Promotion effects of alkali- and alkaline earth metals on catalytic activity of mesoporous Co₃O₄ for 4-nitrophenol reduction

Mesoporous cobalt oxides doped with alkali (Li, Na, K, Cs) and alkaline earth (Mg, Ca) metals were synthesized and evaluated for their catalytic activity in the reduction of 4-nitrophenol. The prepared materials were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Brunauer–Emmet–Teller (BET) and hydrogen temperature programmed reduction (H₂-TPR) analyses. The characterization techniques used showed the materials to consist of mono-dispersed nanoparticle aggregates with connected, well defined intra-particle voids and the crystalline phase of the cobalt oxide to be cubic Co₃O₄, while the pore diameters ranged from 12.1 to 19.2 nm depending on the metal ion dopant. The reduction of 4-nitrophenol was chosen as a well-controlled model reaction allowing us to determine the catalytic activity as a function of alkali or alkaline earth dopant. Calcium doped cobalt oxide was found to be the most catalytically active with an apparent rate constant of 3.76 × 10⁻³ s⁻¹ and the order with respect to dopant was Ca > Cs > Mg > Li > K > Na. From characterization of the catalysts by SEM, TEM and H₂-TPR promotion effects of the dopants were found to be due to electronic changes in the catalysts as a result of doping rather than structural changes. The kinetics of the most active calcium doped catalyst was modeled in terms of Langmuir–Hinshelwood kinetics. The Langmuir–Hinshelwood surface rate constant for Ca-doped Co₃O₄ was 5.47 × 10⁻⁵ mol m⁻² s⁻¹ compared to the undoped Co₃O₄ at 5.33 × 10⁻⁶ mol m⁻² s⁻¹. Activation energies were calculated to be 51.3 kJ mol⁻¹ and 50.7 kJ mol⁻¹ for undoped and Ca-doped Co₃O₄.