Processing and properties of high-purity micro-lamellate (NH₄)₂RuCl₆ particles

2.1 Materials

All reagents and solvents were used as received without further purification. Crude ruthenium powder (≤99.9 wt%) was purchased from Sino Platinum Metals (Yimen) Co., Ltd. (Yimen, Yunnan, P.R. China), and the high purity chlorine (Cl₂) came from Pengyida Co., Ltd. (Kunming, Yunnan, P.R. China). Guarantee-grade sodium hydroxide (NaOH), ammonium chloride (NH₄Cl), hydrochloric acid (37.5 wt%, HCl), and ethanol came from Sigma (St. Louis, MO, USA). The self-made distilled water was used as the solvent.

2.2 Synthesis of (NH₄)₂RuCl₆ powder

To avoid the interferences of contaminations, high-purity (NH₄)₂RuCl₆ powder was synthesized from crude Ru powder by chemical refining method (oxidation distillation+chemical precipitation). The starting Ru powder was heated (85°C) and dissolved in NaOH solution, and a stream of Cl₂ passed through it, causing the ruthenium to distill as the tetroxide (RuO₄). The product was then collected in a solution of hydrochloric acid (6 mol/l), from which H₂RuCl₆ can be obtained. Then, the saturated NH₄Cl solution was slowly added into the H₂RuCl₆ solution with high-speed magnetic stirring. The high-purity micro-lamellate (NH₄)₂RuCl₆ particles were achieved.

2.3 Analysis and characterization

The ruthenium content and purity of as-synthesized powder was measured by inductively coupled plasma-atomic emission spectrometry (ICP-ASE, Optima 5300 DV, PerkinElmer, USA). The oxygen-nitrogen analyzer (G8 GALILEO ON/H, Bruker, USA) was employed to measure the nitrogen content in the as-synthesized compound. The solid phases in different stages were characterized by high temperature X-ray diffraction (HTXRD, 9-KW SmartLab^®, Rigaku, Japan). The thermal decomposition behavior and released gas of (NH₄)₂RuCl₆ powder during the heat treatment was measured by the thermogravimetric analysis-mass spectrum coupling (TG-MS, QMS 403D Aëolos^®, Netzsch, Germany) in He atmosphere and at 5°C/min heating rate. The microstructure of (NH₄)₂RuCl₆ particles and thermal decomposed products were observed by scanning electron microscopy (SEM, S-3400N, Hitachi, Japan).

3.1 Elemental analysis

The element analysis results of starting Ru sponge and as-synthesized (NH₄)₂RuCl₆ powder are shown in Table 1, where Ru, N, and several impurity elements are listed. The impurities in the starting Ru sponge exceeded 500 ppm, thus indicating that the purity of starting Ru sponge was lower than 99.95 wt%. The ruthenium and nitrogen contents in the compound were 28.89 and 8.00 wt%, respectively, which coincided with the theoretical stoichiometric ratio of the (NH₄)₂RuCl₆ compound. After the chemical refining processes described in this study, all the element contents of the impurities effectively decreased. In the as-synthesized (NH₄)₂RuCl₆ powder, the Na, Ca, and Si contents were 1.3, 1.2, and 2.9 ppm, respectively. The purity of refined (NH₄)₂RuCl₆ powder was higher than 99.999%. Thus, the impurities in the raw Ru sponge could be effectively reduced through the chemical refining process.

3.2 Thermal analysis

The thermal decomposition behavior and released gases of the (NH₄)₂RuCl₆ powder during the heat treatment was measured by TG-DTA-MS in He atmosphere at 5°C/min heating rate. The thermograms recorded for (NH₄)₂RuCl₆ are shown in Figure 1. As can be seen, a slight weight loss (2.191 wt%) occurred between room temperature and 255°C. The (NH₄)₂RuCl₆ compound is hygroscopic, which means it can readily absorb moisture in air. Thus, when the (NH₄)₂RuCl₆ powder was heated, the moisture evaporated. The thermal decomposition of (NH₄)₂RuCl₆ took place in two consecutive stages with weight losses, for which the inflection point coincided with the temperature corresponding to the endotherms or exotherms in DTA trace in the presence of He. When the heat was applied, the weight loss was initiated, and the first intense endothermic peak was observed at 309.4°C, corresponding to the loss of the evaporation of gas from the precursor, i.e. (NH₄)₂RuCl₆. Further, the precursor should be changed into other Ru compound(s). This was followed by another weight loss. A quick weight loss was also observed in the temperature range of 314.0°C–352.7°C, which appeared in the TG curve due to the evolution of gases by the decomposition of the Ru compound(s). After 352.7°C, the TG trance became stable with no further weight loss; there were also no peaks to indicate the formation of Ru metal.

Figure 1:

TG-DTA analysis of the as-synthesized (NH₄)₂RuCl₆ powder.

The theoretical MS analysis data of HCl, NH₃ and N₂ are shown in the left side of Figure 2. The released gases of (NH₄)₂RuCl₆ in thermal decomposition process were recorded by mass spectrum (MS), as shown in Figure 2 (right side). According to the theoretical data of MS, HCl and NH₃ were analyzed at 313.7°C (in the 1st decomposed stage), indicating that the original (NH₄)₂RuCl₆ powder was transformed into other Ru compounds as well as released HCl and NH₃ gases. As the temperature increased, HCl and N₂ were detected at 352.2°C, indicating that the 2nd thermal decomposed stage of (NH₄)₂RuCl₆ powder released HCl and N₂ gases (Figure 2B). By referring to the result of TG-DTA analysis, no gas signs were measured, (Figure 1), suggesting that the thermal decomposition process of (NH₄)₂RuCl₆ powder was completed.

