Photoacoustic Spectroscopic Study of Cobalt Containing Molecular Sieves

J. Weitkamp, H.G. Karge, H. Pfeifer and W. HSldcrich (Eds.) Zeolites and Related Microporous Materials: State of the Art 1994 Studies in Surface Science and Catalysis, Vol. 84 0 1994 Elsevier Science B.V. All rights reserved.

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Photoacoustic Spectroscopic Study of Cobalt Containing Molecular Sieves Hyun-Sik Han and Hakze Chon Department of Chemistry Korea Advanced Institute of Science and Technology, Taejon, Korea ABSTRACT Photoacoustic (PA) spectroscopy was applied to obtain depth-resolved information. The depth-profiling capability of PA spectroscopy was investigated varying the sampling depth of cobalt containing molecular sieves by changing the modulation frequency. PAS results indicate the presence of cobalt species associated with a template molecule located mainly in the inner part of the crystallite of as-synthesized CoAPO-44 and having lower symmetry than framework cobalt ions. 1. INTRODUCTION

Photoacoustic (PA) spectroscopy has proved to be a useful tool to study heterogeneous catalyst (1-7). In photoacoustic spectroscopy, the light penetrating the sample deeper than the sampling depth, determined by the thermal diffusion length, does not contribute to the generation of the PA signal as the periodicity is damped out before the heat flows to the surface. The sampling depth is inversely proportional to the square root of the modulation frequency. The PA spectrum of samples with a concentration gradient of absorbing chromophores along the depth shows a dependence on the modulation frequency (8-10). In this work, PA spectroscopy was used to investigate the depth profile of cobalt ions in CoAPO-44.

2. EXPERIMENTAL Figure 1 shows the schematic diagram of the UV-VIS PA spectrometer system equipped with a PA cell allowing the in-situ treatment of the sample under vacuum

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at high temperature (7). A 1kW Xe arc lamp (Oriel Model 6269) was used as the light source. The PA signal was detected by a microphone connected to the PA cell body through a wave guide giving the form of a Helmholtz resonator. The cell body was made of a stainless steel block and was sealed by a quartz window. The output of the monochromator (Oriel Model 77250) with a spectral resolution of 8 nm was modulated by a mechanical chopper controlled by a chopper controller (SRS Model 540). The PA signal was introduced to a two-channel lock-in amplifier (SRS Model 510) through a preamplifier (SRS Model 550). Photoacoustic signal was normalized with a signal obtained from carbon black. PA spectra in the infrared region were obtained using a FT-IR spectrometer (Bomem, MB102) equipped with a photoacoustic unit ( MTEC 200) at the resolution of 4 cm-1.

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Figure 1. Schematic diagram of UV-VIS spectrometer CoAPO-44 molecular sieve was synthesized following the procedure described in the literature (1 1). The mixture of hydrated aluminium oxide (Conoco, 85%) and orthophosphoric acid (Kanto, 85%) diluted with water was stirred vigorously until a homogeneous gel was obtained. Then cobalt(I1) acetate tetrahydrate (Fluka) was added to the gel and finally cyclohexylamine (Aldrich) was added dropwise to the homogeneous gel with a continuous stirring. The reaction mixture, having a composition CgHl-jN : 0.40 COO : 0.80 A1203 : P2O5 : 50 H20, was placed in an autoclave for 24 hours at 150 O C .

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The chemical analyses of the synthesized samples were performed by electron microprobe analysis (Philips) and X-ray photoelectron spectroscopy (XPSLAB MK-2). SEM image and X-ray diffraction patterns were obtained with Philips (Model 535-M) and Rigaku (D/MAX-3) instruments, respectively. The CoAPO-5 sample synthesized using Pr3N template had the composition Co:Al:P = 0.035: 0.46:0.50 (7). 3. RESULTS Synthesized CoAPO-44 (average size 30pm ) showed the same X-ray diffraction pattern as that reported in the literature (1 1). The morphology was rhombohedral, typical of molecular sieves of chabazite structure. Figure 2 shows the PA spectra of as-synthesized CoAPO-44 and CoAPO-5 samples at modulation frequency of 25 Hz. The absorption spectrum of CoAPO-5, showing the characteristic triplet pattern, was typical of cobalt-containing aluminophosphate molecular sieves obtained by other investigators (7,12- 14).

