(114b) Utilizing a Modular Induction Reactor, on-Demand Ammonia Synthesis Harmonizes Seamlessly with Intermittent Renewable Energy Patterns.

Ammonia (NH₃) is distinguished as an ideal hydrogen carrier by its high volumetric hydrogen energy density of 10.7 kg H₂/100 l. This property, coupled with its ease of liquefaction, handling, storage and transport, makes ammonia a promising component of a sustainable energy ecosystem. Despite these advantages, about 90 % of the world's ammonia production still relies on the centuries-old Haber-Bosch process, a thermocatalytic conversion of nitrogen (N₂) and hydrogen (H₂) at high temperatures (above 300 °C) and high pressure (above 15 MPa). The Haber-Bosch process is one of the largest industrial chemical processes in the world and has been hailed as a groundbreaking invention of the 20th century, winning three Nobel Prizes in Chemistry.

In our quest to modernise and optimise ammonia synthesis, we present a breakthrough approach focused on on-demand, delocalised ammonia synthesis. This innovative process exploits the remarkable hydrogen storage capacity and favourable physical properties of ammonia to address the challenges associated with renewable energy integration, hydrogen storage and efficient energy conversion.

The breakthrough in our approach is the operation of the reactor where the reactants and products are kept at near room temperature, only 3 cm from the catalyst bed. This proximity not only contributes to safety and efficiency, but also minimises energy losses throughout the synthesis process. By operating the reactor under low hydrogen conditions at 65 bar and 600 °C, we achieved an impressive ammonia production rate of over 10 mmol_NH3/(g_cath). These results are proof of the efficiency and potential of our approach.

Key to our approach is the development of a customised reactor equipped with an alternating magnetic field for catalyst heating. The catalyst at the heart of our system consists of a ferromagnetic alloy of cobalt (Co) and nickel (Ni) surrounded by an alumina shell and decorated with 0.8 wt% ruthenium (Ru). This novel catalyst design improves reaction kinetics and ensures selectivity of ammonia synthesis, which is a significant difference from conventional catalytic systems.

Our catalysts represent a significant advance in ammonia synthesis. They are characterised by an exceptionally fast heating capability, which enables us to reach temperatures of over 800°C in less than 5 minutes. This fast temperature behaviour is crucial for the precise control of catalytic reactions and the improvement of overall efficiency. In addition, the composition of the ferromagnetic core in our catalysts is tunable, giving us the flexibility to modulate the Curie temperature of the material. This tunability allows us to optimise the performance of the catalyst under different conditions, adding a new dimension to our innovative approach. By tuning the material properties to the specific reaction requirements, we open up a range of possibilities for fine-tuning the synthesis process and improving the sustainability of ammonia production overall.

To investigate the temperature-dependent formation of the CoNi alloy, we performed temperature-programmed X-ray diffraction (XRD) analysis. Our results show that the maximum achievable temperature for heat treatment of the samples is 400 °C in an air atmosphere. Beyond this threshold, we observe the formation of Ni and Co aluminate phases together with the thermodynamically stable form of alumina, α-Al₂O₃. This means that homogeneous distribution and the formation of a CoNi alloy are no longer possible, as the fusion of these metals only occurs at significantly higher temperatures, usually above 850 °C.

Furthermore, when measuring in a hydrogen environment, we observed the formation of the CoNi alloy at about 500 °C, followed by the growth of nanoparticles (NPs) at temperatures of 850 °C and above. Despite the XRD results, H₂-TPR (hydrogen temperature programmed reduction) measurements and VSM (vibratory sample magnetometry) data suggest that reduction at temperatures below 850 °C is not sufficient to achieve the desired results.

We performed hydrogen temperature-programmed reduction to investigate the reduction profiles of bimetallic nanoparticles. The aim was to determine the optimal reduction temperature of 850 °C and to find out whether it ensures complete reduction of CoNi nanoparticles. In addition to varying the ratio of Co:Ni and CoNi:Al₂O₃ in the samples, samples with pure Co and pure Ni NPs embedded in alumina were also prepared. The latter samples should provide information on how the alloying of the metals influences the reduction profiles.

