Characterization of an Al-, Ga-based catalyst by Ga NMR and XAS

Characterization of an Al-, Ga-based catalyst by Ga NMR and XAS

Solid State Nuclear Magnetic Resonance 16 Ž2000. 103–108 www.elsevier.nlrlocatersolmag Characterization of an Al-, Ga-based catalyst by Ga NMR and XA...

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Solid State Nuclear Magnetic Resonance 16 Ž2000. 103–108 www.elsevier.nlrlocatersolmag

Characterization of an Al-, Ga-based catalyst by Ga NMR and XAS Dominique Massiot a,) , Renaud Revel b,1, Claire Magnenet a , Dominique Bazin b a

b

Centre de Recherche sur les Materiaux a` Haute Temperature, CNRS, 1D AÕenue de la Recherche Scientifique, ´ ´ 45071 Orleans ´ cedex 2, France Laboratoire pour l’Utilisation du Rayonnement Electromagnetique, Bat 209D, UniÕersite´ Paris Sud, 91405 Orsay cedex, France ´ Accepted 21 December 1999

Abstract Both Nuclear Magnetic Resonance ŽNMR. and X-Ray absorption spectroscopies characterize local order around the observed nuclei. With the recent progresses of 71 Ga solid state NMR, it has become possible to take advantage of the complementary information that can be obtained using these two methods. This opens the possibility of a more thorough description of the first coordination shells of Ga in oxide. We present and discuss the example of an Al-, Ga-based catalyst. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Solid state NMR; MAS; MQ-MAS; QPASS; 27Al;

71

Ga; Catalyst

1. Introduction Both high-resolution solid state Nuclear Magnetic Resonance ŽNMR. and X-Ray Absorption Spectroscopy ŽXAS. techniques are able to selectively characterize the local environment of the observed atom or nucleus in crystalline, disordered, amorphous and glassy materials. They provide different but complementary insights into the local structure through chemical shifts and electric field gradients at the different sites observed by NMR or electronic

structure and the pseudo-radial distribution function derived from XAS at the absorption edge. Whereas NMR may be able to resolve the different sites for the observed nuclei, XAS can directly describe the inter-atomic distances. Given the recent methodological developments of 71 Ga solid state NMR spectroscopy w1–4x, we have a rather unique opportunity to benefit from both spectroscopic approaches and to give a better description of the structural roles of gallium in complex materials such as Al-, Ga oxidebased catalysts.

2. Experimental )

Corresponding author. Tel.: q33-238-25-55-18; fax: q33238-63-81-03. E-mail address: [email protected] ŽD. Massiot.. 1 Now at: Institut Franc¸ais du Petrole, 1 et 4, Avenue de ´ Bois-Preau, ´ 92852 Rueil Malmaison Cedex, France.

The studied sample, SnO 2 :ŽZn,Ga.rAl 2 O 3 Ž0.5 Sn, 3.3 Ga, 1.7 Zn, 34.7 Al, 59.8 O at.%., is an industrial oxide material investigated for its catalytic

0926-2040r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 2 0 4 0 Ž 0 0 . 0 0 0 6 0 - 6

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properties. A conventional X-Ray diffraction study shows a dominant contribution of spinel type structure ŽZnAl 2 O4 , g-Al 2 O 3 . with remaining SnO 2 Žrutile structure.. The analysis of the powder diagram by Scherrer w5x formula and numerical simulation reveals a finely divided microstructure with grain ˚ These spinel-type sizes in the range of 40–50 A. structures imply two possible structural positions for Al or Ga in four- and six-fold coordination and possible vacancies.

