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Acid induced oligomerization of aurocyanide adsorbed on carbon
118
Surface
ACID INDUCED ON CARBON
OLIGOMERIZATION
Science 203 (1988) 118~128 North-Holland, Amsterdam
OF AUROCYANIDE
ADSORBED
C. KLAUBER CSIRO, Division of Mineral Products, c/o Curttn Unwersity G. P.O. Box U1987, Perth, WA 6001, Australiu Received
19 February
1988; accepted
for publication
of Technolow,
22 April 1988
Aspects of the adsorption of aurocyanide anions Au(CN); onto an activated carbon from alkaline media and the effects of subsequent acid reaction have been examined using X-ray photoelectron spectroscopy (XPS). The results indicate that the linear Au(CN); anion adsorbed intact on and parallel to the graphitic planes of the carbon, with the graphitic n-electrons taking part in a donor bond to the central gold atom, stabilized by charge transfer to the terminal nitrogen atoms. This bonding mechanism was maintained after acid reaction induced oligomerization of the adsorbed Au(CN); producing the tetramer ALI,( which attached to the graphitic plane via four identical T-donor bonds. These additional bonds necessitated the terminal nitrogen atoms to accept even further charge transfer so as to accommodate the larger carbon-anion complex.
1. Introduction The selective adsorption of aurocyanide Au(CN); onto the surface of activated carbons is of interest due to its widespread industrial use in enriching solutions Although
of dissolved
the phenomenon
has the adsorption
chemistry
gold
as part
of the gold ore processing
has been known
circuit.
for some time [l], only recently
begun to be understood
[2,3]. This understanding
has principally relied upon deductions from the observed behaviour of adsorption from solution under varying conditions. Rationalization favours the adsorption
mechanism
to involve
the adsorption
of [M”+][Au(CN);],,
ion
pairs [2,3]. Although the early XPS work by McDougall et al. [4] proved inconclusive as to the identity of the gold cyanide adsorbate, recent Mossbauer [S], adsorption and XPS (61 evidence firmly indicates that the solution anion Au(CN)i adsorbs intact. Whilst the solution cations M”- do adsorb on the carbon to maintain surface charge neutrality [6], there is no direct spectroscopic evidence for intimate ion pair association on the surface. Recently however, Adams and coworkers [7,8,2] have strengthened indirect support for the ion pair through a series of solvent, polymer and carbon extraction experiments. 0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
C. Klauber / Acid induced oligomerization
ofAuf CT?);
on C
119
In contrast, evidence as to how and where the aurocyanide unit bonds onto the activated carbon surface has hitherto been lacking. Spectroscopic evidence is provided herein for the Au(CN); unit bonding via a-electron donation from the graphitic planes of the activated carbon to the central gold atom. This occurring with a charge redistribution to the terminal nitrogen atoms. Adams et al. [2j have independently speculated of a charge transfer interaction from the delocalized electrons of a condensed aromatic carbon structure (skeletal density similar to that of graphite) to the vacant orbitals in the gold complex. Curiously however, Adams et al. also maintain that pure graphite does not adsorb aurocyanide due to its lack of pore structure, though the methods used /7,8,2] would fail to detect adsorption due to pure graphite’s low specific surface area. The adsorption bonding mechanism described in this work for Au(CN); has been found to apply equally well to the unusual tetramer anion Au,(CN); formed by acid induced oligomerization.
2. Experimental The activated carbon used here, Calgon GRC22, was loaded with aurocyanide to an equilibrium level of 1853 pmol gg’ of carbon by agitation in a borate buffered (pH 10) solution of KAu(CN), kept at 303 K For 24 h. Solution concentration of the aurocyanide at equilibrium was 0.1 mol dme3. The extent of loading was determined by atomic absorption of the residual gold in solution. Although the aurocyanide coverage level investigated here is well above that reached in the industrial process (typically about 20 pmol g-l), XPS investigation over a wide range of coverages indicated a consistent adsorption state [9]. After removal from the gold solution the carbon was washed with gold-free borate buffer chilled to 273 K to remove the possibility of entrained aurocyanide solution prior to drying. In addition three sub-samples of this gold loaded carbon were treated under increasingly vigorous acid conditions. Respectively washing in 1M HCl for 15 min and 180 min at 298 K, and for 15 min at 373 K. All samples were dried, crushed and then pressed into indium foil for analysis. XP spectra were obtained using a VG ESCALAB MkII (base pressure < 1 X lo-“’ mbar) with Mg Ka radiation (1253.6 eV) at 300 W, 6 mm slits and a constant analyser pass energy of 50 eV. All samples were maintained at about 150 K during analysis so as to minimize any possible radiation damage. Careful time dependent comparisons of the Au 4f peaks indicated that under these conditions the gold species adsorbed on the carbon did not suffer any disproportionation within the time interval that all data was acquired. Gold adsorbed on carbon appeared particularly stable. The Au 4f,,,,, 4f,,,, C Is, N Is and K 2p,,,, 2p,,, photoelectron peaks were recorded, counts for each element being accumulated respectively for total times of 10, 5, 40 and 5 s per
120
C. Klauber / Acid induced oligomerizmon
ofAu(CN),
on C
50 meV channel. At the coverage level examined only about 2% of the C Is signal was due to the cyanide carbon atoms, the remainder coming from the substrate. However, with careful spectral subtraction the cyanide C Is could be observed for the case of adsorption from alkali. Spectroscopically of principal interest are the Au 4f and N Is peaks for monitoring both the overall stoichiometry of the adsorbed aurocyanide and indicating the likely nature of the bonding mechanism. As there was no experimental indication of any breakup of cyanide units as a consequence of adsorption, the N 1s signal was thus used to monitor cyanide. As a useful binding energy (BE) comparison the two gold compounds KAu(CN), and AuCN were also examined. The KAu(CN), was Mattheyplate grade from Johnson Matthey Chemicals, whereas the AuCN was precipitated from the former by heating in the presence of excess HCl acid. The KAu(CN), as the bulk salt displayed a sensitivity toward X-ray or secondary electron induced degradation of the nominal Au(I) state. Consequently, whilst the initial BE values from KAu(CN), were useful, stoichiometric calibrations were based on measurements from the AuCN [9]. Structurally KAu(CN), consists of nearly linear Au(CN); anions with both carbon atoms bound to the central gold atom, the nitrogen atoms being in terminal positions [lo]. In contrast AuCN consists of infinite linear polymer chains . . . AuCNAuCNAuCNAuCN . . with the nitrogen atoms occupying only bridging positions [ll]. Thus, in each compound the nitrogen atoms exist in only a single structural environment.
