Abstract:

Tungsten carbide (WC) is commonly used as a photocatalytic material for hydrogen production via water reduction. However, it is often combined with an effective photoabsorber to provide sufficient photoactivity. This is attributed to the narrow band gap of WC, which leads to an inadequate redox capability for water reduction. Notably, this limitation was overcome using a novel solid-liquid photocatalytic system that compliments bare WC photocatalysts with liquid-phase photosensitizing erythrosine B (ErB). The proposed concept eliminates the need to couple WC with photoabsorbing semiconductors, which often requires tedious procedures for the proper functionalization of photocatalytic composites. The experimental results indicated significant hydrogen production from the proposed solid-liquid photocatalytic system under irradiation with visible light (λ = 520 nm); however, only in the presence of triethanolamine (TEOA) as a sacrificial reagent. Evidently, a blank experiment with only WC and ErB under typical photoreaction conditions exhibited nearly zero photoactivity and the production of H₂ was undetected. Similarly, nonactivity was observed for the photoreaction in the presence of ErB or WC in the irradiated TEOA solution. These blank experiments confirmed the significance of all three components, namely WC, ErB, and TEOA, which functioned as the photocatalyst, photoabsorber, and sacrificial reagent, respectively, for suitable H₂ production in the proposed system. The effects of three critical parameters, such as pH, ErB concentration, and WC concentration, were systematically investigated. The optimum pH for H₂ production was 8, with a slight variation to more basic or acidic conditions reducing the photoactivity of the system. At pH < 8, part of TEOA undergoes partial protonation, thereby losing its activity as a sacrificial reagent in the photocatalytic system. As the pH increased to > 8, the low proton concentration in the reaction medium perturbed the thermodynamic drive, leading to suppressed H₂ production. The optimum ErB concentration was 1 mmol·L^-1, and decreasing or increasing the ErB concentration from the optimal point was detrimental to H₂ production. The diluted system (ErB concentration < 1 mmol·L^-1) provided insufficient sensitizing agents, whereas the concentrated system (> 1 mmol·L^-1 ErB) induced significant scattering effects that prevent light from penetrating into the reactive liquid phase. Conversely, the WC concentration exhibited a positive correlation with H₂ production in a steady manner, and the highest H₂ production measured by the system was at a WC concentration of 12 mmol·L^-1. Under optimum conditions, 66 μmol∙h^-1 of H₂ was successfully produced, with a slightly higher apparent quantum efficiency (AQE) of 6.6% at 520 nm, which was attributed to the synergism of ErB-TEOA-WC in the proposed system. The photoelectrochemical evaluation confirmed the positive interactions between ErB, TEOA, and WC, which caused reduced impedance while improving charge utilization in the system. Consequently, an excellent H₂ turnover number (TON) of 15 was achieved with negligible activity decay for at least 20 h of reaction. Density functional theory (DFT) calculations confirmed the major roles of W- and C-vacant sites in H₂ production, which were attributed to their enhanced product desorption that facilitates high turnover rates during photoreactions. In conclusion, the proposed novel liquid-solid photocatalytic WC/ErB/TEOA system provides more facile photo-derived H₂ energy from water, which circumvents the tedious photoabsorber coupling of metal carbide photocatalysts.

Key words: Liquid-phase, WC, Erythrosine B, Triethanolamine, Photocatalysis, H₂ production