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XPS characterization of supported Ziegler–Natta catalysts
Applied Surface Science 253 (2006) 753–756 www.elsevier.com/locate/apsusc
XPS characterization of supported Ziegler–Natta catalysts§ V.K. Kaushik a,*, V.K. Gupta b, D.G. Naik b a
b
Research Centre, Indian Petrochemicals Corporation Limited, Vadodara 391346, India R&D Centre, Reliance Industries Limited, Village-Mora, Post-Bhatha, Surat-Hazira Road, Surat 394510, Gujarat, India Received 29 September 2005; received in revised form 27 December 2005; accepted 4 January 2006 Available online 14 February 2006
Abstract Surface analytical technique ESCA (electron spectrometer for chemical analysis) has been used for analysis of catalysts used in propylene polymerization. As a result of this analysis it has been shown that productivity of a catalyst can be correlated to Ti/Mg atomic ratio that indicates dispersion of titanium atoms on magnesium support. A quantitative indicator of productivity, i.e. ‘‘titanium index’’ has also been evaluated for studied catalysts. # 2006 Elsevier B.V. All rights reserved. Keywords: Ziegler–Natta catalyst; Propylene polymerization; ESCA; Dispersion; Titanium index
1. Introduction The development of supported Ziegler–Natta catalyst opened a new era in the polyolefin technology both from an industrial and scientific viewpoint. The use of magnesium dichloride as support for fixation of titanium complexes has significantly increased the complexity of the catalyst. Catalyst performance and the nature of active titanium species is mainly dependant on the preparation methodology and characteristics of each component used in catalyst synthesis [1,2]. ESCA being a surface sensitive analytical technique probes only top atomic layers in heterogeneous catalyst systems. The surface composition and electronic environment of active species in these layers are responsible for the performance of a catalyst. Although few research groups have reported ESCA studies for different Ziegler–Natta catalyst systems [3–7], we have used this technique for correlating performance of catalysts used in various petrochemical processes [8–12]. The present investigation deals with the XPS characterization of supported Ziegler–Natta catalysts for polypropylene polymerization. One of the important parameter useful in correlating performance of these catalysts is Ti:Mg atomic ratio, which indicates dispersion of titanium
§
IPCL Communication No. 503. * Corresponding author. Tel.: +91 265 3062255x3416; fax: +91 265 3062104. E-mail address: [email protected] (V.K. Kaushik).
0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.01.002
species on magnesium based support. The study has also been extended for evaluation of a parameter ‘‘titanium index’’ and correlation of this parameter with the productivity of several catalysts used in slurry phase polymerization. 2. Experimental Since the chemicals used are highly sensitive to moisture, all experiments were carried out under moisture free and pure nitrogen atmosphere. The atmosphere bag and dry box was also used for transfer of chemical components. 2.1. Glassware All the glassware used for doing the experiments were heated at 120–150 8C and then degassed prior to their use in experiments. 2.2. Preparation of catalyst The solid catalyst preparation was carried out using magnesium ethoxide as support material [13,14]. The catalyst preparation involves treatment of magnesium ethoxide with titanium tetrachloride:chlorobenzene mixture at elevated temperature in the presence of ethylbenzoate as internal donor. This was followed by two more treatment of TiCl4:chlorobenzene mixtures followed by isolation of solid product by hydrocarbon washing.
