- Email: [email protected]
Al2O3 Catalysts: Ftir and Hrtem Studies
Guczi, L.et ul. (Editors), New Frontiers in Caalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary Q 1993 Elsevier Science Publishers B.V.All rights reserved
COKE DEPOSITS ON Pt/AI,O, CATALYSTS: FTIR AND HRTEM STUDIES L. Marchese, E. Borello, S. Coluccia, G. Mama and A. Zecchina Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, Via P. Giuria 7, 10125 Torino, Italy
Abstract Graphitic and amorphous coke deposits on Pt/A1203 catalysts are evidenced by HRTEM. Their interaction with the metal phase and the support is studied by the spectra of CO adsorbed at 300 and 77K. A band due to li uid-like CO stabilized on coke at 77K is observed at 2136 cm-P
.
1. INTRODUCTION
Coking and sulfidation are two main deactivation processes for reforming catalysts [l]. It is relevant to determine the structure of the deposits, their location, the treatments to remove them and the structure of the metal phase after reactivation treatments. These points are examined in this report by Infrared Spectroscopy and High Resolution Transmission Electron Microscopy. CO adsorption at 300K and 77K has been adopted as a test for measuring the overall surface activity both of the metal phase and the support. It is found that CO also interacts with the coke phase. 2. EXPERIMENTAL
The results refer to a Pt/A1203 sample (CK 306, Pt 0.6 wt % by Cyanamid Ketjen) which, after catalytic runs, showed a 6.7 wt % of coke content. The reforming feed was a H /n-eptane mixture (molar ratio =2) ; the n-eptane contained g.4 ppm of sulfur. Infrared spectra were obtained by Bruker IFS 48 and the HRTEM micrographs by a Jeol 2000EX instruments. 3. REBULTS AND DISCUSSION
Electron Microscopy Carbonaceous compounds on a particle of the coked catalyst are visible in the high resolution micrograph in Fig. 1. Curved fringes 3.6 apart in zone A are associated with graphitized carbon black (interplanar spacing d 002 - 3.4 A ) . Coke deposits with amorphous structure (as aigueh-by their 3.1.
2524
Figure 1. High resolution electron micrograph showing graphitized ( A ) and amorphous (B) coke deposits on Pt/A1203 catalyst.
disordered arrangement [2]) are present in the zones B. HRTEM analysis of the Pt phase is not discussed in this short report. However, Pt particles size was determined to be 10-15 A. IR spectra of t h e a a t a l y a t Figure 2 A shows the IR spectra of the coked Pt/A1203 catalyst reactivated by gradual oxidation (150 Torr 0 ) at increasing temperature. Disc ete, though broad, absorpsions are observed at 1700-1100 cm-' in the spectrum of the original coked sample (curve a). The overall transparency in t h e IR region increases gradually as the oxidizing temperature increases (curves b-d) and the sample turns rom black to whitish. Moreover, the bands at 1700-1100 cm-' are strongly reduced in intensit and in spectrum d only weak components in the 1600-1450 cm-' ra ge are left. By contrast, the intensity of a band at 1370 cm-' increases as the oxidizing temperature increases up to 773K. This band disappears upon heating in H2 at 973K. The bands which disappear by oxidation are certainly associated with oke. In particular, two correlated bands at 1580 and 1470 cm-' are due to the asym. and sym. stretching vibrations of carboxylate groups [3], while the other components are due to deformation modes of residual CH groups as found in carbonaceous deposits with llmolecularll form and with high H/C ratio [4]. Noticeably, bands in this region were also attributed t o carbon-carbon stretching vibrations in distorted graphitic structure in dust carbon particles and in soccerball shaped st uctures (c60, fullerene-like molecules) [5]. The 1370 cm-' band is due to sulfate groups [6] which originate from oxidation of the sulfur containing compounds which are fed into the reactor during the catalytic runs. 3.2.
