Influence of the hydration by the environment humidity on the metallic speciation and the photocatalytic activity of Cr/MCM-41

V.R. ELIAS; E.V. SABRE; N.I. CUELLO; E. WINKLER; L. ANDRINI; S.G. CASUSCELLI; G.A. EIMER

Conferencia; 5th Czech-Italian-Spanish Conference on Molecular Sieves and Catalysis; 2013

M41S family of mesoporous silicates and their transition metal derivatives, have been extensively studied in order to enhance their properties as environmentally compatible catalysts. The molecular structure of metal oxides deposited on inorganic supports (as MCM-41) is of fundamental importance to understanding the behavior of these resulting composites as heterogenous catalysts. Particularly, Cr, a transitional metal of 3d series has a notable variability on their oxidation states and coordination number and thus, in their activity for different catalytic reactions. Recently, different metals were used in order to synthesize solids with photocatalyst properties and Cr has shown a good performance for organic compound degradation under visible radiation.In this work, MCM-41 silicates were modified with different Cr loadings by the wet impregnation method and then with Ti. Since the molecular structure of Cr on mesoporous molecular sieves is strongly dependent on the environmental conditions, here it is analyzed the influence of the environmental water presence. Several spectroscopic characterization techniques as UV-Vis DR, ESR and XAS were employed in order to infer about the Cr speciation. It could be observed that different period of exposure on hydrated environmental conditions have a remarkable influence on the Cr speciation and, as a consequence, on the activity of Cr/MCM-41 solids in the Acid Orange 7 photodegradation. On the contrary, the Cr speciation and the photoactivity of the samples after Ti loading(TiO2/Cr/MCM-41) were practically not influenced by the environmental conditions. These observations prove the shield effect of the TiO2 cover on the Cr species, which protects them from water presence. The several spectroscopic analyses allow us to conclude about the importance of the Ti presence on the active Cr species stabilization.