Alternative fuels are essential to enable the transition to a sustainable and environmentally friendly energy supply. Synthetic fuels derived from renewable energies can act as energy storage media, thus mitigating the effects of fossil fuels on environment and health. Their economic viability, environmental impact, and compatibility with current infrastructure and technologies are fuel and power source specific. Nitrogen-based fuels pose one possible synthetic fuel pathway. In this review, we discuss the progress and current research on utilization of nitrogen-based fuels in power applications, covering the complete fuel cycle. We cover the production, distribution, and storage of nitrogen-based fuels. We assess much of the existing literature on the reactions involved in the ammonia to nitrogen atom pathway in nitrogen-based fuel combustion. Furthermore, we discuss nitrogen-based fuel applications ranging from combustion engines to gas turbines, as well as their exploitation by suggested end-uses. Thereby, we evaluate the potential opportunities and challenges of expanding the role of nitrogen-based molecules in the energy sector, outlining their use as energy carriers in relevant fields.

In this work, a series of VO x -loaded In2O3 catalysts were prepared, and their catalytic performance was evaluated for CO2-assisted oxidative dehydrogenation of propane (CO2-ODHP) and compared with In2O3 alone. The optimal composition is obtained on 3.4V/In2O3 (surface V density of 3.4V nm–2), which exhibited not only a higher C3H6 selectivity than other V/In catalysts and In2O3 under isoconversion conditions but also an improved reaction stability. To elucidate the catalyst structure–activity relationship, the VO x /In2O3 catalysts were characterized by chemisorption [NH3-temperature-programmed desorption (TPD), NH3-diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), CO2-TPD, and CO2-DRIFTS], H2-temperature-programmed reduction (TPR), in situ Raman spectroscopy, UV–vis diffuse reflectance spectroscopy, near-ambient pressure X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and further examined using density functional theory. The In–O–V structure and the extent of oligomerization, which play a crucial role in improving selectivity and stability, were identified in the VO x /In2O3 catalysts. In particular, the presence of surface VO x (i) inhibits the deep reduction of In2O3, thereby preserving the activity, (ii) neutralizes the excess basicity on In2O3, thus suppressing propane dry reforming and achieving a higher propylene selectivity, and (iii) introduces additional redox sites that participate in the dehydrogenation reaction by utilizing CO2 as a soft oxidant. The present work provides insights into developing selective, stable, and robust metal-oxide catalysts for CO2-ODHP by controlling the conversion of reagents via desired pathways through the interplay between acid–base interactions and redox properties.

The nature of surface oxygen species on/in a silver powder catalyst and their reactivity with ethylene were systematically investigated with multiple spectroscopies and DFT calculations. Unique quasi in situ HS-LEIS, in situ NAP-XPS, and in situ Raman spectroscopy demonstrated that the silver surface is covered by a thin oxide layer (1–3 nm) after an oxidation treatment and during ethylene oxidation. Periodic DFT models allowed assignments of oxygen species detected by in situ Raman spectroscopy with detailed structures on p(4 × 4)–O–Ag(111) surfaces. In situ NAP-XPS and Raman spectroscopy suggest that Ag4–O2 on oxidized Ag is the active oxygen species for ethylene epoxidation. The experimental and theoretical methodologies developed in the present work serve as an efficient toolbox for the scientific investigation of the silver–oxygen system for selective oxidation reactions and rational guidance of computational calculations coupled with experimental findings.

Supported V2O5–WO3/TiO2 materials are employed as selective catalytic reduction (SCR) catalysts for NO x emission control from power plants. Fresh SCR catalysts usually receive exposure to harsh treatments in the industry to accelerate catalyst activation (calcination in air at 650 °C) and catalyst aging (hydrothermal aging at 650 °C) in a way that represents various points in the catalyst/product lifetime. The present study investigates the catalyst structural and chemical changes occurring during such harsh treatments. Three series of supported V2O5–WO3/TiO2 catalysts were prepared by incipient-wetness impregnation of aqueous ammonium metavanadate and metatungstate precursors. The catalysts were subsequently dried and calcined at 550 °C in O2, 650 °C in O2, and hydrothermal conditions (10% O2, 8% H2O, 7% CO2, and 75% N2) at 650 °C. The resulting catalysts were physically characterized by numerous techniques (in situ Raman; in situ IR; in situ high-field–high-spinning solid-state 51V MAS NMR; in situ electron paramagnetic resonance; X-ray diffraction; Brunauer, Emmett, and Teller surface area; and inductively coupled plasma) and chemically probed with adsorbed ammonia, SCR–TPSR, and the SCR reaction. The surface WO x sites on the TiO2 support behave as a textural promoter that stabilizes the TiO2 (anatase) phase from sintering and transforming to the undesirable crystalline TiO2 (rutile) phase that can lead to formation of a Ti1–x V x O2 (rutile) solid solution with reduced V4+ cations (∼7–15%). The surface VO x sites are mostly oligomerized as surface V5+O x sites (∼50–85% oligomers) and the extent of oligomerization tends to increase with surface WO x coverage and calcination temperature. A major difference between the calcined and hydrothermally treated catalysts was the low concentration of surface NH3 * species on Lewis acid sites for the hydrothermally treated catalysts, yet the SCR activity was almost comparable for both catalysts. This finding suggests that surface NH4 +*, primarily associated with the surface VO x sites, are able to efficiently perform the SCR reaction. Given that multiple catalyst parameters were simultaneously varying during these treatments, it was difficult to correlate the SCR activity with any single catalyst parameter. A correlation, however, was found between the SCR TOF/activity and the sum of the surface NH3 * and NH4 +* species, which is dominated by the surface NH4 +* species.

Bimetallic alloy catalysts frequently demonstrate distinct performances that are superior to their monometallic counterparts, yet their surface chemistry needs to be carefully studied to understand their structure−activity relationships. The nanoporous Ag 0.03 Au 0.97 alloy catalyst becomes highly active and selective for oxidative methanol coupling to methyl formate after O 3 activation. HS-LEIS reveals the O 3 treatment results in enrichment of Ag (>30%) on the outermost surface layer, while oxygen treatment additionally leads to segregation of a larger portion of Cu impurity on the surface. A series of characteristic Raman bands at 395, 577, 867, and 904 cm −1 only form under oxidative methanol coupling reaction on O 3 -activated AgAu catalyst. These bands correspond to Ag 3 −O* (395 cm −1 ), M−O* on O−Au(111) and AgAu alloy (577 cm −1 ), CH 3 OH* (867 cm −1 ), and HOOH* (904 cm −1 ), as revealed by DFT calculations. The cyclic in situ Raman and reactivity studies indicate the detected oxygen species could be related to a "memory effect" of the catalyst upon pretreatment. The current study highlights the importance of applying surface-specific techniques for investigation of compositions of outermost surface layers of alloy catalysts, as well as integration of in situ spectroscopies and computational investigations for understanding surface structures at the molecular level under reaction conditions.

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