2019 Volume 60 Issue 9 Pages 1964-1967
Nanoporous CeO2 was prepared implementing the dealloying method on precursors consisting of Ce–Al alloys characterized by different atomic arrangements. In fact, the atomic arrangement of the precursor alloy strongly influence the surface area of CeO2 in the final product. Nanoporous CeO2 with quite high surface area were formed when an amorphous Ce–Al alloy was used as the precursor. The catalytic performance of the catalyst that Ni was supported on CeO2 prepared from amorphous alloy were evaluated based on the reaction whereby molecular hydrogen is released from ammonia borane. A high level of catalytic activity was observed due to that fine Ni particles were dispersed on CeO2 prepared from amorphous alloy with quite high surface area.
Amorphous alloys have a disordered atomic arrangement unlike their crystalline counterparts, and they have been attracting the attention of researchers in various fields due to their unique properties derived from the atomic arrangement.1–3) In recent years, it has been reported that amorphous alloys exhibit excellent catalytic performances in comparison with those of the corresponding crystalline alloys.4–7)
CeO2 is widely used as a catalyst support in, for instance, three-way catalysis, CO oxidation, and the water gas shift reaction, due to its excellent durability and oxygen-storage capacity.8–11) Recently, we reported that nanoporous CeO2 prepared by extracting Al species from the Ce–Al amorphous alloys show excellent catalytic support properties such as a high surface area and small pore size.12–14) The catalyst supporting Ru on the nanoporous CeO2 prepared from the amorphous alloys as precursors showed a high catalytic activity in hydrogen generation from ammonia borane. However, Ru is one of the noble metal and expensive for practical use.
Although hydrogen is one of the most promising next-generation energy sources, hydrogen is difficult to store and transport. As a solution to this problem, hydrogen-generating systems that use high-hydrogen-containing compounds such as ammonia borane, hydrazine, or formic acid have been the focused by much researcher’s attention.15–21) In particular, ammonia borane is a stable solid characterized by a high hydrogen content (19.6 mass%); moreover, molecular hydrogen can be obtained from it with high efficiency by way of catalytic hydrolysis.
In this study, to obtain an excellent catalytic support property, we investigate the optimum condition for extracting Al species from the Ce–Al amorphous alloy. To clarify the influence of the precursor’s atomic arrangement on the properties of porous CeO2, porous CeO2 was also prepared using a crystalline Ce–Al alloy as a precursor. Ni was used as a supported metal on the obtained nanoporous CeO2 prepared from the amorphous alloy. In addition, it was found that the CeO2 prepared from amorphous alloy in this study has a much higher surface area than the CeO2 prepared by a general preparation method such as hydrothermal synthesis method and dry decomposition methods.8,22)
The mother alloy Ce8Al92 was prepared starting from a mixture of pure Ce and Al lumps using the arc-melt technique in a highly purified Ar atmosphere.23) Amorphous Ce8Al92 (denoted as a-CeAl) in the form of ribbons (1-mm wide and 10–20-µm thick) were produced from master ingots through liquid quenching by a single-roller melt-spinning method using a copper wheel. The a-CeAl thus obtained was stored in a vacuum desiccator until use. The Al species were selectively extracted from a-CeAl by immersing the mother alloy in a 1 M NaOH aqueous solution at 343 K for various periods of time (t = 6, 8, 10, and 12 h). The product of this extraction process was washed with distilled water and was dried to obtain nanoporous CeO2, denoted as a-CeO2(t), where t represents the time for which the mother alloy was kept immersed in aqueous NaOH. For the preparation of the crystalline Ce–Al alloy (c-CeAl), a-CeAl was heated at 573 K under vacuum for 2 h. The nanoporous CeO2 samples prepared from c-CeAl through treatment with an NaOH solution, are denoted as c-CeO2(t).
Ni/a-CeO2 and Ni/c-CeO2 (Ni loading: 2 mass%) were prepared by depositing Ni on the as-synthesized a-CeO2 and c-CeO2 by hydrogen reduction. In particular, 0.1 g of CeO2 were mixed with 37.6 mL of an aqueous solution of NiCl3·6H2O (2.067 mmol/L, 2.4 mL), and the resulting mixture was stirred at room temperature for 1 h. The water in the suspension was evaporated under vacuum, and the powder thus obtained was dried overnight.
2.2 Catalyst characterizationX-ray diffraction (XRD; Rigaku, Ultima IV) measurements were performed to analyze the crystallinity of the samples. Field emission-scanning electron microscopy (FE-SEM; JEOL, JSM-7001) was used to determine the surface morphology of the samples. A sputtering instrument was used to coat the sample surfaces with carbon. Energy dispersive X-ray spectrometry (EDX, EDAX Ltd. DX-4) measurements were performed to analyze the bulk atomic ratio. X-ray photoelectron spectroscopy (XPS) was performed to evaluate the Al and Ce surface atomic ratios at the sample surface. The surface areas of the samples were estimated by the Brunauer-Emmett-Teller (BET) method using nitrogen physisorption isotherms obtained at 77 K (MicrotracBEL Corp. BEL-SORP mini).
