12936
J. Phys. Chem. C 2008, 112, 12936–12942
Reductive Adsorption of Cd2+ on a Nickel Surface in Aqueous Solution: Characterization of Surface Adatoms by in Situ EXAFS Gregory Godard, Aimery de Mallmann,* Jean-Pierre Candy, Steven Fiddy,† and Jean-Marie Basset* UniVersite´ de Lyon, Institut de Chimie de Lyon, France, UniVersite´ Lyon 1, CPE Lyon, CNRS, UMR 5265 C2P2, LCOMS, Baˆtiment 308 F-CPE Lyon, 43 BouleVard du 11 NoVembre 1918, F-69616, Villeurbanne, France ReceiVed: February 1, 2008; ReVised Manuscript ReceiVed: April 23, 2008
Cd2+ can be removed from water by reaction with hydrogen adsorbed on the surface of alumina supported nickel particles. A redox reaction between Cd2+ and adsorbed hydrogen occurs with formation of Cd(0) and two protons. The in situ EXAFS studies were carried out in water at the Cd K-edge during the grafting reaction, and at various nickel coverages by cadmium. At low coverage, the Cd(0) is interacting with ca. four surface nickel atoms at a distance of 2.64 Å. Cd completes its coordination sphere by interacting with ca. two water molecules at a distance of 2.27 Å. At higher coverage, Cd is bonded to only three surface nickel at a distance of 2.61 Å and to ca. three water molecules, at a distance of 2.25 Å. The rather small Cd-Ni distance found by EXAFS may result from a slight polarization of the Cd-Ni bond. Cadmium deposition is thermodynamically driven by the formation of these Cd-Ni bonds (underpotential deposition) and is then limited to a monolayer of cadmium atoms deposited onto the nickel surface. Introduction For public health and environment protection, heavy metals have to be removed from water. Some of them such as lead, mercury, or cadmium are highly toxic leading to cancers, kidney disorders, or bone diseases.1–3 Among the removal technologies, the adsorption on a solid adsorbent is one of the most described and studied. Most of the adsorptions are based on ionic exchange on activated charcoal4 or polymeric adsorbent.5 Heavy metals can also be absorbed on metallic surfaces by redox reaction: metallic ions are reduced by an electrical current6,7 or by the oxidation of a sacrificial metal such as zinc, iron or aluminum (cementation).8–10 It was recently proposed to remove heavy metals such as cadmium from contaminated aqueous effluents by adsorption on nickel particles covered with hydrogen.11 This adsorption is based on a redox reaction between Cd2+ and Ni-H leading to H+ and Cd(0) but differs from cementation since the reduction of cadmium ions is not carried out by a sacrificial metal (Ni) but by hydrogen adsorbed on the nickel particles (Scheme 1). Cadmium was successfully removed from water. For each cadmium ion removed, two hydronium ions were produced showing the oxidation of adsorbed hydrogen. Since nickel dissolves in acidic media, an accurate pH regulation at a neutral level is required. After treatment, the remaining Cd2+ concentration in the polluted water can be lower than the limiting ICPAES detection value (20 ppb). Cadmium removal occurs until there is a full monolayer of cadmium atoms deposited on the nickel surface. This new water treatment process allows cadmium removal at a level lower than that required by European legislation. Moreover, hydronium ion is the only product of the redox reaction. * To whom correspondence should be addressed. E-mail: demallmann@ cpe.fr;
[email protected]. Tel: +33 (0)4 72 43 17 94. Fax: +33 (0)4 72 43 17 95. † Present address: Synchrotron Radiation Department, CCLRC Daresbury Laboratory, Warrington WA44AD, United Kingdom.
