X-ray diffraction studies of catalysts

$Page 1: X-ray diffraction studies of catalysts$
Ultramicroseopy 34 (1990) 41-46 41 North-Holland

X-ray diffraction studies of catalysts

J.B. C o h e n Department of Materials Science and Engineerin~ The Robert 1L McCormick School of Engineering and Applied Science, The Technological Institute, Northwestern University, Evanston, IL 60208, USA

Received 3 January 1990; at Editorial Office 28 May 1990

With the new high-intensity X-ray sources, by using anomalous scattering and glancing incidence, it is now possible to obtain considerable statistical information on catalysts-supported metals (size distribution and perfection), micron-sized single-crystal particles (structure), and the nature of surfaces. Examples of work on catalyst materials is given to illustrate these possibilities.

1. Inlroduetion

There is no question that to fully answer the many questions we all have about catalysts - how they actually function on an atomic scale - the various microscopes offer the most likely avenues. Nevertheless, X-ray diffraction can provide val- uable, statistically sound, information, which is often difficult to obtain with these more local tools. In fact, X-ray diffraction has been employed for a long time to characterize catalysts, via struct- ural analysis (primarily with powders), or to examine the perfection of the catalyst. The recent availability of synchrotron radiation has provided new impetus for such investigations in a number of ways. In this paper we will briefly review studies in four main areas: (1) powder patterns of supported metal catalysts, (2) new developments in small-angle scattering, (3) surface structures, (4) structures of microcrystals. While EXAFS and XANES are closely connected to diffraction, we shall comment only briefly on these tools.

2. Supported metal catalysts

2.1. Experimental techniques

It is often desirable to examine catalysts after various treatments, or during a reaction, and

without exposure to the environment. For this purpose, we have used a series of simple cells for wide-angle diffraction [1,2]. In the first of these, a large shallow circular dish of glass is sealed with mica, and attached to long glass tubes, which can be connected to a gas train. The cell is easily placed on a diffractometer, and the catalyst can be moved to the tubular section by tilting, for inser- tion in a furnace or a gas train. A smaller version can be used to examine the pat tern during a

reaction; diminished size keeps the catalyst amount small enough for proper gas-flow conditions.

We have also constructed a cell that permits close temperature control, even for exothermic reactions. In it, a sample can be heated to as high as 800 K, treated and then cooled to as low as 140 K, all in situ on a diffractometer [3].

Highly dispersed catalysts are difficult to examine with ordinary X-ray sources. With a rotating anode X-ray generator (or the new high-power sealed X-ray tubes for standard generators), it is possible to obtain data down to about an average size, ( D ) ~ 25 A (40% metal exposed). (The percent metal exposed is calculated from the particle sizes, as described in refs. [1,2].) With the intensity available at a synchrotron source, multiple peaks can be observed up to - 60% metal exposed, and we have obtained interpretable patterns to ( D ) -- 13 A ( - 80% metal exposed). At still smaller sizes, the pat tern is so extended that analysis is better

0304-3991/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)

42 J.B. Cohen / X-ray diffraction studies of catalysts

done by considering the scattering in terms of the radial density distribution, as is done for liquids or amorphous solids. (But this does not imply that the particles are indeed amorphous!) We have generally found that a short wavelength ( - 0.7/k) is useful because the penetration is deeper and hence the sampling is better. Also, the peaks are at lower angles and less broad.

With some supports such as alumina (with Pt or Pd), there may be only a single peak that is independent of the strong peaks of the support. (More about what can be done in such a case below.) Another procedure for examining such catalysts is via small-angle scattering. Brumberger and co-workers [4] have shown that it is possible to measure the surface area between support and catalyst, catalyst and air, and support and air, without pore-filling and with very simple procedures. With a rotating anode, and a position-sensitive detector, an entire pattern can be obtained and processed in a couple of hours, and in seconds at a synchrotron source. The wide range of wavelengths available at a synchrotron or storage ring permits the use of two additional methodologies. The atomic form factor changes drastically near an absorption edge for any of the electron shells. If the pattern is recorded at any energy very close

to, but just below, the L edge of Pt (or Pd) and also well below this edge, only the Pt scattering will change, not that of the silica or alumina support. By taking the difference, or even better yet, evaluating the derivative with respect to energy, the contribution of the support can be mini- mized. In addition, it can be shown that in such a difference, the scattered intensity for Pt decreases much less with scattering angle than in the normal case, thus emphasizing higher-order peaks. We show an example of this, in fig. 1 from ref. [5], Pt on Al203. In the patterns for each wavelength, the Pt peaks are largely obscured by those from Al 203, but in the difference the metal peaks are quite clear. Note also the "flatness" of the background after the subtraction. We have used this procedure successfully down to sizes of 13-14 ,~.

