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Synthesis and spectroscopic characterization of Pt and Pd silica supported catalysts
]OURNA
Journal of Non-Crystalline Solids 147&148 (1992) 753-757 North-Holland
L OF
NON-CRYSTALLINE SOLIDS
Synthesis and spectroscopic characterization of Pt and Pd silica supported catalysts T. L d p e z a, M. Morfin b, j. N a v a r r e t e b, L. H e r r e r a
a
and R. G d m e z a
Universidad Autdnoma Metropolitana (Iztapalapa), A.P. 55-534, Depto. De Qu[mica, Mdxico, D.F. 09340, Mexico b Instituto Mexicano del Petr61eo; Apartado Postale 14-805 M&ico, D.F. 07730, Mexico
Platinum and palladium silica supported catalysts were prepared by the sol-gel method using as metallic precursors the square planar complexes t[MCI2(NH3)2] with M = Pt or Pd. The catalysts were synthesized at pH = 3 and 9. If the preparation occurs in acid medium, the resulting solids show uniform microporous structure with BET areas > 500 m 2 / g . However, if the experiment is made in a basic medium, the resulting solids show non-homogeneous pore size distribution and BET areas < 100 m 2 / g .
1. Introduction
The sol-gel process has had a great success in the preparation of catalytic materials, because the physical and chemical properties can be controlled during the hydrolysis reaction. The incorporation of active metals (Pt, P d ) i n the sol during the gelation allows the metal to have a direct interaction with silica. Carturan and co-workers [1-3] have studied Pt and Pd catalysts supported on vitreous materials, obtaining high metal dispersions, metal particle sizes < 30 A. These catalysts in the phenylacetylene hydrogenation reaction show high activity and selectivity to styrene formation. Ldpez and co-workers [4-8], using different metallic precursors (PdC12, H 2 P t C I 6 . 6 H 2 0 , RuC13-3H20) in the preparation of sol-gel catalysts, found that a fraction of the metal content is incorporated into the support network, and the remaining fraction is strongly anchored on the surface through the OH groups of the support. The textural, structural and catalytic properties of the sol-gel catalysts strongly depend upon the interactions produced during the gelation between the gel and the metallic precursor [7]. The present work examines the interactions in a new type of catalysts, synthesized from the square planar complexes
t[M(NH3)2C1] where M = Pt or Pd and the tetraethoxysilane in the post-gelation step.
2. Experimental
P d - 0 . 5 O H - catalyst was prepared by refluxing 0.099 g of [Pd(NH3)2C12] (ICN K & K Inc., 99.9%) at 76°C with 24 ml H20, 2 ml NH4OH (Baker, 33.0%) (pH = 9) and 48 ml ethanol (Baker, 99.9%) for 10 rain with constant stirring; 37.18 ml of tetraethoxysilane (TEOS) was added to the solution driping for 4 h. Reflux continued until gelation. The final metal concentration was 0.5%. Catalysts P d - I . 0 O H and P d - 2 . 0 O H - are prepared in a similar way increasing only the metal concentration in the reaction. Pd-0.5H + catalyst was prepared by refluxing 0.099 g of t[Pd(NH3)2C12] (ICN K & K Inc., 99.9%) at 76°C with 24 ml H 2 0 , 2 ml HC1 (Baker, 36.5%) (pH = 3) and 48 ml ethanol (Baker, 99.9%) 37.18 ml of tetraethoxysilane (Alfa Products, 99%) was added to the solution driping for 4 h. Reflux continued until gelation. The palladium content in the final solid is 0.5%. Catalysts P d - I . 0 H + and Pd-2.0H + were prepared in a similar way, increasing the metal concentration to obtain 1% and 2% of palladium, respectively.
