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Structure and catalytic properties of Pt–Ir catalysts as a function of different substrate
Applied Surface Science 182 (2001) 1±11
Structure and catalytic properties of Pt±Ir catalysts as a function of different substrate A. Zecua-FernaÂndez, A. GoÂmez-CorteÂs, A.E. Cordero-Borboa, A. VaÂzquez-Zavala* Instituto de FõÂsica UNAM, Apdo, Postal 20-364, MeÂxico City 01000 D.F., Mexico Received 26 January 2001; accepted 26 May 2001
Abstract A series of Pt±Ir catalysts prepared by the impregnation method in three different supports and different atomic concentrations were characterized by conventional transmission electron microscopy (TEM), high resolution electron microscopy (HREM), X-ray diffraction (XRD) and n-hexane contact reaction as a catalytic probe. The reaction products were grouped in linear (hydrogenolysis products), isomers (2- and 3-methilpentane), cyclic (cyclohexane) and aromatic (benzene) hydrocarbons. An increment in hydrogenolysis products was observed as the amount of Ir was increased. This observation was extreme when Al2O3 and SiO2 were used as supports, but it was not so for the case of TiO2. From TEM results, it was concluded that the metallic particle size is a function of Ir content. From X-ray diffraction analysis, it was found that a unique crystalline phase containing atoms of Pt and Ir precipitates into the specimens under study. This phase was found to have a face-centered cubic structure and a lattice parameter whose value changes as long as the relative amounts of Pt and Ir does. A linear relationship was observed to hold between the lattice parameter of the precipitated phase and the relative concentration of Pt for specimens with metallic weights of 4 and 8% in a SiO2 support. This result may be taken as a direct experimental evidence of the fact that the crystalline structure of the phase-centered cubic phase found to precipitate in amorphous silica obeys the Vegard's law and therefore, the character of a solid solution Pt(x) Ir(1 x), where x is the Pt molar fraction, can be assigned to the structural nature of this phase. This new phase and the changes recorded in particle size could explain the changes in selectivity and catalytic activity observed in the n-hexane contact reaction. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Pt±Ir catalyst; X-ray diffraction; TEM
1. Introduction Tyson and Miller [1] estimated the surface energies of solid metals from measurements of free energies on liquids; in particular, they obtained the following surface energies for pure Pt and Ir: jPt 700 K 2430 erg=cm2 ; jIr 700 K 3010 erg=cm2 *
Corresponding author.
In connection with these measurements, it is known that in the case of a metallic melt, the metal with a lower melting point is segregated to the surface of the particles resulting from cooling the melt. Active surface area is the ®rst consideration in catalytic activity and, in general, a high metal surface area is generally required for a bimetallic catalysts to be of practical interest. Platinum is currently so expensive that bimetallic catalysts using this metal are of economical use only in the case of having a signi®cant fraction of the metallic atoms on the surface. Both, the surface
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composition and the size of the metallic particles is commonly correlated with catalytic activity and selectivity of the catalysts. In the case of alloys obtained from melts containing atoms of Pt and Ir, very little is known about the resulting crystalline phases and the surface composition of particles made of these alloys. This makes any attempt to interpret the data taken out during bimetallic Pt±Ir catalysts reactions very dif®cult. In reforming process carried out by using this sort of catalysts a rapid deactivation occurs due to the fact that the concentration of carbon residues can block both the metal and the acid sites and then the catalyst must be early regenerated [2]. In particular, in naphtha feedstocks reforming process the catalysts made of the noble metal on an alumina support deactivates during this process by a consequence of carbonaceous residues building upon the catalyst surface [3]. Sintering can occur during both reforming and regeneration operations at temperatures near to 770 K. Regeneration is commonly carried out with the purpose of burning the coke formed on the catalyst surface. It has been mentioned in several works [4,5] that Ir has the ability to destroy coke precursors by hydrogenolysis. Also it has been observed [6] that Ir could destroy coke deposits under a hydrogen atmosphere. In the work made by Barbier et al. [7], it was found that the addition of Ir to Pt acts in the same way on the amount of deposited coke at atmospheric pressure as all Pt does under increasing overall pressure for the coking reaction on SiO2. When coke is burned in an oxygen ¯ow, in the case of Pt±Ir, Ir is more susceptible towards oxidative sintering than Pt [8,9]. It has been observed [10] in several reactions that a Pt±Ir presul®dation reduced the hydrogenolysis activity of Ir, but sulfur addition increases the dehydrogenation rates for the monometallic and bimetallic surfaces. The aim of this work is to observe the changes in all morphology, size and crystalline structure, suffered by the Pt±Ir alloy particles found to precipitate in an amorphous SiO2 as a function of the Pt±Ir concentration ratio, and to observe also the changes in both activity and selectivity suffered by the Pt±Ir catalysts. 2. Experimental A series of catalysts with different relative atomic concentrations of Pt and Ir at 1% in metallic weight
Table 1 Catalysts series prepared by impregnation method Series
Support
Atomic percentage
Metallic weight (%)
1
Al2O3
Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
1
2
TiO2
Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
1
3
SiO2
Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
1
4
SiO2
Pt Pt±Ir Pt±Ir Pt±Ir Pt±Ir Pt±Ir Ir
(80±20) (60±40) (50±50) (40±60) (20±80)
4
5
SiO2
Pt Pt±Ir Pt±Ir Pt±Ir Pt±Ir Pt±Ir Ir
(80±20) (60±40) (50±50) (40±60) (20±80)
8
were prepared from aqueous solutions of H2PtCl6 and H2IrCl6 (Aldrich) and amorphous SiO2 and crystalline both Al2O3 and TiO2 supports as it is described in Table 1. In the case of the catalysts supported on amorphous SiO2, we also prepared another two series of samples at metallic weights of 4 and 8% in order to improve intensity measurements during X-ray diffraction analyses. Two new different atomic relative concentrations of Ir (40 and 60%) were added to Pt and included in both of these two series in order to make easier to ®nd any experimental correlation between the lattice parameter of the Pt±Ir crystalline phase forming the precipitated bimetallic particles and the relative atomic concentrations of Pt and Ir used to prepare the catalysts. In all the cases, the classic method of impregnation was used to prepare the
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catalysts. After impregnation, the supports were stirred for 1 h at 298 K and then evaporated and dried at 373 K for 12 h. The samples were then reduced by ¯owing hydrogen for 2 h at 773 K with a ¯ow rate of about 60 ml/min. Catalytic measurements were carried out under atmospheric pressure by using a continuous ¯ow laboratory-made microreactor coupled with a gas chromatograph Perkin-Elmer equipped with a ¯ame ionization detector. The reaction gas mixture was obtained by passing hydrogen through a saturator device containing n-hexane at 273 K. The reactor temperature was about 673 K and a continuous hydrogen ¯ow of about 20 ml/min was used. Due to the fact that the catalytic activity of the Ir catalysts is quite high its mass was varied in order to maintain the differential reactor behavior. The experiment was initiated by gently rising the temperature of the sample from 293 to 673 K while passing the hydrogen ¯ow through it. This ®nal temperature was maintained for 2 h time in order to reactivate the catalyst, and then the reaction was started. As usual, the catalytic selectivity is de®ned as moles of a particular product per moles of the total product times 100%. Conventional electron microscopy was done by means of a JEOL-100CX electron microscope, with Ê point to point resolution, equipped with a sidea 3.5 A enter goniometer. High resolution electron microscopy was performed with a JEOL-4000EX electron Ê point to point resolution. The microscope with a 1.7 A catalysts were ground to powder in an agate mortar and then mixed with ethanol to form a suspension. A drop of this suspension was deposited on a commercial copper grid, previously coated with a thin ®lm of carbon, and then transported to the microscopes under an inert atmosphere of N2 (99.99%). Some specimens were kept in the microscope overnight before analysis to minimize possible contamination.
