Characterization by UV-vis-NIR reflectance spectroscopy of the exchange sites of nickel on silica

Characterization by UV-vis-NIR Reflectance Spectroscopy of the Exchange Sites of Nickel on Silica L A U R E N T BONNEVIOT, 1,2 OLIVIER LEGENDRE, M A G G Y KERMAREC, DANII~LE OLIVIER, 3 AND M I C H E L CHE 1 Laboratoire de R~activitd de Surface et Structure, URA1106 CNRS, UniversiM P. et M. Curie, 75252 Paris Cedex 05, France Received January 13, 1989; accepted April 10, 1989 The coordination of Ni 2+ ions in Ni/SiO2 catalysts has been studied by UV-vis-NIR reflectance spectroscopy at various stages of the preparation, drying, and calcination followed by rehydration. The catalysts were prepared by competitive cation exchange from an ammoniacal nickel solution. The nickel is first exchanged as a hexammine complex on the silica surface via a purely electrostatic mechanism at pH 9.8 or 10.4. Upon washing with water, substitution reactions occur in the coordination sphere of Ni 2+ ions, which remain in octahedral symmetry and become grafted to the silica surface via ligand displacement with surface oxygen. The so formed [Ni(~SiO)2L4] surface complexes contain ligands L from the solution (H20 or NH3 ) and ~ SiO mononegative ligands from the support forming ionic bonds with Ni 2+. Distortion of the grafted nickel from octahedral symmetry increases when H20 substitutes for NH3 ligands. This is attributed to hydrogen bonds between the ligands L and the unexchanged ~-~SiO- surface groups. From these results, a model of the exchange sites and the exchange mechanism are proposed. © 1990AcademicPress,Inc. INTRODUCTION

Among all transition metals, nickel is one of the most important in homogeneous (1) and heterogeneous (2) catalysis. Although heterogeneous nickel catalysts are used in many industrial reactions and conditioned in many different ways, their preparation is in most cases largely empirical. Thus the improvement o f the preparation techniques and the characterization of the catalytic sites are two objectives of primary importance. In this way we have shown that Ni + ions can be obtained by thermal reduction of highly dispersed Ni/SiO2 catalysts prepared by competitive cation exchange (3, 4). This result is very important since these Ni ÷ ions are able to catalyze olefin oligomerization ( 5 - 9 ) . The formation of such an unusual oxidation state 1TO whom all correspondence should be addressed. 2 Present address: Drpartement de Chimie, Universit6 Laval, Qurbec, Canada G1K 7P4. 3 Present address: Institut de Recherches sur la Catalyse, CNRS, 2 rue Albert Einstein, 69626 Villeurbanne, France. 0021-9797/90 $3.00 Copyright© 1990by AcademicPress,Inc. All rightsofreproductionin any form reserved.

has been related to the high dispersion of the Ni 2+ precursor ions achieved on the surface (3). It is well known that, for platinum ( 10, 11 ), the exchange method can lead, after the reduction of the precursor ions, to a highly dispersed metal phase. Depending on the support and the charge of the metal complex, anion or cation exchange may proceed on oxide surfaces (12). Silica is a slightly acidic support and behaves as a very poor anion exchanger, so that cation exchange will be the preferred method on this support. The silica surface is mainly composed of siloxane (~Si--O--Si~---) and of silanol groups ~-----SiOH (13, 14). The silanol groups can be involved in an acid-base equilibrium allowing, in a basic medium in presence of M q÷cations, cation exchange to occur as -~- SiOH ~ ~- SiO- + H ÷

[ 1]

M q+ + m ( ~ S i O - ) * - ~ [M(~SiO)m] (q-m)+.

[2]

534 Journal of Colloidand InterfaceScience. Vol.134,No. 2, February1990

UV-VIS-NIR

OF EXCHANGE

According to the metal ( 15 ) or the ligand steric hindrance (16) in the complex to be exchanged, the silica may act as a bi- or tridentate ligand. Burwell et al. (17) observed that one CI- of ion exchange Co (en)2 (C1) ~- complex was slowly substituted by a ~---SiO- group, the silica surface acting as a monodentate ligand. In the case of Ni 2÷ ions, Berton (18) found by chemical analysis that the hexammine complex looses two ammonia molecules when it is adsorbed on silica. According to the fact that the initial complex was a nickel hexammine, Burwell et al. (17) proposed that the change of color of the catalyst is due to the formation of the tetrammine complex which is transformed into diammine complex by washing with water. Anderson (19) showed that the Ni 2÷ exchanged ions are still in an octahedral surrounding and concluded that NH 3 ligands are substituted by H20 or ~ S i O ligands. It appears thus reasonable to write the tetrammine and the diammine complexes as [Ni(~---SiO)2(NH3)4] and [Ni(FSiO)2 (NH3)2(HaO)2], but this has not yet been demonstrated. In this work, the Ni/SiO2 catalysts have been prepared by an exchange process in an ammoniacal medium. The changes occurring in the coordination sphere of Ni 2÷ ions are followed at different steps of the preparation and drying treatment using UV-visNIR reflectance spectroscopy. Our purpose is to verify the existence of the diammine and tetrammine complexes by the determination of (i) the number of ammonia molecules in the adsorbed complex after each step, (ii) the number and the nature of the bonds involving the oxide support. In a forthcoming paper (20), we will study the decrease of the coordination number of Ni 2+ ions produced by thermal treatment in oxygen or in vacuo. EXPERIMENTAL

