Catalytic decomposition of ammonia on tungsten (100), (110), and (111) crystal faces

Abstract

The rates of catalytic decomposition of ammonia on (100), (110), and (111) single crystal faces of tungsten were measured over the temperature range 800–970 °K for ammonia pressures ranging from (0.5–100)× 10−3torr and for nitrogen and hydrogen pressures varying from (0–50) × 10−3torr. In all cases the rate of decomposition was of the form, rate=A+BPNH3(2/3), and was independent of nitrogen and hydrogen partial pressures. The constants A and B varied substantially with crystal face; the values of B for the (111), (100), and (110) faces were in approximate ratio 8.4:1.55:1 and these substantially established the decomposition rate for PNH3>5 × 10−3torr. Rate forms for NH3 and ND3 were compared on the (111) face at 860 °K, with results ANH3=AND3,BNH3≈ 1.47 BND3. The observed rate form is derived from a model involving nearly complete surface coverage by the species WN and small surface coverages by species W2N, W2N3H2, and WNH. The A term in the rate law is generated by the reaction, 2WN→W2N+(1/2)N2. This process was advocated as rate limiting in ammonia decomposition by Matsushita and Hansen and their rate law for this process obtained on polycrystalline tungsten furnishes an estimate of the A parameter in the same order of magnitude as that observed. The BPNH32/3 term results from a steady state balance of W2N3H2 and WNH decomposition reactions. Peng and Dawson have advocated decomposition of W2N3H as rate limiting in the decomposition of ammonia, and have also suggested WN dissociation may be rate limiting at low pressures, W2N3H dissociation at higher pressures. The present model therefore resembles that of Peng and Dawson in concept but not in detail.