The importance of the Phillips catalyst in the industrial PE production makes Cr/SiO2 one of the most investigated systems [1] however, the structure of CrVI/SiO2 and of its relative CrII/SiO2 is still unresolved. This is mainly due to the amorphous nature of the support that prevents the use of diffraction techniques and is responsible of heterogeneity in the grafted species. Up to now also EXAFS spectroscopy, often used to investigate the local structure of amorphous systems, still didn’t fully elucidate the structure of CrII sites due to the low Cr loading (0.5 wt%) and to its high reactivity toward O2/H2O poisons. Highly diluted systems should be measured in fluorescence mode, a request that increases the acquisition times, making contamination problems more severe [1,2]. Finally, the absence of a structural model and the difficulty in treating open shell systems makes a computational approach not straightforward [3]. In situ, temperature dependent, XAFS experiment allowed us to prove that CrII species grafted on SiO2 surface are significantly extracted from the surface upon interaction with CO at 100 K, being the Cr-O bond distance elongated of 0.08 Å [4]. This first direct structural evidence quantifies the findings previously obtained via vibrational (IR and Raman) studies. All data allow to conclude that the absorption evolves into two coverage dependent steps: (i) displacement of weak ligands (surface siloxanes), resulting in non classical carbonyls; (ii) relaxation of the Cr-O surface bonds with subsequent optimization of the Cr•••CO distance and transformation into classical carbonyls [4]. [1] E. Groppo, C. Lamberti, S. Bordiga, G. Spoto and A. Zecchina, Chem. Rev., 105 (2005) 115. [2] E. Groppo, et al. J. Catal, 230 (2005) 98; G. Agostini, e al. J. Phys. Chem. C, 111 (2007) 16437. [3] A. Damin, et al. J. Phys Chem. A, 113 (2009) 14261. [4] D. Gianolio, E. Groppo, J. G. Vitillo, A. Damin, S. Bordiga, A. Zecchina, and C. Lamberti, Chem. Commun., 46 (2010) 976.
Direct evidence of adsorption induced CrII mobility on the SiO2 surface upon complexation by CO
Jenny G. Vitillo;
2010-01-01
Abstract
The importance of the Phillips catalyst in the industrial PE production makes Cr/SiO2 one of the most investigated systems [1] however, the structure of CrVI/SiO2 and of its relative CrII/SiO2 is still unresolved. This is mainly due to the amorphous nature of the support that prevents the use of diffraction techniques and is responsible of heterogeneity in the grafted species. Up to now also EXAFS spectroscopy, often used to investigate the local structure of amorphous systems, still didn’t fully elucidate the structure of CrII sites due to the low Cr loading (0.5 wt%) and to its high reactivity toward O2/H2O poisons. Highly diluted systems should be measured in fluorescence mode, a request that increases the acquisition times, making contamination problems more severe [1,2]. Finally, the absence of a structural model and the difficulty in treating open shell systems makes a computational approach not straightforward [3]. In situ, temperature dependent, XAFS experiment allowed us to prove that CrII species grafted on SiO2 surface are significantly extracted from the surface upon interaction with CO at 100 K, being the Cr-O bond distance elongated of 0.08 Å [4]. This first direct structural evidence quantifies the findings previously obtained via vibrational (IR and Raman) studies. All data allow to conclude that the absorption evolves into two coverage dependent steps: (i) displacement of weak ligands (surface siloxanes), resulting in non classical carbonyls; (ii) relaxation of the Cr-O surface bonds with subsequent optimization of the Cr•••CO distance and transformation into classical carbonyls [4]. [1] E. Groppo, C. Lamberti, S. Bordiga, G. Spoto and A. Zecchina, Chem. Rev., 105 (2005) 115. [2] E. Groppo, et al. J. Catal, 230 (2005) 98; G. Agostini, e al. J. Phys. Chem. C, 111 (2007) 16437. [3] A. Damin, et al. J. Phys Chem. A, 113 (2009) 14261. [4] D. Gianolio, E. Groppo, J. G. Vitillo, A. Damin, S. Bordiga, A. Zecchina, and C. Lamberti, Chem. Commun., 46 (2010) 976.
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