J. Phys. Chem. 1995,99,14437-14443
14437
Silica-Supported Zirconia. 1. Characterization by Infrared Spectroscopy, Temperature-Programmed Desorption, and X-ray Diffraction Z. Dang,P B. G. Anderson, Y. Amenomiya, and B. A. Morrow* Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada KIN 6N5 Received: May I, 1995;In Final Form: July 22, 1995@
Silica-supported zirconia samples have been prepared by aqueous impregnation of a nonporous fumed silica with zirconyl nitrate giving 5, 10, or 20 wt % ZrO2 on Si02 after calcining at 450 "C. The surface areas were comparable to that of the parent silica (200 m2/g), and IR spectra showed OH peaks characteristic of Si02 and of monoclinic ZrOz. The intensity of the SiOH peak decreased with Zr02 loading, and a new IR band near 945 cm-' was attributed to the formation of ZrOSi linkages. The ZrO2 is dispersed as clusters on SiOz, and free SiOH is still available even at the highest Zr02 loading. X-ray diffraction shows either that SiOd ZrO2 is amorphous or that the Z r 0 2 crystallites are too small to be detected. Infrared spectroscopy of adsorbed ammonia and pyridine have shown that sites for coordination exist on all mixed oxides and on pure Zr02. The number of sites for coordination of NH3 increases as the ZrOz loading increases. Moreover, there are very weak Bronsted acid sites on SiOdZrO2 which can protonate NH3; these are absent on ZrOz. Temperatureprogrammed desorption of adsorbed NH3 has shown that the number of sites available for coordination, protonation, or H-bonding to ZrOH is about 2.5-3-fold greater.than the number of sites for H-bonding to SiOH groups on pure Si02. Finally, IR of adsorbed C02 (a probe of surface basicity) has shown that Si02/ ZrO2 is less basic than of pure Zr02.
Introduction Zirconium dioxide has received considerable attention as a catalyst because it has moderate strength acid and base sites, as well as reducing and oxidizing properties.'-3 Therefore, it is suitable for a wide variety of catalytic and its surface properties have been c h a r a c t e r i ~ e d . ~ *Particular ~-~ attention has been directed to sulfated zirconia which is claimed to exhibit superacid properties which can make this material suitable for CH activation and alkane i s o m e r i ~ a t i o n . ' ~ - ~ ~ Zirconia is difficult to produce in a high surface area form,3 and it is relatively more expensive than the more common oxide materials such as alumina or silica. Accordingly, we have prepared some silica-supported zirconia catalysts with the view that (a) silica is a very thermally stable support and is readily available as a nonporous material of high surface area and (b) a binary Si02/ZrO2 oxide catalyst would be expected to be more acidic than either material alone according to theoretical considerations.I In this paper, we describe the surface characterization of Si021 Z r 0 2 catalysts which contain 5, 10, and 20 wt % zirconia which have been prepared by wet impregnation of zirconyl nitrate on a nonporous fumed silica. The catalysts have been characterized using infrared spectroscopy and X-ray diffraction, and the acid properties have been studied using IR spectroscopy of adsorbed ammonia and pyridine and temperature-programmed desorption of ammonia. In a subsequent paper,24the acid properties of the 10% zirconia catalyst after sulfation with varying quantities of sulfate will be described, and its potential as a catalyst for the direct coupling between olefins and methane will be assessed. Although the methods used in this paper have been previously used to study pure zirconia and sulfated zirconia, and several other groups have prepared ZrO~/Si02mixed o ~ i d e s , ~ ~this -~O s2
'
On leave from Lanzhou Institute of Chemical Physics, P.O. Box 97, Lanzhou, Gansu 73000, P. R. China. Abstract published in Advance ACS Absrracts, September 1, 1995. @
is the first characterization of supported ZrO2 on Si02 catalysts using IR spectroscopy of adsorbed base probe molecules. Miejers et al.25have studied some Si02/ZrO2 catalysts prepared by incipient wetness impregnation of zirconyl nitrate on silica and of SiOz/ZrOz prepared by the deposition of zirconium ethoxide on silica, but their study was mainly concemed with the precursor stages during the calcining steps leading to the final product. The same g r o ~ p has ~ ~ also , ~ ~studied the deposition of ZrO2 on thin films deposited in Si02/Si(100) surfaces. Sohn and JangI8 have also carried out a limited characterization of some SiO2/ZrO2 catalysts prepared by coprecipitation, and they studied its activity for 2-propanol dehydration and cumene dealkylation.
