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Heterogeneity of deposited phases in supported transition metal oxide catalysts: reversible temperature-dependent evolution of molecular structures and configurations

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Abstract

In situ high-temperature Raman spectroscopy under steady state oxidative dehydrated conditions was used for determining the temperature dependence of the molecular structures and configurations of (MOx)n (M = Re, Mo, W) sites supported at low submonolayer loadings on TiO2(P25). Prior to the Raman analysis, the studied catalyst samples underwent calcination at 450–480 °C for 4–5 h. Regularly repeated random sequences of heating and cooling under flowing 20%O2/He (in the absence of incoming water vapor) in the 35–430 °C temperature range were shown to cause drastic changes in the vibrational properties of the 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. A heterogeneity of the deposited oxometallic phase was evidenced with three distinctly different species (i.e., MOx-I, MOx-II, and MOx-III) present in each system, each one prevailing in a particular temperature range. It was shown that the temperature could tune the molecular structure of the deposited oxometallic phase presumably on account of minima in the surface free energy. In the direction of temperature lowering, a mechanism leading to a hydrolysis-like of the anchoring bonds by activation of the surface hydroxyls and/or water molecules extant on the uncovered TiO2(P25) surface took place. In situ FTIR spectroscopy under identical conditions and similar temperature sequence protocols complemented the Raman results and corroborated the proposed prevailing configurations and pertinent band assignments.

Graphical abstract: Heterogeneity of deposited phases in supported transition metal oxide catalysts: reversible temperature-dependent evolution of molecular structures and configurations

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Publication details

The article was received on 27 Oct 2017, accepted on 04 Dec 2017 and first published on 06 Dec 2017


Article type: Paper
DOI: 10.1039/C7CP07286D
Citation: Phys. Chem. Chem. Phys., 2018, Advance Article
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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., 2018, Advance Article , DOI: 10.1039/C7CP07286D

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