J. Phys. Chem. 1995,99, 5556-5567
5556
Formation of the MoV1Surface Phase on MoOJZrOz Catalysts Federica Prinetto, Giuseppina Cerrato, Giovanna Ghiotti," and Anna Chiorino Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universitir di Torino, Via P. Giuria 7, I-10125 Torino, Italy
Maria Cristina Campa, Delia Gazzoli, and Valerio Indovina Dipartimento di Chimica, Universitir di Roma "La Sapienza", p.le A. Mor0 5, 00185 Roma, Italy Received: November 2, 1994; In Final Form: January 20, 1 9 9 9
The characterization (chemical analysis, XRD, TEM, HTREM, XPS, ESR, and IR) of MoOdZrO2 is reported. The samples were prepared by (i) equilibrium adsorption (ammonium heptamolybdate solution, AHM, at pH = 1, 2, or 8), (ii) dry impregnation, or (iii) mechanical mixing of Zr02 and MoO3. Samples were studied as prepared (a.p.), after evacuation at increasing temperature, or after heating in 0 2 at 773 K (s.o.). With dilute AHM (0.002 M), equilibrium adsorption led to a plateau of about 5 Mo atoms nm-2 (low loading). More concentrated solutions (0.05 M) yielded samples richer in Mo (high loading). No differences due to the preparation method were found at low loading. X P S showed MoV1only, uniformly spread on ZrO2. On evacuation of a.p. samples up to 423 K, surface molybdates lost water from their outer coordination sphere. At this stage, interaction with the surface was still weak. After evacuation at 573 K, Mo species anchored to the surface. On evacuation at 773 K, ESR and IR showed reduction, and the subsequent titration with 0 2 suggested the presence of Mo'" in addition to MoV. ESR, titration with 02, and IR showed that, in a subsequent treatment in vacuo at 773 K, species anchored to the surface by S.O. were not reduced, thus showing a strong anchorage of molybdates to the surface. The IR results for the low-loading samples showed low nuclearity molybdates (possibly mononuclear) and polymolybdates. As the Mo content increased, the low-nuclearity species concentration increased slightly, whereas the polymolybdate concentration increased markedly. Samples prepared by equilibrium adsorption or impregnation, but having a comparable Mo concentration, had the same (molybdates):(polymolybdates) ratio. For high-loading samples, after s.o., IR showed the presence of other bands probably arising from Moo3 and ZrMonOs. These two compounds were revealed by the XRD and HRTEM analysis.
Introduction Supported molybdenum species, mainly studied on Si02, Al203, and Ti02,'-4 show important catalytic properties. The genesis of Mo-based hydrodesulfurization catalysts has been reviewed by Knozinger,' and a model for the structure of the molybdenum species on A1203 has been developed by Weig01d.~ The dispersion, oxidation state, and structural features of the supported ions may strongly depend on the support. It is therefore of interest to study the nature of molybdenum species on other supports, in addition to those listed above, and, in particular, on Zr02. Zirconia is very stable to thermal treatments, has an amphoteric surface endowed with weakly acid and basic sites,6 and is able to maintain a high specific surface area up to about 1000 K, if transition metal ions are In this context, the convenience of using Zr02 as a support for chromia9has been shown in dehydrogenation For these reactions, the catalytic activity of CrOJZrO2 has been found to be superior to that of both CrO,/SiO;? and CrOJ y-A1203.I2 Supported molybdenum oxide catalysts are generally prepared by coprecipitation or wet impregnation. An altemative way to prepare MoOJZrOz catalysts is the equilibrium adsorption method. Wang and HalIl3 have suggested that the nuclearity of the adsorbed Mo species can be controlled by adjusting the pH of the ammonium heptamolybdate solution used for adsorp-
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Absrracts, March 15. 1995.
tion. For the various supports, the influence of the pH in determining both nuclearity and uptake of the Mo species has been discussed in detail in reviews by Knozinger' and Kim et a1.I4 Another preparation method is the mechanical mixing of Moo3 with the oxide support, showing spontaneous spreading of MOO, on y-Al203 above 670 K.' In previous papers we have reported the catalytic activity of MoO,/ZrO2 for the hydrogenation of propene and the concomitant metathe~is,'~ the characterization by XPS, and preliminary i-IR results.I6 In this paper, we describe the preparation and the characterization of MoOJZrO2 catalysts in their oxidized state. With the aim of assessing the influence of the preparation method on the supported molybdenum species, we used various preparation methods, as specified below. For characterization of samples, we used chemical, XRD, TEM, HRTEM, XPS, ESR, and FTIR techniques. In agreement with the view expressed by Che," we extended characterization of the catalyst from the very early stages of the preparation (sample as prepared, after thermal treatments at 383 K) to the final stage, when the Mo species anchor to the ZrO2 support. We did this in an attempt to understand the ion-support interaction and how it develops through the various stages of catalyst preparation. Although a subsequent paper will report on the surface species formed by H2 reduction of the oxidized Mo precursors, and their characterization by ESR and FTIR, using CO and NO as probe molecules, H2 reduced sample data, necessary to understand the results of present paper, will be mentioned here.
