Development of high-performance (photo)catalyst for the efficient solar hydrogen peroxide production

Abstract

The worldwide population is gradually increasing and fossil fuel as energy resources is limited. Furthermore, dramatic climate change has increased due to many emissions of carbon dioxide (CO2) gas. Many countries have tried to overcome these environmental problems by declaring the Paris Agreement in 2015, sustainable economy paradigm, etc. Because of these activities, a research and development (R&D) have much attention today to replace fossil fuel with other renewable energies such as hydropower, geothermal, solar, and biomass. Hydrogen is a green resource because it only generates a water molecule as by-products. However, the storage of hydrogen gas to hydrogen tank is an enormous issue. For example, it needs a high pressure, almost 700 bar, to inject into the storage tank. Hydrogen peroxide (H2O2) is eco-friendly and important chemical due to oxidizing abilities. Also, H2O2 can be used for the future energy carrier because the energy density of H2O2 is comparable to hydrogen gas. The H2O2 is usually produced by the anthraquinone oxidation (AO) process. However, it requires a high energy input, high installation cost, toxicity, and a danger of explosion due to a mixture of H2/O2. Thus, many researchers have developed any other greener methods for H2O2 generation such as electrochemical and photocatalytic synthesis. Although the present developed process has much developed now, it still has some drawbacks like cost, low efficiency to generate H2O2, and others. Therefore, the research will be much focused on increasing a high concentration of H2O2 and fabricating an efficient catalyst. In this dissertation, two different catalysts have been studied for the high performance of H2O2 generation. One research is that controllable cobalt on layered double hydroxide (LDH) is used for electrocatalyst and another catalyst is that MOF-derived sulfur-doped carbon/CdS nanocomposite is fabricated as a photocatalyst. From these catalysts, inorganic or organic or those mixtures have been utilized for a practical application. In chapter 2, cobalt-MgAl based LDH has been synthesized to improve the H2O2 selectivity from oxygen reduction reaction (ORR)by controlling the amount of cobalt. For a usual electrochemical synthesis, it requires precious noble metals slike Pd and Pt as electrocatalysts. In contrast, Mg and Al atoms are earth-abundant materials and a low dispersed transition metal can overcome the aspect of cost. The cobalt is controlled to 1 Co, 10-1 Co, 10-2 Co, and 10-3 Co, respectively, to observe the effects of cobalt. Compared to CoAl LDH which is not used Mg atoms for cations, the H2O2 selectivity is gradually increased when the amounts of cobalt are decreasing by suppressing the cleavage of O=O during ORR. The (10-3 Co)-MgAl LDH shows a higher H2O2 selectivity, over 90%. Furthermore, the theoretical calculations demonstrate that the cobalt compositions play an enormous role for improving the H2O2 selectivity. All Co-MgAl LDH has a thermodynamically favorable to H2O mechanism which is competitive to H2O2 mechanism. However, the low dispersed Co-MgAl LDH exhibits lower free energies barriers which determine the cleavage of oxygen bond. Thus, it can increase the H2O2 selectivity during electrochemical H2O2 synthesis. Moreover, the CoMgAl LDH is combined with a rutile TiO2 as a photoanode with a copper wire in two-compartment system. The system can provide H2O2 generation without any external voltage. It requires only solar energy as energy sources, so it can be deserved for renewable methods. In chapter 3, to maximize the H2O2 efficiency from photocatalytic synthesis, sulfur-doped carbon/CdS nanocomposite has been fabricated. This nanocomposite is derived from MOF structure at a high temperature under an O2 atmosphere. The generated carbon matrix from MOF precursors encapsulates CdS composite and is confirmed by x-ray analysis and electron microscopic instruments. The carbon layer can suppress the re-adsorption of photogenerated H2O2 on its surface, showing a high H2O2 generation rate due to the decrease of decomposition rate. It produces a high concentration of H2O¬2 (2.65 mM) which is 5 times than commercial CdS (0.55 mM). Furthermore, the CdS photocatalyst is optimized to maximize the efficiency of H2O2 generation. Thus, the photocatalytic reaction is also conducted in 1 M KOH with a sacrificial agent and photocatalytic reaction time is extended from 3 h to 24 h to determine the saturation point. However, the H2O2 formation isn’t saturated after 24 h, but continuously increased, reaching almost 17.1 mM which is the highest concentration until reported. In summary, the encapsulated carbon matrix on the CdS surface can inhibit the adsorption of H2O2, so it could improve the production of H2O2. Therefore, it suggests the possibilities of practical application by the simple fabrication method of MOF-derived nanocomposite.