8504
J. Phys. Chem. 1993,97, 8504-8511
Reactions of Bivalent Metal Ions with Borohydride in Aqueous Solution for the Preparation of Ultrafine Amorphous Alloy Particles Jianyi Shen,’ Zhiyu Li, Qijie Yan, and Yi Chen Department of Chemistry, Nanjing University, Nanjing 21 0008, China Received: February 5, 1993; In Final Form: May 25, 1993
The reactions of Fe2+, Co2+,and Ni2+ with borohydride in aqueous solutions for the preparation of M-B (M = Fe, Co, and Ni) ultrafine amorphous alloy particles (UFAAP) have been studied. A reaction mechanism is proposed, according to which the composition of the M-B products can be predicted and some preparation parameters reported in this paper and in the literature can be explained. The products were characterized and confirmed to be UFAAP by Miissbauer spectroscopy, electron diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy.
I. Introduction The hydrolysis of alkalineborohydride was studied as a potential sourcc of limited amounts of hydrogen gas in the 1950~.l-~ Different compounds have been used to accelerate the hydrolysis of sodium borohydride, and CoCl2 has been found to be the most effective.’ The promotion effect of metal salts such as chlorides of Fe2+, Co2+, and Ni2+ was found to be due to the formation of black precipitates, which were called metal borides by the authors, and catalyzed the hydrolysis of borohydride.’ People later turned to study these metal borides as catalysts for many chemical reactions and found that they were active and selective for the hydrogenationof olefins and organic functional group^.^-^ Wade et ai. have reviewed extensively the borohydride-reduced nickel and cobalt catalysts for different reaction systems.6 The knowledge and applications of noncrystalline alloys developed rapidly since Duwez et al. first prepared the quenched amorphous alloys,7 and the features of small particles that were different from the bulk were realized.8 Therefore, studies of borohydride-reduced metal powders have again received considerable attention.%25 Because of the wide applications of quenched amorphous alloys,2”28 the ultrafine amorphous alloy particles (UFAAP) are expected to be useful since they have advantages in that they can be compacted to any shape to serve different purposes as bulk amorphous alloys. Furthermore, they may be used as ferrofluids, catalysts, and magnetic recording materials.25 As catalysts, amorphous alloys were found to be more active and to exhibit different selectivity as compared to corresponding metals,28and the UFAAP may be more active than quenched ribbons because the small particles have high surface areas.29 The UFAAP can be produced by vapor deposition and chemical reduction. The chemical reduction method can be easily used for large-scale production of UFAAP. Many studies have focused on the preparation and characterization of Fe-B and Fe-M-B (M = Co, Ni, erc.) UFAAP produced by the reduction of metal salts with borohydride in aqueous solution. Several preparation parameters such as pH,23 molar ratio of the reactants,24 concentration,17and the ways of mixing the reactants25 have been investigated. When borohydride solution was added into Fez+ solution, theboron content in Fe-B products was found to increase when the concentration of borohyride decreased from 1 to 0.1 M.17 A range of molar ratios of borohydride to metal salts from 1 to 20 had been applied.24.30 However, as pointed out by Wade et al. in their review article: although it is desirable to establish To whom correspondence should be addressed. Present address: Department of Chemical Engineering, University of Wisconsin-Madison, Madison, WI 53706.
Figure 1. Schematicof the apparatus used for the preparation of FeB, Co-B, and Ni-B UFAAP in this study: (1) nitrogen gas, (2) pressure regulator and meter, (3) micrometering valve, (4) float flow meter, ( 5 ) volumetric reservoir, (6) jacketed beaker, (7) inlet andoutletof isothermal water, (8) electrodes for pH, (9) magnetic stirrer, (10) EA-920ion analyzer, (11) X-Y recorder, and (12) wet-test meter.
