Nanocatalysts: Something Old or New?

Quiz a dozen catalysis researchers about the role of nanotechnology in catalysis and at least a few of them are sure to roll their eyes. One reason for the disdain expressed by some scientists for terms such as "nanotechnology" and other oft-used "nano words" is that while the nanometer scale may represent new and exciting territory for many areas of science, in heterogeneous catalysis it's old hat. Industry has been carrying out some chemical transformations on nanosized particles for decades.

Yet given the seemingly endless stream of nanoscience advances nowadays, surely the burgeoning field has contributed something new to catalysis. Hasn't it? That question--and others regarding new instrumentation capabilities--were among the questions put to a group of catalysis experts last month at the American Chemical Society national meeting in Philadelphia. The issues were addressed in a panel session that was included in a symposium on nanotechnology in catalysis (see page 23). The symposium was sponsored by the Catalysis & Surface Science Secretariat and cosponsored by the Division of Colloid & Surface Chemistry and the ACS Petroleum Research Fund.

Kicking off the panel discussion, Phillip J. Bond, undersecretary of technology for commerce, pointed out that among Washington, D.C., lawmakers, nanotechnology already enjoys broad-based support. As proof, he noted that key congressional bills that fund research in nanoscale science have been passed with overwhelming majorities.

Why, Bond asked, is nanotech so popular among politicians--especially considering that many of them have little formal science education? "It's due to a belief that nanotechnology represents the future for economic benefits, job creation, energy independence, and advances in defense," he answered.

"It also gets support because of the buzz--the newness of the term," Bond acknowledged. And although excitement surrounding nanotechnology may be stirring up some overly optimistic expectations, "the hype is deserved," Bond argued. "We're beginning to see economic benefits and commercial products improved through nanotechnology," he contended. Supporting his claim, Bond pointed to stain-resistant clothing, automobile moldings, decontamination materials, and even tennis balls--all of which are made with nanoparticles or nanocomposites.

"NANOTECHNOLOGY is in its infancy," Bond noted, and breakthroughs are needed to help accelerate innovation. In order for policymakers to continue supporting basic research enthusiastically, Bond told attendees, scientists need to help them understand in some detail how nanotechnology will benefit their districts, the economy, and people's lives.

The other panel members addressed the questions regarding the newness and role of nanotechnology in catalysis more directly. For example, Harold H. Kung, a Northwestern University professor of chemical engineering, commented that although some noble-metal catalysts used in petrochemical processing were prepared in nanoparticle form more than 40 years ago, recent advances have led to improvements in catalyst design and synthesis.

"Now, we talk about controlling each and every nanoparticle--making particles with uniform size and composition," Kung said. That degree of control wasn't available a few decades ago, he added.

f nanoscale materials has progressed recently, one attendee questioned whether the new catalyst preparation methods are amenable to scale-up from laboratory quantities. Panel members responded by noting that a few companies specializing in scale-up of various types of nanocatalysts have just been formed.

Scale-up of catalysts with uniformly sized nanoparticles certainly would be advantageous, agreed Texas A&M University chemistry professor D. Wayne Goodman. "But it's exponentially more important to understand how catalysts work," he argued. With that information, researchers can prepare new catalysts that are more active and selective than those available today.

Pointing to the example of nanometer-sized gold particles that function as selective oxidation catalysts, Goodman emphasized that the recent discovery of new details of that system's reaction mechanism can be attributed to scanning microscopy methods and other experimental techniques that probe matter at the nanometer scale. Some of these tools weren't available just 10 years ago, he said.

Indeed, advances in instrumentation in the past several years have provided scientists with the ability to manipulate, scrutinize, and characterize catalysts on a finer scale than has ever been possible. Yet according to some researchers, instrumentation capabilities are still inadequate.

"Do we really have the tools to study the active sites of catalysts at the nanometer scale under reaction conditions?" asked Hans-Joachim (Hajo) Freund, a professor of chemical physics at Fritz Haber Institute, Berlin. Such tools are still generally unavailable, Freund noted. Instrumentation development is important, he said, but unfortunately most scientists shy away from the field because it's too risky--meaning it's difficult to show progress in the few-year time frame typically required by funding agencies.

Alexis T. Bell, a professor of chemical engineering at the University of California, Berkeley, pointed out that in addition to progress in synthesis and laboratory analysis, computational science is also adding new knowledge to catalysis on materials with nanometer dimensions.

"We're starting to see examples in which theoreticians are using quantum mechanics to make predictions in 'what-if' scenarios in catalysis," Bell said. For example, what if a reaction is carried out on a nanoparticle of a particular composition and shape? What if certain structures and atoms are present on the surface and others are absent?

"It's very exciting and represents a real advance in scientific capabilities," Bell commented. With further advances in "high-throughput computational techniques, we'll be able to provide additional guidance to experimentalists."

In follow-up discussions, symposium organizer Bing Zhou, vice president of catalyst developer Headwaters NanoKinetix, Lawrenceville, N.J., commented that nanotechnology stands to make a much bigger contribution to catalysis than simply providing methods for synthesizing and characterizing nanoparticles. But still greater control over materials' properties is needed before the impact will be felt throughout the catalysis industry, Zhou said.

FOR EXAMPLE, further advances are needed to improve stability and uniformity in particle size and morphology, Zhou remarked. And additional progress is needed to control the composition and surface structure of individual nanoparticles in multicomponent catalysts, he added.

"We also need greater control over the environment near the catalyst's active center," Zhou asserted. The idea is to prepare catalysts in which the structure surrounding the active center can guide reagent molecules toward the center in a particular configuration, thereby steering the reaction toward a preferred product.

Some progress in that area has been made recently. For example, Kung pointed out that Robert Raja, Sir John Meurig Thomas, and their colleagues at the University of Cambridge used the confining environment of certain sized silica pores to force reagent molecules into a particular configuration. The molecules then reacted stereoselectively with organometallic catalysts anchored in the pores (C&EN, March 15, page 37). Similar types of experiments have been reported, Zhou acknowledged, but much more progress along those lines is needed, he said.

"Nanotechnology is not going to change catalysis suddenly," Zhou argued. "Years from now when we look back to this period of research, I think we will consider it a revolution--but one that came about rather slowly."