Surface-Enriched Boron-Doped TiO2 Nanoparticles as Photocatalysts for Propene Oxidation

Abstract

A series of nanostructured boron-TiO₂ photocatalysts (B-X-TiO₂-T) were prepared by sol-gel synthesis using titanium tetraisopropoxide and boric acid. The effects of the synthesis variables, boric acid amount (X) and crystallization temperature (T), on structural and electronic properties and on the photocatalytic performance for propene oxidation, are studied. This reaction accounts for the remediation of pollution caused by volatile organic compounds, and it is carried out at low concentrations, a case in which efficient removal techniques are difficult and costly to implement. The presence of boric acid during the TiO₂ synthesis hinders the development of rutile without affecting the textural properties. X-ray photoelectron spectroscopy analysis reveals the interstitial incorporation of boron into the surface lattice of the TiO₂ nanostructure, while segregation of B₂O₃ occurs in samples with high boron loading, also confirmed by X-ray diffraction. The best-performing photocatalysts are those with the lowest boron loading. Their high activity, outperforming the equivalent sample without boron, can be attributed to a high anatase and surface hydroxyl group content and efficient photo-charge separation (photoelectrochemical characterization, PEC), which can explain the suppression of visible photoluminescence (PL). Crystallization at 450 °C renders the most active sample, likely due to the development of a pure anatase structure with a large surface boron enrichment. A shift in the wavelength-dependent activity profile (PEC data) and the lowest electron-hole recombination rate (PL data) are also observed for this sample.

Scheme 1. Illustration of the Photocatalysts Preparation and Composition/Structure

Scheme 2. Drawing of the Variation of the Propene Concentration under Illumination (in Presence of a Photocatalyst), Parameters as Indicated Above.

Figure 1

XRD spectra for B-X-TiO₂-550 samples. Inset image: amplification of Figure 1 in the 24–30° 2θ range.

Figure 2

XPS data of (a) B 1s, (b) O 1s, and (c) Ti 2p for the B-X-TiO₂-550 samples. The original data are plotted for B 1s, whereas for O 1s and Ti 2p, normalized data are presented (to better observe the shifts).

Figure 3

O 1s XPS deconvoluted spectra for the B-X-TiO₂-550 samples.

Figure 4

Propene conversion (%) at 30 and 60 mL/min for B-X-TiO₂-550 photocatalysts and P25.

Figure 5

CV scans in the dark (a) and under full solar irradiation (b) using various TiO₂ specimens as working electrodes. Scan rate = 20 mV s^–1, irradiation intensity ∼100 mW cm^–2.

Figure 6

(a) PL spectra of B-0-TiO₂-550, B-5-TiO₂-550, and TiO₂-P25. (b) Expansion of the spectral range between 405 and 485 nm, showing the Raman scattering spectra in detail (the scale has been changed to wavenumbers, and intensity is represented in the logarithmic scale, for clarity).

Figure 7

(a) Transient photocurrent measurements at a constant potential of 0 V vs Ag/AgCl and solar irradiation power of 70 mW cm^–2 for electrodes containing P25, B-5-TiO₂-450, B-0-TiO₂-550, and B-5-TiO₂-550. (b) Normalized photocurrent as a function of the irradiation wavelength at a constant potential of 0.4 V vs Ag/AgCl.

Figure 8

PL spectra of B-5-TiO₂-550, B-5-TiO₂-450, and B-5-TiO₂-350 photocatalysts. The intensity of the spectra were normalized using the intensity of the anatase TiO₂ Raman peaks, which should be proportional to the amount of excited material.