3. Results and discussion
3.1 Synthesis and characterization of
CoFe-CoxN@NC
The CoFe-CoxN@NC catalyst was synthesized by
self-assembly and pyrolysis process, as shown in Fig. 1a. In brief,
biorefinery lignin was carboxylated to improve its solubility and
coordinated with Co2+ and Fe3+ to
form the lignin-based CoFe supramolecules, followed by co-doping with
urea and in situ pyrolyzing at high temperature to obtain the
CoFe-CoxN heterojunction on the nitrogen-doped
lignin-derived carbon.
The
crystal phase structure and graphitization degree of
CoFe-CoxN@NC samples were shown in Fig. 1 and Fig.
S1. From the XRD patterns of the
metal free nitrogen-doped lignin-derived carbon materials (NC) and
CoFe-Co5.47N@NC, it could be observed that the
characteristic peaks of NC at the position of 24° and 43°, respectively,
which belonged to the (002) and (100) crystal planes of graphitic carbon53. When the Fe/Co ratio gradually decreased from 7:1
to 1:7, the XRD peaks of (110) and (200) planes of Fe (PDF#06-0696)
moved to a smaller angle and disappeared until the (111) and (200)
planes of Co (PDF#15-0806) appeared, and all the samples had a broad
peak around 26° attributed to the (002) plane of graphitic carbon, as
shown in Fig. 1b. The dominant phase is bimetallic CoFe alloy with
diffraction peaks centered at 44.9° and 65.3°, which can be ascribed to
the (110) and (200) crystal planes of CoFe alloy (PDF#49-1568). The
diffraction peaks of 43.7°, 50.8° and 74.9° corresponding to the (111),
(200) and (220) crystal planes of Co5.47N (PDF#41-0943)
were also observed 54. The XRD peak of
CoFe-Co5.47N@NC moved towards a higher angle
(~ 0.4°) relative to Co5.47N, which
meant that the lattice expansion was due to the partial substitution of
Co by Fe 55. With the increase of pyrolysis
temperature, the diffraction peak intensity belonging to the (002)
crystal plane of graphitic carbon increased, the graphitization degree
gradually increased as well as the electronic conductivity. With the
increase of the urea dosages during the in situ pyrolysis process, the
diffraction peak intensity of 43.7° belong to the (111) plane of
Co5.47N increased continuously, indicating that the
coordination between metal and nitrogen gradually increased, and
nitrogen was successfully introduced into the lignin-derived carbon
support. The optimal Fe/Co ratio and urea dosage was beneficial to the
formation of CoFe alloy and CoxN heterojunction.