We have known for decades that catalysts accelerate a reaction that reduces harmful industrial emissions. And now, we know exactly how they do it.
A recent book by Israel Wachs, Professor of Chemical and Biomolecular Engineering, G. Whitney Snyder, at the University of P.C. The Rosin College of Engineering and Applied Sciences describes the mechanism and is the inside history of the back cover on September 2, 2019, issue of Angewandte Chemie magazine of the German Chemical Society.
Power plants are a major source of toxic emissions from climate change. When fossil fuels such as coal and natural gas are burned, they produce dangerous pollutants, in particular a group of harmful gases called nitrogen oxides (or NO x ) that contribute to acid rain, the formation of ozone on earth, ie and greenhouse gases.
"The combustion process for generating energy requires very high temperatures that cause molecular nitrogen (N 2 ) and oxygen (O 2 ) to be present in the air. or a crack, "says Wachs. "Then the N and O atoms recombine and make NO x which today is considered to be the biggest pollution problem because it is very difficult to control."
As early as the 1970s, the Japanese developed NO x emissions control technology by reacting NO x with ammonia to form harmless nitrogen (N 2 ) and water (H 2 O).
"It's a beautiful chemical reaction that turns something very harmful into something very benign," says Wachs, who runs the Lehigh Operando Laboratory for Molecular Spectroscopy and Catalysis.
NOx emissions are now highly regulated and one common strategy to reduce emissions is the selective catalytic reduction (SCR) of nitric oxides by ammonia. Catalysts accelerate the SCR reaction and control the reaction products (such as the formation of N 2 and H 2 O), which means that the catalyst ensures that the reaction does not produce unwanted harmful gases (hence "selective ").
An SCR catalyst widely used by power plants is titanium-supported vanadium oxide.
"The catalyst consists of vanadium oxide and tungsten oxide dispersed on the surface of a titanium carrier (TiO 2 ). Vanadium oxide is the active component carrying out the selective catalytic 19459  9 ] formation, not adverse reaction products that can be toxic, "says Wachs. "There has been a great debate in the literature for 40 years, since the beginning of the development of this technology, over the question of what exactly is a component of tungsten oxide?"
The research community knows from experience that tungsten oxide thermally stabilizes the titanium support, which is vital since these catalysts can spend years at high temperatures during operation. They also knew that adding tungsten oxide makes vanadium oxide much more active, which is also important because the more active the catalyst is, the less you need it. But why did tungsten oxide have such an effect on the reactivity of vanadium oxide?
Three theories have dominated the years, Wachs says. One claims that tungsten oxide has an acidic character that enhances the chemical reaction. The second states that tungsten oxide somehow shares electrons with vanadium oxide, and the third states that tungsten oxide alters the structure of vanadium oxide.
Wachs and his associates used a cutting-edge tool called high-field magnetic resonance (HF) NMR in conjunction with reaction studies to test each theory.
"There are only a few of these HF NMR spectrometers in the world and their magnetic fields are so sensitive that they give all the fine molecular details of what's going on in the material," he says.
These molecular details appear as signals that Wachs and his team then interpret using theoretical calculations (functional theory of density).
"It turns out that the amount of vanadium oxide is very low in the catalyst, which makes vanadium oxide present as isolated species or monomers," Wachs says. "When you add tungsten oxide, vanadium oxide changes from monomers to oligomers or polymers, so now all of the vanadium oxide is linked as a chain or island of the titanium support. We have conducted independent studies and found that these vanadium oxide oligomers are 10 times more active than the isolated sites of vanadium oxide. Thus, tungsten oxide really changes the structure of vanadium oxide from a less active form to a highly active form. "
This fundamental understanding of how catalyst works will help guide future designs of enhanced SCR catalysts," says Wachs, who was recently selected as a Fellow of the National Academy of Inventors and recognized internationally for his innovative contributions to fundamental catalysis, have been applied in the production of chemicals and air pollution control.
"Now that we know what is going on, there will be no trial and error to improve it as we use a scientific approach to design and to the catalyst. "
And that will have huge implications for the industry and control of air pollution, he says.
" A more active catalyst has significant advantages. First, these systems are huge, almost in size. of a small house, and many of these plants were built before the term of this technology, so space in the plants is limited, so if you have a more active catalyst, you need a smaller footprint. They are also expensive, so if the catalyst is more active you don't need that much. Lastly, as we also think that they will last longer, this will limit the time that an entity needs to stop to install a new catalyst. "
But for Wachs, the public health effect is the most meaningful – and most beneficial – outcome.
" Easily, 40,000 to 50,000 people in the United States die annually due to complications from poor air quality. This is why catalysis and the studies around it have a huge social impact. It is very satisfying when you can solve a problem. this is about 40 years, which will improve technology and address these health problems. "
Identified: A molecular structure that breaks down an important component of smog
Nicholas R. Jaegers et al, Inner Back Cover: A mechanism by which tungsten oxide promotes the activity of supported V2 O5 / TiO2 catalysts to reduce NO X: Structural effects revealed by 51 V MAS NMR spectroscopy (Angew. Chem. Int. Ed. 36/2019), Angewandte Chemie International Edition (2019). Doi: 10.1002 / anie.201908846
Nicholas R. Jaegers et al. A mechanism by which tungsten oxide promotes the activity of supported V2 O5 / TiO2 catalysts to reduce NO X: structural effects revealed by 51 V MAS NMR spectroscopy, Angewandte Chemie International Edition (2019). Doi: 10.1002 / anie.201904503
Finally, the answer to the burning 40-year-old question (2019, October 24)
retrieved October 24, 2019
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