Electron Paramagnetic Resonance, Scanning Electron Microscopy

CNR, Research Area of Rome, Monterotondo Stazione, 00016 Rome, Italy, ... firing process as a preliminary step to get information on ancient ceram...
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J. Phys. Chem. B 2005, 109, 22147-22158

22147

Electron Paramagnetic Resonance, Scanning Electron Microscopy with Energy Dispersion X-ray Spectrometry, X-ray Powder Diffraction, and NMR Characterization of Iron-Rich Fired Clays Federica Presciutti,† Donatella Capitani,*,‡ Antonio Sgamellotti,† Brunetto Giovanni Brunetti,† Ferdinando Costantino,† Ste´ phane Viel,‡,§ and Annalaura Segre‡ Chemistry Department and Center SMAArt, UniVersity of Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy, Institute of Chemical Methodologies, CNR, Research Area of Rome, Monterotondo Stazione, 00016 Rome, Italy, and UniVersity of Aix Marseille I & III, JE 2421 Traces, Site de Saint-Jerome, SerVice 511, 13397 Marseille Cedex 20, France ReceiVed: July 1, 2005; In Final Form: September 28, 2005

The aim of this study is to clarify the structure of an iron-rich clay and the structural changes involved in the firing process as a preliminary step to get information on ancient ceramic technology. To this purpose, illiterich clay samples fired at different temperatures were characterized using a multitechnique approach, i.e., by electron paramagnetic resonance, scanning electron microscopy with electron dispersion X-ray spectrometry, X-ray powder diffraction, magic angle spinning and multiple quantum magic angle spinning NMR. During firing, four main reaction processes occur: dehydration, dehydroxylation, structural breakdown, and recrystallization. When the results are combined from all characterization methods, the following conclusions could be obtained. Interlayer H2O is located close to aluminum in octahedral sites and is driven off at temperatures lower than 600 °C. Between 600 and 700 °C dehydroxylation occurs whereas, between 800 and 900 °C, the aluminum in octahedral sites disappears, due to the breakdown of the illite structure, and all iron present is oxidized to Fe3+. In samples fired at 1000 and 1100 °C iron clustering was observed as well as large single crystals of iron with the occurrence of ferro- or ferrimagnetic effects. Below 900 °C the aluminum in octahedral sites presents a continuous distribution of chemical shift, suggesting the presence of slightly distorted sites. Finally, over the whole temperature range, the presence of at least two tetrahedral aluminum sites was revealed, characterized by different values of the quadrupolar coupling constant.

I. Introduction Clays form a class of technologically important materials having widespread industrial applications, including catalyst support in petrochemical industries. Clays are the major components of white-ware products, bricks, roof tiles, and cements. Firing involves breaking down some phases and forming new ones. Consequently clay artifacts are considered as artificial rocks formed in a kiln.1 The thermal decomposition of clays is one of the most studied ceramic reactions. Moreover clays constitute an important part of new synthetic materials such as nanocomposites. The growth of new phases, during and after firing, is related both to the firing temperature and to the composition of the microsites in which the phases grow. To optimize the processing of clays, the thermal decomposition mechanisms must be fully understood. In particular, as the clay structure breaks down, details on the structure and on the distribution of atomic species in the intermediate amorphous states should be obtained. To study the modification induced by firing, an illite-rich clay from Deruta, a town in the region of Umbria (Italy), was chosen. In fact, Deruta has been for many centuries one of the most important centers for the production of ceramics in Italy. The quality of these ceramics is well-known, with historical wares from Deruta, especially those belonging to the Renaissance * Corresponding author: e-mail, [email protected]; (+39).06.90672385; fax, (+39).06.90672477. † University of Perugia. ‡ CNR. § University of Aix Marseille.

period, shown in most relevant museums all over the world. Several factors encouraged Deruta to become a major producer of ceramics, one of these is the availability of suitable earth from which to form clay. In fact, the hills around Deruta are particularly rich in a pure strain of clay which also washes up along the shores of the nearby Tiber River. Therefore the aim of this study is to clarify the structure of this clay and to explain the structural changes involved in its firing, as a preliminary step to obtain information about the firing processes of the ancient ceramic technology. To this purpose illite-rich clay samples fired at different temperatures were characterized using a multitechnique approach, i.e., by electron paramagnetic resonance (EPR), scanning electron microscopy with energy dispersion X-ray spectrometry (SEM-EDS), X-ray powder diffraction (XRPD), and magic angle spinning (MAS) and multiple quantum magic angle spinning (MQ-MAS) NMR. Previous studies on illite-rich clays2 using various techniques3 showed that four main reaction processes occur during firing: dehydration, dehydroxylation, structural breakdown, and recrystallization. In the 350-400 °C temperature range, the interlayer water is driven off, dehydroxylation occurs between 450 and 700 °C, and the irreversible structural breakdown occurs between 800 and 900 °C, with the formation of new phases at about 900 °C. For the sake of clarity, the results obtained from each technique will be discussed separately.

