Electrochemistry of carbonaceous materials

Electrochemistry

of carbonaceous

3. Reactivity of redox couples phosphoric acid media* Renato

Tomat,

Romeo

Salmaso

with

and Sandro

materials

coal slurries

in 85%

Zecchin

CNR-lstituto di Polarografia ed Elettrochimica Preparativa, 35020 Padova, Italy (Received 2 December 1991; revised 11 January 1993)

Corso Stati Uniti 4,

Sardinian subbituminous coal slurries were oxidized in 85% H,PO, by metal ions (Ce4+, V5+, V4+, Mn3+ and Fe3 ‘) and the effects of metal ion concentration, coal content of slurry, coal particle size and temperature were examined. (Keywords: coal; oxidation; kinetics)

Several studies’-l4 have been made of electrochemical techniques for coal conversion, since such processes may be controlled continuously. In the electrolytic oxidation of acidified coal slurries, currents are quite low (0.025 mAcmp2 for a slurry concentration of 10gl-‘)6, mainly because of the oxidation of ferrous species trapped in the coal and acting as charge carriers between the anode and the coal particles. Hence in the electrolytic oxidation of coal slurries, acidic slurries containing redox mediators are generally used in order to obtain a reasonable current density. This paper reports measurements of the rate of reaction of Sardinian coal slurries with oxidizing ions in concentrated H3P04. In this medium good manipulation is possible at high temperatures, and a wider working range is achieved. To monitor the decrease in concentration of the oxidizing ions, an electrochemical method was used, because of its reproducibility and convenience.

EXPERIMENTAL Reagents

The coal used was from the Sulcis basin (Sardinia). It was mechanically powdered and then separated into four fractions passing sieves of apertures 74, 105, 177 and 250 pm. The smallest fraction was boiled under reflux in 2M H,SO, for 18 h to remove all leachable inorganic material, in particular iron-containing species. The coal was then filtered and repeatedly washed with distilled water, dried under vacuum at 50°C and kept in stoppered bottles in a desiccator. Solutions were prepared using reagent-grade 96% H,SO, and 85% H,PO, and distilled water. The inorganic salts of the metals examined (Ce, V, Mn and Fe) were pure reagents. Solutions of FeNH,(SO,),. 12H,O and Ce(S0,),.4H,O were prepared by dissolving the salts directly in H,P04. V5+ solutions were prepared by dissolving V,O, in 1~ H,SO, to 0.08~. Solubilization *Part

2: ref. 19

00162361/94/02/0211~3 0 1994 Butterworth-Heinemann

Ltd.

rapid and the solution was stable indefinitely at 25°C. A similar stock in 85% H,PO, could not be prepared because solubilization required heating and resulted in a mixture of IV and V oxidation states. V4+ solutions were prepared by dissolving V,O, in 85% H,PO, to 0.08~. For experiments with Mn3+-Mn2 + couples, a solution of Mn3+ in 85% H,PO, was prepared by electrolytic oxidation at room temperature of Mn2+ acetate tetrahydrate at 1.2V at a Pt working electrode, because solubilization of Mn3+ salts in this medium is difficult. After electrolysis, the Mn3+ was titrated with a solution of Fe(I1) sulfate in 3M H,SO, with ferroin as indicator. The faradaic yield was -90%. was

Apparatus and procedure

Voltammetric and chronoamperometric measurements on the coal slurries were carried out at 45, 70 and 100°C in a three-electrode thermostated cell containing 25 cm3 of H,PO,. The working electrode was a 0.52 cm2 Pt plate and the counter-electrode a Pt plate. A standard calomel electrode was used as a reference. A reflux column connected to the cell was used to condense vapour evolved at high temperatures. All experiments were performed under inert gas (argon) atmosphere. The electrochemical apparatus was an electronic potentiostat associated with an x-y or y-t recorder. Voltammograms were run before and after the reactions at a potential scan rate of 5 mV s- ’ using a programmable function generator together with the above instruments. Experimental current-time curves at different coal slurry concentrations, coal particle sizes and temperatures were obtained with stirring at a controlled speed of 2000 rev min- ’ at the stationary working electrode, after addition to the coal slurry in 85% H,PO, of the appropriate quantity of the oxidizing metal ions, by monitoring the current at potentials on the plateau of the reduction wave. Under the conditions of vigorous stirring used for homogeneous dispersion of the solid, a constant flux of the oxidizing ions to the electrode was obtained, allowing their concentration in the bulk of the slurry to be monitored.

