Molecular charge transfer and quantum efficiency analyses of a photochemical reactor for hydrogen production

This paper examines a lab-scale, batch-type photochemical reactor design for hydrogen generation from water. The reactor comprises a transparent cylindrical reaction vessel of 100 ml volume, which is illuminated with monochromatic LED arrays from 4 sides. The reaction is kept at constant pressure by a mechanism which enlarges the volume above the liquid in correspondence to the rate of hydrogen production. The measurement of volume, pressure and temperature is used to determine the number of moles of gas generated. The vessel is filled with a non-aqueous solution containing 0.62 M water and a supramolecular device (SMD), namely {[Ru(bpy)2dpp]2Rh(OH)2}5+ (with bpy = 2,2′–bipyridine and dpp = 2,3–bis(2–pyridil)pyrazine)), which reduces water photochemically under the presence of DMA as sacrificial electron donors (DMA = n,n-dimethylaniline), according to the following reaction: 2H2O(l)+2DMA→2hνH2(g)+2OH−+2DMA+. The equations for modelling of the photo-catalytic process are identified and used for simulations and analyses to improve the apparatus design and find the best operating conditions. The molecular simulations with Discovery Studio software are used for 3D representation of the SMD to identify its fundamental properties in the relevant energetic states, i.e., ground, metal to ligand charge transfer (MLCT), metal to metal charge transfer (MMCT) and the photo-activated catalyst states. The quantum efficiency of each sub-process is determined based on specified assumptions and subsequent analysis. It shows that the quantum efficiency is 0.01, which is consistent with experimental results from past studies. Two routes of the catalysis were identified: one short (300 ns), where the catalyst is activated from the 3MLCT state, and one long (400 ns) where the catalyst is activated via a 3MMCT state. The turnover frequency is mostly influenced by the rate of incident photons. A mathematical model of five differential equations predicts the molar concentration of the relevant species in the batch reactor. The model is used to determine the variation of concentration, hydrogen production and quantum efficiency in time. It is found that the process has an initial phase, when the concentration of hydrogen dissolved in the solution increases up to saturation; thereafter it remains constant. This result is also consistent with past literature experimental results. Because of other species production (e.g., hydroxyl, DMA+), water and DMA consumption, the quantum efficiency of the process decreases in time. Also the pressure in the head space influences the process. If the reactor operates with a full head space (50 ml), the pressure increases by ∼20 kPa in 12 h. The study provides useful input to the design of the experiment and also its application to the modelling of photochemical hydrogen production.

► Lab-scale photochemical reactor design for photo-catalytic hydrogen generation. ► Cylindrical reaction vessel of 100 ml volume with transparent glass. ► Reaction is kept at constant pressure by a mechanism which enlarges the volume. ► Use of supramolecular complexes for the photochemical water reduction. ► Molecular simulation and kinetics modelling applied to determine the performance.