Highly dispersed rhodium atoms supported on defect-rich Co(OH)2 for the chemoselective hydrogenation of nitroarenes

Abstract

Catalysts of 0.54% (Pt, Rh, Pd or Ru)/Co(OH)₂ and 1.23% Rh/Co(OH)₂ were successfully synthesized via simple hydrothermal and precipitation methods for the selective hydrogenation of nitroarenes. It was found that 0.54% Rh/Co(OH)₂ outperformed other catalysts, for example, 2-chloronitrobenzene hydrogenation (with a turnover frequency (TOF) of 575 h⁻¹) under 3.0 MPa H₂ at 100 °C for 1 h (much higher than that of 1.23% Rh/Co(OH)₂; TOF, 398 h⁻¹). Through characterization of the nanostructure (XRD, TEM, HRTEM, AC-STEM (spherical aberration-corrected scanning transmission electron microscopy), AC-STEM-EDX elemental mapping and line-scanning, H₂-TPD, etc.), it was confirmed that Rh was dispersed in the form of single atoms and ultrasmall nanoclusters on Co(OH)₂ for 0.54% Rh/Co(OH)₂. Moreover, 0.54% Rh/Co(OH)₂ also exhibited high selectivity for –NO₂ hydrogenation for other selected nitroarene substrates hydrogenation reactions. This was mainly attributed to the presence of Rh as single atoms or super tiny nanoclusters in 0.54% Rh/Co(OH)₂, leading to a significant increase in the number of active sites, beneficial adsorption of the –NO₂-group at defect-rich Co(OH)₂, and an interfacial synergy effect of Rh–Co(OH)₂, thus improving the catalytic performance.