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Aggregation State of Pt-Au/C Bimetallic Catalysts Prepared by Surface Redox Reactions P. Del Angel,*,†,‡ J. M. Dominguez,‡ G. Del Angel,† J. A. Montoya,‡ E. Lamy-Pitara,§ S. Labruquere,§ and J. Barbier§ Departamento de Quı´mica, Universidad Auto´ noma Metropolitana-Iztapalapa, Apdo. Postal 55-534, C.P.09340 Me´ xico, D.F.; Instituto Mexicano del Petro´ leo, Programa de Simulacio´ n Molecular, Eje Central L. Ca´ rdenas 152, 07730 Me´ xico, D.F.; and LACCO, Unite´ de Recherche Associe´ e au CNRS, DO 350, Universite´ de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Ce´ dex, France Received December 9, 1999. In Final Form: May 1, 2000 The bimetallic catalysts Pt-Au/C (graphite) were prepared by selectively depositing Au on supported monometallic Pt/C catalysts by means of the reduction “in situ” of AuCl4-. The parent metal (Pt) was used as the reducing agent for the direct redox reactions (“DR”), while the second redox method used was the refilling method (“RE”), which consisted in adsorbing hydrogen first on the parent metal (Pt) and subsequently reducing the AuCl4- species by contact with the Pt-H interface at low temperature. The catalysts PtAu/C were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and high-resolution electron microscopy (HREM). The interaction between Pt and Au was explored at the nanometer scale by means of EDS and HREM, with the aim of characterizing the aggregation state of the metals in the Pt-Au/C systems prepared by the novel redox methods. The composition of the individual metal particles of the solids “as-prepared” (i.e., DR-A and RE-B) indicated the presence of both metals, Pt and Au. However, a thermal reduction at 300 °C under H2 seems to redistribute the Au phase in the DR-A solids, then the particles remain bimetallic, but in the RE-B solids the Au concentration increases substantially; Au migrates toward the small particles (i.e., 5 e D e 7 nm), while the bigger particles (i.e., D g 10 nm) show almost pure Pt. A partial Au coating of certain Pt facets is probable, which indicates that the coating mechanism is selective and could influence the catalytic properties of the bimetallic Pt-Au/C catalysts.
Introduction The supported bimetallic systems Pt-Re/Al2O3 and PtSn/Al2O3 have been the industrial catalysts of choice for obtaining high octane gasoline and hydrogen by means of the naphtha reforming process.1,2 Also, the Pt-M/graphite systems (M ) metal from groups VIII or IB) are potential electrocatalysts for the new fuel cells technology.3 From a fundamental viewpoint, the addition of a second metal to the Pt phase in supported catalysts improves the selectivity, the resistance to poisons,4,5 and the capacity for limiting undesirable reactions or enhancing the more interesting ones.6 At the microscopic scale, the second metal phase may block the active sites present on the surface of the metal (Pt) particles, or it can redistribute the surface sites arrays. Thus, any two metals M1 and M2 can interact in several ways, associating each other in simple bimetallic aggregates, where each metal keeps their own character, or they may form true homogeneous solid solutions with unexpected properties. The traditional methods used for preparing supported bimetallic catalysts consist of the simultaneous or sequential impregnation of * To whom correspondence should be addressed. E-mail: pangel@ www.imp.mx. † Universidad Auto ´ noma Metropolitana-Iztapalapa. ‡ Instituto Mexicano del Petro ´ leo. § Universite ´ de Poitiers. (1) Davis, B. H. J. Catal. 1977, 46, 348. (2) Thornton, D. P. Pet. Chem. Eng. 1969, 41 (5), 21. (3) Sinfelt, J. H. US Patent 3 953 368, 1976, assigned to Exxon. (4) Sinfelt, J. H. Bimetallic Catalysts: Discoveries, Concepts and Applications; John Wiley and Sons: New York, 1983. (5) Weisang, J. E.; Engelhard, P. US Patent 3 700 588, 1972, assigned to Companie Franc¸ aise de Reffinage (C.F. R.). (6) Barbier, J. Advances in Catalyst Preparation, Study No. 4191 CP, Catalytica Studies Division. Mountain View, 1992.
