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 (CO_Oxid) is not emphasized enough. Herein, we tailored the fabrication of Ni-MOF-HNC encapsulated Pd nanocrystals (Pd/Ni-MOF-HNC) for efficient CO_Oxid 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–N_x 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 CO_Oxid 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–N_x, 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 CO_Oxid.