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 ${\text{(0.5–100)× 10}}^{\text{−3}}\mathrm{torr}$ and for nitrogen and hydrogen pressures varying from ${\text{(0–50) × 10}}^{\text{−3}}\mathrm{torr}$ . In all cases the rate of decomposition was of the form, $\mathrm{rate}{\mathrm{=A+BP}}_{{\mathrm{NH}}_3}^{\text{(2/3)}}$ , 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 $P_{{\mathrm{NH}}_3}{\text{>5 × 10}}^{\text{−3}}\mathrm{torr}$ . Rate forms for NH₃ and ND₃ were compared on the (111) face at 860 °K, with results $A_{{\mathrm{NH}}_3}{\mathrm{=A}}_{{\mathrm{ND}}_3}{\mathrm{,B}}_{{\mathrm{NH}}_3}{\text{≈ 1.47\hspace{0.17em}B}}_{{\mathrm{ND}}_3}$ . 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, $2\mathrm{WN}\to {\mathrm{W}}_2\mathrm{N}\text{+(1/2)}{\mathrm{N}}_2$ . 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 ${\mathrm{BP}}_{{\mathrm{NH}}_3}^{\text{2/3}}$ 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.