the support material with metallic salt solutions, but these procedures lead to uncontrolled interactions between both metals, leading most of the time to inhomogeneous multiphase domains. Alternatively, new methods have been devised recently6-17 for preparing bimetallic catalysts by means of redox type reactions, which have the advantage of reducing the second metal “in situ”.6 Using these methods, Barbier et al.17 reported the preparation of Pt-Au/SiO2 catalysts by means of the selective deposition and reduction of AuCl4- species on the parent monometallic Pt/SiO2 catalyst, which is exposed or not to hydrogen, depending on the method used. This might lead to selective interactions between the metals involved in the synthesis, and that is why the present work focused (7) Margitfalvi, J.; Szabo´, S.; Nagy, F.; Go¨bo¨los, S.; Hegedu¨s, M. In Preparation of Catalysts III; Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier: Amsterdam, 1983; Vol 16, p 473. (8) Szabo´, S.; Nagy, F.; Mogor, D. Acta Chim. Hung. 1977, 1, 93 (1), 33. (9) Bakos, I.; Szabo´, S. J. Electroanal. Chem. 1993, 344, 303. (10) Menezo, J. C.; Denanot, M. F.; Peyrovi, S.; Barbier, J. Appl. Catal. 1985, 15, 353. (11) Barbier, J.; Menezo, J. C.; Montassier, C.; Naja, J.; Del Angel, G.; Domı´nguez, J. M. Catal. Lett. 1992, 14, 37. (12) Barbier, J.; Boitiaux, J. P.; Chaumette, P.; Leporq, S.; Menezo, J. C.; Montassier, C. Eur. Patent 380 402, 1990, assigned to Institut Franc¸ ais du Pe´trole. (13) Marecot, P.; Barbier, J.; Mabilon, G.; Durand, D.; Prigent, M. Eur. Patent 92 90516-2, 1992, assigned to Institut Franc¸ ais du Pe´trole. (14) Dumas, J. M.; Geron, C.; Hadrane, H.; Marecot, P.; Barbier, J. J. Mol. Catal. 1992, 77, 87. (15) Menezo, J. C.; Hoang, L. C.; Montassier, C.; Barbier, J. React. Kinet. Catal. Lett. 1992, 46 (1), 1. (16) Barbier, J.; Dumas, J. M.; Geron, C.; Hadrane, H. Appl. Catal. 1990, 67, L1. (17) Barbier, J.; Mare´cot, P.; Del Angel, G.; Bosch, P.; Boitiaux, J. P.; Didillon, B.; Dominguez, J. M.; Schifter, I.; Espinosa, G. Appl. Catal. A: Gral. 1994, 116, 179.
10.1021/la9916171 CCC: $19.00 © 2000 American Chemical Society Published on Web 08/09/2000
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Table 1. Metal Content Determined by Atomic Absorption Spectroscopy of the Catalysts Pt/C and Pt-Au/C catalysta
Pt wt %
Au wt %
preparation conditions
MC DR-A DR-AR RE-B RE-BR
3.61 3.72 3.54 3.61 3.47
0.46 0.32 0.42 0.33
reduced non reduced reduced non reduced reduced
a
MC ) monometallic catalyst, DR ) direct redox; RE ) refilling.
on the study of the aggregation state of the metals Pt and Au in a bimetallic Pt-Au/C system prepared by those redox methods.17 In the present case, the graphite support was chosen because its relative transparency to the electron beam,18 which was used as a probe for the characterization of the metal phase. Also, the conductive properties of graphite might influence the deposit of the second metal (i.e., Au), leading to deposition of gold on the support phase, away from the Pt particles, due to the even potential of the Pt/C system and the modifier represented by the second metal (Au). Previous reports19 on the Pt-Au/SiO2 bimetallic catalysts prepared by the traditional incipient wetness techniques indicated that at low Au concentrations (i.e., 0.3 wt % Au-1 wt % Pt) Pt and Au do not interact each other, but at higher concentrations, i.e., 0.7 wt % Au-1 wt % Pt, they do interact, coexisting for most of the size ranges, especially in the particles having diameters of less than 10 nm, approaching the composition Au3Pt. Other alternative methods for preparing unsupported Pt, Au, and Pt-Au nanoparticles have been reported recently20 using microemulsions, which lead to monodispersed colloidal metallic particles. In this case the size of the small metal particles depends on the micellar aggregates, which act as “reaction cages” where the metallic nuclei are formed. In the present work, the bimetallic catalysts Pt-Au/C (graphite) were prepared by selectively depositing Au on Pt/C using the parent metal (Pt) as the reducing agent (i.e., direct redox reaction “DR”). A second method was used (i.e., refilling method “RE”), which consisted of adsorbing hydrogen first on the parent metal (Pt); then the second metal (Au) was added and subsequently reduced by contact with the Pt-H interface at low temperature, leading to the selective deposition of the second metal (Au) on specific Pt surface sites, i.e., the more active sites for hydrogen adsorption. Therefore, the possible interaction between Pt and Au as well as the possible influence of the graphite support was explored at the nanometer scale by means of energy dispersive spectroscopy (EDS) and high-resolution electron microscopy (HREM), with the aim of characterizing the aggregation state of the metals in the Pt-Au/C system prepared by the aforementioned methods.21 Experimental Methods Monometallic Catalysts. The preparation of the monometallic Pt/C catalysts (i.e., the parent catalysts labeled MC in Table 1) was carried out by means of the impregnation method, using a high-purity (i.e., 99.9% C) powder graphite support LONZA (18) Phillips, J.; Weigle, J.; Herskowitz, M.; Kogan, S. Appl. Catal. A: Gen. 1998, 173, 273. (19) Sachdev, A.; Schwank, J. J. Catal. 1989, 120, 353. (20) Nagy, J. B.; Barette, D.; Fonseca, A.; Jeuniecu, L.; Monnoyer, Ph.; Piedrigross, P.; Ravet-Bodart, I.; Verfuillie, J. P.; Wathelet, A. In Nanoparticles in Solids and Solutions; Fendler, J. H., De´ka´ny, I., Eds.; Kluwer Academic Publishers: Dordrecht, 1996; Vol. 18, p 71. (21) Williams, D. B.; Carter, C. B. Transmission Electron Microscopy; Plenum Press: New York, 1996.
