Combustion engines are still required in power-generation and automobile industry for more decades even in switching to electrification. For the sustainable development, lean combustion at larger air-to-fuel ratios suppresses fuel consumption due to higher heat efficiency. Its exhaust gas is cooler accordingly, so that catalytic conversion must be accomplished at lower temperatures. We are tackling this crucial issue with new materials, i.e. Pt cluster disks chemically bound to a Si semiconductor substrate, PtN/Si (N=5-60) , where electrons are accumulated at their sub-nano interface [2,3]. In this talk, their prominent low-temperature catalytic activity is unveiled on a basis of surface-chemistry measurements.
We have found that CO oxidation by atomic O species on the Pt30/Si disks starts at 130 K , which is lower by 150 K than that on the Pt(111) single-crystal surface . Furthermore, NO reduction proceeds at lower by 100 K than on supported Pt nano-particles. This particularity was observed also in turnover rates (TOR) under a steady-state condition of continuous supply of CO and O2 . Furthermore, hysteresis in the TOR was discernible in the heating and subsequent cooling periods due to the bistability switching between O- and CO-rich regimes on PtN/Si, while no hysteresis for Pd nano-particles on an MgO substrate . Considering a report that a smaller CO-oxidation rate than the dissociative adsorption rates of O2 damps the hysteresis of nano-reactors , the atomic O species produced by PtN/Si are highly reactive to reduce the reaction temperature as well as to maintain the hysteresis even in sub-nano ranges. This is also true in the NO reduction, in which reactive atomic N species recombine into N2 at lower temperatures. It is probable that the low-temperature and highly-efficient catalytic activities of PtN/Si derive from their electron accumulation.
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