the mechanisms of the reductions to determine the best conditions suitable for the preparation of various products, relatively little systematic work has been done in this area. In the case of metal boride formation, such studies would be very difficult because of the extreme rapidity with which the borides are generated. Thus, it is necessary to understand the related chemical reactions so that different kinds of UFAAP with desired properties may be produced. We have made some efforts to obtain insight into the reactions, and a reaction mechanism is proposed in this paper. Some preparation parameters25 influencing the properties of the products of UFAAP can then be explained, and the composition of M-B (M = Fe, Co, and Ni) UFAAP produced by the reactions can be predicted at some reaction conditions according to the mechanism. 11. Experimental Section
The reactions for the preparation of Fe-B, Co-B, and Ni-B UFAAP are carried out by adding at constant rate a solution of potassium borohydride into the solution of FeS04, CoC12, and NiC12, respectively. The apparatus used for the reactionsis shown schematically in Figure 1. Solutions of potassium borohydride were prepared freshly as needed and were adjusted at pH = 12 with sodium hydroxide solution to prevent hydro1ysisn2In each test, 142 mL of KBHd solution (about 0.5 M) was filled into the volumetric reservoir. The solution of a metal salt (200 mL, 0.1 M) was placed in the jacketed beaker, and water at constant temperature of 293 K was cycled in the outer jacket. The solution in the beaker was stirred with a magnetic stirrer. The KBH4 solution was then added into the metal salt solution through a dropper by nitrogen gas at 0.15 MPa. The flow rate of nitrogen
0022-3654/93/2091-8504%04.00/0 0 1993 American Chemical Society
Reactions of Bivalent Metal Ions with Borohydride
The Journal of Physical Chemistry, Vol. 97, No. 32, 1993 8505
TABLE I: Experimental Results for Preparing Fe-B UFAAP with Different Addition Rates (Time/142 mL) of KB)4 solutions time (min)
A
B
5.00 9.43 23.45 31.43 40.50 55.10
3.126 3.252 3.194 3.213 3.128 3.152
2.069 1.910 1.838 1.921 1.869 1.925
-2.0I l l ( l l l j 0 2 4 6 8101214 PH
-8
Figure 2. Electrode potentials of the related half-reactions (cf. context).
content of B (atom 96) from eq 14 ICP 35.0 22.2 24.3 25.4 29.6 29.7
-4
18.7 22.7 24.8 25.8 28.6 28.5
1/(1+ m+ n) km:n
(%)
1.91:0.93:1 3.93:1.75:1 3.17:1.56:1 3.16:1.47:1 2.26:1.19:1 2.38:1.19:1
49.8 58.9 55.3 56.2 50.8 52.1
0
8
4
Vel oci ty(mmls) Figure 4. Miwsbauer spectrum of the F e B sample prepared with the time of 9.43 min.
residues followed by washing with acetone for drying. The black powder was then soaked in acetone when transferred into a flask for passivation. The passivation usually took 24 h at 298 K in flowing nitrogen (120 mL/min) containing approximately 1% oxygen. The samples thus prepared were stable when exposed to air. The composition of the samples was analyzed by inductively coupled plasma spectroscopy (ICP). The amorphous structures of the samples were identified with S7FeMbsbauer spectroscopy (for Fe-B), X-ray diffraction (XRD), and/or electron diffraction. The morphology and particle sizes of the sampleswere determined by transmissionelectron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) was used to analyze the electronic states of the elements on the surface of the samples. The line of Cis (284.6 eV) was taken as the reference to calculate the binding energies.
Ia6 5 I I
4 u 0 1 2 3 4 5 6 7 8 9
III. Results Electrochemistryof the R ~ c ~ ~ o InMaqueous . solutions,metal ions and H+ can be reduced by borohydride to corresponding metals and hydrogen gas:
+ 2e = Fe Co2++ 2e = Co Ni2+ + 2e = Ni Fe2+
Figure 3. Reaction proccss for the preparation of F e B UFAAP, which is indicated by the change of pH versus time (9.43 min for the addition of 142 mL of KBH4 solution in this test).
was controlled with a micrometering valve and measured with a float flow meter. The flow rate of the solution from thevolumetric reservoir was approximately equal to that of nitrogen gas. The pH value of the reaction mixture in the beaker was measured throughout the reaction by an Orion EA-920 ion analyzer with an accuracy of 0.02 pH unit and recorded by an X-Y recorder. The volume of hydrogen gas evolved during the reaction was measured by a wet-test meter (accuracy: 20 mL) and was calibrated by subtracting the volumes of KBH4 solution added and the saturated water vapor at room temperature. All joints were sealed with silicon rubber to prevent leaks. The precipitate produced in the beaker during reaction was washed thoroughly with distilled water for removal of reaction
2 H + + 2e = H, H,O
E0 = -0.44 V
(a)
If'= -0.29
V
(b)
9 = -0.25
V
(4
E0 = 0
+ e = OH- + OSH,
(4
= -0.83 V
(d')
One possible way for boron to be reduced to its elemental state is the reduction of metaborate ions (BO,-) by borohydride:
+ 3H+ + 3e = B + 2 H 2 0 + 2H,O + 3e = B + 4 0 H -
HBO, BO;
Eo = -0.87
E0 = -1.79
V
V (e) (e')
The standard electrode potentials (Eo) of these half-reactions are
8506
Shen et al.
