Factors influencing selectivity in naphthalene hydrogenation over Au- and Pt–Au-supported catalysts

Applied Catalysis A: General 283 (2005) 165–175 www.elsevier.com/locate/apcata

Factors influencing selectivity in naphthalene hydrogenation over Au- and Pt–Au-supported catalysts B. Paweleca, A.M. Veneziab,*, V. La Parolaa,b, S. Thomasa, J.L.G. Fierroa a Instituto de Cata´lisis y Petroleoquı´mica, CSIC, c/ Marie Curie 2, Cantoblanco, 28049 Madrid, Spain Istituto per lo Studio dei Materiali Nanostrutturati, ISMN-CNR Sezione di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy

b

Received 6 October 2004; received in revised form 22 December 2004; accepted 4 January 2005 Available online 27 January 2005

Abstract The effects of the support (g-Al2O3 and SiO2) and of the preparation method (the reduction of metal precursors by ethanol in the presence of polyvinylpyrrolidone (PVP) versus impregnation (IMP)) on the activity and selectivity of the Au and Pt–Au catalysts in naphthalene hydrogenation (P = 2.0 MPa, T = 448 K, WHSV = 45.7 h
1) were studied. The physico-chemical characteristics of the catalysts were evaluated by X-ray diffraction, N2 adsorption–desorption isotherms, DRIFT spectroscopy of the adsorbed CO, thermogravimetric (TG) and X-ray photoelectron spectroscopy techniques. Activity data for the target reaction showed that the SiO2-supported catalysts are slightly better than the g-Al2O3-supported counterparts. The monometallic Au/Al catalyst prepared by the IMP method displayed comparable selectivity but larger activity (mmol s
1 molesAu(Pt)
1) than its homologue prepared by the PVP method. As derived from X-ray line broadening analysis, this is due to much better dispersion of gold particles when using the IMP method. For SiO2-supported catalysts prepared by PVP method, the initial activity followed the trend: 1Au > 1Pt–1Au > 2Pt. Both the monometallic Au/Si and 2Pt/Si exhibited quick deactivation contrary to the binary 1Pt–1Au/Si-pvp sample exhibiting higher activity at long time on stream. In this sample a synergy effect between Pt and Au was attributed to Pt50Au50 alloy formation. The factors controlling the selectivity in naphthalene hydrogenation on gold catalysts are discussed. # 2005 Elsevier B.V. All rights reserved. Keywords: Naphthalene hydrogenation; Silica; Alumina; Au–Pt catalysts

1. Introduction To comply with new diesel specifications the density of middistillate fuels needs to be lower than 845 kg m
3, whereas cetane number higher than 51 [1]. Saturation of naphthalene (NP) to decalin reduces the density and improves cetane number to some extent [2]. The total naphthalene saturation lowers fuel density to ca. 900 kg m
3 and yield a maximum cetane number of about 38. However, since both parameters are still below the new diesel specifications, the ring opening is practiced in hydrocracking of naphthenic molecules [2]. After the first study of Bond and coworkers in the 1960s on the use of gold catalysts for reactions involving H2 [3–6], only a little attention was paid to such reactions. The * Corresponding author. Tel.: +39 0916809372; fax: +39 0916809399. E-mail address: [email protected] (A.M. Venezia). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.01.004

reason for this lies in the fact that gold, with a completely filled Au d-band, has a limited capacity to dissociate H2 molecules [3]. Still with this unfavourable electron configuration, renewed interest of gold catalysts emerged recently [7–26]. This is because their use may well be profitable when the factors controlling intramolecular selectivity are more important than high hydrogenation (HYD) capability. Thus, the selective hydrogenation of a,b-unsaturated ketones [7], crotonaldehyde [9–12], 1,3butadiene [10,11,13] and acrolein [10] over gold catalysts to produce unsaturated products have been investigated. Concerning the hydrogenation of aromatics, earlier studies on the reaction of cyclohexane with hydrogen on gold films [14] and gold powder [15] demonstrated the simultaneous occurrence of the dehydrogenation and hydrogenation of cyclohexane. Hydrogenation reactions are generally considered structure-insensitive [26]. In case of such reactions, the activity of a metal is expected to

