160 167 173 172 162 172 172 170 168 166 171

22.8 22.4 21.8 20.8 21.2 22.5 16.4 22.1 22.7 21.5 17.8 18 18.6 17.6 18.9 20.4 17 17.5 20.1

a b c d e

BET. N2 adsorption. X-ray fluorescence. TPR. Atomic adsorption.

Cl amount (wt.%)c <0.2 1.29 1.84 2.71 1.18 0.79 0.26 0.28 0.22 0.15 0.97

Consump.of H2 (%)d 78 83 95 88 158

33 130 102 114 115 115 107 1007 165 106

334

H. Matsuhashi et al. / Applied Catalysis A: General 272 (2004) 329–338

moisture in air may prevent the redispersion of platinum precursor.

120 HC500A

5.4. Consumption of H2 in reduction treatment

Intensuty/a. u.

80

40

HC500F

DN500F

0 0

0.5

1 1.5 Distance/mm

2

Fig. 4. Distribution of platinum in diametrical direction in alumina particles.

of [Pt(NH3 )4 ]2+ would be strongly adsorbed on the alumina surface, which was negatively charged. The used alumina contained less than 0.2% chloride (data of supplier). The results of X-ray fluorescence measurement shown in Table 2 indicate that the contents of chloride were 0.15–0.28 wt.% when the platinum source without chloride was used for preparation. Large amounts of chloride (0.79 wt.% or higher) remained on the samples prepared by using a platinum source containing chloride. In particular, the amounts of chloride on Entries 2 and 3 prepared by adding NaOH were much larger than the others. The chloride seems to be left on the surface as NaCl. The spherical-form alumina (1.6 mm in diameter) was used for the preparation of samples Entries 11–19. The distribution of platinum in the direction of the alumina particle diameters was observed by EPMA. The results are shown in Fig. 4. The distribution uniformity is high in all samples. A little tendency of the egg-shell type distribution was observed for samples prepared using H2 PtCl6 ·6H2 O (Entries 11–16). This observation indicates that the adsorption of PtCl6 2− on alumina surface is very strong [7]. This electrostatic interaction partially prevented the diffusion of anion into the inner part of alumina. The uniformity of platinum on alumina in the samples of 1 wt.% platinum loading was as follows; DNx00F > HCx00F > HCx00A DNx00F (Entries 17–19) and HCx00F (Entries 14–16) were calcined in a flow of dried air. The calcination of HCx00A samples (Entries 11–13) was carried out in static air. The calcination in a flow of dried air would be effective to obtain the sample of higher platinum uniformity. The distributions of platinum precursor should be the same on HCx00F and HCx00A. That the uniformity of HCx00F was higher than that of HCx00A implies that redispersion of platinum precursor occurs during calcination in dried air. The

On unreduced samples, amounts of H2 consumption in reduction process were determined from the peak areas in TPR profiles. The data were calculated on the assumption that all platinum loaded was initially Pt(IV) and was finally reduced into Pt(0). Most samples showed the values close to 100%. However, consumed amounts of H2 were much larger than 100% on Entries 6, 17 and 18. These were prepared by using Pt(NO2 )2 (NH3 )2 as platinum source and calcination was carried out at 673 K or lower. In these cases, the thermal decomposition of platinum compounds would not be completed and a large amount of H2 was consumed for the reduction of remaining NOx . This means that the temperature higher than 673 K is necessary for the decomposition of Pt(NO2 )2 (NH3 )2 adsorbed on alumina surface. The precursor decomposition would be complete even at 573 K for the samples prepared by using H2 PtCl6 (Entries 11 and 14). The sample prepared by using [Pt(NH3 )4 ]Cl2 (Entry 10) showed a small amount of H2 consumption. In this sample, Pt(0) was observed by EXAFS. Pt(0) seems to be produced by disproportionation of Pt(II) or by reduction by NH3 evolved in the calcination process. 5.5. Dispersion of platinum Platinum dispersion was determined by assuming the stoichiometric factor of CO/Pt = 1 and H2 /Pt = 2. The obtained dispersions were different from each other on the 4.8 wt.% samples, as shown in Table 3. Entry 1 showed low dispersion by static adsorption of CO and H2 , while it gave high dispersion obtained by CO pulse and CO adsorption isotherms. These are several kinds of CO adsorption species of different adsorption energies [8]. The pretreatment of the samples must be done with care because pretreatment conditions strongly affect CO adsorption. On the whole, no clear relation was observed between platinum dispersion and platinum starting material or preparation conditions. On the other hand, differences of platinum dispersion between measuring methods were relatively small, particularly on the sample of DN300F (Entry 17). There was a small effect of calcination temperature on the platinum dispersion observed on the samples prepared by using H2 PtCl6 ·6H2 O (Entries 11–16). The calcination in the flow of dried air gave relative highly dispersed platinum. Simultaneously, the condensation of platinum precursor seems to be assisted by the moisture in air [9]. In contrast to that, the calcination temperature affected platinum dispersion on the samples prepared from Pt(NO2 )2 (NH3 )2 (Entries 17–19). Platinum dispersion was much lower on the sample calcined at 573 K (ca. 38%); however, it attained ca. 70% by calcination at 673 K. As was explained in the section of H2 consumption, the thermal de-

