Grantee Research Project Results
Final Report: Catalytic Reduction of Nitric Oxide
EPA Grant Number: R823529Title: Catalytic Reduction of Nitric Oxide
Investigators: Chuang, Steven S.C.
Institution: University of Akron
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1995 through September 1, 1998
Project Amount: $280,211
RFA: Exploratory Research - Engineering (1995) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Safer Chemicals , Land and Waste Management
Objective:
Nitric oxide (NO) poses a serious environmental threat because it is a precursor for the formation of smog and acid rain on the ground and tropospheric levels. The chief sources of the NO on the ground level are automobile exhausts and flue gas from coal-fired boiler plants. Growing concerns for the environment have promoted the legislature to install stringent regulations of pollutant emissions. The reduction of NO to N2 to meet environment regulations has been a major challenge for coal-fired power plants, process industries, and motor vehicles. A catalytic approach is the most direct method for reduction of NO to N2. However, the current catalysts under development and those reported in the patent literature did not exhibit sufficient activity and deactivation-resistance for control of NO emissions from lean-burn engines and coal-fired power plants.
Most studies on NO reduction focused on catalyst screening; they provided little understanding of the NO reduction mechanism. Lack of mechanistic understanding has greatly hindered the development of effective catalysts. The first objective of this project was to investigate the NO decomposition and reduction mechanisms by determining the reactivity of various forms of adsorbed NO and CO on a series of catalysts under reaction conditions. The catalysts selected for this study include supported Rh, Cu-ZSM-5, and Cu-Al2O3 that show activities for NO reduction from net reducing to net oxidizing conditions. An In situ infrared (IR) method coupled with various transient approaches was employed to study the structure of various adsorbates and their reactivities during NO reduction and decomposition. Information obtained from this study was used to elucidate NO decomposition and reduction mechanisms.
The major problems in the current catalytic converters and further development of Cu-based catalysts are hydrothermal stability and the sintering of metal during use. Inhibition of the sintering process and improvement of its hydrothermal stability will prolong the life of the catalytic converters and may put the Cu-based catalysts into practice for NO removal. One possible approach to inhibit the sintering of metal crystallites on the support surface is the enclosure of metal particles with organometallic precursors such as silane. Silane species can selectively react with the OH group on the support surface and further react with air to form silicon oxide, which may serve as a wall to enclose the metal crystallites. The second objective of this project was to investigate the feasibility of using silane to enclose Cu particles, thus inhibiting the sintering of metal particles on supported catalysts and also improving the hydrothermal stability of Cu-ZSM-5 catalysts during NO decomposition and reduction. Fundamental understanding of the NO decomposition and reduction mechanism as well as development of silane modification may provide new ways for the scientific design and preparation of highly active and selective catalysts and for the effective and economic control of NO emissions.
Summary/Accomplishments (Outputs/Outcomes):
NO is a major component in air pollution that is detrimental to our respiratory system. The most effective approach for control of NO emissions from automobiles and stationary coal-fired power plants is the use of highly active and poison-resistant catalysts that provide the low activation energy pathways to facilitate the conversion of NO to N2. This study addressed critical issues of environmental catalysis of NO emission control by investigating adsorbate reactivities and reaction pathways of NO decomposition and reduction. The mechanistic information obtained from this study can be used to develop effective approaches to guide catalyst development.