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change very little with the atomic structure of the surface and moderately with the nature of metal or as a result of alloy formation [27]. However, a marked structuresensitivity of the hydrogenation of acrolein, crotonaldehyde and 1,3-butadiene was observed by Claus and coworkers [10] with the extremely small gold particles. This was attributed to structural and electronic properties due to the quantum-size effect of sufficiently small gold particles [10]. Similarly, for hydrogenation of crotonaldehyde the structure-sensitivity of the Au/TiO2 catalysts possessing gold particles of ca. 2 nm was reported by Zanella et al. [12]. The dissociation of hydrogen on lowcoordinated sites of gold particles was proposed to be the rate-determining step of this reaction [12]. Contrary to the study by Claus and coworkers [10], Okumura et al. [11] observed that the hydrogenation of 1,3-butadiene over Au catalysts was almost structure-insensitive reaction in terms of the size effect of Au particles and the influence of metal oxides supports such as Al2O3, SiO2, TiO2 [11]. Moreover, it is known that addition of second metal can change drastically the catalytic properties of Au catalysts. In particular, the alloy formed between a platinum/ palladium and gold, changes the catalytic behaviour compared with the monometallic catalysts, increasing the selectivity toward desired products and/or decreasing the deactivation rate [28]. Thus, our recent study confirmed the inhibition of naphthalene over-hydrogenation after employing Pd–Au formulation [8]. This inhibition was attributed to the particular role of Pd [8], which is known to stop naphthalene hydrogenation to tetralin formation [4]. Similarly to bimetallic Pd–Au catalysts [8], the strong inhibition of the tetralin conversion to decalin in the presence of naphthalene in the feed was recently reported for Pt–Pd systems [29,30]. Tetralin conversion on Pt–Pd catalyst was only achieved after essentially the 2- and 3ring aromatics were converted [30]. The enhancement of activity in aromatics hydrogenation on the Pd–Au/g-Al2O3 system was explained assuming involvement of a geometric effect resulting from the dilution of the Pd ensemble in the AuPd alloy [31]. Moreover, even in case of no alloy formation, the Au can play a very important role in a bimetallic formulation by stabilising smaller particle of the more active metals from a group-VIII [8]. After addition of Au to Pt the formation of two kinds of particles, one with Au and Pt atoms and another with Au atoms only, were observed by Galvagno and Parravano [32]. Similarly, for the catalysts prepared by simultaneous impregnation of Al2O3 and SiO2 carriers with Au and Pt precursors Va´ zquez-Zavala et al. [28] concluded that the Au partially blocks the surface Pt atoms, changing the small metal particle morphology. As a continuation of our previous study reporting the inhibiting effect of Pd on the over-hydrogenation of naphthalene in Pd–Au systems [8], we investigate in here the effect of different supports and different preparation methods on the activity and selectivity in NP hydrogenation

of the gold catalysts, extending the study to binary Pt–Au formulation. In particular, as large amount of decalin improves density and cetane number in diesel fuels, the objective was to determine the factors influencing the naphthalene selectivity to decalin on Au catalysts at high hydrogen pressure (P = 2 MPa). To this aim, the Au catalysts were prepared by reduction of metal precursors by ethanol in the presence of the polyvinylpyrrolidone and by impregnation, using g-Al2O3 and SiO2 supports. The g-Al2O3 support was chosen in order to elucidate the influence of a possible metal-support interaction on the morphology of the formed Au0 phases, whereas the inert character of silica allowed for a better understanding of possible PtAu alloying effect. The structural and surface characterization of the samples was performed by means of X-ray diffraction (XRD), N2 adsorption–desorption isotherms, DRIFT spectroscopy of the adsorbed CO (DRIFT-CO), thermogravimetric (TG) and X-ray photoelectron spectroscopy (XPS) techniques.

2. Experimental 2.1. Catalyst preparation Commercial high-purity g-Al2O3 (Condea, Puralox NWa-155; pre-shaped granulates of ca. 1 mm) and sodium-free SiO2 (Aldrich, point of zero charge 3.8) materials were used as carriers. 2Au/Al-pvp, 1Au/Si-pvp, 2Pt/Si-pvp and 1Pt–1Au/Si-pvp catalysts were prepared by alcohol-reduction of metal precursors in the presence of the polymer poly(N-vinyl-2-pyrrolidone (PVP) (named hereafter as PVP-method) [33]. The preparation procedure has been described elsewhere [16,17]. In short, the appropriate quantities of AuCl3 (Aldrich) or PtCl2 (Alfa-Aesar) were dissolved in 400 ml of an ethanol water (1/1 v/v) solution containing the PVP (MW = 10,000). In the binary 1Pt–1Au/ Si-pvp sample, the simultaneous reduction of both metal precursors was employed. The weight ratio of the PVP over the metal precursor was about 5. After adding the support to the solution, the suspension was stirred and refluxed at 363 K for 5 h under nitrogen. The excess of liquid was removed in a rotary evaporator and the solid was washed several times to eliminate the free PVP and the chloride ions. Finally the samples were dried in an oven at 343 K and then calcined in air at 673 K for 1 h. At this temperature PVP decomposes completely. 2Au/Al-imp and 5Au/Al-imp catalysts were prepared by impregnation of the alumina carrier (coded hereafter as IMP-method) with AuCl3 reagent (Aldrich) dissolved in distillate water and diluted HCl (5%), respectively. After reaching adsorption equilibrium, the excess of water was evaporated in a rotary evaporator till dryness, followed by drying in air at 383 K overnight and final calcination in air at 673 K for 3 h. The two samples contained 2 and 5 wt.% gold loading.

