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Environment

Conference Delves into Catalysis Secrets

Advances reported in modeling catalytic behavior and cleaning up crude oil and diesel emissions

by MITCH JACOBY, C&EN CHICAGO
June 27, 2005 | A version of this story appeared in Volume 83, Issue 26

CITY TRAFFIC
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Credit: PHOTO BY MITCH JACOBY
Just a stone's throw from Philadelphia City Hall, researchers discussed a variety of catalysis issues, including cleanup of exhaust emissions from trucks, buses, and other diesel-powered vehicles.
Credit: PHOTO BY MITCH JACOBY
Just a stone's throw from Philadelphia City Hall, researchers discussed a variety of catalysis issues, including cleanup of exhaust emissions from trucks, buses, and other diesel-powered vehicles.

Nearly 1,000 catalysis aficionados flocked to Philadelphia last month to attend the 19th meeting of the North American Catalysis Society. Held every other year, the conference is the premier catalysis meeting of the Americas. From the U.S. to the U.K. and from Turkey to Thailand, scientists from more than 35 countries gathered at this year's meeting to catch up and share the latest news in catalysis.

With some 750 presentations in the technical sessions, the get-together provided attendees with an opportunity to discuss a rich assortment of topics in heterogeneous and homogeneous catalysis. The program included symposia on photocatalysis, electrocatalysis, and developments in surface science and spectroscopy. The meeting also featured various topics in environmental catalysis, such as gasoline- and diesel-engine-emissions cleanup.

Henrik Topsøe opened the meeting by delivering one of the conference's two plenary lectures, in acknowledgement of his receipt of the 2005 Eugene J. Houdry Award in Applied Catalysis. Sponsored by Süd-Chemie (formerly United Catalysts), in Munich, the award recognizes Topsøe "for his significant contribution to the understanding of hydrotreating catalysts." Topsøe is manager of strategic research at catalyst-manufacturer Haldor Topsøe, Lyngby, Denmark.

ALL IN THE FAMILY
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Credit: PHOTO BY MITCH JACOBY
Several members of the Topsøe family are active in catalysis research, including (from left) Henrik; his father, Haldor; and wife, Nan.
Credit: PHOTO BY MITCH JACOBY
Several members of the Topsøe family are active in catalysis research, including (from left) Henrik; his father, Haldor; and wife, Nan.

"When we started this work, we really didn't have in situ techniques," Topsøe recalled, referring to analytical methods for probing catalysts under working conditions. "So, out of necessity, we worked on modifying existing characterization methods and developing new ones." Developing those kinds of procedures is a challenge, Topsøe stressed, because the high pressures and temperatures typically used in industrial catalytic processes generally preclude using conventional laboratory tools for studying catalysts.

NONETHELESS, Topsøe and colleagues in several institutions developed in situ catalyst probes based on X-ray diffraction and absorption methods; infrared and Mössbauer spectroscopy; and, most recently, high-resolution transmission electron microscopy. The procedures have been used to uncover atomic-scale details of active-site structures and the effects of catalyst promoters and poisons. And they have been applied to various types of catalysts, including ones used for methanol and ammonia synthesis and for hydrotreating crude oil derivatives.

Catalytic hydrotreating is a key petroleum-refining process in which hydrogenation reactions are used to rid crude oil fractions of contaminants, such as compounds of sulfur and nitrogen. On the basis of projected changes in fuel market conditions, Topsøe contended that hydrotreating is poised to assume even greater importance in the near future than it does today.

"World demand for oil products is predicted to increase by more than 50% over the next 20 years," Topsøe noted, citing a U.S. Department of Energy report. The increased demand is expected to come primarily from the transportation sector and mainly for additional diesel and jet fuel, both of which are produced through hydrotreating processes.

But it's not just additional processing capacity that will be needed. New environmental regulations are forcing manufacturers to come up with very efficient methods of ridding fuels of sulfur compounds. In the U.S., for example, by 2006, sulfur levels in diesel fuel will need to be reduced from today's 500-ppm standard to just 15 ppm, Topsøe pointed out. Similar requirements are in place in Europe and Japan.

