Cobalt impregnated porous catalyst promoting ammonium sulfate recovery in an ammonia-based desulfurization process

Oxidation of ammonium sulfite plays an important role in the ammonia-based wet flue gas desulfurization. In this work, a porous molecular sieve (4A type) supported cobalt catalyst, named as Co-MS-4A, was developed to expedite the oxidation of ammonium sulfite. The experimental results revealed that the oxidation rate was increased by 1.7 to 6.0 times in comparison with the uncatalyzed scenario. The Co-MS-4A catalyst with a nominal Co loading of 1.2 wt% exhibited the highest oxidation rate of (NH₄)₂SO₃ among the tested catalysts. It was characterized by Brunauer–Emmett–Teller (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and Energy Dispersive Spectrometer (EDS). The BET data showed that the support retained its mesoporous structure after the cobalt impregnation. Due to the low cobalt loading in the catalyst, no obvious Co diffraction peaks was observed by the XRD. The presence of the cobalt species including Co²⁺ and Co³⁺ in the catalyst was confirmed as depicted by the XPS spectra. The TEM-EDS spectra indicated that cobalt particles distributed uniformly in the tunnels of MS-4A support. Furthermore, the effects of operation parameters, such as ammonium sulfite concentration, catalyst concentration, pH, oxygen partial pressure, and temperature, on the oxidation rate of ammonium sulfite were investigated in a bubbling tank. The apparent activation energy of the ammonium sulfite oxidation catalyzed by Co-MS-4A was 17.67 kJ·mol⁻¹. The kinetic analysis implied that the catalyzed oxidation of ammonium sulfite is rapid with a Hatta number higher than ∼4.15, and that the overall reaction was controlled by the mass transfer of oxygen. The work provides a new approach for recycle of byproduct in ammonia desulfurization without transition metal consumption and pollution.