SCHEME 1: Cadmium Reduction by Adsorbed Hydrogen
The reduction of metallic ions by the oxidation of chemisorbed hydrogen was first described by Szabo et al.12–16 and widely used by Barbier et al.17–25 for the preparation of bimetallic catalysts. It involved the reduction of Sn2+, Cu2+, ReO4-, ReCl62-, PtCl62-, or AuCl4- ions on platinum surfaces covered with hydrogen. In our case, we used a nickel catalyst to remove cadmium from water mostly because nickel also chemisorbs hydrogen26 and is cheaper than noble metal catalysts. Even if the cadmium adsorption on nickel particles has been clearly demonstrated as being the result of Cd2+ reduction to Cd(0),11 the true nature of the interaction between the nickel surface and the adsorbed cadmium atoms is not fully understood. In particular, it is not clearly established if cadmium is deposited on the nickel particle as “adatoms” well dispersed on the surface or as cadmium clusters. Besides, one does not know the type of bonding between the nickel surface and the “adatoms of cadmium”. In this article, we present the in situ EXAFS characterization at the cadmium K-edge of the surface species formed and demonstrate that the cadmium atoms are “isolated” on the nickel surface. Depending on coverage, they are linked to three (high coverage) or four (low coverage) nickel atoms and to three (high coverage) or two (low coverage) oxygen atoms (presumably coming from water coordinated to the adatoms of Cd(0)).
10.1021/jp800973g CCC: $40.75 2008 American Chemical Society Published on Web 07/26/2008
Reductive Adsorption of Cd2+
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12937
Figure 1. Apparatus used to study in situ cadmium adsorption on nickel particles by EXAFS in the fluorescence mode.
Experimental Section Water was filtered on activated carbons and deionized with cationic and anionic exchange resins provided by Elvetec Services before distillation and stripping under argon. Distilled water is then stored under argon. Alumina supported nickel was used as a trapping agent of Cd2+. The Ni/Al2O3 solid used as adsorbent is an industrial catalyst, widely used in hydrogenation reactions. The gamma alumina with a surface area of about 200 m2 g-1 was used as a support in an extruded form. The Ni/ Al2O3 was obtained by the incipient wetness method already described by Merlen et al.27 The nickel loading measured by elemental analyses is 58.9% wt%. The dispersion of the metallic particles (number of surface nickel atoms per total number of nickel atoms) was measured by hydrogen chemisorption, assuming that one surface nickel atom chemisorbs one hydrogen atom.26 The value obtained is 0.08 Nis/Ni (Nis defines a nickel atom at the surface of nickel particles, i.e., nickel atom able to chemisorb one hydrogen atom). Hydrogen adsorption was performed in a static volumetric apparatus.28 Before adsorption measurements, the Ni/Al2O3 sample was reduced under flowing hydrogen at 723 K during 5 h. Then, the exhaust is sealed, and the sample is treated under vacuum at 723 K during 3 h. Hydrogen adsorption measurements are performed at room temperature and for an equilibrium pressure of 100 mbar. The reaction of Cd2+ on the nickel is performed in a closed vessel, magnetically stirred and equipped with a pH gauge and two syringe pumps, as described in previous work.11 The modification made on this equipement was the following: a window made with a Kapton film was added on the side of the reactor to allow the X-ray beam to penetrate in the suspension (Figure 1). Before cadmium adsorption, a mass of 2 or 3 g of Ni/Al2O3 is reduced under flowing hydrogen at 723 K for 5 h and cooled under H2 to room temperature. The reduced Ni/Al2O3 sample is introduced under H2 into a Schlenk tube filled with 20 mL of distilled water, and then, the suspension of the reduced catalyst in water is introduced under argon into the reactor filled with 30 mL of water. The reactor is closed and purged with Ar (15 min) and filled with H2. The suspension is stirred for 1 h first at 1000 rpm and then at 700 rpm. A solution of Cd2+ (0.15 M) (coming from CdCl2, 2.5H2O) is progressively (0.16 mL min-1) introduced into the reactor maintained at a constant pH (7.0 ( 0.1) by a regulated addition of a NaOH solution (0.15 M). The kinetics of the reaction is followed by the amount of OH- introduced. All of the results are related to the final volume of solution into the reactor or to the number of surface nickel atoms (Nis). When addition of a given amount of Cd/Nis is reached (corresponding to an expected value ranging typically
Figure 2. X-ray absorption spectra at the Cd K edge. A: Initial CdCl2 solution under stirring at 500 rpm; B: After 9 h under stirring at 700 rpm in the presence of the Ni/Al2O3 catalyst (Cd/Nis ) 0.5); C: After the turn off of the stirring and decantation of the sample.
