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Solar-driven photocatalytic conversion of CO2 into fuels has attracted a lot of interest; however, developing active catalysts that can selectively convert CO2 to fuels with desirable reaction products remains a grand challenge. For instance, complete suppression of the competing H2 evolution during photocatalytic CO2-to-CO conversion has not been achieved before. We design and synthesize a spongy nickel-organic heterogeneous photocatalyst via a photochemical route. The catalyst has a crystalline network architecture with a high concentration of defects. It is highly active in converting CO2 to CO, with a production rate of ~1.6 × 104 μmol hour−1 g−1. No measurable H2 is generated during the reaction, leading to nearly 100% selective CO production over H2 evolution. When the spongy Ni-organic catalyst is enriched with Rh or Ag nanocrystals, the controlled photocatalytic CO2 reduction reactions generate formic acid and acetic acid. Achieving such a spongy nickel-organic photocatalyst is a critical step toward practical production of high-value multicarbon fuels using solar energy.
Rapid fossil fuel consumption induces environmental burden and energy crisis (
). Excessive anthropogenic CO2 emission is a significant concern because of its hastening impact on climate change (
), acidification of ocean (
), crop yield reduction (
), extinction of animal species (
), and damage to human health (
). Removal of excessive CO2 from the atmosphere (
), particularly converting CO2 to fuels using solar energy, is currently a global research endeavor (
). Discovering novel catalysts that can reduce the stable CO2 molecules and convert them to liquid fuels with high activity and selectivity is essential (
). To date, despite the progress that has been made in investigating the photocatalytic reduction of CO2 (
), controlling the reaction to yield a specific product among many possible reaction species, including CO, H2, CH4, and formic acid, remains a great challenge (
). Finding photocatalysts that can efficiently convert CO2 to CO and largely suppress other competing photocatalytic reactions, such as H2 evolution, would be a critical step forward toward practical solar-to-fuels conversion for the production of high-value multicarbon fuels (
We recently developed a laser-chemical method and synthesized active transition metal hydroxide catalysts with a high concentration of defects for water oxidation (
). Specifically, we used an unfocused infrared laser to initiate the reactions between transition metal ions and triethylene glycol (TEG) and obtained a series of metal hydroxide–TEG composites with a distorted layered structure (
). This disordered structure enhances the accessibility of water molecules to the active sites and enables efficient electrocatalysis of alkaline water oxidation (
). Such a laser-chemical strategy may be applied to the discovery of many other catalysts, for instance, novel nanostructured metal-organic heterogeneous catalysts for CO2 reduction reaction.
When designing catalysts for CO2 reduction, the material’s ability to capture the CO2 molecules is another significant consideration (
). Metal-organic frameworks (MOFs) with high surface area and tunable pores have been used for gas capture and heterogeneous catalysis (
). Typically, MOFs have highly ordered crystalline structures constructed by coordinating metal ions or clusters with rigid organic linkers, most often the aromatic carboxylic acid molecules (
), such as terephthalic acid (TPA). In light of MOF structure design, we replace part of the rigid linkers (for example, TPA) in traditional MOFs with soft molecules (for example, TEG) by laser, considering the comparable molecular length of TEG to TPA (fig. S1). When the TEG molecules, which lack essential carboxylic groups for the perfect framework construction, are weaved into the metal-TPA framework, their substitution of TPA linkers may frustrate the growth of highly ordered MOF crystals, resulting in disordered and defective metal-organic hybrids for effective CO2 fixation.
Here, we design a model metal-organic CO2 reduction catalyst, with Ni2+ ions as active metal centers, TPA as a rigid linker, TEG as a soft linker, and dimethylformamide (DMF) as a solvent, via laser-induced solution reactions. The as-synthesized catalyst, labeled as Ni(TPA/TEG), has a crystalline network architecture with considerable defects and performs nearly 100% selective gas production (CO over H2 evolution) with a high CO production rate of ~1.6 × 104 μmol hour−1 g−1. Further metal decorations (that is, Rh and Ag) of the Ni(TPA/TEG) catalyst lead to controlled photocatalytic CO2 reduction reactions that generate formic acid and acetic acid.
RESULTS AND DISCUSSION
Structure determination of the Ni(TPA/TEG) catalyst
As shown in
, the Ni(TPA/TEG) composite forms a disordered spongy network structure, in which Ni, O, and C are uniformly distributed (fig. S2). In comparison, the solution without TEG, Ni(TPA) only, forms large particles (
). A three-dimensional electron tomographic reconstruction of the spongy Ni(TPA/TEG) architecture reveals various mesopores in the structure (
and movie S1), which closely resembles the pore features identified from the N2 physisorption measurements (fig. S3).
shows a typical transmission electron microscopy (TEM) image of the spongy Ni(TPA/TEG) composite, where defective lattices with a d-spacing of 1.02 nm are captured. To further interpret the structure of Ni(TPA/TEG) composite, we acquire a scanning nanobeam diffraction data set using an electron beam with a size of ~3 nm, a total beam current of ~5 pA, and an exposure time of 0.5 s, where the electron beam damage to the metal-organic material has been evidently minimized (
). Single-crystalline diffraction patterns along the  and  orientations of the Ni(TPA/TEG) composite are captured from two different regions of the spongy network (
and movies S2 and S3), showing an orthorhombic structure similar to that of the Ni(TPA) particles (fig. S4). Changes of the diffraction patterns are observed from movies S2 and S3, indicating defects (that is, grain boundaries) in the spongy Ni(TPA/TEG) catalyst (fig. S5).