J. Phys. Chem. 1993,97, 1671-1677
1671
Pt/SiOz Sol-Gel Catalysts: Effects of pH and Platinum Precursor T.Lopez,'*+ P. Bosch,' M. M0-t
OMd R. Gomezt Universidad Autanoma Metropolitana- Iztapalapa, A.P. 55-534 MPxico D.F.09340, Mexico, and Instituto Mexican0 del Petroleo. Eje Central No. 152 MPxico D.F. 07730, Mexico Received: Nwember 17, 1992
Pt/SiOl catalysts were synthesized by the sol-gel method using a square planar metal precursor and tetraethoxysilane. The solids were characterized by FTIR, UV-vis (diffuse reflectance), X-ray diffraction, scanning and transmission electron microscopy, and gas adsorption. The results were compared with those obtained in conventionally impregnated catalysts. It is shown that in the sol-gel catalysts an unusual interaction between the metal and the support exists, which is not present in impregnated catalysts.
IntrOdUCti0n
The continuing need for selective and active catalysts had led to research on nonconventional synthesis techniques,l-s for instance, the sol+ method. The advantages of sol-gel processing include purity, homogeneity, and controlled porosity, along with the formation of high surface area materials at low temperatures. Since metastable porous structures are created in solution, the supported catalyst can be prepared from a homogeneoussolution which includes not only the metal oxide support precursor but also the metal precursor. Metal catalysts are often supported on silica. The preparation of silica aerogels is not difficult if the sol-gel technique is chosen.*-1° Acidic or basic conditions can be used to control the hydrolysis and condensation reactions.'l-14 Along with effects of acids and bases, it is expected that a metal salt, incorporated in the solution, during the sol-gel synthesis, will have a metal support interaction. Furthermore, this interaction may be influenced by the initial symmetry and coordination of the metal atom. If the initial complex symmetry is planar the interaction with silica should be favored and a metal-support interaction would occur even in a support as inert, in this sense, as silica. The performance of such a catalyst should be very different from the corresponding impregnation prepared one. This work studies the effect of the metallic complex precursors on the Pt/SiOzsol-gel synthesizedcatalysts. The characterization techniques are infrared spectroscopy, X-ray diffraction, gas adsorption, and transmission and scanning electron microscopy. ExperiwnW Section
Pt-O.5-OH. rrans-[Pt(NH3)2C12] (0.0768 g, Aldrich, 99.99%), was refluxed at 76OC with 24 ml of water, 2 ml of NH4OH (Baker, 33% of NH3 in water) and 48 mL of ethanol (Baker, 99.9%) for 10 min. Tetraethoxysilane (37.18 mL, Aldrich, 98%) was added in drops to the refluxing solution. The reflux was maintained until gel formation. The final metal content was 0.5%. To prepare the Pt-1.0-OH and the Pt-2.0-OH catalysts, the same procedure was followed increasing the amount of platinum complex: 0.153 and 0.307 g respectively. Pt-0.5-H. t r ~ n s - [ P t ( N H ~ ) ~ C (0.0768 l ~ ] g) was refluxed at 76 OC with 24 mL of water and 2 mL of HCI (Baker, 36.5% in water), pH = 3. 37.18 mL of tetraethoxysilane were dripped into the refluxing solution. The reflux was maintained until gel formation. The platinum final content was 0.5%. +
Universidad AutOnoma Metropolitana-Iztapalapa. Mexican0 del Petroleo.
1 lnstituto
0022-3654/93/2097-1671S04.00/0
To prepare the Pt-1.0-H and the Pt-2.0-H catalysts the complex concentration was increased and the same procedure was followed: 0.153 and 0.307 g respectively. Pt-1.0-ImpOH. The silica support was prepared by the solgel method from tetraethoxysilane at pH 9, it was dried at 70 and treated at 200 OC. The sol-gel silica was mixed with the rrans[Pt(NH&C12] complex dissolved previously in distilled water. The complex concentration was such that the final platinum content was 1%. It was maintained under stirring at 60 OC until dried. Pt-1.0-ImpH. To obtain pH 3, HCl was used in the hydrolysis step of silica synthesis. The metal-supported catalyst was, then, prepared as described previously. All the gels were dried at 70 OC for 15 h. The calcined samples are the gels dried and treated at 450 OC for 4.5 h.
