19632
J. Phys. Chem. 1996, 100, 19632-19635
Use of Ti(III) Complexes To Reduce Ni, Co, and Fe in Water Solutions Vadim V. Sviridov, Gvidona P. Shevchenko,* Andrei S. Susha, and Nahla A. Diab Physico-Chemical Research Institute, Belarusian State UniVersity, Minsk 220080, Belarus ReceiVed: September 20, 1996X
The possibility of using Ti(III) complexes to produce metals of the iron subgroup in aqueous solution at room temperature has been studied. The influence of the solution composition and of the deposition conditions on the process of the reaction has been investigated. The metal reduction was shown to be accompanied by hydrogen generation via the reaction of water reduction with Ti(III) complexes, which is catalyzed with the metal particles formed. The metal powders produced were investigated by X-ray diffraction and transmission electron microscopy methods. The peculiar feature of the reductant used, when compared to conventional hydrogen-containing ones, is that upon employing it ultrafine (10-30 nm) particles of pure polycrystalline highly monodisperse nickel, cobalt, and iron powders are produced, which form chain structures.
2Ti(III) + Me(II) ) Me + 2Ti(IV)
Introduction Amorphous and crystalline powders of iron subgroup metals generate interest among researchers due to their magnetic and catalytic properties. Borohydride, hypophosphite and hydrazine can be used to produce metal Ni, Co, and Fe powders by electroless deposition in solutions.1-4 These compounds, being strong reductants, have a number of drawbacks: when using borohydride or hypophosphite, it is impossible to produce pure powders containing no boron or phosphorus impurities, the reagents are unstable, and higher temperatures and pH are necessary. It is known4-7 that redox couples of variable-valence metals, such as Fe2+/Fe3+, Ti3+/Ti4+, Cr2+/Cr3+, etc., can be used as the reducing agents in the reactions of electroless metal deposition. The redox couples make it possible to obtain pure metallic products, and the thermodynamic conditions of the reaction, as well as the kinetics thereof, can be changed through complexation. Moreover, the said reductants, in contrast to those containing hydrogen, can be electrochemically regenerated and reused in the process. There is information on the use of Ti3+ ions to produce silver, copper, and bismuth powders and coatings in acid medium.6-8 As for the complex Ti(III) compounds, their reducing activity is higher.9-11 Thus, we found that Ni(II) complexes can be reduced with those of Ti(III) in water solution, yielding highly dispersed metal.10 We did not find any information on the Ti(III)/Ti(IV) couple being used to reduce cobalt and iron in aqueous solutions, though Ti(III) in glycerol was reported to be employed as a titrant to analytically determine a wide range of ions, including those of such active metals as nickel, cobalt, and iron.9 The present paper gives the results of the investigation of the possibility of using Ti(III) complexes to produce powders of iron subgroup metals in water solutions, as well as the results of the investigation of the properties of the powders formed. Experimental Section The reaction was carried out by mixing under intensive stirring equal volumes of the reductant and the compound to be reduced taken in the stoichiometric ratio according to the equation X
Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)02897-3 CCC: $12.00
