x-ray diffraction and electron microscopy studies of platinum-tin-silica catalysts

$: X-ray diffraction and electron microscopy studies of platinum-tin-silica catalysts$
Applied Catalysis A: General, 87 (1992) 45-67

Elsevier Science Publishers B.V., Amsterdam

45

APCAT A2280

X-ray diffraction and electron microscopy studies of platinum-tin-silica catalysts

Ram Srinivasan and Burtron H. Davis Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, KY 40511 (USA)

(Received 13 January 1992, revised manuscript received 1 April 1992)

Abstract

Platinum and tin were added to a silica support by impregnation. The platinum content was kept

constant at 1.0 wt.-%, and the amount of tin varied. X-ray diffraction (XRD), transmission electron

microscopy (TEM), electron microdiffraction and energy dispersive spectroscopy (EDS) data revealed

that the dominant fraction of the crystalline platinum was present as metallic platinum for catalysts

containing Pt: Sn= 1: 0.5 mol ratio, PtSn alloy for catalysts containing Pt : Sn= 1: 5 and non-crystalline

platinum or Pt/Sn alloy and metallic tin for catalysts containing Pt: Sn = 1: 8. It appears that the pres-

ence of tin in this bimetallic system aids the dispersion of platinum particles. However, the form of

platinum depends upon the Sn/Pt ratio, and no single structure will adequately describe all Pt-Sn-SiO,

catalysts.

Keywords: bimetallic, microdiffraction, platinum/tin, tin, transmission electron microscopy, X-ray

diffraction.

INTRODUCTION

Pt-Sn bimetallic catalysts are known for their improved aging characteris- tics. The Pt-Sn combination becomes more important as continuous regen- eration processes are utilized for naphtha reforming. These lower pressure processes require introduction of the regenerated catalyst into the reaction zone without special treatment. For this process the Pt-Sn combination has been found to be a good candidate, and it is now receiving renewed attention in commercial units.

The catalytic behavior of the PtSn combination depends upon the structural properties of the catalysts and attention was focussed on understanding this phenomenon. Various characterization techniques have been utilized in at- tempts to identify the crystal structures that are present in this bimetallic

Correspondence to: Dr. B.H. Davis, Center for Applied Energy Research, University of Kentucky,

3572 Iron Works Pike, Lexington, KY 40511, USA. Tel. (+ l-606)2570251, fax. ( + l-606)2570302.

0926-3373/92/$05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.

46 R. Srinivasan, B.H. Davis/Appl. Catal. A 87 (1992) 45-67

system. The temperature-programmed reduction (TPR) technique provides an indirect method of identifying the chemical state of the tin species. Based on the data obtained using TPR, it was claimed that tin was not reduced to zero valent state [l-4]. Without reduction to the Sn” state, PtSn alloys could not be formed as has been claimed’ (for example, see refs. 5-7).

Studies using electron spectroscopy for chemical analysis (ESCA) or X-ray photo-electron spectroscopy (XPS ) indicated that tin was present only in an oxidized state [8-lo]; later one of these groups reported XPS data for Sn” [ 11,121. Hoflund and coworkers [13-161 reported the formation of Sn” and PtSn alloy, using XPS, ion scattering spectroscopy (ISS ) and Auger electron spectroscopy (AES), when platinum was supported on Sn02.

X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) are techniques that provide data that rep- resents a diagnostic of the bulk structure. These techniques were used to study alumina and silica supported catalysts, each containing 1 wt-% platinum, but with variable tin content [ 17,181. These catalysts were prepared by impregnation from acetone solution to minimize hydrolysis of the tin compound. The results revealed a radial distribution peak that could be assigned to Pt-Sn scattering, but this by itself is not sufficient to prove the development of alloy clusters upon reduction. Meitzner et al. [5] used EXAFS to study a Pt-Sn- SiO, catalyst prepared by impregnation and platinum on a coprecipitated Sn- A1203 catalyst. The Pt-Sn-Al,O, material was obtained by first preparing alumina-tin hydrogel so that tin was uniformly dispersed throughout the material; platinum was then added by impregnation with hexachloroplatinic acid. According to these authors, platinum is more highly dispersed on the tin-containing support than on alumina alone even though hydrogen chemisorption indicated that essentially all the platinum atoms in both catalysts were surface atoms. Furthermore, platinum in Pt-Sn-A&O, is more electron deficient than platinum in Pt-SiO, or Pt-Sn-Si02. They concluded that the EXAFS data show that the catalyst consists essentially of platinum clusters on alumina containing Sn2+ at the surface.