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II. Experimental Section Firing Procedure. The samples were prepared using the following procedure. To easily model the clay, it was plunged

10.1021/jp0536091 CCC: $30.25 © 2005 American Chemical Society Published on Web 11/02/2005

22148 J. Phys. Chem. B, Vol. 109, No. 47, 2005 in water and then dried at room temperature for 1 day. Samples were fired in an oxidant atmosphere at different temperatures, 600, 700, 800, 900, 1000, and 1100 °C, in a kiln of a ceramist’s workshop. After an 8-h heating cycle, the temperature was kept constant for 2 h at the final value. Finally, the samples were slowly cooled to room temperature inside the kiln. Analytical investigations were performed using atomic absorption spectroscopy, SEM-EDS, and X-ray fluorescence spectrometry (XRF). Compositional Analysis. The chemical composition of the samples was determined by X-ray fluorescence spectrometry using a Philips, PW 1400 with Rh KR radiation. The measurements were carried out on compressed powder pellets. The loss on ignition (LoI) was determined by heating at 1000 °C for 12 h. Intensities were processed according to previously reported methods.4 A quantitative determination of the clay iron content was also carried out by a Perkin-Elmer atomic absorption spectrophotometer, model 1100B. Acid digestion was performed using mixtures of HClO4/HF. The iron content of all samples was about 5.4 ( 0.6%. SEM-EDS. Samples were cut to obtain thin cross sections which were coated with carbon at high pressure to make the surface conductive. Scanning electron microscopy investigation was performed using a Philips XL 30 operating at 20 kV and equipped with an EDAX DX4 energy dispersive spectrometer (EDS). Elements with atomic number higher than 10 were identified. The counting rate was kept close to 1500-1800 counts/s over the whole energy range. After peak acquisition, the intensity values were used for quantification, according to the ZAF procedure and to the “standardless” approach (namely, using computed pure element intensity factors). Analytical accuracy was checked using secondary standards, i.e., samples with known composition; an agreement better than 10% was achieved. The analytical precision was better than 0.5% for major elements and better than 20% for minor elements (with a concentration ranging from 0.3 to 3-5 wt %). To study the sample morphology, backscattered images (BSE) were collected. In addition, EDS compositional studies were carried out on the samples, both on the bulk and on a few grains. To characterize the bulk, 10 measurements were performed by scanning different areas with dimensions about 800 × 800 µm2 and avoiding iron-rich grains, and then an arithmetic average was performed. The areas used for the compositional studies of the grains depended on the dimensions of the grains. XRPD. X-ray powder diffraction patterns were collected in the 3-100° 2θ range according to the step scanning procedure with Cu KR radiation on a Philips X’PERT APD diffractometer; a PW3020 goniometer equipped with a bent graphite monochromator on the diffracted beam, 0.5° divergence and scatter slits, and a 0.1 mm receiving slit were used. The LFF tube operated at 40 kV and 30 mA. To minimize preferred orientations, the sample was carefully side loaded into an aluminum sample holder with an oriented quartz monocrystal underneath. The diffraction patterns for quantitative analysis were collected in the 3-100° 2θ range with a 13 s/step data collection time. The diffraction patterns for qualitative analysis were collected in the 3-60° 2θ range with a 3 s/step data collection time. Microcrystalline silicon powder (200 mesh) was used as a reference standard, and it was added to the samples in a Si/ sample ) 1/10 weight fraction ratio. EPR Spectroscopy. Electron paramagnetic resonance spectra were recorded at the X-band frequency (9.1 GHz) at 297 K using a Bruker EMX EPR spectrometer, equipped with the EMX high sensitivity probehead. Samples were finely powdered,

Presciutti et al. weighted (124 mg), and inserted into 4 mm EPR quartz tubes. To perform the Fe3+ quantitative analysis, six samples were prepared containing alum (KAl(SO4)2‚12H2O)/ferric-alum (Fe(NH4)(SO4)2‚12H2O) mixtures with known amounts of Fe3+, namely, 9.65%, 7.72%, 5.79%, 3.86%, 1.93%, and 0.37%. NMR Spectroscopy. Samples (130 mg) were inserted in 4 mm zirconia rotors sealed with Kel-F caps. All spectra were recorded on a Bruker ASX200 NMR spectrometer. The spinning rate was always 12 kHz. 27Al NMR. 27Al MAS spectra were recorded at 52.15 MHz. The π/2 pulse length was 3 µs, and 800 transients made of 512 data points were collected with a recycle delay of 60 s. In fact, measurements of spin-lattice relaxation time (T1) indicated the presence of two relaxing components of about 0.05 and 12 s, respectively. T1 measurements were performed after a careful match of the magic angle by using the saturation recovery sequence and repeated twice at two different spinning rates, 5 and 12 kHz. The ppm scale was referenced to Al(H2O)63+. 27Al {1H} CP-MAS NMR Spectra. The cross polarization (CP) was performed by applying the variable spin-lock scheme “RAMP”.5 Specifically, the RAMP causes one of the channels to be spin-locked slightly off the Hartmann-Hahn condition, except in the middle of the RAMP where the spin-lock is exactly matched. In this way the motional modulation of the 27Al-1H dipolar coupling caused by spinning the sample at high rate can be overcome. Experimentally, the RAMP can be applied on either the X or the 1H channel; here the RAMP was applied on the 1H channel and, during the contact time τ, the amplitude of the spin lock was increased from 50% to 100% of its maximum value. All spectra were recorded using the TPPM15 1H decoupling scheme,6 with a decoupling field strength of 75 kHz. CP-MAS spectra were recorded at different contact times τ, with τ ranging from 0.2 to 20 ms. The signal in the crosspolarized spectra showed a maximum at τ ) 0.5 ms; thus this contact time was used for all spectra. 27Al 3Q MAS NMR spectra7-10 were obtained with a twopulse sequence: p1-t1-p2-aq; the phase cycling was composed of six phases for the selection of triple-quantum coherences. The pulse lengths p1 and p2 were optimized to 3 µs and 7 µs, respectively. In the following, the single-quantum (1Q) and the triple-quantum (3Q) dimensions are referred to as the f2 and f1 dimensions, respectively. In f1, 100 regularly spaced (∆t1 ) 6 µs) t1 increments were acquired according to the TPPI acquisition scheme and, for each t1 increment, 732 transients made of 256 data points were collected in f2. The ppm scale was referenced to ν0 frequency in f2 and to 3ν0 in f1 using Al(H2O)63+ as an external reference. The 3Q MAS spectra were zero filled and Fourier transformed using 512 × 512 data points. The centers of gravity δG1 and δG2 of the two-dimensional (2D) spectral ridges in f1 and f2, respectively, can be used for estimating the isotropic chemical shift δiso and the second-order quadrupolar effect Pq according to eqs 2.1, 2.2, and 2.3