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
754
T. Ldpez et al. / Pt and Pd silica supported catalysts
P t - 0 . 5 O H - catalyst was prepared using the same procedure of Pd catalysts (pH9), but using the metallic precursor t[Pt(NH3)2C12] (ICN K & K Inc., 99.9%). The following metallic quantities were used for these catalysts: P t - 0 . 5 O H - (0.0768 g of complex); P t - I . 0 O H - (0.153 g) and for P t 2 . 0 O H - (0.307 g). P t - 0 . 5 H + catalyst was p r e p a r e d in the same way as the Pd catalysts synthesized at p H = 3, using the necessary quantity of the trans Pt complex to obtain 0.5%, 1% ( P t - I . 0 H ÷) and 2% ( P t - 2 . 0 H +) of metallic concentration in the final solid. The resulting gels were dried at 70°C for 15 h and then thermally treated at 450°C for 4½ h. The solid catalysts were characterized by F T I R spectroscopy with a 170-SX Nicolet equipment, using transparent pellets of unmixed catalysts. The U V - V i s (diffuse reflectance) spectra were obtained using a Cary 17 D Varian spectrophotometer; self-supporting pellets were prepared to obtain the spectra. The B E T areas were measured in a Micromeritics ASAP 2000.
2%
z
O.5%
WAVENUMBER (cm"
I
Fig. 1. FTIR spectra of Pd/SiO 2 catalysts calcined at 450°C. synthesized in basic medium, two small shoulders at 780 and 750 cm -1 appear, due to the N H 3 rocking vibration [9].
3. Results 3.1. P d / S i O 2 catalysts
During the gelation at p H = 3 the color goes from yellow (initial solution), to violet-blue at the gel point. During the drying and calcination it changes to brown. U n d e r basic pH, the initial solution is yellow-orange, which turns to a g r e e n - g r e y at the gel point and to a light brown after calcination. The infrared (IR) spectra of Pd vibrations are assigned as follows: 3465 cm -~ to O H stretching, 3453 cm -1 to silanol terminal groups and 3294 cm -~ to O H stretching of water and ethanol. The silica bands (1080 c m - t stretching O - S i - O ; 950 cm -~ stretching S i - O H ; 801 cm -1 flexion -O-Si-Oand 463 cm -1 flexion S i - O - ) are lightly shifted when the palladium is present. An important region to discuss is the one of 800-350 cm -1 (fig. 1). The I R spectra of P d / S i O 2 show a band at 496 cm -1 characteristic of stretching vibration P d - N and another one at 350 c m - a due to the Pd-C1 stretching frequency. In the samples
(f)
(e)
o
z
m nO
I
Idl
m <
(b)
2
2~
3~o WAVELENGTH
s~o (nm)
4bo
Fig. 2. UV-Vis (diffuse reflectance) spectra of Pd/SiO 2 catalysts calcined at 450°C; (a) Pd-0.5H+,, (b) Pd-0.5OH , (c) Pd-I.0H +, (d) Pd-I.0OH-, (e) Pd-2.0H + and (f) Pd2.0OH -.
T. Ldpez et aL / Pt and Pd silica supported catalysts
In all calcined P d / S i O 2 samples, an intense band at 260 nm (fig. 2) and another of lower intensity at 320 nm were observed. They are due to the charge transfer transitions (CT) between the metal and the ligands [10]: Pd ~ OHsupport [rr(OH) --+ t 2 Pd], Pd --+ Cl['rr(e u) --+ big] and Pd --+ NH3[rr(NH 3) ~ tzg Pd]. W h e n the samples have not received thermal treatment the band at 260 nm is not observed and the other one appears at 310 nm. In the visible region, several low-intensity bands are observed assigned to the d - d transitions of a square planar complex with symmetry D 4 h : 5 8 0 nm [1Alg--+3Blg]; 5 0 0 nm [1A1_-+ • r(OH)]. 460 nm [1Alg-+ 1Aeg] and 420 nm [~Aag -+ 3Eg]. At 600 and 710 nm, two low-intensity bands appear; due to the interactions between the metal and the O H of the support [t2g(Pd) --+ ,n-(OH)], which cause a distortion in the D4h symmetry of the complex. Table 1 shows that an acid m e d i u m produces a narrow microporous structure (maximum at 15 A) and a surface area up to 550 m 2 / g , but a basic medium generates a broad pore size distribution (up to 400 A) and a low surface area. 3.2. Pt / SiO 2 catalysts The gels obtained are yellow powders due to the presence of the Pt, and they turn to a grey
