4290
I n d . E n g . C h e m . Res. 1995,34, 4290-4296
Influence of Magnetic Field on the Autocatalytic Reduction of Nickel Ion Supported by pAl203 Powder Chia-Chien Lee and Tse-Chuan Chou* Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, ROC 701
The magnetic field effects on the autocatalytic reduction of nickel ion by the hypophosphite ion in aqueous solution were explored. The reaction behaviors of the autocatalytic reduction of nickel ion were well interpreted by the Langmuir-Hinshelwood adsorption model. The results indicated that both the rate constant of the free-radical recombination reaction and the rate constants of adsorption and desorption reactions were affected by the magnetic field. At fwed initial pH 5.5 and 0.037 M succinic ion, the theoretical analyses correlated well with the experimental results and a semiempirical rate equation for the steady rate was obtained as rs = 0.0482 exp(-14090B/ R2')[1.0178 exp(+12960B/R2')[Ni2+y(l 1.0178 exp(+12960B/RT)[Ni2+1)1where B is the magnetic flux density in units of tesla and T is the absolute temperature of 304 K. Both the apparent frequency factor and the apparent activation energy decreased with the magnetic field. The reaction behaviors of the autocatalytic reduction of nickel ion were also affected by the initial pH and initial concentration of the succinic ion of the deposition solution; however, the magnetic field effects with respect to the initial pH and initial concentration of the succinic ion were insignificant.
+
Introduction As mentioned in our previous study (Lee and Chou, 1994), magnetic field affects the reaction kinetics of electroless nickel deposition based on the theory of magnetic spin rephase. The results indicated that the reaction order with respect to the hypophosphite ion was significantly affected by the magnetic field. However, the reaction order with respect to the nickel ion was affected by the magnetic field insignificantly. This result was caused by the interaction of the complex agent and the properties of the substrate as mentioned in our previous study (Lee and Chou, 1994) which was unsuccessful to obtain the rate constants for adsorption and desorption reactions with respect t o the nickel ion. The composition as well as properties of the substrate change during the electroless nickel deposition at the initial stage. Nevertheless, the classical LangmuirHinshelwood adsorption model was applied to the reaction mechanism of electroless nickel deposition (Lee and Chou, 1994). The magnetic field effects on the electroless nickel deposition were preliminary studied (Lee and Chou, 1994)based on the theories of adsorption and magnetic spin rephase. Analog t o the electroless deposition of metal films, the effects of the magnetic field on the electrodeposition (Chiba et al., 1986; Chiba et al., 1988; Dash and King, 1972) of metal ions were reported. The magnetic field effects of electrodeposition are attributed t o the magnetohydrodynamic effect (Fahidy, 1983; Hubbard and Wolynes, 1981; Mohanta and Fahidy, 1978) in which the motions of charged ions are influenced by the electric and magnetic fields simultaneously. The magnetic field effects on the crystal orientations of metal films were emphasized (Mohanta and Fahidy, 1978). The magnetic field effects are due to the Lorentz force (Hubbard and Wolynes, 1981) on the ions in which visible convection of electrolyte occurs. In spite of the magnetohydrodynamics, magnetic field effects on the electrodeposition of metal films (Danilyuk et al., 1990) were examined with respect to the electrochemical kinetics and the
* Author to whom correspondence should be addressed. 0888-5885/95/2634-4290$09.00/0
magnetic spin effects. A theoretical model based on the spin conversion of chemisorbed paramagnetic particles (Danilyuk et al., 1990) was proposed to interpret the oscillatingbehaviors of cathode potential. Similar to our previous study (Lee and Chou, 1994),the magnetic field effects were caused by the spin conversion of a radical pair. However, it was unclear on the reaction kinetics and magnitude of the magnetic flux density. Accordingly, magnetic field effects on the electrochemical reactions consist of four relative aspects: electrochemically, electrode kinetics and adsorption and, magnetically, magnetohydrodynamicsand magnetic spin effects. Electroless deposition of metal films is similar to the electrodeposition of metal films except that the electrons are generated from the reductants in electroless deposition and from the electrodes in electrodeposition, respectively. The reports on the effects of magnetic field on electroless deposition of metal films were few. Chiba and Ogawa (1989) and Osaka et al. (1992) reported the magnetic field effects on the material properties of electroless depositions of nickel and cobalt films, respectively. However, the effects of the magnetic field on the reaction kinetics of electroless depositions were not mentioned. Recently, Tabulina et al. (1993) reported that the nickel-plating rate decreases with the magnetic field in a hypophosphite ion solution. Unfortunately, the studies with respect to the reaction kinetics are still unclear. The phenomena of growth of the electroless metal deposit in the initial stage were reported (Honma and Noguchi, 1990, 1972; Marton and Schlesinger, 1968; Watanabe, 1990) and indicated that the initial growth of the electroless metal deposit strongly depended on the properties of the substrate and the nucleation of the electroless metal deposit. Based on o u r previous study (Lee and Chou, 19941, a surface renew technique was developed in which a steady deposition rate appeared. At the steady rate stage, the nucleation of the electroless metal deposit disappeared and the influences of the substrate surface were minimized. Consequently, the surface concentrations of active sites for adsorption were almost constant when nickel was electrolessly and continuously deposited. The reaction behaviors of the
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4291 autocatalytic reduction of nickel ion by hypophosphite ion in an aqueous solution a t a steady state in the presence of a magnetic field were studied in this work. In addition, the factors affecting deposition in the presence of a magnetic field were also explored.