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Electrochemistry

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RESULTS AND DISCUSSION The rate constant of the metal ioncoal reaction was measured in concentrated H,PO, as electrolyte, since it allows relatively high temperatures to be used without water loss and at the same time the electrolysis potential to be extended to more anodic values. Currents are lower than in sulfuric acid. The viscosity of concentrated H,PO, is much higher than that of water and of dilute solutions of acids; values are 0.59mPa s for water and 20mPas for 85% H,PO, at 45°C. This means that, according to Walden’s law, the diffusion coefficients in H,PO, as electrolyte are lower, since for most substances the product of the diffusion coefficient in a medium and the dynamic viscosity, Dq, is a constant’ 5. The possibility of ion complexation also leads to further current reduction, although the conductivity of H,PO, is increased by working at temperatures of N 100°C or above. To exclude the possibility of oxidation in the homogeneous phase at the expense of dissolved coal species, a portion of the coal was repeatedly washed in hot H,PO, and the filtrate was tested for reaction with the oxidant metal ion: reactivity was practically negligible. As the stationary current due to reduction of the oxidizing ions under the same conditions as in the kinetic experiments but in the absence of coal was N 100 ,uA, and the reaction times between the ions and coal were at most a few minutes, the contribution to the disappearance of these ions by their reduction at the electrode was also negligible. Figure I shows the rate constant of the Ce4+ ion-coal reaction obtained from current-time measurements at +6OOmV, as a function of coal concentration, at 45°C. The i-t curve fitted first-order kinetics with respect to both metal ion and coal concentration. The same linear dependence on both coal and Ce4+ concentration was found at 70 and 100°C. The rate increased with increasing temperature, as shown in Figure 2. The slope of this line gave an activation energy of 39.4 kJ mol- ‘, in agreement with the literature9 and indicating the good oxidizing power of Ce4+. The coal particle size also influenced the rate of coal oxidation by metal ions, as shown in Figure 3. This may

I

I

I

I

I

2.7

2.8

2.9

3.0

3.1

l/T

(1O-3 K-l)

Figure 2 Arrhenius plot of experimental rate constants (pseudo-firstorder conditions) for reaction between Ce4+ and <74pm washed coal slurries in 85% H,PO,. Initial Ce4+ concn 1.57 x 1O-aM; coal contents 5g1-’

L 0

I

I

I

I

1

2

3

4

r-1

[mm

-1)

Dependence on coal particle size (radius r of equivalent sphere) of experimental rate constants for reaction between unwashed coal and Ce4+ in 85% H,PO, at 100°C. Initial Ce4+ concn 1.57 x 10m3M; coal content 1Ogl-i; coal particle sizes ~74, <105, ~177 and <250pm Figure 3

be due to the increased overall surface area and thus of the number of active oxidizable sites when the particle size is reduced. In fact the reaction rate was found to depend linearly on the reciprocal of the radius of the coal particles, assuming them to be spherical. To study the reactivity of other ionic species with high redox potentials, similar measurements were carried out with V5+ and Mn3+, standard potentials of which in 85% H,PO, are respectively +970 and + 1020mV (compared with + 930 mV for the couple Ce4+-Ce3 ‘) at 45°C. For vanadium, there are two reactions:

Figure 1 Experimental rate constants (pseudo-first-order conditions) for reaction between Ce4+ and <74pm washed coal slurries in 85% H,PO, as a function of coal content at 45°C. Initial Ce4+ concn 1.57 x 10-3M

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V5 + + coal - k;

V4+ + products

V4 + + coal - kl,

V3+ + products

where k; and k; represent the experimental rate constants measured using a sufficiently large excess of coal to obtain pseudo-first-order conditions. The first couple in H,PO, is reversible; the second is not. The formal redox potential

Electrochemistry Table 1 Experimental rate constants (pseudo-first-order conditions) for reaction between V‘+ ( 1 57 x 10-3M) and washed coal slurries in 85% H,PO, at 100°C Coal (gl-‘)

:;o- zs-1 )

5 7.5 10 15 30 45

0.19 0.30 0.41 0.55 1.17 1.68

of carbonaceous

materials:

R. Tomat et al.

at 100°C (unpublished data); since for Fe3+ they are of the order of 10-ss-’ for slurries containing 4OglI’ of the same coal in dilute H,SO, at 25”C”, in H,PO, they will be lower. It was therefore impossible to obtain reproducible data by chronoamperometry at + 50 mV. Studies are in progress on the optimization of massive electrolysis in H,PO, for oxidative degradation of coal mediated by the above metal ions, together with characterization of the resulting products. CONCLUSIONS

Table 2

Experimental rate constants (pseudo-first-order conditions) for reaction between Mn3+ (1.57 x 10e3M) and washed coal slurries in 85% HJPO, at 45°C

