Langmuir 1988,4, 385-391 Hz)16s(CHJ &0zH, 11250540-7; CH3(CHz)I,S(CHz)loCOzH, 112505-01-8;CHg(CHz)l*S(CHz)loCOzH,112505-02-9; CHS(CHZ)~~S(CH~)&OZH, 112505-03-0; CH3(CH2)15S(CHz)l5COzH, 112505-04-1;CO3(CDz)15S(CH2)loC02H,112505-05-2;[CHs(CHz)e]ZS, 693-83-4; [CH3(CH2)111ZS, 2469-45-6; [CH3(CHZ)171ZS, 1844-09-3;[CH3(CH2)21]zS,106683-08-3;[H02CCHz]zS,123-93-3; [HOzC(CHz)z]2S, 111-17-1;[HOzC(CHz)5]zS, 10341-18-1;[HOZC-
385
(CHz)101ZS, 29265-73-4; [HO2C(CHZ)151ZS, 112505-06-3;[HOZC(CHz)z1]29,112505-07-4;[HO(CHz)11]2S,112505-08-5;[CHSOZC(CHz)15]2S,112505-09-6;Au, 7440-57-5;Si, 7440-21-3. Supplementary Material Available: Further details of preparation of dialkyl sulfides not described above (13 pages). Ordering information is given on any current masthead page.
Spectroscopic Characterization of Pt-Sn Bimetallic Catalysts Prepared by Solvated Metal Atom Dispersion (SMAD) Yong-Xi Li, Yong-Fu Zhang,+and Kenneth J. Klabunde* Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received May 18,1987. In Final Form: September 11,1987 Earlier results demonstrated that Pt-Sn/support catalysts prepared by solvated metal atom dispersion (SMAD) have unique catalytic properties. Spectral characterization of these starting catalysts using Mossbauer, X-ray diffraction,and X-ray photoelectron spectroscopy is reported herein. Results show that supported Pt catalysts, when treated with solvated Sn atom solutions,become doped with SnO. Treatment of Pt-Sn/A1203 at 500 "C with €&/hydrocarbon caused no change in the spectral characteristics. Thus, the SMAD materials appear "catalyticallyready" upon preparation. Based on the spectral data, a model of a catalytic particle is proposed that suggests a Pt particle is partially coated with Sn to form patches of Pt-Sn alloy.
Introduction Since the discovery and commercial use of bimetallic reforming catalysts in the 196O's, numerous studies of their composition and properties have been reported. In particular, the Pt-Sn/support combination compared with monometallic Pt/support has been found to possess better stability, better selectivity, but lower initial There is considerable disagreement regarding the valence state of tin in these catalysts. One view is that addition of Sn4+compounds to Pt/A1203followed by H2 reduction results in the formation of Pto-Sno alloys so that the Pt particle surfaces are diluted by SnO, thereby yielding smaller ensembles. This could explain why cracking and hydrogenolysis reactions are inhibited and thereby selectivity to aromatics However, recent studies by Burch and co-workerss and Volter and co-workersghave described temperature-programmed reduction (TPR) of Pt-Sn catalysts and show that it is not possible t o reach the SnO state with Pt-Sn/A1,03 but is possible with PtSn/Si02 and Pt-Sn/C. Adkins,lo Sexton," and their coworkers, using XPS, have supported these findings, as one of us has.12 Thus, conventionally prepared Pt-Sn/A1203 catalysts apparently do not contain SnO. We have reported that the solvated metal atom dispersion (SMAD) procedure can directly yield highly dispersed catalysts with significant portions of the metals in their zero valent states.l3-l6 In particular we have prepared Pt-Sn/A1203 SMAD systems that show high activity and good selectivity ~r0perties.l~ Herein we report XPS, NMR, XRD, and Mossbauer characterization of these interesting catalysts. Experimental Section Catalysts. First, platinum monometallic catalysts were prepared by using a conventional method. The supports we used 'Current address: Institute of Shanghi Petroleum Complex, Shanghi, China.