So the push is on to improve sulfur-cleanup technologies such as hydrodesulfurization (HDS) catalysis, one of Topsøe's longtime focus areas. HDS is a hydrotreating process in which sulfur compounds are reacted with hydrogen in the presence of Co-Mo-S or Ni-Mo-S catalysts to produce volatile H2S, which is removed from the feed material.

During the past several years, Topsøe and his coworkers have conducted numerous in situ investigations of HDS catalysts to understand their nature and structure at the atomic scale and to learn how to improve them. For example, early experiments using X-ray absorption spectroscopy and infrared spectroscopy methods showed that molybdenum is present in the form of MoS2 nanoclusters and in some cases is confined to a single MoS2 slab or layer.

In Co-Mo-S catalysts, cobalt is present in low concentrations and serves as a promoter that boosts catalytic activity. From experiments based on Mössbauer spectroscopy, a nuclear method, the Topsøe group found that cobalt atoms often reside along the edges of MoS2 nanoclusters. That observation turned out to be crucial because it was shown that HDS catalysts that have promoter atoms along the particle edges are especially active. Topsøe stressed that scientific insight gleaned from in situ studies led to new strategies for preparing catalysts that outperform the ones made by the classic trial-and-error approach.

ON THE EDGE
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Credit: COURTESY OF HENRIK TOPSØE
Thiophene molecules (yellow) bind readily to metallic brim sites in MoS2-based catalyst particles--a key step in hydrodesulfurization.
Credit: COURTESY OF HENRIK TOPSØE
Thiophene molecules (yellow) bind readily to metallic brim sites in MoS2-based catalyst particles--a key step in hydrodesulfurization.

OTHER INSIGHTS in HDS catalysis have come about through scanning tunneling microscopy (STM) studies coupled with quantum-mechanical calculations. Recently, a number of surprising features were uncovered in studies carried out in collaboration with the research groups of physics professors Jens K. Nørskov at the Technical University of Denmark, Lyngby, and Flemming Besenbacher at Aarhus University, in Denmark.

For example, the Danish scientists found that some of the tiny MoS2 clusters have a triangular shape with a brim just inside the outer edge of the particles that appears quite bright in STM images. The brightness corresponds to small regions with high densities of electronic states, Topsøe noted, which indicates that the sulfide structures, which are semiconductors in bulk form, contain surface metallic (molybdenum) sites.

Additional work combining STM analysis and computations shows that thiophene molecules, which are model sulfur compounds used in HDS studies, bind readily (but not very tightly) to the brim sites, where they are hydrogenated to butene thiolates and undergo C-S bond activation. The fairly mobile molecules can then move about the surface until coming in contact with edge vacancies (lattice dislocations) where hydrogenolysis strips the molecules of sulfur.

Topsøe pointed out that properties of the metallic brim sites explain a number of features observed over the years in HDS studies. A key observation--one that is now understood on the basis of theoretical computations--is that aromatic compounds do not poison the HDS catalysts because they bind weakly to the brim sites. In contrast, basic nitrogen compounds bind tightly to the brim and consequently poison the catalysts.

Topsøe remarked that these newly understood features of hydrodesulfurization chemistry have led the company to develop its new line of BRIM Technology catalysts, which includes a product for refining ultralow sulfur diesel.

Another series of in situ investigations that the Topsøe group has used to uncover catalysis secrets is based on transmission electron microscopy (TEM). Just a couple of years ago, the researchers designed a new TEM facility and procedures that enable specimens to be examined with angstrom-level resolution while exposed to elevated temperatures and pressures of reactive gas mixtures.

