XRD characterization of highly dispersed metal

XRD characterization of highly dispersed metal catalysts on carbon support Paolo Scardf) Dipartimento di Ingegneria dei Materiali, Universita di Trento, 38050 Mesiano, TN, Italy

Pier Luigi Antonuccib) CNR-Istituto di Ricerche sui Metodi e Processi Chimici per la Trasformazione e I'Accumulo deU'Energia, Via Salita S. Lucia sopra Contesse, 39-98126 S. Lucia, ME, Italy (Received 14 December 1992; accepted 7 April 1993)

Carbon-supported Pt catalysts were prepared from H 2 PtCl 6 or K2PtCl6 aqueous solutions. Particle size and structure after several thermal activation treatments were studied by X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Cyclic Voltammetry (CV), and the results of the three techniques were compared. As the catalysts were highly dispersed on an amorphous support, a conventional XRD profile analysis for crystallite size determination could not be performed properly, because of the strong overlapping between the broad Pt peaks superposed to the halos of the amorphous phase. Thus, a new procedure of whole XRD pattern fitting, based on the Rietveld method, was used to have reliable data of Pt particle size (surface area) and lattice parameter. All structural and microstructural parameters were refined within the same procedure, also considering the transparency of the carbon supported catalysts and minimizing the effect of the amorphous background. The method can also take into account the presence of bimodal particle size distributions, which is difficult to study by CV or TEM.

I. INTRODUCTION Solid-state characterization of supported metal crystallites is of paramount significance for the knowledge of the structure-property relationships, with particular emphasis to that directly linked to the catalytic phenomenon. Information on crystallites' particle size and distribution is, in this respect, essential as these parameters strongly influence the degree of metal-support interaction due to structural sensitivity effects, especially when said catalytic materials are employed for demanding (structure sensitive) reactions1'2; in this case, a specific reaction rate is observed, depending upon particle size and other structural parameters, such as the lattice parameter of the active component. Carbon-supported Pt catalysts find large application for a number of such structure sensitive reactions3'4; thus, information related to the bulk properties of the material is a necessary complement to the more widespread knowledge obtained by surface spectroscopy techniques. Particle size of supported catalysts is usually determined by selective gas chemisorption (GC) methods, transmission electron microscopy (TEM), electrochemical techniques (CV, cyclic voltammetry), or by x-ray,

^Address correspondence to this author. b) Current address: Facolta di Ingegneria, Istituto di Chimica, Via E. Cuzzocrea 48, 89100 Reggio Calabria, Italy. J. Mater. Res., Vol. 8, No. 8, Aug 1993

from diffraction line broadening or low angle scattering.3 As is well known, some drawbacks are connected with each of the above-mentioned techniques: a rather poor statistical significance is inherent to TEM, whereas experimental conditions can sometimes affect the results obtained by CV 5 ; on the other hand, the use of the GC requires the knowledge of the chemisorption stoichiometry at monolayer coverage.3 Conventional x-ray diffraction (XRD) profile analysis is based on the Fourier transform of diffraction peaks. Several problems in the evaluation of Fourier coefficient, influencing the reliability of the method, arise in the case of highly dispersed metal particles onto amorphous supports. Even though profile fitting techniques have been devised,6"8 broad overlapping peaks and irregular background, due to the spectrum of the amorphous support, are often an insurmountable difficulty. In the present study, XRD patterns of highly dispersed, carbon-supported Pt catalysts have been analyzed using a procedure based on the Rietveld method.9"13 The proposed procedure, that permits one to calculate simultaneously lattice parameter and crystallite size, consists of the modeling of a large angular range of the XRD pattern, with the intensity directly connected with the structure of the Pt phase. In this way much more reliable results can be obtained even for samples made of very small metal particles and in the presence of a noncrystalline phase. © 1993 Materials Research Society

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P. Scardi and P. L. Antonucci: XRD characterization of highly dispersed metal catalysts on carbon support