0022-3654/95/2099-5556$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 15, 1995 5557
MoV*Surface Phase on MoOJZrO2 Catalysts
TABLE 1: MoOJZrOz Samples and Their Molybdenum Surface Density surface area surface density sample (m2g-9 (Mo atoms nmW2) ZrO2(823) 40 Zr02(923) 25 ZMoO,40(923,a)pHl 1.20 ZMo0,69(923,a)pHl 21 2.08 ZMo2.09(923,a)pHl 24 6.45 0.74 ZMo0,47(823,a)pH2 ZMol.O2(823,a)pH2 1.63 3.20 ZMol.98(823,a)pH2 ZMo2,17(823,a)pH2 3.52 ZMo2.33(823,a)pH2 3.79 ZMo2,49(823,a)pH2 37 4.06 4.64 ZMo2.83(823,a)pH2 ZMo2.89(823,a)pH2 4.74 ZMo5.64(823,a)pH2 46 9.67 19.63 ZMo10.53(823,a)pH2 ZMol.l1(923,a)pH2 2.83 ZMol,36(923,a)pH2 23 3.49 1.29 ZMo0.8 1(823,a)pH8 1.84 ZMol. 15(823,a)pH8 ZMo0.4l(923,a)pH8 25 1.04 0.76 ZMo0.48(823,i) 1.53 ZMo0.96(823,i) 2.82 ZMol.75(823,i) 3.15 ZMol.95(823,i) 4.69 ZMo2.86(823,i) 5.2 ZMo2.0(923,m) 2.4 ZMo1.4(823,m) 10.4 ZMo6.0(823,m) Experimental Section Sample Preparation. The zirconia support was prepared by hydrolysis of zirconium oxychloride with ammonia, as already de~cribed.~ Before its use as support, the material was calcined in air generally at 823 K or, in some cases, 923 K. The MOO,/ ZrO2 samples were prepared by (i) equlibrium adsorption, (ii) dry impregnation, or (iii) mechanical mixing of ZrOz with Moos. For equilibrium adsorption, the ZrOz support was shaken for 72 h at room temperature (RT) with a solution of ammonium heptamolybdate (AHM, Carlo Erba, R.P.) at pH = 2. A few samples were also prepared at pH = 1 or 8. The pH values were fixed by nitric acid or ammonia. For impregnation, the AHM solution was prepared at pH = 2. The catalysts were then dried at 383 K for 24 h, ground into fine powder, placed in polyethylene sample holders, and left in the ambient atmosphere. For mechanical mixing pure ZrO2 and Moo3 were ground together in a agate mortar for 20 min. The catalysts were studied as prepared (a.p.), after heating in dry 0 2 at 773 K (s.o.), or by other thermal treatments in vacuum or in a controlled atmosphere, as specified. The MoOJZrOz catalysts are designated as ZMox(T,a), ZMox(T,i), or ZMox(T,m), where x gives the analytical Mo content (weight percent) and T (K) the calcination temperature of the ZrO2 used as support. The symbols a, i, and m specify the preparation method: equilibrium adsorption, impregnation, and mechanical mixing, respectively. For samples prepared by equilibrium adsorption, the pH of the AHM solution is also specified as ZMox(T,a)pH 1,2, or 8. The Moo3 (purity 99.99%) was from Aldrich. The Mo content was determined by atomic absorption after dissolution of a sample in concentrated (40%) HF solution and subsequent dilution. High-purity 0 2 and HI (Matheson C.P.) were used without further purification. BET surface areas measurements were carried out using N2 as adsorbate at 78 K, on both pure ZrO2 and MoO.JZrO2 catalysts. Table 1 lists the samples investigated, their surface area, and Mo surface density (calculated assuming that all Mo is dispersed on the surface).
Procedures and Characterization Techniques. TEM and HRTEM. Transmission electron microscopies of a.p. and S.O. samples were taken with a JEOL 2000 EX electron microscope equipped with a top entry stage, working at 200 kV. The powder was ultrasonically dispersed in 2-propanol, and the suspension was deposited on a copper grid coated with a holey carbon film. The instrumental magnification (3 x lo4 to lo6) is specified in the captions to Figures 2-4. XRD Measurements. The X-ray diffraction analysis of a.p. and S . O . samples was carried out by a Philips PW 1710 diffractometer at 45 kV and 20 mA using the Cu K a radiation. XPS Measurements. X-ray photoelectron spectra were obtained with a Leybold-Heraeus LHS 10 spectrometer (A1 Ka, 1486.6 eV, 12 kV-30 mA) operating in the FAT mode (50 eV pass energy) and interfaced with a Hewlett-Packard 2113B computer. The samples were pressed onto a gold decorated tantalum plate attached to the sample rod. The analysis chamber was evacuated to
