Decomposition studies of NH₃ and ND₃ in presence of H₂ and D₂ with Pt/Al₂O₃ and Ru/Al₂O₃ catalysts

In the fusion reactor ITER, ammonia will be produced as a result of the interaction between the hydrogen isotopes used as fuel and nitrogen used to spread the power loads of a larger area. As part of the fuel management in ITER, NQ3 (NQ3, Q = H, D, T) will have to be decomposed using a palladium membrane reactor. The decomposition of pure NH3 and ND3 was studied in this work using commercial platinum (Pt) and ruthenium (Ru) catalysts on alumina (0.5 wt% loading), in a conventional reactor configuration (i.e., without a palladium membrane). With Pt/Al2O3, decomposition fractions larger than 90% were achieved with NH3 above 800 K using the lowest flow-to-mass ratio (/g-cat) of 0.015 sccm g−1. However, with the increase of /g-cat to 0.220 sccm g−1, similar decompositions were achieved only at  K. In contrast, with Ru/Al2O3 decomposition fractions above 90% were attained already below 700 K, regardless of /g-cat. With both catalysts the decomposition of NH3 was found to be more efficient than that of ND3 at a wide range of temperatures, thus evidencing the existence of isotopic effect. A strong inhibition of both NH3 and ND3 in presence of, respectively, H2 and D2 with Pt/Al2O3 was observed. This effect was stronger at lower temperatures and larger hydrogen partial pressures. The inhibition effect with Ru/Al2O3 was less pronounced and it was suppressed at 629 K. Isotopic exchange reactions with equimolar mixtures of NH3-D2 and ND3-H2 revealed that the most and least abundant isotopologue are, respectively, NH2D and ND3. At the relevant temperature window in which the PMR will be operated (673–823 K), the Ru-based catalyst exhibits superior performances in terms of decomposition rates, negligible isotopic and inhibition effects. A slight reduction of the performances with this catalyst was observed with 0.200 sccm g−1. This work suggests that 0.5 wt% Ru/Al2O3 is the most suitable catalyst to be used during ITER operation.