Alloy composition dependence of formation of porous Ni prepared by rapid solidification and chemical dealloying

doi:10.1016/j.jallcom.2008.04.017

Journal of Alloys and Compounds

Volume 472, Issues 1–2, 20 March 2009, Pages 71-78

https://doi.org/10.1016/j.jallcom.2008.04.017 Get rights and content

Abstract

In this paper, the effect of alloy composition on the formation of porous Ni catalysts prepared by chemical dealloying of rapidly solidified Al–Ni alloys has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray (EDX) analysis and N₂ adsorption experiments. The experimental results show that rapid solidification and alloy composition have a significant effect on the phase constituent and microstructure of Al–Ni alloys. The melt spun Al–20 at.% Ni alloy consists of α-Al, NiAl₃ and Ni₂Al₃, while the melt spun Al–25 and 31.5 at.% Ni alloys comprise NiAl₃ and Ni₂Al₃. Moreover, the formation and microstructure of the porous Ni catalysts are dependent upon the composition of the melt spun Al–Ni alloys. The morphology and size of Ni particles in the Ni catalysts inherit from those of grains in the melt spun Al–Ni alloys. Rapid solidification can extend the alloy composition of Al–Ni alloys suitable for preparation of the Ni catalysts, and obviously accelerate the dealloying process of the Al–Ni alloys.

Introduction

It has been known that nanoporous metals can be self-assembled by dealloying or selective dissolution of a less noble component from a binary alloy [1], [2], [3]. Besides touching on fundamental issues of alloy thermodynamics and corrosion science [4], [5] and providing a means for obtaining high surface area granular metal catalysts such as Raney nickel [6], the process offers the potential to produce monolithic metal bodies with a nanoscale pore structure. Raney Ni with a sponge-like structure is a well-known catalyst used in several hydrogenation processes [7].

Rapid solidification (RS) was first introduced by Duwez et al. [8], and melt spinning is the most commonly used technique to achieve rapid solidification and to produce long and continuous ribbons [9]. Rapid solidification processing involves exceptionally high cooling rates (10⁴ to 10⁸ K s⁻¹) during solidification from the molten state [10]. The levels of undercooling achievable at such high cooling rates lead to significant and often potentially beneficial modifications to rapidly solidified microstructures compared with those produced under conventional conditions. These include the refinement of the as-solidified microstructure, the refinement of the scale of segregation, the increase in the equilibrium solid solubilities of solute elements and the formation of novel metastable crystalline and amorphous phases [11]. Therefore, rapid solidification has had a substantial impact on our fundamental understanding of materials synthesis by solidification, as well as on our ability to develop materials for technological applications.

Raney Ni catalyst precursors, with the Al–50 wt.% Ni alloy as a representative, are conventionally solidified naturally. Al, in the form of intermetallic compounds or eutectic, is leached by alkali, and then porous catalysts with surface areas as high as ∼100 m² g⁻¹ are obtained [12]. Incorporation of the rapid solidification technique to the preparation of the Raney-type alloys is expected to retain the merits of the rapidly quenched alloys, while circumventing the drawback of low surface area [13]. Recently, attention has been paid to novel skeletal Ni catalysts (designated as RQ Ni) prepared by alkali dealloying of rapidly quenched Al–Ni alloys [13], [14], [15], [16]. Lei et al. [14] reported that melt quenching and pretreatment with hydrogen can effectively enhance the catalytic activity of Raney Ni. The RQ Ni catalyst exhibited higher selectivity in hydrogenation of 2-ethylanthraquinone to “active quinones” than that of the Raney Ni catalyst prepared from a conventional alloy [15]. More notably, the composition, texture, structure, and consequently activity and selectivity of the RQ Ni catalyst deviate markedly from the conventional Raney Ni [13], [16].

To date, however, less attention has been paid to the influence of alloy composition on the microstructure of rapidly solidified Al–Ni alloys and on the formation of porous Ni catalysts by dealloying. The present work aims to investigate the alloy composition dependence of formation of porous Ni prepared by rapid solidification and chemical dealloying.

Section snippets

Experimental

In this work, the Al–Ni alloys with nominal compositions of Al–20, 25 and 31.5 at.% Ni were prepared from pure Al (99.9 wt.%) and pure Ni (99.9 wt.%). The charges were melted in a quartz crucible by high frequency induction heating and were cast into ingots in an iron chill mould. The ingots obtained are rod-like with a diameter of 10 mm. Using a single roller melt spinning apparatus, the pre-alloyed Al–Ni ingots were remelted in a quartz tube by high-frequency induction heating and then rapidly

Results and discussion

Fig. 1 shows the microstructures of three Al–Ni alloy ingots solidified under conventional conditions. It is obvious that alloy composition has a significant influence on the microstructures of the Al–Ni alloy ingots. For the Al–20 at.% Ni alloy, columnar dendrites can be observed in the microstructure, as shown in Fig. 1a. In local areas, the trunks of the dendrites are parallel to each other. Fig. 1b clearly shows the microstructure of the alloy in detail at a higher magnification. It can be

Conclusions

Rapid solidification and alloy composition have a significant influence on the phase constitution and microstructure of Al–Ni alloys. The Al–Ni alloys solidified under conventional conditions are composed of α-Al, NiAl₃ and Ni₂Al₃. The amount of the α-Al phase decreases and that of the Ni₂Al₃ phase increases with increasing Ni contents from 20 to 31.5 at.%. The melt spun Al–20 at.% Ni alloy consists of α-Al, NiAl₃ and Ni₂Al₃, while the melt spun Al–25 and 31.5 at.% Ni alloys comprise NiAl₃ and Ni₂

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 50701028) and Excellent Middle-age and Young Scientists Research Award Foundation of Shandong Province (No. 2007BS04024). The SEM experiments from Ruhr University Bochum (Bochum, Germany) are also acknowledged. Z.H. Zhang gives his thanks for the financial support from DFG-SFB 459 for guest visit. We thank Dr. Wenhua Zhang for his help of N₂ adsorption/desorption experiments.

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¹: Present address: Department of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xueyuang Road, Haidian District, Beijing 100083, PR China.

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Alloy composition dependence of formation of porous Ni prepared by rapid solidification and chemical dealloying

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Appl. Catal.

Mater. Sci. Eng. A

J. Catal.

J. Catal.

Appl. Catal. A

J. Catal.

J. Catal.

Nature

J. Mater. Res.