Robert L. Whetten Professor Office: L1-242 ES&T Phone: 404-894-8255 Fax: 404-894-7452 E-mail Robert L. Whetten

B.S., 1980, Weber State University; M.S., 1982, Cornell University; Ph.D., 1984, Cornell University

Alexander von Humboldt Senior Award, 1993

Research Interests

Size-selected Cluster Models for nm-scale Catalysts. Adsorption & reactions on Au- ,VO_x- and carbon- cluster catalysts in the atmospheric-pressure flow reactor.

Clusters as Molecule-scale Surfaces and Catalysts. This report describes gas-phase experiments on the interactions of small molecules with discrete elemental or compound clusters, which have been performed because they offer the potential for benchmark-level understanding of many important gas-solid processes, including those implicated in operating catalysts. Real-world heterogeneous catalysis is highly complex, typically involving intrinsic surface defects and high surface coverage, even for simple reactions. Gas-phase clusters generated in advanced sources provide a good match to this complexity, because they can be generated and studied over a wide range of size-, composition-, coverage-, and charge-states. Besides adsorption-desorption isotherms and kinetic parameters, the properties of clusters and their cluster-adsorbate complexes can be determined by infrared and photoelectron spectroscopy. Two points can be stressed in particular: The role of the support can be assessed, because the same processes can be compared for clusters entirely without support or with small elements of the support incorporated. Second, the observed adsorption and catalytic activity can be compared with those of high-level theoretical calculations (or simulations), where the composition of the system is completely defined. These points are all brought out in discussion of active systems below.

Chemical Equilibria and Kinetics in the High-pressure Flow Reactor. The favored method involves a fast-flow reactor under high-pressure conditions, because it ensures efficient thermalization of clusters as well as reaction products over the full range of coverages. The reactor methodology developed by Livingston et al.(1) has demonstrated how adsorption kinetics and equilibria can be distinguished, over the range T = 200 to 330 K. Recent work has extended this range, e.g. to over 420 K, with ongoing improvements aiming to reach 700 K. Clusters generated by laser-ablation and thermalized in helium adsorb diverse small gas molecules in one or two reactor stages. The effluent (cluster-adsorbate complexes in carrier helium) is analyzed mass-spectrometrically.

Gold-cluster Oxidation and Hydrogen-generation Catalysis. Oxidation reactions employing molecular oxygen (O₂) are among the most important of environmental, biological, and industrial processes requiring efficient catalysts for reduced-temperature and high-selectivity operation. Catalysts consisting of gold nanoparticles supported on certain metal-oxide supports have been shown to have exceptional activity and robustness even at reduced temperatures and humid environment. We have investigated the adsorption of various gases (O₂, CO, NO_x, SO₂, D₂O) on smaller gold-cluster anions, Au_N(-), N to 20 atoms, at near-ambient temperatures. Activation of O₂ to the superoxo form may be essential to oxidation catalytic activity, and is shown to be a highly selective process. Results from recent research on (CO, O₂) coadsorption and reaction on gold-cluster anions, are presented and interpreted. Results from support the steps in the full catalytic cycle illustrated below. We also demonstrate a novel source of hydrated gold-clusters or their hydroxides, that makes it possible to study a vital range of catalytic processes, including the water gas shift reaction, methanol reforming to (CO₂, H₂), and related electrocatalytic reactions.

Carbon Clusters as Model Soot-catalysts-Adsorption and Reaction with SO₂. Carbon clusters serve as appropriate model compounds for black-carbon soot, a particularly active form of elemental carbon that has been shown to be responsible, in part, for catalyzed atmospheric reactions in polluted atmosphere. We have investigated the reactions of negatively charged carbon clusters, C_N-, 5 < N < 60, with a range of atmospheric gases, including NO_x and SO_x compounds. The adsorption of SO₂ molecules on the larger carbon clusters is particularly facile at room temperature and up to 140^oC. The smaller clusters, up to N = 25, are also found to undergo efficient reactions leading to the oxidized form of carbon clusters, C_NO_1,2, and even to partial combustion (CO₂ loss). These processes may help account for the health consequences of the coincidence of soot and SO 2emissions in certain locations. The soot-catalyzed oxidation of SO₂ by O₂ to SO₃ (and hence H₂SO₄), is also considered to be important in such atmospheric settings, and can be investigated by the same co-adsorption approach as described above for gold-cluster catalyzed reactions.