cbrvrcteriutioa FI'IR and UV-Vis (Diffuse Reflectance). The catalysts were characterized by IR spectroscopy with a 170-SX Fourier transform Nicolet spectrometer. Pressure was applied to the samplepowder (no KBr wasused) until the pellet wastransparent. The UV-vis (diffuse reflectance) spectra were obtained with a Cary 17 D Varian spectrograph, with integrating sphere. To obtain the spectra, self-supporting pellets were prepared. The reference was a 100% reflectance sample. X-ray Diffraction. The X-ray diffraction measurements were performed in a D 500 Siemens Diffractometer coupled to a copper X-ray tube. A diffracted beam monochromator selected the K& radiation. From the peak profiles the crystallite size distribution was obtained.15 Cas Adsorption and Tnasmissioa Electron Microscopy. The surface areas were measured by the BET method in a Micromeritics ASAP 2000. The bright field images were obtained in a JEOL 100 CX electron microscope. The particle size distribution was estimated counting the metal particles in the micrographs. Results and LXscussion pH Effect 011 the Sol. Gelling time in the various samples is different, it depends on pH and metal concentration, Table I. In pH 9 samples, the solution is, initially, pale yellow as the metallic salt, but when pH is adjusted to 9, it turns colorless. If tetraethoxysilane is added, a white suspension is observed. After the gelation, the color goes from white to gray, and it is not altered bydrying. Dependingon the metal content, when calcined, the powder changes from gray to black. Note that such an effect cannot be due to the presence of organic residues, the gelled tetraethoxysilane is, indeed, white. Q 1993 American Chemical Society
1672 The Journal of Physical Chemistry, Vol. 97, No. 8, 1993
Lopez et al.
TABLE I: Gelation Time of the Pt/SiO2 SOl-Gel Catalvsts gelation area BET catalyst Pt-0.5-H Pt-1 .O-H Pt-2.0-H
Pt4.5-OH Pt-1 .O-OH Pt-2.0-OH Pt-1 .O-ImpH Pt-1.0-ImpOH
time (h)
morphology
20 18 15 25 32 72
crystalloid crystalloid crystalloid powder powder powder powder powder
color
(m2/n) 490 590 622 49 41 63 558 118
In pH 3 preparations, the solution is first pale yellow. When the HCl is added, the solution turns colorless and finally the gel is colorless and crystalline. If dried, the powder is pale gray, but if calcined, the solid turns black. As a general tendency, the gelling time is shorter if the synthesis is performed in acid medium instead of basic medium. Metal content (0.5, 1.0, or 2.0%) does not affect gelling time in pH 3 samples, but, if the metal content is increased, in pH 9, samples the gelling time also increases. BET Surface Area Values. BET equation was used to interpret the Nz physisorption measurements. If catalysts are sol-gel prepared at pH 3, Table I, the surface area value is 10 times higher than if prepared at pH 9. In the first case, isotherms are type I and do not show any hysteresis, i.e., the adsorption is limited to a few molecular layers and the solid is microporous. The pore size is 25 A. In the base catalysts isotherms are type V, these solids sh0w.a type A hysteresis loop,16pores are, then, cylindrical and open in both ends. The pore size is not uniform, and the pore distribution, then, is broad. A clear correlation exists between pH and surface area in solgel prepared catalysts. The same effect is observed if the catalysts are impregnated as in the conventional process. TheitH 3 impregnated catalyst surface area is 5 times larger than the pH 9 impregnated catalyst. It seems, then, that the sol-gel techniquC . is more sensitive to pH than impregnation, as far as surface areas are concerned. If the pore size distributionsof the Pt-1 .O-Imp-H and Pt-1.0-ImpOH catalysts are compared, at pH 3 