Siting and coordination of cobalt in ferrierite: XRD and EXAFS studies at different Co loadings

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Microporous and Mesoporous Materials 62 (2003) 191–200

Siting and coordination of cobalt in ferrierite:XRD and EXAFS studies at different Co loadings

M.C. Dalconi a,*, A. Alberti a, G. Cruciani a, P. Ciambelli b, E. Fonda c

a Dipartimento di Scienze della Terra, Sez. di Mineralogia, Universit�aa di Ferrara, Corso Ercole I d’Este 32, 44100 Ferrara, Italyb Dipartimento di Ingegneria Chimica e Alimentare, Universit�aa di Salerno, Salerno, Italy

c Dipartimento di Scienze Chimiche, Universit�aa di Trieste, Trieste, Italy

Received 29 January 2003; received in revised form 7 May 2003; accepted 10 May 2003

Abstract

The crystal structure of three synthetic ferrierite samples loaded with different amounts of Co cations (2.5 wt.%, 4.3

wt.% and 13.7 wt.% respectively) were refined by means of synchrotron X-ray powder diffraction data. The unit cell

content of cobalt exchanged ferrierites resulted: jNa0:18K0:03Co1:12H1:36(H2O)17:2j[Al3:81Si32:19O72]––FER, jNa0:18K0:03-Co1:89(H2O)17:2j[Al3:81Si32:19O72]––FER, and jNa0:18K0:03Co1:89(H2O)17:2j[Al3:81Si32:19O72]––FER respectively. In the

overloaded sample the excess of Co (9.4 wt.%) is located outside the zeolite structure as cobalt oxide phases. After

cation exchange the space group, which was P21=n in the as-synthesised form, becomes Immm. The crystal structure of

the three synthetic samples is characterised by the presence of a Co(H2O)2þ6 polyhedron inside the ferrierite cage. The

occupancy of this polyhedron varies as a function of the Co amount in the extra-framework sites of the ferrierite

structure: up to 50% for a complete ion-exchange. The residual cobalt occupies two further cation sites.

� 2003 Elsevier Inc. All rights reserved.

Keywords: Ferrierite; Metal ion-exchanged zeolite; Co2þ ions; Rietveld refinement

1. Introduction

In the last decades, zeolites exchanged with

transition metal ions have received increasing in-terest as promising catalysts for a variety of im-

portant reactions [1]. Zeolites can serve as hosts to

activate transition metal ions, offering a unique

ligand system with multiple types of coordination

for cations. Zeolite topology, type of cation, its

location and coordination are among the factors

* Corresponding author. Tel.: +39-532-293-752; fax: +39-

532-293-760.

E-mail address: [email protected] (M.C. Dalconi).

1387-1811/$ - see front matter � 2003 Elsevier Inc. All rights reserve

doi:10.1016/S1387-1811(03)00404-9

controlling catalytic activity. It has recently been

shown that transition metal ion-exchanged zeolites

with low Al content and pentasil rings in their

frameworks (FER, MOR, MFI) display high ac-tivity and selectivity towards nitrogen in selective

catalytic reduction of nitrogen oxides (NOx) with

methane (CH4-SCR) [2]. Co-exchanged ferrierites

in particular have been found to exhibit high ac-

tivity towards the decomposition of NOx with

methane in the presence of excess oxygen [3].

Ciambelli et al. (personal communication) showed

that Co-ferrierite displays different catalytic ac-tivity depending on the loading of Co ions in zeo-

lite. Stakheev et al. [4] reported that the presence

of dispersed cobalt oxide species in addition to the

d.

mail to: [email protected]

192 M.C. Dalconi et al. / Microporous and Mesoporous Materials 62 (2003) 191–200

Co ions increase the rate of the CH4-SCR of NO

by enhancing the rate of NO2 formation. The Co-

ferrierite catalyst and its interaction with nitrogen

oxides, CH4, and SO2 has been studied by several

spectroscopic techniques (FTIR, UV–vis spec-

troscopy, EXAFS, temperature programmed de-sorption, X-ray photoelectron spectroscopy,

magnetic susceptibility measurements) [5–7], but

there are no reported diffraction studies on the

locations of extra-framework cobalt ions in fer-

rierite.