0743-7463/88/2404-0385$0l..50/0
Table I. Physical Properties of the Supports surface area, pore volume, density, support
m2/g
207 A1203-Si02
SiOz
100 300
mL/g 0.45 0.28 1.00
g/mL 0.62 0.75 0.40
in the preparations were Alto3,Al2O3-SiO2,and SiOz (Davison Speciality Chemical Co.). Their properties are summarized in Table 1. When A1203 was the support, it was impregnated with an aqueous solution of chloroplatinic acid (Strem Chemical Inc.,) for 24 h followed by drying at 120 "C overnight. Then the catalysts were caicinated in flowing air at 500 "C for 4 h. The reduced (1) Kluksdahl, M. E. US.Patent 3415737,1968. (2)Sinfelt, J. H., AIChE J. 1973,19,678. (3)Roberti, A.; Ponec, V.; Sachtler, W. M. H. J. Catal. 1973,28,381. (4)Dautzenberg, F.M.;Helle, J. N.; Biloen, P.; Sachtler, W. M. H. J. Catal. 1980,63,119. (5)Sachtler, W.M. H.; van Santen, R. A. Adu. Catal. 1977,26,69. (6)Bacaud, R.; Bussiere, P.; Figueraa, F. J. Catal. 1981,69,399. (7)Muller, A. C.;Engelhard, P. A.; Weisang, 3. E. J. Catal. 1979,56, 65. (8)Burch, R.J. Catal. 1981,71,348.Burch, R.;Garla, L. C. J. Catal. 1981,71,360. (9)Volter, J.; Lieske, H.; Lietz, G. React. Kinet. Catal. Lett. 1981,16, 87. (10)Adkins, S.R.; Davis, B. H. J. Catal. 1984,89,371. (11)Sexton, B. A.; Hughes, A. E.; Foger, K. J. Catal. 1984,88,466. (12)Li, Y. X.; Hsia, Y. F. In Oil and Gas in China;1985;Vol. 1,p 161. (13) Li, Y.X.; Klabunde, K. J. Langmuir 1987,3,558. (14)Matsuo, K.;Klabunde, K. J. J. Catal. 1982,73,216. (15)Klabunde, K. J.; Imizu, Y. J. Am. Chem. SOC. 1984,106,2721. (16)Imizu, Y.;Klabunde, K. J. In Catalysis of Organic Reactions; Augustine, R. L.; Ed.; Marcel Dekker: New York, 1985. (17)Cohen, R. C.; McMullin, P. G.; Westhein, G. K. Reu. Sci. Instrum. 1963,34, 671. (18)Purcell, K. F.; Edwards, M. P.; Currutte, B.; Eck, J. S. Rev. Sci. Instrum. 1986,56,108. (19)Powder Diffraction File Seta 1-5,Inorganic Volume; Joint Committee on Powder Diffraction Standards: 1601,Park Lane, Swarthmore, Pennsylvania 19081. Third Printing, Printed in Philadelphia, PA (1974), PDF 4-0673.
0 1988 American Chemical Society
Li et al.
