Direct conversion of methane to value‐added chemicals with high selectivity under mild conditions remains a great challenge in catalysis. Now, single chromium atoms supported on titanium dioxide nanoparticles are reported as an efficient heterogeneous catalyst for direct methane oxidation to C1 oxygenated products with H2O2 as oxidant under mild conditions. The highest yield for C1 oxygenated products can be reached as 57.9 mol molCr−1 with selectivity of around 93 % at 50 °C for 20 h, which is significantly higher than those of most reported catalysts. The superior catalytic performance can be attributed to the synergistic effect between single Cr atoms and TiO2 support. Combining catalytic kinetics, electron paramagnetic resonance, and control experiment results, the methane conversion mechanism was proposed as a methyl radical pathway to form CH3OH and CH3OOH first, and then the generated CH3OH is further oxidized to HOCH2OOH and HCOOH.
Single-atom
catalysts (SACs) often exhibit superior activity and
selectivity in heterogeneous catalysis because of their maximized
atom utilization and unique coordination environments. However, most
reported studies about SACs in heterogeneous catalysis focus on model
reactions with simple molecules. In addition, many reported single
atoms are confined in microporous structures, hindering the mass transfer
of molecules with large sizes, thus limiting their practical applications
in industry. In this study, we report a molten salt-assisted method
to synthesize metal single atoms anchored on a hierarchical porous
nitrogen-doped carbon support (denoted as M1/h-NC, M includes
Co, Fe, Ni, Mn, and Cu). Taking Co1/h-NC as an example,
compared to the control sample which has Co single atoms being encapsulated
in a microporous N-doped carbon support (denoted as Co1/m-NC), Co1/h-NC exhibits significantly higher catalytic
activity in the selective hydrogenation of large-sized pharmaceutical
molecules, such as nimodipine (calcium channel blocker) and 2-(3′,4′-methylenedioxyphenylethyl)quinoline
(antispasmodic natural alkaloid intermediate). The superior catalytic
performance of Co1/h-NC is directly ascribed to the integration
of the advantages of single-atom active sites and hierarchical mesoporous
structure, which is beneficial for the mass transfer of molecules
with large sizes and enables nearly all the Co single atoms to be
accessible for catalytic reactions.
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