The aim of this study is to investigate the

structural modifications undergone by ferrierite

when loaded with different amounts of Co2þ ca-tions, and to localize the extra-framework sites by

high-resolution X-ray powder diffraction.

2. Experimental

2.1. Sample preparation

The synthetic ferrierite Engelhard EZTM-500

jK2:7Na1:1(H2O)12j[Al3:8Si32:2O72]––FER was ion-

exchanged to ammonium form by exhaustive ex-

change with aqueous solutions of ammonium

nitrate. Co-ferrierite samples (Co-FER) were pre-

pared by exchanging, at 80 �C, the ammoniumform of ferrierite with Co(CH3COO)2 aqueous

solutions. By varying the exchange time (18, 46 and302 h for samples named Co-FER18, Co-FER46

and Co-FER302 respectively) and renewing the

exchange solution many times, different cobalt ion

contents were obtained: 2.5 wt.% in sample Co-

FER18, 4.3 wt.% in Co-FER46 and 13.7 wt.% in

Co-FER302. After calcination at 550 �C for 2 h,the three samples were rehydrated at room condi-

tions. The cation content after exchange was de-

Table 1

Chemical formulae of as-synthesised (AS-FER) and Co-exchanged fe

Sample Co wt.%

AS-FER –

Co-FER18 2.5

Co-FER46 4.3

Co-FER302 Extra-zeolite Co 4.3+ 9.4 tot¼ 13.7*E% ¼ f½Al� ðNaþKþ 2 CoþHÞ�=ðNaþKþ 2 CoþHÞg 10

termined by atomic absorption spectroscopy

(AAS), whereas the water content was determined

by thermogravimetric analysis only for sample Co-

FER46; the same water content was attributed to

the other two samples. Assuming the framework

composition of the as-synthesised ferrierite(Engelhard EZTM-500), in the Co-FER18 sample

the Co2þ cations are not sufficient to counterbal-

ance the framework charge (see Table 1); the re-

mainder of the charge is probably compensated by

the presence of Brønsted acid sites produced during

thermal decomposition of the residual NHþ4 ca-

tions [8]. In the case of the Co-FER46 sample the

ion-exchange is complete, confirming an almostmonocationic Co-form (Table 1). The Co-FER302

sample indicated an overloading of cobalt ions

which could only in part (1.89 cations per unit cell)

be hosted in the extra-framework sites of ferrierite;

the excess of cobalt (9.39 wt.%) is probably located

outside the zeolite structure (see Table 1).

2.2. X-ray diffraction

High-resolution, room temperature synchrotron

X-ray powder diffraction data of the hydrated

samples were collected at the Swiss–Norwegian

beamline (SNBL) at ESRF (Grenoble), using a

Si(1 1 1) monochromator and a Si(1 1 1) analyser,

with a scintillation counter as detector. The sam-

ple, placed in a capillary tube 0.7 mm in diameter,was spun during data collection to minimise its

preferred orientation. Experimental and refine-

ment details are reported in Table 2.

Close inspection of the diffraction patterns of

Co-exchanged ferrierite did not show the presence

of any reflections of the type hþ k þ l ¼ 2nþ 1,which are forbidden in the Immm space group; this

space group was therefore adopted in the refine-

rrierites

Chemical formula E (%)�

jNa1:10K2:71(H2O)11:8j[Al3:81Si32:19O72] 0.0

jNa0:18K0:03Co1:12H1:36(H2O)17:2j[Al3:81Si32:19O72] 0.0

jNa0:18K0:03Co1:89(H2O)17:2j[Al3:81Si32:19O72] )4.5jNa0:18K0:03Co1:89(H2O)17:2j[Al3:81Si32:19O72] )4.5

0.