Ni/SiO2 catalysts were prepared by competitive cation exchange from an ammoniacal

SITES I N S I L I C A

535

solution of Ni(NO3)2 and NH4NO3 using a process described recently for platinum catalysts (21 ). The amount of nickel is determined by the relative concentration of the competitor cations, Ni2+ and NH ~ ions, and by the number of exchangeable surface silanol groups (22). The metal diffusion problem which usually leads to a concentration gradient of the metal within a pellet (21 ) is avoided since after a sufficient time the thermodynamic equilibrium is achieved over the whole surface of the catalyst. In this way a good dispersion is obtained both at a microscopic and macroscopic scale. The silica is, in a first step, stirred in an ammoniacal solution at pH 11 for 24 h at 293 K and then centrifuged. In a second step, the ammoniated silica is stirred in the exchange solution for 60 h at 293 K. In Process I, 30 g of silica are treated in 1 liter of a solution containing 0.05 mole of Ni(NO3)2, 0.3 mole of NH4NO3, and 1.5 moles of ammonia. In Process II, 17 g of silica are treated in 1 liter of a solution containing 0.1 mole ofNi(NO3)2, 0.6 mole of NH4NO3, and 4 moles of ammonia. The pH values are 9.8 and 10.4 for the competitive ion exchange Processes I and II, respectively. Three types of silica were used in this work. The Aerosil 380 (nonporous, 350 m2/g) was provided by Degussa, and the PBM 500 and the Sphrrosil XOA 400 (5 and 8 nm mean pore diameter, respectively, both with a 400 m2/g specific surface area) were provided by Rhrne-Poulenc. After exchange, the nickel content in the catalysts was determined by chemical analysis (Service Central d'Analyse du CNRS, Vernaison). Table I gives the results obtained using the two exchange processes. The UV-vis-NIR spectra were recorded on a Beckman 5270 spectrometer working in the 200-2500 nm range and equipped with an integration sphere, the inside wall of which is covered by a layer of barium sulfate used as a reference for all the experiments. The spectra are given in log(I/Io) and log(R~) versus the wavelength Xin transmittance and reflectance experiments, respectively. R~ is the reflectance Journal of Colloid and Interface Science, Vol. 134, No. 2, February 1990

536

BONNEVIOT ET AL. TABLE I Characteristics of the Various Ni/SiO2 Catalysts

Sample E E E E E

1.1 1.7 2.7 4.3 4.9

Ni (wt %)

Process

Silica

1.1 1.7 2.7 4.3 4.9

I I II II II

Aerosil Degussa 380 PBM 500 Aerosil Degussa 380 PBM 500 Sphrrosil X O A 400

To rationalize these results and those found in the literature (17-19), we propose the following mechanism: after ammoniation of silica, adsorption of nickel hexammine occurs in the first step via a properly so-called exchange process. It is followed by a ligand displacement or grafting to the support via substitutions of two NH3 ligands by two surface m-SiO- groups: --silica is first ammoniated,

--~-SiOH + NH3 '--+ {~-SiO-, NH 4+ } [3] of a sample with an infinite thickness (-~4 --the exchange by the hexammine complex mm) relative to a reference. Quantitative proceeds within surface ion pairs, measurements are obtained from the reflectance spectra, log(R~) = f ( ~ ) , by the use of 2{~-~SiO-, NH~-} + [Ni(NH3)6] 2+ the remission function (23) which is equiva{2(~-SiO-), [Ni(NH3)6] 2+ } + 2NH~- [4] lent to the absorption coefficient deduced from the Beer-Lambert Law (see Appendix II). --the tetrammine complex is obtained by ligand displacement, POSSIBLE E X C H A N G E M E C H A N I S M

The chemical analyses given in Table I show that, for the same process, the amount of nickel exchanged varies from one silica to the other. This amount depends on the number of exchangeable ~ S i O H silanol groups, i.e., on the surface group density and the surface area of the support. The specific surface area of silica is known to decrease in basic media due to the support particle growth (14). This would lead to a decrease of the total silanol groups and to a reduction of the exchange capacity. This loss of area has been measured by a BET technique to be 30% for the silica PBM 500 using a similar process for copper exchange (24). The area loss, indeed, occurs during the ammoniation step at pHI 1 (25), and a 40% loss was measured for the Aerosil silica. The loss of surface area of the silicas can account for their different exchange capacity. Moreover, in the same conditions of preparation, the amount of exchange is smaller for Ni 2+ than for Cu 2+ or Co 2+ ions (26). This fact, in agreement with the literature (15), suggests that the adsorption of the metal at the surface depends not only on the electrical charge but also on the nature of the complex in solution or in the adsorbed state. Journal of Colloid and Interface Science, Vol. 134,No. 2, February 1990

{2(~---SiO-), [Ni(NH3)6] 2+ } *-' [Ni(~--m-SiO)2(NH3)4] + 2NH3.