X-ray diffraction is another bulk characterization technique. Pt-Sn-AIBO, catalysts prepared from the complex [ Pt3Sn8C120] ‘- were examined using an in-situ XRD technique. For catalysts containing either 0.6 or 5 wt.-% platinum, PtSn alloy, and no other alloy, was observed [ 61. In another XRD study [ 71, it was observed for a series of catalysts containing 1 wt.-% platinum on alumina and increasing amounts of tin to give Pt : Sn mol ratios from 1: 1 to 1: 10 that in all of these samples the alloyed platinum was present as Pt:Sn=l:l.

“‘Sn Mijssbauer data also provide a bulk diagnostic and this method has been utilized in a number of studies (see for example refs. 19-33). Direct evidence for Pt-Sn alloy formation has been obtained from at least some of these Mijssbauer studies (see for example refs. 19,26,27,30,31). However, many of

R. Srinivasan, B.H. Davis/Appl. Catal. A 87 (1992) 45-67 47

these studies were at high metal loadings, and even then a complex spectrum was obtained, thus, there was some uncertainty in assigning Sn” to the exclu- sion of other tin states. Miissbauer results clearly show changes upon reduction in hydrogen but the width of the peaks for the reduced sample prevents, in general, a specific assignment for some states of tin. This is exemplified by the results of Kuznetsov et al. [ 291 who reported that Pt-Sn-y-A1,OB catalyst samples prepared by conventional impregnation techniques are multicomponent; that is, they have highly dispersed species that are a result of chemical interactions of Sn4+, Sn’+, and Sn” with both support and platinum. Platinum according to these authors, forms nearly all possible alloys with tin. Recent Mossbauer data [32] for a catalyst series similar to those used in our XRD studies [6,7] have shown evidence for PtSn alloy formation. The latter data showed that the extent of alloy formation on an alumina support, at constant platinum loading, increased with increasing tin loading. Also, it was shown that Sn” was formed more readily with the silica support than for the alumina support.

As noted by Meitzner et al. [ 51, tin in a Pt-Sn catalyst is more easily reduced to a zero valence when it is present on a silica support than when an alumina support is employed. The ease of reduction was also noted by Chojnacki and Schmidt [33]. Silica has been frequently used as a support for platinum and Pt-Sn catalysts, especially when a nonacidic support is desired (for example, see refs. 34 and 35). Since these characterization and activity measurements for the silica supported catalysts will undoubtedly be compared to those with an alumina support, it is desirable to obtain characterization data for a platinum and Pt-Sn-SiOz catalyst series that is similar to those obtained with alumina [6,7,10-12,16,17,36]. Furthermore, the present study would add to our understanding of similarities and differences in the Pt-Sn system where one of the components, tin, is expected to interact much more strongly with one support (alumina) than with the other (silica).

Thermodynamically, alloy formation between platinum and tin is favored. As the degree of tin reduction has been found to be dependent on the extent of tin loading, catalysts were made containing Pt : Sn mol ratios from 1: 0.5 to 1: 12. Two silica supports were used; one of intermediate surface area and another one of high surface area. Since XRD is a bulk characterization technique, it was used to characterize the Pt-Sn-SiO, catalysts after reduction in hydrogen. Electron microscopy and electron microdiffraction techniques have also been utilized to obtain data to complement the XRD data.

EXPERIMENTAL PROCEDURES

Preparation of catalysts

Pt-Sn catalysts were prepared on two silica supports. One was Davison grade No. 15 silica with a surface area of 800 m2/g and the other was Davison silica

48

TABLE 1

R. Srinivasan, B.H. DavisjAppl. Catal. A 87 (1992) 45-67

Chemical state and average crystal size determined by XRD

Pt:Sn Davison SiO, (340 m’/g) Davison SiO, (800 m’/g) mol ratio

Phase Crystal size (nm) Phase Crystal size (nm)

l:o Pt 10” 1:0.5 - - Pt 7 1:l b.c b.c

1:2.7 PtSn 10 Pt b

1:5 Pt 3 PtSn 12 1:5 Sn 40 1:8 Sn 47 Sn 40 1: 12 Pt 9 1:12 Sn 14

“The intensity of the XRD peaks is not as great as, for example, the Pt : Sn = 1: 8 catalyst; similar crystal sizes were obtained from XRD line broadening and measurements from TEM pictures. ‘The peak is too broad to calculate the crystal size from the full width at half maximum intensity. ‘Trace amounts of Pt and PtSn alloy phases.