LT10, which has a particle diameter between 2 and 20 µm, and a BET surface area equal to 12 m2/g. The platinum was deposited from a solution of hexachloroplatinic acid, H2PtCl6 (Aldrich, 99.9%), in absolute ethanol (Merck, 99.9%). The required volume of solution was taken to have a concentration of 4 wt % Pt with respect to the support, and then an additional volume of ethanol was added to complete 50 mL of ethanol by gram of catalyst. The impregnating solution was put in contact with the graphite support under vigorous stirring for 30 h at room temperature, bubbling nitrogen until complete dryness, and then the solvent excess was eliminated by heating mildly at 70 °C for 15 h. Finally, the Pt/C catalysts were treated thermally at 300 °C under H2, for 3 h, until complete reduction. The TPR (10 °C/min in pure H2) profile indicates that the main hydrogen consumption occurred between 100 and 200 °C, indicating the complete reduction of the metal phase. This was verified by means of DTA and TGA together, showing that above 200 °C the mass loss variation is only 0.3%, which is very different to the 5.04% variation registered below 200 °C. This indicates that the solids are almost completely reduced, i.e., 93.4%, below 200 °C. The metal dispersion was determined using the metal content data as an initial parameter, and this allowed the calculation of the amount of the second metal needed for preparing the Pt-Au bimetallic catalysts. In the present case the Pt and Au content was determined by atomic absorption spectroscopy (AAS), leading to 3.61 wt % Pt0 for the monometallic Pt/C (labeled MC in Table 1). Using the hydrogen pulse chemisorption method as reported previously,22 with stoichiometric ratio H/Pt ) 1, the Pt dispersion in the monometallic Pt/C catalysts was equal to 12.5%. Bimetallic Catalysts. The parent catalyst Pt/C (MC) was modified by the addition of gold using two methods: (a) the “refilling” method (RE), in this case the reducer is preadsorbed on the metallic surface (Pt) of the parent catalyst (hydrogen is most commonly used), and (b) the direct redox (DR) reaction, the reducer of the modifier is the parent metal (Pt). Thus, a series of Pt-Au/C bimetallic catalysts were made by modifying the initial Pt/C solids with a second metal, i.e., Au, according to the data shown in Table 1. The experimental procedure was similar to the one reported previously.17 (a) Refilling (RE) Method. The Pt-Au/C catalyst was obtained by the addition of the HAuCl4 solution to the reduced monometallic Pt catalyst, according to the following reaction:17
3PtH + AuCl4- f Pt3Au + 4Cl- + 3H+
(1)
The first step consisted of introducing a hydrogen flow for 1 h and then a nitrogen flow for 0.5 h; this treatment was enough to eliminate the hydrogen reversibly adsorbed, and finally the HAuCl4 solution was added, keeping the pH ) 1, by means of HCl diluted in H2O. The Au concentration was calculated in function of the amount needed to form two monolayers of Au on top of the parent metal (Pt), i.e., 0.3 wt % Au. The Au monolayer corresponds to the amount equivalent to one monolayer of hydrogen (i.e., H/Pt ) 1) adsorbed on the Pt surface, according to eq 1. The reason for introducing the Au amount needed for two layers was justified in terms of the possible spreading of the Au metal on the graphite support, due to the conducting properties of the latter. The mass balance was verified using atomic absorption spectroscopy (AAS) by means of the analysis of the final solids and the remaining solution. The label RE-B means the bimetallic Pt-Au catalyst prepared by reducing “in-situ” the Au phase, while the second one, RE-BR, means the same catalyst treated under H2 at 300 °C, 1 h, after the deposition of the Au phase. (b) Direct Redox (DR) Method. In this case, the parent metal (Pt) may act as the reducing agent for the second metal species (i.e., Au3+).17,23 The solids DR-A were prepared only by contacting the reduced Pt/C with the HAuCl4 solution, while the DR-AR solids were obtained by reducing the former DR-A at 300 °C under H2 for 1 h. All these materials contained a nominal concentration of 0.3 wt % Au, which was verified by means of (22) Freel, J. J. Catal. 1972, 25, 139. (23) Handbook of Chemistry and Physics, 62nd ed.; CRC Press: Boca Raton, FL, 1982.
