Kinetic Characteristics of the Process of Synthesis of Nickel Nanopowder by the Chemical Metallurgy Method
T. H. Nguyena,b, V. M. Nguyenc,*, V. N. Danchuka, M. H. Nguyenb, H. V. Nguyena, and X. D. Tangd
a National University of Science and Technology “MISiS,” Moscow, 119991 Russia
b Le Quy Don Technical University, Hanoi, 100000 Vietnam
c Institute of Research and Development, Duy Tan University, Danang, 550000 Vietnam
d Vietnam–Russia Tropical Centre, Hanoi, 100000 Vietnam
*e-mail: nguyenvanminh15@duytan.edu.vn; chinhnhan88@gmail.com Received May 18, 2020; revised May 18, 2020; accepted May 29, 2020
Abstract—The kinetic characteristics of the process of synthesis of Ni nanopowder (NP) by the chemical met- allurgy method are studied. Nickel NP is obtained by reduction of NiO nanopowder with hydrogen in a tubu- lar furnace at temperatures in the range from 240 to 280°C. Nickel oxide nanopowder is prepared by thermal decomposition of nickel hydroxide Ni(OH)2 at 300°C, which has been synthesized in advance by chemical precipitation from aqueous solutions of nickel nitrate 10 wt % and alkali NaOH 10 wt % with pH 9 at room temperature. It is found that NiO NP is more readily reduced at temperatures above 250°C. The rate constant of the reduction process at 280°C is about 2.5 times higher than in the case of reduction at 240°C. The dura- tion of the reduction process at 280°C is shorter by a factor of more than two in comparison with the case of reduction at 240°C. Based on the results of calculation of the activation energy of the reduction process from isothermal data, an assumption is made about the kinetically controlled rate-limiting regime of the reduction of NiO NP. It is revealed that Ni nanoparticles obtained by hydrogen reduction of nickel oxide have an aver- age size in the range of 60–120 nm, and each of them is connected to several adjacent particles by necks.
DOI: 10.1134/S1995078020020160
INTRODUCTION
At present, the production of Ni nanopowder (NP) with the desired properties is of great practical impor- tance [1]. Nickel NP is widely used in various fields of technology and industry, for example, the creation of magnetic materials; preparation of elastic layered elec- troconductive materials; creation of fine coatings on ceramic, quartz, metal, plastic, and composite items of any complex shape; activation of sintering processes for powder materials; creation of effective catalysts and adsorbents; and production of capacitors, elec- tronic microcircuits, etc. [2–12]. Nickel NP are pro- duced using various mechanical and physicochemical methods, most of which are characterized by high energy consumption and low productivity. The chem- ical metallurgy method consisting in the chemical deposition of oxygen-containing metal compounds with their subsequent thermal decomposition and reduction has a number of advantages, such as low costs, environmental friendliness, and the possibility of controlling the properties of products during their syntheses [13–15].
Reduction is the longest and most energy-consum- ing stage of the process owing to the need to maintain a given temperature until completion of the reactions.
An increase in the reduction temperature leads to acceleration of the sintering of nanoparticles (NPs) and the formation of micrometer-sized powders [6, 13, 14].
It should be noted that the widespread application of Ni NPs is restrained by their high cost caused by the fact that the final stage of reduction should be con- ducted very slowly at low temperatures to form nano- sized particles. Therefore, study of the kinetic charac- teristics of the reduction process in order to establish rational time and temperature regimes during the syn- thesis of Ni NP is an important fundamental and applied problem [16–18].
Considering the above, this work was aimed at syn- thesizing Ni NPs by the chemical metallurgy method, studying the properties of the initial, intermediate, and final products, and calculating the kinetic parameters of the process of preparation of Ni NPs by hydrogen reduction.
MATERIALS AND TECHNIQUES
Nickel oxide NPs synthesized by thermal decom- position of nickel hydroxide nanopowders that were prepared in advance by chemical precipitation from FUNCTIONAL AND CONSTRUCTION
NANOMATERIALS
aqueous solutions of nickel nitrate Ni(NO3)2 (10 wt %) and alkali NaOH (10 wt %) with pH = 9 at room tem- perature and under continuous stirring were used as materials for the preparation of Ni NPs. Nickel hydroxide is obtained by the following reaction:
The pH value was monitored with an Expert 001 pH meter; the measurement error was ± 0.03. Using a centrifuge, the obtained Ni(OH)2 precipitate was washed until the ions of the dissolved salt were com- pletely washed off, which was controlled by the pH of the solution above the precipitate. Then the precipi- tate was dried at room temperature for two days.
