Development of highly efficient platinum nanocatalysts for the dehydrogenation of ammonia borane via rational design of graphitic carbon nitride-based heterojunction photocatalysts

In the last decade, the use of hydrogen as an efficient energy carrier has already been started in fuel cells, portable devices, public transportations, and grid systems thanks to the advancements in the related technologic/scientific area. However, the safe storage and delivery of hydrogen are still on-going challenges in front of the hydrogen energy-based society. The implementation of solid chemical hydrogen storage compounds as the hydrogen energy carrier systems is one of the most promising solutions. Among the solid chemical hydrogen storage materials, ammonia borane (NH3BH3, AB) arises as one of the most convenient material for the chemical hydrogen storage owing to its advantageous properties. Among the several methods applied for H2 generation from AB, the hydrolysis route which results in the release of 3 equivalents of H2 gas in the presence of a suitable metal catalyst has been mostly preferred one for the mobile applications. Up to date, numerous catalysts have been tested in the HAB, among which platinum nanoparticles (Pt NPs) supported on high-surface-area and porous materials have been reported with one of the highest catalytic activities for the hydrolysis of AB (HAB). However, those Pt NPs were generally synthesized by following the methods comprising high-temperature surfactant-assisted decomposition and reduction of Pt precursors, and then the obtained highly monodisperse Pt NPs were deposited on a suitable support material to prevent their agglomeration. Supported Pt NPs can also be synthesized by the in-situ synthesis method involving the impregnation of Pt precursor into the support material followed by their reduction in catalytic reaction medium, which has distinct advantages over the initial one such as being sustainable, practical, green, atom-economical, cost-effective, and timesaving. Here, the selection of support material is so crucial to obtain stable and reusable nanocatalysts. Graphitic carbon nitride (g-CN) has been suggested as an appropriate substrate to stabilize Pt NPs due to its nitrogen-rich "six-fold interstices" between tri-s-triazine units, having capability of strong interaction with incorporated metal, and its adjustable surface area and morphology obtained by following different synthesis procedures. Besides, g-CN is a visible light active semiconductor material with suitable band positions for the construction of heterojunctions with Pt NPs and other suitable semiconductor materials. In this thesis, it was aimed to enhancement of the catalytic activity of Pt nanocatalysts in the HAB via the rational design of g-CN based heterojunction photocatalysts. To fulfill this aim, firstly, a novel method for the in-situ generation of Pt NPs supported on mesoporous g-CN (m-g-CN/Pt) during the catalytic HAB was realized under the white-light irradiation. Secondly, U-g-CN, which was synthesized by using urea as an amine precursor via an easy and green synthesis approach compared to m-g-CN, was used as a support material for the in-situ generation of Pt NPs. U-g-CN/Pt showed better catalytic performance than m-g-CN/Pt nanocatalysts under the same conditions. Next, to further enhancing the catalytic activity of the Pt-based nanocatalysts in the HAB, we turned our attention to the in-situ synthesis of the bimetallic MPt NPs supported on U-g-CN (U-g-CN/MPt). Indeed, U-g-CN/Pt92Au8 and U-g-CN/Pt85Au15 provided a higher catalytic activity (hydrogen production rate of 68.9 and 67.4 L H2 gPt-1 min-1, respectively) than the monometallic counterpart (60.4 L H2 gPt-1 min-1), which was attributed to the alloy effect, surface plasmon resonance (SPR) contribution, and the heterojunction formation. Thirdly, on the way to thesis's main goal, we tried to further enhance the activity of Pt nanocatalysts in the HAB by supporting them on U-g-CN/WOx binary nanocomposites by considering the optical properties and charge kinetics of U-g-CN can be improved via the construction of heterojunction with other semiconductors having proper band gap and band potentials. The results revealed that U-g-CN/a-WOx/Pt nanocatalysts showed a higher catalytic activity than U-g-CN/Pt by providing a maximum hydrogen production rate of 48.1 L H2 gPt-1 min-1. All the yielded Pt nanocatalysts (m-g-CN/Pt, U-g-CN/Pt, U-g-CN/MPt, and U-g-CN/a-WOx/Pt) were characterized via many advanced analytical techniques including TEM, HR-TEM, XPS, XRD, ICP-MS, FTIR, PL, and TRES techniques to clarify the reasons behind the enhanced photocatalytic activities in each step. Moreover, the rate law and the activation parameters were also derived by using the data of kinetic studies. Additionally, a reusability test for each developed Pt based nanocatalyst in HAB was also reported. In summary, this thesis demonstrates that g-CN is a appropriate substrate as support and stabilizer for the in-situ synthesis of catalytically active Pt or MPt NPs in hydrogen production from the HAB due to its availability for rational design of heterostructures.