Particle Size Effect on Hydrogen Cyanide Synthesis with CH4 and NO over an Alumina-supported Platinum Catalyst

The effect of particle size of Pt nanoparticles on the conversion of CH₄ into hydrogen cyanide (HCN) using NO as an oxidant over the Pt supported (γ + θ)-Al₂O₃ was studied. The Pt catalysts with various average particle size in the range of 1.6 to 4.1 nm were obtained by controlling the loading amount and calcination temperature. The NO conversion in the catalysts having small particle size (1.6-3.2 nm) of Pt was lower than the larger one. The catalysts having large particle size (4.1-4.2 nm) showed high selectivity to HCN reaching 53.5% at 1.3% C-based yield at 400°C over 10 wt.% Pt/Al₂O₃. The higher activity with larger Pt particles is attributed to the suppression of the sequential reaction of HCN to CO₂ and NH₃ which likely proceeded at the interface between metal and support. Pt L₃-edge XAFS spectra showed that the small particle catalysts were covered with Pt-CN species after the reaction test. Meanwhile, Pt-CO was observed as main adsorbed species on the large particle catalysts, suggesting that HCN desorption process was facile for the larger Pt particle case.