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Metal-support interaction in mixed oxide supports
T.S.R. Prasada Rao and G. Murali Dhar (Editors) Recent Advances in Basic and Applied Aspects of Industrial Catalysis Studies in Surface Science and Catalysis, Vol. 113 9 1998 Elsevier Science B.V. All rights reserved
957
M e t a l - s u p p o r t interaction in mixed oxide supports R. Malathi, P. Madhusudhan Rao and R. P. Viswanath Department of chemistry, Indian Institute of Technology, Madras, Chennai-600036. Supported metals are used extensively in heterogeneous catalysis. In the present investigation platinum is loaded onto titania and titania-alumina supports to study the SMSI effects in detail. The catalysts were characterized by X-ray Diffraction(XRD), Stepwise Temperature Programmed Reduction (STPR) and chemisorption measurements. All the samples exhibit characteristic behaviour showing SMSI effect after HTR, though there is only moderate interaction in the mixed oxide sample. From STPR studies, the reducibility of platinum and the support in supported platinum systems is shown to depend on the extent of the interaction at the interface. Keywords: metal dispersion, chemisorption, mixed oxide, temperature programmed reduction. 1. INTRODUCTION Supported metals form an important class of heterogeneous catalysts. Though the support was originally considered to be an inert carrier that dispersed and stabilized the metal particles, it has been observed that interactions do occur between the metal and the support[ 1]. These catalysts have found application in fine chemical industry and several other industrial processes. In the present investigation, platinum supported on titania and titania-alumina were studied. It is well known that a strong interaction exists between platinum and titania, however, the extent of this interaction will depend on the interfacial characteristics. This can be modified by varying the metal and the support precursors, method of preparation and pretreatment conditions. In this investigation, an attempt is made to delineate the differences existing at the platinum-titania interface by XRD, chemisorption and Stepwise Temperature Programmed Reduction (STPR) studies.
2. EXPERIMENTAL The catalysts were prepared using titania and mixed oxide supports. The titania supports used were commercial grade (Baker, UK) and the gel, prepared by the hydrolysis of titanium tetrachloride. These were designated as TiO2(C) and TiO2(G), respectively. The mixed oxide, titania-alumina (T-A), was prepared by the co-hydrolysis of titanium isopropoxide and aluminium isopropoxide in isopropanol solution [2]. Platinum was then impregnated on these supports by wet-impregnation method. The miffed oxide support was estimated gravimetrically by the oxine method after extracting the support from the catalyst
958 [3]. The percentage of titania and alumina was found to be 22% and 78% respectively. The platinum loading was 1% and 5%, estimated by spectrophotometric method. The catalysts were then characterized by XRD, chemisorption and surface area measurements and stepwise temperature programmed reduction (STPR) in hydrogen. All the catalysts were reduced in flowing hydrogen at 573K (low temperature of reduction, LTR) and 773K (high temperature of reduction, HTR), respectively, prior to these measurements.
2.1 X-ray Diffraction Measurements The freshly reduced catalysts were characterized by X-ray diffraction using a Rigaku Miniflex X-ray diffractometer(1800) with CoK~ radiation. Metal and metal oxide phases were obtained and peaks corresponding to the different supports were also observed. The titania support was found to be of pure anatase form. The titania-alumina support showed no characteristic peak of either of the oxides indicating the formation of the mixed oxide. BET surface area measurements were carried out using Carlo Erba sorptometer with liquid nitrogen at 77K. The samples were degassed at 393K prior to all the experiments. The BET surface areas of the gel, commercial and the mixed oxide samples were found to be 100, 10 and 250m2/g respectively.
2.2 Chemisorption measurements The room temperature hydrogen chemisorption measurements were carried out using the conventional all-glass static vacuum system. The dynamic vacuum obtained was ultimately around 10-6 Torr. About 0.5g of the sample was taken in the reactor and fused to the system. The measurements were then carried out by reducing the samples in hydrogen for 24 hours at LTR and HTR. The H/M ratios were calculated by assuming that the ratio H/Pt is equal to 1 as reported in literature [4]. The particle sizes were calculated using the expression ds(nm) = 108 / %D, where D is the dispersion given by, D = number of surface metal atoms / total number of metal atoms [5].
