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Al2O3 catalysts
Applied Catalysis A: General 201 (2000) 241–251
Dispersion stability and methylcyclopentane hydrogenolysis in Pd/Al2 O3 catalysts A.B. Gaspar, L.C. Dieguez∗ NUCAT-PEQ-COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, CEP 21945-970, Rio de Janeiro, Brazil Received 24 September 1999; received in revised form 5 January 2000; accepted 6 January 2000
Abstract Pd/Al2 O3 catalysts with different palladium contents and precursor salts have been submitted to a reduction–oxidation cycle and their dispersions have been measured. The catalysts presented a decrease in dispersion, except for the catalyst with 1 wt.% Pd prepared with PdCl2 , which remained constant. The presence of complex species of palladium and chlorine, identified by diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) analyses, can be responsible for the stability of the dispersion. The precursor salt affected the activity and selectivity in the methylcyclopentane reaction, but the selectivity was rather constant for different palladium contents in the catalysts prepared with PdCl2 . These last catalysts also showed hydrocracking reactions, attributed to the significant content of residual chlorine. These reactions changed considerably the distribution of the methylcyclopentane hydrogenolysis products compared to the literature. The catalyst with 1 wt.% Pd, prepared with Pd(NO3 )2 , showed less activity and a distinct distribution of the products, in which no light products were observed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Pd/Al2 O3 ; Dispersion; TPR; XPS; DRS; Methylcyclopentane hydrogenolysis
1. Introduction Palladium-based catalysts have been used in the petrochemical (hydrogenation, dehydrogenation and oxidation) [1] and automotive (autoexhaust catalytic converters) industries [2]. Recent developments about the latter application have focused on the replacement of the noble metals, i.e. platinum and rhodium, by palladium, which is less expensive and more abundant. However, the use of such catalytic system in drastic conditions, like high temperatures, can cause a loss of activity due to sintering of the metallic particles. Several authors [3–9] have studied the behavior of metal∗ Corresponding author. Fax: +55-21-5907135. E-mail address: [email protected] (L.C. Dieguez)
lic particles of palladium and platinum supported on alumina when submitted to reduction and oxidation, aiming either to redisperse sintered catalysts or to increase the dispersion of new ones. Ruckenstein and Chen [3,4] studied the behavior of thin films of Al2 O3 supported palladium crystallites heating in O2 and H2 at 623, 773, 1023 and 1193 K. When heated in oxygen, spread palladium particles showed to be dependent on temperature and crystallite size. At 623 K, small crystallites spread, but the large ones formed toruslike shapes and no spread was observed. At higher temperatures, size effect seems to be less important. At 773 K, almost all the crystallites spread to irregular shapes and contain cavities of different sizes. At 1023 K, the effect of spreading was similar but not at the same extent as that heated at
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773 K. The authors explained the results by using the interfacial phenomena. The spreading, more effective at 773 K, occurs by the formation of pits and cavities on the crystallites. This formation is caused by a difference of interfacial tension between the palladium oxide and the unoxidized metal. In other study, using hydrogen at 923, 998 and 1073 K, Ruckenstein and Chen [5] observed that sintering of the palladium particles might occur via migration and coalescence, direct ripening and/or Ostwald ripening mechanisms. Finally, the authors [6] exposed the alumina-supported palladium crystallites to an alternative heating procedure in O2 at 1023 K, and H2 at 823 or 873 K. The phenomena observed was reported as spreading when heating in O2 , and sintering when heating in H2 . Spreading and sintering were explained as interfacial tension gradient and migration-coalescence processes, respectively. Lieske and Völter [7] studied a 0.6 wt.% Pd/Al2 O3 catalyst prepared with Pd(NO3 )2 and treated in oxygen in the temperature range of 773–1173 K, after a severe presinterization in argon. Redispersion was observed at about 973 K, but a more pronounced effect was reported at 1173 K, which is 30 K greater than the temperature of PdO decomposition in O2 [8]. The phenomenon was explained by the formation of a two-dimensional surface phase [PdO]sc during oxidation at high temperatures. This species would be responsible for the spreading. Similar works have been reported for platinum catalysts. Lieske et al. [9] prepared Pt/Al2 O3 catalysts with and without chlorine and observed that redispersion only occurs in the presence of chlorine. The catalyst, initially chlorine-free, only redisperses when chlorine is added. Surface complex species formed during oxidation step, like [PtIV Ox Cly ]s, identified by DRS analysis, seem to be the redispersing agent of the molecular migration mechanism. Lee and Kim [10] prepared Pt catalysts over ␥-Al2 O3 , TiO2 and SiO2 and carried out a reduction– oxidation cycle, using H2 and O2 . For Pt/Al2 O3 samples the redispersion was more effective at 793 K in O2 . The addition of chlorine brought about a significant increase in dispersion. The molecular migration mechanism was used to explain the redispersion, where surface complex species are the agents of the process.
The hydrogenolysis of methylcyclopentane is well characterized for Pt/alumina catalysts [11–14], but fewer works have been published about Pd/alumina, mainly for catalysts with large particles [15–19]. This reaction has also been used in the characterization of the Pd/zeolite system [20–22], but the detection of benzene and cycloexane, as products of a distinct route of ring enlargement reaction, makes a comparative analysis difficult. For platinum catalysts, this reaction produces 2-methylpentane (2MP), 3-methylpentane (3MP) and n-hexane (nH), but both selectivity and activity depend on the size of metal particles. Gault [11] showed that, for high dispersion Pt/Al2 O3 catalysts, the mechanism of the hydrogenolysis is non-selective and the product distribution is 40% of nH, 40% of 2MP and 20% of 3MP. For low dispersion catalysts, the mechanism is selective, resulting in 67% of 2MP and 33% of 3MP. In this case, no n-hexane is formed. The product distribution on palladium catalysts differs from platinum ones, being intermediate between the two extreme mechanisms [15]. On Pd/Al2 O3 catalysts the reaction gives 42–49% 2MP, 25–28% 3MP, 21–24% nH and 2–4% CP (cyclopentane). This distribution is rather insensitive to the calcination temperature and metallic dispersion. However, the influence of acidity and nature of the support are reported [16,17]. Hydrocracking reaction has been noticed in Pt/Al2 O3 and associated with the presence of chlorine in the reactional medium [12,13]. This specific vicinity causes the production of acidic sites in the support and Pt–Cl bonds instead of Pt–O ones. Extensive cracking in Pd/Al2 O3 catalysts is reported to be negligible, being around 3% of the products, even in high conversion levels [15–17,19]. The objective of this work was to investigate the behavior of the dispersion of Pd/alumina catalysts when submitted to a reduction–oxidation-reduction cycle. A wide range of metallic dispersions was obtained varying the Pd content and precursor salt, maintaining pre-treatment conditions constant. H2 chemisorption experiments were used to measure the dispersion after each reduction step, and diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) to characterize the palladium species. The catalysts were tested in the methylcyclopentane hydrogenolysis and the distribution of the products was also evaluated.