The Journal of Physical Chemistry, Vol. 97,No.32, 1993 and (g’), we obtain a half-reaction:
BH; = B
+ 2H, + e
I? = -1.31 V
(h)
which produces elemental boron. Thus, elemental boron may be formed through the disproportionationof BHd-, Le., the reduction of B3+ by H- within BH4- ions. Mechanism of the Reactions. In early studies, the hydrolysis reaction of borohydride for the generation of Hz was established:
BH;
+ 2H20 = BO; + 4H,t
(1) which can be greatly accelerated by H+ or metals such as Fe, Co, and Ni.1 Thus, this hydrolysis reaction may accompany reactions between borohydride and metal ions in aqueous solutions for the production of M-B (M = Fe, Co, Ni, etc.) UFAAP. Assuming that the reductions of metal ions and B3+are independent of each other, the formation processes of metals and boron in the products can be expressed as below according to the mass and charge balances:
+ 2M2++ 2H20 = 2M1+ BO; + 4H’ + 2Hzt
BH;
725 715 705 Bi nd i ng Energy(eV)
BH;
+ H,O = B 1 + OH- + 2.5H2t
(2)
(3)
Equation 3 is actually a combination of the half-reactions (h) and (d’). In the literature, different overall reactionsfor producing Co-B and Ni-B have been suggested.3.32 Here, we list the same equationsrepresentingthe possible overall reactions for producing M-B UFAAP:
196 192 188 184 Bi ndi ng Energy(eV) Figures. XPSspectraofFe2pandBlslevelsin theFe-Bsampleprepared with the time of 9.43 min: (a, b) Fe 2p and B 1s levels in the as-prepared sample, (a’, b’) Fe 2p and B 1s levels after bombardment with argon ions.
-
taken from the CRC Handbook of Chemistry and Physics.31 According to the literature,2 BH4- will give 8e (HH+) in reactions in alkaline solutions:
BH;
+ 80H- = BO; + 6H,O + 8e
J? = -1.25 V
(f)
-
However, the H+ formed may react with H- in BH4- to give HZ rapidly. So, we suppose that BH4- gives 4e (H- H), instead of 8e, as follows:
BH;
+ 2H,O = HBO, + 3H+ + 2H2 + 4e ,!? =-0,98V (g)
or
BH4-
+ 40H- = BO,- + 2H20 + 2H, + 4e ,!? = -1.67 V (g’)
Since the half-reaction (g’) is equal to ( f ) plus (d’) times 4, the standard electrode potentials of (g) and (g’) are calculated to be -0.98 and -1.67 V, respectively. By assuming the unit activity of the ions and species except for H+ in the solutions, the pH dependence of the electrode potentials of these half-reactions is plotted in Figure 2. It is seen from Figure 2 that the electrode potentials of (e) and (e’) are close to those of (g) and (8’) in the whole pH range. So, the elemental boron cannot be reduced from the metaborate ions. But by combining (e) and (g), or (e’)
BH;
+ 4MZ++ 80H-= 4M1+ BO; + 6H,O
(6)
2BH;
+ 4MZ++ 60H- = 2M,B3 + 6H,O + H,t
(7)
Taking the ions and species in the reaction equationsas unknowns, the reaction eqs 1-7 can be written into
-1
0 2 2 0 0 1 2 -2 2 -2 0 1 0 1 0 0 -1 1 2 -3 0 -2 -1 4 2 3 6 -2 -1 1 4 - 6 2 - 4 0 24-60-4-2
- 1 4 -1 -2 0 -2.5 0 -0.5 -3 -12.5 -10 0 -1
BH4* M2+ H+ OH- = 0 (8) M B BO,H, By elementary row operation, the matrix of the coefficients becomes
-
1 0 2 2 0 0-1-4 0 1 - 2 0 - 1 0 0 1
0 0 0 0 0 0 0 0 which has a rank of 3, indicating that only three of the above equations are linearly independent. Hence, any eqs 4-7 can be obtained by the linear combination of the eqs 1-3. Let a1,az, and a3 represent the coefficient vectors of equations 1-3, respectively, and an overallreaction can then be simply expressed
The Journal of Physical Chemistry, Vol. 97, No. 32, 1993 8507
Reactions of Bivalent Metal Ions with Borohydride
TABLE II: Experimental Results for Preparing Fe-B UFAAP with Different Concentrations of KBIt Solutions (142 mL) content of B (atom %) from time (min)
n(KBH4) (mol)
A
B
21.37 21.35 19.76 23.45 20.64 19.54
0.14000 0.10500 0.08873 0.07000 0.04662 0.02597
3.314 3.275 3.254 3.194 3.191 3.084
1.984 1.980 1.890 b.838 1.888 1.743
~~
~
~
eq i4
ICP
R = dV(H2)/dr (L/min)
19.4 22.5 21.5 24.3 26.0 28.4
18.9 21.7 19.4 24.8 25.8 28.4
0.503 0.360 0.326 0.221 0.164 0.094
€ 9
-1
-
i
>
I , , , , ,
5
0
I
Time (min.)