[5]

This sequence is summarized by 2 {~---SiO-, NH~- } + [Ni(NH3)6] z+ [Ni(~SiO)2(NH3)4 + 2NH3 + 2NH~-. [6] It is not clear at this point when the nickel hexammine complex is grafted to the silica surface. This may occur either (i) in the exchange solution just after exchange, (ii) during filtration and washing, or (iii) during the drying step. RESULTS

Choice of Samples The NH3 displacement in the coordination sphere of Ni 2+ ions which lies at the silicasolution interface depends on the NH3 concentration in the solution that wets the silica and on the diffusion rate of NH3 in the porosity of the silica. The effect of washing has been studied on the nonporous Aerosil silica to avoid inhomogeneity during the process which could result from the diffusion problems and pore size distribution. The Aerosil has the further advantage, unlike Sphrrosil XOA 400 and

UV-VIS-NIR OF EXCHANGE SITES IN SILICA

537

~-/Kc m-1

PBM 500, not to absorb in the UV range where a characteristic band of NO ~ anions appears in the spectra (vide infra). The intensity of this band permits following the removal of NO ~ anions during the washing treatment:

50

25

15

12.5

(1) after centrifugation of the sample in presence of the exchange solution, (2) after a first washing step with a 1.33 M NH3 solution, (3) after a second washing step with a 0.44 M NH3 solution, (4) after a third washing step with a 0.148 M NH3 solution. The sample E 2.7 has been investigated in each of the preceding states. The drying of the Aerosil gel leads to a rapid change of the sample color from blue to green. It was, thus, impossible to study the intermediate steps of drying. In contrast this change of color was very slow on the mesoporous PBM 500 which stays blue after drying at 20°C. This can be attributed to the high porosity of this silica in which NH3 molecules are trapped more efficiently than on the other. It has been found easier to monitor this change with the PBM 500. In fact, the sample E 1.7 turns green only after 15 h at 353 K in air. The samples have been investigated in either one of the following states: ( 1 ) wet after centrifugation of the exchange solution (blue) (2) filtered and dried at 293 K in air for 15 h (light-blue) (3) filtered and dried at 353 K in an oven for 15 h (light-green) (4) calcined at 773 K in oxygen and rehydrated at 293 K for 1 year (white).

UV- Vis-NIR Band Attribution The U V - v i s - N I R spectra of the E 1.7 and E 2.7 catalysts, in the states given above, are represented in Figs. 1 and 2, respectively. Three ranges can be distinguished: (i) a UV range (50,000-33,000 cm-X) corresponding to charge transfer transitions

g Cz:

o ._J

200

300 4.00

500

600

700

800

X/nm

FIG. 1. UV-vis spectra of sample E 2.7: (a) after exchange, (b) after a first washing with a 1.33 M NH3 solution, (c) after a second washing with a 0.44 M NH3 solution, and (d) after a third washing with a 0.148 M NH3 solution. All the spectra were recordedafter centrifugation.

(ii) a U V - v i s - N I R range (33,000-8000 cm -1 ) which is characteristic of d - d transitions of Ni2+ ions (iii) an NIR range (8000-4000 cm - l ) where several bands appear due to water or ammonia in exchanged or unexchanged silica. Two bands at 48,000 and 40,500 cm -1 are observed for unexchanged silica (Fig. 2e). The 40,500 cm -1 band absent in the Aerosil silica is attributed to impurities contained in the PBM 500 silica. The 33,000 cm -1 band, observed only in the centrifuged sample (Figs. 1 and 2a) and which disappears after washing Journal of Colloid and Interface Science, Vol. 134, No. 2, February t 990

538

BONNEVIOT ET AL. o'-/103c rn-1 4.0 30 25

20

o'-/103c m-1 15

11 10 9

8

7

6

,2,,,

5

4-,5

blO '

'

" z 3g ,-, "

/

)

*

2

@ ® "3

"i

;

;

-~

8

7

X/lOOnm

9

11

13 15 17 X/lOOnrn

19

21

23

25

FIG. 2. UV-vis-NIR spectra of the sample E 1.7 after different steps of preparation: (a) wet after centrifugation, (b) filtered and dried at 20°C in air for 15 h, (c) filtered and dried at 80°C in an oven during 15 h, (d) calcined at 500°C in oxygen and rehydrated for 1 year, and (e) unexchanged ammoniated silica dried at 80°C in an oven for 15 h.

with water, has been attributed to NO ~ anions. This is in agreement with literature data (27) which indicate that this band, due to the (~r* ~-- n) transition, is characteristic of uncomplexed nitrate ions. All the spectra, except those of unexchanged silica (Fig. 2e), exhibit broad bands around 10,000, 13,500, 15,000, and 25,000 cm -1 which are characteristic of octahedral Ni 2÷ ions; similar sets of bands have, indeed, been observed for [Ni(H20)6] 2+ and [Ni(NH3)6] 2+ complexes (28, 29). On this basis, the bands Vl, Vsf(sf = spin forbidden), rE, and v3 can be attributed to the following electronic transitions of octahedral Ni 2+ ions such as (30): Vl

3A2g(F) --~ 3T2g(F )

[71

vsf

3A2g(F) ~

lEg(D)

[8]

3Tlg(F )

[9]