between 0.05 and 0.8 Cd/Nis), the addition is stopped and the EXAFS data are recorded. At the end of the experiments, samples of the reaction media are taken and analyzed in order to determine the amount of cadmium remaining in solution. We checked that, for less than 2500 ppm of cadmium introduced in water (this value corresponds to 0.8 Cd2+/Nis), Cd(OH)2 precipitation does not occur at pH 7. EXAFS data were acquired on beam-line ID26 at ESRF (Grenoble, France) at the Cd K-edge (26.711 keV), using a Si(220) monochromator in the fluorescence mode with a silicon diode type detector. The calibration in energy was performed with a silver foil (E0 ) 25.514 keV). Three spectra are shown in Figure 2. Spectrum A corresponds to an aqueous solution of CdCl2 under stirring without nickel catalyst. Spectrum B was recorded under stirring during the equilibrium of the cadmium adsorption reaction on nickel for a ratio Cd2+/Nis ) 0.5, and spectrum C was obtained once the stirring was turned off. Except for the lower noise of spectrum C, no main difference can be observed between the general shape of spectra B and C. In order to obtain high quality EXAFS spectra, the magnetic stirrer was thus shut off and the catalyst decanted before acquiring the EXAFS data. The comparison between A (CdCl2 in solution) and C (CdCl2 in the presence of supported Ni particles covered with hydrogen) shows that the intensity of the white line has decreased and modifications in the first oscillations of the spectrum. This indicates that a reaction has occurred. The EXAFS spectra were analyzed by standard procedures using the suite of programs developed by Alain Michalowicz, in particular the EXAFS fitting program RoundMidnight using calculations with spherical waves.29 The postedge background subtraction was carefully conducted using polynomial or cubicspline fittings and the removal of the low-frequency contributions was checked by further Fourier transformation. The fitting of the spectrum was done with the k3 weighted data using the following EXAFS equation, where S02 is a scale factor;30 Ni is the coordination number of shell i, rc is the total central atom loss factor, Fi is the EXAFS scattering function for atom i, Ri is the distance to atom i from the absorbing atom, λ is the photoelectron mean free path, σi is the Debye-Waller factor, Φi is the EXAFS phase function for atom i, and Φc is the EXAFS phase function for the absorbing atom:
12938 J. Phys. Chem. C, Vol. 112, No. 33, 2008 n
χ(k) = S02rc(k)
∑ i)1
NiFi(k, Ri) 2
kRi
Godard et al.
( )
exp
-2Ri exp(-2σi2k2) × λ(k)
sin[2kRi + Φi(k, Ri) + Φc(k)] FEFF831
The program was used to calculate theoretical values for rc, Fi, λ, and Φi + Φc based on model clusters of atoms. The refinements were performed by fitting the structural parameters Ni, Ri, σi, and the energy shift, ∆E 0 (the same for all shells). The fit residue, F (%), was calculated by the following formula:
F )