H. Matsuhashi et al. / Applied Catalysis A: General 272 (2004) 329–338

335

Table 3 Platinum dispersion supported on alumina Entry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Sample

Hakodate-0 Hakodate-8 Hakodate-11 Kyoto Kitami Kobe Ichikawa Nagoya-1 Nagoya-2 Hachioji HC300A HC400A HC500A HC300F HC400F HC500F DN300F DN400F DN500F a b c d e f g h i j k

Pt dispersion (%) COa

COb

COc

COd

11.9 19.3 14.3 35.3 38.3 58.1 38.9 50.5 16.4 14.8

75 42 46 51 68 71 42 65 17 21

85 64 52 52 83 66 45 59 16 27

85.6 51.0 46.5 61.0 89.6 80.1 72.5 98.6 30.6 27.6 59.3 49.5 56.8 72.0 63.6 73.0 40.3 68.2 63.1

COe

Cof

Cog

H2 h

H2 i

H2 j

18.6 22.7 23.1 53.4 56.4 75.1 47.8 64.6 22.3 24.5 82 49 71 85 73 47 42 46 53

72 75 83 63 51 66 37 72 62

55.3 46.9 52.5 63.2 66.8 65.4 31.9 40.9 43.2

32.3 53.2 54.9 77.0 74.8 74.1 40.7 70.3 50.4

70 66 68 42 70

H2 –O2 k 66.1 53.6 55.1 53.4 64.8 64.5 47.6 52.8 19.8 24.3 65.6 60.5 68.8 65.4 66.6 70.9 47.2 69.6 64.8

CO static adsorption. CO pulse. CO pulse. CO adsorption isotherm. CO static adsorption. CO pulse. CO isotherm. H2 adsorption isotherm. H2 static adsorption. H2 static adsorption. pulse H2 –O2 titration.

composition of platinum compounds was not completed at 673 K. This may be a reason for lower platinum dispersion obtained on the sample calcined at lower temperature. Although, the dispersion on this sample decreased by the calcination at 773 K, the dispersion of HC samples remained at a higher level. The effect of remaining chloride may be the reason for this behavior [10]. However, details of the effect of chloride are not clear. The particle size of platinum calculated by use the data of CO adsorption and pulse H2 –O2 titration ((d) and (k) in Table 3), TEM, and EXAFS are summarized in Table 4. Good agreement exists between these data, without so many exceptions. 5.6. Relation between platinum dispersion and turnover frequency The relation between the platinum dispersion and TF of several catalytic reactions requires discussion. So combustion of propane (Fig. 5) and oxidation of CO (Figs. 6 and 7) were preformed as the oxidation reactions, and hydrogenations of ethylene (Fig. 8) and naphthalene (Fig. 9), hydrodechlorination of trichloroethane (CH3 CCl3 ) (Fig. 10), hydrodesulfurization of thiophene (Figs. 11 and 12), and

Table 4 Change of platinum particle size Entry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Sample

Hakodate-0 Hakodate-8 Hakodate-11 Kyoto Kitami Kobe Ichikawa Nagoya-1 Nagoya-2 Hachioji HC300A HC400A HC500A HC300F HC400F HC500F DN300F DN400F DN500F

Pt size (nm) CO adsorption

H2 –O2 titration

TEM

EXAFS

1.3 2.1 2.4 1.8 1.2 1.3 1.5 1.1 3.6 4.0 1.8 2.2 1.9 1.5 1.7 1.5 2.7 1.6 1.7

1.6 2.0 2.0 2.0 1.7 1.7 2.3 2.1 5.6 4.6 1.7 1.8 1.6 1.7 1.6 1.5 2.3 1.6 1.7

37 2.2 2.2 0.7 0.9 0.9 1 2.4 29 8.5 0.7 0.9 0.9 0.7 0.7 0.7 1.6 2.1 1.1

1.9 4.2 2.8 2.2 1.9 1.9 3.4 – 2.1

336

H. Matsuhashi et al. / Applied Catalysis A: General 272 (2004) 329–338

8

100 HC300F

90 DN300F

Ea/kJ mol-1

TF/s-1

6

DNx00F

4

80

HC400F

DN400F TA300F

70

TA400F DN500F

2

60 HCx00F

HCx00A

50

0

0

20

40

60

80

Pt dispersion/% Fig. 5. The relationship between the TF and the platinum dispersion in combustion of propane.