3 T l g ( P ).

[10]

u2

3Azg(F) ~ v3

3A2g(F) ~

Journal of Colloid and Interface Science, Vol. 134, No. 2, February 1990

3A2g(F) is the ground state and, in O h symmetry, vl is equal to the crystal field value 10 Dq. vsf is the only spin forbidden transition. Apart from the latter, all the bands undergo a shift toward low energies when one goes from spectrum a to d (Fig. 2). This bathochromic effect on spin allowed transitions is related to a decrease of the crystal field experienced by Ni 2÷ ions in agreement with the Tanabe Sugano diagrams (31 ). The spin forbidden transition is found to be rather insensitive to crystal field changes (31 ) in agreement with our experimental data. Moreover, as expected from the theory (32), one observes an increase of intensity for the spin forbidden transition when the v2 and vsfbands (transitions [8] and [9]) are brought nearer to one another. This observation about the spin forbidden transition of the octahedral Ni 2+ ion is important, since it could be erroneously assigned to a transition of the ion in the tetrahedral symmetry. Such misinterpretation has been made

UV-VIS-NIR OF EXCHANGE SITES IN SILICA for clay minerals (33) as pointed out recently (34) in a system where there was no evidence oftetrahedral nickel (35). As indicated in Table II, all the spectra can be correctly interpreted assuming an octahedral crystal field. The narrow bands between 1300 and 2500 n m (7700 and 4000 cm -~ ) correspond to the 2v harmonic and v + ~ combination bands where v and ~ are the stretching and deformation vibrations respectively of the N - H and O - H vibrators related to H20, NH3, N H ~ , and ~ S i O H groups (36). The attribution, indicated in Fig. 2, is based on literature data ( 19, 36, 37). The spectrum of a wet silica (Fig. 2a) shows the presence of a significant amount of water and ammonia. The spectrum of the unexchanged dried silica (Fig. 2e) or of the exchanged silica calcined at 773 K (Fig. 2a) is the only one which does not exhibit the bands of ammonia. DISCUSSION

Efficiency of the Washing Treatment During washing, the disappearance of the NO~ band at 33,000 cm -1, while those of Ni 2+

539

ions are still present in the spectrum of the gel or of the dry catalysts, illustrates one of the exchange characteristics which consists of the replacement of the counterion NO 3 by silica surface groups as (Ni 2+, 2NO3) + 2 { N H ~ , - ~ - S i O - } *-* 2 ( N H ~ , NO~) + {Ni 2+, 2F~-SiO-},

[11]

where Ni 2+ represents the hexammine complex. The trace of NO g in the gel is due to the exchange solution trapped in the silica gel. The efficiency of the washing can be monitored by the disappearance of the NO 3 band at 33,000 cm -~. The height of this band and the band at about 28,200 cm -~, characteristic of Ni 2+ ions (transition [l 0 ]), can give the fraction Xe (Appendix I) of unexchanged nickel in the gel. The calculation is based on the following hypotheses: (i) since NO3- anions can be removed by washing, they must originally belong to the solution trapped within the pores, (ii) from the electroneutrality rule, the

TABLE II Interpretation of the ElectronicSpectraof Nickel ExchangeSilica (Sample E 1.7) (Transitionin cm-1) Spectra:

i

2a

Nature of the sample:

Ammoniacal solution

Color:

Purple blue

Magenta

vl vsf v2 v3 (ny a Species

10,750 13,150 17,600 28,200 6 Hexarnmine

10,200 13, 350 17,250 27,600 iii Mixed

9,400 13,300 15,750 26,700 4.0 iv Tetrammine

0.86

0.87

0.90

Wet silica after centrifugation

2b

2c'

2c

2d

i

Dried at 293 K for 15 h

Like 2b, then at 293 K for 1 year

Dried at 353 K for 15 h

Calcined at 773 K and rehydrated

Aqueous solution

Light blue

Light green blue

Light green

White

Green

8,600 13,400 14,900 24,600 0 iv Tetraaquo

8,500 13,500 15,400 25,300 0 Hexaaquo

8,850 8,800 13,300 13,300 15,100 15,000 25,800 25,600 2.3 i~ 1.9iv Diarnmine 0.89

0.89

0.85

0.91

i See Refs. (22) and (23). ii Averagenumber of ammonia moleculesin the complex. i~i(n) is equal to 4.75 from Eq. [ 13] assumingno grafting,while for the exchangesolution (n) equals 5.32; see Table IV. iv(n) taken from Eq. [14], assuming a completegrafting;see Fig. 3. Journal of Colloid and Interface Science, Vol. 134, No. 2, February 1990

540

BONNEVIOT ET AL.

concentration of the Ni 2+ ions in the solution tions [7 ] and [9]) are not significantly affected trapped in the gel but not exchanged at the by the two first washing steps suggesting that surface is half that of NO ~ anions since the the nickel hexammine complex is still present proton concentration is negligible at such pH at the silica surface. A shift occurs at the third and the charges of exchanged nickel ions are washing step indicating that, at this stage of neutralized by the surface (Eq. [ 11] ), preparation, the coordination sphere of Ni 2+ (iii) the peak at about 28,000 cm -~ con- ions changes according to Eq. [ 5 ] (Fig. 1). tains the contribution due to the exchanged Ni 2+ ions and unexchanged Ni 2+ ions which can be assumed to have, to a reasonable approximation, the same extinction coefficient, (iv) the relative concentrations of NO~ and Ni 2+ ions are deduced from the transmittance spectra of the exchange solution using absorbance values and from the reflectance spectra of the gel using the remission function (23), respectively. After centrifugation, this calculation gives 49% of effectively exchanged nickel. This is in agreement with the 48% obtained from the concentration of the exchange solution and its proportion by weight of 5/6 in the gel. Therefore, UV-vis data provide reasonably quantitative information despite the presence of the solution in the gel which could modify the conditions of validity of the remission function (increase of the contribution of the specular component in the reflected waves). The details of the calculations are given in Appendix I, while Table III reports the results. Two washing steps eliminate the exchange solution trapped in the gel as indicated by the total removal of NO~ ions. At the same time, the w and v2 spin allowed d-d bands (transi-

