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Synthesis

Probing Catalysis on the Nanoscale

Studies reveal that catalytic properties of small particles depend on their size and imperfections

by MITCH JACOBY, C&EN CHICAGO
September 27, 2004 | A version of this story appeared in Volume 82, Issue 39

Reduce the size of tiny chunks of solids until the pieces reach the nanometer-scale regime, and curious properties start to pop up. Semiconductor dots exhibit size-dependent colors, for example, and metals melt at much lower temperatures than their chunkier counterparts.

Catalytic properties of solids also change with size. Researchers are trying to understand the basis of such effects to control them and prepare catalysts that mediate chemical transformations efficiently and selectively. Some of the latest results of investigations of reactions on nanometer-scale catalysts were presented last month at the American Chemical Society national meeting in Philadelphia. The symposium, which focused on nanotechnology in catalysis, was sponsored by the Catalysis & Surface Science Secretariat and cosponsored by the Division of Colloid & Surface Chemistry and the ACS Petroleum Research Fund.

Enrique Iglesia, a professor of chemical engineering at the University of California, Berkeley, reported on the structure and catalytic function of small domains of metals and metal oxides. He opened the presentation by referring to semiconducting quantum dots, in which electronic properties are known to change with particle size. The situation is similar with oxides, he said, in which changes in the size and structure of minuscule domains alter the materials' electronic properties and hence their catalytic function.

As an example of size-dependent catalytic properties of metal-oxide domains, Iglesia reported on a study of o-xylene isomerization on acidified (protonated) tungsten oxide supported on porous zirconium oxide. The catalysts are prepared by anchoring tungsten oxide precursors on a zirconium oxyhydroxide material.

Iglesia noted that the concentration of WOx species on the support surface, which determines the size and structure of the domains, is easily controlled experimentally by adjusting the quantities of the reagents or the temperature of the heat treatment. Using those methods, Iglesia and coworkers prepared a collection of catalyst samples in which the extent of tungsten aggregation ranged from isolated WO3 molecules to two-dimensional monolayers to crystalline clusters.

The Berkeley group observed that samples having a tungsten concentration (surface density) of about 10 tungsten atoms per nm2 provide much higher catalytic activity than samples with higher or lower tungsten densities. And based on analysis via Raman and UV-Vis spectroscopy and other methods, the group concluded that the most active phase consists of 2-D monolayers constructed from W­O­W linkages unavailable in isolated tungstate structures.

"BEING TOO SMALL is not good for acid catalysis on tungsten oxide structures," Iglesia concluded. Isolated species are accessible to the reagents, but they are unreactive because they are too small to delocalize the charge--meaning, to stabilize the acidic protons needed for the reaction, he explained. Larger 3-D nanometer-sized crystallites are also inactive as catalysts because the would-be catalytic centers are inaccessible in the crystal's interior and the crystallites are oxygen deficient and unable to support acid catalysis.

Iglesia
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Credit: PHOTO BY MITCH JACOBY
Credit: PHOTO BY MITCH JACOBY

Iglesia reasoned that similar arguments can be extended to oxidation catalysis. As in the acid catalysis study, a number of samples of metal oxides covering a range of metal concentrations and domain sizes were examined. Specifically, oxides of vanadium, niobium, molybdenum, and tungsten were used to catalyze oxidative dehydrogenation of alkanes. As in the o-xylene isomerization study, oxide domains of intermediate size were found to be the most active oxidation catalysts.

According to Iglesia, the rate-limiting step in the oxidative dehydrogenation reaction is a reduction process, which requires activating a C­H bond in the alkane. The bond is activated on a pair of lattice oxygen atoms that reside at the surface of an oxide domain. Iglesia explained that this sequence leads to a transition state in which substantial electron density is transferred from the oxygen atoms to metal centers in the oxide domains. As in the earlier study, Iglesia concluded that domains that are too small to stabilize charged species are inactive catalytically.

Freund
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Credit: PHOTO BY MITCH JACOBY
Credit: PHOTO BY MITCH JACOBY

"Both types of chemistry require the metal-oxide domains to be large enough to accommodate charge," Iglesia said, but not so large that metal atoms become inaccessible inside the oxide domains.

Switching gears, Iglesia reported on recent work on small metal and carbide clusters that highlights the importance of particle size in catalytic reactions. He showed, for example, that in methane-activation reactions the tendency to form desired products (CO and H2) and undesirable carbon filaments depends on the metal cluster size. Iglesia also noted that methane can be converted selectively at high temperature to compounds such as benzene or naphthalene in the presence of approximately 0.5-nm metal-carbide clusters encapsulated in the pores of zeolite ZSM-5. The size requirement in that case is simply that the catalyst particles fit in the zeolite channels, he explained. The resulting confinement prevents the system from producing undesired higher molecular weight products.