grade No. 12 with a surface area of 340 m2/g. The concentration of platinum was kept constant at 1 wt.-%. Appropriate amounts of H,PtC1,.5H,O and SnC14.5H20 were dissolved in acetone so as to make different mol ratios of Pt/Sn, as shown in Table 1. The acetone solution containing the salts of platinum and tin was then impregnated onto the silica support by an incipient wetness technique. The catalyst was then dried in air at 120°C overnight. A portion of the dried catalyst was reduced in a glass reactor in flowing hydrogen at 450’ C and atmospheric pressure for about 16-18 h. The reduced powder was cooled to room temperature in hydrogen flow and then passivated in a flow of 1%-oxygen in helium at room temperature for 25-30 h. This passivated material was utilized for XRD and TEM studies.

XRD

The crystal structures developed in the reduced catalysts were investigated using a Rigaku X-ray diffractometer, which was operated at 40 kV and 20 mA with a Cu target. A pyrolytic graphite crystal monochromator was used on the diffracted beam path. The X-ray data were acquired in a PDP 11/23 computer at a step width of 0.02’) counting 10 s at each step.

TEM and electron microdiffraction

The reduced and passivated powder was suspended in absolute ethanol. This suspension was then agitated in an ultrasonic bath; a drop of the suspension


was placed on a carbon coated copper grid. The alcohol evaporated leaving a film of catalyst particles on the grid, which was then loaded into a Hitachi H800 NA scanning transmission electron microscope (STEM). The operating voltage was 200 kV. Microdiffraction and energy-dispersive X-ray (EDX) analysis were carried out in the nanoprobe mode. The nanoprobe used was 2 and 5 nm in diameter. This microscope is equipped with a silicon-lithium diode detector (Link) and a multi-channel analyzer (Tracer 500). The X-rays emitted from the specimen upon electron irradiation were collected in the range O- 20 keV for 60 s.

RESULTS

First, XRD data for the samples prepared using the silica support with a surface area of 800 m2/g will be described. The XRD pattern from the Pt : Sn = 1: 0.5 catalyst (Fig. 1) indicates the presence of metallic platinum that is present in the face-centered cubic form. The peaks, with their respective (hkl) planes, are identified in this figure. The average crystallite size calcu- lated from the full width at half maximum intensity from the Pt (111) profile is 6.5 nm. The large and very broad peak in the neighborhood of 28= 22.4” is due to the anomalous scattering from the silica support. This scattering was observed for both silica supports.

The XRD pattern obtained from the catalyst with Pt : Sn = 3 : 8 (Fig. 2 ) shows that only a small amount of crystalline platinum is present and no other alloy phase was identified. The 28 range from 37-47” where the Pt (ill), PtSn (lOi2), PtSn (1120), Sn (220), and Sn (211) profiles fall, isplottedon alarger scale in the inset of Fig. 2. It is clear that only Pt (111) is present, and that the

J

20 30 40 50 60 70 60 90

28

Fig. 1. X-ray diffraction pattern from the catalyst Pt : Sn = 1: 0.5 on 800 m*/g silica support.

50 R. Srinivasan, B.H. Dauis/AppL. Catal. A 87 (1992) 45-67

20 30 40 50 60 70 80 90

2%

Fig. 2. X-ray diffraction pattern from the Pt : Sn=3 : 8 on 800 m2/g silica. The intensity of the platinum peak can be seen reduced. The 28 range from 37-47” is plotted on an enlarged scale in the inset. It is believed that platinum has been utilized to form PtSn phase of discrete particles that cannot yield an X-ray peak.

I 20 30 40 50 60 70 80 90

20

Fig. 3. X-ray diffraction pattern from the Pt : Sn = 1: 5 on 800 m’/g SiO,. The only crystalline phase formed is PtSn alloy phase.

peak is too broad to permit a calculation of average crystallite size. Further- more, the average crystallite sizes in Table 1 indicate that the platinum dispersion is greater for this catalyst than for the one with Pt: Sn= 1: 0.5 mol ratio.

INTE

NS

ITY

IN

TEN

SIT

Y

2

Sn

(2

00)

P

N

E I

iz

sn

(1

01)

g

2

INTE

NS

ITY

. .