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atomic absorption spectroscopy. The experimental compositional data are reported in Table 1. X-ray Diffraction. The X-ray diffraction (XRD) patterns were obtained in a Siemens D-500 diffractometer fitted with a Cu tube (35 kV, 25 mA) and a graphite monochromator for eliminating the Kβ lines. The identification of the X-ray peaks corresponding to Au0 and Pt0 was performed using the JCPDS database. The Rietveld analysis was made using the program FULLPROF.98, which was applied subsequently for determining the cell parameters and the mean crystallite diameter, i.e., the diameter of the small metal particles. The initial set of parameters included the positions reported for the FM3M space group with the m3m Wyckoff position for both Au0 and Pt0 systems. In this way, some important parameters were determined, as for example the background in the region of interest, the cell parameters, the crystallite (i.e., metal particle) diameters, the peak profile, i.e., Pt0(111) and Au0(111), and the temperature factor (Debye-Waller). Electron Microscopy and EDS Characterization. The mean particle diameters and the structural and compositional properties of the solids were determined by means of a TEMSCAN Phillips CM120 electron microscope and a EDS Edax system fitted to the former instrument. In addition, a JEOL 3010 highresolution electron microscope fitted with an EDAX unit was used at 300 kV for obtaining the lattice images, as well as the compositional analysis of the individual metal particles. The surface (Ds) metal particle diameter was determined according to the statistical equation:
Ds )
∑n D /∑n D 3
i
i
i
i
2
i
(2)
i
where Di is the diameter measured directly from the electron micrographs and ni is the number of particles having the diameter D i. The metal thin film approximation (MTF) described by CliffLorimer24 was applied for calculating the relative concentration of Pt and Au, using the integrated intensities of the platinum and gold X-ray emission lines, by means of the following equation:
CPt/CAu ) kPt-Au(IPt/IAu)
(3)
where kPt-Au is the compositional constant, approximately equal to 1, CPt and CAu are the relative metals compositions expressed by their relative percent, and IPt and IAu are the corresponding EDS peak intensities. The capability of the STEM unit for focusing the electron beam down to 2 nm allowed the compositional analysis of individual Pt metal particles in the size range between 2.5 and 20 nm. The minimal mass fraction of Au detected was around 1.5% of the total mass irradiated within the interaction volume,24 that is, around 10 -18 g; this is enough to detect a few atoms of Au coating the surface of Pt particles in the size range between 2.5 and 20 nm. Further details on the minimal mass fraction are reported elsewhere.24 Figure 1 illustrates the experimental energy resolution level used in this work for differentiating the Pt and Au EDS peaks. The close vicinity of the Pt-LR and Au-LR emission lines leaves a difference of only 270 eV between two maxima, i.e., EPt-LR ) 9.44 keV and EAu-LR ) 9.71 keV, which was resolved in this work using the EDAX detector with an energy resolution of about 130 eV.
Results X-ray Diffraction. The analysis of the whole series of bimetallic catalysts by means of X-ray diffraction (XRD) is condensed in Figure 2, where the peaks corresponding to Pt, i.e., (111), (200), and (220), are clearly outlined against the typical graphite peaks. The Pt diffraction peaks appear rather broad, which is typical of the low metal particle size in the range between 2 and 25 nm. One observes from Figure 2 that the X-ray pattern corre(24) Domı´nguez, J. M.; Simmons, G. W.; Klier, K. J. Mol. Catal. 1983, 20, 369.