After that, the dried Ni(OH)2 was ground in a Fritsch Pulverisette 2 special mill. The resulting pow- der was calcined in a SNOL 10/11 muff le furnace at a temperature of 300°C for 5 h, in accordance with the following reaction:
The obtained NiO NP was used in further studies.
The kinetics of the reduction of NiO NP was stud- ied in a SNOL 0.2/1250 tubular furnace in a hydrogen atmosphere at different temperatures. The following reaction took place during the process:
The hydrogen source was a CAM-1 hydrogen gen- erator; the relative humidity of the obtained hydrogen did not exceed 1%. Based on the data of thermogravi- metric analysis (TGA) [18], the reduction tempera- tures were selected in the range from 240 to 280°C.
The phase composition (qualitative and quantita- tive) of powder samples was determined by powder X-ray diffraction (XRD) analysis on a Difrei-401 X-ray diffractometer (Russia) with a CrKα radiation source at room temperature.
The specific surface area (Ssp) of the samples was measured using the Brunauer–Emmett–Teller (BET) method by low-temperature nitrogen adsorption on a NOVA 1200e analyzer (United States). The measure- ment accuracy was ± 5%. The mean particle size of the powders was calculated from the data of the Ssp value measurements by the following formula:
where ρ is the pycnometric density in kg/m3, Ssp is the specific surface area in m2/kg, and Dm is the mean diameter of particles in m.
The size characteristics and morphology of the nanoparticle powders obtained were investigated by the electron microscopy method on a JSM 6700F scanning electron microscope (SEM) (Japan).
The degree of conversion (α, i.e., the ratio of the amount of the reagent consumed in the reaction to its
+ = ↓ +
3 2 2 3
Ni NO( ) 2NaOH Ni OН( ) 2NaNO.
= +
2 2
Ni OH( ) NiО HO.
+ 2 = + 2
NiO H Ni H O.
m =
sp
6 , D ρ
S
initial amount) was calculated by the following for- mula:
where m0 is the initial weight of the oxide NiO in g, and mt is the weight of NiО consumed in the reaction for the time period t in g.
The kinetics of the processes of synthesis of Ni NPs by hydrogen reduction was studied using the contract- ing sphere model [19, 20]. The use of this model makes it possible to calculate the rate constants of the process (k) by the following formula:
(1) where k is the rate constant in s–1, t is the reaction time in s, and α is the degree of conversion in unit fraction (u.fr.).
The activation energy (Ea) was calculated from the experimental data obtained under isothermal condi- tions by using the integral form of the Arrhenius equa- tion:
(2) where k is the reaction rate constant, A is a constant called the pre-exponential factor, Еa is the activation energy in J/mol, T is the temperature in K, and R is the universal gas constant in J/(mol K).
RESULTS AND DISCUSSION
The structure and morphology of the starting material NiO NP for the preparation of Ni NPs by hydrogen reduction are shown in Fig. 1.
The powder X-ray diffraction analysis (Fig. 1a) showed that the initial sample contains a purely crys- talline nickel oxide phase, and no other phases were found. As can be seen from Fig. 1b, NiO nanoparticles have a disk-like shape (f lakes) with a very small thick- ness. Such particles subsequently form dense aggre- gates. The specific surface area of NiO NP was 35.6 m2/g, which corresponds to a mean particle size of about 25 nm.
To study the kinetic characteristics of the reduction of the NiO NP under isothermal conditions, the time dependences of the degree of conversion α were obtained at different temperatures, namely, at 240, 250, 260, 270, and 280°C. These temperatures are in the range of intensive reduction processes, as shown by the TGA data published in [18]. As shown in [7], reduction at a temperature higher than 280°C leads to an undesirable acceleration of the processes of aggregation and sintering of the forming metal nanoparticles.
Figure 2 shows the dependences of the degree of conversion α on reduction time t at different tempera- tures. As can be seen from Fig. 2, the reduction of NiO
=
0
α mt , m
⋅ = − − α1 (1 ) , 13 k t
=− a1 +
ln E ln ,
k A
R T
NP begins to accelerate strongly at temperatures above 250°C. The reduction at 240°C proceeds rather slowly.
