Abstract
High-purity ultrafine platinum particles are widely used to fabricate platinum electrode oxygen sensors for automobiles and thick-film platinum resistance temperature elements. In this study, the near-spherical ultrafine Pt particles of high purity were synthesized by chemical purification, spray-drying, and ignition from crude Pt powder. Impurities in the initial Pt powder were eliminated by the strong acid cation resin exchange column and precipitation treatment. Near-spherical (NH4)2PtCl6 particles were obtained after spray-drying, and then the microstructure and size of as-synthesized Pt particles were controlled by the ignition process. The influences of different heating temperatures during ignition treatment on the microstructure and size of Pt particles were investigated. The purity of as-synthesized Pt particles was higher than 99.999 wt%, and the average size was about 1.12 μm. The results indicate that high-purity ultrafine Pt particles can be efficiently synthesized by chemical refining.
1. Introduction
Ultrafine platinum powder is a kind of material with high chemical activity. The application of ultrafine Pt powder as a microelectronic material (platinum electronic paste) and catalytic material is expanding rapidly [1–7]. Pt electronic paste is mainly used to prepare platinum electrode oxygen sensors for automobiles and thick-film platinum resistance temperature elements [8–10]. It is known that the Pt electronic paste is composed of ultrafine Pt particles, organic carriers, and binders [9]. Therefore, the properties of ultrafine Pt particles have a great influence on the properties of Pt materials fabricated by Pt electronic paste. In general, the preparation of Pt electronic paste has higher requirements for Pt particles [9, 11], such as the narrow average distribution ranging from 0.1 to 3 μm, homogeneous and near-spherical grain morphology, good dispersion and no agglomeration, low impurity content, and high density, which are strictly required. So, a suitable Pt powder plays a key role in preparing Pt electronic paste. At present, a suitable Pt powder for Pt electronic paste is mainly synthesized by the liquid chemical reduction method [6, 7, 12]. However, the liquid chemical reduction method has several limitations in terms of agglomeration, the purity of Pt powder, and the production of a large amount of waste liquor. Hence, it is very important to develop an appropriate combined approach (chemical method combined with other methods) to solve the above drawbacks.
Spray-drying is a known method for producing spherical particles due to the instantaneous transition from the liquid (solvent) to the solid (solute) phase [13–19]. Compared with other drying technologies, spray-drying is clean, rapid, reproducible, inexpensive, and easily scaled up, which is why it is widely used in industrial milling [14, 17, 18]. During the process of spray-drying, the solution (solute-solvent) is completely atomized and dispersed into the chamber (hot gas) when interacted with the high-pressure gas at the nozzle. Afterward, the atomized fogdrops evaporate in the drying chamber and form spherical or near-spherical solid particles simultaneously. The powder prepared by spray-drying has high dispersibility and good sphericity and homogeneity [13, 18, 19]. Moreover, it is of high purity since no impurity is introduced during the spray-drying stage. The particle size distribution ranging from 0.5 to 5 μm can be obtained by controlling spray-drying parameters, because the result is heavily affected by the parameters [19]. Therefore, spray-drying is an effective way to prepare a high-purity ultrafine powder [18, 19].
In this study, the high-purity ultrafine Pt particles were prepared by the spray-drying method combined with chemical purification from crude platinum powder (≤99.9%). The purity of the Pt powder separated by chemical purification was higher than 99.999 wt%, and an ultrafine Pt powder was obtained after the spray-dried ammonium hexachloroplatinate ((NH4)2PtCl6) particles as intermediates were ignited in a pipe furnace in N2/H2 (vol. 5 : 5). To the best of our knowledge, there are no reports about preparing high-purity ultrafine Pt particles by spray-drying. So, in the present work, the parameters of the calcination process are intensively studied to understand the synthesis mechanism of high-purity ultrafine Pt particles in depth.
2. Materials and Methods
All reagents and solvents were used as received without further purification. Crude platinum powder (≤99.9%) was purchased from Sino-Platinum Metals Co. Ltd. (Yimen, Yunnan, P.R. China); strong acid cation resin was purchased from Langfang City Nanda Resin Co. Ltd. (Langfang, Hebei, P.R. China); and guaranteed high-grade sodium hydroxide (NaOH), sodium chloride (NaCl), ammonium chloride (NH4Cl), hydrochloric acid (37.5 wt%, HCl), and nitric acid (66.5 wt%, HNO3) were purchased from Sigma-Aldrich (St. Louis, MO). Deionized water was used as the solvent.
