Recover the activity of sintered supported catalysts by nitrogen-doped carbon atomization

The sintering of supported metal nanoparticles is a major route to the deactivation of industrial heterogeneous catalysts, which largely increase the cost and decrease the productivity. Here, we discover that supported palladium/gold/platinum nanoparticles distributed at the interface of oxide supports and nitrogen-doped carbon shells would undergo an unexpected nitrogen-doped carbon atomization process against the sintering at high temperatures, during which the nanoparticles can be transformed into more active atomic species. The in situ transmission electron microscopy images reveal the abundant nitrogen defects in carbon shells provide atomic diffusion sites for the mobile atomistic palladium species detached from the palladium nanoparticles. More important, the catalytic activity of sintered and deactivated palladium catalyst can be recovered by this unique N-doped carbon atomization process. Our findings open up a window to preparation of sintering-resistant single atoms catalysts and regeneration of deactivated industrial catalysts.

calculated energies along the pyrolysis pathway of Pd atom for Pd10 cluster to Pd-N3 and Pd-N4 defect respectively and their corresponding configurations of initial and final states. Pd, N, and C atoms are marked in green, blue, and grey, respectively. The activation energy Ea represents the difficulty for this pyrolysis process. The total energy difference ΔE represents the stability of the final states compared with initial states which means the Pd-N3 defect is more stable than Pd-N4 defect.

Supplementary Notes
We used N-free Resorcino-formaldehyde (RF) resin as the coating polymers Quantitative analysis of the sample after annealing at 900°C for 3h (Pd SAs/TiO2@C) was carried out through least square EXAFS fitting. The fitting curves are displayed in Supplementary Figure 18, and the corresponding structure parameters are listed in Table   S2. According to the fitting results, the proposed local structure of Pd SAs involves coordination by four N.
Furthermore, density functional theory (DFT) calculations were performed to simulate the process of thermal atomization (Supplementary Figure 19). Both N3 (a) and N4 (b) defects were considered. We adopted Pd10 as an example and found that spliting a single  Table   S3). Anatase TiO2 used in our experiment would completely transform into rutile TiO2.
As the temperature was elevated up to 800°C, only rutile phase was observed from XRD pattern, indicating the phase transformation had been finished. However, when the anatase TiO2 was coated by carbon layers, the phase transformation indeed was not TEM and STEM images of Pd NPs/TiO2-900 in Supplementary Figures 39 and 2 showed that many Pd NPs sintered severely with average diameter up to ~14.53 nm.  Figures 43e and 44e). These results all reveal that the supported Au/Pt NPs can be re-dispersed to atomic metal species by our strategy.

Supplementary methods
Computational methods. The Vienna Ab-initio Simulation Package (VASP) code was used to perform density functional theory (DFT) calculations [1][2][3] . The projector augmented wave (PAW) method was used to describe the interaction between the valence electrons and ionic cores 4,5 . The optB88-vdW method was utilized as the exchange-correlation functional that accounts for the long-range van der Waals (vdW) forces [6][7][8][9] . This functional is a revised version of the vdW-DFT functional, which has been demonstrated to improve the accuracy for a variety of molecules on transitionmetal surfaces.
The equilibrium lattice constants of anatase which has a tetragonal structure are = where the ER@NC and EA@NC represent the total energy of supercells of anatase and rutile covered with a NC layer. The EG which represents the total energy of a graphene containing 12 atoms was included for a more reliable comparison between the different chemical forms 12 . For the structures covered with pure carbon layers, the equation can be modified as: where the ER@C and EA@C represent the total energy of supercells of anatase and rutile covered with a carbon layer. The EG which represents the total energy of a graphene containing 12 atoms was included for a more reliable comparison between the different chemical forms.
We use the optB88-vdW method to do the TS search. Brillouin zone was sampled with a 3×3×1 k-point mesh. We established slab models based on the 8×8 graphite, and a 15 Å vacuum was set to eliminate the interaction between slabs. In order to ensure the stable adsorption position of Pd10 cluster, we tested the different positions around the defect. The results show that the displayed structure possesses a stable state. The structures of Pd-N3 defect and Pd-N4 defect contain 137 atoms (124 C atoms) and 136 atoms (122 C atoms) respectively. The energy cutoff of 500 eV and a residual force threshold of 0.001 eV/Å was used for geometry optimizations.