Pd nanocrystals encapsulated in MOF-derived Ni/N-doped hollow carbon nanosheets for efficient thermal CO oxidation: unveiling the effect of porosity

Abstract

Rational synthesis of Ni-metal-organic-framework (MOF)-derived hollow N-doped carbon (Ni-MOF-HNC) nanostructures has garnered great attention in various catalytic reactions due to their outstanding catalytic and physicochemical merits, but their activity toward thermal CO oxidation (COOxid) is not emphasized enough. Herein, we tailored the fabrication of Ni-MOF-HNC encapsulated Pd nanocrystals (Pd/Ni-MOF-HNC) for efficient COOxid at low temperature, driven by microwave-irradiation, annealing at 900 °C and chemical etching to form Ni-MOF-HNC that is used as a support for the growth of Pd nanocrystals under microwave-irradiation. The obtained Pd/Ni-MOF-HNC possesses hollow carbon sheets with a great surface area (153.05 m2 g−1), pore volume (0.12 cm3 g−1), rich Pd/Ni-Nx active sites, Ni-metal defects, rich N-content (7.53 at%), mixed Pd/Ni-oxide phases, and uniformly distributed ultra-small Pd nanocrystals (7.03 ± 1.10 nm); meanwhile, Pd/Ni-MOF-NC formed without etching had no porosity and less Ni-metal defects. The thermal COOxid activity of Pd/Ni-MOF-HNC was significantly superior to Pd/Ni-MOF-NC and commercial Pd/C catalysts. This is evidenced in the great ability of Pd/Ni-MOF-HNC to utterly oxidize CO at a lower complete conversion temperature (T100) of 114.5 °C compared with Pd/Ni-MOF-NC (153.8 °C) and Pd/C (201.5 °C) under atmospheric pressure. Conspicuously, the T100 of Pd/Ni-MOF-HNC was lower than those of most previously reported Pd-based catalysts due to the high porosity, surface area, and electronic interaction of Pd/Ni-Nx, and Ni-metal defects, which promote the adsorption/activation of reactants (CO + O2), decrease the activation energy to 73.1 kJ mol−1 and enhance the reaction rate at the same CO conversion percentage. Thus, this study may open the gates for the utilization of MOF-HNC as a support for Pd-based catalysts for thermal COOxid