Selective Methane Oxidation to Methanol on ZnO/Cu₂O/Cu(111) Catalysts: Multiple Site-Dependent Behaviors

Abstract

Because of the abundance of natural gas in our planet, a major goal is to achieve a direct methane-to-methanol conversion at medium to low temperatures using mixtures of methane and oxygen. Here, we report an efficient catalyst, ZnO/ Cu 2 O/Cu(111), for this process investigated using a combination of reactor testing, scanning tunneling microscopy, ambientpressure X-ray photoemission spectroscopy, density functional calculations, and kinetic Monte Carlo simulations. The catalyst is capable of methane activation at room temperature and transforms mixtures of methane and oxygen to methanol at 450 K with a selectivity of ∼30%. This performance is not seen for other heterogeneous catalysts which usually require the addition of water to enable a significant conversion of methane to methanol. The unique coarse structure of the ZnO islands supported on a Cu 2 O/ Cu(111) substrate provides a collection of multiple centers that display different catalytic activity during the reaction. ZnO−Cu 2 O step sites are active centers for methanol synthesis when exposed to CH 4 and O 2 due to an effective O−O bond dissociation, which enables a methane-to-methanol conversion with a reasonable selectivity. Upon addition of water, the defected O-rich ZnO sites, introduced by Zn vacancies, show superior behavior toward methane conversion and enhance the overall methanol selectivity to over 80%. Thus, in this case, the surface sites involved in a direct CH 4 → CH 3 OH conversion are different from those engaged in methanol formation without water. The identification of the site-dependent behavior of ZnO/Cu 2 O/Cu(111) opens a design strategy for guiding efficient methane reformation with high methanol selectivity.

Platinum‐Catalysed Selective Aerobic Oxidation of Methane to Formaldehyde in the Presence of Liquid Water

“…The obtained maximum rate of methane oxidation over Pt/Al 2 O 3 was lower at 16 μmol g −1 cat h −1 (albeit at a lower inlet molar ratio of H 2 O/CH 4 of 136). The role of water in the aerobic, selective oxidation of methane was highlighted previously, [9, 13–17, 20] but here we show unequivocally the need to perform the reaction in the presence of liquid water to accelerate the aerobic oxidation of methane. The increase in the activity upon entering the trickle bed regime changes gradually, possibly due to the incomplete wetting of the catalyst by liquid water [28] .…”

“…Some success in the selective oxidation of methane has been achieved using sulfuric acid, [6] H 2 O 2 [7] or N 2 O [8] as the oxidant. However, the use of oxygen [9][10][11][12][13][14][15][16][17][18][19] or even air as the oxidant is necessary to make the process economically viable. It should be noted further that heat is an important side product in the selective methane oxidation (the oxidation of methane to formaldehyde releases 34 % of the lower heating value of methane), which will need to be recovered to obtain an energy-efficient process.…”

Recent progress of catalytic methane combustion over transition metal oxide catalysts

“…TMOs are one of the most important classes of solid catalysts that have been extensively used in a variety of important industrial processes, such as the water gas shift reaction, CO oxidation, methanol synthesis, etc., owing to their excellent thermal stability, variable valence states, and Earth abundance ( Tepamatr et al, 2016 ; Arena et al, 2017 ; Huang et al, 2021 ). In water-containing feed, the surface of TMOs undergo acid–base reaction to form surface hydroxyl groups, which react with other reactants via proton or electron transfer ( Yang et al, 2022 ).…”

“…However, such biomimetic strategies are often limited by industrial-scale reactions. Some TMOs can dissociate CH 4 at room temperature, which offers the possibility for the direct conversion of CH 4 ( Huang et al, 2021 ). Zuo et al found that an inverse CeO 2 /Cu 2 O/Cu (111) catalyst is able to bind and dissociate CH 4 at room temperature by mimicking the function of the CH 4 monooxygenase.…”

Advanced Kinetic Characterization

“…Further details on other heterogeneous phenomena such as island formation can be found in the archived literature. 13,[31][32][33][34] The dynamic nature of this catalogue framework is highlighted by a study by Ertl and co-workers 13 .…”