Grantee Research Project Results
1997 Progress Report: Novel Catalysts for Lean-NOx Reduction by Methane
EPA Grant Number: R825430Title: Novel Catalysts for Lean-NOx Reduction by Methane
Investigators: Flytzani-Stephanopoulos, Maria
Institution: Tufts University
EPA Project Officer: Hahn, Intaek
Project Period: December 5, 1996 through December 4, 1999
Project Period Covered by this Report: December 5, 1996 through December 4, 1997
Project Amount: $479,533
RFA: Exploratory Research - Air Engineering (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Land and Waste Management , Air , Safer Chemicals
Objective:
The objectives of this project are to prepare, characterize and evaluate novel zeolite catalysts for the selective catalytic reduction (SCR) of NOx with methane in excess oxygen-containing exhaust gas streams. The new catalysts comprise bi-functional metal ion-exchanged zeolites with one metal in higher loading (primary metal) and a secondary metal ion in small amounts (promoter). A main objective of the project is to study the promotion mechanism and the SCR reaction kinetics.Progress Summary:
During this year, systematic screening tests, including sample preparation and activity evaluation, were conducted in this laboratory to identify the primary and the secondary metal ions using ZSM-5 zeolite as the support. A new bi-cation catalyst system, namely cerium and silver ion-exchanged zeolites, was identified to be very active and selective for the reduction of NO by methane in the presence of excess oxygen at dry and SO2-free conditions as well as in the presence of water vapor and SO2. Composition optimization and parametric studies were carried out over selected catalyst samples. Several bulk and surface analysis techniques, such as XRD, XPS, STEM/EDS and UV-VIS, were employed to characterize fresh and used catalyst samples and to investigate the chemical and textural functions of cerium and silver.
Accomplishments and Research Results
A significant promotion effect was found over cerium-modified silver ion-exchanged or impregnated zeolite catalysts for the reduction of NO by methane in the presence of excess oxygen at both dry and SO2-free conditions as well as in the presence of water vapor and SO2. In this system, silver is the primary metal catalyst, while cerium is the promoter. Neither Ag-ZSM-5 nor Ce-ZSM-5 alone is a good NOx SCR catalyst. The presence of oxygen in the feed gas stream enhances the catalyst activity and excess oxygen (~8 mol%) has no adverse effect on catalyst selectivity. Composition optimization studies showed that a small amount of cerium (~1 wt%) can provide the maximum promotion effect. Both the conversion and reaction rate of NO to N2 increased with silver loading up to 5.5-6.5 wt%. Higher silver loading only facilitates methane oxidation by oxygen, thus, decreasing the catalyst selectivity. The activity tests, parametric studies and characterization of fresh and used catalyst samples have demonstrated the following chemical and textural functions of cerium and silver in the system: i) cerium sites catalyze the oxidation of NO to NO2, which is the preferred oxidant of CH4. Activated methane over the silver sites reacts with NO2 to form N2 (and CO2 and H2O); ii) cerium suppresses the direct methane oxidation by oxygen both in the presence and in the absence of NO which was found to occur on silver sites; iii) silver clusters in Ag-ZSM-5 enhance the direct CH4 oxidation. The structural function of Ce in the Ce-Ag-ZSM-5 samples is to keep Ag+ ions dispersed by inhibiting the migration of Ag+ ions and preventing the formation of large Ag clusters. Highly dispersed Ag+ ions are the preferred sites for the SCR reaction.
Future Activities:
Future research will focus on measurement of the SCR reaction kinetics as well as the kinetics of the independent methane oxidation at both dry and wet conditions. Further structural characterization will be performed to correlate the reaction rate with the state of silver in the zeolite.Journal Articles:
No journal articles submitted with this report: View all 19 publications for this projectSupplemental Keywords:
Selective catalytic reduction, nitrogen oxides, lean-NOx reduction, methane, silver, cerium, ZSM-5, zeolites, RFA, Scientific Discipline, Air, Toxics, Waste, INDUSTRY, Ecology, Environmental Chemistry, HAPS, mobile sources, tropospheric ozone, Industrial Processes, Incineration/Combustion, Engineering, Engineering, Chemistry, & Physics, Nitrogen Oxides, Nox, nitrous oxide, urban air, hydrocarbon, industrial waste, mass spectrometry, emission control technologies, emissions measurement, air pollution control, combustion emissions, Ammonia, electric utilities, emission controls, catalyst formulations, smog, methane, combustion technology, industrial boilers, methane , combustion, catalytic combustion, incineration, urban air , atmospheric deposition, acid rainProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.