∑ [kχexp(k)-kχcal(k)]2 k
∑ [kχexp(k)]2
100 Figure 3. Hydroxide ions added versus cadmium introduced.
k
As recommended by the Standards and Criteria Committee of the International XAFS Society,32 an improvement of the fit took into account the number of fitted parameters. The number of statistically independent data points or maximum number of degrees of freedom in the signal is defined as Nidp ) (2∆k∆R/ π) + 2. The inclusion of extra parameters were statistically validated by a decrease of the quality factor, (∆χ)2/ν and the values of the statistical errors generated in RoundMidnight were multiplied by [(∆χ)2/ν]1/2 in order to take the systematic errors into account, since the quality factors exceeded one. The error bars thus calculated are given in parentheses after each refined parameter. The scale factor, S02 ) 0.95, was determined from the analysis of the spectra of the reference compound CdO (6 O at 2.30(2) Å, 12 Cd at 3.30(1) and 8 Cd at 4.15(5) Å)33,34 and was kept constant in all of the fits. For 0.05 and 0.15 M CdCl2 aqueous solutions, it was thus found that dissolved Cd(II) was surrounded by 6.2(9) O at 2.27(2) Å, in agreement with the Cd(H2O)62+ formula. Chlorine coordination to cadmium could not be clearly detected by EXAFS for these CdCl2 aqueous solutions since any attempt to introduce Cl in the coordination sphere of Cd was not statistically validated because the quality factor increased tremendously. As in the Mosselmans et al. paper,35 which studies particular a 0.1 M CdCl2 solution at room temperature, the results obtained suggest that the majority of cadmium atoms are only coordinated by oxygen atoms (neither [CdCl4]2– nor [Cd(OH2)5Cl]+ complexes detectable by EXAFS) in these solutions. Moreover, it must be emphasized that the solutions considered in our study present much lower Clconcentrations, from 0.002 to 0.033 M (i.e., 100 to 6 times less chlorine in solution) at the end of the addition of the CdCl2 solution, and this disfavors the complexation of Cd(II) by chlorine in solution according to the law of mass action. In conclusion, since the EXAFS data provide only an average picture of the metal-ligands bonding environment it is not possible to completely rule out chloride coordination to some of the cadmium in the solutions studied here at the beginning or at the end of the CdCl2 addition, but the results suggest that the vast majority of cadmium in these solutions are not coordinated by chlorine, as Cd(H2O)62+. Results and Interpretation Increasing amounts of cadmium chloride were added at a constant pH of 7, to a suspension of Ni/Al2O3 under H2 (1 atm) at room temperature in water in the in situ EXAFS apparatus (see the Experimental Section). After each cadmium addition, the amount of hydroxide ions at equilibrium was always twice the amount of Cd2+ (Figure 3) in agreement with the simple
reduction of Cd2+ to Cd(0) by two surface nickel hydrides leading to two H+ in solution. We have to note that, in a first approximation and taking into account the atomic radius of the two metals, namely 0.125 nm for Ni(0) and 0.149 nm for Cd(0), no more than 0.7 Cd per surface Ni atoms (Nis) can be deposited as a monolayer of Cd on a nickel surface. Two situations have been considered: Cd/Nis < 0.6 and Cd/Nis ) 0.8. A. For Cd/Nis < 0.6. Typically, when the cadmium amount introduced is such that the Cd2+/Nis ratio is lower than 0.6, the concentration of Cd2+ remaining in solution after equilibrium of adsorption is lower than 1 ppm. In proportion, 99.9% of the cadmium introduced is adsorbed on the catalyst and less than 0.1% of the cadmium introduced remains in water.11 Therefore the contribution of the soluble Cd2+ to the EXAFS spectra is negligible. It is known, according to Liu et al.36 that the Cd2+ physisorbs on the alumina surface but the equilibrium of adsorption is such that for Cd2+concentration in solution lower than 1 ppm, no more than 1% of the cadmium introduced can be adsorbed on the alumina support. Therefore, the contribution of Cd adsorbed on alumina to the EXAFS spectra is negligible. A measure of the chloride concentration of the solution after treatment by the Ni/Al2O3 adsorbent and filtration, for a Cd2+/ Nis ratio introduced of 0.34, has shown that the chloride anions brought (124 ppm ( 5%) stay in the solution and are not adsorbed by the solid, while the Cd2+ concentration in water decreased from 197 to 0.02 ppm. Therefore very few chlorine, if any, can be coordinated on the Cd species adsorbed on the solid. For various amounts of cadmium introduced and grafted on the nickel surface (namely 0.05, 0.1, 0.2, 0.4, 0.5, and 0.6 Cd/ Nis), the EXAFS spectra were recorded in situ at the cadmium K-edge in the fluorescence mode. EXAFS spectra recorded at equilibrium, corresponding to three initial concentrations of cadmium (Cd/Nis ) 0.05, 0.1, and 0.5) are represented in Figure 4. The Cd K-edge EXAFS derived parameters are reported in Table 1. These k3χ(k) Cd K-edge EXAFS spectra were fitted with two types of neighbors, a heavy neighbor identified as nickel (main peak in the Fourier transform) and a light atom attributed to oxygen (shoulder at the left of the main peak in the Fourier transform).37 The results concerning the average coordination sphere of cadmium and its evolution with the coverage of nickel surface by cadmium, (up to 0.6 Cd/Nis) are represented in Figure 5. Cadmium atoms are surrounded by ca. three nickel atoms at 2.61(1) to 2.64(2) Å and 2 to 3 oxygen atoms at 2.25(2) to 2.27(3) Å. Any tentative fit considering cadmium instead of nickel atoms in the second shell led to unreasonable Cd-Cd distances, shorter than 2.44 Å. This
Reductive Adsorption of Cd2+
Figure 4. Cd K-edge k3-weighted EXAFS (left) and corresponding Fourier transform (right) for Ni/Al2O3 samples on which (I) 0.05, (II) 0.1, and (III) 0.5 monolayer equiv. of Cd has been deposited (Cd/Nis ) 0.05, 0.1, and 0.5). Solid lines: experimental; dotted lines: spherical wave theory.
