Graphitic carbon nitride/red phosphorus heterojunctions decorated with platinum nanoparticles as catalysts for the photo-assisted hydrolysis of ammonia borane

Increasing usage of fossil fuel as the primary energy source is the main reasoning of global warming due to the undesirable greenhouse gas emissions. To mitigate greenhouse gas emissions and ensure a sustainable future, it is urgent to adopt alternative energy sources. Among the alternatives, hydrogen as an energy carrier, which can be produced from numerous sources at all geographies and emits only water upon its combustion, is one of the most promising one. Therefore, it plays an important role in the energy transition for net-zero emission targets. However, the safe storage and transportation of hydrogen remain significant challenges that need to be addressed. While storage of hydrogen in gas or liquid state presents several risks and is not efficient in terms of the capacity, chemical hydrogen storage in low-volume and lightweight solid materials offers the best solution. Among the chemical hydrogen storage materials, ammonia borane is the most promising one due to its high hydrogen content, stability under ambient conditions, high solubility in water and non-toxicity. However, the dehydrogenation of ammonia borane is a considerably slow process, and an efficient catalyst is needed to make it a viable option for widespread use in hydrogen economy. Among the possible ways of dehydrogenation of ammonia borane, hydrolysis of ammonia borane (HAB) was both theoretically and experimentally shown to be superior to other methods. For this reason, to date, many metal catalysts (Fe, Ni, Co, Pt, Ru, Rh, Au) have been tested in the HAB. However, these metal catalysts tend to agglomerate and suffer from low stability due to their high surface energy. As a solution, the utilization of high surface area support materials for the immobilization of metal NPs has been reported as the best choice. Immobilization of metal NPs onto semiconducting support materials has got particular attention since it is possible to boost the activity of metal catalysts under light irradiation by the heterojunction formation between semiconducting support and metal NPs. Two-dimensional graphitic carbon nitride (g-CN) has gained great attention in photocatalytic hydrogen generation reactions due to to its favorable band positions and visible-light activable bandgap (2.7 eV). However, g-CN suffers from high photogenerated electron-hole recombination and low visible light utilization, which inhibits its widespread application. In this regard, coupling g-CN with another semiconductor material to form heterojunctions is seen as a great option to improve its optical properties and charge dynamics. For this purpose, red phosphorus (RP), a commercially available, low-cost, visible light-active semiconductor material with its wide range of optical absorption and suitable band configuration, can be a proper option for constructing a heterojunction with g-CN. Therefore, g-CN was coupled with RP (RP/g-CN) to reach improved optical properties in this thesis. Since HAB reaction requires a metal catalyst for hydrogen generation, the as prepared RP/g-CN heterojunction was also decorated with Pt nanoparticles (NPs), and the resulting Pt/RP/g-CN ternary composites were tested in the HAB. The Pt/RP/g-CN catalyst was synthesized by using a two-step procedure comprising the liquid-phase impregnation of hydrogen hexachloroplatinate (IV) complex (hexachloroplatinic acid) to as-prepared RP/g-CN binary heterojunctions and then following chemical reduction by NaBH4. We demonstrated that the catalytic activity of Pt/RP/g-CN increased by 33% under visible light irradiation compared to dark one. The highest activity was recorded with the 5.53 wt% Pt loaded optimum RP25/g-CN75 catalyst with the turnover frequency (TOF) of 142 mol H2.mol Pt-1. min-1. The structural, chemical, and optical properties of all synthesized pristine, binary and ternary materials were investigated by using many advanced characterization methods (XRD, TEM, XPS, FTIR, UV-vis DRS, PL, TRPL, EIS). The improved activity of ternary catalyst compared to Pt/RP and Pt/g-CN binary ones was attributed to its increased visible light absorption ability and reduced electron-hole recombination. Reduction in the photogenerated charge recombination was attributed to the Schottky junction formation between both semiconductors and Pt, and heterojunction formation between RP and g-CN. To unveil the mechanism, the band structures and electron flow paths were further investigated by several experiments. RP and g-CN was revealed to have staggered band structure. Investigations revealed type-2 heterojunction formation with no hole flow making it possible to utilize higher oxidation potentials. We called this structure as complex type-2 heterojunction. Mechanistic studies were also performed by performing several trapping experiments and the roles of electrons, holes and OH• radicals were explained. Next, kinetic studies were performed with the optimum catalyst to determine rate equation and activation energy. The photocatalytic HAB was found to be first order in terms of Pt concentration and zeroth order in terms of AB concentration, and activation energy was found as 52.64 kJ/mol. The reusability and stability of the catalyst was also investigated, and activity was recorded for 10 consecutive experiments. After the initial use, the activity was noted to drastically decrease. However, after that it was observed to conserve its activity for at least ten cycles with only a small decrease after each cycle. Post-characterization of the catalyst was revealed Pt NPs agglomeration after the first-use giving rise to decrease in the activity.