Mechanism of methane decomposition with hydrogen addition over activated carbon via in-situ pyrolysis-electron impact ionization time-of-flight mass spectrometry

摘要

A novel in-situ pyrolysis-electron impact ionization time of flight mass spectrometry (Py-EI-TOFMS) combining with isotope tracing was first used to investigate the mechanism of methane decomposition with hydrogen addition over activated carbon (AC). The results show that free radicals produced from decomposition of methane labelled by four deuterium atoms (CD4) are directly detected by in-situ Py-EI-TOFMS. The breakage of first C-D bond is the control step of CD4 decomposition. During CD4 decomposition with hydrogen addition, the added hydrogen takes part in the free radical reaction process, promotes the decomposition and improves catalytic stability of AC. Two reactions happen during the CD4 decomposition with hydrogen addition, including CD4 cracking to free radicals and hydrogen reacting with the free radicals. The former produces D-2 molecules by combining of two D free radicals, while the latter generates more HD molecules than D-2. It is inferred that the addition of H-2 declines the collision between CD4 molecules or CDx free radicals, and stabilizes the produced CDx free radicals from CD4 decomposition over AC, leading to the enhancement of CH4 conversion and catalytic stability. This work provides a deep insight of H-2 promotion on CH4 decomposition over AC.