Mechanism of MoO3 dispergation on SiO2 surface

Molybdena dispergation on silica surface via heat treatment of dry mixtures of MoO3 and the dispersed silica support is a convenient preparative technique of the synthesis of surface Mo(VI) oxo-species. The driving force for the thermally induced dispergation of bulk MoO3 is the decrease in surface energy which is lower for surface Mo(VI) oxo-species. The aim of the present study is a quantum chemical modelling of the mechanism of MoO3 molecule interaction with ?Si–OH groups of silica surface via reaction 2(?Si–OH) + MoO3 ? (?Si–O–)2Mo(=O)2 + H2O resulting in the formation of (?Si–O–)2Mo(=O)2 Mo(VI) oxo-species. Restricted Hartree-Fock method (MO LCAO approximation) using the SBKJC (Stevens-Basch-Krauss-Jasien-Cundari) valence-only basis set was used. Si10O12(OH)16 cluster was used as a model for highly hydroxylated silica surface in which silicon atoms in the core siloxane chains are saturated by four silanol and six silanediol groups. Such a cluster was considered as providing a realistic structure for the SiO2 surface and exhibiting high stability due to its large size. The optimised geometry of MoO3 molecule of C3v symmetry was used in calculations. Quantum chemical simulation of the reaction Si10O12(OH)16 + MoO3 ? Si10O12(OH)14O2MoO2 + H2O accompanied by the formation of surface (?Si–O–)2Mo(=O)2 Mo(VI) oxo-species was undertaken. We considered two structures of (?Si–O–)2Mo(=O)2 Mo(VI) oxo-species attached to Si10O12(OH)16 silica cluster via nearby and distant ?Si–OH groups. At 700 K, when molybdena dispergation over silica surface begins, the Gibbs energy of the formation of (?Si–O–)2Mo(=O)2 Mo(VI) oxo-species via reaction of MoO3 molecule with nearby and distinct ?Si–OH groups of Si10O12(OH)16 cluster was found to be ?260 and ?337 kJ/mol, respectively. The mechanism of the reaction of MoO3 molecule with distant ?Si–OH groups of Si10O12(OH)16 cluster accompanied by the formation of (?Si–O–)2Mo(=O)2 Mo(VI) oxo-species was considered as more favourable energetically. It has been found that at 700 K the reaction proceeds in two stages and requires overcoming of the activation barriers of 161 and 154 kJ/mol.