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XAFS study of Co(II) sorption at the α-Al2O3-water interface
ELSEVIER
Physica B 208&209 (1995) 439-440
XAFS study of Co(II) sorption at the ot-A1203-water interface S.N. Towle*, J.A. Bargar, P. Persson, G.E. Brown Jr., G.A. Parks Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA
Abstract Aqueous Co(II) sorption onto 0t-Al20 3 at pH 8.1 and surface concentrations (F) of 0.16 to 11.3 ~tmol m - 2 has been studied by XAFS spectroscopy. At low F, Co adsorbs in a monodentate fashion or in a bridging bidentate fashion and is present as small oligomers. A Co(OH)2-1ike phase forms at high F.
1. Introduction Knowledge of the stoichiometry of sorption reactions at the oxide-water interface is important in a number of areas. XAFS spectroscopy is one of the few techniques that can provide information about cationic complexes at such interfaces in situ, i.e. with water present. A number of workers have used titration methods to study the reaction stoichiometry of Co(II) ions sorbing on aluminas [1]; however, such studies have failed to derive a unique model of the surface complex. Our group [2, 3] has performed in situ XAFS studies on systems similar to this one. Use of Qt-AI203 as an adsorbent allows direct comparison with previous work on 7-AI2Oa-
2. Experimental Samples were prepared at six F values (0.27 19.6 ~tmol m -2) by equilibrating Linde-A a-A1203 solid in Co(NO3) 2 solutions ranging in concentration from 10 -4 to 10 -2 M. All slurries were then titrated to pH 8.1, centrifuged, and the supernatants analyzed to find the amount of uptake (/> 90% in all cases). XAFS experiments were conducted on the wet pastes at 20 °C at SSRL * Corresponding author.
on beamline IV-3 using Si(2 2 0) monochromator crystals detuned 40% to reject harmonics. Ring energy and current were 3.0 GeV and 50-100 mA, respectively. Fluorescence-yield data were collected with a Ge array detector at count rates of ~ 40 kcps. Data reduction and analysis were carried out using the program EXAFSPAK by G. George (SSRL) in conjunction with F E F F 6 [4]. Eo was fixed at 7720 eV, corresponding to the inflection point in the Co K-edge.
3. Results and discussion Fig. 1 shows deglitched, normalized, background-subtracted EXAFS and non-phase-shifted Fourier transforms (FT) of these data. The FTs of all samples show a 1st shell at R + A of 1.7 and two 2nd shells at 2.8 and 3.3 ~, (all uncorrected for phase shift). Results of our analysis are presented in Table 1. Comparison of FTs for sorption samples and Co(OH)2 (solid) indicates that the 1st and 2nd shells of the former are analogous to similar features in the latter. In Co(OH)2, the 1st shell has six O and the 2nd shell has six in-plane Co atoms. For both shells, Co(OH)2 was used as a model for determining phase and amplitude functions. The FT feature at 3.3 A, though small, appears in all of these sorption samples as well as in other data [2, 3], but not in Co(OH)2. This
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 1 8 - 7
440
S.N. Towle et al./Physica B 208&209 (1995) 439-440
Co
[ '~ ] l l i ~co~o.~ A
uJ t~x
F=O.30
5
7
9
11
13
1
k(l~)
2
3
4
5
6
R+a (A)
Fig. 1. EXAFS and non-phase-shift-corrected Fourier transforms. Table 1 Sample
No
Ro
Nco
Rco
NAI
RAI
F F F F F F
6 6 6 6 6 6
2.09 2.09 2.08 2.09 2.09 2.07
0.6 1.0 1.6 2.5 3.3 4.0
3.12 3.12 3.09 3.10 3.09 3.10
0.7 0.5 0.7 0.6 0.6 not fitted
3.63 3.61 3.56 3.65 3.62
= = = = = =
0.27 0.30 1.14 1.30 3.76 19.6
peak was fit with a F E F F model consisting of cornerlinked A106 + C 0 0 6 octahedra. The phase-corrected distance from the Co absorber to 2nd-neighbor A1 (3.6A) (Table 1) provides information about the mode of sorption. Due to the sizes of the C 0 0 6 and A106 octahedra, the largest possible separation of the Co and AI atoms is ~ 3.1 ,~, if they are edge-linked. Although there is little evidence for edge-shared A106 neighbors, an A1 shell at this distance would overlap strongly with the Co shell, making it impossible to reject edge-shared bidentate complexes. If C 0 0 6 is linked in bidentate fashion to two AIO 6 octahedra (i. e., a bridging bidentate site) in the 0~-A1203 surface, the distance between Co and A1 would be 3.65 A, which is the same as the observed distance. Thus, the CoO6 octahedron must either be bonded to the surface in a monodentate (atop) configuration, or on the bridging bidentate sites of the bulk ~-A1203 structure.