TABLE III Determination of the Fraction x~ of Exchanged Nickel in the Gel During the Washing

The bathochromic effect, occurring on vl, v2, and v3 spin allowed transitions, proceeds even more deeply after drying (Fig. 2). The spectra still characteristic of octahedral Ni 2+ ions suggest that the hexacoordinated nickel must undergo substitution reaction within the coordination sphere; i.e., the NH3 ligands can be replaced by either H20 molecules or ~ S i O - surface groups. The lowering of symmetry during the substitution should involve a splitting of the electronic levels (31 ) and the appearance of new bands. In fact we do not observe this effect because the ligands involved are close to one another in the spectrochemical series. The only exception concerns spectra 2c (Fig. 2c) and 2 c' (Table II) where a shoulder at about 23,800 cm-1 is observed on the side of the main peak at 25,800 cm -1 which could be due to the splitting of the 3Tlg(P) electronic level (transition [10]). Nevertheless, this is a small effect which can be neglected for the sake of simplification. The position of the bands can be given by the rule of the average environment (38). The number p of NH3 ligands substituted by H20 molecules can be monitored by the frequency shift Av3 of the v3 band with a linear approximation calculated from the spectrum of [Ni(H20)6 ] 2+ and [Ni(NH3)6] z+ complexes taken as reference as

Av3 = p[v3 (hexaaquo)

Washed gela Sample state

Solution in Process II

Centrifugated gel

1st

2rid

3rd

ra X~

7.3 0

1.8 0.49

1.3 0.73

0.26 0.99

0 I

a 1st, 2nd, and 3rd refer to the washing steps described in the section Results--Choice of the Samples. Journal of Colloid and Interface Science, Vol, 134, No. 2, February 1990

Measure of the Degree of Substitution

-

v3 (hexammine) ] / 6

= -480p

in cm -1.

[12]

It follows that the calculated frequency v3c~ for the [Ni(NH3)n(HzO)p] 2+ intermediate complex with n + p = 6 is given by

541

U V - V I S - N I R OF E X C H A N G E SITES IN SILICA

V3c~l= 28,200 - 480p

in cm -1,

[13]

where 28,200 represents the transition v3 (expressed in cm -1) of the [Ni(NH3)6] 2+ complex. Their concentration is referred to as Nn in Appendix II. In an ammoniacal solution of nickel, it is possible to obtain the concentrations of the hexa-, penta-, tetra-, and triammine complexes from the pH, NH3, and Ni 2÷ concentrations and the equilibrium constants of each intermediate complex, kn (29). The average number ( n ) of NH3 ligands in the coordination sphere of Ni2+ ions can be calculated (Appendix II) or obtained experimentally from the UV-vis spectra and from Eq. [13 ] and compared with the spectra of the gel as reported in Table IV. This linear approximation could be applied to surface complexes in the gel, i.e., to ~--SiO- ligands if the spectrum of the [Ni(~-~-SiO)6] 4- complex were available as a reference, but this is not the case. We must assume that the final stage of the substitution achieved after the complete departure of the NH3 ligands (spectrum 2d) corresponds to

TABLE IV Comparison of the Calculated and Experimental Values for vs in the Solution and in the Gel after Exchange or During Washing [Ni2+]~ Process I II

(n) b

u3,:,ab

In the exchangesolution 0.05 0.10

5.32 5.88

27874 28075

In the washing~lution lst e 2nd ~ 3rd ~

----

5.6 4.88 4.08

28008 27662 27278

v~p ~

&vsa

In the gel 27600 28170

-274 +95

In the gel 28170 27780 26880

+162 +118 -398

a In moles/liter. b See Appendix II and Table V. c Experimental value for the wet gel after filtration or centrifugation. d Predicted for the solution without exchange: Au3 = V3exp - V3cal. 1st, 2nd, and 3rd refer to the washing steps described in the section Results--Choice of the Samples.

[Ni(~SiO)z(H20)4]. This assumption is based on the fact that ammonia is not detected in spectrum 2 d i n agreement with Anderson's results (19). We can exclude more than two bonds between Ni 2+ and the surface since it would lead to a very distorted symmetry; our results together with those of Anderson (19) do not support this hypothesis. For [ Ni(--~-SiO)2 (H20)4] and [Ni(H20)6] 2+, a shift, Av3, of--300 cm -~ is expected for each H20 molecule substituted by one -~-~SiO- ligand. Consequently, the substitution of one NH3 by one H20 ligand and then by one ~ S i O - ligand, should involve a decrease of 480 and then 300 cm -1, i.e., a total of 780 cm-1. u3c~ can be obtained from a relation similar to relation [ 13 ] v3cal = 28,200 - 780m -480p

i n c m -1,

[14]

where m represents the number ~ S i O - ligands in the [ N i ( ~ S i O )m(NH3)n(H20 )p] (2-m)+ surface complex with n + m + p = 6. In Fig. 3, the shift of u3 is plotted versus two types of substitution sequence. The first one takes into account only the substitution of NH3 by H20 while the second includes a preliminary substitution of NH3 by two ~---SiO- ligands. The experimental data which have been included in Fig. 3 show that spectrum 2b corresponds to the tetrammine, and 2c and 2c' to the diammine complex, postulated earlier by BurweU et aL (17). The tetrammine surface complex seems to be formed during the third washing step in conditions of pH and NH3 concentration corresponding to the stability range of the tetrammine complex in solution. This suggests that the substitutions by ~-SiOligands occur from H20 and not from NH3 ligands. For Process I, the v3 band in spectrum 2a has an intermediate value between that of the exchange solution ( ( n ) = 5.2, see Appendix II) and of the grafted tetrammine complexes. At this stage the silica gel contains ~ 51% of effectively exchanged nickel. From v3 and the Journal o f CotloM a n d Interface Science,