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Figure 2. PA spectra of as-synthesized CoAPO-44, amplitude (a) and phase (b) and CoAPO-5 (c) at 25 Hz.

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On the other hand, a rather flat and broad absorption spectrum was observed in case of CoAPO-44 in the triplet region. Along with the triplet absorption band, a weak absorption band at 350 nm was observed. The phase of PA signals of assynthesized CoAPO-44 at 25 Hz was also shown in Figure 2. The 350 nm band was more conspicuous in the phase spectrum than in the amplitude spectrum. Figure 3 shows the PA spectra of as-synthesized CoAPO-44 at modulation frequencies of 25, 200 and 1600 Hz. Increasing the modulation frequency resulted in a decrease in the bandwidth as well as an appearance of the triplet absorption pattern similar to that of CoAPO-5. The 350 nm band was not observed at 1600 Hz.

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Figure 3. PA spectra of as-synthesized CoAPO-44 at 25 (a), 200 (b) and 1600 Hz (c). Figure 4 shows the PA spectra of calcined CoAPO-44 sample at 25, 200 and 1600 Hz. Contrary to the case of the as-synthesized sample, no change in the absorption pattern with the modulation frequency was observed. Figure 5 shows the FT-IWPAS spectra of as-synthesized CoAPO-44 samples. The IR spectrum shows the absorption bands due to cyclohexylamine (CHA) at

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1350-1600 cm-1 in addition to the framework vibrations. The absorption band due to CHAH+ was observed at 15 10 cm-1 (1 5). At high mirror velocity, the relative intensity of the absorption band at 1452 cm-1 due to C-H motion to that of CHAH+ decreased.

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Figure 4. PA spectra of calcined CoAPO-44 at 25 (a), 200 (b) and 1600 Hz (c). The surface cobalt concentration of as-synthesized CoAPO-44, Co/(Al+P) = 0.053, was lower than that of bulk composition, Co/(Al+P) = 0.087. Calcining the sample resulted in the increase of the surface cobalt concentration to Co/(AI+P) = 0.1 1. 4. DISCUSSION The triplet absorption band at 500-650 nm is responsible for the intense blue color of CoAPO catalysts. The characteristic absorption pattern due to the 4A2 --> 4F(P) transition of Co*+ has been considered as an indication of tetrahedral coordination of cobalt. The spectrum of CoAPO-5 showing the symmetric splitting

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Figure 5 . FT-IWPA spectra of as-synthesized CoAPO-44 at the mirror velocity of 1 (a) and 0.2 cm/min (b). of the triplet is the typical electronic absorption spectra of CoAPOs. In these, the cobalt ions were incorporated in framework position (12- 14). The fact that the PA spectrum of as-synthesized CoAPO-44 recorded at 1600 Hz was similar to that of CoAPO-5 suggests that the cobalt ions within the sampling depth are primarily in framework positions. The presence of the 1510 cm-1 band, due to CHAH+, in the IWPAS spectrum of the as-synthesized sample was another indication of cobalt ions in framework positions ( I 5). A rather broad and less resolved triplet structure observed at 25 Hz suggests the presence of additional cobalt species other than framework cobalt. It is known that the shape of the triplet absorption structure due to cobalt ions in tetrahedral environment is strongly affected by the degree of deviation from tetrahedral symmetry (16). It was noted that the presence of ion-exchangeable non-framework cobalt species present in CoAPO-5 as well as the cobalt ions have C3v symmetry in CoZSM-5 in the presence of a template (13,17). These species may be responsible for the rather broad absorption spectrum observed at 25 Hz. The modulation frequency dependence was also observed in the IR spectra obtained photoacoustically. The sampling depth in FT-IWPA spectroscopy can be varied by changing the mirror velocity. The modulation frequencies at 1452 cm-1

