Abstract
Catalytic properties of the cluster compound (TBA)2[Mo6Ii8(O2CCH3)a6] (TBA = tetrabutylammonium) and a new hybrid material (TBA)2Mo6Ii8@GO (GO = graphene oxide) in water photoreduction into molecular hydrogen were investigated. New hybrid material (TBA)2Mo6Ii8@GO was prepared by coordinative immobilization of the (TBA)2[Mo6Ii8(O2CCH3)a6] onto GO sheets and characterized by spectroscopic, analytical, and morphological techniques. Liquid and, for the first time, gas phase conditions were chosen for catalytic experiments under UV–Vis irradiation. In liquid water, optimal H2 production yields were obtained after using (TBA)2[Mo6Ii8(O2CCH3)a6] and (TBA)2Mo6Ii8@GO) catalysts after 5 h of irradiation of liquid water. Despite these remarkable catalytic performances, “liquid-phase” catalytic systems have serious drawbacks: the cluster anion evolves to less active cluster species with partial hydrolytic decomposition, and the nanocomposite completely decays in the process. Vapor water photoreduction showed lower catalytic performance but offers more advantages in terms of cluster stability, even after longer radiation exposure times and recyclability of both catalysts. The turnover frequency (TOF) of (TBA)2Mo6Ii8@GO is three times higher than that of the microcrystalline (TBA)2[Mo6Ii8(O2CCH3)a6], in agreement with the better accessibility of catalytic cluster sites for water molecules in the gas phase. This bodes well for the possibility of creating {Mo6I8}4+-based materials as catalysts in hydrogen production technology from water vapor.
Highlights
Dihydrogen production from water by sunlight is one of the most satisfactory ways of sustaining worldwide energy production and solving the looming environmental crisis [1–3]
Ligand exchange between the carboxylate apical groups and the oxygen functionalities of the GO nanosheets takes place in the immobilization process. This hybrid nanomaterial was characterized by means of Fourier transform infrared spectroscopy (FTIR), Raman, UV–Vis, photoluminescence, x-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), inductively coupled plasma atomic emission spectrometry (ICP-AES), and combustion analysis
Once the (TBA)2[Mo6Ii8(O2CCH3)a6] precursor was solubilized in water/acetone/TEA (50/45/5% v/v), it was subjected to vacuum during 2 h at room temperature and a mixture of red polyhedral and laminar crystals were formed in a transparent solution after overnight
Summary
Dihydrogen production from water by sunlight (hydrogen evolution reaction, HER) is one of the most satisfactory ways of sustaining worldwide energy production and solving the looming environmental crisis [1–3]. We studied the photocatalytic activity and stability of the octahedral molybdenum (II) iodide cluster compound (TBA)2[Mo6Ii8(O2CCH3)a6] and of the hybrid (TBA)2Mo6Ii8@GO nanocomposite in the production of hydrogen as one of the most representative reactions in the field of renewable energy. It was recently reported that the apical acetate ligands can be replaced by other carboxylates such as isonicotinate ligands [69] where the coordinative anchoring of the highly luminescent {Mo6Ii8}4+ cluster cores through the GO functionalities would closely imitate the molybdenum–acetate bonds in [Mo6Ii8(O2CCH3)a6]2−. Ligand exchange between the carboxylate apical groups and the oxygen functionalities of the GO nanosheets takes place in the immobilization process This hybrid nanomaterial was characterized by means of Fourier transform infrared spectroscopy (FTIR), Raman, UV–Vis, photoluminescence, x-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), inductively coupled plasma atomic emission spectrometry (ICP-AES), and combustion analysis. Catalytic vapor water photoreduction is presented as the optimal methodology to enhance the activity and stability of the {Mo6Ii8}4+ cluster active sites
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