386 Langmuir, Vol. 4, No. 2, 1988 TO
"a'",,-
Table 11. Pt-Sn Bimetallic Catalysts Synthesized in This Work loading, wt% SMAD catalyst support solvent Pt Sn A THF 0.6 0.3 B A1203 THF 0.6 0.5 C THF 0.6 1.0 D A1203 THF 0.6 1.5 E A1Z03 THF 0.6 3.0 F Al2O3-SiO2 THF 0.6 1.0 0.6 1.5 G A1203-Si02 THF H A1203-Si02 THF 0.6 3.0 I SiOz THF 0.6 1.0 0.6 1.5 J SiOa THF K SiOz THF 0.6 3.0 L A1203 toluene 0.6 1.0 M A1203 coimpregnation 0.6 1.0 N A1203 coprecipitation 0.6 1.0
s./ Figure 1. SMAD reactor. (1)reactor chamber; (2) solvent shower head; (3) matrix; (4) liquid Nz; (5) vaporization crucible with metal, (6) the reduced Pt/AlzO3 catalyst; (7)magnetic stir bar; (8) solvent. catalysts were obtained by reduction in flowing Hz at 500 "C for at least 4 h. The reduced catalysts were stored under pure Nz awaiting addition of tin. When AlzO3-SiO2and SiOz were used, the incipient wetness impregnation methodz0was used for preparing platinum-containing catalysts. The dryness, calcination, and reduction procedures were the same. All catalysts prepared contained 0.6 w t % Pt. The addition of Sn to the platinum monometallic catalysts was done by the SMAD procedure. Essentially, the procedure is identical with that described previously.13-*6Briefly, the reduced Pt support catalysts were placed in the chamber of the metal vapor reactor under a pure N, atmosphere (Figure 1). Then the chamber was evacuated to Torr and cooled to liquid nitrogen temperature (-196 "C). While the SMAD solvent (here we used tetrahydrofuran (THF)and toluene) was inletted, electrical power heated a W-Alz03 crucible containing a certain amount of tin metal to vaporize it at a suitable rate. The vapor of the tin metal and solvent vapor were condensed on the walls of the chamber into a frozen matrix (about 2 h, 0.1-0.5, of g Sn, 120 ml of solvent). The matrix exhibited a rainbow of colors from orange-yellow to dark purple to black. After metal evaporation, the matrix was melted and its color became black. This homogeneous black solution flowed down onto the Pt/AlZO3 catalysts and upon warming deposited atoms and clusters of Sno containing some carbonaceous residues from the solvent. After complete warmup the Pt-Sn/Alz03 catalyst became dark gray to black, and excess solvent was water white (1.5-h impregnation time). The slurry was siphoned out and excess solvent removed by decanting and evaporation. The final, dry Pt-Sn/A1203 catalysts were stored under pure NP. Table I1 lists the catalysts prepared. Samples M and N were conventionally prepared Pt-Sn/A1203 systems treated by coimpregnation (with a solution of SnClZ.2Hz0and acetone) and coprecipitation methods, respecti~ely.'~J~ XPS Experiments. X-ray photoelectron spectra were obtained by using an AEI-ES-2OOB spectrometer with Mg radiation (hv > 1253.6 ev). The power was 60 W, and the vacuum of the sample chamber was 1 X lo-* Torr. The catalyst samples were placed on one side of double-sided sticky tape under a flow of argon and inserted into the XPS chamber. For Pt-Sn/A1203 catalysts A-E and M and N (see Table 11) Cis, Alzp,Clzp,Sn3d3jz,and Sn3d5,z ~
(20) Vannice, M. A.; Hasselbring, L. C.; Sen, B.J.Catal. 1985,95, 57.
photoelectron lines were recorded. For Pt-Sn/AlzO,-SiOz (samples F-H), the Sizp line was also monitored. For Pt-Sn/SiOz catalysts (samples I-K), Cls, Sizp,Clzp,Sn3d512, and Ptdn lines were monitored. The Al (74.7 ev) and Si, (,l03.4 ev) p e a k were used as reference lines. T%e deviation of binding energies obtained by using these two reference lines for the same sample was less than 0.05 ev. When tin foil was studied as a standard Snosample, the C1, (284.7 ev) line was used as a reference. Mossbauer Experiments. A locally constructed constantacceleration Mossbauer spectrometer was used for the experiments."Js The Mossbauer source was 10 mCi of Ca11gSn03 (Amarsham), which also produced a strong Sn K a X-ray, only about 1.4 keV over the Mossbauer y-line. In order to filter this X-ray, a 0.05" palladium foil was used in front of the catalyst samples. The output pulses from a Harshaw NaI(T1) scintillation counter were collected in an ADAC system 1000 microcomputer, which also checked drive motion. The microcomputer is capable of foreground/background operation, which allows continuous spectral display (or other operations) and data acquisition. Both positive and negative source acceleration data are collected and subsequently overlaid to obtain the final spectrum. The spectral analysis is performed on an IBM-370 computer, the data being passed from the microcomputer to the IBM-370 by modem.ls The calibration of the velocity scale was accomplished by the use of a 25-pm cy-Fe Mossbauer standard and checked by the resonant absorption of P-Sn standard foil. An Argon-sealed holder with a 450-mg sample was mounted on the instrument for measurement at room temperature. XRD and 119Sn NMR Experiments. The powder XRD pattern was recorded with a Y-4 X-ray diffractometer using monochromatized high-intensity Cu K a radiation (A = 1.5405 A), which worked at a current of 20 mA and voltage of 40 kV. The pattern was recorded at 20" 5 20 5 70° with a scan rate of 1"(20)/min. A Bruker 400 multinuclear FT-NMR spectrometer was used for l19Sn NMR experiments. The resonance frequency was 149.115 MHz. Most of samples were handled under a pure Nz stream and sealed in sample tubes for measurements.