II. EXPERIMENTAL Two different procedures (A and B) were adopted to produce Pt catalysts supported on a high surface area (BET SA = 950 m 2 /g) carbon black (Ketjen-black EC, by AK20 Chemie). Samples K50, K80, K81, and K84 were prepared by the A procedure. The carbon black (10 g • I"1 H 2 O) was mixed with H 2 O and a small amount of C 2 H 5 OH (10 ml • g"1 carbon) to form a suspension. The ethanol improves the wetting of the carbon black. A solution of H2PtCl6 (1 g/50 ml H 2 O) was added to the solution, followed by the addition of 4 ml of 30% H 2 O 2 . The solution was stirred and maintained at 45 °C for 30 min, then 20 ml of Na 2 S 2 O 4 solution (30 g • I"1 H2O) were slowly added. The temperature was kept constant for 15 min, then it was lowered to room temperature, at which the slurry was continuously stirred for 2 h. The supported electrocatalyst was separated from the aqueous solution by filtering, then it was washed with water, and finally dried overnight at 70 °C. Thermal activation of the electrocatalyst was performed in a quartz tube furnace under a flowing nitrogen atmosphere at 350-900 °C for a time ranging from 45 to 240 min (see Table I). Samples K60, K78, and K79 were prepared following the same procedure, except for the use of Na 2 S 2 O 3 instead of Na 2 S 2 O 4 as a reducing agent. According to procedure B (samples ST2A and ST6B), 2 g of carbon were suspended in 25 ml of H 2 O under stirring at 60 °C for 30 min; an aqueous K2PtCl6 solution (1.1 g/25 ml H 2 O) was then added. After 5 min, 55 ml of 0.1 M NaOH were further added, increasing the temperature to 70 °C for 30 min. After cooling at 25 °C, the reduction was performed by 6 ml of 2 M N 2 H 4 at 40 °C for 30 min. The slurry was kept under stirring for 2 h, then it was filtered, washed, and

TABLE I. List of Pt/C catalysts.

Pt (wt. %)

r(°c)

Activation t (min)

K50 K80 K81 K84 K60 K78 K79

17.0 13.7 13.8 14.5 16.4 17.2 17.1

400 900 900 900 350 900 900

240 60 60 45 240 60 60

Procedure B ST2A ST6B

19.0 19.8

600 900

60 60

Activation Sample Procedure A

dried overnight at 70 °C. Thermal activation was made at 600-900 °C for 60 min (see Table I). X-ray diffraction analyses were conducted by a Rigaku D-max III horizontal diffractometer with a graphite curved crystal monochromator in the diffracted beam, using copper radiation produced at 40 kV/30 mA and slits: 1/2° (divergence), 0.15 mm (receiving), and 1/2° (antiscattering). Step scan mode was selected with a 0.1° step and 10 s fixed time in the range 15-130°. A KC1 standard sample was used to determine the instrumental broadening, following a procedure reported elsewhere.14 TEM was performed using a STEM apparatus (JEOL CX 20) whose ultimate spatial resolution is 0.5 nm. To allow a uniform distribution of the Pt crystals, the catalytic layer was first dispersed in isopropyl alcohol, then deposited and dried on a TEM support comprising a Cu grid covered with a carbon film a few tens of nanometers in thickness. Electrochemical measurements of the Pt surface area were performed by conventional CV, using the coulombic charge for hydrogen adsorption in the potential range from 0.4 to 0.05 V (vs RHE) in 50% H 3 PO 4 at room temperature. For these measurements, flooded electrode structures consisting of Pt/C electrocatalyst and a small amount of PTFE (10%), which is necessary to maintain structural integrity, were used.15 III. RESULTS AND DISCUSSION The XRD patterns were analyzed using a procedure based on the Rietveld method.9'10 Regarding the present application, the advantage is that lattice parameter and crystallite size can be calculated simultaneously, using an algorithm very effective for broad and overlapping peaks, also considering sample transparency effects. In the following the method is briefly described; further details can be found in the literature cited. The procedure consists of modeling the whole experimental pattern using analytical functions with the intensity directly connected with the structure factors of the present phases. Usually the method is employed to refine the material structure (atomic positions, occupancy, temperature factors, and lattice parameters), but recently it was improved to refine the material microstructure.11"13 In fact, very precise quantitative phase analysis can be done, including crystallite size and microstrain measurement, also considering preferred orientation and possible anisotropy. The base idea was to introduce profile analysis into the Rietveld algorithm. Considering the Fourier transform of a peak, from the Warren-Averbach theory,16'17 the Fourier coefficients An, corrected for instrumental broadening, can be written as (1)

