Methane has the advantage of releasing less carbon dioxide when it’s burned than do many other hydrocarbon fuels. However, because of the very stable structure of the methane molecule, it can be difficult to access the energy stored within. When unburned methane escapes into the atmosphere, it’s a greenhouse gas 20 times more powerful than carbon dioxide.
Presently available emissions-control catalysts are ineffective at reducing concentrations of methane in exhaust streams, the authors note. Additionally, combustion of methane at high-temperature results in emissions of NOx and CO. However, the combustion of methane promoted by heterogeneous catalysts could utilize the available energy of methane at lower temperatures, thereby increasing system performance and limiting emissions by drastically reducing the required temperatures.
To be suitable for this application, heterogeneous catalysts for methane oxidation must be very active at low reaction temperatures (below at least 400°C) and lso be catalytically and mechanically stable at high reaction and flame temperatures. While palladium (Pd)-based catalysts supported on alumina or zirconia are one of the best catalysts for catalytic CH4 oxidation (3), they tend to deactivate through loss of active surface by sintering and by transformation into metallic Pd at temperatures above 600 °C.
Ceria (CeO2) can improve the catalytic activity of supported Pd by stabilizing PdOx, but pure CeO2 has limited thermal stability.
Materials that could simultaneously enhance the performance of Pd-based catalysts at low temperatures and limit deactivation at elevated temperatures would greatly improve catalytic CH4 combustion processes. The tailored positioning of the building blocks at the nanometer scale can markedly improve the performance of the materials through electronic and steric interactions.
...Core-shell structures are special cases in which metal-support interactions are enhanced by maximizing [he inter- face area between the metal particles and the oxide support.]
...Here, we report on a hierarchical design of core-shell type catalysts inspired by the concepts of supramolecular chemistry in which the building blocks are preorganized in a way to exploit their catalytic interactions to the maximum extent. Supramolecular chemistry concepts have not been widely applied in heterogeneous catalysis because of the difficulty in manipulating the metal-support interaction at the nanoscale.
—Cargnello et al.
The team prepared two active building blocks, Pd and CeO2, were prepared separately, then placed them in solution for self-assembly to form supramolecular core-shell structures held together by metal ion–ligand coordination interactions. The result is PdO encapsulated in a cerium oxide matrix, resulting in enhanced performance relative to the traditional supported PdO catalysts.
Because small particles such as these tend to clump together when heated and because these clumps can reduce a catalyst’s activity, the team deposited them on a hydrophobic surface composed of aluminum oxide to ensure they were evenly distributed.
Testing the material’s activity, the researchers found that their core-shell nanostructure performed 30 times better than the best methane combustion catalysts currently available, using the same amount of metal. It completely burned methane at 400 °C.
The data presented here demonstrate that the Pd@CeO2 structures deposited as single units on the hydrophobic alumina act as supramolecular catalysts. In these structures, the synergy between Pd and CeO2 produces active sites that are equally active in all of the samples, though in different numbers. As a further confirmation, CO chemisorption results demonstrated very similar Pd accessibility for all of the Pd@CeO2 samples prepared, corroborating the defined geometry and morphology obtained through the supramolecular approach.
This approach could potentially be valuable even for three-way catalysts, where the special properties shown here could be important for improving the activity at low oxygen concentrations, enhancing stability against sintering, and protecting against poisoning through the core-shell configuration.
—Cargnello et al.
While enthusiastic about the approach, Dr. Ferrauto, Research Fellow, Hydrogen and Fuel Cells at Columbia’s Department of Earth and Environmental Engineering, noted in his accompanying Perspective that:
...exhaust contains harmful combustion products, such as sulfur oxides and oil-additive elements such as compounds of phosphorus, zinc, and calcium, all of which poison most catalysts, especially PdO and CeO2. The presence of 5% steam in lean-burn engines can also inhibit catalytic reactions and cause excessive sintering of the catalytic components and their carriers. Furthermore, catalysts must be prepared cost-effectively in high volumes as a washcoat on a monolith. It remains to be shown whether Cargnello et al.’s catalyst can survive under these conditions.
...After 40 years of catalyst failures, there is reason to be hopeful. The low-temperature methane catalyst reported by Cargnello et al. is an important step toward two main applications: abatement of methane from internal combustion engines for emission control, and catalytically assisted homogeneous combustion. However, demonstration in real applications will require additional catalytic and system breakthroughs in sintering and poison resistance required for long-term performance. I hope others will take up the challenge to begin addressing these important issues.
Matteo Cargnello, now a postdoctoral fellow in Penn’s Department of Chemistry, joined Raymond J. Gorte, the Russell Pearce and Elizabeth Crimian Heuer Professor in Penn’s Department of Chemical and Biomolecular Engineering and Kevin Bakhmutsky, a former Ph.D. student in Gorte’s lab, in the study.
Their collaborators included Paolo Fornasiero and Tiziano Montini of Italy’s University of Trieste and National Research Council and José J. Calvino, Juan José Delgado and Juan Carlos Hernández Garrido of the Universidad de Cádiz in Spain.
The study was funded by the University of Trieste and Consortium INSTM, the Air Force Office of Scientific Research and Spain’s Ministry of Science and Innovation.
M. Cargnello, J. J. Delgado Jaén, J. C. Hernández Garrido, K. Bakhmutsky, T. Montini, J. J. Calvino Gámez, R. J. Gorte, and P. Fornasiero (2012) Exceptional Activity for Methane Combustion over Modular Pd@CeO2 Subunits on Functionalized Al2O3. Science 337 (6095), 713-717.doi: 10.1126/science.1222887
Robert J. Farrauto (2012) Low-Temperature Oxidation of Methane. Science337 (6095), 659-660. doi: 10.1126/science.1226310
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