754
V.K. Kaushik et al. / Applied Surface Science 253 (2006) 753–756
Catalysts 1–3 were prepared as above while the catalyst 4 was prepared by adding benzoyl chloride at room temperature in third titanation stage. The catalysts 5 and 6 were prepared by increasing the temperature of benzoyl chloride addition to 90 8C. 2.3. Propylene polymerization Dry n-hexane was taken in preheated moisture free SS jacketed reactor fitted with magnetic stirrer. MgCl2 supported catalyst, cocatalyst (triethylaluminium), in cocatalyst/catalyst ratio of 200–250 and external donor (paraethoxy ethylbenozate) in cocataylst/donor molar ratio of 3–5 was taken in decane. Hexane was saturated with propylene. Mixture of catalyst/ cocatalyst and donor was added in reactor and hydrogen was added as chain terminating agent as per requirement of melt flow index. Reactor temperature was maintained at 70 8C by heating/ cooling system. Simultaneously propylene pressure was maintained at 5 kg/cm2. Polymerization reaction was carried out for 1 h. After 1 h of reaction, hexane was removed and polymer was collected/dried. Productivity of catalyst was calculated based on polymer yield and catalyst amount. 2.4. Measurement of surface area Surface area of catalysts was measured on Sorptomatic 1990 instrument by BET method. Catalyst samples were degassed under high vacuum to constant weight and the measurement of adsorption and desorption completed at liquid nitrogen temperature using pure nitrogen gas. At the end of analysis BET surface area was computed using standard software. 2.5. ESCA experiment A vacuum Generator ESCALAB MKII spectrometer equipped with twin X-ray anode (aluminium and magnesium) was used for recording of X-ray photoelectron spectra (XPS). These X-ray anodes were operated at 10 kV 10 mA and the vacuum in the analysis chamber was maintained better than 5 108 mbar during analysis. The spectrometer was calibrated using Ag 3d5/2 photoelectron line [15] at 368.3 eV. Catalyst samples were dusted on double side adhesive tape and mounted on the sample holder. Each catalyst was transferred in argon atmosphere via fast entry lock attached to preparation chamber. Catalyst was kept overnight in the preparation chamber before being transferred to analysis chamber for recording of XPS spectra. The data were collected using the DELL computer-based data system interfacing the spectrometer. Reported values of binding energies are average of at least two different runs and are accurate to 0.2 eV.
Fig. 1. XPS survey scans of catalyst no. 1 and 6.
spectra indicates presence of more titanium species on the surface of catalyst no. 6 compared to catalyst no. 1. Relative line intensities of Ti 2p, Cl 2p and Mg 2s photoelectron lines in catalyst no. 4 are shown in Fig. 2. All XPS spectra were recorded using Mg Ka X-ray source. Binding energy measurements were made using C 1s photoelectron line from aliphatic carbon at 284.8 eV [16] as an internal reference. The Ti 2p3/2 binding energy reported for these catalysts is 459.3 0.1 eV which is higher than TiCl3 (458.3) and lower than TiCl4 (459.8) reported by Sleigh et al. in their studies on organometallics [16]. To confirm reported [16] binding energy data, we have also measured Ti 2p3/2 binding energy for TiO2 at 458.6 0.1 eV [17]. Lowering in Ti 2p3/2 binding energy value of TiCl4 on adsorption at active MgCl2 support indicates transfer of electronic charge from support to titanium atom in supported Ziegler–Natta catalysts. Similar XPS observations on titanium state in supported PP catalysts have been made by Terano’s group [3,18,19]. Somorjai’s group has carried out extensive surface studies [20–23] on model catalysts to correlate catalyst surface
3. Results and discussions XPS survey scans of catalyst no. 1 and 6 are shown in Fig. 1. A comparison of Ti 2p photoelectron line intensity in these
Fig. 2. Ti 2p, Cl 2p and Mg 2s photoelectron lines on the surface of catalyst no. 4.
V.K. Kaushik et al. / Applied Surface Science 253 (2006) 753–756
755
Table 1 Derivation of K value Catalyst no.
Ti index
Log(Ti index)
Observed productivity
K = productivity/(log(Ti index))
Average K value
1 2 3 4 5 6
31.98 32.69 32.61 51.46 82.22 66.16
1.514 1.505 1.513 1.711 1.915 1.821
5.3 5.1 5.0 5.8 6.9 6.6
3.50 3.39 3.30 3.39 3.60 3.62
3.46 0.16
Table 2 XPS resultsa Catalyst no.
