A DFT study of phenol adsorption on a low doping Mn–Ce composite oxide model

Density functional theory calculations (DFT+U) were performed on a low doping Mn–Ce composite oxide prepared from experimental data, including X-ray diffraction (XRD) and temperature-programmed reduction (TPR). We considered a 12.5% Mn–doped CeO2 solid solution with fluorite–type structure, where Mn replaces Ce4+ leading to an oxygen–deficient bulk structure. Then, we modeled the adsorption of phenol on the bare Ce0.875Mn0.125O1.9375(111) surface. We also studied the effect of water adsorption and dissociation on phenol adsorption on this surface, and compared the predictions of DFT+U calculations with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements. The experimental results allowed us to both build a realistic model of the low doping Mn–Ce composite oxide and support the prediction that phenol is adsorbed as a phenoxy group with a tilt angle of about 70° with respect to the surface.Facultad de Ciencias ExactasCentro de Investigación y Desarrollo en Ciencias Aplicada

Density functional theory study of water interactions on Mn-doped CeO2(1 1 1) surface

Spin-polarized density functional theory (DFT + U) periodic calculations have been performed to study water adsorption and dissociation on the 12.5% Mn-doped CeO2(1 1 1) surface. Our results indicated that Mn cation is the surface active site for water adsorption and dissociation reactions. The H2O molecule preferably adsorbs on a Mn cation, causing some relaxation of the surface O-layer and, thus, facilitating the bonding of one of the HH2O with the nearest oxygen atom. After overcoming an energy barrier of 0.46 eV, the water molecule could dissociate into OH and H species. The latter configuration is about 50% more exothermic than the molecular one, suggesting the Ce0.875Mn0.125O1.9375(1 1 1) surface would be easily hydroxylated under reaction conditions. In addition, the calculations showed that water adsorption on the Mn-doped CeO2(1 1 1) surface did not favor the creation of surface oxygen vacancies as it has been reported for pure CeO2(1 1 1). On the other hand, we created a surface oxygen defect in the slab with structural oxygen vacancies and computed water interactions on the reduced surface. Although, the adsorption of OH species in the O-hole caused many surface and subsurface atomic displacements, no changes in the oxidation state of Mn and Ce cations were detected.Fil: García Pintos, Delfina. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Química; ArgentinaFil: Juan, Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto de Física del Sur. Universidad Nacional del Sur. Departamento de Física. Instituto de Física del Sur; ArgentinaFil: Irigoyen, Beatriz del Luján. Universidad de Buenos Aires. Facultad de Ingeniería. Departamento de Ingeniería Química; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentin

Mn-Doped CeO2: DFT+U Study of a Catalyst for Oxidation Reactions

In this work, we performed DFT+U periodic calculations to study the geometric and electronic properties of 12.5% Mn-doped CeO2 solid solution. The doping with Mn allowed some Mn2+ cations to substitute Ce4+ ions into the CeO2 lattice and thus drove the formation of a stable O-deficient bulk fluorite-type structure. The Mn-doped CeO2(1 1 1) surface, generated upon the cleavage of the O-deficient bulk, exhibits Mn cations in a (3+) oxidation state. Spin-polarized energy calculations and charge analysis also evidenced the effect of Mn-dopant in facilitating the creation of surface oxygen vacancies; which reflected in extended surface and subsurface ions relaxation and reduction of Mn atoms located on surface and inner cationic layers. Concerning the oxidation state of Ce, it remained unaltered as Ce4+ when an O atom was removed from the topmost anionic layer of the surface system. Reduction of a Ce4+ cation to Ce3+ was evidenced after the creation of a second surface O-vacancy. Our results indicate facilitated surface oxygen release, Mn3+/Mn2+ redox couples formation, and promoted anionic mobility and can help to better understand the effect of Mn in enhancing Mn-doped CeO2 catalytic performance in oxidation reactions.Fil: Garcia Pintos, Delfina. Universidad de Buenos Aires. Facultad de Ingenierí­a. Departamento de Ingenierí­a Química; Argentina;Fil: Juan, Alfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - CONICET - Bahía Blanca. Instituto de Física del Sur; Argentina;Fil: Irigoyen, Beatriz. Universidad de Buenos Aires. Facultad de Ingenierí­a. Departamento de Ingenierí­a Química; Argentina