Novel MnaZrbCrcOx catalysts for low temperature NH3-SCR derived from high H2O content flue gas via natural gas combustion

Abstract

A series of MnaZrbCrcOx catalysts with different Cr/Mn molar ratios were synthesized via co-precipitation for the selective catalytic reduction of NOx with NH3 (NH3-SCR). It was found that a Mn5Zr2Cr2.5Ox catalyst with a Mn:Zr:Cr ratio of 5:2:2.5 exhibited excellent low-temperature activity and the best levels of H2O resistance at a high weight hourly space velocity (WHSV) of 54,000 ml/(h·g). NO conversion levels were measured as greater than 87% within a temperature range of 150–250 °C, and catalytic performance levels reached 90% even 50 h after reactions occurred in the presence of 15% H2O at 230 °C. The characterization results show that strong interactions occurring between Mn, Zr and Cr oxides and higher quantities of Mn4+ and Lewis acid sites improved the redox properties and facilitated H2O resistance. In situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) show that coordinated NH3 on Lewis acid sites and bidentate nitrates play important roles in SCR reactions and that such reactions follow both Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) mechanisms. The competitive adsorption of H2O and NO may be the main cause of the adverse effects of H2O on catalytic performance.