Silver incorporated into tunneled manganese oxide promotes hydrogen production from aqueous formaldehyde

Abstract

The efficient production of molecular hydrogen from renewable biomass-derived resource is of great significance to the sustainable development of the future. In this work, various Ag/MnO2-X (X indicates the pH value of wetness impregnation, i.e., 1, 5, 9, 13) were prepared and applied to room-temperature formaldehyde reforming reaction. In acidic condition, the Ag species were well dispersed and partially replaced K+ cations in the tunnels of MnO2. With the increase of pH, the Ag species aggregated into clusters (diameter of 2 ∼ 5 nm). The difference in the geometric and electronic structure of Ag species resulted in variable catalytic performance. The turnover frequency of Ag/MnO2-1 (26,382 h−1) for hydrogen production was 4.48-fold to that of Ag/MnO2-13 (5,882 h−1). In Ag/MnO2-1, the formed Ag-O-Mn bond in the tunnels acted like an electron transfer channel, promoting for the reduction of chemisorbed oxygen and its subsequent proton capture from formaldehyde. Furthermore, this unique structure also benefited for the dissociation of water molecules, accelerating the combination of two protons (one is from formaldehyde, the other is from water) for hydrogen production. This work clearly explains the Ag size/structure-dependent effect on formaldehyde reforming at atomic level, providing guidance for the design of high-efficient catalysts for aqueous-phase hydrogen evolution reactions.