Novel reaction could spark alternate approach to ammonia production

May 2, 2018 by Morgan Mccorkle, Oak Ridge National Laboratory
Aberration-corrected scanning transmission electron microscopy (STEM) images of CNSs. (A) The pristine nanospikes exhibit layers of folded graphene with some structural disorder due to nitrogen incorporation in the basal plane. (B) O-etched CNS retains the layered graphene structure but exhibits a much larger radius at the tip, thereby lowering the local electric field present at the tips. Credit: Science Advances (2018). DOI: 10.1126/sciadv.1700336

The search for a more energy efficient and environmentally friendly method of ammonia production for fertilizer has led to the discovery of a new type of catalytic reaction.

Researchers at the Department of Energy's Oak Ridge National Laboratory used nanoscale spikes of carbon to catalyze a reaction that generates ammonia from nitrogen and water, aided by lithium salt and the application of an electric . The study, published in Science Advances, reveals a type of that has been theoretically suggested but never demonstrated.

"It's a catalyst that operates completely based on the electric field; this has never been observed for nitrogen," said ORNL's Adam Rondinone, the study's lead author. "We call it a physical catalyst—normally a catalyst is chemical."

Ammonia, a compound made of one and three hydrogen atoms, is typically produced through the energy-intensive Haber-Bosch approach. This process uses high temperature and pressure to split the stable bonds of , requiring large amounts of natural gas. Industrial production of ammonia consumes an estimated 3 percent of the world's energy and generates 3 to 5 percent of the world's greenhouse gas emissions.

"Ammonia production is a huge problem that we need to find ways to address," Rondinone said. "In pursuit of this goal, we've discovered a reaction mechanism that gives us a new pathway."

Oak Ridge National Laboratory researchers have discovered a new type of catalytic reaction that could lead to more energy efficient and environmentally friendly method of ammonia production for fertilizer. Credit: ORNL

Unlike Haber-Bosch, the team's process occurs at room temperature in a solution of water, dissolved and lithium perchlorate salt, with the help of a unique catalyst in the form of nanoscale carbon spikes. These spikes, only 50-80 nanometers in length and one nanometer wide at the tip, act as hot spots to amplify the electric field and attract positively charged lithium ions. The lithium is hypothesized to drag along nitrogen molecules, which concentrate around the electrified carbon spikes and begin reacting to form ammonia.

"Every normal catalyst operates by forming a chemical bond between the reactive molecule and the catalyst surface. In this case, there's no chemical bond necessary. It's simply the high electric field that allows the reaction to proceed," Rondinone said.

The reaction's low yield - about 12 percent - limits its viability for industrial use, but the discovery of its unique electrochemistry may help develop alternative approaches to generation.

The researchers also used computational modeling and simulation to understand their experimental results. They calculated theoretical predictions of the electric field, the enrichment of ions around the carbon spikes and the molecular orbital energies of nitrogen to describe how the molecules destabilized in the electric field.

"Due to the sharp nanospike tips, the local is indeed very strong, on the order of 10 volts per nanometer," said ORNL theoretician Jingsong Huang. "We performed calculations to study the ionization potential and electron affinity of nitrogen under applied electric fields, and those calculations suggest that otherwise inert becomes reactive."

The study is published as "A Physical Catalyst for the Electrolysis of Nitrogen to Ammonia."

Coauthors are ORNL's Yang Song, Daniel Johnson, Rui Peng, Dale Hensley, Peter Bonnesen, Liangbo Liang, Jingsong Huang, Arthur Baddorf, Timothy Tschaplinski, Nancy Engle, Zili Wu, David Cullen, Harry Meyer III, Bobby Sumpter and Adam Rondinone, Virginia Tech's Fengchang Yang, Fei Zhang and Rui Qiao, and Georgia Tech's Marta Hatzell.

Explore further: Ammonia on-demand? Alternative production method for a sustainable future

More information: Yang Song et al. A physical catalyst for the electrolysis of nitrogen to ammonia, Science Advances (2018). DOI: 10.1126/sciadv.1700336

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