In one study, the group used the new TEM techniques to uncover previously unknown details regarding the location, chemical state, and other properties of barium used to promote a boron nitride-supported ruthenium catalyst. Used for ammonia synthesis, the catalyst's activity increases by some 2.5 orders of magnitude with the addition of just a small amount of barium. And in another TEM study, the Topsøe group tracked--in real time--abrupt changes in size and shape exhibited by 10-nm nickel catalyst particles used to form carbon nanofibers through hydrocarbon decomposition (C&EN, Feb. 2, 2004, page 7). 

TRADING PLACES
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Credit: COURTESY OF MATTHEW NEUROCK
New modeling work shows that a large positive potential can cause oxygen atoms (red) from water molecules to displace metal atoms (blue) in a submerged electrode. The process marks the onset of electrode corrosion.
Credit: COURTESY OF MATTHEW NEUROCK
New modeling work shows that a large positive potential can cause oxygen atoms (red) from water molecules to displace metal atoms (blue) in a submerged electrode. The process marks the onset of electrode corrosion.

THE MEETING'S other plenary talk was presented by Matthew Neurock, a professor of chemical engineering and chemistry at the University of Virginia, Charlottesville. Neurock was honored with the Paul H. Emmett Award in Fundamental Catalysis, which is sponsored by the Davison Chemical Division of W.R. Grace and presented to scientists who are 45 or younger. The award recognizes Neurock for "pioneering contributions to theoretical methods for the analysis and prediction of catalytic rates and selectivities."

Since the mid-1990s, Neurock has worked on developing theoretical methods to probe the role of competing surface processes and the effects of various reaction environments on catalytic behavior. As the Virginia researcher noted, at that time very little theoretical catalysis work was being carried out by chemical engineering academics. Many engineers and scientists, including anonymous reviewers of his early grant proposals, were highly skeptical that quantum-mechanical calculations would yield useful information in catalysis and answer relevant questions. "Where is the engineering in this?" a reviewer once asked.

"But just five years later, nearly every chemical engineering department in the U.S. had hired people to work on electronic-structure calculations," Neurock asserted. Since that time, a number of chemical companies in the U.S., Europe, and elsewhere have shown significant interest in fundamental studies, he added.

Some of Neurock's work in the past few years has focused on using computational methods to understand catalytic features that control energetics and influence reaction rates of hydrogenation of ethylene and maleic anhydride. Other studies have probed mechanistic details that govern activity and selectivity in vinyl acetate synthesis on palladium surfaces (C&EN, Nov. 29, 2004, page 25).

Recently, Neurock has turned his attention to developing computational methods for modeling the complex reaction environments inherent to electrocatalytic and electrochemical systems. The models predict the behavior of reactive species in water at the solution-electrode interfaces in the presence of an electrochemical potential.

As a prelude to studying electrocatalytic reactions in direct-methanol fuel cells (DMFCs), cells that do not require converting methanol to hydrogen to generate electricity, Neurock and coworkers used the new modeling techniques to probe electrochemical behavior of water near the surface of submerged electrodes. They showed that the model predicts the expected changes in the structure and reactivity of water as a function of applied potential.

For example, as the potential applied to the cathode grows increasingly negative, water molecules become oriented with the hydrogen side toward the electrode and react to form surface hydrides and ultimately liberate H2. As the anode potential becomes increasingly positive, water dissociates, forming surface hydroxides or surface oxygen and H3O+ species in solution. At larger positive potentials, oxygen and surface metal atoms exchange places, which is the onset of metal dissolution and leads to electrode corrosion. Neurock noted that information of this type can be used to establish the stability of materials used in fuel cells.

In recent years, various reaction pathways have been proposed for methanol oxidation to CO2 at the anode of DMFCs. Some researchers have suggested that the reaction proceeds by way of dual reaction paths, one of which leads to CO production and the other, to formaldehyde or formic acid.

Neurock
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Credit: PHOTO BY MITCH JACOBY
Credit: PHOTO BY MITCH JACOBY

By applying the group's new computational technique, Neurock and coworkers find that both reaction pathways are plausible energetically at an anode potential of roughly 0.5 V or greater. They find that the major path proceeds through an initial C­H activation step to form adsorbed CH2OH, which leads to formation of CO. In contrast, the minor path appears to proceed via O-H bond activation, which leads to the production of CH3O and subsequent formation of formaldehyde.