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Powder X-ray diffraction techniques were used to characterize the atomic structure of samples prepared by adding different atomic concentrations of Ir and Pt to amorphous silica for overall metallic weights of about 4 and 8%. The Ir percentage atomic concentrations were of about 0, 20, 40, 50, 60, 80 and 100. This was done by means of a Siemens-D5000 diffractometer operating at working conditions of 20 mA and 30 kV and using Cu Ka-radiation as primary beam. Bragg re¯ections from the samples were monochromatized by using a graphite crystal in order to avoid other than the Ka-radiation component to reach the scintillation detector device of the diffractometer. Specimens for X-ray unit cell measurements were prepared by mixing powder from the samples under study with powder of standard silicon, in concentration of about 20% in overall weight, in order to use the silicon X-ray diffraction signals to calibrate the angular scale of the powder X-ray diffraction patterns. Xray analyses were carried out only on specimens prepared with amorphous silica supports because of the fact that the X-ray re¯ections from Al2O3 and TiO2 crystalline supports interfere with the metallic catalysts diffraction peaks. 3. Results and discussion 3.1. Conventional electron microscopy Particle size of the supported catalysts was measured by using several bright ®eld electron microscope images. These measurements are shown in Table 2. It is observed from this table that the particle size decreases when Ir is added to Pt regardless the support used to prepare the sample. This observation may indicate that re-dispersion of the metallic particles
Table 2 Average particle size of catalysts at 1% in metallic weight obtained by electron microscopy Catalysts
Ê) Particle size (A supported in Al2O3
Ê) Particle size (A supported on TiO2
Ê) Particle size (A supported on SiO2
Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
44 42 39 36 32
59 56 48 45 38
82 71 66 60 51
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is due to the presence of Ir atoms, The corresponding size distribution histograms appear in Fig. 1. In general, the bimetallic particles are observed to have irregular shapes as it is shown in the bright ®eld image of a specimen with a Ir relative concentration of about 50% on an amorphous SiO2 support (Fig. 2a). In the case of all Pt on an amorphous SiO2 support (Fig. 2b), the particles appear to have well developed crystalline habits. 3.1.1. X-ray diffraction Fig. 3 shows a typical powder X-ray diffraction pattern taken in the range from 15±908 in twice the
Bragg's angle for a sample of amorphous silica containing equal parts of Pt and Ir atoms for an overall metallic weight of 4%. In this pattern, a full set of relatively well-de®ned Bragg re¯ections can be observed to peak off from a typical amorphous scattering background pro®le. The presence of these re¯ections make evident that a crystalline phase has precipitated in the amorphous silica matrix of the specimen under study. The powder pattern shown in Fig. 3 was found to be quite well indexed by using any one of the re¯ection index sets corresponding to the pure Pt or Ir chemical phases whose cubic structures were long reported by Swanson and Tatge [11] and by
Fig. 1. Particle size distribution on (a) Pt±Ir/Al2O3, (b) Pt±Ir/TiO2, (c) Pt±Ir/SiO2 catalysts.
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Fig. 1. (Continued ).
Swanson et al. [12], respectively. This fact means that the crystalline phase responsible for the powder pattern under focus posses a cubic structure with a facecentered Bravais lattice. Moreover, the whole set of observed re¯ections was found to match closely in intensities with the expected re¯ections for either the pure Pt or the pure Ir chemical phases. These ®nd is a direct experimental evidence of the fact that the facecentered cubic phase under analysis has structure factors closely related to those of the pure Pt and the Ir phases, and therefore, that the crystalline phase precipitated in the silica amorphous matrix is constituted by Pt and Ir atoms. For sake of comparison, the expected positions corresponding to re¯ections from pure Pt (*) and Ir (*) phases are indicated with vertical lines in Fig. 3. It is feasible to learn from this ®gure that any one of the (h k l)-re¯ections observed to appear in the powder pattern under analysis is located between the expected position for the pure Pt (h k l)re¯ection by his left-hand side and the expected position for the pure Ir (h k l)-re¯ection by his right-hand side. This fact means that the unit cell size of the phase responsible for the observed re¯ections is smaller than the unit cell reported for Pt, but larger than the one reported for Ir. The right lattice parameter value was computed by using the (3 1 1)-re¯ection observed to be centered at about 82.58 in twice the Bragg's angle in the powder pattern under analysis.
