Zn-Cu promoted TiO₂ photocatalyst for CO₂ reduction with H₂O under UV light

Highlights

• TPD-CO₂ results for TiO₂ samples have recorded different strengths of CO₂ adsorption.

• Cu¹⁺ and Cu²⁺ species coexist in CT and CZT photocatalysts.

• CH₄ production achieved the range of 126–184 μmol/g after 24 h-irradiation period.

• CH₄ formation has increased in the following order: TiO₂ (P-25) ∼ TiO₂ < CT < CZT.

• The interaction between CO₂ and photocatalyst has influence on photoactivity.

Abstract

The photocatalytic reduction of CO₂ has been studied aiming to find a useful application for such low cost and abundant raw material. Besides reducing CO₂ in the atmosphere, the process can contribute for the generation of high energy products (CH₄ and CH₃OH). The reaction was performed in liquid phase, batch, at 25 °C, with the photocatalyst (1 g/L) maintained in suspension. UVC lamp (18 W, 254 nm) was chosen as the radiation source. Photocatalysts were prepared using oxides of titanium, copper and zinc. Commercial TiO₂ (P-25, Degussa) was utilized as reference. Techniques such as N₂ adsorption, XRF, SEM-EDS, XRD, XPS, DRS UV–vis and TPD-CO₂ were used for photocatalysts characterization. Catalysts having specific area ranging from 36 to 52 m²/g and bandgap energies varying from 3.0 to 3.3 eV were obtained. TPD-CO₂ results showed different strengths of CO₂ adsorption for each photocatalyst. In the performance tests, CH₄ production achieved the range of 126–184 μmol/g_cat after a 24 h-irradiation period. Regarding the photocatalysts tested, it was observed increased CH₄ formation in the following order: TiO₂ (P-25) ∼ TiO₂ < 2%CuO/TiO₂ < 2%CuO–19%ZnO/TiO₂. Results indicate that the interaction between CO₂ and the photocatalyst influences the photocatalytic activity.

Introduction

As a consequence of new modes of production, since the Industrial Revolution, technology has had a profound effect on socioeconomic and environmental conditions. Atmospheric concentrations of CO₂ have grown exponentially since that period and nowadays it is considered the gas that mostly contributes to the greenhouse effect [1], [2], [3]. This assertion is questioned in numerous studies linking global warming to Earth’s natural aging process and not to human activities. Even with those divergent views, there is a consensus toward the need to reduce CO₂ emissions.

Among the alternatives to decrease atmospheric CO₂ concentrations, CO₂ capture and sequestration is one of the most studied [4]. In this scenario, CO₂ reuse in chemical processes became an important issue and alternatives such as heterogeneous catalytic and electrocatalytic conversions were brought to light. The development of alternative production processes using CO₂ as a non-expensive feedstock or co-feeding may actually help to reduce the inconvenience caused by the uncontrolled release of gas.

The artificial photosynthesis process was named as such because of the similarity to nature’s photosynthesis performed by plants [5], [6]. While plants transform solar energy in chemical, with the help of chlorophyll, generating O₂ and glucose, CO₂ photocatalytic reduction is assisted by a photocatalyst and UV irradiation, transforming water and CO₂ into organic compounds of high energy level. Therefore, researchers dealing with artificial photosynthesis have great interest in environmental issues [7], [8], [9]. The choice for CO₂ photocatalytic reduction represents a sustainable and economically viable alternative: that is, it enables the use of both sunlight as a radiation source and CO₂ as raw material. The most employed photocatalysts in that type of reaction are semiconductor materials, such as TiO₂, ZnO and ZrO₂ [7], [10], [11], [12], [13].

Titanium dioxide (TiO₂) and zinc oxide (ZnO) are the most common materials used in heterogeneous photocatalysis for bringing the features of low cost, low toxicity, photostability, high catalytic activity [14], activation by sunlight and, finally, chemical stability over a wide pH range. The energy required to activate both oxides, TiO₂ and ZnO, by light is approximately 3.2 eV, which corresponds to UV radiation of wavelength shorter than 387 nm [15]. That characteristic enables the use of sunlight as the radiation source, since the wavelengths in this range represent approximately 3% of the solar spectrum that reaches the Earth́s surface. Watanabe [16] has shown that ZnO can reduce CO₂ with a reductant molecule – H₂O or H₂ – under high pressures of 25–35 kg/cm² of CO₂ gas generating oxygenated compounds. Pérez-Larios et al. [17] have used TiO₂–ZnO mixed oxides to improve from water splitting. The results showed an activity six times higher for TiO₂–ZnO-X mixed oxides than that for bare TiO₂ semiconductor.

Theoretical and experimental studies have shown that the crystal phase of TiO₂ and the defect disorders in TiO₂ have influence on CO₂ adsorption, activation and dissociation processes [13]. According to Rodriguez et al. [18], the presence of defects on TiO₂ surface induces the formation of new adsorption configurations, in which CO₂ is bonded at the defect sites. Electrons stored in the oxygen-deficient (V_O) can be spontaneously transferred to CO₂ and once the CO₂⁻ is formed, the radical may decompose into CO through the occupation of one oxygen atom into the V_O site.

Regarding photoreduction and photooxidation applications, the effect of metallic dopants, such as Ce, Cu, CuO, Pt, Au and Ag, in TiO₂ has also been reported [19], [20]. However, a high concentration of metal/metal oxide can generate recombination centers of electron-hole pairs, leading to a reduction of photocatalytic efficiency. Another possibility is the use of nonmetals as doped and co-doped materials (for instance, C, N, S, F, etc.), which results in a significant narrowing of the bandgap, if compared to metal doping, leading to high photocatalytic efficiency under visible light irradiation [21], [22], [23], [24], [25], [26].

Copper oxide (CuO) has been extensively studied in CO₂ photocatalytic reduction for several reasons. First, the presence of copper associated to the photocatalyst has an important role in CH₃OH production, increasing significantly its production if compared to a photocatalyst without promoter. Another reason for using CuO is the redistribution of electric charge on the surface of the semiconductor support [10], [19], [27]. According to Tseng et al. [28], copper plays the role of a trapper of electrons preventing the recombination electron-hole pair and consequently promoting significant increase in photoefficiency. They have observed that the highest CO₂ conversion occurred when Cu content was 2% and that large amounts of photocatalyst in the reaction may hinder the diffusion of UV irradiation [28]. Slamet et al. [15] found that the photocatalysts promoted by CuO showed better performance than the materials promoted by the species Cu⁰ and Cu⁺¹.

In the present study, the influence of Zn–Cu deposition on the photoactivity of TiO₂ has been investigated in the photocatalytic reduction of CO₂ with H₂O to produce C1 fuels, such as CH₄ or CH₃OH. The direct impact of the presence of the brookite phase, instead of rutile phase, on the photocatalytic properties was also investigated. Additionally, CO₂ adsorption capacity of the photocatalysts was measured by temperature programmed CO₂ desorption to evaluate CO₂ interaction with the photocatalysts.