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Table 1 Catalysts data and reduction–oxidation cycle results Catalyst
1PdCl 5PdCl 10PdCl 15PdCl 1PdN
Pd content (wt.%)
0.91 5.09 9.32 14.52 0.85
Treatment cycle Pd reduction at 298 K (%)
Pd dispersion (%)
1st step
2nd step
1st step
2nd step
0 2 84 100 100
39 98 99 100 100
71 62 43 17 19
74 48 38 11 10
2. Experimental 2.1. Catalyst preparation Pd/Al2 O3 catalysts were prepared by the incipient wetness impregnation method using ␥-Al2 O3 Engelhard AL-3916P (156 m2 /g and 0.46 cm3 /g). Catalysts with 1, 5, 10 and 15 wt.% Pd were prepared using PdCl2 (Riedel de-Haën, 99.99%) as precursor salt. The Pd(NO3 )2 (Sigma, 99.9%) was used for the preparation of 1 wt.% Pd catalyst. After impregnation, the catalysts were dried at 393 K for 16 h and calcined at 773 K for 4 h. Table 1 summarizes the catalysts, metal contents and sample codes. 2.2. Reduction–oxidation cycle and H2 chemisorption The reduction–oxidation cycle was performed in a Pyrex U-tube reactor with an on line thermal conductivity detector. The catalysts were submitted to a sequence of reduction (TPR) from 298 to 773 K (10 K/min) with H2 /Ar stream (1.6% H2 , 30 cm3 /min — first step). In an intermediate procedure, the sample was oxidized by passing a mixture of O2 /He (2.2% O2 , 30 cm3 /min) from 298 to 773 K (10 K/min). Finally, a second reduction was done (second step). Pd reduction at 298 K was calculated discounting the peak at 343 K, ascribed to -PdH desorption [23], from the reduction peak at 298 K. The amount of catalyst for each experiment was calculated in order to have 10 mg of palladium. Under these conditions, the analyses were not limited by diffusional effects. After each reduction cycle, the sample was outgassed under argon flow at 773 K for 30 min, cooled
to 343 K and then H2 chemisorption measurements were performed. The amount of irreversible adsorbed H2 was measured using the frontal method at 343 K in order to avoid -PdH phase formation. Pure gases (99.5%) were passed through a molecular sieve filter before entering the reactor. 2.3. Characterization by XPS and DRS X-ray photoelectron spectroscopy (XPS) experiments were carried out in a Perkin–Elmer 257 spectrometer using Mg K␣ (E=1253.6 eV) as source. Vacuum in the spectrometer chamber was about 10−8 Torr. The C 1s signal at binding energy 284.6 eV was taken as the internal standard for reference. The order of the analysis was Pd 3d, C 1s, Al 2p, Cl 2p and O 1s. In order to verify the palladium photoreduction, the Pd 3d spectrum region was analyzed using 50 and 200 scans, corresponding to 12 and 48 min, respectively. Finally, the N 1s spectrum region was scanned for the catalyst prepared with Pd(NO3 )2 . The XPS spectra were adjusted by Gaussian curves to evaluate the relative concentration species. The intensities were corrected using sensibility factors from the literature [24]. The relative accuracy of the equipment was ±0.2 eV. Diffuse reflectance spectroscopy (DRS) was carried out in a Varian Cary 5 Spectrometer. The analyses were recorded in the 200–800 nm range (UV–VIS). The catalysts were diluted with Al2 O3 support to obtain the same Pd content. The reference for the experiments was the support of the catalysts. The standard used was PdO, obtained by decomposition of Pd(NO3 )2 in a furnace at 673 K.
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2.4. Activity measurements The methylcyclopentane (MCP) hydrogenolysis was investigated at 1 atm and 573 K in a Pyrex U-tube reactor, using 200 mg of catalyst. Before the reaction, the samples were reduced using a H2 /N2 (18% H2 , 30 cm3 /min) mixture at 773 K for 1 h (first step). A reaction mixture with H2 :MCP ratio of 18 was obtained by passing H2 (99.9%) through a saturator with MCP (Aldrich, 98%), maintained at 273 K by an ice/water bath. The overall time of reaction was 100 min. The reaction products were analyzed on line, each 25 min of reaction, by a Varian 2440 chromatograph equipped with a FID detector and a polypropileneglycol (25%) in Chromosorb W column.
3. Results 3.1. Reduction–oxidation cycle and H2 chemisorption The Pd contents, reduction–oxidation cycle results and Pd dispersion are presented in Table 1. The H2 consumption results are presented as the fraction (%) of Pd reduction at 298 K. However, all the catalysts presented total H2 consumption corresponding to the complete reduction of PdO to metallic Pd. The TPR profiles of the two steps are showed in Fig. 1a and b. In the first step, Fig. 1a, the catalyst 1PdCl did not present any reduction at 298 K, but a peak at 430 K was observed. The reduction profile of the 5PdCl sample displayed a small reduction peak at 298 K and two other peaks at 350 and 618 K. The catalyst 10PdCl showed a similar profile to 5PdCl, but with more H2 consumption at 298 K. The catalysts 15PdCl and 1PdN were very similar, exhibiting only one reduction peak at 298 K. Fig. 1b shows the reduction profiles of the second step. It showed quite similar profiles presenting reduction at 298 K. However, the catalyst 1PdCl presented reduction at 298 and also at 423 K. The reduction profiles of the 5PdCl, 10PdCl, 15PdCl and 1PdN were very similar, with H2 consumption at 298 K and H2 desorptions at 331, 331, 327 and 341 K, respectively. The 5PdCl and 10PdCl samples also presented very small peaks at 353 and 345 K. For the catalysts 15PdCl and 1PdN, the reduc-
tion was complete at 298 K, as observed in the first step of the cycle. Table 1 presents the fraction of Pd reduction at 298 K. In the first step, the catalysts 1PdCl and 5PdCl presented a similar behavior, without H2 consumption at 298 K, or a negligible one, respectively. The other catalysts presented almost total reduction in this stage. In the second step, while the catalysts 5PdCl, 10PdCl, 15PdCl and 1PdN showed total reduction at 298 K, the catalyst 1PdCl showed a distinct behavior, with only 39% of reduction in the same condition. The dispersion of the catalysts, obtained by H2 chemisorption, is presented in Table 1 and, as expected, the greater the Pd content, the lower the dispersion. Concerning on the precursor salt, the catalyst 1PdCl showed 3.5-fold more dispersion than 1PdN. When both steps were compared, two distinct groups of catalysts appeared. All the catalysts, except 1PdCl, presented decrease in dispersion between the two steps of the reduction–oxidation cycle. The values in Table 1 showed that the change of dispersion between steps was influenced by the Pd content, and the catalysts 15PdCl and 1PdN presented the largest differences. For the catalyst 1PdCl, the dispersion was not altered, despite the change observed in the TPR profiles between the first and the second cycle steps. Some catalysts were also analyzed by static H2 chemisorption in an ASAP 2000C Micromeritics equipment, in the first reduction–oxidation cycle. The dispersions were 43, 33 and 14% for the catalysts 5PdCl, 10PdCl and 1PdN, respectively. The results were about 25% smaller than the dynamic method (Table 1). 3.2. X-ray photoelectron spectroscopy (XPS) XPS results of the catalysts 1PdCl, 1PdN and 15PdCl are presented in Table 2, including values for Pd 3d5/2 binding energies, full width at half medium (FWHM) and atomic ratios of Pd 3d/Al 2p and Cl 2p/Pd 3d. The spectra of the catalysts are shown in Fig. 2. Alumina presented an atomic ratio Cl/Al of 1.16×10−2 . It explains the chlorine observed for the catalyst 1PdN, prepared with Pd(NO3 )2 . The binding energy of Pd 3d5/2 was compared with values from the literature. In the catalyst 1PdN it was near metallic palladium (335.2 eV), according to Kumar et al. [25], indicating that partial reduction
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Fig. 1. TPR profiles for the reduction–oxidation cycle (a: 1st and b: 2nd step) CS∗ =chart speed.
occurred during the analysis, after 50 scans. For the catalyst 15PdCl, the binding energy was similar to PdO (336.3 eV), indicating that no palladium reduction occurred at the same time of analysis. The differ-
ent behavior of the catalysts must be due to the surface chlorine contents (Cl/Pd ratio) in both catalysts. The catalyst 1PdCl presented a binding energy between PdO (336.3 eV) and PdCl2 (337.8 eV),
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Table 2 XPS analyses for 50 and 200 scans Catalyst
Pd 3d/Al 2p (×102 )
Pd 3d5/2 (eV) Binding energy
1PdCl 15PdCl 1PdN a
FWHM
Cl 2p/Pd 3d (×102 )
50 scans
50 scans
200 scans
50 scans
200 scans
336.9 336.5 335.6
336.0 336.0 335.2
2.9 2.4 1.9
2.7 2.7 2.1
0.58 3.25 0.57
4.00 1.46 1.00a
Alumina: Cl/Al=1.16×10−2 .