2525
CO adsorption at 300K on Pt. Figure 2B shows the IR spectra of CO adsorbed (80 Torr) at successive stages of removal of coke from the surface. Before adsorption experiments, samples were treated in H2 t o reduce the metal phase to%--' and fhen outgassed at 773K. The bands i the 2120-1950 cm- range monitor CO linearly adsorbed on Pt8 sites [7]. These absorptions are very weak in curve a, showing that on the original sample the Pt sites are almost completely poisoned by coke. They are much more intense in curve b, indicating that, by oxidation at 473K, a large fraction of Pt sites are freed from carbon deposits though the majority of these contaminants are still present on the sample (cfr. Fig. 2A,b). Some increase of the intensity of the is observed in the last stages (Fig. 2B,curves c-d). Apparently, the bonds between Pt sites and carbonaceous contaminants are destroyed in the first steps of oxidation, whereas the bulk of coke deposits burn off at higher temperature. The spectrum of CO adsorbed on the sample regenerated at 773K (Fig. 2B,d) is similar to that of the fresh uncoked catalyst [7] and this suggests that dimension and morphology of the metal particles did not change significantly upon the catalytic runs and regeneration procedures. 3.3.
CO adsorption on the AlZ03 support t 300K. Only a small fraction of cationic All' sites stabilize CO at 300K €;I. The two bands at the highest frequencies (21802250 cmin the spectra at 300K are due to CO adsorbed on the alumina support [8]. They are already present in Fig. 2B,a, showing that surface A13+ sites are available for CO adsorption even pn a heavily coked sample. Only the very we k band at 2235 cm- , due to CO adsorbed on the most acidic A1 9+ sites, appears after the last stage of regeneration (oxidation at 773K, Fig. 2B,d), indicating that such sites were previously interacting with carbonaceous deposits. 3.4.
CO adsorption at 77K on A1 O3 and on Coke. Pratically all A13+ sites a8sorb CO at 77K [ 8 ] . The spectrum in the inset in Fig. 2B refers to CO adsorption on the original coked sample. The bands at 2185 and 2160 c m ' l are due to CO on tetrahedral and octahedral A13+ sites respectively [8]. Their intensity slightly increases upon regeneration, confirming that only a small fraction of the surface sites of the support are involve in the stabilization of coke. The band at 2136 cm-' is assigned to liquid-like CO stabilized on the coke phase. In fact, its intensity progressively declines as the carbonaceous contaminants are gradually burnt off and, consequently, CO appears to be a useful probe molecule also for characterizing carbonaceous contaminants. 3.5.
Acknowledgement. The authors are grateful to CEE and ASPPiemonte for financial support and t o prof. G. Froment for providing the coked sample.
2526
I
I
I
I
2200 1800
'
I
L
1400' 100
Wavenumbers cm
-1
Wavenumbers cm-
'
Figure 2. Infrared spectra of coked Pt/A1203 catalyst gradually regenerated by oxidation at increasing temperature. Sect. A : background spectra (transmittance) of the original coked sample (curve a) and after oxidation at 473,573 and 773K respectively (curves b-d). Sect. B: spectra (absorbance) of CO adsorbed at 300K on the sample at the successive stages of regeneration quoted in Sect. A. Inset: CO adsorbed at 77K on the original coked sample. 4.
1
2 3 4
5 6
REFERENCES J. Oudart and H. Wise (eds.), Deactivation and Poisoning of Catalysts, Marcel Dekker, Inc., New York, 1985. P. Gallezot, C. Leclercq, J. Barbier and P. Marecot, J. Catal. 116 (1989) 164. J. Najbar and R.P. Eischens, in M.J. Phillips and M. Ternan (eds.), Proceed. 9th ICC, Calgary 1988, vol. 3, p. 1434. M.J.D. Low and C. Morterra, Structure and Reactivity of Surfaces, C. Morterra, A. Zecchina and G. Costa (eds.), Elsevier, Amsterdam, 1989. W. Krkitschmer, K. Fostiropoulos and D. R. Huffman, Chem. Phys. Lett., 170 (1990) 167. C.R. Apesteguia, T.F. Garetto and A. Borgna, J. Catal., 106 (1987) 73.