Experimental Section The substrate for electroless nickel deposition was y-alumina powder (Merck) and the pretreatments of y-alumina powder were described previously (Lee and Chou, 1994). The reactor was water-jacketed to keep the desired temperature within k0.5 "C by a water bath. The electroless nickel deposition solution was prepared by mixing nickel sulfate, sodium succinate, and sodium hypophosphite. The pH of the deposition solution was adjusted to 5.50 by sulfuric acid. The solid-liquid ratio of y-alumina powder to the solution is 1.6 g of pretreated powder per 100 mL of the deposition solution. A mechanical agitation of 250 rpm was applied to make the reaction under kinetic control (Lee and Chou, 1994). The concentration of the nickel ion was determined by a W spectrometric method. The magnitude of the electric magnetic field was controlled by the input electric current from the power supply.
Results and Discussion Adsorption of Nickel Ion. The reaction mechanism of the autocatalytic reduction of nickel ion based on the adsorption was proposed in our previous study (Lee and Chou, 1994). Autocatalytic reduction of nickel ion was considered t o be a heterogeneous reaction that nickel ions and hypophosphite ions were proposed to adsorb on two different active sites (KOand Chou, 1993, 1994; KOet al., 1995). The overall reaction for the autocatalytic reduction of nickel ion is
Ni
+ H2P0,- + 2H+ (1)
The concentration change of water is negligible in an aqueous solution. Accordingly, the reaction rate r is
r = kenlep2 where 81, and 1 3 ~ 2are the fractions of the first and second types of active sites which are occupied by nickel ions and hypophosphite ions, respectively. Based on the classical Langmuir-Hinshelwood adsorption model, and at steady state, the rates of adsorption and desorption of reactive species are equal. Consequently, the fractions of active sites occupied by nickel ions and hypophosphite ions, respectively, are
K"[Ni2'l
+
= 1 K"[Ni2+1
(3)
where subscript s denotes the steady state. In our previous study (Lee and Chou, 1994), the initial rate was chosen to evaluate the magnetic field effects; however, the magnetic field effect with respect to the nickel ion was unclear. The initial concentration of the hypophosphite ion, [H2P02-10, in this study was kept constant to evaluate the magnetic field effect with respect to the nickel ion. Accordingly, the steady rate became K"[Ni2+l rs = (1 K"[Ni2+l)
+
where K = KlP[H2P02-ld(l 6 can be rewritten as
+ Ke[HzP02-10). Equation (7)
The values of K and K" were obtained experimentally by plotting the reciprocal of rs versus the reciprocal of the concentration of the nickel ion which resulted in straight lines with intercepts of 1/K and slopes of 1IKK" at different magnetic flux densities. According t o eq 7, the steady rate depends on the values of both K and K" strongly in the presence of a magnetic field. The observable deposition of nickel occurred after an induction period which depended on the deposition temperature and concentrations of reactants as reported in our previous study (Lee and Chou, 1994). The concentration of Ni2+determined by a W spectrometer changed linearly with reaction time after the induction period. The existing time of the steady rate depended on the experimental conditions. Consequently, the steady rate of nickel deposition was determined by the linear changes of the concentration of Ni2+ with respect to time. The magnetic field effect on the autocatalytic reduction of nickel ion is the effect of the magnetic field on the rate constants of both K" and K. The experimental results with respect to nickel ion correlates well with eq 7 as shown in Figure 1. The straight lines are observed by plotting the reciprocal of the steady rate versus the reciprocal of the concentration of nickel ion. Table 1 shows the values of K and K" under different magnetic flux densities which are calculated from the intercepts and slopes of those straight lines in Figure 1. The K" is defined to be the ratio of the adsorption rate constant to desorption rate constant with respect to nickel ion. Consequently, the fraction of active sites occupied by nickel ion, O,l, is obtained (Lee and Chou, 1994) from K" according to eq 8,
K"[Ni2+l
+
= 1 K^[Ni2+l (4)
where K" and Ke are the ratios of adsorption rate constant to desorption rate constant for nickel ion and hypophosphite ion, respectively. Combining eqs 2-4, the steady rate rs is
