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Metal Atoms and Clusters on 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 few-atom metal clusters bound on oxide matrices catalyze certain 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 or the methanol steam reforming reactions. 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, Au/ZnO, Au/La2O3, and Au/TiO2 catalysts for the water-gas shift reaction. The maximum activity is realized when gold is atomically dispersed on any of these oxides. Reaction in H2-rich gases induces some clustering of gold accompanied by loss of activity. The gold clusters (<1.5 nm dia) can be atomically re-dispersed 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. Extension to Pt supported on carbon nanotubes was shown in a recent publication.

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.


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. We have recently shown a similar shape effect for the methanol steam reforming reactions on Au/CeO2 and Au/ZnO nanoshapes. The chemical effect of the support, however, is indirect, as similar values of the apparent activation energies for different shapes and different supports demonstrate. 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.


Single Atom Alloy Catalysts

Individual atoms of palladium, represented by the yellow peaks, in the surface of copper help break hydrogen molecules into two atoms, facilitating hydrogenation reactions. Image: Courtesy of Sykes Lab.

We have recently reported that when single atoms of palladium are added to copper, the resulting "single atom alloy" is active for selective hydrogenation reactions; e.g. of acetylene to ethylene or of styrene to ethylbenzene.

Using atomic-resolution scanning tunneling microscopy, our collaborator Prof. Charles Sykes (Tufts, Chemistry) and his group have shown that the hydrogen adsorption and dissociation takes place on the Pd atoms, followed by spillover to the Cu (111) sites where adsorbed H atoms are weakly adsorbed. In the new paper STM along with TPD were used to show how the Cu(111) surface now becomes active for the selective hydrogenation of unsaturated hydrocarbons.

Selective hydrogenation of phenylacetylene to styrene
P = 100 psi H2; T = 25 oC; solvent: n-hexane

Reactant : catalyst ratio = 10:1
PCCP 2013.

In follow up work, we have extended the surface chemistry findings to the synthesis of copper nanoparticles decorated with small amounts of palladium. These were found to catalyze the selective hydrogenation of phenylacetylene to styrene at ambient conditions. This opens the possibility to design other bimetallic catalysts as Single Atom Alloys for a variety of selective hydrogenation and dehydrogenation reactions.


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