indicating the presence of complex species, PdO+ [Pdx Oy Clz ]. This assignment was also supported by the large full width at half medium (FWHM) of Pd 3d5/2 in this catalyst (2.9 eV). Finally, the chlorine enrichment of the 1PdCl catalyst surface was remarkable compared with other catalysts, and it may explain its different behavior. Another procedure of XPS analysis was to evaluate the behavior of the catalysts with prolonged exposition to X-rays. The results of these studies are presented in Table 2 and Fig. 2. Comparing the values for 50 and 200 scans, it can be observed a decrease in the binding
energy of Pd 3d5/2 with the time of exposition for all samples. The binding energy for the catalyst 1PdN (335.2 eV) was the same obtained by Kumar et al. [25] for metallic palladium, showing that palladium was completely reduced after greater time of analysis (200 scans for Pd 3d). For the catalysts 1PdCl and 15PdCl, the energies were very close to the one obtained for palladium oxide (336.3 eV). However, a shift in the binding energy of Pd 3d5/2 towards smaller values was verified. Thus, a modification of the palladium species occurred during the analysis. This hypothesis was supported by
Fig. 2. XPS analysis of Pd/Al2 O3 catalysts after 50 and 200 scans.
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Fig. 3. DRS analysis of the Pd/Al2 O3 catalysts.
the width of both samples (2.7 eV), bigger than that one observed for the catalyst 1PdN (2.1 eV). 3.3. Diffuse reflectance spectroscopy (DRS) Fig. 3 presents DRS spectra of the catalysts diluted with the support. Gaussian deconvolution was used in order to discriminate the bands, reported with the respective assignments in Table 3. The standard PdO showed a band in the charge transfer region at 332 nm (Pd→O), and two other bands in the d–d transition region at 404 and 490 nm. This spectrum resembled that obtained by Lyubovsky and Pfefferle [26], with a sample of Pd(NO3 )2 deposited over a pure ␣-alumina plate and calcined.
The catalysts 5PdCl, 10PdCl and 15PdCl showed similar spectra, resembling that of bulk PdO. The increase in Pd content did not change the spectra significantly. These catalysts showed bands around 330–346 and 399–408 nm, ascribed to PdO, in the charge transfer and d–d transition regions, respectively. The band at 483/490 nm, observed only in the catalyst 15PdCl and in the standard PdO, can be ascribed to bulk PdO particles without interaction with the support. The catalyst 1PdCl showed a distinct spectrum from other samples. The band at 284 nm is ascribed, according to Bozon–Verduraz et al. [27–29], to a superficial complex of Pd and chlorine, that can be reported as Pdx Oy Clz . This spectrum also presented a band at 206 nm and a shoulder at 246 nm. The former was
Table 3 DRS results of Pd/Al2 O3 catalysts Catalyst
Dilution catalyst:support
Wavelength (nm)
Attribution [27–29]
1PdCl 5PdCl 10PdCl 15PdCl 1PdN PdO bulk
1:1 1:9 1:19 1:29 1:1 –
206, 331, 346, 330, 314, 332,
Pdx Oy Clz , PdO PdO PdO PdO PdO Bulk PdO
(246), 284, 403 399 408 406, 483 411 404, 490
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observed but not assigned by Rakay et al. [28] and it can be associated to the charge transfer (Pd→Cl) of PdCl2 . This author also reported the shoulder at 246 nm after heating a PdCl2 /Al2 O3 catalyst in oxygen, assigned as the charge transfer of Pd→O. The catalyst 1PdCl also presented a band in the d–d transition region at 403 nm attributed to the PdO, following the standard spectrum. Finally, the catalyst 1PdN presented two bands, at 314 and 411 nm. According to the standard PdO spectrum, the second band is associated with PdO. The other one can be associated to the contribution of the band of PdO (332 nm) and of the nitrate ions adsorbed on the support, which gives a strong band at 300 nm [24]. In fact, this catalyst presented a N/Al ratio of 0.86×10−2 in the XPS analysis, showing that, even after calcination, the nitrogen from the precursor salt can be detected. 3.4. Methylcyclopentane hydrogenolysis The results of methylcyclopentane (MCP) hydrogenolysis are presented in Table 4. The behavior of catalytic activity versus H2 chemisorption can be observed in Fig. 4. The reaction was sensitive to the precursor salt and to the Pd content in PdCl2 /Al2 O3 .
Table 4 Selectivities for 2MP, 3MP, nH, HC and nH/2MP ratio Catalyst
1PdCl 5PdCl 10PdCl 15PdCl 1PdN a
TONa (h−1 )
141 39 28 28 24
Selectivity (%)
nH/2MP
2MP
3MP
nH
HC
28 26 32 32 51
43 44 41 39 35
17 15 17 18 14
11 15 10 11 0
0.6 0.6 0.5 0.6 0.3
Initial activity per site.
The order of activity per mass of Pd was 1PdCl>5PdCl>10PdCl>15PdCl∼1PdN (Fig. 4). It can be observed that the deactivation occurred for all catalysts. However, this effect was more prominent for the first 25 min of reaction, being decreased until the end of the analysis. Thus, the data of selectivity were compared after 50 min of reaction, when stability was achieved. At this point, the catalysts prepared with PdCl2 presented the same conversion, around 20%. The catalysts 15PdCl and 1PdN, which presented the same dispersion, did not show distinction in the catalytic activity normalized by palladium content. In the reaction with Pd/Al2 O3 , besides methylcyclopentane (MCP) hydrogenolysis products, 2-methyl-
Fig. 4. Methylcyclopentane hydrogenolysis with Pd/Al2 O3 .
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pentane (2MP), 3-methylpentane (3MP) and n-hexane (nH), the formation of hydrocracking (HC) products was also observed. We did not observe cyclopentane or products of ring enlargement, such as cycloexane and benzene. The selectivity results for the catalysts prepared with PdCl2 presented a ratio nH/2MP lower than 1, suggesting an intermediate products distribution between the selective (nH/2MP=0) and the non-selective (nH/2MP=1) mechanisms observed in Pt/Al2 O3 catalysts [11]. The main product obtained was 3MP, followed by 2MP, nH and hydrocracking products (HC). The diversity in particle size for the Pd catalysts did not result important changes in the distribution of the products. The amount of hydrocracking products (HC) was very considerable in this study. In the temperature of 573 K, the selectivity of HC was about 12% for the catalysts prepared from PdCl2 . The catalyst 1PdN showed the lowest activity per active site (TON) in the same reaction conditions (Table 4). Despite the difference of activity between catalysts from nitrate and chloride, the catalyst 1PdN did not present hydrocracking products and 2MP was the main product.
