Acceleration of Liquid-Solid Redox Reaction with a Magneto

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Acceleration of Liquid-Solid Redox Reaction with a Magneto-Catalyzed Method Caixing Liu, Yang Yang, Yuecheng Bian, Zongwei Ma, Chun Zhou, Qianwang Chen, Yuping Sun, and Zhigao Sheng J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08514 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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The Journal of Physical Chemistry

Acceleration of liquid-solid redox reaction with a magneto-catalyzed method Caixing Liu,ab Yang Yang,a Yuecheng Bian,ab Zongwei Ma,a Chun Zhou,a Qianwang Chen,ab Yuping Sun,ac Zhigao Sheng*,a

a

Anhui Province Key Laboratory of Condensed Matter Physics at Extreme Conditions, High

Magnetic Field Laboratory, Chinese Academy of Science, Hefei 230031, China b

University of Science and Technology of China, Hefei 230026, China

c

Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of

Sciences, Hefei 230031, China

ACS Paragon Plus Environment

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ABSTRACT: To accelerate the chemical reaction is a key issue in the studies of catalytic chemistry. Here, by taking liquid-solid redox reaction Zn/CuSO4 as a model system, we present a remote and non-touched magneto-catalyzed method that can accelerate the chemical reaction efficiently. The effects from intensity (B) and intensity × gradient (B∇B) of applied magnetic field are distinguished and the dominant role played by the B has been confirmed. With B increasing, the more of Zn-Cu galvanic cells and the bigger area of Cu/Cu2+ interfacial could be realized via a magnetohydrodynamics effect, which were proved by both optical and electron microscopic observations. It was found that 22 times enhancement of reaction rate and 7700 J/mol reduction of activation energy were achieved when an 8.4 T magnetic field was applied. These observations provide a magneto-catalyzed method to modulate the chemical reaction.


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INTRODUCTION Controlling of chemical kinetics has attracted many attentions due to their wide applications in industrial manufactures. To speed up a reaction, catalyst is the most popular choice. In a conventional catalyzed reaction, the catalyst takes part in the reaction, but it is chemically unchanged at the end of the reaction. In spite of this, the catalysts are generally difficult to be separated from the reaction products1-2. In this context, finding a noncontact catalytic method holds a great promise for chemical industry. As one of examples, electrostatic catalysis has been developed at first. This is because that all chemical reactions can be viewed as the movement of electrons and/or nuclei and then one could expect that the external electric field can affect their kinetics efficiently. The electrostatic catalysis is widely harnessed by enzymes3-4 and has also been applied to a Diels–Alder reaction recently5. In addition to the electric field, another remote and noncontact method is the magnetic field. With the development of superconducting magnets, magnetic field effects on physical or chemical process have attracted much attention recently6-14. For instance, magnetic isotope effects in organic photochemical reactions were found6-7. J. Turr et al. found that the magnetic field could enhance the rate and polymer molecular weight of polyreaction8. Hirota, N et al. found that the magnetic field can significantly enhance the dissolution rate of oxygen into water910

. Moreover, the magnetic field effects on the deposition rate and structure of metals have been

discovered in electrodeposition11-12. Our previous studies also found the magneto-accelerated effects on Ostwald ripening of hollow Fe3O4 nanospheres13 and Kirkendall processes of silicon nanospheres14. Comparing with electric field, which can drive the charges directly in the chemical reactions, the effect caused by the external magnetic field is a lit bit complicated. Indeed, magnetic field is


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a vector field which can be depicted simultaneously by its intensity (B) and gradient (∇B, change of B over distance)15. At first, the external magnetic field can provide an additional energy onto the chemical reaction, which is called magnetic potential energy15 (Um ~ B). Also, similar to the electric field, Lorenz force16 (FL ~ B), exerted on a moving charge through a magnetic field, could affect the moving direction of particles in the chemical reaction. Moreover, for a substance with magnetic moment, an additional magnetic force (Fm ~ B∇B)9, 17 may also alter the reaction kinetics. Accordingly, in the studies of magnetic field effect on the chemical reactions, it is quite important to distinguish the roles played by Um, FL and Fm. Based on their definitions, the Um and FL are decided by B, while the Fm is determined by the product of intensity and gradient (intensity × gradient, B∇B)18. As a result, identifying the contributions from B and B∇B respectively becomes one of the key issues to explore the mechanism of magnetic field effects9, 19

. Nevertheless, compared with the electric field effect case, this issue for the magnetic field

effect on chemical reactions remains largely unexplored. Liquid-solid reaction is one of the most basic chemical reactions. In this work, we showed that the magnetic field could effectively accelerate the liquid-solid reaction between metal and salt solution, such as Zn/CuSO4, Fe/CuSO4 and Zn/AgNO3. The contributions from B and B∇B were distinguished respectively and the key role played by B was identified. Based on both optical and electron microscopic results, the magnetohydrodynamics effects on the formation of metal galvanic cells, reaction interfaces, activation energy, as well as the acceleration effect were discussed.

