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Research:

Metal Atoms and Clusters in Oxide Matrices

Gold atoms and clusters are clearly visible in 5wt%Au/Fe2O3 catalyst particles used for CO oxidation and the water-gas shift reaction. HAADF-STEM picture taken at Oak Ridge National Lab and used in article in Journal of Electron Microscopy
 

A main goal of this research is to understand how metal atoms and metal clusters (few atoms) bound in oxide matrices catalyze certain redox reactions. Reaction pathways on isolated metal centers can be very different from those favored on metal nanoparticles. The binding energy of adsorbates on metal ions and clusters, and spillover effects are widely different than on supported metal nanoparticles. For example, in work carried out in our laboratory, we have shown that gold or platinum metal nanoparticles supported on cerium oxide do not participate in the catalysis of the water-gas shift reaction. Instead, the active sites in these catalysts are isolated gold or platinum species, embedded or otherwise strongly interacting with the Ce-O lattice. These findings were recently extended to Au-FeOx catalysts for the water-gas shift reaction. The maximum activity is realized when gold is atomically dispersed in either of these oxides (ceria, iron oxide). Reaction in H2-rich gases induces clustering of gold accompanied by loss of activity. The gold clusters (<1.5 nm dia) can be atomically re-dispersed in the oxide by oxidation at temperatures of 350-400oC. Deactivation can be suppressed by addition of small amounts of oxygen in the gas stream. Stability and activity can thus be tuned to desired levels by controlling the oxygen potential of the reaction gas mixture. The extension of these findings to other supported metal-oxide catalyst systems and other redox reactions is under investigation in our lab.

Atomic-scale Catalyst Design

The image shows a cluster PtK6O4(OH)2 identified by DFT calculations to be an active site for the low-temperature water gas shift reaction. Experimentally, K- or Na- stabilized Pt (OH)x is highly active and stable in realistic reaction gas compositions and temperatures. Science
 

We recently reported that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction (H2O + CO → H2 + CO2) used for producing H2. The alkali ion–associated surface OH groups are activated by CO at low temperatures (~100°C) in the presence of atomically dispersed platinum. Both experimental evidence and density functional theory calculations suggest that a partially oxidized Pt-alkali-Ox(OH)y species is the active site for the low-temperature Pt-catalyzed WGS reaction. These findings are useful for the design of highly active and stable WGS catalysts that contain only trace amounts of a precious metal without the need for a reducible oxide support such as ceria.

Unsupported Metal Clusters

Unsupported gold nanoparticles in solution are reported here for the first time to catalyze the oxidation of CO at ambient conditions. Gold was stabilized in solution by various polyamidoamine dendrimers. Dendrimer encapsulated gold nanoparticles (DENs) 0.5−2.5 nm in diameter have low initial activity. With storage time, however, the activity of the aged DENs increased and became comparable to a reference Au−TiO2 catalyst with the same gold loading and average gold particle size, which was tested under the same reaction conditions. The activation is attributed to partial hydrolysis of gold as followed by UV−vis spectroscopy. Journal of Physical Chemistry C

Publications:

Hot Gas Desulfurization with regenerable sorbents

Click on diagram to enlarge.

Over the past two decades, a significant effort has been devoted to the development of regenerable sorbents for the desulfurization of coal-derived fuel gas streams at high temperatures. A main barrier has been the low sorbent structural stability in cyclic operation. Presently, a lot of interest in this field derives from the intended use of fuel processing to produce hydrogen for fuel cells. Any sulfur present in the fuel, will be converted to H2S during the autothermal or steam reforming step of fuel processing. The sensitivity of most anode materials to sulfur requires deep desulfurization of the anode feed gas stream. Zinc oxide, the H2S sorbent of choice for low-temperature (<400ºC) operation, is unsuitable for high-temperature fuel cells, such as SOFC (Solid Oxide Fuel Cell), which require that the desulfurization unit be operated at temperatures exceeding 650ºC. At these temperatures, and with highly reducing gas streams, the rare earth oxides, e.g. lanthanum, praseodymium and cerium oxides are among the most promising sorbent materials. Recent work in our lab has identified a novel scheme of cyclic surface adsorption/desorption of H2S, which renders the sorbents regenerable at temperatures as high as 800oC. Removal of H2S from 50-500 ppm down to ppb levels in the presence of water vapor and carbon oxides has been measured over the oxysulfide sorbent surfaces. Regeneration is possible with a variety of purge gas streams. The new technology is suitable for multi-scale applications to effectively desulfurize hot fuel gases derived from various sulfur-laden fuels. Current work in our lab involves sorbent preparation as thin coatings on supports, and adsorption/reaction kinetics of H2S and COS on the rare earth oxysulfide surfaces.

Publications:

Lean-NOX Reduction Catalysts

The selective catalytic reduction (SCR) of NOx by methane in oxygen-rich gas streams is under investigation in our lab. Lean-NOx catalysts are needed to meet both energy efficiency and stringent NOx emission standards from a variety of lean-burn engines. Of particular interest to the after-treatment of CNG-vehicle exhaust gases is a CH4-SCR catalyst. For this application, we use silver as the primary catalyst, stabilized in an alumina matrix. Oxidized silver [Ag-O-Al] species are the active sites for the SCR reaction. When silver nanoparticles co-exist on the alumina surface, the selectivity to the SCR reaction drops, as the direct methane combustion reaction on silver becomes favored. Mechanistic studies of the stability of this type of catalyst in SO2 - laden streams have identified an unexpected structural stability of the dispersed silver which would otherwise sinter into large particles and deactivate. The reaction stability is good in SO2-containing flue gases, as long as the temperature is above ~ 625oC, which keeps part of the surface sulfate-free. In a related interesting finding from this work, SO2 can be used to re-disperse silver in aged Ag-Al2O3 catalysts treated in SO2-free gas where the silver particles had grown to > 100 nm sizes.

Publications:

Nanocatalyst Synthesis

Both the chemical nature and nanoarchitecture of the support can be key contributors to the activity, selectivity, and stability of heterogeneous catalysts. For example, we have recently reported a strong shape (rod, cube, polyhedron) and crystal plane ((100), (110), (111)) effect of nanoscale ceria on the activity of Au-CeO2 catalysts for the water–gas shift reaction. Hydrothermal synthesis techniques can be used to prepare the oxide nanocrystals. Addition of the metal (Au, Pt, Pd, Cu, etc.) can be done in a second step by impregnation, deposition-precipitation and other methods. Alternatively, the metal can be added to a growing oxide crystal under some conditions, e.g. when a surfactant is used. These novel synthesis techniques allow the preparation of single crystals and shapes with moderately high surface areas, such that they can be used under realistic conditions. Hence, they represent a major development in the ongoing effort to bridge the "materials gap" in catalysis. Current work in our lab involves the application of new synthesis protocols to prepare several different oxides and metal-doped oxides at the nanoscale with specific crystal surfaces and evaluate the structure-activity relationships in the reactions under investigation.

Scheme for synthesis of gold catalysts on ceria nano rods, cubes, and polyhedra, and their activity for the WGS reaction. Click on diagrams to enlarge.

Publications:
 
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