hypothesis was then rejected since the shortest known Cd-Cd bond determined by XRD in Cd2(AlCl4)2 is 2.561 Å38 or 2.576(1) Å.39 However, for a high Cd coverage, interaction between two neighbor Cd atoms deposited on the nickel surface cannot be completely excluded. Thus, in the case of Cd/Nis ) 0.5 the spectrum presented a very good signal/noise ratio, due to a high cadmium concentration and to an averaging with a larger number of recorded spectra. In this case the inclusion of a third shell with 0.3(2) Cd atom at a distance of 3.01(3) Å, close to that found in metallic cadmium by XRD (2.989 Å)40 or by EXAFS (2.965(25)41 and 2.97 Å42), without modification of the parameters relative to the two first shells, improved the fit since it decreased the quality factor32 ((∆χ)2/ν) from 2.10 to 2.08.43 Any tentative fit considering chlorine, with oxygen and nickel, in the coordination sphere of cadmium led to low Cl coordination numbers (NCl < 0.12) and was not considered since higher quality factors ((∆χ)2/ν < 2.4) were obtained with unreasonable Cd-Cl distances, larger than 2.86 Å, while 2.55-2.70 Å distances are normally seen in octahedral complexes44 and shorter ones for tetrahedral species.45 The Cd-Ni bond distance found by EXAFS in those samples, 2.61 to 2.64 Å, is intermediate between distances observed in inorganic compounds as Ph3Ge-Cd-(η5-Cp)(GePh3)Ni-CdNi(GePh3)(η5-Cp)-Cd-GePh3, with 2.459(2) and 2.476(2) Å for Cd-Ni bond lengths46 or {µ3-Co(CO)3}Cd3{µ-{O2C(µ3-C)Co3(CO)9}}3(THF)3, with an average Cd-Co distance of 2.573(7) Å in the CoCd3 tetrahedron47 and the sum of the covalent radii of both elements in their metallic structure, 1.25 Å for Ni and 1.49 Å for Cd,48 i.e., 2.74 Å for an unpolarized Cd-Ni bond length. The smaller Cd-Ni distance found by EXAFS compared to this last one, may result from a polarization of the Cd-Ni bond. Indeed, it should be emphasized that, following the
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12939 respective work function of nickel (5.15 eV) and cadmium (4.22 eV),49 the Cd-Ni bond should be polarized, cadmium atoms deposited on the nickel surface being under the form of Cdδ+ and the nickel atoms negatively charged, Niδ′-. These EXAFS data thus confirm that the reaction between Cd2+ and hydrogen adsorbed occurs on the nickel surface, in agreement with the fact that Cd2+ is effectively chemisorbed on nickel after reduction by surface hydrogen (Scheme 2), with a small polarization of the Ni-Cd bonds. B. For Cd/Nis ) 0.8. Let us recall that for such a concentration some Cd2+ remains in solution. For this case, the situation is more complex. The first peak at 1.5-2 Å in the Fourier transform increases dramatically while the second peak at 2-2.5 Å decreases (comparison of I and IV in Figure 6). This phenomenon is mainly due to an increase in the number of light scatterers at 2.25 Å(oxygen) from 2.8 (for Cd/Nis ) 0.6) to 3.4 (for Cd/Nis ) 0.8) and a decrease of the number of heavy scatterers at 2.61 Å from 2.7 for (for