Results Chemical State of Sn in Pt-Sn/Alz03 SMAD Catalysts. 1. XPS. T h e Sn3d3 and Sn3d5/2photoelectron lines for standard samples of dnOz and ground-up mixtures of Sn02/A1,03/Si0, (200 mesh) a r e shown in Figure 2 (spectra 1 and 2). Their SnM5/2 binding energies are 486.4 and 486.5 eV with peak half-widths (AE) of 2.2 and 2.6 ev, respectively (Table 111). These peaks are considered t o be due to Sn4+oxides. T i n metal foil polished under a flow of argon was placed in t h e XPS chamber, and Sn3d3 and Sn3d5,2binding energies were obtained as shown in & w e 2. As expected, some tin oxides were also present. T h e spectra for SnM5/2 showed two peaks with binding energies of 485.4 and 483.6 eV, respectively. T h e r e is more than a 1-eV difference between t h e peak at 485.4 a n d that of
Spectroscopy of Pt-Sn Bimetallic Catalysts
Langmuir, Vol. 4, No. 2, 1988 387
494
490 486 B i n d i n g enerqy, eV
482
Figure 3. XPS spectra of Sn3d312and SnsdS for (1)catalyst E; (2) oxidized catalyst E in air; (3) catalyst C; (4) oxidized catalyst C in air.
Figure 2. XPS spectra of Sn3d3/2 and Sn3d5,2for standard samples and the SMAD Pt-Sn/A1203 catalysts: (1) SnO,; (2) SnO,/ Si02/A1203;(3) polishing tin foil under Ar stream; (4) catalyst E; (5) catalyst D; (6) catalyst C; (7) catalyst B; (8) catalyst A.
Table 111. Half-Width of Sn3d5,zPeaks for Some Standard Samples and Catalysts and Their Binding Energies for the Two Deconvoluted Peaks peak binding half-width samples m, eV Ebl Eb2 SnOP 2.2 486.4 SnOz + A1203+ SiOz 2.6 486.5 polishing tin foil 3.5 485.4 483.9 under Ar catalyst A 3.5 485.7 483.9 B 3.5 485.7 483.9 C 4.0 485.5 483.7 D 4.0 485.5 483.7 E 4.0 458.3 483.9 F 4.0 485.3 483.9 G 4.2 485.5 483.7 H 3.5 485.9 484.1 I 3.2 485.1 483.3 J 3.3 485.1 483.3 K 3.5 485.6 483.8 M 2.6 485.9 N 2.5 485.9 Sn4+at 486.4-486.5 eV. The width of the peak at 485.4 eV is larger than those at 486.41486.5 eV. We believe the
peak at binding energy 485.4 eV is due to Sn4+ with Sn2+ and intermediate oxidation states contributing. The peak at 483.6 eV is characteristic of the SnO state. If area ratios are measured, the SnO component is about two-fifths that of the total Sn3d5/2absorption. The Sn3d312 and Sn3d5/2 lines for the Pt-Sn/AlzO, SMAD catalysts (samples A-E, Table 11)are also shown in Figure 2. Apparently the amount of Sn added has little effect on the final chemical state of the surface tin. Note the shoulder at lower binding energy and the broad nature of the peaks. Their peak half-widths are much larger than standard widths of 2.2-2.6 eV. Therefore, two peaks must be involved, and these are shown with broken lines in Figure 2. Other evidence supports this conclusion; Figure 3 shows spectra where the SMAD catalysts C and E have been exposed to air. Note the disappearance of the peak due to Sno. Also, after 10 or 60 min of argon ion sputtering of oxidized catalysts C and E, the Sn3d5/2 peak width increased 0.4 and 0.8 eV, respectively. We conclude that there are at least two chemical (valence) states of Sn present in SMAD catalysts A-E. The Sn3d512 component at 485.5 eV is mainly due to Sn4+plus mixed oxides, while the component at 483.6 eV is due to SnO, which appears to be the major component. The area ratio between these two varies little with percent of Sn in the catalyst and is similar to that found for polished tin foil (under argon). Since XPS is a probe of the surface of tin particles, it could be concluded that as the tin content goes up in these SMAD catalysts most of it remains in the SnO state. Only the surface is partially oxidized (probably during handling even though we attempted to rigorously exclude 02,as with the tin foil experiments), and thus a surface with Sn4+, Sn2+,and SnO is observed. Note that SnO is definitely present though; this is a new finding for Pt-Sn/A1203
Li et al.