ξδG2 - |p|δG1C ξ - |p| δG1 ξG1C ) |p|

δiso )

PQ ) ν0

(

(2.1) (2.2)

)

C 2 9 [4I(2I - 1)] (δG1 - δG2) 10 p2 - 1 106

1/2

(2.3)

where ξ is the slope of the quadrupolar axis QIS. Here p ) 3, I ) 5/2, ν0 ) 52.15 MHz, and ξ ) 3/4. In the 3Q MAS spectra,

Characterization of Iron-Rich Fired Clays

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the A, QIS, and CS axes are defined by eqs 2.4, 2.5, and 2.6, respectively11

19 ν 12 2

( ) 3 ν ) ( )ν 4

ν1 )

1

2

ν1 ) 3ν2

TABLE 1: Composition (wt %) of Different Grains Typically Found in the Samples aluminum silicate

(2.4) (2.5) (2.6)

Spectral Deconvolution. The deconvolution of 27Al MAS and 3Q MAS NMR spectra was performed using the dm2004 program.12 Deconvolution of 27Al 1D MAS Spectra. In the dm2004 program the Q-MAS 1/2 model was selected to model the central transition under infinitely fast MAS spinning. Each resonance was characterized by the following parameters: the isotropic chemical shift δiso, the amplitude, the quadrupolar coupling constant Cq, the asymmetry parameter ηq, and EM, a parameter used to apodize the theoretical line shape. Initial guesses for δiso and Cq were extracted from the 3Q-MAS spectra, and then the best fit procedure was applied. Specifically, by using the Czjzeck model of the dm2004 software,13 it was possible to fit the resonances due to the octahedral site by taking into account the presence of a chemical shift distribution. In fact, according to this model, the line to be fit was characterized by the following parameters: δiso, Cq, ηq, EM, the amplitude, the width of the isotropic chemical shift distribution ∆CS, and the parameter d. d ranges from 1 to 5, 5 being for a Gaussian isotropic distribution of the electric field gradient (EFG).14 Deconvolution of 27Al 3Q MAS Spectra. The deconvolution of 3Q-MAS spectra was performed for deconvoluting the spectral ridges due to 27Al in tetrahedral sites. Before the spectra were deconvoluted, a shearing transformation was applied to align the anisotropic lines parallel to the f2 axis.15 In modeling multidimensional experiments some problem may arise because of the large number of adjustable parameters to be taken into account. Therefore we started fitting the corresponding 1D MAS spectrum and then we used the obtained parameters as initial guesses in the deconvolution of the 3Q MAS spectrum. In addition to the parameters which characterize a resonance in a 27Al 1D MAS spectrum, a parameter was added to compute the two-dimensional spectrum, that is, the line width in f1. 29Si NMR. MAS NMR spectra were recorded at 39.76 MHz. The π/2 pulse length was 4 µs, and 4000 transients made of 512 data points were collected with a recycle delay of 60 s. CP-MAS spectra were obtained using the following parameters: a π/2 pulse length of 3.5 µs, a contact time of 1 ms, and a recycle delay of 3 s. The ppm scale was referenced to tetramethylsilane (TMS). To model each resonance, the deconvolution of 29Si MAS spectra was performed using the dm2004 program.12 The Gaussian/Lorentzian model was selected. Each resonance was characterized by the following parameters: amplitude, position, width at half-height and a function giving the Gaussian/Lorentzian ratio; here we only used Lorentzian line shapes. 23Na NMR. MAS NMR spectra were recorded at 52.94 MHz. The π/2 pulse length was 4.5 µs, and 12000 transients made of 512 data points were collected with a recycle delay of 1 s. The ppm scale was referenced to a 0.1 M NaCl H2O solution. III. Results and Discussion XRF. The chemical composition of the samples was obtained by X-ray fluorescence analyses. The composition of the main

other crystals

W (%) W (%) W (%) W (%) W (%) W (%) W (%) W (%) Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO BaO TiO2 MnO FeO a

0.6 1.3 4.4 89.7 a 0.2 0.6 0.9 a 0.4 a 1.8

13.6 1.7 19.9 63.8 a 0.5 0.1 0.1 a a a 0.3

1.6 0.9 20.9 59.1 a 0.5 15.8 1.0 a a a 0.4

0.6 3.3 1.8 24.8 0.9 12.7 0.2 53.8 a. 0.5 a 1.6

2.2 6.8 20.0 35.5 a 0.5 2.7 5.0 a a 24.0 3.4

1.7 1.9 4.0 40.0 46.5 2.6 0.5 1.4 a 0.4 0.2 1.0

0.9 0.6 3.0 7.0 a 26.2 0.3 1.5 59.5 a a. 1.0

1.4 4.4 3.4 7.7 a 0.8 0.4 23.3 a a 53.6 4.9

Not detected.

