2025 AIChE Annual Meeting

Rapid Measurement of Interfacial pH By ATR–Seiras for Nitrate Reduction

Manufacturers produce over a million tons of ammonia each year for nitrogen-based fertilizers, which creates two environmental issues. First, the Haber-Bosch process—the industrial method of producing ammonia—generates carbon dioxide (CO2) at rates four times higher than ammonia production, thus polluting the air and massively contributing to climate change. In addition, around 25% of the nitrates within the fertilizer leach into groundwater, contaminating it and causing various health issues from human consumption as well as decimating aquatic life. However, direct electrochemical reduction of nitrate to ammonia is an environmentally sustainable way to mitigate these environmental issues. Also, electrochemistry presents an attractive opportunity to use green energy to power chemical reactions.

While the validity of this reaction has been proven to be effective through electrochemical means, the efficiency and selectivity still need to be improved to make it a cost-effective solution for the rural communities most affected by groundwater pollution. Specifically, a slightly acidic electrolyte solution has improved the selectivity of nitrate reduction to ammonia. While earlier studies had focused on the bulk pH role on selectivity, very few studies have tried to elucidate the role of interfacial pH on selectivity. Previous work done by our group has developed a protocol to measure the interfacial pH (10 nm from the electrode surface) by performing ATR–SEIRAS and reported the drastic swings in pH that were particularly dramatic in the first minute.

Our FTIR with rapid scanning capabilities allows us to track these dynamic and swift changes in interfacial pH. We performed calibration between 1 to 13 pH using a phosphate buffer system, recording a spectrum for every 0.2 pH increment. The ratio of the peak areas from the calibration data at known pH values is then used to identify the in situ pH under reaction conditions. The pH was measured before and after each in situ experiment at 0.1 V vs. the reversible hydrogen electrode (VRHE) to ensure reproducibility. These insights into interfacial pH dynamics have paved the way for future work to improve the selectivity of electrochemical nitrate reduction. This will advance the development of cleaner, more efficient ammonia production methods to benefit both the environment and vulnerable communities.