Charge Transfer and Synergy on Mn-Mn Dimer Sites in Manganese Oxides: Activity for NO Oxidation

The Mn-Mn dimer has been found to be catalytically active in various manganese oxides for NO oxidation. However, to date, it remains unclear how the dimer determines catalytic performance. Herein, we employed a combination of DFT theoretical calculations and an experimental approach to investigate the O2 dissociation capability and NO oxidation activity of single Mn sites and Mn-Mn dimer sites with varying bond lengths. Our results indicate that Mn-Mn dimer sites outperform single Mn active sites in both O2 activation and NO oxidation. This enhancement is primarily attributed to the short-range ordered geometry of the Mn-Mn dimers, which suppresses the formation of NO3* intermediates and promotes NO2* desorption. Among the three types of Mn-Mn dimers examined, the Mn-Mn dimer in BaMnO3, with the shortest Mn-Mn bond length, aligns most favorably with O-O, supporting the most efficient O2 activation. Conversely, MnO2, characterized by the longest Mn-Mn bond length, exhibits greater charge transfer and synergistic effects at the local active site, achieving the highest NO catalytic activity. Furthermore, we found that dual-site exposure of Mn-Mn dimers is more effective for catalytic reactions than single-site exposure. This study provides important insights into the structure–activity relationship between the geometric structure of catalytic active sites and the adsorption of intermediates.