755
color if they are calcined (0.5% Pt) and black (2.0% Pt) because of the metal oxidation. All the F T I R spectra present a wide band at 3429 cm-1, which has two small shoulders at 3630 and 3300 cm -1, all being due to the stretching vibrations of ethanol, water and silanol groups. The silica bands are observed in the same positions as those found in P d / S i O 2 catalysts. At 495 cm -1, the P t - N H 3 stretching vibration is observed. At 377 and 363 cm -1, two characteristics bands of the Pt-C1 stretching vibration are observed which are more intense in the catalysts p r e p a r e d with HC1. At 385 cm-1, a flexion vibration band due to the P t - O bond (fig. 3) and a wide shoulder at 375 c m - 1 assigned to the P t - O H flexion vibration [11] are observed. This indicates the interaction that exists between the metal with the support through the O H groups. In all the spectra, a small band of charge transfer transition (CT) between 220 and 230 nm is observed, due to the m e t a l - l i g a n d electronic transition [ltzu(~)--+Zeg ( z 2, x Z - y 2 ) ] . At 310 nm, an intense band appears, assigned to CT transition [1Alg ~ 1Aig] from the amino ligands to Pt. Besides, in the acid catalyst a band around 330 nm appears due to CT Pt--+ O H electronic transition [ e g ( P t ) + ,rr(OH)]. This band increases in intensity when the metal content is increased. Meanwhile, in the catalyst p r e p a r e d at p H = 9, a band at 270 nm appears, which corresponds to
Table 1 BET areas of P t / S i O 2 and P d / S i O 2 catalysts (calcined at 45°C for 41 h) Catalyst
BET area (mZ/g)
Catalyst
Pd-0.5H + Pd-I.0H + Pd -2.0H + Pd-0.5OHPd-I.0OHPd-2.0OH-
457 537 395 188 16
Pt-0.5H + Pt-I.0H + Pt-2.0H + Pt-0.5OHPt-I.0OH Pt-2.0OH-
489 589 622 48 14 63
0.5 1.0 2.0 0.5 1.0 2.0
P d / S i O 2 (pH9) (ref. [12]) P d / S i O 2 (pH9) (ref. [12])
889
P t / S i O 2 (pH9) (ref. [7]) P t / S i O 2 (pH9) (ref. [7])
614
0.1
1134
0.5
SiO 2 (pH9) This work
333
634
SiO 2 (pH3) This work
BET area (mZ/g)
580
[M]
-
756
T. Ldpez et al. / Pt and Pd silica supported catalysts
/•
~ (e}2.0%(?0oC
its ligands during the gelation reaction and the solution changes its color: Acid medium t[MC12(NH3)2] + HCl ~ [MC1B(NH3) ] HCI> [ M C 1 4 1 2 - ;
Basic medium
t[MC12(NH3)2] + N H 4 O H ~ [MCI(NH3)3] + NH4OH [ M ( N H 3 ) 4 ]
z o2 1--
i¢.
0.5% [45o ~¢)
WAVENUMBER (era'l)
Fig. 3. FTIR spectra of P t / S i O 2 catalysts: (a) Pt-0.5H + (450°C), (b) P t - I . 0 H + (450°C), (c) Pt-2.0H + (450°C), (d) P t - I . 0 H + (70°C) and (e) Pt-2.0H + (70°C).
;
M = Pt or Pd. The square planar symmetry (D4h) is not altered during the process, it only suffers an important distortion when the metal interacts with the support. Fourier transformation infrared (FTIR) spectroscopy shows that the sol-gel silica is highly hydroxylated and the silanol terminal groups are found in high-energy regions and tend to be very reactive. Besides, typical bands of M-C1, M - N H 3 and M - O are observed; therefore, the metal does not lose either the C1 ligand or the amino ligand, but incorporates in its coordination sphere oxygen from the support: 2 - Si-OH + [M(CI)x(Nn3),]
[1Alg ~ leg], which is a CI ~ Pt charge transfer transition. In the visible region around the 412-420 nm, the d - d characteristic transition [1Alg ~ 3Eg] of the complex t[Pt(NH3)2C12] is observed. At 710 nm, a very intense band is seen in the Pd and Pt catalysts. In catalysts prepared at pH = 3 (table 1), the surface areas are high (480-600 m2/g). These catalysts have a uniform microporous structure. Instead, the BET areas of catalysts prepared at pH = 9 fall to values < 50 m 2 / g and these catalysts have a macroporous structure.
