Oxidation of methanol on Ru catalyst: Effect of the reagents partial pressures on the catalyst oxidation state and selectivity

Abstract

In situ core level photoelectron spectroscopy and mass spectrometry have been utilized to study the methanol oxidation on a model RuO2 catalyst at pressures ranging from 10 -6 to 10 -1 mbar. The experiments were carried out varying the O 2 /CH 3 OH molecular mixing ratio from 0.25 to 3.3 and the reaction temperature from 350 to 720 K. The Ru 3d 5/2 and O 1s core level spectra were used to characterise the dynamic changes in the Ru oxidation state by exposing the oxide pre-catalyst to different reagents partial pressures and temperatures. Full oxidation to CO 2 + H 2 O or partial oxidation to CO + H 2 O + H 2 have been observed in the whole pressure range for specific reaction conditions, which preserve the oxide catalyst state or reduce the oxide to metallic Ru. The selective oxidation to formaldehyde is observed only at pressures in the 10 -1 mbar range, catalyzed by a RuOx surface oxide formed by partial reduction of the oxide pre-catalyst.

Methanol oxidation over model cobalt catalysts: Influence of the cobalt oxidation state on the reactivity

“…At high x valence of the CoO x oxide total oxidation is favoured, on the CoO partial oxidation to CH 2 O and on metallic Co decomposition to CO and H 2 . Analogous behaviour was also found for methanol oxidation on ruthenium [7]. High CO 2 production rates on Co 3 O 4 are related to equally high formate coverages on CoO x surface (see figure 5) …”

“…The stoichiometric Co 3 O 4 oxide was only observed in pure oxygen atmosphere, while in the presence of methanol a mixture of CoO/Co 3 O 4 was always present. This is in fully analogy with the observations on ruthenium where a transient ruthenium oxide and not RuO 2 was the active phase for methanol oxidation [6,7]. A simple comparison of the geometric structures of spinel (Co 3 O 4 ) and rocksalt (CoO) oxides can provide additional insights into the reaction mechanism.…”

“…Compared to previous methanol oxidation studies on copper and ruthenium catalysts [4,7], the coverage of formate and methoxy species on cobalt oxide is greatly enhanced (from <2 on Cu to ca. 7 % on Co).…”

“…These are clear indications for partial reduction of Co 3 O 4 under working conditions [24,26]. In pure methanol stream (bottom spectrum), the Co 2p 3/2 peak at 778.3 eV is indicative of cobalt in the metallic state [22,32], verifying that methanol is a very effective reducer for cobalt oxides [7]. The XAS spectra presented in Fig.…”

“…These studies showed that the structure of a heterogeneous catalyst is dynamic and depends intimately on the reaction conditions. Such dynamic changes have, for example, been observed on a Cu and Ru catalyst during methanol oxidation, indicating that the active state of the catalyst exists only during the process of catalysis [4,5,6,7].…”