Synthesis, characterization and photocatalytic evaluation of MWO4 (M = Ni, Co, Cu and Mn) tungstates

doi:10.1016/j.ijhydene.2016.10.117

International Journal of Hydrogen Energy

Volume 41, Issue 48, 28 December 2016, Pages 23312-23317

https://doi.org/10.1016/j.ijhydene.2016.10.117 Get rights and content

Highlights

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Photocatalysts for the production of H₂ from water splitting.
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CuWO₄, CoWO₄, NiWO₄, and MnWO₄ photocatalysts for H₂ production under visible light.
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Photocatalytic performance to H₂: MnWO₄ > CoWO₄ > NiWO₄ > CuWO₄.

Abstract

Tungstates have been very scarcely studied as photocatalysts for the production of hydrogen from the dissociation of the water molecule. The aim of the present study is the synthesis and characterization of family tungstate MWO₄ (M = Co, Cu, Mn and Ni) materials to evaluate their photocatalytic activity towards the production of hydrogen within the visible light range. CuWO₄, CoWO₄, NiWO₄, and MnWO₄ were characterized by XRD, BET, UV–Vis and SEM. Band gap energies of these tungstates fell within the visible light spectrum from 2.24 eV for CoWO₄ to 2.56 eV for MnWO₄, respectively. Maximum hydrogen production was achieved by MnWO₄ with 139 μmolH₂/gcat, while the lowest production was observed for sample CoWO₄ with 24 μmolH₂/gcat. From these results it can be inferred that metal transition tungstates can be considered as potential photocatalysts for H₂ production via the splitting of the water molecule under visible light irradiation.

Introduction

Catalysis is defined as a change in reaction rate due to an agent called catalyst. This catalyst not only makes possible the chemical reaction to occur at a faster rate, but also lowers its activation energy, while not being part as a reactant or product. When the catalytic reaction is activated through light absorption it is called photocatalysis [1], and this is defined as a chemical reaction induced by photo-irradiation in the presence of a photocatalyst [2]. Such a material facilitates chemical reactions without being consumed or transformed. This process starts when a semiconductor material is illuminated with a suitable wavelength light that must be equal or higher to the width of the band gap energy of a semiconductor. The general idea of using a photocatalyst to split the water molecule consists in using the oxidizing and reducing effect of the charges that are generated in a semiconductor [1]. Photocatalysis can be applied to solve environmental and energy problems [3], [4], [5], which include hydrogen production from water splitting. Some of the advantages of the photocatalysis are: low processing costs, the evolution of hydrogen and oxygen during the water splitting reaction and suitable small reactor systems for domestic applications, thus providing a huge potential market [2]. Scheelite and wolframite type compounds such as tungstates (AWO₄) are part of an important family of materials from a technological point of view. According to the literature, Rajagopal et al. [6] report tungstates with different and interesting properties, that draw attention, because they exhibit potential applications in different technological areas [7], such as; state solid laser [8], microwave, scintillation [9], optoelectronic devices, optic fibers [6], humidity sensors and fluorescent lamps due to their photoluminescence appealing properties, meanwhile some others tungstates are of special interest due to their conductivity and electrical properties [10]. Considering their optical and electronic properties, these materials can be seen as potential photocatalysts towards the hydrogen production.

Moreover, the coprecipitation synthesis route is a simple, fast and a soft chemical method. Furthermore, it has many advantages such as; low calcination temperature, low cost, and above all it does not require special operating conditions [11]. This technique is probably the most used to prepare nanometric powders [12]. Garcia-Perez et al. [13] carried out the synthesis of bivalent transition metal tungstates such as; Co²⁺, Cu²⁺, Mn²⁺ and Ni²⁺ prepared through coprecipitation, where CuWO₄ series presented the greatest photocatalytic activity. Therefore, the objective of the present research is the synthesis, characterization and photocatalytic evaluation of materials of the tungstate family MWO₄ (M = Cu, Co, Ni and Mn) through the water splitting reaction towards the hydrogen production under the visible light irradiation.

Section snippets

Synthesis of precursor solutions

The synthesis of MWO₄ (M = Co²⁺, Cu²⁺, Mn²⁺ and Ni²⁺) compounds was carried out by the reaction in solution method. This technique consists in a dissolution-precipitation reaction of salts containing the metals of interest; this method may lead, sometimes, to amorphous materials that by an adequate thermal treatment, can be transformed into crystalline materials with an optimal morphology and microstructure. Initially, the equivalent amount of 0.4 mol of Na₂WO₄·2H₂O were weighed in grams and

X-ray diffraction

Bivalence metal tungstate powders derived from the synthesis by the dissolution-precipitation technique were structurally characterized using XRD. Fig. 2 shows the diffractogram peaks of the synthetized MWO₄ photocatalysts. XRD technique showed that for the four thermally treated samples at 400 °C for 4 h, the obtained products were crystalline, meaning, that the XRD spectrum of every sample showed the corresponding JCPDS reflections reported in the literature [11], [13], [15]. The XRD

Conclusions

Divalent tungstates of transition metals were successfully synthetized using an effective synthesis method, followed by calcination at 400 °C. The photocatalytic activity of CoWO₄, CuWO₄, NiWO_4, and MnWO₄ were investigated. Rating photocatalytic performance under visible light spectrum according to experimental results, the following order can be established: MnWO₄ > CoWO₄ > NiWO₄ > CuWO₄. The photocatalyst with the highest H₂ production was MnWO₄. The dissolution-precipitation method allowed

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View full text

Synthesis, characterization and photocatalytic evaluation of MWO4 (M = Ni, Co, Cu and Mn) tungstates

Highlights

Abstract

Introduction

Section snippets

Synthesis of precursor solutions

X-ray diffraction

Conclusions

J Mol Catal A Chem

Catal Today

Solid State Sci

J Lumin

Chem Phys Lett

Ceram Int

Electrochim Acta

Appl Surf Sci

Nuevos Materiales para el Desarrollo de Celdas de Combustible en México

Heterogeneous photocatalysis: state of the art and present applications

Top Catal

Electrochemical photolysis of water at a semiconductor electrode

Nature

Synthesis, characterization and photocatalytic evaluation of MWO₄ (M = Ni, Co, Cu and Mn) tungstates