388 Langmuir, Vol. 4, No. 2, 1988
.< : . .. i
-3
:
0 +3 Velocity, mm/s
+6
Figure 6. Mhbauer spectra of ll%n for various catalysts at room temperature: (1)THF Pt-Sn/A1203SMAD catalyst; (2) toluene Pt-Sn/Alz03SMAD catalyst; (3) THF Pt-Sn/A1203SiO, SMAD catalyst; (4) THF Pt-Sn/SiOz SMAD catalyst.
Table IV. Parameters from Computer Fittings for the Pt-Sn Catalysts B i n d i n q energy, eV
XPS spectra for various Pt-Sn/Alz03CE dysts: (1) the coprecipitated catalyst; (2) the coimpregnated catalyst; (3) the SMAD catalyst. Figure
lines figure IS, width, QS, area, predominate in text catalysts mm/s mm/s mm/s % Sn species (spectra) C 2.65 1.20 0 100 Sno 6 (1) L 2.69 1.17 0 100 Sno 6 (2) SnO F 2.66 1.19 0 100 6 (3) I 2.66 1.19 0 100 Sno 6 (4) N 0.0 0.81 0 68 SnOz 7 (1) 0.9 12 SnO2.Al2O3 0.2 0.80
M
60
55
50
40
45
35
30
25
20
28
Figure 5. XRD spectrum for THF Pt-Sn/Al,O, SMAD catalyst.
catalysts, and this is clearly shown by comparison of XPS spectra with conventional catalysts (Figure 4). 2. X-ray Diffraction (XRD). A typical XRD spectrum for the THF Pt-Sn/A1203 SMAD catalyst is shown in Figure 5. There are three sharp diffraction peaks at 26 = 30.660°, 32.050°, and 43.971°, respectively, and their d values are 2.91, 2.79, and 2.06 (A). Relative intensities of each peak ( I / O are 100, 90.6, and 73.8. All of these parameters are almost identical with those for SnO metal.Ig Clearly, this XRD result shows that the Sn in the PtSn/A1,03 SMAD catalyst is present in the SnO metallic state. It should be noted also that XRD is a bulk amlysis. The XRD spectrum does not show peaks for tin oxides, indicating that only a small amount of these could be present in the SMAD catalyst. Therefore, the tin oxides shown in the above XPS results must be concentrated on the surface of the tin particles.
