A single-atom catalyst (SAC) is pretty much what it says on the tin; individual metal atoms that act as active sites to speed up a reaction, dispersed on a supporting material. SACs are desirable in heterogeneous catalysis as they make use of every metal atom, leading to greater possible catalytic efficiencies and activities. Metal-organic frameworks (MOFs), porous and crystalline materials that are useful in their own right, have been identified as useful precursors for pyrolysis to form nanostructures and single-atom catalysts. Researchers in Switzerland and China have now demonstrated this strategy for the formation of single-atom nickel species for electrocatalysis, where their MOF-derived material showed excellent activity, efficiency and durability for the electrochemical CO2 reduction reaction (CO2RR).
Whilst MOF-derived nanostructures exist, single-atom sites are typically harder to achieve by pyrolysis. Most organic linkers within MOFs typically contain oxygen coordination sites to bind the metal, but this oxygen content is lost in pyrolysis due to CO2 formation. Without other Lewis-basic coordination sites within the MOF, pyrolysis can often lead to metal aggregation rather than dispersed single-atom sites for catalysis. The researchers overcame this challenge by adding both a polymer and a secondary nitrogen-rich compound to the MOF before pyrolysis, aiding in the formation of dispersed and stable single-atom metal sites.
Figure 1: The preparation of the nickel-containing nitrogen-doped carbon catalysts
The researchers selected Ni2(NDISA) as the MOF to study, with nitrogen-containing naphthalene diimide salicylic acid (NDISA) linkers. They introduced a polydopamine (PDA) polymer into the porous channels of the MOF structure to create Ni2(NDISA)-PDA, and then further subjected the MOF/polymer composite to melamine as an additional nitrogen source. The researchers then subjected the MOF, the MOF/polymer composite and the MOF/polymer composite with melamine to pyrolysis, followed by etching with acid to remove any unbound nickel particles, to form the nickel-containing nitrogen-doped-carbon catalysts Ni/NC, Ni/NC-D and Ni/N-CNT, respectively (Figure 1). The MOF/polymer composite with melamine went on to form carbon nanotubes (CNTs) after pyrolysis as an effect of decomposition of the melamine.
The researchers used a range of techniques to characterise the materials. They found that the Ni/N-CNT material had the highest nickel loading and therefore the greatest number of dispersed single-atom nickel sites, owing to both the addition of the polymer that prevented aggregation of the metal and the addition of the nitrogen-rich melamine to aid nickel binding to the surface. All three materials were tested for electrochemical CO2 reduction, an important carbon neutral cycle. The three materials all showed selective production of CO and H2, with the Ni/N-CNT material showing the greatest faradaic efficiency and stability owing to the greater amount of nickel active sites. Overall, this simple strategy of combining a MOF precursor with a polymer and a nitrogen-rich source successfully enhanced the performance of the MOF-derived material with single-atom nickel sites, and has future potential in a wider variety of electrochemical applications using a range of MOF and polymer building blocks.
To find out more, please read:
Shuliang Yang, Jie Zhang, Li Peng, Mehrdad Asgari, Dragos Stoian, Ilia Kochetygov, Wen Luo, Emad Oveisi, Olga Trukhina, Adam H. Clark, Daniel T. Sun and Wendy L. Queen
Chem. Sci., 2020, 11, 10991-10997
About the blogger:
Dr. Samantha Apps recently finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.
