Transition metal nitride catalysts for selective conversion of oxygen-containing molecules.

Earth abundant transition metal nitrides (TMNs) are a promising group of catalysts for a wide range of thermocatalytic, electrocatalytic and photocatalytic reactions, with potential to achieve high activity and selectivity while reducing reliance on the use of Pt-group metals. However, current fundamental understanding of the active sites of these materials and the mechanisms by which selective transformations occur is somewhat lacking. Recent investigations of these materials from our group and others have utilized probe molecules, model surfaces, and in situ techniques to elucidate the origin of their activity, strong metal-support interactions, and unique d-band electronic structures. This Perspective discusses three classes of reactions for which TMNs have been used as case studies to highlight how these properties, along with synergistic interactions with metal overlayers, can be exploited to design active, selective and stable TMN catalysts. First, studies of the reactions of C1 molecules will be discussed, specifically highlighting the ability of TMNs to activate CO2. Second, the upgrading of biomass and biomass-derived oxygenates over TMN catalysts will be reviewed. Third, the use of TMNs for H2 production via water electrolysis will be discussed. Finally, we will discuss the challenges and future directions in the study of TMN catalysts, in particular expanding on opportunities to enhance fundamental mechanistic understanding using model surfaces, the elucidation of active centers via in situ techniques, and the development of efficient synthesis methods and design principles.

Controlling Product Distribution of Polyethylene Hydrogenolysis Using Bimetallic RuM3 (M = Fe, Co, Ni) Catalysts.

Plastic hydrogenolysis is an attractive approach for producing value-added chemicals due to its mild reaction conditions, but controlling product distribution is challenging due to the formation of undesired CH4. This work reports several bimetallic RuM3/CeO2 (M = Fe, Co, Ni) catalysts that shift the product of low-density polyethylene hydrogenolysis toward longer-chain hydrocarbons. These catalysts were characterized by using X-ray absorption fine structure spectroscopy, electron microscopy imaging, and H2 temperature-programmed reduction. The combined catalytic evaluation and characterization results revealed that the product distribution was regulated by the formation of bimetallic alloys. A model compound, n-hexadecane, was selected to further understand the differences in hydrogenolysis over the Ru-based catalysts. Although a longer reaction time shifted the product toward smaller molecules, the bimetallic (RuCo3/CeO2) catalyst limited the further conversion of C2-C5 into CH4. This work highlights the role of bimetallic alloys in tailoring the interaction with hydrocarbons, thereby controlling the product distribution of polymer hydrogenolysis.

Plasmon-Enhanced Greenhouse Selectivity for High-Temperature Solar Thermal Energy Conversion.

The greenhouse effect arises when thermal radiation is forced to undergo absorption and re-emission many times before escaping, while sunlight transmits largely unimpeded. Although this effect is responsible for global warming, it is generally weak in solid-state materials because radiation can be easily overpowered by other modes of heat transfer. Here, we report on the use of infrared plasmonic nanoparticles to enhance the greenhouse effect in transparent mesoporous materials. Local surface plasmon resonances in transparent conducting oxide nanoparticles (TCO NPs) selectively shorten the mean free path of thermal photons while maintaining high solar transmittance. The addition of a small amount of TCO NPs (<0.1% by volume) nearly halves the heat losses at 700 °C. This leads to an experimentally demonstrated effective thermal emittance of ∼0.17 at 700 °C, which is the lowest reported value to date, among all selective surfaces and transparent insulating materials measured at 650 °C or above. Our results show that plasmon-enhanced greenhouse selectivity (PEGS) is a promising mechanism for spectral control of radiative heat transfer, and more specifically, for conversion of minimally concentrated sunlight into high-temperature heat.