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are 290 Hz and 1452 Hz when the mirror velocity is 0.2 and 1 cm/sec, respectively. The absorption due to CHAH+ species represents the amount of cyclohexylamine bonded with cobalt ions in framework positions. On the other hand, the C-H vibration at 1452 cm-1 can be a measure of total cyclohexylamine, CHAH+ and CHA forms. The IR and UV-VIS PAS results suggest that the additional cobalt species having lower symmetry than framework cobalt ions may be cobalt ions with cyclohexylamine as the ligand. The generation of PA signals requires the propagation of the thermal wave originating from light absorption to the solid-gas interface. The phases represent a time delay of the PA signal detected in the surrounding gas to the incident modulated light and contain the information associated with the location of absorbing species (8-9). The fact that the 350 nm band observed in the amplitude spectrum of as-synthesized CoAPO-44 at 25 Hz was conspicuous in the phase spectrum suggests that the species responsible for this absorption band are located in a deeper site of the crystallite. This may be attributed to a cobalt-cyclohexylamine interaction, i.e., charge transfer between them. The chabazite cage has sufficient space to occlude two cyclohexylamines (1 8). Modulation frequency dependence was not observed in the case of the calcined sample as shown in Figure 4 suggesting that the situation of cobalt distribution observed in as-synthesized samples has changed by calcination. Increase in surface cobalt concentration the calcined sample indicates migration of cobalt ions to the surface of the crystallites, resulting in a rather uniform distribution of cobalt ions. PAS results indicate the presence of cobalt species having lower symmetry than framework cobalt ions, mainly located in the inner part of the crystallite of the as-synthesized sample. REFERENCES 1. G.F. Kirkbright, J. de Physique, 44 (1983) C6-99. 2. L.W. Burggraf, D.E. Leyden, R.L. Chin and D.M. Hercules, J. Catal., 78 (1982) 360. 3. S.M. Riseman, S. Bandyopadhyay, F.E. Massoth and E.M. Eyring, Appl. Catal., 16 (1985) 29. 4. J.G. Highlight and J.B. Moffat, J. Catal., 98 (1986) 248. 5 . N.L. Rockley and M.G. Rockley, Appl. Spectrosc., 41 (1987) 471.

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6. J. Philippaerts, E.F. Vansant, G. Peeters and E. Vanderheyden, Anal. Chem. Acta, 195 (1987) 237. 7. K.Y. Lee and H. Chon, J. Cata1.,126 (1990) 677. 8. A. Rosencwaig, and A. Gersho, J. Appl. Phys., 47 (1976) 64. 9. R.M. Miller, in P. Hess(Ed.), "Photoacoustic, Photothermal and Photochemical Processes at Surfaces and in Thin Films", Springer-Verlag, Heidelberg, 1989, p.171. 10. Michael L. Mackenthun, Roderick D. Tom and Thomas A. Moore, Nature, 279 (1979) 265 1 1 . US patent, 4,567,029 (1986). 12. B. Kraushaar-Czarnetzki, Wilma G.M. Hoogervorst, Ronald R. Andrea, Cees A. Emeis and Wim H.J. Stork, J. Chem. SOC.Faraday Trans., 87 (1991) 891. 13. Consuelo Montes, Mark E. Davis, Brendan Murray and Mysore Narayana, J. Phys. Chem., 94 (1 990) 6425. 14. M.P.J. Peeters, J.H.C. van Hooff, R.A. Sheldon, V.L. Zholobenko, L.M. Kustov and V.B. Kazansky, Proceedings of the 9th IZC, 1993, p.65 1. 15. J. Batista, V. Kaucic, N.Rajic and D. Stojakovic, Zeolites, 12 (1992) 925 16. K. Klier, R. Kellerman, and P.J. Hutta, J. Chem. Phys., 61 (1974) 4224. 17. R. Mostowicz, A.J. Dabrowski and J.M. Jablonski, Stud. Surf. Sci. Catal., 49 (1989) 249. 18. G. Nardin, L. Randaccio, V. Kaucic and N. Rajic, Zeolites, 1 1 (1991) 192.