Total pressure variation determination of reaction rates at constant reactant pressure Catalytic decomposition of ammonia by tungsten

Confirmation of a mechanism for the catalytic decomposition of ammonia by tungsten deduced from thermal desorption results requires measurements of the overall rate of reaction over a wide range of reactant pressures. Total pressure variation measurement of the rate is complicated by slow desorption of the reactant from the reaction vessel wall. A manostat is described which eliminates these problems. The rate of reaction has been determined for constant ammonia pressures in the range 9.10−3 to 6 torr and temperatures varying from 825 ° to 1050 °K. The reaction is zero order for nitrogen and hydrogen. At 1050 °K, the order for ammonia is zero at low pressure with a transition at 10−1 torr, as expected for a surface saturated with δ-nitrogen, WsN, at low pressure and formation of η-species, WS2N3H, above 10−1 torr. However, for pressures above 10−1 torr the order remains constant at 13 even above the pressures required for η-saturation. It is proposed that absorbed hydrogen, H(∗), participates in desorption of the η-species, H(∗) + WS2N3H(η) → WS2N(β) + N2(g) + H2(g) and that the absorbed hydrogen is in equilibrium with gas phase ammonia, to NH3(g) + 2Wb WB2N + 3H(∗) account for the 13 order for the reaction. The activation energy for the reaction has been determined at pressures varying from 10−2 to 2 torr and all values fall in the range 35.9 to 39.7 Kcal mole−1 in agreement with the activation energies for δ-and η-desorption. The activation energy falls, as predicted, at the transition pressure of 10−1 torr. An increase at high pressure can be explained if the hydrogen absorption reaction is endergonic. Reasons are suggested for the lack of agreement between these results and recently published data.