Application of microfabricated reactors for operando Raman studies of catalytic oxidation of methanol to formaldehyde on silver

Operando Raman-GC studies of the catalytic oxidation of methanol to formaldehyde on silver using a microfabricated reactor as a reactor cell are presented. The microreactor is made of silicon and glass with a wide reaction channel of 8 mm and a channel depth of 120 mu m. The silver catalyst is incorporated into the microchannel by sputter coating. The reaction is performed at atmospheric pressure, temperature between 723 and 813 K with a feed containing 8.75% CH(3)OH, 3.5% O(2) and 6.63% H(2)O (He as balance) at residence time of 6-7 ms at reaction temperature. Raman spectra of the silver catalyst after exposure to 4.1% O(2) (He as balance) at 773 K show the presence of subsurface (at 640 cm(-1)) and surface (at 810 cm-1) atomic oxygen species. During an activation procedure consisting of repeated oxidation/reaction cycles at 773 K, the 810 cm-1 band disappears immediately after introducing a CH(3)OH/O(2)/H(2)O/He mixture, indicating that this surface atomic oxygen species participates in the reaction. The 810 cm-1 band is not observed in the subsequent oxidation/reaction cycles; instead a broad feature between 400 and 800 cm-1 appears which may be associated with a severe restructuring of the catalyst by methanol oxidation. The catalyst stabilizes after three oxidation/ reaction cycles as shown by improved and approximately constant CH(2)O selectivities. This is accompanied by Raman spectra with sharp definition of the 400-800 cm(-1) band, indicating the importance of stabilization of subsurface oxygen species in obtaining high CH(2)O selectivity. Deactivation of the catalyst due to carbon deposition is observed when a feed without H(2)O is introduced into the reactor, as demonstrated by bands at 1350 and 1585 cm-1 in the Raman spectra and by decreased conversion of CH(3)OH with reaction time. The work presented demonstrates that microfabricated reactors can be easily integrated with Raman spectroscopy and GC for operando studies. (c) 2006 Elsevier B.V. All rights reserved.