Image courtesy of Charles Sykes, Tufts University
Scanning Tunneling Microscope Image Showing Atomically Dispersed Palladium Atoms in a Copper Surface. The palladium atoms activate hydrogen, and the copper sites insert it into acetylene, thus allowing the industrially important conversion of acetylene to ethylene to proceed with 100% selectivity.
Researchers demonstrated for the first time how single Pd atoms can convert the otherwise catalytically inert surface of an inexpensive metal, Cu, into an ultra-selective catalyst. High-resolution imaging allowed for characterization of the active sites in the bimetallic surfaces, and temperature-programmed reaction spectroscopy assessed the chemistry.
Binding single metal atoms strongly in crystalline surfaces of a different metal offers a general strategy for designing novel bifunctional heterogeneous catalysts for the production of fuels, agricultural chemicals and medicine. By controlling changes to the amount and geometry of the atomically dispersed metal, catalyst selectivity and activity can be fine-tuned with ultimate precision.
Catalytic hydrogenations are critical to many industries including fuels, agriculture, and pharmaceuticals. In the petroleum refining industry, for instance, catalytic hydrogenations are performed to produce hydrogen-rich products like gasoline. Typical heterogeneous hydrogenation catalysts involve nanoparticles composed of expensive noble metals or alloys based on platinum, palladium (Pd), rhodium, and/or ruthenium. To reduce the cost and potentially increase reactivity, BES-funded researchers at Tufts University coated the catalytically inert surface of an inexpensive metal, copper (Cu), with single Pd atoms which somewhat surprisingly yielded an ultra-selective catalyst. Using high-resolution imaging and temperature-programmed reaction spectroscopy, researchers determined that the mechanism involved facile dissociation of molecular hydrogen at individual Pd atoms followed by spillover onto the Cu surface. This weak binding to Cu enabled the ultra-selective hydrogenation of both alkenes and alkynes. The reaction selectivity was much higher than that measured on pure Pd, illustrating the unique synergy of this bimetallic system. This surface science and kinetics experimentation shows that atomic dispersion of transition metals in other metal surfaces may lead to the development of novel catalysts and potentially more efficient chemical conversions.
Maria Flytzani Stephanopoulos
DOE Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division
National Science Foundation
G. Kyriakou, M. B. Boucher, A. D. Jewell, E. A. Lewis, T. J. Lawton, A. E. Baber, H. L. Tierney, M. Flytzani-Stephanopoulos, and E. C. H. Sykes. "Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations." Science 335, 1209-1212 (2012). [DOI: 10.1126/science.1215864]
Matthew B. Boucher, Branko Zugic, George Cladaras, James Kammert, Matthew Marcinkowski, Timothy J. Lawton, E. Charles H. Sykes, and Maria Flytzani-Stephanopoulos. "Single Atom Alloy Surface Analogs in Pd0.18Cu15 Nanoparticles for Selective Hydrogenation Reactions." Phys. Chem. Chem. Phys., 15 (29), 12187–12196 (2013). [DOI: 10.1039/C3CP51538A]
Tufts Nano Catalysis & Energy Laboratory
Tufts Nanoscale Imaging and Surface Analysis Laboratory