Synthesis, Characterization, and Sulfur Tolerance of PtâMoOx

14790

J. Phys. Chem. C 2007, 111, 14790-14798

Synthesis, Characterization, and Sulfur Tolerance of Pt-MoOx Catalysts Prepared from Pt-Mo Alloy Precursors Polina A. Zosimova,† Andrey V. Smirnov,† Sergey N. Nesterenko,† Valentina V. Yuschenko,† Wharton Sinkler,‡ Joseph Kocal,‡ Jeniffer Holmgren,‡ and Irina I. Ivanova*,† Chemistry Department, M. V. LomonosoV Moscow State UniVersity, Leninskie Gory, 1, Building 3, 119991, Moscow, Russia, and UOP LLC, 50 East Algonquin Road, Des Plaines, Illinois 60016 ReceiVed: May 4, 2007; In Final Form: July 14, 2007

Novel Pt-MoOx catalytic systems were prepared by the oxidation-reduction of Pt-Mo alloy precursors with different metal contents. According to XRD, SEM, and TEM data, oxidation at 550 °C led to the destruction of initial alloys and the formation of molybdenum oxide and metallic platinum particles. As evidenced by TPR H2, adsorption measurements, and in situ XRD analysis, during reduction in the presence of Pt, molybdenum oxide can trap hydrogen already at ambient temperature with the formation of molybdenum oxide hydroxide or molybdenum bronzes. In contrast to initial alloys, the oxidized systems showed significant catalytic activity in toluene hydrogenation. The results suggested that both metallic platinum and molybdenum oxide contribute to this activity. The sulfur resistance test revealed that Pt-MoOx systems had an enhanced tolerance to sulfur contained in the feed.

1. Introduction Binary systems containing a noble metal and another metal or metal oxide are of great interest since the catalytic and chemisorption properties of noble metals could be significantly altered due to interactions with other components. These effects have been extensively studied for Pt-ZrO2, Pt-supported zeolites, Pt-Pd/SiO2-Al2O3, and other compositions.1-3 Bimetallic systems containing noble metals and molybdenum/ molybdena have also been mentioned as catalysts for desulfurization,4 hydrogenation,5 reduction of NOx,6 etc. For the preparation of binary Pt-Mo systems, the impregnation of MoO3 with Pt-containing salts solutions was used. However, the very low surface area of MoO3 (a few m2/g) is a great disadvantage of this preparation procedure. Besides that, to reach a Pt content of 0.5-1 wt %, several consecutive impregnations should be performed, resulting in the formation of large Pt aggregates. Some techniques for the preparation of molybdenum oxide with a large surface area such as evaporation of the oxide aerosol in a frame reactor7,8 or partial reduction of MoO3 in the presence of Pt9 have been reported, but they are not widely used. Therefore, to investigate the interfering effects, Pt and MoO3 dispersed over alumina,10-12 silica,13 zeolites,14 or other supports were usually applied. But, it is necessary to note that the interaction of Pt with a support can be stronger than with molybdenum oxide,15 which makes the investigation of direct Pt-MoO3 interactions difficult. These problems can be avoided with the application of a method based on the oxidation of Pt-Me alloy precursors. Such a method of preparation of Pt-oxide systems was reported for Pt-CeOx catalysts, which showed a noticeable catalytic activity in CO oxidation.16 The possibility of the formation of TiO2 and TiAlOx phases during oxidation of the Ti3Al alloy was also * Corresponding author. Tel.: +7-495-9392054; fax: +7-495-9393570; e-mail: [email protected]. † M. V. Lomonosov Moscow State University. ‡ UOP LLC.