Cd/Nis ) 0.6) to 2.2 for (for Cd/Nis ) 0.8) (Table 1). This result obtained at high loading may have two origins: Cd2+ remaining in water or Cd2+ adsorption on alumina. After the removal of the aqueous phase and replacement by fresh water, to eliminate the hypothesis of a contribution of soluble Cd2+, the spectrum does not change much (comparison of I and II in Figure 6 and of Entries 7 and 8 in Table 1). In order to see if some Cd2+ was adsorbed on the support, the sample was washed with a concentrated solution of Mg2+ (ca. 5 M in MgCl2) in order to exchange Cd2+ by Mg2+. Effectively the first peak at 1.5-2 Å decreased and the second at 2-2.5 Å increased in intensity (III in Figure 6 and entries 9 in Table 1: 3.0(4) O at 2.25(1) Å and 2.6(7) Ni at 2.61(1) Å). The number of light scatterers and the number of heavy scatterers become closer to those observed in the case of cadmium adsorbed on the nickel particles with Cd/Nis ) 0.6 (comparison of entries 9 and 6 in Table 1). In conclusion, for 0.8 Cd/Nis introduced, a part of the cadmium adsorbed on the solid can be exchanged by Mg2+ cations. This exchangeable cadmium is attributed to Cd2+ cations adsorbed on the alumina support (e.g., Al-O-Cd+ 50–52). Assuming that the cadmium adsorbed on the nickel surface is linked in average to 2.7 Nis (value obtained for Cd/Nis ) 0.6), the part of the Cd linked to the alumina surface for Cd/Nis ) 0.8 (II in Figure 6) may correspond to ca. 11 % of the total. This result is in good accordance with the values obtained by Liu et al.36 Discussion It is necessary to examine the data by considering the behavior in solution of Cd2+ in the context of its redox properties. According to Nernst equation applied to (Cd/Cd2+) and (H2/ H+), Cd2+ cannot be reduced to Cd(0) aggregate in a system having less than the theoretical concentration of 30,000 ppm of Cd2+ in solution.53 Effectively EXAFS shows that during cadmium reduction by hydrogen adsorbed on nickel, Cd-Ni bonds are selectively formed instead of Cd-Cd bonds. The Nernst equation cannot be simply applied to bimetallic systems (such as Cd grafted on Ni) but must be corrected by the semiempirical Kolb equation.54 It has been reported that a monolayer of metallic ions (M)x+ could be deposited onto the surface of a different metal (M′) at a potential more positive than the Nernst potential.24,55,56 This phenomenon has been called “underpotential deposition” (UPD). According to Kolb et al.,54 the ionic contribution to the chemical M-M′ bond due to the difference of work functions between M and M′ can account for the shift
12940 J. Phys. Chem. C, Vol. 112, No. 33, 2008
Godard et al.
TABLE 1: Cd K-Edge EXAFS Derived Parameters for the Cd/Ni Samplesg first shell (O) of Cd Cd/Nis 1 2 3 4 5 6 7 8 9
b,d
0.05 0.1b,d 0.2b,d 0.4b,d 0.5c,d 0.6b,d 0.8b,d,e 0.8b,d,f 0.8b,d,g
N
R (Å)
2.0(6) 2.5(7) 2.7(8) 2.6(7) 2.5(3) 2.8(5) 3.4(4) 3.2(4) 3.0(4)
2.27(3) 2.26(2) 2.26(2) 2.26(2) 2.252(8) 2.25(2) 2.25(1) 2.25(1) 2.25(1)
second shell (Ni) of Cd
D.W.
