Gas-solid photocatalytic oxidation of dinitrogen to nitrogen oxides

Abdelkarim Zaim,^a Jonathan Z. Bloh^a

^a DECHEMA Research Institute, Frankfurt am Main, Germany

In recent years, there has been a notable increase in the demand for sustainable fertilizer production, driven primarily by rising energy costs and concerns about climate change. Additionally, as the global population continues to expand, there is a greater demand for agricultural commodities. While nitrogen is the most abundant element in the atmosphere, only a tiny fraction is fixed in the soil as bioavailable nitrogen, highlighting the necessity of nitrogen-based fertilizers in modern agriculture [1].

In the present era, nitric acid utilized in fertilizer manufacturing predominantly originates from the Ostwald process, wherein ammonia is oxidized. In this highly exothermic process, most of the energy contained in ammonia is converted to heat. The primary source of ammonia is derived from the Haber-Bosch process, which is responsible for approximately 1-2 percent of global CO₂ emissions and is regarded as one of the largest global energy consumers [2]. A more efficient solution to this problem is the direct oxidation of nitrogen to nitric acid, as this process does not require the enormous energy input associated with the formation of NH₃ (Δ_RH = 383 kJ mol^-1), but instead only necessitates the energy required for the formation of NO (Δ_RH = 91 kJ mol^-1) from the elements [3].

Photocatalysis represents a viable method for the direct generation of nitric acid from its elemental constituents. Under mild conditions, the process requires only light, water, and air. As a preliminary demonstration, the production of nitrates was shown in a gas phase reaction from water and air with the use of titanium dioxide (TiO₂) as a photocatalyst. It was determined that water is an essential component of the reaction, and the primary product is NO₂; this can be converted to nitric acid in water [4].

Although the mechanism of this reaction remains to be fully elucidated, we present an initial investigation of the influences of the reaction conditions on the kinetics and product composition. For example, the effects of the reactant ratio, temperature, and light intensity are considered, and all products of the reaction are analyzed both qualitatively and quantitatively. This contributes to a more comprehensive understanding of the mechanism and kinetics of the reaction.

In conclusion, a methodology for enhancing the photocatalytic oxidation of nitrogen will be presented, with the objective of achieving higher product concentrations and selectivities. This will contribute to a comprehensive understanding of the photocatalytic synthesis of nitric acid from dinitrogen.