On the active oxygen in bulk MoO3 during the anaerobic dehydrogenation of methanol

Abstract

The oxidation of methanol under anaerobic reaction conditions over MoO3 has been studied using an in situ approach, combining ultraviolet−visible (UV−vis), Raman, wide-angle X-ray scattering (WAXS), and online mass spectroscopy (MS) techniques. Comparison of the UV−vis and MS data reveals that during the initial stages of the reaction methanol is chemisorbed onto the oxide’s surface, primarily at defect sites. Reaction then begins, producing formaldehyde, dimethyl ether, and water. At low temperatures, CO and MoO2 are also produced, as reoxidation of the reactive sites cannot occur rapidly enough to avoid additional reduction. After the initial heating, continued reduction of the bulk oxide by Mars−Van Krevelen oxygen transfer to the active surface sites is observed as a change in the total Raman intensity. Most significantly, after 125 min of reaction, bulk MoO2 is observed and here a Rietveld analysis of the MoO3 WAXS data indicates qualitatively that one of the three unique oxygen environments (O1) becomes more active than the others. This is confirmed by changes in the Raman data, indicating that the Mo−O1 bond to this oxygen is broken more quickly. This work has therefore identified the oxygen most likely to be transferred through the bulk of the oxide during the Mars−Van Krevelen oxygen transfer. However, a comparison with previous works and our MS and UV−vis data indicates no particular relationship between the bonding in the bulk oxide and its surface reactivity. Therefore, although O1 is abstracted and transferred through the bulk, it may be replacing other oxygen atoms (i.e., O2 or O3) at the oxide surface. This work then also demonstrates that to fully understand a parent oxide we cannot rely on a bulk view of the entire system, but must obtain separate details about both the surface sites (responsible for selectivity) and the bulk sites (that maintain catalytic activity by oxygen transfer), particularly under oxygen-free conditions.