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Associate Professor Krista Walton directs the Department of Energy-funded Energy Frontier Research Center that is investigating how acid gases affect materials used in pollution control.

Acid Test

Earth’s air pollution and climate change issues are linked to combustion and its detrimental byproducts: greenhouse gases such as carbon dioxide (CO2) and gases that pollute the atmosphere such as nitrogen oxides.

The good news is that today’s advanced materials can trap or neutralize these acid gases right in the smokestack, or even capture CO2 straight from the atmosphere. Multiple research teams are working to increase the efficiency of these important materials; the Department of Energy (DOE) is currently funding a number of such projects under its Energy Frontier Research Center (EFRC) program.

But a key question remains: How do acidic gases affect materials designed to lower their emissions? How durable, for instance, will these advanced materials be when subjected to real-world environments like the hot exhaust flues of a power plant?

“There’s a knowledge gap here — scientists don’t yet understand the fundamentals of how acid gases like carbon dioxide, nitrogen oxides, and sulfur oxides interact with important classes of materials,” said Krista Walton, an associate professor in the Georgia Tech School of Chemical & Biomolecular Engineering (ChBE). “If you create a new material that separates CO2 with record efficiency in the lab, but it only lasts a few days in an industrial environment, then it’s not a useful advance.”

The DOE recently awarded a four-year $11.2 million grant to Georgia Tech to lead an EFRC that studies materials degradation caused by acid gases. Directed by Walton, the new center involves research teams from six universities and a government laboratory. Collaborating with Georgia Tech are researchers from Lehigh University, University of Alabama, University of Florida, University of Wisconsin, Washington University in St. Louis, and Oak Ridge National Laboratory.

Dubbed the Center for Understanding and Control of Acid Gas-Induced Evolution of Materials for Energy (UNCAGE-ME), the Georgia Tech-led effort is one of 10 new EFRCs recently funded by the DOE.


MULTIPLE RESEARCH THRUSTS

The seven partners are investigating a range of solid materials — including metals, polymers, ceramics, and composites — that have the ability to trap or chemically alter acid gases via separations/ catalysis techniques. The overall study is divided into several research thrust areas, and the goal in each case is to understand, down to the molecular level, exactly what’s taking place as acid gases interact with a given material.

The EFRC is multifaceted, Walton explained. Unlike many materials efforts that focus on designing a single material for a target application, this center covers numerous materials and employs a wide range of research techniques. Moreover, the research process itself is highly integrated — most of the principal investigators from the seven partner institutions are involved in two or more projects.

The center is tackling four major research thrusts, all concerned with materials relevant to industry. The work focuses on acid-gas interactions with:

  • Model nonporous oxide-based solids, such as copper and titanium oxides.
  • Ordered (crystalline) porous materials, such as metal organic frameworks.
  • Disordered porous materials, including carbons and amine/oxide composites.
  • External surfaces of porous materials.

“The multiple partner structure of this EFRC fits our culture at Georgia Tech very well, because we’re accustomed to collaborating,” said David Sholl, a ChBE professor who is an EFRC deputy director and leader of the thrust investigating external surfaces of porous materials. “It means we can do things that no individual person can do alone; typically three, four, or even five different research groups are contributing their techniques to each thrust.”

Professor Christopher W. Jones of ChBE is leading the thrust on disordered porous materials, while Professor Sankar Nair of ChBE is leading the ordered porous materials thrust. Assistant Professors Michael Filler and Ryan Lively of ChBE, as well as Professor Thomas Orlando of the Georgia Tech School of Chemistry and Biochemistry, are also principal investigators in the center. Zili Wu of the Oak Ridge National Laboratory is leading the research thrust that is addressing model nonporous oxide-based solids.

Georgia Tech researchers (left to right) David Sholl, Christopher Jones, and Sankar Nair are part of a team investigating the effects of acid gases on a variety of materials used in pollution control.

Georgia Tech researchers (left to right) David Sholl, Christopher Jones, and Sankar Nair are part of a team investigating the effects of acid gases on a variety of materials used in pollution control.

BROAD-BASED CENTER

Besides acid gases such as carbon dioxide, nitrogen oxides, and sulfur oxides, industrial exhausts also contain smaller amounts of other problematic constituents like hydrogen sulfide, hydrogen chloride, and chlorine. Though these chemicals usually occur only in trace amounts, their effect on materials is also poorly understood and could be significant.

Capturing these gases, or changing them into less-harmful chemicals, is accomplished via two main approaches:

Adsorption separations or membrane separations are leading methods for capturing CO2. Adsorption divides a target molecule from other chemicals by making it adsorb — stick — to the surfaces of a material. Alternatively, membranes can allow a given chemical species to pass through while blocking others.

Catalysis increases the rate of a chemical reaction by utilizing an additional ingredient known as a catalyst. In the catalytic converter of a gasoline-fueled vehicle, for example, this approach is used to generate several changes, including converting carbon monoxide into CO2 by exposing it to platinum or other elements.

“We’ve sought to position our center to be both a catalysis center and a separations center,” said Jones, an EFRC deputy director. “Until now, the constituents of industrial exhaust have rarely been studied by a large collection of people who are all focused on understanding how they work.”


ATOMIC-LEVEL ANALYSIS

The EFRC’s numerous research teams are utilizing a wide range of investigative approaches and techniques to study how acid gases interact with and degrade materials.

A typical first step might involve developing an entirely new material in a given class, so researchers can fully map and understand its molecular structure and composition, explained Nair, who is leading the thrust on ordered (crystalline) porous materials such as metal organic frameworks. An in-depth knowledge of the material’s makeup allows the researcher to chart the locations and the concentration of its defects and impurities.

Once such fully mapped materials are available, researchers can perform experiments on them aimed at analyzing gas/material interactions at the molecular level. Such complex investigations are conducted at several length scales using multiple techniques — including scanning tunneling microscopy that allows researchers to view individual atoms, nuclear magnetic resonance spectroscopy, infrared or raman spectroscopy, X-ray crystallography and diffraction, and neutron scattering.

“One of our hypotheses is that acid gases primarily interact with a material at so-called defect sites — also called heterogeneous sites — within the material,” Nair said. “Now it’s quite possible that degradation could be taking place at these same sites. If so, we want to pinpoint how the interaction and the degradation are related.”


COMPUTER MODELING OF MOLECULES

The EFRC, which involves some 50 researchers, including graduate students, is emphasizing its education mission along with its scientific one. Every student spends time at other institutions, becoming familiar with the other researchers and their investigative methods.

The center’s experimental studies are being supported by extensive computational modeling efforts. Combining experimental approaches and computational techniques is key to achieving the fundamental knowledge that the center is working toward, said Sholl, whose own research focuses on computer modeling. Calculations made using computers are able to interpret experimental results in unique ways, and, in turn, experiments can help validate computer data.

“What we ultimately want to know is, which atoms are on the surface of these materials and exactly where they are,” Sholl said. “There’s no experiment alone that can tell you that. We have to gradually deduce that information over time via a process of give-and-take that’s based on both experimental and computational methods.”

Rick Robinson is a science and technology writer in Georgia Tech’s Institute Communications. He has been writing about defense, electronics, and other technology for more than 20 years.

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