Grantee Research Project Results
Final 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 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:
This project involved activity and stability studies of novel catalysts for the selective catalytic reduction (SCR) of NOx by methane. High-activity CH4-SCR catalysts will find application in both mobile (NGV) and stationary (gas turbines, diesel engines, coal power plants) combustion sources. The challenges for SCR are to successfully react CH4 with (NO + O2), rather than with oxygen only, to the preferred reduction product N2, with adequate resistance to water vapor and sulfur dioxide and over a wide temperature window. Development of CH4-SCR catalysts of good activity and stability would provide an attractive alternative to NH3-SCR, presently the only commercially practiced technology for NOx reduction in gas turbine exhausts.
Several metals (Co, Pd, Ni, Mn, Fe, In) in atomic dispersion, i.e., as ions stabilized in zeolites, have been reported in the literature to possess good activity for the reduction of NOx to nitrogen with CH4 in the presence of excess oxygen. The Ag-zeolite system had not been recognized as a catalyst for CH4-SCR of NOx until recently, when it was found in this laboratory that cerium-promoted Ag-NaZSM-5 was active for this reaction over the temperature range of 400-650?C. The coexistence of Ce and Ag in the zeolite was crucial for high conversion of NO to N2 and changed the nature of the catalyst from an oxidation one (i.e., Ag-NaZSM-5) to a good CH4-SCR catalyst, Ce-Ag-ZSM-5.
The following four project objectives were successfully met:
1. To use rare earth ions as promoters of metal ion exchanged zeolite catalysts for the SCR reaction of NO by methane. The promotion effect was studied on a series of Ag- and Pd-ZSM-5 catalysts of different SCR activity and selectivity.
2. To characterize the effect of the promoter on the catalyst activity and selectivity for the SCR reaction. A variety of analytical techniques, including TPD/MS, TPR, UV-VIS diffuse reflectance spectroscopy, HREM, and XPS were used to meet this objective.
3. To carry out kinetic measurements and activity tests under demanding conditions, including the effect of practical impurities, to evaluate the potential of selected catalysts for industrial applications.
4. To transfer knowledge gained from the Ce-Ag-zeolite catalyst system to non-zeolitic matrices of higher hydrothermal stability. This was accomplished by the development of new Ag-alumina catalysts with high activity and stability for the SCR of NOx by methane.
Summary/Accomplishments (Outputs/Outcomes):
Testing of the metal-zeolite and the Ag-alumina catalysts was conducted in flow reactors over a wide parametric range. All tests were run with very lean gas mixtures containing 0.25 percent NO, 0.25-2 percent CH4 and 2.5-10 percent O2. The effect of H2O and SO2 gas impurities on the catalyst activity and stability was evaluated with selected catalyst compositions. Aqueous ion exchange procedures were used to prepare the metal-loaded zeolites. Ag-ZSM-5 and Ce-Ag-ZSM-5 were prepared by exchanging Na-ZSM-5 zeolite (Davison, Si/Al=13.8 or 21.5), while Pd-HZSM-5 and Pd-Ce-HZSM-5 were prepared by aqueous ion exchange of NH4-ZSM-5 (PQ, Si/Al=25) in dilute solutions of the metal nitrates. Cerium was exchanged first (80?C, 2h), followed by washing, drying (110?C, 10-12 h) and heating in air (500?C, 2 h). Exchange of silver and palladium took place at room temperature for 24 hours (for Ag - in the dark). After exchange of the metal ions, the sample was filtered, washed, dried and calcined. The Ag-Al2O3 catalysts were prepared by a co-gelation procedure using an aqueous solution of tetra-methylammonium hydroxide (25 percent) and metal nitrates. The resultant gelatinous precipitate was aged for 8-10 h at room temperature, filtered and washed several times with deionized water, dried under vacuum at 100?C for 8 h, followed by calcination at 650 and 800?C (in two parts) for 10 h (heating rate 2oC/min).
