TiO2/LaFeO3 Composites for the Efficient Degradation of Benzoic Acid and Hydrogen Production

Abstract

LaFeO₃/TiO₂ composites were prepared in the range 0-12.2 wt% of LaFeO₃, characterized, and tested for both benzoic acid (BA) and 4-methoxycinnamic acid (MCA) degradation in aqueous solution, and hydrogen evolution. The preparation method was via ball-milling without thermal treatment. The composite materials presented agglomerates of LaFeO₃ with an average size from 1 to 5 μm, and the TiO₂ powder was well dispersed onto the surface of each sample. They showed varying activities for BA degradation depending on composition and light wavelength. The 6.2 wt% and 12.2 wt%-LaFeO₃/TiO₂ composites exhibited the highest activity under 380-800 nm light and could degrade BA in 300 min at BA concentration 13.4 mg L^-1 and catalyst 0.12 g L^-1. Using a 450 nm LED light source, all composites degraded less than 10% of BA, but in the presence of H₂O₂ (1 mM) the photocatalytic activity was as high as 96% in <120 min, 6.2 wt%-LaFeO₃/TiO₂ composite being the most efficient sample. It was found that in the presence of H₂O₂, BA degradation followed first order kinetic with a reaction rate constant of 4.8 × 10^-4 s^-1. The hydrogen production rate followed a classical volcano-like behavior, with the highest reactivity (1600 μmol h^-1g^-1 at 60 °C) in the presence of 3.86%wt- LaFeO₃/TiO₂. It was also found that LaFeO₃/TiO₂ exhibited high stability in four recycled tests without losing activity for hydrogen production. Furthermore, a discussion on photogenerated charge-carrier transfer mechanism is briefly provided, focusing on lacking significant photocatalytic activity under 450 nm light, so p-n heterojunction formation is unlikely.

Keywords: benzoic acid degradation; lanthanum ferrite; photocatalytic hydrogen evolution; semiconductor photocatalyst.

Figure 1

(a) XRD patterns of B-D-E nanocomposites and the reference compounds TiO₂ and LaFeO₃; (b) SEM image of C; (c) SEM image of D; (d) SEM image of E; (e) SEM image of LaFeO₃.

Figure 2

Photocatalytic degradation of BA in the presence of samples A (TiO₂), B (1.3 wt%-LaFeO₃/TiO₂), C (3.9 wt%-LaFeO₃/TiO₂), D (6.2 wt%-LaFeO₃/TiO₂), E (12.2 wt%-LaFeO₃/TiO₂) and LF (LaFeO₃) from HPLC analysis. Experimental conditions: 0.120 g L⁻¹ catalyst loading, [BA] = 1 × 10⁻⁴ M, λ = 380–800 nm light.

Figure 3

Pathway of degradation of benzoic acid following ref. [28].

Figure 4

Fluorescence spectral patterns against UV irradiation time (t1 = 1 h) in the decomposition of benzoic acid in the presence of (a) sample A (TiO₂), (b) sample E, and (c) sample F (LaFeO₃). The excitation wavelength was 230 nm. Experimental conditions: 0.120 g L⁻¹ catalyst loading, [BA] = 1 × 10⁻⁴ M, λ = 380–800 nm radiation. The measured solutions were obtained from the supernatant after centrifugation of the corresponding suspensions containing the photocatalyst. The peaks at ca. 460 nm and 690 nm are instrumental background, as evidenced by the spectrum of pure water excited in the same conditions, which is reported in each panel (^….. line) to guide the eye. The expected location of the signals from benzoic and salicylic acids is indicated by yellow lines.

Figure 5

Photocatalytic degradation of BA in the presence of samples A (TiO₂), B (1.3 wt%-LaFeO₃/TiO₂), C (3.9 wt%-LaFeO₃/TiO₂), D (6.2 wt%-LaFeO₃/TiO₂), E (12.2 wt%-LaFeO₃/TiO₂) and LF (LaFeO₃), without H₂O₂ (a) and with H₂O₂ (b). Data from HPLC analysis. Experimental conditions: 0.120 g L⁻¹ catalyst loading, [BA] = 1 × 10⁻⁴ M, [H₂O₂] = 1 mM, under monochromatic 450 nm light. For reference, the degradation of BA in the presence of H₂O₂ without catalyst is shown (blue dotted line).

Figure 6

Photocatalytic hydrogen evolution rate (HER) at varying LaFeO₃ (%wt.) with respect to P25. Methanol was used as a model scavenger. Experimental conditions: [Methanol] = 2.5 M; 0.5 g L⁻¹ catalyst loading (Sample C); V = 0.3 L; T = 35 °C; natural pH of the solution.

Figure 7

Photocatalytic hydrogen evolution rate (HER) at varying temperature of the system. Methanol was used as a model scavenger. Experimental conditions: [Methanol] = 2.5 M; 0.50 g L⁻¹ catalyst loading (Sample C); V = 0.3 L; natural pH of the solution.

Figure 8

(A) Photocatalytic hydrogen evolution rate (HER) in successive photocatalytic tests. (B) Photocatalytic hydrogen evolution rate (HER) during a single photostability test. Methanol was used as a model scavenger. Experimental conditions: [Methanol] = 2.5 M; 0.50 g L⁻¹ catalyst loading (Sample C); V = 0.3 L; natural pH of the solution; T = 35 °C.

Figure 9

Scheme of the energy band structures and the correspondingly transfer process of photogenerated electron–hole pairs of semiconductor photocatalysts under UV light.