Abstract
Abstract Catalytic capacity of ceria mainly stems from a facile switch in the Ce oxidation states from +4 to +4 − x. While various experimental and computational studies pinpoint the reduction chemistry of Ce atom through the creation of oxygen vacancies, the analogous process when ceria surface is decorated with cations remains poorly understood. Where such results are available, a synergy between experimental and first principle calculation is scarce. Niobium materials are evolving and their use in catalysis is being widely investigated due to their high surface acidity and thermal and chemical stability. This study aims to report structural and electronic properties of various configurations of mixed Ce–Nb oxides and elaborates on factors that underpin potential catalytic improvements. Evaluations of the samples through X-ray diffraction (XRD), Fourier transform infrared (FTIR), N2-adsorption–desorption, scanning electron microscope (SEM), energy dispersive spectroscope (EDS), and thermogravimetric (TGA) analyses are examined and discussed. First principles density functional theory (DFT) calculations provide structural features of the Ce–Nb solutions at low concentration of Nb via computing atomic charge distribution. Contraction in the lattice parameter after Nb doping was confirmed with both XRD and DFT results. SEM analysis reveals particle growth at the loading of 50 wt%. FTIR results established the Ce–Nb–O bond at 1,100 cm−1 and the TGA analysis confirms the thermal stability of Nb-doped ceria. Tetrahedral O atoms demonstrate an increase in electronegativity and this in turn facilitates catalytic propensity of the material because the O atoms will exhibit higher affinity for adsorbed reactants. Cerium oxide (CeO2) after Nb doping displays a noticeable band gap narrowing, confirming the possible improvement in the catalytic behavior. The 4d states of the Niobium pentoxide (Nb2O5) is found to fill up the 4f states of CeO2 around the Fermi energy level promoting electrons excitation in the CeO2. Reported electronic, structural, and thermal characteristics herein indicate promising catalytic applications of niobium-promoted ceria.
Highlights
Doped or pure metal oxides have been widely deployed to improve the catalytic activity and selectivity of both oxidation and reduction reactions through substitution of the cation of the base metal oxide with the cation of a second metal oxide [1]
Nb2O5 is found to improve the properties of CeO2 in terms of the narrowed band gap and the electronic states of the Ce and O atoms
Experimental results of X-ray diffraction (XRD) confirm the formation of new peaks associated with Nb at high loading, and the energy dispersive spectroscope (EDS) analysis detects the presence of Nb
Summary
Doped or pure metal oxides have been widely deployed to improve the catalytic activity and selectivity of both oxidation and reduction reactions through substitution of the cation of the base metal oxide with the cation of a second metal oxide [1]. The promotional effect of Nb2O5 as a second metal or metal oxide in operations such as catalysis is gaining attention due to the relative ease of their reaction with other metal oxides to form mixed metal oxide phases with a complex structure [6]. This complex structure could result from the distortion of niobium oxide deposited on the parent metal or originated from the actual disruption of the chemical bonds at the surface of the parent oxide [1,7]. Electron transfer occurs, and the active centers are the dopants or the oxygen atoms close to the dopants, leading to a material behavior that departs from the pure metal oxides [1]
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