2020 Volume 61 Issue 5 Pages 1037-1040
Single phase intermetallic RENi2Si2 (RE = Y, La) nanoparticles were successfully prepared at as low as 600°C in molten LiCl–CaH2 system. The BET surface areas were 40.0 m2/g and 14.4 m2/g corresponding to the particle sizes of 26.0 nm and 65.5 nm for YNi2Si2 and LaNi2Si2, respectively. The improved surface areas are much higher than those prepared by a conventional arc-melting of <2 m2/g. The novel preparation approach presented in this study is a simple scalable chemical method and shown to be applicable to prepare some rare earth metal doped intermetallic nanoparticles by directly reducing the oxide precursors.
Nickel silicide is an attractive catalyst to show high performances in CO methanation,1,2) hydrogenation,3–9) hydrodesulfurization,10–12) photocatalytic decomposition of nitrogen monoxide13) and electro-chemical hydrogen evolution.14) In addition, rare earth metal doped intermetallic silicide catalysts have been reported to exhibit tremendous improvement in the catalytic activity of transition metals, such as LaCu0.67Si1.33,15) LaCoSi,16) LnRuSi (Ln = La–Nd)17,18) and LnNiSi,19) due to the electron donation from the rare earth metals. The metal silicide catalysts are so fascinating that they are inexpensive and chemically stable. However, it is not an easy task to prepare the metal silicide nanocomposites20) in order to increase the surface areas following the improvement in the catalytic activities. Reported nickel silicide nanocomposites are mostly prepared by silaneous compound treatment based on a chemical vapor deposition technique.2–8,10–12,21–30) Other reported unique chemical approaches are high temperature hydrogen reduction,9) solid-state exfoliation,31,32) electrochemical technique,33) sodium-assisted synthesis as a reductant/molten salt34,35) and so on. These previous methods are actually effective approaches to prepare nickel silicide nanocomposites, but they require severe experimental conditions, such as use of flammable silaneous gases, high temperature, electric supply and so on. Particularly, as for rare earth metal doped ternary intermetallic silicides, the strong oxygen affinity and low potential reduction of rare earth metals36) make it much harder to synthesize the rare earth metal doped nanocomposites. The BET surface areas of bulk powder of rare earth metal doped ternary intermetallic silicides prepared by a conventional arc-melting are commonly as very low as ≦1.8 m2/g.15–17) The improvements have been successfully made by only a limited number of reports via the post-treatments of a physical Ar/H2 arc evaporation for LaCu0.67Si1.33 (65.0 m2/g)37) and a chemical etching for LnRuSi (1.5–4.9 m2/g).18) However, the physical approach requires expensive facilities and the chemical-etched samples have still low surface areas, and thus simpler and more scalable methods are preferable for commercial application.
Previously, we have successfully prepared Ni3Al nanocomposites by directly reducing the Ni–Al oxide precursors in a molten LiCl–CaH2 mixture at 600°C.38) The oxides were totally reduced at such a low temperature due to high reduction capacity of the molten salt with an assistance of a reducing agent of CaH2, resulting in the low temperature synthesis of Ni3Al nanocomposites. The required chemicals of LiCl and CaH2 are inexpensive and also handled in air easily due to their relative chemical stabilities. Therefore, the developed chemical method is a scalable simple approach to prepare intermetallic nanocomposites including non-reducible metals. In this study, we attempted to prepare the nanocomposites of ThCr2Si2-type intermetallic YNi2Si2 and LaNi2Si2 via the chemical method. Intermetallic ThCr2Si2 type compounds39) are attractive materials due to the unique properties of superconductivity,40–42) magnetism43–45) and heavy fermion.46) Thanks to the developed method, single phase YNi2Si2 and LaNi2Si2 with the size of <100 nm were synthesized, resulting in the high BET surface areas of 40.0 m2/g and 14.4 m2/g, respectively.