10
15
Figure 6. Rates of hydrogen evolved during the reactions for the preparation of Fe-B UFAAP with different concentrations of KBH4 solutions as indicated.
.2
Log (Ci of KBH4) Figure 7. Logarithmic plots of reaction rates, dV(Hl)/dt, with respect to the initial concentration of KBH4 solutionsfor the preparation of Fe-B UFAAP.
by
la, + maz
+ naj = 0
(9)
where I , m, and n are numbers, denoting the factors of the reactions (l), (2), and (3), respectively, in the overall reaction. Since all possible species of the reactants and products in the reactions are included in the vector of unknowns in eq 8, we think that eq 9 expresses any of the possible overall reactionsbetween borohydride and metal ions in aqueous solutions. Therefore, if the ratio 1:m:n is known, the preparation reactions for M-B UFAAP will be clear. According to eqs 1-3 and 9, if 1 mol of BH4- consumed during the reactions releases A mol of HI, then A=
41
+ 2m + 2.5n
l+m+n 2m From eqs 10 and 11, the ratio km:n is found to be B=
+
(2A- 5)B 0.5, lS :1 (8 - 2A)B- 2 '(8 - 2A)B- 2 The composition of the M-B UFAAP will be -M= 2 - =m B n
(12)
3 (8-2A)B-2
or
B%=-
M+B
x 100%-
2 0
m
Time average concentration of KBH4 (M/min.)
l+m+n and if 1 mol of Mz+reduced consumes B moles of BH4-, then
1:m:n=
c
(8 - 2A)B - 2 X 100% (14) (8 - 2A)B + 1
The relative amount of hydrolysis of BH4- in the reactions is
1 =-A+--2 1 5 l+m+n 3 6B 3 Preparation of Fe-B UFAAP. The F e B UFAAP were prepared by the reduction of ferrous sulfate with potassium borohydride in aqueous solutions. The typical reaction process is shown in Figure 3, which is indicated by the change of pH value of the reaction mixtures during the reaction. Since the solution
Figure 8. Boron content in the Fe-B UFAAP as a function of C,, as defined by eq 22. of FeS04 is acidic (pH = 3.76) and the electrode potential of 2H+/Hz is much higher than that of FeZ+/Fe at this pH, only H+ ions are reduced when the borohydride solutionis first added. The rapid reduction of H+ causes an abrupt increase of pH to about 7 (Figure 3), at which both electrode potentials of 2H+/H2 and FeZ+/Feare equivalent (see Figure 2) and the reduction of Fez+ begins. The reduction of Fe2+ in turn sharply decreases the pH value of the reaction solutions (Figure 3) since it releases H+ as given by eq 2. Afterward, the pH value of the reaction mixtures remains almost constant at about 4 during the whole reaction period. This is because if the pH value were lowered, the reduction of H+ would prevail over the reduction of Fez+,which would in turn increase the pH value, and uice versa. When all Fez+ ions in the solutionshave been reduced, the pH value increases again, indicating that the reaction between FeS04 and KBH4 has been complete. So, the value of B for each test can be obtained according to the amount of KBH4 consumed up to the equivalent point as shown in Figure 3. The amount of H2 evolved during the reaction to the equivalent point was measured by a wet-test meter, from which the value of A can be evaluated. The experimental values of A and B as well as the boron content of the Fe-B products produced with different addition rates of KBH4 solutions derived from formula 14 and from the determination of ICP are listed in Table I.
