Abstract
The overall objective of GREEN Hydrogen frOm Urea rich waStewater using novel catalytic processEs (GREENHOUSE) is to develop a novel proof-of-concept catalytic process for the production of green hydrogen (H2) from urea-rich wastewater. The project addresses two major challenges: 1) the sustainable production of green H₂ using a renewable, non-critical and nitrogen-based raw material, and 2) the valorisation of highly polluting livestock-derived urea waste, which is detrimental to soil pH, groundwater quality and agricultural productivity. H₂ is one of the most promising alternatives to fossil fuels. However, the green credentials of H₂ strongly depend on the method used for its production. Currently, steam reforming of carbon-rich fossil fuels still dominates industrial H₂ production. Despite the significant progress achieved in water splitting using photons, electrons, thermal energy, or their combination, these technologies still account for only a limited fraction of total H₂ production and face important challenges before large-scale implementation. Therefore, it is critical to develop novel and complementary processes to produce green H₂ and meet the anticipated future demand. GREENHOUSE proposes a catalytic waste-to-energy strategy for converting livestock-derived nitrogen-containing waste into green H2. Urea, (NH2)2CO, one of the main components of agricultural and farm wastewater, hydrolyses to ammonium ions, which may further lead to nitrous oxide formation through nitrification–denitrification processes. These products contribute to soil acidification, smog formation in densely urbanised regions, and environmental degradation. They also affect agricultural productivity, leading to government regulations controlling the amount of ammonium in soil. The project will therefore develop novel catalytic reforming technologies to convert this polluting nitrogen-containing waste into green H2, which can be used as fuel and/or as a feedstock for the production of chemicals. Recent advances in single-atom catalysis, controlled synthesis of supported multi-metallic clusters and nanoparticle-based catalysts provide the scientific basis for this ambitious catalytic waste-to-energy project. GREENHOUSE is expected to deliver a step-change in addressing energy and environmental challenges by using urea-rich animal farm waste as a non-critical feedstock for green H2 generation. The outcome of the project will significantly contribute to the decarbonisation of the energy sector, while mitigating the negative impact of animal waste on the environment in general and agricultural soil in particular. Results Achieved The GREENHOUSE project has first established the analytical and experimental framework required to investigate urea decomposition and its potential use as a route toward ammonia and hydrogen production. The project is coordinated by Prof. Nikolaos Dimitratos at the University of Bologna, together with Prof. Daniele Caretti, and involves the University of Milan unit led by Prof. Alberto Villa. A first important achievement was the development of reliable analytical protocols for the quantification of ammonia, isocyanic acid, urea and possible by-products. UV–Vis spectroscopy, standard-addition experimental procedures and HPLC analysis were implemented to ensure reproducible product detection, reliable mass balances and robust interpretation of the reaction pathway. These methods represented an essential basis for all subsequent catalytic experiments. Systematic studies were then performed in batch autoclave reactors to evaluate the effect of temperature, reaction time and catalyst loading. The results demonstrated that urea hydrolysis is strongly temperature dependent. Urea conversion and ammonia yield increased clearly between 135 °C and 165 °C, with conversion values exceeding 86–93% at 165 °C. These data confirmed that higher temperature significantly accelerates both urea thermolysis and the hydrolysis of isocyanic acid into ammonia. An important activity of the project was the direct comparison between TiO2-P25 and γ-Al2O3 under identical reaction conditions as reference materials. XRD, BET and NH₃-TPD characterisation allowed the identification and elucidation of structure–property–activity relationships. TiO2, mainly promoted HNCO hydrolysis and helped reduce the accumulation of this intermediate. γ-Al2O3, characterised by higher surface area and stronger acidity, showed a more complex catalytic behaviour. At low catalyst loading, its acidic sites appeared to stabilise isocyanic acid, whereas at higher catalyst mass it significantly improved both urea conversion and ammonia yield. At 150 °C and 1.25 g catalyst loading, γ-Al2O3 reached 66% ammonia yield and 70% urea conversion. One of the most significant achievements was the higher turnover numbers obtained in comparison with literature values. γ-Al2O3 reached a TON of 15 at 165 °C, while TiO2 reached a TON of 12 under similar conditions. These values, obtained using relatively low catalyst loadings, demonstrate the high catalytic potential of the studied oxides under concentrated urea-feed conditions. The project also generated important mechanistic insights. The non-linear effect of catalyst loading was attributed to competitive adsorption between ammonia and isocyanic acid on catalyst surfaces. The progressive increase in ammonia concentration during the reaction may influence local pH and surface speciation, directly affecting intermediate stability and conversion. These findings are particularly relevant for real urea-rich wastewater streams, where concentration effects and surface interactions may strongly influence catalytic performance. In parallel, hydrous hydrazine was used as a model nitrogen-containing molecule to develop catalysts capable of producing hydrogen. Coprecipitated cobalt–ceria oxides doped with iridium were synthesised and tested as the chosen materials in hydrous hydrazine liquid phase decomposition using alkaline conditions. CoO was identified as the active phase, while the presence of Ir induced a five-fold increase in catalytic activity and improved H2 selectivity. The addition of NaOH further enhanced the catalytic activity and increased H2 selectivity from 15% to 68%. Advanced characterisation by XRD, TEM, XPS, XAFS and CO adsorption, combined with DFT calculations and operando ATR-IR, elucidated the role of the Ir/CoO interface and the cooperation between Brønsted basic and Lewis acidic sites. The project also opened a new direction through exploratory photoreforming experiments using gold nanoparticles supported on titania. These tests qualitatively confirmed the formation of both ammonia and hydrogen, suggesting that a two-stage strategy may be more realistic than direct thermochemical hydrogen production from urea: first, thermochemical hydrolysis of urea to ammonia; then, photocatalytic reforming of ammonia to hydrogen. Although quantitative analysis is not yet possible with the current photoreactor and analytical setup, these preliminary results provide a promising basis for the next phase of the project. The project has achieved significant dissemination through poster presentations at the XII Workshop del Gruppo Interdivisionale Green Chemistry – Chimica Sostenibile in Bologna, at the Waste-Energy-Water Nexus: Materials & Processes – WEW-ICS25 in Rimini and Marina di Ravenna, and at the Japan-Italy Workshop on Sustainable Chemistry in Bologna. In addition, the results on hydrous hydrazine decomposition were published in ACS Applied Materials & Interfaces, ensuring international visibility of the project outcomes. Overall, the scientific outcomes include the development of robust analytical protocols, the identification of TiO2 and γ-Al2O3 as relevant model catalysts for urea hydrolysis, the clarification of the role of catalyst acidity and surface properties, the achievement of high TON values for ammonia production, and the identification of photoreforming as a promising complementary route for H₂ generation. Future outputs will include publications on thermochemical urea hydrolysis, ammonia photoreforming using Au/TiO2 catalysts, and a mini-review on catalytic conversion of urea and ammonia to hydrogen.
Project details
Unibo Team Leader: Nikolaos Dimitratos
Unibo involved Department/s:
Dipartimento di Chimica Industriale "Toso Montanari"
Coordinator:
ALMA MATER STUDIORUM - Università di Bologna(Italy)
Total Eu Contribution: Euro (EUR) 203.430,00
Total Unibo Contribution: Euro (EUR) 109.986,00
Project Duration in months: 24
Start Date:
16/10/2023
End Date:
25/02/2026