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2.2. Catalyst characterization 2.2.1. N2 adsorption–desorption isotherms Textural properties of the bare SiO2 and g-Al2O3 supports and calcined catalysts were determined by isothermal adsorption of nitrogen at 77 K with a Micromeretics TriStar 3000 apparatus. The samples were previously degassed at 423 K for 24 h under a vacuum of 1.33  10
2 Pa to ensure a clean dry surface, free of any loosely bound adsorbed species. The specific areas of the samples were determined by using the nitrogen adsorption data from the relative equilibrium pressure interval of 0.03 < P/P0 < 0.3 according to the standard BET procedure, and using a value of a 0.162 nm2 for the cross-section of the nitrogen molecule. Pore distributions were calculated from the desorption branch of the corresponding nitrogen isotherm employing the BJH method. 2.2.2. X-ray diffraction (XRD) The X-ray diffraction measurements were carried out on bare supports and pre-reduced at 573 K samples with a Seifert 3000P diffractometer using Cu Ka (l = 0.15406 nm) radiation, equipped with a bent graphite monochromator in the diffracted beam and an automatic primary slit. The patterns were collected using a 2u step of 0.028, with an accumulation time of 5 s, and 2u scanning angles ranging from 58 to 708. The Au0 and Pt0 particle sizes were calculated from the line broadening of the Au(1 1 1) and Pt(1 1 1) reflections at 38.18 and 39.88, respectively, using the Scherrer equation [34]. For 1Pt–1Au/Si-pvp sample, the molar composition of AuPt alloy was obtained from the lattice parameter shift, calculated from the angular position of the (1 1 1) alloy reflections, according to Vegard’s law [35]. 2.2.3. DRIFTS spectra of adsorbed CO DRIFTS spectra were collected on a Nicolet 510 FT-IR spectrophotometer, using a Harrick HVC-DRP cell that allows in situ treatments with different gases at temperatures up to 773 K. OMNIC software was used for data processing. The interferograms consisted of 500 scans and the spectra were collected using a KBr spectrum as background. About 30 mg of the finely ground sample was placed in a sample holder and pretreated in situ in the DRIFT cell under a H2 flow at ambient pressure. For sample reduction, temperature was increased at a rate of 10 K min
1 up to 573 K and then kept at this temperature for 0.5 h. After reduction, the samples were cooled under a flow of He and then treated with a flow of 5% CO in He mixture at room temperature and ambient pressure for 20 min. 2.2.4. X-ray photoelectron spectroscopy (XPS) The spectra were acquired with a VG Escalab 200R spectrometer equipped with a hemispherical electron analyser and an Al Ka (hn =1486.6 eV) X-ray source. The binding energies (BEs) of Au 4f5/2, Pt 4d5/2, Si 2p and Al

167

2p core-level spectra were recorded and the corresponding binding energy was referenced to the C 1s line at 284.9 eV (accuracy within 0.1 eV). Peak intensities were estimated by calculating the integral of each peak after smoothing and subtraction of the ‘‘S-shaped’’ background. Atomic surface contents were estimated from the areas of the peaks, corrected using the corresponding sensitivity factors [36]. The catalysts subjected to on-stream operation were withdrawn from the reactor without exposure to air and then analyzed by X-ray photoelectron spectroscopy (XPS). 2.2.5. Thermogravimetric analysis (TGA) The amount of coke deposited on the catalysts was determined with thermogravimetric TGA/SDTA851e equipment (Mettler Toledo), measuring the weight change in the coked catalysts during oxidation. The burning of coke was carried out by raising sample temperature to a final temperature of 1073 K at a rate of 10 K min
1 in a 20% O2/N2 mixture. 2.2.6. Temperature programmed reduction TPR measurements were carried out in a Micromeritics TPD/TPR 2900 apparatus. TPR profiles were recorded by passing a 10%H2/Ar gas mixture (60 ml (STP) min
1) through the sample heated at a constant rate of 10 K min
1 1 up to 800 K. The effluent gas was passed through a cold trap placed before the tehrmoconductivity detector (TCD) to remove water from the exit stream. 2.3. Activity measurements Naphthalene hydrogenation was performed in a continuous-down-flow catalytic reactor. A model feed, consisting of naphthalene (NP) dissolved in hexadecane was prepared. The molar flow rates of naphthalene was 2.2 mmol h
1. Activity tests were performed using 0.25 g of catalyst (sieved fraction, 250–300 mm) diluted with inert particles of SiC at a volume ratio of 2:1 in order to limit the radial thermal gradient. Al2O3supported catalysts were sieved directly from the as-prepared catalysts whereas the SiO2-supported catalysts were pelleted before crushing and sieving. The procedure for catalyst activationinvolvedheatingtoareductiontemperatureof573 K for 4 h in an H2/N2 mixture (ratio 1:10 vol.) at atmospheric pressure. Catalytic activities were measured at T = 448 K, 2.0 MPa total pressure, a weight hourly space velocity (WHSV) of 45.7 h
1, and a H2/feed ratio of 220 l(N) l
1. Liquid samples were analysed by GC with FID (Varian Model Star 3400 CX chromatograph) equipped with a 30 m  0.53 mm DB-1 column (J&W Scientific). Besides unreacted model feed compounds (NP), tetralin and decalin were the only products detected. Taking into account that there was an excess of hydrogen in the reactor and that decalin dehydrogenation by heterogeneous catalysts occurs at temperatures as high as 713 K [37], the reversible decalin dehydrogenation to naphthalene was neglected. The reaction activities (mmol s
1 moles of metal
1) were calculated

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according to the formula: A = X F/vAu(P), were X is the conversion of naphthalene, F the naphthalene flow rate (mol s
1), and v the moles of metal(s) (calculated from nominal metal loadings).

3. Results and discussion 3.1. X-ray diffraction (XRD) Fig. 1(a) and (b) shows powder X-ray diffraction patterns of the g-alumina and SiO2-supported catalysts, respectively, pre-reduced at 573 K. Table 1 compiles crystallite sizes as calculated by Debye–Scherrer equation from XRD line broadening of the most intense peaks. As suggested from the absence of any metal related diffraction peak, the 2Au/Al-imp catalyst is likely to contain small metal particles of crystal sizes below the detection limits of the XRD. The increase in Au content from 2 to 5 wt.% led to the formation of large crystals (49.7 nm) of the Au0 particles (JCPDS 4-784). Moreover, the size of the Au0 nanoparticles formed on g-Al2O3 carrier is governed by the preparation method, as indicated by the different Au0 particle sizes obtained in the 2Au/Alimp and 2Au/Al-pvp catalysts, below 4 and 32 nm, respectively. Thus, contrary to what could be expected from the presence of the organic stabiliser on the catalyst surface [16,17], the PVP method led to a poorer Au dispersion than the IMP method. Other studies reported that the gold could not be highly dispersed over metal oxide supports when the classical impregnation method is used [8,4–6,11]. This was first observed by Bond with Au/ SiO2 and Au/Al2O3 catalysts [4–6]. The explanation of the formation of large Au particles when the impregnation method was employed resides in the low melting point of gold and to the sintering effect favoured by the presence of chloride ions.

