Low-Temperature Initiation Enhances HCN and NH3 Formation From Methane and NO Over Pt/Al2O3.

The low-temperature activation of methane remains challenging due to its high C─H bond dissociation energy, yet it offers an attractive route for hydrogen cyanide (HCN) synthesis under milder conditions. We investigated HCN and NH3 formation from CH4 and NO over a commercially available Pt/Al2O3 catalyst, focusing on the influence of NO concentration, reaction temperature, and reaction-start conditions. Increasing the NO concentration up to 4.5% significantly enhanced HCN production, achieving a maximum carbon-based yield of 3.5%. Elevating the temperature further improved activity, with the highest HCN yield of 4.8% obtained at 475°C. Comparison between ramping and constant-temperature conditions demonstrated that the initial reaction temperature strongly influenced product distribution, with ramping from 300°C enabling substantially higher CH4 conversion and HCN formation. In situ Fourier Transform Infrared (FTIR) spectroscopy identified Pt-CO, Pt-CN, Pt-NCO, and Al-NCO species, among which Pt-CO showed the strongest correlation with HCN production, whereas strongly adsorbed Pt-CN behaved predominantly as an inhibiting spectator species. Temperature-swing operation effectively promoted desorption of such inhibiting species, thereby enhancing HCN and NH3 yield, a trend supported by in situ dispersive x-ray absorption fine structure (DXAFS) analysis. These findings highlight the critical role of dynamically controlling surface intermediates in improving low-temperature HCN synthesis.