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Heterogeneity of Deposited Phases in Supported Transition Metal Oxide Catalysts: Reversible Temperature-Dependent Evolution of Molecular Structures and Configurations

Abstract

In situ high temperature Raman spectroscopy under steady state oxidative dehydrated conditions is used for capturing the temperature dependence of molecular structures and configurations of (MOx)n (M = Re, Mo, W) sites supported at low loadings on TiO2(P25). Prior to Raman analysis, the studied catalyst samples have undergone calcination at 450–480oC for 4–5 h. Regularly repeated random sequences of heating and cooling under flowing 20%O2/He (in absence of incoming water vapors) in the 35 – 430oC temperature range are shown to cause drastic changes in the vibrational properties of M–O stretching modes and in the molecular structures and configurations of the deposited ReOx, MoOx and WOx sites in a reversible and reproducible manner. Three distinctly different species (i.e. MOx-I, MOx-II and MOx-III) are present in each system, each one prevailing at a particular temperature range. It is shown that temperature can tune the molecular structure of the deposited oxometallic phase, presumably on account of minima in surface free energy. In the direction of temperature lowering, a mechanism leading to hydrolysis of anchoring bonds by activation of surface hydroxyls and/or water molecules extant on the uncovered TiO2(P25) surface takes place. In situ FTIR spectroscopy under identical conditions and similar temperature sequence protocols complements the Raman results and corroborates the proposed prevailing configurations and pertinent band assignments

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Publication details

The article was accepted on 04 Dec 2017 and first published on 06 Dec 2017


Article type: Paper
DOI: 10.1039/C7CP07286D
Citation: Phys. Chem. Chem. Phys., 2017, Accepted Manuscript
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    Heterogeneity of Deposited Phases in Supported Transition Metal Oxide Catalysts: Reversible Temperature-Dependent Evolution of Molecular Structures and Configurations

    C. Andriopoulou and S. Boghosian, Phys. Chem. Chem. Phys., 2017, Accepted Manuscript , DOI: 10.1039/C7CP07286D

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