Single-Atom Cationic Ag and Metallic Ag Nanoparticles Supported on Al2O3 as Catalysts for Water-Resistant and CO2-Selective HCHO Oxidation

Abstract

Ag-based catalysts have been extensively investigated in HCHO oxidation on account of the low price of the Ag precursor and their considerable low-temperature activity. It is known that H2 pretreatment has a remarkable promotion effect on Pt- and Pd- catalysts for HCHO oxidation, whereas the effect of H2 reduction on Ag-based catalysts has been rarely reported, probably due to the coexistence of multiple states of Ag species resulting from the high Ag loading. Herein, we prepared a 1% Ag/Al2O3 (Ag/Al-fresh) catalyst with Ag species in a uniform state and then reduced Ag/Al-fresh with H2 at 600 °C to obtain the Ag/Al-600H2 catalyst. We observed that H2 reduction had a significant influence on the performance of Ag/Al2O3 nanocatalysts in HCHO oxidation. In a dry reaction atmosphere, Ag/Al-fresh and Ag/Al-600H2 exhibited comparable HCHO conversion levels, but Ag/Al-600H2 showed much higher CO2 selectivity than Ag/Al-fresh. The presence of water vapor severely suppressed the HCHO conversion of the Ag/Al-fresh catalyst, while Ag/Al-600H2 showed high water resistance, maintaining high HCHO conversion and CO2 selectivity at 35% RH. Characterization results showed that the Ag species on Ag/Al-fresh and Ag/Al-600H2 were single-atom cationic Ag and metallic Ag nanoparticles, respectively. It was revealed that the Ag0 nanoparticles were more conducive than single-atom Ag+ to formate decomposition, as well as O2 and H2O activation, thus accelerating the conversion of HCOO– and CO intermediates to CO2 and contributing to the higher activity and water resistance of the Ag/Al-600H2 catalyst.