mentioned.17 In the present paper, this approach was used for the first time for the preparation of Pt-MoOx systems. The close contact of platinum with molybdenum oxide changes the electronic and ensemble properties of Pt due to metal-support interaction effects. The existence of such an interaction was shown by many examples.11,18 According to the data obtained by TPR experiments,19-21 the presence of Pt significantly lowers the reduction temperature of molybdena. The investigation of Pt-Mo bimetallic structures after the reduction of Pt-MoO3 at 500-550 °C revealed the formation of bimetallic clusters, in which molybdenum was involved in the bimetallic particle at the zero valence state; dispersed molybdenum atoms in a low valence state were also observed. Heretofore, deVries et al., based on XPS measurements, found the reduction of Mo6+ to Mo5+ in the presence of PtO2 in dispersed MoO3 over Al2O3 below 300 °C.12 The reduction of molybdenum oxide with hydrogen results in the formation of nonstoichiometric molybdenum bronzes HxMoO3.22 In the case of bulk MoO3, this process begins above 200 °C and typically leads to the material with H0.34MoO3 composition.23 But, in the presence of Pt or Pd, the bronze formation begins already at ambient temperature and provides a higher degree of hydrogen incorporation (1.55 < x < 1.72).22,24 The dissociation of hydrogen molecules proceeds on Pt particles, and then hydrogen is transferred to MoO3 by a spillover mechanism, promoting the formation of bronze compounds.11,25 These molybdenum bronzes can be considered as hydrogen traps, and it is possible to use hydrogen from this reservoir to carry out the hydrogenation processes. As an example, the hydrogenation of ethylene by HxMoO3 was described.26,27 Jackson et al. reported that after reduction, platinum supported on molybdena is positively charged (δ+) due to a metal-support interaction.18 This phenomenon can play an important role in the modification of the chemisorption properties of Pt, in particular, in resistance to sulfur poisoning. It is well-known that the high sulfur tolerance of Pt and Pd supported on acidic oxides or zeolites arises owing to the formation of electron-

10.1021/jp073410j CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007

Pt-MoOx Catalysts Prepared from Pt-Mo Alloy Precursors deficient metal particles Ptδ+ and Pdδ+.1 It can be expected that a similar behavior can be observed for Pt-MoO3 systems. The aim of the present work was to develop an experimental procedure for the preparation of Pt-MoO3 systems, which will ensure intimate contact between Pt and Mo and allow us to avoid interactions with the other supports. The novel method based on the alloying of Pt with Mo and consecutive oxidation of alloys to form Pt-MoO3 systems was applied. The effect of Pt-Mo interactions on catalytic properties and sulfur tolerance was studied in toluene hydrogenation. 2. Experimental Procedures The Pt-MoOx systems were synthesized via the oxidation of Pt-Mo alloy precursors. Three alloys, namely, Pt75Mo25, Pt30Mo70, and Pt2Mo98 with Pt/Mo atomic ratios of 75:25, 30:70, and 2:98, respectively, were prepared by arc-melting of stoichiometric mixtures of pure components Pt (99.99%) and Mo (99.98%) on a water-cooled copper hearth under 5 × 104 Pa of argon (99.95%). Each sample was turned over and remelted 2 times and then was annealed for 300 h at 900 °C in order to ensure homogeneity. Before characterization and catalytic testing, the alloys were converted into powder by mechanical grinding with diamond abrasive instruments and sieved to yield a particle with a size of 0.01-0.12 mm. Pt-Mo alloys were oxidized in a flow of dry air at 550 °C for 10 h to yield Pt-MoO3 catalysts; this procedure is denoted as Ox. Standard catalyst pretreatment before catalytic experiments consisted of the reduction of oxidized samples in hydrogen (99.95%) at 450 °C for 1 h; this procedure is denoted as OxRed. The samples were characterized using XRD, SEM/EDS, TEM/EDS, TGA, N2 adsorption measurements, and TPR H2 techniques. X-ray powder diffraction spectra were recorded in the range of 2θ ) 20-80° using a DRON-4 diffractometer with Cu KR radiation. In situ high temperature XRD experiments on oxidation and reduction procedures were performed using a Theta Bruker D-500 diffractometer with a Cu anode X-ray source and Kevex Si(Li) solid state detector. Sample oxidation was studied during heating in a flow of synthetic air (mixture of O2 (20%) and N2 (80%)) in the temperature range of 20600 °C with an increment of 100 °C; the duration of heating in each step was 20 min. The last spectrum was taken after cooling the sample down to ambient temperature. The reduction was investigated in a flow of hydrogen (4%) and helium (96%) in the same temperature range. The quantitative XRD analysis using the Rietan 2000 program was performed for the determination of phase content. The average Pt particle size was estimated by the Scherrer equation. The oxidation of alloys in a flow of dry air was also studied by TG measurements using a TA SDT Q600 instrument. About 30 mg of alloy was used for analysis. The oxidation was carried out in a flow of dry air starting from ambient temperature to 800 °C with a heating rate of 10°/min. The oxidation of alloys was continuously monitored by following the gain of sample weight with time. The SEM images were obtained on a 5900 LV JEOL electron microscope combined with EDS analysis. The TEM examination was performed using JEOL 3000F field-emission TEM equipment operated at 300 kV. For TEM analysis, the samples were prepared by being embedded in LR White epoxy and sectioning with a 45° diamond knife. The specific surface area was measured by an ASAP 2010M Micromerictics analyzer. The data were obtained from the N2

