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

Methane (CH4) was converted to hydrogen cyanide (HCN) at temperatures from 300 to 425 & DEG;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 & DEG;C, whereas the yield was increased to 3.2% at 425 & DEG;C but rapidly decreased, resulting in 0.24% after 90 h at 425 & DEG;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 & DEG;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 & DEG;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.