Figure 1. Typical EDS spectra: (a, top) Pt particle, (b, middle) Au particle, (c, bottom) Pt-Au particle.
sponding to DR-AR shows a small peak appearing around 38.18° (2Θ), which is clearly resolved from the Pt (111) peak; this small peak corresponds to Au(111) having a d ) 2.355 Å, and it appears more clearly outlined for DRAR with respect to the other patterns; for example, in the RE-BR samples (Figure 2) a small shoulder is barely detached from the more prominent Pt(111) peak. The blank consisting of pure graphite or Pt/C did not show any diffraction peak around this position. Therefore, this lead us to conclude that in the samples reduced at high temperatures, DR-AR and RE-BR, the metallic gold atoms meet together and form crystallites, which are big enough for contributing to the diffraction peak pattern.
Pt-Au/C Bimetallic Catalysts
Figure 2. X-ray diffraction patterns corresponding to the monometallic Pt/C (MC) and the Pt-Au/C bimetallic catalysts: DR-A, DR-AR, RE-B, and RE-BR.
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Figure 5. Particle size distribution of the catalyst Pt/C (MC.). Table 2. Determination of the Mean Particle Diameters from TEM and XRD TEM
Figure 3. Bright field micrograph of Pt/graphite catalysts (MC).
Figure 4. Bright field micrograph of Pt-Au/C catalyst (REBR).
Bright Field Images. Figure 3 shows the TEM bright field image of a monometallic Pt/C catalyst (MC), where one observes the Pt particles size distribution on the support and their morphology constituted by the typical hexagonal outlines. These elongated and regular profiles correspond to various crystallographic orientations of the fcc Pt particles, i.e., [111] and [110] axis parallel to the electron beam,25 respectively. Various Moire´ type patterns are clearly observed across the Pt particles, due to the superposition of the Pt and graphite crystalline lattices, which form a bicrystal system. (25) Dominguez, J. M.; Yacaman, M. J. J. Catal. 1980, 64, 223.
XRD
catalyst
Ds (nm)
Pt (nm)
Au (nm)
MC DR-A DR-AR RE-B RE-BR
11 8 10 9 10
8 10 11 10 12
7 16 6 5
Figure 4 shows a TEM bright field image of the bimetallic Pt-Au/C catalyst prepared by the “refilling” method and reduced under hydrogen at 300 °C (RE-BR). A preferential distribution of the Pt metal particles is observed around the rims of the graphite flakes. Also, the composite particle at the center of the micrograph shows an apparent variation of density that could lead to misleading information concerning the metal density variation in a single bimetallic aggregate, but in fact this can only be confirmed by means of the EDS chemical analysis at the particle scale. Figure 5 shows the histogram corresponding to the particle size distribution for the monometallic Pt/C catalysts, while Figures 6-9 contain the histograms for the Pt-Au/C catalysts series. As observed, there is a trend in the particle size distribution for the samples treated at 300 °C under H2, toward the coalescence of the metallic particles, while the untreated solids showed a behavior similar to the monometallic catalyst. The mean particle diameters were calculated using the statistical equation for the surface (Ds) diameters, and the results are reported in Table 2. EDS Analysis. The quantitative compositional analysis of individual metal particles was realized by means of EDS and the results are shown in Figures 10-13 for a series of particles in each sample. As expected, the catalysts DR-A and RE-B showed the higher Pt content with respect to Au. In contrast to this behavior, the catalysts reduced at 300 °C, DR-AR and RE-BR, show a substantial increase in their Au content. This is clearly shown by the analysis of the RE-BR catalysts (Figure 13), which were constituted by two kinds of particles, the first one consisting of small rounded metal particles, having a particle diameter between 5 and about 7 nm, and a high Au content (Figure 13), while the second kind were particles in the middle size range (D ≈ 10 nm), presenting elongated hexagonal shapes and being constituted of almost pure Pt. Then, the main compositional difference between the nonreduced samples, i.e., DR-A and RE-B, and those reduced at the final stage, DR-AR and RE-BR, seems to be the higher Au content for the
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Figure 6. Particle size distribution of the catalyst Pt-Au/C (DR-A).
Del Angel et al.
Figure 9. Particle size distribution of the catalyst Pt-Au/C (RE-BR.).
Figure 7. Particle size distribution of the catalyst Pt-Au/C (DR-AR.).
Figure 10. EDS microanalysis of DR-A catalyst.
Figure 8. Particle size distribution of the catalyst Pt-Au/C (RE-B.).