Our observations revealed a common pattern in all the materials we processed, characterised by two distinct clusters of reduction peaks. The low temperature (Low T) cluster generally ended before 400 °C, while the high temperature (High T) cluster ended before 900 °C. Remarkably, these observed reduction temperatures exceeded expectations for Co and Ni NPs. This discrepancy can be attributed to strong metal-support interactions (SMSI), a phenomenon that is well documented in the literature.

Our results suggest that Co-NPs require a much higher reduction temperature compared to Ni-NPs. Interestingly, alloying these metals lowered the temperature required for complete reduction by almost 100 °C, from 950 °C for pure Co nanoparticles to less than 850 °C for the Co₆₇Ni₃₃-A50 material.

We hypothesise that the low-T peak cluster is associated with CoNi-NPs that are not completely covered by alumina, leading to weak interactions. In contrast, the high-T peak cluster likely represents NPs that are closely associated with alumina and possibly fully embedded in the alumina matrix. Within the high-T cluster, the peaks below 650 °C are associated with the initial reduction step of Ni²⁺ and Co^2+/3+. The peak at about 650-700 °C probably corresponds to the complete reduction of Ni, while the peak between 750 and 800 °C is probably related to the complete reduction of Co.

Interestingly, our experiments with pure CoNi precipitate (without Al₂O₃) showed complete reduction at temperatures below 500 °C. This result is consistent with our hypothesis that the increased reduction temperature is caused by SMSI. This conclusion is further strengthened when comparing the Co₆₇Ni₃₃-A70 and -A50 materials, where the peaks of the high reduction temperatures are up to 70 °C lower in the latter material, mainly due to the lower alumina content and the less importance of SMSI interactions.

We investigated the magnetic properties of the synthesised materials using two vibrating sample magnetometers. The measurements were performed in air at room temperature and in an H2 atmosphere at elevated temperatures. It is found that increasing the Co content in the alloys leads to higher values of Ms (saturation magnetisation), Mr (residual magnetisation) and Hci (coercive field), independent of the VSM used. In particular, the difference in magnetisation (M), measured at 100 °C and 500 °C with an applied field of 400 Oe, becomes less pronounced with higher Co content.

X-ray photoelectron spectroscopy (XPS) was used to determine the oxidation states of the metals in the pCN-BZ and CxN1-x-AZ-850-1 particles. The focus was on Co 2p_3/2 and Ni 2p_3/2. We performed peak deconvolution according to the spectral fitting parameters established by Biesinger et al. (10.1016/j.apsusc.2010.10.051).

From the primary peak at 856.0 eV, it is evident that the oxidised Ni species is Ni(OH)₂. In the case of Co, the predominant surface species prior to reduction appears to be Co₃O₄, as indicated by the triple peak at about 780.3, 781.6 and 782.9 eV. After reduction and subsequent reoxidation, Co(OH)₂ is the predominant species, as evidenced by the formation of a distinct peak at about 786.5 eV. The coexistence of the two metals leads to a slight increase in the binding energy of Co by about 0.4 eV, while Ni remains relatively unaffected. In particular, the presence of alumina appears to shift the peak positions to slightly lower binding energies, which are about 0.4 eV lower.

When exposed to environmental conditions, the materials undergo a reoxidation process. During this phase, we observe some accumulation of cobalt on the surface of the nanoparticles (NP). However, in all cases we can detect traces of metallic Ni (peak at 853.4 eV) and Co (peak at 778.2 eV). It is worth noting that the amount of metal detected is consistently less than 10% in all cases.

Comparing the theoretically calculated Ms values with the measured results, significant differences can be seen, which are primarily due to the rapid passivation of the metal. As expected, no significant difference is found between the results obtained at RT in different atmospheres, as the reduction at room temperature is minimal. However, notable differences become apparent when the samples are heated to 400 °C in an H2 atmosphere and subsequently cooled, resulting in increased Ms values for all samples.

Our research highlights the feasibility and potential of demand-controlled ammonia synthesis as a key element of a sustainable energy infrastructure. By exploiting the hydrogen storage properties of ammonia and its ease of handling, combined with a customised reactor that uses induction heating and a high-performance catalyst, we present a practical solution for volatile renewable energy sources. This work contributes to ongoing efforts to advance the field of sustainable energy conversion and storage and paves the way for a cleaner and more reliable energy future.