Solution pH was adjusted with NaOH (for the titanium component) or HCl (for the solution of the metal to be reduced). The reagents of reagent grade (chemically pure, analytically pure) were used without any further purification. Particle size and shape were determined by transmission electron microscopy (TEM). The X-ray study of the nickel, cobalt, and iron powders formed was carried out using Co KR and Mo KR radiation. The potentials of the Ti(III)/Ti(IV) system were measured on the platinum electrode using the saturated silver chloride electrode as the reference one. The polarization curves were obtained using a three-electrode cell, the solution being first deaerated by bubbling argon gas. Electrodes of Ni, Co, and Fe produced in galvanostatic mode by deposition of the corresponding metal ion on the titanium substrate with a surface area of about 0.5 cm2 from the solution of the following formulationsNi(Co,Fe)SO4, 0.025 M; Na3Cit, 0.075 M; NH3, 0.25 M; pH ) 9.0; current, 25 mA; deposition time, 5 minswere used as working electrodes. The saturated silver chloride electrode was used as reference electrode, and the platinum electrode was used as an auxiliary one. Potential sweep rate was 10 mV/s. Results and Discussion When the standard reduction potentials of Ni(II) (ENi2+/Ni ) -0.257 V), Co(II) (ECo2+/Co ) -0.277 V), and Fe(II) (EFe2+/Fe ) -0.44 V) and the standard oxidation potential of Ti(III) (ETi3+/Ti4+ ) -0.04 V) are compared, it is apparent that the reduction of those metals with the Ti(III)/Ti(IV) couple is thermodynamically impossible, ∆E of the reaction being less than zero. The introduction of ligands into the solution provides the possibility of affecting the redox couple potential significantly. We showed (Table 1) that in the presence of a number of acidic ligands the redox potential of Ti(III)/Ti(IV) couple shifts into the negative region to a greater extent than EMe2+/Me so that the reduction of the complex ions of the metals in question becomes thermodynamically possible (Table 2). The preliminarily investigation showed that to achieve the reduction of iron subgroup metals with Ti(III) complexes it is essential that ammonia or amines be present in the solution together with the acidic ligands. Scarcely affecting the redox potentials of Ti(III)/Ti(IV) and Me(II)/Me(0) and ∆E of the reduction, ammonia and amines exert a substantial influence on the process kinetics. © 1996 American Chemical Society
Using Ti(III) Complexes to Reduce Ni, Co, and Fe
J. Phys. Chem., Vol. 100, No. 50, 1996 19633
Figure 1. Polarization curves of (1) cathodic and (1Å) anodic processes in supporting electrolyte; (2) cathodic process in the solution including Me(II) ions; (3) anodic process in the solution including Ti(III) ions, on nickel (a, b), cobalt (c), and iron (d) electrodes in solutions of the following formulation: NiSO4 (a, b), CoSO4 (c), FeSO4 (d), 0.025 M; TiCl3, 0.05 M; TiCl4, 0.002 M; Na3Cit, 0.075 M; NH3, 0.25 M (solutions b-d), without NH3 (solution a); pH ) 9.0; t ) 20 °C.
Figure 2. Kinetic curves for metal and water reduction with Ti(III) complexes in (a) nickel, (b) cobalt, and (c) iron deposition process. Solution formulation: MeSO4, 0.025 M; TiCl3, 0.05 M; TiCl4, 0.002 M; Na3Cit, 0.075 M; NH3, 0.25 M; pH ) 8.0; t ) 20 °C.
TABLE 1: Relationship between Redox Potential of the Ti(III)/Ti(IV) Couple and Solution Composition: TiCl3, 0.05 M; TiCl4, 0.002 M potential, V ligand concentration
pH ) 5.0
pH ) 7.0
pH ) 9.0
Na3Cit, 0.05 M Na3Cit, 0.05 M; NH3, 0.25 M EDTA, 0.05 M EDTA, 0.05 M; NH3, 0.25 M
-0.42 -0.42 -0.45 -0.45
-0.57 -0.55 -0.59 -0.59
-0.66 -0.68 -0.71 -0.71
TABLE 2: Relationship between Ti(III)/Ti(IV), Me(II)/Me, and ∆E of the Reduction Process on Electrodes of Different Nature in the Solution of the Following Formulation: MeSO4 0.025 M; TiCl3, 0.05 M; TiCl4, 0.002 M; Na3Cit, 0.075 M; NH3, 0.25 M; pH ) 9.0, t ) 20 °C potential, V electrode material
Me(II)/Me
Ti(III)/Ti(IV)
∆E
Ni Co Fe
-0.75 -0.82 -0.89
-1.03 -0.95 -0.94
0.28 0.13 0.05
Electrochemical study of the partial reactions of Ti(III) oxidation and Ni, Co, and Fe reduction showed that in the presence of acidic ligands without ammonia in weakly alkaline medium there is a region of the coupled proceeding of oxidation and reduction processes, but the mixed currents are very low (nickel reduction being an example presented in Figure 1a. With ammonia, the presence thereof results in a significant growth of the currents of Ti(III) oxidation and Me(II) complex (Me ) Ni, Co, Fe) reduction (Figure 1b-d) and, consequently, will cause the mixed currents of partial electrode reactions to increase. According to the electrochemical mechanism of electroless metal deposition, that should be manifested as the growth of the electroless nickel, cobalt, and iron deposition rates in the ammonia-containing solutions, conforming to experimental results. In the case of cathodic polarization of the electrodes, the process of hydrogen ion reduction proceeds in the supporting electrolyte (containing neither Me(II) nor Ti(III)). The growth