Figure 11. EDS microanalysis of DR-AR catalyst.
latter case. This lead us to conclude that the final stage of reduction at 300 °C provokes a substantial modification of the relative metal content of the particles, which is related to the higher mobility of the Au species and the lower melting point of Au0 (i.e., Tmp ) 1064 °C) with respect to Pt (i.e., Tmp ) 1772 °C). All the samples showed a certain proportion of particles containing only Pt, i.e., about 15%, but a minor proportion of particles contained only Au (i.e., about 5%). This might
indicate that not all the Au phase was in contact with the Pt phase initially, because the hydrophobic properties of the graphite support might enhance the diffusion of the aqueous species in solution. Also, the more labile Au species could form isolated islands during the thermal reduction, especially in the DR-AR solids. Table 3 compares the compositional values obtained from atomic absorption spectroscopy (AAS) and those obtained by EDS. The AAS results are “bulk” analyses, and they are similar
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Figure 12. EDS microanalysis of RE-B catalyst.
Figure 13. EDS microanalysis of RE-BR catalyst. Table 3. Chemical Analysis of Au and Pt by Means of Atomic Absorption (AAS) and Energy Dispersive (EDS) Methods (wt %) AAS metal contents
EDSa metal contents
AAS
EDS
samples
Au (%)
Pt (%)
Au (%)
Pt (%)
Au/Pt
Au/Pt
DR-A DR-AR RE-B BE-BR
0.46 0.32 0.42 0.33
3.72 3.54 3.61 3.47
19.5 46.1 18.0 43.8
80.5 53.9 82.0 56.2
0.12 0.09 0.12 0.09
0.24 0.85 0.22 0.78
a
Carbon not included.
each other, regardless of the treatment, i.e., before or after reduction. However, the results obtained by EDS are variable; for example, the content of Au in DR-AR and RE-BR solids was about twice with respect to the DR-A and RE-B samples. This is more evident from the comparison of the Au/Pt ratios for DR-A and RE-B, as determined by AAS and EDS, showing that the ratios detected by EDS are about twice with respect to the results obtained by AAS. This indicates a preferential deposition of Au on the metal particles in the size range between 5 and 7 nm. In contrast to these results, DR-AR and REBR show that their Au/Pt ratios, determined by EDS, increase about 8 times with respect to the AAS ratios. These are extremely high ratios that can only be explained in terms of the type of analysis performed by EDS and which will be discussed further in the sections below.
Figure 14. (a) HREM of DR-A bimetallic catalyst. (b) (2-D)FFT (Crisp program) of the lattice shown in (a). (c) Theoretical calculation of the (2-D)-FFT (CaRIne program) corresponding to (a).
High-Resolution Electron Microscopy. The samples DR-A and RE-BR were studied by HREM in the JEOL 3010 machine after their EDS analysis, to make sure that the particles in the region of interest were bimetallic. The HREM images belonging to the sample DR-A showed that most of the particles were identified as platinum (i.e., d(111) ) 2.26 Å), after the diffraction analysis shown in Figure 14b,c. These are the 2-D fast Fourier transforms obtained in the computer by means of the Crisp program,26 from the lattices displayed in Figure 14a. Figure 14c is the diffraction pattern simulated by means of the CaRIne program (i.e., version 3.1); the comparison between those 2-D optical transforms lead us to conclude that there are not strains proper of lattice variations. However, the EDS analysis indicated that both particles contained both metals, Pt and Au, but the high resolution images do not indicate any influence of the latter metal upon the former. The relatively small gold content is not enough to form extended planes, and the thermal conditions are not too strong for modifying the Pt lattice parameters, i.e., by (26) Weirich, F. E.; Ramlav, R.; Simon, A.; Hovmoleer, S.; Zou, X. Nature 1996, 382, 144.