components ranges as follows: Na2O ) 2.8-3.1%, MgO ) 3.3-3.5%, Al2O3 ) 15.9-16.7%, SiO2 ) 56.7-58.4%, P2O5 ) 0.1-0.2%, K2O ) 2.2-2.4%, CaO ) 5.2-5.4%, TiO2 ) 0.6-0.7%, MnO ) 0.1-0.2%, Fe2O3 ) 5.5-5.7%, L.o.I ) 3.8-6.8%. The composition of all samples was practically the same except for, as expected, the loss on ignition. SEM-EDS. The SEM-EDS was used for determining microtextural and microchemical features of the samples. All samples showed the same microtexture which consisted of grains (from 10 to several hundred µm) within a fine-grained porous matrix. The pore size was variable and could reach up to 100 µm. The EDS analysis of the grains showed different compositions. Many grains were quartz grains,16 whereas others were aluminum-silicate of alkaline metals (Na, K, Ca, and Mg); these minerals belonged to the unfired clay or were formed after the structural breakdown of the phyllosilicates. Other grains were rich in Ba and S, or in Mn, P, or Fe (Table 1).17 A more detailed treatment was required to describe the iron content observed both in the grains and in the matrix of the samples. The dimension of the iron-rich grains ranged from 50 µm2 to several hundred µm2. In the samples fired at T < 900 °C, the iron content was lower than 60 wt % (Table 2), whereas in the samples fired at T g 900 °C it could be as high as 90 wt % (Table 3). In the sample fired at 1100 °C some particular grain, consisting of both octahedral and tetrahedral crystals, with dimension of about 150 × 200 µm2 and an iron content of about 90 wt %, was observed (Figure 1). The iron content of the matrix of the samples was studied by considering the average composition of 10 areas of about (800 × 800 µm2); the selected areas did not contain any iron grain. The iron content in the sample matrix decreased as the firing temperature increased (Figure 2). This observation suggested that the iron migrates from the matrix toward preferential sites of crystallization. In fact the amount of iron in grains increased from 60% to 90%. XRPD: Qualitative Phase Analysis. In all samples the mineralogical phases were identified with a search/match on the PDF database on the basis of the characteristic reflections. The unfired clay contained quartz, (Qz, SiO2, PDF no. 46-1045), K-feldspar (Kfs, (K,Na)(Si,Al)4O8, PDF no. 84-0710), Na-rich plagioclase (Napg, NaAlSi3O8, PDF no. 18-1202), illite 2M (Ill, (K)(Al,Mg,Fe)2(Si,Al)4O10(OH)2‚n(H2O), PDF no. 26-0911), calcite (Cc, CaCO3, PDF no. 13-0198), and ordered anorthite (An, Ca2Al(AlSi)O7, PDF no. 71-0788). The basal reflections of montmorillonite 15A (Mm, (Na,Ca)0,3(Al,Mg)2Si4O10(OH)2‚ n(H2O), PDF no. 29-1498) and chlorites (Chl, Na0,5(Al,Mg)6(Si,Al)8O18(OH)12‚5(H2O), PDF no. 40-0744) were also iden-

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TABLE 2: Composition (wt %) of Some Iron-Rich Grains in the Clays Fired at T e 800 °C wt % at 600 °C Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 MnO FeO a

wt % at 700 °C

wt % at 800 °C

50 × 10 µm

20 × 20 µm

20 × 10 µm

60 × 100 µm

70 × 130 µm

40 × 60 µm

30 × 20 µm

20 × 15 µm

20 × 10 µm

2.4 5.5 9.7 18.7 0.8 0.2 0.9 3.0 a 1.6 57.3

2.5 12.8 20.2 36.1 0.8 1.0 1.1 2.0 a a 23.6

0.9 2.9 5.8 12.5 1.7 12.0 1.2 11.0 a 0.6 51.3

2.1 6.5 16.3 31.9 a 0.7 1.9 2.0 0.7 a 38.0

1.6 2.6 7.1 14.0 a 1.0 0.5 14.1 0.4 a 58.7

1.5 2.9 8.3 19.4 a 1.2 0.9 10.4 0.4 10.2 44.8

2.4 5.0 10.9 23.5 a 1.3 0.7 12.0 0.1 a 44.1

1.8 2.4 17.5 24.6 a 0.8 1.0 1.4 0.7 a 49.9

1.6 4.9 12.1 17.0 a 0.8 1.4 2.1 0.9 a 59.3

Not detected.

TABLE 3: Composition (wt %) of Some Iron-Rich Grains in the Clays Fired at T g 900 °C wt % at 900 °C

wt % at 1000 °C

wt % at 1100 °C

elements 20 × 50 µm 80 × 120 µm 60 × 120 µm 30 × 30 µm 100 × 30 µm 100 × 60 µm 150 × 200 µm 60 × 40 µm 80 × 80 µm Na2O MgO Al2O3 SiO2 P2O5 SO3 K2O CaO TiO2 MnO FeO a

3.4 4.9 7.4 12.2 a 1.7 0.7 3.3 0.7 a 65.8

2.8 4.2 3.8 5.2 2.7 2.1 0.1 6.6 0.5 0.7 71.4

2.2 4.1 7.2 17.2 a 2.0 0.4 5.4 0.2 2.4 58.9

a 4.9 7.5 9.4 3.7 0.4 0.5 2.9 0.4 a 70.4

2.2 3.8 5.5 13.2 2.0 1.2 0.2 5.4 0.5 1.2 64.8

2.2 2.6 4.9 14.4 0.5 0.5 0.4 2.5 0.3 0.7 71.2

0.8 1.1 2.1 3.1 0.4 0.7 0.4 0.4 0.8 a 90.3

1.1 1.2 3.6 9.7 a 0.7 0.4 1.0 0.5 a 81.8

1.9 3.9 6.0 18.0 0.7 a 0.2 8.5 0.4 a 60.4

Not detected.

