Hydrogen cyanide synthesis on polycrystalline platinum and 90:10 platinum[ndash ]rhodium surfaces

The synthesis of HCN from methane-based feedstocks has been studied at relatively low temperatures (up to 780°C) and pressures (0.02–0.15 Torr) over both pure Pt and Pt–10%Rh alloy model catalysts. Steady-state synthesis of HCN is observed over Pt from CH4–NH3–O2 feed mixtures with a maximum in activity observed using a 1:1:0.5 mixing ratio. The activity of the Pt–10%Rh alloy under comparable conditions is significantly lower, indicating that Rh has an inhibiting effect on the synthesis reaction at these temperatures and pressures. Comparable steady-state levels of activity were not observed over Pt for pure CH4–NH3, CH4–NH3–CO2 or CH4–NO feed mixtures; in particular, for CH4–NH3 feeds in the absence of oxygen a high initial activity was seen to decay rapidly over time. The effects of various surface pretreatments of the Pt were found to be transitory, with a gradual return of the HCN activity to its normal level in CH4–NH3–O2 (1:1:0.5) mixtures in all cases, and the most beneficial pretreatment effect was observed for a simple physical surface roughening induced by mild ion bombardment. By contrast, pre-exposure of the catalyst surface to ethylene or ammonia at elevated temperatures led to an initial inhibition or decrease in the synthesis activity.