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Figure 15. HREM of RE-BR bimetallic catalyst. This particle was identified as a composite of Pt (d ) 0.22 nm) and Au (d ) 0.235 and 0.206 nm). Table 4. Interplanar Distances (d) for Au, Pt, and Au3Pt (Ref: JCPDS Database) d (nm) (hkl)
Au
Pt
Au3Pt
111 200 220
0.235 0.204 0.144
0.226 0.196 0.138
0.225 0.196 0.139
intercalation of Au atoms between the Pt crystal planes. Then, the planes running parallel to the electron beam have their interplanar distance right as indicated in Figures 2 and 14a. Similarly, the HREM images of the sample RE-BR showed the evidence of further interaction between the metal particles, as shown in Figure 15, where two particles are in close interaction each other, one containing only Pt (i.e., d ) 0.222 nm, on the left-hand side) and the other containing only Au (i.e., d ≈ 0.235 nm, on the right-hand side); there, the grain boundary is indicated by arrows. This particle belongs to the sample submitted to thermal reduction at 300 °C, after the deposit of the second metal (Au); then the interaction between the metals is stronger than the case of particles belonging to the untreated samples. However, in both cases there is no evidence of platinum-gold solid solutions, and the lattice parameters of Au and Pt remain the same in the particle size range below 10 nm. Then, the characterization of the solid solutions is elusive, because the close vicinity of the (111), (020), and (110) interplanar distances for Au, Pt, and Au3Pt, as shown in Table 4, thus making it possible that the small difference, if any, falls within the experimental error. Discussion The compositional and structural analyses of small platinum and Pt-Au metal particles were performed, together with the analysis of the particle interaction derived from the preparation conditions proper of the new redox methods DR and RE. The physical methods such as energy dispersive spectroscopy (EDS) were applied at the subparticle level using a 2.0 nm electron beam diameter. The EDS detector allowed to distinguish between the EPt-LR ) 9.44 keV and EAu-LR ) 9.71 keV emission lines, leading to individual particle analysis below the 10 nm range. Also, the X-ray diffraction techniques showed clearly the presence of the main crystallographic planes in the metallic Pt particles, i.e., (111)/(200)/(220), which indicates the formation of extended facets on the highly crystalline Pt particles. This was confirmed by the TEM images, which showed the classical fcc-structured Pt
particles orientated with [111] and [110] axis parallel to the electron beam. Also, a small diffraction peak corresponding to Au(111) appeared in the XRD pattern of the reduced solids, i.e., DR-AR and RE-BR, giving a strong evidence on the mobility of the Au0 species, which seem highly labile on the graphite surface. The Au0 crystallites are big enough to contribute to the X-ray diffraction pattern. In parallel, the high-resolution electron microscopy methods having a resolution limit of about 0.17 nm were applied for obtaining the lattice images of the small metal particles. The metal particle size distribution was determined from the transmission electron micrographs, showing a trend toward the higher particle diameters, apparently caused by the coalescence of some particles, especially in the solids treated under H2 at 300 °C, i.e., Pt with Pt and Pt with Au, showing again that Pt particles originally anchored to the graphite support could have a relative mobility across the surface, especially under the reduction conditions at 300 °C in the presence of H2 (Figures 7 and 9). The characterization studies of the nonreduced solids, i.e., DR-A and RE-B, showed a poor interaction between the two metals. The XRD patterns of the DR-A and RE-B solids show a small peak like a shoulder, around 2Θ ) 38.18°, which corresponds to the Au0 (111) position, close to the Pt0 (111) peak appearing at 2Θ ) 39.76° (i.e., labeled MC in Figure 2). However, the shoulder is not defined as well as the diffraction peak corresponding to the DR-AR solids. Then, on the basis of the XRD results, it seems that the number and size of the Au0 crystallites in the nonreduced solids, i.e., DR-A and RE-B, are less important with respect to the reduced solid DR-AR. This result is compatible with the determinations made by EDS, which ruled out the presence of isolated monometallic particles but indicated always the presence of both metals in a single particle, as indicated in Figures 10 and 12. In addition, the high-resolution electron microscopy studies on the nonreduced solids DR-A and RE-B did not show a particular interaction between both metals; i.e., there is not a lattice distortion, neither apparent overgrowths on top of the Pt particles. Instead of this, Figure 14a shows the lattice resolution image of the DR-A solids with a clear definition of the main lattice planes of Pt across the particles, i.e., d(111) ) 0.226 nm and d(200) ) 0.196 nm. As observed in this typical image, there is no additional lattices superimposed on the original Pt lattice, indicating that Au atoms do not form periodic structures on top of the Pt particles. However, the EDS methods indicated the presence of Au in most of the particles belonging to DR-A and RE-B solids, but it might not form long-range ordering or specific structures on the Pt particles surface. Then, the presence of Au “adatoms” on the top surface of the Pt particles might not be discarded; in fact, the highresolution images such as Figure 14a show high contrast points on the top of some Pt particles, as the ones arrowed in Figure 14a, which might arise from very small Au aggregates on top of the Pt particles, i.e., Au dimers, trimers, etc. Further work is underway to verify this point and will be reported in a forthcoming publication. The possible formation of a Pt-Au bimetallic alloy with a defined structure in the nonreduced solids, DR-A and RE-B, is discarded from the evidence shown by the lattice resolution images and their corresponding Fourier transforms (i.e., Figure 14a-c); then, it is clear that there is neither lattice strains nor interplanar distances variation that could be attributed to the formation of a solid solution. This result coincides with previous studies on Pt-Au/ SiO2,19 reporting that on the 1 wt % Pt-0.3 wt % Au