Table 2

Experimental and refinement details for Co-exchanged ferrierite

Co-FER18 Co-FER46 Co-FER302

k (�AA) 0.64791 0.64791 0.64791

Min/max 2h (�) 3/42.6 3/42.6 3/42.6

Rwp (%) 4.4 4.4 4.4

Rp (%) 3.5 3.5 3.5

Red-v2 3.5 2.8 3.5

R2F (%) 5.0 4.5 7.5

Nvar 69 69 69

Nobs 3975 4066 3934

Zero-shift (�) 0.0014 0.0022 0.0049

Background function Shifted Chebyshev

24-coefficients

Shifted Chebyshev

24-coefficients

Shifted Chebyshev

36-coefficients

Profile function pseudo-Voigt Pseudo-Voigt pseudo-Voigt

Rp ¼ R½Yio � Yic�=RYio; Rwp ¼ ½RwiðYio � YicÞ2=RwiY 2io�0:5; Red-v2 ¼ RwiðYio � YicÞ2=Nobs � Nvar); R2F ¼ RjF 2o � F 2c j=RjF 2o j.

M.C. Dalconi et al. / Microporous and Mesoporous Materials 62 (2003) 191–200 193

ments. The Immm space group was also assumed

in the structure refinement of the NH4 form, which

is the precursor of the Co-exchanged form,whereas the Na-, K- as-synthesised form was re-

fined in the monoclinic P21=n space group [9]. It isto be noted that the structure of the natural Mg-

rich ferrierite was refined in the Immm space group

[10,11], whereas the natural Na-, K rich, Mg-poor

ferrierite from Altoona (Washington) was refined

in the P21=n space group [12].Rietveld structures refinements were performed

using the GSAS package [13]. The profiles of the

Co-ferrierite patterns showed signs of peak broad-

ening due to particle size and stacking faults: this

was accounted for by including terms in the

pseudo-Voigt peak shape function. Table 3 reports

the refined values of the lattice parameters; the

parameters of as-synthesised ferrierite (AS-FER)

are also reported for comparison. The most evi-dent effect is the variation of the unit cell para-

meter a; this aspect will be discussed later.

Table 3

Refined values of the unit cell parameters of Co-exchanged ferrierite

AS-FER Co-FER18

Space group P21=n Immm

a (�AA) 18.8500(4) 18.9903(3)

b (�AA) 14.1361(1) 14.1161(1)

c (�AA) 7.4439(1) 7.4621(1)

V (�AA3) 1983.54(4) 2000.36(3)

DV (%) +0.85

The unit cell of as-synthesised ferrierite is reported for comparison. A

2.3. EXAFS

Co K-edge (7709 eV) spectra were collected atthe DCI XAFS 13 (Lure, Orsay, France) at liquid

nitrogen temperature in transmission mode. The

samples were prepared by grinding the Co-FER46

and Co-FER302 powders together with polyeth-

ylene in a mortar and then pressed into pellets. The

EXAFS data were analysed using the UWXAFS

package, applying theoretical phase functions and

amplitudes calculated by FEFF7 [14] and using thestructural model of the cobalt oxide phase Co3O4.

Quantitative data analysis was performed in the

ranges Kmin ¼ 2 to Kmax ¼ 12 �AA�1 and Rmin ¼ 1:1to Rmax ¼ 3:1 �AA using the FEFFIT routine.

3. Results and discussion

Ferrierite is a pentasil zeolite characterised by

the presence of a two-dimensional channel system

Co-FER46 Co-FER302

Immm Immm

19.0254(3) 19.0332(4)

14.1205(1) 14.1183(1)

7.4692(1) 7.4700(1)

2006.59(3) 2007.23(4)

+1.16 +1.19

S-FER¼ as-synthesised ferrierite.


parallel to the (1 0 0) plane. An 8-membered ring

channel develops along the [0 1 0] direction,

whereas a 10-membered ring channel is parallel to

the [0 0 1] direction and alternates along the b axiswith the so-called ferrierite cage, namely a

[82626458] cage.