Table 1 Crystal phases of the mono- and bimetallic catalysts after reduction at 573 K as determined by XRD Catalyst

Crystal sizes (nm)

Crystal phases

JCPDS

5Au/Al-imp 2Au/Al-imp 2Au/Al-pvp 1Au/Si-pvp 2Pt/Si-pvp 1Pt-1Au/Si-pvp

49.7 <4 32 29.5 8.8 10 (Au) 4 (PtAu)

Au0 No detected Au0 Au0 Pt0 Au0 Pt50Au50

4-784 – 4-784 4-784 4-802 4-784 (Au0) 4-802 (Pt0)

Both 1Au/Si-pvp and 2Au/Al-pvp catalysts prepared with the same PVP method showed peaks corresponding to Au0 phase (JCPDS 4-784). The sizes of gold particles in both catalysts are very close to each other (29.5 nm versus 32 nm). This excluded the support effect when PVP method is employed. Because of the lower melting point of Au as compared to Pt [11], the Pt0 particles (JCPDS 4-802) formed on SiO2 carrier were smaller than Au0 particles formed on this support (8.8 nm versus 29.5 nm). In addition, the binary 1Pt–1Au/Si-pvp catalyst showed gold individual Au0 nanoparticles and also PtAu alloy (Pt50Au50). Similarly, as it was reported previously, the use of the direct reduction of adsorbed chloro-precursors [38] and PVP as a stabilizer allowed to prepare AuPd alloy crystallites on a SiO2 substrate [16,17]. 3.2. Textural properties The textural properties of the calcined catalysts were evaluated from the nitrogen adsorption–desorption isotherms recorded at 77 K. The BET specific area, total pore volume and average pore diameter are summarized in Table 2. Data collected for silica and alumina substrates indicate that the bare SiO2 shows a larger specific BET area (546 m2 g
1 versus 147 m2 g
1) and lower average pore

Fig. 1. X-ray powder diffractograms of the pre-reduced (573 K) g-Al2O3 (a) and SiO2 (b) supported catalysts.

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169

Table 2 Textural propertiesa of the calcined Au and Pt–Au catalysts and bare SiO2 and Al2O3 supports Catalyst

BET (m2 g
1)

dp (nm)

VTot (cm3(CN) g
1)

VN2 at P/P0 = 0.2 (cm3(CN) g
1)

Bare g-Al2O3 5Au/Al-imp 2Au/Al-imp 2Au/Al-pvp Bare SiO2 1Au/Si-pvp 2Pt/Si-pvp 1Pt-1Au/Si-pvp

147 139 141 142 556 515 528 312

12.3 11.9 12.0 12.0 6.10 7.0 6.2 7.2

0.45 0.41 0.43 0.42 0.85 0.67 0.82 0.67

42 36 39 41 155 143 147 103

a BET specific area, average pore diameter (dp), total pore volume (VTot) at P/P0 = 0.98 and volume of N2 adsorbed at P/P0 = 0.2 as measured by N2 adsorption–desorption isotherms at 77 K.

diameter (5.0 nm versus 12.3 nm) than its bare Al2O3counterpart. The nitrogen adsorption–desorption isotherms of both bare Al2O3 and SiO2 supports are shown in Fig. 2(a). According to the IUPAC classification, the N2 isotherm of Al2O3 substrate is of type IV whereas the adsorption isotherm of SiO2 has the shape in-between isotherm types IV and I [39]. The type IV is characteristic for a mesoporous material whereas type I reveals the presence of micropores [39]. The hysteresis loops of both bare Al2O3 and SiO2 supports belong to type H2 and H1, respectively [40], which are characteristic of the pores with nonuniform and uniform, respectively, size and shape. In both cases, the shape of hysteresis loop corresponds to open-ended cylindrical pores. The pore size distributions, expressed as the plot of dV/ d log (D) versus D applied to the desorption branch of the nitrogen isotherms, where V is the pore volume (ml g
1) and D the pore diameter (nm), are presented in the inset of Fig. 2(a). A single peak for both carriers is observed. As compared to silica, alumina support shows a marked drop in peak intensity, which is accompanied by a broadening and shift of the maximum of the pore size distribution towards higher pore size. The average pore diameter of the bare SiO2 and Al2O3 supports are 6.1 and 12.3 nm, respectively. The

minor changes in the textural properties (Table 2), observed for the silica and alumina-supported monometallic samples, indicate that the metal particles are deposited mainly on the external surface of the SiO2 particles. The strong decrease in BET specific area after incorporation of both Pt and Au on SiO2 (from 556 to 312 m2 g
1) is surprising considering the low metal loading (2 wt.%). However, the decrease of the volume of N2 adsorbed at relative pressure P/P0 = 0.2 (Table 2) and the shape of hysteresis loops (Fig. 2(b)) with respect to the bare silica may suggest the formation and growth of PtAu alloy (d = 4 nm) within the silica pores. 3.3. Temperature programmed reduction (TPR) TPR profiles of the bare SiO2, PVP polymer (as supplied by Aldrich) and calcined 2Pt/Si-pvp and 1Pt–1Au/Si-pvp catalysts are shown in Fig. 3. As no H2-consumption peaks were found in the TPR profiles of 1Au/Si-pvp catalyst, its TPR profile was not included in this figure. Likewise for the alumina-supported monometallic gold catalysts. No peaks corresponding to reduction of redox impurities were observed in the bare SiO2. The lack of a low temperature peak, in the 2Pt/Si-pvp and 1Pt–1Au/Si-pvp catalyst

