Thermal Dissociation of Hydrogen

Abstract

This paper describes ``thermal dissociation'' or ``atomization,'' $${\mathrm{H}}_2\text{⇌2\hspace{0.17em}}\mathrm{H},$$ on a Mo or W catalyst, from 1200° to 2500°K. We emphasized transient behavior of individual species (especially H) caused by changing catalyst temperature. Reaction rates, and equilibria derived from them, confirm Langmuir's results of 50 years ago, and disagree with more recent authors who published reaction rates 200‐fold faster. These authors did not emphasize chemical identification of both reactants and products, but attributed a measured decrease of total pressure to H, produced as in (1) and abstracted either chemically or by refrigeration. Reactions unrelated to atomization can decrease total pressure. Transient behavior above 1200°K was dominated by diffusion; this suggests that neither surface atomization mechanism usually favored is important. Our transient methods permit evaluation of parameters fundamental for diffusion and for solution of hydrogen in W or Mo, from 10^—7 to 10^—4 Torr, well below the range of the Fowler—Smithells theory. These may be the first quantitative diffusion data published for the tungsten—hydrogen system. Using D=D₀ exp(—Q/RT) we obtained: $$\begin{array}{ccc}& \mathrm{Tungsten}& \mathrm{Molybdenum}\\ \mathrm{D\hspace{0.17em}}\mathrm{at}{\text{\hspace{0.17em}1985}}^{\circ }\mathrm{K}& {\text{2.2×10}}^{\text{−8}}& {\text{2.8×10}}^{\text{−7}}\\ D_0& {\text{7.25×10}}^{\text{−4}}& \\ Q& \text{41.5\hspace{0.17em}}\mathrm{kcal}/{\mathrm{mole\; H}}_2& \\ \mathrm{Heat\; of\; solution}& \text{64\hspace{0.17em}}\mathrm{kcal}/{\mathrm{mole\; H}}_2& \text{16–22\hspace{0.17em}}\mathrm{kcal}/{\mathrm{mole\; H}}_2{\text{\hspace{0.17em}(1360–1980}}^{\circ }\mathrm{K})\\ \mathrm{Solubility}& \mathrm{One\; atom\; of\; H\; per}\text{\hspace{0.17em}400\hspace{0.17em}}\mathrm{atoms\; of\; W\; at}{\text{\hspace{0.17em}2440}}^{\circ }\hspace{0.17em}\mathrm{and}{\text{\hspace{0.17em}1.2×10}}^{\text{−4}}\hspace{0.17em}\mathrm{Torr}& \mathrm{One\; atom\; of\; H\; for}{\text{\hspace{0.17em}10}}^4\hspace{0.17em}\mathrm{atoms\; of\; Mo\; at}{\text{\hspace{0.17em}1985}}^{\circ }\mathrm{K\; and}{\text{\hspace{0.17em}10}}^{\text{−6}}\hspace{0.17em}\mathrm{Torr}\end{array}$$