Catalytic hydrogenations are critical steps in many industries including agricultural chemicals, foods and pharmaceuticals. In the petroleum refining industry, for instance, catalytic hydrogenations are performed to produce light and hydrogen rich products like gasoline. Typical heterogeneous hydrogenation catalysts involve nanoparticles composed of expensive noble metals or alloys based on platinum, palladium, rhodium, and ruthenium. We have demonstrated for the first time how single palladium atoms can convert the otherwise catalytically inert surface of an inexpensive metal into an ultraselective catalyst.(1) High resolution imaging allowed us to characterize the active sites in single atom alloy surfaces, and temperature programmed reaction spectroscopy to probe the chemistry.(1-5) The mechanism involves facile dissociation of hydrogen at individual palladium atoms followed by spillover onto the copper surface, where ultraselective catalysis occurs by virtue of weak binding.(5) The reaction selectivity is in fact much higher than that measured on palladium alone, illustrating the system’s unique synergy.
Potential energy landscape showing how single atom alloys offer completely different reaction kinetics and thermodynamics to the parent metals.
Our single atom alloy approach may in fact prove to be a general strategy for designing novel bi-functional heterogeneous catalysts in which a catalytically active element is atomically dispersed in a more inert matrix. Moreover, some of the best industrial alloy catalysts to date may already be operating via this mechanism, but there is currently no method to directly probe the atomic geometry of an alloyed nanoparticle in a working catalyst. From a practical application standpoint, the small amounts of precious metal required to produce single atom alloys generate a very attractive alternative to traditional bimetallic catalysts. While we have used a surface science approach to directly visualize the atomic-scale structure of the active sites and relate this information to hydrogenation reactivity/selectivity, we pose a challenge to the catalysis community to synthesize metal nanoparticles with trace amounts of an active element in order to generate single atom alloy surfaces capable of similar efficient and selective hydrogenation chemistry.
1) "Controlling the Spillover Pathway with the Molecular Cork Effect" M. D. Marcinkowski, A. D. Jewell, M. Stamatakis, M. B. Boucher, E. A. Lewis, C. J. Murphy, G. Kyriakou and E. C. H. Sykes - Nature Materials 2013, 12, 523-528. (link)
2) "Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations" 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 - Science 2012, 335, 1209-1212. (link)
3) "An Atomic-Scale View of Palladium Alloys and their Ability to Dissociate Molecular Hydrogen" A. E. Baber, H. L. Tierney, T. J. Lawton and E. C. H. Sykes - ChemCatChem 2011, 3, 607-614. (link)
4) "Atomic-Scale Geometry and Electronic Structure of Catalytically Important Pd/Au Alloys" A. E. Baber, H. L. Tierney and E. C. H. Sykes - ACS Nano 2010, 4, 1637-1645. (link)
5) "Hydrogen Dissociation and Spillover on Individual Isolated Palladium Atoms" H. L. Tierney, A. E. Baber, J. R. Kitchin and E. C. H. Sykes - Physical Review Letters 2009, 103, 2461021-2461024. (link)