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The Journal of Physical Chemistry, Vol. 97, No. 32, 1993 I
The TEM graphs34of the Fe-B samples prepared with times from 9 to 60 min showed that the particles are spherical with an average particle size of about 80 nm. The particles form chains, as reported by Oppegard et al.,30probably because of the selfmagnetization of the ultrafine iron-boron particles in single magnetic domains.
I 6 8 10 12 14 16 18 Time(mi n.) Figure 9. Reaction processfor the preparation of Co-B UFAAP,indicated by the change of pH versus time (21.55 min for the addition of 142 mL of KBH4 solution in this test). I .
0
2
4
Except for the test with an addition time of 5.00 min, the theoretical values of the boron content calculated from formula 14 are in good agreement with the experimental values obtained by ICP. This result implies that the reaction mechanism we proposed above is reasonable and that the overall reactions between borohydride and the metal ions in aqueous solutions proceed according to eqs 1-3. The calculated value of atom % B for the test with addition time of 5.00 min is much greater than that for ICP analysis because of the unexpected larger value of B in this test than in others. This indicates that in this test the addition rate of KBH4 solution was too fast, so the added KBH4 did not react with Fez+ in time and accumulated in the reaction mixture. It is also seen from Table I that during the reduction of Fe2+ about half of the BH4- ions are hydrolyzed, and as an overall reaction about 1.9 mol of BH4- are needed for the equivalent reduction of 1 mol of Fez+ (the values of B in Table I). Thus, it is generally not necessary to use a ratio of BH4- to metal ions of much greater than 2 for the preparation of M-B UFAAP, but ratios less than 2 might not be sufficient to reduce all the metal ions in the solutions.30 Error analysis showed that the errors for the determination of the values of A and B mainly depend on the accuracy of the measurements of the volume of H2 evolved and the volume of the KBH4 solution consumed at the equivalent points. When the deviations for the volumes of HZand KBH4 solution are within 30 mL (usually 3 L in total) and 2 mL (142 mL in total), respectively, the error for the calculation of atom % B can be within 1%,which is in the same accuracy as ICP. This accuracy can be satisfied with the apparatus used in this study (see Figure 1). The Mossbauer spectra of the Fe-B UFAAP samples exhibit broadened sextets, reflecting the random arrangement of iron atoms.25 Figure 4 shows the Mossbauer spectrum of the F e B sample prepared with the time of 9.43 min. The spectrum was fitted with a broadened sextet, with a mean hyperfine field of 246 kOe, by using the method described by Le Ca&rand Dubois,33 which is also an indication that iron existed in the metallic state in this sample. No iron oxides could be fitted from the Mossbauer spectrum. In fact, it should be mentioned that the ICP analysis accounted for more than 98% total weight of iron and boron (98.7-99.8%) in the samples prepared as listed in Table I. The Mossbauer spectra for the F e B samples prepared with times longer than 9.43 min are similar to the spectrum shown in Figure 4. However, the mean hyperfine field decreases linearly with the increase of boron content in the samples, which is consistent with the result obtained by Linderoth et ~ 1and. further ~ ~ confirms the ICP results in this study.
Figure 5 shows XPS spectra of the F e B sample prepared with the time of 9.43 min. The spectrum of the as-prepared sample exhibits a broad peak with binding energy around 710.0 eV for the Fe 2p3p level (Figure 5a), which can be attributed to the oxidized iron species on the surface. After the sample was bombarded with 8-kV argon ions for 3 min, a sharp peak with binding energy of 707.4 eV appeared (Figure Sa'), corresponding to the metallic iron in the sample.35 Two XPS peaks are observed for the B 1s level with binding energies near 188 and 192 eV. These peaks are attributed to elemental and oxidized boron atoms on the surface of the sample, respectively. After bombardment with argon ions, the area of the peak near 188 eV increases while that of the peak near 192 eV decreases (Figure 5, b and b'). Taking into consideration the results from ICP and Mossbauer spectroscopy discussed above and the fact that the surface atoms are much easier oxidized than those in the bulk, the XPS results indicate that the as-prepared sample was covered with a thin layer of oxides of iron and boron on the surface. This oxides layer was formed during passivation, which can prevent the elemental iron and boron under it from oxidation, and hence the Fe-B samples prepared were stable when exposed to air. The oxides layer can be eliminated by bombardment with argon ions. Therefore, the Fe-B particles prepared are further confirmed to be composed of elemental iron and boron under the thin oxides layer. Table I also shows that the boron content of the F e B samples increases with theaddition timeof the KBH4solutions. Toexplain this result, another group of tests was performed with varied concentrations and the similar addition rate of KBH4 solutions. Table I1 gives the results. The addition time for 142 mL of KBH4 solutions was maintained at about 20 min. The boron content of the samples calculated from equation 14 is again consistent with the ICP results and increases with the decrease of the concentration of the KBH4 solutions used. The same phenomenon has been observed by Wells et al.17 We define the initial concentration of KBH4 solutions as
