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KINETICS, CATALYSIS, AND REACTION ENGINEERING Effect of Chromate Addition on the Structure and Kinetics of Leaching for the Formation of Promoted Skeletal (Raney) Cobalt Catalysts Andrew J. Smith,*,† Leonito O. Garciano II,† and Tam Tran‡ School of Chemical Sciences and Engineering, UniVersity of New South Wales, Sydney 2052, Australia, and NUEnergy Pte Ltd., 25 International Business Park, Unit #03-25, Singapore 609916
The internal porous structure of a chromia-promoted skeletal cobalt catalyst was directly and clearly imaged for the first time. The structure was made finer by the addition of sodium chromate to the leaching solution during preparation, and this resulted in a higher BET (Brunauer, Emmett, and Teller) surface area. Leaching proceeded with a sharp reaction front and followed an Avrami-Erofe’ev kinetic equation. A full quantitative model including parameters for particle size, chromate concentration, and temperature was derived. The activation energy of 75 kJ mol-1 was not significantly different from that for unpromoted cobalt. The mechanism of producing the finer structure in light of the kinetic information is discussed in terms of the exposed aluminum surface area. Introduction The effect of chromia promotion, either through alloying before leaching, soaking after leaching, or by adding chromate to the leach liquor during the preparation of skeletal catalysts, is well-known to increase the catalytic activity of both skeletal copper and nickel.1-6 Studies of the skeletal copper system have revealed that chromate slowed the leaching kinetics7,8 while creating a much finer porous structure and correspondingly higher surface area.3,7 A mechanism for this was proposed.9 A detailed structural and kinetic study of the effect of chromate addition to the leaching of skeletal cobalt has not been undertaken so far, and it is useful to explore the fundamental effects of this important additive in this commercial catalyst. This work is a direct extension to our previous paper on the unpromoted cobalt catalyst,10 confirming and extending the kinetic model and ideas to the chromium-promoted form of the catalyst. This work also allows the mechanistic theories proposed for the copper catalyst to be tested in a different but comparable system. Method Alloy. Precursor Co-Al alloy was kindly donated by the Davison Division of W. R. Grace & Co. Baltimore, Maryland. The alloy was crushed in a mortar and pestle and screened to narrow particle size ranges using a series of laboratory test sieves. The precursor alloy materials tested were in the (-211 + 160), (-160 + 125), and (-125 + 50) µm ranges. Characterization. The particle size distribution was determined by a Coulter LS230 and a Malvern Mastersizer. Crystal identification was confirmed by X-ray diffraction on a D5000 diffractometer. A Nikon model Epiphot 200 optical microscope was used for imaging the reaction front of partially leached alloy. Electron microscopy images were obtained on an xT Nova NanoLab 200, which combines a dual-beam high-resolution * Corresponding author. Phone: +61 2 9385 4319. Fax: +61 2 9385 5966. E-mail:
[email protected]. † University of New South Wales. ‡ NUEnergy Pte Ltd..
focused ion beam (Ga FIB) and a high-resolution scanning electron microscope. The composition of acid-digested catalyst samples was analyzed using a VISTA AX CCD simultaneous ICP-AES (inductively coupled plasma atomic emission spectroscopy). The BET surface area was determined by a singlepoint BET (Brunauer, Emmett, and Teller) nitrogen adsorption method, and the pore volume was determined by the BJH (Barret, Joyner, Halenda) method on a Micromeritics system. Leaching Procedures. The leaching experiments were conducted using a 0.6 L round reactor. The temperature during leaching was maintained by a Haake (FISONS) k20 refrigeration unit with DC3 circulator. In each experiment, 5.0 g of the alloy was weighed accurately and added to 0.5 L of sodium hydroxide + sodium chromate maintained at the test temperature. The sodium chromate concentration was one of either 0.001, 0.005, 0.010, or 0.025 mol L-1, the temperature was set at either 4.7, 11, 20, or 40 °C, and the NaOH concentration was either 1.5, 2.7, 6.0, or 10.4 mol L-1, as indicated in the results. The volume of hydrogen evolved during leaching ((0.1%) was measured by a precalibrated displacement wet-type gas flow meter (DM3A) from Alexander Wright Division. As reported earlier,10 the measurement of evolved hydrogen was confirmed as valid for tracking the reaction progress. Results and Discussion Structure. The chromia-promoted skeletal cobalt catalyst has a structure similar to the unpromoted catalyst but on a finer scale (Figure 1). This effect of chromia promotion on making a finer structure is consistent with that seen in the skeletal copper system.7 The chromia-promoted cobalt catalyst has pores that are smaller than those of unpromoted cobalt,10 unpromoted copper,3,11 or chromia-promoted copper.3,7 Leaching proceeds with a sharp reaction front (Figure 2). Changes to the catalyst surface area, pore volume, and average pore diameter with varying amounts of hydroxide or chromate concentration, or with changes in temperature, are shown in Figure 3. Hydroxide concentration has a minimal impact above 2 mol L-1, while an increasing concentration of chromate gives
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Leaching at lower temperatures promotes a finer structure and, hence, larger surface area. Kinetics. The Avrami-Erofe’ev kinetic model12-14 with a time exponent of unity
-ln(1 - R) ) Koverallt
Figure 1. Internal porous structure of chromia-promoted skeletal cobalt. Leaching conditions: 6.0 mol L-1 of NaOH + 0.025 mol L-1 of Na2CrO4 at 4.7 °C.
