Development of visible light active heterojunction photocatalysts for production of hydrogen and value-added chemicals

Abstract

The rise in the world’s population, rapid industrialization, and excessive use of fossil fuels have led to undesirable side effects of environmental pollution and climate change. Consequently, sustainable production of clean fuel by using renewable energy sources has attracted global attention. In this context, hydrogen (H2) is considered an environmental-friendly alternative to fossil fuels that can meet the growing energy demands of the world. At present, various sources such as fossil fuels, biomass, and water are utilized for the generation of H2. Particularly, H2 generation by water splitting has gained significant interest since it does not produce any unwanted side products. The water-splitting consists of two half-cell reactions, reduction half-cell where H2 is produced and oxidation half-cell where oxygen is produced. In photocatalytic H2 production from water, the photogenerated electrons are utilized in reducing protons of water into hydrogen and the holes are quenched by addition of sacrificial electron donors. On the other hand, biomass is considered one of the most abundant renewable sources of energy produced by plants through photosynthesis. The controlled degradation of raw biomass can yield various important compounds such as ethanol, furans, 5-hydroxymethylfurfural (HMF), lactic acid, succinic acid, etc., which can be further converted into more value-added chemicals. The direct oxidation of biomass derivatives requires harsh conditions like high temperature, and some harmful solvents, which often results in poor selectivity of the oxidized product. Recent studies have shown that photocatalytic H2 generation from water can be coupled with the valorization of biomass derivatives to achieve simultaneous hydrogen generation combined with the production of value-added chemicals in high selectivity. In this regard, visible light-responsive semiconductor nanostructures have attracted immense attention due to their size-dependent unique optical and photocatalytic properties. Keeping these points, we sought to develop visible-light-active photocatalysts supported by noble-metal-free co-catalysts for simultaneous production of hydrogen and value-added chemicals from biomass derivatives. Further to achieve enhanced photocatalytic H2 production combined with high yield generation of oxidized products of biomass derivatives, various heterojunction photocatalysts composed of inorganic metal sulfide/oxide hybrid nanostructures were developed and their photocatalytic applications were studied. The thesis has been divided into six chapters, as summarized below. Chapter 1: This chapter introduces natural and artificial photosynthesis, the basic principle of water splitting into H2 production. A brief introduction to biomass valorization, semiconducting nanomaterials, and heterojunction formation is also given. Various synthesis methods of CdxZn1-xS based photocatalysts and their applications for visible-light-driven hydrogen generation from water are discussed. Various semiconductor photocatalysts employed for simultaneous H2 generation combined with oxidation of biomass derivatives is also discussed. Chapter 2: In this chapter, syntheses of Zn0.5Cd0.5S/Ni2P heterostructures composed of Zn0.5Cd0.5S photocatalyst supported by non-noble metal-based co-catalyst, Ni2P and its photocatalytic activity for efficient visible-light-driven H2 production from water is studied. The photocatalytic studies revealed higher H2 generation activity by Zn0.5Cd0.5S/1wt%Ni2P (S2) heterostructure over pristine Zn0.5Cd0.5S and CdS/1wt%Ni2P. The enhanced H2 generation activity of S2 has been attributed to efficient spatial separation of photogenerated charge carriers and the presence of highly reactive Ni2P sites on the surface of Zn0.5Cd0.5S microspheres. Furthermore, the catalyst S2 can be recycled for several cycles without significant loss of catalytic activity and photostability. This study highlights the importance of Zn0.5Cd0.5S/Ni2P heterostructure containing non-noble metal-based co-catalyst for efficient visible-light-driven H2 production from water. Chapter 3: In this chapter, synthesis of heterostructure photocatalysts composed of twin Zn0.5Cd0.5S nanorods (NRs) decorated with noble-metal-free co-catalyst, Ni(OH)2 nanoparticles (NPs) for efficient visible-light-assisted H2 generation from water is carried out. The optimized heterostructure, Zn0.5Cd0.5S/7wt%Ni(OH)2 (CN-7), showed excellent catalytic activity with an H2 generation rate of 139 mmol g-1 h-1 which was found to be about 1.5 and 9.4 times higher than those of pristine Zn0.5Cd0.5S NRs (C-1) and CdS/7wt%Ni(OH)2, respectively. Further, the photocatalytic activity was extended for reduction of 4-nitrophenol from water using in situ generated hydrogen as a reducing agent. This work represents a unique demonstration of the use of water, a green solvent, as a source of reducing agent for visible-light-driven reduction of environmental pollutants from water. Chapter 4: In this chapter, rational design of Zn0.5Cd0.5S/MnO2 heterostructure having ideal band alignment for H2 production coupled with oxidation of biomass-derived HMF to DFF with high yield and selectivity is presented. Photocatalytic investigations revealed higher activity by Zn0.5Cd0.5S/1%MnO2 exhibiting the DFF yield of 46% and a simultaneous H2 generation rate of 1322 mol g-1 in 24 h. The heterostructure exhibits good catalytic activity even under natural sunlight irradiation affording DFF with 14% yield and H2 generation rate of 152.6 μmol g-1 in 6 h. The high catalytic activity of the heterostructure over the parent materials has been attributed to the efficient separation of photogenerated charge carriers with the aid of Z-scheme mechanism and synergistic catalysis between Zn0.5Cd0.5S and MnO2. This work represents a unique demonstration of noble metal-free selective oxidation of HMF to DFF integrated with H2 production under mild reaction conditions. Chapter 5: In this chapter, rational construction of Z-scheme CoTiO3/xZn0.5Cd0.5S heterostructures with varying composition, featuring suitable band structure for efficient photocatalytic reduction of protons of water into H2 combined with selective oxidation of furfuryl alcohol (biomass derivative) to a value-added product, Furfuraldehyde is presented. The photocatalytic studies showed higher catalytic performance of CoTiO3/10wt%Zn0.5Cd0.5S heterostructure for selective oxidation of furfuryl alcohol to Furfuraldehyde with 95% yield coupled with an H2 generation rate of 1929 mol g-1 h-1 which is about 4-fold higher than that of pristine Zn0.5Cd0.5S. The enhanced catalytic activity of the heterostructure has been ascribed to synergistic interaction aided by Z-scheme heterojunction. Chapter 6: In chapter 6, we developed NiTiO3/ZnIn2S4 heterojunction photocatalysts by varying the NiTiO3:ZnIn2S4 ratio. The heterostructure with NiTiO3:ZnIn2S4 ratio of 1:1 showed the best photocatalytic activity with a high H2 generation rate of 4.43 mmol g-1 h-1. Further, the photocatalytic activity was extended to reduce biomass-derivative, HMF into value-added 2, 5-bis(hydroxymethyl)furan (BHMF) from water using the in situ generated hydrogen as a reducing agent. The NiTiO3:ZnIn2S4 heterostructure with 1:1 composition exhibited the best catalytic activity with BHMF yield of > 99% along with 100% selectivity using water as a source of reducing agent. This work represents a rare demonstration of simultaneous H2 generation combined with the reduction of biomass derivative, HMF into high-value chemical, BHMF in water.