ci=
n(KBH,)/t X 1000 V(KBH4,aq)/t + V(M2+,aq)
(16)
where n(KBH4) is the total moles of KBH4 used in each test, t is the addition time for 142 mL of KBH4 solution in seconds, and V(KBH4,aq) and V(M2+,aq)are the volumes of KBH4 and metal salt solutions (142 and 200 mL in this study), respectively. Figure 6 shows the amount of H2 evolved during the reactions with different concentrations of KBH4 solutions. The slopes of the lines are the rates of Hz evolved during the reactions, which are taken as the reaction rates as collected in Table 11. The rate equation is
R = k[BH;]a[M2+]B (17) For the constant concentration of metal salts, it becomes R = kTBH;]* (18) This is the rate of overall reaction. For the individual reactions 1-3, the rate equations should be
The Journal of Physical Chemistry, Vol. 97, No. 32, 1993 8509
Reactions of Bivalent Metal Ions with Borohydride
TABLE III: Experimental Results for Preparing Co-B UFAAP with Different Addition Rates (Time/142 mL) of KBI& Solutions content of B (atom %) from time (min)
A
B
5.75 9.27 21.55 32.16 42.45
3.044 3.012 3.063 3.073 3.080
1.855 1.777 1.877 1.932 1.919
eq 14 34.0 33.5 33.6 34.5 33.8
ICP
1:m:n
33.1 34.7 34.0 35.6 33.7
1.63:0.97:1 1.53:0.99:1 1.72:0.99:1 1.72:0.95:1 1.78:0.98:1
1/(1+ m
+ n) (%)
45.3 43.5 46.4 46.8 41.4
dV(Hz)/dt (L/min) 0.925 0.458 0.248 0.158 0.103
0 -0.4
2 -0.8 s h
g
-1.2
-1
-1.6
-4
-3.8
-3.6 -3.4 -3.2 Log (Ciof KBHJ
-3
-2.8
Figure 10. Logarithmic plots of reaction rates, dV(Hz)/dt, with respect to the initial concentration of KBH4 solutions for the preparation of Co-B UFAAP.
c t
Figure 12. Transmission electronmicrograph of the Co-B sampleprepared with the time of 21.55 min.
KBH4 solutions. In fact, when we make a plot of dV(H2)ldt versus Ci for all the tests with varied addition time and concentration of the KBH4 solutions, all the points locate on the same linear line. The average concentration can also be defined as
1 cav =-
n(KBH4) x 1000 V(M2+,aq)
t V(KBH4,aq)
Figure 11. Typical selected-area electron-diffraction image of the Co-B UFAAP.
R, =
Rm = Rn=
41
+ 2m41+ 2.5nkTBH,]"'
41
+ 2m + 2.5n kTBH,]"*
41
+ 2m + 2% k TBH,]
2m
2.51
a3
Figure 7 gives the logarithmic plots of R, Rl, R,, and R, with respect to Ci for the preparation of Fe-B, from which the KBH4 orders for the overall reaction and reactions 1-3 are determined to be 1.04, 1.14,0.92, and 0.62, respectively. Since m/n = R m / Rn a [KBH4]a2-Q3= [KBH4]0.3,we see that B% = 1/(2m/n + 1) 0: 1/(2[KBH4]0.3+ 1) (cf. eq 13); Le., the boron content will decrease with the increase of KBH4 concentrations. Therefore, we conclude that because the reaction order in KBH4 for the reduction of Fe2+is greater than that for the reduction of B3+, the boron content in the Fe-B products depends on the KBH4 concentration. The reactions with different addition rates of KBH4 solutions can be analyzed in the same way. This means that the nature of the addition rate influencingthe boron content of the Fe-B samples is due to the change of concentration of the
+
(22)
when KBH4 solutions are added into metal salt solutions with different original concentrations and addition times. Here t is in minutes. Figure 8 shows a plot of boron content in the Fe-B samplesversus Cava All the points derived from different addition times and original concentrations of KBH4 solutions locate on the same curve, which further confirmsthat the change of addition time changes the concentrationof KBH4 solution in the reactions. Because Cav includes two factors of original concentration and addition rate of the KBH4 solution, according to Figure 8 and eq 22, one can estimate the preparation conditions that should be used to obtain the Fe-B UFAAP with desired composition for boron content from 16% to 28%. It should be noted that the boron content may not be higher than 30% for the preparation of Fe-B UFAAP by adding KBH4 solution to the Fe2+solution as indicated by the linear fit of the points with lower Cavin Figure 8. In this study, the highest values of boron content obtained are around 29%. Preparation of Co-B UFAAP. The Co-B UFAAP were prepared by the reduction of cobalt chloride with potassium borohydridein aqueoussolutions. The reaction process, as shown in Figure 9, is similar to that between FeS04 and KBH4 but with a lower plateau (3.5) in the pH versus time curve. The experimental results for the preparation of Co-B UFAAP are collected in Table 111. The molar ratio of BH4- to Co2+for the equivalent reaction is 1.8. The values of boron content in Co-B samples are higher than those in Fe-B samples and do not change with the addition rates of KBH4 solutions. Figure 10 shows the logarithmic plots of the rates of the reactions for the preparation of Co-B versus Ciof KBH4solutions. All the overall and individual
Shen et al.
