R.K. Grasselli and A.W. Sleight (Editors),Structure-Actiuity and Selectiuity Relationshlps in Heterogeneous Catalysis 0 1991 Elsevier Science Publishers B.V., Amsterdam
SUPPORTED METAL CATALYSTS PREPARED FROM AMORPHOUS METAL
A. Baikerl, J. De Pietrol, M. Maciejewskil and B. Wdz2 1Department of Industrial and Engineering Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland 2University of Basel, Institute of Physics, CH-4056 Basel, Switzerland
SUMMARY NifZI-02 and Pd/ZrO2 catalysts have been prepared by controlled oxidation of amorphous Ni64Z1-36 and Pd33Zr67 metal alloys in oxygen containing atmospheres. The oxidation which largely influences the morphological, structural and chemical properties of as-prepared catalysts has been studied using thermoanalytical methods (TG,DTA), XRD, XPS, gas adsorption and electron microscopy. The catalysts derived from the metallic glasses exhibit some unique structural and chemical properties which are discussed. Their potential for the liquid phase hydrogenation of organic compounds is illustrated using the hydrogenation of trans-Bhexene-1-a1 as a n example. Hydrogenation over Ni/ZrO2 yielded hexane-l01, whereas over PdZI-02 hexane-1-a1 could be produced selectively.
INTRODUCTION Amorphous metal alloys have gained interest in catalysis research due to their potential as model catalysts and as catalyst precursors. Progress in this field has been discussed in two recent reviews [1,21. Here we report the preparation of zirconia supported nickel and palladium catalysts from corresponding metallic glass precursors. The major aim was to learn more about the chemical and structural changes the metallic glass precursors undergo during their transformation to the active catalysts and about the suitability of as-prepared catalysts for liquid phase hydrogenations of organic compounds. E X P m r M E N T A L The metallic glass precursors, Ni64Zr36 and Pd33zi-67,were prepared from the pure metals using the technique of melt spinning. Before use a s precursor materials the ribbons were ground in liquid nitrogen to flakes of about 0.5 - lmm size. Catalysts were prepared by oxidizing the precursor materials in a n oxygen containing atmosphere under appropriate conditions and subsequent reduction in hydrogen a t 600 K. The structural and chemical changes the metallic glass precursors underwent during their transformation to the active catalysts have been studied using powder X-ray diffraction (XRD), thermal analysis (TG,DTA), X-ray photoelectron spectroscopy (XPS),gas adsorption and scanning and transmission electron microscopy. Catalytic tests were performed in a 500 ml
autoclave under constant hydrogen pressure using an agitator speed of 1500 rpm. Products were analyzed by gas chromatography using a HP 5890A GC equipped with a HP-FFAP capillary column (30m x 0.53mm x lpm).
REsuLTsANDDISC~ION Nickel on zirconia from Ni64Zr36 Nickel on zirconia was prepared by controlled oxidation of the amorphous Ni-Zr alloy in air and subsequent reduction. The oxidation in air in the temperature range 570-750K resulted in solids containing ZrO2 and metallic nickel besides unreacted amorphous metal alloy. Significant Oxidation of Ni t o NiO was only observed aRer almost complete oxidation of the zirconium in the alloy. Figure 1 depicts the XRD patterns of the amorphous Ni64Zr36 alloy corresponding to different degrees of oxidation (a)of the amorphous metal alloy. a was measured gravimetrically and denotes the fraction oxygen consumed divided by the amount of oxygen required to convert Zr to ZrO2. The XRD patterns indicate the built-up of small crystalline particles of tetragonal and monoclinic ZrO2 and metallic nickel upon oxidation.
Fig.1 XRD patterns of amorphous Ni6G1-36alloys of different degree of oxidation a A - as quenched alloy. Reflections of Ni are shaded, arrows indicate positions of main reflections of tetragonal (T)and monoclinic (M) Z1-02.
The crystallization behavior of as-prepared samples investigated by DSC measurements under an inert gas atmosphere is shown in Fig. 2. Note that the temperature range of crystallization did not depend significantly on the degree of oxidation of the alloy. This behavior was hrther supported by the observation that the specific heat of crystallization referred to the unreacted core of the alloy was constant ca. 40 J/g, regardless of the degree of oxidation of the alloy sample (3). Thus the presence of zirconia in the oxidized alloy did virtually not influence the crystallization behavior of the unreacted part of the alloy.
Fig. 2 Crystallization behavior of amorphous and partially oxidized Ni64Z1-36alloy investigated by DSC measurements. (A) corresponds to as-quenched amorphous alloy, the degree of oxidation a is indicated on curves. Heating rate 5Wmin. The chemical and structural changes of the bulk were accompanied by similar drastic changes in the textural properties of the alloy. The BET surface area of the precursor material (0.02 m2/g) increased to 10 - 25 m2/g depending on the oxidation conditions used. The surface oxidation behavior of the amorphous precursor alloy was investigated by means of XPS and UPS (4). Oxygen doses up to 2000L were used to study the initial stages of the oxidation of the clean surfaces in the temperature range from room temperature to 570K.