The crude Pt powder was dissolved in aqua regia solution (HCl/HNO3, vol. 3 : 1) and heated from room temperature to 90°C, from which the H2PtCl6 solution can be obtained. Then, hydrochloric acid (6 mol/L) was constantly added into the H2PtCl6 solution, and the mixed solution was heated to boiling repeatedly to remove nitrate, by which the pH value of the solution was adjusted to 1.5. The base metal cation impurities were removed after the nitrate-removed H2PtCl6 solution (pH ) slowly passed through the strong acid cation resin exchange column. The process of removing impurities was repeated after the pH value of the H2PtCl6 solution was adjusted to 3, by which the noble metal impurities could be removed. High-purity (NH4)2PtCl6 powder was precipitated by adding a saturated NH4Cl solution to the H2PtCl6 solution. Then, the (NH4)2PtCl6 precipitation dissolved in distilled water was spray-dried to microspherical particles. Subsequently, high-purity microspherical (NH4)2PtCl6 particles were ignited in N2/H2 (vol. 5 : 5) at 500°C for 30 min followed by cooling under N2/H2, and the high-purity ultrafine Pt particles were synthesized.
The purity of as-synthesized (NH4)2PtCl6 powder was measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Optima 5300 DV, PerkinElmer, USA). A commercial spray-drying machine (B290, BUCHI, Switzerland) was used to synthesize high-purity microspherical (NH4)2PtCl6 particles. The purity of crude Pt powder and high-purity ultrafine Pt particles were analyzed by inductively coupled glow discharge mass spectrometry (GD-MS, GD Plus GD, Element™, Thermo Fisher Scientific, USA), respectively. The solid phases in different stages were characterized by X-ray diffraction (XRD, Empyrean, PANalytical, the Netherlands, CuKα radiation at 40 kV). The microstructures of (NH4)2PtCl6 particles and thermally decomposed products were observed by field emission scanning electron microcopy (FE-SEM, S-4800, Hitachi, Japan). The average particle size and size distribution of the as-synthesized Pt powder were obtained by a laser particle size analyzer (Mastersizer 3000, Malvern, UK).
3. Results and Discussion
3.1. Elemental Analysis
The elemental analysis results of crude Pt powder, chemically precipitated powder of (NH4)2PtCl6, and as-synthesized Pt particles are shown in Table 1, where Pt and several impurity elements are listed. The purity of crude Pt powder was lower than 99.9 wt%, because the impurities in the initial Pt powder exceeded 500 ppm. In the chemical precipitation powder of (NH4)2PtCl6 obtained by the chemical purification method, all the element contents of the impurities were effectively decreased to lower than 10 ppm (the Na, Ca, Fe, and Si contents were 1.1, 1.7, 1.2, and 2.9 ppm, respectively). The Pt content in the compound was 43.95 wt%, which coincided with the theoretical stoichiometric ratio of the (NH4)2PtCl6 compound. As a result, the purity of as-synthesized Pt particles was higher than 99.999%, and the impurity contents of Na, Ca, Fe, and Si were 1.9, 2.6, 1.8, and 1.9, respectively. The impurity contents of as-synthesized Pt particles combined with the chemical precipitation powder of (NH4)2PtCl6 were slightly increased, because the amount of impurities did not change and the total mass was reduced when (NH4)2PtCl6 particles were sintered. Therefore, the impurities in the raw Pt powder could be effectively reduced through the chemical purification process.
3.2. As-Precipitated (NH4)2PtCl6 Analysis
The microstructure of the precipitates is shown in Figure 1(a), where a lot of blocks consisting of three-dimensional irregular polyhedrons of micron size are observed. In order to control the high-purity property of precipitation, no other reagent was added. Thus, the as-precipitated (NH4)2PtCl6 shows its natural microstructure. The XRD pattern of the chemical sediment (after the precipitation process) is shown in Figure 1(b). All diffraction peaks of the chemical sediment can be found in the International Centre for Diffraction Data (formerly Joint Committee on Powder Diffraction Standards JCPDS 7-240 database) [20], and the chemical sediment is identified as (NH4)2PtCl6.