4. Discussion The study of reduction–oxidation cycle resulted in sintering of the catalysts prepared from chloride and nitrate of palladium, except for the catalyst 1PdCl. The stability of the dispersion in this case can be explained by the formation of surface complex species, like Pdx Oy Clz , detected by XPS and DRS. These species have more interactions with the support than the initial PdO [30,31], stabilizing the dispersion. The results of characterization suggested the presence of surface complex species, like Pdx Oy Clz , in the catalyst 1PdCl. In the XPS analysis, the binding energy of Pd 3d5/2 was situated at 336.9 eV, an intermediate value between the binding energies of PdO (336.3 eV) and PdCl2 (337.8 eV). In addition, the full width at half medium (FWHM) of this band, the largest of all samples analyzed, supports the presence of Pdx Oy Clz complex species in this catalyst. In the analysis of DRS, catalyst 1PdCl showed a distinct spectrum with bands at 206, (246), 284 and 403 nm. The assignment of these bands was done by
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many authors [27–29]. Thus, the bands in the charge transfer region, 206 and 284 nm, are associated to complexes where chlorine is in the co-ordination sphere of the palladium oxide forming species, represented by Pdx Oy Clz . Complex species of Pdx Oy Clz has been reported in the literature [27,31] in palladium catalysts prepared using PdCl2 and its formation has been associated to an increase in dispersion. Shen et al. [30] observed the formation of a bridged complex structure of chlorine and palladium in Pd–Mg/SiO2 catalysts, ascribing the improved dispersion to this complex, followed by the stabilization of the palladium surface structure, when compared to the catalysts without chlorine. Simone et al. [31] verified, by electronic microscopy, that the Pd/Al2 O3 catalysts prepared with PdCl2 presented lower particle size (20–40 Å) than that one prepared with Pd(NO3 )2 ·2H2 O (33–100 Å). Another important occurrence was noted in the XPS analysis about the tendency of photoreduction of palladium oxide to the metallic form, reflected in the shift of the binding energy and in the large FWHM of the samples, as a result of the prolonged exposition to the X-rays and/or high vacuum. The results suggested that the catalysts 1PdCl, 1PdN and 15PdCl showed distinct photoreduction behavior. The catalyst 1PdN presented partial reduction to Pd0 (335.6 eV, FWHM=1.9 eV) after 50 scans and total reduction (335.2 eV, FWHM=2.1 eV) after 200 scans. For the catalyst 15PdCl, the reduction of PdO was not verified after 50 scans (336.5 eV, FWHM=2.4 eV). However, a partial reduction occurred after 200 scans (336.0 eV, FWHM=2.7 eV). The surface chlorine content can answer the difference between catalysts, detected by XPS. In the 1PdN catalyst, the atomic ratio Cl/Al was 0.57×10−2 , while for 15PdCl this value was 4.73×10−2 . Thus, the greatest amount of surface chlorine in the catalyst 15PdCl can stabilize palladium species. Nevertheless, the presence of complex species Pdx Oy Clz was not verified by DRS or XPS for this catalyst. The catalyst 1PdCl presented a distinct behavior in the XPS analysis. For 50 scans, the reduction of PdO was not verified (336.9 eV, FWHM=2.9 eV). Probably, the presence of surface complexes Pdx Oy Clz , more stable in the alumina surface than the palladium oxide, increases the resistance to reduction. In fact, in the TPR analyses this catalyst always presented
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reduction at higher temperatures than the other ones, in the first and in the second cycle of reduction–oxidation. For 200 scans, the catalyst 1PdCl presented a similar result to the 15PdCl. The binding energy (336.0 eV) and the FWHM (2.7 eV) were the same in both samples, allowing one to verify the species composition (Pd0 +PdO). So, the prolonged exposition to the radiation and/or vacuum can decompose the Pdx Oy Clz species in the catalyst 1PdCl, and partial reduction can take place. Bozon–Verduraz et al. [27] observed that, during XPS analysis, the reduction of palladium chloride to Pd0 occurred after some hours. Légaré et al. [32] and Peuckert et al. [33], working with palladium films, verified the decomposition of PdO–Pd0 after hours in high vacuum conditions. While the first authors observed partial decomposition at room temperature after 12 h, Peuckert only verified such result up to 420 K. This author reported that PdO was stable at high vacuum at 420 K. Between 420 and 720 K, an intermediate stoichiometry was obtained: PdO0.7 for films and PdO0.4 for powder. The results of the methylcyclopentane hydrogenolysis, classified as sensible to the structure for Pt/Al2 O3 , revealed a difference in the turnover frequencies of 5-fold between the catalysts 1PdCl and 15PdCl (Table 4). For the catalysts prepared with PdCl2 , the predominant product was 3-methylpentane. In the dispersion range of 17–71% (Table 2), the distribution of the products did not change significantly in comparable conversion conditions. Similarly, other authors [15,18] did not observe change on the products distribution for Pd/Al2 O3 catalysts with the metallic dispersion. The selectivities for the three main products of MCP hydrogenolysis, 2MP, 3MP and n-hexane, resulted in an intermediate distribution between the two extreme mechanisms on Pt/Al2 O3 [11]. However, we observed 3MP as the main product, suggesting that other reactions than hydrogenolysis are taking place, like isomerization and cracking. Indeed, our results suggest that three main reaction pathways provide the product distribution observed: the methylcyclopentane hydrogenolysis, the 2-methylpentane isomerization and hydrocracking (Scheme 1). The hydrocracking products distribution, reported in Table 5, contributes to explain the total distribution of products observed (Table 4). Le Normand et al. [15] and Hajek et al. [19] reported n-pentane as the main
Scheme 1. MCP and 2MP reaction pathways.
product of the hydrocracking of 2-methylpentane and n-hexane, while i-pentane was the main product of 3-methylpentane hydrocracking. We observed mainly methane, n-butane and n-pentane as hydrocracking products, but i-pentane was not observed in the same conditions. In this case, there was not hydrocracking of 3-methylpentane. It is possible that isomerization of 2-methylpentane to 3-methylpentane also contribute to the high 3-methylpentane selectivity. In fact, Le Normand [15] observed a small enhancement in 3-methylpentane at the expenses of 2-methylpentane, relative to the theoretical distribution (50% 2MP, 25% 3MP and 25% n-hexane). This fact was explained by the higher stability of the intermediary complex π -olefin-σ alkyl 1,2-5 of 3-methylpentane than that of 2-methylpentane. The presence of chlorine is also related to the hydrocracking reaction, resulting in the formation of C1 –C5 products [12,13], although some authors [16,30] reported that this reaction is negligible at low conversions and temperatures lower than 573 K. However, the presence of chlorine causes the formation of acidic sites, that can enhance the cracking activity. The precursor salt affected activity and selectivity. The catalyst 1PdN, prepared with palladium nitrate, Table 5 Distribution of the hydrocracking products (HC) Catalyst Methane Ethane Propane i-Butane n-Butane n-Pentane 1PdCl 5PdCl 10PdCl 15PdCl
6.2 7.6 6.0 6.0
0.1 0.2 – –
– 0.1 – –
0.2 0.3 – –
2.4 2.7 1.8 1.9
2.5 4.0 2.3 3.1
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presented 2MP as the main product (Table 4). In this case, no hydrocracking products or cyclopentane were observed. Le Normand et al. [15,16] compared the activity for MCP hydrogenolysis and 2MP isomerization in catalysts prepared with palladium chloride and nitrate as precursors. The results showed that the activity for these reactions was greater for catalysts prepared with chloride. This behavior was explained in terms of the formation of a distinct Pd site in the presence of chlorine. The authors suggested that this site was settled at the interface of Pd and the alumina support with a chlorine bridge [16]. On the other hand, in the catalyst from nitrate it would just exist the classic sites of palladium in the surface of the support.
5. Conclusions Pd/Al2 O3 catalysts submitted to a reduction– oxidation cycle revealed sinterization of the palladium particles in the 5PdCl, 10PdCl, 15PdCl and 1PdN. These catalysts presented substantial reduction at 298 K and lower dispersion, comparing the results of the first and the second steps of the cycle. The catalyst 1PdCl did not show change of dispersion, when submitted to the cycle. The stability of this sample was attributed to the presence of complex species Pdx Oy Clz , identified by XPS and DRS. In this catalyst, palladium is well dispersed in the surface, influenced by the formation of the complex. The interaction of the metal particles with the support in this catalyst was well characterized by TPR, with a distinct behavior from other catalysts. The behavior of the catalysts prepared with PdCl2 to methylcyclopentane hydrogenolysis did not change with metallic dispersion in terms of selectivity. The relative high selectivity to 3MP was explained in terms of hydrocracking and isomerization of 2-methylpentane. The activity and selectivity of the catalysts prepared with chloride were distinct from that prepared with nitrate.
Acknowledgements A.B. Gaspar is grateful to CNPq (Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnológico, Brazil) for the scholarship received during this work.
