Molybdena deposited on titania by equilibrium deposition filtration: structural evolution of oxo–molybdenum(vi) sites with temperature

Abstract

The equilibrium deposition filtration (EDF) method, an advanced catalyst synthesis route that is based on a molecular level approach, can be used for tailoring the oxometallic phase deposited on a porous oxide support. Here, the EDF method is used for synthesizing (MoO_x)_n/TiO₂ catalysts. In situ Raman spectroscopy in the temperature range of 25–450 °C, low temperature (77 K) EPR spectroscopy and DR-UV spectroscopy are used for studying the evolution of the structural configuration of oxo–Mo^VI species on TiO₂ with increasing temperature as well as the influence of the supported (MoO_x)_n species on the photo-generation of electrons and holes of TiO₂. This study concerns (MoO_x)_n/TiO₂ samples in which the surface densities after calcination are 0.3, 2.6 and 3.9 Mo per nm², thereby covering a very wide range of submonolayer coverage. The gradual heat treatment of the catalysts results in a transformation of the initially (prior to drying) deposited species and the pertinent species evolution at the nano-level is discussed by means of a number of mechanisms including anchoring, association, cleavage and surface diffusion.