Nanocasting synthesis of chromium doped mesoporous CeO₂ with enhanced visible-light photocatalytic CO₂ reduction performance

Highlights

• Chromium doped mesoporous CeO₂ catalysts were synthesized by a nanocasting route.

• The Cr doped CeO₂ catalysts have a high surface area and uniform pore structure.

• Induced Cr³⁺ into CeO₂ can greatly extend the spectral response property.

• Enhanced surface chemisorbed oxygen species were observed in Cr doped CeO₂.

• The Cr doped CeO₂ catalysts exhibited an enhanced activity for CO₂ photoreduction.

Abstract

Chromium doped mesoporous CeO₂ catalysts were synthesized via a simple nanocasting route by using silica SBA-15 as the template and metal nitrates as precursors. The effect of Cr doping concentration (5%, 10%, 15% and 20% of the initial Cr/(Cr + Ce) molar percentage) on the structures of these catalysts and their photocatalytic performances in reduction of CO₂ with H₂O were investigated. The results indicated that the introduction of Cr species could effectively extend the spectral response range from UV to visible light region (400–700 nm) and improve the electronic conductivity for the mesoporous CeO₂ catalysts which exhibited an enhanced photocatalytic activity in the reduction of CO₂ with H₂O when compared with the non-doped counterpart. The highest CO and CH₄ yield of 16.2 μmol/g-cat. and 10.1 μmol/g-cat., respectively, were acquired on the optimal chromium doped CeO₂ catalyst with the initial Cr(Cr + Ce) molar percentage of 15% under 8 h visible-light irradiation, which were more than twice as high as that of bare CeO₂. The remarkably increased photocatalytic performance should be attributed to the advantageous structural and compositional features of the chromium doped mesoporous CeO₂.

Introduction

Carbon dioxide (CO₂), which accounts for more than 63% of the long-lived greenhouse gases (e.g. CH₄, N₂O), has been released tremendously from fossil fuel combustion in the past decades [1], [2]. The conversion of atmospheric CO₂ into industrially favorable chemicals is highly desired, for the sake of environmental protection and human society sustainability. Recently, a newly-found sustainable path is the photocatalytic reduction of CO₂ with H₂O into hydrocarbon fuels by semiconductors, since it helps with atmospheric CO₂ reduction and partly solves energy problems [3], [4]. Up to now, great amount of studies have been reported for the increased CO₂ photocatalytic conversion efficiency using various semiconductor photocatalysts. Among them, TiO₂-based photocatalysts are more attractive because titania is featured with high oxidation ability, low cost, non-toxicity and long-term photostability [5], [6], [7], [8]. However, the practical applications of TiO₂-based photocatalysts in the reduction of CO₂ with H₂O are still limited by the wide band gap and relative slow carrier transport of titania [9], [10], [11], [12].

It is well known that n-type semiconductor CeO₂ has some titania-like properties such as low cost, chemical stability, anti-photoirradiation stability and non-toxicity [13], [14], [15]. Recent progress shows that CeO₂ can absorb a larger fraction of solar spectrum compared to TiO₂ and its photoinduced electron-hole pairs have much longer lifetime [16], [17], [18]. Therefore, CeO₂ is a potentially efficient photocatalyst in many environmental applications [19], [20], [21]. We previously investigated the photocatalytic behavior of ordered mesoporous CeO₂ in the reduction of CO₂ with H₂O [22]. We found that the mesostructure can enhance its CO₂ adsorption ability, but its photocatalytic CO₂ conversion efficiency is still restricted by the rapid recombination of photoinduced electron-hole pairs for pure CeO₂, especially under visible-light irradiation. Foreign ion doping, which accelerates the charge separation and extends the energy range of photoexcitation, is an effective way to enhance the performances of semiconductor photocatalysts [23], [24], [25]. Among various elements, the transition metals such as Co, Fe, Cr, Cu and Mn are most widely-used dopants for optical and photo-electrochemical modification of semiconductors [26], [27], [28], [29], [30]. Theoretically, transition-metal dopants can improve the light-absorbing properties by introducing extra energy levels into the band gap of TiO₂ [31]. Moreover, the existent metal oxide cluster, due to the formation of heterojunctions, could accelerate the separation efficiency of photoinduced electron-hole pairs and thereby facilitate the electrons transfer from one to another.

In this work, we demonstrated the synthesis of a series of chromium doped mesoporous CeO₂ catalysts with 2D ordered hexagonal mesostructures and different Cr doping concentrations through the nanocasting route, by using ordered mesoporous silica SBA-15 as a template, and chromic and cerium nitrates were used as the Cr and Ce precursor, respectively. The textural and structural properties of the prepared catalysts were fully characterized via N₂ adsorption-desorption isotherms, transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and UV–vis diffuse reflectance spectroscopy (UV-vis DRS). Their photocatalytic performances for the reduction of CO₂ with H₂O under visible-light irradiation were thoroughly examined.