Unraveling the Origin of Photocatalytic Deactivation in CeO2/Nb2O5 Heterostructure Systems during Methanol Oxidation: Insight into the Role of Cerium Species

Abstract

The study provides deep insight into the origin of photocatalytic deactivation of Nb₂O₅ after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO₂/Nb₂O₅ heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV-visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb₂O₅ and CeO₂) and as-prepared CeO₂/Nb₂O₅ heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by operando-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO₂/Nb₂O₅ heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb₂O₅ to CeO₂. However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.

Figure 1

(a) Activity of catalysts in methanol photooxidation at the steady state for 2 h of reaction. (b) Graph presenting differences between activity of ceria-modified catalysts prepared by wet impregnation, mechanical mixtures of metal oxides, and theoretical methanol conversion expected at the given concentration of Nb₂O₅ and CeO₂ in the composite materials (theoretical conversion of methanol was estimated by summing up activity of given amounts of commercial CeO₂ and parent Nb₂O₅ in the heterostructures). A mechanical mixture of metal oxides was prepared using parent Nb₂O₅ and commercial CeO₂ (see Section 2).

Figure 2

FTIR spectra of catalyst surfaces at the steady state during methanol adsorption under dark conditions.

Figure 3

(a) FTIR spectra of catalyst surfaces recorded at the beginning of photocatalytic oxidation of methanol under UV light (λ = 365 nm). (b) Graph presenting changes in intensity of the most intense IR band typical for adsorbed formate species (i.e., band at 1577 cm^–1 for Nb₂O₅ and Ce1/Nb₂O₅; band at 1584 cm^–1 for Ce5/Nb₂O₅ and Ce10/Nb₂O₅; and band at 1565 cm^–1 for commercial CeO₂) at the beginning of photocatalytic process. (c) Relationship between the activity of catalysts and increase in intensity of the most intense IR band typical of adsorbed formate species. The variations of the IR band’s intensities (in panels (b) and (c)) were measured by the dedicated tool in OMNIC software, after having subtracted the IR spectrum of the catalyst at the equilibrium steady state during methanol adsorption under dark conditions, from the IR spectrum after about 25 min of photocatalytic reaction.

Figure 4

PXRD patterns of catalysts normalized to the intensity of the (001) peak.

Figure 5

(a) Bright-field low magnification TEM image of the representative ceria-modified sample prepared by wet impregnation. (b) Low magnification HAADF-STEM image and simultaneously acquired EDX elemental mapping of Nb L, Ce L, and O K. (c) HRTEM and (d) high-resolution HAADF-STEM images of the representative Ce/Nb₂O₅ sample and corresponding ring ED pattern indexed based on the orthorhombic Cmmm Nb₂O₅ structure (a = 6.62 Å; b = 3.60 Å, c = 3.91 Å) obtained from PXRD.

Figure 6

HAADF-STEM (left panel) and simultaneously acquired ABF-STEM (right panel) high-resolution images along the two main crystallographic zone axes, (a) [010] and (b) [110], of the orthorhombic Cmmm Nb₂O₅ structure (a = 6.62 Å; b = 3.60 Å, c = 3.91 Å). The magnified [010] HAADF-STEM and ABF-STEM images together with the overlaid structural model are given as an inset in (a) (Nb atoms, orange spheres; O atoms, blue spheres).

Figure 7

(a) Low-temperature nitrogen adsorption–desorption isotherms of catalysts. (b) Pore size distribution estimated for the catalysts from the adsorption branch of N₂ isotherms using the BJH method.

Figure 8

(a) Diffuse-reflectance UV–vis spectra of catalysts. (b) Results of band gap estimation using the Tauc plot method for selected catalysts.

Figure 9

Nb 3d + Ce 4p (left) and Ce 3d (right) XP spectra of different catalysts studied in this work.

Figure 10

Photoluminescence spectra of catalysts.

Figure 11

Schematic representation of charge transfer process resulting in deactivation of ceria-modified Nb₂O₅ samples prepared by wet impregnation; CB: conduction band, VB: valence band, V₀: energy level of defects (mainly oxygen vacancies, O_V) localized below the Ce 4f band of highly deficient ceria species (CeO_2–x) loaded on Nb₂O₅.