VoL 134, No. 2, February 1990

542

BONNEVIOT ET AL.

But the grafting is not evidenced in Process II, where the proportion of pentammine in the Subsfitution of NH3 bLj Ice//I~ exchange solution is not negligible (23.2%) while there is only 1.4% of tetrammine complex (Table V). The grafting occurs in Process 27.0 I, via the tetrammine complex representing Id 9.5% of solvated nickel. Nickel, rather, is ad"7 26.5 E sorbed as a mixture of exchanged hexammine u and grafted [ Ni(~--- SiO)2 (NH3)4] surface . . 26~0 ¢n complex. In conclusion, the grafting to the • I Substitutio r~ 2~5 surface takes place via the tetrammine com/ /2c I in f h e / ~wet ~ t plex and occurs during the exchange in Process I 25.0 I and not during the washing as in Process II /2d J where a higher concentration of ammonia faI I I ! I 2 3 /~ 5 vors the stability of the hexammine complex. number of NH3 ligonds The tetrammine surface complex is found FIG. 3. Frequency shift of v3 for octahedral Ni 2+ ions to readily change into the diammine complex exchanged on silica versus the number of NH3 ligands which is particularly stable since it is not deremaining in the complex after substitution by ~--~SiOstroyed after 15 h at 353 K. i

u

l

l lkm~

28.0 -

//i

surface groups or H20 molecules. Empty circles stand for [Ni(H20)6] 2+ and [Ni(NH3)6] 2+ complexes in solution, full circles represents the data taken from Fig. I concerning nickel exchanged in the wet gel (sample E 2.7), and the full squares correspond to the data of Fig. 2 (sample E 1.7).

proportion of solution trapped in the gel, the calculated value for the adsorbed nickel corresponds to a pentammine grafted complex.

Nature of the Bonding to the Surface Information on the covalent nature of the bonds created with surface groups can be obtained from the calculation of the electronic repulsion parameter, the so-called Racah parameter. The latter decreases from the value B0 for the free ion to a value B when this ion is complexed. It is more convenient to use the

TABLE V Characteristics of the Solutions Process

[NH3],aaa

[NH3]~-,~b

pH c

N6

N5

In mol/liter

N3

(n)

%

c . _ _ ~ . ~

lst 2nd 3rda

N4

Exchange solutions

1.5 4.0

0.8 2.6

9.8 10.5

1.33 0.44 0.148

1.23 0.39 0.116

11.7 11.2 10.6

43.9 75.3

45.6 23.2

Washing solutions 53.3 40.2 21.8 50.0 4.0 28.2

9.5 1.4

1.0 0.04

5.32 5.74

6.03 23.0 39.8

0.46 5.2 28.0

5.6 4.88 4.08

o NH3 added in the exchange solution to obtain the ammine complex. b Calculated free NH3. c N. = [Ni(NH3)~(H20)p]2+, with n = 6 to 3 and n + p = 6. a 1st, 2nd, and 3rd refer to the washing steps described in the section Results--Choice of the Samples. Journal of Colloid and Interface Science, Vol. 134, No. 2, February 1990

UV-VIS-NIR

OF EXCHANGE

543

SITES I N S I L I C A

nephelauxetic parameter/3 = B/Bo measured Model for the Exchange Site from spin allowed transitions (30); the more Figure 4 represents the model for the struccovalent the complex, the smaller /3. Nevture of the grafted complex which emerges ertheless, ifa distortion in the complex leading from the above discussion. The octahedron to a lower symmetry displaces the electronic around Ni 2+ is bound to the surface by one levels, then the determination of/3, valid for edge via two Ni--OSi-~- bonds and two suma given symmetry, is no longer rigorous. mits via hydrogen bonds. Thus, Ni 2+ ions exAssuming an octahedral symmetry,/3 is deperience two different distortions of bond antermined from the diagram (30) which gives gle. The first one (Fig. 4a), imposed by the the ratio Dq/B versus v3/v,. B and /3 are distance d between two --~-SiO- groups, readily obtained since vl is equal to 10 Dq and Bo to 1041 cm -I for Ni 2÷ ions (30). The values changes the angle a = ~SiO--Ni--OSi~--which is equal to 90 ° in a regular octahedron. of/3 have been included in Table II. This process of calculation avoids the use of the v2 The second one, due to hydrogen bonding, transition which is perturbed by the vicinity distorts the octahedron via the two ligands located in trans, lowering the angle 0 = L - N i o f the b'sf transition (32). When 6 NH3 are substituted by 6 1-120 li- L ( L = NH3 or H20) from its original value, 180 °, to a smaller one. gands, the increase of 13 from 0.86 to 0.91 is The angle c~ depends on the nature of the due to an increase of ionicity of the complex. surface --~SiO- groups which could either When the hexammine transforms into the teoriginate from geminal, vicinal, or other types trammine by substitution of 2 NH3 by 2 of pairs o f silanol groups depicted as ~---SiO- ligands, /3 increases similarly from 0.86 to 0.90. Thus the ionic nature of the d = 0.262 nm d = 0.304 nm bonds increases following the sequence OH ~

Ni-NH3 < N i - H 2 0 < N i - - O S i = .