PiS

n

(107

2)

3F

Pts

n

(112

0)

G

INTE

NS

ITY

INT

EN

SIT

Y IN

TEN

SITY

LS

n

(220

)

-Sn

(2

11)


Fig. 6. A bright field electron micrograph obtained from the catalyst Pt : Sn = 1: 0.5 on 800 m’/g silica support.

The XRD pattern from the catalyst containing Pt : Sn = 1: 5 composition (Fig. 3) shows that the only crystalline phase detected was the PtSn (hcp); evidence for metallic platinum or any other Pt-Sn alloy phase was not obtained. The average crystallite size is estimated to be 12 nm. However, the XRD pattern from the Pt : Sn = 1: 8 catalyst (Fig. 4) exhibited peaks only for a crystalline j?-Sn phase. The inset in Fig. 4 shows tin peaks, and that only a very broad platinum profile, if any, is present. The average crystallite size of tin particles is estimated to be 40 nm.

In summary, for the catalysts prepared using the high surface area silica support containing low concentrations of tin, only the metallic platinum phase was observed, as the tin concentration was increased to a mol ratio of Pt : Sn = 1: 5, a PtSn alloy phase developed. Further increase in tin concentration (Pt : Sn = 1: 8) developed the metallic /?-tin phase. It is evident that the


Fig. 7. Microdiffractionpatterns using a 5 nm probe obtained from three representative, individual particles in Fig. 6. The crystallographic orientations of these platinum particles are: (i) (100)) (ii) (EO), and (iii) (110).

type of crystalline structure developed in these reduced catalysts is a function of the concentration of tin.

The XRD patterns obtained from the materials prepared using Davison silica of 340 m2/g surface area are shown in Fig. 5. The XRD pattern for the Pt- SiO, catalyst without tin indicated that the average size of the platinum crystals was about 10 nm. The diffraction pattern from the Pt : Sn = 1: 1 catalyst exhibits peaks indicating the presence of trace amounts of crystalline platinum

2 4 6 8 10 12 14 16 18

PARTICLE SIZE (nm)

PARTICLE SIZE (nm) PARTICLE SIZE (nm)

Fig. 8. A particle size distribution histogram obtained from TEM for platinum on Sio, (340 m”/g) (a);Pt:Sn=l:OJ (b);Pt:Sn=1:5 (c)andPt:Sn=1:8 (d)onthe800m2/gsupport.

and the PtSn alloy phase. For the Pt : Sn = 3 : 8 catalyst, the PtSn (hcp) alloy phase was formed and no other crystalline phase could be observed (Fig. 5a). The PtSn alloy crystals for the Pt : Sn = 3 : 8 mol ratio are larger on the silica support of 340 m2/g surface area than they are for the one with 800 m2/g surface area. The diffraction pattern from the Pt : Sn = 1: 5 catalyst shows that a crystalline P-tin phase is present (Fig. 5b); also, a trace of platinum was observed as a broad peak. The intensity of the crystalline tin phase increased dramatically as the tin concentration was increased further; this can be seen in the pattern from the Pt : Sn = 1: 8 catalyst (Fig. 5~). For the catalyst with the highest tin loading (Pt : Sn = 1: 12) on this silica support, the XRD pattern (Fig. 5d) shows a trace of the Pt ( 111 )fCC profile in addition to the presence of metallic tin. The observed crystalline phases and the crystallite sizes for these catalysts are summarized in Table 1. It may be noted that the onset of PtSn alloy formation begins with lower tin concentration on the 340 m2/g silica support than for the 800 m2/g silica support.

For electron microdiffraction studies the following four catalysts were se- lected: (i) Pt-Si0,/340 m2/g; (ii) Pt : Sn = 1: 0.5/800 m2/g Si02, which showed metallic platinum by XRD; (iii) Pt : Sn= 1: 5/800 m2/g Si02, which showed PtSn alloy and (iv) Pt : Sn = 1: 8/800 m2/g Si02, which showed metallic tin.