3. Results Shown in figs. la-ld are the N 1s peaks for the aurocyanide ion adsorbed on carbon from alkaline solution and after the increasingly severe acid reactions. As a consequence of acid reaction the nitrogen (and hence cyanide) to gold ratio decreased, from an initial value of 2.00 [9] in fig. la to an experimentally determined value of 1.26 in fig. Id. At the same time the peak width broadened dramatically from an initial full width at half maximum (FWHM) of 1.86 k 0.05 eV to 2.86 _t 0.05 eV with an indication of at least two distinct nitrogen states. The channel data displayed is after removal of a Shirley background [12]. To facilitate peak decomposition a 23-point (1.15 eV wide interval) quadratic/cubic smoothing polynomial [13] was passed through the data five times (fine solid line in fig. 1). Using the experimental peak shape of fig. la for a single N 1s state, the remaining spectra lb, lc and Id have been decomposed in terms of a diminishing coverage of the species represented by la, (i.e. adsorbed Au(CN);) and an increasing presence of two other nitrogen states. These are seen to occur at binding energies of 399.0 and 397.5 eV in fig. Id. In the curve fit the BE location of the high and low states were constrained by the width of the overall N Is peak shape, with the only effective variable
ofAu(CN);
C. Klauber / Acid induced oligomerization
721
on C
A I
I
I
N
I
I
I Is
: effect
Of
acid
on
I
I
I
I
C-Au(CN)2-
399.0
, terminal
I
I
I
I
I
402
400
396
396
394
A binding
energy
I
I
I
I
I
402
400
396
396
394
N
J
(eV1
Fig. 1. N 1s photoelectron peak for (a) Au(CN); adsorbed from alkali then subjected to increasingly severe acid (1M HCl) reaction, (b) 15 min at 298 K, (c) 180 min at 298 K and (d) 15 min at 373 K. Curves normalized to constant gold coverage. Counts scale applies directly only to curve (a).
being the intensities of the components. All residual Au(CN); can be seen to have disappeared by curve lc. It was further found that the low BE component at first increased in intensity and then decreased, whilst the high BE component continued to increase in intensity. The ratio of the high to low BE nitrogen states thus increased with acid reaction, tapering off (fig. Id) to an observed value of 1.48. Although some gold desorbed during the acid reactions, neither the Au 4f,,,, 4f,,, peak shape nor BE position were observed to alter. The Au 4f,,, FWHM marginally increased outside of experimental error from 1.94 + 0.05 eV in case la to 2.06 k 0.05 eV in case Id. As expected, the potassium signal disappeared with acid reaction as H+ displaced adsorbed K+.
4. Discussion The formation of the polymeric species AuCN by heating a solution of Au(CN); in the presence of acid is well known [14], the reaction proceeding
122
C. Klauber
/ Acid induced olipmerrration
ojAu(CN);
Table I Expected nitrogen/gold and bridging nitrogen/terminal nitrogen aurocyanide anionic complexes of stoichiometry Au,(CN);+,
ratios
Species
N/Au
N,/N,
C-Au(CN); C-Au,(CN)j C-Au,(CN), C-Au,(CN); C-Au,(CN),
2.00 1 so 1.33 1.25 1.20
0.50 1 .oo 1.50 2.00
on C
for possible
adsorbed
by the evolution of HCN. A variety of temperatures for this to occur have been reported from 323 K [15] to 383 K [14], the variability probably a consequence of heterogeneous catalysis effects [8]. However, neither the CN : Au ratio changing from 2.00 to 1.26 nor the evolution of high BE : low BE nitrogen states in a ratio of 1.48 are consistent with the aurocyanide Au(CN); fully converting to AuCN. Instead, the possibility of polymerization suggests oligomerization as an alternative, i.e. that limited chain length anions of stoichiometry Au x (CN);, , might instead form on the carbon under acid conditions. Two points are apparent, firstly, such a species will have a CN : Au ratio greater than unity, and secondly, it will possess cyanide units (and hence nitrogen atoms) in two distinct chemical environments. A nitrogen atom (Nh) will either bridge a carbon and gold atom or it will terminate the chain (N,). Here it is assumed that terminal cyanides in Au,(CN),+, will bond carbon end to the gold as in Au(CN); [lo]. Table 1 illustrates the variation in the ratios N/Au and N,/N, for possible carbon adsorbed species C-Au .(CN);+ I. x -=c6. Since a bridging nitrogen would be expected to donate its lone pair toward the adjacent positive gold atom whilst a terminal nitrogen would retain that lone pair, N, would exhibit a higher N 1s BE than N,. Having made that assignment, the interpretation of the spectral decomposition in fig. Id is perfectly consistent with the aurocyanide having been converted to the adsorbed tetramer Au,(CN);. The experimental ratios of N/Au of 1.26 and N,/N, of 1.48 are in excellent agreement with the 1.25 and 1.50 that would be expected from C-Au,(CN); (table 1). This interpretation is reinforced since the observed growth in N, and dimution in N, is itself indicative of increasing polymerization. Although fig. 1 is consistent with the existence of C-Au(CN); prior to acid reaction and C-Au,(CN); after acid reaction little else can be evaluated from the results regarding any intermediates with x < 4. These would no doubt exist but the unknowns are not adequately overdetermined so as to provide a unique solution. As will become apparent from the BE discussion below, as the N, BE is sensitive to the value of x, slightly different N, states would
C. Klauber / Acid induced oligomerization
of Au(CN);
on C
123
strictly be required for each intermediate. This can be seen in fig. lb with the newly formed terminal nitrogen peak shifted from that for the residual Au(CN); species. Neither the instrumental resolution nor the signal-to-noise of the nitrogen spectra warrant any extended analysis. To the author’s knowledge there is no evidence for anionic complexes such as Au,(CN);+, for x > 1 existing in solution, either as transients or as stable entities. A further conclusion can be reached regarding the adsorption geometry and location of the adsorbed aurocyanide species. For C-Au(CN); and AuCN the and N 1s peaks displayed virtually identical shapes and widths. Au 4f,,, 94f,,, This observation implies that both nitrogen atoms for C-Au(CN); exist in identical environments and thus eliminates the supposition (e.g. mechanistic summaries in refs. [2,4]) that the linear Au(CN); unit interacts with the oxygen functionalities that exist on an activated carbon [16]. Such interaction would ensure an asymmetric environment for the nitrogen atoms of Au(CN); . The same conclusion can be reached on the basis of measured oxygen site being insufficient to explain the quantity of adsorbed concentration, aurocyanide [9]. A single nitrogen atom environment also precludes the Au(CN); unit from interacting preferentially with the activated carbon’s graphitic planes via only one of the cyanide groups (i.e. lying