3.1. Co-FER18

The crystal structure of Ni-exchanged ferrierite

[15] was adopted as starting model for Rietveld

refinement. As for Ni-ferrierite, the framework of

Co-FER18 did not show any noticeable difference

with respect to the framework of natural Mg-rich

Table 4

Atomic coordinates, fractions, isotropic temperature factors and selec

Site x=a y=b

T1 0.1554(2) 0

T2 0.0848(1) 0.2020(2)

T3 0.2735(2) 0

T4 0.3236(1) 0.2008(2)

O1 0 0.2103(8)

O2 0.2430(5) 0

O3 0.0995(3) 0.0884(3)

O4 0.2017(3) 0

O5 0.2500 0.2500

O6 0.1565(4) 0.2803(5)

O7 0.1121(2) 0.2531(3)

O8 0.3244(2) 0.0891(3)

Co1 0 0

Co2 0.457(2) 0

Co3 0 0.291(3)

W1 0 0

W2 0.090(1) 0.067(1)

W3 0.033(1) 0.163(2)

W4 0 0.441(3)

W5 0 0.597(2)

W6 0.045(2) 0.5000

T1–O3¼ 1.639(3) [x2] T3–O2¼ 1.628(4) Co1–W1

T1–O4¼ 1.627(3) [x2] T3–O4¼ 1.601(4) Co1–W2

T2–O1¼ 1.614(3) T3–O8¼ 1.622(2) [x2] Co1–W3

T2–O3¼ 1.628(4) T4–O5¼ 1.594(2) Co2–W5

T2–O7¼ 1.630(2) [x2] T4–O6¼ 1.609(2) Co2–W6

T4–O7¼ 1.609(3) Co2–W6

T4–O8¼ 1.612(3) Co3–W3

Co3–W5

W1–O3¼ 2.89(1) [x4] W2–O2¼ 3.07(2) W2–O4¼W2–O6¼ 3.27(2) W3–O6¼ 2.88(2) W3–O7¼W4–O1¼ 3.42(3) W5–O7¼ 3.28(2) [x2] W6–O8¼

ferrierite [10]. As in Ni-exchanged ferrierite, the

structure refinement indicated the presence of

three extra-framework cobalt cation sites and six

water molecule sites (see Table 4 and Fig. 1). One

of these (Co1 cation site in Table 4) is located at

the centre of the ferrierite cage, and is six-coordi-nated to water molecules in a slightly distorted

octahedral configuration. The Co(H2O)2þ6 octahe-

dron has two possible configurations, differing by

their rotation around the z axis, which show strongsimilarities with the Mg(H2O)

2þ6 octahedron in

Mg-rich natural ferrierite and with the Ni(H2O)2þ6

octahedron in Ni-ferrierite. Similarly to the Ni-

exchanged form, the site at the centre of the fer-

ted bond distances (�AA) of Co-FER18

z=c Fractions Uiso ( 100 �AA2)0 1.000 1.6(2)

0 1.000 1.3(1)

0.2960(5) 1.000 1.8(1)

0.2065(3) 1.000 1.7(1)

0 1.000 4.4(5)

0.5000 1.000 4.6(5)

0 1.000 3.5(3)

0.1834(7) 1.000 3.2(3)

0.2500 1.000 4.5(3)

0.5000 1.000 3.7(3)

0.1832(5) 1.000 3.9(2)

0.2513(6) 1.000 3.4(2)

0.5000 0.33(1) 3.1(4)

0 0.07(1) 10.0(3)

0.5000 0.10(1) 7.3(1)

0.239(4) 0.49(1) 9.9(1)

0.5000 0.49(1) 11.0(9)

0.5000 0.38(1) 11.4(1)

0.132(7) 0.31(1) 12.6(1)

0.366(5) 0.40(1) 12.9(1)

0.235(6) 0.27(2) 7.2(1)

