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

Abstract

The aerobic, selective oxidation of methane to C 1 -oxygenates remains a challenge, due to the more facile, consecutive oxidation of formed products to CO 2 . Here, we report on the aerobic selective oxidation of methane under continuous flow conditions, over platinum-based catalysts yielding formaldehyde with a high selectivity (reaching 90 % for Pt/TiO 2 and 65 % over Pt/ Al 2 O 3 ) upon co-feeding water. The presence of liquid water under reaction conditions increases the activity strongly attaining a methane conversion of 1-3 % over Pt/TiO 2 . Density-functional theory (DFT) calculations show that the preferential formation of formaldehyde is linked to the stability of the di-σ-hydroxy-methoxy species on platinum, the preferred carbon-containing species on Pt( 111) at a high chemical potential of water. Our findings provide novel insights into the reaction pathway for the Pt-catalysed, aerobic selective oxidation of CH 4 .

Selective, Aerobic Oxidation of Methane to Formaldehyde over Platinum ‐ a Perspective

“…[99] Fe/ZSM-5 + Cu-ZSM-5 [b] H 2 O 2 323 20 0.013 0.5 92 (CH 3 OH) 75 [d] [100] [d] 0.17 [12] [a] Turnover frequency. [b] Co-feeding water.…”

“…[122,123] The adsorption of oxygen containing species on the various platinum surfaces upon exposure to O 2 and H 2 O has been explored well. The complete coverage of the dominant platinum surface, Pt (111), with oxygen is not easily achieved due to the lateral interactions between adsorbed oxygen species [12,122,[123][124][125][126][127] and a maximum coverage of 0.44 ML is typically achieved upon exposing Pt (111) to O 2 . [126] Co-feeding water (and thus generating surface hydroxyl species and even adsorbed water) will result in fully covered platinum surfaces saturated with oxygen containing species, [12,[128][129][130] such as O, Scheme 4.…”

“…The complete coverage of the dominant platinum surface, Pt (111), with oxygen is not easily achieved due to the lateral interactions between adsorbed oxygen species [12,122,[123][124][125][126][127] and a maximum coverage of 0.44 ML is typically achieved upon exposing Pt (111) to O 2 . [126] Co-feeding water (and thus generating surface hydroxyl species and even adsorbed water) will result in fully covered platinum surfaces saturated with oxygen containing species, [12,[128][129][130] such as O, Scheme 4. Mechanistic pathways for the formation of methyl hydroperoxide and methanol in the oxidative conversion of methane over PdAu-catalysts (based on [105,107,111] ).…”

“…The dense surface Pt (111) exposed to a partial pressure of oxygen of 1 bar and of water of 1 bar at 493 K is expected to be covered with adsorbed OH and H 2 Ospecies. [12] The more open surface Pt(100) exposed to similar reaction conditions will be covered with atomic O and OH species, [124,131] whereas the edge between the Pt(100) and Pt (111) facets will be covered with atomic oxygen at the bridge sites. [122,123] The relative weak adsorption of oxygen-containing species on Pt (111) could favor the desorption of oxygen containing product compounds.…”

“…[136] Changing the titania support serendipitously from the initially used P25 to TiO 2 in the rutile phase (with roughly the same platinum dispersion) resulted in the formation of formaldehyde and its derivatives with a high selectivity (~90 %) in the partial oxidation of methane upon addition of water to the feed. [12] Producing formaldehyde in a laboratory reactor comes with its own problems, as many researchers have noted previously. A notorious problem is the polymerization of formaldehyde resulting in the formation of polyoxymethylene (POM), which will result in blockage of the effluent lines (this has happened several times; this polymer cannot be removed easily thus necessitating the replacement of this line).…”