non-parallel to the graphitic planes). The simplest interpretation that remains is that the aurocyanide ion bonds to activated carbon on and parallel to the graphitic planes. Logically the C-Au,(CN); unit would adsorb in a similar fashion, especially in view of the bonding mechanism similarities discussed below. Some understanding of the bonding mechanism can be ascertained from the Au 4f,,, and N 1s BE’s. Shown in fig. 2 is a BE correlation diagram relating those BE’s for AuCN, KAu(CN),, C-Au(CN); and C-Au,(CN);. The very high electrical conductivity of the substrate carbon ensured no referencing problems for the aurocyanide adspecies. Referencing of the insulating compounds AuCN and KAu(CN), was achieved by aligning the C 1s BE’s for the cyanide carbon of the two insulators and C-Au(CN); . The cyanide C 1s for C-Au(CN); was determined from curve subtraction to lie at 285.5 eV. A tabulation of BE values is also given in table 2. Justification for using the carbon line is based on the carbon atom being bound to both gold and nitrogen in all four cases. Certainly, given the adsorption model proposed here, the cyanide carbons would not be expected to actively participate in the surface bonding. This choice of referencing method is internally consistent since it places the BE for the bridging nitrogens in AuCN in close agreement with the bridging nitrogens in C-Au,(CN),. The formal oxidation state for the gold atoms in AuCN and KAu(CN), is + 1. Fig. 2 shows that there is a substantial charge donation from the graphitic plane toward the gold atom(s) in both C-Au(CN); and C-Au,(CN); as a consequence of the BE decrease of 0.3 to 0.5 eV upon adsorption. An analysis of the Au 4f,,, BE’s for 49 compounds covering the
124
C. Klauher / Acid Induced oligomerization ofAu(CNJlm on C
Au(ll)
Au(l)
I
Au(O)
I C-Au4fCN);
KAu(CN)~
AuCN
399
398
86
397
N 1s
85
84
Au 4’1/2 binding
energy
(eV)
Fig. 2. Binding energy correlation diagram comparing the N 1s and Au 4f,,, BE’s for A&N, KAu(CN),, C-Au(CN); and C-Au,(CN);, Referencing of AuCN and KAu(CN),was achieved by aligning the cyanide C 1s peak with that of C-Au(CN); at 285.5 eV. Expected Au 4f,,, BE’s for + 1 and + 2 formal oxidation states is based upon an average of BE’s from 49 compounds [9].
formal oxidation states of - 1, + 1, + 2 and + 3 indicated a simple linear correlation with the variation of 1 eV in BE per electron formally transferred [9]. Hence, the BE shift would indicate the nett transfer of 0.3 to 0.5 of an electronic charge to the cationic gold centres upon adsorption. Since in Table 2 Observed core level binding energies (BE) for two reference compounds and the two adsorbed aurocyanide species Species
N Is
AuCN
399.1 (b)
K 21)~ _
KAu(CN) z
398.6 (t)
C-Au(CN); C-Au,(CN);
C 1s (cyanide)
Au 4f,,,
285.5 (from 295.0)
85.0
293.1
285.5 (from 292.8)
85.2
398.1 (t)
292.9
285.5
84.7
399.0 (b) 397.5 (t)
_
_
84.7
The reference compounds have had their experimental BE’s shifted to align their cyanide C 1s with that of C-Au(CN);. Values given in eV (0.05 ev) relative to E, of the activated carbon. Bridging and terminal nitrogen are indicated by (b) and (t).
C. Klauber / Acid induced oligomerization of Au(CN);
on C
125
graphite the top of the occupied valence band is dominated by two r-bands [17,18], it is these delocalized m-electrons that will logically form a donor bond with the gold atoms. The gold atom acceptor level possibly being an empty 6s6p hybrid orbital. This is directly analogous to the type of bonding behaviour encountered in sidebound transition metal arene complexes [19]. Interestingly the C-Au,(CN); species exhibits the same Au 4f,,, BE shift as the C-Au(CN),, indicating that the larger anion bonds to the graphite via four r-donor bonds identical to the single bond in the C-Au(CN); case. The Au 4f,,, peak for C-Au,(CN); exhibited a 6% increase in FWHM over C-Au(CN);, but there was no alteration of its shape indicating the absence of any multiple gold states. In addition to the gold centres accepting charge, it is apparent from fig. 2 that the terminal nitrogen atoms in C-Au(CN); have also accepted charge in comparison with the terminal nitrogen atoms in KAu(CN),. Presumably this is a necessary consequence of the n-donor bond to the gold atom and is required for overall stability of the adsorbed complex. Of course any concepts of bonding to the substrate should be considered in the light of what is known of the bonding within the Au(CN); complex. CN- ligands are known to be strong u-donors and strong ?r*-acceptors, so although Au(I) nominally has a closed 5d” shell, the bonding in Au(CN), is described as a delocalization of 5d electrons into the 7~* orbitals of the CN- ligands and u-donation from the CN.- into the normally unoccupied Au 6p, orbital [20]. On that basis the graphitic +electrons might donate into the Au 6pX,,, or to a 6s6p hybrid orbital. The C-Au(CN); N 1s BE decrease (fig. 2) could be a consequence of the adsorption reducing u-donation from the CN- or a result of a further r*-acceptance of charge. Of course this discussion is limited by the more general question of the extent to which extra-atomic relaxation induced shifts or surface layer dipole effects are influencing the measured BE’s in comparison to any chemical shifts. This cannot be answered suffice to point out that ABE between cyanide C 1s and N Is decreased by 0.5 eV between KAu(CN), and C-Au(CN);, whilst noting that both the carbon and nitrogen atoms are the same distance from the substrate surface. In the case of C-Au,(CN); the four donor bonds require the two terminal nitrogens to accept even further negative charge. The bridging nitrogens exhibit the same BE as in AuCN. Although an intuitive response might be to expect the terminal nitrogen atoms with additional valence charge over the normal lone pair to back donate electrons to the graphitic substrate, the cyanide groups are oriented parallel to the graphitic surface and are some distance above it due to the size of the gold atom(s). Shown in fig. 3 are the most likely bonding locations for the anions Au(CN); and Au,(CN); on the graphitic plane. The six-fold sites for the gold atoms would be expected to provide maximum overlap with the graphitic a-orbitals. Other locations are of course possible as the equivalence of the terminal nitrogen atoms only requires
126
C. Klauber / Acid induced oligomeriration 2.456
of Au(CN),-
on C
A
l--i
C-AU(CN)~Au-C
2.12
a
C-N
1.17
w
C-Au4(CN)5Au-Au
5.09
A
Au-N
1.97
a
Au-C
1.97
A
C-Nb
1.15
A
C-Nt
1.17
A
Fig. 3. Schematic depiction of the aurocyanide species Au(CN); and Au,(CN); adsorbed on a graphitic plane. C and N atoms are scaled according to their covalent radii and Au atoms to the hard sphere ionic radius for Au(I) [21] (solid line) and atomic radius [22] (broken line). The listed bond lengths are those from KAu(CN), [lo] and AuCN 1111.