The fractions of active sites occupied by nickel ion a t different magnetic flux densities are calculated based on the results of K" in Table 1and eq 8. The results of Bn1 are listed in Table 1. The values of both Kh and 81, increase with magnetic flux density as shown in Table 1. In addition, the value of 8,1 also increases with the concentration of nickel ion.
4292 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 750 \
-3.2 .
*
-3.6
.
-4.0
.
-4.4
'
h
v
FI
4
0' 5
1
10
I
15
I
20 1/M
I
25
J 30
l/[Ni'*], Figure 1. Plot of the reciprocal of the steady rate versus the reciprocal of the concentration of nickel ion at different magnetic flux densities. [HZPOZ-]= 0.255 M. [C4H4042-] = 0.037 M. Deposition temperature = 31 "C. Initial pH = 5.5. Table 1. Effects of the Magnetic Field on Values of K, ([HzP02-1 = 0.255 M, [C4H4042-l = 0.037 M, Deposition Temperature = 31 "C, Initial pH = 5.5)
Kh, and
enl [Ni2+]= [Ni2+]= [Ni2+]= [Ni2+]= B , T K,M K",l/M 0.038M 0.076M 0.114M 0.152M 0.105 0.135 0.00 0.0470 1.027 0.038 0.072 0.075 0.139 0.195 0.244 0.15 0.0224 2.127 0.248 0.305 0.20 0.0156 2.886 0.099 0.180 0.296 0.359 0.25 0.0116 3.681 0.123 0.219
The results indicate that the adsorption and desorption reactions are affected by both the concentration of nickel ion and the magnetic field. It is easy to realize that the fraction of active sites occupied by nickel ion increases with the concentration of nickel ion. It is more interesting that increasing the magnetic field also increases the fraction of active sites occupied by nickel ion as shown in Table 1. For example, the value of 8,1 increases from 0.038 to 0.123, with the magnetic field increasing from 0.00 to 0.25 T at 0.038 M nickel ion. This result indicates that the magnetic field can enhance the adsorption of nickel ion on the active sites of catalyst. Although the magnetic field can enhance the adsorption of nickel ion as shown in Table 1, there is another important factor to determine the steady rate. Such an important factor is the rate constant K which was defined as the rate constant for the combination reaction of the radical pair of Ni'+ and atomic hydrogen as mentioned in a previous study (Lee and Chou, 1994). Magnetic field af€ectsthe spin rephase of radicals in the radical pairs of Ni'+and atomic hydrogen. The change of steady rate in the presence of magnetic field is due to the spin rephase of the radical pair of Ni'+and atomic hydrogen. The fraction of active sites occupied by nickel ion increases with the magnetic field as shown in Table 1. The steady rate should increase with the magnetic field if the magnetic field effect on the rate constant K is ignored. However, the experimental results of Figure 1show that the steady rate decreases with the magnetic field. The results reveal that increasing the magnetic flux density decreases the rate constant K. The magnetic field inhibited and promoted the rate constants K and K", respectively. The decrease of rate constant K with the magnetic flux density is larger than that of 8,1 at higher concentration of nickel ion by comparing
-4.0
'
I
I
I
I
0.13 0.21 0.29 Magnetic Flux Density, T Figure 2. Plot of ln(K) versus the magnetic flux density. -0.03
0.05
the results of Table 1. Consequently,the net steady rate decreases with magnetic flux density. The magnetic field effect on the steady rate is due to the fact that the surface concentration of the singletphased radical pair is changed by the magnetic field (Lee and Chou, 1994). The magnetic field also affects the rate constant K" as mentioned previously. Consequently, both the rate constants K and K,,are functions of the magnetic flux density. In the absence of the magnetic field, the rate constants K and K" are functions of temperature and characteristics of reactants. However, the external magnetic field is considered to be a factor affecting the autocatalytic reduction of nickel ion. The action of the magnetic field is assumed to be similar to that of thermal energy according t o the consideration of chemical kinetics. Based on the Arrhenius plot, the rate constants K and K" are assumed to be exponential functions of the magnetic flux density. Two exponential functions which are similar to the expression for activation energy are proposed t o correlate the magnetic flux density and the rate constants K and K",
