Selective Catalytic Reduction Of Nitric Oxide Over Cerium-doped Activated Carbons

Abstract

Selective catalytic reduction (SCR) with ammonia for diesel engine NOx reduction using activated carbon (AC) was studied. Comparisons of unmodified and cerium-doped Granular Activated Carbon (GAC), Activated Carbon Fiber (ACF), and Multiwall Carbon Nanotubes (MWCNTs) were conducted. Physical and chemical properties and durability of the catalysts were examined using Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS), carbon hydrogen and nitrogen (CHN) analysis, X-ray Diffraction (XRD) analysis, Raman spectroscopy, X-ray Photoelectron spectroscopy (XPS), Thermo gravimetric analysis (TGA), Braunauer-Emmett-Teller (BET) surface area analysis and density analysis. Experiments were carried in a fixed bed column at various temperatures from 100º to 400º C for low concentration NOx (150 ppm) and high concentration NOx (500ppm) at a total flow rate of 200ml/min, NOx/NH3 ratio of 1:1 and oxygen concentration of 5.6%. The stability of the cerium-doped GAC (CeGAC) was studied by conducting a 12-hour steady-state run. It was found that CeGAC has a high reduction efficiency of about 80% at 300ºC for low concentration NOx. CeMWCNTs have a high reduction of 85% at 300ºC for high concentration NOx. However, the NO oxidation and ammonia slip emission in the exhaust is higher for CeMWCNTs than for other types of catalyst. CeGAC with high space velocity of 288,000 h-1 shows a stable reduction percentage for various temperatures. The 12 hour stability test for CeGAC shows steady-state reduction percentage throughout the test. Reducing the NOx/NH3 ratio to 1:0.9 maintains the reduction percentage and lowers NO oxidation and ammonia slip significantly. This study indicates that CeGAC could be applicable in onboard engines with computerized ammonia injection control systems.