Molecular
hydrogen is one of the essential reactants in the chemical
industry, and its generation from renewable sources such as biomass
materials and water is of great benefit to the future society. Generally,
molecular oxygen should be pre-eliminated in the hydrogen evolution
reactions (HERs) in order to avoid the reverse hydrogen oxidation
reaction (HOR). Here, we report a highly efficient HER from a formaldehyde/water
mixture using MgO supported Ag nanoparticles (AgNPs/MgO) as the catalyst
and molecular oxygen as a promoter. The HER rate depends almost linearly
on the oxygen partial pressure, and the optimal turnover frequency
(TOF) of the silver catalyst exceeds 6,600 h–1.
Based on the experimental and theoretical results, a surface stabilized
MgO/Ag–•OOH complex is suggested to be the
main catalytically active species for the HER.
Efficient
molecular hydrogen generation from renewable biomass-derived
resources and water is of great importance to the sustainable development
of the future society. Herein, ultrasmall Ag nanoclusters supported
on a defect-rich MgO matrix (AgUCs/MgO) are synthesized by a facile
impregnation/calcination method and are applied to robust oxygen-promoted
formaldehyde reforming into H2 at room temperature. Density
functional theory calculations and experimental observations show
that the catalyst spatially builds up a channel for directional electron
transfer from electron-rich Ag sites to the anti-bonding π orbital
of chemisorbed bridged O2 molecules, leading to the implementation
of low-temperature O2 adsorption and activation. The catalytically
active species, •OOH, is thus selectively generated
via a preferential two-electron reduction of O2 with a
low energy barrier on Ag sites, involving an unusual long-range proton-coupled
electron transfer process. The •OOH–AgUCs/MgO
active center is efficient for the subsequent C–H activation
and H2 generation, leading to a 3-fold improvement of the
turnover frequency as compared with its analogous AgNPs/MgO catalyst.
Our atomic-level design and synthetic strategy provide a platform
that facilitates the construction of an electron–proton transfer
channel for catalysis, altered adsorption configurations of activated
reactants, and enhancement of catalytic hydrogen generation activity,
extending a promising direction for the development of next-generation
energy catalysts.
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