Experimental Section The silica support was Cab-0-Si1grade M-5 and was provided by the Cabot Corp. Zirconia-impregnatedsilicas were prepared by mixing 2 g of silica with 6 mL of an aqueous solution of zirconyl nitrate, followed by drying in air ovemight at 110 "C and then calcining in air for 24 h at 450 "C. The calcined samples were ground into a fine powder using a mortar and pestle and were stored for further use. The solution concentration was chosen so as to give 5, 10, or 20 wt % ZrO2 after calcining. Pure zirconia was provided by Degussa. For infrared investigation, about 30 mg of the pure or mixed oxide was compacted at about lo7 Pa in a 25 mm diameter stainless steel die, and the resulting disk was mounted in a previously described 300 mL volume quartz celL3' All samples were heated for 1 h under vacuum at 450 "C prior to use. IR spectra were recorded using a Bomem Michelson MBlOO instrument (DTGS detector) at a resolution of 4 cm-I. The BET surface areas of disks of SiOz,ZrO2, and the various Si02/ZrO2 samples after vacuum activation for 1 h at 450 "C are shown in Table 1. For temperature-programmed desorption (TPD)studies, sample disks of the type which were used for the IR studies were broken up and sieved; granules ranging from 16 to 32 mesh (0.5-1.0
0022-3654/95/2099-14437$09.00/0 0 1995 American Chemical Society
Dang et al.
14438 J. Phys. Chem., Vol. 99, No. 39, I995
TABLE 1: BET Surface Areas (m2/g) after Vacuum Activation at 450 "C sample BET area (m2/g) sample BET area (m2/g) Si02 205 SiO?/Zr0?-20% 189 Si02/ZrO2-5% 20 1 ZrO? 45 SiO2/ZrOZ-l0% 190 mm) were used. The sample holder consisted of a 3/g in. diameter stainless steel tube into which 100 mg of the sieved particles was placed on top of a glass wool support. The resulting bed was 1 cm deep. Samples were typically activated under vacuum at 450 "C for 1 h prior to cooling to room temperature under vacuum. In a typical experiment, the reactant was allowed to contact the evacuated sample at a given pressure for times from 5 to 60 min; then the excess gas was condensed into a liquid nitrogen trap. Helium was then passed through the catalyst bed (30 mL/ min) at room temperature for 30 min. In the TPD experiment, the same flow rate of He was used, and the temperature was increased linearly with time at a rate of 15 "C/min. Desorbing products were detected using a thermal conductivity detector. In some cases the products desorbed were trapped by condensation in a liquid N2 trap placed after the TCD detector. The quantity desorbed was measured from the pressure after the products were expanded into a known volume. In a second method for quantitatively measuring the desorption products, integration of the TPD curves was compared with those obtained when known quantities of effluent were passed through the system. In this way a calibration curve was developed. When both methods were used (integration and expansion), the agreement was within &lo%, which we will take as the accuracy of our quantitative results. In order to compare the rates at which products are removed in the IR and the TPD experiments, it is important to realize that the experimental arrangements are quite different. In the IR experiment, a small sample disk is contained in a large free volume (300 mL), and this is connected by a 20 cm length of 5 mm i.d. Pyrex tubing to a large volume vacuum manifold. Evacuation is fast and efficient. In the TPD apparatus, the reactor is connected by 2 m of '/g in. 0.d. (about '/I6 in. i.d.) stainless steel tubing having multiple bends and constrictions of less than '/I6 in. i.d. where Swagelok fittings are located. Vacuum conductance is about 100-fold lower than that of the IR cell. X-ray diffraction analysis was performed using a Philips Analytical Model PW3710 powder X-ray diffractometer using Cu K a radiation.
Results Basic IR Spectra of Si02 and SiOdZrOz. Infrared spectra of 450 "C vacuum-activated silica and zirconia in the OH stretching region (3900-3600 cm-') are shown in Figure 1, A and E, respectively. The silica spectrum is dominated by an intense sharp peak at 3747 cm-' due to isolated noninteracting surface silanol groups (single SiOH and geminal Si(OH)2) which have been characterized p r e v i o ~ s l y . ~In~the . ~ ~spectrum of Z r O 2 there is a broad peak near 3775 cm-' which is unique to the monoclinic phase and another broad band with maxima at 3680 and 3670 cm-I which are characteristic of the tetragonal or monoclinic phases, r e s p e c t i ~ e l y . ~ , ~For - ~