2.3 Catalytic reactionsSamples (20 mg) of the as-prepared Ni/CeO2 catalysts were placed in a quartz reaction vessel that was sealed with a rubber septum and heated at 673 K under an H2 atmosphere for 1 h to reduce Ni species. Then, the reaction vessel was connected to a gas burette and was purged with Ar. A mixture of ammonia borane (1 mmol) and distilled water (5 mL) was then injected into the reaction vessel to initiate the reaction, without exposing the catalysts to air. The reaction mixture was then magnetically stirred at 303 K.
The crystallinities of Ce–Al alloys prepared by the liquid quenching method were investigated by XRD. The results from these measurements confirmed that a-CeAl obtained by the liquid quenching method consisted of an amorphous alloy (Fig. 1(a)). As can be evinced from the data reported in Fig. 1(b), after thermal treatment of a-CeAl at 573 K, the constituents of crystalline alloys were Al and Ce3Al11. Table 1 reports the percent Ce/Al bulk atomic ratios of the various species before and after undergoing NaOH treatment: before NaOH treatment, the prepared Ce–Al alloy consisted of 9–10% Ce and 90–91% Al; after NaOH treatment, the Al/Ce–Al atomic ratio decreased drastically. As shown in Fig. 1(c), (d), the XRD patterns of the samples after NaOH treatment included broad peaks assigned to CeO2. After Al extraction, the samples exhibited a silver color; however, that color turned yellow once the samples were dried under atmospheric air. These results indicated that selective Al extraction from a-CeAl and c-CeAl was indeed achieved by NaOH treatment and that the samples were oxidized and CeO2 formed after the samples had undergone a drying process in atmospheric air.
X-ray diffraction patterns of (a) amorphous Ce–Al alloy, (b) crystalline Ce–Al alloy, (c) nanoporous CeO2 obtained using amorphous Ce–Al as precursor alloy, and (d) nanoporous CeO2 obtained using crystalline Ce–Al as precursor alloy.
Notably, the samples’ bulk atomic ratios were determined performing EDX measurements. As can be evinced from the data reported in Table 1, Al was present in a-CeO2(6 h) and a-CeO2(8 h) in 8% and 5% Al/Ce–Al atomic ratios, respectively. Moreover, Al was not detected at all in samples that had undergone a 10 h NaOH treatment. On the other hand, c-CeO2 samples displayed an almost constant 6–8% value for the Al/Ce–Al atomic ratio, regardless of the treatment time, between 6 h and 12 h. The samples’ surface atomic ratios were determined performing XPS measurements. 19% of Al was present on the surface of a-CeO2(6 h), whereas small amounts of Al were remained on the surface of the other prepared samples. It is assumed that the extraction of Al from the amorphous alloy by NaOH treatment progresses uniformly and slowly from the surface, whereas Al extraction from the crystalline alloy proceeds rapidly along the grain boundary. These different extraction mechanisms account for the fact that Al is extracted more slowly from the amorphous alloy than from the crystalline one (surface Al/Ce–Al ratio of a-CeO2(6 h): 19%; surface Al/Ce–Al ratio of c-CeO2(6 h): 4%). Furthermore, it is assumed that the fact that a fair amount of Al remained in c-CeO2 even after the NaOH treatment time was increased is due to the coverage with CeO2 resulting from the rapidity of Al extraction. To clarify the chemical states of residual Al species of a-CeO2 and c-CeO2, XPS measurement was carried out. The peaks at 73.8 eV in the Al 2p XPS spectra revealed that Al species exist in the oxidized state (Al2O3, CeAlO3). In the XRD patterns of a-CeO2 and c-CeO2 (Fig. 1(c) and (d)), the peaks derived from Al species was not observed. Therefore, it is suggested that Al species in a-CeO2 and c-CeO2 may be highly dispersed.
Nitrogen adsorption measurements were carried out, and the specific surface area of each sample was calculated implementing the BET method (Table 1). The surface area of a-CeO2 with 232–271 m2·g−1 was approximately two times larger than that of c-CeO2 with 118–147 m2·g−1. c-CeO2 has almost same in surface area with the sample prepared by general preparation methods such as hydrothermal synthesis method and dry decomposition methods (ca. 50–180 m2·g−1).8,22) It is assumed that the CeO2 prepared from the amorphous alloy is advantageous in surface area. No significant difference in pore diameter was observed among the various prepared samples.