Figure 3. Nickel (1) and cobalt (2) reduction rate versus pH of the reaction mixture. Solution formulation: MeSO4, 0.025 M; TiCl3, 0.05 M; TiCl4, 0.002 M; Na3Cit, 0.075 M (Ni), 0.05 M (Co); Na4P2O7, 0.025 M (Co); NH3, 0.25 M; t ) 20 °C.
of the overpotential value and the rate decrease in the sequence Fe, Co, Ni (curves 1 in Figure 1 b-d) are characteristic of the process in the potential region corresponding to the metal formation. The partial curves of the cathodic process in the solution containing Me(II) (curves 2 in Figure 1b-d) show that the potential of the beginning of metal formation on the corresponding electrode shifts into the more negative region as one passes from Ni to Co and Fe, resulting in a decrease in ∆E and mixed currents of the reaction. Comparison of polarization curves for the processes proceeding in the supporting electrolyte and in that containing Me(II) shows that the ratio of the rates of water and metal reduction is substantially governed by the electrode material: the current density for the metal formation is much higher than that for the water reduction at the nickel electrode while at the iron electrode the current density of the hydrogen evolution process exceeds that of the metal formation, and only in the potential range from -1000 to -900 mV (the inset in Figure 1d) is the reduction of iron with Ti(III) complexes thermodynamically possible. The polarization curves of Ti(III) anodic oxidation on Ni and Co electrodes are identical (curves 3, Figure 1b,c): on both
19634 J. Phys. Chem., Vol. 100, No. 50, 1996
Sviridov et al.
Figure 4. TEM micrographs of the powders of iron subgroup metals produced by reduction in water solution with Ti(III) complexes: (a) Ni, (b) Ni replica of the surface, (c) Co, (d) Fe. (e) SEM micrographs of Ni synthesized in magnetic field (B ) 0.1 T).
curves a distinct limiting current is seen in the potential region -800 to -600 mV. At the same time in the case of anodic polarization on the Fe electrode there is no limiting current in the indicated potential region, the reason being that the material of the electrode itself is oxidated in this potential region (curve 1 in Figure 1d). A similar behavior is shown to some extent by the Co electrode too. The difference in the potentials of the beginning of the anodic Ti(III) oxidation process for Ni and for Co or Fe electrodes may be connected with the process being affected by the reaction of hydrogen ion reduction, proceeding as a coupled one at the surface of the electrodes. The reduction of iron subgroup metals in citrate-ammonia solutions with the Ti(III)/Ti(IV) redox couple proceeds at room temperature in the pH range of 7.0-9.5. Nickel and cobalt reduction yields highly dispersed precipitates while in the case of iron a hydrosol is formed, and the metal separation is possible only if the reaction is carried out in the magnetic field. The reduction of all the metals is accompanied by hydrogen formation at the expense of water reduction with Ti(III)