7
8
L. Marchese, M.R. Boccuti, S. Coluccia, S. Lavagnino, A. Zecchina, L. Bonneviot and M. Chef Structure and Reactivity of Surfaces, C. Morterra, A. Zecchina and G. Costa (eds.), Elsevier, Amsterdam, 1989. A. Zecchina, E. Escalona Plater0 and C. Otero Arean, J. Catal., 107 (1987) 244,
COKE DEPOSITS ON Pt/AI,O, CATALYSTS: FTIR AND HRTEM STUDIES L. Marchese, E. Borello, S. Coluccia, G. Mama and A. Zecchina Dipartimento di Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, Universita di Torino, Via P. Giuria 7, 10125 Torino, Italy
Abstract Graphitic and amorphous coke deposits on Pt/A1203 catalysts are evidenced by HRTEM. Their interaction with the metal phase and the support is studied by the spectra of CO adsorbed at 300 and 77K. A band due to li uid-like CO stabilized on coke at 77K is observed at 2136 cm-P
.
1. INTRODUCTION
Coking and sulfidation are two main deactivation processes for reforming catalysts [l]. It is relevant to determine the structure of the deposits, their location, the treatments to remove them and the structure of the metal phase after reactivation treatments. These points are examined in this report by Infrared Spectroscopy and High Resolution Transmission Electron Microscopy. CO adsorption at 300K and 77K has been adopted as a test for measuring the overall surface activity both of the metal phase and the support. It is found that CO also interacts with the coke phase. 2. EXPERIMENTAL
The results refer to a Pt/A1203 sample (CK 306, Pt 0.6 wt % by Cyanamid Ketjen) which, after catalytic runs, showed a 6.7 wt % of coke content. The reforming feed was a H /n-eptane mixture (molar ratio =2) ; the n-eptane contained g.4 ppm of sulfur. Infrared spectra were obtained by Bruker IFS 48 and the HRTEM micrographs by a Jeol 2000EX instruments. 3. REBULTS AND DISCUSSION
Electron Microscopy Carbonaceous compounds on a particle of the coked catalyst are visible in the high resolution micrograph in Fig. 1. Curved fringes 3.6 apart in zone A are associated with graphitized carbon black (interplanar spacing d 002 - 3.4 A ) . Coke deposits with amorphous structure (as aigueh-by their 3.1.
2524
Figure 1. High resolution electron micrograph showing graphitized ( A ) and amorphous (B) coke deposits on Pt/A1203 catalyst.
disordered arrangement [2]) are present in the zones B. HRTEM analysis of the Pt phase is not discussed in this short report. However, Pt particles size was determined to be 10-15 A. IR spectra of t h e a a t a l y a t Figure 2 A shows the IR spectra of the coked Pt/A1203 catalyst reactivated by gradual oxidation (150 Torr 0 ) at increasing temperature. Disc ete, though broad, absorpsions are observed at 1700-1100 cm-' in the spectrum of the original coked sample (curve a). The overall transparency in t h e IR region increases gradually as the oxidizing temperature increases (curves b-d) and the sample turns rom black to whitish. Moreover, the bands at 1700-1100 cm-' are strongly reduced in intensit and in spectrum d only weak components in the 1600-1450 cm-' ra ge are left. By contrast, the intensity of a band at 1370 cm-' increases as the oxidizing temperature increases up to 773K. This band disappears upon heating in H2 at 973K. The bands which disappear by oxidation are certainly associated with oke. In particular, two correlated bands at 1580 and 1470 cm-' are due to the asym. and sym. stretching vibrations of carboxylate groups [3], while the other components are due to deformation modes of residual CH groups as found in carbonaceous deposits with llmolecularll form and with high H/C ratio [4]. Noticeably, bands in this region were also attributed t o carbon-carbon stretching vibrations in distorted graphitic structure in dust carbon particles and in soccerball shaped st uctures (c60, fullerene-like molecules) [5]. The 1370 cm-' band is due to sulfate groups [6] which originate from oxidation of the sulfur containing compounds which are fed into the reactor during the catalytic runs. 3.2.