Figure 2. Iron content in the bulk of the clays fired at different temperatures.

Figure 1. (a) Grain observed in the sample fired at 1100 °C formed by octahedral and tetrahedral crystals with dimension of ≈150 × 200 µm2 and an iron content of ≈90 wt %. (b) Magnification of the same grain showing a nice octahedral crystal.

tified. The reflections of the last two phases were clearly less intense than the others. Therefore montmorillonite and chlorites were neglected in this study. The pattern of the sample fired at 800 °C showed the disappearance of the calcite peak, due to the breakdown of the carbonates, whereas the illite basal reflections were still present. The hematite (Hem, Fe2O3, PDF no. 33-0664), gehlenite (Geh, Ca2Al(AlSi)O7, PDF no. 350755), and diopside (Di, CaMgSi2O6, PDF no. 24-0203) peaks were identified as newly formed phases, and the intensity of their reflections was higher in the samples fired at 1000 and 1100 °C. The illite peaks disappeared above 900 °C whereas those due to Na-Ca plagioclase (27-29° 2θ region) increased and were well defined in the samples fired at 1000 and 1100 °C (Figure 3a). XRPD: Quantitative Phase Analysis. The quantitative mineralogical phase composition of the samples fired at 800,

Characterization of Iron-Rich Fired Clays

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22151 TABLE 4: Percentage of Different Phases Obtained by the Rietveld Method in the Clays Fired at 800, 900, 1000, and 1100 °C phases

800 °C

900 °C

1000 °C

1100 °C

Qz (%) Hem (%) NaPg (%) Geh (%) Di/Augite (%) Kfs (%) An (%) Ill (%) Mont (%) Cc (%) amorphous (%) agreement indicesb

26.9 1.0 5.7 a a 5.6 10.3 21.1 800 °C. This observation may be explained by two effects. In fact, as the firing temperature increases, Fe2+, which preferentially replaces 27Al in Oh environment, is oxidized to paramagnetic Fe3+ which affects the intensity of the signal of 27Al in Oh environment. Moreover, as the firing temperature increases, the illite structure breaks down and the signal due to 27Al in Oh environment does not exist any more. According to XRPD data, as the illite structure breaks down, new phases are observed; all these new phases only contain 27Al in the Td environment.32,33 A possible interpretation of the deconvoluted spectra shown in Figure 6 is that, by rising the firing temperature, one tetrahedral site becomes more crystalline and distorted whereas the other one loses crystallinity. As shown in previously reported 27Al MAS NMR spectra, significant differences exist among different clays, including dehydroxylated fired clays.33 For instance, in the case of kaolinite and hallosyte, besides the resonances due to 27Al in

22154 J. Phys. Chem. B, Vol. 109, No. 47, 2005 Td and Oh environments, a resonance at about 26 ppm has been reported. This resonance was ascribed to 27Al in a pentacoordinated environment. AlO5 has also been found in the 2:1 system pyrophyllite,34 whereas, in other clays, no signal due to AlO5 has been reported.35 In the 27Al spectra (Figure 6) no evidence of resonances due to 27Al in a pentacoordinated environment (20-30 ppm) was observed. However the spectra were too broad for definitely ruling out the presence of 27Al in a pentacoordinated environment, which however might be present in a small amount. 27Al 3Q MAS NMR Spectra. 27Al NMR (3Q) MAS NMR experiments7,10 can refocus second-order quadrupolar effects that broaden the 27Al signals. The 3Q MAS spectra of the clays under study are shown in Figure 7. In the triple-quantum (3Q) dimension of the 2D map (f1), the lines are three times more separated by their isotropic chemical shift than in the MAS single quantum dimension (f2) and are free of second-order quadrupolar effects. In these 2D spectra the dotted lines represent the different orientations of the signals in a 3Q-MAS spectrum: “A” denotes the anisotropic axis, “QIS” is the axis giving the direction of the induced quadrupolar shift, and the “CIS” axis gives the direction of the isotropic chemical shifts. The 3Q-MAS spectra of the S1 samples showed two main well-separated spectral ridges, due to 27Al in Td and in Oh environments, respectively. It must be noted that the spectral ridges of 27Al in a Td environment were rather well aligned along the CS axis, whereas the spectral ridge of 27Al in a Oh environment appeared out of the CS alignment, thus showing the presence of a chemical shift distribution. Note also that the MQMAS peak due to the Td site was along the A axis, whereas the MQMAS peak due to the Oh site appeared not well aligned along this axis. It is worth noticing that the QIS axis is the axis along which the mass centers of the ridges from different species with the same isotropic chemical shift but different quadrupolar constants are located. In the 3Q-MAS spectrum of the unfired clay, at least two spectral ridges due to 27Al in tetrahedral sites, named, Td1 and Td2, aligned along the QIS axis, were clearly observable (Figure 7a), and the isotropic projection along f1 also showed a clear multiplicity. The 3Q-MAS spectra of the samples fired at 600, 700, and 800 °C again showed the presence of spectral ridges due to 27Al in two distinct tetrahedral environments, Td1 and Td2. However, as the firing temperature increased, the chemical shifts of Td1 and Td2 became closer and closer, making the observation of their separation increasingly difficult (Figure 7b-d). The 3Q-MAS spectra of the samples belonging to S2 only showed the spectral ridges due to 27Al in a Td environment (Figure 7e-g). Two spectral ridges, Td1 and T2d, due to 27Al in two slightly different Td environments, both aligned along the QIS axis, were observed: Td1 and Td2 exhibited close values of chemical shifts but rather different quadrupolar coupling constants. According to the literature,36,37 by using the centers of gravity G1 δ and δG2 (in f1 and f2, respectively) of the 3Q-MAS spectra, it was possible to estimate the isotropic chemical shift δiso and the second-order quadrupolar effect Pq of each spectral ridge. The obtained parameters were used as initial guesses in a best fit procedure applied to deconvolute the 1D MAS spectra. The isotropic chemical shifts (δisoTd1 and δisoTd2), the quadrupolar coupling constants (CqTd1 and CqTd2), the asymmetry parameters (ηqTd1 and ηqTd2) of 27Al in Td sites, as well as the isotropic chemical shift (δisoOh) and the quadrupolar coupling constant (CqOh) of 27Al in the Oh site are reported in Table 5. The Czjzeck model13 used for simulating the 27Al resonance in the Oh site