Metallic particles present in the catalyst with Pt : Sn = 1: 0.5 on 800 m2/g


Fig. 9. (a) Bright field image from the catalyst Pt : Sn = 1: 0.5 on 800 m’/g silica. (b) Dark field image obtained by tilting the beam to (O02)p, diffraction spot.

silica are shown in the bright field electron micrograph in Fig. 6. The electron probe was moved from particle to particle and a few of them, which are suitably oriented, diffracted the electron beam. Typical electron microdiffraction patterns using a 5 nm probe are shown in Fig. 7; only a few diffraction spots could be observed for each pattern because of the presence of a limited number of


Fig. 10. A bright field image from the catalyst Pt : Sn = 1: 5 on 800 m2/g silica support.

planes in these particles. The electron microdiffraction patterns shown in Fig. 7 indicate that the particles are metallic platinum oriented in the indicated crystallographic directions. Particle size distributions obtained from TEM are presented in Fig. 8. A platinum-silica (340 m”/g) catalyst that does not contain tin was characterized using TEM. The average platinum crystal size was about 10 nm (Fig. 8) and is in agreement with the data obtained by XRD. The major fraction of particles fall in the range of 4-7 nm for the Pt : Sn = 1: 0.5 catalyst; this size range was also obtained by XRD (Table 1) . A typical bright field image from the Pt: Sn= 1: 0.5 catalyst is shown in Fig. 9a. The corresponding dark field image obtained by tilting the beam to the (002 ) rt diffraction spot is shown in Fig. 8b. It can be seen that only a few particles contribute to the diffraction intensity. Only the bright particles in Fig. 9b yield microdiffraction patterns, and these correspond to metallic platinum.

Electron microdiffraction data were also obtained for the passivated

R. Srinivasan, B.H. DavislAppl. Catal. A 87 (1992) 45-67

Fig. 11. (a) Microdiffraction pattern using a 5 nm probe from the catalyst Pt : Sn = 1: 5 on 800 m*/g silica support. The orientation of the particle is PtSn, (310)f,,. (b) Microdiffraction pattern from another particle; the orientation in this case is PtSn (0221 )hcp.

Pt : Sn = 1: 5 supported on 800 m”/g silica catalyst. A bright field electron micrograph obtained from this catalyst is shown in Fig. 10. Two representative microdiffraction patterns are shown in Fig. lla and b; the pattern in Fig. lla corresponds to ( PtSnz)fC, oriented in the (310) direction and that in Fig. llb

R. Srinivasan, B.H. DavislAppl. Catal. A 87 (1992) 45-67

< 0.3 5.440

ENERGY, kev

10.6 >

Fig. 12. A typical EDX pattern obtained from a particle of 10 nm in diameter using a 5 nm probe. Evidence for platinum and tin can be seen. Copper peak is from the grid and silicon peak is from the support.

Fig. 13. A bright field image from the catalyst Pt: Sn= 1: 8 on 800 m2/g silica support. The area marked “a” containing dispersion of very small particles; the area marked “b” contains larger particles.

R. Srinivasan, B.H. Davis/Appl. Catal. A 87 ’ (I 992) 45 -67

< 0.3 5.440 10.6 > ENERGY, kev

59

5.440 ENERGY, kev

10.6

Fig. 14. (a) EDX patterns obtained from the area marked “a” in Fig. 16. (b) EDX pattern obtained from the area marked “b” in Fig. 16.

corresponds to ( PtSn) hcp oriented in the (0221) direction. Diffraction patterns could also be obtained as well from other thick particles in Fig. 10 which showed both PtSn and PtSn, phases. A typical EDX pattern obtained from one of these particles using a 5 nm probe is shown in Fig. 12, which indicates the presence of platinum and tin. Thus, while XRD data indicate that only the PtSn alloy is formed, the microdiffraction data show the presence of some particles which deviate from the Pt : Sn = 1: 1 mol ratio.

A bright field image from the passivated Pt-Sn = 1: 8 catalyst supported on 800 m2/g silica is presented in Fig. 13. The agglomerate on the left marked “a” contains dispersed small metal particles. The EDX pattern obtained from this agglomerate using a 5 nm electron probe (Fig. 14a) exhibits small platinum and tin peaks. However, if the beam is made to hit the agglomerate marked “b” at the center (Fig. 13 ) containing a dispersion of larger particles, the tin peaks were large (Fig. 14b). This result is in accordance with the bimodal particle size distribution for this catalyst (Fig. Sd). The area marked “a” in Fig. 13 appears to consist of fine PtSn alloy particles and the area marked “b” consists of large tin particles. The bright field and dark field images of another area are


Fig. 15. (a) A bright field image from Pt: Sn= 1: 8 on 800 m*/g silica support. (b) The corresponding dark field image obtained from the Sn (001) diffraction spot.

shown in Fig. 15a and b respectively. A typical electron microdiffraction pattern from a P-tin particle is shown in Fig. 16. The particles that contribute to the diffraction intensity can be clearly seen in the dark field image obtained from the Sn (001) diffraction spot. The resulting image (dark field) would clearly identify those particles which contribute to the diffraction intensity in the Sn (001) orientation. This is complementary information for the microdiffraction patterns.