A slowly-counted pro®le of this re¯ection together with vertical lines marking the expected positions for the pure Pt (3 1 1)-re¯ection and the pure Ir (3 1 1)-re¯ection is shown as an inset in Fig. 3. The (3 3 1)-re¯ection from silicon was used to correct angular position measurements and it is also shown in Fig. 3. By applying the Bragg's law, a lattice Ê was obtained parameter of about 3:907 0:008 A for the face-centered cubic phase under investigation. However, it was observed that this value changes as the relative composition of Pt and Ir does. The sequences of (3 1 1)-re¯ection pro®les corresponding to Ir percentage relative concentrations of about 0, 20, 40, 50, 60, 80 and 100, for samples with metallic weight of 4 and 8% are shown from the top to the bottom in Fig. 4a and b, respectively. These two ®gures show that a smooth shifting is suffered by the unit cell parameter value of the precipitated phase from the one expected for pure Pt to the one expected for pure Ir as the Ir relative concentration moves from 0 to 100%. Unit cell parameter measurements for the samples corresponding to the pro®les shown in Fig. 4a and b are listed down in the second and the third columns of Table 3, respectively. A linear relationship is observed to be hold between the lattice parameter of the precipitated phase and the relative concentration of Pt for specimens with overall metallic weights of 4 and 8%, as can be seen in the plots
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Fig. 2. (a) Bright ®eld image of a Pt±Ir (50±50)/SiO2 catalysts showing irregular shapes. (b) Bright ®eld image of Pt/SiO2 catalysts where the particles appear to have nice crystalline habits.
shown in Fig. 5a and b, respectively. This result is a direct experimental evidence of the fact that the crystalline structure of the face-centered cubic phase found to be precipitated in the amorphous silica matrix obeys the Vegard's law and therefore, the character of a solid solution Pt(x) Ir(1 x), where x
is the Pt molar fraction, can be assigned to the structural nature of this phase. 3.1.2. H2 chemisorption Hydrogen chemisorption was used to measure the dispersion of the active phase, and Langmuir-type
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Fig. 3. Powder X-ray diffraction pattern of a sample of amorphous silica containing equal parts of Pt and Ir atoms for a metallic load of about 4%. Inset: pro®le of the re¯ection centered at about 82.58 in 2y.
Fig. 4. The (3 1 1)-re¯ection pro®les corresponding, from the top to the bottom, to percentual Ir relative contents of about 0, 20, 40, 50, 60, 80 and 100 for samples with metallic loads of about 4% (a) and 8% (b).
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Table 3 Pt±Ir catalysts on SiO2 showing the changes in the unit cell parameters as a function of Ir content Ê) x) phase unit cell parameter 0.005 (A
Relative composition of the metallic load of platinum and iridium (Pt%±Ir%)
Pt(x) Ir(1
For an overall metallic load of about 4%
For an overall metallic load of about 8%
0±100 20±80 40±60 50±50 60±40 80±20 100±0
3.846 3.861 3.881 3.884 3.894 3.906 3.926
3.841 3.864 3.870 3.892 3.899 3.907 3.926
Fig. 5. Plots of the unit cell parameter of the precipitated phase vs. relative Pt content for specimens with overall metallic loads of 4% (a) and 8% (b).
A. Zecua-FernaÂndez et al. / Applied Surface Science 182 (2001) 1±11 Table 4 Total dispersion and particle size obtained by H2 chemisorption for series of catalysts at 1% in metallic weight Catalyst
Dispersion (%)
Ê) Particle size (A
Al2O3 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
25.93 30.69 42.52 56.54 69.15
44 42 39 36 32
TiO2 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
18.20 23.24 32.27 43.69 56.38
55 49 43 39 36
SiO2 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
13.00 16.85 23.23 32.13 41.00
78 67 60 54 49
adsorption isotherms were obtained, and then based on these results total dispersion and average particle size were determined. Similar behaviors are observed for the three series of catalysts as can be inferred from Table 4. The chemisorption of H2 in moles increases when the Ir concentration does. It is known that both the particle size and the metal distribution have a strong dependence on the physical properties of the support as well on the thermal treatments causing larger or lower dispersion. The catalysts supported in alumina have the largest dispersion in comparison with those supported in TiO2 and SiO2. When the support is SiO2 the particle size for Pt catalysts is practically twice than the Ir-supported catalysts, this also happen for TiO2 catalysts. But this observation does not happen for Al2O3 catalysts, the relation of adsorbed moles for Ir is of about two orders of magnitude compared with the one for Pt. For pure Pt on a SiO2 support, an average particle Ê is measured and an homogeneous size of 78 A distribution of the reaction products is found, being the larger (37.4%) or linear hydrocarbons. In this case, the unit cell parameter of the metallic catalyst as Ê . When measured by X-ray diffraction is about 3.926 A
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Ir atoms are added at a relative atomic concentration of about 20%, a decrement in the unit cell parameter Ê is observed for the two metallic value to 3.906 A weights used (4 and 8%) in silica-supported catalysts, but a decrease in the average particle size value to Ê is also observed. Both the average size particle 67 A and the lattice parameter are observed to diminish even more as long as the Ir relative content increases until reach pure Ir. This behavior is observed to hold in all the samples analyzed. For the cases of pure Ir in Al2O3 or SiO2, the selectivity is observed to be almost 100% towards linear hydrocarbons. This is not so for the TiO2-supported catalysts. This difference may be attributed to a different particle support interaction. 3.1.3. Catalytic activity The bimetallic Pt±Ir catalysts prepared by the impregnation method were supported on SiO2 (aerosil 200 m2/g), Al2O3 (g-alumina 170 m2/g) and TiO2 (anatase 100 m2/g). The catalysts were tested in the hydrogenolysis of n-hexane. In order to measure the support acidity contribution to the reaction, the three supports were tested without the metallic phase, and under the same conditions as the metallic catalysts. The results indicated no intrinsic activity of these supports at these conditions, in consequence no contribution to the catalytic activity of the metallic phase is expected. The evaluation of the catalytic behavior was performed in a continuous ¯ow microreactor at atmospheric pressure for a 3 h time period. The evaluation of the catalytic properties was performed from two points of view; ®rst, the total catalytic activity was considered by following the n-hexane consumption during the reaction as a function of the deactivation time. The second point of view considered the distribution of products at the exit of the reactor, which is related with the selectivity of the catalyst. The total activity represented by the percentage of conversion was found to show the catalytic behavior of the supported catalysts at 673 K. When Ir is added to Pt catalysts the catalytic activity was observed to increase considerably due to the fact that Ir has an intrinsic activity of about two orders of magnitude larger than the one for Pt. From the deactivation curves it was observed that both the monometallic catalysts (Pt and Ir) and the
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bimetallic ones behave in a similar way, existing deactivation during the ®rst 2 h of reaction, followed by stabilization and ®nally existing a constant behavior during the last hour under reaction. The most active catalysts were the catalysts supported in alumina, followed by those in titania and then silica. Addition of Ir to Pt catalysts may produce a decrement in the amount of coke deposits in agreement with the reports by Barbier and coworkers [7]. In those works, they propose that this behavior is related with the high hydrogenolytic behavior of the bimetallic catalysts. However, in our study, the Pt±Ir bimetallic catalysts were found to be cokized at atmospheric pressure (independently of the support) and to render a larger amount of light hydrocarbons than of the rest of products, implying that these light products are adsorbed on the metallic phase. From these observations, it is concluded that Ir, independently of the support used, shows an intrinsic activity larger than the one shown by Pt. In this connection, we can do mention that the bimetallic catalysts showed an intrinsic activity closer to Ir than to Pt. Precipitation of the Pt(x) Ir(1 x) solid solution found by X-ray analysis or the dispersion increment suffered by the catalyst particles as we add Ir may explain these changes.