The intermetallic nanoparticles were prepared by directly reducing SiO2-supported oxide precursors in a molten LiCl–CaH2, according to previously reported procedures.38) First, in order to prepare Ni–RE–Si oxide precursors, Ni(NO3)2·6H2O (Wako Pure Chem. Corp.) and RE(NO3)3·6H2O (RE = Y or La, Wako Pure Chem. Corp.) were dissolved in distilled water and after well-mixing the solution, SiO2 nanoparticles (10–20 nm, Sigma-Aldrich Co. LLC.) were suspended in the solution at a molar ratio of RE/Ni/Si = 1/2/2. The nanoparticle was chosen as a silicon source for a purpose of preparing final nano-sized powder. While stirring the suspension, it was kept heating at 110°C overnight. The dried powder was then calcined at 500°C in air for 2 h in order to obtain the oxide precursors. Next, the precursors were mixed with CaH2 (JUNSEI Chem. Co. Ltd.) and LiCl (Wako Pure Chem. Corp.) in a mortar in a weight ratio of precursor/CaH2/LiCl = 0.2/0.8/0.4. The mixed powder was then loaded in a SUS reactor connected with Ar gas flow system and heated at 600°C for 2 h under the gas flow. After cooling, black hardened solid was obtained probably due to the formation of cooled molten salt as similarly to previous report.38) The reduced precursor was then crushed in a mortar and rinsed by 0.1 M NH4Cl aqueous solution in order to remove any calcium species and LiCl. Finally, the precipitate was washed by distilled water several times in order to obtain the final products.
2.2 Materials characterizationMaterial characterization was carried out by X-ray diffraction (XRD) measurements, N2 adsorption experiments, and scanning electron microscope (SEM) observation. The crystal structure was investigated by XRD (SmartLab (3 kW), Rigaku) with CuKα radiation at 40 kV and 45 mA. The porosity was examined by N2 adsorption at −196°C (BELLSORP mini-II, Microtrac-BEL). The sample was pretreated at 200°C for 30 min under vacuum before the measurement. The pore size distribution was analyzed from the measured isotherms using the Barrett, Joyner, and Halenda (BJH) method. The morphology was observed by SEM (JSM-7800F, JEOL Ltd.) with energy dispersive X-ray spectrometry (EDS) for elemental analysis.
Figure 1 shows XRD patterns of the oxide precursors and final products of YNi2Si2 and LaNi2Si2. In the both oxide precursors, observed peaks were mainly of NiO (Fig. 1(a)). Peaks assignable to Y2O3 were also observed in the diffraction patterns of the YNi2Si2 precursor. On the other hand, no clear peaks of lanthanum oxides were detectable in the diffraction patterns of the LaNi2Si2 precursor, but the lanthanum species could exist in the oxide form, such as La2O3, because lanthanum’s oxygen affinity is so strong. After the treatments of reduction and rinsing, definite peaks attributable to YNi2Si2 and LaNi2Si2 phases were clearly observed for the final products (Fig. 1(b)). Some small unknown peaks were also observed, but they were negligible in their intensity. It is noteworthy that no peaks assignable to NiO or Ni metal were observed, and furthermore that no peaks to Y2O3, La2O3, or SiO2 were observed for either of the two samples. These results indicate that the reduction process with a molten LiCl and CaH2 could enable total reduction of silicon, lanthanum, and yttrium oxides to proceed at as low as 600°C for alloying afterwards.
XRD patterns of (a) oxide precursors and (b) final products (1: YNi2Si2, 2: LaNi2Si2), together with references (1: Y2O3 (PDF01-076-8044), 2: La2O3 (PDF01-077-2795), 3: NiO (PDF00-044-1159), 4: YNi2Si2 (PDF01-084-5175), 5: LaNi2Si2 (PDF01-077-2795), 6: Ni (PDF00-004-0850)).