3. XAS experimental The XAS spectra were performed at LURE on the D44 station of the D.C.I. storage ring running at 1.85 GeV with an average current of 300 mA and a time life of 200 h. The X-rays were monochromatized by two Si Ž311. single crystals and the incident I 0 and transmitted I 1 intensities were recorded with two ionization chambers filled with air. The monochromator, operating with 1mm vertical slits, has an energy resolution of 2.3 eV at 10 keV. Calibration of the experiment was made with a reference zinc metal foil. The Extended X-ray Absorption Fine Structure ŽEXAFS. oscillations were extracted according to the previously described protocol w6,9x Žlinear background subtraction, Heilter–Eisenberger normalization after the 5th degree polynomial removal for atomic absorption, Kaiser window with t s 2.5 be˚ y1 .. The Ga K-edge spectrum tween 2.3 and 12.9 A has been recorded for the studied sample SnO 2 :ŽZn,Ga.rAl 2 O 3 and three different reference crystalline compounds, in which Ga occupies octahedral ŽZnGa 2 O4 . tetrahedral ŽGaPO4 . and both octahedral and tetrahedral Žb-Ga 2 O 3 . positions.

4. NMR experimental 71

Ga Žas 27Al. is a quadrupolar nucleus and its NMR spectra are usually broadened to the second order due to quadrupolar interaction Žinteraction of the quadrupolar moment of the observed nucleus with the electric field gradient.. While usual principal field and Magic Angle Spinning ŽMAS. rates give resolved spectra for 27Al, the second order

broadening are usually so large for 71 Ga that the MAS spectra remain unresolved under these conditions in anhydrous oxide materials w1,4x. Two recently developed approaches can partly overcome this difficulty: the use of conventional one-dimensional experiments at the highest available magnetic fields Ž18.8 T, 800 MHz. combined with very high spinning rates Ž; 30 kHz. w3x or the use of the more sophisticated two-dimensional Quadrupolar Phase Adjusted Spinning Sideband ŽQPASS. experiment that allows us to reconstruct a spinning sideband free spectrum with apparent very fast or infinite spinning rate at the cost of much longer acquisition time w2,4x. The 71 Ga and 27Al NMR spectra have been acquired under static- and high-speed MAS conditions at 9.4 and 18.8 T Ž400 and 800 MHz 1 H frequency, respectively.. The 71 Ga 9.4 T static, MAS and QPASS spectra were obtained on a Bruker DSX 400 solid state spectrometer equipped with a Bruker 4-mm double-bearing probehead that achieves spinning rates of up to 15 kHz. The 18.8 T experiments have been acquired on a liquid state Bruker DRX 800 spectrometer equipped with a 2.5 mm MAS probehead with spinning rates of up to 30 kHz. The 27Al spectra have been acquired with a simple one-pulse excitation sequence, using a small pulse angle to ensure quantitative excitation. The broad 71 Ga spectra have been acquired using echo experiments, synchronized with the spinning rate in the case of MAS experiment w1x. The shifted-echo QPASS experiment has been acquired as previously described with a spinning rate of 10 kHz w2x using a selective pr2 pulse width duration of 1ms. One of the major difficulties of this study was the low Ž3 at.%. concentration of Ga in the sample. Thus, the acquisition of a single slice of the QPASS experiment required summation of 16 k transients to achieve an SrN of ; 5r1, which correspond to an acquisition time of 4 h 30 min. A complete acquisition of a QPASS experiment would require typically 32 slices Ž; 160 h. to completely separate the different spinning sidebands, like in the case of b-Ga 2 O 3 w2x. To shorten the acquisition time, we took benefit of the main feature of the PASS idea that makes it possible to tailor the acquisition of the different slices so as to progressively improve the resolution. This is because the pitch dimension acquired in the indirect dimension of the two-dimensional experi-

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Fig. 1. Comparison of 9.4 T 40 and 80 kHz.

