Understanding the co-effects of manganese and cobalt on the enhanced SCR performance for MnxCo1−xCr2O4 spinel-type catalysts

Abstract

A novel ternary Mn_0.1Co_0.9Cr₂O₄ catalyst was identified from a range of chromium-based spinel-type (XCr₂O₄) oxides for the SCR of NO_x in coking flue gas. It exhibited a broad operating window (166–393 °C) under a gas hourly space velocity of 112 000 h⁻¹ with excellent H₂O and SO₂ durability. Compared to MnCr₂O₄ and CoCr₂O₄, the coexistence of Mn and Co cations in Mn_0.1Co_0.9Cr₂O₄ resulted in the enrichment of favorable Co³⁺, Mn⁴⁺, and Cr⁵⁺ species and oxygen vacancies, and enhanced Lewis acidity and redox ability. Mn and Co cations stably exist on the A²⁺ sites of Mn_xCo_1−xCr₂O₄ when x is less than 0.2, and the spinel-type structure preservation was conducive to their increased SCR activity. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) revealed that both CoCr₂O₄ and Mn_0.1Co_0.9Cr₂O₄ followed the Langmuir–Hinshelwood mechanism, while higher amounts of adsorbed ammonia and nitrate species were produced on Mn_0.1Co_0.9Cr₂O₄, and the reaction rates were much higher. In addition, DFT demonstrated that the oxygen vacancy formation energy was lowered by Mn doping, and the enhanced affinity of NH₃ and NO with Mn_0.1Co_0.9Cr₂O₄ helps to form conducive NH₂ and nitrate species which facilitate the SCR reaction.