2.3 Stepwise Temperature Programmed Reduction The fresh samples were subjected to a programmed reduction in the static vacuum system and the hydrogen uptake was measured. An equilibration time of 1 hour between two temperatures was found to be sufficient. The temperature was increased using a variable auto transformer. The working pressure was around 100 Torr. An 'on line' liquid nitrogen cold trap was used to condense the water formed as the by-product of reduction. The reproducibility of the experiment was checked by oxidizing the samples in situ in oxygen and then repeating the experiments. 3. RESULTS AND DISCUSSION The XRD pattems as shown in Fig 1 obtained for all the samples after their reduction in hydrogen indicate the presence of unreduced platinum oxides even after HTR in the supported titania gel and mixed oxide sample. However, complete reduction to metallic platinum was observed on supported commercial titania system. It c a n b e observed that the phase transformation of titania from anatase to rutile does not occur under the experimental
959 conditions in the gel catalyst. However, for a similar system using iron based gel sample, phase transformation was observed [6].
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2 e (Co- I~) Fig 1. XRD patterns of 5% Pt / TiQ(C) reduced at (i) 573K, (ii) 773K, 5% Pt / TiO2(G) at (iii) 573K, (iv) 773K, 1% Pt / T-A at (v) 573K, (vi) 773K and 5% Pt / T-A at (vii) 573K and (viii) 773K. 1. TiO2(A). 2. PtO2 3. Pt.
In the case of titania-alumina sample, the absence of any phase corresponding to either titania or alumina indicates that the support consists of homogeneous mixed oxide phases. Lahousse et al., have reported similar observations in similar systems[2]. From Table 1, it can be observed that the titania supported sample exhibit the characteristic SMSI behaviour after HTR irrespective of the origin of the support. The low platinum containing samples supported on the mixed oxide support shows the highest dispersion. Only a mild suppression in hydrogen chemisorption is observed on the catalysts employing the mixed oxide support. McVicker and Ziemiak have reported a similar observation on this system[7]. However, they have prepared the mixed oxide by hydrolysing titanium isopropoxide on performed alumina. Thus pure titania and alumina phases would be present together with the mixed oxide. The mild suppression has been attributed to partial SMSI formation between the supported TiO2 phase and a fraction of the Pt state. They further suggested th~/t the suppression in chemisorption would increase with increase in the TiOJPt ratio.
960 Table 1 Hydrogen chemisorption results of all the samples. Catalyst Reduction H 2 uptake Temperature (K) ( ~ mole / g ) 5% Pt/TiO 2 (C) 573 4.4 773 0.0 5%Pt/TiO 2 (G) 573 7.5 773 0.0 1%Pt/TiO2-A120 3 573 10.4 773 8.1 1%Pt/YiO 2 (C) 573 7.4 773 0.0 5%Pt/TiO2-A120 3 573 5.3 773 3.9
% Dispersion 4 6 41 32 29 4 3
Particle size (nm) 30 17 2.5 3.5 4 26 35
The STPR profiles show two peaks for all the samples (Fig 2). The first peak has been assigned to the reduction of platinum species and the other to the reduction of the support. Sexton et al.[8], in their study on Pt/TiO2, prepared from HzPtC16, obtained TPR profiles similar to those reported in this investigation. They have observed the reduction of Pt ions at the same temperature for samples prepared by impregnating H2PtC16 on the support (TiO2). They observed a homogeneous distribution of Pt on TiO2 from TPR profiles. From these profiles, it can be observed that the peak temperatures for the reduction of platinum species are different, suggesting that the extent of platinum-support interaction is different. Moreover, the reduction of TiO 2 in the gel sample occurs at a lower temperature (573K) than that observed with other samples. In spite of this, one observes chemisorption of hydrogen to be appreciable. This may be due to the fact that for the generation of the SMSI state, the metal sites are effectively covered by the support species[9,10]. For this surface migration to take place, the high temperature reduction will be necessary. At lower temperature the energetics may not favour such a process. However, the reduction of the pure support was not observed within this temperature range as reported earlier[11]. Surprisingly the two peaks are shifted to higher temperatures in Pt/T-A sample, although, the BET area of the mixed oxide sample is high and the metal crystallites are well dispersed. This may be due to the presence of larger portion of metallic platinum on the alumina rich regions in the mixed oxide which contains 78% A1203. This is also evidenced by the mild suppression in hydrogen chemisorption after HTR as has also been reported in literature[7]. Thus the reduction of the mixed oxide phase occurs at a higher temperature than in the gel sample but similar to commercial sample despite possessing the highest surface area. It can be observed from these profiles that the reduction of platinum oxides was obtained at around 400K in all the samples which is characteristic of supported platinum systems. From XPS studies it was noticed that, unlike the other systems, in the gel sample there exists a homogeneous distribution of Pt in TiO 2 [12]. Thus an intimate contact exists between platinum and the support leading to a facile reduction of both platinum and titania.