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14791 adsorption isotherm at 77 K. The specific surface area was calculated from the BET equation. For TPR studies (TPR H2), 15-30 mg of the alloy precursor thoroughly mixed with porous quartz powder was placed in a tubular quartz reactor and heated in a flow of dry air (20 mL/ min) at 550 °C and was kept at this temperature for 10 h. After cooling to ambient temperature, a sample was purged in 3.5% H2/Ar flow for 0.5 h, and then the temperature was raised to 1000 °C with a rate of 8°/min. The TPR profiles were registered using a TCD. ECOCHROM software was used to collect and process the TPR data. XPS analysis was used for the characterization of the state of elements on the catalyst surface for both as-synthesized and activated materials. The X-ray photoelectron spectra were recorded on an Escalab MK-II spectrometer at an operating pressure of 10-7 Pa. Al KR (hν ) 1486.6 eV) and Mg KR (hν ) 1253.6 eV) radiation and an electron analyzer passing through an energy of 20 eV were used. The binding energy (BE) calibration was effected by assuming that the 4f7/2 peak for pure gold is 84.0 eV and that the 2p and 3p peaks for copper are 932.67 and 75.14 eV, respectively. The BE values were referenced to the C1s line (285.5 eV). Each spectral region was scanned several times to obtain a good signal-to-noise ratio. XPS spectra after polynomial background correction were fitted by means of an iterative least-squares procedure with pseudoVoigt line shapes. The reproducibility of the fitted peak positions was better than 0.2 eV. The catalytic properties of the samples were tested in a toluene hydrogenation reaction at 150 °C under atmospheric pressure and in the range of a weight hourly space velocity (WHSV) of 0.7-20.9 gtoluene/(gcat h). In a typical experiment, 0.01-0.3 g of the catalyst sample thoroughly mixed with porous quartz was placed into the tubular quartz reactor and treated in a flow of H2 at 450 °C for 1 h prior to the catalytic run. For the reaction, a flow of hydrogen was saturated with toluene at 0 °C and then passed through the reactor. The WHSV was varied by adjusting the catalyst loading and the flow of reagents. Reaction products were analyzed on-line on a Crystal 2000M chromatograph equipped with a 50 m SE-30 capillary column. Methylcyclohexane was the major reaction product. Structural isomers of methylcyclohexane and condensed aromatic compounds were also detected, but their total amount was less than 1%. Taking into account that the reaction was carried out in excess of H2, the activities of the catalysts were determined using the first-order rate equation

A ) V0 ln

(1 -1 y)

where ν0 is the toluene feeding rate, and y is the toluene conversion. For the calculation of effective activity Acat, ν0 was normalized to the catalyst weight [gtoluene/(gcat h)] and corresponded to WHSV. For the estimation of specific activity APt, ν0 was normalized to the amount of accessible Pt atoms [gtoluene/ (gPtacc h)]. It should be noted that for all the catalysts, the ν0 values were adjusted in such a way as to achieve low toluene conversions (not higher than 15%). The Acat and APt values were therefore obtained from the initial (linear) regions of the kinetic curves and did not depend on WHSV. The amount of accessible Pt atoms was estimated as the number of atoms on the surface (1.25 × 1019 Pt atoms/m2)11 of spherical Pt particles with diameters obtained from XRD data (Table 1). The accuracy of the determination of the average

14792 J. Phys. Chem. C, Vol. 111, No. 40, 2007

Zosimova et al.