2525
CO adsorption at 300K on Pt. Figure 2B shows the IR spectra of CO adsorbed (80 Torr) at successive stages of removal of coke from the surface. Before adsorption experiments, samples were treated in H2 t o reduce the metal phase to%--' and fhen outgassed at 773K. The bands i the 2120-1950 cm- range monitor CO linearly adsorbed on Pt8 sites [7]. These absorptions are very weak in curve a, showing that on the original sample the Pt sites are almost completely poisoned by coke. They are much more intense in curve b, indicating that, by oxidation at 473K, a large fraction of Pt sites are freed from carbon deposits though the majority of these contaminants are still present on the sample (cfr. Fig. 2A,b). Some increase of the intensity of the is observed in the last stages (Fig. 2B,curves c-d). Apparently, the bonds between Pt sites and carbonaceous contaminants are destroyed in the first steps of oxidation, whereas the bulk of coke deposits burn off at higher temperature. The spectrum of CO adsorbed on the sample regenerated at 773K (Fig. 2B,d) is similar to that of the fresh uncoked catalyst [7] and this suggests that dimension and morphology of the metal particles did not change significantly upon the catalytic runs and regeneration procedures. 3.3.
CO adsorption on the AlZ03 support t 300K. Only a small fraction of cationic All' sites stabilize CO at 300K €;I. The two bands at the highest frequencies (21802250 cmin the spectra at 300K are due to CO adsorbed on the alumina support [8]. They are already present in Fig. 2B,a, showing that surface A13+ sites are available for CO adsorption even pn a heavily coked sample. Only the very we k band at 2235 cm- , due to CO adsorbed on the most acidic A1 9+ sites, appears after the last stage of regeneration (oxidation at 773K, Fig. 2B,d), indicating that such sites were previously interacting with carbonaceous deposits. 3.4.
CO adsorption at 77K on A1 O3 and on Coke. Pratically all A13+ sites a8sorb CO at 77K [ 8 ] . The spectrum in the inset in Fig. 2B refers to CO adsorption on the original coked sample. The bands at 2185 and 2160 c m ' l are due to CO on tetrahedral and octahedral A13+ sites respectively [8]. Their intensity slightly increases upon regeneration, confirming that only a small fraction of the surface sites of the support are involve in the stabilization of coke. The band at 2136 cm-' is assigned to liquid-like CO stabilized on the coke phase. In fact, its intensity progressively declines as the carbonaceous contaminants are gradually burnt off and, consequently, CO appears to be a useful probe molecule also for characterizing carbonaceous contaminants. 3.5.
Acknowledgement. The authors are grateful to CEE and ASPPiemonte for financial support and t o prof. G. Froment for providing the coked sample.
2526
I
I
I
I
2200 1800
'
I
L
1400' 100
Wavenumbers cm
-1
Wavenumbers cm-
'
Figure 2. Infrared spectra of coked Pt/A1203 catalyst gradually regenerated by oxidation at increasing temperature. Sect. A : background spectra (transmittance) of the original coked sample (curve a) and after oxidation at 473,573 and 773K respectively (curves b-d). Sect. B: spectra (absorbance) of CO adsorbed at 300K on the sample at the successive stages of regeneration quoted in Sect. A. Inset: CO adsorbed at 77K on the original coked sample. 4.
1
2 3 4
5 6
REFERENCES J. Oudart and H. Wise (eds.), Deactivation and Poisoning of Catalysts, Marcel Dekker, Inc., New York, 1985. P. Gallezot, C. Leclercq, J. Barbier and P. Marecot, J. Catal. 116 (1989) 164. J. Najbar and R.P. Eischens, in M.J. Phillips and M. Ternan (eds.), Proceed. 9th ICC, Calgary 1988, vol. 3, p. 1434. M.J.D. Low and C. Morterra, Structure and Reactivity of Surfaces, C. Morterra, A. Zecchina and G. Costa (eds.), Elsevier, Amsterdam, 1989. W. Krkitschmer, K. Fostiropoulos and D. R. Huffman, Chem. Phys. Lett., 170 (1990) 167. C.R. Apesteguia, T.F. Garetto and A. Borgna, J. Catal., 106 (1987) 73.
7
8
L. Marchese, M.R. Boccuti, S. Coluccia, S. Lavagnino, A. Zecchina, L. Bonneviot and M. Chef Structure and Reactivity of Surfaces, C. Morterra, A. Zecchina and G. Costa (eds.), Elsevier, Amsterdam, 1989. A. Zecchina, E. Escalona Plater0 and C. Otero Arean, J. Catal., 107 (1987) 244,