the distribution is monomodal and centered at around 24 A. But if the pH is basic the small pores disappear and only pores whose sizes are between 300 and 600 A are observed. Therefore acidity alters the pore size in the impregnated catalysts. Scanning Electron Microscopy. Figure 1 compares the morphology of the various catalysts. pH 9 prepared catalysts are constituted by "spheroidal" aggregates. The reaction mechanism in ammonia medium is thought to be determined by the charges due to silanol groups deprotonation.8 These charges are present in the reaction environment and maintain ammonia ions close together, particles are, hence, negatively charged. Instead, in pH 3 synthesis, large homogeneous particles, "tactoid" type, are observed. TransmissionElectron Microscopy. To characterize the metal phase, transmission electron microscopy was used. From bright field images (Figure 2), metal particle size distributions were obtained. Sol-gel pH 3 prepared catalysts show metal particle, size distributionswhich depend on the metal content. As platinum percentage is increased metal particle size increases (Figure 3a). If platinum content is 0.5% (Pt-0.5-H) the particle size distribution is narrow, monomodal, and centered at 15A. Instead, the Pt-2.0-H catalyst has larger particles; the distribution in this case is bimodal and is centered at 35 and 75 8. Pt-1 .O-ImpH catalyst reproduces the metal particle size distribution found for Pt-O.5-H catalyst. But, if compared with the corresponding sol-gel 1% catalyst, the metal particle size is smaller. Hence, in this case the metal does not seem free to move. Sol-gel pH 9 catalysts show similar particle size distributions for all metal contents (Figure 3b). The distributions are
Figure 1. S E M micrographs of (A) Pt-1.0-OH and (B) Pt-1.0-H
catalysts.
Figure 2. Bright field image STEM of Pt-1 .O-H catalyst (magnification X 400 000).
monomodal and centered at D = 15 A. They are similar to the one obtained for Pt-0.5-H catalyst. Basic environment, hence, inhibits metal atom motion. Particles are stuck on the support. In this case the high metal content does not alter the metal particle size. No differences are observed for impregnated catalysts. X-ray Diffraction Study. Diffraction peaks attributed to platinum were observed in all pH 3 catalysts (Figurep), excepting the case of Pt-1 .O-OH catalyst where the crystallites may have been toosmall togive rise to peaks (Figure 5). The (1 1 1) platinum peak profile was used to estimate the crystallite size distribution. These distributions concern only large crystallites as small crystallites (less than 30 A) are not observed by this technique. The X-ray crystallite size distributions are, then, complementary to the transmission electron microscopy distributions.17J8Figure 6,shows that crystallites present in Pt-1 .O-H catalyst areslightly larger than those found in Pt-1.0-Imp-H catalyst but that Pt1.O-ImpOH preparation has monocrystalline particles as large as 180A. Hence the differencebetween sol-gel and impregnated pH 9 catalysts is the number of large particles. In Pt-1.0-OH
The Journal of Physical Chemistry, Vol. 97, No.8, 1993 1673
Pt/Si02 Sol-Gel Catalysts
'** xxx tt+
080
*I
Pt-0.5-H R-1.0-H Pt-2.0-H Pt-1.0-Imp-H
Pt- 1.0- imp- OH Pt-1.0-OH
* * * Pt-0.5-OH xxx
+++
n
I
PI- 1.0-OH Pt-2.0-OH
I
378
39.4
>
41 .O
(A'
28
CUW)
Figure 5. X-ray diffraction platinum peak observed for basic catalysts. -:Pt-
-:FI. -:Pt-i.O-
eo
I O - imp-H I.O- i mp-OH
H
>
DtA, Figure 3. Metal particle size distributions as determined by STEM of (a) acid and (b) basic catalyst. I5
45
75
I
45.21
80.42 135.63 180.84
Figure 6. Platinum crystallite size volume distribution, F,(D) as defined in ref 22, determined by X-ray diffraction.