The shape of the edge itself examined by XANES (X-ray near-edge spectroscopy) can be employed to reveal information on d-band vacancy concentration versus treatment. The oscillations at energies above the edge (EXAFS) can provide information on near-neighbor atom spacing and some limited information on the chemical environment. The best way to use such tools is to combine them with others, rather than using only one of them.

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J.B. Cohen / X-ray dtffraction studies of catalysts 43

It is true that these X-ray procedures are much less sensitive to sample preparation than chemisorption techniques. Nonetheless, it is desirable to use them in conjunction with such methods. In analysis of chemisorption data, it is often desirable to make an assumption as to the number of gas molecules that attach to each atom in the catalyst. Careful X-ray studies can indicate the validity of such a number and, as well, reveal whether the catalyst particles cluster. In this case, the X-ray size will be much smaller than that indicated by chemisorption. This can also be done by comparing results from X-ray diffraction and electron microscopy [6].

2.2. Analysis of the data

X-ray diffraction has been employed for a long time to attempt to characterize supported catalysts, but until recently for wide-angle diffraction, only the width of a peak has been examined. From the well known Scherrer equation, this width yields a "size". However, it has not often been recognized that this procedure has inherent limitations: (1) The measured "size" is not really a size, but a ratio of two moments in the distribution, ( D 2 ) / (D). (2) The averages are volume averages, whereas the desired weighting for catalytic studies is by surface area. Furthermore, this difference in weighting has not always been recognized in making compari- sons to results from electron microscopy [7]. (3) Other factors contribute to broadening, such as internal strain. Instrumental factors are particu- laxly difficult to remove from a measurement of just the breadth, and the effects of local strain can only be approximated with this single measure.

Instead of this methodology, in our work we have chosen to use Fourier analysis of the entire peak shape. By this procedure, all of the above problems are eliminated. In particular, we focus on the cosine coefficients of the Fourier series representing a peak. The instrumental effects are then readily removed with the Fourier coefficient of peaks from large particle samples, and the remaining coefficient of harmonic number (n),

An, can be written as a product:

_ A s i z e A d i s t o r t i o n A n --..at n ZXn(hkl) . (1)

The first term is related to the (surface weighted) average size, whereas the second is a function of the microstrains. The first term is the same for all peaks, whereas the second depends on the inter- planar spacing, "d ", of the diffraction planes, and is proportional to 1/d 2. The two terms are readily separated by plotting lnA n for a given n versus 1/d 2, for multiple orders of a diffraction peak. The microstrains are obtained from the slope for each n and hence versus distance normal to the diffraction planes. If there is no strain variation from point to point in a particle, this plot is horizontal; that is, the coefficients for multiple orders will superimpose. The initial slope of A~ ~ versus n yields the average size, whereas the second derivative is the size distribution [8]. From the measured size in different crystallographic direc- tions, and from the shape of the size distribution, information on particle shape can be obtained. Tested analytical procedures are also available when only a single peak can be measured [9].

Other information that can be obtained from a wide-angle diffraction pattern includes the detec- tion of stacking faults and microstrains, the lattice parameters, and the mean-square amplitude of vibration.

