(617b) Yittria-Stabilized Zirconia (YSZ) Supports for Low Temperature Ammonia Synthesis

NH₃ is important as the raw material for fertilizer production and high hydrogen density (17.7 wt. %) energy carrier. Conventionally, NH₃ is synthesized through the well-known Haber-Bosch process at 400-500°C and P~150 bar. Both critical reaction conditions and massive production (145 mt NH₃ in 2014 globally) make it one of the most energy extensive process, consuming 1-2% of the world’s total energy expense. Here we introduce YSZ as a more active Ru catalyst support than traditionally used supports such as Al₂O₃. The addition of Cs promoter increased rates an order of magnitude higher by reducing the apparent activation energy from 103 kJ/mol to 65 kJ/mol. The rate enhancement is largely insensitive to the amount of promoter addition, with Cs outperforming Ba and K by a factor of 2. At 400°C under 1.0 MPa, the synthesis rate was comparable with that of most active oxide-supported Ru catalysts. At low temperature (< 350°C) the rate becomes inhibited by H₂ absorption, but the use of lower H_2:N₂ ratios enables the rate to remain comparable to what is observed in stoichiometric mixtures at temperatures > 400°C. Relative to a normal ratio H₂:N₂ = 3, operating at an optimized H₂:N₂ enables a positive pressure dependence of the reaction rate especially at temperature < 350°C. The superior performance of YSZ supported Ru catalyst suggests that it is promising for innovative NH₃ synthesis processes operating at reduced temperatures.