ESR Study of the Pyrolysis of Hydrogen on Tungsten

Abstract

Electron spin resonance (ESR) techniques have been adapted to the study of the atomization of hydrogen occurring on the surface of heated filaments. The method involves heating an axial metal filament in a circular cavity to temperatures of 1000–2000°K and measuring the steady-state atom concentration produced in the gas. Heating is accomplished by DC current pulsed at 6000 pulses/sec with a 15% duty cycle. Under these conditions, the sample space is free a relatively high proportion of the time from current-induced field inhomogeneities. The sensitivity and spectral line shapes observed compare favorably with those observed in control experiments in the same cavity in the absence of the heated filament. The atomization of hydrogen on tungsten filaments can be conveniently studied by ESR at hydrogen pressures between 0.1 and 100 torr; thus, this method complements previously developed techniques that are most useful at lower pressures (10−6-10−2 torr). The observed pressure and temperature dependences of the H atom concentration are interpreted in terms of a mechanism involving the production of hydrogen atoms on the filament, their diffusion into the gas, and recombination on the cavity wall. The surface reaction appears to be complex, changing character at an H2 pressure somewhere around 0.5 torr. Extrapolation of the ESR data to very low H2 pressures yields results consistent with previous low pressure studies. At higher H2 pressures the rate of H atom production decreases due to surface saturation effects.