K = KOexp(-Ea/RT) exp(+mB/R!l')
K"
= KOnexp(-Ean/RT) exp(+m"B/RT)
(9)
(10)
where B is the magnetic flux density and m and m" are the magnetic constants representing the magnetic effect. At the right-hand side of eqs 9 and 10, the terms KO and KO"and the terms E, and Eanare the frequency factors and the activation energies, respectively. The magnetic effects are contributed by the last terms at the right-hand side of eqs 9 and 10. To evaluate the magnetic field effect, eqs 9 and 10 are rewritten to the logarithmic forms,
+ mB/RT ln(K") = ln(Kon)- Ea"/RT + mnB/RT ln(K) = ln(Ko) - Ea/RT
(11) (12)
At constant temperature, the activation energies due to the thermal effect, E, and Ea", and the frequency factors, KOand KO", are constants by assuming that they are not functions of both temperature and magnetic flux density. Plots of logarithms of rate constant K listed in Table 1versus the magnetic flux density are shown in Figure 2. Furthermore, plots of logarithms of rate constant K" listed in Table 1versus the magnetic flux density are shown in Figure 3. Straight lines are
.
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4293
1.4 I
1.0
I
;6; P
r l Y
v
0.6
-0.2
1
-0.03
-0.8
I
1
I
Ya#naUc PIUI Density
'
,
-4.0 2.96
0.13 0.21 0.29 Magnetic Flux Density, T Figure 3. Plot of ln(K") versus the magnetic flux density. 0.05
rs = 0.0482 exp(-14090B/RT) x 1.O178 exp(+ 12960B/RT)[Ni2+l (1+ 1.0178 e~p(+12960B/RT)[Ni~~] where B is the magnetic flux density in units of tesla. Deposition Temperature. In the previous section, the magnetic field effects on the rate constants Kh and K were discussed at constant temperature. The rate constants K and K" were also functions of temperature because they were the rate constants of free-radical recombination and adsorption and desorption reactions, respectively. The overall rate constant of the Arrhenius plot cannot proceed directly from eq 6 . However, an approximate method is used here to determine the apparent frequency factors and the apparent activation energies of the overall rate constant at different magnetic flux density. According to the results of Table 1, the maximum value of K"[Ni2+l is 0.279 which is assumed t o be negligible with respect to the unity of the denominator of eq 6 . Accordingly, eq 6 is approximated t o be a pseudo rate equation, 2+
I
1
I
I
3.12
3.20
3.28
3.38
1/T x lo', 1/K
observed in both Figures 2 and 3 which indicate that the form of the action of magnetic energy on the reactions is similar t o that of thermal energy. In general, the values of the rate constants of chemical reactions increase with temperature. However, the magnetic field can increase or decrease the rate constant according to the signs of magnetic constants m and mn. The values of m and m n are determined by using the slopes of straight lines in Figures 2 and 3 and are -14.09 and +12.96 kJ/mol.T, respectively. The signs of magnetic constants m and m n represent that the values of the rate constants K and K" increase and decrease with the magnetic flux density, respectively. Consequently, the magnetic field can retard the geminate recombination reaction of the radical pair of Ni'+and atomic hydrogen or accelerate the adsorption reaction of nickel ion in the autocatalytic reduction of nickel ion. Based on the theoretical analysis as shown in eq 6 and the experimental results of Table 1,a semiempirical rate equation a t 31 "C is obtained,
rs K0,,[Ni ]
I
3.04
(14)
Since the initial concentrations of nickel ion were the same, Kobs was obtained. The Arrhenius plot of Kobs is shown in Figure 4, and the apparent frequency factors
Figure 4. Plot of ln(K,b,) versus the reciprocal of the absolute temperature at different magnetic flux densities. [Ni2+]= 0.076 M. [H2P02-] = 0.255 M. [C4H4042-l = 0.037 M. Initial pH = 5.5
Table 2. Effect of the Magnetic Field on the Frequency Factor and Activation Energy (INPI = 0.076 M, IHzP02-1 = 0.255 M, [Cfi042-l = 0.037 M, Initial pH = 5.5)
B, T 0.00 0.15 0.20 0.25
frequency factor 5.37 x 10s 2.44
107 1.07 x 107 8.45 x lo6
activation energy, kJ/mol 58.82 51.26 49.30 48.93