A main goal of the project was to elucidate how Ag-ZSM-5, a low-activity catalyst, was considerably promoted by the presence of small amounts (<1.5 wt percent) of cerium in the zeolite. Ce-Ag-ZSM-5 has comparable activity and stability, respectively, to Pd-ZSM-5 and Co-ZSM-5, the top two contenders for CH4-SCR of NO. A series of parametric studies was conducted to meet this goal. The major finding from this part of the project was that the presence of cerium in Ag-ZSM-5 stabilized silver in the form of isolated silver ions and oxidized silver clusters in the zeolite matrix. These are the active sites for CH4-SCR of NOx. On the contrary, metallic silver nanoparticles are active for methane combustion under these conditions. Cerium was present in the zeolite both as Ce3+ and as CeO2 clusters (1-3 nm). Limited silver migration and particle formation, accompanied by a corresponding loss of SCR activity, took place in the presence of 5 percent H2O and 50 ppm SO2 in the feed gas at 600?C. Reaction kinetics were measured on a high Ag-content (8.7 wt percent) Ce-Ag-ZSM-5 catalyst. A strong dependence of the reaction rate on PNO (0.8 order) and a weak dependence on PCH4 (0.3 order) was found over the temperature range 450-600oC.
In following work, the potential of using cerium as a promoter of other metal-loaded zeolites was examined. Addition of cerium in Pd-ZSM-5 promoted the high temperature activity of the latter and stabilized it against deactivation, which is a well-known drawback of Pd-ZSM-5 catalysts. Less than 2 wt percent cerium was adequate as a promoter. The presence of cerium prevented the migration of palladium and the agglomeration of PdO particles, which are inactive for CH4-SCR, catalyzing instead the competing reaction of methane with oxygen. Similar to the case of Ce-Ag-ZSM-5, the role of cerium is to stabilize the isolated Pd2+ ions, which are the active sites for the SCR reaction. Thus, this project has validated the use of cerium as a structural promoter of metal loaded-zeolite catalysts.
Another major goal of the project was to explore the possibility of using open supports of inherently better hydrothermal stability than zeolites. Thus, we attempted to prepare highly dispersed Ag-alumina catalysts as alternatives to the Ag-zeolites developed in the project. Using a single-step cogelation technique, Ag-alumina materials were prepared containing atomically dispersed silver and silver clusters. These silver structures were very stable under reaction conditions up to 700?C and in cyclic reduction/oxidation experiments up to 600?C monitored by UV-VIS diffuse reflectance spectroscopy. The alumina surface was totally modified by the presence of silver as shown by TPD of NO. This type catalyst showed comparable activity to the Ce-promoted Ag-ZSM-5 catalyst, but better stability than the latter. Parametric studies were conducted with a 3.6 wt percent Ag-alumina. Good and stable activity over a period of 80 h was found under demanding SCR conditions in a gas mixture containing 0.25 percent NO, 2 percent CH4, 10 percent O2, 10 percent H2O, 30 ppm SO2, bal. He at 600?C and at a space velocity of 38,000 h-1 (NTP). A number of fundamental studies of the interaction of NO, H2O and SO2 with the Ag-alumina surfaces was undertaken in the project to elucidate the unique properties of these materials. Further development of the Ag-alumina catalysts is strongly recommended for application to a variety of lean-burn engine exhaust gas streams.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
Other project views: | All 19 publications | 4 publications in selected types | All 4 journal articles |
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Keshavaraja A, She X, Flytzani-Stephanopoulos M. Selective catalytic reduction of NO with methane over Ag-alumina catalysts. Applied Catalysis B: Environmental 2000;27(1):L1-L9. |
R825430 (Final) |
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Li ZJ, Flytzani-Stephanopoulos M. On the promotion of Ag-ZSM-5 by cerium for the SCR of NO by methane. Journal of Catalysis 1999;182(2):313-327. |
R825430 (1998) R825430 (Final) |
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Li Z, Flytzani-Stephanopoulos M. Selective catalytic reduction of nitric oxide by methane over cerium and silver ion-exchanged ZSM-5 zeolites. Applied Catalysis A: General 1997;165(1-2):15-34. |
R825430 (Final) |
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Li Z, Flytzani-Stephanopoulos M. Effects of water vapor and sulfur dioxide on the performance of Ce-Ag-ZSM-5 for the SCR of NO with CH4. Applied Catalysis B: Environmental 1999;22(1):35-47. |
R825430 (Final) |
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Supplemental Keywords:
methane combustion, lean-NOx catalysts, NOx removal, lean-burn engines, silver catalysts, palladium catalysts, cerium oxide., RFA, Scientific Discipline, Air, Toxics, Waste, INDUSTRY, Ecology, Environmental Chemistry, HAPS, mobile sources, Industrial Processes, tropospheric ozone, Incineration/Combustion, 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.