Direct observation of double-layered reaction intermediates at the atomic scale during methane pyrolysis

Abstract Direct observation of reaction intermediates at atomic scale is the most desired way to understand reaction mechanism. However, these transient intermediates during catalytic processes have never been directly observed thus far. Herein, we use in situ electron microscopy with high spatio-temporal resolution to directly probe reaction intermediates during methane pyrolysis over nanoporous Au. A double-layered reaction intermediate was discovered, comprising a chemisorbed –CH2 inner layer on the Au catalytic sites with a physisorbed CH4 outer layer, combined with electron energy-loss spectroscopy and in situ infrared spectroscopy. This structure was also confirmed by density functional theory calculations and Ab initio molecular dynamics. Further, we propose a CH4 substitution mechanism for the double-layered intermediates, wherein the physisorbed outer CH4 directly replenishes the chemisorbed inner –CH2 upon its consumption, while gaseous CH4 physisorbs on –CH2/Au to rebuild the double-layered intermediate structure. This provides unprecedented insights unraveling the molecular pathway of heterogeneous catalytic processes.