Improving water splitting using RuO₂-Zr/Na₂Ti₆O₁₃ as a photocatalyst

Highlights

• Hydrogen production by water splitting was reported using RuO₂-Zr/Na₂Ti₆O₁₃ as a photocatalyst.

• 2%Ru:Zr/Na₂Ti₆O₁₃ shows a good activity and stability for hydrogen evolution.

• The improvement on the catalytic activity of the impregnated materials suggested a synergistic effect between the Zr and RuO₂, which acted like an electron trap.

Abstract

Na₂Ti₆O₁₃ and Zr/Na₂Ti₆O₁₃ prepared by the sol–gel method and impregnated with different amounts of RuO₂ (0.1–10 wt%) were evaluated as photocatalysts for water splitting. The calcined materials at 800 °C were characterized by X-ray diffraction (XRD) analysis, Raman spectroscopy, UV–vis spectroscopy with diffuse reflectance spectroscopy (DRS), N₂ physisorption, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The physicochemical characterization confirmed that both materials had the tunnel structure. However, the Zr/Na₂Ti₆O₁₃ sample revealed high Zr dispersion. The ideal amount of RuO₂ to load the samples with was shown to be 2.0 wt %, which produced hydrogen at a rate of 265 μmol h⁻¹. The apparent quantum yield efficiency was 21%. The improvement on the catalytic activity of the impregnated materials suggested a synergistic effect between the Zr and RuO₂, which acted like an electron trap.

Introduction

In the next 40 years, it will be necessary to produce 50% more energy [1]. To reach this goal, natural sources as water, solar light, etc. will be used instead of fossil fuels in accordance with international environmental agreements to avoid emitting pollutants during the combustion process [2], [3].

Hydrogen has been considered seriously as a potential candidate to provide clean energy in the green energy research field because of its high energy content per unit mass. That is, 1 kg of hydrogen gas can produce an equivalent amount of energy as 1 gallon of gasoline [4]. Ninety-five percent of hydrogen (H₂) is produced industrially by reforming the steam of hydrocarbons, such as methane, and 5% is produced by the electrolysis of water [5], [6]. However, electrolysis consumes large amounts of energy. Therefore, recent research has been focused on producing hydrogen using economical and sustainable processes [7]. One way to approach this challenge has taken advantage of two of the main resources available on the planet: water and solar energy. The photo-induced decomposition of water into H₂ and O₂ is a potential technology to convert the energy of photons into chemical energy [8], [9], [10], [11], [12], [13].

Since Honda–Fujishima used TiO₂ in the electrochemical photolysis of water splitting in 1972 [14]; many efforts have been devoted to finding new photocatalytic materials. In 1991, Inoue et al. [15] investigated the photoexcitation ability of alkaline metal hexatitanates (M₂Ti₆O₁₃, M = Na, K and Rb) with the tunnel structure and that of the hexatitanates where M = Cs with the layered structure. It was found that the photocatalytic activity decreased in the order of Na > K > Rb > Cs, and the best photocatalytic performance was below 15 μmol h⁻¹ under visible light [15]. The photoactivity of the titanates has been attributed to the presence of distorted TiO₆ octahedra whose stronger dipole moment allows electron-hole separation. The authors correlated the increase in activity with the decreasing size of the cation. One of the limitations of titanates as photocatalysts is that they generally have very low specific surface areas and are prepared by the solid state method. To increase the specific surface areas, Na₂Ti₆O₁₃ and Na₂Ti₃O₇ have been prepared by different soft chemistry methods. However, only Na₂Ti₆O₁₃ has shown attractive photocatalytic properties [16]. On the other hand, Zang et al. [17] have reported promising photocatalytic results using BaTi₄O₉/RuO₂ as photocatalysts for water splitting.

To improve the photoactivity of the titanates, a large variety of substituent ions has reportedly been used: (i) the transition metal ions: Ti, Zr, Nb, Ta and W with d⁰ electronic configuration; (ii) the rare earth metal ion Ce and (iii) the typical metal ions: Ga, In, Ge and Sb [18], [19], [20]. The metal ions with d⁰ or d¹⁰ electronic configurations have been reported as promising materials for water decomposition [21]. With this in mind, Na₂Ti₆O₁₃ and Zr/Na₂Ti₆O₁₃ semiconductors were prepared by soft chemistry (sol–gel method) and impregnated with RuO₂ (0.1–10 wt.%) in the present work to improve the photoactivity of titanates. The as-prepared solids were characterized by XRD, nitrogen adsorption, Raman and UV–vis spectroscopy and observed using TEM.