Using spilled over hydrogen in NH3 synthesis over supported Ru catalysts

• In the absence of gas phase hydrogen, ammonia synthesis at room temperature and atmospheric pressure was demonstrated using spilled over hydrogen.
• Spilled over hydrogen atoms on the support migrate back to the metal to react with dissociated nitrogen.
• Deliberate use of spilled over hydrogen, with a controlled hydrogen feeding strategy can eliminate hydrogen poisoning.

The role of spilled over hydrogen in ammonia synthesis reaction was monitored over Ru/SiO2, Ru/SBA15 and Ru/CNF using an in-situ DRIFTS cell reactor. Strongly bound hydrogen trapped on the metal and spilled over hydrogen trapped on the support due to a diffusion barrier remained on the surface after the overnight purge followed by hydrogen exposure over a reduced catalyst. When N2 flow was introduced at room temperature, IR characteristic signals of NH3 were observed, indicating the formation of ammonia from hydrogen trapped on the surface. Total and weak hydrogen adsorption isotherms measured at different temperatures indicated that at low coverages spilled over hydrogen is trapped on the support in a kinetically metastable state. These results indicated the possibility of ammonia synthesis with the irreversibly bound hydrogen present on the support. A new hydrogen feeding strategy was proposed after a microkinetic model was used to estimate the rates in a plug flow reactor. An incremental hydrogen feeding strategy in the earlier phases of the reaction was proposed to eliminate hydrogen poisoning. The model predictions revealed that the alternative feeding strategy increased the rates by two orders of magnitude at the early stages of the reaction.

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