4. Discussion
T h e results show that the complexes t[PdC12(NH3) 2] and t[PtClz(NH3) e] interact with the silica gel when they are prepared by the sol-gel method. First, the metal tends to change
[SiO 2 ] - O - [ M C l x ( N H 3 ) y ( O H ) z ] . The [MCIx(NH3)y(SiO).] species can be formed. The nucleophiles (=- S i - O - , pH = 9) or the electrophiles ( = S i - O +-2H, pH = 3) are then incorporated into the metal coordination sphere. In table 1, two important effects are shown: (i) an effect of the complex symmetry, (ii) an effect of the gelation medium acidity. The BET areas are higher when the catalysts are prepared at pH = 9 with an octahedral complex (H2PtC12) [7] or with a planar salt (PdC12) [12]. However, when the catalysts are synthesized with square planar complexes at pH = 3, the acidity varies the gelation mechanism, the areas are high and the catalysts have uniform microporous structure. In basic medium the areas are smaller and the pore diameters are not homogeneous (macroporous and microporous). The bands observed by U V - V i s spectroscopy in the catalysts show that the metal does not lose
T. L6pez et al. / Pt and Pd silica supported catalysts
its s y m m e t r y D4h. T h e b a n d s shift to lower energy r e g i o n s b e c a u s e t h e s y m m e t r y is d i s t o r t e d by the silanol g r o u p s i n c o r p o r a t e d to t h e c o o r d i n a tion s p h e r e of t h e m e t a l . Two i m p o r t a n t b a n d s t h a t do n o t b e l o n g to t h e t r a n s c o m p l e x e s can b e o b s e r v e d at 260 n m (very i n t e n s e in Pd catalysts) a n d a n o t h e r at 710 rim. B o t h a r e a s s i g n e d to an e l e c t r o n i c t r a n s i t i o n b e t w e e n t h e m e t a l a n d t h e O H of t h e s u p p o r t .
5. Conclusions (1) T h e p H of g e l a t i o n r e a c t i o n is i m p o r t a n t b e c a u s e b o t h ( p H = 9 a n d p H = 3) t e n d to interc h a n g e t h e l i g a n d s of t h e p r e c u r s o r c o m p l e x d u r ing the gelation. (2) T h e s y m m e t r y o f t h e c o m p l e x has a strong effect u p o n t h e specific surface a r e a s a n d p o r e size d i s t r i b u t i o n . (3) A m e t a l s u p p o r t i n t e r a c t i o n exists, a n d o r i g i n a t e s a n i m p o r t a n t d i s t o r t i o n in t h e s q u a r e p l a n a r s y m m e t r y of t h e m e t a l l i c p r e c u r s o r . T h e a u t h o r s a r e i n d e b t e d to C O N A C Y T P R O I D E S - S E P for financial s u p p o r t .