Fig. 2. N2 adsorption–desorption isotherms at 77 K of the bare SiO2 and Al2O3 supports (a) and calcined SiO2-supported catalysts (b). In the inlets of both figures the pore distributions are given.

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Fig. 3. TPR profiles of the calcined 1Pt–1Au/Si-pvp and 2Pt/Si-pvp catalysts, bare SiO2 carrier and PVP precursor (heating temperature ramp 10 K min
1).

profiles, ascribable to re duction of platinum oxides indicates that reduced platinum is formed after the high temperature calcinations. For both samples the presence of a broad and weak TPR peak with maximum at about 510 K may be attributed to the reduction of some residual platinic acid [19]. In the high temperature region, the TPR profile of 2Pt/ Si-pvp sample shows two overlapping peaks about 808 and 863 K. Considering the TPR profile of the PVP polymer, both peaks could be associated to hydrogenation of pyrolyzed residues, still remaining on the catalyst surface once the catalyst was calcined at 673 K. For 1Pt–1Au/Si-pvp sample, the peaks associated to hydrogenation of PVP residues moves to 714 and 970 K. The observed differences in the high temperature peaks in 1Pt–1Au/Si-pvp with respect to the 2Pt/Si-pvp sample may reflect the different structure and/or location of the pyrolyzed PVP residues. As the peaks indicative of the reduction of PVP residues are irrelevant from the catalytic point of view no further attention is given to these peaks. 3.4. In situ DRIFT spectra of adsorbed CO DRIFT spectra of adsorbed CO on the catalysts were recorded with the aim to determine the nature of exposed Au and Pt atoms on the surface of pre-reduced (573 K) 1Pt– 1Au/Si-pvp sample (Fig. 4). For comparison reason the spectra of pre-reduced 2Pt/Si-pvp and of silica and PVP were also measured. Taking into account that CO chemisorption on the metal surfaces is reversible [41] and the assignment of irreversible bands of CO adsorption is an arbitrary one [42], only the reversible CO adsorption is reported in Fig. 4. The spectrum of CO adsorbed on the 1Pt– 1Au/Si-pvp catalyst displayed bands at 2177, 1982, 1866 (very intense) and 1666 cm
1. All these bands were easily removed by flushing the adsorbate/adsorbent system with He for 3 min at room temperature, thus confirming a very weak interaction between CO and the metal surface [8]. The

Fig. 4. DRIFTS spectra (at room temperature) of the CO adsorbed to the pre-reduced (at 573 K) monometallic 1Pt/SiO2 and binary 1Pt–1Au/Si-pvp catalyst (reversible CO adsorption) and to the pure silica and PVP ligand.

band ca. 2177 cm
1, which is slightly above that of free CO molecule (2143 cm
1) is assigned to CO adsorbed on partially reduced gold sites [43]. The other small band at about 2112 cm
1 is produced by CO adsorption on finely dispersed gold particles [44,45]. Furthermore, no peaks assignable to CO adsorbed on Pt0 species (CO linear band at 2070 cm
1) [44] and platinum species in an electrondeficient state (CO linear band at 2080 cm
1) [46] were observed. Based on literature data [43] and in accord with the XRD data (Table 1), the bands at 1982, 1866 (very intense) should be ascribed to multibonded CO adsorbed on different metal sites such as terraces, steps or kink sites of the Pt50Au50 alloy and/or Au0 sites. The band at low wavenumber, 1666 cm
1, could be assigned to bidentate carbonate species [43]. The strong peak at around 1700 cm
1 from CO adsorbed on PVP is absent in the catalysts. 3.5. X-ray photoelectron spectroscopy (XPS) Photoelectron spectroscopy was used to determine the chemical state of the elements and their surface proportions after catalyst calcination at 673 K and on-stream reaction. For the calcined samples, the values of the binding energies of the most intense peaks of platinum (Pt 4f7/2) and gold (Au 4f7/2) are listed in Table 3. The Au 4f core level spectra of the used 5Au/Al-imp, 2Au/Al-pvp and 2Au/Al-imp are shown in Fig. 5, whereas of the calcined and used 1Au/Si-pvp and 1Pt–1Au/Si-pvp catalysts are shown in Fig. 6(a) and (b), respectively. For all gold catalysts, the Au 4f7/2 binding energy found at ca. 84.0 eV is typical of metallic gold [16– 18]. The Pt 4f core levels spectra of the calcined and used

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171

Table 3 XPS data of the Au, Pt and Pt–Au catalysts after calcination at 673 K Catalyst

Al (Si) 2p (eV)

Au 4f7/2 (eV)

Pt 4f7/2 (eV)

Au/Al(Si)

