2.5. Photocatalytic H2 Evolution Procedure

UV–Vis irradiations were carried out with a spot light Hamamatzu Xe lamp (Lightnincure LC8 model, 800–200 nm, 1000 W/m², fiber optic light guide with a spot size of 0.5 cm). For the photocatalytic reactions performed in the presence of an aqueous solution, the chosen photoreactor was a cylindrical Pyrex vessel with a total volume of 55 mL and was 140 mm in diameter, with an inlet and outlet with independent valves equipped with a manometer to determine the pressure. Figure 2a illustrates the experimental setup for liquid water photoreduction. In a typical reaction, the photoreactor was initially charged with 15 mL solvent mixture of Milli-Q water (with or without an organic co-solvent) and TEA (5% v/v) or methanol (30% v/v) as electron donors. The whole system was purged with a nitrogen flow for 30 min. The amount of photocatalyst (30 and 0.54 mg of (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆] for standard and comparative studies, respectively; 30 and 11 mg of (TBA)₂Mo₆Iⁱ₈@GO for standard and comparative studies, respectively) was added to the solution under a N₂ atmosphere, and the photoreactor was re-purged (10 min) under vigorous magnetic stirring to ensure the absence of oxygen in the system. The reaction vessel was sealed and pressurized with N₂ up to 0.5 bar. This was kept 1.5 cm away from the light source and was immersed into a thermostatic bath at 25 °C during irradiation (5 h of standard time). The gas phase samples (100 μL) were collected with a Samplelock Hamilton syringe. The amount of hydrogen evolved in the photoreactor during irradiation was determined by gas chromatography (GC) with an Agilent 6850 GC system (Santa Clara, CA, USA) equipped with a bonded polystyrene–divinylbenzene HP-PLOT Q column (30 m length, 0.53 mm inner diameter, 40 μm film thickness; Agilent J&W) and a thermal conductivity detector (TCD). Helium was the carrier gas, and the flow rate was set to 5 mL/min. The temperatures of the injector and detector in GC analysis were 53 and 220 °C, respectively, and the isothermal oven temperature profile was set to 50 °C.

Experimental setups for liquid water (a) and vapor water (b) catalytic photoreductions.

For the photocatalytic reactions carried out in the presence of aqueous mixtures in vapor phase, the photoreactor was a double cylindrical quartz reactor (110 mL of total volume) in which the two vessels were connected with a quartz bridge (2 cm length, see Figure S1). Figure 2b illustrates the experimental layout for vapor water photoreduction. Water (30 mL Milli-Q water) and the sacrificial electron donor (methanol, 30% v/v) were loaded into the reactor vessel (a) and purged with Ar (30 min). The photocatalyst (10 and 5 mg of (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆]; 10 and 30 mg of (TBA)₂Mo₆Iⁱ₈@GO) was loaded into the reactor vessel (b). The reactor was sealed and pressurized with argon up to 0.5 bar and connected to an electrical heating ribbon that allowed the reactor vessel (a) to be heated to 70 °C in order to achieve the evaporation of the water/sacrificial mixture. The vessel (a) was irradiated for 24 h (standard irradiation time). The H₂ generation was determined by an Agilent 490 Micro GC equipped with a molecular-sieve-coated CP-Molsieve 5Å column, Agilent J&W) and a thermal conductivity detector (TCD). Ar was taken as the carrier gas, and the flow rate was set to 5 mL·min⁻¹. The temperatures of the injector and detector in GC analysis were 110 and 220 °C, respectively, and the isothermal oven temperature profile was set to 62 °C with an initial column pressure of 15 psi.

The hydrogen peak area was converted to the corresponding concentrations, based on the standard calibration curve. The moles of hydrogen generated were calculated by using the ideal gas law (n = PV/RT). Control experiments were performed, one under UV–Vis irradiation without a photocatalyst, and another under dark conditions using the cluster materials at the standard experimental conditions. In the reactions performed with water in the vapor phase, the use of TEA as a sacrificial electron donor was discarded because the control reactions, in the absence of the molybdenum photocatalyst, afforded methane as a subproduct due to the decomposition of TEA in the reaction conditions. In contrast, during the photocatalytic experiments executed with the halogenated hexanuclear molybdenum complexes described in this work, only hydrogen, atmospheric nitrogen, and oxygen were detected by GC-TCD analyses, independently of the sacrificial donor used. Measurements from the control experiments showed the detection of the atmospheric gases exclusively, which confirmed the selective water to hydrogen photoreduction.

Reuse experiments of the (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆] complex and (TBA)₂Mo₆Iⁱ₈@GO nanocomposite were done under standard catalytic conditions after recovering the solid material by evaporation (cluster complex) or filtration (hybrid material) under vacuum.