Fig. 3 compares the XPS Zr 3d spectra of the fresh amorphous Ni64zr36 alloy, the sample after exposure to 80 L 02, and a Zr02 reference sample. The Zr 3d levels of the alloy cleaned by argon ion bombardment are located at Eb = 179.4 eV. The shift compared to clean metallic Zr (Eb= 179.0 eV) is due to alloying (5). After exposure to 80 L an additional doublet can be seen which is attributed to Zr in an oxidized state shifted by 3.1 eV with respect to pure Zr. The different shifts in the Z r 3d core levels of the ZrO2 reference sample and the sample obtained by exposure to 80 L 0 2 indicate a different stoichiometry of these zirconium oxides. Comparison of the observed shifts with literature data indicated that the zirconia formed upon oxygen exposure was deficient in oxygen Z1-01<~<2. It appears that oxygen deficient zirconia is formed predominantly in the initial stage of the oxidation, i.e. when metallic zirconium is still present in the sample. With higher degree of oxidation (i.e. when the bulk of the alloy is oxidized) stoichiometric zirconia (Zr02) becomes prevalent, as the XRD patterns (Fig. 1) indicate.
BINDING E N E R G Y
Fig. 3 XPS core level spectra of zr 3d levels of Ni64zr36 after exposure to 80 L 0 2 at 420K.Reference spectra of ZrO2 (obtained after exposure of Zr to 1000 L 0 2 ) and clean metallic alloy (0 L 0 2 ) are shown for comparison. Zr 3d5/2 core level positions of Zr and ZrO2 are indicated by vertical lines.
Palladium on zirconia from Pd33zr67 Figure 4 shows the XRD patterns of the amorphous Pd33Zr67 alloy after oxidation in air a t 590K for different times. In contrast to the behavior observed with the Ni-Zr alloy significant oxidation of the group VIII transition metal occurs already at relatively low degree of oxidation a. Note that the degree of oxidation a was defined here as the fraction oxygen consumed divided by the amount of oxygen required to convert the alloy to PdO and ZrO2. The bulk concentration of metallic Pd first increases and then decreases with increasing degree of oxidation a. The fully oxidized sample contained ZrO2, PdO and a little Pd. Although monoclinic and tetragonal ZrO2 exist i n the oxidized samples, the monoclinic phase is dominant independent of the degree of oxidation.
Fig. 4 XRD patterns of amorphous Pd33zx-67 after different degree of oxidation a. Oxidation carried out in air a t 590K.A - as-quenched alloy; reflections of Pd are shaded, asterisks indicate reflections due to PdO, arrows indicate positions of main reflections of monoclinic (M) and tetragonal (T) ZrO2.
The crystallization behavior of the oxidized samples (Fig. 4) investigated using DSC (Fig. 5.)
Fig. 5 Crystallization behavior of the Pd33Zr67 alloy after oxidation in air a t 590K investigated by DSC under inert gas atmosphere. The degree of oxidation a is marked on the curves. Trace (A) corresponds to as-quenched amorphous alloy. Inset in upper right corner represents overview of DSC curve for a = 0.3 and illustrates the occurrence of the solid state reduction 2 PdO + Zr + 2 Pd + 21-02 (broad signal at 500-700K)previous to crystallization of the unreaded alloy a t higher temperature. Heating rate 5Wmin. The DSC curves shown in Fig. 5 indicate that the as-quenched sample starts to crystallize around 700K under the conditions used. Most interesting is that the partially oxidized samples show an additional exothermal process at significantly lower temperature. Simultaneous TG measurements revealed that during both thermal events the sample weight did not change. The exothermal process occurring a t lower temperature is attributed to the solid
state reduction of PdO by metallic Zr: 2 PdO + Zr + 2 Pd + ZrO2 which occurred in the partially oxidized samples. For the sample with a = 0.3 this reaction was not complete before the crystallization of the unreacted amorphous alloy occurred. The superposition of the two thermal events is even more pronounced for the sample with a = 0.5. Note that the larger the degree of oxidation (a)of the sample was, the higher was the temperature of the reduction of PdO by metallic zirconium. As a result of the drastic chemical and structural changes of the bulk material the BET surface area of the amorphous precursor increased from 0.025 to about 60 m2Ig depending on the oxidation conditions used. Pore size distribution measurements using nitrogen capillary condensation indicated that the material contained mainly pores of 2 - 4 n m size besides some larger pores. The morphological changes are illustrated by the scanning electron micrograph presented in Fig. 6. The initially flat surface of the precursor alloy changed to a rough surface which was built up of small agglomerates containing zirconia and palladium as evidenced by