Pt/Si

Pt/Au

5Au/Al-imp 2Au/Al-imp 2Au/Al-pvp 1Au/Si-pvp 2Pt/Si-pvp 1Pt-1Au/Si-pvp

74.5 74.5 74.5 103.4 103.4 103.4

84.1 84.0 84.1 83.9 – 84.1

– – – – 71.4 (70), 73.3 (30) 71.1 (77), 73.3 (23)

0.0059 0.0010 0.0008 0.0058 – 0.0009

– – – – 0.104 0.0012

– – – – – 1.33

2Pt/Si-pvp and 1Pt–1Au/Si-pvp catalysts are shown in Fig. 7(a) and (b), respectively. The Pt 4f7/2 profile of both samples displayed peaks at ca. 71.0 and 73.0  0.3 eV, associated with Pt0 [47,48] and PtO [36] species, respectively. As seen in Table 3, the differences between the Au 4f7/2 binding energies in the monometallic and bimetallic samples are within the experimental error. However, the small shift (0.2 eV) between 1Au/Si-pvp and 1Pt–1Au/Si-pvp catalysts may be indicative of an electron transfer between gold and platinum in an alloyed phase. From XRD data, the composition of this alloy is close to Pt50Au50. It is worth noticing that only the calcined 5Au/ Al-imp sample showed the presence of a very small proportion of chloride species (BE of the Cl 2p3/2 peak at 198.1 eV; Cl/Al atomic ratio of 0.03) on the catalyst surfaces. Quantitative XPS data of calcined catalysts are presented in Table 3. Considering the Pt/Si atomic ratio derived from XPS spectra, the 2Pt/Si-pvp sample showed a much higher platinum surface concentration as compared with the binary 1Pt–1Au/Si-pvp sample. This result, in accord with the N2 adsorption–desorption data could be due to the burial of the alloy particles (Pt50Au50) inside the support pores. The gold surface exposure, as measured by the Au/Al(Si) atomic ratios (Table 3), depends on the support, metal concentration and preparation method and follows the order:

Fig. 5. Au 4f core level XPS spectra of the used Al2O3-supported catalysts tested in NP HYD.

5Au/Al-imp  1Au/Si-pvp  2Au/Al-imp  1Pt–1Au/Sipvp > 2Au/Al-pvp. 3.6. Thermogravimetric analysis (TGA) The amount of coke deposited on the catalysts was determined by TGA analysis by measuring the weight change of the coked catalysts during oxidation in 20% O2/N2 mixture. The TGA and the dw=dT profiles of the used catalysts are shown in Fig. 8, respectively, and the amount of coke (%) is given in Table 4. For all catalysts the coke is burnt at ca. 497 K. No weight loss feature at ca. 700 K, which is usually associated to adsorbed naphthalene [49], was observed. This means that in spite of the low naphthalene conversion (at TOS = 2 h ca. 27.5%), the naphthalene did not adsorb on the catalyst surface. From Table 4, the amount of coke follows the trend: 1Pt–1Au/Sipvp > 2Pt/Si-pvp > 1Au/Si-pvp  5Au/Al-imp > 2Au/ Al-pvp > 2Au/Al-imp. Thus, independently of the preparation method, a larger amount of coke is formed on SiO2supported catalysts than on Al2O3-supported catalysts. 3.7. Catalyst structure-activity correlation 3.7.1. Factors influenced the catalytic behaviour of the monometallic Au catalysts The influence of the support (g-Al2O3 versus SiO2), the catalyst preparation method (IMP versus PVP) and Auloading (2Au wt.% versus 5Au wt.%) on the catalytic behaviour of monometallic Au catalysts in naphthalene hydrogenation are summarized in Table 4 where the specific rates (expressed as mmole of NP per s and per mole of metal) after 2.3 h of time on stream are reported. It is clear that the activity of Au catalysts depends on the preparation method and support employed, being the IMP method more effective than PVP method, and SiO2 support more effective than Al2O3. Considering Al2O3 series, the largest activity of 2Au/ Al-imp catalyst is due to the better dispersion of Au0 particles on this carrier as derived from XRD (Table 1). The increase in Au-loading from 2 to 5 wt.% led to decrease in activity, which could be linked with the marked decrease of gold dispersion with increasing Au-loading (see XRD data, Table 1). The 1Au/SiO2-supported gold catalyst was more effective than its 2Au/Al2O3 counterpart (Fig. 9). This is surprising because the SiO2-supported catalysts are generally considered to be less active than g-Al2O3 catalysts for

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Fig. 6. Au 4f levels XPS spectra of the calcined (a) and used in NP HYD (b) 1Au/Si-pvp and 1Pt–1Au/Si-pvp catalysts.

hydrotreating reactions [50]. This is because the metal oxides interact very weakly with the amorphous SiO2 support, and as a result very poor dispersion of the active ingredient is reached [50,51]. However, in the present work the Au dispersion achieved on SiO2 support employing PVP method was close to that achieved on Al2O3. In addition, the DRIFT-NH3 spectra (not presented here) revealed a very weak acidity of all the catalyst and hence no correlation between acidity and activity was observed. This is in agreement with our previous study [16,17], which showed that only medium strength acid sites influence the aromatic hydrogenation on silica–alumina-supported Au–Pd catalysts. The 1Au/Si-pvp catalyst although more active than its Al2O3-supported counterpart produced a larger amount of coke (Table 4). Since acidic supports favour deactivation by coking [52] and the density of hydroxyl groups in SiO2 is nearly one-half of that in Al2O3 [53], the larger coke formation on the SiO2 than on Al2O3 catalysts could be interpreted on the basis that a lower surface concentration of dissociated hydrogen is present on the silica-supported

