Morphology Engineering toward Highly Efficient NiO Nanocatalysts for Oxygen-Promoted Hydrogen Production from Formaldehyde Solution

Renewable biomass solutions for hydrogen production are a sustainable technology to boost the hydrogen economy, in which formaldehyde reforming into hydrogen using cost-effective catalysts has garnered significant attention. In this study, we successfully fabricated two stable NiO nanostructures with distinct morphologies, namely nanospheres and nanosheets, through a simple two-step process. The morphology-dependent efficiency of hydrogen production under oxygen-promoted conditions was investigated. The nanosphere-shaped NiO (NiO–NP) exhibited a distinctive three-dimensional network and showed a much higher turnover frequency (TOF: 277.2 h–1) for H2 production from alkaline formaldehyde (HCHO) solution at room temperature, in contrast to NiO nanosheets (NiO–NH, TOF: 153.3 h–1). Scanning electron microscopy and thermogravimetric tests revealed that the porous structures consisting of nanoribbons in NiO–NP catalysts facilitated the exposure of the active sites and mass transfer processes. Moreover, X-ray photoelectron spectroscopy indicated a larger peak area ratio of defect sites/lattice oxygen in NiO–NP, suggesting a higher number of oxygen vacancies (Vo). This ensured favorable chemisorption of molecular O2 on the catalyst surface, establishing a channel for electron delivery to O2 species, which provided evidence for the more efficient hydrogen production of NiO–NP catalysts under oxygen partial pressure (pO2). Electron paramagnetic resonance results indicated that the breaking of the C–H bond of HCHO was coupled with the O2 reduction, forming •OOH through preferential two-electron reduction. Notably, •OOH played a crucial role as reactive oxygen species in dehydrogenation reactions, with the •OOH–NiO complex serving as the active center. These findings not only offer possibilities for enhancing catalytic performance through nanostructure engineering but also provide insights into hydrogen production from renewable biomass using cost-effective catalysts.