Manufacturing Bits: Aug. 26

Smokestack metrology; converting pollution into clean air; seeing gold nanoparticles.


Smokestack metrology
The National Institute of Standards and Technology (NIST) has nearly completed the construction of one of the world’s more unusual measurement systems. The organization is devising a 50-meter horizontal smokestack at its campus in Gaithersburg, Md.

The new facility will accurately determine the amount of gases discharged from smokestacks at coal-fired power plants and other industrial sites. At present, for a typical coal-fired power plant, measurements of carbon dioxide (CO2) flux in smokestacks have uncertainties that may be as large as 10% to 20%.

Diagram of the SMSS facility under construction on NIST's Gaithersburg campus. (Source: NIST)

Diagram of the SMSS facility under construction on NIST’s Gaithersburg campus. (Source: NIST)

“For both current mitigation efforts and future emissions monitoring, the nation needs to improve CO2 measurement in smokestacks of coal-burning power plants,” said Aaron Johnson of PML’s Fluid Metrology Group, on NIST’s Web site. “And if you improve measurement of total flow, you will improve measurements of other pollutants as well, such as sulfur compounds and nitrous oxide. So we’re constructing a scale-model smokestack simulator here at NIST to better understand and monitor gas flow. The goal of the project is to find a way to get CO2 flux measurements with a 1% uncertainty at a reasonable cost.”

Today, measurements are made by using a so-called continuous emission monitoring system (CEMS) in each smokestack. About two-thirds of these units are ultrasonic systems made up of two transmitter/receiver units placed at different elevations in the stack.

It appears likely that placing one or more additional pairs of ultrasonic units that sample different paths could produce more accurate measurements. So one of the goals of the new Scale-Model Smokestack Simulator (SMSS) project at NIST, which includes a section fitted with multiple separate ultrasonic transmitter/receiver units, is “to understand how much bang for the buck you get by using multi-path metering,” Johnson said.

High-accuracy monitoring became even more important on June 2, 2014, when the Environmental Protection Agency announced draft plans to reduce greenhouse gas emissions from power plants by as much as 30% by 2030.

Converting pollution into clean air
Carnegie Mellon University and the Department of Energy’s Oak Ridge National Laboratory have found a new gold molecule, a catalyst that consists of 25 gold atoms. The technology catalyzes the conversion of a variety of molecules, including the transformation of poisonous carbon monoxide into harmless carbon dioxide, according to researchers.

There are some roadblocks, however. One of the challenges with gold nanoparticles is to make them both stable and active. Researchers have made tiny clusters of gold atoms that are stabilized by compounds called ligands. But the ligands block the very sites needed to catalyze the conversion of carbon monoxide into carbon dioxide.

“The ligands are double-edged swords,” said Zili Wu of Oakridge, on the agency’s Web site. “We’re interested in using gold clusters as catalysts or catalyst precursors. Ligands on the one hand stabilize the gold particle structure, but on the other hand decrease their catalytic performance. Balancing those two factors is the key to creating a new catalytic system. One way is to utilize a metal oxide (here, cerium oxide) as an inorganic ligand to stabilize the gold clusters when the organic ligand has to be removed for catalysis.”

In the lab, researchers synthesized gold clusters and made the cerium oxide rods. The gold clusters were heated. Then, the ligands started to come off and gold’s catalytic activity increased. The optimal temperature for producing gold nanocluster catalysts for carbon monoxide oxidation is 498 Kelvin, Wu said.

Seeing gold nanoparticles
For centuries, artists have used gold nanoparticles in various art forms, such as staining glass. But more recently, the electronic properties of gold nanoparticles make these materials ideal for a number of applications.

In the electronics world, gold nanoparticles could be used as conductors, printable inks, photonics and molecular electronics. The materials could also be used for the development of sensors, drug delivery devices and biological contrast agents. For example, gold nanoparticles are also used to detect biomarkers in the diagnosis of heart diseases, cancers, and infectious agents, according to Sigma-Aldrich, a life science and high-technology company.

“Gold nanoparticles interaction with light is strongly dictated by their environment, size and physical dimensions. Oscillating electric fields of a light ray propagating near a colloidal nanoparticle interact with the free electrons causing a concerted oscillation of electron charge that is in resonance with the frequency of visible light. These resonant oscillations are known as surface plasmons,” according to Sigma-Aldrich.

“As particle size continues to increase toward the bulk limit, surface plasmon resonance wavelengths move into the IR portion of the spectrum and most visible wavelengths are reflected, giving the nanoparticles clear or translucent color. The surface plasmon resonance can be tuned by varying the size or shape of the nanoparticles, leading to particles with tailored optical properties for different applications,” according to the firm.

But understanding the physical properties of these materials is challenging. Stanford University, the University of Jyväskylä and others have devised a high-resolution electron microscopy technology that enables images at the atomic resolution in 3D.

Using the technology, researchers have been able to see a gold nanoparticle at 1.1nm in diameter, with 68 gold atoms organized in a crystalline fashion at the center of the particle.

Visualization of the atomic structure of a gold nanoparticle determined by electron microscopy. The colored spheres denote gold atoms in different crystal shells around the central axis (red). The background shows a collection of real-life electron microscopy data from which the single structure shown was reconstructed. (Source: Stanford) University.

Visualization of the atomic structure of a gold nanoparticle determined by electron microscopy. The colored spheres denote gold atoms in different crystal shells around the central axis (red). The background shows a collection of real-life electron microscopy data from which the single structure shown was reconstructed. (Source: Stanford) University.

Researchers were able to determine the structure of gold nanoparticles by aberration-corrected transmission electron microscopy (TEM). Researchers used a TEM with a minimal electron dose.

The structure was supported by small-angle x-ray scattering and by comparison of observed infrared absorption spectra. The result was supported by small-angle X-ray scattering done in Lawrence Berkeley National Laboratory, and by mass spectrometry done at Hokkaido University.