Enhanced Photocatalytic Activity and Stability in Hydrogen Evolution of Mo6 Iodide Clusters Supported on Graphene Oxide

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.

Nanostructured Hybrids Based on Tantalum Bromide Octahedral Clusters and Graphene Oxide for Photocatalytic Hydrogen Evolution

“…The surface of GO has different oxygen donor functionalities, mainly represented by hydroxyl, carboxyl, and epoxy groups, that are suitable for coordinative metal cluster anchoring [ 35 , 36 ]. Octahedral tantalum clusters are suitable candidates for immobilization since they have a high affinity towards oxygen-donor ligands.…”

“…Their immobilization avoids metal aggregation and suppresses the charge recombination by channeling electrons into the GO substrates. Recently, we immobilized molybdenum octahedral clusters, which are optimal photosensitizers and electron transfer catalysts, onto GO surfaces in order to obtain nanohybrids which resulted in efficient water reduction photocatalysts [ 35 , 36 ]. In this work, the [{Ta 6 Br i 12 }Br a 2 (H 2 O) a 4 ]·4H 2 O cluster was systematically immobilized on GO nanosheets by taking advantage of the labile nature of the apical ligands of the cluster and the coordinating ability of the oxygen functionalities of the GO support.…”

Molybdenum-Containing Metalloenzymes and Synthetic Catalysts for Conversion of Small Molecules

“…The TOF values with respect to atomic molybdenum were 5 × 10 −6 s −1 and 3 × 10 −4 s −1 for the heterogeneous and the homogeneous materials, respectively. Recently, the (TBA) 2 [Mo 6 I i 8 (O 2 CCH 3 ) a 6 ] was also coordinatively immobilized onto GO to give (TBA) 2 Mo 6 I i 8 @GO [184]. To assure the cluster stability of the cluster units under photocatalytic conditions, both materials were tested in vapor water photoreduction.…”

Octahedral Molybdenum Cluster-Based Single Crystals as Fabry–Pérot Microresonators

“…These clusters have outstanding optical properties such as absorption of light in the UV–vis range, bright emission in the red/near-IR region with high quantum yields, and PL lifetimes up to several hundreds of microseconds. − It is also well-known that these clusters act as efficient photosensitizers in processes related to the generation of singlet oxygen. These properties make these inorganic compounds particularly attractive in design of functional hybrid nanomaterials with potential applications in nanoarchitectonics, , optoelectronics, − lighting, hydrogen storage, biomedicine, ,− and catalysis. − Surprisingly, only scarce studies about the optical and electronic properties of octahedral molybdenum cluster-based materials at the single crystal level and at the micrometer scale are reported, and those are limited to Cs 2 [Mo 6 X i 8 X a 6 ] (X = Cl, Br, I) compounds. − …”