σ2
2
(Å )
0.010(4) 0.010(4) 0.011(4) 0.012(5) 0.010(2) 0.010(3) 0.010(2) 0.009(2) 0.010(2)
N
R (Å)
D.W. σ2 (Å2)
F (%)
3.8(8) 3.6(4) 3.5(5) 2.9(6) 2.8(4) 2.7(5) 2.2(5) 2.4(5) 2.6(7)
2.64(2) 2.62(1) 2.63(1) 2.62(1) 2.618(6) 2.61(2) 2.61(1) 2.61(1) 2.61(1)
0.015(2) 0.015(2) 0.014(4) 0.014(4) 0.014(2) 0.015(4) 0.016(4) 0.016(4) 0.015(3)
13.5 9.4 8.6 8.4 2.5 8.5 3.3 5.2 3.8
a N, coordination number; R, distance from Cd atom; D.W. σ2, Debye-Waller factor; F, fit residue (single scattering theory; values in parentheses represent the errors generated in RoundMidnight29). b ∆k ) 2.1 - 11.9 Å-1; ∆R ) 0.8 - 3.4 Å. c ∆k ) 2.1 - 12.6 Å-1; ∆R ) 0.8 - 3.4 Å. d Number of parameters fitted: P ) 7; overall scale factor: S02 ) 0.95; energy shift: ∆E0 ) 1.8 ( 0.8 eV, the same for both shells. e Sample obtained after the introduction of 0.8 Cd/Nis, spectrum I in Figure 6. f After washing by water, spectrum II in Figure 6. g After washing with a saturated solution of MgCl2 under a H2 atmosphere, spectrum III in Figure 6.
Figure 5. Evolution of the cadmium coordination sphere with the amount of cadmium grafted on the nickel particles (Cd/Nis). (A) Number of bonded oxygen atoms (open circles); (B) number of bonded nickel atoms (gray squares).
SCHEME 2: Cadmium Reaction with Hydrogen Adsorbed on a Nickel Surface
Figure 6. Fourier transform (without phase correction) of the Cd K-edge k3-weighted EXAFS of Ni/Al2O3 samples. (I) Cd/Nis ) 0.8; (II) sample I washed with distilled water; (III) sample II washed with a saturated solution of MgCl2 and then with distilled water; (IV) Cd/ Nis ) 0.6. Solid lines: experimental; dotted lines: spherical wave theory.
of the potential of M deposition. The authors showed that the potential shift (∆Up) is related to the difference in work functions (∆Φ) by the following semi-empirical equation:
∆Up ) R∆Φ
with R ) 0.5 V(eV)-1
In our case, since the work function of Cd and Ni are ΦCd ) 4.22 eV and ΦNi ) 5.15 eV respectively,57 ∆Φ ) Φ Ni - ΦCd is equal to 0.93 eV and ∆Up would represent 0.46 V. Thus, cadmium monolayer deposition could occur to a potential 0.46 V higher than that of bulk deposition (Nernst potential) (Figure 7). The potential of cadmium becomes higher than that of hydrogen even at cadmium concentrations as low as 1 ppb.58 Scheme 3 summarizes the behavior of Cd2+ in the reduction deposition on hydrogen covered nickel. Cadmium deposition is thermodynamically driven by the formation of Cd-Ni bonds (underpotential deposition) and is then limited to a monolayer of cadmium atoms deposited on the nickel surface (path 2 in Scheme 3). When nickel is fully covered by a monolayer of cadmium, hydrogen cannot be adsorbed anymore due to the poisoning of cadmium, toward hydrogen adsorption by nickel. Cadmium deposition can no longer occur. Regarding the real structure of the adsorbed cadmium, about 2 to 3 light atoms, very likely oxygen atoms, are observed in the first shell, at 2.24-2.26 Å from the cadmium atom. Since, after equilibrium, there are negligible amounts of Cd2+ remain-
Figure 7. Redox potential of the couple (Cd/Cd2+) and (Cd-Ni/Cd2+) versus Cd2+ concentration in water. Comparison with the redox potential of the couple (H2/H+) at pH ) 7.