VO_x Clusters with Tunable Oxidation States-selective Adsorption of O₂ and SO₂. Supported vanadium-oxide catalysts, V₂O_5-y, are among the most widely used of transition-metal oxide catalysts. In particular, they are employed for selective oxidation reactions, including the large-scale industrial production of sulfuric acid from SO₂. The predicted structures of nm-scale vanadium-oxide particles include a series of vanadyl network (cage) structures of very high stability, where reactivity can be expected at the reduced-valent defect sites. Our vanadium-oxide cluster source generates V_NO_M(-) clusters with a mean vanadium oxidation-state that can be tuned sensitively from +5 (fully oxidized) to +4, confirms that the adsorption and reactions of molecules (O₂, H₂O, SO₂) depends crucially on this parameter.

Representative Publications

Adsorption of carbon monoxide on smaller gold-cluster anions in an atmospheric-pressure flow-reactor: temperature and humidity dependence, W. T. Wallace, R. B. Wyrwas, A. J. Leavitt, R. L. Whetten,Phys. Chem. Chem. Phys., 2005, 7 (5): 930-937.

Scanning tunneling microscopy determination of single nanocrystal core sizes via correlation with mass spectrometry, T. P. Bigioni, T. G. Schaaff, R. B. Wyrwas, L. E. Harrell, R. L. Whetten, P. N. First, J. Phys. Chem. B, 2004, 108 (12): 3772-3776.

Oxygen Adsorption on Hydrated Gold Cluster Anions: Experiment and Theory, William T. Wallace, Richard B. Wyrwas, Robert L. Whetten, Roland Mitric, Vlasta Bonacic-Koutecky, J. Am. Chem. Soc. (2003), 125(27), 8408-8414.

Coadsorption of CO and O₂ on Selected Gold Clusters: Evidence for Efficient Room-Temperature CO₂ Generation, William T. Wallace, Robert L. Whetten, J. Am. Chem. Soc. (2002), 124(25), 7499-7505.

Transition from nanoparticle to molecular behavior: a femtosecond transient absorption study of a size-selected 28 atom gold cluster, Stephan Link, Mostafa A. El-Sayed, Gregory T. Schaaff, Robert L. Whetten, Chem. Phys. Letts. (2002), 356(3,4), 240-246.

Visible to Infrared Luminescence from a 28-Atom Gold Cluster, Stephan Link, Andrew Beeby, Simon FitzGerald, Mostafa A. El-Sayed, Gregory T. Schaaff, Robert L. Whetten, Robert L., J. Phys. Chem B (2002), 106(13), 3410-3415.

Metastability of gold-carbonyl cluster complexes, AuN(CO)M-, W. T. Wallace, Robert L. Whetten, Euro. Phys. J. D: Atomic, Molecular and Optical Physics, (2001), 16(1-3), 123-126.

Properties of a Ubiquitous 29 kDa Au:SR Cluster Compound, Gregory T. Schaaff, Marat N. Shafigullin, Joseph T. Khoury, Igor Vezmar, Robert L. Whetten, J. Phys. Chem. B (2001), 105(37), 8785-8796.

Low-temperature activation of molecular oxygen by gold clusters: a stoichiometric process correlated to electron affinity, B. E. Salisbury, W. T. Wallace, Robert L. Whetten, Chem. Phys. (2000), 262(1), 131-141.

Carbon monoxide adsorption on selected gold clusters. Highly size-dependent activity and saturation compositions, William T. Wallace, Robert L. Whetten, J. Phys. Chem. B (2000), 104(47), 10964-10968.