TABLE 1: Characteristics of Pt-Mo-Containing Samples initial sample

phase composition (unit cell parameters, Å)

Pt75Mo25

fcc Pt; a ) 3.912

Pt30Mo70

bcc Mo; a ) 3.157 Mo3Pt2; a ) 5.560 and c ) 4.490 bcc Mo; a ) 3.144

Pt2Mo98

a

Ox

OxRed


Pt particle sizea (nm)


fcc Pt; a ) 3.921 MoO3; traces fcc Pt; a ) 3.919 orthorhombic MoO3; a ) 13.916, b ) 3.694, and c ) 3.958 fcc Pt; a ) 3.917 orthorhombic MoO3; a ) 13.920, b ) 3.700, and c ) 3.969

35

fcc Pt; a ) 3.921

32

23

fcc Pt; a ) 3.921

24

19

fcc Pt; a ) 3.922

20-24

Pt particle sizea (nm)

Calculated from XRD data using the Scherrer equation.

experiment was carried out up to complete deactivation of the sample. Then, for each run, the amount of sulfur required for catalyst poisoning was estimated. 3. Results and Discussion

Figure 1. XRD patterns of Pt-Mo alloys (o: Pt, ∆: Mo; and a: Mo3Pt2).

Figure 2. TG curves obtained during oxidation of Pt-Mo alloys.

size of the Pt particles coincides with the precision of calculation of the Pt crystallite size using the Scherrer equation. It should be noted that H2 and CO adsorption techniques, which are usually used for the determination of the amount of accessible Pt atoms, were not applicable in our case since H2 and CO interact with MoO3 and both methods give higher values of adsorption than theoretically possible for Pt.11 To study the sulfur tolerance of the catalysts, H2S was added to the feed by the pulse injection of the mixture of N2 and H2S (970 ppm); each pulse contained 0.47 µg of H2S. If the activity of the sample did not change after 10-15 pulses, the pulses were replaced by continuous addition of H2S to the reagent flow, the content of H2S in the flow being 0.018-0.085 µg/mL. The

3.1. Phase Composition of Initial Alloys. The alloying of Mo and Pt led to the formation of solid solutions and Pt-Mo intermetallic compounds (Figure 1, Table 1). Because of the close atom size and parameters of crystal structure (fcc Pt and bcc Mo), Pt and Mo can easily form solid solutions in a broad range of concentrations. The XRD patterns of the sample with a high content of Pt (Pt75Mo25) corresponded to the formation of a single phase with a fcc crystal structure typical for pure Pt. The XRD data obtained for the Pt30Mo70 sample revealed the formation of two phases: bcc Mo (about 85%) and the intermetallic phase Mo3Pt2 (about 15%). The further increase of the Mo/Pt ratio (Pt2Mo98) led to the formation of a solid Pt-Mo solution with a structure typical for metallic Mo. 3.2. Alloy Oxidation. The TG experiments, performed in a flow of air, showed a significant increase of sample weight during heating of alloys above 500 °C (Figure 2). These changes were due to the oxidation of Mo into MoOx. In the case of the Pt30Mo70 and Pt2Mo98 samples, the increase in weight was close to the amount of oxygen needed for the transformation of Mo into MoO3, while for Pt75Mo25, the x value was equal to 0.8. This observation points out that not all Mo was oxidized into MoO3 in the Pt-rich sample. At temperatures higher than 650 °C, the decrease of sample weight occurred due to the evaporation of molybdenum oxide.28 Thus, 550 °C was chosen as an optimal temperature for the oxidation of the materials studied. To follow the changes in phase composition during the oxidation procedure, the in situ high temperature XRD experiments were performed on the Pt30Mo70 sample (Figure 3). After heating to 500 °C, a number of signals appeared on the XRD pattern. According to the XRD ICSD database, the intensive peaks at ca. 39, 46, and 67° (2θ) corresponded to metallic Pt; all other signals corresponded to molybdenum oxide in different modifications with the domination of the orthorhombic phase MoO3. The complete destruction of the alloy was observed between 500 and 600 °C. The SEM images of the initial and treated alloys are presented in Figure 4. The oxidation of Mo may lead to a more than 3-fold increase of the sample volume (Vsp(Mo) ) 0.098 m3/g and Vsp(MoO3) ) 0.32 m3/g).29 This peculiarity of Mo caused significant changes in the morphology of the samples upon oxidation. Thus, in the case of the Pt75Mo25 sample with the lowest content of Mo, the needle-shaped crystals up to 40 µm in length appeared in the SEM images. A similar shape of MoO3 crystals was described.30 The initial alloy granules were

Pt-MoOx Catalysts Prepared from Pt-Mo Alloy Precursors

J. Phys. Chem. C, Vol. 111, No. 40, 2007 14793

Figure 3. XRD patterns obtained during in situ oxidation of Pt30Mo70 sample (*: orthorhombic MoO3; o: Pt; ∆: Mo; and a: Mo3Pt2).