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J. Mater. Res., Vol. 8, No. 8, Aug 1993


where n is the harmonic number; Asn and A% are the size and strain coefficients, respectively, representing the effect of crystallite size and lattice distortions on profile broadening. It can be shown that a surface weighted mean crystallite size, (Ds), can be obtained from an Asn versus L plot (L = n • dhU is the column length and dhkl is the interplanar distance), taking the intercept with the abscissa of the tangent at L = 0. Actually, the program gives the distribution of crystallite column size p(L), obtained from the second derivative of the size Fourier coefficients, Asn. p(L) can be used for the calculation of (Ds) or other types of mean values. (Ds) is used for surface area calculation; it can be easily shown that, for a polydisperse system of spheres, the surface area S is given by18: S =

4-M p{Ds)

(2)

where M and p are mass and density of a particle, respectively. The S values were calculated taking p = 21.44-10 6 g/m 3 . Crystallite mean size obtained by XRD cannot be directly compared with particle size obtained by other techniques such as TEM or CV without assuming a suitable model for particle shape. In fact, it must be stressed that (Ds) is the mean length of the cell columns making up coherently diffracting domains. It can be easily demonstrated that for a monodisperse system of spheres a simple relation holds18:

d = \{DS)

(3)

were d is the mean particle diameter. In the present work, comparison with other size measurements will be done on the basis of this model. The experimental problem was quite difficult to handle because of the presence of a large fraction of carbon black, whose amorphous pattern interferes with a correct background modeling. Moreover, as shown in the patterns of Fig. 1, a noticeable peak broadening was present, for all the samples, indicating that the metal is present in the form of very small particles. To reduce the effect of the support and to achieve a better reliability also in the measurement of the Pt lattice parameter, only the high angle 60-130° range was considered, where the carbon contribution to the pattern can be accounted for by the polynomial form fitting the background. Sample transparency constitutes a source of systematic error in peak position, i.e., lattice parameter. Thus, a suitable correction was adopted11: A0, =

sin (20) R-fi

'

(4)

50

SO 70 B0 90 2-Thsta [degrees]

100

110

120

FIG. 1. XRD experimental pattern for samples K60 and K78.

where R is the goniometer radius (185 mm) and yu is the linear absorption coefficient: — I • Pc • wc + I —

We

vt>pt

(5)

VP

where /x/p, p, and w are mass absorption coefficient, density, and weight fraction, respectively. The values calculated for our samples ranged between 600 and 850 cm"1. The computer code LSI19 was used to perform the refinements. As an example, the graphical output for sample K84 is shown in Fig. 2. The residual, i.e., the difference between experimental (• • •) and modeled (—) intensity, is reported below. Figure 3 shows the crystallite size distributions, p(L) and L- p(L), for the same sample, from which a mean size (Ds) — 17.5 A was obtained. The results for all samples studied are summarized in Table II, including lattice parameter, crystallite mean size, and surface area [in parentheses is reported the product S X Pt (wt. %)]. Also, the quality factors, expressing the goodness of the fit, are reported: —11/2

(6)

**-wp

1/2

(N - P)

GoF =

R wp

Rexp

(7)

(8)

where w,- is the weight of the data point, Yio and Yic the experimental and calculated intensities, respectively,


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**M^^ 90 2-Theta

100 [degrees]

110

120

FIG. 2. Experimental (•••) and modeled (—) pattern for K84. The residual is reported below. The whole (10-130°) experimental spectrum is reported in the inset.

N the number of data, and P the number of fitting parameters. Temperature factors found from refinement of powder diffraction pattern are often unreliable as they tend to correlate with other fitting parameters and to compensate errors affecting peak intensity. Thus, the accuracy of the present modeling is also attested to by the refined values of the temperature factors, between 0.25 and 0.40 A 2 , that are plausible for atoms as heavy as platinum. Two samples (K50 and K60) gave only meaningless, negative values (—0.04, - 0 . 0 8 A 2 ; these samples, both treated at the lowest activation temperature, showed a larger noncrystalline fraction, as shown in Fig. 1, making the modeling

FIG. 3. Crystallite size distributions p(L) (• • •) and p(L) • L (—) for sample K84. 1832