Mechanism of NO Decomposition. Decomposition of NO to N2 and O2 involves N-O bond dissociation as well as N-N, and O-O bond formation. The focus of this study was to determine the type of adsorbed species involved in these bond-breaking and bond-formation processes on the catalyst surface. In situ IR and mass spectroscopy (MS) coupled with temperature programmed reaction (TPR), isotopic temperature programmed desorption (TPD), and step transient and pulse transient techniques have been used to study the dynamic behavior of adsorbed species in the NO decomposition reaction on over- and under-exchanged Cu-ZSM-5. This study revealed Cu2+(NO), Cu+(NO), bridging Cu2+(NO3-), and NO+ as the major adsorbates and N2, N2O, O2, and NO2 as the products. N2 formation accompanied by the presence of Cu+(NO) suggests that Cu+ initiates the NO decomposition process. However, no direct correlation between Cu+(NO)2/Cu+(NO) intensity and N2 formation was observed. Adsorbed oxygen from dissociated NO changes the oxidation state of Cu+ ions, causing the formation of Cu2+(NO3-). While Cu2+(NO3-) decomposes to N2, N2O, NO2, and O2 during TPD, it is only partially responsible for the formation of O2 during NO decomposition. Isotopic study shows that adsorbed oxygen on Cu-ZSM-5 desorbs during the pulse NO reaction. These results demonstrate the presence of two pathways for O2 formation: oxygen produced from the decomposition of Cu2+(NO3-) and oxygen from the desorption of adsorbed oxygen on Cu-ZSM-5. Detailed analysis of adsorbate reactivities has led to the proposed Reaction Scheme I (see figure).
The analysis of the Reaction Scheme I and the reactivity of adsorbates revealed that desorption of oxygen (i.e., steps 3a and 5) is the rate-limiting step. Comparative studies of NO and N2O decomposition on Cu-ZSM-5 show that NO decomposition produced adsorbed O2-; N2O decomposition produced adsorbed O-. O2- desorbed at a significantly lower rate than O-. Furthermore, O- desorption is not inhibited by the presence of O2. Because promotion of oxygen desorption led to enhancement of NO decomposition activity, efforts in catalyst development for NO decomposition should focus on identification of an effective promoter in accelerating oxygen desorption and/or search for an effective approach for converting O2- to O-.
I
Reaction Scheme I
Silane Modification. Silanation of Cu-ZSM-5 by SiH4 removed the OH group from the ZSM-5 surface, thus improving hydrothermal stability of ZSM-5 catalysts. Silanation also caused a slight decrease in Cu+(NO) intensity and in the rate of N2 formation, but severely inhibited Cu2+(NO3-) and O2 formation. Improvement of the hydrothermal stability of the catalyst by silanation did not offset the severe inhibition of O2 and Cu2+(NO3-) formation by silanation.
Catalytic Reduction of NO by C3H6 in the Presence of O2 Over CuO/Al2O3. Infrared study of the catalytic reduction of NO by C3H6 in presence of O2 on CuO/Al2O3 showed that the reaction produced adsorbed C3H5-NO2, C3H5-ONO, Cu+-NCO, Cu0-CN, and Cu+-CO species as well as N2, CO2, and H2O products. Transient infrared study of adsorbate dynamics revealed that the reaction proceeds via adsorbed C3H5-NO2 and Cu0-CN intermediates on Cu0/Cu+ surfaces. Detailed analysis of adsorbate reactivities has led to the proposed Reaction Scheme II.
Comparative studies of the reaction on CuO/Al2O3 and CuO showed that the oxidation of C3H6 to CO2 and H2O occurred mainly on the Al2O3 surface. Thus, improving the selectivity to N2 and inhibiting oxidation requires suppression of the oxidation site on the Al2O3 surface. This result pointed out an important direction in the development of selective catalysts for NO reduction.
Reaction Scheme II
Catalytic Reduction NO by CO on Rh/Al2O3 Under Net-Oxidizing Conditions. Infrared study of the dynamic behavior of adsorbates over a 0.2 weight percent Rh/Al2O3 catalyst under net-oxidizing conditions shows that Rh2+(CO), Rh-NO+, and Al-NCO are spectator adsorbates whose responses lag behind that of gaseous CO2 product. The observed transient responses of Rh+(CO)2, Rh-NO-, and gaseous CO2 support the proposed Reaction Scheme III. Rh-NO- dissociates to adsorbed nitrogen and adsorbed oxygen, which further reacts with Rh+(CO)2 to produce CO2. Addition of air to the NO-CO stream produced adsorbed oxygen, which reacted with Rh-NO+ producing the nitrato species and providing more Rh sites for the NO-CO reaction at 473 K. Adsorbed oxygen was found to inhibit the NO conversion and N2O formation by blocking reduced Rh0 sites for Rh-NO-, and oxidizing Rh0 and Rh+ to Rh2+ sites. Rh sites are readily reduced and returned to their initial state and the catalyst returned to its initial activity when air was withdrawn from the reactant stream. Variation of the NO/CO ratio in the reactant stream shows that high concentration of NO (i.e., oxidant) enhances the extent of oxidation of Rh0 to Rh+, resulting in low catalyst activity for NO reduction. Keeping the Rh surface in the Rh0 state by lowering the NO/CO ratio decreases Rh+(CO)2 intensity and shifts the light-off to a lower temperature. Presence of O2, H2 C3H8 in simulated gas competes with NO and CO for the Rh site, lowering NO reduction activity. Preservation of reduced Rh sites for Rh-NO- is required to maintain the catalyst activity for NO-CO reaction under net-oxidizing conditions.