3.1.4. Selectivity of the n-hexane reaction The distribution and the mol% of products at the reactor exit as a function of time are shown in Table 5. The average selectivity for each product from hydrogenolysis of n-hexane for the three different supports used can be seen in this table. The n-hexane reaction carried out in presence of an atmosphere of hydrogen renders hydrogenolysis products as well as isomers and cyclic and aromatic products. The reaction products were identi®ed from interpretation of the chromatogram. Products identi®cation was done in agreement with residence times and response factors using standard hydrocarbons. The products were classi®ed in the following four groups: hydrogenolysis products (linear hydrocarbons C1±C5), isomers (2- and 3-methilpentane), cyclic (cyclohexane) and aromatic (benzene). Other products were canalized as a secondary hydrogenolysis products. From Table 5, changes in selectivity of the catalysts as Ir is added is observed. In the case of pure Pt on an Al2O3 support, the products distribution is more or less homogeneous, but it suffers a radical change when Ir is added even in a small amount. In presence of Ir, hydrogenolysis products increase considerably and they reach maxima when there exist pure Ir on Al2O3. This behavior is
Table 5 Selectivity (%) and products distribution in hydrogenolysis of n-hexane at 673 K for the three series of catalysts Catalysts
Hydrogenolysis products (linear hydrocarbons)
2- and 3-Methilpentane (isomers)
Cyclohexane (cyclic)
Benzene (aromatic)
Al2O3 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
37.69 57.84 87.55 88.97 93.20
17.02 10.42 2.65 2.57 1.18
25.23 20.98 8.32 7.42 5.16
20.06 10.76 1.48 1.04 0.46
TiO2 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
43.36 49.45 50.79 54.72 59.81
22.78 16.81 16.30 15.75 14.41
8.57 8.90 9.35 10.19 10.41
25.29 24.84 23.56 19.34 15.37
SiO2 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
37.41 48.44 69.01 72.58 93.00
22.62 14.97 10.67 8.03 1.59
20.12 19.40 12.98 12.63 3.89
19.85 17.19 7.34 6.76 1.52
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observed to be similar to the behavior of SiO2-supported catalysts, but it is observed to be quite different from the behavior shown by TiO2-supported catalysts. The last observation may be explained by taking into account the existence of a strong metal±support interaction. 4. Summary The addition of a second metal to Pt it is known to cause considerably changes in its catalytic behavior. In particular, the changes on Pt behavior due to the presence of Ir atoms has been scarcely reported in the literature. For this reason, it is interesting to know how this metal is incorporated to Pt ensembles. From our study, it was observed that Ir divide the large platinum ensembles into smaller ones and we think that this phenomenon could explain the inhibition of the coking reaction and also be involved in the changes in catalytic activity and selectivity. The selectivity behavior of the Pt±Ir catalysts was found to be similar for the cases of Al2O3 and SiO2 supports, but completely for the TiO2, case. We attributed the observed differences to a different interaction particle±support. The crystalline phase found in this work having a solution character Pt(x) Ir(1 x) and a fcc structure is certainly involved on the catalytic behavior under study, and the presence of this new phase could
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afford another possible explanation to the observed drastic changes. Acknowledgements We would like to express our appreciation to Mr. Pedro Mexia and Mr. Carlos Flores for the photographic work, and Physicist E. Hernandez-Juarez for his academic help to do the X-ray analyses. References [1] W.R. Tyson, W.A. Miller, Surf. Sci. 62 (1977) 267. [2] K.K. Kearby, J.P. Thorn, J.A. Hinlicky, US Patent 3,134,732 (1964). [3] L.J. Daniels, P. Sperling, A.G. Rouquier, Oil Gas J. 78 (1972). [4] J.L. Carter, G.B. McVicker, M. Weisshan, W.S. Hmak, J.H. Sinfelt, Appl. Catal. 3 (1982) 327. [5] J. Margitfalvi, S. GoÈboÈloÈs, E. Kwaysser, M. Hegedus, F. Nagy, L. Koltai, React Kinet. Catal. Lett. 24 (1984) 315. [6] R.T.K. Baker, R.D. Sherwood, J. Dumesic, J. Catal. 66 (1980) 56. [7] J. Barbier, E. Churin, P. Marecot, J. Catal. 126 (1990) 228. [8] G.B. McVicker, R.L. Garten, R.T.K. Baker, J. Catal. 54 (1978) 129. [9] J.H. Sinfelt, US Patent 3,953,368 (1976). [10] A.L. Bonivardi, F.H. Ribeiro, G.A. Somorjai, J. Catal. 160 (1996) 269. [11] H.E. Swanson, E. Tatge, National Bureau of Standards, Circular 539,1,31-2 (1953). [12] H.E. Swanson, R.K. Fuyat, G.M. Ugrinic, National Bureau of Standards, Circular 539,4,9-10 (1955).