/ OH

~H

o"Si~

.S

o" A subsequent substitution of NH3 ligands should involve a new decrease of/3. A reverse effect is observed (/3 = 0.85) when a complete substitution leads to the formation of tetraaquo complex. This can be attributed to a distortion of the complex due to hydrogen bonds set up between the ligands and surface groups. The surface groups involved in the hydrogen bond are probably ~-~-SiO- groups which are much more basic than ~---SiOH groups (17). The hydrogen bond is stronger with water than With ammonia (39), so the distortion increases when water substitutes ammonia. The greater the shift from a pure octahedral symmetry, the farther the overstep from the limit of validity of/3 calculation. This explains, contradicting the trend of higher ionicity observed in the first steps of substitution, the surprising decrease of/3 as the substitution by water proceeds.

Geminal

l ~

o"~ ~ 0

O[H

o/

_Si

o

0

Silanols

4

Vicinal

d = 0.526

Silanols

nm D

OH

OH

I

I

o ,.s,~

/ s,~,. ° Si

© Adjacent

Silanols

SCHEME |. Silano] groups on the su~acc of silica.

From their experimental data, Peri and Hensley (43) proposed as a model for a silica surface, a partially dehydrated/3 cristobalite (100) face containing 15.4 and 84.6% of gemJournal of Colloid and Interface Science, Vol. 134, No. 2, February 1990

544

BONNEVIOT ET AL.

y "-...

./.,

HO . . . . . . .

•, .

>.S i

..........

OH

S i ..,.

. . . . . . . . . . . .

0

® X

y.,,,,,

........o.=

,o
,.~S

i~

0

,+i+,,.,.

/

® FXG. 4. Schematic representation of the surface diammine complex (thick dash lines represent hydrogen bonds): (a) view along the z axis, setting aside NH3 ligands, (b) view in perslx~cfive.

inal (scheme la) and vicinal (scheme lb) pairs. Recent 29Si solid state N M R experiments on silica (44) have broadly confirmed these values but have shown that the actual surface is a mixture of (100) and ( 111 ) cristobalite faces. In spite of these results, the grafting of VC14 on silica has been found to occur preferentially on vicinal silanol pairs using the EPR of V 4+ to characterize the surface (45). This probably applies to Ni 2+ exchange since after drying and calcination in oxygen the sites are highly distorted as expected for vicinal silanol pairs (20). From the comparison of the distances dbetween oxygen in silanol pairs (scheme a, b, and c) and the distance d = 0.290 nm between Journal of Colloid and Interface Science,

Vol. 134,No. 2, February1990

two cis-oxygens in the octahedron of nickel (calculated from the N i - O bond length = 0.205 nm (31)), one can find the best pairs of silanols to anchor Ni 2÷ ions at the surface of silica by considering a site with a minimum distortion. This distortion can be estimated by the angle a = O - N i - O (Fig. 4a) in the anchored complex which is equal to 90 ° in the regular octahedron. The distance d between hydroxyls in a vicinal group has been estimated to be in the range 0.30-0.32 nm from the size of MClx metal chloride molecules which bind via a double bond onto a silica surface (40, 41). For adjacent silanols (scheme lc), d can be taken to 0.526 n m from the (001) face of a tridymite (13). From Si-O bond length (=0.161 nm) and the angle O - S i - O (=109 ° 24') in the tetrahedron, the distance d is calculated to be 0.262 nm for geminal groups and 0.304 nm on a fiat surfae (from 0.294 to 0.315 nm on concave to convex surfaces within the range of curvature for silica surfaces) for vicinal groups. The distance d cannot be greater than twice the N i - O bond which eliminates adjacent silanol pairs. For geminal (scheme 1a) or vicinal (scheme lb) pairs, a takes on values of 80 ° or about 95 ___2 °, respectively, which slightly favors the vicinal pairs as exchange sites for Ni 2÷ ions. Furthermore, due to its longer O O distance, the vicinal pairs experience weaker hydrogen bonds than geminal pairs which can be calculated (39, 42) to be 4-5 and 7-8 Kcal/ mole, respectively. Therefore vicinal pairs are expected to be more active than geminal pairs toward cation exchange. The second distortion, due to hydrogen bonding (Fig. 4b), involves those ligands which are close to the surface. Two hydrogen bonds per ligand can possibly be created and their strength is determined by the N - - - O or O - - - O length in the NH3--HOSi~--~- or H 2 0 - - H O S i - - systems (39, 42). This length can be roughly estimated to be about 0.32 nm from the density of OH groups on silica ( 13, 14). The hydrogen bond energy is anticipated