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Dispersion stability and methylcyclopentane hydrogenolysis in Pd/Al2 O3 catalysts A.B. Gaspar, L.C. Dieguez∗ NUCAT-PEQ-COPPE, Universidade Federal do Rio de Janeiro, C.P. 68502, CEP 21945-970, Rio de Janeiro, Brazil Received 24 September 1999; received in revised form 5 January 2000; accepted 6 January 2000
Abstract Pd/Al2 O3 catalysts with different palladium contents and precursor salts have been submitted to a reduction–oxidation cycle and their dispersions have been measured. The catalysts presented a decrease in dispersion, except for the catalyst with 1 wt.% Pd prepared with PdCl2 , which remained constant. The presence of complex species of palladium and chlorine, identified by diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) analyses, can be responsible for the stability of the dispersion. The precursor salt affected the activity and selectivity in the methylcyclopentane reaction, but the selectivity was rather constant for different palladium contents in the catalysts prepared with PdCl2 . These last catalysts also showed hydrocracking reactions, attributed to the significant content of residual chlorine. These reactions changed considerably the distribution of the methylcyclopentane hydrogenolysis products compared to the literature. The catalyst with 1 wt.% Pd, prepared with Pd(NO3 )2 , showed less activity and a distinct distribution of the products, in which no light products were observed. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Pd/Al2 O3 ; Dispersion; TPR; XPS; DRS; Methylcyclopentane hydrogenolysis
1. Introduction Palladium-based catalysts have been used in the petrochemical (hydrogenation, dehydrogenation and oxidation) [1] and automotive (autoexhaust catalytic converters) industries [2]. Recent developments about the latter application have focused on the replacement of the noble metals, i.e. platinum and rhodium, by palladium, which is less expensive and more abundant. However, the use of such catalytic system in drastic conditions, like high temperatures, can cause a loss of activity due to sintering of the metallic particles. Several authors [3–9] have studied the behavior of metal∗ Corresponding author. Fax: +55-21-5907135. E-mail address: [email protected] (L.C. Dieguez)
lic particles of palladium and platinum supported on alumina when submitted to reduction and oxidation, aiming either to redisperse sintered catalysts or to increase the dispersion of new ones. Ruckenstein and Chen [3,4] studied the behavior of thin films of Al2 O3 supported palladium crystallites heating in O2 and H2 at 623, 773, 1023 and 1193 K. When heated in oxygen, spread palladium particles showed to be dependent on temperature and crystallite size. At 623 K, small crystallites spread, but the large ones formed toruslike shapes and no spread was observed. At higher temperatures, size effect seems to be less important. At 773 K, almost all the crystallites spread to irregular shapes and contain cavities of different sizes. At 1023 K, the effect of spreading was similar but not at the same extent as that heated at
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773 K. The authors explained the results by using the interfacial phenomena. The spreading, more effective at 773 K, occurs by the formation of pits and cavities on the crystallites. This formation is caused by a difference of interfacial tension between the palladium oxide and the unoxidized metal. In other study, using hydrogen at 923, 998 and 1073 K, Ruckenstein and Chen [5] observed that sintering of the palladium particles might occur via migration and coalescence, direct ripening and/or Ostwald ripening mechanisms. Finally, the authors [6] exposed the alumina-supported palladium crystallites to an alternative heating procedure in O2 at 1023 K, and H2 at 823 or 873 K. The phenomena observed was reported as spreading when heating in O2 , and sintering when heating in H2 . Spreading and sintering were explained as interfacial tension gradient and migration-coalescence processes, respectively. Lieske and Völter [7] studied a 0.6 wt.% Pd/Al2 O3 catalyst prepared with Pd(NO3 )2 and treated in oxygen in the temperature range of 773–1173 K, after a severe presinterization in argon. Redispersion was observed at about 973 K, but a more pronounced effect was reported at 1173 K, which is 30 K greater than the temperature of PdO decomposition in O2 [8]. The phenomenon was explained by the formation of a two-dimensional surface phase [PdO]sc during oxidation at high temperatures. This species would be responsible for the spreading. Similar works have been reported for platinum catalysts. Lieske et al. [9] prepared Pt/Al2 O3 catalysts with and without chlorine and observed that redispersion only occurs in the presence of chlorine. The catalyst, initially chlorine-free, only redisperses when chlorine is added. Surface complex species formed during oxidation step, like [PtIV Ox Cly ]s, identified by DRS analysis, seem to be the redispersing agent of the molecular migration mechanism. Lee and Kim [10] prepared Pt catalysts over ␥-Al2 O3 , TiO2 and SiO2 and carried out a reduction– oxidation cycle, using H2 and O2 . For Pt/Al2 O3 samples the redispersion was more effective at 793 K in O2 . The addition of chlorine brought about a significant increase in dispersion. The molecular migration mechanism was used to explain the redispersion, where surface complex species are the agents of the process.
The hydrogenolysis of methylcyclopentane is well characterized for Pt/alumina catalysts [11–14], but fewer works have been published about Pd/alumina, mainly for catalysts with large particles [15–19]. This reaction has also been used in the characterization of the Pd/zeolite system [20–22], but the detection of benzene and cycloexane, as products of a distinct route of ring enlargement reaction, makes a comparative analysis difficult. For platinum catalysts, this reaction produces 2-methylpentane (2MP), 3-methylpentane (3MP) and n-hexane (nH), but both selectivity and activity depend on the size of metal particles. Gault [11] showed that, for high dispersion Pt/Al2 O3 catalysts, the mechanism of the hydrogenolysis is non-selective and the product distribution is 40% of nH, 40% of 2MP and 20% of 3MP. For low dispersion catalysts, the mechanism is selective, resulting in 67% of 2MP and 33% of 3MP. In this case, no n-hexane is formed. The product distribution on palladium catalysts differs from platinum ones, being intermediate between the two extreme mechanisms [15]. On Pd/Al2 O3 catalysts the reaction gives 42–49% 2MP, 25–28% 3MP, 21–24% nH and 2–4% CP (cyclopentane). This distribution is rather insensitive to the calcination temperature and metallic dispersion. However, the influence of acidity and nature of the support are reported [16,17]. Hydrocracking reaction has been noticed in Pt/Al2 O3 and associated with the presence of chlorine in the reactional medium [12,13]. This specific vicinity causes the production of acidic sites in the support and Pt–Cl bonds instead of Pt–O ones. Extensive cracking in Pd/Al2 O3 catalysts is reported to be negligible, being around 3% of the products, even in high conversion levels [15–17,19]. The objective of this work was to investigate the behavior of the dispersion of Pd/alumina catalysts when submitted to a reduction–oxidation-reduction cycle. A wide range of metallic dispersions was obtained varying the Pd content and precursor salt, maintaining pre-treatment conditions constant. H2 chemisorption experiments were used to measure the dispersion after each reduction step, and diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS) to characterize the palladium species. The catalysts were tested in the methylcyclopentane hydrogenolysis and the distribution of the products was also evaluated.
