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

Abstract

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 19.2nm 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 3.76×10−3s−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 5.47×10−5molm−2s−1 compared to the undoped Co3O4 at 5.33×10−6molm−2s−1. Activation energies were calculated to be 51.3kJmol−1 and 50.7kJmol−1 for undoped and Ca-doped Co3O4.