and
757
References [1] G. Carturan, G. Cocco, L. Schiffini and G. Strukul, J. Catal. 65 (1980) 359. [2] G. Carturan and G. Strukul, J. Catal. 57 (1979) 516. [3] G. Carturan, G. Facchin, G. Cocco, S. Enzo and G. Navazio, J. Catal. 76 (1980) 405. [4] T. Ldpez, A. Lopez-Gaona and R. Gdmez, Langmuir 6 (1990) 1343. [5] T. Ldpez, A. Lopez-Gaona and R. Gdmez, J. Non-Cryst. Solids 108 (1989) 45. [6] T. Ldpez, P. Bosch and R. Gomez, React. Kinet. Catal. Lett. 41 (1990) 217. [7] T. Ldpez, A. Romero and R. Gdmez, J. Non-Cryst. Solids 127 (1991) 105. [8] T. Ldpez, M. Villa and R. Gdmez, J. Phys. Chem. 95 (1991) 1690. [9] R. Layton, D.W. Sink and J.R. During, J. Inorg. Nucl. Chem. 28 (1966) 1965. [10] A.B.P. Lever, Inorganic Electronic Spectroscopy, 2nd Ed. (Elsevier, Amsterdam, 1984). [11] G.L. Morgan, R.D. Rennick and C.C. Soong, Inorg. Chem. 5 (1966) 372. [12] T. Ldpez, M, Asomoza, P. Bosch, E. Garcia-Figueroa and R. Gdmez, J. Catal., in press.
Journal of Non-Crystalline Solids 147&148 (1992) 753-757 North-Holland
L OF
NON-CRYSTALLINE SOLIDS
Synthesis and spectroscopic characterization of Pt and Pd silica supported catalysts T. L d p e z a, M. Morfin b, j. N a v a r r e t e b, L. H e r r e r a
a
and R. G d m e z a
Universidad Autdnoma Metropolitana (Iztapalapa), A.P. 55-534, Depto. De Qu[mica, Mdxico, D.F. 09340, Mexico b Instituto Mexicano del Petr61eo; Apartado Postale 14-805 M&ico, D.F. 07730, Mexico
Platinum and palladium silica supported catalysts were prepared by the sol-gel method using as metallic precursors the square planar complexes t[MCI2(NH3)2] with M = Pt or Pd. The catalysts were synthesized at pH = 3 and 9. If the preparation occurs in acid medium, the resulting solids show uniform microporous structure with BET areas > 500 m 2 / g . However, if the experiment is made in a basic medium, the resulting solids show non-homogeneous pore size distribution and BET areas < 100 m 2 / g .
1. Introduction
The sol-gel process has had a great success in the preparation of catalytic materials, because the physical and chemical properties can be controlled during the hydrolysis reaction. The incorporation of active metals (Pt, P d ) i n the sol during the gelation allows the metal to have a direct interaction with silica. Carturan and co-workers [1-3] have studied Pt and Pd catalysts supported on vitreous materials, obtaining high metal dispersions, metal particle sizes < 30 A. These catalysts in the phenylacetylene hydrogenation reaction show high activity and selectivity to styrene formation. Ldpez and co-workers [4-8], using different metallic precursors (PdC12, H 2 P t C I 6 . 6 H 2 0 , RuC13-3H20) in the preparation of sol-gel catalysts, found that a fraction of the metal content is incorporated into the support network, and the remaining fraction is strongly anchored on the surface through the OH groups of the support. The textural, structural and catalytic properties of the sol-gel catalysts strongly depend upon the interactions produced during the gelation between the gel and the metallic precursor [7]. The present work examines the interactions in a new type of catalysts, synthesized from the square planar complexes
t[M(NH3)2C1] where M = Pt or Pd and the tetraethoxysilane in the post-gelation step.
2. Experimental
P d - 0 . 5 O H - catalyst was prepared by refluxing 0.099 g of [Pd(NH3)2C12] (ICN K & K Inc., 99.9%) at 76°C with 24 ml H20, 2 ml NH4OH (Baker, 33.0%) (pH = 9) and 48 ml ethanol (Baker, 99.9%) for 10 rain with constant stirring; 37.18 ml of tetraethoxysilane (TEOS) was added to the solution driping for 4 h. Reflux continued until gelation. The final metal concentration was 0.5%. Catalysts P d - I . 0 O H and P d - 2 . 0 O H - are prepared in a similar way increasing only the metal concentration in the reaction. Pd-0.5H + catalyst was prepared by refluxing 0.099 g of t[Pd(NH3)2C12] (ICN K & K Inc., 99.9%) at 76°C with 24 ml H 2 0 , 2 ml HC1 (Baker, 36.5%) (pH = 3) and 48 ml ethanol (Baker, 99.9%) 37.18 ml of tetraethoxysilane (Alfa Products, 99%) was added to the solution driping for 4 h. Reflux continued until gelation. The palladium content in the final solid is 0.5%. Catalysts P d - I . 0 H + and Pd-2.0H + were prepared in a similar way, increasing the metal concentration to obtain 1% and 2% of palladium, respectively.