hydrocracking of hexane (Fig. 13) were performed as the reduction reactions. Combustion of propane, hydrogenation of ethylene, hydrodechlorination of trichloroethane, and hydrocracking of hexane were carried out in a conventional flow reaction system. A batch system was used for hydrogenation of naphthalene. Hydrodesulfurization of thiophene was carried out in both a flow and a re-circulation system. These reactions were performed on catalysts of 1 wt.% platinum-loaded catalyst to obtain a common understanding about the relation between the TF of each reaction and the platinum dispersion on alumina. The effect of platinum starting materials was additionally investigated. As shown in Figs. 5, 6, and 8–13, negative relations were observed between the platinum dispersion and TF of all reactions except hydrocracking of hexane. Fig. 7

20

40 60 80 Pt dispersion/%

100

Fig. 7. Change of the activation energy of CO oxidation. TAx00F: prepared from [Pt(NH3 )4 ]Cl2 .

showed the change of activation energy of CO oxidation on platinum catalysts against the platinum dispersion. The activation energy of oxidation increased with increasing of platinum dispersion. The trend indicates that the performance of one active site was reduced; the metallic character of platinum particle is being lost as it becomes a smaller particle. The effect of platinum starting materials was also observed simultaneously on combustion of propane, hydrogenation of naphthalene and hydrodesulfurization of thiophene. HCx00F and HCx00A prepared using H2 PtCl6 showed low activity for propane combustion, as shown in Fig. 5. The low activity of these catalysts can be attributed to the chloride remaining on the catalyst surface because combustion of propane is strongly interfered with by chloride [11].

6

0.4

TA400F

0.3

DNx00F

DN500F DN400F

DN300F

2

TF/min-1

TF/min-1

4

HCx00F

0.2

HC300F

TA300F

HCx00A

0.1

HC400F

0

0 0

20

40

60

80

100

Pt dispersion/% Fig. 6. The relationship between the TF and the platinum dispersion in oxidation of CO with O2 . TAx00F: prepared from [Pt(NH3 )4 ]Cl2 .

20

40 60 Pt dispersion/%

80

Fig. 8. The relationship between the TF and the platinum dispersion in hydrogenation of ethylene.

H. Matsuhashi et al. / Applied Catalysis A: General 272 (2004) 329–338

100

8

80

HC (Flow) HCx00F

6 DN (Flow)

60

TF/h-1

TF/h-1

337

40 4 DNx00F

20

0

2 70

80 90 Pt dispersion/%

100

60

70 80 Pt dispersion/%

90

Fig. 9. The relationship between the TF and the platinum dispersion in hydrogenation of naphthalene.

Fig. 11. The relationship between the TF and the platinum dispersion in hydrodesulfurization of thiophene performed in flow reaction system.

In hydrodesulfuriztion of thiophene, the effect of starting materials was observed when it was carried out in a flow reaction system. It was not observed when the reaction was carried out in a re-circulation system, as presented in Fig. 12. The effect of starting materials was notable in hydrogenation of naphthalene (Fig. 9). The TF and activity were higher on the catalysts prepared from H2 PtCl6 . The difference of TF between HCx00F and HCx00A samples was not clear,

though HCx00F samples showed higher hydrogenation activities than HCx00A samples. The hydrodechlorination of 1,1,1-trichloroethane over alumina-supported platinum catalysts gave mainly 1,1-dichloroethane and ethane. The selectivity toward chloroethane over all of the catalysts was less than 2%. The overall activity (TF) decreased with increasing platinum dispersion as shown in Fig. 10a. This result resembles the situation in the hydrodechlorination of dichlorodifluoromethane [12] and dichloromethane [13] over alumina-supported palladium catalysts investigated previously. The selectivity to 1,1-dichloroethane decreased and that to ethane increased with increasing platinum dispersion (Fig. 10b). Mori et al. [5] have observed a similar effect of platinum dispersion

0.05 (a)

0.04 TF/s-1

DNx00F

0.03 0.02

HCx00A

8

HCx00F

DN (Circulation)

(b)

60

DNx00F

40

8

CH3-CH3

6

6

HCx00A

CH3-CHCl2

4

HCx00F

20

TF/h-1

0 80

Selectivity/%

Selectivity/%

0.01

4 HC (Circulation)

2 CH3-CH2Cl

0 20

40

60

80

0 100

Pt dispersion/% Fig. 10. The relationship between the TF and the platinum dispersion in dechlorination of 1,1,1-trichloroethane.(䊉, 䉱, 䊏, 䉬) DNx00F, (䊊, , 䊐, 䉫) HCx00A, (䊊, , 䊐, 䉫) HCx00F.