Presciutti et al.

Figure 7. Unsheared 27Al 3Q MAS NMR spectra of (a) the unfired clay and of the clays fired at (b) 600 °C, (c) 700 °C, (d) 800 °C, (e) 900 °C, (f) 1000 °C, and (g) 1100 °C. The MAS spectrum and the isotropic projection are shown in the f2 (horizontal) and f1 (vertical) axes, respectively. The CS, QIS, and A axes are defined in the Experimental Section.

accounted for a chemical shift distribution;14 the width of the chemical shift distribution ∆CS is also reported in Table 5. The dm2004 program also gave the area of each 27Al resonance. It was then possible to obtain the percentage of 27Al in the Oh and Td sites. However, it must be pointed out that, due to the presence of Fe3+, this percentage is not quantitative. In fact, the resonances due to 27Al atoms within the wipeout sphere of Fe3+ are completely lost, and hence the obtained values only referred to the sites outside the Fe3+ wipeout sphere.28 In the unfired clay 27Al was present in an almost equal amount in Oh and Td sites. As the firing temperature increased, the amount of 27Al in an Oh site progressively decreased. In addition, in the samples fired at T > 800 °C, the resonance at about 4.5

Characterization of Iron-Rich Fired Clays

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22155

Figure 8. Experimental (left) and simulated (right) Td region of the sheared 27Al 3Q MAS NMR spectra of (a) the unfired clay and of the clays fired at (b) 600 °C, (c) 700 °C, (d) 800 °C, (e) 900 °C, (f) 1000 °C, and (g) 1100 °C.

TABLE 5: 27Al NMR Parameters of All Clays Obtained by Deconvoluting 1D MAS and 2D 3Q MAS NMR Spectra with the Dm2004 Software: δiso ) Isotropic Chemical Shift; Cq ) Quadrupolar Coupling Constant; ηq ) Asymmetry Parameter; ∆CS ) Width of the Chemical Shift Distributiona T (K) unfired 600 700 800 900 1000 1100 a

δisoT1 CqT1 δisoT2 CqT2 δisoOh CqOh ∆CS (ppm) (kHz) ηqT1 (ppm) (kHz) ηqT2 (ppm) (kHz) (ppm) 72 71 66 66 63 64 64

2065 2683 2373 2373 2160 3266 3298

0.1 0.9 0.8 0.8 0.0 0.0 0.0

62 64 61 61 60 61 57

3099 3371 3227 3227 3096 2431 1992

0.5 0.6 0.6 0.6 0.0 0.0 0.0

4.7 4.6 4.6 4.5

2451 1711 1841 1708

37 18 19 17

ηqOh was fixed to 0.61.

ppm fully disappeared, which reflected the structural breakdown of the illite structure and agreed with the XRPD data reported above. The expanded tetrahedral region of all spectra, after applying the shearing procedure,15 is shown in Figure 8, with the

experimental and simulated spectra reported on the left and right side, respectively. It is worth noting that the set of parameters obtained for the Td sites from the simulation of 1D-MAS spectra were very similar to the corresponding set obtained from the best fit of the 3Q-MAS spectra. On samples fired at T < 900 °C, two Td sites were always found. As previously observed, these two sites gave rise to NMR signals with very similar chemical shifts whereas the quadrupolar coupling constants were different.38 A possible interpretation of the structural difference of these two environments is that slightly different distortions may change the electric field gradient (EFG) distribution and, as a consequence, the value of the quadrupolar coupling constant. In fact the quadrupolar coupling constant Cq is proportional to the largest of the three principal axes of the EFG tensor at the nucleus.35 It has been previously shown that, in some cases, either the distortion of the local environment or long-range lattice effects may affect the magnitude of the EFG and, therefore, the magnitude of Cq.39 Therefore two Td sites may show resonances with very similar isotropic shift but different Cq values. In samples fired at T > 800 °C, 27Al spectra

22156 J. Phys. Chem. B, Vol. 109, No. 47, 2005

Presciutti et al.