¼ 1.94(2) [x2]¼ 1.95(1) [x4]¼ 2.38(2) [x4]¼ 1.89(2) [x4]¼ 1.94(4) [x2]¼ 2.56(5) [x2]¼ 1.90(3) [x2]¼ 1.88(3) [x2]

3.32(1) [x2]

3.09(2) [x2]

2.78(2) [x2]

Fig. 1. Cobalt cation sites in Co-exchanged ferrierite (all three

samples) at room temperature. Water molecule sites are not

reported in the picture.


rierite cage in the Co-exchanged forms is only

partially occupied by Co2þ ions (33%). A second

Co2þ site (Co2) was detected near the centre of the

10-membered ring, corresponding to the equiva-

lent Ni2 site determined in the Ni-exchanged form,

whereas the third site (Co3) is situated not far

from the centre of the 8-membered ring window

delimiting the ferrierite cage. Both Co2 and Co3sites are scarcely occupied (7% and 10% respec-

tively) and exclusively coordinated to water mol-

ecules. The Co3 site corresponds to the Ni3 site of

the Ni-exchanged form. On the whole, the extra-

framework cation content (�1.3 atoms per unitcell) given by the structure refinement is in ac-

ceptable agreement with the value given by the

chemical analysis.

3.2. Co-FER46

The structure refinement was carried out start-

ing from the atomic parameters of Co-FER18. The

two crystal structures are very similar; the onlynoticeable differences are in the higher occupancies

of the cobalt sites in Co-FER46 (see Table 5). In

particular, the Co1 site is 50% occupied, the same

occupancy as found for this site in Ni-exchanged

ferrierite. In the case of Co-FER46, the extra-

framework cation content (�2 atoms in unit cell) isin acceptable agreement with the chemical analy-

sis. It is to be noted that the Co2þ amount given bythe chemical analysis is slightly higher than that

necessary to counterbalance the framework charge

(Co=Al ffi 0:5); this discrepancy can be attributedeither to intrinsic error in the measurement or to

an incipient formation of cobalt oxides (see also

the EXAFS results). In any case, the same Co

content was assumed for the Co-FER302 sample

(see Table 1).

3.3. Co-FER302

The structure refinement of Co-FER46 provided

the starting parameters to refine the Co-FER302

structure. No remarkable differences were detected

with respect to the Co-FER46 structure, especially

in the extra-framework cation site occupancies (seeTable 6). This means that the excess of cobalt in-

dicated by chemical analysis (6.3 atoms per unit

cell, 1.9 of which located in the extra-framework

sites of Co-FER302) is situated outside the fer-

rierite structure in a cobalt oxide phase. The

presence of a second, poorly crystalline phase in

the Co-FER302 sample is accounted for by

(a) the presence of wide ‘‘bumps’’ around d ¼2:66–2.33 �AA (2h range 14�–16�) and d ¼1:50–1.53 �AA (2h range 24�–26�) in the diffrac-tion pattern of this sample;

(b) the values of the unit cell parameters;

(c) the results of EXAFS analysis.

(a) The bumps found in the Co-FER302 diffrac-tion pattern were not detected in either Co-FER18

or Co-FER46 patterns (see Fig. 2). Considering the

strong similarities between the refined structures of

Table 5

Atomic coordinates, fractions, isotropic temperature factors and selected bond distances (�AA) of Co-FER46

Site x=a y=b z=c Fractions Uiso ( 100 �AA2)

T1 0.1557(3) 0 0 1.000 0.7(2)

T2 0.0831(2) 0.2018(3) 0 1.000 1.6(1)

T3 0.2725(2) 0 0.2948(6) 1.000 1.7(1)

T4 0.3239(2) 0.2010(2) 0.2067(4) 1.000 1.4(1)

O1 0 0.2122(1) 0 1.000 4.9(5)

O2 0.2455(6) 0 0.5000 1.000 4.2(5)

O3 0.1024(5) 0.0899(6) 0 1.000 3.1(3)