962 REFERENCES
1. G. M. Schwab, Adv. Catal., (1978) 1. 2. C. Lahousse, A. Aboulyt, F. Mauge, J. Bachelier and J. C. Lavalley, J. Mol. Catal., (1993) 283. 3. A. Classen, and L. Bastings, Analyst, 92 (1967) 614. 4..N.E. Bogdanchikova, N. E. Buyanova, V. I. Zaikonskii, O. F. Zapreeva and A. P. Karnaukhov, Kinet. Katal., 30 (1989) 999. 5. S. C. Fung, J. Catal., 76 (1982) 225. 6. P. Madhusudhan Rao, V. Sudharshan and R. P. Viswanath, Internl. Series in Chem. Engg., (1995), 219 eds., P. Kanta Rao and R. S. Beniwal, Wiley Eastern, New Delhi. 7. G. B. McVicker and J. J. Ziemiak, J. Catal., 95 (1985) 473. 8. B. A. Sexton, A. E. Hughes and K. Foger, J. Catal., 77 (1982) 85. 9. M. A. Vannice and S. Y. Wang, J. Phys. Chem., 85 (1981) 2543. 10. B. J. Tatarchuk and J. A. Dumesic, J. Catal., 70 (1981) 308. 11. S. J. Tauster and S. C. Fung, J. Catal, 55 (1978) 29 12. R. Malathi, P. Madhusudhan Rao, B. Viswanathan and R. P. Viswanath (unpublished results).
957
M e t a l - s u p p o r t interaction in mixed oxide supports R. Malathi, P. Madhusudhan Rao and R. P. Viswanath Department of chemistry, Indian Institute of Technology, Madras, Chennai-600036. Supported metals are used extensively in heterogeneous catalysis. In the present investigation platinum is loaded onto titania and titania-alumina supports to study the SMSI effects in detail. The catalysts were characterized by X-ray Diffraction(XRD), Stepwise Temperature Programmed Reduction (STPR) and chemisorption measurements. All the samples exhibit characteristic behaviour showing SMSI effect after HTR, though there is only moderate interaction in the mixed oxide sample. From STPR studies, the reducibility of platinum and the support in supported platinum systems is shown to depend on the extent of the interaction at the interface. Keywords: metal dispersion, chemisorption, mixed oxide, temperature programmed reduction. 1. INTRODUCTION Supported metals form an important class of heterogeneous catalysts. Though the support was originally considered to be an inert carrier that dispersed and stabilized the metal particles, it has been observed that interactions do occur between the metal and the support[ 1]. These catalysts have found application in fine chemical industry and several other industrial processes. In the present investigation, platinum supported on titania and titania-alumina were studied. It is well known that a strong interaction exists between platinum and titania, however, the extent of this interaction will depend on the interfacial characteristics. This can be modified by varying the metal and the support precursors, method of preparation and pretreatment conditions. In this investigation, an attempt is made to delineate the differences existing at the platinum-titania interface by XRD, chemisorption and Stepwise Temperature Programmed Reduction (STPR) studies.