2.21 3.27 0.0 3.30
0.83 1.09 1.0 1.09
0
1.80 0 1.64
8 12 23 77
PtSnd SnO Sn02 SnO
3. Mossbauer Spectroscopy. The Mossbauer spectrum for the THF Pt-Sn/A1203 SMAD catalyst is shown in Figure 6 (spectrum 1). A large absorption peak appears at about IS = 2.6-2.7 mm/s. The Mossbauer parameters from computer fitting are listed in Table IV. They indicate that the Sn in the Pt-Sn/A1203 SMAD catalyst is in the SnO state. No evidence for tin oxides was found, and this is consistent with the XRD observation. This is reasonable after considering the fact that the Mossbauer spectroscopy (transmission) is also a bulk material analysis technique. Again, we can conclude that Sn4+ or Sn2+ species which were detected by XPS must reside on the surface of the tin particles and make up less than 5% of the total Sn (according to Mossbauer and XRD analytical limits). For comparison, Mossbauer spectra of conventional catalysts M and N are shown in Figure 7 (spectra 1 and 2). For the coprecipitated Pt-Sn/Alz03 catalyst, the Mossbauer spectrum indicates that most of the Sn, even after the H2 treatment, remains in the Sn4+state (IS = 0-0.2 mm/s, QS = 0.80-0.81 mm/s). A small portion may be present as a Pto-Sno alloy (IS = 2.21 mm/s, QS = 0.80-0.81 mm/s) and as Sn2+(IS = 3.23 mm/s, QS = 1.09 mm/s). For the Pt-Sn/A1203 catalyst prepared by coimpregnation with a solution of H2PtC1, and SnCl2.2Hz0-
Langmuir, Vol. 4,No. 2,1988 389
Spectroscopy of Pt-Sn Bimetallic Catalysts
Table V. l19Sn NMR Data for Various Samples chemical shift, peak width,
sample
:* :. .. .. .
I
e
.
? ! I. :
A -3
+3
0
e6
Velosity, mm/s
Figure 7. Miissbauer spectra for varioul Pt-Sn/A1203 catalysts after reduction: (1) the coprecipitated catalyst; (2) the coimpregnated catalyst (with acetone solution); (3) the SMAD
catalyst.
l
....., ... ...... .............. ....
.:.
.. .. . . .;. . . . 2
-3
V e l o c i t y , mm/s
Figure 8. Moasbauer spectra for Pt-Sn/A1203SMAD Catalysts: (1) before use; (2) after use as a catalyst at 500 "C for n-heptane conversion (on stream over 8 h); (3) after air exposure for 2 h at 25 "C;(4) after air exposure for 8 h at 25 "C.
acetone, the Mossbauer spectrum shows that the main portion of tin is represented by a doublet of Sn2+(77% of the spectrum area) and also a peak a t IS = 0.0 mm/s, which represents Sn4+. It is clear that there are significant differences among the three Pt-Sn/A1203 catalysts. Parameters from computer fitting and possible Sn species are listed in Table IV. Important spectra are shown in Figure 8. A SMAD Pt-Sn/A1203 catalyst was treated with H2/n-heptane at 500 "C for 8 h. From comparison with Figure 6, it can be seen that no change in the Sn Mossbauer spectrum was
ppm 0
ppm
BuSnC13 (in acetone) SnC14(liquid) SnC14 (in acetone) SnC14-5Hz0(in acetone) SnC14/A1203(impregnated with
-25.7 -437.4 -443.4 -497.3
2.0 2.0 2.0 20.0
SnC1,-acetone solution) SnCl4/AlZO3 (impregnatedwith
-519.0
10.0
SnC14-HC1 solution in water) SnC12.2Hz0(in acetone) SnC12-2H20 (solid) SnOz (solid) ground mixture of Sn02+ Al2O3
-162.8 -385.2 -358.3 -360.6
2.0 6.0 22.0 22.0
0.5
detected. This indicates that even at 500 OC the PtSn/AI2O3catalysts remained unchanged. Since our catalytic reactions were generally carried out below 500 "C in an environment of hydrocarbon/H2,these results indicate that SMAD catalysts prepared without 500 "C/H2 treatment are in their "catalytically ready" state. 4. '19Sn NMR Experiments. In Table N , tin chloride is ruled out as a possible component in the assignment of tin species; this is based on lI9Sn NMR experiments. Under the experimental conditions we employed, we observed a large chemical shift between SnC1, (liquid) and alumina-supported SnC1,. However, when different solutions were used for impregnating alumina (SnC1,-acetone or SnC1,-HC1 water), the resultant alumina-supported SnGl, (dried under vacuum) showed l19Sn NMR absorptions at -497.3 to -519.0 ppm. The difference between their chemical shifts was very small, and peak widths were about 10-20 ppm (Table V). After calcination of SnCl4/AI2O3in air at 500 "C for 24 h, the Mossbauer spectrum showed that it had been completely converted to Sn02/A1203.However, we were not able to observe the l19SnNMR peak within a *3000 ppm range. Perhaps this is because the peak was seriously broadened so that it disappeared into the background. For all Pt-Sn/A1203 catalysts in the present study, SMAD or conventional samples, no l19Sn NMR peaks could be detected within a *3000 ppm range. If the Sn were present as SnC1, on alumina in any of the Pt-Sn/A1203 catalysts, NMR detection should have been pos.sible under our experimental conditions. Chemical State of Sn in the Pt-Sn/A1203-Si02 SMAD and Pt-Sn/SiO, SMAD Catalysts. 