A schematic model of Pd catalysts on alumina supports with and without ALD alumina overcoat during the reaction at 675°C to convert ethane to ethylene. (A) The uncoated Pd/Al2O3 catalyst. (B) During reaction using uncoated catalysts, carbon filaments (thick black lines) plug the reactor, causing substantial sintering and leaching of Pd nanoparticles from the support (barred white lines). (C) The Pd catalyst with an ~8-nm ALD alumina overcoat, containing ~2-nm micropores after activation. (D) The ALD alumina-overcoated catalyst during reaction. The microporous alumina overcoat trapped and stabilized the Pd nanoparticles, inhibiting Pd nanoparticle leaching and coke formation, while greatly enhancing ethylene formation. Credit: Lu, et al.; AAAS, Science, March 9, 2012
Coking and sintering are terms that usually conjure up images from iron and steelmaking and the heavy industrial equipment like blast furnaces and coke ovens. The delicacy of nanomaterials tends not to be one’s first thought. However, coking and sintering, it turns out, are big problems for high-temperature catalysis processes and the essential nanoscale catalyst particles.
Palladium nanoparticles are used to catalyze the oxidative dehydrogenation reaction of ethane to ethylene at 650°C, but they tend to “deactivate” over time, largely owing to coking and sintering. Coking is the accumulation of carbon on the metal, which blocks the metal surface from the reactants. Sintering is the formation of larger metal particles, which reduces the catalyst surface area and overall activity.
Previous efforts to avoid catalyst deactivation have focused on either coking or sintering, even though they often are simultaneous problems. A new paper published in Science by a team from Northwestern University, Argonne National Laboratory and Southeast University (China) reports that coatings of alumina nanolayers on palladium nanoparticles effectively addresses both problems.
Using atomic layer deposition, the group deposited 45 layers of alumina, making an amorphous coating thickness about eight nanometers thick. ALD has the advantage, according to the paper, of being “a self-limiting growth process for depositing highly conformal thin films on surfaces regardless of whether the materials are flat or possess high-aspect-ratio features, high surface area, or high porosity.” Forty-five layers were necessary because some sintering still occurred with 30 layers.
The catalytic reaction that converts ethane to ethylene is known to be susceptible to heavy coke formation, and the authors report, “the overcoating greatly reduced catalyst deactivation by coking and sintering at high temperatures.” Even better, the ethylene yield increased by more than tenfold. (The authors make no comment, but increasing the yield of an industrial-scale process by an order of magnitude would be huge.)
The palladium catalyst nanoparticles are mesoporous, with pore sizes averaging 6.6 nanometers. The mesoporosity disappears with the deposition of the coating, but calcining at 700°C (slightly above process temperatures) brings back the mesoporosity and new, two-nanometer-wide pores form in the coating. The coating porosity comes about from structural changes in the coating from dehydration, removal of carbon residues left by the ALD process and dewetting of the alumina from the palladium nanoparticles.
The coating porosity is one key to the system’s efficacy. The paper reports, “These pores made it possible for the embedded PD NPs to be accessible to reagents, while the overcoat imparted high thermal stability.”
A second mechanism is also suggested. The paper cites work by other researchers studying nickel catalysts, who found that carbon nanofibers (coking) initiate at the step edges of the nickel particle surfaces and that restructuring the step edge aided fiber growth. Ostwald ripening, too, is a factor, and therefore, “the edge and corner atoms play a central role in both sintering and coking,” and the coking and sintering resistance appears to happen “because the edge and corner atoms are selectively blocked and stabilized by alumina overcoats.”
Finally, the authors suggest that the alumina overcoat divides the palladium nanoparticle’s surfaces into “ensembles of Pd atoms that are too small to support coke formation.”
The work was supported by the Dow Chemical Company and DOE. International and domestic patents for “Metal Catalyst Composition” have been filed.
The paper is “Coking- and Sintering-Resistant Palladium Catalysts Achieved Through Atomic Layer Deposition,” by Junling Lu, Baosong Fu, Mayfair C. Kung, Guomin Xiao, Jeffrey W. Elam, Harold H. Kung, Peter C. Stair. DOI: 10.1126/science.1212906.