Results of this study: (1) unravel the limitation of the current catalysts (i.e., Rh catalysts) and those (i.e., Cu-based catalysts) under development; (2) reveal the fundamental scientific principles governing NO decomposition and reduction; and (3) provide a basis for guiding development of effective catalysts for NO emission control. This study is expected to benefit the U.S. auto- and coal-fired power industries in the development of effective catalytic converters to meet the increasingly stringent regulations on NOx emissions.
Reaction Scheme III
I. Adsorption
Step 1 | Rh0 + CO (g) | → | Rh0?CO | (linear CO) |
Step 2 | 2Rh0 + CO (g) | → | (Rh0)2-CO | (bridged CO) |
Step 3 | Rh+ + 2CO (g) | → | Rh+(CO)2 | (gem-dicarbonyl) |
Step 4 | Rh0 + NO (g) | → | Rh0-NO- | (bent NO) |
Step 5 | Rh+ + CO (g) | → | Rh+-CO | (linear CO) |
Step 6 | Rh+ + NO (g) | → | Rh-NO+ | (cationic NO) |
Step 7 | Rh0-CO + NO (g) | → | Rh0-NO- + CO (g) | |
Step 8 | Rh+(CO)2 + NO (g) | → | Rh-NO+ + 2CO (g) |
II. Oxidation
Step 9 | Rh0-NO- + Rh0 | → | Rh0-N + Rh0-O |
Step 10 | Rh0-O + 2Rh0 | → | (Rh+)2O2- |
III. Reduction
Step 11 | 2Rh+(CO)2 + O2- | → | 2Rh0-CO + CO2(g) + CO (g) |
Step 12 | Rh0-CO + Rh0-O | → | 2Rh0 + CO2(g) |
Step 13 | (Rh+)2O2- + Rh0-CO | → | 2Rh0 + CO2(g) |
IV. Formation of N2, N2O, and ?NCO
Step 14 | Rh-NO + Rh0-N | → | 2Rh0 + N2O (g) |
Step 15 | Rh0-N + Rh0-N | → | 2Rh0 + N2(g) |
Step 16 | Rh0-N + CO | → | Rh0-NCO |
Step 17 | Rh0-NCO + Al | → | Al-NCO + Rh0 |
Rh0-NO- as a whole should be considered a neutral species.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 5 publications | 2 publications in selected types | All 2 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Konduru MV, Chuang SSC. Investigation of adsorbate reactivity during NO decomposition over different levels of copper ion-exchanged ZSM-5 using in situ IR technique. Journal of Physical Chemistry B 1999;103(28):5802-5813. |
R823529 (1998) R823529 (Final) |
Exit |
Supplemental Keywords:
nitric oxide, emission control, selective catalytic reduction, SCR, adsorbate reactivity, reaction intermediates, reaction pathway, NO decomposition,, Scientific Discipline, Air, Environmental Chemistry, Engineering, Chemistry, & Physics, Environmental Engineering, metal catalysts, nitrous oxide, catalytic oxidation, characterization of catalysts, catalyst formulations, catalysts, air pollution, silination studies, Nitric oxide, catalytic reductionProgress 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.