Figure 2 shows the adsorption/desorption isotherms of nitrogen and the corresponding pore size distributions of the final products. The calculated physical values are summarized in Table 1, together with crystallite sizes calculated from XRD measurements by the Scherrer equation. The BET surface areas were 40.0 and 14.4 m2/g for YNi2Si2 and LaNi2Si2, respectively. Hystereses, that are used to confirm an existence of mesopores (<50 nm) in samples, were hardly observed between adsorption and desorption isotherms and the pore volumes were so small of 0.046–0.135 cm3/g in comparison with common porous materials.47) Therefore, these observed results indicates that the obtained powders were nearly non-porous nanoparticles. With an assumption that the obtained powders were composed of non-porous spheres, the average particle sizes were estimated from the surface areas as 26.0 nm and 65.5 nm for YNi2Si2 and LaNi2Si2, respectively. These values are close to those of the crystallite sizes estimated by the Scherrer equation, which supports our assumption that the prepared sample powders were non-porous. Commonly, metal silicide powder prepared by arc-melting and subsequent crushing in a mortar exhibits very low surface area of ca. 1–2 m2/g with a micro-level particle size.15,16) Hence, the chemical synthesis method demonstrated in this study is highly advantageous in preparing metal silicide nanoparticles with high surface area in a simpler manner.
(a) Adsorption and desorption isotherms of nitrogen and (b) the pore size distributions (Top: YNi2Si2, Bottom: LaNi2Si2).
Figure 3 shows SEM images of the prepared YNi2Si2 and LaNi2Si2. The results of the SEM-EDS for elemental analysis are listed in Table 2. It can be recognized in the images that the nanoparticles aggregated and seem to have a spherical morphology for both samples. The particle size distributions are relatively wide, but the nanoparticles of estimated sizes by N2 adsorption and XRD (26.0–65.5 nm) are clearly seen on the images. So, these results of N2 adsorption, XRD and SEM observation confirmed the successful formation of YNi2Si2 and LaNi2Si2 nanoparticles. There is no previous report of preparing RE–Ni–Si ternary nanopowder by using molten salts. As for Ni–Si binary system, the nanopowder was prepared by using melt Na at 850°C.35) In the previous report, a mixed-phase powder of Ni3Si and Ni31Si12 has been prepared at 850°C in a Na melt and the specific surface area has been 3.11 m2/g corresponding to the particle size of 250 nm. In comparison with the previous report, our method gave much smaller nickel silicide nanoparticles with higher surface areas at a lower temperature of 600°C. One main difference between our method and the previous method is the addition of CaH2 with a molten salt. So, although it is not easy to lead to a clear conclusion only from the comparison, CaH2 could mainly contribute to the formation of small nanoparticles observed in this study. From the elemental analysis, the molar ratios of RE/Ni/Si were 1/2.1/1.5 and 1/1.6/1.2 for YNi2Si2 and LaNi2Si2, respectively. These obtained ratios are slightly different from the stoichiometric ratios, but because clear formation of YNi2Si2 and LaNi2Si2 were observed by XRD, the results were qualitatively acceptable. Negligible amounts of calcium were detected in the final products, and it indicates that most of the calcium species were effectively removed from the final products by NH4Cl solution rinsing treatment. In addition, considerable amounts of oxygen were detected for both samples, so it is probable that the sample surfaces could be oxidized to form Y2O3, La2O3, SiO2 and NiO, although these phases were not detected by XRD measurements.
SEM images of final products of (a) YNi2Si2 and (b) LaNi2Si2.
Single-phase intermetallic YNi2Si2 and LaNi2Si2 nanoparticles were successfully prepared at 600°C in a molten LiCl–CaH2 system. The prepared silicide nanoparticles exhibited high BET surface areas (14.4–40.0 m2/g), indicating the formation of very small nanoparticles (26.0–65.5 nm). Calcium hydride was suggested to be a key compound to form such small nanoparticles. These nanoparticle formation was succeeded through the low temperature direct reduction of Y2O3, La2O3 and SiO2 by high reduction capacity of the molten LiCl–CaH2 mixture. The chemical preparation method demonstrated in this study required inexpensive chemicals and very simple experimental procedures only, so it could be a very promising scalable approach to prepare rare earth metal doped ternary intermetallic silicide nanoparticles, such as ThCr2Si2-type intermetallic compounds.
A part of this work was conducted at Advanced Characterization Nanotechnology Platform of the University of Tokyo, supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