less reliable. This was especially true for K50, which has peaks broader than K60. This resulted in a very poor refinement of lattice parameter for K50, which was omitted from Table I and the following discussion. For three samples (ST2A, ST2B, and K79) a single Pt phase could not account for the experimental profile. This was understood as due to the presence of a complex size distribution; for this reason, two Pt phases were introduced to model the pattern. In this way a bimodal size distribution was obtained, as shown in Fig. 4. Table II reports the mean values of the two phases together with the relative phase percentage; S was calculated taking into account the presence of two distributions. The lattice parameters, especially for the small fraction of large crystallites, are probably not reliable because of correlation between parameters; on the contrary, from the viewpoint of the profile analysis the result is good because the only important condition to be satisfied is a good modeling (Fig. 5), as confirmed by the quality factors reported in Table II. The correlation between lattice parameter and crystallite size is shown in Fig. 6. Samples with bimodal size distribution were not included in the plot. As known from the literature,20 the Pt lattice parameter decreases by reducing the crystal size because of compressive forces exerted by surface stress for small particles. Although the lattice parameter values here measured shift aside from that reported in the literature20'21 (related to particle diameters larger than 40 A), the trend of the data appears to confirm the established behavior. On the other hand, we are not aware of previous investigations dealing with analogous measurements for particle sizes


P. Scardi and P. L Antonucci: XRD characterization of highly dispersed metal catalysts on carbon support

TABLE I I . Results of XRD pattern refinement. Sample

K50 K80 K81 K84 K60 K78 K79 ST6B ST2A

a (A)

5 (m 2 /g)

(Ds) (A)

89.7 (15.2) 109.1 (14.9) 56.5 ( 7.8) 106.0 (15.4) 43.4 ( 7.1) 111.7 (19.2) 84.9 (14.5)

20.8 17.1 33.0 17.6 43.0 16.7 21 (95.1%) 243 ( 4.9%) 18 (98.2%) 490 ( 1.8%) 12 (97.4%) 224 ( 2.6%)

3.9187 3.9189 3.9197 3.9212 3.9165 3.9173 3.9184 3.924 3.924 3.926 3.928

101.8 (20.1) 151.6 (28.8)

in the range here examined (20-70 A). The scatter observed in Fig. 6 is likely due to the interactions occurring between such highly dispersed particles and the amorphous substrate. The lowering of lattice parameter for carbonsupported Pt catalysts has been previously associated with the formation of platinum carbide alloy22; although it has not yet been observed in bulk, its existence has been reported in thin-film form.23 On the other hand, solubility and diffusivity phenomena of carbon in platinum are known to occur in the range of temperature (700-800 °C)24 here adopted to activate the investigated catalysts; thus, as it is very unlikely to have the alloy formation under the present preparation conditions, the observed differences with the values reported for bulk metal are to be attributed to some degree of interaction

RwP

GoF

8.54 8.32 10.60 9.09 8.60 8.48 7.75

1.17 1.16 1.23 1.05 1.30 1.14 1.09

7.56

1.10

8.43

1.11

between the dispersed crystallites and the amorphous carbon support. Finally, it is very interesting to compare the particle size obtained by XRD, TEM, and CV. As shown in Table III, the agreement, on the whole, is satisfactory. Two points must be emphasized. First, XRD data are almost always between TEM and CV data: TEM < XRD < CV. Considering the particles as spheres, TEM observation performed on a bidimensional section tends to underestimate particle diameter. On the contrary, CV measurement cannot consider clustering effects, and the size tends to be overestimated. Second, it is worth noting that neither TEM nor CV can give detailed information on the particle size distribution, in particular regarding the occurrence of bimodal distributions. While CV is intrinsically unsuitable to

o.io 50 FIG. 4. Crystallite size distributions p(L) ported in the inset.

100

150

(•••) and p(L)-L

200

250

300

350

400

450

(—) for sample K79; the detail of the large crystallites' fraction is re-


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'

^

^

^

^

^ BO

90 2-Theta

100 [degrees]

110

ISO

FIG. 5. Experimental (•••) and modeled (—) pattern for sample K79. The residual is reported below. The whole (10-130°) experimental spectrum is reported in the inset.

detect bimodal distributions, TEM is limited only by statistical sampling. Though, considering K79, less than one particle out of 10000 belongs to the fraction of larger particles. Such details can hardly be appreciated by TEM, whereas the statistical basis of XRD makes this technique more complete in this sense. IV. CONCLUSION Conventional XRD profile analysis cannot operate in the presence of broad overlapping peaks superposed to an amorphous background. This condition is frequently met with when studying highly dispersed catalysts on amorphous support. For this reason, a new procedure of whole pattern fitting, which gives a reliable measurement of crystallite size distribution and lattice parameter, was devised. The obtained values of particle size lie well

between those found by TEM and CV, confirming the validity of the method. Moreover, it was shown that the proposed procedure is effective also for samples with bimodal particle size distribution, that are typical of several highly dispersed catalysts. This occurrence usually escapes both CV and TEM observation. Finally, using our results, the correlation between Pt lattice parameter and particle size was extended down to sizes as small as 2.5 nm. ACKNOWLEDGMENTS The authors wish to thank Dr. Luca Lutterotti and Dr. Nicola Giordano for their helpful discussions and Dr. Lidia Pino for supplying the catalyst samples. TABLE III. Mean particle diameter (A). Sample

