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Liu et al. Chem Synth 2023;3:24 https://dx.doi.org/10.20517/cs.2023.13 Page 5 of 14

RESULTS AND DISCUSSION
The synthetic strategy of Pd nanoclusters supported on nitrogen/amino co-functionalized carbon supports
is illustrated in Scheme 1. First, the NC support was prepared through a soft nitriding technique as
reported [39,40] . After that, APTS was fully mixed with NC aqueous solution under ultrasonic and stirred to
modify the supports with amino groups. Then, quantitative Na PdCl aqueous solution was added to the
4
2
above mixture with stirring for 2 h. After that, NaBH solution was introduced into the above suspension
4
and stirred for another 2 h to reduce Pd precursor. Finally, the obtained Pd@NH -NC was centrifuged and
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repeatedly washed with water. For comparison, the reference Pd@NH -C, Pd@NC, and Pd@C were also
2
prepared in the same strategy.
As shown in Figure 1a, the Pd nanoclusters with ultrasmall size are uniformly dispersed on the nitrogen/
amino co-functionalized carbon surface without any aggregation. The HR-TEM image shown in Figure 1b
reveals a lattice spacing of 1.94 Å, corresponding to the (200) crystal face of Pd. The uniform small size (1.42
± 0.3 nm) and good dispersion of Pd nanocluster are clearly shown in the HAADF-STEM image and
particle size distributions histogram in Figure 1c and d. The TEM images of reference catalysts are also
shown in Supplementary Figure 2. Obviously, Pd nanoparticles show larger size and poorer dispersion on
supports for Pd@NH -C, Pd@NC, and Pd@C. The results indicate that the nitrogen/amino co-
2
functionalization can provide abundant anchor sites for Pd active centers, resulting in the reduced size and
uniform distribution of Pd nanoclusters on carbon supports. Furthermore, the STEM-EDS analysis in
Figure 1e-h clearly demonstrates the distribution of Pd and N on carbon supports. The content of Pd in
different samples is evaluated by ICP-MS, and the mass loadings of Pd in the Pd@NH -NC, Pd@NH -C,
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2
Pd@NC, and Pd@C are estimated to be 1.6 wt. %, 1.6 wt. %, 1.3 wt. %, and 2.1 wt. %, respectively.
Meanwhile, the N sorption isotherms of all sample show type I and type IV isotherms, suggesting that there
2
are micropores (1.0-1.6 nm) and mesopores (larger than 4 nm) in the samples [Supplementary Figure 3].
The BET surface areas of C, Pd@C, Pd@NC, Pd@NH -C, Pd@NH -NC are measured as 238.6, 184.6, 90.4,
2
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93.2, 78.0 m ·g , respectively.
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-1
The structural characteristics of catalysts were studied by XRD first [Figure 2a]. All catalysts reveal two
broad peaks at 25º and 43º ascribed to the (002) and (101) crystal planes of amorphous carbon . In
[41]
addition, three clear diffraction peaks positioned at 40º, 47º, and 68º can be observed in Pd@C catalyst,
which correspond to the (111), (200) and (220) planes of Pd, respectively. Notably, no significant diffraction
peaks of Pd were observed for Pd@NH -NC, Pd@NH -C and Pd@NC due to the ultrasmall size of the Pd
2
2
over the supports, which agrees well with the TEM characterization. Furthermore, Raman spectra display
two distinct peaks at 1,338 cm and 1,588 cm , which are known as D and G bands, corresponding to
-1
-1
disordered and graphitic carbon, respectively [Figure 2b]. The Pd@NH -NC shows a high I /I value of
D
2
G
1.00, which is higher than Pd@NH -C (0.98), Pd@NC (0.99) and Pd@C (0.91), suggesting more defects
2
introduced by nitrogen/amino co-functionalization of carbon supports. The FT-IR spectra were performed
to confirm the successful nitrogen/amino co-functionalization [Figure 2c]. As compared with Pd@C,
-1
distinguished new peaks appear at 1193 and 1721 cm for Pd@NH -NC, Pd@NH -C, and Pd@NC, which
2
2
are ascribed to the stretching of C-N and C=O groups, respectively, suggesting the successful modification
-1
of surface nitrogen [39,42] . Moreover, for the Pd@NH -NC and Pd@NH -C, the peak at 1,121 cm originates
2
2
from the asymmetric stretching of the Si-O-C bonds, and the peaks at 2,870 cm and 2,932 cm originate
-1
-1
from the -CH - stretching from APTS, indicating the efficient graft of amino group on carbon. The
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photographs in Figure 2d (below) display the hydrophilicity and dispersibility of the samples in water. It is
clear that Pd@NH -NC and Pd@NC are well dispersed in the water and remained for 60 min. In
2
comparison, the Pd@NH -C and Pd@C show different degrees of sedimentation. Meanwhile, the contact
2
angle measurements were performed to investigate the surface wettability of samples. As shown in Figure 2d