As F increases, the number of Co 2nd-neighbors increases. Even at the lowest F, however, the number of Co 2nd-neighbors is easily measurable, suggesting that adsorption sites adjacent to an existing Co adion are preferred over other sites. At higher F, the clusters of hydrated Co ions increase in size until for the sample with F of 19.6 lamolm -2 the adsorbed species seems to be a hydrous, highly disordered solid. Even for this coverage, though, solid Co(OH)2 is more than two orders of magnitude below the equilibrium solubility at pH 8.1. Since the n u m b e r of Co 2nd-neighbors is only four, the stoichiometry of this surface phase can be written as CO(I _x)(OH)(2_zx)(H20)2x, with x ~ 0.3. The main difference between these data and those for Co(II)/y-Al203 I-2] is that the Co-A1 distance is much smaller (3.3 A) in the case of 7-A1203. This difference may result from the way the fits were done, rather than from intrinsic differences in the data. The curve fitting technique used here better accounts for the overlap between shells than does the back-transform fitting technique used in the earlier study [2]. Also, if Co and A1 were at a distance of 3.3,g,, oxygens of the adjacent octahedra would be separated by just 2.3/~, a shorter O - O distance than found in any oxide. Co(II) adsorbs at the at-Al203-water interface as a monodentate species or a bidentate species. Co(II) tends to cluster on the surface, resulting in a disordered Co(OH)2-1ike phase at high F. The local environment of Co(II) on 0t-Al203 is similar to that on 7-A1203.
Acknowledgements This work was supported by D O E grant DE-FG0393ER14347-A002 and was carried out at SSRL, supported by D O E and NIH.
References I-1] P.H. Tewari and W. Lee, J. Colloid Interface Sci. 52 (1975) 77. [2] C.J. Chisholm-Brause, P.A. O'Day, G.E. Brown Jr. and G.A. Parks, Nature 348 (1990) 528. [3] P.A. O'Day, G.E. Brown Jr. and G.A. Parks, J. Colloid Interface Sci. 165 (1994) 269. 1-4] J.J. Rehr, S.I. Zabinsky and R.C. Albers, Phys. Rev. Lett. 69 (1992) 3397.
Physica B 208&209 (1995) 439-440
XAFS study of Co(II) sorption at the ot-A1203-water interface S.N. Towle*, J.A. Bargar, P. Persson, G.E. Brown Jr., G.A. Parks Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA
Abstract Aqueous Co(II) sorption onto 0t-Al20 3 at pH 8.1 and surface concentrations (F) of 0.16 to 11.3 ~tmol m - 2 has been studied by XAFS spectroscopy. At low F, Co adsorbs in a monodentate fashion or in a bridging bidentate fashion and is present as small oligomers. A Co(OH)2-1ike phase forms at high F.
1. Introduction Knowledge of the stoichiometry of sorption reactions at the oxide-water interface is important in a number of areas. XAFS spectroscopy is one of the few techniques that can provide information about cationic complexes at such interfaces in situ, i.e. with water present. A number of workers have used titration methods to study the reaction stoichiometry of Co(II) ions sorbing on aluminas [1]; however, such studies have failed to derive a unique model of the surface complex. Our group [2, 3] has performed in situ XAFS studies on systems similar to this one. Use of Qt-AI203 as an adsorbent allows direct comparison with previous work on 7-AI2Oa-
2. Experimental Samples were prepared at six F values (0.27 19.6 ~tmol m -2) by equilibrating Linde-A a-A1203 solid in Co(NO3) 2 solutions ranging in concentration from 10 -4 to 10 -2 M. All slurries were then titrated to pH 8.1, centrifuged, and the supernatants analyzed to find the amount of uptake (/> 90% in all cases). XAFS experiments were conducted on the wet pastes at 20 °C at SSRL * Corresponding author.
on beamline IV-3 using Si(2 2 0) monochromator crystals detuned 40% to reject harmonics. Ring energy and current were 3.0 GeV and 50-100 mA, respectively. Fluorescence-yield data were collected with a Ge array detector at count rates of ~ 40 kcps. Data reduction and analysis were carried out using the program EXAFSPAK by G. George (SSRL) in conjunction with F E F F 6 [4]. Eo was fixed at 7720 eV, corresponding to the inflection point in the Co K-edge.
