In the Laboratory
Simple Synthesis and Characterization of Several Nickel Catalyst Precursors
W
Yolanda Cesteros* and Pilar Salagre Facultat de Química, Universitat Rovira i Virgili, Tarragona 43005, Spain; *
[email protected] Francisco Medina and Jesús E. Sueiras Escola Tècnica Superior d’Enginyeira, Universitat Rovira i Virgili, Tarragona 43005, Spain
Bulk and supported nickel catalysts are widely used in heterogeneous catalysis in such reactions as hydrogenation, oxidation, and steam reforming (1–3). Nevertheless, students in laboratory courses are seldom introduced to the synthesis of these catalysts, even though the reactants required are not expensive and the laboratory material is very simple. It is interesting that students in advanced inorganic chemistry courses learn to prepare and characterize several nickel catalyst precursors, which directly or after reduction can lead to active nickel catalysts. We found almost no references to the preparation of nickel catalysts or nickel catalyst precursors in this Journal (4), but there are reports on the preparation of nickel compounds (5, 6 ). The one-week experimental series described here is a simple means of explaining differences in the preparation procedures and properties of bulk and supported catalyst precursors. Our aims are the following: (i) to use the impregnation to incipient wetness method followed by calcination as a simple procedure for preparing supported nickel catalyst precursors and the calcination of nickel nitrate to obtain a nonstoichiometric NiO bulk catalyst precursor; (ii) to compare the reducibility of bulk and supported nickel catalyst precursors by using the temperature-programmed reduction technique (TPR); (iii) to study how the support affects the particle size (visualized by scanning-electron microscopy) and the crystallinity (estimated by X-ray diffraction) of the supported catalyst precursors. This will affect the interaction between the NiO and the support and, consequently, their reducibility (for this study, a commercial γ-Al2O3 and a synthesized NiAl2O4 spinel are proposed as supports); and (iv) to introduce students to some solid-state characterization techniques: X-ray diffraction (XRD), temperature-programmed reduction (TPR), and scanning electron microscopy (SEM). These experiments can also be adopted if only one or two of the characterization techniques described here are available. It will still be a useful exercise. Experimental Procedure
Preparation of the Supports Two different supports were used. One is a commercial γ-Al2O3. The other is a NiAl2O4 spinel, which is prepared as follows. A nickel aluminate precursor is synthesized by coprecipitating, at room temperature, a stoichiometric mixture (Ni:Al = 1:2) of 100 mL of nickel nitrate and aluminium nitrate solution (where [Ni2+]= 0.17 M and [Al3+] = 0.34 M) using the ammonium hydroxide method (7 ). The base (0.6 M NH3 solution) is added dropwise with constant stirring until
pH 8 (pH meter). The precipitate is filtered, washed with deionized water, and dried overnight in an oven at 120 °C. The NiAl2O4 is obtained by calcining the dried sample at 500 °C for 5 hours.
Impregnation of the Supports Incipient-wetness impregnation is a common procedure for introducing the active phase in the support. Several nickel nitrate hexahydrate solutions are prepared on the basis of the pore volume of each support so that the final composition is 0.27 g of NiO per gram of support. The pore volume of each support is found by measuring the volume of deionized water required to impregnate a known amount of sample until wetness. In our experiment, the pore volume of the γ-Al2O3 and NiAl2O4 samples has been 0.9 mL/g and 0.8 mL/g, respectively. The corresponding nickel nitrate hexahydrate solution is added dropwise to the support and the sample is homogenized with a spatula. Ni(NO3)2–γ-Al2O3 and Ni(NO3)2–NiAl2O4 samples are dried in an oven at 120 °C overnight. Preparation of the Nonstoichiometric NiO Catalyst Precursors Calcination of 1 g of nickel nitrate hexahydrate, 1 g of Ni(NO3)2–γ-Al2O3, and 1 g of Ni(NO3)2–NiAl2O4 in a furnace at 350 °C for 30 min leads to the corresponding catalyst precursors, NiO, NiO–γ-Al2O3, and NiO–NiAl2O4, which are referred to as A, B, and C, respectively. Hazards Nickel salts and their solutions will irritate eyes and some people’s skin, if they come into contact. It should be assumed that these substances are poisonous if ingested and their solutions irritate the eyes and the respiratory tract. Repeated or prolonged contact may cause sensitization of the skin. Repeated or prolonged inhalation may cause asthma. These substances have been investigated for their possible carcinogenic effect on humans (8, 9). Aluminum nitrate and γ-Al2O3 can irritate the eyes and the skin slightly. Ammonia is corrosive and can cause serious skin burns, blisters, and blurred vision; the vapors should not be inhaled. When ingested, ammonia can cause abdominal cramps, sore throat, and vomiting. The NO2 gas formed during the calcination of the catalyst precursors is very corrosive and should be avoided, since it can irritate the eyes, nose, and throat and decrease pulmonary function. When these samples are being prepared, it is important to use protective gloves, protective clothing, safety goggles and breathing protection. You should also work in a hood.