Figure 9. 27Al NMR MAS spectra of the unfired clay recorded (a) with and (b) without cross polarization.

showed only two tetrahedral environments. The parameters obtained for the two 27Al resonances were very close in chemical shifts whereas again they showed different Cq values. Dehydration and Dehydroxylation of Fired Clays. To best describe the effect of the firing temperature, the presence in clays of water and of OH groups must be taken into account. To obtain this information, a 27Al CP-MAS study was also performed. Many factors may affect the efficiency of the cross-polarization process of the resonances of quadrupolar nuclei under MAS conditions. Thus resonances of two distinct sites may display markedly different cross-polarization properties, exhibiting matching conditions at very different field strength. By carefully taking into account these points, we may assess that the efficiency of the magnetization transfer from 1H to 27Al is closely related to the total proton concentration in the sample. According to the literature,40 in hydrous aluminates such as gibbsite, the interlayer water is associated to 27Al in Oh environment. Moreover, earlier studies on illite-rich clays32,41 showed that, in the 350-400 °C range of firing, the interlayer water is driven off; therefore, only the protons of the hydroxyl groups participate in the CP process. The dehydroxylation takes place between 450 and 700 °C. In Figure 9, the 27Al MAS (b) and CP-MAS (a) spectra of the unfired clay are shown. According to the literature,40 the CP-MAS NMR spectra showed that the available protons were closely associated with 27Al in Oh site, as this site provided a considerable CP-MAS signal intensity whereas no CP-MAS intensity was observed for the resonance of 27Al in Td sites. The intensity of the CP-MAS signal decreased in the sample fired at 600 °C and, due to the dehydroxylation, the signal fully disappeared in the sample fired at 700 °C. Although the previous information seems reliable, its absolute validity cannot be ascertained because of the presence of iron which could hamper the transfer of magnetization from nearby protons to aluminum. TABLE 6:

29Si

Figure 10. 29Si MAS NMR spectra and their deconvolution of (a) the unfired clay and of the clays fired at (b) 600 °C, (c) 700 °C, (d) 800 °C, (e) 900 °C, (f) 1000 °C, and (g) 1100 °C. The frequency axis is expressed in ppm upfield with respect to TMS. 29Si

MAS Spectra. In Figure 10, the 29Si MAS NMR spectra of the unfired and fired clays are shown. The corresponding deconvoluted spectra are superimposed to the experimental ones, along with the single components obtained from the deconvolution procedure. In all spectra a resonance centered at about 107 ppm due to quartz was observed; this resonance was much sharper in the spectra of clays fired at 600, 700, and 800 °C, thus showing a certain amount of short-range disorder in the clays fired T g 900 °C. The resonance centered at about -93 ppm could be easily ascribed to the Q3 units of illite; this resonance was observed in the spectra of samples fired at temperatures lower than 900 °C; at higher temperatures, when the structural breakdown of illite occurred, this resonance was no more observable. In agreement with XRPD data, the resonances centered at about -96 and -101 may be attributed to the presence of KFeldspars.42 The peak broadening and the extra intensity in the -86 to -100 ppm range may be tentatively attributed to the resonances of a range of silicon environments in strained regions between albite and anorthite-rich domains.42,43 In some samples a shoulder centered at about -114 ppm was also observable (Table 6); this shoulder might be due to silica polymorphs or to silica glass with a broad distribution of the SiOSi bond angles. In fact, according to the literature,42 the resonances of silica polymorphs appear at the highest field of the 29Si chemical shift range of silicates, from -107 (quartz) to -121 ppm, whereas silica glass may show a broad resonance

Chemical Shifts and Relative Integrals Obtained by Deconvoluting 1D MAS Spectra with the Dm2004 Software

sample

δ (ppm)

%A

δ (ppm)

%A

δ (ppm)

%A

δ (ppm)

%A

δ (ppm)

%A

δ (ppm)

%A

unfired 600 °C 700 °C 800 °C 900 °C 1000 °C 1100 °C

-110.1 -107.6 -107.3 -108.1 -108.0 -108.4 -108.2

49 16 15 21 33 46 42

-100.8 -100.5 -101.0 -101.8 -100.8 -100.7

14 2 5 12 22 28

-97.4 -96.6 -96.9 -97.3 -96.8 -95.0 -95.0

27 21 13 28 21 15 13

-87.3 -87.2 -86.7 -89.1 -91.0 -89.7 -88.9

13 17 35 9 33 8 8

-92.4 -92.7 -92.6 -93.2

11 32 35 16

-112.9

20

-115.6 -113.3

10 10

Characterization of Iron-Rich Fired Clays

Figure 11. 29Si NMR MAS spectra of the unfired clay recorded (a) with and (b) without cross polarization. The frequency axis is expressed in ppm upfield with respect to TMS.

J. Phys. Chem. B, Vol. 109, No. 47, 2005 22157 required to avoid any saturation of the signal due to the crystalline quartz. Therefore, in the crystalline quartz domain, no paramagnetic iron was present and the observed line broadening was possibly due to disorder. The 29Si MAS and CP-MAS spectra of the unfired clay are shown in Figure 11; as expected, the quartz spectral resonance detected in the MAS spectrum fully disappeared in the CP spectrum. 23Na MAS NMR Spectra. The 23Na MAS NMR spectra of all clays are shown in Figure 12. For the clays fired at the highest temperatures, the spectrum showed a single broad line, rather symmetric and unstructured, centered at about -27 ppm from NaCl. This line agreed very well with literature data and was due to albite glasses44 and/or Na-rich plagioclase.35 The spectra of the clays fired at the lowest temperatures appeared complex and poorly resolved and could not be assigned from literature data. A further study at higher field and higher spinning rate will be performed. IV. Conclusion

Figure 12. 23Na MAS NMR spectra of (a) the unfired clay and of the clays fired at (b) 600 °C, (c) 700 °C, (d) 800 °C, (e) 900 °C, (f) 1000 °C, and (g) 1100 °C.

centered at about -110 ppm. Note that no evidence of gehlenite, observed by XRPD, was found in the 29Si NMR spectra. Interestingly, although the samples contained a large amount of paramagnetic iron, a recycle delay of at least 60 s was