I
I
37.8
30.4
41.0
'28 ( X=Cu K s 1
assigned to a dxy-dxz-y'(IBlg)transition. The band found at 272 nm is due to a dy2-dx2-y'(IB2g) transition. Another transition is present at 371 nm which is spin forbidden, it is attributed to a d,y-dxz-yz (3Blg)transition. The bands at 215 and 200 nm are due to "d" to "p" orbitals:
Figure 4. X-ray diffraction (1 11) platinum peak observed for acid catalysts.
catalysts only very small particles are observed, but in Pt-l.0ImgOH catalyst a high amount of large particles is found. pH 3 sol-gel catalysts and impregnated catalysts are similar as far as large particles are concemed but they differ in the number of small particles. UV-Via (Muse Refleeme) Spectroscopy. The Pt(I1) complexes are generally square-planar complexes and they are diamagnetic.19 Such a geometrical configuration can only be due to a ds, and it is stabilized through small volume ligands as in the case of trunr-[Pt(NH&Cl2]. These ligands form r bonds which may compensate the destabilization, four is preferred instead of six as coordination number. Chat et aI.*O studying the tranr-[Pt(NH&CI2] spectra, assuming a simple correlation between the PtC142-bands and the Pt trans isomer, assigned the d-d transitions. Extended Huckel molecular orbital calculations by Patterson et aI.l9 showed, in agreement with the results of Chat, that the D4h symmetry complex becomes D2h. As a reference pattem the UV-vis spectrum of the complex in solution was studied, a band at 3 15 nm appears which has been
If the spectrum of the complex is measured in solid state, it is slightly different, the d-p transitions are not observed. The bands present in the sol-gel prepared catalysts are shifted. In Figure 7,the spectra of the pH 3 synthesized solids (Figure 7a-c) are compared to the pH 9 preparations (Figure 7d,e). A small charge-transfer band appears between 220 and 230 nm, it is due to the metal-ligand [*(Cl)-d,~y' (Pt)] electron transition. The intense band at 310 nm has been attributed to a transition [lA~gTIB~,] from the amine ligands to the metal orbitals. The high-intensity band found at 330 nm which is not present in the spectrum of the complex is assigned to the transition from the "d" platinum orbitals to the r orbitals of the OH groups of the silica [d,(Pt)-II(OH)]. This band increases with metal concentration. Therefore, a strong metal support interaction through silanol groups is evident, the platinum incorporates OH groups to its coordination sphere, the metal symmetry is distorted, and the other bands are shifted. In the Pt-0.5-OH and Pt-1.0-OH the band at 270 nm corresponds to the transition [IAI,-~EJ.
Lopez et al.
1674 The Journal of Physical Chemistry, Vol. 97, No. 8, 1993
200
A0
360
so0
4;o
WAVELENGTH (nml
Figure 9. UV-vis (diffuse reflectance) spectra of (a) Pt-1 .O-ImpH and (b) Pt-1 .O-Imp-OH catalysts.
If the interaction is a strong bulk interaction the following reactions are proposed: 3bO
250
350
400
[=Si-O-Si=]-OH
+ trans-[Pt(NH,),Cl,]
WAVELENGTH
(nm ) Figure 7. UV (diffuse reflectance) spectra of (a) Pt-O.5-H, (b) Pt-l.0H, (c) Pt-2.0-H, (d) Pt-O.S-OH, and (e) Pt-1.0-OH catalysts.
PH3
-.
[~Cl,(OH),(SiO),l where x
+ y + z = 4 and
[=Ji-O-Si=]-OH
+ trans-[Pt(NH,),Cl,]
-
PH9
[~(NH3),(0H),(SiO),l
+
I
soo
roo B;O (nm 1 Figure 8. UV-vis (diffuse reflectance) spectra of (a) Pt-O.5-H, (b) Pt1.0-H, (c) Pt-2.0-H, (d) Pt-0.5-OH, and (e) Pt-1.0-OH catalysts. 400
WJ
WAVELENGTH
In the visible region (Figure 8) a small shoulder at 415 nm is clear due to a d-d transition [1Alg-3EI,]characteristic of the complex. A band appears at 710 nm which was not observed in the complex spectum, which is attributed to the interaction between silica and platinum through the OH groups of e i - O H species. The following reactions are proposed if the interaction is only a weak surface interaction: Si(OEt),
+H20
-
[=Si-O-Si=]-OH
[=SiUSi=]-OH
+ trans-[Pt(NH3),C12] [Si02]-O-
where x
+ y + z = 4.
