Insights into Pt–CN species on an alumina-supported platinum catalyst as active intermediates or inhibitors for low-temperature hydrogen cyanide synthesis from methane and nitric oxide

Abstract

Methane (CH₄) was converted to hydrogen cyanide (HCN) at temperatures from 300 to 425 °C using a commercial 5 wt% Pt/Al₂O₃ catalyst with nitric oxide (NO) as an oxidant. HCN yield of ca. 1% was maintained after 100 h at 400 °C, whereas the yield was increased to 3.2% at 425 °C but rapidly decreased, resulting in 0.24% after 90 h at 425 °C. In situ X-ray absorption fine structure (XAFS) and Fourier-transform infrared (FTIR) measurements showed that Pt–CN species emerged and the extent of the adsorbed species roughly correlated with the production of HCN. However, the Pt–CN species continuously accumulated at higher temperatures than 400 °C regardless of the catalytic activity. The experiment of purging using helium gas followed by hydrogen after the reaction of CH₄ and NO at 425 °C showed that the Pt–CN species were strongly adsorbed on the Pt catalyst and removed from the catalyst by hydrogen treatment with simultaneous formation of HCN and NH₃. This study revealed that the Pt–CN species can function not only as important reaction intermediates, but also as inhibitors of the reaction. An appropriate balance of Pt–CN species and hydrogen species over the Pt surface is required to produce HCN continuously.