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Table 1 Catalysts data and reduction–oxidation cycle results Catalyst
1PdCl 5PdCl 10PdCl 15PdCl 1PdN
Pd content (wt.%)
0.91 5.09 9.32 14.52 0.85
Treatment cycle Pd reduction at 298 K (%)
Pd dispersion (%)
1st step
2nd step
1st step
2nd step
0 2 84 100 100
39 98 99 100 100
71 62 43 17 19
74 48 38 11 10
2. Experimental 2.1. Catalyst preparation Pd/Al2 O3 catalysts were prepared by the incipient wetness impregnation method using ␥-Al2 O3 Engelhard AL-3916P (156 m2 /g and 0.46 cm3 /g). Catalysts with 1, 5, 10 and 15 wt.% Pd were prepared using PdCl2 (Riedel de-Haën, 99.99%) as precursor salt. The Pd(NO3 )2 (Sigma, 99.9%) was used for the preparation of 1 wt.% Pd catalyst. After impregnation, the catalysts were dried at 393 K for 16 h and calcined at 773 K for 4 h. Table 1 summarizes the catalysts, metal contents and sample codes. 2.2. Reduction–oxidation cycle and H2 chemisorption The reduction–oxidation cycle was performed in a Pyrex U-tube reactor with an on line thermal conductivity detector. The catalysts were submitted to a sequence of reduction (TPR) from 298 to 773 K (10 K/min) with H2 /Ar stream (1.6% H2 , 30 cm3 /min — first step). In an intermediate procedure, the sample was oxidized by passing a mixture of O2 /He (2.2% O2 , 30 cm3 /min) from 298 to 773 K (10 K/min). Finally, a second reduction was done (second step). Pd reduction at 298 K was calculated discounting the peak at 343 K, ascribed to -PdH desorption [23], from the reduction peak at 298 K. The amount of catalyst for each experiment was calculated in order to have 10 mg of palladium. Under these conditions, the analyses were not limited by diffusional effects. After each reduction cycle, the sample was outgassed under argon flow at 773 K for 30 min, cooled
to 343 K and then H2 chemisorption measurements were performed. The amount of irreversible adsorbed H2 was measured using the frontal method at 343 K in order to avoid -PdH phase formation. Pure gases (99.5%) were passed through a molecular sieve filter before entering the reactor. 2.3. Characterization by XPS and DRS X-ray photoelectron spectroscopy (XPS) experiments were carried out in a Perkin–Elmer 257 spectrometer using Mg K␣ (E=1253.6 eV) as source. Vacuum in the spectrometer chamber was about 10−8 Torr. The C 1s signal at binding energy 284.6 eV was taken as the internal standard for reference. The order of the analysis was Pd 3d, C 1s, Al 2p, Cl 2p and O 1s. In order to verify the palladium photoreduction, the Pd 3d spectrum region was analyzed using 50 and 200 scans, corresponding to 12 and 48 min, respectively. Finally, the N 1s spectrum region was scanned for the catalyst prepared with Pd(NO3 )2 . The XPS spectra were adjusted by Gaussian curves to evaluate the relative concentration species. The intensities were corrected using sensibility factors from the literature [24]. The relative accuracy of the equipment was ±0.2 eV. Diffuse reflectance spectroscopy (DRS) was carried out in a Varian Cary 5 Spectrometer. The analyses were recorded in the 200–800 nm range (UV–VIS). The catalysts were diluted with Al2 O3 support to obtain the same Pd content. The reference for the experiments was the support of the catalysts. The standard used was PdO, obtained by decomposition of Pd(NO3 )2 in a furnace at 673 K.
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2.4. Activity measurements The methylcyclopentane (MCP) hydrogenolysis was investigated at 1 atm and 573 K in a Pyrex U-tube reactor, using 200 mg of catalyst. Before the reaction, the samples were reduced using a H2 /N2 (18% H2 , 30 cm3 /min) mixture at 773 K for 1 h (first step). A reaction mixture with H2 :MCP ratio of 18 was obtained by passing H2 (99.9%) through a saturator with MCP (Aldrich, 98%), maintained at 273 K by an ice/water bath. The overall time of reaction was 100 min. The reaction products were analyzed on line, each 25 min of reaction, by a Varian 2440 chromatograph equipped with a FID detector and a polypropileneglycol (25%) in Chromosorb W column.
3. Results 3.1. Reduction–oxidation cycle and H2 chemisorption The Pd contents, reduction–oxidation cycle results and Pd dispersion are presented in Table 1. The H2 consumption results are presented as the fraction (%) of Pd reduction at 298 K. However, all the catalysts presented total H2 consumption corresponding to the complete reduction of PdO to metallic Pd. The TPR profiles of the two steps are showed in Fig. 1a and b. In the first step, Fig. 1a, the catalyst 1PdCl did not present any reduction at 298 K, but a peak at 430 K was observed. The reduction profile of the 5PdCl sample displayed a small reduction peak at 298 K and two other peaks at 350 and 618 K. The catalyst 10PdCl showed a similar profile to 5PdCl, but with more H2 consumption at 298 K. The catalysts 15PdCl and 1PdN were very similar, exhibiting only one reduction peak at 298 K. Fig. 1b shows the reduction profiles of the second step. It showed quite similar profiles presenting reduction at 298 K. However, the catalyst 1PdCl presented reduction at 298 and also at 423 K. The reduction profiles of the 5PdCl, 10PdCl, 15PdCl and 1PdN were very similar, with H2 consumption at 298 K and H2 desorptions at 331, 331, 327 and 341 K, respectively. The 5PdCl and 10PdCl samples also presented very small peaks at 353 and 345 K. For the catalysts 15PdCl and 1PdN, the reduc-
tion was complete at 298 K, as observed in the first step of the cycle. Table 1 presents the fraction of Pd reduction at 298 K. In the first step, the catalysts 1PdCl and 5PdCl presented a similar behavior, without H2 consumption at 298 K, or a negligible one, respectively. The other catalysts presented almost total reduction in this stage. In the second step, while the catalysts 5PdCl, 10PdCl, 15PdCl and 1PdN showed total reduction at 298 K, the catalyst 1PdCl showed a distinct behavior, with only 39% of reduction in the same condition. The dispersion of the catalysts, obtained by H2 chemisorption, is presented in Table 1 and, as expected, the greater the Pd content, the lower the dispersion. Concerning on the precursor salt, the catalyst 1PdCl showed 3.5-fold more dispersion than 1PdN. When both steps were compared, two distinct groups of catalysts appeared. All the catalysts, except 1PdCl, presented decrease in dispersion between the two steps of the reduction–oxidation cycle. The values in Table 1 showed that the change of dispersion between steps was influenced by the Pd content, and the catalysts 15PdCl and 1PdN presented the largest differences. For the catalyst 1PdCl, the dispersion was not altered, despite the change observed in the TPR profiles between the first and the second cycle steps. Some catalysts were also analyzed by static H2 chemisorption in an ASAP 2000C Micromeritics equipment, in the first reduction–oxidation cycle. The dispersions were 43, 33 and 14% for the catalysts 5PdCl, 10PdCl and 1PdN, respectively. The results were about 25% smaller than the dynamic method (Table 1). 3.2. X-ray photoelectron spectroscopy (XPS) XPS results of the catalysts 1PdCl, 1PdN and 15PdCl are presented in Table 2, including values for Pd 3d5/2 binding energies, full width at half medium (FWHM) and atomic ratios of Pd 3d/Al 2p and Cl 2p/Pd 3d. The spectra of the catalysts are shown in Fig. 2. Alumina presented an atomic ratio Cl/Al of 1.16×10−2 . It explains the chlorine observed for the catalyst 1PdN, prepared with Pd(NO3 )2 . The binding energy of Pd 3d5/2 was compared with values from the literature. In the catalyst 1PdN it was near metallic palladium (335.2 eV), according to Kumar et al. [25], indicating that partial reduction
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245
Fig. 1. TPR profiles for the reduction–oxidation cycle (a: 1st and b: 2nd step) CS∗ =chart speed.
occurred during the analysis, after 50 scans. For the catalyst 15PdCl, the binding energy was similar to PdO (336.3 eV), indicating that no palladium reduction occurred at the same time of analysis. The differ-
ent behavior of the catalysts must be due to the surface chlorine contents (Cl/Pd ratio) in both catalysts. The catalyst 1PdCl presented a binding energy between PdO (336.3 eV) and PdCl2 (337.8 eV),
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Table 2 XPS analyses for 50 and 200 scans Catalyst
Pd 3d/Al 2p (×102 )
Pd 3d5/2 (eV) Binding energy
1PdCl 15PdCl 1PdN a
FWHM
Cl 2p/Pd 3d (×102 )
50 scans
50 scans
200 scans
50 scans
200 scans
336.9 336.5 335.6
336.0 336.0 335.2
2.9 2.4 1.9
2.7 2.7 2.1
0.58 3.25 0.57
4.00 1.46 1.00a
Alumina: Cl/Al=1.16×10−2 .