At a temperature of 280°C at which the reduction reaction has the maximum rate, the time of complete reduction was 42 min; i.e., it is shorter by a factor of more than two in comparison with the case of reduc- tion at 240°C (at this temperature, the time of com- plete reduction was 94 min).
The values of the reduction rate constants k for the processes of reduction of NiO NP at different tem- peratures were calculated using the contacting sphere model expressed by Eq. (1). The results of determining the values of the degree of conversion α (depending on time) and reduction rate constants k (depending on temperature) are given in Table 1.
Fig. 1. (a) X-ray diffraction pattern and (b) SEM image of the initial sample of NiO NP.
120 100
2T, deg 80
60 40
2500
(a) (b)
I, pulses
2000
1500
1000
500
0
100 nm
Fig. 2. (Color online) Dependence of the degree of conversion on the time of reduction at different temperatures.
t, min100 90 80 70 60 240qC
250qC 260qC
270qC 280qC
50 40 30 20 10 1.0D, u.fr.
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1
0
As can be seen from Table 1, the reduction rate constant at 280°C is about 2.5 times higher than the value obtained for the reduction at 240°C.
To determine activation energy Ea of the reduction process under isothermal conditions, the curves of the dependence of the rate constant logarithm on the inverse temperature were plotted using Eq. (2) (Fig. 3).
The Ea value calculated on the basis of the data given in Fig. 3 for the processes of reduction of NiO NP was about 54 kJ/mol. Comparing the Ea value obtained with the data published in [20], it can be con- firmed that the reduction of NiO NP proceeds in the mode of the kinetic reaction control. In this mode, to increase the temperature is a rational way to accelerate the process.
It should be noted that reduction at high tempera- tures can lead to accelerating the processes of aggrega- tion and sintering of the obtained metal nanoparticles, albeit the elevated temperature in this case is an expe- dient way to accelerate the reduction of NiO NP.
Below, the results of studying the properties of Ni NPs obtained at the temperature of 280°C corre- sponding to the maximum reduction rate are given.
The samples were qualitatively and quantitatively investigated by the XRD method. Figure 4 shows the X-ray diffraction patterns of the intermediate and final products of reduction of NiO NPs with hydrogen at 280°C.
The results of the quantitative phase analysis of the samples by studying the dependence of the intensities of diffraction ref lections on the contents of phases in the test material are given in Table 2.
The XRD data given in Fig. 4 and Table 2 for the samples agree well with the kinetic data. It was shown Table 1. Values of the degree of conversion α and rate con-
stant k
Reduction temperature, °С t, min α, u.fr. k × 104, s–1
240
40 0.22
1.582
60 0.58
80 0.86
94 1
250
20 0.12
2.015
40 0.48
60 0.87
82 1
260
20 0.15
2.447
40 0.63
50 0.89
66 1
270
20 0.23
3.135
30 0.62
40 0.90
52 1
280
10 0.13
3.990
20 0.40
30 0.88
42 1
Fig. 3. (Color online) Calculation of the activation energy in the coordinates of the Arrhenius equation.
8.9 lnk 8.7 8.5 8.3 -8.1 7.9
y = 6498.2x + 3.9061 R2 = 0.9973
Ea= 6498.2 R
Ea = R u 6498.2 = 54026, J/mol
7.7 0.00197
1/T, K1 0.00194
0.00191 0.00188
0.00185 0.00182
that the intensity of the peaks of the metal phase in the samples increases with an increase in the reduction time (Fig. 4), and the intensity of the peaks of the oxide phase decreases continuously and, as a result, the amount of the Ni metal phase increases. The results of the quantitative XRD analysis (Table 2) revealed that the content of metallic Ni was only 40 vol % after the first 20 min of reduction, and hold- ing for the next 20 min makes it possible to reduce the NiO sample almost completely. The reduction process was completed in 42 min with the formation of 100 vol % of the metal phase, i.e., Ni NP. It should be noted that the amount of Ni formed in 20 min in the middle of the reduction stage (from 10 to 30 min) is about 75 vol %, i.e., about three times more than the total amount of Ni formed in the initial (from zero to 10 min) and final (from 30 to 42 min) time intervals.
Thus, the autocatalytic (accelerating) stage of the pro-
cess of reduction of NiO NP at 280°C occurs approx- imately in the period from 10 to 30 min of exposure time.