2. EXPERIMENTAL The catalysts were prepared using titania and mixed oxide supports. The titania supports used were commercial grade (Baker, UK) and the gel, prepared by the hydrolysis of titanium tetrachloride. These were designated as TiO2(C) and TiO2(G), respectively. The mixed oxide, titania-alumina (T-A), was prepared by the co-hydrolysis of titanium isopropoxide and aluminium isopropoxide in isopropanol solution [2]. Platinum was then impregnated on these supports by wet-impregnation method. The miffed oxide support was estimated gravimetrically by the oxine method after extracting the support from the catalyst
958 [3]. The percentage of titania and alumina was found to be 22% and 78% respectively. The platinum loading was 1% and 5%, estimated by spectrophotometric method. The catalysts were then characterized by XRD, chemisorption and surface area measurements and stepwise temperature programmed reduction (STPR) in hydrogen. All the catalysts were reduced in flowing hydrogen at 573K (low temperature of reduction, LTR) and 773K (high temperature of reduction, HTR), respectively, prior to these measurements.
2.1 X-ray Diffraction Measurements The freshly reduced catalysts were characterized by X-ray diffraction using a Rigaku Miniflex X-ray diffractometer(1800) with CoK~ radiation. Metal and metal oxide phases were obtained and peaks corresponding to the different supports were also observed. The titania support was found to be of pure anatase form. The titania-alumina support showed no characteristic peak of either of the oxides indicating the formation of the mixed oxide. BET surface area measurements were carried out using Carlo Erba sorptometer with liquid nitrogen at 77K. The samples were degassed at 393K prior to all the experiments. The BET surface areas of the gel, commercial and the mixed oxide samples were found to be 100, 10 and 250m2/g respectively.
2.2 Chemisorption measurements The room temperature hydrogen chemisorption measurements were carried out using the conventional all-glass static vacuum system. The dynamic vacuum obtained was ultimately around 10-6 Torr. About 0.5g of the sample was taken in the reactor and fused to the system. The measurements were then carried out by reducing the samples in hydrogen for 24 hours at LTR and HTR. The H/M ratios were calculated by assuming that the ratio H/Pt is equal to 1 as reported in literature [4]. The particle sizes were calculated using the expression ds(nm) = 108 / %D, where D is the dispersion given by, D = number of surface metal atoms / total number of metal atoms [5].
2.3 Stepwise Temperature Programmed Reduction The fresh samples were subjected to a programmed reduction in the static vacuum system and the hydrogen uptake was measured. An equilibration time of 1 hour between two temperatures was found to be sufficient. The temperature was increased using a variable auto transformer. The working pressure was around 100 Torr. An 'on line' liquid nitrogen cold trap was used to condense the water formed as the by-product of reduction. The reproducibility of the experiment was checked by oxidizing the samples in situ in oxygen and then repeating the experiments. 3. RESULTS AND DISCUSSION The XRD pattems as shown in Fig 1 obtained for all the samples after their reduction in hydrogen indicate the presence of unreduced platinum oxides even after HTR in the supported titania gel and mixed oxide sample. However, complete reduction to metallic platinum was observed on supported commercial titania system. It c a n b e observed that the phase transformation of titania from anatase to rutile does not occur under the experimental
959 conditions in the gel catalyst. However, for a similar system using iron based gel sample, phase transformation was observed [6].
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2 e (Co- I~) Fig 1. XRD patterns of 5% Pt / TiQ(C) reduced at (i) 573K, (ii) 773K, 5% Pt / TiO2(G) at (iii) 573K, (iv) 773K, 1% Pt / T-A at (v) 573K, (vi) 773K and 5% Pt / T-A at (vii) 573K and (viii) 773K. 1. TiO2(A). 2. PtO2 3. Pt.
In the case of titania-alumina sample, the absence of any phase corresponding to either titania or alumina indicates that the support consists of homogeneous mixed oxide phases. Lahousse et al., have reported similar observations in similar systems[2]. From Table 1, it can be observed that the titania supported sample exhibit the characteristic SMSI behaviour after HTR irrespective of the origin of the support. The low platinum containing samples supported on the mixed oxide support shows the highest dispersion. Only a mild suppression in hydrogen chemisorption is observed on the catalysts employing the mixed oxide support. McVicker and Ziemiak have reported a similar observation on this system[7]. However, they have prepared the mixed oxide by hydrolysing titanium isopropoxide on performed alumina. Thus pure titania and alumina phases would be present together with the mixed oxide. The mild suppression has been attributed to partial SMSI formation between the supported TiO2 phase and a fraction of the Pt state. They further suggested th~/t the suppression in chemisorption would increase with increase in the TiOJPt ratio.