the species to be parallel to the exposed basal graphitic plane. Interestingly the periodicity of the polymer chains in AuCN of 5.09 A [II] isOonly 3.6% larger than twice the periodicity of a single graphite plane (4.912 A [23]). Thus for the C-Au,(CN), complex shown in fig. 3, the maximum mismatch between a gold atom and an underlying graphitic ring would be only about 0.27 A (on forming AuCN from Au(CN); a significant shortening of the intercyanide distance occurs [lO,ll]). The r-donor mechanism for the C-Au(CN); system in conjunction with the parallel adsorption geometry has important implications for the comparative adsorption behaviour of the various transition metal cyanide complexes on activated carbons. Any ligand geometry that precludes the graphitic n-bands from donating charge to the central transition metal cation will inhibit adsorption. In the cyanocuprate(1) series the strengths of adsorption have been established as [3]: Cu(CN),
> Cu(CN);-
> Cu(CN);-.
In solution the geometry of the anions Cu(CN); and be linear and tetrahedral respectively [24]. On steric anion could not bond via the a-donor mechanism. does not require an anionic complex as a prerequisite
Cu(CN)iare known to grounds the Cu(CN)iFurther the r-donation so the mechanism is also
C. Klauber / Acid induced oligomerization
of Au(CN);
consistent with the known adsorption behaviour Hg(CN),, which can even displace Au(CN); (41.
on C
of the neutral
121
complex
5. Conclusions XPS studies of aurocyanide species adsorbed onto an activated carbon indicated that from alkaline solution Au(CN); adsorbed on and parallel to the graphitic planes. The likely bonding mechanism being a simple r-donor bond from the graphitic structure to the central cationic gold atom. Nett charge transfer was estimated to be 0.3 to 0.5 of an electronic charge. Additional charge was also transferred to the terminal nitrogen atoms on the cyanide units, however, the nitrogen atoms did not appear to be directly involved in the adsorption. Although solution cations did coadsorb, whether this was as a strict ion pair is unknown since it is quite plausible for the cation-anion interaction to occur via the graphitic valence electrons. One implication of this adsorption geometry is that some transition metal cyanides would be sterically hindered from adsorbing in the same manner as Au(CN); . This bonding mechanism was found to similarly apply to an unusual aurocyanide tetramer Au,(CN); formed by acid induced oligomerization of C-Au(CN);. The C-Au,(CN); adspecies being well characterized structurally via the existence of bridging and terminal nitrogen positions, the former displaying a 1.5 eV higher N Is BE. This complex bound to the graphitic planes by four rr-donor bonds to the four gold centres, each of identical charge transfer to the single bond in C-Au(CN),. The additional bonds caused even more charge transfer to the two terminal nitrogen atoms, although again these did not appear to be directly involved in the adsorption bond.
Acknowledgement The author would like to thank Warren Jones for preparing the carbons.
References [I] [2] [3] [4]
W.N. Davis. U.S. Patent 227, 963 (1880). M.D. Adams, G.J. McDougall and R.D. Hancock, Hydrometallurgy 19 (1987) 95. C.A. Fleming, 26th Annual Conference of Metallurgists, Winnipeg (August 1987). G.J. McDougall, R.D. Hancock, M.J. Nicol, O.L. Wellington and R.G. Copperthwaite, African Inst. Mining Met. 80 (1980) 344. [5] J.D. Cashion, D.J. Cookson, L.J. Brown and D.G. Howard, in: Industrial Applications Mtissbauer Effect, Eds. G.J. Long and J.G. Stevens (Plenum, New York, 1986). 16) C. Klauber, H. Linge and W. Jones, 194th ACS, New Orleans (1987).
J. S. of the
128 [7] [8] [9] [lo] [ll] [12] [13]
[14] [15] [16] [17] [lS] [19] [20] [21] [22] [23] 1241
C. Klauber / Acld induced oligomerizatton
oJAu(CN);
on C
G.J. McDougall, M.D. Adams and R.D. Hancock, Hydrometallurgy 18 (1987) 125. M.D. Adams, G.J. McDougall and R.D. Hancock, Hydrometallurgy 18 (1987) 139. H. Linge, W. Jones and C. Klauber, to be published. A. Rosenzweig and D.T. Cromer, Acta Cryst. 12 (1959) 709. H. Zhdanov and E. Shugam, Acta Physicochim. URSS 20 (1945) 253. D.A. Shirley, Phys. Rev. B 5 (1972) 4709. (a) A. Savitzky and M.J.E. Golay, Anal. Chem. 36 (1964) 1627; (b) J. Steiner, Y. Termonia and J. Deltour, Anal. Chem. 44 (1972) 1906; (c) H.H. Madden, Anal. Chem. 50 (1978) 1383. B.F.G. Johnson, in: Comprehensive Inorganic Chemistry, Vol. 3, Eds. J.C. Bailar, H.J. EmelCus, R. Nyholm and A.F. Trotman-Dickensen (Pergamon, New York, 1973) p. 145. M.O. Faltens and D.A. Shirley, J. Chem. Phys. 53 (1970) 4249. J.T. Cookson, Jr., in: Carbon Adsorption Handbook, Eds. P.N. Cheremisinoff and F. Ellerbusch (Ann Arbor Science, Ann Arbor, 1978) p. 241. Z. Zupan, Phys. Rev. B 6 (1972) 2477. F.R. McFeeley, S.P. Kowalczyk, L. Ley, R.G. Cavell, R.A. Pollak and D.A. Shirley, Phys. Rev. B 9 (1974) 5268. J.P. Collman and L.S. Hegedus, Principles and Applications of Organotransition Metal Chemistry (University Science Books, Mill Valley, 1980) pp. 113, 107. H. Prosser, G. Wortmann, K. Syassen and W.B. Holzapfel, Z. Phys. B 24 (1976) 7. R.D. Shannon and C.T. Prewitt, Acta Cryst. B 25 (1969) 925. C.F. Fischer, At. Data Nucl. Data Tables 4 (1972) 301. R.W.G. Wychoff, Crystal Structures, Vol. 1, 2nd ed. (Interscience, New York, 1963) p. 26. G.W. Chantry and R.A. Plane, J. Chem. Phys. 33 (1960) 736.
