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Scientists have long sought to understand how nitrogen atoms (N) are incorporated into carbon materials—such as graphene, graphite, and nanodiamonds—because the modes of N-doping strongly influence their catalytic and electronic properties. Developing a reliable method for both qualitative and quantitative characterization of nitrogen bonding states is essential for improving materials used in batteries, fuel cells, and sensors.

However, common probing techniques such as X-ray photoelectron spectroscopy (XPS) and CHN elemental analysis provide only surface-level or bulk composition data at limited sensitivity. They often fail to distinguish between different nitrogen environments—pyrrolic, pyridinic, and graphitic—especially when the N content is at trace levels below 1,000 ppm.

The lack of accurate, high-resolution methods for nitrogen speciation has hindered the rational design of N-doped materials and the resolution of long-standing industrial problems related to trace nitrogen in carbon.

In a 2024 article, Takeharu Yoshii, Hirotomo Nishihara, and co-workers from the Advanced Institute for Materials Research (AIMR) at Tohoku University addressed this challenge with a novel analytical approach¹. Using vacuum temperature-programmed desorption (TPD) capable of heating carbon samples to 2,100 °C, the team achieved complete desorption of N species from designed carbon structures and measured the evolved gases—NH₃, HCN, and N₂—with unprecedented sensitivity.

Unlike conventional surface-sensitive methods, this destructive approach enables full quantitative analysis of nitrogen within the bulk.

“Our key innovation is combining an ultra-high-temperature vacuum TPD system with mass spectrometric (MS) detection and a purpose-designed N-doped carbon material (N-CMS) composed of rarely stacked, curved graphene sheets,” explains Yoshii. “This integration enabled complete desorption of nitrogen species and a direct correlation between the evolved gases and specific nitrogen environments.”

Using complementary density-functional theory modeling to analyze the MS results, the team showed that in N-CMS, pyrrolic-N releases NH₃ and HCN, pyridinic-N produces HCN and N₂, and graphitic-N yields only N₂ at higher temperatures.

The technique also provided precise qualitative and quantitative determination of nitrogen in diverse carbon materials, including industrial coke, where XPS failed to detect buried nitrogen atoms.

“When we applied the technique to nitrogen-containing coke, we detected only N₂ desorption above 1,200 degrees Celsius,” says Yoshii. “This indicated that nearly all the nitrogen existed in stable graphitic environments, which conventional XPS could not capture because much of it was buried deep within the carbon structure.”

By achieving two orders of magnitude higher sensitivity than previous methods, this work establishes ultra-high-temperature TPD as a transformative and complementary tool for understanding and controlling nitrogen chemistry in carbon materials, with broad implications for catalysis, energy storage, and materials manufacturing.

A personal insight from Takeharu Yoshii

What part of this project gave you the greatest sense of accomplishment, and why?

For me, the biggest sense of accomplishment came when we finally proved that nitrogen dopants in carbon materials can be identified using a destructive analytical method. Reaching 2,100 °C to achieve complete desorption and quantification of all nitrogen species was something no one had done before. Getting there wasn’t easy—we had to design hardware that could survive ultra-high temperatures, calibrate toxic HCN gas, and separate CO and N₂ signals that share the same mass. After years of trial and error, seeing clear evidence of each nitrogen bonding state was an incredibly satisfying moment.

(Author: Patrick Han)

This research highlight has been approved by the authors of the original article and all information and data contained within has been provided by said authors.