Ti/Mg atomic ratio
Titanium index
BET surface area (m2/gm)
Ti 2p3/2 BE (eV)
Cl 2p BE (eV)
Mg 2s BE (eV)
Laboratory determined productivity (kg.PP/gm.Cat)
Calculated productivity with estimated error
1 2 3 4 5 6
0.122 0.123 0.124 0.235 0.299 0.261
32.69 31.98 32.61 51.46 82.22 66.16
268 260 263 219 275 253
459.2 459.2 459.2 459.2 459.4 459.2
199.3 199.4 199.3 199.4 199.5 199.4
90.2 90.3 90.2 90.1 90.3 90.3
5.3 5.1 5.0 5.8 6.9 6.6
5.2 0.2 5.2 0.2 5.2 0.2 5.9 0.3 6.6 0.3 6.3 0.3
a
C 1s photoelectron line at 284.8 eV was taken as internal reference in binding energy measurements.
Fig. 3. Relative titanium dispersion on the surface of three catalysts showing large variation in productivity.
and Mg 2s photoelectron lines to evaluate this ratio. These values shown in Table 2 indicate qualitative correlation of surface titanium dispersion with the productivity of the catalyst. Relative titanium dispersion on the surface of catalysts 3–5 indicating large variations in productivity are shown in Fig. 3. As surface area of a catalyst play an important role in its performance, a parameter ‘‘titanium index’’ which is a function of titanium dispersion and catalyst surface area was also evaluated for these catalysts. Looking into the productivity value of these catalysts and evaluated titanium index shown in Table 1, it was observed that productivity varies as log of titanium index. Hence, [productivity/log(Ti index)] ratio was estimated for all these catalysts. This estimation gave a constant value for this ratio as 3.46 0.16. So an empirical relation: productivity ¼ ð3:46 0:16Þlogðtitanium indexÞ
structure and polymer properties. Although slight difference1 in Ti 2p3/2 value has been noticed, their observations on TiCl4 deposited on MgCl2 [21] also indicate lowering in Ti 2p3/2 binding energy value for TiMgxCly (interacting state of TiCl4 with MgCl2—called mixed titanium/magnesium chloride by Magni and Somorjai [21]) compared to TiCl4. This observation therefore also confirms our results on supported Ziegler– Natta catalysts. Dispersion of active titanium species on support is important in qualitative evaluation of catalyst performance. This can be represented by surface Ti/Mg atomic ratio for these catalysts. We have used published sensitivity factors [24,25] for Ti 2p3/2
1 Magni and Somorjai [21,22] have used Au 4f7/2 at 84.0 eV as binding energy reference in their measurements.
was derived to correlate observed productivity of catalyst with the estimated titanium index. Calculated productivity in kg/gm catalyst using this relation is given in Table 2 along with the estimated error. These results show very good agreement (within 5%) between calculated and observed productivity values for these catalysts in slurry phase polymerization. 4. Conclusions Based on the present study it can be concluded that surface analytical technique ESCA can be used for correlating performance of supported PP catalysts. Two important parameters used in such correlation process are ‘‘titanium index’’ and electronic environment of titanium atoms on the surface of these catalysts.
756
V.K. Kaushik et al. / Applied Surface Science 253 (2006) 753–756
Acknowledgements Authors acknowledge the support provided by IPCL and RIL management for carrying out this study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
V.K. Gupta, J. Polym. Mater. 16 (1999) 97. S. Bhaduri, V.K. Gupta, Curr. Sci. 78 (2000) 1082. K. Hasebe, H. Mori, M. Terano, J. Mol. Catal. A: Chem. 124 (1997) L1. J. Tian, B.T. Huang, Chin. Chem. Lett. 7 (1996) 67. P.G. Gassman, M.R. Callstrom, J. Am. Chem. Soc. 109 (1987) 7875. M. Furuta, J. Polym. Sci., Polym. Phys. Ed. 19 (1981) 135. S.H. Kim, D. Gnani, G.A. Somorjai, Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 42 (2001) 331. V.K. Kaushik, M. Ravindranathan, Appl. Catal. 47 (1989) 343. V.K. Kaushik, Ch. Sivaraj, P. Kanta Rao, Appl. Surf. Sci. 51 (1991) 27. A. Wali, S.M. Piallai, V.K. Kaushik, S. Satish, Appl. Catal. 135A (1996) 83. A. Wali, S. Unnikrishnan, S.M. Pillai, V.K. Kaushik, S. Satish, J. Catal. 173 (1998) 84.