EXPERIMENTAL SECTION Zinc granules (Acros Organics, 20 mesh), iron powder (Aladdin, 100 mesh), cupric sulfate anhydrous (Macklin, 99%), silver nitrate (General-Reagent) and hydrochloric Acid (Adamas)


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were used in this work. Both zinc granules and iron powder were activated chemically before using. The reactants were sealed into a small vial (13 × 23 mm, Figure S1a), which was studied at room temperature without magnetic field and with magnetic field, respectively. The microstructure of reaction products were measured by both optical and electron microscopes. The scanning electron microscopic images were obtained on a FEI Helios Nanolab 600i. The optical microscopy images were obtained on a MV3000 microscope.

RESULTS AND DISCUSSION The magnetic field effects were studied between CuSO4 solution and zinc at first with a superconducting split magnet20 (Figure S1). The reaction rates were characterized by the solution color and the concentration change of Cu2+ ions (ΔC), which were measured via spectrophotometry method (Figure S2). As shown in the Figure 1a, 2.7 ml CuSO4 (1 M) and 0.24 g zinc granules (1.36 eq. based on the Cu2+ ions) were sealed into two small vials separately. The original solution was bright blue because of Cu2+ ions. One vial was put into a magnetic field (B = 4 T, B∇B = 64 T2/m) and another was not. After 1 hour, it was found that the color was still blue (ΔC = 0.37 mol/L) for the vial without magnetic field and the solution became almost colorless (ΔC = 0.85 mol/L) for the vial in the magnetic field. This indicates that the magnetic field can accelerate the reaction of Zn/CuSO4 efficiently. Similar magneto-acceleration phenomena were also found in other liquid-solid reactions between metal and salt solution, such as Fe/CuSO4 and Zn/AgNO3 (Figure S3). By put five same Zn/CuSO4 reactants at different places of the superconducting magnet, the magnetic field effect with different B and B∇B can be explored (Figure 1b). One hour later, it was found that the colors of the reacted solutions change with vials’ positions and it seems that such variation is synchronous with the variation of the B and not with the B∇B. This feature was


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also confirmed by the position dependence of ΔC ions as shown in Figure 1d. It was found that the ΔC of Cu2+ (red squares) has almost same tendency with the variation of B (solid blue line) and it is quite different from that of B∇B (dashed green line). This implies that the magnetoacceleration effect is mainly related to B rather than B∇B.

Figure 1. (a) The photos of Zn/CuSO4 reaction products without and with magnetic field. (b) and (c) are the magnetic field parameters and their corresponding photos of Zn/CuSO4 reaction products obtained at five places of the superconducting magnet. (d) The position dependences of ΔC, B and B∇B. To further distinguish the contributions of B and B∇B in the magneto-acceleration effect, the B dependence of reaction rate with fixed B∇B or vice versa were studied. On the one hand, the Zn/CuSO4 reactions were carried on by varying the intensity B individually at fixed position x = 470 mm in the magnet, at which B∇B is zero. For each external field B, the time evolutions of Cu2+ ions concentration were monitored and the results were presented in Figure 2. When B = 0 T, around 6.5 h is needed to finish the reaction. With increasing of B, the reaction time is


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reduced. It decreases dramatically to 3 h at B = 2.5 T, 1 h at B = 5.5 T and even down to 0.3 h at B = 8.4 T. That is to say the reaction can be accelerated around 22 times by an 8.4 T magnetic field. Moreover, it was found that the average reaction rate, defined as VB = ΔC/t (ΔC = 1 mol/L), is exponentially proportional to the B as shown in the inset of Figure 2. The VB-B curve can be well fitted by the relation VB = V0exp(a·B) with V0 = 0.15 mol·L-1h-1 and a = 0.368. Combining with Arrhenius equation analysis1, 8, the reduction of activation energy (∆E) was calculated as 7700 J/mol for B = 8.4 T, which is much larger than the magnetic energy Um ~0.5 J/mol (see Supporting Information (SI) for details).