+ EtOH
-
[Pt(NH3),C1,,(OH),]
where x y + z = 4. Duringgelation stepat pH 3 electrophiles(Si+OH2)are formed but at pH 9 nucleophiles (Si-0-) are present. In each case they incorporated to the platinum coordination sphere as shown in a previous work! The conventionally impregnated catalysts (Pt-1.0-ImpH and Pt-1 .O-ImgOH) show different spectra, Figure9. Theabsorption bandat 370nmand thesmallshoulderat 385 nmarenotobserved. No absorption is found in the visible region of the spectrum, the interaction with the support is not nearly as strong as in the sol-gel preparation. Infrared Spectroscopy. The infrared spectrum of the trans[Pt(NH3)2C12]complex shows the following bands: 506 (Pt-N assymetricstretching), 330 (assymetric stretching), and 365 cm-I (Pt-CI symmetric stretching). The Pt-N and Pt-Cl vibrations involve charges in the plane.21 The vibration spectra of the Pt/SiOz catalysts were studied in the 4000-350-~m-~region where the characteristic bands of the support (SiOz) are evident. In Figure 10 the dried and calcined Pt-0.5-H, Pt-1 .O-OH, and Pt-2.0-OH catalysts are compared. In the 400&2800-~m-~region there is a wide band centered at 3429 cm-1. This band has two shoulders, the first one at 3630 cm-1 and the other at 3300 cm-1 (Figure 11). These bands are assigned to the fundamental 0-H vibrations of the various hydroxyls present either as silanols (=&-OH), water (HOH) or residual ethanol (EtOH). The higher energy bandis attributed to terminal silanol groups which are not lost even with thermal treatment as shown by Figure 5: the shape of the band is the same for the dried and the calcined samples. These groups cause the short-range ordering during the postgelation step.22.23 Such a band can be explained as a lengthening Si-OH and Si0-H vibration, whose intensity depends on the thermal treatments. Most hydroxyl groups are partially reversible at 400 OC, this property produces a noticeablesupport networkcontraction. When the solids are treated thermally, the impurities occluded in the gel (water and ethanol, which are reaction products and which
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1675
Pt/SiO2 Sol-Gel Catalysts
'.->O
3iOQ
2600
2200
7
Id00
IO00
400
WAVENUMBER k " l I
Figure 10. RTIR spectra of (a) calcined Pt-0.5-H, (b) calcined Pt1.O-H, (c) calcined Pt-2.0-H, (d) dried Pt-1 .O-H, and (e) dried Pt-2.0-H catalysts.
I Boo
V 725
650
675
500
WAVENUMBER lcm9a
XQ
Figure 12. The 800-350-cm-' region FTIR spectra of (a) calcined Pt0.5-H, (b) calcined Pt-l .O-H, (c) calcined Pt-2.0-H, (d) dried Pt-l .O-H, and (e) dried Pt-2.0-H catalysts.
363 cm-I, characteristic of Pt-Cl stretching vibrations, are observed, they are more intense in pH 3 (HCI) prepared catalysts. At 385 cm-I an important peak due to a bending vibration Pt-O appears, and a small shoulder at 575 cm-1 is assigned to a Pt-OH bending vibration.21 These bands are not present in the complex. The presence of these two bands at 385 and 575 cm-I shows a metal-support interaction through silanols, due to the incorporation of the hydroxyl groups in the coordination sphere of the metal, as suggested previously:
-
Hl0
tranr-[Pt(NH,),Cl,]
[Pt(NH3),Cly(H20),] where x
+y + z = 4
-+OH
trans-[Pt(NH,),Cl,]
W 3800
3600
5400
3200
30QO
,
2800
WAVENUMBER Icm") Figure 11. The 4000-2800-cm-1 region FTIR spectra of (a) calcined Pt-0.5-H, (b) calcined Pt-1.0-H, (c) calcined Pt-2.0-H, (d) dried Pt1.0-H, and (e) dried Pt-2.0-H catalysts.