Figure 4. SEM images of Pt-Mo samples: initial and after Ox treatment.

preserved, and probably some amount of the molybdenum (or molybdena) was still in the sample particles. On the contrary, the oxidation of the Mo-rich Pt30Mo70 and Pt2Mo98 samples led to the partial destruction of particles due to the growth of molybdenum oxide crystals, which occurred preferably on the edges of the particles (Figure 4). According to low temperature N2 adsorption measurements, these changes in morphology did not affect significantly the surface area of the samples leading to its slight increase from 1 to 3-5 m2/g. To study the bulk distribution of the components, TEM micrographs of the Pt30Mo70 samples after oxidation were obtained (Figure 5). Examination of Figure 5a suggests that epoxy was incorporated inside the sample granules. This points to the existence of large voids inside the particles. According to electron diffraction patterns, well-ordered metallic platinum and molybdena phases were formed after oxidation. The HRTEM image (Figure 5b) displays planes of the molybdenum

oxide crystalline structure. The precise analysis of interplanar spacing from the image revealed the presence of three Mocontaining phases: orthorhombic phase MoO3, β phase MoO3, and Mo4O11. The platinum particles in the oxidized sample had an average size of 20 nm (Figure 5), which is in good agreement with XRD data (Table 1). However, larger and smaller Pt particles were also observed. This nonuniformity can be explained by different heat transfer from the bulk and from the surface during oxidation, resulting in the formation of larger platinum particles in the bulk and smaller particles on the surface of the grains. Thus, the oxidation of Pt-Mo alloys at 550 °C leads to the formation of materials with molybdena on platinum grains as in the case of the platinum-rich Pt75Mo25 sample and the dispersed on platinum particles on molybdena as in the case of the Pt30Mo70 and Pt2Mo98 samples. Even though the prepared systems have a low surface area, this method of synthesis


Zosimova et al.

Figure 5. TEM images of Pt30Mo70-Ox (a and b) and Pt30Mo70-OxRed (c and d) samples.

permitted us to obtain the Pt-MoOx samples in a wide range of Pt contents and with a high dispersion of Pt, which is hardly possible by conventional wet impregnation procedures. 3.3. Reduction of Oxidized Pt-Mo Alloys. According to TEM data, the reduction of the Pt30Mo70-Ox sample in hydrogen at 450 °C led to the transformation of molybdena crystal phases to a quasi-amorphous state (Figure 5d). The evolution of the phase composition of the Pt30Mo70-Ox sample during reduction at different temperatures was studied by in situ XRD analysis (Figure 6). In the presence of hydrogen, the peaks corresponding to the MoO3 phase (see Figure 3) disappeared at ambient temperature, while a new intensive peak in the region of 2θ ) 24-26° was observed. According to the XRD ICSD database and literature data,23 this peak could be attributed either to molybdenum oxide hydroxide phase Mo5O7(OH)8 or to molybdenum bronzes, but the precise phase determination demands additional investigation. During heating up to 300 °C, this new phase disappeared, and above 300 °C, only a metallic platinum phase was detected. On the HRTEM image (Figure 5d), the planes of the crystalline lattice of molybdenum oxide also were not observed. The disappearance of the crystalline Mo-containing phases could be due to further hydrogen consumption and the formation of amorphous HxMoO3 species.

The reducibility of the samples was also studied by TPR H2 (Figure 7). On the reduction profile of bulk molybdena, used as a reference, three main peaks at 520, 620, and 845 °C, corresponding to a sequential reduction of molybdenum oxides, were observed. The total amount of hydrogen uptake was 3.0 mol of H2/mol of Mo, confirming the complete reduction of MoO3 into Mo. The TPR profiles of oxidized Pt-Mo alloys were quite different from those of bulk molybdena. A new peak at 230 °C appeared. In contrast, the peak at 620 °C disappeared, and the maxima of the high temperature peaks shifted toward lower temperatures. On the Pt-rich sample Pt75Mo25, the hydrogen consumption began already at ambient temperature and increased gradually up to 400 °C. The hydrogen uptake calculated from TPR spectra in all cases was lower than needed for the complete reduction of MoO3 to metallic molybdenum and was between 1.5 and 1.8 mol of H2/ mol of Mo. We suppose that oxidized Pt-Mo alloys accumulate hydrogen at ambient temperature. The consumption of H2 at ambient temperature cannot be detected in TPR experiments and can account for the underestimation of H2 uptake over Pt/ MoOx samples. To fix the amount of hydrogen adsorbed at ambient temperature, a vacuum static volumetric setup was used. The results confirmed that a significant consumption of hydro-



Figure 6. XRD patterns obtained during in situ reduction of Pt30Mo70-Ox sample (o: Pt; b: Mo5O7(OH)8; and *: unknown phase).

