Hollow mesoporous atomically dispersed metal-nitrogen-carbon catalysts with enhanced diffusion for catalysis involving larger molecules

Single-atom catalysts (SACs) show great promise in various applications due to their maximal atom utilization efficiency. However, the controlled synthesis of SACs with appropriate porous structures remains a challenge that must be overcome to address the diffusion issues in catalysis. Resolving these diffusion issues has become increasingly important because the intrinsic activity of the catalysts is dramatically improved by spatially isolated single-atom sites. Herein, we develop a facile topo-conversion strategy for fabricating hollow mesoporous metal-nitrogen-carbon SACs with enhanced diffusion for catalysis. Several hollow mesoporous metal-nitrogen-carbon SACs, including Co, Ni, Mn and Cu, are successfully fabricated by this strategy. Taking hollow mesoporous cobalt-nitrogen-carbon SACs as a proof-of-concept, diffusion and kinetic experiments demonstrate the enhanced diffusion of hollow mesoporous structures compared to the solid ones, which alleviates the bottleneck of poor mass transport in catalysis, especially involving larger molecules. Impressively, the combination of superior intrinsic activity from Co-N4 sites and the enhanced diffusion from the hollow mesoporous nanoarchitecture significantly improves the catalytic performance of the oxidative coupling of aniline and its derivatives.


Synthesis of s-CoNC:
The synthesis of s-CoNC was similar to that of h-CoNC, except solid ZnCo-BZIF without etching process was used as a precursor to anneal in the tube furnace.

Synthesis of h-NC:
The synthesis of h-NC was similar to that of h-CoNC, except only 300.0 mg of Zn(CH3COO) 2· 2H2O was used as the reactant. measurements were conducted on a Thermo ESCALAB spectrometer using a monochromated Al Kα radiation (hv=1486.6 eV). The energy calibration of the spectrometer was performed using the C 1s peak at 284.8 eV. Powder X-ray diffraction patterns (XRD) were recorded on a Bruker D8 Advance X-ray diffractometer equipped with a Cu-Kα radiation source (λ = 1.5418 Å) and operated at a scan rate of 10° min -1 .

Characterizations
The inductively coupled plasma optical emission spectrometer (ICP-OES) measurements were conducted on an iCAP7600 spectrometer for metal elemental analysis.
XAFS measurement: AFS spectra at the Co K-edge, Ni K-edge, Mn K-edge, and Cu K-edge were measured at BL14W1 station in Shanghai Synchrotron Radiation The k 3 -weighted EXAFS spectra in the k-space ranging from 2-10.5 Å -1 were Fouriertransformed to real (R) space using a hanning window.
Dispersion-corrected density functional theory calculations (DFT-D3) 6 were performed for group adsorbed on Co-N4/G and Co cluster) surfaces. We set an energy cutoff, a convergence criterion for self-consistent iteration and ionic relaxation to be 480 eV, 10 -5 eV and 0.05 eV Å -1 , respectively. The k-space integration was performed using a 3 × 3 × 1 Monkhorst-Pack grid. For the catalytic reaction, the model of Co-NC is built by coordinating Co atoms with four nitrogen atoms to form a square planar Co-N4 structure on nitrogen-doped graphene, as determined from the fitting results of EXAFS data. The cluster model with ten Co atoms placed on nitrogen- Supplementary Fig. 16 The Arrhenius plots for the aniline coupling over different catalysts. a h-CoNC, b s-CoNC, c h-CoNPNC. Considered the same atomically dispersed catalytic sites of h-CoNC and s-CoNC, the lower Ea means the reaction suffers more internal diffusion barriers. Compared with the same structure of h-CoNC and h-CoNPNC, lower Ea of h-CoNC means atomically dispersed catalytic site has a lower reaction barrier than particles site.
Supplementary Fig. 17 The conversion of different catalysts of coupling of aniline at different stirring rates. a h-CoNC, b s-CoNC, c h-CoNPNC. The conversions increase with the increasing stirring rates because external diffusion was gradually excluded. When the stirring rates were larger than 400 rpm, the effect of external diffusion can be regarded as completely excluded.
Supplementary Fig. 18 GC spectra of the products of oxidative coupling of 4chloroaniline using a h-CoNC and b s-CoNC as catalysts. Only 1,2-bis(4chlorophenyl)diazene (with the retention time of 10.4 min) was observed as the product (The peak at the retention time of 4.7 min was the reactant 4-chloroaniline). c Mass spectrum of the product 1,2-bis(4-chlorophenyl)diazene.

Supplementary Tables
Supplementary S0 2 is the amplitude reduction factor; CN is the coordination number; R is the interatomic distance (the bond length between central atoms and surrounding coordination atoms); σ 2 is Debye-Waller factor (a measure of thermal and static disorder in absorber-scatter distances); ∆E0 is edge-energy shift (the difference between the zero kinetic energy value of the sample and that of the theoretical model). R factor is used to value the goodness of the fitting.