2 60

70 80 Pt dispersion/%

90

Fig. 12. The relationship between the TF and the platinum dispersion in hydrodesulfurization of thiophene performed in closed re-circulation system.

338

H. Matsuhashi et al. / Applied Catalysis A: General 272 (2004) 329–338

6. Conclusions

120 DNx00F

100

TF/h-1

80 HCx00A

60 40 20 0 60

70 80 Pt dispersion/%

90

Fig. 13. The relationship between the TF and the platinum dispersion in hydrocracking of hexane.

on the product distribution, but the TF do not depend on the dispersion in the same reaction over silica-supported platinum catalysts. Although the discrepant effect of platinum dispersion on the TF is not well understood at this stage of investigation, we predict that sites for ethane formation exist on the edges or corners of platinum surfaces, the number of which increases with increasing dispersion. Fig. 10 also indicates that no effect of starting materials of platinum appears on hydrodechlorination of 1,1,1-trichloroethane. The effect of starting materials was not observed on TF for ethylene hydrogenation or for hydrodechlorination of trichloroethane. As is shown in Fig. 8, TF data lie on a straight line. This observation on ethylene hydrogenation is explained differently from the hydrodechlorination of trichloroethane. It had been reported that this reaction is structure-insensitive [14]. The activity depends on the active metal surface area measured by the adsorption amounts of H2 and CO. The TF of hexane hydrocracking had no clear relation with the platinum distribution nor with the starting materials. This reaction is probably structure-sensitive and the activity depends on not the platinum dispersion but on the exposed surface structure. Additionally, hydroisomerization and hydrocracking proceeded on the same active sites [15]. So the relation between the TF and platinum dispersion might be very complicated. The platinum dispersion gave no information on the exposed surface structure.

Platinum-supported on alumina catalysts was prepared by impregnation methods under different conditions. The results showed the following: (i) high dispersion of platinum was achieved without extra modification for the catalyst preparation method; (ii) an attentive pretreatment brought a highly reproducible result in platinum dispersion measurements; (iii) the catalytic activities for combustion of propane, hydrodesulfurization of thiophene and hydrogenation of naphthalene were affected by the starting materials of platinum; (iv) the relation between the TF and platinum dispersion was negative in combustion of propane, oxidation of CO, hydrogenations of ethylene and naphthalene, hydrodechlorination of trichloroethane (CH3 CCl3 ), and hydrodesulfurization of thiophene. There was no clear relation in hydrocracking of hexane.

Acknowledgements This work has been carried out as a research project of the Reference Catalyst Committee of the Catalysis Society of Japan.

References [1] N. Katada, H. Igi, J.-H. Kim, M. Niwa, J. Phys. Chem. B 101 (1997) 5969. [2] Reference Catalyst Committee, Shokubai (Japanese) 28 (1986) 41. [3] Reference Catalyst Committee, Shokubai (Japanese) 31 (1986) 317. [4] Y. Yazawa, H. Yoshida, N. Takagi, S. Komai, A. Satsuma, T. Hattori, Appl. Catal. B 19 (1998) 261. [5] T. Mori, Y. Nakamura, T. Takahashi, K. Gotoh, Y. Morikawa, J. Ion Exchange 14 (2003) 401. [6] T. Kurosaka, M. Sugioka, H. Matsuhashi, Bull. Chem. Soc. Jpn. 74 (2001) 757. [7] B.N. Shelimov, J.-F. Lambert, M. Che, B. Didillon, J. Mol. Catal. A 158 (2000) 91. [8] A. Bourane, O. Dulaurent, D. Bianchi, J. Catal. 196 (2000) 115. [9] S.E. Wanke, P.C. Flynn, Catal. Rev. 12 (1975) 92. [10] K. Foger, H. Jaeger, Appl. Catal. 56 (1989) 137. [11] P. Marecot, A. Fakche, B. Kellali, G. Mabilon, M. Prigent, J. Barbier, Appl. Catal. B 3 (1994) 283. [12] W. Juszczyk, A. Malinowski, Z. Karpinski, Appl. Catal. A 166 (1998) 311. [13] A. Malinowski, D. Lomot, Z. Karpinski, Appl. Catal. B 19 (1998) L79. [14] J. Grunes, J. Zhu, E.A. Anderson, G.A. Somorjai, J. Phys. Chem. B 106 (2002) 11463. [15] K.-C. Park, S.-K. Ihm, Appl. Catal. A 203 (2000) 201.