Nb-Doped TiO2 Photocatalysts Used to Reduction of CO2 to Methanol

Received Jun 13, 2017; Revised Sep 26, 2017; Accepted Oct 15, 2017

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1. Introduction

The development of photocatalysis began in 1972 [1], aiming to create efficient water treatment systems and to produce H₂ to use as fuel. Various researchers have conducted investigations in many different fields, such as semiconductor photoelectrochemistry [2], photocatalysis [3, 4], and photoreduction of CO₂ in aqueous environments, to produce methanol [5]. The TiO₂ produced by Evonik, known as TiO₂–P25 (consisting of 75% anatase phase and 25% rutile phase), is the mostly used for this purpose due to its excellent photoactivity and is also used as a reference to compare results with others news materials. TiO₂–P25 has high surface area (∼50 m²/g), and its complex crystalline microstructure is directly related to the synthesis method [6].

The photocatalysis process consists in the exposition of semiconductor powder to ultraviolet light radiation which will create then the electron-hole pair from the electronic excitation. The hole (h⁺) is formed in the valence band, and the electrons (e⁻) are formed in the conduction band. These species can react with adsorbed water or hydroxyl group and produce hydroxyl (${\text{OH}}^{•}$) radical and superoxide anion (${\mathrm{O}}_2^{•-}$) radical, starting an oxidation and reduction process and then degrading the component of interest [7–9].

The addition of transitional metals to TiO₂ seeks to increase the recombination time of electron/hole pair during electronic excitation [10, 11]. It is believed that these transitional metals create acceptor/donator electron centers, influencing the electronic recombination. Perazolli et al. [12] synthesized a ternary mixture of Ti, Sn, and Ag oxides that obtained better Rhodamine B discoloration results than P25 due to the heterojunction TiO₂/SnO₂ and formation of metallic Ag reduced from Ag₂O increasing the recombination time. The utilization of heterostructures has been studied in degradation of organic compounds. Through the solvothermal synthesis, Li et al. [13] synthesized pure anatase, pure rutile, and mixed anatase-rutile cake-like particles and observed that anatase-rutile particles reached higher...