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3.3. Spray-Drying Process Analysis
The precipitated (NH4)2PtCl6 powder was dissolved in deionized water with a certain concentration and heated to 85°C to dissolve sufficiently. The schematic representation of the typical process wherein the solution of the precursors is atomized into fogdrops by a two-fluid nozzle is shown in Figure 2(a). The solution and high-pressure gas (air or nitrogen) interact at the nozzle due to their high relative speed, and then the solution is crashed into a lot of small fogdrops in the drying chamber. Subsequently, the atomized fogdrops are instantaneously dried into microsized near-spherical solid particles in the spray-drying chamber where the temperature is 180°C. As shown in Figure 2(b), the microsized near-spherical (NH4)2PtCl6 powder was prepared by spray-drying. The microstructure of the spray-dried (NH4)2PtCl6 powder contains a lot of pores and regular blocks, because during the spray-drying process, the crystallized (NH4)2PtCl6 on the surface is broken by the water vapour in the fogdrops, and then the broken crystal (NH4)2PtCl6 grows again. All diffraction peaks of the spray-dried powder (in Figure 2(c)) can be found in the International Centre for Diffraction Data and can be identified as (NH4)2PtCl6, which shows that all the spray-dried (NH4)2PtCl6 particles are crystallized perfectly. The particle size distribution of the spray-dried (NH4)2PtCl6 powder was between 0.5 and 3.5 μm, and the mean size was about 1.85 μm (in Figure 2(d)).
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3.4. The XRD Pattern Analysis
The thermal behavior of (NH4)2PtCl6 was studied in depth by Yılmaz and Içbudak [21]. Yılmaz and Içbudak had pointed out that (NH4)2PtCl6 decomposes in two stages: the first stage takes place when the temperature ranges from 279°C to 345°C and the second stage takes place when the temperature ranges from 345°C to 410°C. In this study, the spray-dried (NH4)2PtCl6 powder was ignited in N2/H2 (vol. 5 : 5) at different temperatures (300°C, 345°C, 380°C, and 410°C, respectively) for 30 min with a heating rate of 5°C/min, and the solid phases in different stages were characterized by X-ray diffraction. Figures 3(a)–3(d) show the X-ray diffraction patterns of the products at four different temperatures of 300°C, 345°C, 380°C, and 410°C, respectively. The product ignited at 300°C for 30 min consisted of both (NH4)2PtCl6 and Pt(NH3)2Cl4 phases, which can be seen in Figure 3(a). The newly generated Pt(NH3)2Cl4 phase is very weak, and the (NH4)2PtCl6 can decompose at 300°C but only a little. When the temperature reached 345°C, the phases of (NH4)2PtCl6, Pt(NH3)2Cl4, Pt, and PtCl2 could be found as shown in Figure 3(b), respectively. In addition, the phase of PtCl2 was very weak compared with the other phases, and the phase of Pt(NH3)2Cl4 became stronger. It is indicated that the thermal decomposition of (NH4)2PtCl6 is very intense at 345°C, and Pt(NH3)2Cl4 as an intermediate product might decompose into Pt and PtCl2 phases. When the temperature increased to 385°C, the phases of Pt and PtCl2 were still observed, but the phases of (NH4)2PtCl6 and Pt(NH3)2Cl4 disappeared (in Figure 3(c)). At this stage, (NH4)2PtCl6 and Pt(NH3)2Cl4 were completely transformed into Pt and PtCl2, and the remaining Pt compounds were mostly Pt and a little PtCl2 phase. All the diffraction peaks of the samples treated at 410°C for 30 min presented in Figure 3(d) were the typical Pt XRD peaks, and no other peaks existed. It is concluded that the Pt compounds can be completely transformed into a Pt metal at 410°C.