8510 The Journal of Physical Chemistry, Vol. 97, No. 32, 1993 I
borohydridein aqueous solutions. The reaction process is similar to that between CoCl2 and KBH4. The experimental results for the preparation of Ni-B UFAAP are given in Table IV. The molar ratio of BH4- to Ni2+for the equivalentreaction is 1.9.The values of boron content in the Ni-B samples are similar to those in the Co-B samples but seem to increase with the addition rate of KBH4 solutions. The logarithmic plots of the rates of the reactions for the preparation of Ni-B versus Ci of KBH4 solutions show that the reaction orders in KBH4 are 1.01,0.97,1 .OO,and 1.17 for the overall reaction and reactions 1-3, respectively. The small difference between the reaction orders for the reductions of Ni2+and B3+may explain why the boron content in the Ni-B samples increases with the increase of the addition rate of KBH, solutions. Physical characterizations similar to those for Co-B samples indicate that the Ni-B samples are in an amorphous state with an avergae particle size of about 40 nm.
n
3
d
U
>r
.-
.cI
v)
C c Q, C U
795 705 775 Bi ndi ng Energy(eV)
-
IV. Discussion If km:n = 2:1:1,then the linear combination of eqs 1-3 gives eq 5, which may be the best approximation of the expression in eqs 4-7 for the overall reactions between metal salts and borohydride in aqueous solutions. However, only few data in Tables I, 111, and IV meet this ratio. Thus, there does not seem to be only one overall reaction equation for the reaction between M2+ and BH4- in an aqueous solution. Three independent reactions, as expressed by eqs 1-3, take place when mixing a metal salt solution with borohydride solution. Only reactions 2 and 3 are useful for the preparation of M-B UFAAP. The hydrolysis of B h - (reaction 1) should be considered to be an undesirable reaction and decresaed as much as possible for the economic use of borohydride. If the hydrolysis of B H i is excluded, the overall reaction for the production of M-B UFAAP will be
196 192 188 184 Bi ndi ng Energy(eV)
Figure 13. XPS spectra of Co 2p and B 1s levels in the Cc-B sample prepared with the time of 21.55 min: (a, b) Co 2p and B 1s levels in the as-prepared sample, (a', b') Co 2p and B 1s levels after bombardment with argon ions.
