Recent Progress on Ammonia Cracking Technologies for Scalable Hydrogen Production

The global energy transition necessitates the development of technologies enabling cost-effective and scalable conversion of renewable energies into storable and transportable forms. Green ammonia, with its high hydrogen storage capacity, emerges as a promising carbon-free hydrogen carrier. This article reviews recent progress in industrially relevant catalysts and technologies for ammonia cracking, which is a pivotal step in utilizing ammonia as a hydrogen storage material. Catalysts based on Ru, Ni, Fe, Co, and Fe-Co are evaluated, with Co-based catalysts showing exceptional potential for ammonia cracking. Different reactor technologies and their applications are briefly discussed. This review concludes with perspectives on overcoming existing challenges, emphasizing the need for catalyst development, effective reactor design, and sustainable implementation in the context of the energy transition

Solid waste as a renewable resource

The volume of waste produced by human activity continues to grow, but steps are being taken to mitigate this problem by viewing waste as a resource. Recovering a proportion of waste for re-use immediately reduces the volume of landfill. Furthermore, the scarcity of some elements (such as phosphorous and the rare-earth metals) increases the need for their recovery from waste streams. This volume of Issues in Environmental Science and Technology examines the potential resource available from several waste streams, both domestic and industrial. Opportunities for exploiting waste are discussed, along with their environmental and economic considerations. Landfill remains an unavoidable solution in some circumstances, and the current situation regarding this is also presented. Other chapters focus on mine waste, the recovery of fertilisers, and the growing potential for compost. In keeping with the Issues series, this volume is written with a broad audience in mind. University students and active researches in the field will appreciate the latest research and discussion, while policy makers and members of NGOs will benefit from the wealth of information presented

Renewable Electricity Generation in Small Island Developing States: The Effect of Importing Ammonia

Recently, we demonstrated for Curaçao that renewable electricity generation from wind combined with energy storage in the form of ammonia is competitive with imported fossil fuels, such as LNG, oil, and coal. In the current work, we have expanded the model by considering imported green ammonia as an alternative to local electricity generation and storage. Local production of ammonia as an energy storage medium was compared with imported ammonia to make up the electricity produced from onshore wind, for Curaçao and Fiji’s largest island Viti Levu. Curaçao and Viti Levu have been selected as two interesting extremes with favorable and non-favorable wind conditions, respectively. Assuming a market price of 500 USD/t NH3, it is found that importing ammonia is the most feasible solution for both islands, with a levelized cost of electricity (LCOE) of 0.11 USD/kWh for Curaçao and 0.37 USD/kWh for Viti Levu. This compares to 0.12 USD/kWh for Curaçao; however, for Viti Levu, this value increases to 1.10 USD/kWh for a completely islanded system based on onshore wind and imported ammonia. These islands represent two extreme cases in terms of wind load factor and load consistency, as Curaçao has a high and consistent wind load factor when compared to Viti Levu. Thus, the conclusions obtained for these locations are expected to be applicable for other small island developing states

Green ammonia enables sustainable energy production in small island developing states: A case study on the island of Curaçao

Small Island Developing States (SIDS) have a high dependency on fossil fuels for energy, water, and food production. This has negative implications on the carbon footprint and resilience of the SIDS. Wind power is one of the most promising options for renewable energy in the coastal areas of the SIDS. To account for the seasonal intermittent nature of wind energy, ammonia can be used for energy storage. In this paper, ammonia as an energy vector, is examined to reduce the costs and carbon footprint of energy on the island of Curaçao as a showcase for Caribbean SIDS. The levelized cost of electricity (LCOE) for the combined wind and ammonia energy storage system is 0.13 USD/kWh at a discount rate of 5%. This is cost competitive with the LCOE of 0.15–0.17 USD/kWh from heavy fuel oil, which is the main electricity source in the Caribbean SIDS. In Curaçao, the LCOE from LNG and coal without carbon capture and storage (CCS) is 0.07–0.10 USD/kWh and 0.09–0.14 USD/kWh, respectively. When CCS is applied, the LCOE from LNG and coal is 0.10–0.13 USD/kWh and 0.14–0.21 USD/kWh, respectively. This suggests that the LCOE of the combined wind and ammonia energy storage system can be competitive with fossil-based alternatives with carbon capture and storage (CCS) in a decarbonized energy landscape. The CO2-footprint of the combined wind energy and ammonia energy storage system is 0.03 kg CO2/kWh, compared to 0.04 kg CO2/kWh and 0.12 kg CO2/kWh for LNG-/coal-based energy generation with CCS, respectively

Elucidating transport and reaction processes in solid–liquid interfaces using attenuated total reflectance infrared spectroscopy (ATR-IR)

ATR-IR spectroscopy has become an ubiquitous tool for the study of heterogeneous catalysts, probing selectively the chemistry of the solid-fluid (gas and liquid) interfaces under reaction conditions in real-time. By placing a thin catalyst layer on top of an ATR crystal, one can restrict the analysis volume to the solid–liquid interface, effectively reducing the interference of the liquid in the final spectra. The present contribution is providing a tutorial overview of ATR-IR spectroscopy for the study of solid–liquid interfaces of heterogeneous catalysts. First, we review the fundamentals of ATR-IR spectroscopy and the importance of the flow cell design, transport processes, concentration gradients, deposition of stable catalyst layers, spectra acquisition, and data interpretation. Then, we explore the utilization of the technique to study catalysts operating in liquid phase at steady-state, transient, and dynamic conditions. Finally, we provide a brief analysis of the latest developments in the field and the outlook of the technique in the context of the energy transition. Graphical abstract: ATR-IR spectroscopy probing liquid–solid catalyst interfaces.</p

