Interaction of ammonia and hydrogen with tungsten at elevated temperature studied by gas flow through a capillary

The interaction of ammonia and hydrogen (H-2 and D-2) was studied by flowing pure gas or gas mixture through a hot tungsten capillary. The composition of the gas after passing the capillary was analyzed by mass spectrometry as a function of capillary temperature. Specific temperatures were identified where changes in mass spectra take place indicating thermal decomposition and isotope exchange channels. Measurements with pure ammonia and deuterium provided new data for the thermal decomposition of these molecules on hot tungsten. Ammonia gets effectively decomposed at around 900K in the tungsten capillary, and only 7% of ammonia survives through the capillary at temperatures above 1100 K. By studying the production and desorption of HD in ammonia and deuterium mixture, the authors show that D-2 molecules can get adsorbed on tungsten dissociatively only at temperatures above 1300K in the presence of nitrogen or ammonia fragments. An adsorption barrier of 0.17 eV is determined for this case. A comparison of adsorption barrier of tungsten with that of native oxide on the surface is given when the mixture of H-2 and D-2 was flown through the hot tungsten capillary. Above 1300K capillary temperature, H/D isotope exchange took place in the ammonia molecule on the surface yielding 1% of exchange as derived from the increase in ion current of ND3 in comparison to NH3.