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Ga spectra: static, MAS 10 kHz and QPASS for two, four and eight slices with fictive spinning rates of 20,

ment is directly encoding the spinning sideband number w7,8x. Consequently, the acquisition of two slices at pitches u s 0 and u s 1r2 enables the reconstruction of a MAS spectrum with twice the actual spinning rate. Four slices at u s nr4, n s 0–3, Ž u s nr8, n s 0–7. give fictive spinning rate of four times Žeight times. the actual spinning rate and so on. The spectra obtained for our samples are presented in Fig. 1 and correspond to 10, 20 and 40 h of acquisition for QPASS two, four and eight slices, respectively, for the same data set. It appears that at least 32 steps would have been necessary to completely separate all the spinning sidebands but also that the eight-step experiments Žfictive spinning rate of 80 kHz. already provide most of the resolution with a clear definition of the central n s 0 spinning

sideband that does not overlap any more with the remaining spinning sidebands.

5. Results and discussion The shape of the Ga K-edge ŽFig. 2a. provides direct information on the dominant geometry around Ga atoms by comparison to crystalline reference compounds Žtetrahedral GaO4 in GaPO4 , octahedral GaO6 in ZnGa 2 O4 and coexisting GaO4 and GaO6 in b-Ga 2 O 3 .. For the studied sample, the Ga main geometry is clearly of the tetrahedral GaO4 type. The fit of the first peak in the Fourier Transform of the EXAFS oscillations cannot be modeled simply with a single contribution: while the average distance of

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Fig. 2. Ža. Comparison at Ga K edge for b-Ga 2 O 3 Ž ` ., ZnGa 2 O4 Ž –o– ˘ ., GaPO4 Ž — — — . and SnO 2 :ŽZn,Ga.rAl 2 O 3 Ž – – – .. Žb. Best fit of EXAFS oscillations above the Ga K edge for SnO 2 :ŽZn,Ga.rAl 2 O 3 .

˚ Žclose to the the first oxygen shell is of 1.84 A typical distance in a tetrahedron. the computed coordination number is of 4.8, higher than expected. Moreover, the EXAFS modulations coming from higher distances cannot be refined with our chosen model compounds. As in the case of Al NMR w10x, the Ga NMR chemical shift position is known to be directly characteristic of the coordination state of Ga in oxide compounds w4,11x ŽGaO4 ; 200 ppm, GaO5 ; 75 ppm, GaO6 ; 0 ppm.. The 71 Ga spectra obtained in static conditions are broad and featureless, and MAS does not improve resolution because of numerous broad overlapping spinning sidebands. The chemical shift cannot be measured or even estimated with enough accuracy from these experiments. The eightslice QPASS spectrum, with an 80 kHz separation of the spinning sidebands, gives more information ŽFig. 1.. Its asymmetric shape Žsharp low field and trailing high field edge. is characteristic of a distribution of the quadrupolar interaction parameters as in the case of 27Al MAS NMR spectra w12x. The dominant chem-

ical shift position is consequently located at its rising edge. This partly resolved spectrum unambiguously shows that most of the observed Ga has a chemical shift of the order of ; 200 ppm characteristic of a tetrahedral coordination state GaO4 . Furthermore, the observed width of the spectrum appears to be compatible with the possible existence of Ga in higher coordination states ŽGaO5 andror GaO6 . that could not be resolved at 9.4 T. As recently shown for crystalline b-Ga 2 O 3 , it is possible to gain a factor of four in resolution and in sensitivity for Ga NMR Ždue to reduced quadrupolar broadening and enhanced chemical shift separation. by using a very high magnetic field Ž18.8 T. and very high spinning rate Ž30 kHz. w3x. The spectrum presented in Fig. 3 was acquired under these conditions at Bruker Karlsruhe ŽGermany.. It clearly shows the presence of two overlapping contributions ascribed to GaO4 Ž; 200 ppm. and GaO6 Ž; 30 ppm.. The simulated spectrum ŽFig. 3. assumes an average quadrupolar coupling of 12 MHz Žclose to that observed for GaO4 site of b-Ga 2 O 3 . with a Gaussian distribution Ž"3 MHz.. The presence of a significant amount of intermediate GaO5 coordination state, which could be proposed for this disordered material, is unlikely, as it would appear between the two resolved GaO4 and GaO6 lines leading to an unresolved spectrum. The relative populations can be tentatively quantified, giving 80–90% of GaO4. The simulated QPASS spectrum has been obtained with the parameters derived from the analysis of the high field spectrum, assuming a perfect irradiation of the spin system. This leads to an increased line width of the center band and increased intensities of the spinning sidebands, linked to the limited irradiation bandwidth of the QPASS experiment that chains 9 p pulses to achieve the spinning sideband modulation. This was already remarked in the study of the Magnesium Gallium spinel MgGa 2 O4 where the 71 Ga EFG tensor Žquadrupolar coupling parameter. is clearly underestimated w4x. A more conventional 27Al study, carried out at 9.4 and 18.8 T, clearly resolves two aluminum sites AlO4 and AlO6 in a 30:70 ratio, typical of a g-Al 2 O 3 spinel type phase. Both the static and MAS spectra obtained at 18.8 T are shown in Fig. 4. It is interesting to note that, while the MAS spectrum easily resolves the AlO4 and AlO6 contributions, they are