provides an excellent approximation for the leaching kinetics of the chromia-promoted catalyst, up to overall conversions of ∼70-80% (Figures 4-7). This is the same kinetic model that was found to best describe the leaching of the unpromoted cobalt catalyst.10 Thus, it is likely that the same reaction mechanism is acting here and that the presence of chromate has not changed the underlying mechanism. The overall rate constant is inversely proportional to the average particle size, consistent with that seen for the unpromoted cobalt catalyst, and the effect of hydroxide concentration again causes a peak in the leaching rate at a concentration of ∼6 mol L-1 of hydroxide (Figure 6), but the influence is weak. Addition of sodium chromate to the leach liquor decreased the leaching rate (Figure 4), with increasing concentrations of chromate leading to further reductions in the leaching rate. A decrease was also seen with the copper catalyst7,8 and is probably related to the formation of the finer, stabilized structure hindering access for aluminum dissolution.7,9 The inverse of the overall rate constant of the kinetic model was proportional to the concentration of chromate in the leach liquor (Figure 8). In comparison to Figure 8, the skeletal copper system was reported as showing a logarithmic relationship.7,8 However, the data for the skeletal copper system could be graphed at least as adequately, if not more so, as the same inverse relationship found for the cobalt system (Figure 9). The cobalt system would not fit the logarithmic relationship. It is noted here that the comparison of these two systems is limited by the different range of chromate concentrations investigated. An Arrhenius plot (Figure 10) gave the activation energy for leaching as 75 ( 2 kJ mol-1. This compared with 72 ( 4 kJ mol-1 for the unpromoted cobalt catalyst. There was no significant difference between these two values, and the same rate equation applies, which means that the rate-limiting step is probably the same with or without the presence of chromate. Even so, the overall rate slows in the presence of chromate. This may be interpreted as being related to the exposed surface area during dissolution. That is, the mechanism and rate-limiting step are unchanged, but the amount of exposed aluminum for reaction is significantly reduced with increasing chromate in the system. In the skeletal copper system, chromate is known to aid nucleation of the product copper and hinder its agglomeration into larger crystallites.9 Hindering the agglomeration would help keep more of the aluminum in the alloy covered from hydroxide attack, slowing the reaction but not changing the mechanism or rate-limiting step. The same is probably occurring in the skeletal cobalt system when chromate is added and would explain the slowed kinetics but similar activation energy. For comparison, the activation energy values obtained for the copper system with and without promotion by chromia were 72 and 69 kJ mol-1,8 respectively, again with no significant difference caused by the chromia. As with the unpromoted cobalt catalyst, an overall kinetic model can be proposed that describes the rate of leaching in the presence of sodium chromate,
Figure 2. Sharp reaction front during leaching.
an increasing catalyst surface area over the range 0-0.025 mol L-1, corresponding with a decrease in the average pore diameter.
(1)
-ln(1 - R) )
( )
-Ea k 0t exp RT s0([CrO24 ] + 0.025)
(2)
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Figure 3. Effect of temperature, hydroxide concentration, and chromate concentration on surface area, pore volume, and pore diameter of chromia-promoted skeletal cobalt. Conditions held constant were 6.0 mol L-1 of NaOH, 0.025 mol L-1 of Na2CrO4, and 4.7 °C.
Figure 4. Kinetic model fit at different concentrations of sodium chromate (6.0 mol L-1 of NaOH, 4.7 °C, and (-125 + 50) µm particle size).
Figure 5. Kinetic model fit at different temperatures (6.0 mol L-1 of NaOH, 0.025 mol L-1 of Na2CrO4, and (-125 + 50) µm particle size).
where R ) overall conversion, k0 ) 2.1 × 1014 (µm‚mol‚L-1‚h-1), Ea ) activation energy (75 × 103 J‚mol-1), R ) gas constant (8.314 J‚K-1‚mol-1), T ) temperature (K), so ) initial average particle size (µm), [CrO24 ] ) concentration of chromate in leach liquor (mol L-1), and t ) time (h).
When inserting a zero value for chromate concentration in eq 2, the rate constant is of the same order as the value obtained from the non-chromate equation presented in the previous paper. While the values differ, they are within error given the empirical nature of the derivation. One could
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Figure 6. Kinetic model fit at different concentrations of sodium hydroxide (0.025 mol L-1 of Na2CrO4, 4.7 °C, and (-125 + 50) µm particle size).
Figure 9. Reanalyzed kinetic data for chromium-promoted skeletal copper as an inverse relationship (left axis) and logarithmic relationship (right axis). Data from ref 7.
Figure 7. Kinetic model fit at different initial particle size ranges (6.0 mol L-1 of NaOH, 0.025 mol L-1 of Na2CrO4, and 4.7 °C). Figure 10. Arrhenius plot (6.0 mol L-1 of NaOH + 0.025 mol L-1 of Na2CrO4 and (-125 + 50) µm particle size).
Acknowledgment Financial support from the Australian Research Council is gratefully acknowledged. Literature Cited
Figure 8. Effect of sodium chromate on the rate of leaching (6.0 mol L-1 of NaOH, (-125 + 50) µm particle size, and 4.7 °C).
try to average the values to obtain a universal model with or without chromium presence (averaging the rate constant and activation energy values); however, this increases the error present in each model. Given the effect of even small amounts of chromium on the way the leaching proceeds, it is probably best to leave the two models separate for now for improved accuracy. Conclusion The internal structure of a chromate-promoted skeletal cobalt catalyst has been imaged and presented for the first time. The kinetics of leaching are slowed with the addition of chromate, but the activation energy is unchanged. A full quantitative kinetic model for the leaching of chromia-promoted skeletal cobalt catalysts has been developed, and the probable mechanistic action of chromate has been discussed.
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ReceiVed for reView June 12, 2007 ReVised manuscript receiVed February 6, 2008 Accepted February 8, 2008 IE070809L