complexes, the further reaction being catalyzed with the metal particles formed. Note that in the absence of those particles Ti(III) complexes reduce water only at pH > 9.8. The kinetic curves for metal and water reduction are presented in Figure 2. The extent of Ti(III) consumption (R) in the reactions of Me(II) and water reduction is used to characterize the process. Metal formation proceeds with an induction period (from several seconds to minutes), the duration thereof being governed by the composition of the solution. During that period metal particles of colloid dimensions are formed, initiating the reaction of water reduction. The induction period being over, nickel and cobalt reduction proceeds at a high rate, which is essentially constant in the time range of 5-15 min (depending on the solution composition), whereupon the rate is increased drastically to the point of cessation of the reaction (Figure 2 a,b). Iron is reduced at a low rate (Figure 2c). The pH value of the system and the nature of the ligands used for Me2+ and Ti3+ complexation exert the greatest influence on the reaction rate (Figure 3). The maximum reduction degree
Using Ti(III) Complexes to Reduce Ni, Co, and Fe which could be achieved was 85-90% for Ni, 40-50% for Co, and 3-5% for Fe. A rather fast decrease of the rate of Me(II) reduction seems to be mainly due to the substantial (about 1.0-1.5) drop in pH of the system during the reaction and, to a smaller extent, to the additional consumption of Ti(III) in the concurrent reaction of water reduction catalyzed with the metal particles formed. The kinetic curves of water reduction with Ti(III) complexes also show a time lag stemming from the necessity of the metal particles to be formed (Figure 2), following which the reaction proceeds without a sharp rate increase, in contrast to the reaction of metal reduction. Hydrogen is observed to be evolved at a rather high rate when the main reaction (metal reduction) essentially stops. The reaction rate of water reduction grows with the pH of the solution and with the concentration of the components. The consumption of Ti(III) complexes for the reaction of water reduction can amount to as much as 20-30%, depending on the conditions of the experiment. A comparison of the rates of the reduction of iron subgroup metals and water with Ti(III) complexes suggests that in the case of nickel the reduction thereof proceeds at a significantly higher rate than the concurrent reaction of water reduction (Figure 2a), whereas in the case of iron reduction with Ti(III) the latter reaction dominates over the reaction of metal formation (Figure 2c). In the case of cobalt, the ratio of metal and water reduction rates is intermediate between those for the processes of nickel and iron reduction. These results are in good agreement with those obtained in the electrochemical study. X-ray analysis showed that the precipitates formed during the reaction and dried in the inert atmosphere are highly dispersed polycrystalline nickel, cobalt, and iron powders. As Co and Fe powders are quite reactive and tend to oxidize in air, the X-ray patterns show the presence of oxide phases Co3O4, Fe3O4, and Fe2O3 in the air-dry powders. There is no X-ray evidence of any nickel oxide phase. Electron microscopic study of the metal powders formed showed that they are comprised of particles assembled in chains 0.1-0.3 µm in diameter and several micrometers long (Figure 4). Oriented systems of chains are formed in the case of synthesis in external magnetic field (Figure 4e). For all the three metals the particles comprising the chain structures are highly monodisperse. Nickel chain structures have the maximal (as compared with cobalt and iron) length, while in the case of iron the shortest chains are formed (the length to diameter ratio in the latter case can be lower than 10). It should be noted that if borohydride is used as the reducing agent, individual Ni and Co are unlikely to form any chain structures1 and it is only for Fe and alloys thereof that such structures could be formed.1,12 Using the replica method (Figure 4), it was found that the particles composing chain structures are aggregates of smaller particles 10-20 nm in size, which matches the size of the coherent scattering regions calculated from the broadening of metal diffraction peaks in X-ray patterns. Electron microscopic study of the initial stages of the reaction of Ni (similarly for Co and Fe) reduction with Ti(III) complexes showed that already during the induction period (the first 5-6 s after the solutions are mixed) individual spherical nuclei 1020 nm in size are formed in the system. Secondary particles spherical in shape, of size 30-40 nm, are formed through nuclei growth and aggregation. According to ref 13, those secondary particles should already exhibit ferromagnetic properties. Aggregation of secondary particles yields short chains, which next