Structure and catalytic properties of Pt±Ir catalysts as a function of different substrate A. Zecua-FernaÂndez, A. GoÂmez-CorteÂs, A.E. Cordero-Borboa, A. VaÂzquez-Zavala* Instituto de FõÂsica UNAM, Apdo, Postal 20-364, MeÂxico City 01000 D.F., Mexico Received 26 January 2001; accepted 26 May 2001
Abstract A series of Pt±Ir catalysts prepared by the impregnation method in three different supports and different atomic concentrations were characterized by conventional transmission electron microscopy (TEM), high resolution electron microscopy (HREM), X-ray diffraction (XRD) and n-hexane contact reaction as a catalytic probe. The reaction products were grouped in linear (hydrogenolysis products), isomers (2- and 3-methilpentane), cyclic (cyclohexane) and aromatic (benzene) hydrocarbons. An increment in hydrogenolysis products was observed as the amount of Ir was increased. This observation was extreme when Al2O3 and SiO2 were used as supports, but it was not so for the case of TiO2. From TEM results, it was concluded that the metallic particle size is a function of Ir content. From X-ray diffraction analysis, it was found that a unique crystalline phase containing atoms of Pt and Ir precipitates into the specimens under study. This phase was found to have a face-centered cubic structure and a lattice parameter whose value changes as long as the relative amounts of Pt and Ir does. A linear relationship was observed to hold between the lattice parameter of the precipitated phase and the relative concentration of Pt for specimens with metallic weights of 4 and 8% in a SiO2 support. This result may be taken as a direct experimental evidence of the fact that the crystalline structure of the phase-centered cubic phase found to precipitate in amorphous silica obeys the Vegard's law and therefore, the character of a solid solution Pt(x) Ir(1 x), where x is the Pt molar fraction, can be assigned to the structural nature of this phase. This new phase and the changes recorded in particle size could explain the changes in selectivity and catalytic activity observed in the n-hexane contact reaction. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Pt±Ir catalyst; X-ray diffraction; TEM
1. Introduction Tyson and Miller [1] estimated the surface energies of solid metals from measurements of free energies on liquids; in particular, they obtained the following surface energies for pure Pt and Ir: jPt 700 K 2430 erg=cm2 ; jIr 700 K 3010 erg=cm2 *
Corresponding author.
In connection with these measurements, it is known that in the case of a metallic melt, the metal with a lower melting point is segregated to the surface of the particles resulting from cooling the melt. Active surface area is the ®rst consideration in catalytic activity and, in general, a high metal surface area is generally required for a bimetallic catalysts to be of practical interest. Platinum is currently so expensive that bimetallic catalysts using this metal are of economical use only in the case of having a signi®cant fraction of the metallic atoms on the surface. Both, the surface
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 3 6 1 - 0
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composition and the size of the metallic particles is commonly correlated with catalytic activity and selectivity of the catalysts. In the case of alloys obtained from melts containing atoms of Pt and Ir, very little is known about the resulting crystalline phases and the surface composition of particles made of these alloys. This makes any attempt to interpret the data taken out during bimetallic Pt±Ir catalysts reactions very dif®cult. In reforming process carried out by using this sort of catalysts a rapid deactivation occurs due to the fact that the concentration of carbon residues can block both the metal and the acid sites and then the catalyst must be early regenerated [2]. In particular, in naphtha feedstocks reforming process the catalysts made of the noble metal on an alumina support deactivates during this process by a consequence of carbonaceous residues building upon the catalyst surface [3]. Sintering can occur during both reforming and regeneration operations at temperatures near to 770 K. Regeneration is commonly carried out with the purpose of burning the coke formed on the catalyst surface. It has been mentioned in several works [4,5] that Ir has the ability to destroy coke precursors by hydrogenolysis. Also it has been observed [6] that Ir could destroy coke deposits under a hydrogen atmosphere. In the work made by Barbier et al. [7], it was found that the addition of Ir to Pt acts in the same way on the amount of deposited coke at atmospheric pressure as all Pt does under increasing overall pressure for the coking reaction on SiO2. When coke is burned in an oxygen ¯ow, in the case of Pt±Ir, Ir is more susceptible towards oxidative sintering than Pt [8,9]. It has been observed [10] in several reactions that a Pt±Ir presul®dation reduced the hydrogenolysis activity of Ir, but sulfur addition increases the dehydrogenation rates for the monometallic and bimetallic surfaces. The aim of this work is to observe the changes in all morphology, size and crystalline structure, suffered by the Pt±Ir alloy particles found to precipitate in an amorphous SiO2 as a function of the Pt±Ir concentration ratio, and to observe also the changes in both activity and selectivity suffered by the Pt±Ir catalysts. 2. Experimental A series of catalysts with different relative atomic concentrations of Pt and Ir at 1% in metallic weight
Table 1 Catalysts series prepared by impregnation method Series
Support
Atomic percentage
Metallic weight (%)
1
Al2O3
Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
1
2
TiO2
Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
1
3
SiO2
Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
1
4
SiO2
Pt Pt±Ir Pt±Ir Pt±Ir Pt±Ir Pt±Ir Ir
(80±20) (60±40) (50±50) (40±60) (20±80)
4
5
SiO2
Pt Pt±Ir Pt±Ir Pt±Ir Pt±Ir Pt±Ir Ir
(80±20) (60±40) (50±50) (40±60) (20±80)
8
were prepared from aqueous solutions of H2PtCl6 and H2IrCl6 (Aldrich) and amorphous SiO2 and crystalline both Al2O3 and TiO2 supports as it is described in Table 1. In the case of the catalysts supported on amorphous SiO2, we also prepared another two series of samples at metallic weights of 4 and 8% in order to improve intensity measurements during X-ray diffraction analyses. Two new different atomic relative concentrations of Ir (40 and 60%) were added to Pt and included in both of these two series in order to make easier to ®nd any experimental correlation between the lattice parameter of the Pt±Ir crystalline phase forming the precipitated bimetallic particles and the relative atomic concentrations of Pt and Ir used to prepare the catalysts. In all the cases, the classic method of impregnation was used to prepare the
A. Zecua-FernaÂndez et al. / Applied Surface Science 182 (2001) 1±11
catalysts. After impregnation, the supports were stirred for 1 h at 298 K and then evaporated and dried at 373 K for 12 h. The samples were then reduced by ¯owing hydrogen for 2 h at 773 K with a ¯ow rate of about 60 ml/min. Catalytic measurements were carried out under atmospheric pressure by using a continuous ¯ow laboratory-made microreactor coupled with a gas chromatograph Perkin-Elmer equipped with a ¯ame ionization detector. The reaction gas mixture was obtained by passing hydrogen through a saturator device containing n-hexane at 273 K. The reactor temperature was about 673 K and a continuous hydrogen ¯ow of about 20 ml/min was used. Due to the fact that the catalytic activity of the Ir catalysts is quite high its mass was varied in order to maintain the differential reactor behavior. The experiment was initiated by gently rising the temperature of the sample from 293 to 673 K while passing the hydrogen ¯ow through it. This ®nal temperature was maintained for 2 h time in order to reactivate the catalyst, and then the reaction was started. As usual, the catalytic selectivity is de®ned as moles of a particular product per moles of the total product times 100%. Conventional electron microscopy was done by means of a JEOL-100CX electron microscope, with Ê point to point resolution, equipped with a sidea 3.5 A enter goniometer. High resolution electron microscopy was performed with a JEOL-4000EX electron Ê point to point resolution. The microscope with a 1.7 A catalysts were ground to powder in an agate mortar and then mixed with ethanol to form a suspension. A drop of this suspension was deposited on a commercial copper grid, previously coated with a thin ®lm of carbon, and then transported to the microscopes under an inert atmosphere of N2 (99.99%). Some specimens were kept in the microscope overnight before analysis to minimize possible contamination.