Enhanced transport in Gas-Liquid-Solid catalytic reaction by structured wetting properties: nitrite hydrogenation

This work presents a new approach to improve mass transfer in and around catalyst particles in three-phase operation with micro-structured catalysts, containing hydrophilic and hydrophobic domains. Partially hydrophilic catalysts were prepared via physical mixing of hydrophobic perfluorinated octyltrichloro silane (FOTS)/γ-Al2O3 domains and hydrophilic Pd/γ-Al2O3 domains, resulting in manipulation of water wetting, both at the external surface and the pores inside the support particles. The modified catalysts were characterized with elemental analysis, XRF, N2 physisorption and light microscopy after selective dyeing hydrophobic and hydrophilic domains. The catalysts are tested for hydrogenation of nitrite in water, which is an extremely fast reaction whereas the product distribution (N2 versus NH4+) is also easily influenced by internal concentration gradients. Noticeably, the partially hydrophilic catalyst is more active and produces more ammonium compared to hydrophilic catalyst. This work demonstrates that this way of structuring the catalyst enables influencing the internal concentration gradients for aqueous systems. For the case of nitrite hydrogenation, we show that structured catalysts achieve the same rate per gram Pd at lower hydrogen pressure compared to classical hydrophilic catalysts. This results in formation of less ammonia, which is of practical importance for cleaning of drinking water

In-situ ATR-IR Spectroscopy Reveals Complex Absorption-Diffusion Dynamics in Model Polymer-Membrane-Catalyst Assemblies (PCMA)

Molecular transport in porous media plays an essential role in heterogeneous catalysis. Here, we have studied a Polymer-Membrane-Catalyst Assembly (PCMA) system consisting of PET membranes with a well-defined pore structure coated with thermo-responsive polymer (poly(n-isopropylacrylamide) or p-NIPAM) coupled to an in-situ ATR-IR cell containing palladium supported on γ-Al 2O 3. This PCMA model is designed to mimic the structure of a core-shell catalyst coated with stimulus responsive polymer. At low temperatures (32 °C), we observed faster CO−Pd saturation. Our multi-physics model of the CO transport and chemisorption on the PCMA suggests that the delay is caused by the strong affinity of the CO molecules with the polymer brushes. When stimulus responsive polymers are present, a complex interplay between diffusion and absorption determines the dynamic behavior of the system. Furthermore, we demonstrate that this behavior could be reversed by reducing the ratio of polymer inside the pores and the surface, in which the delay observed is now above the LCST. This new insights on the dynamics of transport, absorption, and chemisorption processes occurring on so-called “nano-reactors” provide new opportunities for developing new self-regulating catalysts with faster transitions between ON- and OFF- states

N-isopropylacrylamide polymer brushes alter the micro-solvation environment during aqueous nitrite hydrogenation on Pd/Al2O3 catalyst

Nitrite contaminants in freshwater streams, resulting from run-off of fertilizers and livestock farming, are a major ecological challenge. Here, we have developed a new family of catalysts based on Pd/Al2O3 coated with N-isopropylacrylamide polymer (p-NIPAM) brushes that increase N-N bond formation over N-H bond formation, promoting nitrogen selectivity by 3-fold, reaching >99% for the Pd/Al2O3 containing 20 wt% carbon in the form of p-NIPAM, without significant drops in catalytic activity (TOF of c.a. 6.8 ± 1.1 min−1). Strikingly, rigorous mass transport studies revealed that the presence of p-NIPAM does not limit the transfer of molecules during the hydrogenation of nitrites in aqueous phase. These observations were corroborated by detailed reaction kinetics in which similar activation barriers for nitrites disappearance of 30–34 kJ mol−1 were obtained regardless the polymer content. The observed reaction orders for nitrites were similar on the coated and un-coated catalysts, indicating that the rate determining step, most likely NOX-H bond formation, remained unaltered. The apparent barriers for ammonia formation, however, drastically increased from 41 ± 3 kJ mol−1 on Pd/Al2O3 to 63 ± 4 kJ mol−1 and 76 ± 5 kJ mol−1 on the 7 and 20 wt% C counterparts, respectively. Contrary to the widely accepted operation mode of thermo-responsive catalysts containing p-NIPAM brushes, we demonstrated that these polymers modify the chemical environment near the active site as shown by in-situ ATR data, tuning the catalyst selectivity without altering the molecular transport. These results will facilitate the development of more selective catalysts for liquid phase reactions relevant for drinking water purification

The onset of mass transport limitations triggers the stimulus responsiveness of polymer coated catalysts

A series of Pd/Al2O3 catalysts coated with N-isopropylacrylamide polymer (p-NIPAM) brushes with increasing particle size of the support were prepared to study the interplay between mass transport limitations and the ability of the thermo-responsive catalysts to modify reactivity during reaction. Nitrite hydrogenation in water was chosen as probe reaction, which is a key step in the reduction of nitrites and nitrates from drinking water. The results show that the apparent activation energy decreases on increasing temperature above the LCST, i.e. the temperature where the polymer undergoes a phase change, for catalyst with particle sizes between 38 and 100 μm, alluding to a significant increase in mass transfer limitations. In sharp contrast, both p-NIPAM modified catalysts with smaller particle size and uncoated catalysts do not show this change in activation energy with temperature. Detailed multi-physics mass transport and reaction modelling indicated that the transport of the limiting reagent, hydrogen in this case, is severely reduced when the p-NIPAM collapses at temperatures above its LCST. It is concluded that effective reaction control using stimulus responsive polymers requires the system to be close to the mass transport limitation regime, to maximize the effect of the conformation change of the polymer on the catalyst performance and achieve sharp and reversible transitions from active to inactive