Fig. 4. X-ray diffraction patterns of intermediate and final samples, namely, the samples obtained after (a) 10, (b) 20, (c) 30, and (d) 42 min of reduction.
140 120
2T, deg 100
80 60
40 8000
(a)
NiO Ni I, pulses
6000
4000
2000
0 120 140
2T, deg 100
80 60
40 8000
(b)
NiO Ni I, pulses
6000
4000
2000
0
140 120
2T, deg 100
80 60
40 8000
(c) NiO
Ni I, pulses
6000
4000
2000
0
2T, deg
70 80 90
60 12000
(d) Ni
I, pulses
9000
6000
3000
0
Table 2. Results of the quantitative X-ray diffraction analy- sis of the samples obtained
Sample
Phase content, at %
NiO Ni
NiO NP, starting material 100 0
After 10 min of reduction 87 13
After 20 min of reduction 60 40
After 30 min of reduction 12 88
After 42 min of reduction, pure Ni NP 0 100
The powder X-ray diffraction analysis of the final sample (Fig. 4d) showed that it contains only the fcc phase of Ni.
Figure 5 shows SEM images of the intermediate and final samples of reduction of NiO NP at 280°C.
One can see that the reduction proceeds fraction-by- fraction for the sample obtained after 10 min of reduc- tion (Fig. 5a), while the reduced fraction of the sample is comprised of very small nanoparticles of metallic nickel, which tend to form aggregates in the form of f lakes (white-colored objects in the microphoto- graph). For the sample obtained after 20 min of reduc- tion (Fig. 5b), the reduction proceeds almost over the entire surface. In this case, metallic nanoparticles begin to grow and acquire a certain shape. In the sam- ple obtained after 30 min of reduction (Fig. 5c), a small unreduced fraction remained, while metallic nanoparticles continue their growth and most of them have a rounded shape. The sample obtained after 42 min of reduction (Fig. 5d) is practically reduced.
Compared to the previous samples, Ni nanoparticles in the final product sample have a much larger size (about 60–120 nm) because of the longer time of their growth under prolonged exposure. In this case, the nanoparticles are mainly in a sintered state, and each of them is connected to several neighboring particles by necks.
The BET data on the specific surface area of NP in the obtained samples (Table 3) are in good agreement with the results of microscopic analysis. It is shown that the specific surface area of NP in the obtained samples rapidly decreases with an increase in the time of reduction; accordingly, the mean size of nanoparti- cles increases. It was found that the processes of sin- tering and aggregation of the metallic nanoparticles formed in the course of reduction lead to a significant decrease in the Ssp value of the final product compared to the starting material (from 35.6 m2/g for NiO NP to 8.8 m2/g for Ni NP).
Fig. 5. SEM images of intermediate and final samples, namely, the samples obtained after (a) 10, (b) 20, (c) 30, and (d) 42 min of reduction.
100 nm
(a) (b)
(c) (d)
100 nm 100 nm
100 nm
Table 3. Data on Ssp value measurements of the samples obtained by the BET method
Sample Ssp, m2/g Dm, nm NiO NP, starting material 35.6 ± 1.8 25 After 10 min of reduction 25.2 ± 1.3 34 After 20 min of reduction 20.5 ± 1.0 39 After 30 min of reduction 11.8 ± 0.6 59 After 42 min of reduction, pure Ni NP 8.8 ± 0.4 77
CONCLUSIONS
The kinetic characteristics of the reduction of NiO NP with hydrogen at temperatures of 240 to 280°C are investigated. It is found that the reduction rate con- stant at 280°C is approximately 2.5 times higher than that in the case of reduction at 240°C; accordingly, the duration of the process is reduced by more than half.
A strong acceleration of the reduction of NiO NP starts at temperatures above 250°C.
The activation energy of the processes of reduction of NiO NP is around 54 kJ/mol, which indicates a kinetically controlled reaction regime. In this regime, the process can be accelerated in rational way by increasing the temperature.
Apparently, the autocatalytic (accelerating) stage of the reduction of NiO NP at 280°C occurs in the period from 10 to 30 min of exposure.
It is revealed that Ni nanoparticles obtained by the reduction of nickel oxide with hydrogen at 280°C have sizes in the range of 60–120 nm with a mean particle size of 77 nm and are in a sintered state, and each of them is connected to several neighboring particles by necks.
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Translated by O. Kadkin