Surface
ACID INDUCED ON CARBON
OLIGOMERIZATION
Science 203 (1988) 118~128 North-Holland, Amsterdam
OF AUROCYANIDE
ADSORBED
C. KLAUBER CSIRO, Division of Mineral Products, c/o Curttn Unwersity G. P.O. Box U1987, Perth, WA 6001, Australiu Received
19 February
1988; accepted
for publication
of Technolow,
22 April 1988
Aspects of the adsorption of aurocyanide anions Au(CN); onto an activated carbon from alkaline media and the effects of subsequent acid reaction have been examined using X-ray photoelectron spectroscopy (XPS). The results indicate that the linear Au(CN); anion adsorbed intact on and parallel to the graphitic planes of the carbon, with the graphitic n-electrons taking part in a donor bond to the central gold atom, stabilized by charge transfer to the terminal nitrogen atoms. This bonding mechanism was maintained after acid reaction induced oligomerization of the adsorbed Au(CN); producing the tetramer ALI,( which attached to the graphitic plane via four identical T-donor bonds. These additional bonds necessitated the terminal nitrogen atoms to accept even further charge transfer so as to accommodate the larger carbon-anion complex.
1. Introduction The selective adsorption of aurocyanide Au(CN); onto the surface of activated carbons is of interest due to its widespread industrial use in enriching solutions Although
of dissolved
the phenomenon
has the adsorption
chemistry
gold
as part
of the gold ore processing
has been known
circuit.
for some time [l], only recently
begun to be understood
[2,3]. This understanding
has principally relied upon deductions from the observed behaviour of adsorption from solution under varying conditions. Rationalization favours the adsorption
mechanism
to involve
the adsorption
of [M”+][Au(CN);],,
ion
pairs [2,3]. Although the early XPS work by McDougall et al. [4] proved inconclusive as to the identity of the gold cyanide adsorbate, recent Mossbauer [S], adsorption and XPS (61 evidence firmly indicates that the solution anion Au(CN)i adsorbs intact. Whilst the solution cations M”- do adsorb on the carbon to maintain surface charge neutrality [6], there is no direct spectroscopic evidence for intimate ion pair association on the surface. Recently however, Adams and coworkers [7,8,2] have strengthened indirect support for the ion pair through a series of solvent, polymer and carbon extraction experiments. 0039-6028/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
C. Klauber / Acid induced oligomerization
ofAuf CT?);
on C
119
In contrast, evidence as to how and where the aurocyanide unit bonds onto the activated carbon surface has hitherto been lacking. Spectroscopic evidence is provided herein for the Au(CN); unit bonding via a-electron donation from the graphitic planes of the activated carbon to the central gold atom. This occurring with a charge redistribution to the terminal nitrogen atoms. Adams et al. [2j have independently speculated of a charge transfer interaction from the delocalized electrons of a condensed aromatic carbon structure (skeletal density similar to that of graphite) to the vacant orbitals in the gold complex. Curiously however, Adams et al. also maintain that pure graphite does not adsorb aurocyanide due to its lack of pore structure, though the methods used /7,8,2] would fail to detect adsorption due to pure graphite’s low specific surface area. The adsorption bonding mechanism described in this work for Au(CN); has been found to apply equally well to the unusual tetramer anion Au,(CN); formed by acid induced oligomerization.
2. Experimental The activated carbon used here, Calgon GRC22, was loaded with aurocyanide to an equilibrium level of 1853 pmol gg’ of carbon by agitation in a borate buffered (pH 10) solution of KAu(CN), kept at 303 K For 24 h. Solution concentration of the aurocyanide at equilibrium was 0.1 mol dme3. The extent of loading was determined by atomic absorption of the residual gold in solution. Although the aurocyanide coverage level investigated here is well above that reached in the industrial process (typically about 20 pmol g-l), XPS investigation over a wide range of coverages indicated a consistent adsorption state [9]. After removal from the gold solution the carbon was washed with gold-free borate buffer chilled to 273 K to remove the possibility of entrained aurocyanide solution prior to drying. In addition three sub-samples of this gold loaded carbon were treated under increasingly vigorous acid conditions. Respectively washing in 1M HCl for 15 min and 180 min at 298 K, and for 15 min at 373 K. All samples were dried, crushed and then pressed into indium foil for analysis. XP spectra were obtained using a VG ESCALAB MkII (base pressure < 1 X lo-“’ mbar) with Mg Ka radiation (1253.6 eV) at 300 W, 6 mm slits and a constant analyser pass energy of 50 eV. All samples were maintained at about 150 K during analysis so as to minimize any possible radiation damage. Careful time dependent comparisons of the Au 4f peaks indicated that under these conditions the gold species adsorbed on the carbon did not suffer any disproportionation within the time interval that all data was acquired. Gold adsorbed on carbon appeared particularly stable. The Au 4f,,,,, 4f,,,, C Is, N Is and K 2p,,,, 2p,,, photoelectron peaks were recorded, counts for each element being accumulated respectively for total times of 10, 5, 40 and 5 s per
120
C. Klauber / Acid induced oligomerizmon
ofAu(CN),
on C
50 meV channel. At the coverage level examined only about 2% of the C Is signal was due to the cyanide carbon atoms, the remainder coming from the substrate. However, with careful spectral subtraction the cyanide C Is could be observed for the case of adsorption from alkali. Spectroscopically of principal interest are the Au 4f and N Is peaks for monitoring both the overall stoichiometry of the adsorbed aurocyanide and indicating the likely nature of the bonding mechanism. As there was no experimental indication of any breakup of cyanide units as a consequence of adsorption, the N 1s signal was thus used to monitor cyanide. As a useful binding energy (BE) comparison the two gold compounds KAu(CN), and AuCN were also examined. The KAu(CN), was Mattheyplate grade from Johnson Matthey Chemicals, whereas the AuCN was precipitated from the former by heating in the presence of excess HCl acid. The KAu(CN), as the bulk salt displayed a sensitivity toward X-ray or secondary electron induced degradation of the nominal Au(I) state. Consequently, whilst the initial BE values from KAu(CN), were useful, stoichiometric calibrations were based on measurements from the AuCN [9]. Structurally KAu(CN), consists of nearly linear Au(CN); anions with both carbon atoms bound to the central gold atom, the nitrogen atoms being in terminal positions [lo]. In contrast AuCN consists of infinite linear polymer chains . . . AuCNAuCNAuCNAuCN . . with the nitrogen atoms occupying only bridging positions [ll]. Thus, in each compound the nitrogen atoms exist in only a single structural environment.