3. Results and discussion Fig. 1 shows deglitched, normalized, background-subtracted EXAFS and non-phase-shifted Fourier transforms (FT) of these data. The FTs of all samples show a 1st shell at R + A of 1.7 and two 2nd shells at 2.8 and 3.3 ~, (all uncorrected for phase shift). Results of our analysis are presented in Table 1. Comparison of FTs for sorption samples and Co(OH)2 (solid) indicates that the 1st and 2nd shells of the former are analogous to similar features in the latter. In Co(OH)2, the 1st shell has six O and the 2nd shell has six in-plane Co atoms. For both shells, Co(OH)2 was used as a model for determining phase and amplitude functions. The FT feature at 3.3 A, though small, appears in all of these sorption samples as well as in other data [2, 3], but not in Co(OH)2. This
0921-4526/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 7 1 8 - 7
440
S.N. Towle et al./Physica B 208&209 (1995) 439-440
Co
[ '~ ] l l i ~co~o.~ A
uJ t~x
F=O.30
5
7
9
11
13
1
k(l~)
2
3
4
5
6
R+a (A)
Fig. 1. EXAFS and non-phase-shift-corrected Fourier transforms. Table 1 Sample
No
Ro
Nco
Rco
NAI
RAI
F F F F F F
6 6 6 6 6 6
2.09 2.09 2.08 2.09 2.09 2.07
0.6 1.0 1.6 2.5 3.3 4.0
3.12 3.12 3.09 3.10 3.09 3.10
0.7 0.5 0.7 0.6 0.6 not fitted
3.63 3.61 3.56 3.65 3.62
= = = = = =
0.27 0.30 1.14 1.30 3.76 19.6
peak was fit with a F E F F model consisting of cornerlinked A106 + C 0 0 6 octahedra. The phase-corrected distance from the Co absorber to 2nd-neighbor A1 (3.6A) (Table 1) provides information about the mode of sorption. Due to the sizes of the C 0 0 6 and A106 octahedra, the largest possible separation of the Co and AI atoms is ~ 3.1 ,~, if they are edge-linked. Although there is little evidence for edge-shared A106 neighbors, an A1 shell at this distance would overlap strongly with the Co shell, making it impossible to reject edge-shared bidentate complexes. If C 0 0 6 is linked in bidentate fashion to two AIO 6 octahedra (i. e., a bridging bidentate site) in the 0~-A1203 surface, the distance between Co and A1 would be 3.65 A, which is the same as the observed distance. Thus, the CoO6 octahedron must either be bonded to the surface in a monodentate (atop) configuration, or on the bridging bidentate sites of the bulk ~-A1203 structure.
As F increases, the number of Co 2nd-neighbors increases. Even at the lowest F, however, the number of Co 2nd-neighbors is easily measurable, suggesting that adsorption sites adjacent to an existing Co adion are preferred over other sites. At higher F, the clusters of hydrated Co ions increase in size until for the sample with F of 19.6 lamolm -2 the adsorbed species seems to be a hydrous, highly disordered solid. Even for this coverage, though, solid Co(OH)2 is more than two orders of magnitude below the equilibrium solubility at pH 8.1. Since the n u m b e r of Co 2nd-neighbors is only four, the stoichiometry of this surface phase can be written as CO(I _x)(OH)(2_zx)(H20)2x, with x ~ 0.3. The main difference between these data and those for Co(II)/y-Al203 I-2] is that the Co-A1 distance is much smaller (3.3 A) in the case of 7-A1203. This difference may result from the way the fits were done, rather than from intrinsic differences in the data. The curve fitting technique used here better accounts for the overlap between shells than does the back-transform fitting technique used in the earlier study [2]. Also, if Co and A1 were at a distance of 3.3,g,, oxygens of the adjacent octahedra would be separated by just 2.3/~, a shorter O - O distance than found in any oxide. Co(II) adsorbs at the at-Al203-water interface as a monodentate species or a bidentate species. Co(II) tends to cluster on the surface, resulting in a disordered Co(OH)2-1ike phase at high F. The local environment of Co(II) on 0t-Al203 is similar to that on 7-A1203.
Acknowledgements This work was supported by D O E grant DE-FG0393ER14347-A002 and was carried out at SSRL, supported by D O E and NIH.
References I-1] P.H. Tewari and W. Lee, J. Colloid Interface Sci. 52 (1975) 77. [2] C.J. Chisholm-Brause, P.A. O'Day, G.E. Brown Jr. and G.A. Parks, Nature 348 (1990) 528. [3] P.A. O'Day, G.E. Brown Jr. and G.A. Parks, J. Colloid Interface Sci. 165 (1994) 269. 1-4] J.J. Rehr, S.I. Zabinsky and R.C. Albers, Phys. Rev. Lett. 69 (1992) 3397.