TEM a

K50 K80 K81 K84 K60 K78 K79

18 22 38 30 25 38 38

XRD

CV

3.922 3.920 -

3.918 3.917 -

3.915

ST6B ST2A 10

20

30 40 50 60 Mean particle diameter (A)

70

FIG. 6. Platinum lattice parameter versus particle diameter. 1834

80

19

31 25.5 49.5 26.5 64.5 25 31.5 365 27 735 18 336

(95.1%) ( 4.9%) (98.2%) ( 1.8%) (97.4%) ( 2.6%)

"Mean particle size calculated as length-number.3


49 25 54 34 83 37 47 39 24

P. Scardi and P. L Antonucci: XRD characterization of highly dispersed metal catalysts on carbon support

REFERENCES 1. M. Boudart, Adv. Cat. 20, 153 (1969). 2. G.A. Somorjai, Adv. Cat. 26, 2 (1977). 3. J. R. Anderson, Structure of Metallic Catalysts (Academic Press, London, 1975). 4. K. Kinoshita, J. Electrochem. Soc. 137, 845 (1990). 5. M. Hayes and A.T. Kuhn, Appl. Surf. Sci. 6, 1 (1980). 6. G. Cocco, S. Enzo, S. Galvagno, Z. Poltarzewski, and R. Pietropaolo, J. Chem. Soc. Faraday Trans. 81, 321-333 (1985). 7. S. Enzo, A. Benedetti, and S. Polizzi, Z. Kristallogr. 170,275-287 (1985). 8. R. Campostrini, G. Carturan, S. Dire, and P. Scardi, J. Mol. Catal. 53, L13 (1989). 9. H.M. Rietveld, Acta Crystallogr. 22, 151-152 (1967). 10. H.M. Rietveld, J. Appl. Cryst. 2, 65-71 (1969). 11. L. Lutterotti and P. Scardi, J. Appl. Cryst. 23, 246-252 (1990). 12. P. Scardi, L. Lutterotti, R. Di Maggio, and P. Maistrelli, Mater. Sci. Forum 79-82, 233-238 (1991). 13. P. Scardi, L. Lutterotti, and R. Di Maggio, in Advances in Xray Analysis, edited by C. S. Barrett, J. V. Gilfrich, T. C. Huang, R. Jenkins, G. J. McCarthy, P. K. Predicki, R. Ryon, and D. K. Smith (Plenum Press, New York, 1992), Vol. 35A, pp. 69-76. 14. P. Scardi, L. Lutterotti, and P. Maistrelli, in Accuracy in Powder Diffraction II, edited by E. Prince and J. K. Stalick (NIST Special Publication 846, Gaithersburg, MD), p. 216.

15. N. Giordano, E. Passalacqua, L. Pino, A. S. Aricd, V. Antonucci, M. Vivaldi and K. Kinoshita, Electrochimica Acta 36, 1979 (1991). 16. B.E. Warren and B.L. Averbach, J. Appl. Phys. 21, 595-599 (1950). 17. H. P. Klug and L. E. Alexander, X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed. (John Wiley & Sons, New York, 1974), pp. 643-655. 18. W. L. Smith, J. Appl. Cryst. 5, 127 (1972). 19. L. Lutterotti, P. Scardi, and P. Maistrelli, J. Appl. Cryst. 25, 459-462 (1992). 20. K. Kinoshita, in Modern Aspects of Electrochemistry, edited by J.O'M. Bockris, B.E. Conway, and R.E. White (Plenum Press, New York, 1982), pp. 557-637. 21. H.J. Wasserman and J.S. Vermaak, Surf. Sci. 32 168 (1974). 22. Development of Ternary Alloy Cathode Catalysts for Phosphoric Acid Fuel Cells, Final Report, United States Department of Energy Contract DE-AC21-82MC24261, prepared by Giner, Inc., Waltham, MA (1988). 23. M. J. Witcomb, V. Dahmen, and K. H. Westmacott, Acta Metall. 31, 743 (1983). 24. R.T. Yang, P.J. Goethel, J.M. Schwartz, and C.R.P. Lund, J. Catal. 122, 206 (1990).


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