3. Results Shown in figs. la-ld are the N 1s peaks for the aurocyanide ion adsorbed on carbon from alkaline solution and after the increasingly severe acid reactions. As a consequence of acid reaction the nitrogen (and hence cyanide) to gold ratio decreased, from an initial value of 2.00 [9] in fig. la to an experimentally determined value of 1.26 in fig. Id. At the same time the peak width broadened dramatically from an initial full width at half maximum (FWHM) of 1.86 k 0.05 eV to 2.86 _t 0.05 eV with an indication of at least two distinct nitrogen states. The channel data displayed is after removal of a Shirley background [12]. To facilitate peak decomposition a 23-point (1.15 eV wide interval) quadratic/cubic smoothing polynomial [13] was passed through the data five times (fine solid line in fig. 1). Using the experimental peak shape of fig. la for a single N 1s state, the remaining spectra lb, lc and Id have been decomposed in terms of a diminishing coverage of the species represented by la, (i.e. adsorbed Au(CN);) and an increasing presence of two other nitrogen states. These are seen to occur at binding energies of 399.0 and 397.5 eV in fig. Id. In the curve fit the BE location of the high and low states were constrained by the width of the overall N Is peak shape, with the only effective variable
ofAu(CN);
C. Klauber / Acid induced oligomerization
721
on C
A I
I
I
N
I
I
I Is
: effect
Of
acid
on
I
I
I
I
C-Au(CN)2-
399.0
, terminal
I
I
I
I
I
402
400
396
396
394
A binding
energy
I
I
I
I
I
402
400
396
396
394
N
J
(eV1
Fig. 1. N 1s photoelectron peak for (a) Au(CN); adsorbed from alkali then subjected to increasingly severe acid (1M HCl) reaction, (b) 15 min at 298 K, (c) 180 min at 298 K and (d) 15 min at 373 K. Curves normalized to constant gold coverage. Counts scale applies directly only to curve (a).
being the intensities of the components. All residual Au(CN); can be seen to have disappeared by curve lc. It was further found that the low BE component at first increased in intensity and then decreased, whilst the high BE component continued to increase in intensity. The ratio of the high to low BE nitrogen states thus increased with acid reaction, tapering off (fig. Id) to an observed value of 1.48. Although some gold desorbed during the acid reactions, neither the Au 4f,,,, 4f,,, peak shape nor BE position were observed to alter. The Au 4f,,, FWHM marginally increased outside of experimental error from 1.94 + 0.05 eV in case la to 2.06 k 0.05 eV in case Id. As expected, the potassium signal disappeared with acid reaction as H+ displaced adsorbed K+.
4. Discussion The formation of the polymeric species AuCN by heating a solution of Au(CN); in the presence of acid is well known [14], the reaction proceeding
122
C. Klauber
/ Acid induced olipmerrration
ojAu(CN);
Table I Expected nitrogen/gold and bridging nitrogen/terminal nitrogen aurocyanide anionic complexes of stoichiometry Au,(CN);+,
ratios
Species
N/Au
N,/N,
C-Au(CN); C-Au,(CN)j C-Au,(CN), C-Au,(CN); C-Au,(CN),
2.00 1 so 1.33 1.25 1.20
0.50 1 .oo 1.50 2.00
on C
for possible
adsorbed
by the evolution of HCN. A variety of temperatures for this to occur have been reported from 323 K [15] to 383 K [14], the variability probably a consequence of heterogeneous catalysis effects [8]. However, neither the CN : Au ratio changing from 2.00 to 1.26 nor the evolution of high BE : low BE nitrogen states in a ratio of 1.48 are consistent with the aurocyanide Au(CN); fully converting to AuCN. Instead, the possibility of polymerization suggests oligomerization as an alternative, i.e. that limited chain length anions of stoichiometry Au x (CN);, , might instead form on the carbon under acid conditions. Two points are apparent, firstly, such a species will have a CN : Au ratio greater than unity, and secondly, it will possess cyanide units (and hence nitrogen atoms) in two distinct chemical environments. A nitrogen atom (Nh) will either bridge a carbon and gold atom or it will terminate the chain (N,). Here it is assumed that terminal cyanides in Au,(CN),+, will bond carbon end to the gold as in Au(CN); [lo]. Table 1 illustrates the variation in the ratios N/Au and N,/N, for possible carbon adsorbed species C-Au .(CN);+ I. x -=c6. Since a bridging nitrogen would be expected to donate its lone pair toward the adjacent positive gold atom whilst a terminal nitrogen would retain that lone pair, N, would exhibit a higher N 1s BE than N,. Having made that assignment, the interpretation of the spectral decomposition in fig. Id is perfectly consistent with the aurocyanide having been converted to the adsorbed tetramer Au,(CN);. The experimental ratios of N/Au of 1.26 and N,/N, of 1.48 are in excellent agreement with the 1.25 and 1.50 that would be expected from C-Au,(CN); (table 1). This interpretation is reinforced since the observed growth in N, and dimution in N, is itself indicative of increasing polymerization. Although fig. 1 is consistent with the existence of C-Au(CN); prior to acid reaction and C-Au,(CN); after acid reaction little else can be evaluated from the results regarding any intermediates with x < 4. These would no doubt exist but the unknowns are not adequately overdetermined so as to provide a unique solution. As will become apparent from the BE discussion below, as the N, BE is sensitive to the value of x, slightly different N, states would
C. Klauber / Acid induced oligomerization
of Au(CN);
on C
123
strictly be required for each intermediate. This can be seen in fig. lb with the newly formed terminal nitrogen peak shifted from that for the residual Au(CN); species. Neither the instrumental resolution nor the signal-to-noise of the nitrogen spectra warrant any extended analysis. To the author’s knowledge there is no evidence for anionic complexes such as Au,(CN);+, for x > 1 existing in solution, either as transients or as stable entities. A further conclusion can be reached regarding the adsorption geometry and location of the adsorbed aurocyanide species. For C-Au(CN); and AuCN the and N 1s peaks displayed virtually identical shapes and widths. Au 4f,,, 94f,,, This observation implies that both nitrogen atoms for C-Au(CN); exist in identical environments and thus eliminates the supposition (e.g. mechanistic summaries in refs. [2,4]) that the linear Au(CN); unit interacts with the oxygen functionalities that exist on an activated carbon [16]. Such interaction would ensure an asymmetric environment for the nitrogen atoms of Au(CN); . The same conclusion can be reached on the basis of measured oxygen site being insufficient to explain the quantity of adsorbed concentration, aurocyanide [9]. A single nitrogen atom environment also precludes the Au(CN); unit from interacting preferentially with the activated carbon’s graphitic planes via only one of the cyanide groups (i.e. lying