Pore Development during Selective Leaching

Abstract

The leaching kinetics of a Cu-50 weight percent A1 alloy in aqueous NaOH have been studied. The product of the reaction, known as Raney copper, an important industrial catalyst, consists almost entirely of highly porous copper. In forming this material, the A1 is selectively dissolved from the alloy, the rate of the leaching reaction being controlled by liquid-phase diffusion within the pores of the leach residue. At constant temperature, the spacing of these pores increases as the leaching rate decreases. As the temperature decreases, so does the pore spacing. The conversion of the original alloy phase, CuAI~, to porous copper is considered as an example of a phase transformation, and the pore spacing is taken as a measure of the distance over which Cu-A1 segregation must occur within the alloy. Models based on volume diffusion adjacent to the reaction front and on boundary diffusion at the front are used to describe the relationship between pore spacing and reaction rate. Application of these models to the experimental data, for a temperature of 20~ yields estimates of D = 6 x 10 -lz cm2s -1 with an activation energy of 53 -+ 2 k J for volume diffusion and DB = 5 X 10 -I0 cm2s-' with an activation energy of 62 -+ 4 k J for boundary diffusion. It is concluded that alloy segregation takes place at the leach reaction front by a mechanism of boundary diffusion.The selective leaching in alkaline solution of aluminum from alloys to leave a porous, spongy residue is a wellknown, but incompletely understood, phenomenon. Reactions of this type have been exploited since the days of Raney (1, 2) in the manufacture of high surface-area metal catalysts. The widespread use of Raney catalysts in a variety of chemical processes adds practical interest to fundamental questions about the nature of the leaching mechanism. Since the leach reaction determines the product morphology, it is possible that an understanding of the leaching mechanism will lead to the ability to control the morphology and, hence, the catalyst properties. The purpose of this study is to examine the relationships between leaching kinetics and product morphology.A series of studies (3-6) has shown that caustic leaching of aluminum and zinc from alloys based on copper, nickel, and cobalt proceeds according to parabolic kineticswhere X is the thickness of the reaction-product rim formed in time, t. The magnitude of k o is found to be independent of the orientation of the parent alloy with respect to reaction direction. From these observations, it is concluded that the leach reaction rate is controlled by diffusion through the liquid occupying the pores of the reaction product. This conclusion is consistent with the magnitude of k, and its activation energy.During these reactions, the microstructure of the precursor alloy is reproduced in the reaction product. An example is shown in Fig. 1, where a Cu-A1 alloy consisting of CuAI~ grains separated by frozen A1-CuA12 eutectic is seen to produce grains of porous copper separated by void space. This observation, to...

Selective Dissolution below the Critical Potential and Back Alloying in Copper-Gold Alloy

“…15 Kaiser 15 calculated a diffusivity of 5 ϫ 10 Ϫ12 cm 2 s Ϫ1 from the measured In concentration profile in the In-rich product layer of a Sn-In alloy. Diffusivities of a similar magnitude were reported to explain the selective dissolution behavior that produced the product layers in Cu-Au, 12 Cu-Al, 26,37 Cu-Ni, 38 and Cu-Zn 36 alloys. These diffusivity values are a few orders of magnitude larger than the value (ϳ10 Ϫ18 cm 2 s Ϫ1 ) calculated at the end of a 10 h recovery period that follows the transformation.…”

“…The room temperature diffusivities obtained by extrapolating high temperature data are of the order of 10 Ϫ30 cm 2 s Ϫ1 for Cu-Au, 41 10 Ϫ18 cm 2 s Ϫ1 for In-Sn, 42 and 10 Ϫ23 cm 2 s Ϫ1 for Cu-Al. 26 These extrapolated values are based on the assumptions of an equilibrium concentration of vacancies in the crystalline solid, similarity of the diffusion mechanisms at the high and low temperatures, and a crystalline phase through which the diffusion process occurs. Although the nature of the Cu-depleted product layer is not well defined, one might safely argue that these assumptions cannot be expected to hold during selective dissolution at room temperature.…”

“…Although the nature of the Cu-depleted product layer is not well defined, one might safely argue that these assumptions cannot be expected to hold during selective dissolution at room temperature. In addition to the above-mentioned literature 12,15,26,[36][37][38] reporting extremely high diffusion rates, compared to the extrapolated high temperature values, there is also a large volume of literature revealing similarly high three-dimensional diffusion rates which result in alloy formation during underpotential and overpotential deposition 39,40,[43][44][45][46] and during epitaxial growth [47][48][49][50][51][52][53][54] at room temperature. Furthermore, there are the findings that during dealloying ͑above E c ͒ the surface area increased steeply by up to 70 times, 18 that the Au-rich product layer was in a semisolid state, different from that of the crystalline parent alloy 19 and that the lattice parameter of the Au-rich porous structure varied between that of the alloy and the more noble metal.…”

“…17 On the other hand, the mechanisms that lead to selective dissolution from a planar surface below E c , and dealloying above E c , are the subject of considerable debate. [2][3][4][5][6][7][8][9][10][11][12]15,[18][19][20][21][22][23][24][25][26] The subject has been reviewed before [4][5][6] and is still attracting considerable attention.…”

Structure and Kinetics of Leaching for the Formation of Skeletal (Raney) Cobalt Catalysts

“…Leaching at lower temperatures gave significantly higher surface areas with a correspondingly larger pore volume and smaller average pore diameter. Surface areas are in general larger, and pore size is smaller, than those for skeletal copper leached under similar conditions 4 Effect of varying sodium hydroxide concentration (at 20.6 °C) and temperature (at 6.0 M NaOH) on (a) the total surface area, (b) the average pore diameter, and (c) the total pore volume of skeletal cobalt catalysts (particle size = −165 + 125 μm). …”