JChemEd.chem.wisc.edu • Vol. 79 No. 4 April 2002 • Journal of Chemical Education
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In the Laboratory
Figure 1. Powder X-ray diffraction patterns of the catalytic precursors. (a) NiO; (b) NiO–γ-Al2O3; (c) NiO–NiAl2O4. (*) NiO phase; ( 䊉 ) γ-Al2O3 phase; ( 䊏 ) NiAl2O4 phase.
Discussion The samples are characterized by XRD, TPR, and SEM. Powder X-ray diffraction patterns of the samples were obtained with a Siemens D5000 diffractometer using nickel-filtered CuKα radiation. The crystalline phases were identified using files from the Joint Committee on Powder Diffraction Standards (JCPDS). The supports were identified as γ-Al2O3 (10) and NiAl2O4 (11). Figure 1 shows the powder diffraction patterns obtained for the three catalyst precursors. The peaks in sample A are characteristic of the NiO phase (12), whereas samples B and C have two phases: NiO and γ-Al2O3 phases for sample B and NiO and NiAl2O4 phases for sample C, as expected. During calcination, nickel nitrate decomposes to NiO. The reaction is 350 °C
Ni(NO3)2⭈6H2O → NiO + 2NO2 + O2 + 6H2O The varying sizes of the NiO particles lead to several differences in the crystallinity of the samples. The NiO of the bulk sample (A) is more crystalline than the NiO of supported samples (B and C). This is because the support has a dispersant role and therefore the NiO particles of the supported samples are smaller. The NiO is less crystalline in sample B than in sample C. These results suggest that the sizes of the NiO particles are ordered in the following way: A > C > B. The NiO–support interaction is also related to the size of the NiO particles in the supported samples. This interaction can be estimated by TPR. Temperature-programmed reduction studies were performed in a Perkin-Elmer TGA 7 microbalance. Each sample (30 mg) was heated in a 5 vol % H2–Ar flow (80 cm3/min) from 120 to 900 °C at 5 °C/min. These TPR studies were carried out to find differences in the reducibility of the three catalyst precursors. Reaction 1 takes place in all three samples, and in sample C reaction 2 takes place as well. NiO + H2 → Ni + H2O
(1)
NiAl2O4 + H2 → Ni + Al2O3 + H2O
(2)
Ignoring the initial drift due to losses of constitutional water, we can see from the TPR profiles (Fig. 2) that the NiO sample loses 22.41% of weight through a single acceleratory rate step occurring at 344–406 °C. The theoretical weight loss 490
Figure 2. TPR plots of the three catalytic precursors.
due to the total reduction of stoichiometric NiO to Ni is 21.41%. This 1% of difference is due to the nonstoichiometry of the prepared NiO. Sample B (NiO–γ-Al2O3), however, shows two weight losses at 195–300 °C and 350–900 °C. They are smaller than those observed for sample A. This suggests that there are two reduction steps caused by a different NiO–γ-Al2O3 interaction. First, the NiO that interacts least with the support is reduced (first reduction step) and then the NiO that interacts more strongly with the support is reduced at higher temperatures (second reduction step). The total weight loss is much higher (12.3%) than the reduction of the NiO to Ni (4.5%) leads us to expect. This is because the γ-alumina loses a considerable amount of water, mainly above 350 °C. Lastly, the NiO–NiAl2O4 sample has three weight losses at 190–325 °C, 340–430 °C and 445–900 °C. The first two steps correspond to the reduction of the NiO that has a different degree of interaction with the spinel (similar to the two steps observed for sample NiO–γ-Al2O3). These two reduction steps almost reached the theoretical weight loss of 4.5%. The third weight loss (≅10%) is due to the reduction of the spinel and matches the theoretical value fairly well. The initial reduction temperature decreases in the order sample A (≈344 °C) > sample B (≈195 °C) ≅ sample C (≈190 °C). This is due to two different contributions: (i) the small NiO particles favor its reduction (this explains the higher initial reduction temperature found for sample A and (ii) high NiO– support interaction inhibits NiO reduction. This interaction is higher when the NiO particles are smaller. This explains why the initial reduction temperature of sample B, whose NiO particles are smaller than those of sample C (deduced by XRD), is similar to that of sample C. SEM is used to confirm the differences in the particle sizes mentioned above and to observe their morphology. Scanning electron micrographs were obtained with a JEOL JSM-35C scanning microscope operating at an accelerating voltage of 35 kV, a work distance of 16 mm, and magnification in the range 40,000–50,000×. Figure 3 shows the scanning electron micrographs of the three samples. In contrast to the welldefined octahedral particles of around 300 nm observed for the bulk NiO compound (Fig. 3, left), the NiO–γ-alumina precursor has an amorphous surface (Fig. 3, center) and the
Journal of Chemical Education • Vol. 79 No. 4 April 2002 • JChemEd.chem.wisc.edu
In the Laboratory
Figure 3. Scanning electron micrograph taken from the surface of the three catalytic precursors. Left: NiO; center: NiO–γ-Al2O 3; right: NiO–NiAl2O 4.
NiO–NiAl2O4 sample (Fig. 3, right) shows octahedral NiO particles that are smaller (