960 Table 1 Hydrogen chemisorption results of all the samples. Catalyst Reduction H 2 uptake Temperature (K) ( ~ mole / g ) 5% Pt/TiO 2 (C) 573 4.4 773 0.0 5%Pt/TiO 2 (G) 573 7.5 773 0.0 1%Pt/TiO2-A120 3 573 10.4 773 8.1 1%Pt/YiO 2 (C) 573 7.4 773 0.0 5%Pt/TiO2-A120 3 573 5.3 773 3.9
% Dispersion 4 6 41 32 29 4 3
Particle size (nm) 30 17 2.5 3.5 4 26 35
The STPR profiles show two peaks for all the samples (Fig 2). The first peak has been assigned to the reduction of platinum species and the other to the reduction of the support. Sexton et al.[8], in their study on Pt/TiO2, prepared from HzPtC16, obtained TPR profiles similar to those reported in this investigation. They have observed the reduction of Pt ions at the same temperature for samples prepared by impregnating H2PtC16 on the support (TiO2). They observed a homogeneous distribution of Pt on TiO2 from TPR profiles. From these profiles, it can be observed that the peak temperatures for the reduction of platinum species are different, suggesting that the extent of platinum-support interaction is different. Moreover, the reduction of TiO 2 in the gel sample occurs at a lower temperature (573K) than that observed with other samples. In spite of this, one observes chemisorption of hydrogen to be appreciable. This may be due to the fact that for the generation of the SMSI state, the metal sites are effectively covered by the support species[9,10]. For this surface migration to take place, the high temperature reduction will be necessary. At lower temperature the energetics may not favour such a process. However, the reduction of the pure support was not observed within this temperature range as reported earlier[11]. Surprisingly the two peaks are shifted to higher temperatures in Pt/T-A sample, although, the BET area of the mixed oxide sample is high and the metal crystallites are well dispersed. This may be due to the presence of larger portion of metallic platinum on the alumina rich regions in the mixed oxide which contains 78% A1203. This is also evidenced by the mild suppression in hydrogen chemisorption after HTR as has also been reported in literature[7]. Thus the reduction of the mixed oxide phase occurs at a higher temperature than in the gel sample but similar to commercial sample despite possessing the highest surface area. It can be observed from these profiles that the reduction of platinum oxides was obtained at around 400K in all the samples which is characteristic of supported platinum systems. From XPS studies it was noticed that, unlike the other systems, in the gel sample there exists a homogeneous distribution of Pt in TiO 2 [12]. Thus an intimate contact exists between platinum and the support leading to a facile reduction of both platinum and titania.
962 REFERENCES
1. G. M. Schwab, Adv. Catal., (1978) 1. 2. C. Lahousse, A. Aboulyt, F. Mauge, J. Bachelier and J. C. Lavalley, J. Mol. Catal., (1993) 283. 3. A. Classen, and L. Bastings, Analyst, 92 (1967) 614. 4..N.E. Bogdanchikova, N. E. Buyanova, V. I. Zaikonskii, O. F. Zapreeva and A. P. Karnaukhov, Kinet. Katal., 30 (1989) 999. 5. S. C. Fung, J. Catal., 76 (1982) 225. 6. P. Madhusudhan Rao, V. Sudharshan and R. P. Viswanath, Internl. Series in Chem. Engg., (1995), 219 eds., P. Kanta Rao and R. S. Beniwal, Wiley Eastern, New Delhi. 7. G. B. McVicker and J. J. Ziemiak, J. Catal., 95 (1985) 473. 8. B. A. Sexton, A. E. Hughes and K. Foger, J. Catal., 77 (1982) 85. 9. M. A. Vannice and S. Y. Wang, J. Phys. Chem., 85 (1981) 2543. 10. B. J. Tatarchuk and J. A. Dumesic, J. Catal., 70 (1981) 308. 11. S. J. Tauster and S. C. Fung, J. Catal, 55 (1978) 29 12. R. Malathi, P. Madhusudhan Rao, B. Viswanathan and R. P. Viswanath (unpublished results).