Adsorption and Decomposition of Ammonia on a Single-Crystal Tungsten (100) Surface

Abstract

The interaction of ammonia with a single-crystal tungsten (100) surface has been investigated at pressures below 10−7 torr with a combination of techniques, including low-energy electron diffraction (LEED), flash desorption, mass spectrometry, and work-function measurements. The results indicate that adsorption of NH3 at room temperature is nondissociative, occurs with a sticking probability >0.45, and lowers the work function φ by ∼1 eV. Raising the temperature to ∼800°K after saturation results in dissociation of NH3 and desorption of H2, leaving behind half a monolayer of NH2 in a C(2 × 2) structure with a Δφ of ∼−0.4 eV. This structure can be destroyed rapidly at ∼1375°K, with desorption of H2 and N2. The NH2 density can be increased to ∼1 monolayer by exposure to ammonia at ∼800°K. During this process Δφ returns to a value of ∼0, the structure changes from a C(2 × 2) to a(1 × 1), and there is evidence that the rate of hydrogen desorption is controlled by the breaking of a N–H bond. Adsorption and desorption of NH3 occurs readily, but without decomposition, when all the NH2 sites have been filled. The (1 × 1)–NH2 layer desorbs as N2 and H2 in two stages, at ∼1150°K and ∼1375°K, each with the equivalent of half a monolayer of NH2. There are no indications that adsorbed NH or N are formed as intermediate products.