Fig. 16. A microdiffraction pattern using a 5 nm probe obtained from a p-Sn particle. B = (110).

The particle size distributions obtained from TEM are quite informative. For Sn: Ptz0.5: 1 the average particle size (Pt) peaks at about 5 nm. For the Sn : Pt = 5 : 1 catalyst where PtSn is present, the particle size peaks at about 8 nm. For the Sn: Pt =8: 1 material, the metal particle size distribution is bimodal; it is presumed that the first peak at about 3 nm is due to PtSn alloy particles and the second peak is for tin metal particles. The agreement between the size range obtained by TEM and XRD is reasonable.

DISCUSSION

When alumina is used as a support for Pt-Sn catalysts, several problems are encountered in the characterization of crystalline phases using XRD and/or electron diffraction. The major XRD profiles of Pt (fee) phase are masked by the peaks corresponding to A1203 and it becomes very difficult to detect the platinum phase. On the other hand, if a silica support is used, it is easy to distinguish the crystalline phases as the silica is amorphous and causes only diffuse scattering at low 28 angles. No XRD peaks from the silica support interfere with the peaks corresponding to Pt, PtSn, PtSn,, Sn, etc. Also, it is much easier to reduce tin to the zero valent state when silica is used as a support [ 51. There is therefore a tendency to characterize Pt-Sn-SiO, and to extrapolate the results to the Pt-Sn-Al,O, catalysts.

For the catalysts used in the present investigation, the platinum content was kept at 1.0 wt.-% and the tin loading was varied from Pt : Sn= 1: 1 to 1: 12 mol ratio. The XRD results indicate that the crystalline phase that is formed pro- gresses from metallic platinum to PtSn (hcp ) to metallic tin as the amount of tin is increased. It is also observed that the formation of crystalline tin or PtSn

62 R. Srinivasan, B.H. DavisjAppl. Catal. A 87 (1992) 45-67

alloy occurs at a lower Sn/Pt ratio on the 340 m2/g silica than on 800 m2/g silica. This is reasonable in view of the higher dispersion of the atoms or small particles on the higher surface area support. Thus, for Pt : Sn = 1: 5, PtSn (hcp) phases was formed on 800 m2/g silica while the crystalline P-tin phase was formed on 340 m2/g silica. Actually, at the reduction temperatures (400” C or higher) the Sn” is presumably present as molten tin spread upon the silica surface. Upon cooling, the Sn” agglomerates and solidifies to form crystalline P-tin. Also, for Pt : Sn = 3 : 8 a trace of crystalline platinum was present on 800 m2/g silica but on the 340 m”/g silica support PtSn (hcp) alloy phase was formed. The XRD peaks corresponding to Pt or PtSn (hcp) are broad indicating a crystallite size of about 4-6 nm. However, for the XRD profiles for the higher tin containing catalysts, the peaks corresponding to the p-Sn phase are sharp, indicating larger crystal size ( > 40 nm). Although tin is present in ex- cess and exhibits the p-Sn phase, a broad diffraction profile from metallic platinum is also observed. Presumably the presence of metallic tin at higher tin loading ties up a significant fraction of Sn” and this, in fact, decreases the fraction of platinum present in an alloy form, or allows larger platinum crystals to form.

Electron microdiffraction and EDX analysis offer information complementary to the XRD data. From the electron micrographs, the particle dispersion can be ascertained and compared to XRD data. Sometimes, because of the small particles due to high dispersion, XRD may not offer valid crystallographic information. At these times, the electron microdiffraction technique may be a useful tool. For example, for a coprecipitated Pt-Sn-A120Z catalyst, we were unable to get any useful XRD pattern, while electron microdiffraction results indicated the presence of PtSn and PtSn, alloy phases [ 361. Handy et al. [ 371 recently obtained evidence for PtSn and PtSn, and PtSn, phases with this technique for Pt/Sn on a very low area support. However, both the XRD and TEM techniques will encounter some crystallite size below which it is impossible to resolve crystalline material.