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3.5. Microstructural Characterization
According to XRD analysis, the microstructures of the thermally decomposed products of (NH4)2PtCl6 powder at different calcination temperatures (300°C, 345°C, 380°C, 410°C, 450°C, and 500°C, respectively) are shown in Figure 4. The typical FE-SEM images of the microstructures of Pt compounds with high magnification are observed. In this part, the reduction process had the same parameters as the thermal decomposition process, and two experiments at two different igniting temperatures (450°C and 500°C, respectively) were added. After treatment at 300°C for 30 min, the (NH4)2PtCl6 powder just began to decompose and its microstructure was almost the same as that of spray-dried (NH4)2PtCl6 particles (in Figure 4(a)). However, the surface of products with many nanopores changed slightly because of the release of thermally decomposed gases (NH3 and HCl), as shown in Figure 4(a) (top left corner). When the temperature reached 345°C, the thermal decomposition of Pt compounds was very intense, and more gas was rapidly released. The volume of Pt compounds compared with products ignited at 300°C shrank and became more densified, and there were many nanopores and numerous nanoparticles in the surface of Pt compounds as can be seen in Figure 4(b). According to XRD analysis, the Pt compound contained the (NH4)2PtCl6, Pt(NH3)2Cl4, Pt, and PtCl2 phases at 345°C. The newly generated Pt compounds gathered to form a compact structure, and especially newborn Pt atoms agglomerated into nanoparticles due to the high energy of newborn Pt atoms. As the heating temperature increased to 380°C, the products were composed of Pt and a little PtCl2 according to XRD analysis. At this stage, as (NH4)2PtCl6 and Pt(NH3)2Cl4 were transformed into Pt and PtCl2 and the newborn Pt atoms grew into nanoparticles simultaneously, a microsized Pt metal framework composed of Pt nanoparticles (approximately 15 nm) with a large number of mesoporous material was formed, as shown in Figure 4(c). When the temperature was continually being heated to 410°C, the remains were totally transformed into pure Pt according to XRD analysis. Figure 4(d) shows the microstructure of the Pt particles at 410°C. The microsized Pt metal framework with mesoporous materials became more clearly observable, and the Pt nanoparticle size became enlarged to approximately 40 nm because the number of newly generated Pt atoms increased and the growth of Pt nanoparticles continued. Simultaneously, a few amounts of Pt lumps (approximately 1 μm) were formed by the gathering and growth of newly generated Pt nanoparticles which were unstable and had high activity at a higher sintering temperature. When the igniting temperature increased to 450°C, there was no chemical reaction at this stage but there were physical changes. Obviously, the number of Pt metal frameworks with mesoporous materials decreased, while the quantity of microsized Pt particles increased, as shown in Figure 4(e). This was due to the unstable Pt nanoparticles which had higher activity at a higher temperature (450°C) and were easier to sacrifice to form microsized Pt particles. Finally, the unstable Pt nanoparticles were completely transformed into microsized Pt particles which were near spherical, well dispersed, and densified (in Figure 4(f)). The particle size distribution of Pt powder is shown in Figure 5, and the average size is about 1.12 μm with a particle size distribution between 0.2 and 1.8 μm. It is indicated that the near-spherical ultrafine Pt particles with good dispersity were obtained.
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4. Conclusions
In summary, the high-purity (99.999 wt%) (NH4)2PtCl6 particles can be obtained from crude Pt powder (99.9 wt%) by the chemical purification method. Then, the microsized near-spherical (NH4)2PtCl6 particles can be achieved by spray-drying. The mean particle size of the spray-dried (NH4)2PtCl6 powder is about 1.85 μm, and the particle size distribution ranges from 0.5 to 3.5 μm.
The microsized near-spherical (NH4)2PtCl6 particles were totally transformed into Pt metal at 410°C, and the microstructure of thermal decomposition products was seriously influenced by different igniting temperatures. The results of this study show that the near-spherical high-purity ultrafine Pt particles with good dispersity and high density can be obtained by being ignited in N2/H2 (vol. 5 : 5) with a heating rate of 5°C/min at a temperature of 500°C for 30 min. The purity of the ignited ultrafine Pt particles was higher than 99.999 wt%, and the average size was about 1.12 μm. As-synthesized Pt particles meet the requirements of Pt paste fabrication. A further study will focus on the mechanism of the near-spherical ultrafine Pt particles formed from a Pt metal framework with mesoporous materials to the microsized lump Pt particles.
Data Availability
Data is provided by the Northwest Institute for Non-ferrous Metal Research and Kunming Institute of Precious Metals.
Conflicts of Interest
All authors declare that we have no competing interests.
Authors’ Contributions
This article was written by Panchao Zhao. Panchao Zhao, Wei Yi, and Qigao Cao gave ideas. The experiment was performed by Bosheng Zhang, Kunkun Chen, and Panchao Zhao.
Acknowledgments
This research was supported by funds from the National Natural Science Foundation of China (51864022), the Science and Technology Project of Yunnan Province-New Products (2016BA001), the State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals (SKL-SPM-2018011), and Shaanxi Province Innovative Talent Promotion Program-Young Science and Technology Nova Project (2018KJXX-055). The authors thank the facilities and technical assistance of the Centre for Microscopy, Characterization, & Analysis at the Kunming Institute of Precious Metals.