non-parallel to the graphitic planes). The simplest interpretation that remains is that the aurocyanide ion bonds to activated carbon on and parallel to the graphitic planes. Logically the C-Au,(CN); unit would adsorb in a similar fashion, especially in view of the bonding mechanism similarities discussed below. Some understanding of the bonding mechanism can be ascertained from the Au 4f,,, and N 1s BE’s. Shown in fig. 2 is a BE correlation diagram relating those BE’s for AuCN, KAu(CN),, C-Au(CN); and C-Au,(CN);. The very high electrical conductivity of the substrate carbon ensured no referencing problems for the aurocyanide adspecies. Referencing of the insulating compounds AuCN and KAu(CN), was achieved by aligning the C 1s BE’s for the cyanide carbon of the two insulators and C-Au(CN); . The cyanide C 1s for C-Au(CN); was determined from curve subtraction to lie at 285.5 eV. A tabulation of BE values is also given in table 2. Justification for using the carbon line is based on the carbon atom being bound to both gold and nitrogen in all four cases. Certainly, given the adsorption model proposed here, the cyanide carbons would not be expected to actively participate in the surface bonding. This choice of referencing method is internally consistent since it places the BE for the bridging nitrogens in AuCN in close agreement with the bridging nitrogens in C-Au,(CN),. The formal oxidation state for the gold atoms in AuCN and KAu(CN), is + 1. Fig. 2 shows that there is a substantial charge donation from the graphitic plane toward the gold atom(s) in both C-Au(CN); and C-Au,(CN); as a consequence of the BE decrease of 0.3 to 0.5 eV upon adsorption. An analysis of the Au 4f,,, BE’s for 49 compounds covering the
124
C. Klauher / Acid Induced oligomerization ofAu(CNJlm on C
Au(ll)
Au(l)
I
Au(O)
I C-Au4fCN);
KAu(CN)~
AuCN
399
398
86
397
N 1s
85
84
Au 4’1/2 binding
energy
(eV)
Fig. 2. Binding energy correlation diagram comparing the N 1s and Au 4f,,, BE’s for A&N, KAu(CN),, C-Au(CN); and C-Au,(CN);, Referencing of AuCN and KAu(CN),was achieved by aligning the cyanide C 1s peak with that of C-Au(CN); at 285.5 eV. Expected Au 4f,,, BE’s for + 1 and + 2 formal oxidation states is based upon an average of BE’s from 49 compounds [9].
formal oxidation states of - 1, + 1, + 2 and + 3 indicated a simple linear correlation with the variation of 1 eV in BE per electron formally transferred [9]. Hence, the BE shift would indicate the nett transfer of 0.3 to 0.5 of an electronic charge to the cationic gold centres upon adsorption. Since in Table 2 Observed core level binding energies (BE) for two reference compounds and the two adsorbed aurocyanide species Species
N Is
AuCN
399.1 (b)
K 21)~ _
KAu(CN) z
398.6 (t)
C-Au(CN); C-Au,(CN);
C 1s (cyanide)
Au 4f,,,
285.5 (from 295.0)
85.0
293.1
285.5 (from 292.8)
85.2
398.1 (t)
292.9
285.5
84.7
399.0 (b) 397.5 (t)
_
_
84.7
The reference compounds have had their experimental BE’s shifted to align their cyanide C 1s with that of C-Au(CN);. Values given in eV (0.05 ev) relative to E, of the activated carbon. Bridging and terminal nitrogen are indicated by (b) and (t).
C. Klauber / Acid induced oligomerization of Au(CN);
on C
125
graphite the top of the occupied valence band is dominated by two r-bands [17,18], it is these delocalized m-electrons that will logically form a donor bond with the gold atoms. The gold atom acceptor level possibly being an empty 6s6p hybrid orbital. This is directly analogous to the type of bonding behaviour encountered in sidebound transition metal arene complexes [19]. Interestingly the C-Au,(CN); species exhibits the same Au 4f,,, BE shift as the C-Au(CN),, indicating that the larger anion bonds to the graphite via four r-donor bonds identical to the single bond in the C-Au(CN); case. The Au 4f,,, peak for C-Au,(CN); exhibited a 6% increase in FWHM over C-Au(CN);, but there was no alteration of its shape indicating the absence of any multiple gold states. In addition to the gold centres accepting charge, it is apparent from fig. 2 that the terminal nitrogen atoms in C-Au(CN); have also accepted charge in comparison with the terminal nitrogen atoms in KAu(CN),. Presumably this is a necessary consequence of the n-donor bond to the gold atom and is required for overall stability of the adsorbed complex. Of course any concepts of bonding to the substrate should be considered in the light of what is known of the bonding within the Au(CN); complex. CN- ligands are known to be strong u-donors and strong ?r*-acceptors, so although Au(I) nominally has a closed 5d” shell, the bonding in Au(CN), is described as a delocalization of 5d electrons into the 7~* orbitals of the CN- ligands and u-donation from the CN.- into the normally unoccupied Au 6p, orbital [20]. On that basis the graphitic +electrons might donate into the Au 6pX,,, or to a 6s6p hybrid orbital. The C-Au(CN); N 1s BE decrease (fig. 2) could be a consequence of the adsorption reducing u-donation from the CN- or a result of a further r*-acceptance of charge. Of course this discussion is limited by the more general question of the extent to which extra-atomic relaxation induced shifts or surface layer dipole effects are influencing the measured BE’s in comparison to any chemical shifts. This cannot be answered suffice to point out that ABE between cyanide C 1s and N Is decreased by 0.5 eV between KAu(CN), and C-Au(CN);, whilst noting that both the carbon and nitrogen atoms are the same distance from the substrate surface. In the case of C-Au,(CN); the four donor bonds require the two terminal nitrogens to accept even further negative charge. The bridging nitrogens exhibit the same BE as in AuCN. Although an intuitive response might be to expect the terminal nitrogen atoms with additional valence charge over the normal lone pair to back donate electrons to the graphitic substrate, the cyanide groups are oriented parallel to the graphitic surface and are some distance above it due to the size of the gold atom(s). Shown in fig. 3 are the most likely bonding locations for the anions Au(CN); and Au,(CN); on the graphitic plane. The six-fold sites for the gold atoms would be expected to provide maximum overlap with the graphitic a-orbitals. Other locations are of course possible as the equivalence of the terminal nitrogen atoms only requires
126
C. Klauber / Acid induced oligomeriration 2.456
of Au(CN),-
on C
A
l--i
C-AU(CN)~Au-C
2.12
a
C-N
1.17
w
C-Au4(CN)5Au-Au
5.09
A
Au-N
1.97
a
Au-C
1.97
A
C-Nb
1.15
A
C-Nt
1.17
A
Fig. 3. Schematic depiction of the aurocyanide species Au(CN); and Au,(CN); adsorbed on a graphitic plane. C and N atoms are scaled according to their covalent radii and Au atoms to the hard sphere ionic radius for Au(I) [21] (solid line) and atomic radius [22] (broken line). The listed bond lengths are those from KAu(CN), [lo] and AuCN 1111.
