Single Atom Modified Covalent Triazine Frameworks As Electrocatalysts
Authors: Kamiya Kazuhide , Kamai Ryo , Hashimoto Kazuhito , Nakanishi Shuji 
Published: February 2020

Covalent organic frameworks (COFs) are cross-linked porous polymers, which have great potential in catalysis area because of their unique porous structure (i.e. high surface area), mechanical robustness and high design flexibility. Especially, covalent triazine frameworks (CTFs), which have 1,3,5-triazine structure as units, possess the high density of nitrogen (N) atoms in their organic pore for metal coordinations [1,2]. Recently, Palkovits et al. reported that platinum-modified CTF (Pt-CTF) functioned as a solid catalyst for the selective low-temperature oxidation of methane to methanol [3]. In general, the oxidation of methanol can easily proceed compared to methane oxidation because the C-H bond energy of methanol is much smaller than that of methane. Thus, the catalytic activity for methane partial oxidation to methanol let us assume that Pt-CTF has no activity to oxidize methanol (i.e. methanol tolerance).The methanol tolerance is the essential property for the cathode catalyst of direct methanol fuel cells (DMFC) as the methanol-crossover effect is one of the issues to be addressed. However, the application of the frameworks as electrocatalysts has not been achieved to date because of their poor electrical conductivity. In the present work, we have successfully applied CTF-based materials to electrocatalysts by hybridizing non-conductive CTFs with conductive carbon particles (CPs).

The hybridized material was obtained by the polymerization of CTFs on CPs in molten ZnCl2. Then, Pt atoms were grafted in the pore of CTFs by the impregnated with platinum chloride.Then, Pt atoms were grafted in the pore of CTFs by the impregnated with platinum chloride (Pt-CTF/CP). The resulting catalyst showed apparent electrocatalytic activity for oxygen reduction reaction (ORR). In case of a commercial 20 wt% Pt/C, after the addition of 1 M CH3OH, the onset potential of the cathodic current shifted approximately 200 mV in the negative direction in the presence of oxygen (Figure, inset). In contrast, surprisingly, the overlap of the CH3OH oxidation current with that of the ORR was almost negligible for Pt-CTF/CP even in the presence of 1 M CH3OH as shown in the Figure [4]. These results clearly showed that Pt-CTF/CP exhibited a high methanol tolerance during the ORR, as we expected. This is the first demonstration of the potential for CTF-based materials to serve as electrocatalysts.

The extended X-ray absorption fine structure (EXAFS) and transmission electron microscopy (TEM) results demonstrated that the Cu atoms in Cu-CTF/CP are singly isolated. It has been reported that at least two adjacent Pt sites are required for methanol oxidation. Actually, to the best of our knowledge, there are no works that reported the possibility of the methanol oxidation on single Pt-atom site. Thus, the structural feature of single Pt atoms, which were stabilized by N atoms in the CTF pores, is likely the origin of the methanol-tolerance.

[1] K. Iwase, K. Kamiya
et al., Angew. Chem. Int. Ed.
38, 11068, (2015)

[2] T. Yoshioka, K. Kamiya
et al., J. Phys. Chem. C in press. 10.1021/acs.jpcc.5b10962

[3] Palkovits et al. Angew. Chem. Int. Ed.
48, 6909, (2009)

[4] K. Kamiya
et al. Nat. Commun. 5, 5040, (2014)

Figure 1