Figure 2. The reaction time dependence of the Cu2+ ions concentration (C) for the reactions with different B. The inset shows the B dependence of the average reaction speed (VB). On the other hand, the magneto-acceleration effects were also studied with fixed B and varied B∇B (see SI for method). The ∆C of Cu2+ ions in the reaction products was recorded as a function of B∇B and the typical results for B = 2.3, 3.8, and 6.4 T were shown in the Figure 3a. For B = 2.3 T, the ∆C was almost constant and the colors of the reaction vials didn’t change with B∇B (Figure 3a). Similar tendency could also be found for the cases with B = 3.8 T and 6.4 T


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(Figure S4). Such B∇B insensitive feature is different from the previous report17. As mentioned above, a magnetic force Fm can be induced by B∇B for a substance with non-zero magnetic susceptibility χm10,16. For a paramagnetic Cu2+ ion with χm = 1500 × 10-6 cm3/mol21, the Fm is around 0.8 N/mol for the B∇B=105 T2/m (see SI for details). Such force is too small to modulate the reaction rate in a megascopic system22. To explore the B∇B contribution further, another typical liquid-solid reaction, Zn/AgNO3, was studied in the magnetic field (Figure S3a). Contrary to the paramagnetic Cu2+, both Zn2+ and Ag+ are diamagnetic ions19 with much smaller χm = -10 × 10-6 cm3/mol and -24 × 10-6 cm3/mol, respectively. The two orders reduction of χm suggests that the magnetic force Fm effect caused by B∇B should be negligible in the Zn/AgNO3 reaction. And then, no magneto-acceleration effect should be expected if the B∇B is dominant. Figure 3b presents the typical results for the magnetic field effect on Zn/AgNO3 reaction (B = 5.5 T, B∇B = 0 T2/m). It was interesting to find that an obvious magneto-acceleration effect still existed and ~50% enhancement of Ag+ ions ∆C is achieved in 0.5 h. These results confirm again that the Um and Fm aren’t dominant in the magneto-acceleration effects. The kinetics process of the liquid-solid reaction between metal and salt solution has mainly three steps2. For Zn/CuSO4 reaction (Figure 4a), as an example, the first step is the migration of Cu2+ ions to the surface of zinc. The second step is the electron transformation from zinc to Cu2+. After reaction, the solid zinc becomes Zn2+ ions and the Cu2+ ions turn to be solid copper simultaneously. The last step is the migration of Zn2+ ions into solution. Both step 1 and step 3 are related to the ion diffusion, which only be influenced by the magnetic force (Fm ~ B∇B) in the magnetic field19, 21. But according to the above discussion, the magneto-acceleration effect is insensitive to the B∇B. Hence, the magnetic field effect on the step 1 and step 3 is negligible in our case. Then, the magnetic field effect on the step 2 should be carefully examined.


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Figure 3. (a) The B∇B dependence of the Cu2+ ions ΔC. The reaction time for 6.4 T case is 0.5 h, and the others are 1 h. The photos corresponded 2.3 T are shown in the inset. (b) The Ag+ ions ΔC in Zn/AgNO3 reactions obtained without and with magnetic field. (c) The mass change (Δm) of zinc in Zn/HCl reactions obtained without and with magnetic field It was reported that the electron transformation from zinc to Cu2+, i.e., the step 2, has two paths23. One is the direct electron transformation from solid zinc to the Cu2+ ions. Another is an indirect path through the Zn-Cu galvanic cells (Figure 4a). In detail, the products of first path, solid copper, are attached onto the solid zinc and form tiny Zn-Cu galvanic cells in the presence of an electrolyte (CuSO4), which can lower the activation energy of the charge transformation process. And then, the Cu2+ ions can get electrons easily from the surface of solid copper than


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that from the solid zinc24-25. As a result, this indirect path becomes dominant in the step 2 of the Zn/CuSO4 reaction, which has been confirmed by both theoretic and experimental reports23-25.