1. XPS. Figure 9 shows SnM3d3/2and SnM5 X P S lines for the SMAD Pt-Sn/A1203-Si02 (samples F-fI) and SMAD Pt-Sn/Si02 (samples I-K) catalysts, respectively. The spectra are very similar for the SMAD Pt-Sn/A1203 catalysts shown in Figure 2, and we conclude that the chemical state of tin is very similar. Thus, peak shape and half-widths are nearly the same except for peak intensities that vary with tin loading. Apparently, when tin is added by the SMAD procedure, the chemical states are unaffected by the supports used. These findings are different from conventional preparations, where tin states are different depending on the supports used.8-11 2. Mossbauer Spectra. In Figure 6 the spectra for THF Pt-Sn/A120,-Si02 and Pt-Sn/Si02 SMAD catalysts show no differences in peak shape, compared to those for the THF Pt-Sn/A1203 SMAD catalyst, except that the peak widths are slightly changed. This result again indicates Sno to be predominant. XPS shows the presence of some oxidized forms of Sn due to surface oxidation, but the total amount is very small compared with SnO. Thus for SMAD catalysts, the tin is mainly present as Sno regardless of support.
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390 Langmuir, Vol. 4, No. 2, 1988
496
492
488
484
480
496
492
488
484
480
B i n d i n g enerqy, eV
Figure 9. XPS spectra of S ~ M ,and , ~ SnM5,i for (1) catalyst H; (2) catalyst G; (3) catalyst F; (4) catalyst K; (5) catalyst J; (6) catalyst I. Table VI. Binding Energies of Clapfor Some Standard Samdes and Catalysts sample SnClz/A1203 SnC14/A1203 NaCl HC1 catalyst M N C D E F G
H
Clzl,binding energy, eV 200.4 200.4 198.6 198.6 198.6 coimpregnated Pt-Sn/AlZO3 198.4 coprecipitated Pt-Sn/A1203 198.4 SMAD Pt-Sn/Alz03 198.2 SMAD Pt-Sn/A1203 198.3 SMAD Pt-Sn/A1203 198.3 SMAD Pt-Sn/A1203-Si02 198.4 SMAD Pt-Sn/Al2O3-Si0, 198.3 SMAD Pt-Sn/AlZO3-SiOz
Chemical State of Chlorine in the Pt-Sn SMAD Catalysts. Since we add the Pt to our systems in a conventional way (starting with the chloride), some C1 residues could be present and can be detected by XPS. It can be seen from Table VI that the binding energies for ClZPare the same for our SMAD catalysts compared with conventional systems, at 198.4-198.6 eV. This indicates that the C1 environments are the same and that the chlorine is not present as tin chlorides since standard samples show ClZP at 200.4 eV, which is consistent with the NMR results discussed earlier. However, it is interesting to compare the Clzpspectra for Pt-Sn SMAD catalysts with different supports (figure 10). Clearly, the intensity of the C1, peak strongly depends on the support and is highest for A1203. A noticeable peak is also observed for A1203-Si02but cannot be observed at all when the support is SiOz. I t could be concluded that A1203strongly adsorbs or incorporates chloride into its lattice, whereas SiOz does not (and it is probably lost as HC1). Chemical State of Pt in Pt-Sn/Si02 SMAD Catalysts. Due to overlap of the Alzppeak with the Pt4fpeak,
Table VII. Binding Energies of Ptln in Some Standard Samdes and the SMAD Pt-Sn/SiO, Catalysts sample Pt4n,zbinding energy, eV reference Pt metal foil 70.9 catalyst I 71.0 72.0 71.2 J 71.8 K 71.0 71.8
the chemical state of Pt in the A1203-supportedcatalysts could not be studied. We were able to study Pt in the Si02-supportedsystems, however, and binding energies are listed in Table VII. The Si, peak was used as a reference. It can be seen that the Ptm/2binding energy is due to Pto, and, as would be expected, the SMAD procedure does not affect the valence state of Pt. Since others10J1,20have shown Pt to be in the Ptostate in conventional systems, it can be seen that in this aspect SMAD and conventional systems do not differ.