Coal (gl-‘1

;-1,

10 20 35

0.11 0.21 0.37

of the V’ ‘-V4+ couple depends on the medium: in 85% H,PO, (14.7M) it is N +550mVi6. As the standard potential of the V 5+-V4+ redox couple is + 970 mV, the coal reduces V5+ to V4+ and V3+ at two different rates. The experimental value of k; is at least 100 times that of k; at the same temperature and coal concentration; k; = 13 x 10e2 s-l was obtained at a coal concentration of 5g1-’ at 70°C and the results of a series of measurements of k’, at 100°C with various coal contents (the current being monitored at 0 V after addition of V4+) are shown in Table I. Current-time measurements after addition of Mn3+ to the slurry showed first-order kinetics with respect to the metal and coal (Table 2). It should be noted that the reaction rate was again higher than with cerium, as predicted on the basis of the standard potential values of the Mn3+-Mnzf and Ce4+ -Ce3 + couples. For Fe3 +, the results were strongly dependent on oxygen present as an impurity, since the standard potential of the Fe3 +-Fe’+ couple in concentrated H3P04 is + 150mV vs. SCE at 25”Ci6, i.e. it undergoes a considerable shift towards more cathodic values in comparison with the data obtained in dilute H2S04 (+ 540mV vs. SCE4). Consequently, Fe2 + may be oxidized by traces of oxygen which may still be present in the slurry and which is irreversibly reduced at a potential of - +750mV vs. RHE”. This is due to the slowness of the Fe3+-coal reaction in H,PO,: in fact for Ce4+ the rate constants are higher in dilute H,SO, at 25°C than in 85% H,PO,

Kinetic measurements on metal ion-coal slurry reaction in 85% H3P04 gave values of rate constants for Fe, Ce, V and Mn species at various temperatures and coal particle sizes. The kinetics were first-order with respect to both ions and coal, at least in the initial reaction phase, when, as pointed out by Thomas et al.14, oxidation of more superficial sites on the coal particles takes place. The good linear dependence on reagent concentrations justifies this statement. The order of reaction rates directly follows that of the redox potentials of the metal couples used, i.e. Mn3+ >V5+>Ce4+>V4+ >Fe3+.

REFERENCES

7 8

9

10 11 12 13 14 15 16 17 18 19

Coughlin, R. W. and Farooque, M. Nature 1979, 279, 301 Farooque, M. and Coughlin, A. W. Nature 1979, UIO, 666 Coughlin, R. W. and Farooque, M. J. Appl. Electrochem. 1980, 10, 129 Rallo, F. Paper to ISE Meeting, Venice, Sept. 1980 Lynch, C. S. and Collett, A. R. Fuel 1932, 11, 408 Swinkels, D. A. J., Anthony, K. E., Trau, T. and Linge, H. G. NERDDC Research Report, Contract No. 80/0210, Department of National Development and Energy, Canberra, 1981 Okada, G., Guruswamy, V. and Bockris, J. O’M. J. Electrochem. Sot. 1981, 128,2097 Swinkels, D. A. J., Anthony, K. E., Linge H. G. and Valentine, N. B. NERDDC Research Report, Contract No. 80/0210, Department of National Development and Energy, Canberra, 1982 Dhooge, P. M., Stillwell, D. E. and Park, S. M. J. Electrochem. Sot. 1982, 129, 1719 Dhooge, P. M. and Park, S. M. J. Efectrochem. Sot. 1983, 130, 1029 Anthony, K. E. and Linge, H. G. J. Elecrrochem. Sot. 1983,130, 2211 Kreisa, G. and Kochanek, W. J. Electrochem. Sot. 1985, 132, 2084 Kawakami, K., Okumura, T., Kusunoki, K., Kusakabe, K., Morooka, S. and Kato, Y. J. Chem. Eng. Japan 1986, 19, 134 Thomas. G.. Chettiar. M. and Birss. V. I. J. Ad. I. Efectrochem. ~-~ 1990,20,941 Bard, A. J. and Faulkner, L. R. in ‘Electrochemical Methods’, Wiley, New York, 1980, p. 154 Rao, G. and Dikshitulu, L. S. A. TuZu~tn 1963, 10, 295 Glass, J. T., Cahen, G. L., Jr and Stoner, G. E. J. Electrochem. Sot. 1989, 136, 656 Tomat, R., Salmaso, R. and Zecchin, S. Fuel 1992, 71, 459 Tomat, R., Salmaso, R. and Zecchin, S. Fuel 1992, 71, 463

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