ing in solution,11 the presence of light atoms at 2.27-2.25 Å from Cd must be attributed to the adsorbed species. Since the Cd-Ni bonds of 2.63 Å excludes cadmium deposition by ion exchange on the alumina, coordinative adsorption of water on Cd/Ni is the only reasonable interpretation. A quantum chemical approach of water adsorption undertaken by Kuznetsov et al.,59
Reductive Adsorption of Cd2+ SCHEME 3: Cadmium Underpotential Deposition on Nickel Surface as a Monolayer
J. Phys. Chem. C, Vol. 112, No. 33, 2008 12941 ecules. All distances observed agree with metal-metal Cd-Ni bonding mode. At higher coverage of nickel surface by Cd only 3 surface nickel are “sigma” bonded to Cd. There are apparently three water molecules which coordinate to the cadmium at a very short distance of 2.25 Å. Conclusion Metal ion deposition on a metallic surface (Pt, Rh, Pd) through reduction by adsorbed hydrogen was widely used in the past to prepare bimetallic catalyst in water. However, all the surface characterizations were carried out on dry samples usually reduced under hydrogen. The in situ EXAFS studies carried out here on the cadmium deposition on a nickel surface from aqueous solution, demonstrate that for nickel coverage greater than 0.1 and lower than 0.6 Cd/Nis, each cadmium atom removed is directly bonded to about three surface nickel atoms at a Nis-Cd distance of 2.61-2.64 Å and about three water molecules are linked to the cadmium atom, at a Cd-O distance of 2.25-2.27 Å. Therefore, the removal of cadmium ions from water effectively occurs by their reduction by hydrogen adsorbed on the nickel surface and not through a side reaction of ion exchange on the alumina support.
SCHEME 4: Proposed Surface Species
Acknowledgment. The authors gratefully acknowledge Armida Sodo, Laurent Alvarez, Olga Safonova, Sigrid Eeckhout and Thomas Neisus for their help in carrying out this study on ID26 beam line at ESRF (project # ME600), Grenoble, France. The authors also thank Robert Chareyron and Marie-Jose´ Arnoux from the Laboratoire d’Analyse Industrielle for ICP-AES. Ecole Supe´rieure de Chimie Physique Electronique de Lyon (CPE Lyon), Centre National de la Recherche Scientifique (CNRS) and Re´gion Rhoˆne-Alpes are acknowledged for the financial support. References and Notes
using CNDO/2 method, concluded that water molecules are chemisorbed via the oxygen atom without dissociation on the low index faces of metallic cadmium, in hollow position, with a Cd-O distance ranging from 2.14 to 2.25 Å. More recently, using ab initio functional density theory, it was proposed that on several close-packed group VIII metal surfaces (Pd, Ru, Rh, and Pt) H2O binds preferentially at an atop adsorption site with the molecular dipole plane nearly parallel to the surface at a O to metal distances ranging from 2.28 to 2.43 Å.60,61 The adsorption energies were found to range from 0.1 to 0.4 eV and to increase following the sequence: Pd ≈ Pt > Ru ≈ Rh. Using density-functional total-energy calculations within the generalized gradient approximation, it was demonstrated that, on the CdTe(001) surface, water is adsorbed molecularly on Cd atoms with a Cd-O distance of 2.5 Å.62 From the X-ray powder diffraction structure of a cadmium aminophosphonate Cd2Cl2(H2O)4(H2L), the Cd-OH2 distances were measured at 2.30 and 2.36 Å.63 In his review on the interaction of water with solid surfaces, Henderson64 points out that molecular adsorption of water on metal surfaces occurs usually with the oxygen end down and one or both O-H bonds oriented away from the surface, water molecules forming a monolayer. These results are in agreement with our EXAFS parameters. We therefore propose the following model (Scheme 4) for the coordination sphere of adsorbed cadmium atoms depending on the surface coverage At low cadmium coverage the Cd(0) is interacting with 4 surface nickel atoms. Cd completes its coordination sphere to 6 by interacting with two water mol-
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