2.3. Results

2.3.1. Pt catalysts In a series of studies of carefully prepared

catalysts of Pt on silica gel [1,10-12] we have shown that the Pt particles are equl-axed (and definitely not cuboidal as has often been assumed), that the size or percent metal exposed agrees with results from hydrogen chemisorption, and that the particles are free of microstrain, faults, or micro- twins, except when the average size is similar to the pore size of the support. In this latter case, the particles are elongated, and there is microstrain, probably due to differential contraction of the catalyst and support on cooling from the reduction treatment. The reactivity of these strained particles for methylcyelopropane hydrogenolysis is


very high, a fact which we do not yet fully under- stand; it is possible that the rnlcrostrain is connected with this behavior. At least up to 60% metal exposed, and even in air, the particles are not heavily oxidized, and the lattice parameter is that of bulk Pt. The size distribution is sharper when the catalyst is prepared by ion-exchange, rather than impregnation, and the distributions suggest coalescence during preparation, not Ostwald ripening. The hydrogenolysis activity is proportional to the mean-square amplitude of vibration. As the size is reduced still further, for catalysts stored in air the Pt mass is slowly converted to (crystalline) Pt304, a process which is complete at - 80% metal exposed. There is little contact area between the metal and support as determined by small-angle X-ray scattering [10].

Reduction by hydrogen completely alters the chemical reactivity and its variation with size. At the same time, the Pt particl e size is reduced.

A study of tiie area of Lni absorption edge resonance shows a correlation with chemical activity. This implies a correlation with the number of d-band vacancies. This occurred when Pt oxide was reduced by hydrogen or when the particle size was decreased [13].

2.3.2. P d catalysts [2,14] The catalytic activity of Pd/SiO 2 of low per-

centage metal exposed (for methylcyclopropane (MCP) hydrogenolysis at 0°C) is less for catalysts cooled from 723 K in H 2 than for the same material cooled in He. We have shown that this is due to hydride formation (when cooling in H2). Hydride formation becomes more difficult with decreasing particle size up to -30% metal exposed. Exposure to hydrogen results in nearly complete conversion of hydride, but purging with He (even at 0 ° C) reconverts the hydride to metal, although there is some induction period. Passing hydrogen plus MCP over the catalyst also results in the conversion into hydride of a substantial portion of the catalyst originally present as pal- ladium metal. The lattice parameter is the same as for bulk Pd, at least for particles of 45 A or larger.

No hydride forms for very small particles. Yet when catalysts with such a small metal particle size are stored in air, they are converted to (crys-

talline) PdO. Reduction of this oxide with hydrogen produces Pd metal, not hydride.

3. Structure of small particles

Kvick and Kansikas [15] have recently been able to determine the structures of individual zeolite crystals - 3 0 - 7 0 #m in size by using standard single-crystal techniques, employing the high intensity at the National Synchrotron Light Source. In this work, they were able to locate the positions of the atoms in adamantine molecules in the cages in a ZSM-39 structure, and ethylene glycol in a sodalite cage. In the latter case, they could locate the 1.5% Na incorporated in the structure to compensate for the charge on the small amount of A1 in the framework. With the next generation of synchrotrons (like the Ad- vanced Photon Source at Argonne National Laboratory) crystals much less than 1/~m in size will be possible, perhaps as small as 1000 ~,! The difficulty in interpreting powder patterns from these materials is well known, and this is a major step forward (although with synchrotron radiation the improved resolution greatly facilitates powder analysis).

4. Glancing-angle techniques

X-rays have an index of refraction slightly less than unity, and therefore exhibit total external reflection at low glancing angles of the order of 0.1°; this angle increases with increasing electron density. Below such an angle the refracted beam penetrates only 50 A or so and is a source for diffraction from the near-surface regions. The resultant scattering can generally be interpreted with simple kinematic theory. Because of the weak scattering from such thin layers, even with synchrotron radiation, such studies have been limited so far to relatively heavy metals. However, with patience the intensity is useful for more general situations. TiO 2 is a good example of an interesting catalystic oxide, and Dr. P. Zschack has recently completed a detailed study of the (100) surface of the stoichiometric oxide [16]. (A UHV

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J.B. Cohen / X-ray diffraction studies of catalysts 45

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chamber was built to fit on a diffractometer, with digital LEED, Auger sputtering and heating capa- bilities [17].) The resultant structure is shown in fig. 2. The surface is "corrugated" with {110} facets. A study of the reduced form of this oxide would now be particularly interesting because it is known to be a good photo-activated catalyst, perhaps useful in the conversion of light and water into gaseous hydrogen [18].