indicating the presence of complex species, PdO+ [Pdx Oy Clz ]. This assignment was also supported by the large full width at half medium (FWHM) of Pd 3d5/2 in this catalyst (2.9 eV). Finally, the chlorine enrichment of the 1PdCl catalyst surface was remarkable compared with other catalysts, and it may explain its different behavior. Another procedure of XPS analysis was to evaluate the behavior of the catalysts with prolonged exposition to X-rays. The results of these studies are presented in Table 2 and Fig. 2. Comparing the values for 50 and 200 scans, it can be observed a decrease in the binding
energy of Pd 3d5/2 with the time of exposition for all samples. The binding energy for the catalyst 1PdN (335.2 eV) was the same obtained by Kumar et al. [25] for metallic palladium, showing that palladium was completely reduced after greater time of analysis (200 scans for Pd 3d). For the catalysts 1PdCl and 15PdCl, the energies were very close to the one obtained for palladium oxide (336.3 eV). However, a shift in the binding energy of Pd 3d5/2 towards smaller values was verified. Thus, a modification of the palladium species occurred during the analysis. This hypothesis was supported by
Fig. 2. XPS analysis of Pd/Al2 O3 catalysts after 50 and 200 scans.
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247
Fig. 3. DRS analysis of the Pd/Al2 O3 catalysts.
the width of both samples (2.7 eV), bigger than that one observed for the catalyst 1PdN (2.1 eV). 3.3. Diffuse reflectance spectroscopy (DRS) Fig. 3 presents DRS spectra of the catalysts diluted with the support. Gaussian deconvolution was used in order to discriminate the bands, reported with the respective assignments in Table 3. The standard PdO showed a band in the charge transfer region at 332 nm (Pd→O), and two other bands in the d–d transition region at 404 and 490 nm. This spectrum resembled that obtained by Lyubovsky and Pfefferle [26], with a sample of Pd(NO3 )2 deposited over a pure ␣-alumina plate and calcined.
The catalysts 5PdCl, 10PdCl and 15PdCl showed similar spectra, resembling that of bulk PdO. The increase in Pd content did not change the spectra significantly. These catalysts showed bands around 330–346 and 399–408 nm, ascribed to PdO, in the charge transfer and d–d transition regions, respectively. The band at 483/490 nm, observed only in the catalyst 15PdCl and in the standard PdO, can be ascribed to bulk PdO particles without interaction with the support. The catalyst 1PdCl showed a distinct spectrum from other samples. The band at 284 nm is ascribed, according to Bozon–Verduraz et al. [27–29], to a superficial complex of Pd and chlorine, that can be reported as Pdx Oy Clz . This spectrum also presented a band at 206 nm and a shoulder at 246 nm. The former was
Table 3 DRS results of Pd/Al2 O3 catalysts Catalyst
Dilution catalyst:support
Wavelength (nm)
Attribution [27–29]
1PdCl 5PdCl 10PdCl 15PdCl 1PdN PdO bulk
1:1 1:9 1:19 1:29 1:1 –
206, 331, 346, 330, 314, 332,
Pdx Oy Clz , PdO PdO PdO PdO PdO Bulk PdO
(246), 284, 403 399 408 406, 483 411 404, 490
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observed but not assigned by Rakay et al. [28] and it can be associated to the charge transfer (Pd→Cl) of PdCl2 . This author also reported the shoulder at 246 nm after heating a PdCl2 /Al2 O3 catalyst in oxygen, assigned as the charge transfer of Pd→O. The catalyst 1PdCl also presented a band in the d–d transition region at 403 nm attributed to the PdO, following the standard spectrum. Finally, the catalyst 1PdN presented two bands, at 314 and 411 nm. According to the standard PdO spectrum, the second band is associated with PdO. The other one can be associated to the contribution of the band of PdO (332 nm) and of the nitrate ions adsorbed on the support, which gives a strong band at 300 nm [24]. In fact, this catalyst presented a N/Al ratio of 0.86×10−2 in the XPS analysis, showing that, even after calcination, the nitrogen from the precursor salt can be detected. 3.4. Methylcyclopentane hydrogenolysis The results of methylcyclopentane (MCP) hydrogenolysis are presented in Table 4. The behavior of catalytic activity versus H2 chemisorption can be observed in Fig. 4. The reaction was sensitive to the precursor salt and to the Pd content in PdCl2 /Al2 O3 .
Table 4 Selectivities for 2MP, 3MP, nH, HC and nH/2MP ratio Catalyst
1PdCl 5PdCl 10PdCl 15PdCl 1PdN a
TONa (h−1 )
141 39 28 28 24
Selectivity (%)
nH/2MP
2MP
3MP
nH
HC
28 26 32 32 51
43 44 41 39 35
17 15 17 18 14
11 15 10 11 0
0.6 0.6 0.5 0.6 0.3
Initial activity per site.
The order of activity per mass of Pd was 1PdCl>5PdCl>10PdCl>15PdCl∼1PdN (Fig. 4). It can be observed that the deactivation occurred for all catalysts. However, this effect was more prominent for the first 25 min of reaction, being decreased until the end of the analysis. Thus, the data of selectivity were compared after 50 min of reaction, when stability was achieved. At this point, the catalysts prepared with PdCl2 presented the same conversion, around 20%. The catalysts 15PdCl and 1PdN, which presented the same dispersion, did not show distinction in the catalytic activity normalized by palladium content. In the reaction with Pd/Al2 O3 , besides methylcyclopentane (MCP) hydrogenolysis products, 2-methyl-
Fig. 4. Methylcyclopentane hydrogenolysis with Pd/Al2 O3 .
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pentane (2MP), 3-methylpentane (3MP) and n-hexane (nH), the formation of hydrocracking (HC) products was also observed. We did not observe cyclopentane or products of ring enlargement, such as cycloexane and benzene. The selectivity results for the catalysts prepared with PdCl2 presented a ratio nH/2MP lower than 1, suggesting an intermediate products distribution between the selective (nH/2MP=0) and the non-selective (nH/2MP=1) mechanisms observed in Pt/Al2 O3 catalysts [11]. The main product obtained was 3MP, followed by 2MP, nH and hydrocracking products (HC). The diversity in particle size for the Pd catalysts did not result important changes in the distribution of the products. The amount of hydrocracking products (HC) was very considerable in this study. In the temperature of 573 K, the selectivity of HC was about 12% for the catalysts prepared from PdCl2 . The catalyst 1PdN showed the lowest activity per active site (TON) in the same reaction conditions (Table 4). Despite the difference of activity between catalysts from nitrate and chloride, the catalyst 1PdN did not present hydrocracking products and 2MP was the main product.
