Direct Measurements of the Reaction NH2 + H2 → NH3 + H at Temperatures from 1360 to 2130 K

Abstract

The reaction NH2 ϩ H2 → NH3 ϩ H (11) was investigated behind shock waves in the temperature range from 1360 to 2130 K. Time histories of NH2 were measured by frequency modulation (FM) spectroscopy. The rate constant was determined to be k11 ϭ 4.5 · 10 13 · exp[Ϫ61 kJ mol Ϫ1 /RT] cm 3 mol Ϫ1 s Ϫ1 (∆log k 11 ϭ Ϯ0.15, ∆E a ϭ Ϯ9 kJ/mol).According to recent literature values the reverse reaction NH 3 ϩ H → NH 2 ϩ H 2 (Ϫ11) can be described by kϪ11 ϭ 1.5 · 10 7 · T 2.0 · exp(Ϫ47 kJ mol Ϫ1 /RT) cm 3 mol Ϫ1 s Ϫ1 for the temperature range of 400 to 2500 K. On the basis of kϪ11 and by using the enthalpy of formation of ∆fH o 298 K(NH2) ϭ 190 kJ/mol, which was determined in a previous study [1], via detailed balance the following expression for the forward reaction (11) was obtained for a temperature range of 400 to 2200 K: k 11 ϭ 1.5 · 10 4 · T 2.6 · exp(Ϫ25 kJ mol Ϫ1 / RT) cm 3 mol Ϫ1 s Ϫ1 . This expression is in very good agreement with the reported experimental values for k11.Die Reaktion NH 2 ϩ H2 → NH3 ϩ H (11) wurde bei Temperaturen von 1360 bis 2130 K hinter Stoßwellen untersucht. NH 2 -Konzentration-Zeit-Profile wurden mittels frequenzmodulierter (FM) Spektroskopie aufgenommen. Die Geschwindigkeitskonstante wurde zu k11 ϭ 4.5 · 10 13 · exp[Ϫ61 kJ mol Ϫ1 /RT] cm 3 mol Ϫ1 s Ϫ1 (∆log k11 ϭ Ϯ0.15, ∆Ea ϭ Ϯ9 kJ/mol) bestimmt.Auf Grundlage neuerer Literaturwerte kann die Geschwindigkeitskonstante der Rückreaktion, NH 3 ϩ H → NH 2 ϩ H 2 (Ϫ11), im Temperaturbereich von 400 bis 2500 K durch k Ϫ11 ϭ 1.5 · 10 7 · T 2.0 · exp(Ϫ47 kJ mol Ϫ1 /RT) cm 3 mol Ϫ1 s Ϫ1 beschrieben werden. Ausgehend von k Ϫ11 und von einer Bildungsenthalpie des NH2-Radikals von ∆fH o 298 K (NH2) ϭ 190 kJ/mol, die in einer vorhergehenden Arbeit [1] ermittelt wurde, kann über die Gleich-

Energetics and Mechanism of the Hydrogenation of XHnfor Group IV to Group VII Elements X

“…The NH 2 + H 2 reaction has been studied both by theory ,− and by experimentation, where the focus has been more on the energetics and dynamics of the reaction rather than its mechanistic details. It is noteworthy that the reverse reaction NH 3 + H plays an important role in the chemistry of the ammonia pyrolysis at high temperatures.…”

Ab initio thermal rate coefficients for H + NH3 ⇌ H2 + NH2

“…In this work, we calculate the thermochemistry and the thermal rate constants for the similar H + NH 3 ⇌ H 2 + NH 2 reaction using the same theoretical approach as that taken previously . The H‐abstraction reaction H + NH 3 ⇌ H 2 + NH 2 has been fairly well studied both experimentally and theoretically . In addition, a nine‐dimensional global potential energy surface of NH 4 and quantum dynamic calculations have been reported .…”