FE-SEM measurements were performed to observe the surface morphology of the various samples (Fig. 2). Nanoporous structures were observed on the surface of a-CeO2 and c-CeO2. Distorted spherically shaped ligaments were observed on the surface of a-CeO2, whereas rod-like ligaments were observed on the surface of c-CeO2. In the case of CeO2 preparation by the dealloying method, it is assumed that the difference in the atomic arrangement of precursor alloy greatly affects the shape of the ligaments observed on the surface of CeO2. A homogeneous structure of the amorphous precursor alloy might lead to spherical CeO2 formation. Whereas Ce3Al11 is tetragonal structure24) which might lead to the formation of the rod shape. Table 1 shows the ligament diameter of a-CeO2, the ligament diameter and length of c-CeO2. Ligament diameters of a-CeO2 and c-CeO2 showed nearly equivalent values (39–58 nm). On the other hand, the ligament length was about 5 times longer than the ligament diameter (237–308 nm). This long ligaments length might result in the lower surface area of c-CeO2 compared with of a-CeO2. As can be evinced from the data reported in Fig. 2 and Table 1, as the NaOH treatment time increased, the ligament size increased and the surface area decreased. These trends indicate that increasing the length of the immersion in NaOH aqueous solution resulted in CeO2 aggregation.
Field-emission scanning electron microscopy images of (a)–(d) nanoporous CeO2 obtained after treating amorphous Ce–Al with NaOH (for (a) 6 h, (b) 8 h, (c) 10 h, and (d) 12 h), (e)–(h) nanoporous CeO2 obtained after treating the crystalline Ce–Al alloy with NaOH (for (e) 6 h, (f) 8 h, (g) 10 h, and (h) 12 h).
The catalytic properties of Ni/a-CeO2 and Ni/c-CeO2 were evaluated based on the reaction whereby molecular hydrogen is released from ammonia borane. As can be evinced from the data reported in Fig. 3, Ni/a-CeO2 displayed much higher catalytic activity than Ni/c-CeO2. Furthermore, although the precursor was the same amorphous alloy, the catalytic activity greatly varied depending on the length of the NaOH treatment. It found that high activity can be obtained using as catalyst support porous CeO2 with a large surface area and a small pore diameter. The XRD pattern of Ni/c-CeO2 included a small broad peak attributed to the presence of NiO (111) (37.2°) which caused by the exposure to air before XRD measurement. However, such peak was not observed in the XRD pattern of Ni/a-CeO2 (Fig. 4) despite the fact that almost the same surface Ni/Ni–Ce ratios were observed on a-CeO2 and c-CeO2 (7.3 at%Ni and 7.2 at%Ni, respectively) in the XPS measurement. Therefore, it is suggested that finer Ni particles were formed on a-CeO2 than on c-CeO2. Considering the influence of residual Al, Ni/a-CeO2(6 h) exhibited lower catalytic activity than Ni/a-CeO2(10 h) despite having almost same pore size and surface area. Many Al species remain on the surface in Ni/a-CeO2(6 h) compared with Ni/a-CeO2(10 h). Thus, it is assumed that the catalytic activities affect not only surface area and pore diameter but also amount of surface residual Al species.
Turnover frequency (TOF) of hydrogen released from ammonia borane placed either over Ni on a CeO2 support obtained treating amorphous Ce–Al alloy with aqueous NaOH (indicated by “amorphous” in the chart) for different lengths of time or over Ni on a CeO2 support obtained treating a crystalline Ce–Al alloy with aqueous NaOH (indicated by “crystalline” in the bar chart) for different lengths of time.
X-ray diffraction patterns of (a) Ni on a CeO2 support obtained using amorphous Ce–Al alloy as precursor alloy and (b) Ni on a CeO2 support obtained using a crystalline Ce–Al alloy as precursor alloy.
The present catalyst that Ni was supported on a-CeO2 was quarter in hydrogen evolution rate compared with one that Ru/a-CeO2. Whereas the cost of Ru is approximately 100 times more expensive than that of Ni. The Ni/a-CeO2 might be more appropriate than Ru/a-CeO2 for practical use. Furthermore, interestingly, the ligament shape of a-CeO2 and c-CeO2 was spherical and rod shape, respectively. The effects of the ligament shape of CeO2 on the catalytic properties were now under investigation and would be reported elsewhere.
Nanoporous CeO2 catalyst supports were prepared by the dealloying method using Ce–Al alloys as precursors. The precursor’s atomic arrangement and the length of its NaOH treatment time affected the shape and surface area of nanoporous CeO2. Use of an amorphous precursor instead of a crystalline one resulted in the generation of finer nanoporous CeO2 structures; Ni/CeO2 prepared using these structures as support, notably, displayed substantially increased catalytic activity.
We conclude that use of a high-surface-area support prepared from an amorphous alloy is an effective method for preparing catalytically active fine Ni particles.
This study was partially supported by a Grant-in-Aid for Young Scientists (B) (No. 17K14835) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. A. N. also acknowledges Kansai Research Foundation for Technology Promotion, Izumi Science and Technology Foundation (No. H29J035) and Japan Association for Chemical Innovation, Japan.