4. Discussion The study of reduction–oxidation cycle resulted in sintering of the catalysts prepared from chloride and nitrate of palladium, except for the catalyst 1PdCl. The stability of the dispersion in this case can be explained by the formation of surface complex species, like Pdx Oy Clz , detected by XPS and DRS. These species have more interactions with the support than the initial PdO [30,31], stabilizing the dispersion. The results of characterization suggested the presence of surface complex species, like Pdx Oy Clz , in the catalyst 1PdCl. In the XPS analysis, the binding energy of Pd 3d5/2 was situated at 336.9 eV, an intermediate value between the binding energies of PdO (336.3 eV) and PdCl2 (337.8 eV). In addition, the full width at half medium (FWHM) of this band, the largest of all samples analyzed, supports the presence of Pdx Oy Clz complex species in this catalyst. In the analysis of DRS, catalyst 1PdCl showed a distinct spectrum with bands at 206, (246), 284 and 403 nm. The assignment of these bands was done by
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many authors [27–29]. Thus, the bands in the charge transfer region, 206 and 284 nm, are associated to complexes where chlorine is in the co-ordination sphere of the palladium oxide forming species, represented by Pdx Oy Clz . Complex species of Pdx Oy Clz has been reported in the literature [27,31] in palladium catalysts prepared using PdCl2 and its formation has been associated to an increase in dispersion. Shen et al. [30] observed the formation of a bridged complex structure of chlorine and palladium in Pd–Mg/SiO2 catalysts, ascribing the improved dispersion to this complex, followed by the stabilization of the palladium surface structure, when compared to the catalysts without chlorine. Simone et al. [31] verified, by electronic microscopy, that the Pd/Al2 O3 catalysts prepared with PdCl2 presented lower particle size (20–40 Å) than that one prepared with Pd(NO3 )2 ·2H2 O (33–100 Å). Another important occurrence was noted in the XPS analysis about the tendency of photoreduction of palladium oxide to the metallic form, reflected in the shift of the binding energy and in the large FWHM of the samples, as a result of the prolonged exposition to the X-rays and/or high vacuum. The results suggested that the catalysts 1PdCl, 1PdN and 15PdCl showed distinct photoreduction behavior. The catalyst 1PdN presented partial reduction to Pd0 (335.6 eV, FWHM=1.9 eV) after 50 scans and total reduction (335.2 eV, FWHM=2.1 eV) after 200 scans. For the catalyst 15PdCl, the reduction of PdO was not verified after 50 scans (336.5 eV, FWHM=2.4 eV). However, a partial reduction occurred after 200 scans (336.0 eV, FWHM=2.7 eV). The surface chlorine content can answer the difference between catalysts, detected by XPS. In the 1PdN catalyst, the atomic ratio Cl/Al was 0.57×10−2 , while for 15PdCl this value was 4.73×10−2 . Thus, the greatest amount of surface chlorine in the catalyst 15PdCl can stabilize palladium species. Nevertheless, the presence of complex species Pdx Oy Clz was not verified by DRS or XPS for this catalyst. The catalyst 1PdCl presented a distinct behavior in the XPS analysis. For 50 scans, the reduction of PdO was not verified (336.9 eV, FWHM=2.9 eV). Probably, the presence of surface complexes Pdx Oy Clz , more stable in the alumina surface than the palladium oxide, increases the resistance to reduction. In fact, in the TPR analyses this catalyst always presented
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reduction at higher temperatures than the other ones, in the first and in the second cycle of reduction–oxidation. For 200 scans, the catalyst 1PdCl presented a similar result to the 15PdCl. The binding energy (336.0 eV) and the FWHM (2.7 eV) were the same in both samples, allowing one to verify the species composition (Pd0 +PdO). So, the prolonged exposition to the radiation and/or vacuum can decompose the Pdx Oy Clz species in the catalyst 1PdCl, and partial reduction can take place. Bozon–Verduraz et al. [27] observed that, during XPS analysis, the reduction of palladium chloride to Pd0 occurred after some hours. Légaré et al. [32] and Peuckert et al. [33], working with palladium films, verified the decomposition of PdO–Pd0 after hours in high vacuum conditions. While the first authors observed partial decomposition at room temperature after 12 h, Peuckert only verified such result up to 420 K. This author reported that PdO was stable at high vacuum at 420 K. Between 420 and 720 K, an intermediate stoichiometry was obtained: PdO0.7 for films and PdO0.4 for powder. The results of the methylcyclopentane hydrogenolysis, classified as sensible to the structure for Pt/Al2 O3 , revealed a difference in the turnover frequencies of 5-fold between the catalysts 1PdCl and 15PdCl (Table 4). For the catalysts prepared with PdCl2 , the predominant product was 3-methylpentane. In the dispersion range of 17–71% (Table 2), the distribution of the products did not change significantly in comparable conversion conditions. Similarly, other authors [15,18] did not observe change on the products distribution for Pd/Al2 O3 catalysts with the metallic dispersion. The selectivities for the three main products of MCP hydrogenolysis, 2MP, 3MP and n-hexane, resulted in an intermediate distribution between the two extreme mechanisms on Pt/Al2 O3 [11]. However, we observed 3MP as the main product, suggesting that other reactions than hydrogenolysis are taking place, like isomerization and cracking. Indeed, our results suggest that three main reaction pathways provide the product distribution observed: the methylcyclopentane hydrogenolysis, the 2-methylpentane isomerization and hydrocracking (Scheme 1). The hydrocracking products distribution, reported in Table 5, contributes to explain the total distribution of products observed (Table 4). Le Normand et al. [15] and Hajek et al. [19] reported n-pentane as the main
Scheme 1. MCP and 2MP reaction pathways.
product of the hydrocracking of 2-methylpentane and n-hexane, while i-pentane was the main product of 3-methylpentane hydrocracking. We observed mainly methane, n-butane and n-pentane as hydrocracking products, but i-pentane was not observed in the same conditions. In this case, there was not hydrocracking of 3-methylpentane. It is possible that isomerization of 2-methylpentane to 3-methylpentane also contribute to the high 3-methylpentane selectivity. In fact, Le Normand [15] observed a small enhancement in 3-methylpentane at the expenses of 2-methylpentane, relative to the theoretical distribution (50% 2MP, 25% 3MP and 25% n-hexane). This fact was explained by the higher stability of the intermediary complex π -olefin-σ alkyl 1,2-5 of 3-methylpentane than that of 2-methylpentane. The presence of chlorine is also related to the hydrocracking reaction, resulting in the formation of C1 –C5 products [12,13], although some authors [16,30] reported that this reaction is negligible at low conversions and temperatures lower than 573 K. However, the presence of chlorine causes the formation of acidic sites, that can enhance the cracking activity. The precursor salt affected activity and selectivity. The catalyst 1PdN, prepared with palladium nitrate, Table 5 Distribution of the hydrocracking products (HC) Catalyst Methane Ethane Propane i-Butane n-Butane n-Pentane 1PdCl 5PdCl 10PdCl 15PdCl
6.2 7.6 6.0 6.0
0.1 0.2 – –
– 0.1 – –
0.2 0.3 – –
2.4 2.7 1.8 1.9
2.5 4.0 2.3 3.1
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presented 2MP as the main product (Table 4). In this case, no hydrocracking products or cyclopentane were observed. Le Normand et al. [15,16] compared the activity for MCP hydrogenolysis and 2MP isomerization in catalysts prepared with palladium chloride and nitrate as precursors. The results showed that the activity for these reactions was greater for catalysts prepared with chloride. This behavior was explained in terms of the formation of a distinct Pd site in the presence of chlorine. The authors suggested that this site was settled at the interface of Pd and the alumina support with a chlorine bridge [16]. On the other hand, in the catalyst from nitrate it would just exist the classic sites of palladium in the surface of the support.
5. Conclusions Pd/Al2 O3 catalysts submitted to a reduction– oxidation cycle revealed sinterization of the palladium particles in the 5PdCl, 10PdCl, 15PdCl and 1PdN. These catalysts presented substantial reduction at 298 K and lower dispersion, comparing the results of the first and the second steps of the cycle. The catalyst 1PdCl did not show change of dispersion, when submitted to the cycle. The stability of this sample was attributed to the presence of complex species Pdx Oy Clz , identified by XPS and DRS. In this catalyst, palladium is well dispersed in the surface, influenced by the formation of the complex. The interaction of the metal particles with the support in this catalyst was well characterized by TPR, with a distinct behavior from other catalysts. The behavior of the catalysts prepared with PdCl2 to methylcyclopentane hydrogenolysis did not change with metallic dispersion in terms of selectivity. The relative high selectivity to 3MP was explained in terms of hydrocracking and isomerization of 2-methylpentane. The activity and selectivity of the catalysts prepared with chloride were distinct from that prepared with nitrate.
Acknowledgements A.B. Gaspar is grateful to CNPq (Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnológico, Brazil) for the scholarship received during this work.
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