electron dispersive X-ray analysis. High resolution electron microscopy as well as electron diffraction showed that the agglomerates were made up of intimately mixed intergrown small crystallites of zirconia and palladium. Due to this particular structural property as-prepared catalysts exhibit an extremely large interfacial area between the active metal species and the oxidic support material, as has been demonstrated in detail elsewhere (6). Similar large interfacial areas between the metal and the oxidic support are generally not observed in conventionally prepared supported metal catalysts. Figure 7 depicts the XPS core level spectra of Zr 3d levels of Pd33Zr67 measured after different exposures of the alloy to oxygen at 420K. Similar characteristics were observed as with the Ni-Zr alloy. With increasing oxygen exposure the minimum between the two Zr 3d peaks decreases and a shoulder grows at the higher binding energy side of the Zr 3d3/2 peak indicating the formation of zirconium oxide. Again the zirconium oxide formed is non-stoichiometric, i.e. deficient in oxygen, as emerges from the observed shifts of the Zr 3d peaks (compare with reference in Fig. 3). The XRD and XPS investigations indicate that oxygen deficient ZrO2 is prevalent in the initial stage of the oxidation (surface and subsurface region), whereas in completely oxidized samples (bulk oxidation) stoichiometric ZrO2 prevails. This behavior was found t o be characteristic for Ni64Zr36 as well as Pd33Zr67 alloys.
Fig. 6 Scanning electron micrographs illustrating the morphology of the amorphous Pd33Zr67 alloy after oxidation in oxygen atmosphere (0.9 bar 0 2 ) during 5 hours. White bare on left bottom side corresponds to 1 pm.
Binding Energy teV1
Fig. 7 XPS core level spectra of Zr 3d levels of Pd33Z1-67after different exposure to oxygen at 420K (4).
Catalvtic DroDerb‘es of catalvsta Figure 8 compares the catalytic properties of the metalfzirconia catalysts derived from the amorphous Ni-Zr and Pd-Zr alloys for the liquid phase hydrogenation of trans-2-hexene-1-al. Note that over NilZrO2 the hydrogenation occurred in two consecutive steps, finally producing hexane-1-01 (C). In contrast, over Pd/ZrO;! the consecutive hydrogenation step is not occurring and hexane-1-a1(B) is formed with high selectivity.
TIME (min) Fig. 8 Hydrogenation of trans-2-hexene-1-al. Composition of reactant mixture versus time. Conditions: NilZrOp (45g/l), 400 K, 10 bar Hz; Pd/ZrO2 (6OgA). 370 K, H2 1.2 bar.
CONCLUSIONS Ni/ZrOz and PdZrO2 catalysts with interesting chemical and structural properties were prepared from corresponding metal-zirconium alloys by controlled oxidation in an oxygen containing atmosphere and subsequent reduction in hydrogen. The interplay between the simultaneously occurring oxidation and crystallization processes during the oxidation of the precursor alloy was found to be crucial for the development of the structural and chemical properties of the final catalysts. Ni-Zr and Pd-Zr alloys exhibit significantly different oxidation behavior. With Ni-Zr alloys the zirconium is almost selectively oxidized and a solid containing metallic nickel and zirconia is formed. The oxidation of nickel is only
observed after depletion of the metallic zirconium in the alloy or when the oxidation is carried out at high temperatures. In contrast, with Pd-Zr alloys both components are oxidized simultaneously and the resulting solid contains PdO, Pd and ZrO2. In the presence of metallic zirconium (i.e. with partially oxidized samples) the solid state reduction 2 PdO + Zr + Pd + ZrOz was found to occur when samples were heated in a n inert gas atmosphere. This phenomenon indicates that the metallic zirconium present in partially oxidized alloys can act as oxygen scavenger which suppresses deactivation of the active metal species by oxygen contamination. The active catalysts are porous and exhibit BET surface areas in the range of 1060 m2lg depending on the precursor alloy and the oxidation conditions used. A characteristic structural feature of as-prepared catalysts is that the active metal particles are intimately associated with the zirconia phase resulting in unusually large interfacial areas between the metal and the oxidic phases. This characteristic structural property is likely to be of importance for all phenomena where the interfacial area plays a role such a s metal-support interaction, and adsorption and spillover of hydrogen. Another peculiarity of the catalysts derived from the metallic glasses is the fact that ZrO2 is present in monoclinic and tetragonal form. Furthermore, the XPS investigations indicated that part of the zirconia (surface and subsurface region) exists as non-stoichiometric, i.e. oxygen deficient ZrOi