## 3 Results and Discussion
### Chemical Composition of the Catalyst  
The chemical composition of the as-synthesized M@CN was examined by powder XRD, and the recorded patterns are presented in Figure 2a. Two peaks at 27.6 and 13.0 degrees were observed, which represented (002) and (100) facets of conjugated aromatic complexes, respectively [20]. After metal loading, the basic structure of g-C\({}_{3}\)N\({}_{4}\) remained, but the graphitization degree of FeNi-Pd@CN was lower than two other M@CN. There are no obvious peaks related to metal crystal, perhaps due to the low content or small size of the metal species. These results were further confirmed by FT-IR spectra in Figure 2b. All three spectra show the characteristic peaks of g-C\({}_{3}\)N\({}_{4}\): the peak at around 810 cm\({}^{-1}\) is attributed to the vibration of tris-s-triazine units [21], and the peaks in the region of 1100-1700 cm\({}^{-1}\) are attributed to the typical bands of aromatic C-N heterocycles containing either trigonal N(-C)\({}_{3}\) (full condensation) or bridging C-NH-C units (partial condensation), evidencing the formation of extended networks of C-N-C bonds [22]. The peaks between 3000 and 3600 cm\({}^{-1}\) are contributed by N-H stretching [21], and the much broader peaks of FeNi-Pd@CN verified that more free amino groups remained than Fe-Pd@CN and Ni-Pd@CN. The peak of FeNi-Pd@CN at 417.36 cm\({}^{-1}\) attributed to the stretching vibration of Fe-O [23] was also weaker than that of Fe-Pd@CN. It can be speculated that the addition of both Fe and Ni lower the graphitization degree, resulting in more defects in g-C\({}_{3}\)N\({}_{4}\), and in the meantime, large aggregation of Fe species was avoided due to the combination of Fe precursor with amino groups. Previous literature [8] also pointed out the addition of Ni led to the small size and well-dispersed nanoparticles of Fe species.  
The specific bonds and chemical states of FeNi-Pd@CN were further studied using X-ray photoelectron spectroscopy. The XPS survey spectrum (Figure 3a) of the M@CN composite displays the coexistence of C, N, O, Fe, Ni, and Pd elements. The C 1s region of  
Figure 1: (a) SEM image of FeNi-Pd@CN; (b,c) HRTEM image of FeNi-Pd@CN; (d) HADDF images and (e–j) elemental mapping of FeNi-Pd@CN, C, N, O, Fe, Ni, and Pd.  
the spectra (Figure 3b) was fitted into three peaks. The main peak at 288.2 eV was assigned to sp\({}^{2}\) hybridized carbon corrected with N (C-N=C) [24], and two other peaks at 284.8 and 286.4 eV were typical signals of C-C and C-O components. Another \(\pi\)-\(\pi^{\circ}\) satellite 6 eV from the main C1s peak could be observed, indicating the extended delocalized electrons structure of the g-C\({}_{3}\)N\({}_{4}\). The deconvolution of the N 1s XPS signals (Figure 3c) shows three major peaks at 398.9, 399.9, and 401.0 eV, which arise from sp\({}^{2}\) N atoms in triazine rings (C-N-C), sp\({}^{3}\) N atoms (HN-(C)\({}_{3}\)), and the marginal amino(-NH\({}_{2}\)), respectively [17, 25]. The Ni 2p spectrum consists of a relatively narrow spin\(-\)orbit doublet (Figure 3d) characterized by binding energies of the Ni 2p\({}_{3/2}\) and Ni 2p\({}_{1/2}\) core levels of 855.7 and 873.4 eV, respectively [26, 27, 28], verifying the bipositive nature of Ni. Fe 2p spectrum shows a mixture of core level and satellite features due to the existence of Fe\({}^{2+}\) and Fe\({}^{3+}\)[29]. Pd was partly reduced by NaBH\({}_{4}\); thus, its 3d spectrum displays two doublets caused by the Pd\({}^{0}\) and Pd\({}^{2+}\). The Pd 3d core level in this study (336.1 and 338.0 eV for 3d\({}_{5/2}\) of Pd\({}^{0}\) and Pd\({}^{2+}\), respectively) shifted higher binding energy than values in NIST X-ray photoelectron spectroscopy database. This result manifests that Pd with small size supported on carbonaceous materials according to previous reports [30, 31, 32].