Discussion The most straightforward conclusions concerning the SMAD method of preparation are as follows: (1) The SMAD procedure ensures that tin is added as SnO, and >95% remains as SnO. (2) Some surface tin oxides are found due to adventitious oxygen, but only relatively small amounts. (3) The tin is present as mainly SnO regardless of tin loadings and support used. (4)The chemical environment of chloride does not vary from conventional systems, but more chloride is retained with A1203 as a support vs Si02. ( 5 ) Pt is present as PtO. The effect of Sn on the Pt catalytic properties vis-5-vis its chemical state will now be discussed. The spectroscopic methods we used did not allow resolution of Pt or Sn in an alloyed form (Pt-Sn alloy).21 Therefore, we do not have
Langmuir, Vol. 4,No. 2, 1988 391
Spectroscopy of Pt-Sn Bimetallic Catalysts
Pt-Sn/Si02
7 0
9
I 14 202
I 200
I 1 I I 198 196 194 204
I I 202 200
1
I I I 198 196 194 204
1
I
202
200
I
1
lg8
lg6 lg4
B i n d i n q energy, eV
Figure 10. XPS spectra of Clapfor different catalysts: (1)catalyst C; (2) catalyst D; (3) catalyst E; (4) catalyst F; (5) catalyst G; (6) catalyst H; (7) catalyst I; (8) catalyst J; (9) catalyst K.
direct physical evidence for such an alloy. However, it would be expected that the method of preparation would surely yield some coating of SnO on the preexisting Pt particles. Our earlier catalytic evaluation of these catalysts showed a significant effect of Sn on selectivity, and we suggested than an ensemble effect of SnO and Ptoparticles was re~ponsible.'~ This paper reports results that tend to support that conclusion. That is, tin is clearly present as SnO. This coupled with the catalytic selectivity changes indicates the best possible explanation is an alloy/ensemble effect where SnO on PtO particles are important. However, a bulk alloy would not be expected because of the method of preparation. Instead, a surface alloy would be expected, and this probably could not be spectroscopically detected by a bulk analytical technique like Mossbauer. The most likely particle structure then is
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( 2 1 ) Kanekar, C. R.; Rao, K. R. P.M.; Rao, V. U.S.Phys. Lett. 1965, 19, 95.
We cannot necessarily conclude, however, that conventional Pt-Sn/A1203 catalysts are similar in particle structure. In the conventionally prepared A1,03-supported systems no spectral evidence for SnO has been found. And yet tin addition does have a catalytic effect, as reviewed in the Introduction. The catalytic effects are similar but not as pronounced as with the SMAD systems. Taking all this information together, it seems likely (although certainly not proven) that conventional systems may also involve Sno-Pto but that only small amounts of Sno are present, perhaps only on the Pt particles, and may be formed by reduction of Sn4+J2+ to SnO just in the vicinity of the Pt particles. In other words, essentially the same model of the Pt-Sn particle shown above may be applicable. Our next effort will be to prepare bulk Pt-Sn alloys and study them catalytically and spectroscopically as we have these current SMAD systems.
Acknowledgment. The generous support of the National Science Foundation is acknowledged with gratitude. We thank Professor K. Purcell for assistance in carrying out the Mossbauer experiments. Registry No. Pt, 7440-06-4; Sn, 7440-31-5.
