An unexpected new catalyst promoter: ‘inert’ nitrogen gas!

Molecular nitrogen (N₂) is widely used as a carrier or protective gas in many catalytic reactions because of its chemical inertness and large availability in nature. Up to now, N₂ has not been recognized as a promoter or an active component to enhance catalytic performance.

However, the textbook description of inert N₂ has been rewritten in a recent paper published in Nature Catalysis by Duan et al., reporting that N₂ can dramatically promote biomass hydrodeoxygenation (HDO) over ruthenium (Ru)-based catalysts [1]. As exemplified by the HDO of p-cresol to toluene, a representative model reaction for upgrading the lignin-rich biomass, the presence of N₂ led to a 4.3-fold increase in HDO activity over Ru clusters (with an average diameter of 1.2 nm) dispersed on titanium oxide (Ru/TiO₂) in a batch reactor (at 160°C, 1 bar H₂ and 6 bar N₂). Similar promoting effects of N₂ were also observed when applying other Ru catalysts in the HDO reaction, confirming the ability of N₂ to unprecedentedly act as a catalytic promoter.

Detailed studies [1], collaboratively carried out by the research groups of Jun Li from Tsinghua University and Edman Tsang and Dermot O’Hare from University of Oxford with complementary expertise in computational catalysis, surface catalysis and biomass conversion, have clarified the mechanism of N₂ promotion (Fig. 1). In situ X-ray absorption near edge structure and in situ Fourier-transform infrared spectroscopy show that hydrogenated nitrogen species (N_xH_y, x = 1, 2, y = 1, 2) form on the Ru surface under the aforementioned reaction conditions. Density functional theory calculations show that the activation of N₂ to NNH and subsequent N_xH_y species follows an associative reduction mechanism, which had been previously proposed in theoretical studies on other supported metal clusters under mild conditions [2,3]. These N_xH_y species offer protic hydrogens to assist the removal of –OH groups adsorbed on the Ru surface—a rate-determining step for the HDO of p-cresol that consequently lowers the HDO activation energy and enhances its activity (Fig. 1).

Figure 1.

Schematic illustration of the combination of N₂ activation and HDO reaction on Ru/TiO₂ (a revised scheme based on Fig. 5 in [1]). N₂ is converted into NNH and HNNH species on Ru surfaces, which provide protic hydrogen for converting p-cresol into toluene. Color code of the spheres: Ru (green), Ti (light gray), O (red), N (blue), H (white).

This pioneering work provides solid evidence that N₂ can be activated under less severe conditions via an associative reduction mechanism with the formation of NNH as the initial step. It demonstrates that N₂ can no longer be considered as an inert carrier gas, since it can act as a catalytic promoter in HDO reactions, which are of practical importance for upgrading biomass-derived oxygenated feedstocks [4–7]. Furthermore, the work provides a potentially general strategy for the rational tuning of catalyst performance under actual reaction conditions.