[12] S.C. Roy, H.L. Prasad, P. Dutta, A. Bhattacharya, B. Singh, S. Kumar, S. Mahajan, V.K. Kaushik, S.M. Pillai, Ravindranathan, Appl. Catal. 220 (2001) 153. [13] V.K. Gupta, S. Satish, I.S. Bhardwaj, J. Macromol. Sci: Pure Appl. Chem. A3 (1994) 451. [14] B.L. Goodall, A.A. Vander Nat, W. Sjardijn, U.S. Patent No. 4,414,132 (1983). [15] V.K. Kaushik, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 273. [16] C. Sleigh, A.P. Pijpers, A. Jaspers, B. Coussens, R.J. Meier, J. Electron Spectrosc. Relat. Phenom. 77 (1996) 41. [17] P.B. Joshi, G.R. Marathe, N.S. Murthy, V.K. Kaushik, P. Ramakrishna, Mater. Lett. 56 (2002) 322. [18] H. Mori, K. Hasebe, M. Terano, J. Mol. Catal. 140A (1999) 165. [19] H. Mori, K. Hasebe, M. Terano, Polymer 40 (1999) 1389. [20] S.H. Kim, G.A. Somorjai, Surf. Interface Anal. 31 (2001) 701. [21] E. Magni, G.A. Somorjai, Catal. Lett. 35 (1995) 205. [22] E. Magni, G.A. Somorjai, J. Phys. Chem. 100 (1996) 14786. [23] A. Opdahl, S. Hoffer, B. Mailhot, G.A. Somorjai, Chem. Rec. 1 (2001) 101. [24] C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.M. Raymond, L.H. Gale, Surf. Interface Anal. 3 (1981) 211. [25] C.D. Wagner, in: D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis, John Wiley & Sons, 1990.
XPS characterization of supported Ziegler–Natta catalysts§ V.K. Kaushik a,*, V.K. Gupta b, D.G. Naik b a
b
Research Centre, Indian Petrochemicals Corporation Limited, Vadodara 391346, India R&D Centre, Reliance Industries Limited, Village-Mora, Post-Bhatha, Surat-Hazira Road, Surat 394510, Gujarat, India Received 29 September 2005; received in revised form 27 December 2005; accepted 4 January 2006 Available online 14 February 2006
Abstract Surface analytical technique ESCA (electron spectrometer for chemical analysis) has been used for analysis of catalysts used in propylene polymerization. As a result of this analysis it has been shown that productivity of a catalyst can be correlated to Ti/Mg atomic ratio that indicates dispersion of titanium atoms on magnesium support. A quantitative indicator of productivity, i.e. ‘‘titanium index’’ has also been evaluated for studied catalysts. # 2006 Elsevier B.V. All rights reserved. Keywords: Ziegler–Natta catalyst; Propylene polymerization; ESCA; Dispersion; Titanium index
1. Introduction The development of supported Ziegler–Natta catalyst opened a new era in the polyolefin technology both from an industrial and scientific viewpoint. The use of magnesium dichloride as support for fixation of titanium complexes has significantly increased the complexity of the catalyst. Catalyst performance and the nature of active titanium species is mainly dependant on the preparation methodology and characteristics of each component used in catalyst synthesis [1,2]. ESCA being a surface sensitive analytical technique probes only top atomic layers in heterogeneous catalyst systems. The surface composition and electronic environment of active species in these layers are responsible for the performance of a catalyst. Although few research groups have reported ESCA studies for different Ziegler–Natta catalyst systems [3–7], we have used this technique for correlating performance of catalysts used in various petrochemical processes [8–12]. The present investigation deals with the XPS characterization of supported Ziegler–Natta catalysts for polypropylene polymerization. One of the important parameter useful in correlating performance of these catalysts is Ti:Mg atomic ratio, which indicates dispersion of titanium
§
IPCL Communication No. 503. * Corresponding author. Tel.: +91 265 3062255x3416; fax: +91 265 3062104. E-mail address: [email protected] (V.K. Kaushik).