Moving to even smaller catalytic centers, Iglesia reported on very recent work conducted with fellow Berkeley chemical engineering professor Alexander Katz and graduate student Justin Notestein. In that study, the group prepared titanium-calixarene complexes for use as epoxidation catalysts and immobilized them on silica surfaces. By using bulky calixarene substituents, the team ensured that the titanium centers remained isolated from one another during reaction with alkenes.

COMPARED WITH the solution-phase catalyst, the single-metal-center immobilized catalyst was found to be orders of magnitude more active and much more selective in epoxidation reactions, Iglesia noted. The results will be published in a forthcoming issue of the Journal of the American Chemical Society.

Hans-Joachim (Hajo) Freund addressed the reactivity of facets, edges, and corners of metal nanoparticles on oxide surfaces. The professor of chemical physics at the Fritz Haber Institute, Berlin, explained that he focuses on those types of crystal features "in an attempt to close some of the gaps between surface science, in which idealized and model systems are used to understand reactions in complex situations, and realistic catalysis," which typically involves irregularly shaped high-surface-area particles.

The Berlin group prepares model catalysts in vacuum by growing thin films of oxides on metal crystals and then using vapor deposition methods to grow metal particles on top of the oxides. The meticulous procedure allows the group to examine the model systems and reactions occurring on their surfaces with a battery of surface science tools.

For example, by exposing samples of bare alumina and Pd/alumina to nitric oxide and monitoring formation of nitro, nitrito, and other surface species using an infrared spectroscopy method, Freund and coworkers observed that the reactions occur solely at crystal imperfections such as domain boundaries. In contrast, terraces--the flat and crystallographically "perfect" regions--are chemically inactive.

In related work, the Berlin researchers studied the role of defect sites in multifaceted palladium nanocrystals used to catalyze methanol dehydrogenation reactions. Freund noted that two reaction pathways are possible: One leads to gas-phase CO; the other, to surface-bound carbonaceous deposits that can poison catalysts. Using isotopically labeled reagents and molecular beam techniques, the group determined that carbon accumulates at the crystal edges, corners, and small (100) facets, while the CO-forming reaction occurs on the crystal's (111) facets.

Other types of reactions also depend on crystal size and imperfections. Freund showed that under high-vacuum conditions, 3-D palladium nanoparticles are active alkene hydrogenation catalysts, unlike macroscopic Pd(111) single crystals. The single crystals' inactivity is unexpected, Freund remarked, because the palladium nanoparticles have many exposed (111) surfaces.

The difference in reactivities is rooted in the finite size of the nanoparticles, Freund said. Based on thermal desorption experiments and other measurements, he explained that exposing the metal samples to hydrogen causes some of the gas to be absorbed into the bulk of the samples. Unlike large single crystals, when it comes to nanoparticles, which have tiny internal volumes, hydrogen has nowhere to go, and so hydrogen residing just below the particle surface migrates back to the surface and participates in hydrogenation reactions.

Freund added that, for a small range of nanoparticle sizes, little difference is observed in ethylene hydrogenation activity. It's a different story for longer alkenes such as pentene. The size dependence is related to differences in crystal structure and hydrocarbon bonding geometry, Freund asserted.

GOING FOR THE GOLD
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Credit: PHOTO BY MITCH JACOBY
Oak Ridge chemists Yan (left) and Schwartz study catalytic properties of nanosized gold clusters.
Credit: PHOTO BY MITCH JACOBY
Oak Ridge chemists Yan (left) and Schwartz study catalytic properties of nanosized gold clusters.

Larger nanocrystals have larger and more orderly terraces than crystals measuring just a few nanometers. The greater degree of order favors hydrogenation of pentene, which proceeds by way of a di--bonded intermediate, Freund pointed out. The di- species requires a pair of adjacent surface sites to anchor the molecule. In contrast, ethylene attaches to the surface via -bonding, which requires only a single surface site and hence less surface order.

Nanocrystalline gold has drawn considerable attention in recent years since the discovery by Japanese researchers that when the ordinarily inert metal is prepared as nanosized clusters on an oxide support such as TiO2, the precious metal can exhibit high catalytic activity for some reactions.

Oak Ridge National Laboratory chemists Viviane Schwartz and Wenfu Yan reported on studies conducted with Steven H. Overbury and Sheng Dai that aim to uncover the basis of gold's unexpected reactivity. By varying the nature of the support material, gold concentration, treatment temperatures, and other variables, the researchers prepared a variety of catalysts and compared their activity, stability, and other properties.

Yan compared the anatase, rutile, and brookite phases of titania and found that gold particles supported on brookite were more stable during high-temperature oxygen treatments than gold deposited on the other materials. And based on X-ray absorption methods and other measurements, Schwartz reported that on titania, gold is readily reduced in hydrogen and remains in a metallic state during oxidation reactions. She added that 1- to 3-nm-sized particles are most active for CO oxidation and that exposure to temperatures above 300 °C results in larger and less active particles.

"It's not easy being small," declared Iglesia, à la Kermit the Frog. But in catalysis, being small is often crucial. So researchers continue to focus on the nanometer scale as they strive to improve the properties of catalysts.

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