3
Powder X-ray diffraction techniques were used to characterize the atomic structure of samples prepared by adding different atomic concentrations of Ir and Pt to amorphous silica for overall metallic weights of about 4 and 8%. The Ir percentage atomic concentrations were of about 0, 20, 40, 50, 60, 80 and 100. This was done by means of a Siemens-D5000 diffractometer operating at working conditions of 20 mA and 30 kV and using Cu Ka-radiation as primary beam. Bragg re¯ections from the samples were monochromatized by using a graphite crystal in order to avoid other than the Ka-radiation component to reach the scintillation detector device of the diffractometer. Specimens for X-ray unit cell measurements were prepared by mixing powder from the samples under study with powder of standard silicon, in concentration of about 20% in overall weight, in order to use the silicon X-ray diffraction signals to calibrate the angular scale of the powder X-ray diffraction patterns. Xray analyses were carried out only on specimens prepared with amorphous silica supports because of the fact that the X-ray re¯ections from Al2O3 and TiO2 crystalline supports interfere with the metallic catalysts diffraction peaks. 3. Results and discussion 3.1. Conventional electron microscopy Particle size of the supported catalysts was measured by using several bright ®eld electron microscope images. These measurements are shown in Table 2. It is observed from this table that the particle size decreases when Ir is added to Pt regardless the support used to prepare the sample. This observation may indicate that re-dispersion of the metallic particles
Table 2 Average particle size of catalysts at 1% in metallic weight obtained by electron microscopy Catalysts
Ê) Particle size (A supported in Al2O3
Ê) Particle size (A supported on TiO2
Ê) Particle size (A supported on SiO2
Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
44 42 39 36 32
59 56 48 45 38
82 71 66 60 51
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is due to the presence of Ir atoms, The corresponding size distribution histograms appear in Fig. 1. In general, the bimetallic particles are observed to have irregular shapes as it is shown in the bright ®eld image of a specimen with a Ir relative concentration of about 50% on an amorphous SiO2 support (Fig. 2a). In the case of all Pt on an amorphous SiO2 support (Fig. 2b), the particles appear to have well developed crystalline habits. 3.1.1. X-ray diffraction Fig. 3 shows a typical powder X-ray diffraction pattern taken in the range from 15±908 in twice the
Bragg's angle for a sample of amorphous silica containing equal parts of Pt and Ir atoms for an overall metallic weight of 4%. In this pattern, a full set of relatively well-de®ned Bragg re¯ections can be observed to peak off from a typical amorphous scattering background pro®le. The presence of these re¯ections make evident that a crystalline phase has precipitated in the amorphous silica matrix of the specimen under study. The powder pattern shown in Fig. 3 was found to be quite well indexed by using any one of the re¯ection index sets corresponding to the pure Pt or Ir chemical phases whose cubic structures were long reported by Swanson and Tatge [11] and by
Fig. 1. Particle size distribution on (a) Pt±Ir/Al2O3, (b) Pt±Ir/TiO2, (c) Pt±Ir/SiO2 catalysts.
A. Zecua-FernaÂndez et al. / Applied Surface Science 182 (2001) 1±11
5
Fig. 1. (Continued ).
Swanson et al. [12], respectively. This fact means that the crystalline phase responsible for the powder pattern under focus posses a cubic structure with a facecentered Bravais lattice. Moreover, the whole set of observed re¯ections was found to match closely in intensities with the expected re¯ections for either the pure Pt or the pure Ir chemical phases. These ®nd is a direct experimental evidence of the fact that the facecentered cubic phase under analysis has structure factors closely related to those of the pure Pt and the Ir phases, and therefore, that the crystalline phase precipitated in the silica amorphous matrix is constituted by Pt and Ir atoms. For sake of comparison, the expected positions corresponding to re¯ections from pure Pt (*) and Ir (*) phases are indicated with vertical lines in Fig. 3. It is feasible to learn from this ®gure that any one of the (h k l)-re¯ections observed to appear in the powder pattern under analysis is located between the expected position for the pure Pt (h k l)re¯ection by his left-hand side and the expected position for the pure Ir (h k l)-re¯ection by his right-hand side. This fact means that the unit cell size of the phase responsible for the observed re¯ections is smaller than the unit cell reported for Pt, but larger than the one reported for Ir. The right lattice parameter value was computed by using the (3 1 1)-re¯ection observed to be centered at about 82.58 in twice the Bragg's angle in the powder pattern under analysis.