Figure 4. (a) The kinetic process of the Zn/CuSO4 reaction. (b) The schematic illustration of the Zn/Cu2+ reaction in B = 0 and B ≠ 0. (c) The microscopic images of Cu on Zn with different B. (d) The SEM images of reaction product copper obtained with different B. From the viewpoint of electron transformation, the amount of the Zn-Cu galvanic cells and the Cu/Cu2+ interfacial area play the key roles in the controlling of Zn/CuSO4 reaction rate. Without magnetic field, the Zn-Cu galvanic cells and the Cu/Cu2+ interfacial area increases with time


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slowly. With application of external magnetic field, the micro-convection of the Zn/Cu2+ interface induced by Lorentz force, so called magnetohydrodynamic (MHD) effect16, 22, will affect the formation of Zn-Cu galvanic cells as well as the Cu/Cu2+ interfaces. As shown in the Figure 4b, the micro-convection tends to disturb the linear motion of Cu2+, so the MHD effect could increase the nucleation number of copper and reduce its average grain size, which leads to more Zn-Cu galvanic cells and bigger Cu/Cu2+ interfacial area. Figure 4c presents the optical microscopic images for the coverage ratio of Cu (white dots) on Zn (black background), which are obtained with same reaction time (~10 min) and different external magnetic fields (Figure S5). With the magnetic field increases from 0 to 5.5 T, the coverage ratio increases from 1.8% to 6.3%. That means that the magnetic field increases the number of Zn-Cu galvanic cells as well as Cu/Cu2+ interfacial area at the primary stage of the reaction. Figure 4d shows the scanning electron microscopic (SEM) images of solid copper products obtained with reaction time ~12 h and various magnetic fields. It was found that the average grain size of Cu remarkably decreased from 2 µm at 0 T to 1 µm at 5 T and 0.3 µm at 8.4 T. Apparently, the smaller grain size indicates again the enlargement of Cu/Cu2+ interfacial area induced by the magnetic field. Both the optical and electronic microscopic measurements verified the MHD effects mentioned above. Comparing with the reaction without magnetic field, the MHD induced growing number of the Zn-Cu galvanic cells and an enlargement of Cu/Cu2+ interfacial area would reduce the activation energy and accelerate the reaction rate as observed. One point should be noted that, due to the random distribution of ions’ moving directions in the solution (Brownian motion), the MHD effect as well as the magneto-accelerated effect should be magnetic field direction independent, which has been confirmed by our experiments (Figure S3b and S6). Moreover, as a comparison experiment, the Zn/H+ reaction was studied in the magnetic field, in which no formation of


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galvanic cells due to the product H2. It was found that there was no magneto-acceleration effect even an 8.4 T magnetic field was applied (Figure 3c, more details shown in Figure S7). It confirmed again that MHD effect on the metallic galvanic cells is the key point in the magnetoacceleration process.

CONCLUSIONS In summary, a non-touched magneto-catalyzed method was studied in the liquid-solid reaction between metal and salt solution. Taking the non-magnetic Zn/CuSO4 reaction as a model system, 22 times enhancement of reaction rate was achieved. It was revealed that the role of B∇B is nearly negligible and B is the dominant factor for such magneto-catalyzed effect. With evidenced by microscopic observations and compared experimental results, the MHD effects on the formation of Zn-Cu galvanic cells, the area of Cu/Cu2+ interfaces, as well as the reaction rate have been discovered. Our observations provide a new route for the remoted catalysis of chemical reaction by the magnetic field.


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ASSOCIATED CONTENT Supporting Information.

AUTHOR INFORMATION Corresponding Author [email protected]. ORCID Zhigao Sheng: 0000-0003-3382-5968 Present Addresses Zhigao Sheng. High Magnetic Field Laboratory, Chinese Academy of Science, Hefei 230031, China Notes The authors declare no competing financial interests

ACKNOWLEDGMENT We gratefully acknowledge financial support from the National Key R&D Program of China (Grant No. 2017YFA0303603, 2016YFA0401803), the National Natural Science Foundation of China (NSFC; Grant No. U1532155, 11574316), the Key Research Program of Frontier Sciences, CAS (Grant NO. QYZDB-SSW-SLH011), the Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (Grant No. 2016FXCX002, 2016HSC-IU006), the Instrument Developing Project of the CAS (Grant No. YZ201423), and the One Thousand Youth Talents Program of China.


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Table of Contents (TOC) Image


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Figure 2. The reaction time dependence of the Cu2+ ions concentration (C) for the reactions with different B. The inset shows the B dependence of the average reaction speed (VB). 70x62mm (300 x 300 DPI)


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Figure 3. (a) The B∇B dependence of the Cu2+ ions ∆C. The reaction time for 6.4 T case is 0.5 h, and the others are 1 h. (b) The Ag+ ions ∆C in Zn/AgNO3 reactions obtained without and with magnetic field. (c) The mass change (∆m) of zinc in Zn/HCl reactions obtained without and with magnetic field. 259x389mm (300 x 300 DPI)



TOC Graphic 166x156mm (300 x 300 DPI)


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Acceleration of Liquid-Solid Redox Reaction with a Magneto

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