U V - V I S - N I R OF E X C H A N G E SITES IN SILICA

to be equal to 2 and 4 Kcal/mole for ammonia and water, respectively. One expects that the ligands involved in hydrogen bonding are more difficult to remove or substitute. This could explain the stability of the diammine complex. The same reasoning could also explain that the tetraaquo complex experiences a substantially stronger distortion than does the diammine complex. CONCLUSION

Chemical results and UV-vis-NIR spectroscopy studies show that, for Process II, i.e., for initial concentrations of 0.1 and 4 moles/ liter of Ni 2+ ions and ammonia, respectively, and pH 10.5, the nickel hexammine complex is first adsorbed at the surface of ammoniated silica without decomposition. The grafting to the surface by ligand displacement occurs during the washing which changes the conditions of stability of the hexammine complex. For Process I, at lower pH (9.8) and lower NH3 concentration (= 1.5 moles/liter), Ni 2+ ions are directly grafted onto silica in the exchange solution. The calculation of the concentration of the intermediate ammine complex shows that grafting is only effective when the tetrammine complex concentration is no longer negligible. Since Ni ~+ ions keep an octahedral symmetry and a hexacoordinated state, this suggests that these ions are those involved in the grafting mechanism by substitution of two H20 molecules by surface groups in the coordination sphere of the NiZ+ ions. With the rule of the average environment, the shift of the electronic transition v3 (3AIg(P) --~ 3T2g(P)) has been used to monitor the substitution. Substitution of one NH3 by one 1-120 or one ~ S i O - ligand leads to a shift o f - 4 8 0 or - 7 8 0 c m - ~, respectively, for the v3 transition. In the same way, the Vl transition follows a shift toward the low energies along the same series of ligands. From these two transitions, one can extract the nephelauxetic parameter which is related to the ionicity of the complex and classify the ~--~SiO- surface group within the spec-

545

trochemical series in increasing strength of crystal field or decreasing ionicity as Ni--OSi~-~ < Ni-OH2 < Ni-NH3. The grafted surface complex is a tetrammine species described by the formula [Ni(~SiO)2(NH3)4]. The tetrammine complex readily transforms in very stable diammine complex by washing with water or drying in air. This stability is attributed to hydrogen bonds which retain the two ligands located close to the surface where unexchanged silanol groups are available. The unexpected variation of the nephelauxetic parameter of the complexes, while water further substitutes ammonia molecules, has been interpreted by an increasing distortion also due to these hydrogen bonds whose strength increases when changing from NH3 to H20 molecules. We propose a model for the grafted complex where Ni a+ ions are at the center of an octahedron bound to the surface by one edge via two surface oxygens provided by vicinal silanol groups and by two summits via hydrogen bonds. A P P EN D IX I

Quantitative information has been obtained from the spectrum of Fig. 1, on the ratio of the concentration of NO~ and Ni 2÷ ions by taking the value of log(R~) at the maximum of the bands attributed to these ions and subtracting the contribution of the background and using the remission function (23): F ( R ~ ) = (1 - R ~ ) 2 / 2 R ~ o .

The ratio r of the height of the peaks at 33,000 cm -1 (NO~) and 28,000 cm -1 (Ni 2+) in the exchange solution is referred to as rso~ and is equal to 7.3. In the gel this ratio is taken as

reel = F ( R ~ o f N O j ) / F ( R ~ o of Ni 2+ in the gel).

With the assumption given previously in the text, rsotand rgelare related to the concentration of nickel in the solution trapped in the gel Journal of Colloid and Interface Science, Vol. 134,No. 2, February 1990

546

BONNEVIOT ET AL.

[Ni2+]soX and adsorbed at the silica surface [Ni2+].ds by rg~l = rsol[Ni2+]sol/([Ni2+]~ol + [Ni2+]ads). Since the fraction of nickel adsorbed at the surface is given by X e =

[Ni2+]ads/([Ni2+]sol + [Ni2+]ads),

one obtains Xe = 1 -- (rgel/rsol).

APPENDIX II

In the formula used in this appendix, [Ni ( NH3 )n(H20)p ] 2+ complex concentrations are reported as Nn, where n = 6 to 3. The concentration of free NH3 molecules in solution is obtained from the total number of moles of NH3, the pKa = 9.25, and the pH of the solution after exchange. One must take into account in this total number of moles the NH3 (i) added in solution, (ii) released by silica during the exchange process, and (iii) complexed by Ni 2+ ions. The constants of complexation of Ni 2+ ions in aqueous solution with N H 3 are k6 = 1, k5 -- 5, and k4 = 10 at room temperature (see Refs. (28, 29)), and are expressed as kn = N n / N n - 1 [ N H 3 ] .

Therefore the concentrations of each complex o f N i 2+ ions are given by N6 = N3 [ N H 3 ] 3k4ksk6, N6 = N3 [ N H 3 ] 2k4ks, N4 = N3 [ N H 3 ] k4, N3 -- NT D - l ,

with N r = N3 + N4 + N5 + N6

D = 1 + [NH3]k4 + [NH312k4k5 + [NH313k4ksk6 .

One also defines ( n ) , the average number of NH3 ligand per Ni 2÷ ions, by (n)

= (3N3 + 4N4 + 5N5 + 6 N 6 ) / N T .

Journal of Colloid and Interface Science, Vol. 134, No. 2, February 1990

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UV-VIS-NIR OF EXCHANGE SITES IN SILICA

31.

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Journal of Colloid and Interface Science, Vol. 134, No. 2, February 1990