TABLE 3: Binding Energies from XPS Analysis binding energy (eV) sample

Figure 7. TPR profiles of oxidized Pt-Mo alloys and MoO3.

TABLE 2: Hydrogen Consumption over Pure Molybdena and Pt-Mo Oxidized Samples H2 consumption (mol of H2/mol of Mo) sample MoO3 Pt75Mo25-Ox Pt30Mo70-Ox Pt2Mo98-Ox

30 °C (static volumetric setup) 30-1000 °C (TPR) total 0 0.2 0.8 0.7

3.0 1.5 1.8 1.5

3.0 1.7 2.6 2.2

gen takes place at ambient temperature (Table 2). These observations are in line with XRD experiments, which point to the formation of new molybdenum- and hydrogen-containing phases at ambient temperature. Such processes can take place due to the activation of hydrogen on Pt sites at low temperatures and the transfer of H atoms to molybdena by a spillover mechanism as described in the literature.31 It should be noted, however, that the total H2/Mo molar ratio was still lower than 3: these estimations suggest that a part of molybdenum is not completely oxidized during the oxidation

Pt 4f7/2

Pt Mo MoO3 Pt75Mo25 initial Pt75Mo25-Ox

71.4 71.5

Pt75Mo25-OxRed

71.4

Pt30Mo70 initial Pt30Mo70 initial after sputtering Pt30Mo70-Ox

71.0 71.0 70.9

Pt30Mo70-OxRed

71.0

Pt2Mo98 initial Pt2Mo98-Ox

71.2 71.5

Pt2Mo98-OxRed

71.5

Mo 3d5/2

O 1s

228.0 233.6 228.0 228.2 232.6 229.2 230.9 232.5 232.5 228.0 231.3 232.7 231.3 232.6 232.5 229.9 232.5 230.1 232.6

532.7 531.4

71.0

533.3 532.9 530.7 530.7 530.7 530.7 530.6 530.7 530.7

step or is only partially reduced during the reduction step. Thus, the treatment of Pt/MoO3 systems in H2 results in hydrogen consumption already at ambient temperature, destruction of the MoO3 phase, and formation of molybdenum oxide hydroxide or molybdenum bronzes. Pt promotes the interaction of hydrogen with molybdena, causing its reduction at lower temperatures with respect to pure molybdenum oxide. 3.4. Chemical State of Pt and Mo in Initial and Activated Samples. The XPS analysis of the surface was performed on initial and activated samples. For reference, the spectra for pure platinum and molybdenum were also recorded. Pt4f7/2 BE for all samples were only slightly different from those obtained for bulk platinum (Table 3and Figure 8). These small shifts could be due to some Pt-Mo interactions or to the presence of oxygen on the platinum surface, while deep platinum oxidation is


Zosimova et al.

Figure 8. XP spectra of initial and activated Pt75Mo25 samples.

unlikely as the Pt4f7/2 binding energies of platinum oxide are usually 72.2-73.4 eV.32 Thus, in all cases, platinum was in a metallic state even after oxidative treatment. Mo3d5/2 biding energies of pure Mo and the initial Pt75Mo25 sample were almost identical and equal to 228.0 eV (Table 3). In the case of initial Pt30Mo70 and the Pt2Mo98 samples, the spectra contained duplet signals corresponding to Mo(VI) oxide with a binding energy of 232.5 eV, although after sputtering, the Mo3d spectra reported a BE of 228.0 attributed to Mo(0). This points to the formation of molybdenum oxide layers on the surface of the samples with a high Mo content. Oxidative treatment of all the samples was accompanied by the appearance of a peak at 232.5 eV corresponding to the transformation of Mo(0) into Mo(VI) oxide, which is in agreement with XRD and SEM data (Table 3 and Figure 8). However, for the Pt75Mo25 sample, a part of molybdenum remained in the metallic state, which was confirmed by the presence of a 228.2 eV signal (Figure 8). These results correlate with the H2 consumption measurements, which showed the lowest H2/Mo molar ratio for the Pt-rich sample, and TGA results, which demonstrated a lower increase in weight than was needed for the transformation of Mo into MoO3. Probably, the excess of platinum prevents the neighboring molybdenum from complete oxidation; for the samples with a lower platinum content, this effect was less pronounced. The peaks at 231.3 and 229.9 eV observed for the Pt30Mo70-Ox and Pt2Mo98-Ox samples can be attributed to Mo(V) and Mo(IV), respectively.33 Reduction at 450 °C did not change the XP spectra significantly. Only in the case of the Pt75Mo25-OxRed sample did the new signals with binding energies of 230.9 and 229.2 eV appear, indicating the formation of MoOx (x < 3). The metallic molybdenum may also be present, but a corresponding peak may be masked by other signals. 3.5. Toluene Hydrogenation. The catalytic activity of the Pt-Mo alloy systems was investigated in a toluene hydrogena-