O4 0.2016(4) 0 0.180(1) 1.000 3.4(3)

O5 0.2500 0.2500 0.2500 1.000 4.3(3)

O6 0.1574(4) 0.2812(5) 0.5000 1.000 3.3(3)

O7 0.1136(3) 0.2517(4) 0.1788(8) 1.000 4.1(2)

O8 0.3228(4) 0.0882(4) 0.2536(8) 1.000 3.3(2)

Co1 0 0 0.5000 0.50(1) 4.8(3)

Co2 0.455(1) 0 0 0.13(1) 12.6(1)

Co3 0 0.310(2) 0.5000 0.130(5) 8.2(1)

W1 0 0 0.219(4) 0.51(1) 8.9(1)

W2 0.094(1) 0.068(1) 0.5000 0.42(1) 9.9(1)

W3 0.043(1) 0.159(1) 0.5000 0.51(1) 11.9(8)

W4 0 0.447(2) 0.130(6) 0.36(2) 13.4(1)

W5 0 0.597(2) 0.322(4) 0.31(1) 11.9(1)

W6 0.053(1) 0.5000 0.253(4) 0.32(1) 8.7(1)

T1–O3¼ 1.624(7) [x2] T3–O2¼ 1.617(6) Co1–W1¼ 2.10(2) [x2]T1–O4¼ 1.602(7) [x2] T3–O4¼ 1.598(7) Co1–W2¼ 2.04(2) [x4]T2–O1¼ 1.588(4) T3–O8¼ 1.600(5) [x2] Co1–W3¼ 2.40(1) [x4]T2–O3¼ 1.623(7) T4–O5¼ 1.600(2) Co2–W5¼ 2.09(2) [x4]T2–O7¼ 1.618(5) [x2] T4–O6¼ 1.604(3) Co2–W6¼ 1.86(3) [x2]

T4–O7¼ 1.609(5) Co2–W6¼ 2.62(4) [x2]T4–O8¼ 1.631(4) Co3–W3¼ 2.27(2) [x2]

Co3–W5¼ 1.87(3) [x2]

W1–O3¼ 2.85(1) [x4] W2–O2¼ 3.03(2) W2–O4¼ 3.29(2) [x2]W2–O6¼ 3.24(2) W3–O6¼ 2.77(2) W3–O7¼ 3.03(1) [x2]W4–O1¼ 3.46(3) W5–O7¼ 3.22(2) [x2] W6–O8¼ 2.67(2) [x2]


the Co-FER samples, these bumps can be attrib-uted to the excess of Co2þ ions in the over-loaded

sample which forms cobalt oxide phases. It is to be

noted that both Co3O4 and CoO oxides present

intense reflections in the two bump regions. The

presence of very broad bumps rather than sharp

diffraction peaks can be interpreted as due to the

contributions of one or more cobalt-oxide phases

with low crystallinity.(b) Gramlich-Meier et al. [12] showed that, in

natural ferrierites, there is a linear relationship of

the unit cell parameter a versus the Mg atomscontent per unit cell. As far back as 1966, Vaughan

[16] indicated that in the ferrierite sample fromKamloops Lake, British Columbia, the two Mg

atoms present in the unit cell fully occupy the

ferrierite cage, forming Mg(H2O)6 octahedrons

with two possible configurations which differ de-

pending on their rotation around the z axis. Thesame result was obtained by Gramlich-Meier et al.

[11] and Alberti and Sabelli [10] for ferrierites from

Silver Mountain, California, and from Monastir,Sardinia, respectively. On the contrary, in the Mg-

poor ferrierite from Altoona [12] the ferrierite cage

is only slightly occupied, and the unit cell a pa-rameter is far shorter than in Mg-rich ferrierites. It

Table 6

Atomic coordinates, fractions, isotropic temperature factors and selected bond distances (�AA) of Co-FER302

Site x=a y=b z=c Fractions Uiso ( 100 �AA2)

T1 0.1564(4) 0 0 1.000 0.8(3)