In the present investigation, small metal particles were observed. Some of these particles appear to be thick while some of them appear thin. It was difficult to obtain an electron microdiffraction pattern or EDX signals from the thin particles. Since these thin particles have only a limited number of planes, only a few diffraction spots could be seen in the microdiffraction patterns. Only a few particles contributed to the diffraction intensity, as was demonstrated by comparing the bright and dark field images. Also, the X-rays emitted by these particles during electron irradiation are so limited that getting EDX signals from these particles was difficult, or impossible. We were able to obtain EDX signals and microdiffraction patterns only from particles that are suffi- ciently thick.

The data in this report do not cover all possible catalyst compositions nor the range of pretreatments. However, they should be representative of the


structures formed by the reduction of Pt-Sn-SiO, catalysts. There are differences between Pt-Sn-A120s and Pt-Sn-Si02 catalysts but there appears to be more similarities between the silica and alumina supported materials than there are differences.

There are two significant areas where the silica and alumina catalysts differ. We have reported that while Pt-A1203 and Pt-Sn-Al,Os catalysts retain essentially all of the chloride dehydrocyclization at 480’ C (see for example refs. 10-12). On the other hand, essentially all of the chloride added during preparation of a Pt- or Pt-Sn-Si02 catalyst was lost following reduction (ref. 10). The data for the present study is consistent with this finding. All silica supported catalysts used in this study were analyzed following reduction at 450’ C in hydrogen. Even those catalysts containing 4% or more chloride following preparation were found to contain less than 0.01 wt.-% chloride following reduction. The other major difference for the results for the two supports was the formation of metallic tin at high Sn/Pt ratios for the silica support whereas a detectable amount of metallic tin was not formed when alumina was the support.

The present data and our earlier data obtained for similar Pt-Sn-SiO, catalysts [ 12,17,18] are consistent with tin being present in one or more of three chemical states: (1) combined with platinum to form an alloy with the pre- dominate composition of Pt : Sn = 1: 1, (2) in an oxidized state ( Sn2+ and/or Sn4+ ) that appears amorphous to X-rays, and (3) metallic tin. The present data for the silica support show that the distribution of tin among the three chemical states depends upon the surface area of the support and the Sn/Pt ratio. Our XPS data [ 10,121 indicated that Pt” was present following reduction even when essentially all of the tin was in an oxidized state for alumina, silica or carbon support.

The object of catalyst characterization is to develop a model to represent the working catalyst. Thus, we need to define the distribution of platinum among platinum atoms, platinum clusters and PtSn alloy and the tin among metallic tin, PtSn and oxidized tin.

The structure for the oxidized tin is difficult to define. Oxidized tin compounds, as SnO, for example, have not been detected by XRD. There is no direct evidence for SnO or SnO, apart from Mijssbauer data; unfortunately the Mijssbauer data obtained to date is such that it could be consistent with SnO or SnOz but does not require that it must be present in this form. We prefer to view the oxidized tin as being present as an egg-shell tin silicate type structure. In this connection we view an egg-shell of tin aluminate to be essentially the same as a monolayer raft structure of SnO or Sn02 that strongly interacts with the support, and that data generated to date could not distinguish these two forms of tin.

We propose that the extent of reduction of tin to Sn” on a silica support

64 R. Srinivasan, B.H. DavislAppl. Catal. A 87 (1992) 45-67

attains a pseudo equilibrium state that depends strongly upon tin loading. Thus, we can write

Sn”/SnSiO, = k

where x is determined by the valence of tin and one of the variables that k depends upon in this catalyst series is the loading of Sri/m’ of support. For impregnated catalysts the SnSiO, is considered to be predominantly confined to the surface layer(s); however, at some tin loading the surface will become “saturated” with SnSiO, so that additional tin can be present only as Sn’, bulk SnSiO,, or SnO,. Sn” may be present in two forms: Sn” and PtSn alloy. The fraction present as PtSn alloy will depend upon the Pt and Sn” concentration/ m2 of support. At the reduction temperature (450” C), as well as the usual temperatures used for naphtha reforming, tin metal is above the normal melt- ing point and will not be crystalline. In situ XRD studies showed that crystalline tin is not present on the catalyst at 450°C. It appears that there must be significant interaction between Sn” and the support, otherwise the molten tin would coalesce and drip from the catalyst bed during reduction or use. As the catalyst is cooled from the reduction temperature, a temperature is reached where “massive” crystalline Sn” particles will form. We also view the Sn” to attain a pseudo equilibrium state with respect to PtSn alloy; thus,

Sn”/PtSn = k' .