Rice and Bienenstock [19] have recently shown that by using anomalous dispersion and small-an-

gle scattering they could examine the distribution of solute in chalcogenide glasses. The difference in measurements taken at two wavelengths close to the absorption edge of the solute eliminates all effects but those associated with the solute, and any clustering of the solute would produce a small-angie scattering pattern. Such a technique might be particularly useful for studies of surfaces, because Levine [20] has demonstrated that it is possible to do small-angle scattering in glancing- incidence geometry - GISAXS, fig. 3. Levine was able to readily record small-angle patterns from -40% coverage of Au particles on glass (fig. 4) prepared and measured in a UHV chamber. Thus, model catalyst particles on a surface can be char- acterized or the arrangement of solute on the

detector

Fig. 3. Glancing-incidence small-angle X-ray scattering (GISAXS), ref. [20].


surface o f an alloy catalyst can be probed in particles as small as a few microns, by combining this glancing-angle method with anomalous scattering.

Acknowledgements

Our research in this area was supported by DOE (Grant No. DE-AC02-77ER04254 and Grant No. DE-FG02-85ER451 83). The X-ray measurements were performed either at the CHESS synchrotron facility, ComeU University, NSLS, Brookhaven National Laboratory, or in the X-ray Diffraction Facility of Northwestern University's Materials Research Center, supported in part by NSF under Grant No. DMR-MRL-76-80897. We particularly thank Drs. A. Kvick, J. Kansikas and Professor A. Bienenstock for allowing us to men- tion their latest research, prior to its publication.

References

[1] ILK. Nandi, F. Molinaro, C. Tang, J.B. Cohen, J.B. Butt and ILL. Burwell, Jr., J. Catal. 78 (1982) 298.

[2] R.K. Nandi, R. Pitchai, S.S. Wong, J.B. Cohen, R.L. Burwell, Jr. and J.B. Butt, J. Catal. 70 (1981) 298.

[3] J. Pielaszek and J.B. Cohen, in: Proc. 1983 Denver Conf. on Applications of X-Ray Analysis, Vol. 27 of Advances in X-Ray Analysis (Plenum, New York, 1984) p. 293.

[4] J. Goodisman, H. Brumberger and R. Cupelo, J. Appl. Cryst. 14 (1981) 305.

[5] P. Georgopoulos and J.B. Cohen, J. Catal. 92 (1985) 211. [6] J.A. Smelse, K.B. Arcuri, J.B. Butt, R.J. Matyi, L.H.

Schwartz and A. Shapiro, J. Phys. Chem. 85 (1981) 708. [7] R. Matyi, PhD Thesis, Northwestern University, Evan-

ston, IL, 1983. [8] J. Pielaszek, J.B. Cohen, R.L. Burwell, Jr. and J.B. Butt, J.

Catal. 89 (1983) 479. [9] R.K. Nandi, H.K. Kuo, W. Schlosberg, G. Wissler, J.B.

Cohen and B. Crist, Jr., J. Appl. Cryst. 17 (1983) 22. [10] T. Uchijiima, J.M. Herrmann, J.Y. Inouye, R.L. BurweU,

Jr., J.B. Butt and J.B. Cohen, J. Catal. 50 (1977) 464. [11] S.R. Sashital, J.B. Cohen, R.L. Burwell, Jr. and J.B. Butt,

J. Catal. 50 (1977) 479. [12] P.H. Otero-schipper, W.A. Wachter, J.B. Butt and R.L.

Burwell, Jr., J. Catal. 50 (1977) 494. [13] E. Rorris, PhD Dissertation, Northwestern University,

Evanston, IL, June 1983. [14] R.K. Nandi, P. Georgopoulos, J.B. Cohen, J.B. Butt and

R.L. Burwell, Jr., J. Catal. 77 (1982) 421. [15] A. Kvick and J. Kansikas, private communications, ACA

Meeting, July 1989. [16] P. Zschack, PhD Thesis, Northwestern University, De-

cember 1989. [17] P. Zschack, Y.W. Chung and J.B. Cohen, J. Appl. Cryst.

21 (1988) 466. [18] A. Fujishima and K. Honda, Nature 238 (1972) 37. [19] M.B. Rice and A. Bienenstoek, private communications,

ACA Meeting, July (1989). [20] J. Levine, Y.W. Chung and J.B. Cohen, J. Appl. Cryst. 22

(1989) 528.

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X-ray diffraction studies of catalysts