catalyst, and therefore it is unable to hydrogenate the adsorbed residues formed by side reactions. On the contrary, hydrogen dissociation occurs on the Al2O3 surface [54], which can be effective in the hydrogenation of residues. 3.7.2. Influence of the Pt promotion The SiO2 substrate and PVP method were selected to investigate the effect of Pt–Au formulation on the catalytic behaviour in NP HYD. Fig. 9 compares the specific activity versus time on stream (TOS) of monometallic 1Au/Si-pvp and 2Pt/Si-pvp catalysts with that of the binary sample. The binary sample showed a clear synergy effect between Au and Pt at steady-state conditions. As inferred from combined XRD, XPS and N2 isotherm data, this binary sample possesses both Pt50Au50 alloy phase (4 nm) probably inside the pores of the silica and isolated Au0 particles (10 nm) located on the external catalyst surface. Thus, in agreement with our previous study [31], the synergy effect observed for binary sample could be explained as due to the presence of Pt50Au50 alloy. Additionally, the DRIFT-CO spectra of this

Fig. 7. Pt 4f core levels XPS spectra of the calcined (a) and used in NP HYD (b) 2Pt/Si-pvp and 1Pt–1Au/Si-pvp catalysts prepared by PVP method.

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173

Fig. 9. Comparison of the NP conversions for bimetallic 1Pt–1Au/Si-pvp and monometallic 2Pt/Si-pvp and 1Au/Si-pvp catalysts (T = 448 K, P = 2.0 MPa, WHSV = 45.7 h
1).

and hence its specific activity at steady-state conditions was higher than that of the monometallic 1Au/Si-pvp sample (Fig. 9).

Fig. 8. TGA profiles of the Al2O3 and SiO2-supported catalysts.

sample (Fig. 4) show the large amount of low coordination sites of Pt50Au50 alloy (terraces, steps, or kink sites). These sites might contribute to the hydrogen activation [55]. Contrary to the binary 1Pt–1Au/Si-pvp sample, the 1Au/Sipvp catalyst showed a fast deactivation, which was common to the all monometallic gold catalysts. Compared to binary 1Pt–1Au/Si-pvp sample, at TOS = 2.3 h, the monometallic 1Au/Si-pvp catalyst showed larger activity (Fig. 9). Since the 1Au/Si-pvp sample exhibited large Au0 particles (29.5 nm), the explanation of its unexpected large activity might involve a great amount of the fraction of dense (1 1 1) planes, making easier the p-mode of naphthalene adsorption. However, as a consequence of the adsorption of carbon residues on 1Au/Si-pvp sample a quick inhibition of NP adsorption on these gold sites might occur. Conversely, the binary 1Au–1Pt/Si-pvp catalyst showed slower deactivation

3.7.3. Factors influenced the tetralin hydrogenation to decalin HYD of NP proceeds via successive steps: naphthalene ! tetralin (1,2,3,4-tetrahydronaphthalene) ! decalin (cis and trans decahydronaphthalene) [56]. The 1,9-octalin (octa-hydro-naphthalene) is considered as the most reactive intermediate product of the tetralin conversion to decalin [57]. As mentioned in the introduction, we started to investigate factors influencing on selectivity in naphthalene hydrogenation after observation of the exceptionally high decalin formation on Au/g-Al2O3 catalysts tested in simultaneous toluene and naphthalene hydrogenation in the presence of dibenzothiophene [8]. In order to avoid the competitive hydrogenation of toluene/DBT with naphthalene as factor which might influence the naphthalene overhydrogenation, the feed employed in this study contained only naphthalene. Moreover, the excess of hydrogen was diminished employing lower hydrogen pressure (2.0 MPa versus 5.0 MPa). The 5Au/Al-imp sample studied previously in simultaneous aromatics hydrogenation [8] was used as reference of the new prepared Au catalysts. The

Table 4 Activity of the g-alumina and SiO2-supported Au, Pt and Pt–Au catalystsa Catalyst 5Au/Al-imp 2Au/Al-imp 2Au/Al-pvp 1Au/Si-pvp 2Pt/Si-pvp 1Pt-1Au/Si-pvp

Conversion (%) 27.5 18.9 4.5 22.0 4.2 12.8

Rates (mmoles s
1 mole Me
1) 0.17 0.7 0.07 1.2 0.15 0.8

Selectivity to decalin (%) c

44.9 (97.2) 6.8 9.1 5.5 66.4 5.2

Cis/trans decalin ratio c

0.8 (0.6) 0.9 0.4 5.0 1.0 0.8

Cokeb (%) 7.5 6.8 7.3 18.1 20.8 25.7

Tested in the naphthalene hydrogenation. Reaction conditions were: 448 K, P = 2 MPa, WHSV = 45.7 h
1, TOS = 2.3 h. As determined by TGA of the catalysts used in NP HYD. c Tested in simultaneous naphthalene and toluene hydrogenation in the presence of dibenzothiophene (T = 498 K, P = 5.0 MPa, WHSV = 41.2 h
1, steadystate conditions). a

