Microwave-assisted synthesis of Ca1-xMnxMoO4 (x = 0, 0.2, 0.7, and 1) and its application in artificial photosynthesis

Abstract

This work reports, for the first time, to the best of our knowledge, the use of calcium molybdates doped with Mn2+ in the catalytic photoreduction of CO2 to produce compounds with higher added value that have applications in different branches of the chemical industry. The molybdates were prepared at 100 °C by microwave-assisted hydrothermal synthesis and were characterized by X-ray diffraction, Raman vibrational spectroscopy, scanning electron microscopy, and diffuse UV–vis reflectance spectroscopy. The synthesized Ca1-xMnxMoO4 catalysts (x = 0, 0.2, 0.7, and 1) were also calcined at 500 °C, in order to investigate possible phase transitions. For x = 0 and 0.2, the samples crystallized in the tetragonal structure (I41/a, #88) and no phase transitions were observed at 500 °C. For x = 0.7 and 1.0, the phase produced at 100 °C was a hydrated form of manganese molybdate that exhibited triclinic structure (P1‾, #2) and became monoclinic (C2/m, #12) when calcined at 500 °C. The catalysts subsequently investigated were the pure molybdate and the materials with doping contents of 20 and 70 mol% Mn2+, hydrothermally treated at 100 °C and after calcination at 500 °C. The band gap energies ranged from 2.77 to 3.50 eV. In the performance tests, the productions of CO and CH4 after 6 h of irradiation were in the ranges 2.41–19.74 and 0.21–0.82 μmol g−1, respectively. The doped x = 0.2 sample treated at 100 °C exhibited the best performance, producing the highest amounts of CO and CH4. The results indicated that the doping of CaMoO4 with Mn2+ improved the performance of this ceramic for the purpose of artificial photosynthesis. Furthermore, our results support a deep discussion about the role of doping content and crystalline structure of the molybdates on the photocatalytic activity.