In indicating a pseudo equilibrium we are referring to the catalyst state, after one, or a few, days of reduction and/or use; obviously, as the time of use ap- proaches months or years the state of the catalyst may differ from that which we observed.

While other Pt/Sn alloys may form, our data indicate that the dominant Pt/ Sn alloy phase is Pt : Sn = 1: 1 and we shall refer only to this phase in the following discussion.

Tin silicate, or whatever the form of the oxidized tin, is considered to play a role in determining the physical structure of the catalyst. It is viewed that the particle growth of platinum and PtSn is retarded by the presence of SnSiO,

TABLE 2

Variation of states of Pt and Sn in Pt-Sn-SiO, as the Sn/Pt ratio is varied

Sn/Pt ratio on 800 m2/g SiO,

Dominant Pt state Dominant Sn state Sn/Pt on 340 m’/g SiOz

<1

2-5 8-12

Atomic dispersion or small crystals

PtSn (and Pt) Highly dispersed Pt

SnSiO,

PtSn and SnSiO, SnSiO,, Sn” and PtSn

1-3 5-8

R. Srinivasan, B.H. DavislAppl. Catal. A 87 (1992) 45-67 65

0,

I ’ /’ \ \ \ \ /I \ ',pt

pt I /’

\ PtSn \

‘\ Particle ’ /’

\ .- ___--

Size ‘\I ‘\ l .

--.

1.0

Pti

Pt

Sn/Pt -

Fig. 17. Schematics of the distribution of platinum among the states Pti, and the particle size as the Sn/Pt ratio varies (maxima depends upon tin loading, surface area of the support, etc.).

1.0 ----,

‘\ \

\ \

‘\ , ,“‘\,

Sfli ‘X ‘,

s;; ‘\ I’ . .

‘\,PtSn

‘. Sfl

,I ‘\ . . -/4-L

I’ ‘.

‘. /’ / .7---___

/’ .’ SnSiO,

SniPt -

Fig. 18. Schematic representation of the distribution of tin species, Sn;, as a function of Sn/Pt.

sites. Thus, for a catalyst containing 1 wt.-% platinum and varying amounts of tin we identify the dominant crystal structures which are indicated in Table 2. We depict schematically our view of the distribution of the species in Table 2 as shown in Fig. 17. Likewise, the chemical state of the supported tin may be represented as shown schematically in Fig. 18. Thus, as tin is initially added to the catalyst, its role is to react with the support and thereby increase the dispersion of platinum, as the tin concentration is increased further Sn” is formed and this alloys with platinum to form PtSn; as the tin loading increases further Sn” exceeds the mols of platinum and a fraction will therefore be present as Sn” metal as well as dispersed PtSn (or even some PtSn, where x> 1) alloy. Our data [ 11,13,16,17,34,35] for reduced alumina supported Pt-Sn catalysts is also consistent with this model. A difference is that for the alumina


supported catalyst there was strong evidence for some of the tin being present in some form of SnCli- compounds.

CONCLUSIONS

Structural characterization of Pt-Sn-SiO, catalysts have shown that tin loading at a constant 1 wt.-% platinum has a great influence on the structure of the supported metals. For the catalyst containing Pt: Sn= 3: 8 mol ratio, PtSn alloy phase was formed when the support is silica of 340 m2/g surface area, whereas only a high dispersion of platinum particles results if the support is 800 m2/g silica, showing the influence of the surface area of support. For this catalyst series with increasing Sn/Pt ratios, first platinum is formed; as the tin concentration is increased, the PtSn alloy is formed, as the tin concentration is further increased, metallic tin phase is developed. The role of tin in the Pt-Sn bimetallic catalysts appears to aid the dispersion of platinum by interactions of tin with the support. The extent of reduction and dispersion of platinum depends upon the Sn/Pt ratios, surface area of the support and nature of the support material, While there are differences in the reduced catalyst prepared using an alumina and a silica support, particularly in the amount of chloride retained and metallic phases formed, it appears that there are many similarities.

ACKNOWLEDGMENT

The facilities offered by the Materials Characterization Facility (MCF), University of Kentucky, are greatly acknowledged. Help renderedby Mr. Larry Rice with the,TEM examination is also acknowledged.

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