## 3 Results and Discussion
### Enhanced H\({}_{2}\)O\({}_{2}\) Production  
Direct generation of H\({}_{2}\)O\({}_{2}\) from formic acid and O\({}_{2}\) using heterogeneous Pd was illustrated in previous reports. Basically, the process can be divided into two steps: first is the formic acid (FA) decomposition to H\({}_{2}\) (HCOOH\(\rightarrow\)CO\({}_{2}\) + H\({}_{2}\)), and second is O\({}_{2}\) reduction to H\({}_{2}\)O\({}_{2}\) (H\({}_{2}\) + O\({}_{2}\)\(\rightarrow\)H\({}_{2}\)O\({}_{2}\)). FA decomposition begins with the ionization of formic acid and then the cleavage of C-H bond in formate species as a vital step for H\({}^{*}\) forming (\" denotes an active site). [PERSON] et al. [33] summarized that strengthened adsorption of formate species and weakened adsorption of H\({}^{*}\) were favorable for hydrogen production through kinetic analysis. From these points of view, the addition of positively charged Fe and Ni is beneficial to the adsorption of HCOO' intermediates. However, the Pd 3d core level of FeNi-Pd@CN was almost similar to that of Fe-Pd@CN but higher than that of Ni-Pd@CN (Figure S5), suggesting that Fe is more capable than Ni of weakening the H\({}^{*}\) adsorption through the depletion of the d-band electronic state near the Fermi level [34].  
In the second step, O\({}_{2}\) was firstly adsorbed and activated on Pd surface and then reduced to HOO' and H\({}_{2}\)O\({}_{2}\)* by adsorbed H atom. For high H\({}_{2}\)O\({}_{2}\) selectivity and stability, the dissociation of O-O bond should be avoided. DFT results verified that Pd (111) has the lowest crystal surface energy compared to other facets and thus could reduce the dissociation of O-O bond in O\({}_{2}\)', HOO', and H\({}_{2}\)O\({}_{2}\)* [35]. In this study, Pd supported on g-C\({}_{3}\)N\({}_{4}\) exposes many crystal (111) facets and thus has high activity for H\({}_{2}\)O\({}_{2}\) yielding,  
Figure 4: (**a**) Degradation of AR 73 (200 mg/L) and (**b**) TC (10 mg/L) by different catalysts (catalyst dosage: 2 g/L; FA: 46.6 mmol/L; air; 200 mL/min).  
Figure 5: Degradation of AR 73 in multiple uses of FeNi-Pd@CN catalyst (AR 73 200 mg/L; catalyst dosage: 2 g/L; FA: 46.6 mmol/L; air; 200 mL/min).  
An obvious accumulation of H\({}_{2}\)O\({}_{2}\) could be observed even when blowing air by Pd@CN (Figure 6). According to experimental data as well as theory results [36], O\({}_{2}\) is easier to chemically adsorb on Fe and Ni (Fe > Ni > Pd); O\({}_{2}\) temperature-programmed desorption (O\({}_{2}\)-TPD) curves in Figure S6 also suggested that more adsorbed O and lattice O were introduced with the addition of Fe, and thus H\({}_{2}\)O generation would happen with a waste of H\({}^{*}\). It can be speculated from the O\({}_{2}\)-TPD curves and characterization results that the added Ni was most likely embedded in the g-C\({}_{3}\)N\({}_{4}\) framework through coordination with N, preventing O\({}_{2}\) introduction for H\({}^{*}\) consuming. Therefore, H\({}_{2}\)O\({}_{2}\) yielding of Ni-Pd@CN was higher than that of Fe-Pd@CN. Combined with all the merits of Fe and Ni, FeNi-Pd@CN could accumulate 1.69 mmol/L of H\({}_{2}\)O\({}_{2}\) in 150 min. Through boost activity and selectivity, air could be used as the O\({}_{2}\) source instead of pure oxygen in other studies, which could cut down the cost for practical use.