Neurock pointed out that the dual-pathway results of the theoretical work were just confirmed experimentally in a collaborative study with Andrzej Wieckowski, a chemistry professor at the University of Illinois, Urbana-Champaign (J. Phys. Chem. B 2005, 109, 11622).

ONE OF THE hot topics at this year's meeting was diesel-engine-emissions catalysis. Think of diesel engines and probably the first thing that comes to mind is trucks. But as Andrew P. Walker pointed out, diesel engines aren't found just in trucks, buses, and other heavy commercial vehicles. "Increasingly, they are being used to power passenger cars and vans," he stressed. Walker, who is a manager in Johnson Matthey's environmental catalysis division in Malvern, Pa., added that, in Europe, diesel-engine vehicles account for nearly 40% of the light-duty market. And that share is predicted to reach 50% by 2010, he said.

Compared with gasoline engines, diesel engines offer a number of benefits. For example, because of their design, diesel engines "run lean," meaning the fuel is combusted in excess oxygen, which leads to high fuel efficiency. In addition, the excess oxygen (air) leads to cooler operating temperatures and, hence, less thermal stress and greater longevity for diesel engines.

Similar to their gasoline counterparts, diesel engines emit atmospheric pollutants such as CO, nitrogen oxides (NOx), and hydrocarbons. But unlike gasoline engines, they also emit significant quantities of particulate matter (mostly carbonaceous soot), which has been fingered as a health hazard. So legislation has been enacted in Europe, the U.S., and other countries to help curb pollutant levels. In the U.S., for example, diesel-engine manufacturers are required to reduce NOx and particulate-matter emissions by 90% relative to today's limits over the next two to five years. "Emissions legislation is being progressively tightened around the world," Walker observed. And the demands for cleaner air are driving catalysis research efforts.

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Of the various NOx cleanup strategies that have been proposed, selective catalytic reduction (SCR) shows a lot of promise. In SCR, NOx is converted to N2 and water through reaction with a reducing agent, such as hydrocarbons, which may be present in the exhaust stream, or with ammonia, which can be supplied from various sources, including an on-board aqueous urea solution.

At Northwestern University, chemistry professors Wolfgang M. H. Sachtler and Eric Weitz have teamed up to unravel reaction mechanisms that govern NOx reduction. The goal is to understand the chemistry in detail to maximize the effectiveness of NOx cleanup.

Sachtler pointed out that, a few years ago, researchers found that NOx can be reduced efficiently by a Ba/Y-zeolite catalyst and acetaldehyde as the reducing agent at 200 ºC, which is a typical diesel exhaust-gas temperature. So the Northwestern team probed the system by IR spectroscopy, isotopic labeling, and other methods.

According to Sachtler, the investigation revealed a complex mechanism in which acetaldehyde is oxidized to acetic acid, which then reacts with NO2 to form the aci-anion of nitromethane (H2C=NOO­). Nitromethane, in turn, is converted to isocyanic acid (HNCO), which can be hydrated to form ammonia. Ultimately, ammonia reacts with NOx to form ammonium nitrite (NH4NO2), which then decomposes to yield water and dinitrogen.

OH, NO!
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Credit: PHOTO BY MITCH JACOBY
Northwestern's Sachtler (left) and Weitz probe mechanistic details of NOx-cleanup chemistry.
Credit: PHOTO BY MITCH JACOBY
Northwestern's Sachtler (left) and Weitz probe mechanistic details of NOx-cleanup chemistry.

AFTER REVIEWING the main features of the 12-step mechanism, Sachtler remarked that "in 55 years of fundamental research in catalysis, I have never studied a reaction of such complexity in which every step has been reasonably understood and well-documented."