0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.01.002
species on magnesium based support. The study has also been extended for evaluation of a parameter ‘‘titanium index’’ and correlation of this parameter with the productivity of several catalysts used in slurry phase polymerization. 2. Experimental Since the chemicals used are highly sensitive to moisture, all experiments were carried out under moisture free and pure nitrogen atmosphere. The atmosphere bag and dry box was also used for transfer of chemical components. 2.1. Glassware All the glassware used for doing the experiments were heated at 120–150 8C and then degassed prior to their use in experiments. 2.2. Preparation of catalyst The solid catalyst preparation was carried out using magnesium ethoxide as support material [13,14]. The catalyst preparation involves treatment of magnesium ethoxide with titanium tetrachloride:chlorobenzene mixture at elevated temperature in the presence of ethylbenzoate as internal donor. This was followed by two more treatment of TiCl4:chlorobenzene mixtures followed by isolation of solid product by hydrocarbon washing.
754
V.K. Kaushik et al. / Applied Surface Science 253 (2006) 753–756
Catalysts 1–3 were prepared as above while the catalyst 4 was prepared by adding benzoyl chloride at room temperature in third titanation stage. The catalysts 5 and 6 were prepared by increasing the temperature of benzoyl chloride addition to 90 8C. 2.3. Propylene polymerization Dry n-hexane was taken in preheated moisture free SS jacketed reactor fitted with magnetic stirrer. MgCl2 supported catalyst, cocatalyst (triethylaluminium), in cocatalyst/catalyst ratio of 200–250 and external donor (paraethoxy ethylbenozate) in cocataylst/donor molar ratio of 3–5 was taken in decane. Hexane was saturated with propylene. Mixture of catalyst/ cocatalyst and donor was added in reactor and hydrogen was added as chain terminating agent as per requirement of melt flow index. Reactor temperature was maintained at 70 8C by heating/ cooling system. Simultaneously propylene pressure was maintained at 5 kg/cm2. Polymerization reaction was carried out for 1 h. After 1 h of reaction, hexane was removed and polymer was collected/dried. Productivity of catalyst was calculated based on polymer yield and catalyst amount. 2.4. Measurement of surface area Surface area of catalysts was measured on Sorptomatic 1990 instrument by BET method. Catalyst samples were degassed under high vacuum to constant weight and the measurement of adsorption and desorption completed at liquid nitrogen temperature using pure nitrogen gas. At the end of analysis BET surface area was computed using standard software. 2.5. ESCA experiment A vacuum Generator ESCALAB MKII spectrometer equipped with twin X-ray anode (aluminium and magnesium) was used for recording of X-ray photoelectron spectra (XPS). These X-ray anodes were operated at 10 kV 10 mA and the vacuum in the analysis chamber was maintained better than 5 108 mbar during analysis. The spectrometer was calibrated using Ag 3d5/2 photoelectron line [15] at 368.3 eV. Catalyst samples were dusted on double side adhesive tape and mounted on the sample holder. Each catalyst was transferred in argon atmosphere via fast entry lock attached to preparation chamber. Catalyst was kept overnight in the preparation chamber before being transferred to analysis chamber for recording of XPS spectra. The data were collected using the DELL computer-based data system interfacing the spectrometer. Reported values of binding energies are average of at least two different runs and are accurate to 0.2 eV.
Fig. 1. XPS survey scans of catalyst no. 1 and 6.