A slowly-counted pro®le of this re¯ection together with vertical lines marking the expected positions for the pure Pt (3 1 1)-re¯ection and the pure Ir (3 1 1)-re¯ection is shown as an inset in Fig. 3. The (3 3 1)-re¯ection from silicon was used to correct angular position measurements and it is also shown in Fig. 3. By applying the Bragg's law, a lattice Ê was obtained parameter of about 3:907 0:008 A for the face-centered cubic phase under investigation. However, it was observed that this value changes as the relative composition of Pt and Ir does. The sequences of (3 1 1)-re¯ection pro®les corresponding to Ir percentage relative concentrations of about 0, 20, 40, 50, 60, 80 and 100, for samples with metallic weight of 4 and 8% are shown from the top to the bottom in Fig. 4a and b, respectively. These two ®gures show that a smooth shifting is suffered by the unit cell parameter value of the precipitated phase from the one expected for pure Pt to the one expected for pure Ir as the Ir relative concentration moves from 0 to 100%. Unit cell parameter measurements for the samples corresponding to the pro®les shown in Fig. 4a and b are listed down in the second and the third columns of Table 3, respectively. A linear relationship is observed to be hold between the lattice parameter of the precipitated phase and the relative concentration of Pt for specimens with overall metallic weights of 4 and 8%, as can be seen in the plots
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Fig. 2. (a) Bright ®eld image of a Pt±Ir (50±50)/SiO2 catalysts showing irregular shapes. (b) Bright ®eld image of Pt/SiO2 catalysts where the particles appear to have nice crystalline habits.
shown in Fig. 5a and b, respectively. This result is a direct experimental evidence of the fact that the crystalline structure of the face-centered cubic phase found to be precipitated in the amorphous silica matrix obeys the Vegard's law and therefore, the character of a solid solution Pt(x) Ir(1 x), where x
is the Pt molar fraction, can be assigned to the structural nature of this phase. 3.1.2. H2 chemisorption Hydrogen chemisorption was used to measure the dispersion of the active phase, and Langmuir-type
A. Zecua-FernaÂndez et al. / Applied Surface Science 182 (2001) 1±11
7
Fig. 3. Powder X-ray diffraction pattern of a sample of amorphous silica containing equal parts of Pt and Ir atoms for a metallic load of about 4%. Inset: pro®le of the re¯ection centered at about 82.58 in 2y.
Fig. 4. The (3 1 1)-re¯ection pro®les corresponding, from the top to the bottom, to percentual Ir relative contents of about 0, 20, 40, 50, 60, 80 and 100 for samples with metallic loads of about 4% (a) and 8% (b).
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Table 3 Pt±Ir catalysts on SiO2 showing the changes in the unit cell parameters as a function of Ir content Ê) x) phase unit cell parameter 0.005 (A
Relative composition of the metallic load of platinum and iridium (Pt%±Ir%)
Pt(x) Ir(1
For an overall metallic load of about 4%
For an overall metallic load of about 8%
0±100 20±80 40±60 50±50 60±40 80±20 100±0
3.846 3.861 3.881 3.884 3.894 3.906 3.926
3.841 3.864 3.870 3.892 3.899 3.907 3.926
Fig. 5. Plots of the unit cell parameter of the precipitated phase vs. relative Pt content for specimens with overall metallic loads of 4% (a) and 8% (b).
A. Zecua-FernaÂndez et al. / Applied Surface Science 182 (2001) 1±11 Table 4 Total dispersion and particle size obtained by H2 chemisorption for series of catalysts at 1% in metallic weight Catalyst
Dispersion (%)
Ê) Particle size (A
Al2O3 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
25.93 30.69 42.52 56.54 69.15
44 42 39 36 32
TiO2 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
18.20 23.24 32.27 43.69 56.38
55 49 43 39 36
SiO2 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
13.00 16.85 23.23 32.13 41.00
78 67 60 54 49
adsorption isotherms were obtained, and then based on these results total dispersion and average particle size were determined. Similar behaviors are observed for the three series of catalysts as can be inferred from Table 4. The chemisorption of H2 in moles increases when the Ir concentration does. It is known that both the particle size and the metal distribution have a strong dependence on the physical properties of the support as well on the thermal treatments causing larger or lower dispersion. The catalysts supported in alumina have the largest dispersion in comparison with those supported in TiO2 and SiO2. When the support is SiO2 the particle size for Pt catalysts is practically twice than the Ir-supported catalysts, this also happen for TiO2 catalysts. But this observation does not happen for Al2O3 catalysts, the relation of adsorbed moles for Ir is of about two orders of magnitude compared with the one for Pt. For pure Pt on a SiO2 support, an average particle Ê is measured and an homogeneous size of 78 A distribution of the reaction products is found, being the larger (37.4%) or linear hydrocarbons. In this case, the unit cell parameter of the metallic catalyst as Ê . When measured by X-ray diffraction is about 3.926 A
9
Ir atoms are added at a relative atomic concentration of about 20%, a decrement in the unit cell parameter Ê is observed for the two metallic value to 3.906 A weights used (4 and 8%) in silica-supported catalysts, but a decrease in the average particle size value to Ê is also observed. Both the average size particle 67 A and the lattice parameter are observed to diminish even more as long as the Ir relative content increases until reach pure Ir. This behavior is observed to hold in all the samples analyzed. For the cases of pure Ir in Al2O3 or SiO2, the selectivity is observed to be almost 100% towards linear hydrocarbons. This is not so for the TiO2-supported catalysts. This difference may be attributed to a different particle support interaction. 3.1.3. Catalytic activity The bimetallic Pt±Ir catalysts prepared by the impregnation method were supported on SiO2 (aerosil 200 m2/g), Al2O3 (g-alumina 170 m2/g) and TiO2 (anatase 100 m2/g). The catalysts were tested in the hydrogenolysis of n-hexane. In order to measure the support acidity contribution to the reaction, the three supports were tested without the metallic phase, and under the same conditions as the metallic catalysts. The results indicated no intrinsic activity of these supports at these conditions, in consequence no contribution to the catalytic activity of the metallic phase is expected. The evaluation of the catalytic behavior was performed in a continuous ¯ow microreactor at atmospheric pressure for a 3 h time period. The evaluation of the catalytic properties was performed from two points of view; ®rst, the total catalytic activity was considered by following the n-hexane consumption during the reaction as a function of the deactivation time. The second point of view considered the distribution of products at the exit of the reactor, which is related with the selectivity of the catalyst. The total activity represented by the percentage of conversion was found to show the catalytic behavior of the supported catalysts at 673 K. When Ir is added to Pt catalysts the catalytic activity was observed to increase considerably due to the fact that Ir has an intrinsic activity of about two orders of magnitude larger than the one for Pt. From the deactivation curves it was observed that both the monometallic catalysts (Pt and Ir) and the
10
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bimetallic ones behave in a similar way, existing deactivation during the ®rst 2 h of reaction, followed by stabilization and ®nally existing a constant behavior during the last hour under reaction. The most active catalysts were the catalysts supported in alumina, followed by those in titania and then silica. Addition of Ir to Pt catalysts may produce a decrement in the amount of coke deposits in agreement with the reports by Barbier and coworkers [7]. In those works, they propose that this behavior is related with the high hydrogenolytic behavior of the bimetallic catalysts. However, in our study, the Pt±Ir bimetallic catalysts were found to be cokized at atmospheric pressure (independently of the support) and to render a larger amount of light hydrocarbons than of the rest of products, implying that these light products are adsorbed on the metallic phase. From these observations, it is concluded that Ir, independently of the support used, shows an intrinsic activity larger than the one shown by Pt. In this connection, we can do mention that the bimetallic catalysts showed an intrinsic activity closer to Ir than to Pt. Precipitation of the Pt(x) Ir(1 x) solid solution found by X-ray analysis or the dispersion increment suffered by the catalyst particles as we add Ir may explain these changes.
