fig4
![Constructing bimetal, alloy, and compound-modified nitrogen-doped biomass-derived carbon from coconut shell as accelerants for boosting methane production in bioenergy system](https://image.oaes.cc/2581ab7d-d74c-486e-b68c-24d48030e96a/em4062.fig.4.jpg)
Figure 4. (A) XPS measurement spectra, (B and C) Fe 2p XPS spectra of Fe-N-C and Fe/Ni-N-C, (D and E) Ni 2p XPS spectra of Ni-N-C and Fe/Ni-N-C, (F-H) N 1s XPS spectra of Ni-N-C, Fe-N-C, and Fe/Ni-N-C, and (I) schematic diagram of different N types.