and the apparent activation energies Ea,obsare listed in Table 2. Both the apparent frequency factor and the apparent activation energy decrease with the magnetic flux density. As mentioned previously, the steady rate decreases with the magnetic flux density experimentally. Theoretically, the steady rate should be faster at higher magnetic flux density because the lower apparent activation is found at higher magnetic flux density as shown in Table 2. However, the larger negative effects of the apparent frequency factors overcome the smaller ascending effect of the apparent activation energies. For example, the frequency factor decreases from 5.37 x lo8 to 8.45 x lo6, which is about 2 orders, with the magnetic flux density increasing from 0.00 to 0.25 T. Consequently, the net steady rate decreases with the magnetic flux density. The degree of the effect of the magnetic flux density on the apparent frequency factors and the apparent activation energies is small when the magnetic flux density is in the higher range as shown in Table 2. The magnetic field effect is contributed mainly from the spin rephase of the radical pair which is described by the following equation (Steiner and Ulrich, 19891,
&,ob8
where gl and g2 are g-values with respect t o two free radicals in the radical pair, ,ub is Bohr magneton, h is Planck's constant, and B is the magnetic flux density, respectively. Based on eq 15, the period of the rephasing frequency o is 2n. That is to say, the magnetic field effect shows an oscillating function (Danilyuk et al., 1990) on the reaction. The results indicate that the magnetic field effect on the steady rate is part of the oscillating phenomena. Initial pH. The magnetic field effects based on the magnetic rephasing of singlet and triplet radical pairs
4294 Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 0.004
0.004 Mapnetlo Flux Density
.gE 0.003 \
z
6
Y
g 0.002 ah 0)
z 0.001
Magnetlo Flux Deniity 0
'
0.000 4.0
a 0
I
4.5
I
5.0
1
5.5
I
6.0
0.000
6
Initial pH Figure 5. Effects of the initial pH and magnetic flux density on the steady rate. [Ni2+l = 0.076 M. [H~POZ-]= 0.255 M. [C4H4042-l= 0.037 M. Deposition temperature = 31 "C.
were discussed in detail in the Introduction section. The magnetic field effects on the autocatalytic reduction of nickel ion were contributed by both the nickel ion and hypophosphite ion which were the sources of radicals. However, there are also other parameters affecting the autocatalytic reduction of nickel ion without generating the radicals. Two of the important parameters are the initial pH of the deposition solution and the concentration of the complexing agent. However, the two parameters were considered not to be a function of the magnetic field in the theoretical analyses. That is to say, the reaction behaviors of the autocatalytic reduction of nickel ion would differ with the initial pH and concentration of the complexing agent by keeping the temperature constant, but the magnetic field effects should be similar. For completeness of studies on the autocatalytic reduction of nickel ion, effects of the initial pH of the deposition solution and the concentration of the succinic ion on the steady state rate were discussed. The magnetic field effects on the steady rates under different initial pHs of the deposition solution and concentrations of the succinic ion were also explored. Figure 5 shows the effect of the initial pH on the steady rate at different magnetic flux densities. The maxima of the steady rates are obtained a t pH 5.5 as shown in Figure 5. The steady rates at pH > 6.5 or pH .e 4.0 are nominally zero at any magnetic flux density based on the experimental observations that the pale-yellow activated y-Al~Ospowder did not change to black after deposition for 15 min. These results show that the autocatalytic reduction of nickel ion occurs in the pH range from 4.0 to 6.5. A dissociation equilibrium occurs between hypophosphite ion and hypophosphorus acid in an aqueous solution as shown in eq 16 H,PO, =-= H,PO,-
+ Hf (pK = 1.10)
I
0.000
(16)
Hypophosphite ion is the only active reactant (Larson, 1990; Trasatti and Alberti, 1966) for the autocatalytic reduction of nickel ion. The concentration of hypophosphite ion becomes lower when the pH is lower as shown in eq 16. Consequently, the concentration of hypophosphite ion is too low to initialize the autocatalytic reduction of nickel ion when the pH is lower than 4.0. The free nickel ion is another reactant in the autocatalytic reduction of nickel ion. The concentration of free nickel ion is controlled by the dissociation equilibrium of the complex of nickel ion and succinate ion in
0.015
0.00 0.15 0.20 0.25
T T T T
1
I
0.030
0.045
0.
