Materials for Exhaust and Energy Recovery

It is anticipated that diesel engine exhaust aftertreatment will be needed in the 2009-2012 time frame, and possibly for a much longer time in order to meet EPA exhaust emission regulations.  Our goals are to develop NOx catalyst materials and particulate filter technology in concert with diesel engine companies and their suppliers, to facilitate the highest possible fuel efficiency in the resulting commercial vehicle propulsion systems.

Catalyst materials with stable microstructures are needed that can operate at high efficiency over a wide range of exhaust conditions, including low temperatures and varying levels of oxygen and unburned fuel. R&D tasks are expected to include theoretical modeling of catalyst behavior, synthesis and processing studies, bench test and engine exposures, and postmortem analysis of the chemistry and microstructure of the catalyst systems.

Characterization of the effects of exposure in service on the microstructure (including cluster size and distribution) and microchemistry of the aftertreatment systems will be performed. The characterization may lead to the development of more-durable systems and may point to material development paths that result in an optimized temperature window for aftertreatment operation.

Ongoing activities to develop the characterization and life-prediction modeling tools needed to realistically assess the expected performance and life of the particulate filters will be continued. 
Additional engine controls may be required that will depend on the development of new sensors. Our sensor effort this year will focus on development of sensors to detect ammonia which may be an unintended pollutant from urea-injection selective catalytic reduction for NOx reduction.

Milestones:   With input from theory and experimental studies, select and synthesize supported precious metal clusters (Pt, Rh, Pd, Re) to search for optimum catalyst systems for CO, NO, and hydrocarbon  oxidation and NOx reduction. (9/09).  Complete evaluation of feasibility of quantitative analysis of the materials changes underlying the SCR catalyst performance degradation with age.  (9/09).  Determine the change in thermal shock resistance of field tested diesel particulate filters (DPFs) and the thermal shock resistance of one alternate substrate DPF material (09/09).

Related Agrreements:

• Catalyst Characterization
• Durability of Diesel Engine Particulate Filters
• Catalysts via First Principles

Catalyst Characterization
CRADA with Cummins

The objective of this effort is to produce a quantitative understanding of the interdependence between structure and performance to develop options for an exhaust aftertreatment system with improved final product performance in order to meet the US Environmental Protection Agency emissions requirements for 2012 and beyond.

In the FY09, an effort to characterize structural changes upon aging in a commercial zeolite-based urea selective catalytic reduction (SCR) catalyst will be finalized at ORNL using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman Spectroscopy and infrared spectroscopy along with model catalysts representing future SCR commercial systems.  The effort will focus on detailed characterization of the selected catalyst formulations, both powder and honeycomb, aged under the well-controlled conditions.  The work will be extended from initial, exploratory stage to a quantitative correlation between the material changes as identified by various characterization techniques and a measure of performance degradation of urea SCR catalysts. 

Milestone:  Complete evaluation of feasibility of quantitative analysis of the materials changes underlying the SCR catalyst performance degradation with age.  (9/09)

Contact:  Thomas Watkins, Oak Ridge National Laboratory, 865-574-2046, watkinstr@ornl.gov

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Durability of Diesel Engine Particulate Filters
CRADA with Cummins, Inc.

The objectives of this agreement are to identify and implement test techniques to characterize the physical and mechanical properties of ceramic substrates used as diesel particulate matter filters (DPFs), to identify the mechanisms responsible for the degradation and failure of DPFs and to develop analysis tools for predicting their reliability and durability.

The mechanical reliability of engineering systems is determined by the intersection of the operating stress distribution with the mechanical strength distribution of the component material.  DPFs fabricated from porous cordierite have a complex microstructure with microcracks that can heal themselves during high temperature operation and reopen during subsequent cooling.  This microstructural phenomenon makes the mechanical properties of the DPF a dynamic property which can change as a result of the operating service history.  Therefore, it is of interest to characterize the mechanical and thermal shock characteristics of filters that have been returned from field service in order to refine the lifetime prediction procedure for DPFs.  In FY09, the change in thermal shock resistance of DPFs returned from the field or field tested DPFs will be determined. DPF’s fabricated from other materials such as mullite and aluminum titanate have different structural properties than cordierite.  These property differences affect the stress fields generated by the operation of the component.  Also in FY09, the thermal shock resistance of one alternate substrate material will be determined. As needed, elastic modulus, strength, fracture toughness, slow crack growth, density/porosity/microstructure, thermal expansion, fractography, etc. will be characterized to understand the overall mechanical behavior of the field tested DPFs and alternate substrate material DPFs.  In collaboration with Cummins researchers, the results obtained from these tests will be used as input data for the implementation of models to predict the service life of DPFs.  The ultimate goal of this research agreement is to develop a life prediction methodology for porous DPFs because the implementation of such methodology would help minimize the risk of cracking and failure of DPFs in the severe thermal environment in which they will operate.