0022-3093/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
754
T. Ldpez et al. / Pt and Pd silica supported catalysts
P t - 0 . 5 O H - catalyst was prepared using the same procedure of Pd catalysts (pH9), but using the metallic precursor t[Pt(NH3)2C12] (ICN K & K Inc., 99.9%). The following metallic quantities were used for these catalysts: P t - 0 . 5 O H - (0.0768 g of complex); P t - I . 0 O H - (0.153 g) and for P t 2 . 0 O H - (0.307 g). P t - 0 . 5 H + catalyst was p r e p a r e d in the same way as the Pd catalysts synthesized at p H = 3, using the necessary quantity of the trans Pt complex to obtain 0.5%, 1% ( P t - I . 0 H ÷) and 2% ( P t - 2 . 0 H +) of metallic concentration in the final solid. The resulting gels were dried at 70°C for 15 h and then thermally treated at 450°C for 4½ h. The solid catalysts were characterized by F T I R spectroscopy with a 170-SX Nicolet equipment, using transparent pellets of unmixed catalysts. The U V - V i s (diffuse reflectance) spectra were obtained using a Cary 17 D Varian spectrophotometer; self-supporting pellets were prepared to obtain the spectra. The B E T areas were measured in a Micromeritics ASAP 2000.
2%
z
O.5%
WAVENUMBER (cm"
I
Fig. 1. FTIR spectra of Pd/SiO 2 catalysts calcined at 450°C. synthesized in basic medium, two small shoulders at 780 and 750 cm -1 appear, due to the N H 3 rocking vibration [9].
3. Results 3.1. P d / S i O 2 catalysts
During the gelation at p H = 3 the color goes from yellow (initial solution), to violet-blue at the gel point. During the drying and calcination it changes to brown. U n d e r basic pH, the initial solution is yellow-orange, which turns to a g r e e n - g r e y at the gel point and to a light brown after calcination. The infrared (IR) spectra of Pd vibrations are assigned as follows: 3465 cm -~ to O H stretching, 3453 cm -1 to silanol terminal groups and 3294 cm -~ to O H stretching of water and ethanol. The silica bands (1080 c m - t stretching O - S i - O ; 950 cm -~ stretching S i - O H ; 801 cm -1 flexion -O-Si-Oand 463 cm -1 flexion S i - O - ) are lightly shifted when the palladium is present. An important region to discuss is the one of 800-350 cm -1 (fig. 1). The I R spectra of P d / S i O 2 show a band at 496 cm -1 characteristic of stretching vibration P d - N and another one at 350 c m - a due to the Pd-C1 stretching frequency. In the samples
(f)
(e)
o
z
m nO
I
Idl
m <
(b)
2
2~
3~o WAVELENGTH
s~o (nm)
4bo
Fig. 2. UV-Vis (diffuse reflectance) spectra of Pd/SiO 2 catalysts calcined at 450°C; (a) Pd-0.5H+,, (b) Pd-0.5OH , (c) Pd-I.0H +, (d) Pd-I.0OH-, (e) Pd-2.0H + and (f) Pd2.0OH -.