The Northwestern study also addressed a puzzling feature regarding the ratio of reactants. Weitz noted that when ammonia is the reducing agent, NOx can be converted to N2 with nearly 100% efficiency on Ba/Y-zeolite if NO and NO2 are present in equimolar ratios.

To understand the basis for that constraint, Weitz and Sachtler examined the role of NO in NOx reduction. It turns out that NO plays multiple mechanistic roles, Weitz said. For example, NO reduces HNO3 to HNO2 and reduces ammonium nitrate to ammonium nitrite. In addition, NO+ can react with water to form HNO2.

Summarizing the chemical processes in a multistep reaction diagram, Weitz noted that all of the desired reactions lead to ammonium nitrite, which decomposes to form water and N2, the products of NOx abatement. He then explained that if all the reactions are combined to form a net reaction that yields water and N2, the stoichiometry indicates that NO and NO2 combine in a 1:1 ratio (J. Catalysis 2005, 231, 181).

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Credit: PHOTO BY MITCH JACOBY
Fino
Credit: PHOTO BY MITCH JACOBY
Fino

A number of conference attendees presented results of studies aimed at treating soot, the other main diesel-engine pollutant. Debora Fino, materials science and chemical engineering assistant professor at Polytechnic University of Torino, Italy, reported on investigations designed to find suitable catalysts for diesel particulate filters. The filters, which are porous ceramic monoliths (one-piece bricks), trap soot particles on the catalyst-coated filter walls as exhaust gases diffuse through the filter. Fino explained that the traps are regenerated periodically by operating the engine in a way that momentarily boosts the exhaust temperature and burns the soot particles.

Walker
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Credit: PHOTO BY MITCH JACOBY
Credit: PHOTO BY MITCH JACOBY

Fino and coworkers prepared a number of substoichiometric lanthanum -chromium compounds, many of which contain sodium, potassium, or other alkali metals, and assessed their activity for soot combustion. The Torino group found that a lithium-substituted chromite compound, La0.8Cr0.9Li0.1O3, was the most active catalyst and that it regenerated the filter twice as fast and more thoroughly than uncatalyzed filters.

On the basis of a photoelectron spectroscopy investigation, Fino attributed the high activity to weakly adsorbed O­ species on the catalyst surface. She added that, after exposing the catalyst to accelerated aging conditions, the material still exhibited high activity (J. Catalysis 2005, 229, 459).

Yezerets
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Credit: PHOTO BY MITCH JACOBY
Credit: PHOTO BY MITCH JACOBY

Comparing information published in various soot-oxidation studies may lead to contradictory conclusions, according to Aleksey Yezerets, a senior engineer with Cummins, a Columbus, Ind.-based manufacturer of diesel engines. Yezerets, who was one of the symposium organizers, said that widely varying reports of kinetic parameters, including activation energies and reaction orders, are due to differences in sample composition, morphology, combustion conditions, and other experimental factors that vary from study to study.

To bring uniformity to soot-oxidation studies, Yezerets; Neal W. Currier, a technical leader; and coworkers developed an experimental procedure that calls for a commercially available carbon soot standard to be pretreated in a prescribed manner and then oxidized in a stepwise fashion by exposing the sample to metered pulses of oxygen. The rate of soot oxidation is then calculated from the concentration of CO2 at each oxidation step.

According to Yezerets, oxidation results obtained from the new method can be described properly by just a simple kinetic expression at any point in the carbon-oxidation process and across the entire range of experimental conditions. (Appl. Catal. B 2005, 61, 134).

As with any scientific field, popularity of particular catalysis research topics peaks at certain times and falls at others. Diesel-emissions catalysis is a hot topic nowadays, partly because of upcoming legislation deadlines. But no deadlines loom large for nanotechnology in catalysis, operando spectroscopy, fuel processing, and electrocatalysis. Yet symposia on those topics also drew large numbers of attendees and in some cases left the conference organizers scrambling to find seating for overflow crowds. Time will tell which topics will be popular a couple of years down the road.

The next meeting of the North American Catalysis Society is scheduled to take place in Houston in June 2007.

 

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