Kinetic Pathways To Control Hydrogen Evolution and Nanocarbon Allotrope Formation via Thermal Decomposition of Polyethylene

SA Deshmukh and G Kamath and VG Pol and SKRS Sankaranarayanan, JOURNAL OF PHYSICAL CHEMISTRY C, 118, 9706-9714 (2014).

DOI: 10.1021/jp4109317

Polyethylene-based plastic materials are nonbiodegradable in nature and have a profound negative impact on our environment. Efficient disposal of plastic wastes in an efficient, environmental friendly fashion or chemical fixation of plastics into useful intermediates remains an outstanding problem. We employ temperature accelerated reactive molecular dynamics (TARMD) simulations to identify the kinetic pathways during thermal pyrolysis of polyethylene (PE). This allows for attainment of a dual objective viz. (1) clean fuel production via controlled hydrogen evolution and (2) formation of novel nanocarbon allotropes. Detailed atomistic picture of high temperature thermal decomposition that leads to partial or complete dehydrogenation of PE is presented. We identify the various reaction pathways for PE decomposition at high temperatures and demonstrate that a quenching- cooling strategy holds promise for tailoring the degree of graphitic order within the nanocarbon materials while simultaneously fine-tuning the evolution of clean fuel such as hydrogen gas. TARMD simulation trajectories elucidate the effect of simulated kinetic pathways on the reactive decomposition into hydrogen/flue gas/carbon, gas liquid solid phase separation of reaction products, interface dynamics, nucleation, and microstructural evolution of carbon particles. Depending on the quenching rate and the residual hydrogen content, we show that it is kinetically possible to control the reaction pathways and diffusion mechanisms and selectively produce a wide gamut of carbon allotropes (carbon onions, spheres, rods, graphene sheets to name a few). Suitable comparisons are made between simulation and our experimental results. Our simulations illustrate an environmental friendly strategy for controlled synthesis of nanocarbon materials and simultaneous clean energy production from nonbiodegradable products.

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