the species to be parallel to the exposed basal graphitic plane. Interestingly the periodicity of the polymer chains in AuCN of 5.09 A [II] isOonly 3.6% larger than twice the periodicity of a single graphite plane (4.912 A [23]). Thus for the C-Au,(CN), complex shown in fig. 3, the maximum mismatch between a gold atom and an underlying graphitic ring would be only about 0.27 A (on forming AuCN from Au(CN); a significant shortening of the intercyanide distance occurs [lO,ll]). The r-donor mechanism for the C-Au(CN); system in conjunction with the parallel adsorption geometry has important implications for the comparative adsorption behaviour of the various transition metal cyanide complexes on activated carbons. Any ligand geometry that precludes the graphitic n-bands from donating charge to the central transition metal cation will inhibit adsorption. In the cyanocuprate(1) series the strengths of adsorption have been established as [3]: Cu(CN),
> Cu(CN);-
> Cu(CN);-.
In solution the geometry of the anions Cu(CN); and be linear and tetrahedral respectively [24]. On steric anion could not bond via the a-donor mechanism. does not require an anionic complex as a prerequisite
Cu(CN)iare known to grounds the Cu(CN)iFurther the r-donation so the mechanism is also
C. Klauber / Acid induced oligomerization
of Au(CN);
consistent with the known adsorption behaviour Hg(CN),, which can even displace Au(CN); (41.
on C
of the neutral
121
complex
5. Conclusions XPS studies of aurocyanide species adsorbed onto an activated carbon indicated that from alkaline solution Au(CN); adsorbed on and parallel to the graphitic planes. The likely bonding mechanism being a simple r-donor bond from the graphitic structure to the central cationic gold atom. Nett charge transfer was estimated to be 0.3 to 0.5 of an electronic charge. Additional charge was also transferred to the terminal nitrogen atoms on the cyanide units, however, the nitrogen atoms did not appear to be directly involved in the adsorption. Although solution cations did coadsorb, whether this was as a strict ion pair is unknown since it is quite plausible for the cation-anion interaction to occur via the graphitic valence electrons. One implication of this adsorption geometry is that some transition metal cyanides would be sterically hindered from adsorbing in the same manner as Au(CN); . This bonding mechanism was found to similarly apply to an unusual aurocyanide tetramer Au,(CN); formed by acid induced oligomerization of C-Au(CN);. The C-Au,(CN); adspecies being well characterized structurally via the existence of bridging and terminal nitrogen positions, the former displaying a 1.5 eV higher N Is BE. This complex bound to the graphitic planes by four rr-donor bonds to the four gold centres, each of identical charge transfer to the single bond in C-Au(CN),. The additional bonds caused even more charge transfer to the two terminal nitrogen atoms, although again these did not appear to be directly involved in the adsorption bond.
Acknowledgement The author would like to thank Warren Jones for preparing the carbons.
References [I] [2] [3] [4]
W.N. Davis. U.S. Patent 227, 963 (1880). M.D. Adams, G.J. McDougall and R.D. Hancock, Hydrometallurgy 19 (1987) 95. C.A. Fleming, 26th Annual Conference of Metallurgists, Winnipeg (August 1987). G.J. McDougall, R.D. Hancock, M.J. Nicol, O.L. Wellington and R.G. Copperthwaite, African Inst. Mining Met. 80 (1980) 344. [5] J.D. Cashion, D.J. Cookson, L.J. Brown and D.G. Howard, in: Industrial Applications Mtissbauer Effect, Eds. G.J. Long and J.G. Stevens (Plenum, New York, 1986). 16) C. Klauber, H. Linge and W. Jones, 194th ACS, New Orleans (1987).
J. S. of the
128 [7] [8] [9] [lo] [ll] [12] [13]
[14] [15] [16] [17] [lS] [19] [20] [21] [22] [23] 1241
C. Klauber / Acld induced oligomerizatton
oJAu(CN);
on C
G.J. McDougall, M.D. Adams and R.D. Hancock, Hydrometallurgy 18 (1987) 125. M.D. Adams, G.J. McDougall and R.D. Hancock, Hydrometallurgy 18 (1987) 139. H. Linge, W. Jones and C. Klauber, to be published. A. Rosenzweig and D.T. Cromer, Acta Cryst. 12 (1959) 709. H. Zhdanov and E. Shugam, Acta Physicochim. URSS 20 (1945) 253. D.A. Shirley, Phys. Rev. B 5 (1972) 4709. (a) A. Savitzky and M.J.E. Golay, Anal. Chem. 36 (1964) 1627; (b) J. Steiner, Y. Termonia and J. Deltour, Anal. Chem. 44 (1972) 1906; (c) H.H. Madden, Anal. Chem. 50 (1978) 1383. B.F.G. Johnson, in: Comprehensive Inorganic Chemistry, Vol. 3, Eds. J.C. Bailar, H.J. EmelCus, R. Nyholm and A.F. Trotman-Dickensen (Pergamon, New York, 1973) p. 145. M.O. Faltens and D.A. Shirley, J. Chem. Phys. 53 (1970) 4249. J.T. Cookson, Jr., in: Carbon Adsorption Handbook, Eds. P.N. Cheremisinoff and F. Ellerbusch (Ann Arbor Science, Ann Arbor, 1978) p. 241. Z. Zupan, Phys. Rev. B 6 (1972) 2477. F.R. McFeeley, S.P. Kowalczyk, L. Ley, R.G. Cavell, R.A. Pollak and D.A. Shirley, Phys. Rev. B 9 (1974) 5268. J.P. Collman and L.S. Hegedus, Principles and Applications of Organotransition Metal Chemistry (University Science Books, Mill Valley, 1980) pp. 113, 107. H. Prosser, G. Wortmann, K. Syassen and W.B. Holzapfel, Z. Phys. B 24 (1976) 7. R.D. Shannon and C.T. Prewitt, Acta Cryst. B 25 (1969) 925. C.F. Fischer, At. Data Nucl. Data Tables 4 (1972) 301. R.W.G. Wychoff, Crystal Structures, Vol. 1, 2nd ed. (Interscience, New York, 1963) p. 26. G.W. Chantry and R.A. Plane, J. Chem. Phys. 33 (1960) 736.