The use of a large number of physical chemical techniques allowed for a clear description of the effect of firing a complex material, such as an iron-rich clay. When the results from all characterization methods are combined, the following conclusions could be made. Interlayer H2O is located close to aluminum in octahedral sites and is driven off at temperatures lower than 600 °C. Between 600 and 700 °C dehydroxylation occurs, whereas between 800 and 900 °C, the aluminum in octahedral sites disappears, due to the breakdown of the illite structure, and all iron present is oxidized to Fe3+. In samples fired at 1000 and 1100 °C iron clustering was observed as well as large single crystals of iron with the occurrence of ferro or ferrimagnetic effects. Below 900 °C aluminum in octahedral sites presents a continuous distribution of chemical shift, suggesting the presence of slightly distorted sites. Finally, over the whole temperature range, the presence of at least two tetrahedral aluminum sites was revealed, characterized by different values of the quadrupolar coupling. In some sample silica polymorphs or silica glasses with a broad distribution of the SiOSi bond angles were also found. Although the clays under study contained a large mount of iron, no strongly shifted resonances were observed in the 27Al, 29Si, and 23Na NMR spectra. This observation supports the interpretation of clustered iron. In fact, iron clusters are surrounded by few atomic layers, whose resonances are fully wiped out and do not give rise to a significant number of shifted resonances. Other observations could be made by comparing XRPD and NMR data. In fact these data showed a maximum quartz content in the sample fired at 1000 °C. Both techniques confirmed the presence of K-Feldspars.35 In addition, the increase in albite evidenced by XRPD for the samples fired at the highest temperatures was confirmed by 23Na NMR. On the contrary, gehlenite, which was detected by XRPD, could be observed neither in the 27Al nor in the 29Si NMR spectra. However, some extra intensity observed in the 29Si spectra may be tentatively attributed to a range of silicon sites in strained regions between albite and anorthite-rich domains.42,43 Although the set of parameters obtained from 3Q MAS experiments and the deconvolution procedure are coherent, the accuracy in determining the isotropic chemical shifts and quadrupolar coupling constants could be improved by the combined evaluation of MAS and 3Q MAS spectra measured over a range of different Larmor frequencies. Measurements performed at different B0 fields will allow us to assess the role

22158 J. Phys. Chem. B, Vol. 109, No. 47, 2005 of second-order quadrupolar parameters with respect to other line broadening mechanisms in determining the shape and the line width of an NMR signal composed of overlapping resonances due to different sites.45 Moreover, the resolution of the observed Td and Oh 27Al sites might be improved at higher field and spinning rate, and 29Si and 23Na spectra obtained at higher field might surely be more informative. Acknowledgment. We thank Professor D. Massiot for the test version of the dm2004 software, Dr. M. Liberi and Dr. R. Melzi from Bruker Biospin Milano for the use of the Bruker EMX EPR spectrometer, Dr. F. Ziarelli for stimulating discussions, and Dr. A. De Stefanis and Mrs. P. Cafarelli for elemental analysis. This work was performed as part of the Eu-ARTECH project within the VI European Framework. References and Notes (1) Duminuco, P.; Messiga, B.; Riccardi, M. P. Thermochim. Acta 1998, 321, 185. (2) Altaner, S. P.; Weiss, C. A., Jr.; Kirkparick, R. J. Nature 1988, 331, 699. (3) de Arau´jo, J. H.; da Silva, N. F.; Acchar, W.; Gomes, U. U. Mater. Res. 2004, 7, 359. (4) Franzini, M.; Leoni, L.; Saitta, M. Rend. Soc. Ital. Mineral. Petrol. 1975, 31, 365. (5) Metz, G.; Wu, Z.; Smith, S. O. J. Magn. Reson., Ser. A 1994, 110, 219. (6) Bennett, A. E.; Rienstra, C. M.; Auger, M.; Lakshmi, K. V.; Griffin, R. G. J. Chem. Phys. 1995, 103, 6951. (7) Frydman, L.; Harwod, J. H. J. Am. Chem. Soc. 1995, 117, 5367. (8) Fernandez, C.; Amoureux, J.-P. Solid State Nucl. Magn. 1996, 5, 315. (9) Medek, A.; Harwood, J. S.; Frydman, L. J. Am. Chem. Soc. 1995, 117, 12779. (10) Fernandez, C.; Amoureux, J.-P. Chem. Phys. Lett. 1995, 242, 449. (11) Rocha, J.; Esculcas, A. P.; Fernandez, C.; Amoureux, J. P. J. Phys. Chem. 1996, 100, 17889. (12) Massiot, D.; Fayon, F.; Capron, M.; King, I.; LeCalve´, S.; Alonso, B.; Durand, J. O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70. (13) Czjzeck, G.; Fink, J.; Go¨tz, F.; Schimdt, H.; Coey, J. M. D.; Rebouillat, J. P.; Lie´nard, A. Phys. ReV. B 1981, 23, 2513. (14) Neuville, D. R.; Cormier, L.; Massiot, D. Geochim. Cosmochim. Acta 2004, 68, 5071. (15) Man, P. P. Phys. ReV. B 1998, 58, 2764. (16) Deer, W. A.; Howie, R. A.; Zussman, J. An introduction to the rock forming minerals; Prentice-Hall: Englewood Cliffs, NJ, 1992. (17) Viti, C.; Borgia, I.; Brunetti, B.; Sgamellotti, A.; Mellini, M. J. Cult. Herit. 2003, 4, 199.

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