3.1.4. Selectivity of the n-hexane reaction The distribution and the mol% of products at the reactor exit as a function of time are shown in Table 5. The average selectivity for each product from hydrogenolysis of n-hexane for the three different supports used can be seen in this table. The n-hexane reaction carried out in presence of an atmosphere of hydrogen renders hydrogenolysis products as well as isomers and cyclic and aromatic products. The reaction products were identi®ed from interpretation of the chromatogram. Products identi®cation was done in agreement with residence times and response factors using standard hydrocarbons. The products were classi®ed in the following four groups: hydrogenolysis products (linear hydrocarbons C1±C5), isomers (2- and 3-methilpentane), cyclic (cyclohexane) and aromatic (benzene). Other products were canalized as a secondary hydrogenolysis products. From Table 5, changes in selectivity of the catalysts as Ir is added is observed. In the case of pure Pt on an Al2O3 support, the products distribution is more or less homogeneous, but it suffers a radical change when Ir is added even in a small amount. In presence of Ir, hydrogenolysis products increase considerably and they reach maxima when there exist pure Ir on Al2O3. This behavior is
Table 5 Selectivity (%) and products distribution in hydrogenolysis of n-hexane at 673 K for the three series of catalysts Catalysts
Hydrogenolysis products (linear hydrocarbons)
2- and 3-Methilpentane (isomers)
Cyclohexane (cyclic)
Benzene (aromatic)
Al2O3 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
37.69 57.84 87.55 88.97 93.20
17.02 10.42 2.65 2.57 1.18
25.23 20.98 8.32 7.42 5.16
20.06 10.76 1.48 1.04 0.46
TiO2 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
43.36 49.45 50.79 54.72 59.81
22.78 16.81 16.30 15.75 14.41
8.57 8.90 9.35 10.19 10.41
25.29 24.84 23.56 19.34 15.37
SiO2 Pt Pt±Ir (80±20) Pt±Ir (50±50) Pt±Ir (20±80) Ir
37.41 48.44 69.01 72.58 93.00
22.62 14.97 10.67 8.03 1.59
20.12 19.40 12.98 12.63 3.89
19.85 17.19 7.34 6.76 1.52
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observed to be similar to the behavior of SiO2-supported catalysts, but it is observed to be quite different from the behavior shown by TiO2-supported catalysts. The last observation may be explained by taking into account the existence of a strong metal±support interaction. 4. Summary The addition of a second metal to Pt it is known to cause considerably changes in its catalytic behavior. In particular, the changes on Pt behavior due to the presence of Ir atoms has been scarcely reported in the literature. For this reason, it is interesting to know how this metal is incorporated to Pt ensembles. From our study, it was observed that Ir divide the large platinum ensembles into smaller ones and we think that this phenomenon could explain the inhibition of the coking reaction and also be involved in the changes in catalytic activity and selectivity. The selectivity behavior of the Pt±Ir catalysts was found to be similar for the cases of Al2O3 and SiO2 supports, but completely for the TiO2, case. We attributed the observed differences to a different interaction particle±support. The crystalline phase found in this work having a solution character Pt(x) Ir(1 x) and a fcc structure is certainly involved on the catalytic behavior under study, and the presence of this new phase could
11
afford another possible explanation to the observed drastic changes. Acknowledgements We would like to express our appreciation to Mr. Pedro Mexia and Mr. Carlos Flores for the photographic work, and Physicist E. Hernandez-Juarez for his academic help to do the X-ray analyses. References [1] W.R. Tyson, W.A. Miller, Surf. Sci. 62 (1977) 267. [2] K.K. Kearby, J.P. Thorn, J.A. Hinlicky, US Patent 3,134,732 (1964). [3] L.J. Daniels, P. Sperling, A.G. Rouquier, Oil Gas J. 78 (1972). [4] J.L. Carter, G.B. McVicker, M. Weisshan, W.S. Hmak, J.H. Sinfelt, Appl. Catal. 3 (1982) 327. [5] J. Margitfalvi, S. GoÈboÈloÈs, E. Kwaysser, M. Hegedus, F. Nagy, L. Koltai, React Kinet. Catal. Lett. 24 (1984) 315. [6] R.T.K. Baker, R.D. Sherwood, J. Dumesic, J. Catal. 66 (1980) 56. [7] J. Barbier, E. Churin, P. Marecot, J. Catal. 126 (1990) 228. [8] G.B. McVicker, R.L. Garten, R.T.K. Baker, J. Catal. 54 (1978) 129. [9] J.H. Sinfelt, US Patent 3,953,368 (1976). [10] A.L. Bonivardi, F.H. Ribeiro, G.A. Somorjai, J. Catal. 160 (1996) 269. [11] H.E. Swanson, E. Tatge, National Bureau of Standards, Circular 539,1,31-2 (1953). [12] H.E. Swanson, R.K. Fuyat, G.M. Ugrinic, National Bureau of Standards, Circular 539,4,9-10 (1955).