TABLE 4: Toluene Hydrogenationa toluene conversion (%) sample

initial

OxRed-1b

OxRed-2c

Pt Mo Pt75Mo25 Pt30Mo70 Pt2Mo98

46 0 3 0 0

60 0 100 100 13

74 0 100 100 22

WHSV ) 0.7 h-1, 150 °C, 1 h of time-on-stream. b After first oxidation-reduction cycle. c After second oxidation-reduction cycle. a

tion reaction. At the experimental conditions selected, methylcyclohexane was the major reaction product. The pure platinum catalyst showed a noticeable catalytic activity (Table 4). Toluene conversion increased from 46 to 60% after the first OxRed treatment, and the next oxidation-reduction cycle increased the toluene conversion up to 74%. This behavior can be attributed to the formation of microcracks in the platinum particles after consecutive oxidation-reduction treatments, resulting in the increase of a Pt active surface for the molecules of the reagent. After further oxidation-reduction cycles, the changes in toluene conversion were not significant. All as-synthesized Pt-Mo alloys were practically inactive in toluene hydrogenation (Table 4). The absence of activity in spite of the presence of platinum in the sample may have several explanations. On the one hand, after Pt alloys with another metal, the electronic properties of platinum may change in such a way that dissociation (activation) of dihydrogen, which is a necessary step in the hydrogenation process, is not taking place.34 On the other hand, the effect could be due to the smaller size of the Pt sites. As described before,35 the decrease of the metal particle size below 3 nm leads to a significant drop in catalytic activity in benzene and toluene hydrogenation. In the case of the platinum alloyed catalysts, the homogeneity of platinum dis-



Figure 10. Schematic representation of H2 activation over the catalytic systems with low (a) and optimal (b) Pt/Mo ratios.

Figure 9. Specific APt (a) and effective Acat (b) activities of the catalysts in toluene hydrogenation.

tribution is very high, and the number of neighboring platinum atoms could be not sufficient for the formation of catalytically active sites. The activation procedure consisting of oxidation followed by reduction treatment led to a significant increase in the catalytic activity of the samples. The toluene conversion on the Pt75Mo25-OxRed and Pt30Mo70-OxRed samples increased up to 100%, which exceeded the conversion over pure platinum (Table 4). The Pt2Mo98 sample showed a lower conversion of toluene, which reached only 20% after two oxidation-reduction treatments. It is necessary to note that the pure Mo sample was completely inactive both in the metallic state and after the oxidation-reduction cycle. To compare the efficiency of the Pt-Mo systems with different compositions, the effective (Acat) and specific (APt) activities of the catalysts were estimated as described in the Experimental Procedures. The results presented in Figure 9 show that the specific activity of the Pt sites (APt) in the Mo-rich samples Pt30Mo70 and Pt2Mo98 was 2 orders of magnitude higher than in the case of the Pt-rich sample Pt75Mo25. This effect can be attributed either to different activities of Pt sites in the samples or to participation of Mo sites in the catalytic process. The XPS data suggest that the chemical state of platinum (Pt0) is the same in all samples (Table 3); the difference in Pt particle size was also not significant (Table 1). It was therefore proposed that molybdenum oxide may provide for additional active sites, which can participate in toluene hydrogenation. However, the comparison of the effective activities (Acat) revealed significant differences for Mo-rich samples: the Acat value was found to be 10 times higher for the Pt30Mo70 sample with respect to Pt2Mo98 (Figure 9b). This observation suggested that not all molybdenum oxide can participate in toluene

