*3.3. Photocatalytic Activity*

Materials activity in solar fuels production was evaluated by using three different reactor systems (Figure 8) fed with a reagent gas mixture composed of CO2 bubbled in deionized water to saturation. Such gas stream was flown for some minutes through the reactors during their loading operation in order to blow air away. Samples of gas were analyzed at regular intervals of time (30 min) over a period of 3–6 h.

In case (a), 0.1 g of nano-sized photocatalyst was placed in a 40 mL glass reactor, provided with two openings closed by hollow plastic caps. One cap holds a glass tube with a three-way valve for reagent gas mixture loading, the other is provided with a rubber septum for gas sampling using a 250 μL gas-tight syringe. The reactor was loaded with 0.15 MPa of CO2 saturated with H2O and stirred under illumination for 3 h. A gas-sample was withdrawn and analyzed using a GC.

In case (b), CO2 saturated with water at 298 K was flown through a 2 mm i.d. tube (equipped with valves for feeding, sampling and reactor closing and filled with the same nanosized-catalyst) at a rate of 0.1 mL/min under illumination and recycled using a micropump. After three hours cycling the gas was injected into a GC. Alternatively, the tube was filled with CO2–H2O and the closed system illuminated for 3–6 h. The gas was then injected into the GC column.

In case (c), 0.1 g of the nanosized photocatalyst were dispersed on the internal surface of the same vessel used in (a) using a suspension of the catalyst in a volatile liquid (acetone or pentane) that was then slowly evaporated by flowing N2. A thin, homogeneous solid film remained adherent to the surface of the reactor that was charged with CO2–H2O and illuminated. Illumination was applied for 3–6 h and a sample of gas was withdrawn with the gas-syringe at fixed intervals of time and analyzed by GC.