b

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selectivity toward decalin formation is given in Table 4. With exception of 2Pt/Si-pvp and 5Au/Al-imp, the catalysts showed decalin formation lower than 9.1%. Contrary to recent study on the Pt/TiO2 catalysts [29], the 2Pt/Si-pvp showed a large tetralin conversion. This suggests that NP interacts with surface platinum on SiO2 less strongly than on TiO2 [29], thus the hydrogenation of tetralin to decalin is not prevented. This could be expected considering the weak metal-support interaction on inert SiO2 surface. Contrary to SiO2, a stronger metal-support interaction is expected on Al2O3. Considering this work and other previous one [8], the interpretation of the largest over-hydrogenation (44.9% selectivity toward decalin) achieved on the 5Au/Al-imp catalyst is relatively straightforward. From the catalyst characterization (XRD, XPS), the 5Au/Al-imp sample is unique, showing: (i) the largest Au0 particle size (49.7 nm) and (ii) a certain amount of Cl
ions on its surface (Cl/Al atomic ratio = 0.002). Considering the former, it seems that tetralin adsorption is stronger on large Au0 clusters than on small Au0 particles. On the other hand, the possible contribution of Cl
ions to the large tetralin conversion on 5Au/Al-imp sample (Table 4) cannot be ruled out because chloride ions can polarize surface hydroxyl groups of alumina and increase their acidity [58]. Thus, taking into account that the hydrogen addition to tetralin is likely to be metal centered and the hydrogen dissociation occurs on the alumina support [54] and on the Au0 particles [59], the polarized surface hydroxyl groups of alumina may enhance the effect of hydrogen spilled over at the metal interface. The selectivity toward decalin formation of 5Au/Al-imp sample at 448 K (44.9%) was lower than that (94.7%) obtained in our previous work at 498 K [8]. Since degree of hydrogen dissociation is governed by the amount of hydrogen present [60], it appears that the decrease in temperature did not compensate the lower hydrogen pressure employed (5.0 MPa versus 2.0 MPa) [29]. On the other hand, from the study of Sa´ rka´ ny et al. [25] it could be inferred that the carbonaceous deposits adsorbed on active sites decreased the availability of hydrogen for tetralin hydrogenation to decalin. In such case, the lowest naphthalene over-hydrogenation on SiO2-supported samples with large coke formation (Table 4) could be expected. Moreover, the largest coke formation on the binary 1Pt– 1Au/Si-pvp catalyst might account for a very low naphthalene over-hydrogenation on this sample. This is because the carbonaceous deposits decrease the availability of hydrogen on the metal sites, and, as a consequence, the hydrogenation of NP becomes more partial [25]. Contrary to this binary sample, the 2Pt/Si-pvp catalyst with large coke formation also (25.7% versus 20.8%) showed large NP overhydrogenation. The easy hydrogen dissociation on platinum overshadows the effect of coke adsorption. The relatively low NP conversion obtained in this study under the employed reaction conditions (see Table 4), allows us to analyze the cis/trans selectivity in the decalin products in order to infer the possible presence of a 1,9-octalin (octa-

hydro-naphthalene) as intermediate product [57]. According to Weitkamp [57], the preference of the 1,9-octalin to readsorb with the hydrogen atom in position 10 facing the surface or away from the surface led to formation of cis and trans-decalin, respectively. The cis/trans-decalin ratio at TOS = 2.3 h is compiled in Table 4. With exception of 1Au/ Si-pvp sample, the cis/trans decalin ratios for all the catalysts fall in the range 0.4–1.0. For 2Pt/Si-pvp, two decalin isomers are produced at an equimolar ratio, in agreement with the study of Lin and Song [49]. For 5Au/Alimp, the cis/trans-decalin ratio in range 0.6–0.8 indicates that, irrespectively of the feed employed in activity tests, the cis-decalin isomerization was affected by site competition with other molecules present in the feed. On the contrary, the large cis-decalin formation on the 1Au/Si-pvp sample is indicative of the non inhibited presence of 1,8-octalin intermediate adsorbed with the hydrogen atom in position 10 facing the Au0 surface. In summary, these results together with that reported in a previous work [8] allow to outline the factors which influence on the hydrogenation of naphthalene to decalin. These are the following: (i) the stronger adsorption of tetralin than of NP on the surface of the large Au0 particles; (ii) the presence of Cl
ions on the catalyst surface (increasing the acidity of the hydroxyl groups and favouring the hydrogen spill over to the metal); (iii) the competitive naphthalene adsorption with other molecules; (iv) the high H2 pressure.

4. Conclusions Naphthalene hydrogenation was studied over monometallic Au, Pt and bimetallic Au–Pt catalysts supported on gAl2O3 and SiO2. The alumina-supported samples, prepared by IMP method displayed larger activity as compared to the their homologues prepared by PVP method. Moreover, the silica support yielded a gold catalyst more active and more selective to decalin with respect to its homologue on alumina support. The steady state higher activity of the bimetallic 1Pt–1Au/Si-pvp catalyst with respect to the monometallic 1Au/Si-pvp and 2Pt/Si-pvp ones was related to Pt50Au50 alloy formation. The stronger tetralin than NP adsorption on the surface of large Au0 particles, the presence of Cl
ions on the catalyst surface, the high H2 pressure and the competitive naphthalene adsorption with other molecules are important factors influencing on the hydrogenation of NP to decalin. In particular, the selectivity to decalin may be controlled by employing Pt–Au formulation, which enhances the dispersion of gold particles.

Acknowledgements Thanks are due to Dr. M.A. Pen˜ a, Dr. J.M. CamposMartin and Ms. E. Cano-Serrano for technical assistance.

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