spectra indicates presence of more titanium species on the surface of catalyst no. 6 compared to catalyst no. 1. Relative line intensities of Ti 2p, Cl 2p and Mg 2s photoelectron lines in catalyst no. 4 are shown in Fig. 2. All XPS spectra were recorded using Mg Ka X-ray source. Binding energy measurements were made using C 1s photoelectron line from aliphatic carbon at 284.8 eV [16] as an internal reference. The Ti 2p3/2 binding energy reported for these catalysts is 459.3 0.1 eV which is higher than TiCl3 (458.3) and lower than TiCl4 (459.8) reported by Sleigh et al. in their studies on organometallics [16]. To confirm reported [16] binding energy data, we have also measured Ti 2p3/2 binding energy for TiO2 at 458.6 0.1 eV [17]. Lowering in Ti 2p3/2 binding energy value of TiCl4 on adsorption at active MgCl2 support indicates transfer of electronic charge from support to titanium atom in supported Ziegler–Natta catalysts. Similar XPS observations on titanium state in supported PP catalysts have been made by Terano’s group [3,18,19]. Somorjai’s group has carried out extensive surface studies [20–23] on model catalysts to correlate catalyst surface
3. Results and discussions XPS survey scans of catalyst no. 1 and 6 are shown in Fig. 1. A comparison of Ti 2p photoelectron line intensity in these
Fig. 2. Ti 2p, Cl 2p and Mg 2s photoelectron lines on the surface of catalyst no. 4.
V.K. Kaushik et al. / Applied Surface Science 253 (2006) 753–756
755
Table 1 Derivation of K value Catalyst no.
Ti index
Log(Ti index)
Observed productivity
K = productivity/(log(Ti index))
Average K value
1 2 3 4 5 6
31.98 32.69 32.61 51.46 82.22 66.16
1.514 1.505 1.513 1.711 1.915 1.821
5.3 5.1 5.0 5.8 6.9 6.6
3.50 3.39 3.30 3.39 3.60 3.62
3.46 0.16
Table 2 XPS resultsa Catalyst no.
Ti/Mg atomic ratio
Titanium index
BET surface area (m2/gm)
Ti 2p3/2 BE (eV)
Cl 2p BE (eV)
Mg 2s BE (eV)
Laboratory determined productivity (kg.PP/gm.Cat)
Calculated productivity with estimated error
1 2 3 4 5 6
0.122 0.123 0.124 0.235 0.299 0.261
32.69 31.98 32.61 51.46 82.22 66.16
268 260 263 219 275 253
459.2 459.2 459.2 459.2 459.4 459.2
199.3 199.4 199.3 199.4 199.5 199.4
90.2 90.3 90.2 90.1 90.3 90.3
5.3 5.1 5.0 5.8 6.9 6.6
5.2 0.2 5.2 0.2 5.2 0.2 5.9 0.3 6.6 0.3 6.3 0.3
a
C 1s photoelectron line at 284.8 eV was taken as internal reference in binding energy measurements.
Fig. 3. Relative titanium dispersion on the surface of three catalysts showing large variation in productivity.
and Mg 2s photoelectron lines to evaluate this ratio. These values shown in Table 2 indicate qualitative correlation of surface titanium dispersion with the productivity of the catalyst. Relative titanium dispersion on the surface of catalysts 3–5 indicating large variations in productivity are shown in Fig. 3. As surface area of a catalyst play an important role in its performance, a parameter ‘‘titanium index’’ which is a function of titanium dispersion and catalyst surface area was also evaluated for these catalysts. Looking into the productivity value of these catalysts and evaluated titanium index shown in Table 1, it was observed that productivity varies as log of titanium index. Hence, [productivity/log(Ti index)] ratio was estimated for all these catalysts. This estimation gave a constant value for this ratio as 3.46 0.16. So an empirical relation: productivity ¼ ð3:46 0:16Þlogðtitanium indexÞ
structure and polymer properties. Although slight difference1 in Ti 2p3/2 value has been noticed, their observations on TiCl4 deposited on MgCl2 [21] also indicate lowering in Ti 2p3/2 binding energy value for TiMgxCly (interacting state of TiCl4 with MgCl2—called mixed titanium/magnesium chloride by Magni and Somorjai [21]) compared to TiCl4. This observation therefore also confirms our results on supported Ziegler– Natta catalysts. Dispersion of active titanium species on support is important in qualitative evaluation of catalyst performance. This can be represented by surface Ti/Mg atomic ratio for these catalysts. We have used published sensitivity factors [24,25] for Ti 2p3/2