are included in the reaction quilibrium) are desorbed. At 1632 cm-1 (Figure 10) a band attributed to a OH bending vibration, scissor type, appears. The lowest intensity bands of the spectrum are found at 1160 and 1090cm-1. They areattributcd toanantisymmetricstretching vibration of (=Si-O-Si=) siloxane bonds. The band located at 956 cm-I is due to the weakly bonded silanol groups and is associated with the highenergy bands. It disappearswith thermal treatment. Two peaks at 801 and 463 cm-l have been assigned to symmetric and antisymmetric stretching vibration of Si-oSi bonds and to structural deformations of the material. In Figure 12 the 800-350-cm-1 zone of the catalysts prepared in acid medium shows the bands due to the metal. The Pt-N stretching vibration is found at 495 cm-I. Two bands at 377 and
[Pt(NH3),C1y(SiO),l where x + y
+z=4
Silica, as shown by the infrared study, is found to be highly hydroxylated even after calcination. If catalysts are prepared at pH 9, the spectrum is slightlymodified: ThePt-CI bond intensities diminish, Figure 7, and the shoulder at 503 cm-' is observed only in the Pt-2.MH catalyst. However, the Pt-N vibration at 495 cm-I is not found, probably because it is masked by the silica broad band at 471 cm-'. As shown by the Figure 13, thermal treatment does not change the spectra in this region. Note that in these pH 9 catalysts the silanol groups which present high energy bands at around 3660 cm-I are more stable and that the bands are more intense than the ones present in pH 3 catalysts, Figure 14. In the same way, two peaks appear at 2919 and 2891 cm-l due to 0-H group platinum interacting vibrations. In the impregnated catalysts, only the support bands are observed: 3420 (OH stretching), 1632 (OHbending), 1100and 1090 (Si-oSi stretching), 956 (Si-OH stretching), and 801
Lopez et al.
1676 The Journal of Physical Chemistry, Vol. 97, No. 8, 1993
U
Y z
f
(bl
z
z a a !-
*I
z
la )
wo
aw
.
sso
x)o
WAVENUMBER (cnT1
Figure 13. The 800-350-cm-I region FTIR spectra of (a) calcined Pt0.5-OH, (b) calcined Pt-1 .O-OH, (c) calcined Pt-2.0-OH, (d) dried Pt1 .O-OH, and (e) dried Pt-2.0-OH catalysts.
‘0
3;w
2800
Po0
le00
loo0
400 I.
WAVENUMBER (em’’ 1
Figure 15. FTIR spectra of impregnated catalysts: (a) dried Pt-l.0Imp-OH, (b) calcined Pt-1 .O-Imp-OH, and (c) calcined Pt-1.0-Imp-H catalysts.
First, metal reacts exchanging ligands and, hence, a color modification is observed through gelling:
/-
acid environment: HCI
+
Id)
’
(cl
-
HCI
~ ~ u ~ L s - [ R ( N H ~ ) , Ctr~t~-[Pt(NH&C13] ~~] rrunr- [RCl,] base environment: NHdOH
NHdOH
tram- [Pt(NH3),C1,] (bl
4600
xioo
sdoo
si00
si00
WAVENUMBER
(em”)
sdoo
r b
Figure 14. The 400&2800-~m-~region FTIR spectra of (a) calcined Pt-0.5-OH, (b) calcined Pt-1 .O-OH, (c) calcined Pt-2.0-OH, (d) dried Pt-1 .O-OH, and (e) dried Pt-2.0-OH catalysts.
and 460 cm-1 (Si-O-Si symmetric and antisymmetric stretching), as shown in Figure 15. The metal bands are not present in these infrared spectra. Hence, platinum-support interaction is not present. Conclwion
The results show that the rrans-[Pt(NH3)2C12] complex interacts with the silanol groups from the sol step of the process.