[Succinate], M Figure 6. Effects of the concentration of sodium succinate and magnetic flux density on the steady state. [NiZ+l= 0.076 M. [HzP02-] = 0.255 M. Deposition temperature = 31 "C. Initial pH = 5.5.
an aqueous solution,
NiZf
+ C,H,O;-
NiZfC,H,0,2-
+-
(17)
In addition, the dissociation equilibria between succinate ion and succinic acid in an aqueous solution are
HOOCCH,CH,COOH
-
HOOCCH,CH,COOHOOCCH,CH,COO-
-OOCCH,CH,COO-
+ H+ (pK = 4.16) (18)
+ Hf (pK = 5.61) (19)
Formation of hypophosphite ion is favorable at high pH as shown in eq 16; however, more succinate ions are formed similarly by eqs 18 and 19. A higher concentration of succinate ion will reduce the concentration of free nickel ion because free nickel ions form complexes with succinate ions as shown in eq 17. Consequently, the concentration of free nickel ion was too low to continue the autocatalytic oxidation of hypophosphite ion, and no observable nickel ion reduction was found when pH > 6.0. Furthermore, the autocatalytic reduction of nickel ion is no longer stable when the pH is larger than 8.0 because precipitation of nickel hydroxide causes the spontaneous decomposition of the nickel solution. The effect of the magnetic field on the steady rate a t pH 5.5 is larger than other pHs, as shown in Figure 5. A higher steady rate indicates that there are more active sites occupied by nickel ion in the autocatalytic reduction of nickel ion. The magnetic field effect is more significant when more active sites are incorporated into the autocatalytic reduction of nickel ion. The results of the initial pH effect also indicate that the magnetic field does not induce or stop the autocatalytic reduction of nickel ion but just changes the steady rate of the autocatalytic reduction of nickel ion. Complex Agent. Figure 6 shows the effect of the concentration of succinate ion on the steady rate at different magnetic flux densities. The results indicate that the steady rate increases with the concentration of succinate ion significantly when the concentration of succinate ion is lower than 0.04 M. However, such an effect is leveled off when the concentration of succinate ion is larger than 0.04 M, as shown in Figure 6. The magnetic field effects on the steady rate are insignificant
Ind. Eng. Chem. Res., Vol. 34, No. 12, 1995 4295 Table 3. Effect of the Concentration of Succinate Ion and the Magnetic Field on pH Change within Deposition of 15 min ([Nia+l= 0.076 M, [Hfl02-1 = 0.265 M, Deposition Temperature = 31 "C, Initial pH = 6.6) pH change [C4H404'-], M
B = 0.00 T B = 0.15 T B = 0.20 T B = 0.25 T 1.64 1.31 1.21 0.96
0.000 0.019 0.037 0.056
2.01 1.31 1.18 0.92
2.03 1.24 1.16 0.99
1.93 1.24 1.21 1.09
at different concentrations of succinate ion. The steady rate depends significantly on the pH because the initial pH plays an important role in the steady rate as mentioned in the previous section. The results of Figure 6 are due to the degree of buffer. The pH becomes lower as the deposition proceeds because protons are generated when the hypophosphite ion is oxidized as shown in eq 1. Consequently, the steady rate slows down when the pH decreases. The pH change is slowed down by the buffer system, which consists of hypophosphorous acid, hypophosphite ion, succinate ion, and succinic acid as shown in eqs 16, 18, and 19, respectively. The pH change is small when the concentration of succinate ion, i.e., degree of buffer, is high. Consequently, the effect of the concentration of succinate ion on the steady rate is small at a high concentration of succinate ion. At initial pH 5.5, the changes of pH during a run are shown in Table 3. A larger pH change was found at a lower concentration of succinate ion. For example, the averages of pH change at 0.000 and 0.056 M succinate ion are about 1.90 and 0.99, respectively, which correspond to pHs of 3.60 and 4.51 in the solution, respectively. The steady rate of the autocatalytic reduction of nickel ion a t pH 3.60 becomes very low as shown in Figure 5; however, the steady rate a t pH 4.51 is moderate. The pH change is small as the concentration of succinate ion is high as shown in Table 3. However, the magnetic field effects on the pH change during a run of 15 min are insignificant. The insignificant effect of the magnetic field on the pH may be due to the smaller magnetic field effect on the equilibrium constants of ions in the autocatalytic reduction of nickel ion.