1 Magni and Somorjai [21,22] have used Au 4f7/2 at 84.0 eV as binding energy reference in their measurements.
was derived to correlate observed productivity of catalyst with the estimated titanium index. Calculated productivity in kg/gm catalyst using this relation is given in Table 2 along with the estimated error. These results show very good agreement (within 5%) between calculated and observed productivity values for these catalysts in slurry phase polymerization. 4. Conclusions Based on the present study it can be concluded that surface analytical technique ESCA can be used for correlating performance of supported PP catalysts. Two important parameters used in such correlation process are ‘‘titanium index’’ and electronic environment of titanium atoms on the surface of these catalysts.
756
V.K. Kaushik et al. / Applied Surface Science 253 (2006) 753–756
Acknowledgements Authors acknowledge the support provided by IPCL and RIL management for carrying out this study. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
V.K. Gupta, J. Polym. Mater. 16 (1999) 97. S. Bhaduri, V.K. Gupta, Curr. Sci. 78 (2000) 1082. K. Hasebe, H. Mori, M. Terano, J. Mol. Catal. A: Chem. 124 (1997) L1. J. Tian, B.T. Huang, Chin. Chem. Lett. 7 (1996) 67. P.G. Gassman, M.R. Callstrom, J. Am. Chem. Soc. 109 (1987) 7875. M. Furuta, J. Polym. Sci., Polym. Phys. Ed. 19 (1981) 135. S.H. Kim, D. Gnani, G.A. Somorjai, Polym. Prepr. (Am. Chem. Soc. Div. Polym. Chem.) 42 (2001) 331. V.K. Kaushik, M. Ravindranathan, Appl. Catal. 47 (1989) 343. V.K. Kaushik, Ch. Sivaraj, P. Kanta Rao, Appl. Surf. Sci. 51 (1991) 27. A. Wali, S.M. Piallai, V.K. Kaushik, S. Satish, Appl. Catal. 135A (1996) 83. A. Wali, S. Unnikrishnan, S.M. Pillai, V.K. Kaushik, S. Satish, J. Catal. 173 (1998) 84.
[12] S.C. Roy, H.L. Prasad, P. Dutta, A. Bhattacharya, B. Singh, S. Kumar, S. Mahajan, V.K. Kaushik, S.M. Pillai, Ravindranathan, Appl. Catal. 220 (2001) 153. [13] V.K. Gupta, S. Satish, I.S. Bhardwaj, J. Macromol. Sci: Pure Appl. Chem. A3 (1994) 451. [14] B.L. Goodall, A.A. Vander Nat, W. Sjardijn, U.S. Patent No. 4,414,132 (1983). [15] V.K. Kaushik, J. Electron Spectrosc. Relat. Phenom. 56 (1991) 273. [16] C. Sleigh, A.P. Pijpers, A. Jaspers, B. Coussens, R.J. Meier, J. Electron Spectrosc. Relat. Phenom. 77 (1996) 41. [17] P.B. Joshi, G.R. Marathe, N.S. Murthy, V.K. Kaushik, P. Ramakrishna, Mater. Lett. 56 (2002) 322. [18] H. Mori, K. Hasebe, M. Terano, J. Mol. Catal. 140A (1999) 165. [19] H. Mori, K. Hasebe, M. Terano, Polymer 40 (1999) 1389. [20] S.H. Kim, G.A. Somorjai, Surf. Interface Anal. 31 (2001) 701. [21] E. Magni, G.A. Somorjai, Catal. Lett. 35 (1995) 205. [22] E. Magni, G.A. Somorjai, J. Phys. Chem. 100 (1996) 14786. [23] A. Opdahl, S. Hoffer, B. Mailhot, G.A. Somorjai, Chem. Rec. 1 (2001) 101. [24] C.D. Wagner, L.E. Davis, M.V. Zeller, J.A. Taylor, R.M. Raymond, L.H. Gale, Surf. Interface Anal. 3 (1981) 211. [25] C.D. Wagner, in: D. Briggs, M.P. Seah (Eds.), Practical Surface Analysis, John Wiley & Sons, 1990.