+
tram- [F’t (NHJ ,Cl]
+
tram- [R(NH,),]
The square-planar symmetry is not lost during the sol-gel process, but it is highly distorted if the complex interacts with mi-0- nucleophiles (pH 9) or electrophilcs =Si-O+-H2 (pH 3) present in the reaction medium, as shown by the band shifts in the infrared spectra. A small fraction of the initially square-planar compound in sol-gel catalysts is anchored on the silica surface through silanol groups which alters the complex symmetry. Most platinum atoms agglomerate to form FCC Pt crystallites as shown by X-ray diffraction. Pt-OH and Pt-O bonds are present in sol-gel prepared catalysts at pH 3 and 9. Hence an exchange between the complex ligands and the support hydroxyls is expected, and the OH groups should be incorporated into the coordination sphere of the metal. Such an interaction can be interpreted as a metal-support interaction unusual in conventionally prepared catalysts. In impregnated catalysts such an effect has not been observed. An effect of pH in sol-gel prepared catalysts on metal particle size has been observed. In pH 3 catalysts, as the platinum content is increased and the metal particles are larger. In pH 9 catalysts, the metal particle size is 15 A and does not depend on metal
Pt/SiO2 Sol-Gel Catalysts content. Impregnated catalysts acid or basic show a particle size distribution centered also at a diameter of 15 A. In the high metal concentration pH 3 catalysts the presence of large particles can be attributed to the formation of an inhomogeneous 'sol" in the first step of the reaction. Low-pH samples have large surface area values in both preparations sol-gel or impregnation. The use of a sol-gel preparation technique seems to be a valid option to alter the known chemistry in Pt/SiOz catalysts. Indeed by choosing the right precursor (square planar), an interaction not previously reported in these catalysts between the metal and the support has been obtained.
References and Notes (1) Carturan, G.; Cocco, G.;Schiffini, L.;Strukul, G.J. Carol. 1980,65, 359. (2) Ayen, R. J.; Jacobucci, P. A. Rev. Chem. Eng. 1988,5, 157. (3) Gesser, H. D.; Goswami, P. C. Chem. Rev. 1989, 89, 1254. (4) Lopez,T.; Bosch, P.; Asomoza, M.;Gomez,R. J. Catal. 1992,133, 247. ( 5 ) Lopez, T.; Garcia-Cruz, I.; Gomez, R. J. Catal. 1991, 127, 75. (6) Ravidra, S.;Wolf, E. E. J. Cmal. 1988, 110, 249. (7) Raydin, Y.; Stenin, L.; Boronin, A. I.; Bukhtiyarov, A.; Zaikovsky, V. Appl. Caial. 1989, 54, 277.
The Journal of Physical Chemistry, Vol. 97, No. 8, 1993 1677 (8) Brinker, C. J. J. Non-Crysi. Solids 1988. 100, 31. (9) Brinker, C. J.; Scherer,G. W.Sol-ge1Science;AcadcmicPrcss:New York, 1990. (10) Klein, L. C. Annu. Rev. Mater. Sci. 1985, 15, 227. (1 1) Pohl, E. R.;Osterholtz. F. D. Molecularcharacterizationofcompire inierfaces; Plenum: New York, 1979. (12) Timms, R. E. J. Chem. Soc. A 1971, 1969. (13) Iler, R. K. The colloid chemistry of silica and silicates; Cornell University Press: Ithaca, NY, 1955. (14) Brinker, C. J.; Keefer, D. W.; Schaefer, R. A,; Assink, R. A,; Kay, B. D.; Ashley, C. S.J. Non-Cryst. Solids 1984, 63. 45. (1 5 ) Klug, H.; Alexander, L.X-ray Dijjrociion Procedures; Wiley: New York, 1954. (16) Gregg, S.J.; Sing, K. S.W. Adsorption surface area andporosiiy; Academic Press: New York. 1982. (17) Dominguez. J. M.; Bosch, P.; Yacaman, M.J. Revisia del IMP 1983, IS, 66 (in Spanish). (18) Dartigues, J. M.;Chambellan, A.; Corolleur, S.; Gault, F. G.; Renouprez, A.; Morawcck, B.; Bosch, P.; Dalmai-Imelik, G.New J . Chem. 1979, 3, 591. (19) Patterson, H. H.; Tewksbuty, J. C.; Martin, M. Inorg. Chem. 1981, 20, 2297. (20) Chat, J.; Galmen, A.; Orgel, H.J. Chem. Soc. 1958, 486. (21) Morgan, G.L.;Rennick, R. D.; Soong, C. C. Inorg. Chem. 1966,5, 372. (22) Yoshino, H.; Kamiya, K.; Nasu, H. J. Non-Cryst. Solids 1990,126, 68. (23) Segal. D. L. J. Non-Cryst. Solids 1984, 63, 183.