Pt-Au/C Bimetallic Catalysts
catalysts there was not any alloy formation. Then, the formation of a solid solution between Pt and Au in a single particle seems elusive at the concentration levels reported here. The possible surface coating of the Pt particles by a thin layer of gold seems more probable in the case of the catalysts reduced by the RE-BR method; as indicated by its X-ray diffraction pattern in Figure 2, one observes the fixation of the Au0 phase, rather than the formation of big Au crystallites. However, the EDS analysis of the small particles in the size range between 5 and 7 nm of the RE-BR solids showed always the presence of both metals Pt and Au (i.e., left-hand side of Figure 13), while the hexagonal particles having larger diameters, i.e., D > 10 nm, showed a composition based on almost pure Pt. Then, it seems that the RE-BR solids contained small particles having both metals together (Figure 13), while the bigger particles were almost free of gold. In contrast to these results, the EDS analysis of the DR-AR solids indicated always the presence of both metals in the individual particles, forming Pt-Au bimetallic aggregates, but the XRD results (Figure 2) indicates that gold forms independent crystallites, too. In contrast to this result, in the RE-BR solids Au0 is associated with the smaller Pt particles, but the bigger ones are almost free of Au. Also, the EDS analysis of the RE-BR solids showed an unusually high Au content in the smaller particles, which seems too high and unrealistic, because the nominal amount of Au deposited in the solids was only 0.3 wt % Au. This lead us to conclude that the presence of several layers of Au coating the Pt particles could affect strongly the X-ray signal arising from the electron beam penetrating the Au-coated particles. The incoming beam going through the Au layers generate the emission of X-ray radiation from beneath the Au surface layer, in the Pt core; then the radiation can hardly escape from the particle toward the detector, because the strong absorption by the Pt and Au dense layers, which tends to diminish the intensity of the X-ray signal, impeding it to reach the detector. In summary, the Pt X-ray signal arising from the Pt core is attenuated, while the Au X-ray signal arising from the particles top surface is enhanced, because this is not being filtered at all, giving a higher Au X-ray signal with respect to the Pt signal. This in turn makes it that metal particles appear richer in Au, as shown by Figures 11 and 13. Also, in the RE-BR solids, the larger particles show only a small Au contents (i.e., Figure 13), which
Langmuir, Vol. 16, No. 18, 2000 7217
should indicate that those particles are coated by a thin Au layer only on certain facets, i.e., (200) planes but not in others like (111) or (220). Then, the incoming electron beam should hit the Au-coated facets as well as the uncoated ones, thus generating both signals, i.e., Pt and Au, where Pt is now higher than Au because the relative coating on certain facets only (Figure 13). In contrast to this situation, the small metal particles (SMP) in the solids RE-BR are coated completely, which agrees with the fact that SMP have a higher proportion of unsaturated surface sites with respect to the larger particles, making them more reactive. Thus, due to their smaller size range, those particles should have thinner layers of Au coating their surface; that is why the Pt signal arising from those particles is less important, i.e., Figure 13. Conclusions The new redox methods DR and RE lead to weak but selective interactions between two metals in bimetallic Pt-Au supported catalysts. The nonreduced DR-A catalysts form the true bimetallic aggregates, where Au is deposited on top of the Pt particles, forming small aggregates such as dimers, trimers, etc. Upon reduction at 300 °C under H2, most of the Au atoms leave the Pt particles and form crystallites of Au0 having a long-range ordering, but still a small fraction of Au coats the Pt particles. On the other hand, the catalysts prepared by the RE method showed the formation of a bimetallic catalyst just after the deposition and “in-situ” reduction of the AuCl4- species in solution; upon reduction at 300 °C under H2 the Au0 phase is retained on top of the Pt particles, with a marginal migration away of the Pt particles. However, in this case the Au0 phase coats preferentially the smaller particles, i.e., 5 e D e 7 nm, while the bigger ones, i.e., D > 10 nm, remain coated partially, leaving uncoated facets exposed to the electron beam. This could indicate that Au0 selectively covers certain facets of the Pt particles, which opens the possibility for designing specific ways to control the adsorptive and catalytic properties of the Pt-based catalysts. Acknowledgment. This work was partially supported by the Mexico-France collaboration Program PCP 32:IMPUAM-I-Poitiers University-CNRS-Conacyt. LA9916171
