Mechanically Alloyed Nickel-Zirconium as a Heterogeneous Catalyst and a Catalyst Precursor
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Mat. Res. Soc. Symp. Proc. Vol. 497 ©1998 Materials Research Society
metallic glasses still further [6,20,21]. Thus, more recent studies have focused on what the metallic glasses become during reaction conditions. This could be analogous to the "activation" treatments for conventional catalysts. The insightful experiments of the Japanese workers in 1981 [22] led the way to transforming the metallic glasses to active catalysts by a long exposure to reaction conditions. We report here a similar approach, where we have observed the activity of mechanically alloyed NiZr metallic glass as a catalyst for the decomposition of nitric oxide. EXPERIMENTAL PROCEDURE NiZr alloys were milled in a vibratory ball mill in lxl0"6 vacuum as described by Bakker et al. [22]. Surface area and chemisorption measurements were performed on the Quantasorb Chembet 3000. Surface area was measured by physisorption of nitrogen by the BET method. The number of surface metal atoms per gram was determined by selective chemisorption of carbon monoxide at room temperature after cleaning the surface in flowing hydrogen for 2 hours at 673 K. Decomposition kinetics of nitric oxide flowing at 5 cc/min. were measured in a flow reactor containing mechanically alloyed NiZr powder. The decomposition kinetics were studied under both continuous heating and isothermal conditions. For continuous heating experiments, the reactor was heated at a rate of 10 °C/min with a constant flow rate of 5 cc/min of NO in 30 cc/min of He. Isothermal experiments were performed at 673 K with a constant flow rate of 5 cc/min of NO in 18 cc/min of He. Reactants and products were measured in a Gow Mac gas chromatograph equipped with a thermal conductivity detector and a 6 ft x 1/8 inch Hayesep Db column. RESULTS and DISCUSSION Figure 1 shows SEM images of the as milled NiZr powder. The powder is amorphous, as indicated by XRD [17], and exhibits a surface area of 0.1 m2 /g. The surface gives the appearance of severe cold work, characteristic of the milling process, the mechanism for alloy formation at such low temperatures. A rather cracked, porous surface is consistent with the surface area's being several times higher than rapidly quenched metallic glasses. Figure 2 shows the results of NO conversion over NiZr mechanically alloyed powder for continuous heating at 10 °C/min at a constant flow rate of 5 cc/min of NO in 30 cc/min of He. The temperature for 50% conversion, the light off temperature shifts from 292 'C to 393 °C upon cooling and reheating the catalyst. Other examples of this same behavior are shown in Table 1 for Ni 50 Zr 50 and Ni4 5 Zr 55 alloys. It appears from the decreased activity of the catalysts, as indicated by the increased light offtemperatures, that the catalyst is changing and deactivating. The times above light off temperatures for these continuous heating runs were about an hour for each heating run. The initially glassy as-milled structure could be crystallizing during this period. Previous XRD results showed crystallization to a crystalline meta
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