Encouraged by the great promise of metal-nitrogen-carbon (M-N-C) materials in replacing Pt for catalyzing the oxygen reduction reaction (ORR), metal-P species were successfully introduced into carbon matrices in experiments and have exhibited high catalytic activity for the ORR. Here, by means of comprehensive density functional theory (DFT) computations, we investigated the origin and the mechanism of the ORR occurring on Fe- and Co-P-embedded graphenes. Our computations have revealed that the Fe- and Co-P4 moiety-embedded graphenes possess good stability and high chemical reactivity for O2 activation, thus facilitating the subsequent ORR steps, and a more efficient 4e pathway in both acidic and alkaline media is more energetically favorable. Furthermore, by analyzing the computed free energy profiles, the Fe-P4 species-embedded graphene is a more efficient electrocatalyst for the ORR in an alkaline medium than the Co-P4 species-embedded graphene. Our DFT computations will be useful for gaining deeper insight into the high activity of metal-P species.
Abstract:The authors applied one-dimensional (1-D) simulation and 3-D Computational Fluid Dynamics (CFD) simulation to evaluate the potential of in-cylinder control methods on a low-speed 2-stroke marine engine to reach the International Maritime Organization (IMO) Tier 3 NO x emissions standards. Reducing the combustion temperature is an important in-cylinder measure to decrease NO x emissions of marine diesel engines. Miller-cycle and Exhaust Gas Recirculation (EGR) are effective methods to reduce the maximum combustion temperature and accordingly decrease NO x emissions. The authors' calculation results indicate that with a combination of 2-stage turbocharging, a mild Miller-cycle and 10% EGR rate, the NO x emissions can be decreased by 48% without the increased Specific Fuel Oil Consumption (SFOC) penalties; with a medium Miller-cycle and 10% EGR, NO x can be decreased by 56% with a slight increase of SFOC; with a medium Miller-cycle and 20% EGR, NO x can be decreased by 77% and meet IMO Tier 3 standards, but with the high price of a considerable increase of SFOC. The first two schemes are promising to meet IMO Tier 3 standards with good fuel economy if other techniques are combined.
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