Figure 2:

MS analysis of as-synthesized (NH₄)₂RuCl₆ powder in different heating temperatures.

(A) 313.7°C, (B) 352.2°C.

3.3 Microstructural characterization

Based on the TG-DTA-MS analysis, HTXRD was used to characterized the solid phases [original (NH₄)₂RuCl₆ powder thermal decomposition process] in four different temperatures of 20°C (room temperature), 314°C, 352°C, and 500°C, respectively, as shown in Figure 3. The heating rate was 5°C/min, and the XRD characterization was initiated after dwelling at each temperature stage for 30 min. The original powder is high-purity (NH₄)₂RuCl₆, as shown in Figure 3A and Supplemental Table 1. When the temperature reached 314°C, (NH₄)₂RuCl₆ was decomposed into other Ru compounds (Figure 3B). It should be noted that the XRD pattern of the Ru compounds were not listed in the ICDD. When (NH₄)₂RuCl₆ was heated at 314°C, NH₃ and HCl were released, and the other solid phase became a residual. Thus, the solid phase should not be the (NH₄)₂RuCl₆, but must be transformed to other Ru compound(s). However, it was difficult to identify the detailed characteristics of Ru compound(s). The (NH₃)₄Ru₃Cl₁₂ is just the result of theoretical calculation according to the Law of the Conservation of Mass. (NH₃)₄Ru₃Cl₁₂ may comprise one Ru compound or a combination of Ru compounds. Ru metal can be characterized when the temperature increased to 352°C (Figure 3C). Given that Ru was just decomposed from Ru compound(s), the crystallization process was insufficient, and the intensity of the newly generated Ru XRD pattern was very weak. The typical Ru XRD pattern when the temperature reaches 500°C is shown in Figure 3D.

Figure 3:

XRD patterns of as-synthesized (NH₄)₂RuCl₆ powder and their different thermal decomposition products.

(A) (NH₄)₂RuCl₆ powder, (B) heated at 314°C (new Ru compounds), (C) heated at 352°C (newborn Ru), (D) heated at 500°C (Ru).

The microstructure of the (NH₄)₂RuCl₆ powder and its thermal decomposed products are shown in Figure 4. (NH₄)₂RuCl₆ pieces with dimensions of about 2×1.5×0.2 μm (L×W×T) were observed, indicating that well-dispersed (NH₄)₂RuCl₆ particles without the addition of any other regents for the high-purity requirement could be obtained by the chemical precipitation technique (Figure 4A). When the (NH₄)₂RuCl₆ powder was heated to 314°C, those pieces expanded to dimensions of about 3×1.5 μm (L×W), and many residual nano-pores were observed in the surface (Figure 4B). According to the MS analysis and XRD characterization results, HCl and NH₃ were released during the heat treatment, while the (NH₄)₂RuCl₆ was transformed to other Ru compound(s). Therefore, the volume of pieces was enlarged, while many nano-pores remained on the surface. As the heating temperature increased, the new Ru compounds kept decomposing. When the heating temperature reached 352°C, the new Ru compounds were transformed to Ru. Figure 4C shows the irregular shape and dense Ru particles. Although released gases (HCl and N₂) in the 2nd decomposed stage were observed, newly generated Ru decomposed from the Ru compounds agglomerated together due to the high surface energy and atomic diffusion. Therefore, the volume of particles shrank, thus leading to agglomerated Ru particles.

Figure 4:

Microstructures of as-synthesized (NH₄)₂RuCl₆ powder and their different thermal decomposition products.

(A) (NH₄)₂RuCl₆ pieces, (B) after heating at 314°C (new Ru compound particles), (C) after heating at 352°C (Ru agglomerated particles).

According to the TG-DTA-MS analysis, when combined with the HTXRD characterization, (NH₄)₂RuCl₆ powder was decomposed to HCl, NH₃, and other Ru compounds. Then, the Ru compounds continued to decompose to Ru, HCl, and N₂. Hence, the chemical equation of the chemical reaction is described by

3(NH4)2RuCl6→3Ru+18HCl↑+ 2NH3↑+ 2N2↑.

The released gases include N₂, which is different from the preceding report [18]. After the 1st thermal decomposed stage, nitrogen element may have changed from NH₄⁺ to NH₃ because of the heating environment. While some NH₃ were released from the compound, other NH₃ coordinated with Ru to transform to other Ru compounds. The released H⁺ from NH₄⁺ combined with Cl to become HCl. Hence, the chemical formula of the Ru compounds could be (NH₃)₄Ru₃Cl₁₂, and the 1st decomposed stage is given by

3(NH4)2RuCl6→(NH3)4Ru3Cl12+6HCl↑+ 2NH3↑.

Then, because of NH₃-coordinated bonds with Ru, NH₃ was oxidized by Ru (IV) to become N₂, and the final released gases were HCl and N₂. At this point, Cl₂ cannot be released in the (NH₃)₄Ru₃Cl₁₂ compound, because Cl⁻ is more stable than NH₃ in a senior oxidized circumstance (Ru IV). The 2nd decomposed stage is given by

(NH3)4Ru3Cl12→3Ru+2N2↑+ 12HCl↑.