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Abstract

Polycrystalline copper was used as catalyst for the selective oxidation of methanol under stoichiometric reaction conditions
for oxidehydrogenation. Temperature-programmed reaction spectroscopy (TPRS) revealed a broad temperature range of reactivity with two distinct maxima for the production of formaldehyde. Phase analysis with thermogravimetry (TG) and powder X-ray diffraction (XRD) under in situ conditions showed that a phase change occurred between the two maxima for formaldehyde production from bulk Cu₂O to metallic copper. Strongly adsorbed methoxy and formate were detected by X-ray photoelectron spectroscopy (XPS) after prolonged catalytic use. A sub-surface oxygen species and surface OH were identified by XPS. A region of oscillatory behaviour was found in the temperature interval between 623 and 710 K. Multicomponent gas analysis of the reaction products with an ion-molecule reaction mass spectrometer (IMR-MS) allowed to derive a reaction sequence in which both methoxy and formate are necessary as surface species. The most selective state of the catalyst for oxidehydrogenation is the co-adsorption
system methanol-oxygen. Oxidation of the surface by excess molecular oxygen leads to total oxidation. The catalyst is finally
reduced by excess methanol into an inactive pure metallic form. Sub-surface oxygen segregates to the surface and initiates the activity again by enhancing the sticking coefficient for gas phase species.