High-Temperature Hydrogen Adsorption Properties of Precursor- Derived Nickel Nanoparticle-Dispersed Amorphous Silica Yumi H. Ikuhara, w Hiroshi Mori, Tomohiro Saito, and Yuji Iwamoto Japan Fine Ceramics Center, Atsuta-ku, Nagoya 456-8587, Japan Nickel (Ni) nanoparticle-dispersed amorphous silica (Si–O) powders were synthesized from chemical solution precursors. The high-temperature hydrogen adsorption property of the pre- cursor-derived composite powders was investigated in compar- ison with the amorphous Si–O and Ni at 773 K. Among the three powder samples, Ni nanoparticle-dispersed amorphous Si– O exhibited a unique reversible hydrogen adsorption property that was hardly detected on the amorphous Si–O and Ni. The increase amount of the reversibly adsorbed hydrogen was the highest for the composite samples at around the Ni content with a Ni/(Si1Ni) ratio of 0.2–0.3. The results strongly suggested that when the composite material is used in the form of a gas separation membrane, the reversibly adsorbed hydrogen prop- erty is thought to contribute to the additional increase in the number of solubility sites for hydrogen, which leads to a selective enhancement in the high-temperature hydrogen permeance at 773 K. I. Introduction M ICROPOROUS ceramic membranes with molecular sieve-like properties for gas separation have received considerable attention because of their promising application with high dur- ability at an elevated temperature and in a severe corrosive en- vironment compared with polymer membranes. 1,2 Moreover, microporous ceramic membranes can be expected to be used in catalytic membrane reactors for conversion enhancement in de- hydrogenation and methane-reforming reactions for hydrogen production. 3,4 In particular, microporous silica-based ceramic membranes have been reported to have the capability to separate hydrogen from other larger gas molecules by a simple molecular sieving effect. So far, the pore diameter and the thickness of the membranes have been successfully controlled via a wide variety of methods including chemical vapor deposition (CVD), 4–11 sol– gel technique, 12–17 and polymer precursor method. 18,19 Recently, novel nanoparticle-dispersed amorphous silica (Si– O) membranes have been designed and synthesized. 20 The na- noparticles in amorphous Si–O were selected as materials having high-temperature hydrogen affinity. This was expected to be es- sential to enhance the high-temperature hydrogen permselectiv- ity of the amorphous Si–O-based membranes. Based on the design concept mentioned above, Ni was used as a transition metal with high hydrogen affinity. 21,22 The Ni nanoparticle-dis- persed amorphous Si–O membranes were successfully fabricated on a mesoporous anodic alumina capillary (MAAC) tube using chemical processing techniques. 20,23 The MAAC tube was pre- pared by a novel pulse sequential anodic oxidation technique. 24 By changing the pulse voltage in steps, a free-standing MAAC tube composed of amorphous alumina with four layers having highly oriented radial mesopore channels was formed. The chan- nel pore diameters of the four layers from the inner to the outer surface of the MAAC tube were controlled to be approximately 50, 16, 6, and 3 nm, respectively. 23,24 The cross-sectional transmission electron microscopy (TEM) image of the membrane and substrate is shown in Fig. 1. The TEM micrograph reveals that Ni nanoparticles with dark con- trast were highly dispersed in a Si–Ni–O membrane. 23 The re- sults of our previous study on the evaluation of high- temperature hydrogen permselectivity of the novel nanocom- posite membranes are plotted in Fig. 2. 20 All the membranes exhibit a molecular sieve-like property, which is often observed for CVD or sol–gel-derived amorphous Si–O membranes, where the permeances of hydrogen (H 2 ) and helium (He) are higher than those of larger gas molecules such as carbon dioxide (CO 2 ), argon (Ar), nitrogen (N 2 ), carbon monoxide (CO), and methane (CH 4 ). Smaller gas molecules of He and H 2 could mainly per- meate through the intrinsic microporous amorphous Si–O net- work. On the other hand, the permeation of the larger gas molecules is due to the existence of a small amount of defects formed by the connection of meso- and macro-pores, which lead to the degradation of the resulting molecular sieve-like property. However, a unique gas permeation property of the novel nano- composite membranes is that the H 2 permeance selectively increases with increasing Ni content, i.e. the Ni/(Si1Ni) atomic ratio. At a Ni/(Si1Ni) atomic ratio of 0.3, the hydro- gen permeance reaches the highest value of 1.3 Â 10 À7 mol m À2 s À1 Pa À1 , and the permselectivity of H 2 /CO 2 is meas- ured to be 89. This value is much larger than the value calculated based on Knudsen’s diffusion mechanism: 4.67. In addition, it should be noted that the permeance of H 2 is approximately five times higher even in comparison with that of a smaller gas mol- ecule of He. To clarify the mechanism of the unique hydrogen permse- lectivity observed for the novel Ni nanoparticle-dispersed amor- phous Si–O membranes, it is important to understand the high-temperature hydrogen adsorption properties of the nano- composite membrane. So far, hydrogen adsorption on the Ni/Si–O catalysis has been studied at room temperature by the hydrogen adsorption isotherm analysis. 25–27 The total amount of adsorbed hydrogen evaluated by the isotherm anal- ysis included strongly (irreversibly) and weakly (reversibly) ad- sorbed hydrogen as reported on the 20 wt% Ni catalysis supported on Si–O prepared by precipitation–deposition tech- niques. 27 The irreversible hydrogen adsorption on the strong sites complete at lower hydrogen pressure of around 10 kPa on the Ni catalysis supported on Si–O. The metal surface areas of catalysis on silica, alumina, or zeolite at around room tempera- ture were evaluated by measuring the amount of irreversibly adsorbed hydrogen as one hydrogen atom irreversibly adsorbed to the surface of metal atom to form a monolayer. 28,29 In con- trast, reversible hydrogen adsorption would take place on the weaker sites in the composites. 30 There were possibilities that the weaker sites were located at the lattice position as holes in the surface, the deep traps, but accessible by diffusion of the R. Koc—contributing editor This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) as a part of the R&D Project on ‘‘Highly Efficient Ceramic Membranes for High-Temperature Separation of Hydrogen’’ promoted by the Ministry of Economy, Trade and Industry (METI), Japan. w Author to whom correspondence should be addressed. e-mail: yumi@jfcc.or.jp Manuscript No. 22305. Received September 28, 2006; approved October 4, 2006. J ournal J. Am. Ceram. Soc., 90 [2] 546–552 (2007) DOI: 10.1111/j.1551-2916.2006.01434.x r 2006 The American Ceramic Society 546