4X2 cellular specimen cross-sections for evaluation by dynamic mechanical
analysis (DMA). The difference in the shape of the cells for the coated
and uncoated specimens is to be noted.

Right:  The temperature dependence of the elastic modulus of porous cordierite.  The test specimens were subjected to a maximum temperature of 1000oC.  Error-bars represent root mean square errors of fit for one test under each condition.

Left:  The effect of coating on the elastic modulus of cordierite for catalysts.  Coating leads to a small decrease (~10-15%) in the elastic modulus values.  One specimen each was tested for the two short specimens, two specimens for the coated long specimens and three specimens for the uncoated long specimens.  Three repetitions were performed for each specimen and the blue bars represent the average of the values while the error-bars represent the standard deviation of the values.

Milestone:  Determine the change in thermal shock resistance of field tested DPFs and the thermal shock resistance of one alternate substrate DPF material (09/09).

Contact: Thomas Watkins, Oak Ridge National Laboratory, 865-574-2046, watkinstr@ornl.gov

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Catalysts via First Principles

This objective of this agreement is to develop an integrated approach between computational modeling and experimental development, design and testing of new catalyst materials that we believe will rapidly identify the key physiochemical parameters necessary for improving the catalytic efficiency of these materials. The incentive for this work comes from the fact that the development of new catalytic materials is still dominated by trial and error methods, even though the experimental and theoretical bases for their characterization have improved dramatically in recent years.

Our studies clearly show that supported platinum nano-particles are a combination of single atoms, 2-3 atom clusters, and 10-20 atom clusters. While the calculated Pt-Pt bond distances match with observed ones, the three-dimensionality of structure suggests metallic character of the clusters. Our theoretical study also suggests inhibited CO oxidation on metallic clusters, but experimentally observed activity on Pt nanoclusters supported on γ-Al₂O₃ show no inhibition of activity suggesting Pt to be oxidized. We also see no such CO-oxidation inhibition on Pt nanoclusters supported on θ-Al₂O₃. Theoretical modeling studies are in progress to understand this lack of impact of substrate morphology. Another interesting observation is rapid sintering of supported Pt nanoclusters. Even exposure to CO oxidation initiation conditions results in some sintering of Pt clusters. Large particles (12 nm) are more stable and exhibit no sintering after 3 cycles of CO-oxidation.

These results suggest that the experimental and theoretical studies can evaluate a simple catalyst system and reach identical conclusions for the nanostructure and CO-oxidation reactions over supported Pt clusters and provide support to “catalyst by design” concept. We are determining if this conclusion is valid for NO and HC oxidation reactions also. We plan to extend theoretical studies to supported precious metal clusters on alumina and initiate study of intermediates formed during NO and hydrocarbon oxidation reactions. Simultaneously, we will initiate catalytic testing of NO and hydrocarbon oxidation reactions to identify reactive catalyst sites. This will enable us to initiate an iterative process by comparing theoretical and experimental results.

 
DF-STEM images of 2%Pt/g-Al2O3 fresh (left) and after 2 cycles
of NO oxidation to 650°C (White spots represent Pt)

Milestone:  With input from theory and experimental studies, select and synthesize supported precious metal clusters (Pt, Rh, Pd, Re) to search for optimum catalyst systems for CO, NO, and hydrocarbon oxidation and NO_x reduction. (9/09).

Contact:  Chaitanya K. Narula, Oak Ridge National Laboratory, 865-574-8445, narulack@ornl.gov

Complete Project List | Materials for Exhaust and Energy Resources