Conclusions The Langmuir-Hinshelwood adsorption model was used to interpret the reaction behaviors of the autocatalytic reduction of nickel ion by the hypophosphite ion. According t o the theoretical model and the experimental results, the autocatalytic reduction of nickel ion was a heterogeneous catalytic reaction in which the adsorption and desorption reactions were the ratedetermining steps. The results of the magnetic field effects on the autocatalytic reduction of nickel ion by the hypophosphite ion indicated that the rate constant of the free-radical recombination reaction of the radical pair Ni'+ and atomic hydrogen and the rate constants of adsorption and desorption of nickel ion, respectively, were affected by the magnetic field. The relations of the magnetic field and the rate constants were well correlated by two exponential equations as follows,
K = 0.0482 exp(-14090B/RT)
(20)
K" = 1.0178 exp(+12960B/RT)
(21)
where B is the magnetic flux density in units of tesla.
The theoretical analysis of the steady rate correlated well with the experimental results and a semiempirical rate equation was obtained,
r8 = 0.0482 exp(-14090B/RT) x 1.0178 exp(+ 12960B/RT)[Ni2+l (13) 1 1.0178 exp(+12960B/RT)[Ni2+]
(+
)
where B is the magnetic flux density in units of tesla. The apparent frequency factor and the apparent activation energy for the autocatalytic reduction of nickel ion decrease with the magnetic field. The largest steady rate occurred in the pH range from 4.5 to 6.0. The degree of buffer of solution for the change of pH was altered by the concentration of succinate ion. The steady rate increased with the concentration of succinate ion. However, the steady rate slowed down because the change of pH was slower at a higher concentration of succinate ion.
Acknowledgment The authors are thankful for the financial support provided by the National Science Council through Grant NSC 83-0416-E-006-016 and National Cheng Kung University.
Nomenclature B = magnetic flux density, T E , = activation energy for free-radical combination reaction, kJ/mol Ean= activation energy for adsorption and desorption reactions, kJ/mol g = g-values of free radicals h = Planck constant, J s k = rate constant for free-radical combination reaction, Wmin K" = rate constant of adsorption and desorption reactions for nickel ion, 1N Kr, = rate constant of adsorption and desorption reactions for hypophosphite ion, 1N KO= frequency factor for free-radical combination reaction KO" = frequency factor for adsorption and desorption reactions Kobs= observed rate constant, l/min m = magnetic constant for free-radical combination reaction, kJ/(mol T) m n = magnetic constant for adsorption and desorption reactions, kJ/(mol T) R = gas constant, J/(mol K) r = steady rate, Wmin T = absolute temperature, K Subscripts a = adsorption state
n l = first type active site occupied by nickel ion p2 = second type active site occupied by hypophosphite ion s = steady state Greek Letters w = rephasing frequency, rad 19 = fraction of active site p b = Bohr magneton, J s
Literature Cited Chiba, A,; Ogawa, T. Influence of Magnetic Fields on Electroless Nickel Plating. Hyman Ggitsu 1989,40, 568-572.
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Received for review February 14, 1995 Revised manuscript received July 11, 1995 Accepted July 24, 1995@ IE950114X
* Abstract published in Advance ACS Abstracts, November 1, 1995.