T. Ldpez et aL / Pt and Pd silica supported catalysts
In all calcined P d / S i O 2 samples, an intense band at 260 nm (fig. 2) and another of lower intensity at 320 nm were observed. They are due to the charge transfer transitions (CT) between the metal and the ligands [10]: Pd ~ OHsupport [rr(OH) --+ t 2 Pd], Pd --+ Cl['rr(e u) --+ big] and Pd --+ NH3[rr(NH 3) ~ tzg Pd]. W h e n the samples have not received thermal treatment the band at 260 nm is not observed and the other one appears at 310 nm. In the visible region, several low-intensity bands are observed assigned to the d - d transitions of a square planar complex with symmetry D 4 h : 5 8 0 nm [1Alg--+3Blg]; 5 0 0 nm [1A1_-+ • r(OH)]. 460 nm [1Alg-+ 1Aeg] and 420 nm [~Aag -+ 3Eg]. At 600 and 710 nm, two low-intensity bands appear; due to the interactions between the metal and the O H of the support [t2g(Pd) --+ ,n-(OH)], which cause a distortion in the D4h symmetry of the complex. Table 1 shows that an acid m e d i u m produces a narrow microporous structure (maximum at 15 A) and a surface area up to 550 m 2 / g , but a basic medium generates a broad pore size distribution (up to 400 A) and a low surface area. 3.2. Pt / SiO 2 catalysts The gels obtained are yellow powders due to the presence of the Pt, and they turn to a grey
755
color if they are calcined (0.5% Pt) and black (2.0% Pt) because of the metal oxidation. All the F T I R spectra present a wide band at 3429 cm-1, which has two small shoulders at 3630 and 3300 cm -1, all being due to the stretching vibrations of ethanol, water and silanol groups. The silica bands are observed in the same positions as those found in P d / S i O 2 catalysts. At 495 cm -1, the P t - N H 3 stretching vibration is observed. At 377 and 363 cm -1, two characteristics bands of the Pt-C1 stretching vibration are observed which are more intense in the catalysts p r e p a r e d with HC1. At 385 cm-1, a flexion vibration band due to the P t - O bond (fig. 3) and a wide shoulder at 375 c m - 1 assigned to the P t - O H flexion vibration [11] are observed. This indicates the interaction that exists between the metal with the support through the O H groups. In all the spectra, a small band of charge transfer transition (CT) between 220 and 230 nm is observed, due to the m e t a l - l i g a n d electronic transition [ltzu(~)--+Zeg ( z 2, x Z - y 2 ) ] . At 310 nm, an intense band appears, assigned to CT transition [1Alg ~ 1Aig] from the amino ligands to Pt. Besides, in the acid catalyst a band around 330 nm appears due to CT Pt--+ O H electronic transition [ e g ( P t ) + ,rr(OH)]. This band increases in intensity when the metal content is increased. Meanwhile, in the catalyst p r e p a r e d at p H = 9, a band at 270 nm appears, which corresponds to
Table 1 BET areas of P t / S i O 2 and P d / S i O 2 catalysts (calcined at 45°C for 41 h) Catalyst
BET area (mZ/g)
Catalyst
Pd-0.5H + Pd-I.0H + Pd -2.0H + Pd-0.5OHPd-I.0OHPd-2.0OH-
457 537 395 188 16
Pt-0.5H + Pt-I.0H + Pt-2.0H + Pt-0.5OHPt-I.0OH Pt-2.0OH-
489 589 622 48 14 63
0.5 1.0 2.0 0.5 1.0 2.0
P d / S i O 2 (pH9) (ref. [12]) P d / S i O 2 (pH9) (ref. [12])
889
P t / S i O 2 (pH9) (ref. [7]) P t / S i O 2 (pH9) (ref. [7])
614
0.1
1134
0.5
SiO 2 (pH9) This work
333
634
SiO 2 (pH3) This work
BET area (mZ/g)
580
[M]
-
756
T. Ldpez et al. / Pt and Pd silica supported catalysts
/•
~ (e}2.0%(?0oC
its ligands during the gelation reaction and the solution changes its color: Acid medium t[MC12(NH3)2] + HCl ~ [MC1B(NH3) ] HCI> [ M C 1 4 1 2 - ;
Basic medium
t[MC12(NH3)2] + N H 4 O H ~ [MCI(NH3)3] + NH4OH [ M ( N H 3 ) 4 ]
z o2 1--
i¢.
0.5% [45o ~¢)
WAVENUMBER (era'l)
Fig. 3. FTIR spectra of P t / S i O 2 catalysts: (a) Pt-0.5H + (450°C), (b) P t - I . 0 H + (450°C), (c) Pt-2.0H + (450°C), (d) P t - I . 0 H + (70°C) and (e) Pt-2.0H + (70°C).
