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• Posted By : admes liliy
• Posted On : Mar 07, 2023
• Views : 94
• Category : General
• Description : Equations (1)–(5) describe the main steps, including the photocatalytic step for hydrogen production and the thermochemical molten salt step for oxygen production. The main advantage of the proposed cycle is the utilization of a wider part of the solar spectrum, from UV-Vis (photochemical steps) to infrared wavelengths.

Overview

• Equations (1)–(5) describe the main steps, including the photocatalytic step for hydrogen production and the thermochemical molten salt step for oxygen production. The main advantage of the proposed cycle is the utilization of a wider part of the solar spectrum, from UV-Vis (photochemical steps) to infrared wavelengths. This makes it possible for this cycle to achieve higher overall efficiencies compared to past purely photochemical or thermochemical water splitting cycles (Huang et al., 2006). Referring to equation (2), water splitting is achieved by the photochemical oxidation of ammonium sulfite with simultaneous release of hydrogen gas (hydrogen cycle). Then, with a continuous increase in temperature, ammonium sulfate decomposes into ammonia, sulfur dioxide, and oxygen through three thermochemical steps, Eqs(3)–(5) (oxygen subcycle). In the first step (Eq(3)), potassium sulfate is used to suppress the release of sulfur trioxide and separate it from ammonia and remaining water. At higher temperatures (equation (4)), the pyrosulfate formed decomposes back to sulfate. Finally, sulfur trioxide is decomposed into sulfur dioxide and oxygen (equation (5)), and the two gases are subsequently separated using an ammonia absorber (equation (1)). All quoted temperatures are from the corresponding literature.
  SO2 (NH4)2SO3 (NH4)2SO4 K2S2O7 SO3
  + 2NH3 →
  + H2O →
  + K2SO4 → 2NH3 → SO3 + K2SO4 → SO2 + 1/2O2
  Absorption, 25 °C (1) Photochemical, 80 °C (2) Thermochemical, 400 °C (3) Thermochemical, 550 °C (4) Thermochemical, 850 °C (5)
  (NH4)2SO3 (NH4)2SO4 + H2
  + K2S2O7 + H2O
  With regard to the hydrogen subcycle, past studies have mainly focused on the development of photocatalysts. Cadmium sulfide (CdS) is the primary material examined. Muradov et al. (1981) proposed CdS as an ideal photocatalyst for performing the hydrogen generation step. Over the years, a series of sophisticated photocatalysts have been prepared and tested for efficient hydrogen production from sulfide and sulfite solutions. Although many mechanisms have been proposed (Buhlet et al., 1984) and further developed (Tsuji et al., 2004), detailed and comprehensive numerical and experimental analyzes are still lacking. Oxygen cycling, on the other hand, is mainly based on experimental thermal analyzes of the materials used and associated process simulations. Wang (2012) experimentally examined the thermochemical part of molten salts. Subsequently, Littlefield (2012) simulated an integrated process using Aspen Plus and literature on sulfate/pyrosulfate properties (Lindberg 2006). In this case, generalized and simplified thermodynamic approximations are used, which can lead to inaccurate estimates of process conditions and equilibria.
  2.2 Numerical tools
  Several issues requiring further investigation have been identified in previous work (Littlefield et al. 2012). The present work is based on extensive literature research using advanced thermodynamic numerical tools (such as Aspen Plus and FactSAGE) and comprehensive thermodynamic databases (such as FACT and DIPPR project 801). These tools will then help to integrate new materials and processes into existing ones.
  2.3 Experimental tools
  Photocatalytic hydrogen production experiments were performed in a simple bench-scale configuration consisting of a glass vessel (photoreactor), a water bath for temperature control, and a solar simulator (Newport/Oriel Corp., 300 -1,000W output, Model 9119X with Air Mass (AM) 1.5 global filter). Add the synthesized photocatalyst together with the cocatalyst to the aqueous solution containing 1 M (NH4)2SO3 placed inside the photoreactor. Before each experiment, the photoreactor was purged with ultrapure argon (Linde, 99.999%) for approximately 1 h to remove dissolved oxygen from the solution. The evolved hydrogen gas was collected on the surface of the water using an inverted graduated burette.