Figure 11. Amount of H2S needed for total catalyst deactivation during toluene hydrogenation at 150 °C under atmospheric pressure.

hydrogenation. These results can be accounted for by the mechanism of toluene hydrogenation suggested by Wang et al.31 According to this mechanism, the activation of dihydrogen molecules takes place on platinum sites by dissociative adsorption. Hydrogen spillover is further responsible for the migration of active hydrogen to molybdena and the formation of additional active sites for toluene hydrogenation. Our results suggest that not all MoOx particles can participate in hydrogen spillover but only those that are in close vicinity to Pt as is schematically depicted in Figure 10. Thus, in the case of the sample with a low Pt/Mo ratio, most of MoO3 remains inactive (Figure 10a). Therefore, for the optimal catalyst, the ratio between Pt portholes and molybdena should be balanced (Figure 10b). Among the samples studied, Pt30Mo70-OxRed appeared to be the most efficient. 3.6. Sulfur Poisoning. A sulfur tolerance test was performed for all catalysts prepared. The addition of H2S to the feed during toluene hydrogenation over pure Pt led to the rapid decrease of toluene conversion due to Pt active site poisoning. The sulfur tolerance of the Pt-Mo systems was significantly higher. To compare the sulfur tolerance of different catalysts, the amount of H2S required for complete catalyst poisoning was determined in each experiment. These values obtained were normalized to accessible Pt atoms and are presented in Figure 11. It is observed that the amount of H2S needed for poisoning the studied systems increased in the following range of catalysts: Pt < Pt75Mo25 < Pt2Mo98 < Pt30Mo70. The best results were obtained over Pt30Mo70 with a well-balanced amount of Pt and MoOx active

14798 J. Phys. Chem. C, Vol. 111, No. 40, 2007 sites. The noticeable hydrogenation activity in the presence of H2S can be maintained over this catalyst more effectively. The results point to different deactivation mechanisms of Pt sites and Pt-Mo sites in the presence of H2S. Further work is needed to obtain deeper insight into these mechanisms. 4. Conclusion Pt-Mo alloys subjected to oxidative and reductive treatments have been shown to give active catalysts of toluene hydrogenation. According to XRD, SEM, and TEM, the catalysts are composed of well-dispersed Pt particles on molybdenum oxide. The enhanced catalytic activity of these systems could be accounted for by the formation of additional active sites due to the interaction of platinum with molybdenum oxide. Results obtained over the samples with a Pt content in the range of 2-75 atom % suggest that for the optimal catalyst, the Pt and molybdena sites should be balanced. Among the samples studied, Pt30Mo70-OxRed exhibited the best catalytic performance in terms of activity and sulfur tolerance. Acknowledgment. P.Z. thanks the LG Chem Company for a personal scholarship (2006). References and Notes (1) Navarro, R. M.; Pawelec, B.; Trejo, J. M.; Mariscal, R.; Fierro, J. L. G. J. Catal. 2000, 189, 184. (2) Simon, L. J.; van Ommen, J. G.; Jentys, A.; Lercher, J. A. J. Catal. 2001, 201, 60. (3) Szymanski, R.; Charcosset, H.; Gallezot, P.; Massardier, J.; Tournayan, L. J. Catal. 1986, 97, 366. (4) Paal, Z.; Koltai, T.; Matusek, K.; Manoli, J.-M., Potvin, C.; Muhler, M.; Wild, U.; Tetenyi, P. Phys. Chem. Chem. Phys. 2001, 3, 1535. (5) Pinzon, M. H.; Centeno, A.; Giraldo, S. A. Appl. Catal., A 2006, 302, 118. (6) Pereira da Silva, M. A.; Schmal, M. Catal. Today 2003, 85, 31. (7) Hoang-Van, C.; Zegaoui, O. Appl. Catal., A 1995, 130, 89. (8) Hoang-Van, C.; Zegaoui, O. Appl. Catal., A 1997, 164, 91. (9) Matsuda, T.; Hanai, A.; Uchijima, F.; Sakagami, H.; Takahashi, N. Microporous Mesoporous Mater. 2002, 51, 155. (10) Konopny, L. W.; Juan, A.; Damiani, D. E. Appl. Catal., B 1998, 15, 115.

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Synthesis, Characterization, and Sulfur Tolerance of PtâMoOx

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