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Page 10 of 14 Liu et al. Chem Synth 2023;3:24 https://dx.doi.org/10.20517/cs.2023.13

small amount of CO generated when the FA concentration reaches up to 15.0, 20.0, 26.0 M, suggesting that
the dehydration pathways occur under the high FA concentration and thus decrease H selectivity. These
2
results indicate that the water content in the reaction system has a great influence on the catalytic
performance and the H selectivity, which are consistent with previous reports [49,50] . We further evaluated the
2
dehydrogenation of FA under different amounts of catalysts [Figure 5b]. The gas production rate gradually
increases when the Pd concentration increases from 5.75 to 14.00 mM. The normalized plots between In
rate and In [Pd] display a slope value of 1.01, indicating that the dehydrogenation of FA over Pd@NH -NC
2
catalyst is a first-order reaction in Pd concentration. The activation energy (E ) of catalysts was also
a
evaluated by testing the dehydrogenation of FA under elevated temperatures (298, 303, 313, 323, and 333 K).
The time-dependent plots of the generated gas under different temperatures over Pd@NH -NC, Pd@NH -
2
2
C, Pd@NC, and Pd@C are shown in Figure 5c and Supplementary Figure 8. The gas production rates are
obviously promoted with the temperature increases. As shown in Figure 5d, the corresponding TOF initial
-1
values of Pd@NH -NC display as 4892, 5609, 8746, 11947, and 15788 h , which are highest compared with
2
Pd@NH -C, Pd@NC, and Pd@C at different temperatures. By fitting the Arrhenius plots between ln [TOF]
2
−1
and 1/T, the E value of the Pd@NH -NC catalyst is calculated as 28.5 kJ·mol [Figure 5e]. In contrast, E
2
a
a
values of Pd@NH -C, Pd@NC, and Pd@C are 40.5, 36.0, and 32.1 kJ·mol , respectively. The lowest E value
−1
a
2
of Pd@NH -NC for FA dehydrogenation suggests the thermodynamically lowest reaction energy barrier for
2
FA dehydrogenation. Furthermore, the TOF initial value of Pd@NH -NC is compared with other recently
2
reported catalysts for FA dehydrogenation at 298, 323, and 333 K [Figure 5f]. Obviously, the as-prepared
Pd@NH -NC displays higher activity than those of Pd or Pd-based alloy catalysts quoted, and details of the
2
experimental conditions and catalytic performance of these catalysts are listed in Supplementary Table 1.
The recycling stability of the as-prepared Pd@NH -NC catalyst is further evaluated by re-injecting the same
2
amount of pure FA into the previous mixture when the last reaction was completed at 298 K. As shown in
Figure 6a, after five cycles, the complete dehydrogenation of FA can still be achieved with a slight decrease
in gas generation rate. It only takes 10 minutes to generate 146 mL of gas in the fifth cycle, suggesting good
catalytic stability of the Pd@NH -NC catalyst. The XRD results show a small diffraction peak at 40º for
2
spent Pd@NH -NC, which indicates that the Pd nanoclusters suffer a slight aggregation with a larger
2
particle size [Figure 6b]. Meanwhile, the TEM image of spent Pd@NH -NC in Figure 6c indicates that the
2
Pd nanoclusters are still evenly distributed on the carbon surface with a slightly increased size of 1.65 ± 0.5
nm. Additionally, XPS results further show that the nitrogen content of spent Pd@NH2-NC is determined
as 7.9 at. %, which is slightly lower than that of fresh Pd@NH -NC (9.2 at. %) [Figure 6d and Supplementary
2
Figure 9]. There is a small amount of CO (0.23 %) for the fifth dehydrogenation of FA [Supplementary
Figure 10], which may cause catalyst poisoning and lead to the reduction of circulation performance.
Based on the above studies, a possible mechanism for the dehydrogenation of FA over Pd@NH -NC catalyst
2
has been proposed [Figure 7]. FA molecules are firstly absorbed on the surface of Pd@NH -NC, and the
2
nitrogen/amino groups can act as proton scavengers to break the O-H bond and produce H , leaving
+
HCOO on the Pd surface [15,51] . Subsequently, the electron-deficient Pd nanoclusters induced by metal-
-
-[3]
support interaction can be active for the rupture of the C-H bond and the formation of H , followed by the
release of CO molecules in this process. Finally, the H absorbed on amino groups combines with the H
-
+
2
absorbed on Pd nanoclusters to form H . Therefore, the remarkable catalytic performance of Pd@NH -NC
2
2
can be attributed to the synergistic effect of nitrogen/amino co-functionalized carbon support, Pd
nanoclusters, and metal-support interaction.
CONCLUSIONS
In summary, nitrogen/amino co-functionalized carbon-supported Pd nanoclusters have been successfully