TABLE I V Ex d e n t a l Results for Preparing Ni-B UFAAP with Digrent Addition Rates (Time/142 mL) of KB& Solutions content of B (atom 7%) from time (min) A B eq14 ICP I" (7%) (L/min) 4.85 9.77 21.85 40.61
3.090 3.068 3.175 3.145
1.972 1.837 1.910 1.844
34.6 32.2 27.7 27.8
34.0 32.3 28.6 29.5
i.78:0.94:1 1.82:1.05:1 2.67:1.30:1 2.50:1.301
47.8 46.9 53.7 52.0
0.988 0.543 0.178 0.131
reactions 1-3 for Co-B are unit order in KBH4. So, the concentration and addition rate do not influencethe composition of Co-B UFAAP. Figure 1 1 shows a typical electron-diffraction image of the Co-B samples prepared. The broad and diffuse Debye rings in the image indicate the amorphous structure of the samples. The TEM graph of Figure 12 shows that the Co-B sample prepared with the addition time of 21.55 min has a uniform particle size of about 40 nm. The XPS results for this sample, as shown in Figure 13, are similar to those of the Fe-B sample as reported above. The binding energies of Co 2~312and B 1s levels for the metallic cobalt and elemental boron in the sample after bombardment with argon ions are determined to be 778.8 and 188.5 eV, respectively, which are in good agreement with the values reported in the l i t e r a t ~ r e This . ~ ~ result proves that theas-prepared Co-B particles are composed of elemental cobalt and boron under the thin oxides layer on the surface formed by passivation. Preparation of Ni-B UFAAP. The Ni-B UFAAP were prepared by the reduction of nickel chloride with potassium
2BH;
+ 2M2+ + 2H20 = M2B1 + HBO, + 2H' + 4.5H2t (23)
or
5BH;
+ 6M2+ + 6H20 = 2M3B1+ 3HBO2 + 7H+ +
11H2t (24) when m/n = 1 or 1.5, correspondingto the formation of M2Band MgB, respectively. The data in Table I, 111, and IV show 1 < m/n < 1.8for F e B , mln = 1 for C e B , and 1 Im / n < 1.5 for Ni-B. That is, Co-B can be produced stoichiometrically according to eq 23 while both reactions 23 and 24 may take place for the production of Ni-B and Fe-B. Because the boron content in Co-B (33.3%) is higher than that in Fe-B (1 6-29%), it is expected that the boron content in FeCo-B UFAAP will increase with the Co:Fe ratio. This is the result reported by Wells et ~1.1' As shown in Figures 3 and 9, the reductions of metal ions cannot occur at the beginning of the reactionsuntil the pH reaches about 7. However, the reactions take place at a much lower pH (34.5). This indicates that the products, Fe-B, Co-B, and NiB, catalyze the reactions that produce themselves. This catalytic effect may be important because without it the reduction of B3+ as in eq 3 may not take place. In fact, without the existence of the metals, the hydrolysis of BH4- in aqueous solution only produces B02- and H2.1-3 It may be the different catalytic effects of Fe-B, Co-B, and Ni-B that determine the boron content in these products. Our results confirm that the reactions between metal ions and borohydride in aqueous solutions are composed of three independent reactions. However, they m a y not be elementary reactions. These reactions should be further investigated to evaluate how the reaction conditions influence the ratios of
Reactions of Bivalent Metal Ions with Borohydride
The Journal of Physical Chemistry, Vol. 97, No. 32, I993 8511
reactions 1-3 and to find out the best reaction conditions with which the hydrolysis of BH4- may be limited.
References and Notes
V. Summary and Conclusions 1 . The reductions of Fez+,Coz+, and Ni2+with borohydride in aqueous solutions are composed of three independent reactions:
BH; BH;
+ 2H,O
= BO;
+ 4H,t
+ 2M2++ 2H,O = 2 M 1 + BO; + 4H+ + 2H,t BH;
+ H,O = BS + OH- + 2.5H2t
(1)
(2) (3)
The ratios of these reactions depend on metal ions and the reaction conditions, and therefore there is no single reaction equation that can be used to express the overall reactions. Taking reaction 1 as a byproduct reaction, the ratio of reaction 2 to reaction 3 is usually 1-1.5, corresponding to the reductions of 2BH;
Natural Science Foundation of China. Financial support from SINOPEC is also appreciated.
+ 2M2+ + 2H,O = M,B1 + HBO, + 2H+ + 4.5H2t
2. It was found that the overall reactions can maintain the pH of the reaction mixtures at 3-4.5 by themselves because reactions 1 and 2 produce OH- and H+ ions, respectively, and promote each other. The molar ratio of BH4- to metal ions for the equivalent reactions was found to be 1.8-1.9. 3. Both the addition rate and concentration of BH4- solution influence the boron content in Fe-B and Ni-B products but do not affect the boron content in Co-B. This effect is due to the different orders in BH4- of reactions 2 and 3 for the preparation of Fe-B and Ni-B. 4. Except for the thin layer of oxides of metals and boron on the surface (formed by passivation), the F e B , CO-B, and Ni-B particles prepared were in elemental states and consisted of amorphous structures with average particle sizes from 40 to 80 nm.
Acknowledgment. We acknowledge Prof. J. A. Dumesic for helpful discussions when Dr. Jianyi Shen visited his laboratories in the Department of Chemical Engineering, University of Wisconsin-Madison, This work was supported by the National
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