;
M = Pt or Pd. The square planar symmetry (D4h) is not altered during the process, it only suffers an important distortion when the metal interacts with the support. Fourier transformation infrared (FTIR) spectroscopy shows that the sol-gel silica is highly hydroxylated and the silanol terminal groups are found in high-energy regions and tend to be very reactive. Besides, typical bands of M-C1, M - N H 3 and M - O are observed; therefore, the metal does not lose either the C1 ligand or the amino ligand, but incorporates in its coordination sphere oxygen from the support: 2 - Si-OH + [M(CI)x(Nn3),]
[1Alg ~ leg], which is a CI ~ Pt charge transfer transition. In the visible region around the 412-420 nm, the d - d characteristic transition [1Alg ~ 3Eg] of the complex t[Pt(NH3)2C12] is observed. At 710 nm, a very intense band is seen in the Pd and Pt catalysts. In catalysts prepared at pH = 3 (table 1), the surface areas are high (480-600 m2/g). These catalysts have a uniform microporous structure. Instead, the BET areas of catalysts prepared at pH = 9 fall to values < 50 m 2 / g and these catalysts have a macroporous structure.
4. Discussion
T h e results show that the complexes t[PdC12(NH3) 2] and t[PtClz(NH3) e] interact with the silica gel when they are prepared by the sol-gel method. First, the metal tends to change
[SiO 2 ] - O - [ M C l x ( N H 3 ) y ( O H ) z ] . The [MCIx(NH3)y(SiO).] species can be formed. The nucleophiles (=- S i - O - , pH = 9) or the electrophiles ( = S i - O +-2H, pH = 3) are then incorporated into the metal coordination sphere. In table 1, two important effects are shown: (i) an effect of the complex symmetry, (ii) an effect of the gelation medium acidity. The BET areas are higher when the catalysts are prepared at pH = 9 with an octahedral complex (H2PtC12) [7] or with a planar salt (PdC12) [12]. However, when the catalysts are synthesized with square planar complexes at pH = 3, the acidity varies the gelation mechanism, the areas are high and the catalysts have uniform microporous structure. In basic medium the areas are smaller and the pore diameters are not homogeneous (macroporous and microporous). The bands observed by U V - V i s spectroscopy in the catalysts show that the metal does not lose
T. L6pez et al. / Pt and Pd silica supported catalysts
its s y m m e t r y D4h. T h e b a n d s shift to lower energy r e g i o n s b e c a u s e t h e s y m m e t r y is d i s t o r t e d by the silanol g r o u p s i n c o r p o r a t e d to t h e c o o r d i n a tion s p h e r e of t h e m e t a l . Two i m p o r t a n t b a n d s t h a t do n o t b e l o n g to t h e t r a n s c o m p l e x e s can b e o b s e r v e d at 260 n m (very i n t e n s e in Pd catalysts) a n d a n o t h e r at 710 rim. B o t h a r e a s s i g n e d to an e l e c t r o n i c t r a n s i t i o n b e t w e e n t h e m e t a l a n d t h e O H of t h e s u p p o r t .
5. Conclusions (1) T h e p H of g e l a t i o n r e a c t i o n is i m p o r t a n t b e c a u s e b o t h ( p H = 9 a n d p H = 3) t e n d to interc h a n g e t h e l i g a n d s of t h e p r e c u r s o r c o m p l e x d u r ing the gelation. (2) T h e s y m m e t r y o f t h e c o m p l e x has a strong effect u p o n t h e specific surface a r e a s a n d p o r e size d i s t r i b u t i o n . (3) A m e t a l s u p p o r t i n t e r a c t i o n exists, a n d o r i g i n a t e s a n i m p o r t a n t d i s t o r t i o n in t h e s q u a r e p l a n a r s y m m e t r y of t h e m e t a l l i c p r e c u r s o r . T h e a u t h o r s a r e i n d e b t e d to C O N A C Y T P R O I D E S - S E P for financial s u p p o r t .
and
757
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