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Fig. 3. The 71 Ga experimental and modeled spectra of SnO 2 :ŽZn,Ga.rAl 2 O 3 catalyst: Ža. QPASS acquired for an apparent spinning rate of 80 kHz Žactual spinning rate 10 kHz. at 9.4 T, and Žb. MAS spectrum at 18.8 T and 30 kHz spinning rate. The model spectra show the two components, separated at the highest field.

already partly resolved in the static spectrum. In fact, as remarked earlier w3x, the resolution obtained in

Fig. 4. Static and MAS Ž22 kHz. 27Al spectra showing the perfectly resolved AlO4 and AlO6 lines. Note that a partial resolution is already obtained in the static spectrum at this very high field.

static at 18.8 T is close to that obtained in MAS at 9.4 T. This results from increased line separation and decreased quadrupolar broadening at higher field. A more accurate refinement of the EXAFS oscillations included in the first peak of the Fourier transform can now be carried out, based on a dominant tetrahedral site for Ga and excluding five-fold coordinated GaO5 . The obtained results show an important Debye–Waller factor, usually considered as the signature of a disorder Ždistribution of distances of the neighboring oxygen.. This may be analogous to the observed distribution of electric field gradient Žquadrupolar coupling. revealed by the high field NMR study. The final refinement was based on the structure of GaAlO 3 w14x Žisostructural to b-Ga 2 O 3 .. The electronic parameters obtained from a FEFF simulation of the experimental spectrum w13x have been used to refine parameters of the ˚ . EXAFS modulations with FEFfiltered Ž1–3.7 A Ž . FIT15 Fig. 2b . Due to the high number Ž7. of single and multiple scattering paths, the number of refined parameters reaches 14 Ždistances and neighbors numbers. for one site. With 19 independent parameters, it was thus not possible to simultane-

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ously refine the two gallium sites. The best set of parameters Ž x 2 s 3.4%. describes a GaO4- and AlO6-based structure with vacancies and an important disorder arising from dispersion of distances Žmonoclinic structure. and possible AlrGa substitution, directly evidenced by NMR spectra of 71 Ga and 27 Al.

MHz spectrometer and assistance while running experiments. We acknowledge financial support from Region Centre and CNRS Chemistry Department UPR 4212.

References 6. Conclusion 71

With the latest developments, solid state Ga NMR becomes able to evidence different coordination states for this nucleus, even in disordered solids. The obtained results can be used to constrain the interpretation of XAS results and reach a comprehensive interpretation of the whole set of data obtained by these two spectroscopies on a complex real material. On an NMR point of view, very high spinning rates of up to 50 kHz have been shown to be achievable w17x and QPASS can be extended to higher spinning using multiple rotor cycles w16x. Both experiments would enhance resolution and pertinence of the obtained NMR data.

7. Uncited reference w15x

Acknowledgements We thank Bruker Analytik, Stefan Steuernagel, Hans Forster and Detelf Muller for the access to 800

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