J. Phys. Chem., Vol. 100, No. 50, 1996 19635 form long chain structures upon orienting in the geomagnetic field. As the reaction proceeds, the chains grow in diameter. No orientation of individual particles which are not incorporated into chainlike aggregates is observed; that is, the reaction proceeds in the autocatalytic mode at the surface of the aggregates. The observed mechanism of chain structure formation can be related to the peculiarity of Ti(III) complexes used as the reducing agent, that is, to the reversibility of the Ti(III)/Ti(IV) couple. Because of the reductant potential shift (as the reaction proceeds, the Ti(III)/Ti(IV) concentration ratio is decreased and with it the oversaturation of the system in the reductant), the nucleation process occurs at a considerable rate only in the initial stage of the reaction, whereupon becoming energetically unfavorable, and the autocatalytic reaction of metal reduction at the surface of the aggregates already formed begins to proceed in the system. Conclusion The possibility of using the Ti(III)/Ti(IV) redox couple to produce iron subgroup metals in aqueous solution was studied in this work. The reaction was shown to occur in weakly alkaline medium (pH 7.0-9.5) at room temperature in the solution including ligands of acidic type and ammonia. In the absence of ammonia, the reaction is kinetically hindered. The process rate is substantially increased with increasing pH of the solution. The degree of metal reduction is decreased as one passes from nickel to cobalt and iron. The process of metal formation is accompanied by the reaction of water reduction with Ti(III) complexes, the latter reaction being catalyzed with the metal particles formed. The catalytic activity is increased in the sequence nickel, cobalt, iron. Polycrystalline powders of nickel, cobalt, and iron were shown to be formed in the course of the reaction. They are distinguished by forming a welldefined chain structure. Chainlike aggregates of micrometer length were found to be 0.1-0.3 µm in diameter and to comprise 10-20 nm particles. We associate the trend to form chain structures with the nature of the reducing agent used, i.e., Ti(III) complexes. Acknowledgment. This work was supported by the Fundamental Research Foundation of Belarus. References and Notes (1) Kim, S. G.; Brock, J. R. J. Colloid Interface Sci. 1987, 116, 431. (2) Vyshenkov, S. A. Electroless and electrochemical methods of deposition of metallic coatings (in Russian); Machine building: Moscow, 1981. (3) Korovin, N. V. Hydrazine (in Russian); Chemistry: Moscow, 1981. (4) Electroless metal deposition from water solution (in Russian); University Press: Minsk, 1987. (5) Jonker, H.; Molenaar, A.; Dippel, C. J. Photogr. Sci. Eng. 1969, 13, 38. (6) Serdyuk, G. I.; Shevchenko, G. P.; Sviridov, V. V.; AfanasÅeva Z. M. Vestsi AN BSSR Ser. Khim. NaVuk 1988, N2, 112. (7) Shevchenko, G. P.; AfanasÅeva, Z. M.; Sviridov, V. V. Vestsi AN BSSR Ser. Khim. NaVuk 1990, N2, 116. (8) Rutkevich, D. L.; Shevchenco, G. P.; Sviridov, V. V.; Osipovich, N. P. J. Electrochem. Soc. 1993, 140, 3473. (9) Arribas, S.; Moro, R.; Dopico, T. Quim. Anal. 1975, 29, 171. (10) Sviridov, V. V.; Shevchenko, G. P.; AfanasÅeva, Z. M.; Diab, N. A.; Rutkevich, D. L. Vestn. Beloruss. Gos. UniV., Ser 2 1993, N2, 6. (11) SalÅnikov, Yu. I.; Glebov, L. N. Zh. Neorg. Khim. 1987, 32, 2414. (12) Yiping, L.; Hadjipanais, G. C.; Sorensen, C. M.; Klabunde, K. J. J. Appl. Phys. 1991, 69, 5143. (13) Gong, W.; Li, H.; Zhao, Z.; Chen, J. J. Appl. Phys. 1991, 69, 5119.
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