T2 0.0857(2) 0.2023(4) 0 1.000 1.6(2)

T3 0.2740(2) 0 0.2987(7) 1.000 1.7(2)

T4 0.3229(2) 0.2003(3) 0.2065(5) 1.000 1.5(1)

O1 0 0.2102(3) 0 1.000 4.8(7)

O2 0.2398(7) 0 0.5000 1.000 4.3(7)

O3 0.0989(5) 0.0872(5) 0 1.000 3.08(4)

O4 0.2009(4) 0 0.187(1) 1.000 3.4(4)

O5 0.2500 0.2500 0.2500 1.000 4.2(4)

O6 0.1542(5) 0.2790(7) 0.5000 1.000 3.1(4)

O7 0.1117(4) 0.2547(5) 0.1836(9) 1.000 4.1(3)

O8 0.3274(3) 0.0873(4) 0.256(1) 1.000 3.4(3)

Co1 0 0 0.5000 0.47(1) 4.6(4)

Co2 0.442(2) 0 0 0.13(1) 11.8(2)

Co3 0 0.294(3) 0.5000 0.14(1) 6.6(1)

W1 0 0 0.231(5) 0.56(1) 9.0(1)

W2 0.102(3) 0.077(3) 0.5000 0.41(2) 15.8(1)

W3 0.044(2) 0.145(3) 0.5000 0.42(2) 16.5(1)

W4 0 0.452(3) 0.117(8) 0.34(2) 16.4(2)

W5 0 0.595(3) 0.326(5) 0.45(2) 12.0(1)

W6 0.047(4) 0.5000 0.268(1) 0.24(3) 5.5(2)

T1–O3¼ 1.648(2) [x2] T3–O2¼ 1.639(4) Co1–W1¼ 2.02(3) [x2]T1–O4¼ 1.635(2) [x2] T3–O4¼ 1.622(4) Co1–W2¼ 2.24(4) [x4]T2–O1¼ 1.634(4) T3–O8¼ 1.630(2) [x2] Co1–W3¼ 2.20(4) [x4]T2–O3¼ 1.644(4) T4–O5¼ 1.588(2) Co2–W5¼ 2.17(2) [x4]T2–O7¼ 1.635(2) [x2] T4–O6¼ 1.629(3) Co2–W6¼ 1.73(6) [x2]

T4–O7¼ 1.621(3) Co2–W6¼ 2.63(6) [x2]T4–O8¼ 1.639(3) Co3–W3¼ 2.27(4) [x2]

Co3–W5¼ 2.04(4) [x2]

W1–O3¼ 2.82(2) [x4] W2–O2¼ 2.82(3) W2–O4¼ 3.19(2) [x2]W2–O6¼ 3.01(3) W3–O6¼ 2.87(3) W3–O7¼ 3.13(2) [x2]W4–O8¼ 3.48(1) [x2] W5–O7¼ 3.19(2) [x2] W6–O8¼ 2.71(3) [x2]


is evident that parameter a is strongly correlated tothe occupancy of the ferrierite cage and, in ac-

cordance with the correlation pointed out by

Gramlich-Meier et al. [12], the a unit cell para-meter is longer by about 0.36 �AA in Mg-rich fer-rierites, where the cage is fully occupied, with

respect to the ferrierites where Mg is absent and

the ferrierite cage is empty. This relation agrees

well with the results of our structure refinements

(see Fig. 3 and Table 3). In Co-FER46 and Co-

FER302 in particular, where the ferrierite cage is

50% occupied, parameter a is 0.18 �AA longer than inas-synthesised ferrierite, where the cage is empty.

(c) Fig. 4 shows the Co K-edge background-

subtracted EXAFS spectra relative to samples

Co-FER46 and Co-FER302. The spectra were

analysed using two coordination shells around Co,

the first shell consisting of oxygens and the secondone of cobalt atoms. The most evident difference

between the Fourier transform (FT) relative to Co-

FER46 and Co-FER302 is the strong increase in

the latter sample of the peak at R ffi 2:9 �AA (not

corrected for phase shift) (see Fig. 5). This peak

corresponds to a second shell of Co atoms at

about 3.11 �AA, easily referable to a Co–Co distance.As X-ray structure refinements did not show any

Fig. 3. Plot of unit cell parameter a (in �AA) versus number of Mgions per unit cell in ferrierite. For comparison the lattice con-

stants a of Co-ferrierite samples are reported. Symbols:

AS¼ as-synthesised ferrierite; A1, A2 and AL¼Altoona,Washington; SMM¼ Santa Monica Mountains; RM¼RodopeMountains, Bulgaria; K and KL¼Kamloops Lake, BC;SP¼ Sonora Pass, California; MO¼Monastir, Sardinia, Italy;PL¼Pinaus Lake, BC; ML¼Monte Lake, BC; I¼AlberoBassi, Vicenza, Italy (all data from Wise and Tschernich [17]

and references cited therein).

Fig. 4. Co K-edge EXAFS spectra of samples Co-FER46 and

Co-FER302.

Fig. 2. (a) Diffraction patterns of Co-exchanged ferrierite samples; (b) and (c) zoomed regions with ‘‘bump’’ in Co-FER302 pattern.


remarkable differences between Co-FER46 andCo-FER302, and there are no Co–Co distances at

about 3.11 �AA inside the ferrierite structure, this

peak can be attributed only to Co–Co distances

outside the zeolite structure and referable to a

cobalt oxide phase. The weak, poorly defined peak

at 2.9 �AA in the FT of sample Co-FER46 can beeasily attributed to an incipient formation of co-balt oxide. Both the Co-FER46 and Co-FER302

samples show an intense peak at 1.5 �AA (not cor-rected for phase shift) which was modelled by a

shell of 6 oxygens at about 2.08 �AA. This shell maycorrespond to the 6 oxygens of the Co(H2O)

2þ6

octahedron inside the ferrierite cage, as well as to

the first shell of oxygens in a cobalt oxide phase.

Table 7 reports the values obtained from thequantitative data analysis, and Fig. 5 reports the

obtained fits.

Fig. 5. Fit ( ) to the Fourier transform of k3 vðkÞ Co signal(––) for samples Co-FER46 and Co-FER302.

Table 7

Results of the Co K-edge EXAFS analysis for Co-FER46 and

Co-FER302

Sample Co–O Co–Co

N R [�AA] r2 [�AA2] N R [�AA] r2 [�AA2]

Co-

FER46

5.8 2.08(1) 0.007 1.3 3.11(3) 0.008

Co-

FER302

5.2 2.09(3) 0.009 3.6 3.12(4) 0.007


4. Conclusions

Comparison of the crystal structures of ferrierite

loaded with increasing amounts of cobalt ions in-

dicates that ferrierite structures can host cobalt

ions in three different extra-framework sites, even

at a quite low cation exchange. Their occupancy

increases up to the cation content corresponding

to the Co2þ amount necessary to counterbalance

the framework charge (Co=Al ffi 0:5). The mostpopulated site is at the centre of the ferrierite cage,in a quite regular Co(H2O)

2þ6 octahedral coordi-

nation. The refinement results show that the Co

content inside the ferrierite structure strongly in-

fluences unit cell parameter a, so that the amountof cation exchange can be roughly deduced by its

value. It has been demonstrated that an over-

exchange of Co ions gives rise to cobalt oxides

situated outside the zeolite structure.

Acknowledgements

The Ministero della Universit�aa e della RicercaScientifica e Tecnologica is thanked for its finan-

cial support to the research program ‘‘Zeolites,

materials of interest for industry and environment:synthesis, crystal structure, stability and applica-

tions’’ (COFIN 01).

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Siting and coordination of cobalt in ferrierite: XRD and EXAFS studies at different Co loadings

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Transcript of Siting and coordination of cobalt in ferrierite: XRD and EXAFS studies at different Co loadings